Carboxylate Forms of Camptothecins -CPT, SN22 and Karenitecin® Bind Specifically to Site II (Diazepam site) of Human Serum Albumin

Camptothecins are an important class of anti-cancer agents and under physiological conditions exist in equilibrium between the lactone and carboxylate forms. The lactone form of camptothecins is crucial for their anti-tumor effect. Camptothecins, especially carboxylate forms, bind strongly to the plasma protein, human serum albumin (HSA). This binding of the carboxylate form alters the hydrolysis kinetics of the lactone form and may play a significant role in the mechanism of action of this class of anticancer drugs. High-performance affinity chromatography with an immobilized human serum albumin (HSA) stationary phase and a ligand competitive assay were employed to study the binding interactions between camptothecin (CPT), SN22 and a novel highly lipophilic camptothecin, Karenitecin®, with HSA. Employing known Site I (warfarin) and Site II (ibuprofen) binders of human serum albumin in a competitive HPLC assay, we show here that the carboxylate forms of CPT, SN22 and Karenitecin® bind to Site II on HSA but not to Site I on HSA. No direct experimental evidence was obtained to indicate that the lactone forms of these camptothecins bind to either Site I or Site II of HSA. This is the first report of binding interactions for Karentecin® with HSA.