Expanding the Cancer Arsenal with Targeted Therapies: Disarmament

Perspective pubs.acs.org/jmc

Expanding the Cancer Arsenal with Targeted Therapies: Disarmament of the Antiapoptotic Bcl‑2 Proteins by Small Molecules Miniperspective Jeremy L. Yap,† Lijia Chen,† Maryanna E. Lanning,† and Steven Fletcher*,†,‡ †

Department of Pharmaceutical Sciences, University of Maryland School of Pharmacy, 20 N. Pine Street, Baltimore, Maryland 21201, United States ‡ University of Maryland Greenebaum Cancer Center, Baltimore, Maryland 21201, United States ABSTRACT: A hallmark of cancer is the evasion of apoptosis, which is often associated with the upregulation of the antiapoptotic members of the Bcl-2 family of proteins. The prosurvival function of the antiapoptotic Bcl-2 proteins is manifested by capturing and neutralizing the proapoptotic Bcl-2 proteins via their BH3 death domains. Accordingly, strategies to antagonize the antiapoptotic Bcl-2 proteins have largely focused on the development of low-molecular-weight, synthetic BH3 mimetics (“magic bullets”) to disrupt the protein−protein interactions between anti- and proapoptotic Bcl-2 proteins. In this way, apoptosis has been reactivated in malignant cells. Moreover, several such Bcl-2 family inhibitors are presently being evaluated for a range of cancers in clinical trials and show great promise as new additions to the cancer armamentarium. Indeed, the selective Bcl-2 inhibitor venetoclax (Venclexta) recently received FDA approval for the treatment of a specific subset of patients with chronic lymphocytic leukemia. This review focuses on the major developments in the field of Bcl-2 inhibitors over the past decade, with particular emphasis on binding modes and, thus, the origins of selectivity for specific Bcl-2 family members.

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amphipathic α-helix. Four hydrophobic residues on one face of this helix (Val74, Leu78, Ile81, and Ile85) project their side chains into four subpockets (p1−p4) in the hydrophobic groove of Bcl-xL, making contact with multiple hydrophobic residues. Alanine scanning mutagenesis of the Bak peptide confirmed the importance of the aforementioned residues to complex formation. Additionally, charged residues (Arg76, Asp83, and Asp84) on the opposing face also contribute to the stability of the complex, with the most significant being Asp83, consistent with its close proximity to Arg139 of Bcl-xL. Several other heterodimeric Bcl-2 complexes have been solved by NMR and X-ray crystallography,5−8 revealing that the BH3 α-helices of the proapoptotic proteins share a conserved pattern of hydrophobic residues at the i, i + 3/4, i + 7, and i + 11 positions along one face, as well as an aspartate residue at i + 5 on the opposing face, all of which are crucial for the stability of the heterodimeric complexes with antiapoptotic proteins. As typified by the Bcl-xL−Bim-BH3 complex shown in Figure 1, these hydrophobic residues bind the p1, p2, p3, and p4 subpockets of the antiapoptotic protein, while the aspartate binds a conserved arginine located on the other side of the groove. Significantly, the antiapoptotic Bcl-2 proteins exhibit differing binding profiles for their proapoptotic counterparts, which is governed by the precise topologies of the hydrophobic

INTRODUCTION Apoptosis, or programmed cell death, is manifested through two distinct pathways.1 The extrinsic apoptosis pathway is initiated by extracellular ligands that bind cell surface receptors of the tumor necrosis factor receptors (TNFR) superfamily.2 On the other hand, the intrinsic apoptosis pathway occurs entirely within the cell and is regulated by the Bcl-2 family of proteins, which comprises prosurvival and prodeath biomolecules.3 The Bcl-2 proteins all share one or more of the four Bcl2 homology (BH) domains BH1, BH2, BH3 and BH4. The antiapoptotic proteins Bcl-2, Bcl-xL, Bcl-W, Mcl-1, and A1/Bfl-1 each possess BH domains 1 through 4, and promote cell survival through antagonizing the prodeath function of the proapoptotic proteins, which can be subdivided into the BH3only and BH1-3 domain family members. The BH3-only members include Bim, Bad, Bid, Puma, and Noxa, among others, while Bak and Bax are members of the latter subfamily, also dubbed the proapoptotic effectors. The antiapoptotic proteins sequester and neutralize the proapoptotic proteins through seizing their BH3 α-helical “death domains” within their hydrophobic, BH3-binding pockets. The molecular basis for the heterodimerization between antiand proapoptotic Bcl-2 proteins was first elucidated by NMR analysis of a complex between Bcl-xL and a peptide corresponding to the BH3 region of Bak.4 The Bak peptide binds in a hydrophobic groove on the surface of Bcl-xL formed by the BH1−3 domains and concomitantly folds into an © 2016 American Chemical Society

Received: December 6, 2015 Published: October 17, 2016 821

DOI: 10.1021/acs.jmedchem.5b01888 J. Med. Chem. 2017, 60, 821−838

Journal of Medicinal Chemistry

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Figure 1. A crystal structure of the Bcl-xL−Bim-BH3 complex (PDB code 4QVF) with key subpockets and amino acids highlighted. Labels in bold refer to Bcl-xL, and labels in italics refer to the Bim-BH3 α-helix. Left: Connolly surface, colored by atom type (gray = carbon (Bcl-xL); green = carbon (Bim); red = oxygen; blue = nitrogen; yellow = sulfur). Right: cartoon, unsurfaced representation of left structure.

grooves.9 For example, all antiapoptotic proteins can bind Bax, but only Bcl-xL and Mcl-1 can bind Bak. Similarly, there is discrimination among the BH3 proteins: Bim, Bid, and Puma can bind all of the antiapoptotic proteins, while Bad binds only Bcl-2, Bcl-xL, and Bcl-W, and Noxa binds only Mcl-1 and A1. Moreover, some BH3-only proteins, including Bim and Bid, can directly activate Bak and Bax. The release of Bak and Bax allows their oligomerization, which results in the permeabilization of the mitochondrial outer membrane, and a demolition cascade ensues: first, cytochrome c is released, which binds and activates APAF1 leading to apoptosome formation, which activates caspase-9, triggering caspase activation and then cell destruction.10,11 A hallmark of cancer is the evasion of apoptosis, which is attributed mostly to a breakdown in the mitochondrial apoptotic machinery and is often a contributing factor in the resistance to existing chemotherapies.12,13 Particularly, through the neutralization of Bak and Bax, overexpression of the antiapoptotic Bcl-2 proteins is associated with tumor development and progression in a wide range of human cancers, including pancreatic cancer and acute myeloid leukemia.14,15 Accordingly, small molecules, synthetic α-helix mimetics, and stapled peptides capable of mimicking the BH3 domain of proapoptotic Bcl-2 proteins have been developed to antagonize the antiapoptotic proteins, releasing Bak and Bax and thus inducing apoptosis. In this review, we focus on synthetic, nonpeptidic inhibitors of the antiapoptotic Bcl-2 proteins, describing binding modes, selectivities, and in vivo activities for potential cancer therapy. We have selected to present this research in a mostly chronological fashion, as some discoveries were directly built on earlier findings.

Figure 2. Chemical structures of the dual Bcl-xL/Bcl-2 inhibitors 1 and 2.

ring binds in the p4 pocket while the 4-chlorobiphenyl binds in, and considerably opens up (relative to the binding of BH3 ligands), the p2 pocket (Figure 3, PDB code 2YXJ). Although the acylsulfonamide moiety was designed to function as a bioisostere of the side chain of the conserved aspartic acid of the BH3 peptides in addition to serving as the linking point of two fragments, the cocrystal structure in Figure 3 revealed that the acidic acylsulfonamide engages in a hydrogen bond interaction with the backbone amide of Gly138 instead of a salt bridge with Arg139 (Figure 2).17 1 potently inhibits Bcl-2, Bcl-xL, and Bcl-w (all with Ki values of 20 μM). The observed selectivity profile is attributed to the differing plasticities of the p2 pockets on the surfaces of Bcl-2/Bcl-xL/Bcl-w and Mcl-1, as well as to a different protein backbone fold formed by helix α-3 on the side of the p2 pocket of Mcl-1.17 ABT-263 (2 (Figure 2)), or navitoclax, is an orally bioavailable analogue of 1 that has yielded promising phase I/II clinical trial data for various cancers,18 including chronic lymphocytic leukemia (CLL) and small-cell lung cancer.19,20 Like 1, 2 cannot engage Mcl-1; thus,

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DUAL BCL-XL/BCL-2 INHIBITORS: ABT-737 & ABT-263 (NAVITOCLAX) ABT-737 (1, Figure 2), developed by Abbott Laboratories in 2005, is the prototypic BH3 mimetic, which was discovered through the chemical linking of fragments identified to bind Bcl-xL through an NMR-based screen followed by structurebased design.16 Supported by NMR solution structure data, a cocrystal structure of 1 bound to Bcl-xL revealed that the phenyl 822



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Figure 3. A cocrystal structure of 1 (colored by atom type: green = carbon; blue = nitrogen; red = oxygen; yellow = sulfur; brown = chlorine) bound to Bcl-xL (PDB code 2YXJ). Left: Connolly surface, colored by atom type (gray = carbon (Bcl-xL); green = carbon (1); red = oxygen; blue = nitrogen; yellow = sulfur). Right: cartoon, unsurfaced representation of left structure.

Figure 4. Gossypol and derivatives are pan-Bcl-2 inhibitors.

antimalarial.23,24 It was not until this century that it was discovered gossypol also functions as a pan-BH3 mimetic,25 with the synthetic (R)-(−) atropisomer AT101 (4) inhibiting Bcl-2, Bcl-xL, and Mcl-1 with Ki values of 320, 480, and 180 nM, respectively.26 Inspired by gossypol, Wang and colleagues developed TW-37 (5), using structure-based design, which saw a simplification of the molecule, from a synthetic chemistry standpoint, and the removal of the toxic aldehyde groups.26,27 Compound 5 likewise is a pan-BH3 mimetic, with Ki values for Bcl-2, Bcl-xL, and Mcl-1 of 290 nM, 1.11 μM, and 260 nM, and has reached phase I/II clinical trials. Two of the hydroxyls of the pyrogallol motif of 5 were predicted to capture hydrogen bonds with Asn143 and Arg146 analogous to Gln102 and Asp99 of the Bim-BH3 peptide.26 Apogossypolone (ApoG2, 6),

antitumor effects are observed only when Mcl-1 is absent or weakly expressed.21 Unfortunately, the excitement surrounding 2 was dampened somewhat owing to a dose-limiting, transient thrombocytopenia in patients, which occurred as a consequence of the inhibition of Bcl-xL in aging platelets that resulted in their death.22 Despite this on-target toxicity, 2, as an adjuvant therapy, continues to play a prominent role in clinical trials where the thrombocytopenia is mitigated.

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PAN-BCL-2 INHIBITORS: GOSSYPOL AND DERIVATIVES Gossypol (3 (Figure 4)) is a natural polyphenolic aldehyde that has been used in the clinic for a variety of medicinal purposes, including as an oral male contraceptive as well as an 823



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another derivative of gossypol that also lacks the two aldehydes, induces apoptosis in primary CLL cells.28 Sabutoclax (BI-97C1, 7), an apogossypol derivative, appears to bind the Bcl-2 proteins with higher affinities than the other reported gossypol derivatives.29 However, all gossypol derivatives described here exhibit several off-target effects, which may be attributable to their catechol moieties that can lead to the generation of reactive oxygen species30,31 and as such are not authentic BH3 mimetics. (−)-BI-97D6 (8), the (−) atropisomer of a derivative of 6 where the two isopropyl groups are replaced with benzyls, potently disrupts the binding of Bcl-xL, Bcl-2, Mcl-1, and A-1 to a fluorescently labeled Bak-BH3 peptide with IC50 values of 76, 31, 25, and 122 nM, respectively.32 Compound 8 inhibits cell growth in PC-3 human prostate and H23 human lung cancer cell lines in the submicromolar range and also displays limited cytotoxicity to Bax/Bak-deficient cells indicating the gossypol derivative kills cells chiefly dependent on the Bax/Bak pathway. Furthermore, 8 demonstrates in vivo efficacy in a prostate cancer xenograft and Bcl-2 transgenic mouse models.

Figure 6. Pan-Bcl-2 inhibitors 10 and 11 based on an 8-oxo-8Hacenaphtho[1,2-b]pyrrole framework.

which in turn displaces Bak from Mcl-1−Bak complexes, and so it appears that this may be the actual mechanism by which 10 kills cells dependent on Mcl-1.36 As is likely the case with many BH3 mimetics, since Beclin-1 also binds in the BH3-binding groove, 10 disrupts the interaction between Bcl-2 and Beclin1,39 leading to autophagy in addition to intrinsic apoptosis. Second generation derivatives of 10 resulted in more potent pan-BH3 mimetics, for example, 11, which inhibits Bcl-2, BclxL, and Mcl-1 with IC50 values of 20, 18, and 10 nM, respectively.40,41 The carbonyls of 10 and 11 were predicted to bind Arg146 of Bcl-2 and Arg263 of Mcl-1, with the thiomorpholine and thioaryl groups projected into the p2 pockets. Pleasingly, 11 demonstrated improved proapoptotic activity in cancer cell lines.

