Phosphodiesterase inhibitors

A description of various phosphodiesterase inhibitors and their pharmacology.

1. Phosphodiesterase Inhibitors Dr. Mohit Kulmi

2. Introduction A phosphodiesterase is a enzyme that breaks a phosphodiester bond. There are many families of phosphodiesterase including phospholipases C and D, autotaxin, sphingomyelin phosphodiesterase, DNase, RNase and restriction endonucleases. Phosphodiesterases are a diverse family of enzymes that play a key role in regulating intracellular levels of the second messengers cAMP and cGMP, and hence cell function.

3. The cyclic nucleotide phosphodiesterases degrade the phosphodiester bond in the second messenger cAMP and cGMP. PDEs are therefore important regulators of signal transduction mediated by these second messenger molecules. When referring to phosphodiesterases we usually refers to cyclic nucleotide phosphodiesterases which have great clinical implications. PDE family: 11 isoenzymes families (PDE 1-PDE 11) with over 50 isoforms

4. Historical aspects The pionering work on phosphodiesterase began with Henry Hyde Salter in 1886. He was an asthamatic and he noted that when he drank a strong cup of coffee empty stomach his breathing eased, this was a effect attributed to the bronchodilator property of caffeine. Although the mecchanism of action that time was unknown. It has shown now that caffeine act as a weak non-selective PDE inhibitor.

5. The different forms and subtypes of phosphodiesterase were initally isolated from rat brains by Uzunov and Weiss in 1972 and were soon afterwards shown to be selectively inhibited in the brain and in other tissues by a variety of drugs. The potential for selective PDEI’s as theraupetic agents was predicted as early as 1977 by Weiss and Hait. This predicion is now proved to be true in a variety of fields.

6. Classification The PDE superfamily of enzymes is classified into 11 families , namely PDE1 – PDE11. The classification is based on: PDE substrate specificities by enzyme family. Amino acids sequences. Regulatory properties Tissue distribution.

7. PDEs have different substrate specificities. Some are cAMP selective hydrolases – PDE4,7 and 8 Others are cGMP selective – PDE5, 6 and 9 Others can hydrolysis both cAMP and cGMP – PDE1,2,3,10 and 11

8. MAIN TISSUE LOCALIZATION PDE Brain, heart, vascular smooth muscle 1 Adrenal cortex, brain, heart, corpus cavernosum 2 Heart, corpus cavernosum, vascular smooth muscle, platelets, liver pancreas 3 Lung, mast cells, vascular smooth muscle 4 Corpus cavernosum, lung, vascular smooth muscle, platelets, brain, esophagus 5 Retina 6 Skeletal muscle, T cells 7 Testis, thyroid 8 Broadly expressed, not well characterized 9 Brain, testes 10 Skeletal muscle, prostate, liver, kidney, pituitary, testis 11

9. Effect of PDE inhibition PDE cAMP/cGMP AMP/GMP cAMP and cGMP escape from degradation thereby exert many effects. Increased cardiac contractility Smooth muscle relaxation Reduced immune and inflammatory activity of cells.

10. Phosphodiesterase Inhibitors Drugs that block subtypes of the enzyme phosphodiesterase (PDE ). T herefore preventing the inactivation of the intracellular second messengers cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP) by the respective PDE subtype(s ). They are classified into non-selective PDE inhibitors and selective PDE.

11. Classification: A) Nonselective phosphodiesterase inhibitors Caffeine A minophylline IBMX (3-isobutyl-1-methylxanthine) Paraxanthine Pentoxifylline , Theobromine, Theophylline .

12. They act as competitive nonselective phosphodiesterase inhibitors which raise intracellular cAMP, activate protein kinases, inhibit TNF-alpha and leukotriene synthesis, and reduce inflammation.

