Patent Highlights: Recently Approved HCV NS5a ... - ACS Publications

Highlights from the Patents pubs.acs.org/OPRD

Patent Highlights: Recently Approved HCV NS5a Drugs ABSTRACT: Five inhibitors of the NS5a enzyme have been approved as part of oral regimens for the treatment of hepatitis C virus, including daclatasvir (Bristol-Myers Squibb), ledipasvir (Gilead Sciences), ombitasvir (AbbVie), elbasvir (Merck), and velpatasvir (Gilead Sciences). This article reviews worldwide patents and patent applications that have been published on synthetic routes and final forms for these five drugs. ublication policies at pharmaceutical and fine chemical companies vary substantially. While a few companies encourage publication of synthetic routes, many do not. As such, the patent literature may provide the only source of information regarding synthetic routes and final forms for approved drugs, drug candidates, and intermediates. Although patent publications do not offer the same insights and experimental details as a journal article, the patent literature offers useful chemistry nuggets that oftentimes are not disclosed in any other public forum. Some examples of worthwhile chemistry found in process chemistry patents include: novel bond disconnections to a drug candidate or intermediate; improved methodology of known reactions; application of chemistry on highly functionalized compounds; lessons in development, such as practical purification methods, removal of metals, dynamic resolution/racemization, separations, and novel crystallization approaches; crystalline polymorphs, salts, solvates, cocrystals, and methods of preparation. In this article we review the patent literature of five recently approved NS5a inhibitors for the oral treatment of hepatitis C virus (HCV). The rapid development of direct-acting antiviral therapy is an extraordinary medical achievement that has dramatically changed the treatment paradigm for HCV. The crystal structure of the NS5a protein, published in 2005, incorporates two monomers stacked in a dimeric configuration.1 The five approved inhibitors of NS5a are spatially quite similar and are designed to interact with each monomer of the protein. The inhibitors feature a rigid central nonpolar backbone flanked by symmetrical or near-symmetrical hydrophilic side chains that include a dipeptide of Moc-valine− proline or a proline analogue. Bristol-Myers Squibb was an early leader in the development of inhibitors of the NS5a protein.2 In 2008 the company achieved clinical proof of concept for the NS5a mechanism with daclatasvir, an important milestone in the pursuit of an all-oral regimen for HCV.3 This was the third HCV viral enzyme to achieve clinical proof of concept, after NS3/4A protease and NS5B polymerase. The public information available on the manufacturing routes and final forms of these drugs is summarized below. • Daclatasvir (BMS-790052), Bristol-Myers Squibb. A process patent and final form patent have been granted. A book chapter on the discovery and development of daclatasvir offers a brief but insightful overview of the final manufacturing

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© XXXX American Chemical Society

route and the development work that led to final route selection. No other journal articles on the manufacturing route have been published. A European Public Assessment Report (EPAR) provides some information on regulatory starting materials, crystal forms, and formulation. • Ledipasvir (GS-5885), Gilead. The IP portfolio includes one granted process patent, one process patent application, two crystal form patents, and one crystal form patent application. No process chemistry journal articles have been published to date. An EPAR provides some insight on regulatory starting materials, crystal forms, and formulation. • Ombitasvir (ABT-267), AbbVie. No process patents or patent applications have been published, nor any journal

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DOI: 10.1021/acs.oprd.6b00241 Org. Process Res. Dev. XXXX, XXX, XXX−XXX

Organic Process Research & Development

Highlights from the Patents

articles regarding the manufacturing route. The only synthetic information available from the company is found in Medicinal Chemistry publications and the composition of matter patent. One crystal form patent has been granted. One Chinese patent application has been published on an alternate route. An EPAR provides some insight on regulatory starting materials, crystal forms, and formulation. • Elbasvir (MK-8742), Merck. Three process patent applications on the synthesis have published and one patent application on crystal forms. Four journal articles from the process chemistry group have been published. Since elbasvir has not been approved in Europe, an EPAR has not been published. • Velpatasvir (GS-5816), Gilead. One process patent application on the synthesis and one patent application on crystal forms have been published. A Chinese process patent application has published on an alternate route. No process chemistry journal articles have been published to date. Given the recent EU approval, an EPAR has not been published. The order of discussion of the five NS5a inhibitors in this article follows the timing for first approval: Daklinza (daclatasvir), Bristol-Myers Squibb, approved in Japan, July 2014; Ledipasvir, Gilead, approved in the U.S. in October 2014; Ombitasvir, Abbvie, approved in the U.S. in December 2014; Elbasvir, Merck, approved in the U.S. in January 2016; Velpatasvir, Gilead, approved in the U.S. in June 2016.

