RBx 6198: A novel α1-adrenoceptor antagonist for the treatment of benign prostatic hyperplasia

The present study, investigates the effect of RBx 6198, 2-{3-[4-(2-Isopropoxy-phenyl)-piperazin-1-yl]-propyl}-3a, 4, 7, 7a-tetrahydro-isoindole-1, 3,-dione, a novel α1-adrenoceptor antagonist, in both in vitro and in vivo test systems. RBx 6198 is a potent (nanomolar affinity) α1A-adrenoceptor antagonist with demonstrable uroselectivity in anaesthesized dog model. In radioligand binding studies using human recombinant receptors, RBx 6198 exhibited high selectivity (~ 50 fold) for the α1A-adrenoceptor subtype as compared to α1B-adrenoceptor subtype. In order to assess tissue selectivity, the antagonistic effect of RBx 6198 on the phenylephrine induced contractile response of isolated rat prostate, spleen and aorta was characterized. RBx 6198 was 8 fold more potent in inhibiting phenylephrine-evoked contractions of isolated tissues compared to tamsulosin. However, the compound was non-selective for α1Avs. α1D-adrenoceptor like tamsulosin. In anaesthetized beagle dogs RBx 6198 suppressed the intraurethral pressure response to phenylephrine to a greater extent than the mean arterial pressure response thereby demonstrating uroselectivity consistent with in vitro binding and functional data. RBx 6198 was 6.4 fold more uroselective as compared to tamsulosin after i.v. route dose administration. Taken together all results from preclinical studies, it is suggested that RBx 6198 is a novel α1-adrenoceptor antagonist that exhibited improved pharmacological profile over tamsulosin in both in vitro and in vivo.