Design of new beta1-selective adrenoceptor ligands as potential radioligands for in vivo imaging

In general, the failing human heart is characterized by a selective reduction in beta(1)-adrenoceptors (beta(1)-ARs) without change in beta(2)-AR density. Medical imaging techniques, either single photon emission computed tomography (SPECT) or positron emission tomography (PET) with appropriate radioligands, offer the possibility of assessing beta-adrenoceptor density non-invasively in humans. To date, neither a SPECT nor a PET radioligand is available for the selective imaging of cardiac beta(1)-ARs. The aim of this study was to develop potential high affinity beta(1)-selective AR radioligands for the non-invasive in vivo imaging of the beta(1)-AR density in the human heart using SPECT or PET. A variety of racemic N-aryl-N'-[2-[3-aryloxy-2-hydroxy-propylamino]-ethyl]-urea derivatives and chain-elongated analogues, related to the established beta(1)-AR antagonist, ICI 89,406 8i, were synthesized. Competition studies using the non-selective AR ligand, [(125)I]iodocyanopindolol ([(125)I]ICYP), and ventricular membrane preparations of wild-type mice revealed nine ligands with higher beta(1)-AR affinities (up to 76-fold) and beta(1)-AR selectivities (up to 139-fold) than 8i. Mostly, these ligands possess a 2-substituted phenoxy group and a 4-substituted phenyl residue in contrast to the lead compound 8i. The non-radioactive counterparts of the desired SPECT- and PET-radiotracers were synthesized as reference compounds [e.g., 8f, 8g, 8h and 8l as the non-radioactive analogues of the radioiodinated SPECT radioligands, 8e and 8h as the non-radioactive compounds of C-11 labelled PET-tracers (C-11 in the methoxy group)]. The established library of high affinity beta(1)-selective AR antagonists was screened for chemical precursors for the radiosynthesis of the mentioned radioligands. Furthermore, the library consists of some comparison compounds that are unsubstituted, allyl- and alkyl-substituted or chain-elongated (e.g., 8a, 8j, 8o and 8r-t). Future steps will include radiolabelling and pharmacokinetic evaluation of the beta(1)-selective target compounds, which could be applied as sympathetic innervation agents for in vivo investigations and diagnostics in patients suffering from cardiac diseases like heart failure and ventricular arrhythmias.