Sildenafil ODT

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.^[2][3][4]

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.^[5][6]

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.^[2][4][7]

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]

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.^[3]

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.^[3]

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.^[3]

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.^[3]

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.^[3][8]

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.^[3]

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.^[3]

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.^[5]

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]

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. 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/?drugLabelId=Viagra-sildenafil-citrate-471
4. “Sildenafil”, Drug Bank. Available: https://go.drugbank.com/drugs/DB00203
5. 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. Smith, B.P., Babos, M., “Sildenafil”, StatPearls. 2020. Available: https://www.ncbi.nlm.nih.gov/books/NBK558978/