Fukang Yang

Inhibitors of HCV NS5A: From Iminothiazolidinones to Symmetrical Stilbenes

The iminothiazolidinone BMS-858 (2) was identified as a specific inhibitor of HCV replication in a genotype 1b replicon assay via a high-throughput screening campaign. A more potent analogue, BMS-824 (18), was used in resistance mapping studies, which revealed that inhibitory activity was related to disrupting the function of the HCV nonstructural protein 5A. Despite the development of coherent and interpretable SAR, it was subsequently discovered that in DMSO 18 underwent an oxidation and structural rearrangement to afford the thiohydantoin 47, a compound with reduced HCV inhibitory activity. However, HPLC bioassay fractionation studies performed after incubation of 18 in assay media led to the identification of fractions containing a dimeric species 48 that exhibited potent antiviral activity. Excision of the key elements hypothesized to be responsible for antiviral activity based on SAR observations reduced 48 to a simplified, symmetrical, pharmacophore realized most effectively with the stilbene 55, a compound that demonstrated potent inhibition of HCV in a genotype 1b replicon with an EC50 = 86 pM.

Hepatitis C virus NS5A replication complex inhibitors: the discovery of daclatasvir.

The biphenyl derivatives 2 and 3 are prototypes of a novel class of NS5A replication complex inhibitors that demonstrate high inhibitory potency toward a panel of clinically relevant HCV strains encompassing genotypes 1-6. However, these compounds exhibit poor systemic exposure in rat pharmacokinetic studies after oral dosing. The structure-activity relationship investigations that improved the exposure properties of the parent bis-phenylimidazole chemotype, culminating in the identification of the highly potent NS5A replication complex inhibitor daclatasvir (33) are described. An element critical to success was the realization that the arylglycine cap of 2 could be replaced with an alkylglycine derivative and still maintain the high inhibitory potency of the series if accompanied with a stereoinversion, a finding that enabled a rapid optimization of exposure properties. Compound 33 had EC50 values of 50 and 9 pM toward genotype-1a and -1b replicons, respectively, and oral bioavailabilities of 38-108% in preclinical species. Compound 33 provided clinical proof-of-concept for the NS5A replication complex inhibitor class, and regulatory approval to market it with the NS3/4A protease inhibitor asunaprevir for the treatment of HCV genotype-1b infection has recently been sought in Japan.

HCV NS5A Replication Complex Inhibitors. Part 4.1 Optimization for Genotype 1a Replicon Inhibitory Activity

A series of symmetrical E-stilbene prolinamides that originated from the library-synthesized lead 3 was studied with respect to HCV genotype 1a (G-1a) and genotype 1b (G-1b) replicon inhibition and selectivity against BVDV and cytotoxicity. SAR emerging from an examination of the prolinamide cap region revealed 11 to be a selective HCV NS5A inhibitor exhibiting submicromolar potency against both G-1a and G-1b replicons. Additional structural refinements resulted in the identification of 30 as a potent, dual G-1a/1b HCV NS5A inhibitor.

HCV NS5A replication complex inhibitors. Part 2: investigation of stilbene prolinamides.

In a previous disclosure,(1) we reported the dimerization of an iminothiazolidinone to form 1, a contributor to the observed inhibition of HCV genotype 1b replicon activity. The dimer was isolated via bioassay-guided fractionation experiments and shown to be a potent inhibitor of genotype 1b HCV replication for which resistance mapped to the NS5A protein. The elements responsible for governing HCV inhibitory activity were successfully captured in the structurally simplified stilbene prolinamide 2. We describe herein the early SAR and profiling associated with stilbene prolinamides that culminated in the identification of analogs with PK properties sufficient to warrant continued commitment to this chemotype. These studies represent the key initial steps toward the discovery of daclatasvir (BMS-790052), a compound that has demonstrated clinical proof-of-concept for inhibiting the NS5A replication complex in the treatment of HCV infection.

HCV NS5A replication complex inhibitors. Part 3: discovery of potent analogs with distinct core topologies

In a recent disclosure, we described the discovery of dimeric, prolinamide-based NS5A replication complex inhibitors exhibiting excellent potency towards an HCV genotype 1b replicon. That disclosure dealt with the SAR exploration of the peripheral region of our lead chemotype, and herein is described the SAR uncovered from a complementary effort that focused on the central core region. From this effort, the contribution of the core region to the overall topology of the pharmacophore, primarily vector orientation and planarity, was determined, with a set of analogs exhibiting <10 nM EC(50) in a genotype 1b replicon assay.