13. B) Selective phosphodiesterase inhibitors PDE1 selective inhibitors Vinpocetine PDE2 selective inhibitors EHNA ( e rythro-9-(2- h ydroxy-3- n onyl) a denine) Anagrelide PDE3 selective inhibitors Enoximone and milrinone,

14. PDE4 selective inhibitors Mesembrine, Rolipram, Ibudilast, Piclamilast, Luteolin, Drotaverine. PDE5 Inhibitors Sildenafil , Tadalafil, Vardenafil (10 times more potent than sildenafil) Udenafil , Avanafil, Lodenafil

15. T heophylline Apart from inhibiting adenosine receptors, also nonspecificially inhibits PDE enzyme. Wide range of actions CVS – Positive Ionotropic, Chronotropic Smooth Muscles – Relaxation Reduced activity of Inflammatory Cells. Oral theophylline used in patients of COPD and mild to moderate asthma. Act synergistically with β 2 agonists in asthma .

16. Adverse Drug Reactions Narrow margin of safety, theraupeutic range 8-15 mcg/ml Adverse drug reaction Proposed Mechanism Nausea, Vomitting PDE 3 inhibition Headache PDE 3 inhibition Gatric Discomfort PDE 3 inhibition Diuresis Adenosine antagonism Cardiac Arrhythmias PDE 3 inhibition Seizures Adenosine antagonism

17. Drug Interactions Reduced activity in presence of enzyme inducers like rifampicin, carbamazepine et, the dose has to be increased. Increased activity in presence of enzyme inhibitors such as ketoconazole, cimetidine, erythromycin etc, dose must be decreased. Enhanced effect of sympathomimetics, digitalis, hypoglycemic, anticoagulant agents.

18. Pentoxifylline Theobromine analogue, inhibits PDE enzyme. Reduces blood viscosity and improves blood flow in ischemic area through microcirculation. Uses One of the drug of choice for peripheral vascular diseases. Non hemorrhagic stroke Chronic cerebrovascular insufficiency Trophic leg ulcers Diabetic neuropathy. Also inhibits production of TNF-alpha in AIDS patients. Used in dose of 400 mg BD orally .

19. PDE 1 Inhibitor - Vinpocetine Semisynthetic derivative alkaloid of Vincamine, an extract from the periwinkle plant. Still an investigational agent having variety of properties. As vasodilator – By inhibiting PDE type-1 leading to increase in intracellular levels of cGMP, an action that causes the vasorelaxant effects of vinpocetine on cerebral smooth muscle tissue.

20. As anti-inflammatory agent – Reduces the TNF α - induced expression of the mRNA of proinflammatory molecules such as interleukin-1 β , monocyte chemoattractant protein-1 (MCP-1), and vascular cell adhesion molecule-1 (VCAM-1) Vinpocetine’s anti-inflammaory properties would have a protective effect in models of neurodegenerative conditions such as AD and PD.

21. Adverse drug reactions Generally well tolerated Some cases of agranulocytosis reported 2-5 mg tab OD can be inccreased upto 10 mg/day.

22. PDE 2 inhibitors - Anagrelide Drug used for the treatment of essential thrombocytosis or overproduction of blood platelets. Also has been used in the treatment of chronic myeloid leukemia. Mechanism of action – Inhibiting the maturation of platelets from megakaryocytes. Inhibitor of phosphodiesterase-II. It inhibits PDE-3 and phospholipase A2.

23. Uses The combination of anagrelide and aspirin for the initial management of Essential thrombocythemia. Adverse drug reactions – Headache, diarrhoea, unusual weakness/fatigue, hair loss, nausea and dizziness. Less common side effects include: congestive heart failure, myocardial infarction, cardiomyopathy.

24. PDE 3 inhibitors Approved PDE3 inhibitors include the following: Amrinone Cilostazol Milrinone Enoximone A lso referred as nonglycoside nonsympathomimetic ionotropic agents. Positive ionotropic on heart, vasodilatation of vessels.

