Intracavernosal injection of vasoactive intestinal polypeptide (VIP) does not induce erection in man per se

A comprehensive history of injection therapy for erectile dysfunction, 1982-2023

INTRODUCTION

Although oral phosphodiesterase 5 inhibitors represent a first choice and long-term option for about half of all patients with erectile dysfunction (ED), self-injection therapy with vasoactive drugs remains a viable alternative for all those who are not reacting or cannot tolerate oral drug therapy. This current injection therapy has an interesting history beginning in 1982.

OBJECTIVES

To provide a comprehensive history of self-injection therapy from the very beginnings in 1982 by contemporary witnesses and some members of the International Society for Sexual Medicine's History Committee, a complete history of injection therapy is prepared from eyewitness accounts and review of the published literature on the subject, as well as an update of the current status of self-injection therapy.

METHODS

Published data on injection therapy, as a diagnostic and therapeutic tool for ED, were reviewed thoroughly by PubMed and Medline research from 1982 until June 2023. Early pioneers and witnesses added firsthand details to this historical review. Therapeutic reports of injection therapy were reviewed, and results of side effects and complications were thoroughly reviewed.

RESULTS

The pioneers of the first hours were Ronal Virag (1982) for papaverine, Giles Brindley (1983) for cavernosal alpha-blockade (phentolamine and phenoxybenzamine), Adrian Zorgniotti (1985) for papaverine/phentolamine, and Ganesan Adaikan and N. Ishii (1986) for prostaglandin E1. Moxisylyte (thymoxamine) was originally marketed but later withdrawn. The most common side effect is priapism, with the greatest risk of this from papaverine, which has modified its use for therapy. Currently, prostaglandin E1 and trimixes continue to be the agents of choice for diagnostic and therapeutic use in ED. A recent agent is a mixture of a vasoactive intestinal polypeptide (aviptadil) and phentolamine.

CONCLUSIONS

After 40 years, self-injection therapy represents the medication with the highest efficacy and reliability rates and remains a viable option for many couples with ED. The history of this therapy is rich.

Pharmacology and perspectives in erectile dysfunction in man

Penile erection is a perfect example of microcirculation modulated by psychological factors and hormonal status. It is the result of a complex neurovascular process that involves the integrative synchronized action of vascular endothelium; smooth muscle; and psychological, neuronal, and hormonal systems. Therefore, the fine coordination of these events is essential to maintain penile flaccidity or allow erection; an alteration of these events leads to erectile dysfunction (ED). ED is defined as the consistent or recurrent inability of a man to attain and/or maintain a penile erection sufficient for sexual activity. A great boost to this research field was given by commercialization of phosphodiesterase-5 (PDE5) inhibitors. Indeed, following the discovery of sildenafil, research on the mechanisms underlying penile erection has had an enormous boost, and many preclinical and clinical papers have been published in the last 10 years. This review is structured to provide an overview of the mediators and peripheral mechanism(s) involved in penile function in men, the drugs used in therapy, and the future prospective in the management of ED. Indeed, 30% of patients affected by ED are classified as "nonresponders," and there is still an unmet need for therapeutic alternatives. A flowchart suggesting the guidelines for ED evaluation and the ED pharmacological treatment is also provided.

Vasoactive intestinal polypeptide/phentolamine for intracavernosal injection in erectile dysfunction

Erectile dysfunction (ED) is becoming an increasingly common problem and although oral therapies offer first-line treatment for many men, they are contraindicated or ineffective in substantial groups of patients. Intracavernosal injection (ICI) therapy is the most effective nonsurgical treatment for ED and offers an effective alternative to oral therapy. Sufficient arterial blood supply and a functional veno-occlusive mechanism are prerequisites in the attainment and maintenance of a functional erection. Invicorp (Plethora Solutions, London, UK) is a combination of vasoactive intestinal polypeptide (VIP) 25 microg and phentolamine mesylate 1 or 2 mg for ICI in the management of moderate to severe ED. The two active components have complementary modes of action; VIP has a potent effect on the veno-occlusive mechanism, but little effect on arterial inflow, whereas phentolamine increases arterial blood flow with no effect on the veno-occlusive mechanism. Clinical studies showed that Invicorp is effective in >or=80% of men with ED, including those who have failed to respond to other therapies and, unlike existing intracavernosal therapies, is associated with a very low incidence of penile pain and virtually negligible risk of priapism. We estimate that there are >5.9 million men in the USA alone for whom oral ED drugs are not a viable treatment option, and for whom Invicorp might offer a safe and effective alternative.

