Oral phentolamine: an alpha-1, alpha-2 adrenergic antagonist for the treatment of erectile dysfunction

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1 (2000) 12, Suppl 1, S75±S80 ß 2000 Macmillan Publishers Ltd All rights reserved /00 $ Oral phentolamine: an alpha-1, alpha-2 adrenergic antagonist for the treatment of erectile dysfunction 1 1 Department of Urology, Boston University School of Medicine, Boston, MA, USA Phentolamine mesylate is an alpha-1 and alpha-2 selective adrenergic receptor antagonist which has undergone clinical trials for erectile dysfunction treatment. Biochemical and physiological studies in human erectile tissue have revealed a high af nity of phentolamine for alpha-1 and alpha-2 adrenergic receptors. Based on pharmacokinetic studies, it is suggested that 30±40 min following oral ingestion of 40 or 80 mg of phentolamine (Vasomax), the mean plasma phentolamine concentrations are suf cient to occupy the alpha-1 and -2 adrenergic receptors in erectile tissue and thereby result in inhibition of adrenergic-mediated physiologic activity. In large multi-center, placebo-controlled pivotal phase III clinical trials, the mean change in the erectile function domain of the International Index of Erectile Function scores (Questions 1±5 and 15) from screening to the end of treatment was signi cantly higher following use of active drug (40 mg and 80 mg) compared to placebo. Three to four times as many patients receiving phentolamine reported being satis ed or very satis ed compared with those receiving placebo. At doses of 40 mg and 80 mg respectively, 55% and 59% of men were able to achieve vaginal penetration with 51% and 53% achieving penetration on 75% of attempts. The correction of erectile dysfunction or improvement to a less severe category of dysfunction was experienced by 53% of men with the 80 mg dose and 40% with the 40 mg dose of phentolamine. All trends of response were the same regardless of any concomitant medication. There were no severe adverse events. At 40 mg, 7.7% experienced rhinitis and fewer than 3.1% experienced any other side effect of treatment. Phentolamine is safe, well tolerated and ef cacious for the treatment of erectile dysfunction. (2000) 12, Suppl 1, S75±S80 Keywords: penile erection; alpha blocker; oral agents; erectile dysfunction Role of adrenergic agonists Detumescence of the erect penis is initiated by adrenergic agonist stimulation. Adrenergic stimulation results in contraction of the cavernosal arteries with reduced cavernosal arterial in ow as well as contraction of the trabecular smooth muscle leading to collapse of the lacunar spaces. The resultant loss of corporal veno-occlusive function leads to detumescence. The role of the adrenergic neuroeffector system as a mediator of detumescence is noncontroversial. Intracavernosal administration of adrenergic agonists initiate detumescence and have become a routine treatment for prolonged erection. 2 There are several adrenergic agonists with varying af nities for the alpha adrenergic receptors. Correspondence:, 720 Harrison Ave, Boston, MA Received 1 October 1999; accepted 23 ovember 1999 orepinephrine is the physiologic autonomic neurotransmitter and binds both to alpha-1 and alpha-2 adrenergic receptors (Figure 1). Oxymetazoline is another alpha-1 and alpha-2 receptor agonist. Methoxamine, on the other hand, binds primarily to alpha-1 adrenergic receptors while the compound UK-14,304 binds primarily to alpha-2 adrenergic receptors. There are several physiologic activities associated with norepinephrine or catecholamine release at the adrenergic synapse. Sexual intercourse and orgasm is associated with increased heart rate and systolic blood pressure secondary to catecholamine release (Figure 2). 3 Psychologic-mediated release of catecholamines may occur following stressful situations associated with sexual activity such as fear of failure, performance anxiety, anger, shame and embarrassment. Physiologic and=or psychologicmediated catecholamine release may occur secondary to pain associated with Peyronie's disease, prostatitis, epididymitis, or non-genital pain such as headache. All of these above conditions, from

