DIRECT EFFECTS OF SELECTIVE TYPE 5 PHOSPHODIESTERASE INHIBITORS ALONE OR WITH OTHER VASODILATORS ON THE ERECTILE RESPONSE IN CATS

Use of sildenafil citrate in a cat with Eisenmenger’s syndrome and an atrial septal defect

A 2-year-old male neutered Siamese cat presenting with weakness and dyspnoea was diagnosed with an atrial septal defect and pulmonary hypertension, which resulted in right-to-left shunting (Eisenmenger’s syndrome). The cat was treated with sildenafil (0.25–0.6 mg/kg) for 10 months. There were no apparent treatment-related adverse effects. Improvement in clinical signs was noted, although increasing doses of sildenafil were required. After 10 months the cat significantly deteriorated and was euthanased.

Mechanisms of direct relaxant effect of sildenafil, tadalafil and vardenafil on corpus cavernosum

Sildenafil, tadalafil, vardenafil and verapamil induced concentration-dependent relaxation of the rabbit corpus cavernosum muscle precontracted with noradrenaline. The maximal relaxation (%) at 20 microM was 61.4 +/- 6.9, 32.4 +/- 5.4, 100.0 +/- 5.5 and 86.6 +/- 5.1 (n = 5 each) respectively. Pre-incubation of cavernosal muscle strips with N(omega)-nitro-L-arginine or guanylate cyclase inhibitor, 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ) but not adenylate cyclase inhibitor, cis-N-[2-phenylcyclopentyl]-azacyclotridec-1-en-2-amine] (MDL12330A) culminated in only a 20-30% reduction in muscle relaxant action of the 3 phosphodiesterase inhibitors. This suggests that another mechanism of relaxation independent of nitric oxide-cGMP or cAMP pathway was involved. Higher concentrations of sildenafil (100 microM) and vardenafil (10 and 100 microM) produced non-competitive antagonism of noradrenaline-induced contraction characterized by reduced maximal effect. In contrast, tadalafil was devoid of significant effect on noradrenaline. On K(+)-depolarized tissues, sildenafil was as potent as vardenafil whereas tadalafil was the least effective in relaxing K(+)-induced tone. The maximal relaxation (% of K(+)-induced tone) at 20 microM sildenafil, tadalafil and vardenafil was respectively 84.1 +/- 6.5, 9.0 +/- 19.9, and 88.9 +/- 6.2 (n = 5 each). In addition, verapamil, sildenafil and vardenafil were more efficacious than tadalafil in reversing tonic contractions by Ca(2+) channel activator, 1,4,dihydro-2,6-dimethyl-5-nitro-4-[2(triflouromethyl)phenyl]pyridine-3-carboxylic acid methyl ester (BAY K-8644). These results indicate that vardenafil and sildenafil possess direct muscle relaxant potential possibly via inhibiting Ca(2+) influx through both receptor-operated and voltage-dependent Ca(2+) channels whereas tadalafil appears capable of inhibiting receptor-operated transmembrane Ca(2+) entry only.

Future options for combination therapy in the management of erectile dysfunction in older men

The prevalence of erectile dysfunction (ED) has dramatically increased in parallel with the aging of the Western industrialised population. The estimated prevalence of ED worldwide in 1995 was 152 million men. As the population in industrial nations ages, an estimated 322 million men will be affected by ED by the year 2025. Oral drug therapy with the phosphodiesterase (PDE) type 5 inhibitor sildenafil fails in some patients with ED; however, several different classes of drugs demonstrate efficacy in treating ED, creating the potential for pharmacological combination therapy. Pharmaceutical products that lead to the activation of or an increase in cyclic nucleotides (cyclic adenosine monophosphate and cyclic guanosine monophosphate), with or without nitric oxide donors or nitrates, as well as alpha-adrenoceptor antagonists, have been used to treat ED. Sildenafil has been used in combination with alprostadil (prostaglandin E1) and administered via intraurethral or intracavernous route. Successful intercourse using this combination of agents varies from 47% to 100% following failed monotherapy. Various combination therapies for ED are being studied using PDE5 inhibitors, together with other agents, alpha-adrenoceptor antagonists, and testosterone replacement therapy for men with hypogonadism. The combination of centrally acting agents with PDE5 inhibitors, e.g. a regimen of apomorphine plus PDE5 inhibitor, is an attractive approach because the two therapies target different mechanisms. New PDE5 inhibitors such as vardenafil should be tried first as therapy for sildenafil nonresponders before exploring any combination therapy options. Preliminary observations of combination therapy have been encouraging and provide a scientific rationale for prospective, randomised clinical trials with adequate numbers of patients.

Viability and safety of combination drug therapies for erectile dysfunction

PURPOSE

In some patients with erectile dysfunction (ED) oral, topical or intracavernous drug therapy fails. However, several classes of drugs demonstrate efficacy for ED, creating the potential for pharmacological combinations preferable to implantation of a penile prosthesis.

MATERIALS AND METHODS

Preliminary reports suggest that combining oral, topical or intracavernous drugs may salvage patients in whom monotherapy fails.

