Intracellular and plasma pharmacokinetics of efavirenz in HIV-infected individuals

Objectives: The site of action of efavirenz is inside HIV-infected cells. Measurement of intracellular (IC) concentrations of efavirenz may therefore provide further understanding of therapeutic failure, especially where virological rebound occurs despite adequate plasma levels, and a lack of detectable viral resistance. Here, we determined IC and plasma pharmacokinetics of efavirenz and their relationship with plasma protein binding and P-glycoprotein (P-gp, an active drug efflux transporter) expression. Patients and methods: Venous blood samples from 10 HIV-infected patients receiving efavirenz (600 mg once a day plus two nucleoside reverse transcriptase inhibitors) were collected over the 24 h dosing interval. Plasma and peripheral blood mononuclear cells (PBMCs) were isolated. Plasma protein bound and unbound efavirenz were separated using ultrafiltration. IC (or cell-associated), total plasma and unbound plasma efavirenz levels were quantified using HPLC-UV. P-gp expression was measured by flow cytometry. Area under the concentration-time curves (AUC(0-24)) were then calculated using non-compartmental analyses and the IC accumulation expressed as a ratio of IC to plasma AUC(0-24). Results: The median (range) % unbound and IC accumulation ratio was 0.6% (0.4-1.5%) and 1.3 (0.7-3.3), respectively. There was a linear relationship between IC and total AUC(0-24) (r(2) = 0.59, P = 0.01) but not unbound AUC(0-24) (r(2) = 0.13, P = 0.75). An inverse correlation between IC AUC(0-24) and % unbound was observed (r(2) = 41, P = 0.05). There was no relationship between IC AUC(0-24) and P-gp expression on the cell surface (r(2) < 0.01, P = 0.98). Conclusions: There was a direct relationship between % bound efavirenz in plasma and IC accumulation implying that the IC accumulation of efavirenz is related to binding to IC proteins or other cellular constituents. Studies investigating the unbound concentration of antiretrovirals inside the cell are now required.