Synthesis and In Vivo Evaluation of Novel PET Radioligands for Imaging the Endothelin-A Receptor

Authors:
  • NMNatesan Murugesan
    Natesan Murugesan
  • JXJinsong Xia
    Jinsong Xia
  • USUrsula Scheffel
    Ursula Scheffel
  • JHJohn Hilton
    John Hilton
  • HRHayden T. Ravert
    Hayden T. Ravert
  • RDRobert F. Dannals
    Robert F. Dannals
  • ZSZsolt Szabo
    Zsolt Szabo
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Abstract
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 nonradioactive 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-methylindolecarboxamide) (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.

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