The catalytically active form of PDE6 enzyme is a dimer composed of α subunit (PDE6A) and a β subunit (PDE6B) and two identical inhibitory γ subunits. Each of these subunits has two N-terminal GAF domains (GAF-A and GAF-B) and a C-terminal catalytic domain. The GAF-A domain contains a high affinity binding site for cGMP and also probably much of the dimerization interface⁸.

PDE7 contain no known regulatory domains on the N terminus as established for most of the other PDE families, although consensus PKA phosphorylation sites exist in this region. PDE7 mRNA and protein are expressed in a wide variety of immune cells and evidence suggests that PDE7 may play a role in T lymphocyte activation⁸.

PDE8 has two isoforms PDE8A and PDE8B. Each of these two gene products contains two putative regulatory domains of unknown function in their N-terminal region. The first is a “REC domain” homologous to the “receiver” domains of bacterial two-component signaling systems. This is followed closely by a PAS domain (a Period, Arnt, and Sim) first described as a regulatory domain present in several proteins involved in the control of circadian rhythms⁸.

PDE9 has two isoform PDE9A and PDE9B. PDE9A lacks a region homologous to the allosteric cGMP binding site. PDE9 has a significant conserved region of about 270 amino acids common to all PDEs at the carboxy terminal apparently serves as the catalytic domain. The amino-terminal region of this protein is divergent and presumably accounts for the distinctive and regulatory properties unique to the individual PDE families¹⁶.

PDE10 family has only one gene i.e. PDE10A with four variants PDE10A 1-4. PDE10A contains 2 N-terminal GAF domains and hydrolyzes both cAMP and cGMP. Recombinant PDE10A2 is preferentially phosphorylated by PKA in its unique-N terminus, opening a new regulation way of its potential physiological roles, especially in the striatum^{7, 8}.

PDE11family is a dual-substrate PDE family having a catalytic site most similar to PDE5 enzymes. Four splice variants are identified, PDE11A1-4, that contain a conserved carboxyl-terminal, amino-terminal and an amino acid sequence. PDE11A2, PDE11A3, and PDE11A4 contain one or more GAF subdomains⁸.

PDE1 family:

Physiological activity/ functional role:

PDE1 has been implicated to play a role in a number of physiological and pathological processes. PDE1A regulates vascular smooth muscle cells growth and survival of these cells can contribute to the neointima formation in atherosclerosis and restenosis²¹.

PDE1A2 has a potential role in neurodegenerative diseases like parkinsons, axonal neurofilament degradation, motor neuronal degradation, neuronal ischemia, alzheimer’s disease and epilepsy. PDE1B knockout mice have increased locomotor activity and in some paradigms decreased memory and learning abilities²². PDE1B is also involved in dopaminergic signalling and is induced in several types of activated immune cells²³. PDE1B mRNA is induced in phytohemagglutinin (PHA) or anti-CD3/CD28-activated human T-lymphocytes and participates in IL-13 regulation implicated in allergic diseases⁷. PDE1C has been shown to be a major regulator of smooth muscle proliferation in human smooth muscle. Nonproliferating smooth muscle cells (SMC) exhibit only low levels of PDE1C expression but it is highly expressed in proliferating SMCs. It can therefore be speculated that inhibition of PDE1C could produce beneficial effects due to its putative inhibition of SMC proliferation, an event that is responsible for pathophysiology of atherosclerosis²³. Another likely role of PDE1C is in olfaction¹¹, to regulate sperm function and neuronal signaling²³.

PDE1 inhibitors:

Inhibition of PDE1A function significantly attenuates vascular smooth muscle cell growth by decreasing proliferation by affecting G1 phase of cell cycle and induces apoptosis²¹. Vinpocetine is a semisynthetic derivative of vincamine, which is obtained from the periwinkle plant. It increases cerebral blood flow and improves memory and is commercially available as supplement²⁴. In a model of fetal alcohol spectrum disorders, vinpocetine was able to restore neuronal plasticity in visual cortex as well as the functional organization of this area. Vinpocetine may have beneficial effects in conditions such as Alzheimer’s and Parkinson’s where inflammation and poor neuronal plasticity are present due to its potential to enhance neuronal plasticity. The development of more specific drugs may pave the way for the use of PDE1 inhibitors as therapeutic agents in cases of neurodevelopmental conditions such as fetal alcohol spectrum disorders and in degenerative disorders such as Alzheimer’s and Parkinson’s²⁵.

Phosphodiesterases (PDEs) are potential targets for PDE inhibitor-based antiparasitic drugs since genomes of the various agents of human malaria, most notably Plasmodium falciparum, all contain four genes for class 1 PDEs, hinting PDE1 as possible anti-malarial targets²⁶.

PDE2 family:

Physiological activity:

PDE2 is thought to be involved in regulating many different intracellular processes, such as aldosterone secretion from the adrenal gland, cGMP in neurons and effect on long-term memory, cardiac L-type Ca²⁺ current in cardiac myocytes thereby affecting cardiac function, barrier function of endothelial cells under inflammatory conditions⁸.

PDE2 is up regulated when monocytes differentiate into macrophages and thus play a role in inflammatory responses in microvessels. It has been speculated that tumor necrosis factor-alpha (TNFα) might regulate the function of PDE2 in endothelial cells and thereby affecting flow of fluid and cells through the endothelial barrier as in vitro experiments on endothelial cells show up regulation of both PDE2 mRNA and activity⁸.

PDE2 inhibitors:

PDE2 selective inhibitors EHNA (erythro-9-(2-hydroxy-3-nonyl) adenine) and Anagrelide act through cAMP and cGMP and used clinically in Sepsis, Acute respiratory Distress syndrome, Memory loss.

Inhibition of PDE2 by EHNA potentiates NMDA (Nmetyl-D-aspartate) receptor activated increase in cGMP, but has no effect on cAMP concentrations²⁷. Also, EHNA is a potent inhibitor of adenosine deaminase. This dual inhibition leads to the accumulation of the two inhibitory metabolites, adenosine and cGMP, which may act in synergy to mediate diverse pharmacological responses including anti-viral, anti-tumour and antiarrhythmic effects²⁸.

Anagrelide is also a specific inhibitor indicated for the treatment of patients with thrombocythemia, secondary to myeloproliferative disorders, to reduce the elevated platelet count and the risk of thrombosis and to ameliorate associated symptoms including thrombo-hemorrhagic events²⁹.