Scheme 1. Daclatasvir Manufacturing Route

I. DACLATASVIR (BMS-790052), BRISTOL-MYERS SQUIBB Daklinza (daclatasvir) and Sunvepra (asunaprevir) were approved in Japan in July 2014 as that country’s first all-oral HCV therapy. Approval in Europe followed the next month. The FDA in Nov 2014 issued a complete response letter for daclatasvir, citing insufficient information to establish the safety and efficacy of daclatasvir subsequent to the withdrawal of the asunaprevir NDA. After providing additional clinical data for daclatasvir in combination with sofosbuvir, U.S. approval was granted in July 2015. Synthetic Route. A process patent describes the five-step manufacturing route to daclatasvir (Scheme 1).4 A book chapter on the discovery and development of daclatasvir offers an informative perspective on the selection and development of the manufacturing route, which is summarized below.5 The original Medicinal Chemistry route employed a late stage Suzuki-cross coupling (Scheme 2) to generate the biphenyl moiety, taking advantage of the versatility of this reaction to prepare symmetrical and unsymmetrical derivatives.6 The process chemistry approach took advantage of the commercially available bis-acetophenone 1 that incorporates the biphenyl moiety, thereby avoiding a late stage crosscoupling with the associated issues of Pd removal and the need to eliminate boronic acid residues, considered potential genotoxic impurities, to ppm levels. A further key improvement was the discovery that the bis-imidazole 4 could be formed from the corresponding ketoester 3 instead of the ketoamide, a reaction that was higher yielding and used simpler starting materials. This reaction has since been used to form the imidazole moiety common in many NS5a inhibitors, including ledipasvir and velpatasvir (vide infra). While the most convergent route is reaction of the bisbromide 2 with the proline−valine dipeptide, formation of the

imidazole ring subsequent to this resulted in epimerization of the valine chiral center and an intermediate that was not crystalline (Scheme 3), so this approach was abandoned. In the longer route ultimately selected as the manufacturing route (Scheme 1), ketoester 3 is prepared from bis-bromide 2 and Boc-proline, which is not isolated but directly converted to the bis-imidazole 4, isolated as a crystalline free base in high chemical and diasatereomeric purity. Daclatasvir is then formed by Boc-deprotection with HCl to afford 5, which is crystallized as a tetra HCl salt, followed by amide coupling with Moc-Lvaline. Overall yield from bis-acetophenone 1 is reported as 43−58% in the book chapter5 and 26% from the experimental in the patent,4 suggesting the conditions reported in the patent are not those ultimately optimized for manufacture. The B




In summary, a single process patent describes and claims the likely manufacturing route to daclatasvir. No compounds are claimed in this patent. Somewhat surprisingly, the alternate routes that were studied, but ultimately not pursued, were not patented nor even disclosed in the process patent. Nonetheless, since these alternate routes have been described in a book chapter, they cannot be patented by another company and provide freedom to operate.

Scheme 2. Medicinal Chemistry Route to Daclatasvir

II. LEDIPASVIR (GS-5885), GILEAD SCIENCES On October 10, 2014 the FDA approved the fixed-dose combination product ledipasvir 90 mg/sofosbuvir 400 mg, sold under the brand name Harvoni by Gilead Sciences. The combination product was subsequently approved in Europe and Japan. Gilead has entered into a voluntary license agreement with 11 Indian drug manufacturers which allows these companies to manufacture and market generic versions of sofosbuvir and ledipasvir in 101 developing countries, potentially providing access to over 100 million indigent HCV patients.10 Ledipasvir Synthesis. Final Steps. The apparent manufacturing route to ledipasvir (Scheme 4) is described in U.S. Patent 9,056,860, issued June 16, 2015.11a The manufacturing route follows the same sequence as the Medicinal Chemistry route12 but with optimization of reagents and conditions. The process patent only claims the final steps to ledipasvir; the early

Scheme 3. Convergent Route to Daclatasvir

Scheme 4. Manufacturing Route for Ledipasvir

conditions and yields in Scheme 1 are those reported in the patent. According to information provided in the European Public Assessment Report (EPAR) on Daklinza, the API is synthesized in three steps from three regulatory starting materials, with the first step being an alkylation followed by formation of the imidazole ring.7 Therefore, the three regulatory starting materials (RSMs) appear to be dibromide 2, Boc-L-proline, and Moc-L-valine (or L-valine). Final Form of Daclatasvir. A patent has been granted on a single polymorph of the crystalline bis-HCl salt, denoted form N-2, which is crystallized from ethanol.8 According to the FDA biopharmaceutics review of the NDA, daclatasvir is Biopharmaceutical Classification System (BCS) Class 2, (low solubility, high permeability) and has an absolute bioavailability of 67%.9 As reported in the EPAR, one other polymorph of the bisHCl salt was found (N-1), but the N-2 polymorph was determined to be the thermodynamically most stable polymorph.7 No other crystalline salts and no crystalline free base polymorphs have been reported.7 The approved 30 mg and 60 mg immediate release, film-coated tablets are manufactured using a dry granulation process.7 C




