Rapid solid-phase extraction method to quantify [11C]-verapamil, and its [11C]-metabolites, in human and macaque plasma

Introduction: P-glycoprotein (P-gp), all efflux transporter, is it significant barrier to drug entry into the brain and the fetus. The positron emission tomography (PET) ligand, [C-11]-verapamil, has been used to measure in vivo P-gp activity at various tissue-blood barriers of humans and animals. Since verapamil is extensively metabolized in vivo, it is important to quantify the extent of verapamil metabolism in order to interpret Such P-gp activity. Therefore, we developed a rapid solid-phase extraction (SPE) method to separate, and then quantify, verapamil and its radiolabeled metabolites in plasma. Methods: Using high-performance liquid chromatography (HPLC), we established that the major identifiable circulating radioactive metabolite of [C-11]-verapamil in plasma Of humans and the nonhuman primate, Macaca nemestrina, was [C-11]-D-617/717. Using sequential and differential pH elution oil C-8 SPE cartridges, we developed a rapid method to separate [C-11]-verapamil and [C-11]-D-617/717. Recovery was measured by spiking the samples with the corresponding nonradioactive compounds and assaying these compounds by HPLC. Results: Verapamil and D-617/717 recovery with the SPE method was >85%. When the method was applied to PET studies in humans and nonhuman primates, Significant plasma concentration of D-617/717 and unknown polar metabolite(s) were observed. The SPE: and the H PLC methods were not significantly different ill the quantification of verapamil and D-617/717. Conclusions: The SPE method simultaneously processes multiple samples in less than 5 min. Given the short half-life of[C-11], this method provides a valuable tool to rapidly determine the concentration of [C-11]-verapamil and its [C-11]-metabolites in human and nonhuman primate plasma. Published by Elsevier Inc.