Recent advances in the treatment of erectile dysfunction in patients with diabetes mellitus

OBJECTIVE

To present current information on the pathogenesis of and available therapeutic options for erectile dysfunction (ED) in patients with diabetes.

METHODS

We provide a detailed review of the following topics: (1) peripheral and central neurotransmitter pathways involved in the penile erectile process (for example, nitric oxide, acetylcholine, vasoactive intestinal polypeptide, and prostaglandin E(1)), (2) pathogenesis of ED in patients with diabetes (vascular insufficiency, endothelial dysfunction, and autonomic neuropathy), (3) currently available treatment options for ED and their advantages and disadvantages, (4) potential new avenues for future research, and (5) the possibility of preventive treatment.

RESULTS

Clearly a need exists for effective treatment options for ED in patients with diabetes. Because the development of ED in patients with diabetes is often caused by several interrelated mechanisms, including vascular disease, endothelial dysfunction, autonomic neuropathy, hormone imbalance, and certain medications, a thorough understanding of the various pathways involved in penile erection and their modulation in diabetes is essential for physicians to design an effective treatment plan. Interventions that modulate the erectile pathway at different points include therapies that enhance the erectile mechanism (amplification of the nitric oxide pathway), inhibit the detumescence mechanism, or affect the final common pathway by augmenting smooth muscle relaxation. Oral therapy, intracavernosal injections, transurethral pellets, combination therapy, and surgical procedures are available treatment strategies.

CONCLUSION

Despite the availability of many treatment options for ED, early intervention and prevention (by such measures as improved glycemic control and general reduction of associated risk factors) should be emphasized because many of the diabetes-related complications leading to ED are irreversible.

The molecular basis of penile erection

Over the past two decades our understanding of the physiology and the various mediators involved in the pathway of penile erection has greatly increased. This has enabled us to develop effective pharmacological treatments for ED.

Treating men with predominantly nonpsychogenic erectile dysfunction with intracavernosal vasoactive intestinal polypeptide and phentolamine mesylate in a novel auto-injector system: a multicentre double-blind placebo-controlled study

OBJECTIVE

To study the effect of intracorporeal injection (IC) of vasoactive intestinal polypeptide (VIP) and phentolamine mesylate (PM) on men with erectile dysfunction (ED) of nonpsychogenic aetiology.

PATIENTS AND METHODS

The study comprised 236 men with primarily nonpsychogenic ED attending sexual dysfunction clinics at eight institutions. In an initial dose-assessment phase, the men were given IC injections of 25 micrograms VIP combined with PM 1.0 mg (VIP/P-1) or 2.0 mg (VIP/P-2) in a prefilled, single-use auto-injector. The main aetiologies of ED were arteriogenic (38), diabetes mellitus (DM) (39), neurogenic (35), mixed (90), and venous leakage (30). In a placebo-controlled phase, 171 patients were subsequently treated and self-administered up to 12 injections over a 6-month interval.

RESULTS

In the dose-assessment phase there was an overall response rate of 82%, with responses by aetiology as follows: arteriogenic (82%), DM (85%), neurogenic (86%), mixed (80%), and venous leakage (77%). In a subgroup of 159 patients who withdrew from previous IC therapies for ED, 64% responded with an erection suitable for intercourse. Of the 171 patients treated in the placebo-controlled phase, 75% responded to VIP/P-1 and 12% to placebo (P<0.001); 66% responded to VIP/P-2 and 18% to placebo (P<0. 001), with a median duration of erection of 56 min. The principal adverse event was transient facial flushing accompanying 40% of 1711 injections. There was no pain after injection and one episode of priapism (0.06%); only seven patients withdrew because of adverse events. Over 88% and 92% of patients were satisfied with the drug and auto-injector, respectively. More than 85% of patients and 77% of partners reported an improved quality of life.

CONCLUSION

The combination of VIP and PM at the dose used is a safe and effective means of treating male ED of primarily nonpsychogenic aetiology.