PDE3 family:

Physiological activity:

Due to location of PDE3 isoforms in cardimyocytes, they play many roles in cardiac tissues. Apoptosis induced by proapoptotic stimuli such as angiotensin II (Ang II) and isoproterenol is mediated by selective down regulation of PDE3A expression and subsequent induction of inducible cAMP early repressor (ICER). Thus, down regulation of PDE3A observed in failing heart plays a causative role in the progression of heart failure, in part by inducing ICER and promoting cardiomyocyte apoptosis³⁰.

PDE3 is mediator of inflammation in VSMCs through inhibitory effects of S-nitroso-N-acetylpenicillamine (SNAP) and C-type natriuretic peptide (CNP) on NF-κB–dependent transcription, by activation of Protein Kinase A (PKA) via cGMP-dependent inhibition of PDE3 activity³¹.

PDE 3 is involved in the regulation of airway smooth muscle tone. Inhibition of PDE 3 with the selective inhibitors SKandF 94120 or SKandF 94836(siguazodan) results in relaxation of canine and human airway smooth muscle, either on spontaneous tone or tone induced by carbachol^32.

PDE 3 is also involved in platelet aggregation due to location of PDEs in platelets.

In adipocytes, insulin induces formation of macromolecular complexes containing signaling molecules such as Insulin receptor substrate (IRS-1), PI3K ( Phosphatidylinositol 3-kinases) and Protein kinase B (PKB) proteins involved in PDE3B activation/ phosphorylation. This recruitment may be critical for the regulation of cAMP to modulate insulin signaling pathways. PDE3 selectively regulates cAMP synthesis by activation of the prostacyclin receptor ^[33].

PDE3 inhibitors:

Cardiovascular Actions of cAMP-dependent PDE (type3) Inhibitors associated with Systemic Circulation are Vasodilation, increased organ perfusion, decreased systemic vascular resistance and decreased arterial pressure³⁴.

Actions of PDE3 inhibitors on cardiopulmonary system leads to increased contractility and heart rate, increased stroke volume and ejection fraction, decreased ventricular preload, decreased pulmonary capillary wedge pressure³⁴.

The positive ionotropic effect of novel cardiotonic pimobendan and livosimendan is through a combination of phosphodiesterase III inhibition and sensitisation of myocardial contractile proteins to calcium on myofilaments. Selective inhibitors of PDE3 like amrinone, milrinone, olprinone have been used clinically for cardiac contractile dysfunction in acute congestive heart failure and in aggravating phase of chronic heart failure³⁵.

Cilostazol is a potent, reversible PDE3 inhibitor approved clinically for use in patients with intermittent claudication. Cilostazol is involved in inhibition of platelet aggregation by a variety of stimuli, including thrombin, ADP, collagen, arachidonic acid, epinephrine, shear stress and vasodilation in vessels with atherosclerotic plaques ^[36]. Cilostazol has implication in multiple sclerosis through repressive effects on encephalomyelitis by reducing the antigen specific T cell response and decreasing the expression level of adhesion molecule, Inter-Cellular Adhesion Molecule 1³⁷.

Cilostamide reverses leptin signaling through the PDE3B pathway which is responsible for the activation of proopiomelanocortin (POMC) and neurotensin (NT) neurons and regulation of energy homeostasis. Leptin signaling in the hypothalamus is required for normal food intake and body weight homeostasis. Resistance or attenuated response to specific action of leptin, particularly food intake and body weight regulation, may occur due to defect in any of the steps in the signal transduction mechanism and or a defect at downstream of signaling, such as other co-factors/co-activators, resulting in the development of leptin resistance in target neurons. PDE3 inhibitor cilostamide reverses anorectic and body weight reducing effects of leptin through its action in hypothalamus³⁸.

Milrinone has positive inotropic, vasodilating and minimal chronotropic effects through potentiation of the effect of cyclic adenosine monophosphate (cAMP). It is used in the management of heart failure only when conventional treatment with vasodilators and diuretics has proven insufficient due to the potentially fatal adverse effect of milrinone like ventricular arrhythmias. Other PDE3 inhibitors such as Amrinone, Enoximone also have applications in treatment of congestive heart failure due their ionotropic effects and are used only when benefits overweigh risks³⁹.

Newer PDE3 inhibitor like NT-702 has an anti-inflammatory effect as well as a bronchodilating effect and might be useful as a novel potent therapeutic agent for the treatment of bronchial asthma, a new type of agent with both a bronchodilating and an anti-inflammatory effect⁴⁰.

PDE4:

Physiological activity:

PDE4 is expressed in a number of cell types that are considered suitable drug targets for the treatment of respiratory diseases such as asthma and COPD since its inhibition increases intracellular cAMP concentrations, which ultimately results in reduction of cellular inflammatory activity (macrophages, eosinophils, neutrophils)⁴¹.

In the learned helplessness rodent model of depression, PDE4 and adenylate cyclase activity in frontal cortex and hippocampus were decreased in the acute depressive state, while PDE4 activity was increased in the delayed depressive state⁴². PDE4D knockout mice have an antidepressant-like profile, thus implicating that PDE4D-regulated cAMP signaling may play a role in the pathophysiology and pharmacotherapy of depression⁴³. PDE4B interacts directly with another major genetic locus that has been identified as a risk factor for depression, as well as for bipolar disorder and schizophrenia, known as DISC-1 (disrupted in schizophrenia-1)^43,44. DISC-1 and PDE4B form complexes that may result in modulation of PDE4B activity and mutations in DISC-1 thus producing abnormal phenotypes in animals. PDE4 inhibitors might have significant clinical value in the treatment of brain tumors by elevation of cAMP due to higher concentration in tumor cells⁴⁵.

As PDEs are located in smooth muscle cells, differentiation of vascular smooth muscle cells to a proliferative phenotype is associated with a profound up-regulation of specific PDE4 isoforms due to increased histone acetylation. The increased PDE4 activity is seen as preventing cAMP from inhibiting the enhanced proliferation, migration and production of extracellular matrix seen in activated VSMC⁴⁶. Cardiomyocytes and vascular smooth muscle cells selectively vary both the expression and the catalytic activities of PDE4 isoforms to regulate their various functions and altered regulation of these processes influences the development, or resolution, of cardiovascular pathologies, such as heart failure. PDE4 activities alter blood pressure by influencing the functions of contractile VSMC⁴⁷.

