Received for publication January 14, 2009.
Revised May 12, 2009.
Accepted for publication May 12, 2009.

BINDING OF LOPINAVIR TO HUMAN ₁-ACID GLYCOPROTEIN AND SERUM ALBUMIN

Abstract

HIV protease inhibitors are an important component of Highly Active Antiretroviral Therapy used to treat HIV infected pregnant women. They have a low placental transfer and are highly plasma protein bound. This study was carried out to determine the unbound fraction of lopinavir in cord blood and to characterize the binding of lopinavir to 1-acid glycoprotein (AAG) and human serum albumin (HSA), and displacement by ritonavir. Serum was obtained from cord blood from placentae obtained after cesarean section of healthy non-HIV infected women (n=4). The unbound fraction of lopinavir in serum obtained from this cord blood was 2.2 ±1.1%. The unbound fraction of lopinavir in separately obtained maternal serum samples (n=4) was 0.89 ±0.12%, which was not significantly different from that observed with cord serum samples. Varying concentrations of lopinavir, AAG, and HSA in buffer solutions were then used to characterize the lopinavir binding. The data were fit to obtain the number of binding sites (N) and equilibrium dissociation constant (K_D). Binding of lopinavir to AAG (7-23 µM) was saturable with K_D of 5.0 ±1.1 µM and N of 1.2 ±0.2. At low HSA concentrations (15-152 µM), lopinavir binding K_D was 24.3 ±8.7 µM and N was 1.1 ±0.4; however at 758 µM, lopinavir binding was essentially unsaturable. Lopinavir binding to AAG and HSA was not sensitive to ritonavir and thus, efforts to enhance fetal exposure to lopinavir should be focused on other issues such as efflux transporters.

Key words: antivirals, drug distribution, drug-drug interactions, enzyme kinetics, protein binding