25. Inhibition of the PDE isoenzyme 3 leads to an increase of intracellular concentrations of the second messenger cyclic adenosine monophosphate (cAMP). cAMP mediates the phosphorylation of protein kinases, which in turn activates cardiac calcium channels.

26. An increased calcium influx from the sarcoplasmic reticulum (SR) during phase 2 (the plateau phase) of the cardiac action potential leads to a positive inotropic effect of PDE3 inhibitors: they increase the force of cardiac contraction. Increased reflux of calcium into the SR following the plateau phase is responsible for their positive lusitropic effect: they increase relaxation speed. Additionally, PDE3 inhibitors act as vasodilators

27. Kinetics Active orally as well as parenterally Half life is 3-6 hours , excreted via urine. Adverse drug reactions – Nausea, vomitting Cardiac arrythmias Thrombocytopenia Altered liver enzymes Milrinone is less toxic.

28. Uses Only used IV for acute heart failure cases in dose of 100 mg, repeat if necessary. Potentiates action of β 1 agonist. Cilastazole - Raises cAMP in vessel wall leading to vasodilatation and inhibition of platelet aggregation (antiplatelet action) Used as an antiplatelet agent mainly in intermittent claudication. Also used as adjuvant antianginal agent. Metabolized by CYP3A4 hence concurrent use of verapamil, diltiazem and ketoconazole should be avoided. 100 mg OD orally is usual dose.

29. PDE 4 inhibitors Enzyme located in respiratory tract, inhibition of which raises cAMP, causing smooth muscle relaxation and reduced release of cytokines, in turn reduces migration and activation of immune cells. Also potentiates β 2 agonist in asthma. Roflumilast and cilomilast 500 μ g/day oral alone or in combination with β 2 agonist Nausea, vomitting, headache, nasopharyngitis, URTI, decreased appetite.

30. Apremilast , a phthalimide derivative that was approved by the US FDA in March 2014 for use as a treatment for psoriatic arthritis , and in September 2014 for the treatment of plaque psoriasis . Cilomilast , in clinical development for treatment of COPD Ibudilast , a neuroprotective and bronchodilator drug used mainly in the treatment of asthma and stroke. It inhibits PDE4 to the greatest extent, but also shows significant inhibition of other PDE subtypes, and so acts as a selective PDE4 inhibitor or a non-selective phosphodiesterase inhibitor, depending on the dose.

31. Piclamilast , a more potent inhibitor than rolipram . Roflumilast , licensed for the treatment of severe chronic obstructive pulmonary disease in the EU and US.

32. Drotaverine Novel PDE 4 inhibitor, Elevated cAMP leads to relaxation of smooth muscles of GIT. Used as antisposmodic agent in intestinal. Biliary, renal colics as well as irritable bowel syndrome and uterine spasms. Oral as well as parenteral 40-80 mg TDS. Headache, dizziness,, flushing. Fall in BP on IV injection.

33. PDE5 selective inhibitors PDE5 has only one subtype, PDE5A, of which there are 4 isoforms in humans called PDE5A1-4. PDE5 enzyme is specific for cGMP which means it only hydrolyzes cGMP but not cAMP, the selectivity is mediated through network of hydrogen bonding which is favorable for cGMP but unfavorable for cAMP in PDE5. PDE5 is responsible for the degradation of cGMP in the smooth muscle cells lining the blood vessels supplying the corpus cavernosum of the penis, which leads to erectile dysfunction (ED).

34. Mechanism of action: In response to sexual stimulation, Nitric oxide released from nonadrenergic-noncholinergic neurotransmission and the endothelium of the cavernous smooth muscle is probably the principal neurotransmitter for penile erection. Within the muscle, nitric oxide activates a guanylyl cyclase that raises intracellular concentrations of cyclic guanosine monophosphate (GMP).

35. Cyclic GMP in turn activates a specific protein kinase which results in the opening of the potassium channels and hyperpolarization and causes sequestration of intracellular calcium and blocks calcium influx. As a result of this drop in cytosolic calcium, smooth muscle relaxation occurs leading to erection.