PDE4 inhibitors:

Roflumilast is a selective PDE4 inhibitor and it shows a broad pharmacological action across many aspects relevant to COPD pathophysiology, including inflammation, emphysema, lung fibrotic remodelling, pulmonary vascular remodeling and pulmonary hypertension, oxidative stress and mucociliary malfunction. In vitro, roflumilast N-oxide has been demonstrated to affect the functions of many cell types, including neutrophils, monocytes/ macrophages, CD4 and CD8 T-cells, endothelial cells, epithelial cells, smooth muscle cells and fibroblasts. These cellular effects are thought to be responsible for the beneficial effects of roflumilast on the disease mechanisms of COPD, which translate in to reduced exacerbations and improved lung function⁴⁸.

In several phase III trials in patients with moderate to (very) severe COPD and in patients with symptoms of chronic bronchitis and recurrent exacerbations, roflumilast showed sustained clinical efficacy by improving lung function and by reducing exacerbation rates⁴⁹.

Rolipram is another PDE4 inhibitor having potential benefits in depression, memory improvement, neuroprotection and schizophrenia through various mechanisms in animal models⁵⁰. PDE4 is highly expressed in gliomas and thus targeted inhibition with the selective PDE4 inhibitor rolipram inhibits the intra- cranial growth of model glioblastoma multiforme and medulloblastoma xenografts thus having potential in treating brain tumors⁴⁶. PDE4D is an essential mediator of the antidepressant-like effects of rolipram and provides further evidence that PDE4D-regulated cAMP signalling plays a role in the pathophysiology and pharmacotherapy of depression⁵⁰. Rolipram has been implicated in modulating long-term memory through cAMP-response-element-binding- protein (CREB) expression and thus plays role in long-term memory (LTM) formation⁵⁰.

Cilomilast is another PDE4 inhibitor under phase 3 clinical trials for its efficacy evaluation in emphysema and bronchitis. Results from large, phase III COPD studies of cilomilast have been reported; cilomilast was well-tolerated, improved health status, and lung function, and reduced the utilization of healthcare resources and incidence of COPD exacerbations⁴¹.

PDE5:

Physiological activity/ functional Role:

The cyclic nucleotide PDE5 plays a fundamental role in signal translation. It stimulates the relaxation of smooth muscle, the degranulation of neutrophils, inhibits platelet aggregation and initiates translation of the visual signal. PDE5 play role in modulating hemodynamics thus affecting the contracted state of vessel cells by regulating the phasic nature of smooth muscle physiology⁵¹. PDE5 are found to play crucial role in penile erection. During sexual arousal, nitric oxide is released from the nerve endings in the penis and from vascular endothelial cells. Nitric oxide diffuses into the smooth muscle cells of the corpus cavernosum to stimulate guanylate cyclase, which converts GTP into cGMP. This in turn activates a chain of events that include cGMP-dependent protein kinase activation, phosphorylation of several proteins and a decrease in cellular calcium concentration, which eventually triggers relaxation of arterial and trabecular smooth muscle, leading to arterial dilatation, venous constriction, and erection.to penile smooth muscle relaxation⁵².

Phosphodiesterase type 5 (PDE-5) is implicated in endothelial dysfunction by inactivating cyclic guanosine monophosphate, the nitric oxide pathway second messenger, thus play an integral role in pathogenesis of pulmonary arterial hypertension (PAH). PDE5 inhibitors inhibit the degradation of cGMP by PDE 5 and prolong the actions of cGMP to mediate vasodilation and smooth muscle relaxation in PAH. This suggests that PDE5 is a new target for the treatment of pulmonary hypertension and respiratory distress⁵³.

PDE5 inhibitors:

PDE5 inhibitors like sildenafil, tadalafil, vardenafil, are clinically applicable in treatment of chronic renal failure, salt retention in nephritic syndrome, pulmonary hypertension, erectile dysfunction, non-inflammatory chronic pelvic pain syndrome, organ transplantation^{7, 54}.

Phosphodiesterase type 5 (PDE5) inhibitors remain the first-line therapy for most men with erectile dysfunction (ED) of various etiologies. Recent publications suggest PDE5 inhibitors are effective in the treatment of premature ejaculation, prolonged refractory time post-ejaculation, and serve a role in the treatment of lower urinary tract symptoms secondary to benign prostatic hyperplasia⁵⁵. It could be considered as the first-line treatment in the future for the treatment of patients with comorbid benign prostatic hyperplasia and ED⁵⁶. Zaprinast was the first characterized selective PDE5 inhibitor. Currently, PDE-5 inhibitors sildenafil citrate, tadalafil, and vardenafil hydrochloride trihydrate are now approved by the USFDA for the therapy of ED and pulmonary hypertension⁵⁷. The involvement of PDE isozymes in differentially regulating inflammatory cytokines has been reported, but tadalafil is the only PDE5 Inhibitor to show a potentially anti-inflammatory effect (in addition to relaxation) on endothelial cells⁵⁸. Case series and small studies, as well as the large randomized controlled trial, have demonstrated the safety and efficacy of sildenafil in improving mean pulmonary artery pressure, pulmonary vascular resistance, cardiac index, and exercise tolerance in PAH⁵⁹.

Findings from animal studies have shown that in Cystic Fibrosis (CF) PDE5 inhibitors are able to restore the impaired chloride transmembrane transportation by acting directly on cystic fibrosis transmembrane regulator (CFTR) protein without influencing sodium transmembrane transportation. In in vivo CF animal models, PDE5 inhibitors such as sildenafil improved CFTR activity in nasal epithelium and in a P. aeruginosa-susceptible Dilute Brown Non-Agouti (DBA/2) mouse model reduced neutrophil infiltration induced by bacteria⁶⁰. Sildenafil serves as a better graft function after 24 h ischemia when given prior to standard flushing and preservation. This effect is seen when complete/homogenous preservation is done by selective pulmonal vasodilatation⁶¹.

Udenafil and mirodenafil are newer PDE5 inhibitors approved only in South korea for treatment of erectile dysfunction. Lodenafil carbonate and avanafil are also newer PDE5 inhibitors under phase 3 and phase 4 clinical trials⁵³.

