Tadalafil was discovered by Glaxo Wellcome (now GlaxoSmithKline) under a partnership between Glaxo and ICOS to develop new drugs that began in August 1991. In 1993, the Bothell, Washington biotechnology company ICOS Corporation began studying compound IC351, a phosphodiesterase type 5 (PDE5) enzyme inhibitor. In 1994, Pfizer scientists discovered that sildenafil, which also inhibits the PDE5 enzyme, caused penile erection in men participating in a clinical study of a heart medicine. Although ICOS scientists were not testing compound IC351 for treating ED, they recognized its potential usefulness for treating that disorder. Soon, in 1994, ICOS received a patent for compound IC351 (structurally unlike sildenafil and vardenafil), and Phase 1 clinical trials began in 1995. In 1997, the Phase 2 clinical studies were initiated for men experiencing ED, then progressed to the Phase 3 trials that supported the drug's FDA approval. Although Glaxo had an agreement with ICOS to share profits 50/50 for drugs resulting from the partnership, Glaxo let the agreement lapse in 1996 as the drugs developed were not in the company's core markets.