Table 1. Borylation and Suzuki−Miyaura Reaction Conditions medicinal chemistry borylating agent base catalyst solvent temperature isolation

catalyst solvent base temperature isolation overall yield

bis(pinacolato) diboron KOAc PdCl2(dppf)2 (2.4%) dioxane 90 °C isolated by flash chromatography Pd(OAc)2, PPh3 DME/water NaHCO3 93 °C isolated by flash chromatography 85%

Process 1 bis(pinacolato)diboron

Process 2 Borylation bis(pinacolato)diboron

CH3CH2CO2K 5% PdCl2[P(t-Bu)2Ph]2 i-PrOAc 75 °C none

KOAc 5% Pd(OAc)2, 2-dicyclohexylphosphino2′-methylbiphenyl t-amyl alcohol 85 °C none

Suzuki−Miyaura same as borylation same as borylation i-PrOAc, water 2-phase t-amyl alcohol/water K3PO4 K3PO4 75 °C 85 °C remove Pd with aq N-acetyl-L-cysteine remove Pd with aq N-acetyl-L-cysteine wash, isolated as oxalate salt wash, isolated as oxalate salt 81% 77%

Process 3 bis(neopentylglycolato)boron CH3CH2CO2K 5% PdCl2[P(t-Bu)2Ph]2 i-PrOAc 72 °C none

same as borylation i-PrOAc, water 2-phase K3PO4 72 °C remove Pd with aq N-acetyl-L-cysteine wash, isolated as oxalate salt 87%

competitors from using either cross-coupling approach until the compound patent on 11 expires. To complete the synthesis of ledipasvir, deprotection of both Boc groups of 12 is accomplished with HCl in MeCN/water (∼1:1) at 65 °C in 80% yield after isolation as the tetra-HCl salt 13. The final bis-amide coupling with Moc-L-valine, mediated with EDC-HCl, HOBt, and N-methylmorpholine, is described on a 5 g scale with isolation of ledipasvir as an acetone solvate in 73% yield. Alternate Route. A variation to the Gilead route has been published in Chinese patent application CN 104926796 A, in which the imidazole ring-closure is deferred until after crosscoupling.15 As a somewhat less convergent route, the advantage of this approach is not clear. The application includes two other noteworthy variations: (1) Ammonium acetate in DMF at 50 °C is used for imidazole formation, milder conditions that those used by Gilead (toluene at 90 °C, Scheme 5). As noted above, BristolMyers Squibb did not pursue the more convergent route to daclatasvir due to epimerization of the valine chirality under the ammonium acetate conditions at 90 °C (Scheme 3). (2) Deprotection of the Boc groups of 12 is accomplished with MeSO3H and generates the tetra-mesylate salt of 13. Synthesis of Starting Materials 9 and 11. Although not claimed, routes to intermediates 9 and 11 are described in U.S. Patent 9,056,860.11 One route to compound 11 is presented in Scheme 5, consisting of eight steps with a longest linear sequence of six steps. Three routes to cyclopropyl-proline 17 are described, two involving resolution (enzymatic approach shown in Scheme 5) and one via cyclopropanation of N-Boc-4methylene-L-proline (not shown). Perhaps the most interesting reaction in the sequence is the electrophilic difluorination of fluorene 19, which is carried out below −55 °C in THF using a slow addition of LiHMDS to a solution of 19 and 3 equiv of (PhSO2)2NF. The synthesis of compound 9 is accomplished in seven linear steps (Scheme 6). The asymmetry is introduced via an azaDiels−Alder reaction with (S)-α-methylbenzyl amine as chiral auxiliary.16 While the amide coupling of 25 with 4-bromo-1,2diaminobenzene generates two isomers (26 and 27), each of these condense intramolecularly to imidazole 9.

steps are described but not claimed. Compounds 11, 12, and 13 were claimed in U.S. Patent Application 2013/0324740 A1,11b the precursor to U.S. Patent 9,056,860, but these claims were not granted in the issued patent. An additional patent application, 2015/0232453 A1, specifically claims compound 11.13 Three variations of the borylation/cross-coupling sequence are described in the process patent11 and are compared to the Medicinal Chemistry process in Table 1. A key improvement relative to the Medicinal Chemistry process is the telescoping of the borylation and Suzuki−Miyaura steps into a one-pot process, which required development of a catalyst and solvent system suitable for both reactions. Both bis(pincalato)boron and bis(neopentylglycolato)boron are used for the borylation (Scheme 1 only shows the latter). Two different catalyst/ solvent systems are reported for the reactions involving bis(pincalato)boron (processes 1 and 2 in Table 1). For process 1, the borylation of 9 is carried out in i-PrOAc with potassium propanoate, perhaps for improved solubility vs KOAc. The catalyst, chlorobis(di-t-butylphenylphosphine)palladium(II), is used at the 5 mol % level. An equal volume of aq. K3PO4 is added along with 11 for the Suzuki−Miyaura reaction, which is conducted at 75 °C as a two-liquid phase reaction. For process 2, the borylation is carried out with KOAc as base in t-amyl alcohol using a catalyst system of Pd(OAc)2 and 2-dicyclohexylphosphino-2′-methylbiphenyl (5 mol %). The Suzuki−Miyaura reaction is carried out at 85 °C after addition of an equal volume of aq. K3PO4. For all three processes, workup involves extraction with aq. N-acetyl-L-cysteine14 to complex and remove Pd, which remains in the aq layer, and isolation of the crystalline oxalate salt from EtOH/i-PrOAc. The opposite sequence, borylation of fragment 11 and crosscoupling with 9, was noted as a viable alternative, but this approach is not claimed, and no reagents or experimental details are provided.11 Nonetheless, disclosure prevents other companies from patenting the route and provides freedom to operate should Gilead elect to transition to this route in the future. U.S. Patent Application 2015/0232453 A113 includes a claim for compound 11, which, if granted, would preclude D