PDE6 family:

Physiological activity:

The PDE6 family members are better known as the photoreceptor phosphodiesterases.

PDE6 families are expressed in retinal photoreceptor cells, and are involved in mediating the phototransduction cascade. The PDE6 cascade activation is initiated when the protein rhodopsin absorbs a photon. Each activated rhodopsin activates thousands of transducin (a G-protein) by catalyzing the exchange of GDP for GTP. Transducin Tα subunit, with GTP bound activates the catalytic PDE subunits by displacing γ subunits from the active site of the enzyme, thus allowing cGMP hydrolysis. The main function of the rod PDE is to rapidly reduce the steady-state concentration of cGMP in response to light stimulus. This decrease in cGMP concentration causes the closure of cyclic nucleotide gated cationic channels and generates cell membrane hyperpolarization¹⁷. PDE6s are involved in embryonic development and also in transformation to normal birth⁸.

PDE6 inhibitors:

There are no specific PDE6 inhibitors but Sildenafil, vardenafil, and udenafil, inhibit PDE6 with substantially lower affinities than those for PDE5A due to structural similarities of PDE6 subunits with PDE5. Zaprinast, Dipyridamole Vardenafil, Tadalafil are invoved in vision but have adverse effects on vision¹⁷. Electroretinogram studies have shown that PDE5 inhibitors exert a modest effect on visual function⁶².

PDE7 family:

Physiological activity:

A great deal of effort from the pharmaceutical industry has been invested in developing selective PDE7 inhibitors. Selective small molecule inhibitors of this enzyme family provide a novel approach to alleviate the inflammation that is associated with many inflammatory diseases including asthma, chronic obstructive pulmonary disease, atopic dermatitis, psoriasis, lupus, rheumatoid arthritis and multiple sclerosis^{7, 63}. PDE7 is involved in T cell activation thus a dual PDE4-PDE7 inhibitor may be more effective in asthma and COPD⁶⁴.

PDE7 inhibitors:

PDE7A inhibitor SUN11817 suppresses the increase of Alanine Transaminase activity in a dose-dependent manner. PDE7A inhibitor SUN11817 improves liver injury in the Concovalin A model by blocking cytokine production and Fas Ligand expression in Natural Killer T cells (NKT) of mice. PDE7A inhibitor might be a novel pharmaceutical target for hepatitis⁶⁵.

PDE7A inhibitor ASB16165 suppresses cell proliferation and cytokine production of NKT cells by cAMP elevation. Thus PDE7A inhibitor including ASB16165 may be useful for treatment of the diseases like inflammatory bowel disease, asthma, COPD, malaria, HIV in which NKT cells have pathogenic roles^66-69.

YM-393059 is a novel phosphodiesterase (PDE) 7 and PDE4 dual inhibitor that inhibits proinflammatory cytokine production and selectively suppresses the response to the autoantigen without affecting the response to alloantigens. Thus, YM- 393059 is a potent compound for the treatment of autoimmune disorders such as rheumatoid arthritis⁶⁹.

PDE7 is expressed simultaneously on leukocytes and on the brain thus PDE7 play a crucial role as drug target for neuroinflammation. Inhibitors of the activity of PDE7 pathway like VP1.15 and S14 may be useful in the therapy of spinal cord injury, trauma and inflammation, improving motor function ^[70]. Dipyridamole, Thiadiazole are found to be involved in Airway and immunological diseases^17,
71.

PDE8 family:

Physiological activity/functional role and inhibitors:

Phosphodiesterase 8B Gene Variants are Associated with Serum TSH Levels and Thyroid Function. The PDE8B gene modulates circulating TSH levels. Mutation of PDE8B leads to elevation of cAMP levels and thus leads to adrenal hyperplasia and Cushing syndrome. PDE8B upregulation has been implicated in Alzheimer’s disease brain and pituitary adenomas, as well as involvement in a model of modified insulin secretion⁷².

PDE8B promotes steriodogenesis in mouse adrenal gland. Moreover, selective inhibitor (PF04957325) potentiates adrenocorticotropin stimulation of steroidogenesis by increasing cAMP dependent protein kinase activity in primary isolated adrenocortical cells and Y-1 cells⁷³.

Immune system depends upon chemokines to recruit lymphocytes to tissues in inflammatory diseases. PDE8 regulates chemotaxis of activated lymphocytes and thus PDE8 inhibition serves as a therapeutic target in inflammatory diseases⁷⁴. Moreover, PDE8 is a novel target for suppression of effector T cell functions, including adhesion of effector T cells to endothelial cells. Dipyridamole suppresses proliferation and cytokine expression of effector T cells from cAMP responsive element modulator mice lacking inducible cAMP early repressor⁷⁵.

PDE9 family:

Physiological activity/functional role and inhibitors:

PDE9 is highly specific for cGMP and not inhibited by the non selective phosphodiesterase inhibitor 3-isobutyl-1-methyl-xanthine (IBMX) and activity at this enzyme may also contribute to behavioural state regulation and learning⁷⁶. Selective PDE9A inhibitors are involved in prolonging intracellular responses to glutamate and enhance glutamate signaling, and since this process is involved in learning and memory, PDE9A inhibitors have nootropic effect and may be useful in treatment of Alzheimer’s disease⁷⁷.

BAY 73-6691 has shown to improve learning and memory in rats, clinical trials of the compound are under investigation for learning and memory⁷⁸.

Thus Selective inhibitors of PDE9 have demonstrated potentials for treatment of human diseases, cardiovascular diseases, insulin-resistance syndrome and diabetes, obesity and neurodegenerative disorders such as Alzheimer’s disease^{79, 80}.

PDE10:

Physiological activity/functional role and inhibitors:

The high level of expression of PDE10A within medium spiny neurons of the striatum suggests that PDE10A may play a role in this important brain region, dysfunction of which has been implicated in several neuropsychiatric and neurodegenerative disorders including schizophrenia, obsessive–compulsive disorder, Huntington’s disease and Parkinson’s disease^{81, 82}.

The PDE10 family is associated with the progressive neurodegenerative disease like Huntington’s disease (HD), since PDE10A2 mRNA and protein levels in striatum decreases prior to the onset of motor symptoms in transgenic HD mice expressing exon 1 of the human Huntington gene⁸³.

