2-Alkyl analogues of idazoxan (RX 781094) with enhanced antagonist potency and selectivity at central α2-adrenoceptors in the rat

Open Access
Publisher Website
Google Scholar
Abstract
1 Four 2-alkyl (methyl, ethyl, n-propyl and isopropenyl) analogues of idazoxan (RX 781094) have been synthesized and assessed in terms of their central α2/α1-adrenoceptor selectivity and a2-adrenoceptor antagonist potency using both in vitro and in vivo tests in the rat. 2 In cortical binding assays using [3H]-idazoxan and [3H]-prazosin, idazoxan had a 5 times greater α2/α1-selectivity than yohimbine. The 2-alkyl substituted analogues all showed improved selectivity, being between 17 and 29 times more selective than yohimbine for [3H]-idazoxan binding sites. 3 In terms of central antagonist potency in vivo, the most favourable substitutions were 2-ethyl (RX 811033) and 2-n-propyl (RX 811054). Compared with yohimbine, these analogues were, respectively, 36 and 18 times more potent intravenously and 5 and 7.5 times more potent orally in their antagonism of guanoxabenz-induced mydriasis in the pentobarbitone-anaesthetized rat. 4 All the analogues had a duration of action similar to that of idazoxan, which was significantly shorter than that of yohimbine. 5 The results indicate that introduction of alkyl groups in the 2-position of idazoxan greatly increases the α2/α1-adrenoceptor selectivity as measured in binding studies. Improved α2-adrenoceptor affinity and antagonist potency were particularly associated with the 2-ethyl and 2-n-propyl analogues.
Keywords