Overview of Sildenafil Orally Disintegrating Tablets (ODT)
Dosage Strength of Sildenafil ODT
125 mg/ODT (10 ODT/Pack)
Initially developed for the treatment of pulmonary hypertension, angina, and other cardiovascular conditions, sildenafil citrate was accidentally found to be beneficial in males who suffered from erectile dysfunction (ED). Prior to the discovery of its benefits in treating ED, this condition was considered to be an inevitable part of aging in men or due to underlying psychological causes. After its approval in 1998 by the U.S. Food and Drug Administration for the treatment of ED, the popularity of sildenafil citrate has skyrocketed over the past couple of decades as health care providers generally recommend this medication as the first-line therapy in the management of erectile dysfunction in men. Other contributing factors to its appeal and popularity is that sildenafil citrate can be taken orally on demand, is generally well tolerated, with minimal adverse effects.1
Sildenafil citrate is a vasoactive medication belonging to the drug class of phosphodiesterase – 5 enzyme (PDE-5) inhibitors; it is a competitive antagonist of this enzyme. PDE-5 can be found all over the human body especially in the corpus cavernosum within the penis, striated and smooth musculature, as well as in platelets. However, PDE-5 has the largest distribution in the penile corpus cavernosum which is why sildenafil citrate is able to work selectively in this part of the body.2
Sildenafil citrate is generally administered orally. However, it can also be administered intravenously or sublingually. Even though its most popular clinical indication for use is in the management of erectile dysfunction, it is also used in the management of pulmonary hypertension, persistent pulmonary hypertension of the newborn, Raynaud's phenomenon resistant to other vasodilators, as well as in the prevention of pulmonary edema at high altitudes. After oral ingestion, absorption of sildenafil citrate rapidly occurs mainly in the small intestine from where it is then transported in the bloodstream to its area of action. Sildenafil citrate is metabolized in the liver through the action of the hepatic isoenzymes cytochrome P450 3A4 and cytochrome P450 2C9. Following hepatic metabolism, the metabolites are excreted mainly in the stool and, to a lesser degree, in the urine.234
Sildenafil citrate is classified as a pregnancy category B drug by the Food and Drug Administration. Studies have not demonstrated definite risks to fetuses when sildenafil is administered to pregnant mothers. At present, there are no definite clinical indications that warrant the administration of sildenafil citrate in women. Studies done till date have not indicated that sildenafil citrate has comparable benefits in women as they do in men. There are other studies that are still ongoing, however, and their outcomes may provide further insight regarding the utility and benefits of sildenafil in women.56
Mechanism of Action
Sildenafil plays an indirect role in causing penile erection in men suffering from erectile dysfunction. In normal penile erection, sexual stimulation leads to the activation of the non-adrenergic as well as non-cholinergic nerves in the pelvic parasympathetic plexus. Following the activation of these nerves, the neurotransmitter nitric oxide is then released which then transverses the neuromuscular junction of the smooth muscle of the corpus cavernosum and the penile arteries. Nitric oxide then causes an increase in the production of the intracellular second messenger cyclic guanosine monophosphate (cGMP), which is a cyclic nucleotide derived from guanosine triphosphate (GTP). The release of cGMP leads to an increase in blood flow within the penile arteries as well as the relaxation of the smooth muscles of the corpus cavernosum, thereby resulting in penile erection. Penile detumescence is caused by the release of the phosphodiesterase-5 enzyme, which breaks down cGMP and, therefore leads to the contraction of the penile arteries and the corpus cavernosal smooth muscles.
