(R)-[¹¹C]Emopamil as a novel tracer for imaging enhanced P-glycoprotein function

Introduction: 2-Isopropyl-5-[methyl-(2-phenylethyl)amino]-2-phenylpentanenitrile (emopamil; EMP) is a calcium channel blocker of the phenylalkylamine class, with weak substrate properties for P-glycoprotein (P-gp). A weak substrate for P-gp would be suitable for measuring enhanced P-gp function. This study was performed to synthesise (R)- and (S)-[¹¹C]EMP and characterise their properties as P-gp tracers. Methods: We synthesised (R)- and (S)-[¹¹C]EMP and compared their biodistribution, peripheral metabolism, and effects of the P-gp inhibitor cyclosporine A (CsA, 50mg/kg). We compared the brain pharmacokinetics of (R)-[¹¹C]EMP and (R)-[¹¹C]verapamil [(R)-[¹¹C]VER] at baseline and CsA pretreatment with small animal positron emission tomography (PET). Results: (R)- and (S)-[¹¹C]EMP were synthesised from (R)- and (S)-noremopamil, respectively, by methylation with [¹¹C]methyl triflate in the presence of NaOH at room temperature. (R)- and (S)-[¹¹C]EMP yields were ~30%, with specific activity>74GBq/μmol and radiochemical purity>99%. (R)-[¹¹C]EMP showed significantly greater uptake in the mouse brain than (S)-[¹¹C]EMP. Both showed homogeneous non-stereoselective regional brain distributions. (R)- and (S)-[¹¹C]EMP were rapidly metabolised to hydrophilic metabolites. Unchanged plasma (S)-[¹¹C]EMP level was significantly lower than that of (R)-[¹¹C]EMP 15minutes post-injection, whilst>88% of radioactivity in the brain was intact at 15minutes post-injection and was non-stereoselective. CsA pretreatment increased brain activity ~3-fold in mice, but was non-stereoselective. The baseline area-under-the-curve (AUC) of brain radioactivity (0-60minutes) of (R)-[¹¹C]EMP was 2-fold higher than that of (R)-[¹¹C]VER, but their AUCs after CsA pretreatment were comparable. Conclusions: (R)-[¹¹C]EMP is a novel tracer for imaging P-gp function with higher baseline uptake than (R)-[¹¹C]VER.