The discovery of phosphodiesterase 5 inhibitors (PDE5 inhibitors) for causing the erection of the penis was accidental, noticed as a side effect while being administered to investigate their ability to treat hypertension and angina. The problem of erectile dysfunction has become extremely prevalent, with recent figures estimating that about 320 million will encounter the disorder by 2025. There are several available PDE5 inhibitors: sildenafil, vardenafil, tadalafil, and avanafil, which are FDA approved, lodenafil, udenafil, and microdenafil are the other non-FDA commercially available drugs.
Following are the indications where emerging evidence has revealed the advantages of using PDE5 inhibitors, but more research, particularly on human subjects is necessary to understand their efficacy:
To understand the mechanism of action of PDE5 inhibitors, one must first understand the normal physiology of penile erection, an area of medicine where most of the PDE5 inhibitors are currently in use. After sexual stimulation, nerve impulses release neurotransmitters in the corpora cavernosa, causing the production of nitric oxide by endothelial cells, which diffuses into the adjacent smooth muscle cells and stimulates the formation of cGMP, leading to vasodilation and increased penile blood flow.
Mechanism of working of PDE-5 Inhibitors on cGMP
These drugs inhibit the enzyme phosphodiesterase-5 (PDE-5), present in the smooth muscle cells of the vessels. By inhibiting this enzyme, these drugs prevent the degradation of cGMP by PDE-5. cGMP through activation of protein kinase G causes relaxation of the vascular smooth muscle. Prevention of degradation of cGMP by PDE-5 leads to the accumulation of cGMP in the vascular smooth muscle, thereby leading to dilatation of the blood vessels through phosphorylation of different downstream effector molecules. Dilatation of the penile arteries leads to a more prolonged erection. PDE5Is improve endothelial function and reduce apoptosis of vascular smooth muscle cells in corpus cavernosum.
A similar mechanism of action has also shown to result in vasodilation of the pulmonary arteries relieving the symptoms of pulmonary hypertension. They also inhibit the remodeling of the pulmonary vasculature.
When given in subjects suffering from heart failure, they have been known to inhibit the remodeling of the myocardium.
All the PDE5 inhibitors mentioned above have the same mechanism of action, but they differ in their PDE isozyme selectivity with PDE6 (sildenafil), and PDE11 (tadalafil) are usually affected by the cross-reactivity, leading to the presentation of specific side effects associated with PDE5 inhibitor use.
Following oral doses are usually recommended for Erectile dysfunction-
The oral doses of PDE5 inhibitors recommended for the treatment of pulmonary arterial hypertension (PAH):
For patients with lower urinary tract symptoms of benign prostatic hyperplasia (BPH) with concomitant ED - tadalafil 5 mg is preferred.
Characteristics of different types of PDE5Is appear in Table 1
None of the PDE5 inhibitors are genuinely selective for the receptor PDE5, and most of the harmful effects occur due to cross-reactivity with other PDE isoenzymes. These effects are usually dose-dependent. Some of the common adverse effects encountered with the administration of PDE5Is are:
Nevertheless, the adverse outcomes encountered with the use of PDE5 inhibitors rarely lead to their discontinuation.
PDE5Is use has also correlated with specific less known, rarely encountered side effects, and the low frequency of the occurrence of these adverse effects does not necessitate the withdrawal of the drugs from the market-
Caution is necessary for patients with the following relative contraindications:
As mentioned above, certain drugs like nitrates are absolutely contraindicated in patients taking PDE5 inhibitors, and therefore ED patients should be advised about not administering the two drugs concomitantly. Co-administration of PDE5 inhibitors and nitrates is contraindicated, but if the need arises, thorough hemodynamic monitoring is necessary. Some medications like alpha-blockers are needed to relieve concomitant symptoms of BPH in patients with ED and, therefore, require caution with frequent monitoring of blood pressure levels and prompt arrangement of an appointment with a physician if hypotensive symptoms present which may necessitate discontinuation of PDE5 inhibitor. PDE5 inhibitors should only be administered in stable patients taking alpha-blockers. Proper titration and increase dosing in stepwise fashion if co-administered with alpha-blockers.
Patients should consult a physician if they encounter a sustained erection persisting for more than 4 hours as prolonged blood engorgement of blood in the penis can compromise the arterial supply and lead to penile necrosis.
If patients experience an inadequate response to treatment with a PDE5 inhibitor, consider direct injections of PGE1, papaverine, phentolamine, or a combination with improvement in patients with intact vasculature. If still no response, get duplex Doppler ultrasonography, penile cavernosography, and pudendal arteriography to recognize the cause of refractory ED.
All the PDE5 inhibitors undergo extensive tubular reabsorption in the kidney and get extensively excreted as metabolites in the stool. Nonetheless, in patients with severe renal insufficiency (CrCl less than 30 ml/min), where the presence of concomitant ED is fairly common with some studies reporting prevalence rates as high as 80% in patients undergoing dialysis, it is advisable to initiate PDE5 inhibitor therapy at the lowest dose possible with gradual up-titration.
PDE5 inhibitors undergo rapid metabolism in the liver, most notably through the CYP3A pathway. Similar to the recommendations for the usage of PDE5 inhibitors in renal dysfunction, impairment of hepatic function also warrants starting lower doses and gradual up-titration while monitoring for adverse effects. Regularly scheduled follow-ups and frequent monitoring for side effects should take place.