Sildenafil has a chemical structure that is very similar to cGMP and binds competitively to phosphodiesterase-5 enzyme. By binding to the receptors on PDE-5, sildenafil prevents PDE-5 from binding to and degrading cGMP. This competitive antagonist action of sildenafil allows the actions of cGMP in the penile arteries and corpus cavernosum to be prolonged, resulting in the prolongation of penile erection.247
In addition to its location in the penile corpus cavernosum, the phosphodiesterase-5 is also located in large amounts within the pulmonary vasculature. In the lungs, PDE-5 breaks down cGMP, similar to its actions in the corpus cavernosum. Sildenafil also acts as a competitive antagonist to PDE-5 in the pulmonary vasculature preventing the breakdown of cGMP. This has the effect of causing a reduction in pulmonary vascular resistance as well as the mean pulmonary artery pressure. Due to this effect on the pulmonary vasculature, sildenafil has been proposed as an adjunct medication in the treatment of primary pulmonary hypertension, a disease of childhood with a typically poor prognosis. However, studies as to its efficacy in the management of this disorder are still ongoing.2
As earlier stated, sildenafil citrate can be administered orally, sublingually, or intravenously. In the management of erectile dysfunction, however, the oral and sublingual routes are the means by which the medication is commonly administered. After oral ingestion, sildenafil citrate is rapidly absorbed within the small intestine, with a peak plasma concentration time of anywhere between 30 and 120 minutes and a median time of 60 minutes in fasting individuals. It has a bioavailability of approximately 40 percent after oral administration.248
Metabolism of sildenafil citrate occurs primarily in the liver through the action of the hepatic microsomal isoenzymes cytochrome P450 3A4 and 2C9. After the action of the isoenzymes on sildenafil, it is broken down into an N-desmethyl metabolite. Both sildenafil citrate as well as its N-desmethyl metabolite breakdown product are tightly bound to plasma proteins and they have an approximate half-life of four hours. it is estimated that about 96 percent of sildenafil and its metabolite are bound to plasma proteins after oral ingestion. Following its metabolism, the majority of sildenafil citrate is excreted in the stool; a lesser amount, about 13 percent of sildenafil metabolites, is excreted in the urine.48
There are a number of factors that can interfere with the metabolism of sildenafil, thereby increasing or decreasing its concentration in the body. Some of these factors include the following:
- Hepatic impairment: Individuals who have significant hepatic disease such as liver cirrhosis may experience a significant increase in plasma sildenafil levels. This is because the production of the hepatic isoenzymes cytochrome P450 3A4 and 2C9 are markedly diminished in liver cirrhosis. As a result of this diminished production of the hepatic isoenzymes, the body is not able to adequately metabolize sildenafil citrate, resulting in increased plasma levels.
- Renal impairment: Individuals with significant renal disease as evidenced with a creatinine clearance of less than 30mL/min will have a marked increase in plasma sildenafil levels after oral ingestion. Since sildenafil and its metabolites are excreted through the kidneys, renal diseases will impair the ability of the kidneys to perform this task adequately.
- Age: For reasons that are yet to be fully determined, there is a 40 percent increase in the plasma levels of sildenafil and its N-desmethyl metabolite when administered to men greater than 65 years of age.
- Cytochrome P450 3A4 inhibitors: The concomitant use of drugs known to inhibit the hepatic cytochrome P450 enzymes may result in increased plasma levels of sildenafil. Examples of drugs that are known P450 inhibitors are macrolide antibiotics such as erythromycin, antifungal agents such as ketoconazole, and protease inhibitors such as indinavir. By inhibiting the production of cytochrome P450, hepatic metabolism of sildenafil does not occur, leading to its increased levels in plasma.8
There are several clinical conditions that may warrant the use of sildenafil citrate. These include:
- Erectile dysfunction: Especially in men, this is by far the most common reason that people take sildenafil citrate.
- Pulmonary hypertension: This condition may present in childhood, known as primary or idiopathic pulmonary hypertension, or in adulthood. In both idiopathic and adult-onset pulmonary hypertension, the blood vessels in the pulmonary vasculature are constricted and cause a significant increase in pulmonary arterial pressure; this, if not corrected, may result in cardiac failure. By inhibiting the activity of PDE-5, sildenafil allows cGMP to work within the lungs and dilate the pulmonary vessels. This results in a decrease in pulmonary arterial pressure, thereby leading to decreased strain on the heart and a concomitant improvement in cardiac performance.9
- Raynaud's phenomenon: Characterized by transient vasospasm and digital ischemia upon exposure to cold, sildenafil has been shown to be of demonstrable benefit in managing instances of Raynaud's phenomenon that are not responsive to other vasodilator drugs.1011
There are certain conditions under which sildenafil citrate should be administered with caution or outright avoided. Some of these conditions are:
- Hypersensitivity: Sildenafil is absolutely contraindicated in individuals who have a demonstrated hypersensitivity to the drug or any of its components.9
