Phosphodiesterase inhibitors(PDE inhibitors) are a class of agents acting on specific phosphodiesterase enzymes in target cells and are FDA approved for the management of chronic obstructive pulmonary disease, erectile dysfunction, pulmonary arterial hypertension, psoriasis, psoriatic arthritis, and atopic dermatitis. Phosphodiesterase enzymes' essential role serves to separate the phosphate group from the target cell and decrease cyclic adenosine monophosphate (cAMP) or cyclic guanosine monophosphate (cGMP). The inhibition of these enzymes further prevents the degradation of cAMP or cGMP, exhibiting smooth muscle relaxation, vasodilatory, and bronchodilatory effects.
Among the phosphodiesterase inhibitors, sildenafil is the more common agent that acts on PDE-5 and is FDA-approved in 1998 for erectile dysfunction. Sildenafil use is not limited to the treatment of erectile dysfunction(ED) and also obtained FDA approval in 2005 for the treatment of pulmonary arterial hypertension (PAH) in adults. The use of sildenafil in pediatric populations for pulmonary arterial hypertension(PAH) has not received approval and is considered off-label use. Vardenafil and avanafil are other PDE-5 inhibitors, also approved for erectile dysfunction(ED). Tadalafil is a PDE-5 inhibitor approved for the treatment of benign prostatic hyperplasia(BPH) and erectile dysfunction(ED). As both conditions may coincide in the later durations of life, tadalafil may be used as monotherapy for its therapeutic use to treat both conditions.
Cilostazol is a PDE-3 inhibitor that reversibly inhibits platelets from aggregation and is indicated for the treatment of intermittent claudication from peripheral arterial disease(PAD). Dipyridamole is also a PDE-3 inhibitor and is used for postoperative prophylaxis for thrombosis, but its action on the PDE-3 enzyme is relatively weak. Dipyridamole can also be a diagnostic tool as a pharmacologic nuclear stress test. Dypriadamole increases perfusion to the myocardium, revealing reversible and irreversible ischemic areas in the myocardial muscle.
Milrinone and amrinone also inhibit PDE-3 enzymes, but their actions target heart cells, increasing cAMP in the myocardium, producing a positive inotropic effect. Both agents may be used short-term for decompensated heart failure as therapy for greater durations may precipitate ventricular arrhythmias. Roflumilast is a PDE-4 inhibitor that acts on the target cells of lung tissue and is indicated for the treatment of reducing COPD exacerbations. Other PDE-4 inhibitors include apremilast and crisaborole; their uses are nonpulmonary, and indications include treating psoriasis, psoriatic arthritis(PA), and atopic dermatitis(AD) respectively due to their action of decreasing pro-inflammatory chemokines. PDE-4 inhibitors decrease pro-inflammatory mediators and cause bronchodilation by inhibiting the degradation of cAMP. Theophylline is a non-specific PDE inhibitor with weak action on phosphodiesterase enzymes and has approval for symptomatic relief of chronic obstructive pulmonary disease(COPD).
Pentoxifylline is a xanthine analog and is a non-selective phosphodiesterase inhibitor, FDA approved in 1984 for muscle pain, caused by intermittent claudication in chronic peripheral arterial disease. Anagrelide is a PDE-3 inhibitor approved for the treatment of thrombocythemia caused by myeloproliferative disorders. Ibudilast is a non-selective PDE inhibitor that has not secured approval by the FDA for its use. Ibudilast action causes dilations of the bronchioles and has been used to treat asthma in Japan since 1989. The agent is also said to cross the blood-brain barrier, exert anti-inflammatory actions in the central nervous system, and maybe a hopeful agent for inflammatory neurological disorders. Caffeine, one of the most popularly utilized psychoactive compounds used globally, also inhibits phosphodiesterase as one of its three mechanisms of action. It is labeled generally recognized as safe (GRAS) by the FDA since 1958. Caffeine is a CNS stimulant and is used for fatigue, drowsiness, enhancing memory, cognition, and performance. Caffeine citrate has been used to treat apnea of prematurity in neonates gestation age of twenty-two weeks to less than thirty-three weeks and received approval in 1999 for its use.
