Weak Binding of VX-478 to Human Plasma Proteins and Implications for Anti-Human Immunodeficiency Virus Therapy

Abstract

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 α₁-acid glycoprotein (89% at 2 µM total drug concentration, K_d of 4 µM) 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.