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New Prospects for Vinblastine Analogues as Anticancer Agents Romano Silvestri* Dipartimento di Chimica e Tecnologie del Farmaco, Istituto PasteurFondazione Cenci Bolognetti, Sapienza Università di Roma, Piazzale Aldo Moro 5, I-00185 Roma, Italy ABSTRACT: Boger et al. synthesized a series of C20′ urea derivatives of vinblastine that matched or exceeded the potency of vinblastine in cell growth inhibition assays. The studies demonstrated the importance of the H-bond donor on the C20′ position and revealed the presence of a space surrounding the C20′ substituent that tolerates a wide variety of substituents, remarkably enhancing potency of vinblastine analogues. amino acid residues of α- or β-tubulin. The third group (colchicine, combretastatin A-4) includes numerous structurally unrelated small molecules that inhibit tubulin polymerization by binding to the colchicine site. The fourth group consists of compounds that interact at the level of the vinca domain. Besides the clinical importance of the taxoids, additional antimitotic agents might provide benefit to overcome drug resistance, reduce toxicity in patients, and/or show synergy at subtherapeutic doses when used in combination with other MT agents.3 Vincristine and vinblastine are indole-based dimeric alkaloids produced by the leaves of the periwinkle plant Catharanthus roseus (formerly known as Vinca rosea) that inhibit MT assembly by preventing tubulin polymerization. In 1991 Wilson first demonstrated that the antimitotic mechanism of vinca alkaloids does not require MT depolymerization. At high concentrations vinca alkaloids depolymerize MTs and destroy mitotic spindles: the dividing cancer cells appear blocked in mitosis with condensed chromosomes. At low concentrations, they block mitosis more subtly, and cells die by apoptosis. Recently, vinca alkaloids and many colchicine site binding agents have been shown to potently and rapidly induce vascular disruption, leading to tumor necrosis.4 It was proposed that the vinca domain contains both the vinca site, where competitive inhibitors bind, and nearby sites, where noncompetitive inhibitors bind.3 The vinca alkaloids vincristine and vinblastine, maytansine, ansamitocins P-3 and P-4, rhizoxin, and disorazol A1 bind in the vinca site and competitively inhibit each other’s binding to tubulin. Takahashi reported for maytansine and rhizoxin a distinct binding site on tubulin. Noncompetitive inhibitors of vinca alkaloid binding to tubulin include macrocyclic polyethers and peptides/depsipeptides. The former group consists of two families of complex molecules, the spongistatins (also known as the altohytrins) and the halichondrins. Peptides and depsipeptides are a large class of natural noncompetitive inhibitors containing highly modified amino acid residues that bind in the vinca domain. Vincristine and vinblastine have been successfully administered as a single agent and in combination with other drugs in cancer chemotherapy of hematological and solid malignancies.
