Radiosynthesis and in vivo evaluation of 1-[ 18F]fluoroelacridar as a positron emission tomography tracer for P-glycoprotein and breast cancer resistance protein

Abstract

Aim of this study was to label the potent dual P-glycoprotein (Pgp) and breast cancer resistance protein (BCRP) inhibitor elacridar ( 1) with 18F to provide a positron emission tomography (PET) radiotracer to visualize Pgp and BCRP. A series of new 1- and 2-halogen- and nitro-substituted derivatives of 1 ( 4a– e) was synthesized as precursor molecules and reference compounds for radiolabelling and shown to display comparable in vitro potency to 1 in increasing rhodamine 123 accumulation in a cell line overexpressing human Pgp (MDCKII-MDR1). 1-[ 18F]fluoroelacridar ([ 18F] 4b) was synthesized in a decay-corrected radiochemical yield of 1.7 ± 0.9% by a 1-step no-carrier added nucleophilic aromatic 18F-substitution of 1-nitro precursor 4c. Small-animal PET imaging of [ 18F] 4b was performed in naïve rats, before and after administration of unlabelled 1 (5 mg/kg, n = 3), as well as in wild-type and Mdr1a/b (−/−) Bcrp1 (−/−) mice ( n = 3). In PET experiments in rats, administration of unlabelled 1 increased brain activity uptake by a factor of 9.5 ( p = 0.0002, 2-tailed Student’s t-test), whereas blood activity levels remained unchanged. In Mdr1a/b (−/−) Bcrp1 (−/−) mice, the mean brain-to-blood ratio of activity at 60 min after tracer injection was 7.6 times higher as compared to wild-type animals ( p = 0.0002). HPLC analysis of rat brain tissue extracts collected at 40 min after injection of [ 18F] 4b revealed that 93 ± 7% of total radioactivity in brain was in the form of unchanged [ 18F] 4b. In conclusion, the in vivo behavior of [ 18F] 4b was found to be similar to previously described [ 11C] 1 suggesting transport of [ 18F] 4b by Pgp and/or BCRP at the rodent BBB. However, low radiochemical yields and a significant degree of in vivo defluorination will limit the utility of [ 18F] 4b as a PET tracer.