IC50: N/A for Nortadalafil; 5 nM for Tadalafil

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.

In vitro: Tadalafil has been identified as a highly potent PDE5 inhibitor (IC50 = 5 nM) with high selectivity for PDE5 vs PDE1-4 and PDE6. It displays 85-fold greater selectivity vs PDE6 than sildenafil [1].

In vivo: Tadalafil showed profound and long-lasting blood pressure lowering activity (30 mmHg/>7 h) in the spontaneously hypertensive rat model after oral administration (5 mg/kg) [1].

Clinical trial: Tadalafil once-daily provides efficacy comparable to on-demand dosing regimens with PDE-5 inhibitors, is well tolerated and allows patients and their partners to disconnect the administration of medication from sexual activity, thereby enabling them to return to the sex-life they had before the onset of erectile dysfunction [2].

References:
[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.