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PAN-BCL-2 INHIBITOR: OBATOCLAX Although GeminX Pharmaceutical’s obatoclax (GX15-070, 9 (Figure 5)) may be described as a pan-BH3 mimetic, it binds

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MCL-1-SELECTIVE INHIBITOR: MARINOPYRROLE A

Marinopyrrole A (12 (Figure 7),42 or “maritoclax”, was discovered as an Mcl-1-selective inhibitor (IC50 = 10.1 μM) through screening a small compound library of 12 and 23 analogues.43 By interrogating their PPIs with a biotinylated Bim peptide, an ELISA assay indicated that 12 exhibits >8-fold selectivity for Mcl-1 over Bcl-xL. Modeling data suggested that the p4 pocket on Mcl-1 was the likely binding site, although 2D HSQC 1H−15N NMR data agreed only somewhat with this finding. Thus, the origin of Bcl-2 family selectivity remains unknown at this time. In addition to antagonizing Mcl-1’s PPIs, 12 targets Mcl-1 for proteasomal degradation, which is associated with its proapoptotic activity. Although the authors verified the Mcl-1-specificity of 12 in cells through the selective killing of Mcl-1-dependent, but not Bcl-2- or Bcl-xL-dependent leukemia/lymphoma cell lines, this has been disputed.44 12 was shown to synergize with the dual Bcl-2/Bcl-xL inhibitor 1, sensitizing 1-resistant cancer cells to the prototypic BH3 mimetic.43 Acute myeloid leukemia (AML) cells with high levels of Mcl-1 were also killed by 12, once again through Mcl-1 proteasomal degradation.45 Furthermore, 12 demonstrated in vivo efficacy with significant shrinkage of a U937 tumor upon intraperitoneal administration with no observed toxicity. Several studies have identified more potent and selective Mcl1 marinopyrrole derivatives, as well as dual Bcl-xL/Mcl-1 agents.46 Finally, a recent report has disclosed that 12 is capable of inducing cell death by apoptosis in cells that lack Mcl-1,47 echoing concerns44 about earlier claims that maritoclax is an Mcl-1-specific inhibitor.

Figure 5. A pan-Bcl-2 inhibitor.

the Bcl-2 proteins in only the low micromolar range, which is similar to the affinity of 1 for Mcl-1.33 9 kills wild-type cells just as efficiently as Bax/Bak-deficient cells and thus operates through a combination of on- and off-target effects.31,34 Indeed, it has been suggested that 9 causes cell death by inducing mitochondrial swelling and associated damage rather than specifically targeting the Bcl-2 proteins.31 In addition to functioning as a BH3 mimetic, 9 can directly activate Bax in cholangiocarcinoma cells,35 and is able to overcome the resistance of malignant cells to 1, which, as mentioned previously, cannot bind Mcl-1. Rather than this being due to 9 inhibiting Mcl-1, it has been claimed that 9 causes Noxa upregulation and subsequent Mcl-1−Bak dissociation.31,36 Nevertheless, despite these limitations that detract from true BH3 mimicry, 9 entered into multiple clinical trials for a variety of cancers.

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DUAL BCL-2/MCL-1 INHIBITORS: S1 AND DERIVATIVES Song and colleagues discovered that the small-molecule S1 (10 (Figure 6)) from a screening effort and showed that it binds in the nanomolar range to both Bcl-2 and Mcl-1 (Kd = 310 nM (Bcl-2), 58 nM (Mcl-1)), disrupting their PPIs with Bax and Bak, respectively, causes Bax/Bak-dependent apoptosis, and demonstrates efficacy in an animal model.37,38 Later it was shown that 10 induces the BH3-only proteins Noxa and Bim, 824



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Figure 7. Some Mcl-1 selective inhibitors.

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MCL-1-SELECTIVE INHIBITOR: PYROGALLOL DERIVATIVE MIM1 Walensky and colleagues developed a competitive highthroughput screening assay with a fluorescently labeled, Mcl1-derived “stabilized α-helix of Bcl-2 domain” (“Mcl-1 SAHB”) probe to identify small-molecule inhibitors of Mcl-1, which led to the discovery of MIM1 (Mcl-1 inhibitor molecule 1, 13 (Figure 7)), another pyrogallol derivative reminiscent of compound 5.48 Albeit a modest Mcl-1 inhibitor with an IC50 value of 4.8 μM, 13 was the first small-molecule to exhibit selectivity for Mcl-1 over Bcl-xL (IC50 > 50 μM). 1H−15N HSQC NMR studies with 15N-Mcl-1 confirmed 13 binds in the BH3-binding groove. A putative binding mode of 13 was then developed using computational modeling. The pyrogallol (benzene-1,2,3-triol) motif was predicted to engage in hydrogen bonding interactions with Asp256 and Arg263, the latter of which is bound by a conserved BH3 aspartate residue and is often targeted by carboxylic acids in a variety of Mcl-1 inhibitors. The cyclohexyl group was found in the p3 pocket where it was able to make hydrophobic contacts akin to Leu216 of the Mcl-1 SAHB, while the thiazolyl scaffold probed into the p2 pocket. The source of selectivity may be due to the specific occupation of the p2 pocket, akin to Fesik’s work (see below). 13 induces Bak-dependent apoptosis in Mcl-1-independent cells. In addition, cells devoid of the intrinsic apoptosis machinery are unaffected by 13, suggesting its cell activity is on-target. Given the modest activity of 13, it is envisaged that a structure−activity relationship (SAR) campaign is presently underway toward its optimization.

for novel Mcl-1 inhibitors, which resulted in the identification of 8-hydroxyquinoline 14 (Figure 7) with an IC50 of 2.4 μM.49 Elimination of the carboxylic acid, which often compromises cell penetration, and removal of the 4-chloro group, which promotes lipophilicity and lowers solubility, were tolerated. On the other hand, the quinoline nitrogen and the 8-hydroxyl proved indispensible. Further SAR culminated in the discovery of 15, which binds potently to Mcl-1 (IC50 = 0.31 μM) but not to Bcl-xL (IC50 > 40 μM). A computationally derived model proposed that the 8-hydroxyl binds Asn260 through a hydrogen bond, positioning the molecule such that the N-ethylpiperazine and para-CF3-phenyl groups are directed into the p2 and p4 pockets, respectively, although it remains unclear how 15 is able to discriminate Mcl-1 over Bcl-xL. Compound 15 selectively kills Mcl-1 dependent cancer cells via mitochondrial permeabilization, consistent with activation of the intrinsic apoptotic machinery.

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MCL-1-SELECTIVE INHIBITORS: 2-CARBOXY-3-SUBSTITUTED 6,5-FUSED HETEROCYCLES In pioneering work on the rational design of Mcl-1 selective inhibitors and reminiscent of the discovery of 1, Fesik’s group at Vanderbilt University chemically linked two fragments discovered through an NMR-based screen to achieve impressive increases in potency relative to each individual component alone.50 Compounds within one class contained 6,5-fused heterocycles carrying a carboxylic acid at the 2position, for example, 16 (Ki = 131 μM (Figure 7)), while the other class was composed of fragments based on hydrophobic aromatics tethered to polar functionality, often a carboxylic acid, such as 17 (Ki = 60 μM). The authors next performed molecular modeling that was guided by NOESY NMR, which indicated that the former class of fragments bound near Arg263, probably through a salt bridge with the carboxylic acid of the ligand, while the latter class bound in the hydrophobic p2

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MCL-1-SELECTIVE INHIBITORS: 8-HYDROXYQUINOLINES Eutropics Pharmaceuticals and The Scripps Research Institute together conducted a high throughput screen of the NIH Molecular Libraries and Small Molecule Repository (MLSMR) 825



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Figure 8. A cocrystal structure of compound 19 (colored by atom type) bound to Mcl-1 (PDB code 4HW2). Left: Connolly surface, colored by atom type (gray = carbon (Bcl-xL); green = carbon (19); red = oxygen; blue = nitrogen; yellow = sulfur). Right: cartoon, unsurfaced representation of left structure.

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BCL-2-SELECTIVE INHIBITOR: VENETOCLAX The discovery of the dual Bcl-2/Bcl-xL inhibitor 1 was a landmark in medicinal chemistry, but the associated induction of thrombocytopenia by its orally bioavailable analogue 2 prompted the development of a Bcl-2 selective inhibitor. Researchers at AbbVie used reverse engineering of 2 toward the possibility of imparting the small-molecule with selectivity for Bcl-2.52 Small-molecule 21 (Figure 9), a derivative of 2 lacking

pocket. Notably, when fragments from each class both contained carboxylic acids, the binding was mutually exclusive, indicating that the acids were binding the same residue, probably Arg263, and thus that linking two fragments from each class might enhance the binding affinity. Indeed, merging 16 and 17 delivered 18 with a dramatically improved Ki for Mcl-1 of 320 nM and also exhibits a 40-fold selectivity over BclxL. Through SAR studies, compound 19 was subsequently discovered with a Ki of 55 nM and >200-fold selectivity for Mcl-1 over Bcl-xL. Replacement of the benzothiophene with an indole was tolerated, which is noteworthy as the heterocyclic NH provides an opportunity for further structural elaboration. Compound 19 was cocrystallized with Mcl-1: the 4-chloro-3,5dimethylphenyl was found deep in the hydrophobic p2 pocket, as earlier suggested by the NMR experiments with the fragment hits, and the carboxylic acid was engaged in a salt bridge with Arg263 (PDB code 4HW2), as illustrated in Figure 8. Unlike the BH3 peptides, 19 does not bind residues in the p3 or p4 pockets, although the chlorine atom binds in a previously unoccupied pocket formed between A227 and M231, and so it is possible that the affinity of 19 may be enhanced further through building off the indole NH or the indole benzene ring to gain access to one or both of these pockets. It is particularly significant that the crystal structure of 19 bound to Mcl-1 reveals that the p2 pocket has opened up considerably to accommodate the hydrophobic phenyl ring. Although also present in Bcl-xL, the larger and deeper p2 pocket in Mcl-1 observed here may be a source of selectivity that can be exploited in future ligand design. Fesik and co-workers also identified a micromolar fragment hit based on a tricyclic indole 2-carboxylic acid during their original NMR-based screen, which was similarly optimized to yield 20 that binds to Mcl-1 with Ki = 3 nM, with an associated >1700-fold selectivity over Bcl-xL and 100-fold selectivity over Bcl-2.51 Once again, the source of selectivity has been attributed to the occupation of the lower p2 pocket in Mcl-1, which is absent in Bcl-xL and Bcl-2. In human chronic myelogenous leukemia K562 cells, 20 selectively inhibited the pulldown of Mcl-1 by a biotin labeled, Mcl-1-specific peptide, indicating this new class of Mcl-1 inhibitors is functional in cells.

Figure 9. Bcl-2-selective inhibitor 22 (and a precursor) recently approved by the FDA.

the thiophenyl group, cocrystallized with Bcl-2, and significantly, a tryptophan side chain (Trp30) from a second Bcl-2 protein was found in the p4 pocket near to where the smallmolecule bound. In addition to forming a π−π stacking interaction with the nitroaryl moiety akin to the thiophenyl in 2, this tryptophan’s indole nitrogen atom was engaged in a hydrogen bond with Asp103. Notably, the analogous residue in Bcl-xL is Glu96, which represents one of the few amino acid differences present in the BH3-binding grooves of Bcl-2 and Bcl-xL and constituted a breakthrough in the quest for Bcl-2selectivity. The continued fine-tuning of 2 led to the 826



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Figure 10. A cocrystal structure of 23 (colored by atom type) bound to Bcl-2 (PDB code 4MAN). Left: Connolly surface, colored by atom type (gray = carbon (Bcl-xL); green = carbon (23); red = oxygen; blue = nitrogen; yellow = sulfur). Middle: cartoon, unsurfaced representation of left structure.

Figure 11. Bcl-xL selective inhibitors based on 2-carboxy-substituted aza-heterocycles.

