New Competition for Viagra®

Volume VII, Number 2 | March/April 2004
Christina Collins, Pharm.D. Candidate

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Erectile Dysfunction

Erectile dysfunction (ED) is a common disorder that affects up to 30 million men in the United States alone.^1,2 ED is defined as the inability to achieve or maintain a penile erection sufficient for sexual function.³ The prevalence of this disorder increases with age and predominantly affects men over the age of 40.^4,5 In addition to age, there are some disease states that predispose men to develop ED, including hypertension, diabetes mellitus, and atherosclerosis. Smoking, excessive alcohol intake, and some medications (e.g., thiazide diuretics, calcium channel blockers, beta-blockers, digoxin, selective serotonin reuptake inhibitors, and tricyclic antidepressants) may also contribute to the development of ED.^4,6 ED may be categorized into three types based on the causative factors: organic, psychogenic, and multifactorial.^4,6 Organic causes include diabetes, hypertension, spinal cord injuries, and some medications. Depression, psychological stress, relationship problems, and performance anxiety are all of psychogenic origin.⁵ Multifactorial causes are any combination of the above.

Sildenafil (Viagra®), Vardenafil (Levitra®), Tadalafil (Cialis™)

With greater public awareness and discussion of ED, the demand for new and improved treatments has increased tremendously. The phosphodiesterase type 5 (PDE-5) inhibitor sildenafil (Viagra®) has become the drug of choice for treatment of ED since it reached the market in March of 1998. This is due to sildenafil's convenience and tolerability compared to previous therapies.⁵ It has been shown to be beneficial in the treatment of organic and psychogenic ED.^5,6 Sildenafil does have some disadvantages such as side effects and a relatively short duration of action; consequently, the search for the ideal drug to treat ED has continued. This sustained pursuit has led to the development of two new PDE-5 inhibitors: vardenafil (Levitra®; Bayer Pharmaceuticals Corporation in cooperation with GlaxoSmithKline) was approved by the Food and Drug Administration (FDA) on August 19, 2003, and tadalafil (Cialis™; Eli Lilly Corporation) was FDA-approved in November 2003.² These new agents possess distinguishing characteristics that differentiate themselves from each other and sildenafil.

Pharmacology

Sildenafil, vardenafil, and tadalafil are all PDE-5 inhibitors indicated for the treatment of ED. They do not directly cause penile erections, however, they affect the response to sexual stimulation. During sexual stimulation, nitric oxide is produced, which then activates cyclic guanosine monophosphate (cGMP). This results in smooth muscle relaxation and increased blood flow.⁵ Phosphodiesterases, however, catalyze the breakdown of cGMP to its corresponding monophosphate, GMP. There are several PDE isoenzymes, and PDE-5 is the isoenzyme present in highest concentrations in the smooth muscle of the corpora cavernosum of the penis. The PDE-5 inhibitors, sildenafil, vardenafil, and tadalafil mimic the structure of cGMP and competitively inhibit its breakdown by PDE-5 in the corpus cavernosum and related vessels, leading to increased dilatation and blood flow. This allows the induction of an erection during sexual stimulation.⁷

Pharmacodynamics

Sildenafil, vardenafil, and tadalafil all affect ED through the same basic mechanism of inhibiting PDE-5 but have differing potencies and affinities for each of the 11 PDE isoenzymes. Vardenafil is the most potent PDE-5 inhibitor, followed by sildenafil and tadalafil.⁷ However, there is no current evidence that greater drug potency has produced improved clinical efficacy. The affinities vary for the other PDE isoenzymes, and thus may explain the potential differences in their side effect profiles.

Pharmacokinetics

While sildenafil, vardenafil, and tadalafil are all hepatically metabolized, there are variances in their pharmacokinetic profiles. Sildenafil is approximately 75% metabolized, predominantly by cytochrome P450 (CYP) 3A4 with contribution from CYP 2C9.^7,8 Vardenafil is primarily metabolized by CYP 3A4 and secondarily by CYP 3A5 and 2C isoforms.^7,9 Tadalafil is also metabolized by CYP 3A4.^2,10 With CYP 3A4 being the major enzyme responsible for the metabolism of all three agents, they will potentially have many of the same drug interactions with some possible contributions from the minor enzymatic pathways.

Other pharmacokinetic differences include time to maximum concentration and half-life. The slightly faster time to maximum concentration of vardenafil (0.7 hours) compared to sildenafil (0.8 hours) is likely clinically irrelevant, while tadalafil has the slowest onset and may require 2 hours to reach maximum concentration. Tadalafil, however, has the longest half-life (17.5 hours), followed by vardenafil (4 hours) and sildenafil (3.7 hours).^7-10 These differences will account for variation in dosage timing and duration of effect. The considerably longer duration of effect for tadalafil will likely allow less frequent dosing and greater impulsiveness between partners, but also could potentially prolong adverse effects.