Nitrergic relaxation of the horse corpus cavernosum. Role of cGMP

The involvement of nitric oxide (NO) and the mechanisms mediating neurogenic relaxation were investigated in the horse corpus cavernosum. NADPH-diaphorase activity was expressed in nerve fibres around arteries and muscular bundles in the horse trabecular tissue. Relaxations in response to electrical field stimulation were tetrodotoxin (10(-6) M)-sensitive, indicating their neurogenic origin. The NO synthase inhibitor, L-NO-arginine (L-NO-Arg, 3 x 10(-5) M), abolished the electrically induced relaxations, which were significantly reversed by L-arginine (3 x 10(-3) M). Exogenous NO (10(-6)-10(-3) M) evoked relaxations which were unaffected by L-NO-Arg. 1H-[1,2,4]oxadiazolo[4,3,-a]quinoxalin-1-one (ODQ, 5 x 10(-6) M), an inhibitor of guanylate cyclase activation by NO, reduced the relaxations in response to electrical stimulation and exogenous NO. Iberiotoxin (3 x 10(-8) M) or apamin (5 x 10(-7) M), inhibitors of large and small conductance Ca2+-activated K+ channels, respectively, and glibenclamide (3 x 10(-6) M), a blocker of ATP-sensitive K+ channels, failed to modify the relaxations with NO. It is suggested that NO is present in nerve fibres of the horse corpus cavernosum and relaxes smooth muscle through a guanylate cyclase-dependent mechanism. Neither Ca2+-activated nor ATP-sensitive K+ channels seem to be involved in these relaxations.

Medical treatment of erectile dysfunction

There has been a tremendous increase in demand for the treatment of erectile dysfunction in the last 10 years. This has occurred partly because of a greater understanding and awareness by both the general public and clinicians, and also because there now exists a range of effective treatments. The choice of treatments is increasing rapidly and novel delivery systems which may be more patient-friendly than intracavernosal injections are now becoming available. We review the published data on effectiveness and safety of the currently available treatments and discuss recent advances in oral therapy, as these drugs are likely to become available in the near future.

Pituitary adenylate cyclase-activating polypeptide, helospectin, and vasoactive intestinal polypeptide in human corpus cavernosum

1. The distribution and effects of pituitary adenylate cyclase-activating polypeptide (PACAP-27 and -38), helospectin (Hel-1 and Hel-2), and vasoactive intestinal polypeptide (VIP), were investigated in isolated preparations of human corpus cavernosum (CC). 2. Immunohistochemistry revealed coinciding profiles of nerve structures that showed immunoreactivities for VIP and PACAP, and VIP and Hel. Confocal microscopy showed the co-existence of VIP- and PACAP-immunoreactivities, and VIP- and Hel-immunoreactivities in most (90%) varicose nerve structures. 3. As determined by radioimmunoassay, the amounts of VIP, PACAP-27, and PACAP-38 in the preparations were 61.7 +/- 11.6, 0.1 +/- 0.05, and 3.7 +/- 0.5 pmol g-1 wet weight of tissue (pmol g-1 wet wt.), respectively. In tissue from patients with diabetes, the content of VIP was lower (13.7 +/- 0.5 pmol g-1 wet wt.), whereas that of PACAP (-27 and -38) was unchanged. 4. Cyclic nucleotide levels were determined in preparations exposed to PACAP-27, PACAP-38, Hel-1, Hel-2, and VIP. All the peptides, but Hel-2, significantly increased the concentrations of cyclic AMP, whereas the levels of cyclic GMP were unchanged. 5. The peptides concentration-dependently relaxed noradrenaline-contracted preparations. The order of potency was VIP > PACAP 27 > Hel-1 > Hel-2 > PACAP-38. 6. Hel-1, VIP and PACAP-27 effectively counteracted electrically induced contractions. At 10(-6) M, the highest peptide concentration used, the inhibitory effects obtained reached 96 +/- 3%, 87 +/- 6%, and 80 +/- 3%, respectively. 7. The results suggest that PACAP and Hel-1 are co-localized with VIP in nerve structures within the human cavernous tissue, and that the peptides are effective relaxants of CC preparations in vitro. The role of the investigated peptides for penile erection remains to be established.

The role of histamine in human penile erection

OBJECTIVE

To investigate the relaxant action of histamine on human corpus cavernosum in vitro and the erectile response caused by the intracavernous injection of histamine in patients with psychogenic impotence.

PATIENTS AND METHODS

Human corpus cavernosum (HCC) tissue was cut into strips of approximately 2 cm and suspended in a cascade bioassay. The strips were then superfused with oxygenated and warmed Krebs solution and precontracted with noradrenaline (3 microM). Glyceryl trinitrate, acetylcholine and histamine were injected as a single bolus in the absence or in the presence of mepyramine and cimetidine. For the in vivo studies, histamine (30-60 micrograms) was injected intracavernously as a single bolus into the right corpus cavernosum 1 cm from the balamo-preputial sulcus. Similar protocols were carried out for papaverine (50 mg). The erectile response was divided into four grades: no response, tumescence, partial and full erection.

