Cerebrospinal fluid exposure of efavirenz and its major metabolites when dosed at 400 mg and 600 mg once daily: a randomized controlled trial

Background: The optimal penetration of antiretroviral agents into the central nervous system may be a balance between providing adequate drug exposure to inhibit human immunodeficiency virus (HIV) replication while avoiding concentrations associated with neuronal toxicities.

Methods: Cerebrospinal fluid (CSF) exposure of efavirenz and the metabolites 7-hydroxy (7OH) and 8-hydroxy (8OH) efavirenz were assessed after at least 12 weeks of therapy in HIV-infected subjects randomized to commence antiretroviral regimens containing efavirenz at either 400 mg or 600 mg once daily.

Results: Of 28 subjects (14 and 14 on efavirenz 400 mg and 600 mg, respectively), CSF HIV RNA was undetectable in all. Geometric mean CSF efavirenz, 7OH-, and 8OH-efavirenz concentrations (with 90% confidence intervals [CIs]) for the 400-mg and 600-mg dosing groups were 16.5 (13-21) and 19.5 (15-25) ng/mL; 0.6 (.4-.9) and 0.6 (.4-1) ng/mL; and 5.1 (4.0-6.4) and 3.1 (2.1-4.4) ng/mL, respectively. Efavirenz concentration in CSF was >0.51 ng/mL (proposed CSF 50% maximal inhibitory concentration for wild-type virus) in all subjects, and 8OH-efavirenz concentration in CSF was >3.3 ng/mL (a proposed toxicity threshold) in 11 of 14 and 7 of 14 subjects randomized to the 400 mg and 600 mg doses of efavirenz, respectively. Whereas CSF efavirenz concentration was significantly associated with plasma concentration (P < .001) and cytochrome P450 2B6 genotype (CSF efavirenz GG to GT/TT geometric mean ratio, 0.56 [90% CI, .42-.74]), CSF 8OH-efavirenz concentration was not (P = .242 for association and CSF 8OH-efavirenz GG to GT/TT geometric mean ratio, 1.52 [90% CI, .97-2.36]).

Conclusions: With both doses of efavirenz studied, CSF concentrations were considered adequate to inhibit HIV replication, although concentrations of 8OH-efavirenz were greater than those reportedly associated with neuronal toxicity. CSF exposure of 8OH-efavirenz was not dependent on plasma exposure and, as we postulate, may be subject to saturable pharmacokinetic effects.

Clinical trials registration: NCT01011413.