Phosphodiesterases (PDEs) are a superfamily of enzymes. This superfamily is further subdivided into 11 families, PDE1 - PDE11, on the basis of regulatory properties, amino acid sequences, substrate specificities, pharmacological properties and tissue distribution. Their function is to degrade intracellular second messengers such as cyclic adenine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP) which leads to several biological processes like effect on intracellular calcium level by the Ca2+ pathway.
Phosphodiesterase 5 (PDE5) is widely expressed in several tissues in the body for example brain, lung, kidney, urinary bladder, smooth muscle and platelets. It is possible to prevent cGMP hydrolysis by inhibiting PDE5 and therefore treat diseases associated with low cGMP levels; because of this, PDE5 is an ideal target for the development of inhibitors. The therapeutic effects of PDE5 inhibition have been demonstrated in several cardiovascular conditions, chronic kidney disease and diabetes mellitus.
The major PDE5 inhibitors (a subset of the phosphodiesterase inhibitors) are sildenafil, tadalafil, vardenafil, and avanafil, and although all share the same mechanism of action each has unique pharmacokinetic and pharmacodynamic properties which dictate their suitability in various conditions and their side effect profile.
General The human genome contains at least 21 genes involved in determining the intracellular levels of cAMP and cGMP by the expression of phosphodiesterase proteins or PDE's. These PDE's are grouped into at least 11 functional subfamilies, named PDE1-PDE11. PDEs are enzymes that hydrolyze cyclic adenosine 3,5-monophosphate (cAMP) and cyclic guanosine 3,5-monophospahate (cGMP), which are intracellular second messengers, into AMP and GMP. These second messengers control many physiological processes. The cAMP is formed from ATP by the enzyme adenylyl cyclase and cGMP is formed from GTP by the enzyme guanylyl cyclase which are either membrane bound or soluble in the cytosol. When soluble it functions as a receptor for nitric oxide (NO) (see figure 1). Formation of cGMP initiates several reactions in the body including influence on cGMP ion channels, cGMP binding proteins and protein kinase G (PKG). The effect on PKG reduces levels of calcium leading to relaxation of smooth muscles (see figure 2). The PDE5 enzyme is specific for cGMP which means it only hydrolyzes cGMP but not cAMP. The selectivity is mediated through an intricate network of hydrogen bonding which is favorable for cGMP but unfavorable for cAMP in PDE5. By inhibition of PDE5 enzyme the cGMP concentration will be raised and can therefore increase the relaxation of smooth muscles. PDE5 has only one subtype, PDE5A, of which there are 4 isoforms in humans called PDE5A1-4. The difference in PDE5A1-3 isoforms is only in the 5´ end of the mRNA and corresponding N-terminal of the protein.
Distribution of PDE5 in the body In humans the distribution of PDE5A1 and PDE5A2 isoforms is the same and can be found in the brain, lung tissue, heart, liver, kidneys, bladder, prostate, urethra, penis, uterus and skeletal muscles. PDE5A2 is more common than PDE5A1. PDE5A3 is not as widespread as the other two isoforms, and is only found in smooth muscle tissues; it is found in the heart, bladder, prostate, urethra, penis and uterus. Exact distribution of PDE5A4 isoform was not found in the literature. PDE5 enzyme in humans has also been reported in platelets, gastrointestinal epithelial cells, Purkinje cells of cerebellum, corpus cavernosum, pancreas, placenta and colon, clitoral corpus cavernosum as well as vaginal smooth muscle and epithelium.
PDE Structure and SAR PDE enzymes are composed of 3 functional domains: an N-terminal cyclin fold domain, a linker helical domain and a C-terminal helical bundle domain (see figure 3). The active site is a deep pocket at the junction of the 3 subdomains and is lined with highly conserved residues between isotypes of PDE. The pocket is approximately 15 Å deep and the opening is approximately 20 by 10 Å. The volume of the active site has been calculated to be between 875 and 927 Å3. The active site of PDE5 has been described as subdivided into 3 main regions based on its crystal structure in complex with sildenafil: M site: contains both a zinc and magnesium ion. The role of the ions is to stabilize the structure and activation of hydroxide to mediate the reaction. Current PDE5 inhibitors do not interact with the metal ions, in contrast with cGMP. Direct or indirect interactions may improve the potency of future inhibitors. Q pocket: it is believed that the guanidine group of cGMP binds in this region as the Q pocket accommodates the pyrazolopyrimidinone group (see figure 4) of sildenafil. The pyrazolopyrimidinone of sildenafil mimics that of the guanine in cGMP and has the same H-bond donor and acceptor features, forming a bidendrate H-bond with Q817. Card et al. describe the Q pocket as subdivided into 3 parts: A saddle formed by the conserved glutamine (Q817 in PDE5A, Q443 in PDE4B and Q369 in PDE4D) and the P clamp (a hydrophobic clamp at the narrow side of the active sites pocket, formed of invariant purine-selective glutamine and a pair of conserved residues). 2 narrow, hydrophobic pockets, Q1 and Q2, composed mainly of hydrophobic residues flanking the saddle. L region: the methyl piperazine group (see figure 4) of sildenafil is surrounded by Tyr 664, Met 816, Ala 823 and Gly 819 residues, and residues 662-664 form a lid over the pocket narrowing the entrance to the active site of PDE5. Jeon et al. also describe a fourth pocket called the H pocket which is hydrophobic and accommodates the ethoxy phenyl group of sildenafil The 3 PDE5 inhibitors already on the market, sildenafil, tadalafil and vardenafil, occupy part of the active site, mainly around the Q pocket and sometimes the M pocket as well and all 3 interact with the active site in 3 important manners: interaction between the metal ions mediated through water hydrogen bonding with the saddle of the Q pocket hydrophobic interaction with hydrophobic residues lining the cavity of the active site. It has also been described that the hydrophobic interaction with the Q1 and Q2 pockets are important for inhibitor potency and differences between isotypes of PDE in the Q2 pocket can be exploited for selectivity between isotypes.
