Pharmacological analysis of the novel, selective α 1 ‐adrenoceptor antagonist, KMD‐3213, and its suitability as a tritiated radioligand

Pharmacological profiles of tritiated KMD‐3213, a new antagonist of α 1 ‐adrenoceptor (AR), were examined in recombinant and native α 1 ‐AR, and compared with those of prazosin (PZ) and tamsulosin (YM‐617). In saturation experiments, [ 3 H]‐KMD (10–2000 p M ) showed high affinity for α 1a ‐AR (pK D =10.5). However, no significant binding to α 1b ‐AR and insufficient/unsaturated binding to α 1d ‐AR were observed at concentrations up to 2000 p M . In contrast, [ 3 H]‐PZ and [ 3 H]‐YM bound to all subtypes with high affinity (pK D >9). In competition experiments, KMD‐3213 also had higher affinity for α 1a ‐AR than for other two subtypes; p K i =10.4, 8.1 and 8.6 for α 1a ‐, α 1b ‐ and α 1d ‐AR, respectively. [ 3 H]‐KMD also bound to the native α 1A ‐AR (rat submaxillary gland) with high affinity, but not to α 1B ‐AR (rat liver). In rat kidney which expresses α 1A ‐ and α 1B ‐AR, [ 3 H]‐KMD and [ 3 H]‐PZ bound to a single high‐affinity site (pK D =10.8 and 10.1, respectively) with distinct amount of binding sites (B max =159 and 267 fmol mg −1 protein, respectively). [ 3 H]‐PZ binding sites consisted of low‐ and high‐affinity sites for KMD‐3213 (p K i =7.6 and 10.7, respectively), for WB4101 (p K i =8.1 and 10.0) and for YM‐617 (p K i =8.7 and 10.8). The proportion of the high affinity site was approximately 60% in these drugs which was compatible to the ratio between B max of [ 3 H]‐KMD and [ 3 H]‐PZ. [ 3 H]‐KMD binding sites consisted of a single site for these drugs with affinities which were similar to those of the high affinity sites in [ 3 H]‐PZ binding. In functional experiments, KMD‐3213 antagonized the contractile responses to NS‐49 or noradrenaline (NA) with higher affinity in functional α 1A ‐ (rat caudal artery, pA 2 =10.0 against NS‐49) and α 1L ‐AR (dog mesenteric artery, pA 2 =9.9 against NA) than in α 1B ‐ (dog carotid artery, pA 2 =7.7 against NA) and α 1D ‐AR (rat thoracic aorta, pA 2 =8.3 against NA). These results confirm the α 1A ‐AR selectivity and high affinity of KMD‐3213, and indicate that [ 3 H]‐KMD can label selectively α 1A ‐AR.British Journal of Pharmacology (1999) 127 , 19–26; doi: 10.1038/sj.bjp.0702489