Phentolamine : pharmacokinetic aspects and pharmacodynamics in human corpus cavernosum. In vivo and in vitro study

Erectile dysfunction (ED) is defined as the Inability to achieve or maintain an erection adequate for sexual satisfaction. Phentolamine an a-adrenergic antagonist has been used to treat ED since 1994, mostly in combination with other vasoactive agents. More recently oral formulation of Phentolamine mesylate were developed for the disease. The drug is thought to relax penile smooth muscle by a inhibition over a-receptors. Since the paper of Traish (1998) has been speculated that phentolamine may also relax penile smooth muscle by a non-adrenergic mechanism. The aim of this study is to understand the pharmacokinetics aspects of Phentolamine (in vivo study) using the data from a bioequivalence test, and to investigate its pharmacodynamics with the purpose to clear its non-adrenergic mechanism in human corpus cavernosum (in vitro study). Methods (In vivo study): Thirty six healthy male volunteers (mean age 21,5 years old) were enrolled in the study that consisted in a single dose, two-way randomized crossover design comparing one phentolamine formulation (regitinaâ) to one standard phentolamine formulation (Vasomaxâ). In vitro study: A total of 64 isolated human corporeal tissue strips obtained from 16 male donor cadaver (16 –40 years old) were investigate. The pharmacologic activity of phentolamine-mediated relaxation, of pre-contracted erectile tissue strips of human corpus cavernosum were studied in organ bath chambers(non-adrenergic mean). Results: (in vivo study): Regitinaâ 40 mg formulation Cmax geometric mean ratio was 108.29% (90% CI = 98.58 – 118.96 of Vasomax 40 mg formulation. Regitineâ 40 mg formulation [AUC(0-720 min)] geometric mean ratio was 102.33 (90% CI = 97.21 – 19= 07.72) of Vasomaxâ 40 mg formulation. The average phentolamine pharmacokinetics parameters were Cmax 15,4 ng/mL, Tmax 50 min and t1/2 3 h. (In vitro study): Phentolamine caused concentration dependent relaxation in human corpus cavernosum strips pre-contracted with the a-adrenergic agonist phenylephrine as well as with the non-adrenergic serotonin (10-4 M), prostaglandin F2a (10-4 M) and KCl (60 mM) agents, with the best efficacy against phenylephrine (100% of relaxation at 10-3 M - IC50 = 1,5 x10-5M). Tetrodotoxin (TTX – 10-6 M) (Na+ channel blocker) and atropine (10-5 M) (muscarinic receptor inhibitor) did not cause alterations in the phentolamine relaxation of the penile smooth muscle (54,6 ± 4,6% x 48,9 x 6,4%) (52,7 ± 6,5% x 58,6 ± 5,6%) (p > 0,05). The relaxation of phentolamine of the human corpus cavernosum strips pre-contracted with KCl (40 mM) was significantly attenuated by NG-nitro-L-arginine L-NAME (10-4 M) (NO synthase inhibitor) (59,7 ± 5,8% x 27,8 ± 7,1%) (p 0,05). O relaxamento da fentolamina nas tiras de corpo cavernoso humano pré-contraídos com KCl (40 mM) foi significantemente atenuado por NG-nitro-L-arginine L-NAME (10-4 M) ( inibidor da NO sintetase) (59,7 ± 5,8% x 27,8 ± 7,1%) (p < 0,05) e 1H-[1,2,4] Oxadiazole [4,3-a]quinoxalin-1-one ODQ (10-4 M) (inibidor da guanilato ciclase) (62,7 ± 5,1% x 26,8 ± 3,9%) (p < 0,05). O papel dos bloqueadores dos canais de K foram investigados. A glibenclamida (10-4 M) um inibidor do canal de potássio ativado por ATP (inibidor do KATP) causou uma inibição quase completa (90%) do relaxamento da fentolamina em tiras de corpo cavernoso humano pré-contraídas com KCl (40 mM) (56,7 ± 6,3% x 11,3 ± 2,3%) (P < 0,05). Investigação com glibenclamida + L-NAME fez o mesmo efeito (54,6 ± 5,6% x 5,7 ± 1,4%) (p < 0,05). Os bloqueadores do canal de potássio dependente de CA++ (Kca) Charybdotoxina e apamina não modificaram o relaxamento da fentolamina (54,6 ± 4,6% v 59,3 ± 5,2%) Conclusão: Os parâmetros farmacocinéticos médios da fentolamina foram similares aos reportados pela literatura científica. As duas formulações da fentolamina foram consideradas semelhantes. Os resultados dos estudos farmacológicos da fentolamina sugerem que ela relaxa o corpo cavernoso de humanos também por mecanismo não adrenérgico-não colinérgico, ativando o canal de potássio KATP