International Journal of Impotence Research (1998) 10, 215±223 ß 1998 Stockton Press All rights reserved 0955-9930/98 $12.00 http://www.stockton-press.co.uk/ijir

Phentolamine mesylate relaxes penile corpus cavernosum tissue by and non-adrenergic mechanisms

A Traish, S Gupta, C Gallant, Y-H Huang and I Goldstein

Department of Urology, Boston University School of Medicine, Boston, MA 02118

Aim of the study: We investigated the biochemical and physiological mechanisms of action of phentolamine mesylate (VasomaxTM) in regulating erectile tissue smooth muscle contractility in human and rabbit corpus cavernosum. Methods: The binding activity of phentolamine was investigated in a cell-free system by displacement of speci®c and selective radiolabelled ligands to alpha 1 and 2 adrenergic receptors. The physiologic activity of phentolamine-mediated relaxation of adrenergic and non-adrenergic pre-contracted erectile tissue strips of human and rabbit corpus cavernosum were studied in organ bath chambers. Results: In corpus cavernosum membranes, phentolamine displaced binding of the selective alpha 1 receptor antagonists [125I]HEAT and [3H] and the alpha 2 receptor antagonists [3H] and [3H]RX 821002 with relatively high af®nity. Phentolamine caused concentration dependent relaxation in erectile tissue strips pre-contracted with adrenergic agonists , , and UK 14 304, as well as with non- adrenergic contractile agents endothelin and KCl. Biochemical and physiologic studies reveal that the concentration of phentolamine required to displace half maximal binding or to produce half- maximal relaxation was similar to that found in human plasma 30 min after ingestion of 40 mg of VasomaxTM. Reversible inhibition of nitric oxide synthase by L-nitroarginine or mechanical disruption of endothelium diminished non-adrenergic phentolamine-mediated erectile tissue relaxation. Conclusions: Phentolamine mesylate induced relaxation of corpus cavernosum erectile tissue by direct antagonism of alpha 1 and 2 adrenergic receptors and by indirect functional antagonism via a non-adrenergic, endothelium-mediated mechanism suggesting nitric oxide synthase activation.

Keywords: phentolamine mesylate; a-adrenergic receptors; ; corpus caverno- sum smooth muscle; penile erection

Introduction therapy as treatment for male erectile dysfunction, Zorgniotti,2 Gwinup3 and Wagner et al4 have utilized oral and buccal formulations of phentola- Phentolamine mesylate, an , mine mesylate in pilot studies. has been marketed as an injectable substance A new formulation of oral phentolamine mesylate (Regitine, Ciba-Geigy) for over 40 y in the United (VasomaxTM) is currently being investigated for its States. It is approved for the treatment of pheochro- safety and ef®cacy in the treatment of male erectile mocytoma-related hypertension, for the diagnosis of dysfunction.5±8 Pharmacokinetic studies in normal and for norepinephrine-related volunteers have revealed that the mean phentola- dermal necrosis. Since 1984, phentolamine mesylate mine plasma concentration, 30 min after oral inges- has also been successfully utilized, in combination tion of 40 mg VasomaxTM tablets, was 16 ng/ml (42± with other vasoactive agents, for the intracavernosal 50 nM).9 Recent double-blind, placebo controlled, pharmacological treatment of male erectile dysfunc- multi-institutional clinical trials with VasomaxTM 1 tion. Due to inherent problems with self- have documented signi®cant improvement in erec- tile function compared to placebo in patients with minimal erectile dysfunction.5±8 Correspondence: Dr I Goldstein, Department of Urology, The molecular mechanisms of action underlying Doctors Of®ce Building, 720 Harrison Ave, Suite 606, Boston, MA 02118, USA. phentolamine mesylate-induced improvement of Received 29 December 1997; accepted in revised form erectile function are thought to be exclusively 16 March 1998 related to local inhibition of post-synaptic alpha- Phentolamine mesylate in penile erection A Traish et al 216 adrenergic receptors, however, local actions of chemicals were reagent grade and obtained from phentolamine in vascular smooth muscle may also commercial sources. include non-adrenergic pathways.10 The aim of this study was to detail, for the ®rst time, the biochem- ical and physiological mechanisms of action of phentolamine mesylate-mediated relaxation of cor- Biochemical binding studies pus cavernosum by adrenergic and non-adrenergic mechanisms. Membrane preparation and phentolamine mesylate binding to alpha 1 and 2 adrenergic receptors in corpus cavernosum smooth muscle were carried out Materials and methods as described previously.12±14 The data represents the cumulative observations from two or more indepen- dent experiments. Tissue procurement

