Synthesis and in vivo evaluation of ¹⁸F-fluoroethyl GF120918 and XR9576 as positron emission tomography probes for assessing the function of drug efflux transporters.

The purpose of this study was to synthesize two new positron emission tomography (PET) probes, N-(4-(2-(1,2,3,4-tetrahydro-6,7-dimethoxy-2-isoquinolinyl)ethyl)phenyl)-9,10-dihydro-5-[¹⁸F]fluoroethoxy-9-oxo-4-acridine carboxamide ([¹⁸F]3) and quinoline-3-carboxylic acid [2-(4-{2-[7-(2-[¹⁸F]fluoroethoxy)-6-methoxy-3,4-dihydro-1H-isoquinolin-2-yl]ethyl}phenylcarbamoyl)-4,5-dimethoxyphenyl]amide ([¹⁸F]4), and to evaluate the potential of these PET probes for assessing the function of two major drug efflux transporters, P-glycoprotein (P-gp) and breast cancer resistance protein (BCRP). [¹⁸F]3 and [¹⁸F]4 were synthesized by ¹⁸F-alkylation of each O-desmethyl precursor with [¹⁸F]2-fluoroethyl bromide for injection as PET probes. In vitro accumulation assay showed that treatment with P-gp/BCRP inhibitors (1 and 2) enhanced the intracellular accumulation capacity of P-gp- and BCRP-overexpressing MES-SA/Dx5 cells. In PET studies, the uptake (AUC(brain[0-)₆₀ (min])) of [¹⁸F]3 and [¹⁸F]4 in wild-type mice co-injected with 1 were approximately sevenfold higher than that in wild-type mice, and the uptake of [¹⁸F]3 and [¹⁸F]4 in P-gp/Bcrp knockout mice were eight- to ninefold higher than that in wild-type mice. The increased uptake of [¹⁸F]3 and [¹⁸F]4 was similar to that of parent compounds ([¹¹C]1 and [¹¹C]2) previously described, indicating that radioactivity levels in the brain after injection of [¹⁸F]3 and [¹⁸F]4 are related to the function of drug efflux transporters. Also, these results suggest that the structural difference between parent compounds ([¹¹C]1 and [¹¹C]2) and fluoroethyl analogs ([¹⁸F]3 and [¹⁸F]4) do not obviously affect the potency against drug efflux transporters. In metabolite analysis of mice, the unchanged form in the brain and plasma at 60 min after co-injection of [¹⁸F]4 plus 1 were higher (95% for brain; 81% for plasma) than that after co-injection of [¹⁸F]3 plus 1. [¹⁸F]4 is a promising PET probe to assess the function of drug efflux transporters.