Impact of structural alterations on the radiopharmacological profile of 18F-labeled pyrimidines as cyclooxygenase-2 (COX-2) imaging agents

Abstract Introduction Non-invasive imaging of COX-2 in cancer represents a powerful tool for assessing COX-2-mediated effects on chemoprevention and radiosensitization using potent and selective COX-2 inhibitors as an emerging class of anticancer drugs. Careful assessment of the pharmacokinetic profile of radiolabeled COX-2 inhibitors is of crucial importance for the development of suitable radiotracers for COX-2 imaging in vivo . The delicate balance between the selection of typical COX-2 pharmacophores and the resulting physicochemical characteristics of the COX-2 inhibitor represents a formidable challenge for the search of radiolabeled COX-2 imaging agents. Several pyrimidine-based COX-2 inhibitors demonstrated favorable in vitro and in vivo COX-2 imaging properties in various COX-2 expressing cancer cell lines. Here, we describe a comparative radiopharmacological study of three 18 F-labeled COX-2 inhibitors based on a pyrimidine scaffold. The objective of this study was to investigate how subtle structural alterations influence the pharmacokinetic profile of lead compound [ 18 F ] 1a ([ 18 F]Pyricoxib) to afford 18 F-labeled pyrimidine-based COX-2 inhibitors with improved COX-2 imaging properties in vivo . Methods Radiosynthesis of radiotracers was accomplished through reaction with 4-[ 18 F]fluorobenzyl amine on a methyl-sulfone labeling precursor ([ 18 F ] 1a and [ 18 F] 2a ) or late-stage radiofluorination using a iodyl-containing labeling precursor ([ 18 F ] 3a ). Radiopharmacological profile of 18 F-labeled pyrimidine-based COX-2 inhibitors [ 18 F ] 1a , [ 18 F ] 2a and [ 18 F ] 3a was studied in COX-2-expressing human HCA-7 colorectal cancer cell line, including cellular uptake studies in HCA-7 cells and dynamic PET imaging studies in HCA-7 xenografts. Results Cellular uptake of radiotracers [ 18 F ] 2a and [ 18 F ] 3a in HCA-7 cells was 450% and 300% radioactivity/mg protein, respectively, after 90 min incubation, compared to 600% radioactivity/mg protein for radiotracer [ 18 F ] 1a . Dynamic PET imaging studies revealed a tumor SUV of 0.53 ([ 18 F ] 2a ) and 0.54 ([ 18 F ] 3a ) after 60 min p.i. with a tumor-to-muscle ratio of ~1. Tumor SUV for [ 18 F ] 1a (60 min p.i.) was 0.76 and a tumor-to-muscle ratio of ~1.5. Pyricoxib analogues [ 18 F ] 2a and [ 18 F ] 3a showed distinct pharmacokinetic profiles in comparison to lead compound [ 18 F ] 1a with a significantly improved lung clearance pattern. Replacing the 4-[ 18 F]fluorobenzyl amine motif in radiotracer [ 18 F ] 1a with a 4-[ 18 F]fluorobenzyl alcohol motif in radiotracer [ 18 F ] 3a resulted in re-routing of the metabolic pathway as demonstrated by a more rapid liver clearance and higher initial kidney uptake and more rapid kidney clearance compared to radiotracers [ 18 F ] 1a and [ 18 F ] 2a . Moreover, radiotracer [ 18 F ] 3a displayed favorable rapid brain uptake and retention. Conclusion The radiopharmacological profile of three 18 F-labeled COX-2 inhibitors based on a pyrimidine scaffold were evaluated in COX-2 expressing human colorectal cancer cell line HCA-7 and HCA-7 xenografts in mice. Despite the overall structural similarity and comparable COX-2 inhibitory potency of all three radiotracers, subtle structural alterations led to significantly different in vitro and in vivo metabolic profiles. Advances in knowledge Among all tested pyrimidine-based 18 F-labeled COX-2 inhibitors, lead compound [ 18 F ] 1a remains the most suitable radiotracer for assessing COX-2 expression in vivo . Radiotracer [ 18 F ] 3a showed significantly improved first pass pulmonary passage in comparison to radiotracer [ 18 F ] 1a and might represents a promising lead compound for the development of radiotracers for PET imaging of COX-2 in neuroinflammation.