Regulatory Starting Materials (RSMs). The European Public Assessment Report (EPAR) for Harvoni from the Committee for Medicinal Products for Human Use (CHMP), dated 25-Sep-2014,17 offers insights on the definition of regulatory starting materials (RSMs), a topic which has been discussed in a recent series of papers in this journal.18 Regulatory starting materials define the point beyond which cGMP manufacturing practices must be implemented. Gilead’s original proposal for regulatory starting materials (not disclosed) was rejected by the CHMP, who requested Gilead to redefine three starting materials. Two of these were accomplished before the CHMP opinion was issued in Sep2014, while the two sides agreed the third would be completed after European approval. While just speculation, this suggests that the two cross-coupling partners, 9 and 11, were likely not acceptable as RSMs. It is not known from information provided publicly by Gilead if the company had received guidance from the FDA on RSMs, but it is possible that the FDA and CHMP may not be aligned in this case on RSM definition. Interestingly, the CHMP also rejected Gilead’s original proposal for RSM’s for Sovaldi (sofosbuvir).19 Ledipasvir Final Form. U.S. Patent 9,139,570 describes and claims formation of the D-tartrate salt of ledipasvir.20 U.S. Patent Application 2015/0344488 A1 describes the preparation of 15 crystal forms of ledipasvir free base, including 5 anhydrous forms and 10 solvates (Figure 1).21 The anhydrous

Scheme 5. Synthesis of Compound 11

Scheme 6. Preparation of Compound 9

Figure 1. Solvates and anhydrous crystal forms of ledipasvir free base.

crystal forms were created by desolvation of the solvates through heat and/or vacuum drying. The forms which are claimed are the EtOAc and i-PrOAc solvates and anhydrous forms III (from acetone solvate) and V (from EtOAc solvate) (Figure 1, dotted boxes). E




Acetone solvates are claimed in U.S. Patent 8,969,588.22 The diacetone solvate (Form I) crystallizes initially from acetone solution. Drying under vacuum at room temperature produces the monoacetone solvate form (Form II). This form can be dehydrated by vacuum drying at 115 °C. According to European Public Assessment Report (EPAR) for Harvoni,17 the monoacetone solvate is the form of ledipasvir API used in the Harvoni combination product. In the drug product process,17 ledipasvir is dissolved in ethanol and spraydried, making control of the morphology and particle size of the API is nonessential. Based on its low solubility and high permeability, ledipasvir free base is designated as BCS class II.17 The EPAR reports that, for Phase 1 clinical trials, ledipasvir was dosed as an amorphous free base in film-coated tablets.17 The D-tartrate salt was used for phase 2 studies but was found to have significant food effects, which is also described in U.S. Patent Application 20140212487 A1.23 A conventional 30 mg tablet formulation using the tartrate salt had AUC reduced by half when dosed with a high fat food meal vs fasted. In addition, animal data indicated reduced bioavailability of the tartrate salt and amorphous free base when dosed with gastric acid suppressing agents such as famotidine.23 Both effects were mitigated by use of the acetone solvate in a spray-dried dispersion formulation, which showed no food effect and no drug−drug interaction with famotidine.23 The commercial formulation of the fixed-dose combination product with sosfobusvir employs a spray-dried dispersion formulation.17