PDE10A localization to the caudate region of the brain suggests a role(s) in modulating striatonigral and striatopallidal pathways .Thus, PDE10 may be a good therapeutic target for treatment of psychiatric disorders of frontostriatal dysfunction⁸⁴.

Modulation of cAMP levels has been linked to insulin secretion in preclinical animal models and in humans. The high expression of PDE-10A in pancreatic islets suggests that inhibition of this enzyme may provide the necessary modulation to elicit increased insulin secretion. Thus quinoline-based PDE-10A inhibitors showed improvement in glucose tolerance and increase in insulin secretion⁸⁵.

Selective PDE10A inhibitors represent novel therapeutic agents for individuals with schizophrenia. In mice, PDA10A inhibitor, papaverine is associated with increased cGMP levels in the striatum and increased phosphorylation of CREB, which are both crucial for striatal function. Papaverine reduces deficits caused by chronic phencyclidine treatment by alleviating both dopaminergic and glutamatergic dysfunction^86-88. Papaverine has implicated possible clinical use of PDE10A inhibitors as antipsychotics. Papaverine and more selective PDE10A inhibitor MP10 raises striatal cAMP levels and mediates hypothermia, hypoactivity and decreased cardiovascular responses in rats⁸⁹.

PDE11 family:

Physiological activity/functional role and inhibitors:

Preclinical studies on PDE11 knockout mouse model have suggested that PDE11 may be important for sperm development and function. Ejaculated sperm from knockout mice displayed slightly lower sperm concentration and decreased viability compared with controls, and the sperm had a lower rate of forward progression⁹⁰.

PDE11 is suspected of playing a physiological role is in sperm capacitation. Sperm exiting the epididymis are incapable of fertilization until they undergo capacitation in the female genital tract. Several factors influence capacitation, including factors secreted by the prostate. The mechanism of capacitation is also cAMP dependent. Several cytokines cause release of cAMP, which in turn causes an influx of calcium into the spermatozoon triggering capacitation. By keeping cAMP levels low, PDEs are believed to prevent premature capacitation.

Tadalafil is involved in improvement of human testicular functions⁹¹.

CONCLUSION:

The PDE enzymes are now well recognized to be important regulators of many different cellular and molecular functions. The growing knowledge related to the molecular pharmacology of PDEs has already fostered development of selective inhibitors. From the pharmacological point of view, the development of more specific inhibitors for all isoenzymes appears to be a sensible approach to pursue the outstanding clinical needs in CNS disorders (PDE 1/2/3/4/10 inhibitors); sepsis(PDE2 inhibitors); sexual dysfunction in females, cardiovascular disease and pulmonary Hypertension (PDE5 inhibitors); asthma, COPD, allergic rhinitis, psoriasis, multiple sclerosis, depression, Alzheimer's disease and schizophrenia (PDE4/7/8 inhibitors); vision (PDE6 inhibitors); inflammation (PDE7/8 inhibitors); thyroid function (PDE8 inhibitors).

As the study on the physiological roles of the individual PDE isoforms progresses, there is a parallel development of more selective inhibitors of these enzymes, and as a result it is likely that better therapeutically active drugs will emerge. Moreover currently many PDE inhibitors are under clinical trials and in drug company development pipelines for safety, efficacy, and newer indication due to ubiquitous location of phosphodiesterase in humans. The enormous clinical and financial success of the erectile dysfunction drugs has validated the concept that PDE inhibitors can be clinically successful and profitable and has attracted much commercial interest to the PDE superfamily. The future for PDE research seems bright as increased interest from pharmaceutical companies and academic researchers should accelerate the pace of discovery.

REFERENCES:

1. Kopperud R. et al. cAMP effector mechanisms: Novel twists for an ‘old’ signaling system. FEBS Lett. 546; 2003: 121–126.

2. Vaandrager AB and Jonge HR. Signalling by cGMP-dependent protein kinases. Mol. Cell Biochem. 157; 1996: 23–30.

3. Henn V. et al. Compartmentalized cAMP signalling regulates vasopressin mediated water reabsorption by controlling aquaporin-2. Biochem. Soc. Trans. 33; 2005: 1316–1318.

4. Boswell-Smith, V. et al. Phosphodiesterase inhibitors. Br. J. Pharmacol. 147(1); 2006: 252–257.

5. Halene TB and Siegel SJ. PDE inhibitors in psychiatry- future options for dementia, depression and schizophrenia? Drug discovery today. 12(19/20); 2007: 870-878.

6. Conti M and Beavo J. Biochemistry and physiology of cyclic nucleotide phosphodiesterases: essential components in cyclic nucleotide signaling. Annu. Rev. Biochem. 76; 2007: 481–511.

7. Lugnier C. Cyclic nucleotide phosphodiesterase (PDE) superfamily: a new target for the development of specific therapeutic agents. Pharmacol. Ther. 109; 2006: 366–398.

8. Bender AT and Beavo JA. Cyclic nucleotide phosphodiesterase: Molecular regulation to clinical use. Pharmacological reviews. 58; 2006: 488-520.

9. Dousa TP. Cyclic-3',5'-nucleotide phosphodiesterase isozymes in cell biology and pathophysiology of the kidney. Kidney Int. 55 (1); 1999: 29–62.

10. Kakkar R, Raju RV and Sharma RK. Calmodulin-dependent cyclic nucleotide phosphodiesterase (PDE1). Cell. Mol. Life Sci. 55(8-9); 1999: 1164–86.

11. Jeon YH et al. "Phosphodiesterase: overview of protein structures, potential therapeutic applications and recent progress in drug development". Cell. Mol. Life Sci. 62(11); 2005: 1198–1220.

12. Esposito K, et al. Phosphodiesterase genes and antidepressant treatment response: A review. Annals of Medicine. 41; 2009: 177-185.

13. Wechsler J et al. Isoforms of cyclic nucleotide phosphodiesterase PDE3A in cardiac myocytes. J Biol Chem. 277; 2002: 38072–38078.

14. Choi YH et al. Identification of a novel isoform of the cyclic-nucleotide phosphodiesterase PDE3A expressed in vascular smooth-muscle myocytes. Biochem J. 353; 2001: 41–50.