2 76 H 2 Chemical Formulas of orepinephrine and Phentolamine CH orepinephrine CH 3 Phentolamine treatment of erectile dysfunction are as follows. Adrenergic blockade might delay detumescence of the penile erection. Adrenergic blockade might prolong the duration of the erection initiated by sexual stimulation. Adrenergic blockade might potentiate the stimulatory effect of smooth muscle relaxation by removing inhibitory responses mediated by catecholamine release by the sympathetic nervous system. Further research in the effect of alpha adrenergic blockade on the erectile response is needed. Figure 1 Schematic chemical formulas of norepinephrine and phentolamine. sexual intercourse to stress to pain are associated with catecholamine release and an erectolytic effect on penile erection. Principles of adrenergic blockade in the treatment of erectile dysfunction Since activation of adrenergic agonists initiates detumescence, antagonizing the alpha adrenergic contractile response may facilitate the erectile response in men with erectile dysfunction. The principles of adrenergic blockade utilization in the Phentolamine Ð an alpha-1 and alpha-2 adrenergic antagonist Phentolamine mesylate is an alpha-1 and alpha-2 adrenergic antagonist 4±6 which is currently being investigated for its safety and ef cacy as an oral agent in the treatment of male erectile dysfunction. Pharmacokinetics Pharmacokinetic studies in normal volunteers have revealed that the mean phentolamine plasma concentration 30 min after oral ingestion of 40 mg Catecholamine Loading During Sexual Performance mmhg mmhg 125 Blood pressure mmhg 115 beats/min mmhg 85 beats/min 75 mmhg Heart rate 55 beats/min Desire Arousal Orgasm Catecholamine release - causes penile smoothmuscle to contract L. Messenger Sexual Response Cycle Figure 2 Hypothetical release of catecholamine associated with sexual performance from desire to arousal to orgasm. Coital-mediated increases in catecholamine result in an approximate doubling of the resting pulse and a 50% increase in resting systolic blood pressure. Catecholamine release during sexual activity may explain early loss of erection (failure to maintain erectile dysfunction) in some men.

3 phentolamine (Vasomax) tablets was 16±22 ng=ml (42± 50 nm) and after 80 mg was 36±44 ng=ml (80± 100 nm). In 32 individuals, the area under curve (AUC), C max, T max and half-life values for 40 mg and 80 mg phentolamine (Vasomax) tablets were and ngh=ml; and ng=ml; and h; and h, respectively (Figure 3). Plasma phentolamine concentration (ng/ml) Days 1,7,14 (pooled) - 40 mg dose Days 1,7,14 (pooled) - 80 mg dose Time (hours) Figure 3 Pharmacokinetic pro le of phentolamine. Phentolamine represents an agent that has fast onset of therapeutic blood levels (approximately 30±40 min) and fast metabolism (half-life approximately 2 h). Molecular mechanisms The mechanism of action of phentolamine in regulating corpus cavernosum smooth muscle tone has been postulated to be via alpha adrenergic receptor blockade (Figure 4). Detailed biochemical and physiological studies of the mechanism of action of phentolamine mesylate in human corpus cavernosum tissue have recently been reported. 4 Phentolamine mesylate was found to be an effective alpha-1 and alpha-2 adrenergic receptor blocker in corpus cavernosum erectile tissue. Phentolamine competitively displaced speci c alpha-1 receptor ligands (HEAT and prazosin) and speci c alpha-2 receptor ligands (rauwolscine and RX ). The af nity of phentolamine for alpha-1 adrenergic receptors was comparable to the alpha-1a selective receptor antagonist 5-methylurapidil but was less than that of prazosin. The af nity of phentolamine for the alpha-2 adrenergic receptors was greater than that of the alpha-2 receptor agonist UK-14,304 and norepinephrine but was less than that of the alpha-2 receptor antagonists delequamine (RS ) and 77 Phentolamine is an α-selective Receptor Antagonist orepinephrine Phentolamine CH 3 H 2 CH receptor Extracellular Cell membrane α γ Intracellular Gq/11 protein X orepinephrine unable to bind receptor α γ Gq/11 protein Extracellular receptor activated γ α Gq/11 protein activated Cell membrane Intracellular L. Messenger receptor not activated α γ Gq/11 protein not activated Figure 4 Schematic representatation of the mechanism of alpha blockade by phentolamine. The left-hand side of the gure reveals that the sympathetic neurotransmitter alpha-agonist norepinephrine binds to the cell membrane alpha-1 adrenergic receptor initiating receptor conformational changes and activating Gq=11 protein (a critical step to initiate increased intracellular calcium concentration and smooth muscle contraction). The right-hand side of the gure reveals that in the presence of phentolamine, which has high af nity for the alpha receptor, norepinephrine can no longer bind to the oocupied alpha receptor. Since phentolamine binding to the alpha receptor does not induce conformational changes in the receptor, there is no activation of the Gq=11 protein.