Scheme 7. Medicinal Chemistry Route to Ombitasvir

III. OMBITASVIR (ABT-267), AbbVie Viekira Pak, a combination of ombitasvir, paritaprevir, dasabuvir, and ritonavir, was approved in the U.S. in Dec 2014 and in Europe in Jan 2015. Synthetic Routes to Ombitasvir. The only published route to ombitasvir provided by AbbVie is described in the compound patent24 and a publication of the discovery route (Scheme 7).25 In the Medicinal Chemistry route, the chirality of the central pyrrolidine is introduced by asymmetric reduction of the bisketone 30 using an in situ oxazaborolidine catalyst formed with (S)-diphenylprolinol at 17 mol %.26 Two variations are described for the final steps. Either the Moc-L-valine-proline dipeptide 35 can be prepared first, then coupled with bis-amine 34 (Scheme 7), or this can be accomplished sequentially by coupling first with Boc-proline, deprotection of the Boc-groups, then final coupling with MocL-valine (not shown). According to the EPAR,27 ombitasvir is manufactured in seven steps from five regulatory starting materials. Assuming the manufacturing route follows Scheme 7, this suggests a conservative approach to RSM definition with likely RSMs including 28, 29, 4-t-butylaniline, Moc-L-valine (or L-valine), and L-proline (or Boc-L-proline depending on the final steps). An alternate route has been described in a Chinese patent application28 in which a chiral Betti base29 is used as an auxiliary to generate the asymmetric central pyrrolidine (Scheme 8). Intermediate 36 can be prepared in 2 chemical steps followed by a resolution with tartaric acid.30 Conversion to 37 occurs with inversion of stereochemistry at both chiral centers. After deprotection with ceric ammonium nitrate, the tbutylphenyl moiety is then introduced by a C−N cross coupling reaction to generate intermediate 33. The experimental provides no information on control of stereoselectivity.

Scheme 8. Alternate Route to Intermediate 33

F




Final Form. A patent application has been published with a single claim of a hydrate form of ombitasvir free base.31 This form is crystallized from EtOH/water, dried, and then rehydrated using a humid atmosphere. This patent also discloses an anhydrate form and an ethanol−water solvate. According to the EPAR, multiple crystal forms of ombitasvir have been discovered.27 The active substance is Form I, which is a free base with 4.5 mol water,27 and is the apparent form that is claimed in the crystal form patent.31 According to the EPAR, ombitasvir is formulated using a hotmelt extrusion process during which Form I is converted to an amorphous phase.27,32 Therefore, particle size and morphology of the API form are not critical. Crystalline ombitasvir has low aqueous solubility and poor bioavailability.27 For early clinical studies, a spray dried formulation was used.27 A comparison of bioavailability of ombitasvir tablets compounded by spray dry and hot melt extrusion processes showed that both the Cmax (maximum concentration) and total absorption (AUC) from the HME formulation were substantially higher as compared to the spray dried formulation.27,32

A stereoselective process chemistry route is described in a patent application34 and two publications,35,36 as summarized below (Schemes 10 and 11). Scheme 10. First Asymmetric Route to Elbasvir, Early Steps

IV. ELBASVIR (MK-8742), MERCK AND CO., INC. Zepatier, a combination of elbasvir and grazoprevir, was approved in the U.S. in January 2016. Synthetic Routes to Elbasvir. The Medicinal Chemistry route to elbasvir (Scheme 9), while short and efficient, requires a final separation of diastereomers using supercritical fluid chromatography (SFC) with a chiral column.33 Scheme 9. Medicinal Chemistry Route to Elbasvir

A reaction that is taught in textbooks but rarely applied in practice, the Fries rearrangement, is used to prepare 45 by dissolving 44 in neat MeSO3H containing 4% methanesulfonic anhydride at 65 °C. Phenol 45 is directly isolated from the reaction mixture by crystallization after addition of i-PrOH/ water. Treatment of 45 with anhydrous ammonia in MeOH generates imine 46, which crystallizes directly from the reaction mixture. Asymmetric reduction of imine 46 is accomplished via transfer hydrogenation with chiral Ru-DPEN catalyst 47 in dichloromethane, providing 48 in 98% ee in the reaction mixture and >99% ee after crystallization. Ring closure via intramolecular C−N bond formation catalyzed by CuI cleanly forms indoline 49 with no evidence of intermolecular cross coupling, despite the presence of three bromines in the molecule. The chiral center of indoline 49 is then used to induce formation of the new stereocenter of hemiaminal 50. The initial dr of 7:1 at the end of reaction is improved to 99:1 via a crystallization-induced dynamic transformation with TFA in MeCN, wherein the increased stability (lower solubility) of the desired diastereomer vs undesired, coupled with the reversibility of the hemiaminal reaction, drives conversion to the desired diastereomer 50. After finding that many oxidants caused racemization at the aminal center, conversion to indole 51 was accomplished with KMnO4 with no loss in ee. Alternatively, photooxidation G




late stage compound claims would secure IP protection of the process route, preventing simple “work arounds” such as changing solvents or bases that might be outside of the process claims. A patent application39 and publication40 document an alternative route to elbasvir which is highlighted by a C(sp2)N(sp2) cross coupling which affords the indole product directly, avoiding the oxidation (Scheme 13). With use of the chiral bis-