Figure 12. Cocrystal structure of 24 with Bcl-xL (PDB code 3ZLR). Left: Connolly surface, colored by atom type (gray = carbon (Bcl-xL); green = carbon (24); red = oxygen; blue = nitrogen; yellow = sulfur). Right: cartoon, unsurfaced representation of left structure.

development of ABT-199 (22), or venetoclax, which binds potently to Bcl-2 (Ki < 0.010 nM) but more weakly to Bcl-xL (Ki = 48 nM) and Bcl-w (Ki = 245 nM) and, as predicted, does not cause thrombocytopenia.52 22 exhibited no appreciable binding to Mcl-1 (Ki > 444 nM), which may again be due to the slightly different topologies of the p2 pockets of Bcl-2/Bcl-xL/

Bcl-w versus Mcl-1 in tandem with their different plasticities, as outlined earlier in the selectivity profile of 1.17 In addition to recapitulating the hydrogen bond observed between Trp30 and Asp103, the azaindole may also be able to engage in an additional hydrogen bond with Arg107 of Bcl-2, as hinted at by the cocrystal structure of an indole analogue 23 with Bcl-2 827



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shown in Figure 10. As observed with 1, the acylsulfonamide of 23 does not bind the conserved arginine Arg143 but instead binds Gly142. In preclinical studies, venetoclax killed cancer cells directly from patients with AML,53 chronic lymphocytic leukemia (CLL),52 and T-cell acute lymphoblastic leukemia (TALL).54 In fact, venetoclax is presently undergoing clinical trials for AML55 and was recently approved by the FDA under the trade name Venclexta for the treatment of patients with CLL who harbor a 17p chromosomal deletion.56

Figure 13. Mcl-1 selective inhibitors developed from optimized fragments of 10.

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BCL-XL-SELECTIVE INHIBITORS: WEHI-539 AND DERIVATIVES It is generally considered that the selective inhibition of Bcl-2 family members will mitigate toxic side effects of pan-Bcl-2 inhibitors. In this regard, the development of Bcl-xL-specific inhibitors continues to be an active area of research, despite the antiapoptotic protein’s involvement in the survival of mature platelets.22 WEHI-539 (24 (Figure 11)), a doubly substituted thiazole-4-carboxylic acid derivative, was the first highly selective antagonist of Bcl-xL (IC50 = 1.1 nM) boasting an impressive selectivity of >500-fold over Bcl-2 and >400-fold over Bcl-w, Mcl-1, and A1.57 These excellent selectivities have been attributed to the snug fitting of the benzothiazole moiety in the p2 pocket coupled with hydrogen bonding interactions to the main chain amides of Ser106 and Leu108 therein (Figure 12). It is noteworthy that the observed remodeling and deepening of the p2 pocket that results in displacement of Phe105 is distinct from that upon binding 1, likely responsible for the observed selectivities across the Bcl-2 family members. An additional difference is that the carboxylic acid of 24 captures the guanidinium of Arg139, whereas the analogous acidic acylsulfonamide of 1 does not.17 The specificity in vitro is mirrored in cells where 24 has no effect on Bcl-2-expressing cells and causes apoptosis only in Mcl-1-deficient cells via Bakmediated apoptosis.57 Finally, consistent with its affinity to BclxL, 24 killed isolated platelets. Subsequently, fragment-based NMR screening and structure-based design were enlisted towards further optimization, which resulted in the more potent and more druglike Bcl-xL-specific inhibitors A-1155463 (25) and its orally bioavailable version A-1331852 (26) with a picolinic acid core, both of which feature the p2-binding benzothiazole present in 24.58 Although 25 caused a transient thrombocytopenia in mice within 6 h of treatment, platelet numbers had recovered by 72 h. The small molecule also inhibited the growth of a mouse xenograft model of small cell lung cancer.

vitro FP data and indicating an on-target mechanism for cell death. It is noteworthy that the Michael accepting arylamide and acrylonitrile groups do not appear to compromise 27’s selectivity in cells, likely because the Michael addition products would carry an acidic hydrogen flanked by the carbonyl and nitrile functional groups that imparts reversibility. Optimization of the 2-hydroxynicotinonitrile family led to the discovery of 28 with an IC50 of 54 nM for Mcl-1 (ELISA assay).60 An orthogonal ITC assay demonstrated selectivity of 28 for Mcl-1 over Bcl-2, which was mirrored by the greater cell killing effect of the Mcl-1-dependent cell line K562 over the Bcl-2dependent cell line RS4,11.

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DUAL BCL-XL/MCL-1 INHIBITORS: HYBRIDIZATION STRATEGY Researchers at Takeda Pharmaceutical Company Ltd. used structure-guided analysis to merge a selective Mcl-1 inhibitor with a selective Bcl-xL inhibitor to generate hybrid compounds with dual Mcl-1/Bcl-xL inhibitory activities.61 Particularly, a pyrazolo[1,5-a]pyridine-based Mcl-1 inhibitor designed inhouse (Figure 14, colored blue), which is structurally similar

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MCL-1 SELECTIVE INHIBITORS: DERIVATIVES OF S1 Zhang and colleagues recently focused their attention on converting 10 into a more “druglike” and Mcl-1 selective inhibitor, which resulted in the discovery of two new families of small-molecule inhibitors based on cyanoacetamide and 2hydroxynicotinonitrile. An SAR campaign with the former family culminated in 27 (Figure 13) with a Kd for Mcl-1 of 160 nM (fluorescence polarization (FP)) and no affinity for Bcl-2.59 15 N−1H HSQC NMR spectroscopy with 15N-Mcl-1 indicated chemical shift perturbations mostly of side chains in the p2 and p4 pockets and that of Arg263, corroborating the FP data and validating the binding mode hypothesis. Compound 27 selectively killed the Mcl-1 dependent, Bcl-2-independent H23 cell line through inducing apoptosis, reflecting the in

Figure 14. A hybridization strategy resulted in potent dual inhibitors of Bcl-xL and Mcl-1.

to Mcl-1 inhibitors described by Fesik50 and AbbVie (see below), was combined with a fragment of the Bcl-xL inhibitor 2 (colored red) to furnish 29, which inhibits the Mcl-1−Bid and Bcl-xL−Bid PPIs with IC50 values of 610 nM and 4.4 nM, respectively. Cocrystal structures of 29 with Mcl-1 and Bcl-xL validated the authors’ hybridization strategy. As illustrated in Figure 15, while the naphthalene moiety of 29 is deeply nestled in the p2 pocket of Mcl-1, the aryl sulfonamide portion is directed into the solvent. Conversely, both the naphthalene and aryl sulfonamide moieties are found in the p2 and p4 pockets, 828



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Figure 15. X-ray cocrystal structures (Connolly surface) of 29 with Mcl-1 (left, PDB code 3WIY) and Bcl-xL (right, PDB code 3WIZ), colored by atom type (gray = carbon (Bcl-xL); green = carbon (29); red = oxygen; blue = nitrogen; yellow = sulfur).

Figure 16. Novel dual inhibitors of Bcl-xL and Bcl-2 inspired by 1.

inhibits Bcl-2 and Bcl-xL with excellent potencies (Ki < 1 nM) and exhibits >1000-fold selectivity over Mcl-1.65 Although no explanations were provided on the observed selectivity for Bcl2/Bcl-xL over Mcl-1, we surmise the selectivity stems from a combination of specific remodeling of the p2 pockets coupled with the lack of an appropriately positioned acidic group to engage Arg263 of Mcl-1. In addition, 32 demonstrated improved pharmacokinetic properties and solubility over predecessor 31, and completely inhibited the growth of small cell lung cancer xenograft models.

respectively, of Bcl-xL, which likely explains the greater binding affinity of 29 to Bcl-xL versus Mcl-1. In both cases, the carboxylic acid forms a salt bridge with the conserved arginines Arg263 (Mcl-1) and Arg139 (Bcl-xL). Further optimization resulted in compound 30, whose inhibition of Bcl-xL remained constant (IC50 = 3.7 nM), but that of Mcl-1 was improved almost 7-fold (IC50 = 88 nM). It is anticipated that this strategy will likewise afford other rationally designed dual inhibitors that overcome the resistance often encountered by the selective targeting of one protein and the concomitant upregulation of another.62,63

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MCL-1-SELECTIVE INHIBITORS: 3-SUBSTITUTED-N-(4-HYDROXYNAPHTHALEN-1-YL) ARYL SULFONAMIDES A new family of 3-substituted-N-(4-hydroxynaphthalen-1-yl) aryl sulfonamides was developed as potent and selective Mcl-1 inhibitors, following an FPCA-based high throughput screen by the Nikolovska-Coleska laboratory.66 The initial hit was UMI59 (33 (Figure 17)), with a Ki for Mcl-1 of 1.55 μM, and thus it is similar in potency to 13. The authors used computational modeling to develop a binding hypothesis of 33: the naphthalene ring was found near the p3 pocket and the thiophene moiety was projected into the p2 pocket, while the aryl hydroxyl group bound His224, and the carboxylic acid engaged in a network of hydrogen bonds to Asn260 and Arg263. 1H−15N HSQC NMR experiments indicated 33 binds in the hydrophobic groove on the surface of Mcl-1, lending

DUAL BCL-XL/BCL-2 INHIBITORS: NEXT GENERATION Wang and co-workers transformed 1 into a new family of compounds through the replacement of the N-2-(4chlorophenyl)benzyl moiety with an N-(3-pyrrole)phenyl moiety in which the pyrrole unit is tetrasubstituted.64 Another key distinction is that the acylsulfonamide functional group, originally intended as a bioisostere of a carboxylic acid function to bind Arg139 of Bcl-xL, was truncated to a sulfonamide. One of their most potent compounds, BM-957 (31 (Figure 16)), binds to Bcl-2 and Bcl-xL with subnanomolar affinity, akin to its parent compound, and also causes caspase-3-dependent apoptosis in H146 lung cancer cells, and tumor regression in vivo. Further structural optimization of 31 focusing on the pyrrole moiety led to the discovery of BM-1197 (32), which 829



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MCL-1-SELECTIVE INHIBITORS: 3,7,N-TRISUBSTITUTED INDOLE-2-CARBOXYLIC ACIDS AbbVie recently published several selective Mcl-1 inhibitors that are also based on indole-2-carboxylic acids, similar to Fesik’s work.68,69 However, AbbVie’s compounds are more elaborate with additional functionality attached to the indole nitrogen as well as to the benzene ring and, exhibiting picomolar binding to Mcl-1, have 100-fold higher binding affinities. The most potent compound, A-1210477 (36, Figure 17), binds Mcl-1 with Ki = 0.43 nM and exhibits excellent selectivities over Bcl-2, Bcl-xL, Blc-1, and BFL-1 (Ki > 0.66 μM), which is likely due to deep occupation of the remodeled p2 pocket by the naphthalene moiety, as suggested by the cocrystal structure of 37, a close analogue of 36, bound to Mcl1 (Figure 18). Akin to the analogous indoles from Fesik’s laboratory, the carboxylic acid of 37 is recognized by Arg263. Further, 36 disrupts the Mcl-1−Bim PPI in cells and induces apoptosis as a single agent in multiple myeloma and non-smallcell lung cancer (NSCLC) cells. Compound 36 also potently synergized with navitoclax in several tumor cell lines. Although 36 does not exhibit suitable pharmacokinetic properties for in vivo use, it is the first rationally designed BH3 mimetic to selectively target Mcl-1 in cells. Given the picomolar potency of 36 and impressive preclinical profile, it is expected that this tool compound will prove instrumental in the discovery of an inhibitor suitable for further evaluation in mice.69

Figure 17. Some Mcl-1 selective inhibitors with bis-aryl scaffolds.

support to the modeling data. Subsequent structure-based design transformed 33 into 34 with a Ki of 180 nM, representing almost an order of magnitude improvement over the initial hit, and boasts an excellent selectivity profile for Mcl1 over the other Bcl-2 proteins: for example, 9-fold over Bcl-w and 59-fold over Bcl-xL. As with many of the previously described inhibitors, no specific details to rationalize the observed selectivities were provided but it is likely due, at least in part, to the specific interactions in the respective p2 pockets. 34 demonstrated selective inhibition of cell proliferation in the Mcl-1-sensitive leukemia cell lines HL-60 and MV4-11 and selectively caused cell death through a Bak/Bax-dependent mechanism. Analogue UMI-77 (35) was characterized more fully and demonstrated efficacy in a BxPC-3 xenograft model of pancreatic cancer.67

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MCL-1 INHIBITORS: 2-(ARYLSULFONAMIDO) BENZOATES AND 2-HYDROXYBENZOATES (SALICYLATES) In parallel to the development of substituted indoles, AbbVie researchers utilized 1H−13C HSQC NMR to screen a library of 17 000 molecules for affinity to 13C-labeled Mcl-1.70 FPCA was employed as an orthogonal assay; corroboration of the NMR findings coupled with good ligand efficiencies and synthetic tractabilities led to the identification of two chemical families of Mcl-1 inhibitors suitable for further development: aryl sulfonamides and salicylates. Structure-based design of a hit aryl sulfonamide ultimately culminated in 38 (Figure 19), with

Figure 18. A cocrystal structure of 37 bound to Mcl-1 (kindly provided by Dr. Andrew Souers of AbbVie). Left: Connolly surface, colored by atom type (gray = carbon (Bcl-xL); green = carbon (37); red = oxygen; blue = nitrogen). Right: cartoon, unsurfaced representation of left structure. 830



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Figure 19. Mcl-1 inhibitors based on substituted 2-(arylsulfonamido) benzoates and salicylates that were developed from initial HSQC NMR hits.

