PDE5 drug design

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 subfamiles, 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 ų. The active site of PDE5 has been described as subdivided into 3 main regions based on its crystal structure in complex with sildenafil:

Jeon et al. also describe a fourth pocket called the H pocket which is hydrophobic and accommodates the ethoxyphenyl 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:

1. interaction between the metal ions mediated through water
2. hydrogen bonding with the saddle of the Q pocket
3. 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.

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 hypertension

Pulmonary hypertension is the result of upregulation of PDE5 gene expression, causing vasoconstriction in the lung. PDE5 inhibitors are used as potent pulmonary vasodilators reducing Pulmonary hypertension and inhibiting vascular remodelling. 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.

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.

Raynaud's phenomenon

Sildenafil has been shown to be effective in treating severe Raynaud's phenomenon 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.

Heart failure

Sildenafil has shown promise in the treatment of congestive cardiac failure. 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 endothelia receptor antagonist bosentan.

Cardiovascular disease and systemic hypertension

Sildenafil has been shown to improve endothelial function in diabetes and congestive heart failure., It has also been shown to reduce aortic pressure through vasodilation, reduced arterial stiffness and wave reflection and could be used in the management of systemic hypertension.

Vascular disease

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. This data suggest that PDE5 inhibitors may have a role in promoting recovery from stroke.,