T
he discovery of the anticancer properties of vinca alkaloids in the late 1950s was a milestone in the development of cancer chemotherapy. Beside vincristine and vinblastine, the semisynthetic vinorelbine has a major role in chemotherapeutic regimens, and its fluorinated analogue vinflunine has been used in the treatment of carcinoma of the bladder (Figure 1). The paper by Boger et al.1 reported the synthesis of C20′ urea-based analogues of vinblastine that substantially exceeded the potency of the parent compound and also showed activity against a P-gp overexpressing vinblastine-resistant tumor cell line. In contrast to expectations based on the X-ray crystallographic studies, urea derivatives at C20′ bearing large substitutions exhibited potent cytotoxic activity and effective binding to tubulin. The findings in this paper thus alter and broaden our understanding of the mechanism of action of vinca alkaloids. Microtubules (MTs) play a key role in the process of mitosis, during which the chromosomes of a cell are duplicated and separated to form two identical sets, allowing the cell to divide into two daughter cells. MTs are also involved in maintenance of cell shape, cell motility, intracellular transport, and many other cellular functions. MTs are formed from tubulin α,βheterodimers. These structures undergo highly dynamic polymerization and depolymerization transitions by the reversible addition of tubulin dimers at their end. Interference with this dynamic equilibrium, by either inhibiting tubulin polymerization or blocking MT disassembly, prevents proper MT function and ultimately leads to cell death. Because of their importance in mitosis and cell division and to the successful use in cancer therapy of agents interfering with their function, MTs continue to be an important target for the development of new anticancer drugs.2 After treatment with an anti-MT agent, typical hallmarks of mitotic arrest appear at the level of chromosomes, nuclear membrane, and the mitotic spindle, and this results in a sharp increase in the proportion of cells in the G2/M phase of the cell cycle. MT agents may be viewed as falling into five classes.3 The first two classes include the taxoids, epothilones, discodermolide, and eleutherobin, or alternatively, laulimalide and peloruside A. Compounds in these two classes bind to one of two alternative sites on β-tubulin and cause the hyperassembly and stabilization of microtubules. Drugs interacting with tubulin by the other three mechanisms all inhibit microtubule assembly. Compounds t-BCEU, T138067, pironetin, and ottelione A covalently react with © 2013 American Chemical Society
Received: January 2, 2013 Published: January 14, 2013 625
dx.doi.org/10.1021/jm400002j | J. Med. Chem. 2013, 56, 625−627
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Figure 1. Major vinca alkaloids in clinical use.
Figure 2. SAR summary of C20′ derivatives of vinblastine.
side effects are peripheral neuropathy and reversible myelosuppression. The mechanisms underlying vinca-induced neurotoxic effects (much greater for vincristine) to neuronal cell bodies reflect the affinity of the drugs for the axonal MTs and the ability of these to induce formation of tubulin spirals. Vindesine is a semisynthetic analogue of these alkaloids
Despite strong similarity in chemical structure, the different substituent at N1 affects considerably the pharmacological and toxicological profiles. Vincristine (R1 = CHO) is used in the clinical management of leukemias and lymphomas and in many pediatric solid turmors, while vinblastine (R1 = Me) is used to treat metastatic testicular cancer and Hodgkin’s disease. Major 626
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not important to their function. The N-methylurea derivative was notably less potent than the corresponding urea, suggesting that the NH directly attached to the C20′ forms a key H-bond that mimics the alcohol group of vinblastine. In the tubulin binding assay conducted by measuring the competitive displacement of 3H-vinblastine from porcine tubulin, two representative compounds, one containing the large biphenylurea substituent and the other the much smaller N,Ndimethylurea, showed slightly better affinity than vinblastine. The effects of these ureas, to the extent examined in the biological assays, correlated with their tubulin binding affinities and highlighted that the vinblastine interaction with tubulin surrounding the C20′ center is flexible and capable of reorganization to accommodate even a very large substituent. In summary, Boger et al. synthesized a remarkable series of C20′ urea derivatives of vinblastine that matched or exceeded the potency of the parent compound as cell growth inhibitors. These studies demonstrated the key role of the H-bond donor on the C20′ position and revealed the presence of a useful space surrounding the C20′ substituent that tolerates structurally different substituents, some of which led to enhanced potency. These findings open new prospects for vinblastine analogues as anticancer agents.