4 78 rauwolscine. The relatively high af nity of phentolamine mesylate for alpha-1 and -2 adrenergic receptors suggests that 30±40 min following oral ingestion of 40 mg of phentolamine (Vasomax), the mean plasma phentolamine concentrations (40± 50 nm) are suf cient to occupy the alpha adrenergic receptors in corpus cavernosum erectile tissue and thereby result in inhibition of adrenergic-mediated physiologic activity 4 In corpus cavernosum erectile tissue, phentolamine mesylate shifted the dose response of phenylephrine (selective alpha-1 mediated) and UK14,304 (selective alpha-2 mediated) contraction in a concentration-dependent manner. Phentolamine mesylate also induced concentration dependent relaxation in erectile tissue pre-contracted with the selective alpha-1 receptor agonists phenylephrine and oxymetazoline, the selective alpha-2 receptor agonist UK14,304 and the non-selective agonist norepinephrine. These data indicate that phentolamine alpha-adrenergic binding activity mediates the physiologic inhibition of adrenergic induced corpus cavernosum contractile tone and the resulting relaxation response. 4 Contractile responses by 80 mm KCl and endothelin are mediated by nonadrenergic mechanisms. Those mediated by KCl are thought to occur by electromechanical coupling via depolarization of the corpus cavernosum smooth muscle primarily by activation of L-type calcium channels and calcium in ux. Those mediated by endothelin are thought to occur via receptor mediated intracellular calcium release. Phentolamine mesylate, however, induced relaxation of corpus cavernosum strips pre-contracted with 80 mm KCl or endothelin. Such data suggest that phentolamine mediates corpus cavernosum tissue contractility by both adrenergic and non-adrenergic pathways. This relaxation response was attenuated by the competitive nitric oxide synthase inhibitor L-nitroarginine and by the mechanical disruption of the endothelium. One likely possibility to explain phentolamine mesylate-induced relaxation of KCl contracted tissue was an additional non-adrenergic, endothelial-based, nitricoxide mechanism. 4 Indirect antagonism is a recognized complementary pharmacologic mechanism of action of many natural and synthetic agonists and antagonists. Phentolamine mesylate is an adrenergic antagonist to the contractile substance norepinephrine. Phentolamine was found to also possess the agonist function of tissue relaxation via endothelial nitric oxide pathways. In erectile tissue, phentolamine thus was found to elicit activity via a dual mechanism of action. One pathway was by direct antagonism of alpha-1 and -2 receptors agonists, at the receptor level, resulting in reduced intracellular calcium release and inhibition of contractility. The other mechanism was probably by an indirect, functional antagonism, in which phentolamine enhancer nitric oxide ef ciency in relaxing trabecular smooth muscle. This additional pathway was found to result in increased intracellular cyclic nucleotide levels, augmenting reduced intracellular calcium release and corporal smooth muscle relaxation. 4 Clinical Trials The ef cacy and safety of oral phentolamine (40 mg or 80 mg) was examined for the treatment of mild to moderate erectile dysfunction. 5,6 The patient inclusion criteria included a 6 month history of: (1) a consistent change in the quality of erection that adversely affected patient satisfaction with sexual intercourse; (2) reduced but not absent incidence of awakening with a full erection; (3) dif culty achieving an erection prior to intercourse; (4) dif culty maintaining an erection during intercourse; and (5) neutral to extreme dissatisfaction with the current level of sexual function. At visit 1 (screening) patients were instructed to attempt intercourse four times over four weeks and record results on `Sexual Encounter Pro le' cards. A Sexual Encounter Pro le score of 8±16 meant inclusion into the study. Less than 8 or more than 16 meant exclusion from the study. At visit 2 (baseline), the International Index of Erectile Function was administered. A total of 459 patients were randomized into placebo, phentolamine 40 mg or 80 mg. Patients were again instructed to attempt intercourse four times over four weeks and record results on `Sexual Encounter Pro le' cards. A total