To examine the biochemical and physiological mechanisms of action of phentolamine mesylate- Physiological studies in organ bath chambers mediated relaxation, both human and rabbit corpus cavernosum erectile tissues were utilized, as indi- Human and rabbit corpus cavernosum strips were cated in the legend to the speci®c experiments. In mounted to force transducers (Grass Instruments general, initial experiments were performed with FT03, Quincy, MA) in 25 ml organ baths (37C) erectile tissue from the New Zealand White male containing Krebs solution and aerated with 5% CO2/ rabbit. The methodology of tissue harvest was 95% air to maintain a pH of 7.4. Optimal isometric approved by the Institutional Animal Care Commit- 11,12 tension for contraction was attained as described tee and has been previously reported. Data previously.11,13,14 The maximum contraction in- concerning phentolamine mesylate-mediated activ- duced by 80 mM KCl was determined for each strip ity in rabbit erectile tissue were then con®rmed before starting the experimental protocols. Dose using human erectile tissue. Human corpus caver- responses to various adrenergic agonists were nosum erectile tissues were procured during penile obtained by cumulative additions of drugs to the prosthesis implantation as approved by the Institu- chambers in half log increments starting at 1 nM and tional Review Board for Human Studies. In the continuing until the maximal contraction was operating room, erectile tissue biopsies were placed achieved with each treatment. For relaxation stu- in chilled physiological salt solution and immedi- dies, tissues were precontracted with a submaximal ately transported to the laboratory for experimenta- 11,12 concentration of phenylephrine (1 mM), KCl (20± tion as previously reported. 40 mM) or endothelin (30 nM). Once stable contrac- tion was reached, phentolamine dose responses were initiated by the addition of drug in half log Radioisotopes and chemicals increments (0.1 nM±0.1 mM). At the end of the experiment, 100 mM was added to deter- mine maximal relaxation. In experiments in which Zonagen Inc. (Houston, TX) provided phentolamine 3 the strips were pre-contracted with KCl, mesylate. [ H(N)]prazosin (34±45 Ci/mmol) and 2-{b- (10 mM), (10 mM) and (4-hydroxy-3-[125I]iodophenyl)-ethylaminomethyl}- 125 (1 mM) were added to inhibit KCl-induced norepi- tetralone ([ I]HEAT) (  2000 Ci/mmol) were ob- nephrine release.11,13 Relaxation data were normal- tained from New England Nuclear, Boston, MA. [3H] 3 ized as a percent of 80 mM KCl contraction for each RX821002 (54 Ci/mmol) and [ H] rauwolscine concentration. (74 Ci/mmol) were obtained from Amersham, (Ar- lington Heights, IL). Unlabeled prazosin and 5- methylurapidil (5-methyl-6[[3-[4-(2-methoxyphe- nyl)-1-piprazinyl]propyl]amino]-1,3-dimethyluracil) Statistical analysis were obtained from Research Biochemicals Inc., (Natick, MA). Phenylmethylsulfonyl ¯uoride, apro- tinin, L-nitroarginine and pepstatin A were obtained Relaxation of corpus cavernosum tissue to various from Sigma Chemical Company (St. Louis, MO). drugs is expressed as the percent change in tone. Endothelin-1 was obtained from Peninsula Lab. Inc., Maximum tone of the tissue is de®ned as that tone bretylium tosylate was obtained from American between the contraction to phenylephrine (1 mM) Critical Care, guanethidine ismelin monosulfate and the relaxation induced by papaverine (100 mM). was obtained from CIBA-Geigy Corning, RS 15385- Analysis of variance (ANOVA) was used to compare 197 was obtained from Roche Biosciences. All other the responses between control and phentolamine. Phentolamine mesylate in penile erection A Traish et al 217