No evidence has emerged concerning the development of any form of dependence or tolerance with the chronic use of PDE5 inhibitors. Some recent reports have found them being misused concomitantly with alcohol and illicit drugs. A few case reports of seizures and myocardial infarction with the administration of PDE5 inhibitors have come forward, likely due to cerebral vasodilatory and cytochrome P450 inhibition of the illicit drugs that are sometimes taken concurrently with the meds.
A case study conducted on a middle-aged man who consumed 24 tablets of 100 mg sildenafil and presented with transient visual complications except for a visual field defect and annular scotoma that persisted chronically.
Managing the underlying cause
Erectile dysfunction can be due to multiple reasons – psychogenic, neurogenic, vascular, or hormonal. Therapy with symptomatic management with PDE5 inhibitors should be accompanied by simultaneous removal of the etiology causing ED. Hence it is crucial to obtain a relevant history and performing appropriate physical examination.
Patients with psychogenic causes of ED usually have typical early morning and nocturnal erections but are unable to achieve or maintain an erection during a sexual encounter with a specific partner. These patients will benefit from counseling and a referral to a psychiatrist if unremitting symptoms are present.
It is essential to identify atherosclerotic risk factors like smoking, hypertension, diabetes mellitus, and hyperlipidemia in patients presenting with a vascular cause of ED. These patients, in addition to the absence of nocturnal and early morning erections, may present with symptoms of atherosclerosis of major blood vessels, particularly angina, claudication, or a history of a significant cardiovascular event. Therefore, it is recommended to monitor blood pressure, blood glucose, HbA1C, and lipid levels, and initiate treatment for the underlying cause. Patients should be counseled on the importance of smoking cessation, maintaining appropriate dietary and lifestyle habits.
ED patients with concomitant history of long-standing diabetes should be monitored for their diabetic status and through frequent neurological examinations to check for autonomic neuropathy.
Patients with hormonal causes of ED present with reduced libido, suggesting hypogonadism as the cause of ED, which is manageable accordingly.
Treating the underlying cause of ED can have a tremendous impact on the resolution of symptoms without the need to administer PDE5 inhibitors. Concomitant administration of statins, anti-oxidants, and testosterone supplements may aid in alleviating the condition. Careful attention should be given to ED patients with comorbidities like hypertension, diabetes, and hyperlipidemia because it has been observed that strict control of blood pressure, glucose, and lipid profile accentuates the sensitivity of ED to PDE5 inhibitors.
As the above exemplifies, to cover all aspects of ED and use PDE5 inhibitors requires an interprofessional team approach. All the clinicians listed above are candidates for starting therapy in the patient, but they need to work across disciplines with each other to ensure safe treatment and minimal adverse effects. Along this line, the pharmacist should also have significant involvement, checking for drug-drug interactions, verifying dosing based on the issue (ED vs. pulmonary HTN), and counseling the patients on proper dosing and administration. Nursing can also help assess treatment effectiveness and patient compliance. Both pharmacists and nurses need to be able to contact the prescriber with any concerns they may encounter. As clinicians, specialists, nurses, and pharmacists all take an active, collaborative role, PDE5 inhibitor therapy can be most effective for the patient with minimal adverse effects. [Level 5]
Expansion of utilities for PDE5 inhibitors
There appear to be new emerging benefits of PDE5 inhibitors in the management of heart failure, prevention of myocardial remodeling post-infarction. The advantages of using PDE5 inhibitors in these diseases need to be studied extensively for inclusion in the treatment plans for these conditions.
Due to the increased expression of PDE5 receptors on tumor cells, including colon, breast, lung, bladder, and prostate, these drugs have shown a positive response through increasing apoptosis of cancerous cells and blocking one or more Multidrug-resistant transporters such as P-glycoproteins.
One study through the chronic administration of PDE5 inhibitors, particularly tadalafil, has given a promising result of reducing pro-inflammatory cytokine levels in patients with type 2 diabetes.
For patients with co-existent angina, a new drug is available in the market known as ranolazine, which is indicated for prophylactic treatment in chronic angina in patients refractory to first-line antianginal drugs like nitrate and beta-blockers with an added advantage of being safely co-administered with PDE5 inhibitors.
Improving drug administration
The most vital modality to enhance efficacy and compliance of PDE5 inhibitors involves providing instructions to the patients regarding appropriate administration and the associated adverse effects that are possible while taking PDE5 inhibitors. Since about 30 to 35% of patients do not respond to on-demand administration of PDE5 inhibitors, chronic daily administration of these medications can be a consideration as a reasonable alternative to induce normal sexual functioning with the added advantage of not exhibiting tachyphylaxis.
A new type of orodispensable sildenafil formulation is in the market, which disintegrates within seconds of ingestion and hence absorbed quickly in the mouth. This formulation offers an added advantage in patients with dysphagia and improves adherence due to rapid effect.
As an epilogue, the role of PDE5 inhibitors, especially sildenafil and tadalafil, has been well established clinically in erectile dysfunction and pulmonary hypertension. Nevertheless, these drugs offer tremendous potential in other areas of medicine like heart failure, peripheral neuropathy, and peripheral arterial disease, which requires extensive research before they can enter the market for the treatment of these disorders.
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