SAR and pharmacokinetic studies on phenethylamide inhibitors of the hepatitis C virus NS3/NS4A serine protease

High-throughput screening (HTS) and hit validation to identify small molecule inhibitors with activity against NS3/4A proteases from multiple hepatitis C virus genotypes

Development of drug-resistant mutations has been a major problem with all currently developed Hepatitis C Virus (HCV) NS3/4A inhibitors, including the two FDA approved drugs, significantly reducing the efficacy of these inhibitors. The high incidence of drug-resistance mutations and the limited utility of these inhibitors against only genotype 1 highlight the need for novel, broad-spectrum HCV therapies. Here we used high-throughput screening (HTS) to identify low molecular weight inhibitors against NS3/4A from multiple genotypes. A total of 40,967 compounds from four structurally diverse molecular libraries were screened by HTS using fluorescence-based enzymatic assays, followed by an orthogonal binding analysis using surface plasmon resonance (SPR) to eliminate false positives. A novel small molecule compound was identified with an IC₅₀ value of 2.2 µM against the NS3/4A from genotype 1b. Mode of inhibition analysis subsequently confirmed this compound to be a competitive inhibitor with respect to the substrate, indicating direct binding to the protease active site, rather than to the allosteric binding pocket that was discovered to be the binding site of a few recently discovered small molecule inhibitors. This newly discovered inhibitor also showed promising inhibitory activity against the NS3/4As from three other HCV genotypes, as well as five common drug-resistant mutants of genotype 1b NS3/4A. The inhibitor was selective for NS3 from multiple HCV genotypes over two human serine proteases, and a whole cell lysate assay confirmed inhibitory activity in the cellular environment. This compound provides a lead for further development of potentially broader spectrum inhibitors.

Isotopic effect on tautomeric behavior of 5-(2,6-disubstituted-aryloxy)-tetrazoles

Isotopic effect on tautomeric behaviors of the synthesized 5-phenoxy- (1a), 5-(2,6-dimethylphenoxy)-(1b), 5-(2,6-diisopropylphenoxy)-(1c), 5-(2,6-dimethoxyphenoxy)-(1d) and 5-(4-methylphenoxy)-tetrazole (1e) were investigated in DMSO-d6 by adding one drop of D2O. Among 1a-e, 1a, 1d and 1e show small rotational barrier around C5-O1 and O1-C6 while in 1b and 1c there are distinguishable rotational barrier about that bonds. The (1)H NMR spectra of 1b and 1c show slightly different chemical shifts for two methyl and isopropyl groups on those phenyl ring, respectively, while the chemical shifts difference (Δδ) between two methyl and two isopropyl groups were enhanced by adding D2O. The (13)C NMR spectra of 1b show two overlapped singlets for methyl groups after adding D2O. Representatively, the calculations of compound 1c were performed with GAUSSIAN-03 and the rotational barrier about C5-O1 and between isopropyl group and phenyl ring in 1c was calculated with B3LYP/6-31G(d) basis set.

Drug resistance against HCV NS3/4A inhibitors is defined by the balance of substrate recognition versus inhibitor binding

Hepatitis C virus infects an estimated 180 million people worldwide, prompting enormous efforts to develop inhibitors targeting the essential NS3/4A protease. Resistance against the most promising protease inhibitors, telaprevir, boceprevir, and ITMN-191, has emerged in clinical trials. In this study, crystal structures of the NS3/4A protease domain reveal that viral substrates bind to the protease active site in a conserved manner defining a consensus volume, or substrate envelope. Mutations that confer the most severe resistance in the clinic occur where the inhibitors protrude from the substrate envelope, as these changes selectively weaken inhibitor binding without compromising the binding of substrates. These findings suggest a general model for predicting the susceptibility of protease inhibitors to resistance: drugs designed to fit within the substrate envelope will be less susceptible to resistance, as mutations affecting inhibitor binding would simultaneously interfere with the recognition of viral substrates.

