Tadalafil is an orally administered drug for treating impotence (erectile dysfunction), that initially was developed by the biotechnology company ICOS, and then developed and marketed world-wide as Cialis, by Lilly ICOS, LLC, the joint venture of ICOS Corporation and Eli Lilly and Company. Cialis tablets, in 5mg, 10mg, and 20mg doses, are yellow, film-coated, and almond-shaped.

In December of 2003, the Food and Drug Administration approved tadalafil (as Cialis) for sale in the U.S. as the third impotence prescription drug after sildenafil (Viagra) and vardenafil (Levitra). Cialis's 36-hour effectiveness earned it the nickname, "The Weekend Pill"; like sildenafil and vardenafil, tadalafil is recommended as a once-daily medication, only. Moreover, besides impotence, tadalafil currently is in Phase 3 clinical trials for treating pulmonary arterial hypertension.

The history of Cialis cannot be discussed without mentioning Viagra (sildenafil), Pfizer's impotence drug. The FDA's approval of Viagra on 27 March 1998 was a ground-breaking commercial event for the treatment of impotence; sales exceeded a billion dollars. Subsequently, the FDA approved Levitra (vardenafil) on 19 August 2003, and Cialis (tadalafil) on 21 November 2003.

In 2007, Eli Lilly and Company bought the ICOS Corporation for $2.3 billion dollars. As a result, Eli Lilly owned Cialis and then closed the ICOS operations, ending the joint venture and firing most of ICOS's ca. 500 employees, except for 127 employees of the ICOS biologics facility, which subsequently was bought by CMC Biopharmaceuticals A/S(CMC).

Although Sildenafil (Viagra) and Vardenafi (Levitra), and Tadalafil (Cialis) all work by inhibiting PDE5, Tadalafil's pharmacologic distinction is its longer half-life (17.50 hours) — compared to Viagra (4.0–5.0 hours) and Levitra (4.0–5.0 hours) — resulting in longer duration of action, and so partly responsible for "The Weekend Pill" sobriquet. Furthermore, the longer half-life is the basis for current investigation of Tadalafil's daily therapeutic use in relieving pulmonary arterial hypertension. Currently, sildenafil (trade name Revatio) is approved in several world regions as a thrice-daily therapy for pulmonary arterial hypertension.

Moreover, Tadalafil currently is in clinical trials for treating pulmonary arterial hypertension. In some patients, there exists an imbalance of the PDE5/NO system in the pulmonary vasculature that favours selective vasoconstriction of the pulmonary arteries. Investigation of Tadalafil in this disease presumes that inhibiting PDE5 will effect pulmonary artery vasodilation, thus lowering pulmonary arterial pressure and pulmonary vascular resistance. These physiologic changes may then reduce the workload of the right heart's right ventricle. Right heart failure and pulmonary oedema are the principal consequences of pulmonary arterial hypertension.

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