A drug-drug interaction (DDI) study to assess the effect of oral galeterone on the pharmacokinetics (PK) of oral midazolam.

Abstract

316 Background: Galeterone (gal) is a semisynthetic steroid that targets androgen receptor (AR) signaling via increased AR protein degradation, inhibition of CYP17 activity, and inhibition of androgen binding to AR. Gal is a novel potential treatment for prostate cancer. In vitro, gal competitively inhibits CYP3A4 (IC50 = 5.5 μM; midazolam as substrate). This clinical study evaluated whether multiple daily doses of gal alter the single-dose PK of midazolam, a sensitive probe substrate for functional CYP3A4 intestinal and hepatic activity. Methods: In an open-label, fixed sequence, DDI study, 18 healthy male volunteers received midazolam 2 mg Day 1 and Day 5 and gal 2550 mg Days 2-5. Midazolam plasma PK was determined on Days 1 and 5. The effect of gal on the natural log-transformed Cmax, AUC0-inf, and AUC0-t of midazolam was assessed with a linear mixed-effects model; point estimates for geometric means, geometric mean ratios with 90% confidence intervals were determined. Safety was also monitored. Results: The Tmax of midazolam was not altered by gal coadministration. The < 2-fold increases in midazolam Cmax and AUCs were statistically significant. For Cmax, the point estimate of the geometric least squares (LS) mean ratio between the 2 treatments was 1.25 (90% CI: 1.05, 1.48). Higher ratios were observed for the AUC ratios, with LS mean ratio for AUC0-t of 1.57 (90% CI: 1.43, 1.73) and for AUC0-inf of 1.58 (90% CI: 1.42, 1.75). The mean midazolam t1/2 was approximately 76% higher with gal coadministration. Multiple doses of gal were well tolerated with no apparent effect on the safety of midazolam. Conclusions: Per FDA guidance, < 2-fold increases in midazolam Cmax and AUCs observed with gal coadministration support its classification as a weak inhibitor of CYP3A4. Given the similar or higher in vitro IC50’s for CYP2C8 and CYP2C19, respectively, a comparable or lesser degree of enzyme inhibition is expected for gal 2550 mg daily. Based on these results, concurrent CYP3A4, CYP2C8, and CYP2C19 substrates are not contraindicated in patients taking gal; caution is advised in patients receiving or initiating sensitive CYP3A4, CYP2C8, and CYP2C19 substrates, especially those with a narrow therapeutic range.