The binding of HIV-1 protease inhibitors to human serum proteins

The non-specific binding of a drug to plasma proteins is an important determinant of its biological efficacy since it modulates the availability of the drug to its intended target. In the case of HIV-1 protease inhibitors, binding to human serum albumin (HSA) and α 1-acid glycoprotein (AAG) appears to be an important modulator of drug bioavailability. From a thermodynamic point of view, the issue of drug availability to the desired target can be formulated as a multiple equilibrium problem in which a ligand is able to bind to different proteins or other macromolecules with different binding affinities. Previously, we have measured the binding thermodynamics of HIV-1 protease inhibitors to their target. In this article, the binding energetics of four inhibitors currently in clinical use (saquinavir, indinavir, ritonavir and nelfinavir) and a second-generation inhibitor (KNI-764) to human HSA and AAG has been studied by isothermal titration calorimetry. All inhibitors exhibited a significant affinity for AAG ( K a ∼0.5–10×10 5 M −1) and a relatively low affinity for HSA ( K a ∼5–15×10 3 M −1). It is shown that under conditions that simulate in vivo concentrations of serum proteins, the inhibitor concentrations required to achieve 95% protease inhibition can be up to 10 times higher than those required in the absence of serum proteins. The effect is compounded in patients infected with drug resistant HIV-1 strains that exhibit a lower affinity for protease inhibitors. In these cases the required inhibitor concentrations can be up to 2000 times higher and beyond the solubility limits of the inhibitors.