Phosphodiesterase inhibitors block one or more of the subtypes of the phosphodiesterase enzyme, which regulate intracellular levels of cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP). These messenger chemicals communicate cellular surface signals to molecules within the cell, which control the effects of hormones and neurotransmitters that are responsible for cell function. Medications that inhibit phosphodiesterase interfere with the phosphodiesterase 3, 4, or 5 group of enzymes, also known as PDE3, PDE4, and PDE5. Phosphodiesterase inhibitors act as anti-platelet drugs, anti-inflammatory agents, and vasodilators.
Cilostazol and milrinone are examples of phosphodiesterase inhibitors that affect the PDE3 subtype of the enzyme. Physicians may prescribe cilostazol, which prevents platelet adhesion by inhibiting the breakdown of intracellular cAMP, increasing the levels of this chemical messenger, for patients diagnosed with intermittent claudication. The medication also produces some vasodilation, or dilation of the blood vessels. Patients diagnosed with congestive heart failure may receive short-term intravenous therapy with milrinone, which acts on cardiac and smooth muscle. The medication improves the ability of the left ventricle to contract and also relaxes vascular tissue, decreasing the strain on the heart.
Physicians might prescribe the phosphodiesterase inhibitors roflumilast or theophylline as treatment for the chronic obstructive pulmonary diseases (COPD) asthma, chronic bronchitis, or emphysema. Roflumilast inhibits PDE4, which increases cellular cAMP and prevents the inflammatory processes produced by granulated white blood cells. Patients typically use this medication daily for prevention of acute inflammatory attacks. Theophylline inhibits PDE3 and PDE4 enzymes, producing anti-inflammatory and bronchodilating effects. The medication also enhances the strength of the diaphragm muscles by allowing increased calcium ion influx.
Papaverine and sildenafil are phophodiesterase inhibitors that act against the PDE5 subtype of the enzyme, which increases cellular levels of cGMP. Health care providers might prescribe these medications for erectile dysfunction or as treatment for cardiac, cerebral, and peripheral or pulmonary arterial hypertension. The medications generally decrease systemic arterial spasticity and cause vasodilation. This class of medication also relaxes the heart muscle by interfering with conduction pathways. Patients can take papaverine orally, intramuscularly, or intravenously, while sildenafil is administered orally only.
Each group of phosphodiesterase inhibitors has the potential to cause a number of side effects. Medications effective against PDE3 may cause headaches, nausea, and low blood pressure, and come with an increased risk of dangerous ventricular dysrhythmias. PDE4 medications can cause nausea and vomiting while PD5 inhibitors can produce headaches, skin flushing, and low blood pressure.