Figure 20. Cocrystal structures (Connolly surface) of Mcl-1 bound to compounds 39 (left, PDB code 4OQ5) and 41 (right, PDB code, 4OQ6), colored by atom type (gray = carbon (Bcl-xL); green = carbon (39 or 41); red = oxygen; blue = nitrogen; yellow = sulfur).

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an IC50 for the Mcl-1−Bim PPI of 30 nM. A cocrystal structure of the analogue 39 with Mcl-1 (Figure 20, left) revealed that the carboxylic acid forms a salt bridge with Arg263, analogous to Asp67 of the Bim-BH3 peptide, the peripheral phenyl ring of the biphenyl ether reaches into the p1 pocket, and the naphthyl group binds in the p2 pocket far deeper than the corresponding Leu62 of Bim. Likewise, structure-based design, along with computational modeling, was utilized to optimize their lead salicylate, arriving at compound 40 with an IC50 of 0.57 μM, representing a 280fold enhancement in potency.70 A cocrystal structure of the analogue 41 substantiated the predicted binding mode, which involves a salt bridge with Arg263, while the 5-phenyl ring and its n-propyl appendage are located in the p2 and p1 pockets, respectively (Figure 20, right). The Bcl-2 family selectivity for these two families of inhibitors is unknown at this time, since data were only presented for the inhibition of Mcl-1. It will be interesting to ascertain if the ability of the naphthyl moiety of 39 to delve deeply into the p2 pocket of Mcl-1 (and possibly also that of salicylate 40) contributes to protein selectivity as appears to be the case with large, hydrophobic groups, such as the naphthyl moiety in AbbVie’s 3668 and the 4-chloro-3,5dimethylphenyl moiety in Fesik’s work.50 Finally, cell data are yet to be reported, so it remains unclear if these compounds exhibit (on-target) cell activity.

BH3 SYNTHETIC α-HELIX MIMICRY

Many BH3 mimetics have been developed through the generation of synthetic ligands that are complementary to the hydrophobic grooves on the surfaces of the Bcl-2 proteins and can probe into the subpockets. An alternative strategy involves the direct mimicry of the BH3 α-helix itself with non-peptidic small-molecules. These aptly named α-helix mimetics span a range of chemical scaffolds whose purpose is to ensure the correct angular and spatial orientation of their side chains to mimic the i, i + 3/4, and i + 7 (and sometimes i + 11) side chains that are projected from one face of the BH3 α-helix.71−73 In addition to the two original terphenyl-74 and trispicolinamide-based75 α-helix mimetics, terephthalamides,76 phenylpiperazinetriazines,77 and a variety of other scaffolds have all been suitably decorated to mimic one face of the BH3 α-helix, and these have been reviewed more thoroughly elsewhere.71−73 The pioneering terphenyls remain among the most potent BH3 α-helix mimetics with Ki values affinities as low as 114 nM for Bcl-xL.74 Although many helix mimetics are particularly lipophilic, it is possible to achieve selectivity for Bcl-xL over Mcl-1 and vice versa, as well as over other, related α-helix mediated PPIs.77,78 Additionally, utilizing the purine skeleton and the rigid rod diphenylacetylene, we have introduced twofaced, amphipathic BH3 α-helix mimetics that reproduce key hydrophobic functionality from one face and a crucial, conserved aspartate residue from the opposing face.79,80 831



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Two BH3 α-helix mimetics that have been characterized more fully include the terphenyl BH3-M6 (42) and the oligoarylamide JY-1-106 (43), shown in Figure 21. Compound

Mcl-1−Bim PPI in cells. Finally, 44 induces apoptosis in ovarian, lung, and mesothelioma cancer cells when coadministered with 1 or Bcl-xL-targeting siRNA.

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α/β-FOLDAMERS An excellent ligand to inhibit the antiapoptotic Bcl-2 proteins would be the BH3 peptide itself, but in general, peptides penetrate cells poorly and also suffer proteolytic digestion. The incorporation of non-natural amino acids is one strategy by which enzymatic stability might be achieved. Accordingly, the Bim and Puma BH3 peptides have been modified, leading to α/ β-foldamer peptides that are selective for Bcl-xL87 as well as exhibiting dual Bcl-xL/Mcl-1 affinities.88

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STAPLED HELICES A complementary strategy to the incorporation of non-natural amino acids into the native BH3 peptide sequence is the replacement of the side chains of nonbinding amino acids with chemically reactive groups that can be cross-linked akin to a staple. In addition to providing proteolytic stability through the incorporation on non-natural amino acids, this approach has the added benefit of inducing helicity in the peptide, which may offer an entropic advantage since this resembles the conformation of the bound peptide. An SAHB of the BH3 domain of Bid was the first to be developed,89 followed by an SAHB of the Bim-BH3 domain.90 The latter SAHB binds potently to Bcl-xL, Bcl-W, Mcl-1, and A1 and induces intrinsic apoptosis and tumor regression in a murine xenograft model of leukemia.90 In addition, an SAHB of the Mcl-1 BH3 domain was shown to be a selective Mcl-1 inhibitor and killed leukemia cells by mitochondrial apoptosis.91

Figure 21. Synthetic α-helix mimetics 42 and 43 are pan-Bcl-2 inhibitors, while 44 is an Mcl-1 selective inhibitor.

42, projecting isobutyl and benzyl groups in a manner similar to that found on the hydrophobic face of BH3 α-helices, functions as a pan-Bcl-2 inhibitor in vitro (IC50 values of 1.5 μM and 4.9 μM, respectively, for the Bcl-x L−Bak and Mcl-1−Bim complexes) and in cancer cells, freeing up prodeath proteins to induce apoptosis in a caspase- and Bax-dependent manner.81 In addition, it was demonstrated that 42 synergizes with the proteasome inhibitor CEP-1612 by augmenting the killing of A549 cells, presumably in part by inhibiting the degradation of the proapoptotic proteins. The oligoarylamide 43 exhibits similar affinities as terphenyl 42 with IC50 values of 0.394 μM and 10 μM for disruption of the Bcl-xL−Bak-BH3 and Mcl-1− Bak-BH3 PPIs, respectively.82,83 Coimmunoprecipitation assays in the Bcl-xL/Mcl-1-overexpressing REN cell line indicated that 43 can disrupt Bcl-xL−Bak and Mcl-1−Bak PPIs in cells. Compound 43 prompts cell death in tumor cells regardless of Mcl-1 expression level and overcomes the resistance associated with the Bcl-xL-selective inhibitor 1 in cytotoxicity studies, presumably owing to the inherent pan-Bcl-2 specificity of 43. The presence of cleaved PARP suggested cell death was via the intrinsic (mitochondrial) apoptosis pathway, which was further supported by a green shift of the fluorescent signal associated with the JC-1 dye and a dramatic increase in the population of apoptotic cells in a TUNEL assay. These findings are all consistent with the molecular disablement of antiapoptotic Bcl2 proteins. Finally, 43 inhibited tumor growth in a murine lung cancer xenograft model. Reminiscent of Hamilton’s terphenyl and terpyridyl α-helix mimetics,84,85 Poulain’s group developed the novel oligopyridine MR29072 (44), or pyridoclax, which, according to SPR, is selective for Mcl-1 over Bcl-xL.86 The origin of selectivity may be through effective mimicry of the i + 7 phenylalanine of the Mcl-1-selective ligand Noxa, which is not conserved among the BH3-only proteins. Computational modeling of 44 indicated that the styryl mimetic of the i + 7 phenylalanine side chain binds in the p3 pocket, engaging in π−π interactions with His224 and Phe228, and a terminal pyridine is close to Arg263, possibly forming a hydrogen bond. A bioluminescence resonance energy transfer assay confirmed 44 disrupts the

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A BAX AGONIST Walensky and co-workers recently showed that a specific SAHB developed in their group was able to bind to and activate Bax through recognition of a trigger site opposite the BH3 canonical binding groove, which the SAHB also recognizes.92,93 Accordingly, they then used a computational screen to identify small molecules capable of binding this trigger site on the surface of Bax, which led to the discovery of the Bax agonist dubbed BAM-7 (45, Figure 22).94 Based on a pyrazolone core, 45 is the first small-molecule direct agonist of a proapoptotic Bcl-2 protein.

Figure 22. A Bax agonist.

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STRATEGIES TO ACHIEVE BCL-2 FAMILY MEMBER SELECTIVITY The discovery of synthetic agents that are capable of universally antagonizing the antiapoptotic Bcl-2 proteins is a goal that has been met time and again. However, the similar topologies of the BH3-binding grooves of Bcl-2, Bcl-xL, Bcl-w, Mcl-1 and A1/ Bfl-1 render the development of inhibitors that can discriminate 832



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Figure 23. A selection of new Bcl-2 inhibitors published in late 2015/early 2016.

While it must be acknowledged that the p2 pocket appears to be a big player in dictating ligand specificity, the p4 pocket plays a significant role, too. The thiophenyl of 1 (and 2) folds back forming an intramolecular π−π interaction and binds tightly in the p4 pockets of Bcl-2 and Bcl-xL, which appears more open and solvent-exposed in Mcl-1. Indeed, saturation mutagenesis of a Bim-BH3 peptide has shown that the nature of the p4-binding residue had little impact on the binding affinities of the resulting mutants, raising the question of how significant binding the p4 pocket will be in future ligand design towards, and maintaining, selectivity for Mcl-1. Indeed, Tanaka’s work and the recent publications from AbbVie suggest that targeting the p4 pocket can enhance ligand affinity but not selectivity, the latter of which appears to be dictated mostly by binding the p2 pocket. Although Tanaka’s hybrid molecule in which key fragments of Mcl-1 and Bcl-xL selective ligands were merged demonstrated greater selectivity for Bcl-xL, this is due to the acylsulfonamide portion of the ligand being directed into the solvent in the case of Mcl-1, and is thus unable to acquire additional interactions in the p4 pocket, which is in contrast to the binding mode with Bcl-xL. Selectivity for Bcl-2 over Bcl-xL has been accomplished through exploiting one of the few amino acid differences in the hydrophobic grooves of the two proteins: Asp103 in the p4 pocket of Bcl-2 is able to

between Bcl-2 family members a significant challenge. Selective Bcl-2 inhibitors may provide new drugs for cancer chemotherapy or at the very least furnish biochemical tools that can better explore the consequences on cell biology of specific Bcl-2 member inhibition. Along these lines, noteworthy advances have been made in recent years, providing insight into how selectivity might be achieved. A contributing factor in protein family member selectivity lies with the observed remodeling of the p2 pocket in the presence of synthetic ligands. For example, the rigid 4-chlorobiphenyl of 1 and the 4′-chloro-4,4-dimethyl2,3,4,5-tetrahydro-1,1′-biphenyl of 2, bind in the p2 pockets of Bcl-2 and Bcl-xL but are excluded from that of Mcl-1. Conversely, selectivity for Mcl-1 appears to be manifested through especially hydrophobic moieties, such as 4-chloro-3,5dimethylphenyl and naphthyl, often coupled to the main inhibitor scaffold through flexible linkers, which are anchored deeply into the p2 pocket, as exemplified by compounds 19 and 36. It is important to note that the p2 pocket offers more than just hydrophobic interactions to capture synthetic ligands. It has been reasoned that the selectivity for Bcl-xL over Bcl-2 and Mcl-1 with 24 is due to hydrogen bonding to the main chain amide bonds of Ser106 and Leu108 in Bcl-xL near the entrance to the p2 pocket. 833



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hydrogen-bond to the azaindole of 22, while it appears the analogous Glu96 of Bcl-xL cannot. Despite the fact that binding to the conserved Arg139 of Bcl-xL and Arg146 of Bcl-2 is not critical to effective drug development, as exemplified by 1 and 2, respectively, this may not be the case for Mcl-1. To date, Mcl-1 inhibitors whose binding modes have been characterized by crystal structures and/or 2D HSQC NMR spectroscopic analyses have demonstrated that the corresponding Arg263 on the surface of Mcl-1 is engaged, through either salt bridges with acidic functions or multiple hydrogens bonds with polar, neutral moieties. Therefore, this observation could also be exploited toward the design of new, selective ligands.

values were as low as 31 nM and selectivities for Mcl-1 over Bcl-xL were up to 19-fold. Drennen and co-workers developed novel dual Mcl-1/Bcl-xL inhibitors constructed from a 2,6disubstituted nicotinic acid, such as 48, which evolved from the deconstruction of the previously reported synthetic α-helix mimetic 43 from the same group.97 Beekman and colleagues have described a new Mcl-1 inhibitor based on a pyrogallol derivative (49).98 Meanwhile, continued structural elaboration of Fesik’s indoles has led to the discovery of the acylsulfonamide derivative 50 which inhibits Mcl-1 with a Ki of 1 nM and exhibits 3 orders of magnitude selectivity over BclxL.99 Fang’s group have reported some other acylsulfonamides as pan-Bcl-2 inhibitors, including 51, based on an imidazolidine-2,4-dione core,100 and compounds derived from pyrrolidine (e.g., 52) that demonstrate up to 15-fold selectivity for Mcl-1 over Bcl-xL.101 Covalent inhibition of proteins is experiencing a renaissance, particularly through a more targeted strategy, and, in this regard, Mcl-1 has not been neglected. Walensky and colleagues have discovered the covalent inhibitor 53 (Figure 24) that binds Mcl-1 allosterically and selectively through chemical modification of Cys286.102 Finally, researchers at AstraZeneca chemically modified one of AbbVie’s/Fesik’s earlier Mcl-1 inhibitors to include an aryl boronic acid carbonyl warhead, viz. 54.103 An enhancement in inhibitory activity of about two orders of magnitude was observed, which was ascribed to the reversible formation of a covalently-linked iminoboronate complex to Lys234, located in the p1 pocket. It should be noted that no analogous lysine is found on the surfaces of Bcl-2 and Bcl-xL, suggesting this strategy may prove useful in converting pan-Bcl-2 inhibitors into Mcl-1-selective inhibitors.

