Background:  Pharmacokinetic enhancement with ritonavir (RTV), a potent mechanism-based inhibitor of cytochrome P450 3A (CYP3A) is frequently used to increase (boost) systemic exposure of HIV PI. However, chronic use of RTV may cause metabolic adverse effects. Herein we describe the profile of GS-9350, a potent CYP3A mechanism-based inhibitor and pharmaco-enhancer that lacks anti-HIV activity. GS-9350 is formulated as a solid dosage form amenable for co-formulation with other ARV that utilize boosting, including the HIV-1 integrase inhibitor elvitegravir.

Methods:  Antiviral activity was assessed in standard assays. Effect on lipid accumulation and insulin-stimulated glucose uptake were determined in adipocytes. Metabolism and enzyme inhibition studies were performed in vitro with hepatic microsomal fractions and primary hepatocytes. Induction studies were performed by receptor transactivation analysis. The safety, tolerability, and pharmacokinetics of GS-9350 and its ability to boost systemic exposure of the CYP3A probe substrate midazolam (MDZ) vs RTV 100 mg in humans were evaluated in a single- and 14-day multiple-dose escalation study (50, 100, and 200 mg once-daily; n = 18 per cohort).

Results:  GS-9350 showed no antiviral activity at concentrations up to 90 µM and reduced in vitro effects on adipocytes and proteasome activity vs RTV. There was no inhibition of lipid accumulation in adipocyte at 30 µM and <10% inhibition of insulin-stimulated glucose uptake at 10 µM. GS-9350 is a potent (Ki <1 µM) human CYP3A inhibitor; 2-step enzyme inactivation studies showed that GS-9350 was a potent mechanism-based inhibitor of human CYP3A (kinact 0.44 min^–1, Ki 0.94 µM). GS-9350 did not activate the human aryl hydrocarbon receptor and was a very weak agonist (EC₅₀ >30 µM) of the human pregnane X receptor responsible for induction of drug metabolism/transport. In humans, GS-9350 exhibited non-linear pharmacokinetics with respect to dose and time as expected for an mechanism-based inhibitor. Clinical proof-of-concept for boosting in humans was demonstrated; GS-9350 doses of 100 mg and 200 mg and RTV 100 mg exhibited similar inhibition of MDZ apparent clearance (92, 95, and 95%, respectively). All treatments were generally well tolerated. There was no evidence of PR or QTc prolongation.

Conclusions:  GS-9350 lacks antiviral activity, is a potent and selective inhibitor of human CYP3A and may exhibit reduced metabolic adverse events. In humans, GS-9350 effectively inhibits CYP3A metabolism at low doses.