Variation in oral clearance of saquinavir is predicted by CYP3A51 genotype but not by enterocyte content of cytochrome P450 3A5

Objective Saquinavir, a widely prescribed human immunodeficiency virus 1 protease inhibitor, has a low and variable oral bioavailability that has been attributed to extensive first‐pass extraction mediated by hepatic or intestinal cytochrome P450 (CYP) 3A4 and intestinal P‐glycoprotein (P‐gp). The polymorphic CYP3A5 has also been shown to influence the saquinavir metabolite/parent urinary ratio, suggesting a role for CYP3A5. Methods Twenty healthy subjects received a single oral dose of saquinavir (600 mg) with water (control) and, on a separate occasion, with Seville orange juice (a selective intestinal CYP3A4/5 inhibitor). Hepatic CYP3A4 activity was evaluated by use of the erythromycin breath test. Duodenal biopsy specimens were used to assess relative intestinal CYP3A4 and CYP3A5 protein contents. Relative P‐gp content was also assessed in the biopsy specimens and in lymphocytes. Genetic polymorphisms in MDR1 (in exon 21 and 26), CYP3A5 (*1 and *3), and CYP3A4*1B were identified by direct sequencing. Saquinavir plasma concentrations were measured by tandem liquid chromatography–mass spectrometry. Pharmacokinetic parameter estimates (maximum concentration, time to reach maximum concentration, area under the concentration‐time curve, apparent oral clearance [CL/F]) were computed by standard noncompartmental methods. Stepwise multiple regression analysis was used to identify the hepatic or intestinal variables that predicted variation in saquinavir pharmacokinetic measures. Results Baseline saquinavir CL/F was not correlated with liver CYP3A4 activity (the erythromycin breath test result), intestinal CYP3A4 content, or intestinal P‐gp content (r2 = 0.08, 0.08, and 0.007, respectively; P > .2). MDR1 genotype and lymphocyte P‐gp content were also not predictive. Among the 6 subjects expressing intestinal CYP3A5, the mean saquinavir CL/F was almost twice as high as for the 14 nonexpressors (36.7 L/h [95% confidence interval (CI), 18.7–54.6 L/h] and 19.3 L/h [95% CI, 11.2–27.4 L/h], respectively; P = .03). However, among the 6 CYP3A5 expressors, there was an unexpected negative correlation between CL/F and intestinal CYP3A5 content (r2 = 0.58, P = .05). Seville orange juice decreased the mean CL/F in all 20 subjects from 24.5 L/h (95% CI, 16.7–32.3 L/h) to 14.7 L/h (95% CI, 8.4–20.6 L/h) (P = .05). The effect size did not appear to be influenced by CYP3A5 expression. Conclusions The CYP3A5*1 genotype is associated with increased saquinavir CL/F. This does not