obtained by transformation of the ester function to a carboxamide and concomitant hydrolysis of the acetoxy group of the vindoline moiety. It is clinically used to treat melanoma, lung carcinoma, and uterine cancer in combination with other drugs. Modification of the tryptamine ring of anhydrovinblastine to a gramine via a modified Polonovky reaction led to vinorelbine. As with vinblastine and vincristine, vinorelbine causes microtubule depolymerization and mitotic spindle destruction at high concentrations, whereas at lower concentrations, it is able to block mitotic progression by subtle disruptive effects on the spindle. Vinorelbine binds to β-tubulin subunits at the vinca domain near the plus-end of microtubules. The rapid and reversible binding by vinorelbine to soluble tubulin induces a conformational change that increases the affinity of tubulin for itself and reduces the rate of lengthening and shortening in microtubule dynamics. Vinorelbine is used in non-small-cell lung cancer, metastatic breast cancer, and ovarian cancer. Superacid chemistry applied to vinca alkaloids resulted in the synthesis of the fluorinated analogue vinflunine, which had a novel chemical modification at the velbanamime moiety. Vinflunine was introduced in 2009 as an anticancer agent. It is less potent than vinorelbine and other vinca alkaloids in cell culture and in animal models but shows superior antitumor and antimetastatic effects compared to vinorelbine in xenograft models. Vinflunine showed vascular disruption activity and decreased tumor perfusion, suggesting that its antitumor activity is partially mediated by a vascular mechanism. Boger et al.1 reported the synthesis of C20′ amine, urea, and thiourea derivatives of vinblastine. Several of the C20′ ureabased analogues substantially exceed the potency of vinblastine and also showed activity against a P-gp overexpressing, vinblastine-resistant tumor cell line (Figure 2). In contrast to expectations based on the X-ray cocrystal structure of a tubulin bound complex, urea derivatives at C20′ bearing large substituents exhibited potent activity in cell-based proliferation assays and effective binding to tubulin. The vinblastine C20′ urea analogues were obtained by reacting 20′-aminovinblastine with commercially available isocyanates or alternatively with amines via an activated carbamate to provide the desired ureas. The vinblastine C20′ thiourea analogues were also prepared by treatment of 20′-aminovinblastine with isothiocyanates or alternatively of 20′-isothiocyanovinblastine with amines. Introduction of small (C1−C3) alkyl groups at N′ of the unsubstituted urea moiety led to significant enhancements in activity, improving on the potency of the parent compound and providing derivatives that were superior to vinblastine in HCT116 (human colon carcinoma) and L1210 (mouse leukemia) cell lines. Unexpectedly, Boger found that monosubstituted C20′ urea derivatives bearing N′-aryl or -heteroaromatic substituents all exhibited cell growth inhibitory activity at levels exceeding the parent unsubstituted urea, matching or surpassing the potency of vinblastine itself. To further explore the C20′ surrounding area, Boger synthesized the N′biphenylurea derivative. Again, this derivative displayed cell growth inhibitory activity at the same level as vinblastine, thus showing that the C20′ position may accept a wide variety of substituents modulating the chemical and physical properties of the drug. The monosubstituted N′-alkyl- or N′-arylthiourea derivatives were less active than the corresponding ureas. However, this activity difference diminished in the vinblastineresistant cell line. N,N-Disubstituted ureas exhibited potent cell growth inhibitory activity matching or surpassing the activity of vinblastine and indicating that a terminal H-bond donor site is
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REFERENCES
Corresponding Author
*Phone: +39 06 4991 3800. Fax: +39 06 4991 3133. E-mail:
[email protected]. (1) Leggans, E. K.; Duncan, K. K.; Barker, T. J.; Schleicher, K. D.; Boger, D. L. A remarkable series of vinblastine analogues displaying enhanced activity and an unprecedented tubulin binding steric tolerance: C20′ urea derivatives. J. Med. Chem. 2012, DOI: 10.1021/ jm3015684. (2) Jordan, M. A.; Wilson, L. Microtubules as a target for anticancer drugs. Nat. Rev. Cancer 2004, 4, 253−265. (3) Hamel, E.; Covell, D. G. Antimitotic peptides and depsipeptides. Curr. Med. Chem.: Anti-Cancer Agents 2002, 2, 19−53. (4) (a) Carlson, R. O. New tubulin targeting agents currently in clinical development. Expert Opin. Invest. Drugs 2008, 17, 707−722. (b) Biersack, B.; Schobert, R. Indole compounds against breast cancer: recent developments. Curr. Drug Targets 2012, 13, 1705−1719.
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