Second-generation highly potent and selective inhibitors of the hepatitis C virus NS3 serine protease

The hepatitis C virus (HCV) infection is a leading cause of chronic liver disease. The moderate efficacy along with side effects of the current pegylated interferon and ribavirin combination therapy underscores the need for more effective and safer new treatment. In an effort to improve upon our current clinical candidate, Boceprevir (SCH 503034), extensive SAR studies were performed on the P3 capping moieties. This led to the discovery of tert-leucinol derived cyclic imides as a potent series of novel P3 capping groups. Thus, the introduction of these imide caps improved the cell-based replicon EC(90) by more than 10-fold. A number of imides with various substitutions, ring sizes, bicyclic systems, and heterocyclic rings were explored. The 4,4-dimethyl substituted glutarimide emerged as the best cap as exemplified in compound 21 (K(i)* = 4 nM, EC(90) = 40 nM). Systematic optimization of different positions (P', P3, and P1) of the inhibitor resulted in the identification of the lead compound 46, which had an excellent potency (K(i)* = 4 nM, EC(90) = 30 nM) and good pharmacokinetic profile (22% and 35% bioavailability in rats and dogs, respectively). X-ray structure of inhibitor 46 bound to the enzyme revealed that there was an additional hydrogen bonding interaction between one of the imide carbonyls and Cys159.

The introduction of P4 substituted 1-methylcyclohexyl groups into Boceprevir: a change in direction in the search for a second generation HCV NS3 protease inhibitor

In the search for a second generation HCV protease inhibitor, molecular modeling studies of the X-ray crystal structure of Boceprevir1 bound to the NS3 protein suggest that expansion into the S4 pocket could provide additional hydrophobic Van der Waals interactions. Effective replacement of the P4 tert-butyl with a cyclohexylmethyl ligand led to inhibitor 2 with improved enzyme and replicon activities. Subsequent modeling and SAR studies led to the pyridine 38 and sulfone analogues 52 and 53 with vastly improved PK parameters in monkeys, forming a new foundation for further exploration.

Novel potent inhibitors of hepatitis C virus (HCV) NS3 protease with cyclic sulfonyl P3 cappings

Extensive SAR studies of the P3 capping group led to the discovery of a series of potent inhibitors with sultam and cyclic sulfonyl urea moieties as the P3 capping. The bicyclic thiophene-sultam or phenyl-sultam cappings were selected for further SAR development. Modification at the P3 side chain determined that the tert-butyl group was the best choice at that position. Optimization of P1 residue significantly improved potency and selectivity. The combination of optimal moieties at all positions led to the discovery of compound 33. This compound had the best overall profile in potency and PK profile: excellent K(i)(*) of 5.3 nM and activity in replicon (EC(90)) of 80 nM, extremely high selectivity of 6100, and a good rat PO AUC of 1.43 microMh.

The therapeutic potential of NS3 protease inhibitors in HCV infection

Hepatitis C virus (HCV) infection is a serious cause of chronic liver disease worldwide and afflicts > 170 million people. The HCV-encoded NS3 protease is essential for viral replication and has long been recognised as a prime target for antiviral drugs. However, the peculiar active site structure of this enzyme, a shallow dent on the surface of the protein, has rendered the development of small-molecule inhibitors a highly challenging task. Nevertheless, perseverance and creativity has led to significant progress in this field over the last few years resulting in three compounds that are reported to enter the clinic. The impressive reduction of HCV RNA plasma levels observed with two of these inhibitors (ciluprevir and VX-950) in clinical trials has undoubtedly illustrated the potential of this viral enzyme-targeted drug discovery approach.

Novel potent hepatitis C virus NS3 serine protease inhibitors derived from proline-based macrocycles