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CONCLUDING REMARKS The contribution of antiapoptotic Bcl-2 proteins to the survival of cancer cells and as a means of conferring resistance to traditional chemotherapeutics has catapulted this family of proteins to the front line of cancer research. Earlier concerns of disrupting the PPIs of Bcl-2 proteins with small molecules have been obliterated by such notable success stories as navitoclax and venetoclax, the latter of which has now received magic bullet status having recently been approved by the FDA for the treatment of CLL through the disarmament of Bcl-2. Moreover, the selective seizure of individual antiapoptotic Bcl-2 family members has also proven possible. In particular, with the advent of Mcl-1 selective inhibitors, which had lagged behind that of Bcl-xL and Bcl-2, the cancer biology of Mcl-1 can now be closely interrogated; this is a significant step forward in the Bcl2 field, since Bcl-xL/Bcl-2-dependent cancers can evade druginduced reactivation of apoptosis through the upregulation of Mcl-1. This is an exciting time in the realm of Bcl-2 inhibitor research with a large number of ongoing clinical trials, and although the war on cancer is far from over, the small-molecule arsenal is expanding all the time, and our weapons are becoming more deadly. With the recent FDA approval of venetoclax, we foresee a time when Bcl-2 family member inhibitors will become standard chemotherapies of choice to exterminate cancer, both in isolation and in adjuvant therapies.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: steven.fl[email protected]. Phone: 1-410-7066361. Fax: 1-410-706-5017. Notes

The authors declare no competing financial interest.

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Biographies

UPDATE Since this manuscript was initially reviewed, a variety of new Bcl-2 family inhibitors have been published, which are summarized in Figures 23 and 24. The Fletcher group has reported nanomolar inhibitors of Mcl-1 based on 4sulfonamido derivatives of 1-hydroxy-2-naphthoic acids95 and N-sulfamoyl-3-carboxy-substituted tetrahydroquinolines,96 as typified by 46 and ±47, respectively. In the former class, Ki

Jeremy L. Yap received his Ph.D. in Medicinal Chemistry from the University of Maryland School of Pharmacy in 2014. His doctoral work under Prof. Steven Fletcher focused on the design and syntheses of small-molecule inhibitors of oncogenic protein−protein interactions (Bak/Bcl-xL, Bak/Mcl-1, and c-Myc/Max). He was selected as an ACS Predoctoral Medicinal Chemistry Fellow (2012−2013) and an AFPE Predoctoral Fellow (2012−2014) for his work on the design and syntheses of c-Myc specific inhibitors. After his doctorate, he joined Prof. Paul R. Thompson’s group at the University of Massachusetts Medical School where he designed covalent inhibitors of epigenetic “writer” proteins. Currently, he is an investigator with GlaxoSmithKline’s encoded library technologies (ELT) site in Waltham, MA. Lijia Chen studied pharmaceutical sciences and received a B.Sc. degree from China Pharmaceutical University in 2011. He then joined Prof. Steven Fletcher’s group as a graduate student at the University of Maryland School of Pharmacy to pursue a Ph.D. in Pharmaceutical Sciences. Lee’s thesis is focused on the structure-based design, synthesis, and biological evaluation of small molecules as MCL-1 inhibitors. In 2015, Lee won an American Association of Pharmaceutical Scientists Graduate Student Research Award from the Drug Discovery and Development Interface Focus Group. Thus far, Lee has published 13 papers and is the first-author on five of those publications.

Figure 24. Mcl-1 selective covalent inhibitors. 834



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(7) Czabotar, P. E.; Lee, E. F.; van Delft, M. F.; Day, C. L.; Smith, B. J.; Huang, D. C. S.; Fairlie, W. D.; Hinds, M. G.; Colman, P. M. Structural Insights Into the Degradation of Mcl-1 Induced by BH3 Domains. Proc. Natl. Acad. Sci. U. S. A. 2007, 104, 6217−6222. (8) Rajan, S.; Choi, M.; Baek, K.; Yoon, H. S. Bh3 Induced Conformational Changes in Bcl-Xl Revealed by Crystal Structure and Comparative Analysis. Proteins: Struct., Funct., Genet. 2015, 83, 1262− 1272. (9) Llambi, F.; Moldoveanu, T.; Tait, S. W. G.; Bouchier-Hayes, L.; Temirov, J.; McCormick, L. L.; Dillon, C. P.; Green, D. R. A Unified Model of Mammalian BCL-2 Protein Family Interactions at the Mitochondria. Mol. Cell 2011, 44, 517−531. (10) Wei, M. C.; Zong, W. X.; Cheng, E. H.; Lindsten, T.; Panoutsakopoulou, V.; Ross, A. J.; Roth, K. A.; MacGregor, G. R.; Thompson, C. B.; Korsmeyer, S. J. Proapoptotic BAX and BAK: A Requisite Gateway to Mitochondrial Dysfunction and Death. Science 2001, 292, 727−730. (11) Zou, H.; Henzel, W. J.; Liu, X.; Lutschg, A.; Wang, X. Apaf-1, a Human Protein Homologous to C. Elegans CED-4, Participates in Cytochrome C-Dependent Activation of Caspase-3. Cell 1997, 90, 405−413. (12) Hanahan, D.; Weinberg, R. A. Hallmarks of Cancer: the Next Generation. Cell 2011, 144, 646−674. (13) Kelly, G. L.; Strasser, A. The Essential Role of Evasion From Cell Death in Cancer. Adv. Cancer Res. 2011, 111, 39−96. (14) Bold, R. J.; Virudachalam, S.; McConkey, D. J. BCL2 Expression Correlates with Metastatic Potential in Pancreatic Cancer Cell Lines. Cancer 2001, 92, 1122−1129. (15) Del Poeta, G.; Bruno, A.; Del Principe, M. I.; Venditti, A.; Maurillo, L.; Buccisano, F.; Stasi, R.; Neri, B.; Luciano, F.; Siniscalchi, A.; de Fabritiis, P.; Amadori, S. Deregulation of the Mitochondrial Apoptotic Machinery and Development of Molecular Targeted Drugs in Acute Myeloid Leukemia. Curr. Cancer Drug Targets 2008, 8, 207− 222. (16) Oltersdorf, T.; Elmore, S. W.; Shoemaker, A. R.; Armstrong, R. C.; Augeri, D. J.; Belli, B. A.; Bruncko, M.; Deckwerth, T. L.; Dinges, J.; Hajduk, P. J.; Joseph, M. K.; Kitada, S.; Korsmeyer, S. J.; Kunzer, A. R.; Letai, A.; Li, C.; Mitten, M. J.; Nettesheim, D. G.; Ng, S.; Nimmer, P. M.; O’Connor, J. M.; Oleksijew, A.; Petros, A. M.; Reed, J. C.; Shen, W.; Tahir, S. K.; Thompson, C. B.; Tomaselli, K. J.; Wang, B.; Wendt, M. D.; Zhang, H.; Fesik, S. W.; Rosenberg, S. H. An Inhibitor of Bcl-2 Family Proteins Induces Regression of Solid Tumours. Nature 2005, 435, 677−681. (17) Lee, E. F.; Czabotar, P. E.; Smith, B. J.; Deshayes, K.; Zobel, K.; Colman, P. M.; Fairlie, W. D. Crystal Structure of ABT-737 Complexed with Bcl-xL: Implications for Selectivity of Antagonists of the Bcl-2 Family. Cell Death Differ. 2007, 14, 1711−1713. (18) Tse, C.; Shoemaker, A. R.; Adickes, J.; Anderson, M. G.; Chen, J.; Jin, S.; Johnson, E. F.; Marsh, K. C.; Mitten, M. J.; Nimmer, P.; Roberts, L.; Tahir, S. K.; Xiao, Y.; Yang, X.; Zhang, H.; Fesik, S.; Rosenberg, S. H.; Elmore, S. W. ABT-263: a Potent and Orally Bioavailable Bcl-2 Family Inhibitor. Cancer Res. 2008, 68, 3421−3428. (19) Gandhi, L.; Camidge, D. R.; Ribeiro de Oliveira, M.; Bonomi, P.; Gandara, D.; Khaira, D.; Hann, C. L.; McKeegan, E. M.; Litvinovich, E.; Hemken, P. M.; Dive, C.; Enschede, S. H.; Nolan, C.; Chiu, Y.-L.; Busman, T.; Xiong, H.; Krivoshik, A. P.; Humerickhouse, R.; Shapiro, G. I.; Rudin, C. M. Phase I Study of Navitoclax (ABT-263), a Novel Bcl-2 Family Inhibitor, in Patients with Small-Cell Lung Cancer and Other Solid Tumors. J. Clin. Oncol. 2011, 29, 909−916. (20) Roberts, A. W.; Seymour, J. F.; Brown, J. R.; Wierda, W. G.; Kipps, T. J.; Khaw, S. L.; Carney, D. A.; He, S. Z.; Huang, D. C. S.; Xiong, H.; Cui, Y.; Busman, T. A.; McKeegan, E. M.; Krivoshik, A. P.; Enschede, S. H.; Humerickhouse, R. Substantial Susceptibility of Chronic Lymphocytic Leukemia to BCL2 Inhibition: Results of a Phase I Study of Navitoclax in Patients with Relapsed or Refractory Disease. J. Clin. Oncol. 2012, 30, 488−496. (21) van Delft, M. F.; Wei, A. H.; Mason, K. D.; Vandenberg, C. J.; Chen, L.; Czabotar, P. E.; Willis, S. N.; Scott, C. L.; Day, C. L.; Cory,

Maryanna E. Lanning obtained her B.S. degree in Chemistry from the University of North Carolina at Greensboro in 2009, where her research focused on the synthesis and evaluation of metabolic enzyme inhibitors toward aldehyde oxidase and molybdenum oxidase as they relate to antitumor drugs. She began her Ph.D. studies with Prof. Steven Fletcher in 2012 where her research focuses on the design, synthesis and biological evaluation of synthetic α-helix mimetics targeted toward the Bcl-2 family, specifically the Mcl-1−Bim protein− protein interaction. Maryanna is presently an AFPE Predoctoral Fellow (2015−2017). During her time at the University of Maryland, Baltimore, Maryanna has authored or coauthored 15 publications. Steven Fletcher is currently an Associate Professor at the University of Maryland School of Pharmacy (U.S.). He obtained his B.A., M.Sci., and M.A. degrees in Chemistry from the University of Cambridge (U.K.) in 2000, followed by his Ph.D. in Medicinal Chemistry at Imperial College London (U.K.) in 2004, under the supervision of Prof. Andrew D. Miller. Subsequently, Steven completed two postdoctoral positions at Yale University (U.S.) and the University of Toronto (Canada). Steven’s research interests lie in the disruption of helix-mediated protein−protein interactions with small molecules, particularly the Bcl-2 family of proteins, p53/HDM2 and c-Myc/Max, and also in further advances and applications of the Mitsunobu reaction. Steven has authored or coauthored more than 75 publications and several patents in medicinal chemistry and synthetic organic chemistry.

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ACKNOWLEDGMENTS We thank the University of Maryland School of Pharmacy, the American Chemical Society (ACS), the American Association of the Colleges of Pharmacy, and the American Foundation for Pharmaceutical Education (AFPE) for supporting our research in this area.