15. Puzzo D, et al. Role of phosphodiesterase 5 in synaptic plasticity and memory. Neuropsychiatric Disease and Treatment. 4(2); 2008: 371–387.

16. Available from URL: http://www.gencompare.com/pde91.html.

17. Ghosh R. et al. Phosphodiesterase inhibitors: their role and implications; International Journal of PharmTech Research. 1(4); 2009: 1148-1160.

18. Rao YJ and Lei XI. Pivotal effects of phosphodiesterase inhibitors on myocyte contractility and viability in normal and ischemic hearts. Acta Pharmacol Sin. 30(1); 2009: 1–24.

19. Kim KY et al. Discovery of new inhibitor for PDE3 by virtual screening. Bioorganic and Medicinal Chemistry Letters. 21; 2011: 1617–1620.

20. David MG Halpin ABCD of the phosphodiesterase family: interaction and differential activity in COPD. International Journal of COPD. 3(4); 2008: 543–561.

21. Nagel D J, et al. Role of nuclear Ca2+/calmodulin-stimulated phosphodiesterase 1A in vascular smooth muscle cell growth and survival. Circ Res. 98(6); 2006: 777-784.

22. Jeon YH et al. Phosphodiesterase: overview of protein structures, potential therapeutic applications and recent progress in drug development. Cell. Mol. Life Sci. 62 (11); 2005: 1198–220.

23. Bischoff E. Potency, selectivity, and consequences of nonselectivity of PDE inhibition. Int. J. Impot. Res. 16 (1); 2004: S11–4.

24. Sonnenburg WK et al. Identification of inhibitory and calmodulin-binding domains of the PDE1A1 and PDE1A2 calmodulin-stimulated cyclic nucleotide Phosphodiesterases. J. Biol. Chem., 270; 1995: 30989– 31000.

25. Medina AE. Therapeutic utility of phosphodiesterase type I inhibitors in neurological conditions. Frontiers in Neuroscience. 5 (21); 2011: 1-5.

26. Wentzinger L et al. Cyclic nucleotide-specific phosphodiesterases of Plasmodium falciparum: PfPDEα, a non-essential cGMP-specific PDE that is an integral membrane protein. Int. J. Parasitology. 38; 2008: 1625-1637.

27. Tenor H and Schudt C. Analysis of PDE isoenzyme profiles in cells and tissues by pharmacological methods, in PDE Inhibitors (Schudt C, Dent G, and Rabe K eds) London: Academic Press 2004, 21-40.

28. Manganiello VC et al. Diversity in cyclic nucleotide phosphodiesterase isoenzyme families. Arch. Biochem. Biophys. 322; 1995: 1-13.

29. Available from URL: http://dailymed.nlm.nih.gov/dailymed/archives/fdaDrugInfo.cfm?archiveid=9079

30. Bo ding et al. Functional Role of Phosphodiesterase 3 in Cardiomyocyte Apoptosis: Implication in heart failure. Circulation. 111; 2005: 2469-2476.

31. Aizawa T. Role of Phosphodiesterase 3 in NO/cGMP-Mediated Antiinflammatory Effects in Vascular Smooth Muscle Cells. Circulation. 93; 2003: 406-413

32. Barnes PJ. Cyclic nucleotides and phosphodiesterases and airway function. European Respiratory Journal. 8; 1995: 457–462

33. Adderley SP et al. Regulation of cAMP by phosphodiesterases in erythrocytes. Pharmacological reports. 62; 2010: 475-482.

34. Klabunde RE. Cardiovascular Pharmacology Concepts. Available from: http://www.cvpharmacology.com/vasodilator/PDEI.htm

35. Endoh M. Basic and Clinical Characteristics of PDE 3 Inhibitors as Cardiotonic Agents. Cardiovascular Drugs and Therapy. 21(3); 2007: 135-139.

36. Meru AV et al. Intermittent claudication: An overview. Atherosclerosis. 187; 2006: 221–237.

37. Katsuichi M, Juri K and Susumu K. The treatment with selective phosphodiesterase-3 inhibitor cilostazol for experimental autoimmune encephalomyelitis. Journal of Neuroimmunology. 203; 2008: 119–280.

38. Sahu A. A role of phosphodiesterase-3B pathway in mediating leptin action on proopiomelanocortin and neurotensin neurons in the hypothalamus. Neurosci Lett. 479(1); 2010: 18-21.

39. Available from http://www.drugsupdate.com/generic/view/315. Accessed on 10/4/2011.

40. Hori M et al. NT-702 (parogrelil hydrochloride, NM-702), a novel and potent phosphodiesterase 3 inhibitor, suppress the asthmatic response in guinea pigs, with both bronchodilating and anti-inflammatory effects. Eur J Pharmacol. 618(1-3); 2009: 63-69.

41. Swinnen JV, Joseph D and Conti R. Molecular cloning of rat homologues of the Drosophila melanogaster dunce cAMP phosphodiesterase: evidence for a family of genes. Proc. Natl. Acad. Sci. 86; 1989: 5325–5329.

42. Livi GP, et al. Cloning and expression of cDNA for a human low-Km, rolipram-sensitive cyclic AMP phosphodiesterase. Mol. Cell Biol. 10; 1990: 2678– 2686.

43. Bolger G, et al. A family of human phosphodiesterases homologous to the dunce learning and memory gene product of Drosophila melanogaster is potential targets for antidepressant drugs. Mol Cell Biol. 13; 1993: 6558– 6571.

44. Millar JK, et al. Disrupted in schizophrenia 1 and phosphodiesterase 4B: towards an understanding of psychiatric illness. J Physiol. 584; 2007:401-405.

45. Millar JK, et al. DISC1 and PDE4B are interacting genetic factors in schizophrenia that regulate cAMP signaling. Science. 310; 2005: 1187-1191.

46. Sengupta R et al. Treating brain tumors with PDE4 inhibitors. Trends in Pharmacological Sciences 3; 2011: 1–8.

47. Houslay MD. The Long and Short of Vascular Smooth Muscle. Phosphodiesterase-4 As a Putative Therapeutic Target. Mol Pharmacol. 68(3); 2005: 563–567.

48. Houslay MD et al. cAMP-Specific Phosphodiesterase-4 Enzymes in the Cardiovascular System A Molecular Toolbox for Generating Compartmentalized cAMP Signaling. Circ. Res. 100; 2007: 950-966.

