Pharmacological analysis of the novel, selective α1-adrenoceptor antagonist, KMD-3213, and its suitability as a tritiated radioligand

Abstract

1. Pharmacological profiles of tritiated KMD-3213, a new antagonist of α1-adrenoceptor (AR), were examined in recombinant and native α1-AR, and compared with those of prazosin (PZ) and tamsulosin (YM-617). 2. In saturation experiments, [3H]-KMD (10-2000 pM) 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 pM. In contrast, [3H]-PZ and [3H]-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; pK(i)= 10.4, 8.1 and 8.6 for α(1a)-, α(1b)- and α(1d)-AR, respectively. 3. [3H]-KMD also bound to the native α(1A)-AR (rat submaxillary gland) with high affinity, but not to α(1β)-AR (rat liver). In rat kidney which expresses α(1A)- and α(1B)-AR, [3H]-KMD and [3H]-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). [3H]-PZ binding sites consisted of low- and high-affinity sites for KMD-3213 (pK(i) = 7.6 and 10.7, respectively), for WB4101 (pK(i) = 8.1 and 10.0) and for YM-617 (pK(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 [3H]-KMD and [3H]-PZ. [3H]-KMD binding sites consisted of a single site for these drugs with affinities which were similar to these of the high affinity sites in [3H]-PZ binding. 4. 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, pA2 = 10.0 against NS-49) and α(1L)-AR (dog mesenteric artery, pA2 = 9.9 against NA) than in α(1B)- (dog carotid artery, pA2 = 7.7 against NA) and α(1D)-AR (rat thoracic aorta, pA2 = 8.3 against NA). 5. These results confirm the α(1A)-AR selectivity and high affinity of KMD-3213, and indicate that[3H]-KMD can label selectively α(1A)-AR.