Title: ORAL MEDICATIONS FOR ERECTILE DYSFUNCTION: MODE OF ACTION AND TREATMENT ISSUES , By: Casey, Richard, Canadian Journal of Human Sexuality, 11884517, Fall98, Vol. 7, Issue 3
ABSTRACT: This paper describes the physiological processes involved in penile erection and uses this information to explain the mode of action of oral medications used in the treatment of erectile dysfunction (ED). New biomedical treatments for ED, such as sildenafil, present both a challenge and an opportunity for health care professionals to address ED with their patients and clients. An algorithm for the treatment of ED by primary care physicians is proposed and discussed in relation to diagnosis, outcome measurement, referral, and use of sildenafil and other treatment options.
Key words: Erectile dysfunction, Physiology of erection, Oral medications, Treatment Sildenafil
The high prevalence of erectile dysfunction (ED) in North America has been well documented (Feldman et al., 1994), as have the various biomedical treatments used in treatment (Coleman, 1998). The emergence of sildenafil (Viagra[TM]), an effective, easy-to-use, and relatively low risk oral medication (Goldstein et al., 1998; Morales et al., 1998), has given this issue wide publicity and, in the process, altered the landscape for public and professional education about ED and its treatment. Until recently, clinicians estimated that they were seeing only about 10% of men who had symptoms that would fit the standard definition of ED, that is, men who had persistent difficulty in getting or maintaining an erection sufficient for satisfactory sexual performance and who perceived this as a problem. Since ED can cause considerable distress, it was assumed that many men were not seeking treatment due to embarrassment, resignation, acceptance or discomfort with the available treatment methods.
Those of us who treat ED have long sought treatment options that have high efficacy, low adverse effects, and high acceptance by men and their partners. The approval of sildenafil (Viagra[TM]) in the U.S. (pending in Canada) can be expected to have a significant impact on both the number of men and couples seeking treatment, and the delivery of sexual health care in general. A reported 2 million prescriptions for sildenafil were written in the first four months following its approval. Given expanded public access to information (and misinformation) about ED and related sexual problems, it seems likely that physicians and other health professionals will face increasing requests for clear and understandable explanations about new oral medications and other “traditional” treatment options.
While most men with ED do not want or need a detailed lecture on the physiology of erection, they and their partners may nevertheless want to know how pharmacological agents and other biomedical treatments work. This paper therefore describes the process of penile erection, identifies the sites of action of pharmacological agents used in the treatment of ED, and comments on emerging issues in this field.
Our improved understanding of penile physiology has led to better diagnosis and treatment of men with ED. We now recognize that two sets of processes are involved in penile function: one set maintains the flaccid condition, the other facilitates erection.
FACTORS THAT MEDIATE NATURALLY OCCURRING ERECTION Erection occurs when nerve impulses from the brain (psychogenic erection) and from genital stimulation (reflexogenic erection) combine to cause blood to flow faster into than out of the penis. From this limited perspective, the penis can be viewed as a hydraulic organ. It is composed of three sponge-like cylindrical bodies that run the length of the penis (the two corpora cavernosa and the corpus spongiosum) and are supplied with blood by small branches of the penile artery (see Figure 1). These helicine arteries empty into blood spaces (sinusoids) in this spongy tissue. The spaces are lined with vascular epithelial cells and are separated from each other by trabeculae, partitions made of smooth muscle. Blood is carried to the helicine arteries by branches of the pelvic artery, and carded away from the sinusoidal spaces by surface veins that run adjacent to the sheath (tunica albuginea) that surrounds each corporal body.
The nervous input that induces erection is delivered to the penis via the pelvic nerve, which exits the spinal cord at the lower sacral level (S2-S4) and branches into the cavernous nerve that supplies the corporal bodies. Both parasympathetic fibres that release acetylcholine (cholinergic fibres), and other nerves that release nitric oxide (NO), are involved in this process (Figure 2). It is damage to this nerve supply during some rectal and prostate surgeries that can cause post-treatment ED. Nervous input during sexual arousal causes dilation of the cavernosal arteries and results in a 30- to 40-fold increase in the rate of flow of blood through these arteries into the sinusoidal spaces. How does this nervous input cause the increased blood flow that initiates erection?