Figure 1 Phentolamine mesylate displaces the alpha 1 antagonists [125I]HEAT and [3H]prazosin in rabbit corpus cavernosum. Membranes of corpus cavernosum smooth muscle were isolated and aliquots (200 ml, in triplicates) of the membranes were incubated at 25C for 1 h with 0.2 ml of buffer containing [3H] ligand (1 nM) in the absence (total binding) or presence of increasing concentrations (10711±1075 M) of unlabeled ligands. Binding was measured as described in the methods section. Speci®c binding was then determined by subtraction of nonspeci®c binding from total binding. The speci®c binding of each data point was plotted as percent of binding (compared to control) vs log molar concentrations of unlabeled ligands. (a) represents displacement of [125I]HEAT with phentolamine (j), 5 methylurapidil, (s) and prazosin (u). (b) represents displacement of [3H]prazosin with phentolamine (j), 5 methylurapidil, (s) and prazosin (u).

Results adrenergic , 5-methylurapidil. Prazosin was more effective in displacing bound [125I]HEAT and [3H]prazosin than either phentola- mine or 5-methylurapidil (Figures 1a and 1b). Phentolamine mesylate displaces the alpha 1 adrenergic receptor antagonists [125I]HEAT and [3H]prazosin in corpus cavernosum Alpha 2 selective agonists and antagonists do not displace the alpha 1 adrenergic receptor antagonists Phentolamine mesylate displaced the alpha 1 [125I]HEAT and [3H]prazosin in corpus cavernosum adrenergic receptor antagonists [125I]HEAT and [3H]prazosin from alpha 1 adrenergic receptors in corpus cavernosum membranes in a competitive The alpha 2 adrenergic receptor agonist, UK 14 304, manner. Phentolamine mesylate also exhibited a and the alpha 2 adrenergic receptor antagonists, RS similar af®nity to that of the alpha 1A selective 15385-197 and rauwolscine, did not displace

Figure 2 Alpha 2 selective agonists and antagonists do not displace the alpha 1 adrenergic receptor antagonists [125I]HEAT and [3H]prazosin in rabbit corpus cavernosum. Binding studies were performed as described in Figure 1. (a) represents displacement of [125I]HEAT with UK 14 304 (u), rauwolscine, (s) and RS 15385-197 (j). (b) represents displacement of [3H]prazosin with UK 14 304 (u), rauwolscine, (s) and RS 15385-197 (j). Phentolamine mesylate in penile erection A Traish et al 218

Figure 3 Phentolamine mesylate displaces the alpha 2 adrenergic receptor antagonist [3H]rauwolscine in rabbit corpus cavernosum. Binding studies were performed as described in Figure 1 (a) represents displacement of [3H]rauwolscine with phentolamine (j), 5 methylurapidil, (s) and prazosin (u). (b) represents displacement of [3H]rauwolscine with UK 14 304 (u), rauwolscine, (s) and RS 15385-197 (j).

[125I]HEAT and [3H]prazosin binding from alpha 1 adrenergic receptors (Figures 2a and 2b).

Phentolamine mesylate displaces the alpha 2 adrenergic receptor antagonist [3H]rauwolscine in corpus cavernosum

Phentolamine mesylate displaced the alpha 2 adrenergic receptor antagonist [3H]rauwolscine from alpha 2 adrenergic receptors in corpus cavernosum membranes in a competitive manner. Phentolamine mesylate competed for alpha 2 adrenergic binding in a dose dependent manner. The alpha 2-receptor antagonists, RS 15385-197 and rauwolscine, com- peted effectively for binding of rauwolscine to alpha 2 adrenergic receptors. Although prazosin and 5 methylurapidil were thought to be alpha 1 selective, Figure 4 Phentolamine and norepinephrine displace [3H]rau- we observed cross-reactivity with the alpha 2 wolscine and [3H]RX 821002 binding to human corpus caverno- indicating incomplete selectivity in corpus caverno- sum membranes. Binding studies were performed as described in sum smooth muscle (Figures 3a and 3b). Figure 1. Displacement of [3H]rauwolscine or [3H]RX 821002 with phentolamine (d, j, respectively) was compared to that of norepinephrine (r). Phenylephrine did not displace binding of [3H]rauwolscine or [3H]RX 821002 (m, .), respectively). Phentolamine and norepinephrine displace [3H]rauwolscine and [3H] RX 821002 binding to Effects of phentolamine on alpha 1 adrenergic corpus cavernosum membranes receptor mediated responses

