studies in vitro have shown that sildenafil is selective for pde5. its effect is more potent on pde5 than on other known phosphodiesterases (10-fold for pde6, >80-fold for pde1, >700-fold for pde2, pde3, pde4, pde7, pde8, pde9, pde10, and pde11). the approximately 4,000-fold selectivity for pde5 versus pde3 is important because pde3 is involved in control of cardiac contractility. sildenafil is only about 10-fold as potent for pde5 compared to pde6, an enzyme found in the retina which is involved in the phototransduction pathway of the retina. this lower selectivity is thought to be the basis for abnormalities related to color vision observed with higher doses or plasma levels (see pharmacodynamics ).
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