Cyclic adenosine monophosphate(cAMP) and cyclic guanosine monophosphate (cGMP) are intracellular second messenger molecules degraded and inactivated by the enzyme phosphodiesterases(PDE). Phosphodiesterase inhibitors exert their effects on their targeted phosphodiesterase enzymes(PDE-3, PDE-4, PDE-5), preventing cGMP or cAMP degradation, further increasing their levels in smooth muscle cells, causing relaxation and vasodilatory effect in target cells. PDE-3 inhibitors exert their effects by increasing cAMP levels in the myocardium, peripheral vasculature, and platelets. This further causes positive inotropic effects by increasing ionized calcium in the myocardium, vasodilation of the peripheral vessels, and prevents platelet aggregation and can be used for the treatment of decompensated cardiac failure and peripheral arterial disease(PAD).
PDE-4 is an enzyme found in cells of the lungs, and PDE-4 inhibitors inhibit the degradation of intracellular cyclic adenosine monophosphate(cAMP) and increase cAMP levels in target cells, further causing bronchial muscle relaxation. Along with its effects in the lungs, it also decreases pro-inflammatory mediators. These agents are useful in the treatment of COPD, psoriatic arthritis(PA), and atopic dermatitis(AD). Crisaborole is a PDE-4 inhibitor indicated for mild to moderate atopic dermatitis, and its mechanism of action of its therapeutic effect is not fully understood.
PDE-5 inhibitors carry out their action by preventing the breakdown of cyclic guanosine monophosphate (cGMP), further increasing cGMP levels in target cells. The endothelial cells of the penile smooth muscle corpus cavernosum release nitric oxide(NO), which initiates enzyme guanylate cyclase, further enhancing the synthesis of cyclic guanosine monophosphate (cGMP). The prevention of cGMP breakdown further prolongs an erection and is used therapeutically for the treatment of erectile dysfunction(ED). PDE-5 inhibitors also cause pulmonary vasodilation and may be useful for the treatment of pulmonary arterial hypertension(PAH) as PAH may be correlated with the weakened release of nitric oxide in the pulmonary vasculature, resulting in decreased cGMP levels. Nonspecific inhibitors(PDE-3,4,5) exert their action, increasing cAMP levels in the pulmonary vasculature, resulting in bronchial relaxation and further decreasing pro-inflammatory mediators.
Theophylline is a nonspecific phosphodiesterase inhibitor used for the treatment of COPD, and the mechanism of action is not entirely understood. Pentoxifylline's mechanism of action, although not fully understood, is believed to act by the nonselective inhibition of phosphodiesterase enzymes similar to theophylline. This action further enhances blood flow and perfusion in the limbs by increased erythrocyte flexibility, decreased blood viscosity, decreased cell proliferation, and reduced inflammation. This mechanism allows pentoxifylline to be used therapeutically in patients suffering from muscle aches and pain associated with peripheral arterial disease.
Anagrelide exerts its effects by inhibiting PDE-3, decreasing the platelet count and platelet aggregation. The full agent's action is still being studied and is said to interrupt the cell cycle post miosis of megakaryocyte development, decreasing megakaryocyte's size and ploidy. This drug helps to decrease platelet count, further preventing thrombotic complications in myeloproliferative disorders such as essential thrombocytopenia, polycythemia vera, and chronic myeloid leukemia. Ibudilast exerts its actions by nonselective phosphodiesterase inhibition of PDE-3, PDE-4, PDE-10, and PDE-11, decreasing nitric oxide production, pro-inflammatory chemokines, LTB4, and toll-like receptor 4 (TLR4).
Ibudilast's bronchodilator and anti-inflammatory effects have been used therapeutically in Japan to treat bronchial asthma and in post-stroke complications. Caffeine exerts its effects on phosphodiesterase enzymes by nonselective inhibition of PDE1, PDE4, and PDE5, promoting cAMP accumulation. Caffeine also inhibits adenosine and gamma-aminobutyric acid (GABA) receptors and enhances intracellular calcium release, which is used for improving and enhancing cognition, memory, energy, sleep, fatigue, and drowsiness.
Phosphodiesterase inhibitors (PDE inhibitors) are available in oral tablets, injection form, and topical ointment.
These agents can be administered 30 to 60 minutes prior to sexual intercourse.
Benign prostatic hyperplasia (BPH)
Tadalafil is available in 2.5 mg, 5 mg, 10 mg, and 20 mg tablets.
Pulmonary arterial hypertension (PAH):
Sildenafil is available in 20 mg and 10 mg(12.5ml)vial and can be administered orally or via intravenous injection for pulmonary arterial hypertension.