Discovery and Preclinical Evaluation of BMS-955829, a Potent Positive Allosteric Modulator of mGluR5.

Positive allosteric modulators (PAMs) of the metabotropic glutamate receptor subtype 5 (mGluR5) are of interest due to their potential therapeutic utility in schizophrenia and other cognitive disorders. Herein we describe the discovery and optimization of a novel oxazolidinone-based chemotype to identify BMS-955829 (4), a compound with high functional PAM potency, excellent mGluR5 binding affinity, low glutamate fold shift, and high selectivity for the mGluR5 subtype. The low fold shift and absence of agonist activity proved critical in the identification of a molecule with an acceptable preclinical safety profile. Despite its low fold shift, 4 retained efficacy in set shifting and novel object recognition models in rodents.

Identification of Glutathione Conjugates of Acetylene-Containing Positive Allosteric Modulators of Metabotropic Glutamate Receptor Subtype 5

A recent medicinal chemistry campaign to identify positive allosteric modulators (PAMs) of metabotropic glutamate receptor subtype 5 (mGluR5) led to the discovery of potent compounds featuring an oxazolidinone structural core flanked by biaryl acetylene and haloaryl moieties. However, biotransformation studies of some of these mGluR5 PAMs demonstrated the formation of glutathione (GSH) conjugates. The conjugates in question were formed independently of NADPH as the main products in liver microsomes and liver cytosol (rat and human) and exhibited masses that were 307 u greater than their respective substrates, indicating the involvement of a reductive step in the formation of these metabolites. To further characterize the relevant metabolic sequences, GSH conjugates of (4R,5R)-5-(3-fluorophenyl)-4-(5-(pyrazin-2-ylethynyl)pyridin-3-yl)oxazolidin-2-one and (4R,5R)-5-(4-fluorophenyl)-4-(6-((3-fluoropyridin-2-yl)ethynyl)pyridin-2-yl)oxazolidin-2-one were biosynthesized and isolated. Subsequent analysis by NMR showed that GSH had reacted with the acetylene carbon atoms of these mGluR5 PAMs, suggesting a conjugate addition mechanism and implicating cytosolic and microsomal GSH S-transferases (GSTs) in catalysis. Interestingly, five closely related mGluR5 PAMs were not similarly prone to the formation of GSH conjugates in vitro. These compounds also featured acetylenes, but were flanked by either phenyl or cyclohexyl rings, which indicated that the formation of GSH conjugates was influenced by proximal functional groups that modulated the electron density of the triple bond and/or differences in enzyme-substrate specificity. These results informed an ongoing drug-discovery effort to identify mGluR5 PAMs with drug-like properties and a low risk of reactivity with endogenous thiols.

Targeting the BACE1 Active Site Flap Leads to a Potent Inhibitor That Elicits Robust Brain Aβ Reduction in Rodents.

By targeting the flap backbone of the BACE1 active site, we discovered 6-dimethylisoxazole-substituted biaryl aminothiazine 18 with 34-fold improved BACE1 inhibitory activity over the lead compound 1. The cocrystal structure of 18 bound to the active site indicated two hydrogen-bond interactions between the dimethylisoxazole and threonine 72 and glutamine 73 of the flap. Incorporation of the dimethylisoxazole substitution onto the related aminothiazine carboxamide series led to pyrazine-carboxamide 26 as a very potent BACE1 inhibitor (IC50 < 1 nM). This compound demonstrated robust brain Aβ reduction in rat dose-response studies. Thus, compound 26 may be useful in testing the amyloid hypothesis of Alzheimers disease.

Oxazolidinone-based allosteric modulators of mGluR5: Defining molecular switches to create a pharmacological tool box.

Herein we describe the structure activity relationships uncovered in the pursuit of an mGluR5 positive allosteric modulator (PAM) for the treatment of schizophrenia. It was discovered that certain modifications of an oxazolidinone-based chemotype afforded predictable changes in the pharmacological profile to give analogs with a wide range of functional activities. The discovery of potent silent allosteric modulators (SAMs) allowed interrogation of the mechanism-based liabilities associated with mGluR5 activation and drove our medicinal chemistry effort toward the discovery of low efficacy (fold shift) PAMs devoid of agonist activity. This work resulted in the identification of dipyridyl 22 (BMS-952048), a compound with a favorable free fraction, efficacy in a rodent-based cognition model, and low potential for convulsions in mouse.