|Systematic (IUPAC) name|
|Bioavailability||100% (as IV bolus, infusion)|
|Protein binding||70 to 80%|
|Biological half-life||2.3 hours (mean, in CHF)|
|Excretion||Urine (85% as unchanged drug) within 24 hours|
|Molar mass||211.219 g/mol|
|Melting point||315 °C (599 °F)|
|Boiling point||449 °C (840 °F)|
Milrinone, commonly known and marketed under the brand name Primacor, is a medication used in patients who have heart failure. It is a phosphodiesterase 3 inhibitor that works to increase the heart's contractility and decrease pulmonary vascular resistance. Milrinone also works to vasodilate which helps alleviate increased pressures (afterload) on the heart, thus improving its pumping action. While it has been used in people with heart failure for many years, recent studies suggest that milrinone may exhibit some negative side effects that have caused some debate about its use clinically.
Overall, milrinone supports ventricular functioning of the heart by decreasing the degradation of cAMP and thus increasing phosphorylation levels of many components in the heart that contribute to contractility and heart rate. Milrinone use following cardiac surgery has been under some debate because of the potential increase risk of postoperative atrial arrhythmias. However, in the short term milrinone has been deemed beneficial to those experiencing heart failure and an effective therapy to maintain heart function following cardiac surgeries. There is no evidence of any long term beneficial effects on survival.
Contractility in the heart
People experiencing some forms of heart failure have a significant decrease in the contractile ability of heart cells (cardiomyocytes). This impaired contractility occurs through a number of mechanisms. Some of the main problems associated with decreased contractility in those with heart failure are issues arising from imbalances in the concentration of calcium. Calcium permits myosin and actin to interact which allows initiation of contraction within the cardiomyocytes. In those with heart failure there may be a decreased amount of calcium within the cardiomyocytes reducing the available calcium to initiate contraction. When contractility is decreased the amount of blood being pumped out of the heart into circulation is decreased as well. This reduction in cardiac output can cause many systemic implications such as fatigue, syncope and other issues associated with decreased blood flow to peripheral tissues.
Mechanism of action
There are receptors on cardiomyocytes called β-adrenergic receptors. These receptors are stimulated by molecules such as norepinephrine and epinephrine. Stimulation of these receptors causes a cascade of events ultimately leading to increase cyclic adenosine monophosphate (cAMP) within the cell. Cyclic adenosine monophosphate causes increased activation of protein kinase A (PKA). PKA is a protein that phosphorylates many components within the cardiomyocytes and either activates or inhibits their action. Phosphodiesterases are enzymes responsible for the breakdown of cAMP. Therefore, when phosphodiesterases break down cAMP the amount of PKA within the cell decreases as well.
Milrinone is a phosphodiesterase-3 inhibitor. This drug inhibits the action of phosphodiesterase-3 and thus prevents degradation of cAMP. With increased cAMP levels there is an increase in the activation of PKA. This PKA will phosphorylate many components of the cardiomyocyte such as calcium channels and components of the myofilaments. Phosphorylation of calcium channels permits an increase in calcium influx into the cell. This increase in calcium influx permits increased contractility. PKA also phosphorylates potassium channels promoting their action. Potassium channels are responsible for repolarization of the cardiomyocytes therefore increasing the rate at which cells can depolarize and generate contraction. PKA also phosphorylates components on myofilaments allowing actin and myosin to interact more easily and thus increasing contractility and the inotropic state of the heart. Milrinone allows stimulation of cardiac function independently of β-adrenergic receptors which appear to be down-regulated in those with heart failure.
In recent years many studies have been performed showing that milrinone use may present potential adverse side effects in heart failure patients. Following cardiac surgery milrinone has been used as a therapy to maintain ventricular function of the heart. A study conducted by Fleming and colleagues has shown that milrinone use may be associated with increase atrial fibrillation following cardiac surgery. In another study by Smith and colleagues, milirinone appeared to generate a 3-fold increase in tachyarrhythmias following surgery for congenital heart disease. However, other studies suggest that milrinone is extremely beneficial in maintaining heart function in the short term following surgical procedures.
Singh, B.; 1983, U.S. Patent 4,413,127.
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