Synthesis and first in vivo evaluation of new selective high affinity β1-adrenoceptor radioligands for SPECT based on ICI 89,406 Original Research Article

The results of cardiac biopsies suggest that myocardial β1-adrenoceptor (AR) density is reduced in patients with chronic heart failure, while changes in cardiac β2-ARs vary. A technique for visualization and quantification of β1-AR populations rather than total β-AR densities in the human heart would be of great clinical interest. Molecular imaging techniques, either single photon emission computed tomography (SPECT) or positron emission tomography (PET), with appropriate radiopharmaceuticals offer the possibility to assess β-AR density noninvasively in humans, but to date, neither a SPECT nor a PET-radioligand is clinically established for the selective imaging of cardiac β1-ARs. The aim of this study was to design a high affinity selective β1-AR radioligand for the noninvasive in vivo imaging of cardiac β1-AR density in man using SPECT. Based on the well-known selective β1-AR antagonist, ICI 89,406, both the racemic iodinated target compound 11a and the (S)-enantiomer 15a were synthesized. Competition studies using the nonselective AR ligand, [125I]iodocyanopindolol ([125I]ICYP), and ventricular membrane preparations from mice showed that 11a and 15a possess higher β1-AR affinities (up to 265-fold) and β1-AR selectivities (up to 245-fold) than ICI 89,406. Encouraged by these results, the radioiodinated counterparts of racemic 11a (11b: 125I, 11c: 123I) and (S)-configurated 15a (15b: 125I, 15c: 123I) were synthesized. The target compounds were evaluated in rats. Biodistribution and metabolism studies in rats indicated that there is a specific heart uptake of 11b–c and especially 15b–c accompanied by rapid metabolism of the radioligands. Therefore, radioiodinated 11c and 15c appeared to be unpromising SPECT-radioligands for assessing β1-ARs in vivo in the rat. However, the rat may metabolize β-AR ligands more rapidly than other species as demonstrated for (S)-[11C]CGP 12177, a radioligand structurally related to 11a–c and 15a–c. Therefore further studies in a different animal model will be carried out.