Role in diseases
Erectile dysfunction Drugs that inhibit PDE5, sildenafil, tadalafil and vardenafil, have been used as treatment for erectile dysfunction. These inhibitors increase the cGMP, smooth muscle relaxation and consequently cause penis erection during sexual stimulation.
Pulmonary arterial hypertension Upregulation of PDE5 gene expression has been observed in animal models of pulmonary hypertension, and is thought to contribute to vasoconstriction in the lung. Several randomised controlled trials investigating PDE5 inhibitors use in pulmonary arterial hypertension, a subtype of pulmonary hypertension, have demonstrated their potent effects in reducing pulmonary hypertension and vascular remodelling and improving symptoms and mortality in patients with the condition. Long-term treatment with a PDE5 inhibitor has been shown to enhance natriuretic peptide-cGMP pathway, downregulate Ca2+ signaling pathway and alter vascular tone in pulmonary arteries in rat models.
Benign prostatic hyperplasia As of 2011, the long-acting agent tadalafil is licensed for the treatment of urinary symptoms resulting from benign prostatic hyperplasia.
Future indications for PDE5 inhibitors
Cardiovascular diseases PDE5 inhibitors have broad-ranging effects on the cardiovascular system beyond their acute haemodynamic influence. For example, PDE5 inhibitors have been shown to improve several parameters of endothelial function. Increasingly, their use in the management of systemic hypertension (including treatment-resistant hypertension), cardioprotection, heart failure, and peripheral arterial disease are being evaluated.
Heart failure PDE5 inhibitors have shown promise in the treatment of heart failure with reduced ejection fraction through several beneficial effects on lung vasculature, cardiac remodelling and diastolic function. A study showed that effective treatment of pulmonary arterial hypertension with sildenafil improved functional capacity and reduced right ventricular mass in patients. The effects on right ventricular remodeling were significantly greater in comparison with the non-selective endothelial receptor antagonist bosentan. However, PDE5 inhibitors may be harmful in patients with heart failure with preserved ejection fraction due to potential negative inotropic effects.
Chronic kidney disease Experimental studies in animals have shown that PDE5 inhibitors may reverse kidney damage independently of their effects on blood pressure through intra-renal mechanisms. In humans, PDE5 inhibitors have also been shown to reduce proteinuria, a marker of kidney damage. However, the successful introduction of SGLT2 inhibitors and endothelin receptor antagonists to the field of renal therapeutics makes the development of PDE5 inhibitors for this purpose unlikely.
Diabetes mellitus PDE5 inhibitors have been shown to have various macrovascular, microvascular and metabolic benefits in diabetes mellitus, and in a large study of men with type 2 diabetes mellitus the agents were found to significantly reduce patients' risk of death from any cause. It is unclear to what extent this observation reflects the protective effects of PDE5 inhibitors against cardiovascular and renal disease.
Raynaud's phenomenon Sildenafil has been shown to be at least as effective as calcium channel blockers in treating severe Raynaud's phenomenon (RP) associated with systemic sclerosis and digital ulceration. When given sildenafil for 4 weeks subjects had reduced mean frequency and duration of Raynaud attacks and a significantly lowered mean Raynaud's condition score. The capillary blood flow velocity also increased in each individual patient and the mean capillary flow velocity of all patients increased significantly. These results came without significant reductions of the systemic blood pressure. However, the therapeutic effects of PDE5 inhibitors in primary (idiopathic) RP are less well defined.
Stroke Sildenafil has been shown to significantly improve neurovascular coupling without affecting overall cerebral blood flow by increasing brain levels of cGMP, evoking neurogenesis and reducing neurological deficits in rats 2 or 24 hours after stroke. These experimental data suggest that PDE5 inhibitors may have a role in promoting recovery from stroke. However, studies in humans remain inconclusive.
Premature ejaculation Adding PDE5 inhibitors to SSRI drugs (e.g. paroxetine) for the treatment of premature ejaculation could result in better ejaculatory control according to recent studies. Possible mechanism is based on nitric oxide (NO)/cGMP transduction system as a central and peripheral mediator of inhibitory non-adrenergic, non-cholinergic nitrergic neurotransmission in the urogenital system.
Female sexual arousal disorder PDE5 is expressed in clitoral corpus cavernosum and in vaginal smooth muscle and epithelium. Therefore, it is possible that PDE5 inhibitors could affect female sexual arousal disorder but further research is needed. Increased levels of cGMP have been shown to occur in human-cultured vaginal smooth muscle cells treated with a PDE5 inhibitor suggesting involvement of the NO/cGMP axis in the female sexual response.
Sexual Exhaustion Disorder The similarity of many PDE5 inhibitors to the structure of many of the analogs of caffeine that are also adenosine antagonists suggests that in the future, it may be possible to design an PDE5 inhibitor that, like caffeine, is also an adenosine antagonist.
Discovery