of 424 patients completed the study. At visit 3 (end of treatment), the International Index of Erectile Function was again administered. A total of 139, 139 and 146 patients completed placebo, 40 mg and 80 mg oral phentolamine, respectively. The mean age was 57.7, 58.9 and 58.4 y, respectively. The mean height was 69.9, 70.5 and 70.0 inches, respectively. The mean weight was 198.3, and pounds, respectively. The primary ef cacy criteria were the results of the erection domain of the International Index of Erectile Function scores (Questions 1±5 and 15) from screening to the end of treatment and from baseline to the end of treatment. The mean change in the erectile function domain of the International Index of Erectile Function scores (Questions 1±5 and 15) from screening to the end of treatment was: , 2.15 (P < ) and 3.63 (P < ) for placebo, 40 mg and 80 mg, respectively. The mean change in the erectile function domain of the International Index of Erectile Function scores (Questions 1±5 and 15) from baseline to the end of treatment was: 72.3, 5.65 and 6.66 for placebo, 40 mg and 80 mg, respectively. Improvement in erectile function domain scores for placebo, 40 mg

5 and 80 mg were noted in 28%, 53% and 63% of patients, respectively. Additional primary ef cacy criteria included the ability to achieve vaginal penetration and the ability to maintain erection after penetration and achieve ejaculation in at least 75% of attempts. A total of 38%, 51% (P ˆ 0.012) and 53% (P ˆ 0.005) of patients on placebo, 40 mg and 80 mg, respectively, reported the ability to achieve vaginal penetration. The secondary ef cacy criteria were the results of other domain scores of the International Index of Erectile Function scores concerning orgasmic function, sexual desire, intercourse satisfaction, and overall satisfaction. Concerning orgasmic function, the mean change from baseline (visit 2) to end of treatment (visit 3) was: , 0.78 (P ˆ ), 1.05 (P < ) for placebo, 40 mg and 80 mg, respectively. Concerning sexual desire, the mean change from baseline (visit 2) to end of treatment (visit 3) was: 0.05, 0.32 (S), 0.18 (S) for placebo, 40 mg and 80 mg, respectively. Concerning intercourse satisfaction, the mean change from baseline (visit 2) to end of treatment (visit 3) was: 0.23, 0.8 (P ˆ ), 1.1 (P < ) for placebo, 40 mg and 80 mg, respectively. Concerning overall satisfaction, the mean change from baseline (visit 2) to end of treatment (visit 3) was: 7 0.2, 1.3 (P ˆ ), 1.35 (P < ) for placebo, 40 mg and 80 mg, respectively. Other secondary ef cacy criteria were the results of questions 3 and 4 of the erectile function domain concerning frequency of penetration and frequency of maintaining an erection. Concerning frequency of penetration, the mean change from baseline (visit 2) to end of treatment (visit 3) was: , 0.3 (P ˆ ), 0.6 P < ) for placebo, 40 mg and 80 mg, respectively. Concerning frequency of penetration, the mean change from baseline (visit 2) to end of treatment (visit 3) was: , 0.3 (P ˆ ), 0.6 (P < ) for placebo, 40 mg and 80 mg, respectively. Concerning frequency of maintaining erection, the mean change from baseline (visit 2) to end of treatment (visit 3) was: 7 0.1, 0.5 (P ˆ ), 0.8 (P < ) for placebo, 40 mg and 80 mg, respectively. Other secondary ef cacy criteria were the results of overall satisfaction with treatment. The proportion of patients satis ed or very satis ed with the study medication was 10%, 33% and 40% for placebo, 40 mg and 80 mg, respectively. Additional secondary ef cacy criteria were the results of shifts in clinical dysfunction for the erectile function domain score on IIEF at baseline (visit 2) and following end of treatment (visit 3). At baseline, 0± 6 points was considered severe dysfunction, 7±12 was considered moderate-severe dysfunction, 13±18 was considered moderate dysfunction, 19±24 was considered mild dysfunction, and 25±30 was con- 79 Map of Pharmacological First-line Therapies Satisfaction city Town of desire Dysfunction detour Function freeway PDE type 5 inhibitor parkway Choice terrace α blocker boulevard Combination st. Central activator avenue Potency place Topical turnpike L. Messenger Figure 5 Cartoon depiction of potential future options for pharmacologic management of men with erectile dysfunction.

