Cyclic nucleotide phosphodiesterase 5 and sildenafil: promises realized.

PDEs belong to a large superfamily [11 different gene families (PDEs 1–11)] of structurally related, functionally distinct, and highly regulated enzymes (Francis et al., 2001). Most PDE families comprise more than one gene ( 20 PDE genes), which generate multiple protein products ( 50 PDE proteins) via alternative mRNA splicing or use of different promoters/transcription initiation sites. Most cells contain representatives of multiple PDE gene families, but in different amounts, proportions, and subcellular locations. PDE5 isoforms, for example, are relatively abundant in vascular smooth muscle, including the pulmonary vasculature and corpus cavernosum of the penis, where they apparently regulate hydrolysis of cGMP pools that modulate vasodilation (Corbin and Francis, 2002; Corbin et al., 2002). This Perspective will focus on the confluence of our understanding of the biological and pharmacological roles of NO, cGMP, one specific PDE, PDE5, and PDE5 inhibitors in regulation of smooth muscle relaxation, penile erection, and treatment of erectile dysfunction, as well as on important contributions of Jackie Corbin, Sharron Francis, and others in elucidating the complex biology of PDE5 and thus facilitating the bench-to-bedside transition of PDE5 inhibitors (Francis et al., 2001; Corbin and Francis, 2002; Corbin et al., 2002). The emergence of the PDE5 inhibitor, sildenafil (Viagra), as effective therapy for erectile dysfunction finally realizes the promise and potential of specific PDEs to serve as important therapeutic targets and of “family-specific” PDE inhibitors to function as safe and efficacious drugs. Mammalian PDEs exhibit a common structural organization, with a conserved catalytic domain ( 250–300 amino acids) in the C-terminal portion of the molecules and divergent regulatory domains and modules in N-terminal portions. The catalytic core, more highly conserved among members of each gene family than among different gene families, contains a signature motif, common to all PDEs, and consensus metal-binding domains (Francis et al., 2001). In addition to common structural elements, the catalytic core contains family-specific sequences, responsible for differences in substrate affinities, catalytic activities, and sensitivities to specific inhibitors. Some PDE families are relatively specific for cAMP (PDEs 4, 7, 8) or for cGMP (PDEs 5,6,9); others hydrolyze both (PDEs 1, 2, 3, 10, 11) (Francis et al., 2001). Although methylxanthines inhibit almost all PDEs, selective inhibitors that are relatively specific for individual families are available (i.e., for PDEs 1–6). Sildenafil, vardenafil, and tadalafil are potent, selective PDE5 inhibitors (Francis et al., 2001; Corbin and Francis, 2002; Corbin et al., 2002). N-terminal portions of PDE molecules are highly divergent, containing structural determinants that allow different PDEs to respond selectively to specific regulatory signals (Francis et al., 2001). These regulatory regions and modules include, for example, sites and domains that are subject to different types of covalent modification (e.g., phosphorylation) or that interact with allosteric ligands, protein partners, or molecular scaffolds and thereby regulate catalytic activity, protein/protein interactions, and/or subcellular compartmentation. Five PDE families (PDEs 2, 5, 6, 10, 11) contain homologous so-called GAF domains. In three families (PDEs 2, 5, and 6) classified as cGMP-binding PDEs, GAF domains bind cGMP with high affinity but without identical functional consequences (Francis et al., 2001). Signaling pathways, in general, include mechanisms for negative feedback control. From this perspective, PDEs are critical homeostatic regulators of intracellular cyclic nucleotide concentrations. For example, acute increases in cAMP activate cAMP-dependent protein kinase, phosphorylation/ activation of PDE3 and PDE4, and enhanced destruction of cAMP. Long-term elevation of cAMP also provides negative feedback, by increasing transcription of PDE3 and PDE4 genes, indirectly resulting in increased enzymatic activities (Francis et al., 2001). With respect to PDE5, Corbin and Francis, their cowork-