49. Hatzelmann A et al. The preclinical pharmacology of roflumilast - A selective,oral phosphodiesterase 4 inhibitor in development for chronic obstructive pulmonary disease. Pulmonary Pharmacology and Therapeutics. 23; 2010: 235-256.

50. Diamant Z and, Spina D. PDE4-inhibitors: A novel, targeted therapy for obstructive airways disease. Pulmonary Pharmacology and Therapeutics. 5; 2011: 1-8.

51. Tobias B, et al. PDE inhibitors in psychiatry-future options for dementia, depression and schizophrenia?. Drug Discovery Today. 12(19/20); 2007: 870-878.

52. Puzzo D et al. Role of phosphodiesterase 5 in synaptic plasticity and memory. Neuropsychiatric Disease and Treatment. 4(2); 2008: 371–387.

53. Montani D et al. Phosphodiesterase type 5 inhibitors in pulmonary arterial hypertension. Nature. 26(9); 2008: 813-825.

54. Cheng J. and Grande JP. PDE Inhibitors: Novel therapeutic agents for Renal disease, Exp. Biol. Med. 232; 2007: 38-51.

55. Dorsey P et al. Phosphodiesterase type 5 (PDE5) inhibitors for the treatment of erectile dysfunction. Expert Opin. Pharmacother. 11(7); 2010: 1109-1122.

56. Liu L et al. Phosphodiesterase-5 Inhibitors for Lower Urinary Tract SymptomsSecondary to Benign Prostatic Hyperplasia: A Systematic Review and Meta-analysis. Urology. 77 (1); 2011: 123-129.

57. Samuel R, et.al. Structural characterization of sulfoaildenafil, an analog of sildenafil. J. Pharm.Biomed. Anal. 50; 2009: 228-231.

58. Roumeguere T, et al. Effects of Phosphodiesterase Inhibitors on the Inflammatory Response of Endothelial Cells Stimulated by Myeloperoxidase-Modified Low-Density Lipoprotein or Tumor Necrosis Factor Alpha. European Urology. 57; 2010: 522-529.

59. Barnett CF, Machado RF. Sildenafil in the treatment of pulmonary hypertension Vascular Health and Risk Management. Pharmacol. Review. 2(4); 2006: 411–422.

60. Lubamba B et al. Inhaled phosphodiesterase type 5 inhibitors restore chloride transport in cystic fibrosis mice. Eur Respir J 37(1); 2009: 72-78.

61. Pizanis N et al. PDE-5 inhibitor donor intravenous preconditioning is superior to supplementation in standard preservation solution in experimental lung transplantation. European Journal of Cardio-thoracic Surgery. 32; 2007: 42—47.

62. Luu JK et al. Acute effects of sildenafil on the electroretinogram and multifocal electroretinogram. Am J Ophthalmol. 132; 2001: 388–394.

63. Giembycz MA and Smith SJ. Phosphodiesterase 7A: a new therapeutic target for alleviating chronic inflammation? Curr Pharm Des. 12(25); 2006: 3207-20.

64. Goto M, et al. Inhibition of phosphodiesterase 7A ameliorates Concanavalin A-induced hepatitis in mice. International Immunopharmacology. 9; 2009: 1347–1351.

65. Goto M et al. Phosphodiesterase 7A inhibitor ASB16165 suppresses proliferation and cytokine production of NKT cells. Cellular Immunology. 258; 2009: 147–151.

66. Berzofsky JA and Terabe M. The contrasting roles of NKT cells in tumor immunity. Curr Mol Med. 9(6); 2009: 667-72.

67. Vasan S, Tsuji M. A double-edged sword: The role of NKT cells in malaria and HIV infection and immunity. Seminars in Immunology .xxx (2009) xxx–xxx( article in press).

68. Dieren JM, et al. Roles of CD1d-restricted NKT cells in the intestine. Inflamm Bowel Dis. 13(9); 2007: 1146-1152.

69. Yamamoto S, et al. Amelioration of collagen-induced arthritis in mice by a novel phosphodiesterase 7 and 4 dual inhibitor, YM-393059. European Journal of Pharmacology. 559; 2007: 219–226.

70. Paterniti I, et al. PDE 7 Inhibitors: New Potential Drugs for the Therapy of Spinal Cord Injury. Nature. 6(1); 2011: 1-15.

71. Giembycz, MA and Smith SJ. Phosphodiesterase 7 (PDE7) as a therapeutic target. Drugs Fut. 31(3); 2006: 207

72. Arnaud-Lopez L, et al.. Phosphodiesterase 8B Gene Variants Are Associated with Serum TSH Levels and Thyroid Function. The American Journal of Human Genetics.82; 2008: 1270–1280.

73. lisatsai LC, et al. The high affinity cAMP-specific phosphodiesterase8B controls steroidogenesis in the mouse adrenal gland. Molecular pharmacology. 79(4); 2011: 639-648.

74. Dong H, et al. Phosphodiesterase 8(PDE8) regulates chemotaxis of activated lymphocytes. Biochem.biophys Res Commun. 345(2); 2006: 713-719.

75. Amanda G, et al. PDE8 regulates rapid Teff cell adhesion and proliferation independent of ICER. PloS ONE 5(8); 2010: e12011.

76. Andreeva SG, et al. Expression of cGMP-specific phosphodiesterase 9A mRNA in the rat brain. J. Neurosci. 21; 2001: 9068–9076.

77. Wunder F. chacterization of the first potent and selective PDE9 inhibitor using a cGMP reporter cell line. Molecular Pharmacology. 68(6); 2005: 1775-81.

78. Vanderstay FJ, et al. the novel selective PDE9 inhibitor BAY 73-6691 improves learning and memory in rodents. Neuropharmacology. 55(5); 2008: 908-18.

79. Wang H et al. Insight into binding of Phosphodiesterase-9A Selective inhibitors by crystal structures and mutagenesis. J. Med. Chem. 53(4); 2010: 1726-1731.

80. Hou J, et al. Structural Asymmetry of Phosphodiesterase-9, Potential Protonation of a Glutamic Acid, and Role of the Invariant Glutamine. Neuropharmacology. 6(3); 2011: 1-6.

81. Ring HA. and Serra-Mestres J. Neuropsychiatry of the basal ganglia. J. Neurol. Neurosurg. Psychiatry. 72; 2002: 12–21.

