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 (H IV-1) protease (K-i, 0.6 nM) and of HIV-1 replication in antiviral ass ays (IC90, 80 nM). The fractional binding of VX-478 to human plasma an d to purified plasma proteins was determined by equilibrium dialysis a nd difference UV spectrophotometry. Binding to alpha(1)-acid glycoprot ein (89% at 2 mu m total drug concentration, K-d of 4 mu M) accounts f or its fractional binding in plasma (93%). Stopped-flow spectrophotome try methods showed that binding is a reversible two-step process. The measured dissociation rate constant approaches 100 s(-1). The antivira l 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 experim ents in the presence of 15% fetal bovine serum. The effects of protein binding on the antiviral activity of VX-478 are minor, as expected fo r a weak drug-protein interaction.