NITRIC OXIDE AND ERECTION When the penis is flaccid, the smooth muscle fibres that surround the sinusoids are contracted, thus restricting the rate of blood flow into the spaces of the corporal bodies. Erection occurs as a result of the action of nitric oxide (NO) on these smooth muscle fibres. NO activates the enzyme guanylate cyclase that converts guanosine triphosphate (GTP) to cyclic guanosine monophosphate (cGMP). cGMP causes smooth muscle relaxation (probably by decreasing cellular calcium which is needed for contraction) with a resulting increase in blood flow into the sinusoidal spaces (Figure 3). cGMP is inactivated naturally by an enzyme, phosphodiesterase type 5 (PDE-5), which is present in the smooth muscle cells. Sildenafil (Viagra[TM]) inhibits PDE-5, and thus prolongs the effect of cGMP. This is particularly relevant in the case of ED where the natural NO-stimulated production of cGMP is insufficient to cause or maintain an erection.
NO is released from the both the non-adrenergic non-cholinergic (NANC) nerves that supply the penis, and also from the endothelial cells that line the sinusoids. The endothelial cells are triggered to release NO by acetylcholine from parasympathetic nerve fibres that are active during sexual stimulation. This explains why conditions that damage endothelial cells (such as high cholesterol, diabetes, or smoking) may also compromise erection. The mechanisms that mediate NO release and cGMP production and action are thus key features in the process of erection (see Figure 3).
Although it is thought to play a lesser role than NO in facilitating erection, prostaglandin E1 (PGE1) is also released during sexual arousal. Like NO, it activates production of a compound in smooth muscle cells that causes muscle relaxation. In this case, the compound is cyclic adenosine monophosphate (cAMP). PGE1 activates the enzyme adenyl cyclase that converts adenosine triphosphate (ATP) to cAMP. These observations explain why PGE1 injected intercavernosally (Caverject[TM]) or inserted as a pellet intraurethrally (MUSE) can induce erection in men with ED. Other naturally occurring neuropeptides, such as vasoactive intestinal peptide (VIP), may also be involved in the smooth muscle relaxation that leads to erection (see Figure 3).
RESTRICTION ON VENOUS OUTFLOW DURING ERECTION Continued filling of the sinusoids as a result of the above processes eventually compresses the veins that carry blood away from the penis against the tunica that surrounds the corporal bodies. This compression further facilitates erection by blocking blood outflow.
FACTORS THAT MAINTAIN THE NORMAL FLACCID CONDITIONS OF THE PENIS In the flaccid penis, sympathetic nerve fibres release norepinephrine (such nerves are called noradrenergic) which binds to alpha 1 adrenergic receptors on penile smooth muscle to cause intracellular changes that result in contraction. In some men, damage to the endothelial cells may cause release of endothelin, an action which can also cause smooth muscle contraction (see Figure 4). A variety of other factors (not shown) can also regulate the contraction that maintains penile flaccidity.
Our increasing knowledge of the physiological processes involved in erection makes it possible to understand the sites of action of the various medications currently being used or proposed to restore or enhance erectile function in men with ED.
Pharmacological agents can facilitate erection by enhancing the natural processes that induce erection (see Figs 1-3) or by inhibiting the processes that maintain flaccidity (see Fig. 4). The scheme used by Morales (1997) to classify non-invasive pharmacotherapy agents for ED is expanded here in Table 1 to include other compounds currently being considered for either oral or topical use.
ORAL MEDICATIONS
Peripheral Agents
Sildenafil (Viagra[TM]) inhibits PDE-5 and thus blocks degradation of cGMP in the corpus cavernosal smooth muscle when sexual stimulation is present. cGMP causes smooth muscle relaxation and increased blood flow into the sinusoids. This blood flow raises oxygen tension in the endothelial cells and facilitates their production of NO in response to the acetylcholine that is released from parasympathetic fibres in response to sexual stimulation. Sildenafil’s action thus requires that some NO production and release be present.
L-arginine is a precursor in NO synthesis and has therefore been tested as an oral medication for ED because of is possible role in elevating, NO levels (Zorgniotti & Lezzi, 1994). To date, the efficacy of L-arginine remains unproven.
Adrenergic Receptor Agonists
Yohimbine, is a selective inhibitor of alpha adrenergic receptors and produces elevation of blood pressure and heart rate. Despite its use in ED treatment, the efficacy of this compound remains in doubt (see above and Coleman, 1998).
Phentolamine has long been used in combination with other agents in intracavernosal injection (ICI). It blocks both alpha 1- and alpha 2-adrenergic receptors, and thus inhibits the normal contractile effect of noradrenaline on cavernosal smooth muscle. It has also been tested as an oral agent with some success (Goldstein and The Vasomax Group, 1998; Gwinup, 1988; Wagner et al., 1996).