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ABBREVIATIONS USED

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REFERENCES

FP, fluorescence polarization; FPCA, fluorescence polarization competition assay; HSQC, heteronuclear single quantum coherence spectroscopy; NOESY, nuclear Overhauser effect spectroscopy; PDB, Protein Data Bank; PPI, protein−protein interaction; SAHB, stabilized α-helix of Bcl-2; SAR, structure− activity relationship

(1) Strasser, A.; Cory, S.; Adams, J. M. Deciphering the Rules of Programmed Cell Death to Improve Therapy of Cancer and Other Diseases. EMBO J. 2011, 30, 3667−3683. (2) Brenner, D.; Blaser, H.; Mak, T. W. Regulation of Tumour Necrosis Factor Signalling: Live or Let Die. Nat. Rev. Immunol. 2015, 15, 362−374. (3) Youle, R. J.; Strasser, A. The BCL-2 Protein Family: Opposing Activities That Mediate Cell Death. Nat. Rev. Mol. Cell Biol. 2008, 9, 47−59. (4) Sattler, M.; Liang, H.; Nettesheim, D.; Meadows, R. P.; Harlan, J. E.; Eberstadt, M.; Yoon, H. S.; Shuker, S. B.; Chang, B. S.; Minn, A. J.; Thompson, C. B.; Fesik, S. W. Structure of Bcl-xL-Bak Peptide Complex: Recognition Between Regulators of Apoptosis. Science 1997, 275, 983−986. (5) Liu, X.; Dai, S.; Zhu, Y.; Marrack, P.; Kappler, J. W. The Structure of a Bcl-xL/Bim Fragment Complex: Implications for Bim Function. Immunity 2003, 19, 341−352. (6) Denisov, A. Y.; Chen, G.; Sprules, T.; Moldoveanu, T.; Beauparlant, P.; Gehring, K. Structural Model of the BCL-W-BID Peptide Complex and Its Interactions with Phospholipid Micelles. Biochemistry 2006, 45, 2250−2256. 835



Perspective

Homology Domain-3 Mimetic GX15-070 (Obatoclax). Cancer Res. 2008, 68, 3413−3420. (35) Smoot, R. L.; Blechacz, B. R. A.; Werneburg, N. W.; Bronk, S. F.; Sinicrope, F. A.; Sirica, A. E.; Gores, G. J. A Bax-Mediated Mechanism for Obatoclax-Induced Apoptosis of Cholangiocarcinoma Cells. Cancer Res. 2010, 70, 1960−1969. (36) Albershardt, T. C.; Salerni, B. L.; Soderquist, R. S.; Bates, D. J. P.; Pletnev, A. A.; Kisselev, A. F.; Eastman, A. Multiple BH3 Mimetics Antagonize Antiapoptotic MCL1 Protein by Inducing the Endoplasmic Reticulum Stress Response and Up-Regulating BH3-Only Protein NOXA. J. Biol. Chem. 2011, 286, 24882−24895. (37) Zhang, Z.; Jin, L.; Qian, X.; Wei, M.; Wang, Y.; Wang, J.; Yang, Y.; Xu, Q.; Xu, Y.; Liu, F. Novel Bcl-2 Inhibitors: Discovery and Mechanism Study of Small Organic Apoptosis-Inducing Agents. ChemBioChem 2007, 8, 113−121. (38) Zhang, Z.; Song, T.; Zhang, T.; Gao, J.; Wu, G.; An, L.; Du, G. A Novel BH3 Mimetic S1 Potently Induces Bax/Bak-Dependent Apoptosis by Targeting Both Bcl-2 and Mcl-1. Int. J. Cancer 2011, 128, 1724−1735. (39) Zhong, J.-T.; Xu, Y.; Yi, H.-W.; Su, J.; Yu, H.-M.; Xiang, X.-Y.; Li, X.-N.; Zhang, Z.-C.; Sun, L.-K. The BH3 Mimetic S1 Induces Autophagy Through ER Stress and Disruption of Bcl-2/Beclin 1 Interaction in Human Glioma U251 Cells. Cancer Lett. 2012, 323, 180−187. (40) Zhang, Z.; Wu, G.; Xie, F.; Song, T.; Chang, X. 3Thiomorpholin-8-Oxo-8H-Acenaphtho[1,2-B]Pyrrole-9-Carbonitrile (S1) Based Molecules as Potent, Dual Inhibitors of B-Cell Lymphoma 2 (Bcl-2) and Myeloid Cell Leukemia Sequence 1 (Mcl-1): StructureBased Design and Structure-Activity Relationship Studies. J. Med. Chem. 2011, 54, 1101−1105. (41) Song, T.; Li, X.; Chang, X.; Liang, X.; Zhao, Y.; Wu, G.; Xie, S.; Su, P.; Wu, Z.; Feng, Y.; Zhang, Z. 3-Thiomorpholin-8-Oxo-8HAcenaphtho [1,2-B] Pyrrole-9-Carbonitrile (S1) Derivatives as PanBcl-2-Inhibitors of Bcl-2, Bcl-xL and Mcl-1. Bioorg. Med. Chem. 2013, 21, 11−20. (42) Hughes, C. C.; Prieto-Davo, A.; Jensen, P. R.; Fenical, W. The Marinopyrroles, Antibiotics of an Unprecedented Structure Class From a Marine Streptomyces Sp. Org. Lett. 2008, 10, 629−631. (43) Doi, K.; Li, R.; Sung, S.-S.; Wu, H.; Liu, Y.; Manieri, W.; Krishnegowda, G.; Awwad, A.; Dewey, A.; Liu, X.; Amin, S.; Cheng, C.; Qin, Y.; Schonbrunn, E.; Daughdrill, G.; Loughran, T. P.; Sebti, S.; Wang, H.-G. Discovery of Marinopyrrole a (Maritoclax) as a Selective Mcl-1 Antagonist That Overcomes ABT-737 Resistance by Binding to and Targeting Mcl-1 for Proteasomal Degradation. J. Biol. Chem. 2012, 287, 10224−10235. (44) Eichhorn, J. M.; Alford, S. E.; Hughes, C. C.; Fenical, W.; Chambers, T. C. Purported Mcl-1 Inhibitor Marinopyrrole a Fails to Show Selective Cytotoxicity for Mcl-1-Dependent Cell Lines. Cell Death Dis. 2013, 4, e880. (45) Doi, K.; Liu, Q.; Gowda, K.; Barth, B. M.; Claxton, D.; Amin, S.; Loughran, T. P., Jr.; Wang, H.-G. Maritoclax Induces Apoptosis in Acute Myeloid Leukemia Cells with Elevated Mcl-1 Expression. Cancer Biol. Ther. 2014, 15, 1077−1086. (46) Li, R.; Cheng, C.; Balasis, M. E.; Liu, Y.; Garner, T. P.; Daniel, K. G.; Li, J.; Qin, Y.; Gavathiotis, E.; Sebti, S. M. Design, Synthesis and Evaluation of Marinopyrrole Derivatives as Selective Inhibitors of Mcl1 Binding to Pro-Apoptotic Bim and Dual Mcl-1/Bcl-xL Inhibitors. Eur. J. Med. Chem. 2015, 90, 315−331. (47) Varadarajan, S.; Poornima, P.; Milani, M.; Gowda, K.; Amin, S.; Wang, H.-G.; Cohen, G. M. Maritoclax and Dinaciclib Inhibit MCL-1 Activity and Induce Apoptosis in Both a MCL-1-Dependent and -Independent Manner. Oncotarget 2015, 6, 12668−12681. (48) Cohen, N. A.; Stewart, M. L.; Gavathiotis, E.; Tepper, J. L.; Bruekner, S. R.; Koss, B.; Opferman, J. T.; Walensky, L. D. A Competitive Stapled Peptide Screen Identifies a Selective Small Molecule That Overcomes MCL-1-Dependent Leukemia Cell Survival. Chem. Biol. 2012, 19, 1175−1186. (49) Richard, D. J.; Lena, R.; Bannister, T.; Blake, N.; Pierceall, W. E.; Carlson, N. E.; Keller, C. E.; Koenig, M.; He, Y.; Minond, D.; Mishra,

S.; Adams, J. M.; Roberts, A. W.; Huang, D. C. S. The BH3 Mimetic ABT-737 Targets Selective Bcl-2 Proteins and Efficiently Induces Apoptosis via Bak/Bax if Mcl-1 Is Neutralized. Cancer Cell 2006, 10, 389−399. (22) Wilson, W. H.; O’Connor, O. A.; Czuczman, M. S.; LaCasce, A. S.; Gerecitano, J. F.; Leonard, J. P.; Tulpule, A.; Dunleavy, K.; Xiong, H.; Chiu, Y.-L.; Cui, Y.; Busman, T.; Elmore, S. W.; Rosenberg, S. H.; Krivoshik, A. P.; Enschede, S. H.; Humerickhouse, R. A. Navitoclax, a Targeted High-Affinity Inhibitor of BCL-2, in Lymphoid Malignancies: a Phase 1 Dose-Escalation Study of Safety, Pharmacokinetics, Pharmacodynamics, and Antitumour Activity. Lancet Oncol. 2010, 11, 1149−1159. (23) Coutinho, E. M. Gossypol: a Contraceptive for Men. Contraception 2002, 65, 259−263. (24) Razakantoanina, V.; Nguyen Kim, P. P.; Jaureguiberry, G. Antimalarial Activity of New Gossypol Derivatives. Parasitol. Res. 2000, 86, 665−668. (25) Kitada, S.; Leone, M.; Sareth, S.; Zhai, D.; Reed, J. C.; Pellecchia, M. Discovery, Characterization, and Structure-Activity Relationships Studies of Proapoptotic Polyphenols Targeting B-Cell Lymphocyte/Leukemia-2 Proteins. J. Med. Chem. 2003, 46, 4259− 4264. (26) Wang, G.; Nikolovska-Coleska, Z.; Yang, C.-Y.; Wang, R.; Tang, G.; Guo, J.; Shangary, S.; Qiu, S.; Gao, W.; Yang, D.; Meagher, J.; Stuckey, J.; Krajewski, K.; Jiang, S.; Roller, P. P.; Abaan, H. O.; Tomita, Y.; Wang, S. Structure-Based Design of Potent Small-Molecule Inhibitors of Anti-Apoptotic Bcl-2 Proteins. J. Med. Chem. 2006, 49, 6139−6142. (27) Mohammad, R. M.; Goustin, A. S.; Aboukameel, A.; Chen, B.; Banerjee, S.; Wang, G.; Nikolovska-Coleska, Z.; Wang, S.; Al-Katib, A. Preclinical Studies of TW-37, a New Nonpeptidic Small-Molecule Inhibitor of Bcl-2, in Diffuse Large Cell Lymphoma Xenograft Model Reveal Drug Action on Both Bcl-2 and Mcl-1. Clin. Cancer Res. 2007, 13, 2226−2235. (28) Sun, Y.; Wu, J.; Aboukameel, A.; Banerjee, S.; Arnold, A. A.; Chen, J.; Nikolovska-Coleska, Z.; Lin, Y.; Ling, X.; Yang, D.; Wang, S.; Al-Katib, A.; Mohammad, R. M. Apogossypolone, a Nonpeptidic Small Molecule Inhibitor Targeting Bcl-2 Family Proteins, Effectively Inhibits Growth of Diffuse Large Cell Lymphoma Cells in Vitro and in Vivo. Cancer Biol. Ther. 2008, 7, 1418−1426. (29) Wei, J.; Stebbins, J. L.; Kitada, S.; Dash, R.; Placzek, W.; Rega, M. F.; Wu, B.; Cellitti, J.; Zhai, D.; Yang, L.; Dahl, R.; Fisher, P. B.; Reed, J. C.; Pellecchia, M. BI-97C1, an Optically Pure Apogossypol Derivative as Pan-Active Inhibitor of Antiapoptotic B-Cell Lymphoma/Leukemia-2 (Bcl-2) Family Proteins. J. Med. Chem. 2010, 53, 4166−4176. (30) Schweigert, N.; Zehnder, A. J.; Eggen, R. I. Chemical Properties of Catechols and Their Molecular Modes of Toxic Action in Cells, From Microorganisms to Mammals. Environ. Microbiol. 2001, 3, 81− 91. (31) Vogler, M.; Weber, K.; Dinsdale, D.; Schmitz, I.; SchulzeOsthoff, K.; Dyer, M. J. S.; Cohen, G. M. Different Forms of Cell Death Induced by Putative BCL2 Inhibitors. Cell Death Differ. 2009, 16, 1030−1039. (32) Wei, J.; Stebbins, J. L.; Kitada, S.; Dash, R.; Zhai, D.; Placzek, W. J.; Wu, B.; Rega, M. F.; Zhang, Z.; Barile, E.; Yang, L.; Dahl, R.; Fisher, P. B.; Reed, J. C.; Pellecchia, M. An Optically Pure Apogossypolone Derivative as Potent Pan-Active Inhibitor of Anti-Apoptotic Bcl-2 Family Proteins. Front. Oncol. 2011, 1, 28. (33) Nguyen, M.; Marcellus, R. C.; Roulston, A.; Watson, M.; Serfass, L.; Murthy Madiraju, S. R.; Goulet, D.; Viallet, J.; Bélec, L.; Billot, X.; Acoca, S.; Purisima, E.; Wiegmans, A.; Cluse, L.; Johnstone, R. W.; Beauparlant, P.; Shore, G. C. Small Molecule Obatoclax (GX15−070) Antagonizes MCL-1 and Overcomes MCL-1-Mediated Resistance to Apoptosis. Proc. Natl. Acad. Sci. U. S. A. 2007, 104, 19512−19517. (34) Konopleva, M.; Watt, J.; Contractor, R.; Tsao, T.; Harris, D.; Estrov, Z.; Bornmann, W.; Kantarjian, H.; Viallet, J.; Samudio, I.; Andreeff, M. Mechanisms of Antileukemic Activity of the Novel Bcl-2 836