Psoriatic arthritis (PA):
Day 1: 10 mg/day
Day 2: 10 mg twice a day
Day 3: 10 mg (morning) and 20 mg (evening)
Day 4: 20 mg (morning) and 20 mg (evening)
Day 5: 20 mg (morning) and 30 mg (evening)
Day 6: 30 mg twice daily
Apremilast tablets are available in 10 mg, 20 mg, and 30 mg tablets and are administered orally.
Chronic obstructive pulmonary disease (COPD):
Theophylline is available in 200 mg, 400 mg, and 800 mg in 5% dextrose for IV injection.Roflumilast is available in 500mg tablets for oral administration.
Peripheral arterial disease (PAD):
Postoperative thromboembolic prophylaxis:
Decompensated cardiac failure:
Atopic Dermatitis (AD):
Crisaborole can be used in patients age two years or older for topical use only. Should be applied to the affected areas two times daily.
Fatigue and Drowsiness
Should not be consumed six hours prior to bedtime.
Sildenafil, vardenafil, and tadalafil have been associated with hearing loss. Often the hearing loss is unilateral and may occur during the initial 24-hours after administration of the agent and is temporary in almost one-third of patients. Patients should seek immediate medical attention if hearing loss does pursue. Although sildenafil is a PDE-5 inhibitor, it mildly has an inhibitory effect on a phosphodiesterase-6 enzyme found in the retinal rod and cone receptors. This may result in a "blue-tinted" visual disturbance in some subjects taking higher doses. Patients also may experience increased sensitivity to the brightness of lights.
These visual disruptions due to the electrical responses to light are temporary and reversible. Priapism is a reported adverse effect of phosphodiesterase-5 inhibitors. Patients who have erections lasting more than four hours should seek medical attention as permanent damage may result. Apremilast has an increased risk of depression. Patients who have an underlying history of depression should be evaluated thoroughly prior to the initiation of apremilast. Roflumilast may also worsen underlying depression or increased suicidal thoughts; a thorough assessment should be conducted prior to therapy.
Hypersensitivity is an absolute contraindication to all phosphodiesterase inhibitors. Nitrites are contraindicated to use alongside PDE-5 inhibitors: sildenafil, tadalafil, vardenafil, and avanafil as it may result in severe hypotension. Relative contraindication such as pre-existing cardiovascular disease, resting hypotension, and anti-hypertensive use require evaluation before phosphodiesterase inhibitor use.
Heart failure of any sort is a contraindication to cilostazol use. Patients who are taking medications that may cause QT prolongation or have congenital QT syndrome should avoid taking vardenafil. Patients with hemostatic or bleeding disorders are contraindicated to cilostazol use as its mechanism of action inhibits platelet aggregations and may worsen the underlying conditions. Milrinone and amrinone are contraindicated in patients who have a pre-existing valvular disease of the aortic or pulmonic valve as it may cause outflow obstruction. Pentoxifylline is contraindicated in patients with a recent history of cerebral hemorrhage or retinal hemorrhage.
Black box warning:
Medications that cause vasodilatory effects such as antihypertensives, alpha-blockers, and amlodipine, should be avoided with concomitant use with phosphodiesterase-5 inhibitors. If used in sequence, patients should have routine monitoring, and therapy should be started on the lowest possible dosage as it may result in profound hypotension, lightheadedness, and dizziness. Patients who have renal insufficiency and hepatic impairment can result in higher serum concentration of medication; dose adjustment may be necessary. Fluid and electrolytes require monitoring routinely with milrinone use as it may precipitate hypokalemia and induce arrhythmias. Precautions are necessary for patients with underlying ophthalmopathy taking sildenafil, vardenafil, tadalafil, and avanafil as unexpected loss of vision may occur unilaterally or bilaterally, which may be an indication of non-arteritic anterior ischemic optic neuropathy (NAION).
Phosphodiesterase-5 inhibitors are substantially and quickly metabolized by the CYP3A4 enzyme found in the liver. CYP3A4 inhibitors such as ritonavir, ketoconazole, verapamil, itraconazole erythromycin, and grapefruit juice can increase serum concentrations of PDE-inhibitors and require dosing adjustments. Patients taking cilostazol for the management of peripheral arterial disease(PAD) should routinely monitor platelet, and WBC count as thrombocytopenia or leukopenia can develop. Dipyridamole may cause an elevation in liver enzymes and should be monitored routinely and used with precaution in patients with pre-existing liver insufficiency. Amirinone use may cause thrombocytopenia and is hepatotoxic; platelet count and liver enzymes should be monitored routinely. Thrombocytopenia from amrinone may be dose-dependent and is said to be non-immune mediated.