Dopamine Receptor Agonists
Apomorphine This centrally acting drug is thought to excite dopaminergic pathways in the CNS and is proposed for use in treatment of ED that does not have a major organic component. Preliminary testing showed some enhancement of erectile function, with higher doses increasing the severity of such side effects as hypotension and nausea (Heaton, Morales, Adams, Johnston & el-Rashidy, 1995). A more recent placebo-controlled double blind study found that sublingual apomorphine SL administered to men with ED (no major organic component) led to erections firm enough for intercourse in 45.8%, 52.0%, and 59.6% at doses of 2, 4 and 6mg respectively; placebo resulted in similar responses in 32.0%-35.0% (Padma-Nathan et al., 1998). The main side effect of nausea (mainly mild to moderate) was also dose-dependent (2.1%, 19.5% and 39.0% vs. a maximum or 4.9% for controls).
Serotonergic Receptor Agonists
Trazodone, a tricyclic antidepressant, has been associated with the development of prolonged penile erection and priapism. Two recent double blind placebo-controlled studies found that men on trazodone did not differ from the placebo groups in terms of erectile or sexual function (Costabile & Spevak, 1998; Meinhardt et al., 1997). The exact mechanism of action has not been described, and there is little clinical evidence to support its routine use.
INTRAURETHRAL AGENTS
Alprostadil, a synthetic prostaglandin, is inserted into the urethra in pellet form (MUSE, Medicated Urethral System for Erection) from whence it is absorbed into the cavernosal tissue. It is presumed to bind to PGE1 receptors on cavernosal smooth muscle, and probably exerts its muscle relaxant effect, and hence its impact on ED, by causing cAMP elevation (Padma-Nathan et al., 1997).
The preceding overview provides a basis for understanding the rationale for current biomedical treatments for ED, and particularly for oral medications which are the primary focus of this paper.
BACKGROUND Prior to 1980, there were few biomedical treatments for men with ED. The introduction of intracavernosal injection therapy (e.g., papaverine, phentolamine, prostaglandin) in the early 1980s allowed physicians to study erectile activity and to offer patients an effective alternative to penile implantation. This period also lead to an explosion of penile vascular procedures geared to improving blood flow to the penis (arterial surgery) or decreasing venous outflow (dorsal vein ligation). In exceptional cases (traumatic injury to the pudendal artery) these procedures are successful, but most urologists have all but abandoned revascularization/venous ligation procedures due to the limited success in most patients and the difficulty in selecting who may benefit from surgery.
CLINICAL GUIDELINES FOR TRADITIONAL “GOAL-DIRECTED” THERAPY FOR ED In an attempt to standardize patient care, the American Urologic Association (AUA) established a Clinical Guidelines Panel in 1996 to analyze all available literature regarding current methods for treating ED. This resulted in practical recommendations published in December, 1996 (American Urologic Association, 1996). The panel analyzed outcome evidence for five treatment alternatives:
- Oral drug therapy (yohimbine)
- Vacuum Constriction Devices;
- Intracavernosal vasoactive drug injection therapy;
- Penile prosthetic implantation;
- Arterial and venous surgery.
The panel recommended that before treatment is considered, all options should be explained to the patient and, if possible, his partner. Based on a review of the data available at the time, the panel concluded that yohimbine did not appear to be an effective treatment for organic ED, and that the chances of success for arterial and venous surgery did not appear to be high enough to justify the routine use of such therapies. The three recommended options from which patients might choose were thus: Vacuum Constriction Device (VCD) therapy, intercavernosal drug injection therapy (ICI), and penile prosthesis surgery. These methods had been shown to result in sufficiently increased patient and partner satisfaction (and return to intercourse) to warrant continued use (for reviews of these methods and their effects and side effects, see Basson, 1998 and Coleman, 1998).
The panel’s recommendations are consistent with the concept of “goal-directed therapy” first suggested by Lue (1990) and now used by most physicians who treat erectile dysfunction. While this therapeutic approach is often successful, it tends to ignore the individual etiologies for ED because patient choice determines what treatment is used. This approach does not necessarily require that the treating physician make an accurate etiologic diagnosis before deciding on treatment. Furthermore, the treatment options chosen by patients may downplay or ignore other significant psychogenic factors that may be associated with the ED (e.g., the choice of ICI for ED that had its onset secondary to premature ejaculation). Recent developments in treatment of ED have highlighted the need for a rethinking of diagnosis and treatment.
