Inhibition of cyclic GMP-binding cyclic GMP-specific phosphodiesterase (type 5) by sildenafil and related compounds

The cGMP-binding cGMP-specific phosphodiesterase (PDE5) degrades cGMP and r egulates the intracellular level of cGMP in many tissues, including the smo oth muscle of the corpus cavernosum of the penis. Sildenafil (Viagra), a sp ecific PDE5 inhibitor, promotes penile erection by blocking the activity of PDE5, which causes cGMP to accumulate in the corpus cavernosum. In the pre sent study, sildenafil, like other PDE5 inhibitors, stimulates cGMP binding to the allosteric sites of PDES by interacting at the catalytic site of th is enzyme, but the drug does not complete with cGMP for binding at the allo steric sites. Both sildenafil and zaprinast are competitive inhibitors of P DE5, and double-inhibition analysis shows that these two inhibitors added t ogether interact with the catalytic site of PDES in a mutually exclusive ma nner. After site-directed mutagenesis of each of 23 conserved amino acid re sidues in the catalytic domain of PDE5, the pattern of changes in the IC50 values for sildenafil or UK-122764 is similar to that found for zaprinast. However, among the three inhibitors, sildenafil exhibits the most similar p attern of changes in the IC,, to that found for the affinity of cGMP, imply ing similar interactions with the catalytic domain. This may explain in par t the stronger inhibitory potency of sildenafil for wild-type PDE5, compare d with the other inhibitors [sildenafil (K-i = 1 nM) > UK-122764 (K-i = 5 n M) > zaprinast (K-i = 130 nM)]. The affinity of each of these inhibitors fo r PDE5 is much higher than that of cGMP itself (K-m = 2000 nM). It is concl uded that residues such as Tyr(602), His(607), His(643) and Asp(754) may fo rm important interactions for sildenafil in PDE5, but because these amino a cids are conserved in all mammalian PDEs, the selectivity and potency of si ldenafil is likely to be provided by a nonconserved residue or residues in the PDE5 catalytic domain.