Scheme 11. Elbasvir Synthesis, Final Steps

Scheme 13. Second Asymmetric Route to Elbasvir, Early Steps

phosphine ligand 61, the asymmetric center can also be established in the cross coupling step, yielding the indole 60 in high yield and ee. High throughput screening was essential in identifying the optimal ligands and conditions. Another improvement versus the first generation process route is combining ester formation and Fries rearrangement to produce phenol 57 in a single pot reaction mediated by triflic acid. Final Form. A patent application on elbasvir final forms describes and claims eight free base solvates, a free base hydrate, and a salt with 1,5-naphthalene disulfonic acid.41 No information is provided on which is the designated form of the API. A formulation patent application describes solid dispersion formulations for elbasvir suggesting that the API form, morphology, and particle size distribution may not be critical for the solid dosage form.42

catalyzed by an iridium complex and t-butylperoxybenzoate (2 equiv) can be used to effect this transformation.37,38 Cross coupling of indole 51 with imidazole−proline fragment 53 via the bis-Bpin intermediate 52 is accomplished as a one-pot process to afford 54, which is isolated as a bis salt of 4-nitrobenzoic acid (Scheme 11). Salt break and deprotection provide 55 as the tetra HCl salt. The final step involves bis-amide coupling with Moc-L-valine to produce elbasvir, which is crystallized from ethanol. The patent also describes but does not claim, cross coupling of the entire imidazole-proline-Moc-valine fragment 56 which affords elbasvir directly after cross coupling (Scheme 12). While more convergent, this route introduces Pd and boronic acid residues (PGIs) in the final step and would require a robust crystallization to remove process impurities. In addition to the process claims, the patent application claims compounds 48, 50, 51, 52, 54, and 55.34 If granted, the

V. VELPATASVIR (GS-5816), GILEAD SCIENCES Epclusa, a fixed-dose combination of velpatasvir (100 mg) and sofosbuvir (400 mg), was approved in the U.S. on June 28, 2016 and in Europe on July 8, 2016, for treatment of HCV genotypes 1−6. Synthetic RouteFinal Steps. The final steps to velpatasvir from backbone dibromide 62 (Scheme 14) are described and claimed in a process chemistry patent application.43 The bond disconnections are the same as described in the composition of matter patents.44 The phenacyl bromide of 62 is selectively alkylated with the chiral methoxylmethyl proline 63 using K2CO3 in CH2Cl2 to provide intermediate 64. After aqueous workup, the solvent is switched into THF for the alkylation of the secondary bromide with the 2-methylproline-Moc-L-valine dipeptide 65 using Cs2CO3 to afford bis-ester 66. Formation of the bis-imidazole 67 is

Scheme 12. Convergent Cross-Coupling for Elbasvir

H




Dehydrogenation to the aromatic core 68 is accomplished with DDQ and HOAc in 2-MeTHF solvent. Deprotection of the Boc group with HCl/MeOH affords the bis-amine 69 which is crystallized as a triphosphate salt. The final amide coupling with the chiral phenylglycine carbamate fragment 70 mediated by CDMT affords velpatasvir. No yields are provided in the experimental section of the patent and characterization is limited to generalized 1H NMR data, ie, the aromatic region of velpatasvir is reported as δ 8.56−6.67 (m, 13H).43 Synthetic RouteEarly Steps. Multiple routes to intermediates 62, 63, and 65 are described in the process patent application but are not claimed.43 The route to 2-methylproline-Moc-valine fragment 65 is outlined in Scheme 15.45 N-Boc (S)-pyroglutamic acid ethyl

Scheme 14. Final Steps to Velpatasvir

Scheme 15. Route to Intermediate 65

ester is ring-opened with methylmagnesium bromide to form Boc-amine 71. Deprotection with TFA and reductive amination with NaBH(OAc)3 are conducted in a one-pot reaction. Hydride transfer to the intermediate imine occurs on the face opposite to the ethoxycarbonyl group to afford cis-pyrrolidine 72, which is isolated as the tosylate salt. Amide coupling with Moc-L-valine followed by hydrolysis affords the dipeptide 65. Three routes are described for the synthesis of fragment 63. The first route, and the only chemistry in the patent that is described on a multikilo scale, is outlined in Scheme 16. Dimethyl N-Boc-L-glutamate is formylated at low temperature with acetic formic anhydride, which is cyclized to the enamine 73 with TFA. The patent scheme shows both Boc groups present in structure 73, so it is not clear if this is an error or if the Boc groups remain intact upon TFA treatment and whether imine formation can occur with the Boc-protected amine. Hydrogenation of the double bond is carried out with Pd/C; then the ester is reduced to the primary alcohol 74 with NaBH4. Deprotection of both Boc groups is followed by reprotection of the nitrogen to afford 75. After methylation, the dicyclohexylamine salt of 63 is crystallized, presumably to remove the trans-diastereomer formed during the hydrogenation. After salt break, 63 free acid is crystallized from hexane/CH2Cl2. The second approach to 63 starts with N-Boc-cis-4-cyano-Lproline methyl ester and converts the cyano group to the methoxymethyl group in 4−5 steps (Scheme 17). The stereochemistry appears to be maintained at both chiral centers through the sequence. Methanolysis of the cyano group to the methyl ester occurs with concomitant deprotection of the Boc

conducted using NH4OAc in toluene/i-PrOH, conditions similar to those originally described for daclatasvir (Scheme 1) and also used for ledipasvir (Scheme 5) except that i-PrOH is added as cosolvent, likely for increased solubility. I