The introduction of the intraurethral medication alprostadil (MUSE, medicated urethral system for erection) and the oral medication sildenafil (Viagra[TM]) has expanded the range of treatment options and, in so doing, made it more difficult to continue the “goal-directed approach” described above. As other therapeutic options appear (e.g., oral phentolamine, apomorphine, topical agents), physicians will need a better functional classification for the medical treatment of ED. There is presently no “gold standard” and no urologically established therapeutic classification for ED. A newly developed classification scheme (Heaton, 1998; Heaton, Adams & Morales, 1997) may allow clinicians to evaluate new treatments as they are introduced and to better link etiology and treatment method. The scheme identifies four primary modes of action for medications used in the treatment of ED: central initiator, peripheral initiator, central conditioner, peripheral conditioner (local or systemic). The scheme also suggests two subcategories that list routes of delivery and mechanisms of differential sensitivity. The modes of action are briefly summarized below.
Central initiators are medications that have their site of action within the CNS. For example, the neurotransmitter dopamine in the brain plays a role in activating the neural events that lead to erection, and central initiator-type medications would exert their effects at such sites (e.g., apomorphine). Central conditioners improve the internal milieu of the CNS to enable erection, but do not initiate the process (e.g., serotonin plays this role naturally, as does testosterone). Peripheral initiators act outside the CNS and would influence sites directly involved in the initiation of erection (e.g., cholinergic nerves, NO-releasing nerves in the penis play this role naturally; prostaglandin therapy would be expected to work at this level). Peripheral conditioners improve functions in penile tissue or associated systems (nerves, blood vessels) that enhance, but do not cause, erection (e.g., sildenafil acts at this level).
While the Heaton et al. (1997) scheme was developed to indicate where and how medications might be targeted to enhance erectile function, Gajewski (1998) has also used it to explain why ED may be associated with a variety of medical conditions (diabetes mellitus, cardiovascular disease) or a side effect of medications used to treat health problems. Heaton et al. (1997) suggest that the scheme will facilitate a diagnostic classification based on the man’s capacity to respond to particular therapies. In this way, treatment options can be better linked to clinical diagnostic findings. This link between diagnosis and treatment will become increasingly important as compounds with greater specificity make it possible to be more precise and selective in choice of therapy.
INCREASING DEMAND FOR TREATMENT Until now, the absence of a single therapy for ED has meant that the majority of patients are seen in specialized centres that have the resources to offer all, or at least most, forms of biomedical treatment. The average patient probably waited about three years before seeing someone about their ED and then another six months for an appointment with a specialist who could offer this range of treatment.
The present infrastructure of ED treatment in Canada and the U.S. cannot easily meet the high demand for treatment that sildenafil and other emerging treatments are expected to produce. Family physicians will be increasingly involved in diagnosis and prescription, and sex counsellors and therapists who are not physicians will be required to make referrals as they integrate biomedical treatments into other therapeutic approaches.
ROLE OF THE PRIMARY CARE PHYSICIAN
A number of authors have emphasized the role of primary care physicians in sexual health care in general, and in the treatment of ED in particular (Holzapfel, 1998; Basson, 1998). As demand increases, we can expect, in the absence of contraindications, that sildenafil and other oral medications will be widely used in the treatment of ED. The treatment algorithm in Figure 5 outlines how a primary care physician who is not a specialist in sexual medicine might routinely address the needs of men with ED. The first step would involve a medical and sexual history and a physical examination to determine: treatable risk factors for ED (obesity, smoking, etc.); biomedical conditions and medications that might cause or exacerbate ED; level of sexual desire as a contributing factor; and relationship issues that are a cause or consequence of ED.
While a number of methods have been developed to test erectile function (nocturnal penile tumescence testing, duplex Doppler blood flow recordings of the penile arteries, dynamic infusion cavernosometry for assessment of venous outflow, office-based trial with an injectable intracavernosal agent), these are unlikely to be used routinely by primary care physicians. Each method has limitations, even in the hands of a sexual medicine specialist or urologist, and the history and physical can often reveal the information needed for subsequent action (see Fig. 5). Some physicians may use questionnaires, or selected questions from them, to assess ED or sexual functioning. Examples include the International Index of Erectile Function (Rosen et al., 1997), The Brief Male Sexual Function Inventory (BMSFI) and the self-administered Derogatis Inventory for Sexual Functioning (DISF-SF). This information can provide a baseline for assessing treatment efficacy and outcomes.