Perspective

J.; Cameron, M.; Spicer, T.; Hodder, P.; Cardone, M. H. Hydroxyquinoline-Derived Compounds and Analoguing of Selective Mcl-1 Inhibitors Using a Functional Biomarker. Bioorg. Med. Chem. 2013, 21, 6642−6649. (50) Friberg, A.; Vigil, D.; Zhao, B.; Daniels, R. N.; Burke, J. P.; Garcia-Barrantes, P. M.; Camper, D.; Chauder, B. A.; Lee, T.; Olejniczak, E. T.; Fesik, S. W. Discovery of Potent Myeloid Cell Leukemia 1 (Mcl-1) Inhibitors Using Fragment-Based Methods and Structure-Based Design. J. Med. Chem. 2013, 56, 15−30. (51) Burke, J. P.; Bian, Z.; Shaw, S.; Zhao, B.; Goodwin, C. M.; Belmar, J.; Browning, C. F.; Vigil, D.; Friberg, A.; Camper, D. V.; Rossanese, O. W.; Lee, T.; Olejniczak, E. T.; Fesik, S. W. Discovery of Tricyclic Indoles That Potently Inhibit Mcl-1 Using Fragment-Based Methods and Structure-Based Design. J. Med. Chem. 2015, 58, 3794− 3805. (52) Souers, A. J.; Leverson, J. D.; Boghaert, E. R.; Ackler, S. L.; Catron, N. D.; Chen, J.; Dayton, B. D.; Ding, H.; Enschede, S. H.; Fairbrother, W. J.; Huang, D. C. S.; Hymowitz, S. G.; Jin, S.; Khaw, S. L.; Kovar, P. J.; Lam, L. T.; Lee, J.; Maecker, H. L.; Marsh, K. C.; Mason, K. D.; Mitten, M. J.; Nimmer, P. M.; Oleksijew, A.; Park, C. H.; Park, C.-M.; Phillips, D. C.; Roberts, A. W.; Sampath, D.; Seymour, J. F.; Smith, M. L.; Sullivan, G. M.; Tahir, S. K.; Tse, C.; Wendt, M. D.; Xiao, Y.; Xue, J. C.; Zhang, H.; Humerickhouse, R. A.; Rosenberg, S. H.; Elmore, S. W. ABT-199, a Potent and Selective BCL-2 Inhibitor, Achieves Antitumor Activity While Sparing Platelets. Nat. Med. 2013, 19, 202−208. (53) Pan, R.; Hogdal, L. J.; Benito, J. M.; Bucci, D.; Han, L.; Borthakur, G.; Cortes, J.; DeAngelo, D. J.; Debose, L.; Mu, H.; Döhner, H.; Gaidzik, V. I.; Galinsky, I.; Golfman, L. S.; Haferlach, T.; Harutyunyan, K. G.; Hu, J.; Leverson, J. D.; Marcucci, G.; Müschen, M.; Newman, R.; Park, E.; Ruvolo, P. P.; Ruvolo, V.; Ryan, J.; Schindela, S.; Zweidler-McKay, P.; Stone, R. M.; Kantarjian, H.; Andreeff, M.; Konopleva, M.; Letai, A. G. Selective BCL-2 Inhibition by ABT-199 Causes on-Target Cell Death in Acute Myeloid Leukemia. Cancer Discovery 2014, 4, 362−375. (54) Peirs, S.; Matthijssens, F.; Goossens, S.; Van de Walle, I.; Ruggero, K.; de Bock, C. E.; Degryse, S.; Canté-Barrett, K.; Briot, D.; Clappier, E.; Lammens, T.; De Moerloose, B.; Benoit, Y.; Poppe, B.; Meijerink, J. P.; Cools, J.; Soulier, J.; Rabbitts, T. H.; Taghon, T.; Speleman, F.; Van Vlierberghe, P. ABT-199 Mediated Inhibition of BCL-2 as a Novel Therapeutic Strategy in T-Cell Acute Lymphoblastic Leukemia. Blood 2014, 124, 3738−3747. (55) www.clinicaltrials.gov (accessed Jun 30, 2016). (56) FDA News Release. http://www.fda.gov/NewsEvents/ Newsroom/PressAnnouncements/ucm495253.htm (accessed Jun 30, 2016). (57) Lessene, G.; Czabotar, P. E.; Sleebs, B. E.; Zobel, K.; Lowes, K. N.; Adams, J. M.; Baell, J. B.; Colman, P. M.; Deshayes, K.; Fairbrother, W. J.; Flygare, J. A.; Gibbons, P.; Kersten, W. J. A.; Kulasegaram, S.; Moss, R. M.; Parisot, J. P.; Smith, B. J.; Street, I. P.; Yang, H.; Huang, D. C. S.; Watson, K. G. Structure-Guided Design of a Selective BCL-X(L) Inhibitor. Nat. Chem. Biol. 2013, 9, 390−397. (58) Tao, Z.-F.; Hasvold, L.; Wang, L.; Wang, X.; Petros, A. M.; Park, C. H.; Boghaert, E. R.; Catron, N. D.; Chen, J.; Colman, P. M.; Czabotar, P. E.; Deshayes, K.; Fairbrother, W. J.; Flygare, J. A.; Hymowitz, S. G.; Jin, S.; Judge, R. A.; Koehler, M. F. T.; Kovar, P. J.; Lessene, G.; Mitten, M. J.; Ndubaku, C. O.; Nimmer, P.; Purkey, H. E.; Oleksijew, A.; Phillips, D. C.; Sleebs, B. E.; Smith, B. J.; Smith, M. L.; Tahir, S. K.; Watson, K. G.; Xiao, Y.; Xue, J.; Zhang, H.; Zobel, K.; Rosenberg, S. H.; Tse, C.; Leverson, J. D.; Elmore, S. W.; Souers, A. J. Discovery of a Potent and Selective BCL-XL Inhibitor with in Vivo Activity. ACS Med. Chem. Lett. 2014, 5, 1088−1093. (59) Zhang, Z.; Song, T.; Li, X.; Wu, Z.; Feng, Y.; Xie, F.; Liu, C.; Qin, J.; Chen, H. Novel Soluble Myeloid Cell Leukemia Sequence 1 (Mcl-1) Inhibitor (E,E)-2-(Benzylaminocarbonyl)-3-Styrylacrylonitrile (4g) Developed Using a Fragment-Based Approach. Eur. J. Med. Chem. 2013, 59, 141−149. (60) Zhang, Z.; Liu, C.; Li, X.; Song, T.; Wu, Z.; Liang, X.; Zhao, Y.; Shen, X.; Chen, H. Fragment-Based Design, Synthesis, and Biological

Evaluation of N-Substituted-5-(4-Isopropylthiophenol)-2-Hydroxynicotinamide Derivatives as Novel Mcl-1 Inhibitors. Eur. J. Med. Chem. 2013, 60, 410−420. (61) Tanaka, Y.; Aikawa, K.; Nishida, G.; Homma, M.; Sogabe, S.; Igaki, S.; Hayano, Y.; Sameshima, T.; Miyahisa, I.; Kawamoto, T.; Tawada, M.; Imai, Y.; Inazuka, M.; Cho, N.; Imaeda, Y.; Ishikawa, T. Discovery of Potent Mcl-1/Bcl-xL Dual Inhibitors by Using a Hybridization Strategy Based on Structural Analysis of Target Proteins. J. Med. Chem. 2013, 56, 9635−9645. (62) Yecies, D.; Carlson, N. E.; Deng, J.; Letai, A. Acquired Resistance to ABT-737 in Lymphoma Cells That Up-Regulate MCL-1 and BFL-1. Blood 2010, 115, 3304−3313. (63) Hikita, H.; Takehara, T.; Shimizu, S.; Kodama, T.; Shigekawa, M.; Iwase, K.; Hosui, A.; Miyagi, T.; Tatsumi, T.; Ishida, H.; Li, W.; Kanto, T.; Hiramatsu, N.; Hayashi, N. The Bcl-xL Inhibitor, ABT-737, Efficiently Induces Apoptosis and Suppresses Growth of Hepatoma Cells in Combination with Sorafenib. Hepatology 2010, 52, 1310− 1321. (64) Chen, J.; Zhou, H.; Aguilar, A.; Liu, L.; Bai, L.; McEachern, D.; Yang, C.-Y.; Meagher, J. L.; Stuckey, J. A.; Wang, S. Structure-Based Discovery of BM-957 as a Potent Small-Molecule Inhibitor of Bcl-2 and Bcl-xL Capable of Achieving Complete Tumor Regression. J. Med. Chem. 2012, 55, 8502−8514. (65) Bai, L.; Chen, J.; McEachern, D.; Liu, L.; Zhou, H.; Aguilar, A.; Wang, S. BM-1197: a Novel and Specific Bcl-2/Bcl-xL Inhibitor Inducing Complete and Long-Lasting Tumor Regression in Vivo. PLoS One 2014, 9, e99404. (66) Abulwerdi, F. A.; Liao, C.; Mady, A. S.; Gavin, J.; Shen, C.; Cierpicki, T.; Stuckey, J. A.; Showalter, H. D. H.; Nikolovska-Coleska, Z. 3-Substituted-N-(4-Hydroxynaphthalen-1-Yl)Arylsulfonamides as a Novel Class of Selective Mcl-1 Inhibitors: Structure-Based Design, Synthesis, SAR, and Biological Evaluation. J. Med. Chem. 2014, 57, 4111−4133. (67) Abulwerdi, F.; Liao, C.; Liu, M.; Azmi, A. S.; Aboukameel, A.; Mady, A. S. A.; Gulappa, T.; Cierpicki, T.; Owens, S.; Zhang, T.; Sun, D.; Stuckey, J. A.; Mohammad, R. M.; Nikolovska-Coleska, Z. A Novel Small-Molecule Inhibitor of Mcl-1 Blocks Pancreatic Cancer Growth in Vitro and in Vivo. Mol. Cancer Ther. 2014, 13, 565−575. (68) Bruncko, M.; Le Wang; Sheppard, G. S.; Phillips, D. C.; Tahir, S. K.; Xue, J.; Erickson, S.; Fidanze, S.; Fry, E.; Hasvold, L.; Jenkins, G. J.; Jin, S.; Judge, R. A.; Kovar, P. J.; Madar, D.; Nimmer, P.; Park, C.; Petros, A. M.; Rosenberg, S. H.; Smith, M. L.; Song, X.; Sun, C.; Tao, Z.-F.; Wang, X.; Xiao, Y.; Zhang, H.; Tse, C.; Leverson, J. D.; Elmore, S. W.; Souers, A. J. Structure-Guided Design of a Series of MCL-1 Inhibitors with High Affinity and Selectivity. J. Med. Chem. 2015, 58, 2180−2194. (69) Leverson, J. D.; Zhang, H.; Chen, J.; Tahir, S. K.; Phillips, D. C.; Xue, J.; Nimmer, P.; Jin, S.; Smith, M.; Xiao, Y.; Kovar, P.; Tanaka, A.; Bruncko, M.; Sheppard, G. S.; Wang, L.; Gierke, S.; Kategaya, L.; Anderson, D. J.; Wong, C.; Eastham-Anderson, J.; Ludlam, M. J. C.; Sampath, D.; Fairbrother, W. J.; Wertz, I.; Rosenberg, S. H.; Tse, C.; Elmore, S. W.; Souers, A. J. Potent and Selective Small-Molecule MCL-1 Inhibitors Demonstrate on-Target Cancer Cell Killing Activity as Single Agents and in Combination with ABT-263 (Navitoclax). Cell Death Dis. 2015, 6, e1590. (70) Petros, A. M.; Swann, S. L.; Song, D.; Swinger, K.; Park, C.; Zhang, H.; Wendt, M. D.; Kunzer, A. R.; Souers, A. J.; Sun, C. Fragment-Based Discovery of Potent Inhibitors of the Anti-Apoptotic MCL-1 Protein. Bioorg. Med. Chem. Lett. 2014, 24, 1484−1488. (71) Lanning, M. E.; Fletcher, S. Multi-Facial, Non-Peptidic α-Helix Mimetics. Biology (Basel, Switz.) 2015, 4, 540−555. (72) Azzarito, V.; Long, K.; Murphy, N. S.; Wilson, A. J. Inhibition of α-Helix-Mediated Protein-Protein Interactions Using Designed Molecules. Nat. Chem. 2013, 5, 161−173. (73) Lanning, M.; Fletcher, S. Recapitulating the α-Helix: Nonpeptidic, Low-Molecular-Weight Ligands for the Modulation of HelixMediated Protein−Protein Interactions. Future Med. Chem. 2013, 5, 2157−2174. 837



