Binding of Tritiated Sildenafil, Tadalafil, or Vardenafil to the Phosphodiesterase-5 Catalytic Site Displays Potency, Specificity, Heterogeneity, and cGMP Stimulation

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Abstract
Sildenafil, tadalafil, and vardenafil each competitively inhibit cGMP hydrolysis by phosphodiesterase-5 (PDE5), thereby fostering cGMP accumulation and relaxation of vascular smooth muscle. Biochemical potencies (affinities) of these compounds for PDE5 determined by IC50, KD (isotherm), KD (dissociation rate), and KD (½ EC50), respectively, were the following: sildenafil (3.7 ± 1.4, 4.8 ± 0.80, 3.7 ± 0.29, and 11.7 ± 0.70 nM), tadalafil (1.8 ± 0.40, 2.4 ± 0.60, 1.9 ± 0.37, and 2.7 ± 0.25 nM); and vardenafil (0.091 ± 0.031, 0.38 ± 0.07, 0.27 ± 0.01, and 0.42 ± 0.10 nM). Thus, absolute potency values were similar for each inhibitor, and relative potencies were vardenafil ≫ tadalafil > sildenafil. Binding of each 3H inhibitor to PDE5 was specific as determined by effects of unlabeled compounds. 3H Inhibitors did not bind to isolated PDE5 regulatory domain. Close correlation of EC50 values using all three 3H inhibitors competing against one another indicated that each occupies the same site on PDE5. Studies of sildenafil and vardenafil analogs demonstrated that higher potency of vardenafil is caused by differences in its double ring. Exchange-dissociation studies revealed two binding components for each inhibitor. Excess unlabeled inhibitor did not significantly affect 3H inhibitor dissociation after infinite dilution, suggesting the absence of subunit-subunit cooperativity. cGMP addition increased binding affinity of [3H]tadalafil or [3H]vardenafil, an effect presumably mediated by cGMP binding to PDE5 allosteric sites, implying that either inhibitor potentiates its own binding to PDE5 in intact cells by elevating cGMP. Without inhibitor present, cGMP accumulation would stimulate cGMP degradation, but with inhibitor present, this negative feedback process would be blocked.