6 80 sidered normal function. At the end of treatment, the percent of patients categorically improved was 15%, 40% (P ˆ 0.001) and 53% (P ˆ 0.001) for placebo, 40 mg and 80 mg, respectively. Concerning side effects, the proportion of patients reporting treatment-related adverse events during the study period was as follows. A total of 3.8%, 7.7% and 20.9% reported rhinitis for placebo, 40 mg and 80 mg, respectively. A total of 1.7%, 3.1% and 4.5% reported headache for placebo, 40 mg and 80 mg, respectively. A total of 0.2%, 2.0% and 7.0% reported dizziness for placebo, 40 mg and 80 mg, respectively. A total of 0.6%, 1.5% and 7.0% reported tachycardia for placebo, 40 mg and 80 mg, respectively. A total of 0%, 0.7% and 3.5% reported nausea for placebo, 40 mg and 80 mg, respectively. A total of 0%, 0.2% and 2.0% reported hypotension for placebo, 40 mg and 80 mg, respectively. A total of 0.2%, 0% and 2.0% reported hypertension for placebo, 40 mg and 80 mg, respectively. In conclusion, oral administration of phentolamine in doses of 40 mg and 80 mg has been shown to be effective in enhancing erectile function in men with mild to moderate erectile dysfunction. Phentolamine has been found to signi cantly improve the various domains of the International Index of Erectile Function compared to placebo in terms of erectile function, orgasmic function, intercourse satisfaction and overall satisfaction. Three to four times as many patients receiving phentolamine reported being satis ed or very satis ed with study medication compared with those receiving placebo. Partner satisfaction correlated with patient satisfaction which was statistically signi cant. At doses of 40 mg and 80 mg respectively, 55% and 59% of men were able to achieve vaginal penetration with 51% and 53% achieving penetration on 75% of attempts. The correction of erectile dysfunction or improvement to a less severe category of dysfunction was experienced by 53% of men with the 80 mg dose and 40% with the 40 mg dose of phentolamine. All trends of response were the same regardless of any concomitant medication. Phentolamine is a safe and well-tolerated with an adverse event pro le consistent with its vasorelaxant properties. There were no severe adverse events including no major systemic cardiovascular hemodynamic changes over the therapeutic dose range. There is no evidence of dyspepsia and visual disturbances associated with sildena l. 7 At 40 mg, 7.7% experienced rhinitis and fewer than 3.1% experienced any other side effect of treatment. In the step-care management of men with erectile dysfunction, there will likely be several choices of pharmacological rst-line therapy for the management of erectile dysfunction (Figure 5). The mechanism of erectogenic drugs will vary. One class, already FDA-approved for the safe and effective treatment of erectile dysfunction, acts by inhibition of the phosphodiesterase type 5 enzyme. Phentolamine is an example of a drug class that facilitates erections by alpha-1 and alpha-2 adrenergic blockade. 1 Another class of agents activates central mechanisms involved in penile erection. 8 Another class will utilize prostaglandin E1 administered via topical administration. 9 It is likely that phentolamine mesylate will achieve widespread use as an oral alpha-1 and alpha-2 blocking agent for the treatment of erectile dysfunction. References 1 Lue TF, Tanagho EA. Physiology of erection and pharmacologic management of impotence. J Urol 1987; 137: 829± Brindley GS Cavernosal alpha-blockade: A new technique for investigating and treating erectile impotence. Br J Psychiatry 1983; 143: 332± Muller J. JAMA 1996; 275: 1405± Traish A, Gupta S, Gallant C, Huang YH, Goldstein, I. Phentolamine mesylate relaxes penile corpus cavernosum tissue by adrenergic and non-adrenergic mechanisms. Int J Imp Res 1998; 10: 215± Goldstein I, the Vasomax Study Group: Update of Safety and Ef cacy of (Vasomax) in Patients with Erectile dysfnction. Int J Imp Res 1999; 11 (Supple 1): S71. 6 Wyllie M G, Andersson K-E. Oraly Active Agents: The Potential of Alpha-adrenergic Antagonists. In: Carson C, Kirby R, Goldstein, I. eds, Textbook of Erectile Dysfunction. ISIS Medical Media: Oxford 1999; pp. 317± Goldstein I, Lue T, Padma-athan H, Rosen R, Steers W and Wicker P. Oral sildena l in the treatment of erectile dysfunction. ew Engl J Med 1998; 338: 1397± Lal S, et al. Apomorphine-induced penile tumescence in impotent patients- preliminary ndings. Prog europsychopharmacol Biol Psychiatr. 1987; 11: 235± Goldstein I, Payton T, Schechter, P.J. Double-blind, Placebocontrolled Study of Topiglan, a Topical Gel Formulation of 1% Alprostadil for the In-of ce Treatment of Erectile Dysfunction. Int J Imp Res 1999; 11 (Supple 1): S71.