In vivo labeling of endothelin receptors with [¹¹C]L-753,037: Studies in mice and a dog

Endothelin (ET) is a potent mammalian vasoconstrictive peptide and a pressor agent. Its 3 isoforms, ET-1, ET-2, and ET-3, mediate several physiologic actions in several organ systems, binding to 2 major receptor subtypes: ET_A and ET_B. This study was undertaken to evaluate [¹¹C]L-753,037 [(+)-(5S,6R,7R)-2-butyl-7-[2-((2S)-2-carboxy-propyl)-4-methoxyphenyl]-5- (3,4-methylenedioxyphenyl)cyclopenteno [1,2-β]pyridine-6-carboxylate), a new mixed ET receptor A and B antagonist, as a tracer for in vivo labeling of ET receptors in mice and a dog. Methods: [¹¹C]L-753,037 was synthesized, purified, and formulated from a normethyl precursor, L-843,974, and [¹¹C]H₃l. The tracer was studied for its in vivo kinetics, biodistribution, and ET receptor binding characteristics in mice. In the dog, PET imaging was performed to evaluate binding of [¹¹C]L-753,037 to ET receptors in the heart. Specificity of binding was studied in the heart with the selective ET_A antagonist L-753,164. Results: Kinetic studies in mice showed highest tracer uptake at 5 min after injection in liver (25.0 percentage injected dose per gram [%ID/g]), kidneys (18.7 %ID/g), lungs (15.2 %ID/g), and heart (5.6 %ID/g). Initial high uptake in liver, lungs, and kidneys was followed by rapid washout during the next 10 min and a very slow clearance during the time of observation (2 h after injection). By contrast, the radioactivity in the heart remained constant over 2 h. Administration of both ET_A (L-753,164) and mixed ET_A/ET_B (L-753,137) receptor antagonists resulted in dose-dependent inhibition of [¹¹C]L-753,037 binding in mouse heart, lungs, and kidneys but not in the liver. Radioactivity in the brain was very low, indicating that the tracer does not cross the blood-brain barrier. In the dog, a dynamic PET study of the heart showed high tracer accumulation at 55-95 min after injection. Injection of L-753,164 at 30 min before [¹¹C]L-753,037 administration led to a significant reduction in tracer binding. [¹¹C]methyl triphenyl phosphonium was used as a tracer for reference images of the dog heart muscle. Conclusion: The results suggest that [¹¹C]L-753,037 binds to ET receptors in vivo and is, therefore, a promising candidate for investigation of these receptors and their occupancy by ET receptor antagonists using PET.