Scheme 16. Primary Route to Intermediate 63

Scheme 18. Synthesis of Tetracyclic Backbone 62

Scheme 19. Alternate Approach to Velpitasvir

Scheme 17. Alternate Route to Intermediate 63

of velpitasvir alone and as a combination with sofosbuvir,48 suggesting that velpitasvir is rendered amorphous during the formulation process. According to the patent applications, several forms of velpitasvir API are suitable for use in the solid dispersion process although a specific claim is made for a spraydried process of free base in ethanol.48

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SUMMARY Five NS5a inhibitors have been approved over the past 2 years as part of oral regimens for the treatment of HCV. The patent literature discloses the likely manufacturing routes to these five drugs from commercially available raw materials, although the details of the optimized routes are not provided. The patent strategy for most companies is to secure claims for the final steps of a manufacturing route, while disclosing a number of potential routes to earlier intermediates to ensure freedom to operate. Obtaining claims for late stage intermediates is also a key goal, since this prevents a competitor from using a route that intersects with the patented compound as well as preventing “work arounds” of a chemical step, such as changing a solvent or base, until such patent expires. Compound claims are harder to secure if the manufacturing route follows the same bond disconnections as the Medicinal Chemistry route since the intermediates of interest will have already been disclosed in the composition of matter patents. Regarding final forms of an API, companies want to ensure they own patent claims to the form intended to be used commercially. In addition, most companies attempt to secure claims for as many other polymorphs, solvates, cocrystals, and salts as possible. However, with the USPTO requiring demonstration of novelty and usefulness, it has become harder to obtain claims on alternate forms. Nonetheless, disclosure of alternate forms in the published patent application achieves freedom to operate and prevents a competitor from obtaining a patent claim on any disclosed form.

group, so reprotection is necessary. The ester at the 4-position is then selectively hydrolyzed with 1.4 equiv of NaOH in THF at −1 °C to afford ester-acid 76. No yield is provided so no information is available for the selectivity of hydrolysis at the 4position vs the more hindered 2-position, except that hydrolysis later in the sequence requires a temperature of 20 °C. Reduction of the carboxylic acid to the primary alcohol is accomplished with borane−dimethyl sulfide followed by hydrolysis of the methyl ester, then alkylation of the primary alcohol with MeI to afford 63, which is purified by crystallization from i-PrOH/water. Alternatively, hydrolysis of the ester and alkylation can be carried out in a single pot reaction with 63 crystallized from toluene/heptane. A number of routes to backbone 62 are described, but all rely on an alkylation/C−H activation sequence as outlined in Scheme 18. An alternate bond disconnection for the synthesis of velpatasvir is described in a Chinese patent application in which left (77) and right-hand (78) fragments are more fully elaborated and then the tetracyclic backbone is constructed at a late stage.46 This route is more convergent than the Gilead route but overall requires a similar number of steps (Scheme 19). Final Form. A patent application describes and claims 19 crystal forms of velpatasvir, including free base (1 form), bisHCl salt (5 forms), phosphate salt (9 forms), bis-HBr salt (1 form), L-tartrate salt (2 forms), and D-tartrate salt (1 form).47 Two patent applications describe solid dispersion formulations

