Evaluation of oral Ro70-0004/003, an α_1A-adrenoceptor antagonist, in the treatment of male erectile dysfunction

Abstract

α-Adrenoceptor antagonists have been used for the treatment of male erectile dysfunction (MED). Ro70-0004/003 (Ro70-0004) is a selective and orally active α_1A-adrenoceptor antagonist. The objective of this study was to: (1) pharmacologically elucidate the α₁-adrenoceptor subtype mediating norepinephrine-induced contraction of human isolated corpus cavernosal tissue and (2) conduct a clinical proof-of-concept study with Ro70-0004 to test the hypothesis that selective α_1A-adrenoceptor blockade would improve erectile function in patients with MED. In vitro organ bath studies were conducted with strips of human isolated corpus cavernosal tissue obtained from patients undergoing penile prosthesis implantation. Prazosin, cyclazosin, RS-100329 and Ro70-0004/003 antagonized norepinephrine-induced contractile responses with affinity estimates (pK_B or pA₂) of 8.4, 7.3, 9.2 and 8.8, respectively, consistent with the singular involvement of α_1A-adrenoceptor subtype. A clinical study (single center, observer-blind, randomized, placebo-controlled, extended period Latin-Square crossover design) was conducted in 24 male patients (mean age 44 y) with MED of no established organic cause to evaluate the efficacy of a 5-mg oral dose of Ro70-0004. The primary efficacy endpoint was the duration of rigidity >60% at the base of the penis measured between 0.5 and 2.5 h post-dose. Rigidity was assessed by penile plethysmography using the RigiScan Plus® device during visual sexual stimulation. The safety and efficacy of Ro70-0004 was also assessed. A 50-mg dose of sildenafil was included as a positive control. For the primary efficacy endpoint, the mean duration of erection was 9.69 min following administration of placebo, 8.28 min following Ro70-0004, and 22.64 min following sildenafil. Only the difference between sildenafil and placebo reached statistical significance (P<0.05). A similar pattern was observed when measuring a duration of rigidity >80% at the base of the penis (secondary endpoint). Ro70-0004 was safe and generally well tolerated (only two out of 20 patients reported at least one adverse event). The highly selective α_1A-adrenoceptor antagonist, Ro70-0004, given at a single dose of 5 mg, did not improve erectile function when compared to placebo.