82. Graybiel AM. The basal ganglia. Curr. Biol. 10; 2000: R509–R511.

83. Hebb AL, Robertson HA and Denovan-Wright EM. Striatal phosphodiesterase mRNA and protein levels are reduced in Huntington’s disease transgenic mice prior to the onset of neuroscience. Neuroscience. 123; 2004: 967– 981.

84. Seeger TF, et al. (2003) Immunohistochemical localization of PDE10A in the rat brain. Brain Res. 985; 2003: 113–126.

85. Cantin LD, et al. PDE-10A inhibitors as insulin secretagogues. Bioorganic and Medicinal Chemistry Letters. 17; 2007: 2869–2873.

86. Siuciak, JA et al. Inhibition of the striatum-enriched phosphodiesterase PDE10A: a novel approach to the treatment of psychosis. Neuropharmacology. 51; 2006: 386–396.

87. Rodefer JS, et al. PDE10A inhibition reverses subchronic PCP-induced deficits in attentional set-shifting in rats. Eur. J. Neurosci. 21; 2005: 1070–1076.

88. Steven M, et al. Phosphodiesterase 10A Inhibitor Activity in Preclinical Models of the Positive, Cognitive, and Negative Symptoms of Schizophrenia. J. Pharmacol. And Expt. Therapeutics. 331(2); 2009: 574-590.

89. Torremans A, et al. Effects of phosphodiesterase 10 inhibition on striatal cyclic AMP and peripheral physiology in rats. Acta Neurobiol Exp. 70; 2010: 13–19.

90. Wayman C, et al. Phosphodiesterase 11 (PDE11) regulation of spermatozoa physiology. Int J Impot Res. 17; 2005: 216–223.

91. Makhlouf A, Kshirsagar A and Niederberger C. Phosphodiesterase 11: a brief review of structure, expression and function. International Journal of Impotence Research. 18; 2006: 501–509.

Received on 14.06.2011

Accepted on 01.08.2011

Research J. Pharmacology and Pharmacodynamics. 3(5): Sept –Oct. 2011, 223-233

PDE family	Isoform	Regulation	Substrate	Localization		Inhibitors	References
PDE family	Isoform	Regulation	Substrate	Tissue/cellular	intracellular	Inhibitors	References
PDE1	PDE1A	Ca²⁺/calmodulin PKA/PKG	cAMP, cGMP	Smooth muscle, heart, lung, brain, sperm	Predominantly cytosolic	Vinpocetine Nicardipine 8-MeOM-IBMX Nimodipine	[8,17]
	PDE1A1			Lung and heart
	PDE1A2			Brain
	PDE1B1			Neurons, lymphocytes, smooth muscle	Cytosolic
	PDE1B2			Macrophage and lymphocyte
	PDE1C			Brain, spermatids, proliferating human smooth muscles	Cytosolic
PDE2	PDE2A	Stimulated by cGMP	cAMP, cGMP	Adrenal medulla, brain, heart, macrophage, platelet and endothelial cell	PDE2A1is cytosolic; PDE2A2 and PDE2A3 variants are membrane bound	EHNA, anagrelide	[8, 17, 18]
PDE3	PDE3A	Inhibited by cGMP Phosphorylated by PKB	cAMP, cGMP	Heart, vascular smooth muscle, platelet, oocyte, kidney.	Membrane bound or cytosolic	Cilostamide, Milrinone, Cilostazol, Lixazinone	[8, 17, 19]
PDE3	PDE3B		cAMP, cGMP	Vascular smooth muscle, adipocytes, hepatocytes, kidney, T Lymphocyte, macrophages.	Endoplasmic reticulum and microsomal fractions	Cilostamide, Milrinone, Cilostazol, Lixazinone	[8, 17, 19]
PDE4	PDE4A	Phosphorylated by PKA Phosphorylated by ERK	cAMP	Olfactory system, immune cells, testis; brain	Ubiquitous	Rolipram Denbufylline Cilomilast Roflumilast	[8,17,20]
	PDE4B			Immune cells and brain
	PDE4C			Lung, testis, neurons
	PDE4D			Brain and inflammatory cells
PDE5	PDE5A	Binds cGMP Phosphorylated by PKA Phosphorylated by PKG	cAMP	Platelets, brain, lung, heart, kidney, skeletal muscle, penis	Cytosolic	Sildenafil Zaprinast Dipyridamole Ariflo Vardenafil Tadalafil	[8,17, 19,20]
PDE6	PDE6A/ PDE6B	Transducin-activated	cAMP	Rod cells of retina, pineal gland	Membrane bound	Zaprinast Dipyridamole Vardenafil Tadalafil	[7, 8, 17, 19]
PDE6	PDE6C			Cone cells of retina, pineal gland	Cytosolic	Zaprinast Dipyridamole Vardenafil Tadalafil	[7, 8, 17, 19]
PDE7	PDE7A	Rolipram-insensitive	cAMP	Immune cells, heart, skeletal muscle and endothelial cells.	Cytosolic cytosolic	Dipyridamole Thiadiazole	[17]
PDE7	PDE7B	Rolipram-insensitive	cAMP	mRNA found in brain, heart, liver, pancreas, testis, skeletal muscle	Cytosolic cytosolic	Dipyridamole Thiadiazole	[17]
PDE8	PDE8A	Rolipram-insensitive IBMX-insensitive	cAMP	mRNA found in testis, spleen, small intestine, ovary, colon, kidney	Found in cytosolic and particulate fractions	Dipyridamole	[7, 17]
	PDE8B		cAMP	Brain and thyroid	Found in cytosole		[7, 17]
PDE9	PDE9A	IBMX-insensitive	cAMP	Ubiquitous; but higher in kidney, brain, spleen, prostate.	Cytosolic	Zaprinast	[8, 17, 19, ]
PDE 10	PDE10A	Unknown	cAMP, cGMP	Brain, testis, heart, thyroid, pituitary gland, striated and cardiac muscle.		Dipyridamole Papaverine	[8, 17]
PDE 11	PDE11A	Unknown	cAMP, cGMP	mRNA found in salivary and thyroid gland, liver, prostate, testis, developing spermatozoa, skeletal muscle		Tadalafil Zaprinast Dipyridamole	[7,17,19 ]