In our experiments utilizing corpus cavernosum Phenylephrine elicited concentration-dependent smooth muscle cells in culture, phentolamine contractions in rabbit corpus cavernosum. Phentola- mesylate was effective in displacing the alpha 2 mine mesylate in concentrations ranging from 1 nM± adrenergic antagonists [3H]rauwolscine and [3H]RX 10 mM displaced the phenylephrine contractile re- 821002 in a dose dependent manner. Norepinephr- sponse to the right (Figure 5a and 5b). In human ine also displaced [3H]RX 821002 but was less erectile tissues, phentolamine caused dose-depen- effective than phentolamine. The selective alpha 1 dent relaxation in strips precontracted with 3 mM adrenergic receptor agonist, phenylephrine, did not phenylephrine; the half-maximal relaxation response 3 3 compete for either [ H]rauwolscine or [ H]RX (EC50) occurred at 10±30 nM phentolamine, and the 821002 (Figure 4). maximal relaxation response at 1 mM (Figure 6a). Phentolamine mesylate in penile erection A Traish et al 219

Figure 5 Effects of phentolamine on alpha 1 adrenergic receptor mediated responses. (a) Human corpus cavernosum strips or (b) rabbit corpus cavernosum strips, in organ chambers were treated with phenylephrine to determine maximal contractions. Strips were then washed and exposed to various concentrations of phentolamine for 30 min and then subjected to phenylephrine to determine maximal contractions. All data were normalized as described in Methods (for each dose response, n ˆ 3±6 experiments).

Figure 6 Phentolamine mediates relaxation of corpus cavernosum strips pre-contracted with alpha 1 receptor agonists. Human corpus cavernosum strips are shown in (a). Phentolamine induced-relaxation of corpus cavernosum tissue strips precontracted with 3 mM phenylephrine with (EC50) at 10±30 nM phentolamine. Rabbit corpus cavernosum strips are depicted in (b). Phentolamine induced- relaxation of corpus cavernosum tissue strips precontracted with 1 mM phenylephrine (u), 5 mM norepinephrine (j) and 1 mM oxymetazoline (m). For each dose response, n ˆ 4±8 experiments.

Phentolamine caused dose-dependent relaxation in the half-maximal relaxation response (EC50) oc- strips pre-contracted with 1 mM phenylephrine, 5 mM curred at 30±50 nM phentolamine, and the maximal norepinephrine and 1 uM oxymetazoline (Figure 6b). relaxation response occurred at 1 mM (Figure 6a).

Effects of phentolamine on alpha 2 adrenergic Effects of phentolamine on KCl and endothelin-1 receptor mediated responses precontracted corpus cavernosum

The alpha 2 adrenergic receptor agonist, UK 14 304, In rabbit corpus cavernosum strips precontracted caused concentration-dependent contraction in rab- with 80 mM KCl or 30 nM endothelin-1, phentola- bit and human corpus cavernosum. Phentolamine mine mesylate caused concentration-dependent mesylate in concentrations ranging from 10 nM± relaxation (Figures 9 and 10). Since the mechanisms 1 mM displaced the UK 14 304 contractile response of contraction induced by KCl or endothelin-1 are to the right (Figure 7a and 7b). Phentolamine caused not mediated by alpha-adrenergic receptors, an dose-dependent relaxation in strips precontracted additional non-adrenergic mechanism for phentola- with 3 or 10 mM UK 14 304 (Figure 8). In human mine-induced corpus cavernosum relaxation was erectile tissues precontracted with 3 mM UK 14 304, suspected. Phentolamine mesylate in penile erection A Traish et al 220

Figure 7 Effects of phentolamine on alpha 2 adrenergic receptor mediated responses. (a) Human corpus cavernosum strips or (b) rabbit corpus cavernosum strips, in organ chambers were treated with phenylephrine to determine maximal contractions. Strips were then washed and exposed to various concentrations of phentolamine for 30 min and then subjected to UK 14 304 to determine maximal contractions. All data were normalized as described in Methods. For each dose response, n ˆ 4±8 experiments.

