the cgmp-specific pde5 is the major pde isozyme in the corpus cavernosum, and controls penile erection. the pde5 inhibitors used in the clinic amplify the no-ccmp pathway and enhance the normal process leading to penile erection.nortadalafil is demethyl tadalafil, which is a pde5 inhibitor currently marketed in pill form for treating erectile dysfunction under the name cialis and for the treatment of pulmonary arterial hypertension under the name adcirca.
[1] daugan a, grondin p, ruault c, le monnier de gouville ac, coste h, linget jm, kirilovsky j, hyafil f, labaudinière r. the discovery of tadalafil: a novel and highly selective pde5 inhibitor. 2: 2,3,6,7,12,12a-hexahydropyrazino[1',2':1,6]pyrido[3,4-b]indole-1,4-dione analogues. j med chem. 2003;46(21):4533-42.
[2] porst h, hell-momeni k, büttner h. chronic pde-5 inhibition in patients with erectile dysfunction - a treatment approach using tadalafil once-daily. expert opin pharmacother. 2012 jul;13(10):1481-94. doi: 10.1517/14656566.2012.693162.