Efficacy

There are no comparative trials evaluating sildenafil, vardenafil, and tadalafil; therefore, it is difficult to compare the individual trials available due to differences in study design and methodology. However, all three agents have been proven effective in treating ED in clinical trials based on the Erectile Function domain score of the International Index of Erectile Function, a sexual function questionnaire commonly used throughout the studies. Efficacy rates for the PDE-5 inhibitors vary widely throughout the literature with a range of 57% to 83%.^5,11-13 Additionally, they have all shown efficacy in the treatment of ED in men with diabetes mellitus; however, no particular agent has yet to be recommended for patients with cardiovascular disease.^{2,8,10,12,14,15} Comparative trials are required to determine if a specific agent possesses better efficacy.

Drug Interactions

There are drug-drug interactions with each of these medications, many of which involve the same drugs due to CYP 3A4 being the major enzymatic pathway for all of these agents. Due to the potential for severe hypotension, sildenafil and organic nitrates (e.g., nitroglycerin, isosorbide dinitrate [Isordil®, Sorbitrate®], and isosorbide mononitrate [Imdur™, ISMO®, Monoket®]) are contraindicated (i.e., should not be taken together), and sildenafil doses > 25 mg should not be used within 4 hours of taking an alpha-blocker (e.g., prazosin [Minipress®], terazosin [Hytrin®], and doxazosin [Cardura®]). Sildenafil is a substrate of CYP 3A4 and 2C8/9 as well as an inhibitor of CYP 1A2, 2C8/9, 2C19, 2D6, 2E1, and 3A4; therefore, increased effects and possible toxicity of sildenafil occur when used concomitantly with drugs that inhibit these enzymes (e.g., cimetidine [Tagamet®], erythromycin, ketoconazole [Nizoral®], itraconazole [Sporanox®], amprenavir [Agenerase®], indinavir [Crixivan®], saquinavir [Fortovase®], and ritonavir [Norvir®]). Enzyme-inducing agents, such as St. John's wort and rifampin, may have the opposite effect and cause decreased sildenafil levels. Drug-food interactions with sildenafil can also occur. The rate and extent of absorption of sildenafil is reduced when taken with a high-fat meal and grapefruit juice may cause increased serum concentrations of sildenafil, consequently, concurrent use should be avoided.^8-10,16-18

Concomitant use of vardenafil is also contraindicated with alpha-blockers and organic nitrates due to the potential for severe hypotension. Vardenafil is a substrate of CYP 3A4, and to a minor extent the 2C isoforms, leading to interactions with drugs that induce or inhibit these enzymes. Increased serum concentrations of vardenafil may occur with CYP 3A4 inhibitors; therefore, dose reductions of vardenafil are recommended with erythromycin, indinavir (Crixivan®), itraconazole (Sporanox®), ketoconazole (Nizoral®), and ritonavir (Norvir ®). Other enzyme inhibitors that may increase vardenafil levels are amiodarone (Cordarone®), cimetidine (Tagamet®), clarithromycin (Biaxin®), delavirdine (Rescriptor®), diltiazem (e.g., Cardizem®, Cardizem® SR, Cardizem® CD, Cardizem® LA, Cartia XT™, Dilacor® XR, Diltia XT™, Taztia XT™, and Tiazac®), fluoxetine (Prozac®), fluvoxamine (Luvox®), nefazodone (Serzone®), nevirapine (Viramune®), saquinavir (Fortovase®), verapamil (Calan®, Covera-HS™, Isoptin®SR, Verelan®), and grapefruit juice, but there are no current recommendations for dosage adjustments with these agents.^8-10,16-18

Tadalafil is also metabolized by CYP 3A4, resulting in the potential for an interaction with any drug that induces or inhibits this isoenzyme. Ketoconazole (Nizoral®) and ritonavir (Norvir®) have both been shown to increase tadalafil levels; other CYP 450 inhibitors (e.g., erythromycin, itraconazole [Sporanox®], and grapefruit juice), however, have yet to be evaluated, but it is likely that they will also increase tadalafil levels. Patients taking potent inhibitors of CYP 3A4 (e.g., ketoconazole [Nizoral®] or ritonavir [Norvir®]) will require a dose adjustment of tadalafil. The dose should not exceed 10 mg and should not be administered more frequently than every 72 hours. Additionally, tadalafil is thought to increase the hypotensive effect of antihypertensive agents (e.g., calcium channel blockers, diuretics, angiotensin-receptor blockers, ACE-inhibitors, or adrenergic receptor-blocking agents) and therefore should be used with caution. Concomitant administration of tadalafil with the alpha-blockers, except tamsulosin (Flomax®) is contraindicated. Tadalafail was evaluated in combination with doxazosin (Cardura®) and resulted in a significant increase in the blood pressure-lowering effect of doxazosin. Similarly to the other agents, tadalafil is also contraindicated with the use of organic nitrates. This combination can cause severe, possibly fatal, hypotensive episodes.^2,10,18