Figure 8 Phentolamine mediates relaxation of rabbit corpus Figure 9 Effects of phentolamine on KCl precontracted rabbit cavernosum strips pre-contracted with alpha 2 receptor agonist. corpus cavernosum. Corpus cavernosum strips in organ chambers Phentolamine induced relaxation of rabbit corpus cavernosum were precontracted with 80 mM KCl with or without LNNA tissue strips precontracted with 3 (j) or 10 mM (u) UK 14 304. For (30 mM) and the relaxation response to phentolamine was each dose response, n ˆ 3±5 experiments. determined. For each dose response, n ˆ 3±10 experiments.

Non-adrenergic mechanisms of phentolamine-in- LNNA, the phentolamine mesylate-induced relaxa- duced relaxation tion response was shifted to the right (Figure 9). Similarly, the relaxation response of corpus caver- nosum strips precontracted with 30 nM endothelin- To examine if phentolamine mesylate was acting as 1 was reduced in the presence of LNNA (30 mM) an L-type , human corpus (Figure 10). In addition, phentolamine induced cavernosum membranes were incubated with relaxation was reduced in corpus cavernosum strips [3H]PN 200-110, a nitripdine analogue in the in which the endothelium was removed and were presence or absence of phentolamine or nitrip- pre-contracted with 1 mM phenylephrine (data not dine. Phentolamine did not displace binding of shown). [3H]PN 200-110 from corpus cavernosum mem- branes whereas nitripidine competed effectively (data not shown). Discussion To examine if phentolamine mesylate mediated its non-adrenergic relaxation activity via a nitric oxide pathway, rabbit corpus cavernosum strips Phentolamine mesylate is under clinical trials were precontracted with 80 mM KCl with or without testing for potential use as an oral agent in the the reversible nitric oxide synthase inhibitor, L- treatment of erectile dysfunction.5±8 The mechanism nitroarginine (LNNA, 30 mM). In the presence of of action of phentolamine in regulating corpus Phentolamine mesylate in penile erection A Traish et al 221 selective alpha 2 receptor agonist UK 14 304 and the non-selective agonist norepinephrine. These data indicate that phentolamine alpha-adrenergic binding activity mediates the physiologic inhibition of adrenergic-induced corpus cavernosum contrac- tile tone and the resulting relaxation response. Contractile responses mediated by 80 mM 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.15 Endothelin-induced contractions are thought to occur via receptor mediated intracellular calcium release.12 Both of these contractile responses are mediated by non- adrenergic mechanisms. Phentolamine mesylate- Figure 10 Effects of phentolamine on endothelin-1 precon- tracted rabbit corpus cavernosum. The relaxation response of induced relaxation of corpus cavernosum strips corpus cavernosum strips precontracted with 30 nM endothelin-1 pre-contracted with 80 mM KCl or endothelin sug- was determined in the presence or absence of LNNA (30 mM). For gest that phentolamine mediates corpus cavernosum each dose response, n ˆ 3±5 experiments. tissue contractility by both adrenergic and non- adrenergic pathways. One likely possibility to explain phentolamine cavernosum smooth muscle tone has been postu- mesylate-induced relaxation of KCl contracted tis- lated via alpha-adrenergic receptor blockade.10 sue was inhibition of calcium entry via L-type Detailed biochemical and physiological studies of calcium channels by phentolamine. While nitrip- the mechanism