Patients receiving treatment milrinone should be monitored via EKG as it can precipitate ventricular arrhythmias. Electrolytes should also be monitored with milrinone use as it can decrease serum potassium. Patients being treated with roflumilast and apremilast should be followed for alterations in mood, such as worsening depression or suicidal thoughts as it may precipitate underlying or undiagnosed mood disorders. Anagrelide serum concentration can be increased eightfold in hepatic impairment. Liver function tests should be conducted prior to therapy, and platelet count requires routine monitoring for patients who are taking this medication.
PDE-4 inhibitor theophylline serum concentrations of more than 20 mcg/mL are in a toxic range. Theophylline induced seizures and arrhythmias prompt treatment immediately as it may result in mortality from hemodynamic compromise at serum concentrations of 30 mcg/mL or greater. Therapy with anticonvulsants such as benzodiazepine and antiarrhythmic treatment should start immediately. PDE-5 and PDE-3 inhibitors undergo hepatic metabolism, and inhibitors of CYP3A4 or CYP2C19 can increase the serum drug concentrations of therapy. Dose modifications are required before the initiation of treatment to avoid adverse effects.
Hepatoxicity, although uncommon with sildenafil use, has been reported, but studies of the correlation are limited. Anagrelide should not be administered in patients with pre-existing liver disease as it can result in large increases in drug concentration. Drug toxicity may pursue with a dosage of 10 mg per day or greater or 2.5 mg in a single dose of agrenalide. Caffeine toxicities can pursue at 50 mg/L to 350 mg/L, causing arrhythmias, seizures, and mortality. Caffeine toxicity induced seizures can be treated with benzodiazepines, and dysrhythmias have shown successful recovery with the treatment of beta-blockers such as metoprolol or esmolol.
Phosphodiesterase inhibitors(PDE inhibitors) are a class of agents acting on phosphodiesterase enzymes. They are FDA approved for the management of chronic obstructive pulmonary disease, erectile dysfunction, pulmonary arterial hypertension, acute decompensated heart failure, psoriasis, psoriatic arthritis, atopic dermatitis, and neonatal apnea. The management of patients administered PDE inhibitors requires decisive care and decision making from an interprofessional team of healthcare professionals, as the management of each condition can lead to a better quality of life for patients. These healthcare professionals include a primary care physician, a specialist (cardiologist, pulmonologist, urologist, or rheumatologist) depending on the management of the disease, a nurse, and a pharmacist. The primary care and specialist should be familiar with PDE inhibitors, their indications and use, adverse effects, and contraindications. Patients should be fully educated on the side effects of therapy, such as headaches and a decrease in blood pressure that may cause dizziness and lightheadedness.
The patient should also be advised to seek medical attention immediately if more concerning effects such as visual disturbances, hearing loss, and priapism for long periods occur with the use of PDE-5 inhibitors (sildenafil, tadalafil, vardenafil, and avanafil). Elderly population prescribed therapy for erectile dysfunction should be thoroughly evaluated in case dosing modifications may be needed for renal and hepatic impairment in this patient population. The primary care physician should be aware of any other medications the patient may be taking short-term or long-term. Phosphodiesterase inhibitors can cross-react with other agents as they are metabolized by the cytochrome p450 system in the liver resulting in subtherapeutic or toxic serum concentrations.
Cardiac risk factors should be carefully assessed before treatment as this class of medication may cause a decrease in blood pressure and can precipitate or further worsen an underlying cardiac disease. Patients taking antihypertensives should be advised about its concurrent use with phosphodiesterase inhibitors as it can cause symptomatic life-threatening hypotension. PDE-4 inhibitors may worsen or precipitate psychiatric symptoms such as suicidal thoughts and depression in susceptible individuals. The use of this medication class can be discussed with a psychiatrist prior to administration in select individuals.
Nonselective phosphodiesterase inhibitors such as theophylline and caffeine should be used with caution as toxicity can result in seizures, arrhythmias, and mortality. In the event seizures or arrhythmias occur, patients should be treated promptly with benzodiazepines, and beta-blockers as indicated and as needed until the patient is stabilized. The primary care physician should periodically monitor patients as some phosphodiesterase inhibitors may cause elevations in liver enzymes, thrombocytopenia, and electrolyte disturbances. Joint decision making and interprofessional interaction are essential components to decrease adverse effects and enhance patient-centered care to achieve the desired outcome.
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