- Nitrate therapy: Individuals who are not nitrate therapy should not be administered sildenafil citrate. Nitrates are potent vasodilators typically used in the management of cardiac conditions such as angina pectoris. Since sildenafil also a vasodilatory effect through its actions on cGMP, it can potentiate the effects of nitrates when used concurrently which may result in severe hypotension, syncope, and myocardial infarction.9
- Hepatic disease: Since sildenafil is metabolized by hepatic isozymes, hepatic diseases may lead to increased plasma levels of sildenafil and a prolongation of its effects. Care should be exercised when administering sildenafil to individuals with hepatic disease.9
- Renal disease: Similar to hepatic diseases, renal diseases may prolong the effects of sildenafil citrate due to increased plasma levels as a result of diminished renal excretion. Caution should also be exercised when administering sildenafil to individuals with renal diseases.9
- Visual abnormalities: There have been some instances of vision loss in individuals taking sildenafil citrate. The loss of vision is due to a reduction in blood flow to the optic nerve, a condition known as non-arteritic anterior ischemic optic neuropathy (NAION). Individuals with pre-existing visual disturbances may be administered sildenafil only when the benefits clearly outweigh the risks.912
- Cardiovascular disorders: Caution should be exercised when administering sildenafil to individuals with known cardiac disorders such as arrhythmias, aortic stenosis, heart failure, and myocardial infarction, among others.9
Pregnancy / Breastfeeding
Sildenafil citrate is classified as a Food and Drug Administration pregnancy category B drug. Studies have not demonstrated any evidence of birth defects, miscarriages, or adverse fetal or maternal outcomes when used in pregnancy. Insufficient data is available to make a clear determination about whether sildenafil citrate is expressed in breast milk and if there are any untoward effects.5
Adverse Reactions/Side Effects
Toxicity is one of the adverse effects that may be experienced in individuals on sildenafil therapy. The risk of developing toxic effects is especially higher in the presence of hepatic or renal disease, or in individuals on concurrent nitrate therapy. The toxic effects typically present as visual or cardiovascular disturbances. Other adverse reactions that may occur while on sildenafil therapy are tendon rupture, exfoliative dermatitis, hearing loss, seizures, and gastritis, among others.9
- Store this medication at 68°F to 77°F (20°C to 25°C) and away from heat, moisture and light. Keep all medicine out of the reach of children. Throw away any unused medicine after the beyond use date. Do not flush unused medications or pour down a sink or drain.
- 1. Goldstein, I., Burnett, A., Rosen, R.C., Park, P.W., Stecher, V.J.," The Serendipitous Story of Sildenafil: An Unexpected Oral Therapy for Erectile Dysfunction", Sexual Medicine Reviews, vol.7 issue 1, pp. 115-128. 2019. Available: https://www.sciencedirect.com/science/article/pii/S2050052118300830?via%3Dihub
- 2. a. b. c. d. e. McCullough, A.R., "Four-Year Review of Sildenafil Citrate", Reviews in Urology, vol.4 supp 3, S26 – S38. 2002. Available: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1476025/
- 3. "Sildenafil citrate", Prescribers Digital Reference. Available: https://www.pdr.net/drug-summary/Viagra-sildenafil-citrate-471
- 4. a. b. c. d. "Sildenafil", Drug Bank. Available: https://go.drugbank.com/drugs/DB00203
- 5. a. b. Dastjerdi, M.V., Hosseini, S., Bayani, L., "Sildenafil citrate and uteroplacental perfusion in fetal growth restriction", Journal of Research in Medical Sciences, vol.17 issue 7, pp.632-636. 2012. Available: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3685778/
- 6. Monte, G.L., Graziano, A., Piva, I., Marci, R., "Women taking the "blue pill" (sildenafil citrate): such a big deal?", Drug Design, Development, and Therapy. Vol.8, pp. 2251-2254. 2014. Available: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4232035/
- 7. Umrani, D.N., Goyal, R.K., "Pharmacology of Sildenafil Citrate", Indian Journal of Physiology and Pharmacology, vol.43 issue 2, pp. 160-164. 1999. Available: https://www.ijpp.com/IJPP%20archives/1999_43_2/160-164.pdf
- 8. a. b. c. Cheitlin, M.D., Hutter, A. M., Brindis, R.G., Ganz,P., Kaul,S., Russell, R.O., Zusman, R. M., Forrester, J.S., Douglas, P.S., Faxon, D.P., Fisher, J.D., Gibbons, R.J., Halperin, J.L., Hochman, J.S., Kaul, S., Weintraub, W.S., Winters, W.L., Wolk, M.J.," Use of Sildenafil (Viagra) in Patients With Cardiovascular Disease", Circulation, vol.99 issue 1. 1999. Available: https://www.ahajournals.org/doi/10.1161/01.CIR.99.1.168
- 9. a. b. c. d. e. f. g. h. "Sildenafil citrate", Prescibers' Digital Reference. Available: https://www.pdr.net/drug-summary/Viagra-sildenafil-citrate-471
- 10. Levien, T.L., "Advances in the treatment of Raynaud's phenomenon", Vascular Health and Risk Management, vol. 6, pp. 166-167. 2010. Available: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2860448/
- 11. "Sildenafil Effectively Treats Raynaud's Phenomenon", Emergency Medicine News: vol.28 Issue 3, pp. 29. 2006. Available: https://journals.lww.com/em-news/fulltext/2006/03000/sildenafil_effectively_treats_raynaud_s_phenomenon.35.aspx
- 12. Smith, B.P., Babos, M., "Sildenafil", StatPearls. 2020. Available: https://www.ncbi.nlm.nih.gov/books/NBK558978/