David L. Hughes* J



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(10) http://www.gilead.com/~/media/Files/pdfs/other/ HCVGenericAgreementFactSheet.pdf. (11) (a) Scott, R. W.; Vitale, J. P.; Matthews, K. S.; Teresk, M. G.; Formella, A.; Evans, J. W. Synthesis of Antiviral Compound. U.S. Patent 9,056,860 B2, June 16, 2015. (b) Scott, R. W.; Vitale, J. P.; Matthews, K. S.; Teresk, M. G.; Formella, A.; Evans, J. W. Synthesis of Antiviral Compound. U.S. Patent Application 2013/03244740 A1, December 5, 2013. (12) Link, J. O.; Taylor, J. G.; Xu, L.; Mitchell, M.; Guo, H.; Liu, H.; Kato, D.; Kirschberg, T.; Sun, J.; Squires, N.; Parrish, J.; Keller, T.; Yang, Z.-Y.; Yang, C.; Matles, M.; Wang, Y.; Wang, K.; Cheng, G.; Tian, Y.; Mogalian, E.; Mondou, E.; Cornpropst, M.; Perry, J.; Desai, M. C. J. Med. Chem. 2014, 57, 2033−2046. (13) Scott, R. W.; Vitale, J. P.; Matthews, K. S.; Teresk, M. G.; Formella, A.; Evans, J. W. Synthesis of Antiviral Compound. U.S. Patent Application 2015/0232453 A1, August 20, 2015. (14) Villa, M.; Cannata, V.; Rosi, A.; Allegrini, P. Process for the Removal of Heavy Metals. U.S. Patent 6,239,301 B1, May 29, 2001. (15) Li, Z.; Hu, H.; Huang, H.; Qi, X.; Gu, W.; Lin, Y. Preparation of a New NS5a Inhibitor Drug. Chinese Patent Application CN 104926796 A, September 23, 2015. (16) Stella, L.; Abraham, H.; Feneau-Dupont, J.; Tinant, B.; Declercq, J. P. Tetrahedron Lett. 1990, 31, 2603−2606. (17) European Public Assessment Report (EPAR) for Harvoni from the Committee for Medicinal Products for Human Use (CHMP), September 25, 2014. http://www.ema.europa.eu/docs/en_GB/ document_library/EPAR_-_Public_assessment_report/human/ 003850/WC500177996.pdf. (18) (a) Faul, M. M.; Kiesman, W. F.; Smulkowski, M.; Pfeiffer, S.; Busacca, C. A.; Eriksson, M. C.; Hicks, F.; Orr, J. D. Org. Process Res. Dev. 2014, 18, 587−593. (b) Faul, M. M.; Busacca, C. A.; Eriksson, M. C.; Hicks, F.; Kiesman, W. F.; Smulkowski, M.; Orr, J. D.; Pfeiffer, S. Org. Process Res. Dev. 2014, 18, 594−600. (c) Faul, M. M.; Argentine, M. D.; Egan, M.; Eriksson, M. C.; Ge, Z.; Hicks, F.; Kiesman, W. F.; Mergelsberg, I.; Orr, J. D.; Smulkowski, M.; Wächter, G. A. Org. Process Res. Dev. 2015, 19, 915−924. (19) http://www.ema.europa.eu/docs/en_GB/document_library/ EPAR_-_Public_assessment_report/human/002798/WC500160600. pdf. (20) Mogalian, E.; Scott, R. W.; Shi, B.; Wang, F. Solid Forms of an Antiviral Compound. U.S. Patent 9,139,570 B2, September 22, 2015. (21) Mogalian, E.; Scott, R. W.; Shi, B.; Wang, F. Solid Forms of an Antiviral Compound. U.S. Patent Application, 2015/0344488 A1, December 3, 2015. (22) Scott, R. W.; Wang, F.; Shi, B.; Mogalian, E. Solid Forms of an Antiviral Compound, U.S. Patent 8,969,588 B2, March 3, 2015. (23) Mogalian, E.; Oliyai, R.; Stefanidis, D.; Zia, V. Solid dispersion formulation of an antiviral compound. U.S. Patent Application 2014/ 0212487 A1, July 31, 2014. (24) DeGoey, D. A.; Kati, W. M.; Hutchins, C. W.; Donner, P. L.; Drueger, A. C.; Randolph, J. T.; Motter, C. E.; Nelson, L. T.; Patel, S. V.; Matulenko, M. A.; Keddy, R. G.; Jinkerson, T. K.; Gao, Y.; Liu, D.; Pratt, J. K.; Rockway, T. W.; Maring, C. J.; Hutchinson, D. K.; Flentge, C. A.; Wagner, R.; Tufano, M. D.; Betebenner, D. A.; Sarris, K.; Woller, K. R.; Wagaw, S. H.; Califano, J. C.; Li, W.; Caspi, D. D.; Bellizzi, M. E. Anti-viral Compounds. U.S. Patent 8,691,938 B2, April 8, 2014. (25) DeGoey, D. A.; Randolph, J. T.; Liu, D.; Pratt, J.; Hutchins, C.; Donner, P.; Krueger, A. C.; Matulenko, M.; Patel, S.; Motter, C. E.; Nelson, L.; Keddy, R.; Tufano, M.; Caspi, D. D.; Krishnan, P.; Mistry, N.; Koev, G.; Reisch, T. J.; Mondal, R.; Pilot-Matias, T.; Collins, Y. C.; Wagner, R.; Kati, W. M. J. Med. Chem. 2014, 57, 2047−2057. (26) Periasamy, M.; Seenivasaperumal, M.; Rao, V. D. Synthesis 2003, 2507−2510. (27) European Public Assessment Report (EPAR), Viekirax, 2014. http://www.ema.europa.eu/docs/en_GB/document_library/ EPAR_-_Public_assessment_report/human/003839/WC500183999. pdf.

Cidara Therapeutics, 6310 Nancy Ridge Dr., Suite 101, San Diego, California 92121, United States

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected].

ABBREVIATIONS BCS, biopharmaceutical classification system; HCV, hepatitis C virus; EPAR, European Public Assessment Report; CHMP, Committee for Medicinal Products for Human Use; RSM, regulatory starting material; cGMP, current good manufacturing practices; API, active pharmaceutical ingredient; HME, hot melt extrusion; AUC, area under the curve; PGI, potential genotoxic impurity; CDMT, 2-chloro-4,6-bis[3(perfluorohexyl)propyloxy]-1,3,5-triazine; NMM, N-methylmorpholine; DDQ, 2,3-dichloro-5,6-dicyano-1,4-benzoquinone; USPTO, United States Patent and Trademark Office

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REFERENCES

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Patent Highlights: Recently Approved HCV NS5a ... - ACS Publications

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