RESULTS

In vitro studies demonstrated that histamine (3-100 micrograms) caused dose-dependent relaxation of the HCC strips which was significantly inhibited by cimetidine (5-10 microM). The histamine H1 receptor antagonist mepyramine (1 microM) potentiated histamine-induced relaxation. The co-infusion of both mepyramine and cimetidine did not abolish histamine-induced relaxation. When injected intracavernously in humans, histamine (30 micrograms) caused full erection in 13% of the patients, whereas 87% had partial erection or tumescence. A higher dose of histamine (60 micrograms) caused full erection in 26% of the patients and 74% had partial erection or tumescence. Papaverine induced full erection in the majority of patients (66%). In contrast to papaverine, the duration of erection induced by histamine was markedly shorter (mean 200 and 6.5 min, respectively). The penile erections induced by papaverine were associated with complications such as pain, haematoma and priapism. Histamine did not induce any complications. Treatment of eight male patients with psychogenic impotence with the histamine H1 receptor antagonist astemizol (10 mg orally once daily for 1 week) did not affect histamine-induced erectile responses.

CONCLUSION

These results indicate that histamine may play a role in human penile erection. The erection-promoting action of histamine is probably due to H2 receptor activation, although another histamine receptor, possibly H3, also seems to be involved. This study suggests that histamine could be a valuable tool in the diagnosis of erectile dysfunction.

Prostaglandin E1 and the nitric oxide donor linsidomine for erectile failure: a diagnostic comparative study of 40 patients

A total of 40 patients (average age 55.9 years, range 23 to 78 years) with erectile failure was enrolled in a study to compare 1 mg. of the nitric oxide donor linsidomine to 20 micrograms prostaglandin E1 administered for diagnostic purposes. Prostaglandin E1 was considered comparable to linsidomine by 10% of the patients, slightly superior by 15%, moderately superior by 20% and much superior by 55%. No patient rated linsidomine superior to prostaglandin E1. Objective evaluation of erectile response to linsidomine versus prostaglandin E1 by the investigator revealed no response in 7.5% versus 0% of the cases, tumescence in 57.5% versus 17.5%, semirigid erection in 22.5% versus 17.5% and full erection in 12.5% versus 65%, respectively. Duplex sonography after linsidomine and prostaglandin E1 with evaluation of peak flow velocities (centimeters per second) showed that the increase in peak flow velocity after linsidomine was at least a third less than the increase after prostaglandin E1. The appearance after linsidomine often resembled arterial insufficiency, whereas after prostaglandin E1 there was no evidence of arterial insufficiency. The erectile and hemodynamic response to the nitric oxide donor linsidomine was modest compared to that of prostaglandin E1. Therefore, linsidomine is not a genuine alternative to prostaglandin E1 for the diagnosis and treatment of male impotence.

Effects of vasoactive intestinal peptide, peptide histidine methionine, and neuropeptide Y on intracavernous pressure in the rabbit

In anesthesized white rabbits blood was extracorporeally circulated from the left carotid artery into the penile corporeal bodies with a constant flow rate. Corpus cavernosum pressure (CCP) responses to intracavernous injections of drugs were recorded in animals with aortic nonocclusion and aortic occlusion, respectively. Vasoactive intestinal polypeptide (VIP, 1 microgram, 5 micrograms, and 20 micrograms), dissolved in 0.5 mL volumes, induced no significant increases in CCP compared with equivalent volumes of solvent, but the peptide increased the time interval until return of CCP to steady state level. Peptide histidine methionine induced a significant increase in the maximal CCP obtained in nonocclusion, and the time interval until return of CCP to steady state levels was increased in both aortic nonocclusion and occlusion. Neuropeptide Y produced an increase in the maximal CCP in animals with aortic occlusion, and a minor increase in the time interval until return of CCP values to steady state levels in both aortic nonocclusion and occlusion. Thus, all the peptides tested were capable of influencing the smooth muscle tissues involved in penile outflow regulation.

