Binding characteristics of KNI-272 to plasma proteins, a new potent tripeptide HIV protease inhibitor.

The binding characteristics of KNI-272, a potent and selective human immunodeficiency virus (HIV) protease inhibitor, were evaluated in rat and human plasma, and in solutions of human alpha 1-acid glycoprotein (AAG) and human serum albumin (HSA). The unbound fractions (Fu) of KNI-272 were 12.13 and 2.24% in rat and human plasma, respectively, at the drug concentration of 1.0 microgram mL-1. Although KNI-272 binds to both AAG and HSA, the Fu of KNI-272 in AAG solution was 1.83%, and only one-quarter of that in HSA solution (Fu = 6.78%). Binding displacing agents, such as disopyramide, warfarin, diazepam, and digitoxin, were used to determine the binding site of KNI-272 on these plasma proteins. The Fu of KNI-272 in AAG solution increased 14-fold when disopyramide was added to the AAG solution. In addition, warfarin, diazepam, and digitoxin were added to HSA solution as representative drugs bound to distinct binding sites on HSA, namely sites I, II, and III, respectively. The Fu values of KNI-272 in HSA solution significantly increased when warfarin and diazepam were added. In particular, with the addition of warfarin to HSA solution, the Fu of KNI-272 increased to 16%. The modified Scatchard plots of KNI-272 binding to AAG and HSA both showed biphasic curves, and the KNI-272 binding sites at low concentration range on AAG and HSA disappeared with the addition of disopyramide and warfarin, respectively. Therefore, it is considered that KNI-272 binds to the identical site as disopyramide on AAG and site I on HSA in the low KNI-272 concentration range. By comparing the KNI-272 binding parameters obtained in human plasma and these protein solutions, we can assume that KNI-272 binding at low concentration in human plasma is mainly concerned with the binding on AAG. As KNI-272 concentration in plasma increases, HSA becomes concerned with KNI-272 binding.