RESULTS

Agents that lead to activation or increases in cyclic nucleotides (cyclic adenosine monophosphate and guanosine monophosphate) with or without nitric oxide donors or nitrates, or alpha-adrenergic antagonists have been used to treat ED. The phosphodiesterase-5 inhibitor sildenafil has been combined with alprostadil (prostaglandin E1) and administered by either the intraurethral or intracavernous route. Successful intercourse following this combination varies from 47% to 100% when monotherapy with each has failed. The introduction of apomorphine has led to its unapproved use in combination with sildenafil in Europe. Combination strategies may allow lower drug doses and reduced adverse effects.

CONCLUSIONS

The encouraging preliminary observations combined with the potential for adverse events provide a scientific rationale for prospective, randomized clinical trials with adequate numbers of subjects.

Combination therapy for erectile dysfunction: where we are and what's in the future

Penile erection occurs in response to visual, olfactory, imaginative, and tactile stimuli initiated within the brain and/or on the periphery. Responses to these stimuli are mediated by efferent autonomic outflow originating in the sacral spinal cord and transmitted by the cavernosal and penile nerves. A number of neurotransmitters can play an integral role in corpus cavernosum smooth muscle relaxation, in part regulating penile erection through increased smooth muscle synthesis of the secondary messengers cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP). In addition to direct-acting agents, there are indirect-acting smooth muscle-relaxing agents. Phosphodiesterase (PDE) inhibitors such as sildenafil act indirectly and require sexual stimulation and endogenous nitric oxide production to activate the cGMP pathway effectively. In contrast, agents such as prostaglandin E(1) (PGE(1)) act directly on the trabecular smooth muscle, binding to specific e-prostanoid receptors and increasing cAMP synthesis. For this reason the direct-acting agents do not require sexual stimulation for efficacy. Combination pharmacotherapy has been used experimentally to treat erectile dysfunction for 25 years, using combinations of cAMP synthesis augmentors, smooth muscle relaxants and PDE inhibitors, and alpha-blockers administered via intracavernosal injection. The present era of oral pharmacotherapy treatment has resulted in significant awareness in the field of sexual dysfunction; however, a single agent may not be ideal to sustain penile rigidity, especially if comorbidities and severity of erectile dysfunction are accounted for. The rationale for and recent reports on combination therapy are presented in this review.

Different distribution of nitric oxide synthase and soluble guanylyl cyclase activities in the detrusor and proximal urethra of the rabbit

PURPOSE

We investigated the different distributions of nitric oxide synthase (NOS) and soluble guanylate cyclase activities in the detrusor and proximal urethra in the rabbit.

MATERIALS AND METHODS

The detrusor and proximal urethra were excised, the mucosa and muscularis were separated and each layer was homogenized to measure the activities of NOS as well as baseline and sodium nitroprusside (SNP) activated soluble guanylate cyclase. Isometric tension experiments were performed in detrusor and urethral strips with and without mucosa.

RESULTS

In the detrusor NOS and sodium nitroprusside activated soluble guanylate cyclase activities in the mucosa were much higher than in the muscularis. While in the proximal urethra NOS activity in the 2 layers was similar, SNP activated soluble guanylate cyclase activity in the muscularis was 2.8 times higher than in the mucosa. Removing the mucosa reduced nitric oxide (NO) mediated neurogenic relaxation in the proximal urethra, while no relaxation responses could be elicited by electrical field stimulation with or without mucosa in the detrusor. On the other hand, removing the mucosa did not affect SNP or 8-bromo-cyclic guanosine monophosphate induced relaxation in the detrusor and proximal urethra.

CONCLUSIONS

In the detrusor NOS and soluble guanylate cyclase activities were mainly detected in the mucosa, while in the urethra these activities were distributed throughout the mucosa and muscularis. Also, the mucosa in the urethra but not in the detrusor contributed to electrical field stimulation induced, NO mediated relaxation. These findings confirm previous studies demonstrating that NO has a different role in the detrusor than in the proximal urethra.

Synergistic effects of sildenafil on relaxation of rabbit and rat cavernosal smooth muscles when combined with various vasoactive agents

OBJECTIVE

To evaluate which vasoactive agents have synergistic effects on the cavernosal smooth muscles of rabbits and rats when the agents are combined with sildenafil.

MATERIALS AND METHODS

Relaxation responses of cavernosal smooth muscle to single agents (phentolamine, moxisylyte, sodium nitroprusside, forskolin, vasoactive intestinal peptide, VIP, papaverine and sildenafil) in the rabbit, and prostaglandin-E1 and sildenafil in the rat, and to combinations of each agent plus sildenafil, were assessed in vitro. The response to sildenafil of the rabbit strips with and without incubation with l-arginine (1 mmol/L) for 20 min was also evaluated. The effective concentrations for a half-maximal response of single agents and combination solutions were compared.

RESULTS

All single agents induced concentration-dependent relaxation of the rabbit and rat cavernosal smooth muscles. There was significant synergism on rabbit cavernosal smooth muscle when the sildenafil was combined with forskolin, sodium nitroprusside, VIP or phentolamine. There was also significant synergism with sildenafil plus prostaglandin-E1 in rat cavernosal muscles. There were no synergistic effects of combinations of sildenafil plus moxisylyte, papaverine or l-arginine.

CONCLUSIONS

These results suggest potentially effective combined therapies of sildenafil and intraurethral or intracavernosal prostaglandin-E1, intracavernosal forskolin or VIP, or oral phentolamine for patients with erectile dysfunction who have no success after monotherapy with these agents.