Intracavernous self-injection with vasoactive intestinal polypeptide and phentolamine in the management of erectile failure

A total of 52 men, median age 55 years (range 28 to 74 years), with erectile failure was treated with vasoactive intestinal polypeptide and phentolamine. Impotence was classified as psychogenic in 3 patients, psychogenic/arteriogenic in 3, arteriogenic in 25, arteriogenic/neurogenic in 4, neurogenic in 5, venous leakage/psychogenic in 2, venous leakage/neurogenic in 1 and following venous leak surgery in 9. The patients were treated with 30 micrograms vasoactive intestinal polypeptide and 0.5 to 2.0 mg. phentolamine. A total of 1,380 self-injections was given and the number of injections per patient varied from 5 to 245. No patient had priapism, corporeal fibrosis or other serious complications. After sexual stimulation all patients obtained erection sufficient for penetration. Following ejaculation rigidity decreased normally. The median duration of treatment was 6 months (range 1 to 22). Nine patients discontinued treatment. One patient with severe arteriosclerosis experienced decreased effectiveness of the drug and received a penile prosthesis. Five patients elected not to perform self-injection any longer, 1 psychogenic impotent patient was cured, and 1 patient discontinued therapy due to palpitation and sweating. One patient died of a myocardial infarction not associated with this therapy.

Vasoactive intestinal polypeptide (VIP) and peptide histidine methionine (PHM) in human penile corpus cavernosum tissue and circumflex veins: localization and in vitro effects

Localization and functional effects of vasoactive intestinal polypeptide (VIP) and peptide histidine methionine (PHM), two peptides derived from a common precursor molecule, were investigated in isolated preparations from human penile corpus cavernosum (CC) and circumflex vein (CV). VIP- and PHM-immunoreactivity (IR) was demonstrated in both CC and CV. The concentrations of VIP-IR and PHM-IR in CC tissue were 54.4 +/- 15.3, and 42.0 +/- 7.5 pmol g-1 wet weight respectively with a VIP/PHM ratio of 1.5 +/- 0.4 (mean +/- SEM). The corresponding values for CV tissues were 28.0 +/- 7.7 and 9.6 +/- 2.6 pmol g-1 wet weight with a VIP/PHM ratio of 3.1 +/- 0.4. CC and CV displayed VIP- and PHM-IR confined to nerve fibres in close relation to bundles of smooth muscle cells and blood vessels in both tissues. In vitro, VIP and PHM had no effects in unstimulated tissue preparations. Both peptides concentration-dependently (10(-9)-10(-6) M) relaxed CC and CV preparations precontracted with 3 x 10(-6) M noradrenaline. In CC the maximum relaxant effect of VIP and PHM was 22 +/- 11% and 9 +/- 9% and in CV the corresponding values were 82 +/- 8% and 93 +/- 3% respectively. The present study supports the hypothesis of VIP and PHM as neurotransmitters and/or neuromodulators in the nervous control of penile erection.

Effects of pinacidil upon penile erectile tissue, in vitro and in vivo

The effect of pinacidil, a new antihypertensive vasodilator, was studied in vitro upon human and porcine Corpus cavernosum and in vivo upon simian Corpus cavernosum. In vitro, pinacidil incubation (10(-5)-10(-3) M) was found to induce a concentration-dependent inhibition of the tissue response to norepinephrine (NE) and high potassium (K+). Likewise a concentration-related relaxation of the tissue, precontracted with either NE or K+, was seen using pinacidil 10(-5)-10(-3) M. Depending on the concentration applied, spontaneous activity as well as tone and amplitude of the contractions were reduced until total relaxation of the tissue was obtained with a pinacidil incubation of 10(-3) M. In vivo, 5 mg pinacidil in 0.3 ml solution was injected intracavernosally in 17 monkeys. Sixteen monkeys developed tumescence and 10, rigidity of the penis as well. Only one of 5 showed a decrease in the systemic blood pressure. Pinacidil might be of clinical interest as an agent facilitating erection when given intravernosally.

Electrical activity of corpus cavernosum during flaccidity and erection of the human penis: a new diagnostic method?

We investigated 7 normal men and 1 diabetic patient with erectile dysfunction. Electromyography electrodes were placed in the corpus cavernosum of the penis and electrical activity was recorded during flaccidity. With sexual arousal the activity decreased and tumescence was initiated. During tumescence and full erection the electrical activity of the corpus cavernosum almost ceased but in the diabetic patient (neurogenic impotence) an increase was observed. This discoordination might be the cause of the erectile dysfunction. Recording the electrical activity of the corpus cavernosum in patients with suspected neurogenic erectile dysfunction could become clinically valuable, since this is the first test possible to study the function of the autonomic motor system that normally regulates penile function.