Targeting of endothelin receptors in the healthy and infarcted rat heart using the PET tracer ¹⁸F-FBzBMS

The endothelin subtype-A receptor (ET-A) is a promising therapeutic target in cardiovascular disease. We sought to determine the feasibility of an ¹⁸F-labeled ligand, ¹⁸F-(N-[[29-[[(4,5-dimethyl-3- isoxazolyl)amino]sulfonyl]-4-(2-oxazolyl)[1,19-biphenyl]-2-yl]methyl]-N, 4-fluorobenzamide) (¹⁸F-FBzBMS), for imaging ET-A in the healthy and injured rat heart. Methods: Male Wistar rats were used for all experiments. The specificity of cardiac ¹⁸F-FBzBMS uptake was determined in healthy animals (n = 23) using pretreatment with various blocking agents and doses. Myocardial infarction (MI) was induced by permanent left coronary ligation in 32 animals. Autoradiography was conducted to determine regional FBzBMS distribution relative to tissue perfusion at various times after MI. Histology and immunohistochemistry were performed for validation. The feasibility of in vivo detection of the tracer signal was tested using dedicated small-animal PET (n = 6). Results: At autoradiography, intravenous pretreatment with the selective ET-A blocker BMS-207940 reduced myocardial FBzBMS uptake by 93% ± 0.7%. Oral pretreatment with the clinical blocker bosentan resulted in a dose-dependent partial blockade (5 mg/kg, 48% ± 6%; 50 mg/kg, 61% ± 7%; and 100 mg/kg, 88% ± 0.7%). After MI, FBzBMS uptake was preserved in the infarct region from day 1 to month 6, whereas the perfusion tracer ²⁰¹Tl showed a persistent defect (MI-to-remote ratios: ²⁰¹Tl, 0.23 ± 0.28, 0.39 ± 0.07, 0.31 ± 0.07, 0.24 ± 0.12, 0.29 ± 0.10, and 0.23 ± 0.09; and FBzBMS, 0.94 ± 0.28, 0.92 ± 0.20, 0.88 ± 0.13, 0.82 ± 0.12, 0.80 ± 0.11, and 0.84 ± 0.08 at day 1, day 3, week 1, month 1, month 2, and month 6, respectively) (P < 0.01 vs. 201Tl). Ex vivo analysis confirmed ET-A expression in the infarct area, where the signal was partially colocalized with CD31 expression on endothelial cells. In vivo small-animal PET successfully confirmed specific uptake and blockade of FBzBMS in healthy myocardium. Conclusion: Cardiac uptake of the PET tracer ¹⁸F-FBzBMS is specific for ET-A expression in rats, shows infarct-related alterations, and can be imaged noninvasively. Further efforts to establish myocardial ET-A imaging methodology are warranted, with the perspective of determining role, efficacy, and benefit of ET-A targeted drug treatment in cardiovascular disease.