Synthesis and preclinical evaluation of the radiolabeled P-glycoprotein inhibitor [11C]MC113

Objectives: With the aim to develop a PET tracer to visualize P-glycoprotein (Pgp) expression levels in different organs, the Pgp inhibitor MC113 was labeled with C-11 and evaluated using small-animal PET. Methods: [C-11]MC113 was synthesized by reaction of O-desmethyl MC113 with [C-11]methyl triflate. Small-animal PET was performed with [C-11]MC113 in FVB wild-type and Mdr1a/b((-/-)) mice (n=3 per group) and in a mouse model of high (EMT6Ar1.0) and low (EMT6) Pgp expressing tumor grafts (n=5). In the tumor model, PET scans were performed before and after administration of the reference Pgp inhibitor tariquidar (15 mg/kg). Results: Brain uptake of [C-11]MC113, expressed as area under the time-activity curve from time 0 to 60min (AUC(0-60)), was moderately but not significantly increased in Mdr1a/b((-/-)) compared with wild-type mice (mean +/- SD AUC(0-60), Mdr1a/b((-/-)): 88 +/- 7 min, wild-type: 62 +/- 6 min, P=0.100, Mann Whitney test). In the tumor model, AUC(0-60) values were not significantly different between EMT6Ar1.0 and EMT6 tumors. Neither in brain nor in tumors was activity concentration significantly changed in response to tariquidar administration. Half-maximum effect concentrations (IC50) for inhibition of Pgp-mediated rhodamine 123 efflux from CCRFvcr1000 cells were 375 +/- 60 nM for MC113 versus 8.5 +/- 2.5 nM for tariquidar. Conclusion: [C-11]MC113 showed higher brain uptake in mice than previously described Pgp PET tracers, suggesting that [C-11]MC113 was only to a low extent effluxed by Pgp. However, [C-11]MC113 was found unsuitable to visualize Pgp expression levels presumably due to insufficiently high Pgp binding affinity of MC113 in relation to Pgp densities in brain and tumors. (C) 2012 Elsevier Inc. All rights reserved.