(From Wikipedia, the free encyclopedia)
Tadalafil is a drug used to treat male erectile dysfunction (impotence). It was developed by the biotechnology firm ICOS and marketed worldwide by Eli Lilly and Company under the brand name Cialis.
In the United States, tadalafil has Food and Drug Administration approval and became available in December, 2003 as the third impotence pill after sildenafil (Viagra) and vardenafil (Levitra). Due to its 36-hour effect it is also known as the weekend pill. It should be noted that the drug has not been formally studied in regard to multiple sexual attempts during a 36 hour period.
Tadalafil is also currently undergoing Phase III clinical trials for the treatment of pulmonary hypertension.
History
The history of Cialis cannot be discussed without mentioning Pfizer’s drug, Viagra. The FDA’s approval on March 27, 1998, led this prescription drug, Viagra, to a ground breaking success in just the first year of introduction as Pfizer sold drugs worth over a billion dollars. However, things changed considerably for the giant of erectile dysfunction drugs when the FDA also approved Levitra on August 19, 2003, and Cialis on November 21, 2003. In 1993 the drug company Icos began studying IC351, which is a PDE5 enzyme inhibitor, and this is basically the process through which the erectile dysfunction drugs work. In 1994, Pfizer scientists discovered that sildenafil citrate, which is a white crystalline powder that temporarily normalizes erectile function of the penis by blocking an enzyme known to inhibit the production of a chemical that causes erections, caused the heart patients that were participating in a clinical study of a heart medicine to have erections. Although the scientists were not testing the chemical compound IC351 for erectile dysfunction, the compound seemed to have a side effect which could potentially be worth millions, if not billions of dollars. Soon Icos received its very first patent in 1994 on IC351, and the clinical trials of phase 1 took place in 1995. In 1997, phase 2 clinical studies began and Icos performed its first study on patients with erectile dysfunction. Phase 2 lasted about two years, and after that phase 3 began.
In 1998, ICOS Corporation, and Eli Lilly and Company, commercialized the drug for erectile dysfunction, and two years later they filed a new drug application with the U.S. Food and Drug Administration for IC351; the only difference was that this time they decided to call the drug Cialis. In May of 2002, Icos and Eli Lilly and Company reported to the American Urological Association that the phase 3 tests show that Cialis works for up to 36 hours, and one year later Icos and Eli Lilly and Company received the U.S. FDA’s approval for Cialis. One advantage that Cialis has over Viagra is that tadalafil has a half-life of 17.5 hours[2] (and thus Cialis is advertised to work for up to 36 hours, even if by that time there is still about one quarter of the absorbed dose in the body) as compared to 4 hours half-life for sildenafil (Viagra).
Eli Lilly purchased ICOS corporation for $2.1 billion dollars in 2006.
Mechanism of action
Tadalafil works by inhibiting PDE5, an enzyme found primarily in the arterial wall smooth muscle tissue of the penis and the lungs. A 20 mg dose of tadalafil is comparable to a 100 mg dose of sildenafil (Viagra). By inhibiting PDE5, tadalafil relaxes blood vessels in the penis, thereby increasing blood flow and aiding in erection.
Part of the physiological process of erection involves the parasympathetic nervous system causing the release of nitric oxide (NO) in the corpus cavernosum of the penis. NO binds to the receptors of the enzyme guanylate cyclase which results in increased levels of cyclic guanosine monophosphate (cGMP), leading to smooth muscle relaxation (vasodilation) in the corpus cavernosum, resulting in increased inflow of blood and an erection.
Tadalafil is a potent and selective inhibitor of cGMP specific phosphodiesterase type 5 (PDE5) which is responsible for degradation of cGMP in the corpus cavernosum. The molecular structure of tadalafil is similar to that of cGMP and acts as a competitive binding agent of PDE5 in the corpus cavernosum, resulting in more cGMP and better erections. Without sexual stimulation, and therefore lack of activation of the NO/cGMP system, tadalafil should not cause an erection. Other drugs that operate by the same mechanism include sildenafil (Viagra®) and vardenafil (Levitra®).
Tadalafil is currently undergoing clinical trials for the treatment of pulmonary hypertension. The clinical trials are based on tadalafil’s inhibitiong of PDE5. It is hoped that by inhibiting this enzyme, tadalafil will prove effective in opening up blood vessels in the lungs, lowering pulmonary arterial resistance and pressure, and thus reducing the workload of the right ventricle of the heart.
Side effects
The most common side effects when using tadalafil are headache, indigestion, back pain, muscle aches, flushing, and stuffy or runny nose. These side effects usually go away after a few hours. Back pain and muscle aches can occur 12 to 24 hours after taking the drug, and the symptom usually disappears after 48 hours.
In May 2005, the U.S. Food and Drug Administration found that tadalafil (along with other PDE5 inhibitors) could lead to vision impairment in certain patient groups, including diabetics. An investigation is currently ongoing.
Drug interactions
Since PDE5 inhibitors such as tadalafil may cause transiently low blood pressure (hypotension), organic nitrates should not be taken for at least 48 hours after taking the last dose of tadalafil. Using organic nitrates (such as the sex drug amyl nitrite) within this timeframe may increase the risk of life-threatening hypotension.
Since people who have taken tadalafil within the past 48 hours cannot take organic nitrates to relieve angina (such as glyceryl trinitrate spray), these patients should seek immediate medical attention if they experience anginal chest pain. In the event of a medical emergency, paramedics and medical personnel should be notified of any recent doses of tadalafil.
