Background:  HIV PI are co-dosed with ritonavir (RTV) to boost PI exposures by inhibition of CYP3A. SPI-452 is a novel pharmacokinetic enhancer; i.e. a potent inhibitor of CYP3A that lacks antiviral activity. The preclinical pharmacology of SPI-452 was evaluated in vitro and in vivo. SPI-452 tolerability, pharmacokinetics, and ability to boost exposure of HIV PI were evaluated in healthy volunteers in study 1 (single dose) and study 2 (15-day repeat dose).

Methods:  CYP inhibition was tested in human or animal liver microsomes. Inhibition of HIV PI metabolism was assessed in human liver microsomes and hepatocytes. Cell permeability, phosphoglycolate phosphatase (PgP) inhibition, and CYP induction was tested in Caco-2 and hepatocytes. SPI-452 antiviral activity was evaluated in MT4 cells. SPI-452 pharmacokinetic enhancer effect was tested in vivo in rats and dogs with saquinavir (SQV), lopinavir (LPV), and atazanavir (ATV). Placebo-controlled, ascending-dose studies for study 1, phase 1, used escalating doses in 6 cohorts (25, 50, 100, 200, 400, and 600 mg). For phase 2, the following were used:  1000 mg SQV orally on days –1 and 1; SPI-452 (50 or 200 mg) co-dosed on day 1. SPI-452 and SQV pharmacokinetics were evaluated. For study 2, days 1 to 15, SPI-452/placebo and day 15 to 16, PI/placebo. Pharmacokinetics were evaluated on day –7 (for PI), days 1 and 7 (SPI-452), and days 15 to 17 (SPI-452 and PI), and interim pre-dose SPI-452 troughs. PI were darunavir (DRV) and ATV. Tolerability was assessed by physical exam, occurrence of adverse events, telemetry, changes from pre-dose ECG, and safety labs.

Results:  SPI-452 potently inhibited CYP3A (mechanism-based), boosting PI in vitro and in animals. SPI-452 was well tolerated (study 1 n = 48 for phase 1, 20 for phase 2, and 10 in both; study 2, n = 60), with mild adverse events and no severe adverse events. Headache was the most frequent adverse event (study 1:  6 episodes for 5 subjects; study 2:  35 for 17 subjects), then in study 2, nausea/emesis (17 for 8 subjects) and diarrhea (9). No significant changes were seen in safety labs or ECG (both studies), nor mean hematology or liver function tests (study 2) from day –1 to 17 (on drug). SPI-452 exposures were variably dose proportional. SPI-452 markedly boosted SQV. SPI-452 reached steady state by day 14, and boosted ATV and DRV; boosting increased with SPI-452 dose level. PI C24 values were boosted as much as 13- (ATV) and 29-fold (DRV).

Conclusions:  SPI-452 is a potent CYP3A inhibitor and was well tolerated and safe when dosed up to 200 mg once daily for 15 days. SPI-452 reached steady state by day 14, and markedly boosted PI exposures, particularly C24. SPI-452 has satisfied key target criteria for continued development as a novel and promising pharmacokinetic enhancer for HIV therapy.