of action of phentolamine mesylate dine effectively competed for binding of PN 200-110 in human corpus cavernosum tissue have not been in human corpus cavernosum membranes, phento- previously reported. lamine did not, suggesting that phentolamine In this study, we demonstrated that phentolamine mesylate was not acting as L-type calcium channel mesylate was an effective alpha 1 and alpha 2 blocker. adrenergic receptor blocker in both rabbit and Another possibility was that phentolamine acted human corpus cavernosum erectile tissue. Phento- as an indirect antagonist, in that, phentolamine lamine competitively displaced speci®c alpha 1 demonstrated activity at a second receptor site, receptor ligands (HEAT and prazosin) and speci®c eliciting responses which antagonized the electro- alpha 2 receptor ligands (rauwolscine and RX mechanical (KCl) or pharmaco-mechanical coupling 821002). The af®nity of phentolamine for alpha 1 (endothelin). Indirect antagonism is a recognized adrenergic receptors was comparable to the alpha complementary pharmacological mechansim of ac- 1A selective receptor antagonist 5-methylurapidil tion of many natural and synthetic agonists and but was less than that of prazosin. The af®nity of antagonists. In this study, we found that phentola- phentolamine for the alpha 2 adrenergic receptors mine mesylate caused concentration dependent was greater than that of the alpha 2 receptor agonist relaxation of erectile tissue strips pre-contracted UK 13 304 and norepinephrine but was less than with KCl or endothelin. This relaxation response that of the alpha 2 receptor antagonists delequamine was attenuated by the competitive nitric oxide (RS 1385-197) and rauwolscine. The relatively high synthase inhibitor, L-nitroarginine, and by the af®nity of binding of phentolamine mesylate to mechanical disruption of the endothelium. Such alpha 1 and 2 adrenergic receptors suggests that, data suggest that phentolamine mesylate-mediated 30 min following oral ingestion of 40 mg of Vaso- relaxation in corpus cavernosum erectile tissue maxTM, the mean plasma phentolamine concentra- occurred via an additional non-adrenergic, endothe- tions (42±50 nM) will suf®ciently occupy the alpha lial-based, nitric-oxide mechanism. adrenergic receptors in corpus cavernosum erectile Phentolamine mesylate is an adrenergic antago- tissue resulting in inhibition of adrenergic-mediated nist to the contractile substance norepinephrine. physiologic activity. Phentolamine was found to also possess the agonist In rabbit and human corpus cavernosum erectile function of tissue relaxation via endothelial nitric tissue, phentolamine mesylate shifted the dose oxide pathways. In erectile tissue, phentolamine response of phenylephrine (selective alpha 1- thus was found to elicit activity via a dual mechan- mediated) and UK-14 304 (selective alpha 2- ism of action. One pathway was by direct antagon- mediated) contraction in a concentration-dependent ism of alpha 1 and alpha 2 receptor agonists, at the manner. Phentolamine mesylate also induced con- receptor level, resulting in reduced intracellular centration dependent relaxation in erectile tissue calcium release and inhibition of contractility. The pre-contracted with the selective alpha 1 receptor other mechanism was probably by an indirect, agonists phenylephrine and oxymetazoline, the functional antagonism, in which phentolamine Phentolamine mesylate in penile erection A Traish et al 222