The hepatitis C virus (HCV) NS3 protease is essential for viral replication. It has been a target of choice for intensive drug discovery research. On the basis of an active pentapeptide inhibitor, 1, we envisioned that macrocyclization from the P2 proline to P3 capping could enhance binding to the backbone Ala156 residue and the S4 pocket. Thus, a number of P2 proline-based macrocyclic alpha-ketoamide inhibitors were prepared and investigated in an HCV NS3 serine protease continuous assay (K(i*)). The biological activity varied substantially depending on factors such as the ring size, number of amino acid residues, number of methyl substituents, type of heteroatom in the linker, P3 residue, and configuration at the proline C-4 center. The pentapeptide inhibitors were very potent, with the C-terminal acids and amides being the most active ones (24, K(i*) = 8 nM). The tetrapeptides and tripeptides were less potent. Sixteen- and seventeen-membered macrocyclic compounds were equally potent, while fifteen-membered analogues were slightly less active. gem-Dimethyl substituents at the linker improved the potency of all inhibitors (the best compound was 45, K(i*) = 6 nM). The combination of tert-leucine at P3 and dimethyl substituents at the linker in compound 47 realized a selectivity of 307 against human neutrophil elastase. Compound 45 had an IC(50) of 130 nM in a cellular replicon assay, while IC(50) for 24 was 400 nM. Several compounds had excellent subcutaneous AUC and bioavailability in rats. Although tripeptide compound 40 was 97% orally bioavailable, larger pentapeptides generally had low oral bioavailability. The X-ray crystal structure of compounds 24 and 45 bound to the protease demonstrated the close interaction of the macrocycle with the Ala156 methyl group and S4 pocket. The strategy of macrocyclization has been proved to be successful in improving potency (>20-fold greater than that of 1) and in structural depeptization.

Synthesis and biological activity of macrocyclic inhibitors of hepatitis C virus (HCV) NS3 protease

The 17-membered phenylalanine-based macrocycle 6 was prepared starting from 3-iodo-phenylalanine. Macrocyclization of alkene phenyl iodide 5 was effected through a palladium-catalyzed Heck reaction. The macrocyclic alpha-ketoamides were active inhibitors of the HCV NS3 protease, with the C-terminal acids and amides being more potent than tert-butyl esters.

Design, synthesis, and biological activity of m-tyrosine-based 16- and 17-membered macrocyclic inhibitors of hepatitis C virus NS3 serine protease

The limited efficacy and considerable side effects of currently available therapies for the treatment of hepatitis C virus (HCV) infection have prompted significant efforts toward the development of safe and effective new therapeutics. The pentapeptide alpha-ketoamides of type 1 were weak HCV inhibitors with a binding constant, Ki, above 5 microM. We envisioned that cyclization of a P2 phenyl side chain to a P3 capping group could enhance binding through an interaction of the resulting macrocycle with the methyl group of Ala156 on the enzyme backbone. The macrocyclic dipeptide moiety would also decrease the peptidic nature of the inhibitors. The synthesis of macrocyclic HCV inhibitors started from m-tyrosine methyl ester. Two consecutive couplings, first, with Boc-cyclohexylglycine and, then, with hept-6-enoic acid, provided compound 6. The alkene was converted to an alcohol via hydroboration. The key macrocyclization of phenol alcohol 7 was achieved through a Mitsunobu reaction. Both 16- and 17-membered macrocycles (8 and 21) were prepared. After hydrolysis, the macrocyclic acids (15 and 22) were coupled to the right-hand tripeptide (14) to afford alpha-hydroxyamides, which upon Dess-Martin periodinane oxidation furnished the desired alpha-ketoamides. Esters, acids, and amides were incorporated at the C-terminal of these peptides. These inhibitors were tested in an HCV protease continuous assay. The binding constants (Ki) indicated that the 16-membered macrocyclic inhibitors (23 and 24) were less potent than the 17-membered analogues (16-19). It was also evident that C-terminal acids (i.e., 17) and amides (18 and 19) (Ki range: 0.16-0.31 microM) were much better inhibitors than tert-butyl esters (16 and 23). The X-ray crystal structure of compound 17 bound to the enzyme revealed that the macrocycle formed a "donut"-shaped ring around the methyl group of Ala156. P2' phenyl and P1 propyl groups wrapped around the Lys136 side chain, forming a "C"-shaped clamp. The 17-membered macrocyclic inhibitors 17-19 were significantly more potent than the acyclic pentapeptide 1.

Peptide inhibitors of Dengue virus NS3 protease. Part 1: Warhead

Substrate-based tetrapeptide inhibitors with various warheads were designed, synthesized, and evaluated against the Dengue virus NS3 protease. Effective inhibition was achieved by peptide inhibitors with electrophilic warheads such as aldehyde, trifluoromethyl ketone, and boronic acid. A boronic acid has the highest affinity, exhibiting a K(i) of 43 nM.