Received for publication July 26, 2005.
Revised January 19, 2006.
Accepted for publication January 20, 2006.

Dependence of Nelfinavir Brain Uptake on Dose and Tissue Concentrations of the Selective P-gp Inhibitor Zosuquidar in Rats

Abstract

Most reverse transcriptase and protease inhibitors used in highly active antiretroviral therapy for treating human immunodeficiency virus (HIV) infections exhibit poor penetration into the brain, raising the concern that the brain may be a sanctuary site for the development of resistant HIV variants. This study explores the relationship between the dose and plasma and brain concentrations of zosuquidar and the effect of this selective P-gp inhibitor on CNS penetration of the HIV protease inhibitor nelfinavir maintained at steady-state by intravenous infusions in rats. Nelfinavir was infused (10 mg/kg/hr) for up to 10 hours with or without concurrent administration of an intravenous bolus dose of 2, 6, or 20 mg/kg zosuquidar given at 4 hours. Brain tissue and plasma were analyzed for both drug concentrations. Brain tissue:plasma nelfinavir concentration ratios (uncorrected for the vascular contribution) increased nonlinearly with zosuquidar dose from 0.06 ± 0.03 in the absence of zosuquidar and 0.09 ± 0.02 between 2-6 hours after a 2 mg/kg zosuquidar to 0.85 ± 0.19 after 6 mg/kg and 1.58 ± 0.67 after 20 mg/kg zosuquidar. Zosuquidar brain tissue:plasma concentration ratios exhibited a similar abrupt increase from 2.8 ± 0.3 after a 2 mg/kg dose to ~ 15 after the 6 mg/kg and 20 mg/kg doses. The apparent threshold in the plasma concentration of zosuquidar necessary to produce significant enhancement in brain uptake of nelfinavir appears to be close to the plasma concentrations associated with the maximum tolerated dose reported in the literature after repeated dosing of zosuquidar in patients.

Key words: ABC transporters, blood-brain barrier, blood-CNS transport, CNS pharmacokinetics, p-glycoprotein, protein binding, transporters

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