Design of N-Benzoxaborole Benzofuran GSK8175-Optimization of Human Pharmacokinetics Inspired by Metabolites of a Failed Clinical HCV Inhibitor

Abstract

We previously described the discovery of GSK5852 (1), a non-nucleoside polymerase (NS5B) inhibitor of hepatitis C virus (HCV), in which an N-benzyl boronic acid was essential for potent antiviral activity. Unfortunately, facile benzylic oxidation resulted in a short plasma half-life (5 h) in human volunteers, and a backup program was initiated to remove metabolic liabilities associated with 1. Herein, we describe second-generation NS5B inhibitors including GSK8175 (49), a sulfonamide- N-benzoxaborole analog with low in vivo clearance across preclinical species and broad-spectrum activity against HCV replicons. An X-ray structure of NS5B protein cocrystallized with 49 revealed unique protein-inhibitor interactions mediated by an extensive network of ordered water molecules and the first evidence of boronate complex formation within the binding pocket. In clinical studies, 49 displayed a 60-63 h half-life and a robust decrease in viral RNA levels in HCV-infected patients, thereby validating our hypothesis that reducing benzylic oxidation would improve human pharmacokinetics and lower efficacious doses relative to 1.

Figure 1

Metabolites 3 and 4 identified in clinical plasma samples from human volunteers dosed with 1 (GSK5852). Comparison of 1 with a structurally related clinical asset 2 (HCV-796).

Figure 2

Strategic approaches to reduce the propensity for benzylic oxidation associated with N-benzyl boronic acids.

Figure 3

Models of representative target molecules from approaches A–C overlaid with the conformation of 1 (grey) when bound to HCV NS5B GT 1b C316N polymerase (PDB code: 4KAI). Only proposed changes to 1 are shown for clarity. (a) Approach A: the (1R)-isomer of an indane boronic acid (magenta) in a partially eclipsed (nonminimum) conformation (f_{C2–C1–N–S} ≈ −10°). (b) Approach B: the molecular-mechanics-minimized conformation of an oxazolidinone-linked phenyl boronic acid (cyan). (c) Approach C: the molecular-mechanics-minimized conformation of an N-phenyl boronic acid (green) that necessarily displaces the position of the boronic acid relative to 1 (d ≈ 3.7 Å).

Scheme 1. Representative Synthesis of Oxazolidinone 18

Conditions: (i) NBS, CCl₄, benzoyl peroxide, reflux, 4 h, 71%; (ii) aniline 14, DMF, 85 °C, 47%; (iii) LiBH₄, THF, 84%; (iv) triphosgene, Et₃N, DCM, 50%; (v) (BPin)₂, PdCl₂(dppf)·CH₂Cl₂, KOAc, dioxane, 100 °C; and (vi) PS-BBA, HCl, THF, 30% for 2 steps.

Scheme 2. Representative Synthesis of Heteroaromatic Analog (Compound 31)

Conditions: (i) AcOH, 100 °C, 67%; (ii) NaOH, THF, MeOH, H₂O, 100 °C, 100%; (iii) MeNH₂·HCl, HATU, DIEA, DMF, 95%; (iv) (BPin)₂, PdCl₂(dppf)·CH₂Cl₂, KOAc, dioxane, 80 °C; and (v) NaIO₄, NH₄OAc, H₂O, THF, 24% over 2 steps.

Scheme 3. Synthesis of N-Phenyl Boronic Acid Analogs (44–49)

Conditions: (i) Cu(OAc)₂·H₂O, TEA, 4 Å mol. sieves, DCM, 26–39%; (ii) K₂CO₃, DME, H₂O, 80–100 °C or K₂CO₃, HMPA, 60 °C, 29–93%; (iii) SnCl₂ or H₂ with Pd/C or sulfided Pt/C, 50–100%; (iv) NaNO₂, CuBr, HBr, MeCN, H₂O, 0–60 °C, 73–76%; (v) NCS, DMF, 60 °C, 99%; (vi) LiBH₄, THF, MeOH, 5 °C, 100%; (vii) MOMCl, DIEA, THF, 50 °C, 68%; (viii) (BPin)₂, PdCl₂(dppf)-CH₂Cl₂ or Pd(dppb)Cl₂, KOAc, dioxane, 80 °C; (ix) PS-BBA, HCl, H₂O, THF or NaIO₄, HCl, H₂O, THF, 15–74% (2 steps); and (x) HCl, H₂O, THF, MeOH, 71% (2 steps).

Figure 4

X-ray crystal structures of NS5B GT 1b 316N bound with (a) benzyl boronic acid 1 (PDB ID 4KAI); (b) oxazolidinone 25 (PDB ID 6MVK); (c) triazole 31 (PDB ID 6MVQ); and (d) N-phenyl 45 (PDB ID 6MVP). Highlighted are interactions with Arg200 and Tyr448 [residues labeled in panel (a) only for clarity].

Figure 5

X-ray crystal structure of benzoxaborole analog 49 highlighting three ordered water molecules and key interactions to NS5B GT 1a 316Y protein (PDB ID 6MVO).