Figure 11 Hypothetical mechanism of action of phentolamine-induced relaxation of corpus cavernosum smooth muscle. Phentolamine mesylate inhibits alpha 1 adrenerigc receptor activity and diminishes the increase in intracellular calcium via reduced levels of inositol trisphosphate. Phentolamine mesylate inhibits alpha 2 adrenergic receptor activity, removing the inhibitory activity of Gi protein allowing for increase in cyclic AMP and decreased intracellular calcium. Phentolamine mesylate also appears to activate nitric oxide synthase and increase cyclic GMP allowing for an increase in cyclic GMP and decreased intracellular calcium.

bound to an as yet unidenti®ed receptor on the cyclic nucleotide levels by enhancing nitric oxide endothelium and activated nitric oxide synthase. synthase, may be theoretically utilized in conjunc- This additional pathway may be responsible for tion with selective phosphodiesterases to improve corporal smooth muscle relaxation (Figure 11). therapeutic responses in selected patients. Identi®cation of a dual mechanism of phentola- mine-induced corpus cavernosum relaxation im- plies that novel combinations of pharmacologic Conclusions agents with alternative mechanisms of corporal smooth muscle relaxation may be employed in patients with erectile dysfunction unable to restore In summary, a new formulation of phentolamine functional erections with single oral pharmacologi- mesylate is being developed as an oral agent in the cal agents. For example, selective phosphodiester- treatment of male erectile dysfunction. Phentola- ase inhibitors are also being developed for the mine mesylate has been found to modulate human treatment of male erectile dysfunction.16 Such and rabbit corpus cavernosum smooth muscle agents alter intracellular cyclic nucleotide levels contractility by direct alpha 1 and alpha 2 adrener- by diminishing cyclic nucleotide metabolism.16 gic receptor antagonism. The concentration of Phentolamine, which has been found in erectile phentolamine required to displacing half maximal tissue to inhibit adrenergic receptors and increase binding or to produce half-maximal relaxation was Phentolamine mesylate in penile erection A Traish et al 223 similar to that found in plasma 30 min after 5 Porst H et al. Oral phentolamine (Vasomax) in erectile ingestion of 40 mg of VasomaxTM. Phentolamine dysfunctionÐResults of German multicenter study in 177 mesylate also has been found to have indirect patients. Int J Impot Res 1996; 8: 117. 6 Becker AJ et al. Double blind study on oral phentolamine as functional antagonism via an endothelium-mediated treatment for erectile dysfunction. J Urol 1997; 157: 202. nitric oxide pathway (Figure 11). This latter role of 7 Goldstein I and the Vasomax Study Group. Ef®cacy and safety phentolamine suggests that combining oral phento- of oral phentolamine (Vasomax) for the treatment of minimal lamine with an oral cyclic GMP-speci®c phospho- erectile dysfunction. J Urol 1998; 159: 240. diesterase inhibitor will increase cyclic GMP more 8 Becker AJ et al. Oral phentolamine as treatment for erectile dysfunction. J Urol 1998; 159: 1214±1216. than either agent alone. 9 Harris Laboratories Ltd. Study No. 16800. A Single-Center, Seven-Period, Rising-Dose, Tolerance Study of Phentolamine Mesylate Suppositories. November 2, 1994. Acknowledgements 10 Hoffman BB. Adrenoceptor blocking drugs. In: Katzung BG, ed. Basic and Clinical Pharmacology. Appleton and Lang: Norwalk, CT, 1992, pp 132±146. 11 Saenz de Tejada I et al. Regulation of adrenergic activity in This study was supported by NIH grants DK 40025 penile corpus cavernosum. J Urol 1989; 142: 1117±1121. and DK 39080, and by a grant from Zonagen. We 12 Saenz de Tejada I, Carson MP, Goldstein I, Traish AM. acknowledge the assistance of Dr S Gupta in the Endothelin synthesis, activity and receptor types in the human planning of experiments pertaining to organ cham- penis. Am J Physiol 1991; 261: H1078±H1085. ber studies. 13 Traish AM et al. A heterogenous population of alpha-1 adrenergic receptors mediates contraction of human corpus cavernosum smooth muscle to norepinephrine. J Urol 1994; 153: 222±227. References 14 Traish AM, Palmer MS, Goldstein I, Moreland RB. Expression of functional muscarinic acetylcholine receptor subtypes in human corpus cavernosum and in cultured smooth muscle 1 Krane RJ, Goldstein I, Saenz de Tejada I. Medical progress: cells in culture. Receptors 1995; 5: 159±167. Impotence. N Engl J Med 1989; 321: 1648±1659. 15 Rembold CM. Electromechanical and pharmacomechanical 2 Zorgniotti A. Pre-IND Meeting Documents of Gamogen, Inc., coupling. In: Barany M, ed. Biochemistry of Smooth Reference to Bruce F. Mackler letter of March 8, 1994. Muscle Contraction. Academic Press: San Diego, 1996, pp 3 Gwinup G. Oral phentolamine in nonspeci®c erectile insuf®- 227±239. ciency. Ann Int Med 1988; 109: 162±163. 16 Moreland RB, Goldstein I, Traish A. Sildena®l, a novel 4 Wagner G, Lacy S, Lewis R, Zorgniotti A. Buccal phentola- inhibitor of phosphodiesterase type 5 in human corpus mineÐA pilot trial for male erectile dysfunction at three cavernosum smooth muscle cells. Life Sci: Pharmacol Lett separate clinics. Int J Impot Res 1994; 6 Suppl 1: D78. 1998; 62: 309±318.