Synthesis and in vivo evaluation of novel PET radioligands for imaging the endothelin-A receptor

The endothelin subtype-A (ETA) receptor is a member of a family of G-protein-coupled receptors that plays a central role in vasoconstriction, cell proliferation, and hormone production. The aim of this study was to synthesize and evaluate in vivo ¹¹C- and ¹⁸F-labeled analogs of the potent and selective ETA antagonist N-[[2′-[[(4,5-dimethyl-3-isoxazolyl) amino]sulfonyl]-4-(2-oxazolyl) [1,1′-biphenyl]-2-yl]methyl]-N,3,3- trimethylbutanamide (BMS-207940). Methods: Protected precursors and authentic non-radioactive standards were synthesized by reductive amination and subsequent alkylation of protected aldehyde 1. Desmethyl precursor 2 was reacted with ¹¹C-methyl iodide followed by deprotection and high-performance liquid chromatography purification to produce ¹¹C-(N-[[2′-[[(4, 5-dimethyl-3-isoxazolyl)amino] sulfonyl]-4-(2-oxazolyl)[1,1′-biphenyl]-2- yl]methyl]-N,1-methyl-indolecarboxamide) (¹¹C-BMS-5p) 3. Nitro precursor 4 was reacted with ¹⁸F-fluoride and purified by high-performance liquid chromatography to produce ¹⁸F-(N-[[2′- [[(4,5-dimethyl-3-isoxazolyl) amino]sulfonyl]-4-(2-oxazolyl)[1,1′- biphenyl]-2-yl]methyl]-N,4-fluorobenzamide) (¹⁸F-FBzBMS) 5. Biodistribution was determined by injecting male CD-1 mice via the tail vein with either ¹¹C-BMS-5p 3 or ¹⁸F-FBzBMS 5. Specific binding was determined by pretreatment with 1 mg of BMS-207940 per kilogram. PET scanning of a baboon using either ¹¹C-BMS-5p 3 or ¹⁸F-FBzBMS 5 was performed at baseline and 40 min after intravenous administration of 1 mg of BMS-207940 per kilogram. Results: ¹¹C-BMS- 5p 3 was synthesized with 1.5% radiochemical yield in 36 min, with an average specific activity of 1,051 GBq/μmol (28,400 mCi/μmol; n = 5) at the end of synthesis. ¹⁸F-FBzBMS 5 was synthesized with 0.54% radiochemical yield in 130 min, with an average specific activity of 12.9 GBq/μmol (349 mCi/μmol, n = 7) at the end of synthesis. In mice, the highest uptake of both radioligands was in the liver, lungs, and heart. Radioactivity in the liver washed out over time, and uptake in the lungs and heart remained relatively stable. Mice pretreated with 1 mg of BMS-207940 per kilogram showed greater than 64% specific binding in the lungs and kidneys at 60 min. Specific binding in the heart was determined to be 63% for ¹¹C-BMS-5p 3 and 81% for ¹⁸F-FBzBMS 5. PET studies in a baboon showed high uptake of both radioligands in the myocardium. Between 35 and 85 min, the heart-to-blood ratio was 4.7 to 1. Pretreatment with a 1 mg/kg dose of BMS-207940 showed 85% specific binding in the myocardium at 85 min after injection. Conclusion: Both ¹¹C-BMS-5p 3 and ¹⁸F-FBzBMS 5 bind selectively to the ETA receptor in vivo. Further development of these radioligands for imaging the ETA receptor in humans is warranted.