General Information

Cialis (tadalafil) is a selective inhibitor of cyclic guanosine monophosphate (cGMP)-specific phosphodiesterase type 5 (PDE5) for the treatment for erectile dysfunction. Tadalafil along with sildenafil (Viagra, Pfizer) and vardenafil (Levitra, Bayer), belongs to a class of smooth muscle relaxants called phosphodiesterase (PDE) inhibitors.

It is approved for the treatment of erectile dysfunction for up to 36 hours after dosing.

The recommended dose of Cialis is 10 mg, taken prior to anticipated sexual activity. The dose may be increased to 20 mg or decreased to 5 mg.

Clinical Results

FDA approval of Cialis was based on 2 U.S. and 5 international randomized, multicenter, double-blinded, placebo-controlled trials. In total, Cialis has been evaluated in over 4,000 subjects. The primary outcome measures were the Erectile Function (EF) domain of the International Index of Erectile Function (IIEF) and Questions 2 and 3 from Sexual Encounter Profile (SEP). The IIEF is a 4-week recall questionnaire administered at the end of a treatment-free baseline period and at follow-up visits after enrollment. The IIEF EF domain has a 30-point total score, where higher scores reflect better erectile function. SEP is a diary in which subjects recorded each sexual attempt made during the study.

Results of the 2 U.S. trials demonstrated meaningful and statistically significant improvements in all efficacy variables, with positive effects being maintained over time. The trial enrolled 402 subjects, with an average age of 59 years, experiencing erectile dysfunction (ED). Cialis doses ranged from 2.5 to 20 mg, once daily as needed.

Results of the 5 international studies demonstrated statistically significant improvements in all 3 primary efficacy variables. Treatment effects did not diminish over time. In addition, data showed patient-reported improvement in erections across subjects with ED of all degrees of severity while taking Cialis, compared to placebo. The efficacy and safety studies enrolled 1112 subjects, with a mean age of 59. The trials included subjects with ED and with multiple conditions, including diabetes mellitus, hypertension, and other cardiovascular diseases.

Mechanism of Action

Erections are caused by increased blood flow from the relaxation of penile arteries and corpus cavernosal smooth muscle. Cyclic GMP (cGMP) causes smooth muscle relaxation and increased blood flow into the corpus cavernosum. During sexual stimulation, tadalafil will inhibit phosphodiesterase type 5 PDE5, which increases the amount of cGMP in smooth muscles cells thus enhancing erectile function.

Preclinical studies showed that tadalafil is over 10,000-fold more potent for PDE5 than for PDE1, PDE2, PDE4, and PDE7 enzymes, which are found in the heart, brain, blood vessels, liver, leukocytes, skeletal muscle, and other organs.

Literature References

Brock G, McMahon CG, Chen KK, et al: Efficacy and safety of tadalafil in the treatment of erectile dysfunction: results of integrated analyses. J Urol 168: 1332:1336, 2002.

Emmick JT, Stuewe SR, Mitchell M. Overview of the cardiovascular effects of tadalafil. Eur Heart J 2002; 4:H32:47.

Gresser U, Gleiter CH. Erectile dysfunction: comparison of efficacy and side effects of the PDE-5 inhibitors sildenafil, vardenafil and tadalafil. Review of literature. Eur J Med 2002; 7:435-46.

Solomon H, Han JW, Jackson G. Erectile dysfunction and the cardiovascular patients: endothelial dysfunction is the common denominator. Heart 2003; 89:251-4.