Binding of vinblastine to recrystallized human alpha 1-acid glycoprotein

The binding of vinblastine (VLB) to recrystallized human alpha 1-acid glycoprotein (alpha 1-AGP) dissolved in phosphate-buffered saline (pH 7.4) was determined at different drug concentrations using the technique of continuous ultrafiltration. Vinblastine was extremely highly bound (greater than 99.0%) at a drug concentration of 4.0 mumol X l-1, dropping to under 60% at 65.0 mumol X l-1. Binding was best described by a two-class model with higher- (9.4 microM-1) and lower- (0.1 microM-1) affinity sites but with a similar number of binding sites (1.5 as against 1.1 lower-affinity sites). These results strongly suggest that alpha 1-AGP would be a major binding protein for VLB in plasma.