Perspective

(74) Kutzki, O.; Park, H. S.; Ernst, J. T.; Orner, B. P.; Yin, H.; Hamilton, A. D. Development of a Potent Bcl-xL Antagonist Based on α-Helix Mimicry. J. Am. Chem. Soc. 2002, 124, 11838−11839. (75) Ernst, J. T.; Becerril, J.; Park, H. S.; Yin, H.; Hamilton, A. D. Design and Application of an A-Helix-Mimetic Scaffold Based on an Oligoamide-Foldamer Strategy: Antagonism of the Bak BH3/Bcl-xL Complex. Angew. Chem. 2003, 115, 553−557. (76) Yin, H.; Hamilton, A. D. Terephthalamide Derivatives as Mimetics of the Helical Region of Bak Peptide Target Bcl-xL Protein. Bioorg. Med. Chem. Lett. 2004, 14, 1375−1379. (77) Moon, H.; Lee, W. S.; Oh, M.; Lee, H.; Lee, J. H.; Im, W.; Lim, H.-S. Design, Solid-Phase Synthesis, and Evaluation of a PhenylPiperazine-Triazine Scaffold as α-Helix Mimetics. ACS Comb. Sci. 2014, 16, 695−701. (78) Yin, H.; Lee, G.-i.; Sedey, K. A.; Kutzki, O.; Park, H. S.; Orner, B. P.; Ernst, J. T.; Wang, H.-G.; Sebti, S. M.; Hamilton, A. D. Terphenyl-Based Bak BH3 α-Helical Proteomimetics as LowMolecular-Weight Antagonists of Bcl-xL. J. Am. Chem. Soc. 2005, 127, 10191−10196. (79) Lanning, M. E.; Wilder, P. T.; Drennen, B.; Cavalier, M.; Bailey, H.; Chen, L.; Yap, J.; Raje, M.; Fletcher, S. Towards More Drug-Like Proteomimetics: Two-Faced, Synthetic α-Helix Mimetics Based on a Purine Scaffold. Org. Biomol. Chem. 2015, 13, 8642−8646. (80) Jung, K.-Y.; Vanommeslaeghe, K.; Lanning, M. E.; Yap, J. L.; Gordon, C.; Wilder, P. T.; MacKerell, A. D.; Fletcher, S. Amphipathic α-Helix Mimetics Based on a 1,2-Diphenylacetylene Scaffold. Org. Lett. 2013, 15, 3234−3237. (81) Kazi, A.; Sun, J.; Doi, K.; Sung, S.-S.; Takahashi, Y.; Yin, H.; Rodriguez, J. M.; Becerril, J.; Berndt, N.; Hamilton, A. D.; Wang, H.G.; Sebti, S. M. The BH3 Alpha-Helical Mimic BH3-M6 Disrupts BclX(L), Bcl-2, and MCL-1 Protein-Protein Interactions with Bax, Bak, Bad, or Bim and Induces Apoptosis in a Bax- and Bim-Dependent Manner. J. Biol. Chem. 2011, 286, 9382−9392. (82) Yap, J. L.; Cao, X.; Vanommeslaeghe, K.; Jung, K.-Y.; Peddaboina, C.; Wilder, P. T.; Nan, A.; MacKerell, A. D.; Smythe, W. R.; Fletcher, S. Relaxation of the Rigid Backbone of an OligoamideFoldamer-Based α-Helix Mimetic: Identification of Potent Bcl-xL Inhibitors. Org. Biomol. Chem. 2012, 10, 2928−2933. (83) Cao, X.; Yap, J. L.; Newell-Rogers, M. K.; Peddaboina, C.; Jiang, W.; Papaconstantinou, H. T.; Jupitor, D.; Rai, A.; Jung, K.-Y.; Tubin, R. P.; Yu, W.; Vanommeslaeghe, K.; Wilder, P. T.; MacKerell, A. D.; Fletcher, S.; Smythe, R. W. The Novel BH3 α-Helix Mimetic JY-1− 106 Induces Apoptosis in a Subset of Cancer Cells (Lung Cancer, Colon Cancer and Mesothelioma) by Disrupting Bcl-xL and Mcl-1 Protein-Protein Interactions with Bak. Mol. Cancer 2013, 12, 42. (84) Orner, B. P.; Ernst, J. T.; Hamilton, A. D. Toward Proteomimetics: Terphenyl Derivatives as Structural and Functional Mimics of Extended Regions of an α-Helix. J. Am. Chem. Soc. 2001, 123, 5382−5383. (85) Davis, J. M.; Truong, A.; Hamilton, A. D. Synthesis of a 2,3′;6′,3“-”Terpyridine Scaffold as an Alpha-Helix Mimetic. Org. Lett. 2005, 7, 5405−5408. (86) Gloaguen, C.; Voisin-Chiret, A. S.; Sopkova-de Oliveira Santos, J.; Fogha, J.; Gautier, F.; De Giorgi, M.; Burzicki, G.; Perato, S.; Pétigny-Lechartier, C.; Simonin-Le Jeune, K.; Brotin, E.; Goux, D.; N’Diaye, M.; Lambert, B.; Louis, M.-H.; Ligat, L.; Lopez, F.; Juin, P.; Bureau, R.; Rault, S.; Poulain, L. First Evidence That Oligopyridines, α-Helix Foldamers, Inhibit Mcl-1 and Sensitize Ovarian Carcinoma Cells to Bcl-xL-Targeting Strategies. J. Med. Chem. 2015, 58, 1644− 1668. (87) Boersma, M. D.; Haase, H. S.; Peterson-Kaufman, K. J.; Lee, E. F.; Clarke, O. B.; Colman, P. M.; Smith, B. J.; Horne, W. S.; Fairlie, W. D.; Gellman, S. H. Evaluation of Diverse A/B-Backbone Patterns for Functional α-Helix Mimicry: Analogues of the Bim BH3 Domain. J. Am. Chem. Soc. 2012, 134, 315−323. (88) Smith, B. J.; Lee, E. F.; Checco, J. W.; Evangelista, M.; Gellman, S. H.; Fairlie, W. D. Structure-Guided Rational Design of A/B-Peptide Foldamers with High Affinity for BCL-2 Family Prosurvival Proteins. ChemBioChem 2013, 14, 1564−1572.

(89) Walensky, L. D.; Kung, A. L.; Escher, I.; Malia, T. J.; Barbuto, S.; Wright, R. D.; Wagner, G.; Verdine, G. L.; Korsmeyer, S. J. Activation of Apoptosis in Vivo by a Hydrocarbon-Stapled BH3 Helix. Science 2004, 305, 1466−1470. (90) LaBelle, J. L.; Katz, S. G.; Bird, G. H.; Gavathiotis, E.; Stewart, M. L.; Lawrence, C.; Fisher, J. K.; Godes, M.; Pitter, K.; Kung, A. L.; Walensky, L. D. A Stapled BIM Peptide Overcomes Apoptotic Resistance in Hematologic Cancers. J. Clin. Invest. 2012, 122, 2018− 2031. (91) Stewart, M. L.; Fire, E.; Keating, A. E.; Walensky, L. D. The MCL-1 BH3 Helix Is an Exclusive MCL-1 Inhibitor and Apoptosis Sensitizer. Nat. Chem. Biol. 2010, 6, 595−601. (92) Gavathiotis, E.; Suzuki, M.; Davis, M. L.; Pitter, K.; Bird, G. H.; Katz, S. G.; Tu, H.-C.; Kim, H.; Cheng, E. H.-Y.; Tjandra, N.; Walensky, L. D. BAX Activation Is Initiated at a Novel Interaction Site. Nature 2008, 455, 1076−1081. (93) Gavathiotis, E.; Reyna, D. E.; Davis, M. L.; Bird, G. H.; Walensky, L. D. BH3-Triggered Structural Reorganization Drives the Activation of Proapoptotic BAX. Mol. Cell 2010, 40, 481−492. (94) Gavathiotis, E.; Reyna, D. E.; Bellairs, J. A.; Leshchiner, E. S.; Walensky, L. D. Direct and Selective Small-Molecule Activation of Proapoptotic BAX. Nat. Chem. Biol. 2012, 8, 639−645. (95) Lanning, M. E.; Yu, W.; Yap, J. L.; Chauhan, J.; Chen, L.; Whiting, E.; Pidugu, L. S.; Atkinson, T.; Bailey, H.; Li, W.; Roth, B. M.; Hynicka, L.; Chesko, K.; Toth, E. A.; Shapiro, P.; MacKerell, A. D.; Wilder, P. T.; Fletcher, S. Structure-Based Design of N-Substituted 1Hydroxy-4-Sulfamoyl-2-Naphthoates as Selective Inhibitors of the Mcl-1 Oncoprotein. Eur. J. Med. Chem. 2016, 113, 273−292. (96) Chen, L.; Wilder, P. T.; Drennen, B.; Tran, J.; Roth, B. M.; Chesko, K.; Shapiro, P.; Fletcher, S. Structure-Based Design of 3Carboxy-Substituted 1,2,3,4-Tetrahydroquinolines as Inhibitors of Myeloid Cell Leukemia-1 (Mcl-1). Org. Biomol. Chem. 2016, 14, 5505−5510. (97) Drennen, B.; Scheenstra, J. A.; Yap, J. L.; Chen, L.; Lanning, M. E.; Roth, B. M.; Wilder, P. T.; Fletcher, S. Structural Re-Engineering of the α-Helix Mimetic JY-1-106 Into Small Molecules: Disruption of the Mcl-1-Bak-BH3 Protein-Protein Interaction with 2,6-Di-Substituted Nicotinates. ChemMedChem 2016, 11, 827−833. (98) Beekman, A. M.; O’Connell, M. A.; Howell, L. A. Identification of Small-Molecule Inhibitors of the Antiapoptotic Protein Myeloid Cell Leukaemia-1 (Mcl-1). ChemMedChem 2016, 11, 840−844. (99) Pelz, N. F.; Bian, Z.; Zhao, B.; Shaw, S.; Tarr, J. C.; Belmar, J.; Gregg, C.; Camper, D. V.; Goodwin, C. M.; Arnold, A. L.; Sensintaffar, J. L.; Friberg, A.; Rossanese, O. W.; Lee, T.; Olejniczak, E. T.; Fesik, S. W. Discovery of 2-Indole-Acylsulfonamide Myeloid Cell Leukemia 1 (Mcl-1) Inhibitors Using Fragment-Based Methods. J. Med. Chem. 2016, 59, 2054−2066. (100) Wang, G.; Wang, Y.; Wang, L.; Han, L.; Hou, X.; Fu, H.; Fang, H. Design, Synthesis and Preliminary Bioactivity Studies of Imidazolidine-2,4-Dione Derivatives as Bcl-2 Inhibitors. Bioorg. Med. Chem. 2015, 23, 7359−7365. (101) Wan, Y.; Wang, J.; Sun, F.; Chen, M.; Hou, X.; Fang, H. Design, Synthesis and Preliminary Biological Studies of Pyrrolidine Derivatives as Mcl-1 Inhibitors. Bioorg. Med. Chem. 2015, 23, 7685− 7693. (102) Lee, S.; Wales, T. E.; Escudero, S.; Cohen, D. T.; Luccarelli, J.; Gallagher, C. G.; Cohen, N. A.; Huhn, A. J.; Bird, G. H.; Engen, J. R.; Walensky, L. D. Allosteric inhibition of antiapoptotic MCL-1. Nat. Struct. Mol. Biol. 2016, 23, 600−607. (103) Akçay, G.; Belmonte, M. A.; Aquila, B.; Chuaqui, C.; Hird, A. W.; Lamb, M. L.; Rawlins, P. B.; Su, N.; Tentarelli, S.; Grimster, N. P.; Su, Q. Inhibition of Mcl-1 through covalent modification of a noncatalytic lysine side chain. Nat. Chem. Biol. 2016, 12, 931−936.

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