WEAK BINDING OF VX-478 TO HUMAN PLASMA-PROTEINS AND IMPLICATIONS FOR ANTI-HUMAN-IMMUNODEFICIENCY-VIRUS THERAPY

VX-478 is a potent inhibitor of human immunodeficiency virus type 1 (HIV-1) protease (K-i, 0.6 nM) and of HIV-1 replication in antiviral assays (IC90, 80 nM). The fractional binding of VX-478 to human plasma and to purified plasma proteins was determined by equilibrium dialysis and difference UV spectrophotometry. Binding to alpha(1)-acid glycoprotein (89% at 2 mu m total drug concentration, K-d of 4 mu M) accounts for its fractional binding in plasma (93%). Stopped-flow spectrophotometry methods showed that binding is a reversible two-step process. The measured dissociation rate constant approaches 100 s(-1). The antiviral effect of VX-478 was determined in the presence of 45% human plasma, in which the IC90 increased by 1.5-fold compared with control experiments in the presence of 15% fetal bovine serum. The effects of protein binding on the antiviral activity of VX-478 are minor, as expected for a weak drug-protein interaction.