Pharmacokinetics of Co-Formulated Elvitegravir/Cobicistat/Emtricitabine/Tenofovir Disoproxil Fumarate After Switch From Efavirenz/Emtricitabine/Tenofovir Disoproxil Fumarate in Healthy Subjects

Background: Elvitegravir (EVG), a HIV integrase inhibitor, is metabolized primarily by CYP3A, and secondarily by UGT1A1/3; Efavirenz (EFV), a HIV non-nucleoside reverse transcriptase inhibitor, is metabolized by Cytochrome P450 (CYP) 2B6 and induces CYP3A and uridine diphosphate glucuronosyltransferase (UGT) with residual effects post discontinuation because of long T-1/2 (40-55 hours). This study evaluated the pharmacokinetics after switching from efavirenz/emtricitabine/tenofovir disoproxil fumarate (EFV/FTC/TDF) to elvitegravir/cobicistat/emtricitabine/tenofovir disoproxil fumarate (EVG/COBI/FTC/TDF). Methods: Healthy subjects (n = 32 including n = 8 CYP2B6 poor metabolizers) received EVG/COBI/FTC/TDF (150/150/200/300 mg) on days 1-7, and after a washout, received EFV/FTC/TDF (600/200/300 mg) on days 15-28 and switched to EVG/COBI/FTC/TDF (150/150/200/300 mg) for 5 weeks (days 29-62). Pharmacokinetic assessments occurred on days 7, 28, 35, and 42; trough samples (C-trough) were collected periodically until day 63. Safety was assessed throughout the study. Results: Twenty-nine subjects completed with 3 adverse events leading to discontinuation; no grade >= 3 adverse events were reported. Post-EFV/FTC/TDF, mean EVG area under concentration (AUC(tau)) was 37% and 29% lower and mean C-trough similar to 3- and similar to 5-fold above IC95, respectively, on days 35 and 42, and 7-8-fold above IC95 by 5 weeks. COBI AUC(tau) returned to normal by day 42. EVG glucuronide, GS-9200, AUC(tau) was higher (46% and 32% on days 35 and 42, respectively) postswitch. CYP2B6 poor metabolizers displayed higher EFV AUC(tau) and C-max (125% and 91%, respectively) versus non-poor metabolizers, and lower EVG and COBI exposures. EFV C-trough was >IC90 (10 ng/mL) in all subjects postswitch. FTC and tenofovir (TFV) exposures were unaffected. Conclusions: After EFV/FTC/TDF to EVG/COBI/FTC/TDF switch, EVG and/or EFV exposures were in an active range. These findings support further evaluation of switching regimens in HIV-1 patients.