Warnings/Contraindications

All three agents may potentially cause a prolonged erection greater than 4 hours and priapism, a painful erection greater than 6 hours in duration. Medical assistance should be sought if an erection lasts longer than 4 hours. Additionally, if priapism is not immediately treated, penile tissue damage and permanent loss of potency may result.^8-10

There do not appear to be any major differences in contraindications among sildenafil, vardenafil, and tadalafil. Sildenafil is contraindicated with known hypersensitivity to the drug or concurrent use of organic nitrates.^8-10,16-18 Both vardenafil and tadalafil have the same contraindication and, additionally, both are contraindicated with concomitant use of alpha-blockers (with the exception of tamsulosin with tadalafil).^8-10,16-18

Adverse Effects

Due to the presence of PDE-5 in all vascular smooth muscle and inhibition of other PDE isoenzymes, all of these medications may produce adverse effects^.7 The most common adverse effects with sildenafil, vardenafil, and tadalafil are listed in Table 1.

Table 1. Adverse Effects Reported in > 2% of patients.^{8-10,16-18,20}

CPK = creatine phosphokinase

The major adverse effect that may differ between these agents is ocular effects. It is believed that vardenafil and tadalafil may produce less abnormal vision than sildenafil due to their greater selectivity for PDE-5 compared to PDE-6, the isoenzyme responsible for ocular effects. Any clinically significant difference in effect will need to be clarified by post-marketing surveillance of vardenafil and tadalafil.^2,12

Dosing

While sildenafil and vardenafil have similar recommended dosing, the dosing schedule for tadalafil may differ due to its greater time to maximum concentration and longer half-life. Dosing guidelines are listed in Table 2.^2,13

Table 2. Dosage Parameters for Sildenafil, Vardenafil, and Tadalafil ^8-10

Pregnancy

The PDE-5 inhibitors are not FDA-approved for use in women or children. All of these agents are classified as pregnancy-risk category B.^8-10 Pregnancy category B is defined as either animal-reproduction studies have not demonstrated a fetal risk but there are no controlled studies in pregnant women or animal-reproduction studies have shown an adverse effect (other than a decrease in fertility) that was not confirmed in controlled studies in women in the first trimester (and there is no evidence of a risk in later trimesters).²¹

Cost

The price of sildenafil 25- or 50-mg tablets is $90.99 for ten tablets or $259.97 for 30 tablets, while the 100 mg strength of sildenafil is $93.99 for ten tablets or $267.97 for 30 tablets. The price of vardenafil 2.5-, 5-, 10-, or 20-mg tablets is $87.99 for ten tablets or $249.99 for 30 tablets. Finally, the price of tadalafil 5-, 10-, or 20-mg tablets is $89.99 for ten tablets or $259.97 for 30 tablets.^22-24

Investigational Indications

Recently, sildenafil has been investigated for the treatment of pulmonary hypertension.²⁵ It has been shown to decrease pulmonary artery pressure and pulmonary vascular resistance, decrease symptoms, and improve New York Heart Association functional class in patients with pulmonary hypertension. No trials assessing the ability of vardenafil or tadalafil to treat pulmonary hypertension have been reported. The place of sildenafil and the other PDE-5 inhibitors in treating pulmonary hypertension remains to be elucidated.

Researchers have also studied sildenafil for the treatment of sexual dysfunction in women.26,27 While sil-denafil is currently not FDA-approved for this indication, clinical trials have shown improvement of sexual function for some women. Additional studies are necessary to establish clinical efficacy in a larger patient population and to define which female patients may receive benefit from treatment with sildenafil.

Conclusion

Sildenafil, vardenafil, and tadalafil are all effective treatments for ED by inhibiting PDE-5. Some differences exist that may clinically differentiate these agents. Tadalafil has a longer time to maximum concentration and a longer half-life compared to sildenafil and vardenafil. This may allow greater spontaneity and less frequent dosing of tadalafil but may also prolong adverse effects. Additionally, tadalafil and vardenafil have greater selectivity for PDE-5 than sildenafil; this may produce a lower incidence of ocular adverse effects. These differences will need to be substantiated in post-marketing surveillance to provide evidence of their clinical significance. When selecting the most suitable PDE-5 inhibitor for a patient, sildenafil has the greatest amount of data supporting its efficacy and safety long-term and will most likely remain the drug of choice until more long-term data are available for the other agents. Sildenafil, vardenafil, and tadalafil are currently not on the Cleveland Clinic Foundation Formulary of Accepted Drugs.

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References

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