Elevated alpha-1-acid glycoprotein reduces the volume of distribution and systemic clearance of saquinavir.

The purpose of this study was to characterize the relationship between plasma protein binding and the pharmacokinetic disposition of saquinavir during a normal and elevated alpha-1-acid glycoprotein condition. The extent of plasma binding of [14C]saquinavir to human plasma, human albumin, and human alpha-1-acid glycoprotein was also assessed. Transgenic mice, which overexpress plasma alpha-1-acid glycoprotein, and control mice were given a single intravenous injection of saquinavir (10 mg/kg) and plasma samples were harvested as a function of time. The extent of [14C]saquinavir (0.5-30 microg/ml) plasma protein binding in each group of mice was determined by ultrafiltration. Plasma saquinavir concentrations from in vivo administration were determined by high performance liquid chromatography with tandem mass spectrometry. Saquinavir binding in human plasma and control mouse plasma was similar (approximately 3% unbound). In contrast, the extent of binding was significantly increased in transgenic mice (1.5% unbound). Furthermore, saquinavir was more extensively bound to alpha-1-acid glycoprotein than to albumin (2.1 versus 11.5% unbound). The systemic clearance and volume of distribution of saquinavir were significantly reduced in transgenic mice compared with control mice. The results of this study show that alpha-1-acid glycoprotein is the predominant plasma protein to which saquinavir binds. In addition, elevations in plasma alpha-1-acid glycoprotein considerably alter the pharmacokinetic disposition of saquinavir. This is consistent with the observations that systemic exposure to saquinavir in human immunodeficiency virus patients is greater than that in healthy volunteers and that alpha-1-acid glycoprotein levels increase with the degree of HIV infection.