However, before undertaking a trial with sildenafil, physicians and their patients should consider a number of factors. The patient should be aware of treatment options and their possible side effects. Where possible, the partner’s knowledge of and interest and involvement in the treatment should be assessed. The primary goal of safe prescription means that medical contraindications for sildenafil should be ruled out (see Goldstein, 1998; Morales et al., 1998). This would prelude its use by men taking nitrates or those with recent heart attack or chronic heart disease. It would warrant caution for men with angina, recent coronary artery surgery or problems with exercise tolerance. Patients who are reviving sexual activity after a prolonged period of inactivity should be treated as the physician would a patient who requests an assessment before starting an exercise program. A good role of thumb is to exclude patients who cannot easily climb three flights of stairs or who would be referred to a cardiologist prior to such a program.
The predominance of psychological factors in the apparent etiology would warrant referral to a sexual therapist. Attention to these psychological/situational issues should also be part of primary care, since situational ED is often accompanied by anxiety and the concomitant sympathetic nervous system activation that causes peripheral vasoconstriction.
This may explain why patients with a high level of anxiety often experience a suboptimal response to peripheral agents. This would be of particular importance in the case of sildenafil, the action of which requires sufficient sexual response to induce some NO release in the penile tissues. In such high anxiety cases, ICI might therefore be more effective than sildenafil, particularly if psychological counselling to reduce anxiety-provoking expectations is not provided (see Basson, 1998, for discussion of the use of sildenafil in the context of such counselling).
Men who wish to improve “normal” erectile function are probably not candidates for sildenafil. While there are anecdotal reports of patients’ experiencing harder erections and shorter post-ejaculation refractory periods, the exact response of men with uncompromised erectile function is unclear. One would expect that normal erectile function is unlikely to be sufficiently enhanced by sildenafil to warrant prescription.
Having addressed these issues, and in the absence of medical contraindications (see Goldstein et al., 1998; Morales et al., 1998) or other impediments, the patient may opt for a trial of sildenafil (see Fig. 5). As with any such treatment, a key issue will be the choice of a realistic outcome measure. The lIEF is particularly helpful in this regard, because its parameters provide a basis for assessing changes in both erectile function (in sexual situations including intercourse) and psychological well-being (enjoyment, confidence, sexual satisfaction, relationship satisfaction) (Rosen et al., 1998). However, studies that have used this instrument to measure outcome have captured little information on time to onset of erection, duration of erection, or partner reaction and satisfaction. We can anticipate such information in the future but, even then, what constitutes a successful treatment outcome for any method may remain an important and debateable question.
Men who do not have a satisfactory outcome on sildenafil might then be tested with intracavernosal injection or a vacuum erection device (VED). Referral to a sexual medicine specialist might be done before such a trial if the physician is not prepared for this next step or after if it unsuccessful.
The preceding observations indicate that sexual arousal in men involves a complex interaction of physiological and subjective processes. We now have a variety of methods for assessing erectile function (and dysfunction) and the promise of sildenafil and other emerging pharmacological methods for treating the latter. This promise highlights the need for physician training and cross-disciplinary collaboration to meet the sexual health care needs of men and couples seeking treatment for ED.
Correspondence concerning this paper should be addressed to Richard Casey, MD, The Male Health Centres, 1235 Trafalgar Road North Suite 407, Oakville, Ontario, L6H 3P1. Tel: 905-338-1078; Fax: 905-338-3150.
Table 1 Medications for treatment of erectile dysfunction
ORAL MEDICATIONS
Peripheral agents
Adrenergic receptor antagonists
- Yohimbine
- Phentolamine (Vasomax[TM])
Dopamine receptor agonists
• Apomorphine
Serotonergic receptor agonists
• Trazodone
INTRAURETHRAL AGENTS
• Alprostadil (synthetic PGE1)
TOPICAL AGENTS
- Minoxidil
- Prostaglandin E1 (Topiglans)
- Nitroglycerin
UNKNOWN MECHANISMS
• Testosterone
Adapted from Morales (1997)
DIAGRAM: Figure 1. Penile Anatomy and Circulation
DIAGRAM: Figure 2. Key Nerves of the Penis
DIAGRAM: Figure 3. Chemical Messengers and Neurotransmitters Involved in Erection
DIAGRAM: Figure 4. Chemical Messengers Involved in Muscle Cell Contraction
DIAGRAM: Figure 5. Suggested assessment/treatment diagram for PCP
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By Richard Casey, The Male Health Centres, Oakville, Ontario
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