36. PDE-5 inhibitors do not increase the nitric oxide level, but they potentiate the nitric oxide effect to stimulate erection. Without sexual arousal, these inhibitors are ineffective

37. Sildenafil, Tadanafil Indicated in treatment of erectile dysfunction in men due to organic or psychological causes. No effect in absence of sexual stimulation. Can lead to - Headache, nasal congestion, flushing, hypotension. Disturbance in color vision due to inhibition of PDE 6 in retina. Ability to potentiate NO, so should be used cautiously in patients of angina, HTN who are using nitrates.

38. Interactions Reduced activity in presence of enzyme inducers like rifampicin, carbamazepine. Increased activity in presence of enzyme inhibitors such as ketoconazole, cimetidine, erythromycin. Off label use – PDE 5 also locatd in lungs, inhibition leads to raised cAMP leading to vasodilatation. Used in pulmonary hypertension.

39. Future trends in PDE 5 inhibitors Premature ejaulation Adding PDE 5 inhibitors to SSRIs for the treatment of premature ejaculation could result in better ejaculatory control according to recent studies. Possible mechanism is based on nitric oxide/cGMP transduction system as a central and peripheral mediator of inhibitory non-adrenergic, non-cholinergic neurotranmission in the urogenital system.

40. Female sexual arousal disorder – PDE 5 is expressed in clitoral corpus cavernosum and in vaginal smooth muscle and epithelium. Increased levels of cGMP have been shown to occur in human cultured vaginal smooth muscle cells treated with a PDE 5 inhibitor suggesting involvement of the NO/cGMP axis in the female sexual response. Therefore it is possible tat PDE 5 inhibitors could affect female sexual arousal disorder but further research is needed.

41. 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’s attack. The capillary blood flow velocity also increased in each individual patient and the mean capillary flow velocity of all patients increased significantly.

42. Stroke Sildenafil has been shown to significantly improve neurovascular coupling without affecting overll 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 PDE 5 inhibitors may have a role in promoting recovery from stroke.

43. Recent advances Spiroquinazolinones as novel, potent, and selective PDE 7 inhibitors – The latest scientific findings about PDE 7 inhibition provide a novel approach to treat a variety of immunological diseases. Taking this into account, the interesting profile as PDE7 inhibitors makes them as promising drugs to treat a variety of human diseases, predominantly disorders of immune as multiple sclerosis, and ifflammatory systems and also disorders of the CNS such as depression, psychosiss and alzheimer disease.

44. The novel selective PDE 9 inhibitor BAY 73-6691 improves learning and memory in reodents – It may have a role in neurodegenerative diseases like AD.

45. PDE 10 inhibitors PDE 10 is an enzyme that has been shown to be present at high levels in neurons in areas of the brain that are closely associated with many neurological and psychiatric disorders. By inhibiting PDE 10, levels of cAMP and cGMP are increased within neurons and the ability of these neurons to function properly is therby improved. PDE 10 inhibitors have been shown to be as effective in vivo as current anti-psychotic drugs.

46. Conclusion I t is only in the last 10 years, that potent PDE selective drugs have begun to make an impact in the treatment of various diseases. T he worldwide success of sildenafil in treating erectile dysfunction is a an evidence of the same . Selective PDE inhibitors are being investigated in a wide range of diseases including the use of PDE2 inhibitors in sepsis ; PDE5 inhibitors to treat sexual dysfunction in females , cardiovascular disease and pulmonary hypertension ; and PDE4 inhibitors to treat asthma,COPD , allergic rhinitis, psoriasis, multiple sclerosis,depression , Alzheimer's disease and schizophrenia.

47. 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 . A s a result it is likely that better therapeutically active drugs will emerge. A specific inhibitor for each form of PDE could pave the way for basic research on PDE regulation and provide for eventual therapeutic application to control abnormal function.

48. Thank You