Pharmacokinetics
Following oral administration, the drug demonstrates excellent absorption, with up to 90% of the dose absorbed from the gastrointestinal tract. However, despite high absorption rates, systemic bioavailability remains paradoxically low, ranging between 20–35%. The primary reason for this phenomenon is extensive presystemic elimination. During the initial passage of blood from the intestines through the liver (first-pass effect), a large fraction of the active substance is metabolized and degraded before reaching the systemic circulation and target organs.
Temporal parameters are well-defined. Following oral intake, the onset of the therapeutic effect occurs in approximately 1 hour, reaching peak plasma concentration and maximum clinical effect at 2 hours. The standard duration of the antianginal effect is 8 to 10 hours. Extended-release formulations maintain therapeutic blood concentrations for up to 24 hours. The elimination half-life ($t_{1/2}$) ranges from 3 to 7 hours. Metabolites are excreted primarily via the urine and bile, with only a small fraction (3–4%) excreted unchanged.
Mechanism of Antianginal Action
The antianginal mechanism of the drug is multifactorial and relies on the blockade of L-type voltage-gated calcium channels. This process consists of three key components that collectively restore the balance between myocardial oxygen demand and supply.
- Cardiac Effect (Decreased $O_2$ Demand). The drug directly affects the heart by blocking calcium channels in cardiomyocyte membranes. A reduction in intracellular calcium ion concentration leads to decreased myocardial contractility (negative inotropy) and heart rate (negative chronotropy). Consequently, overall myocardial workload decreases, which is critical for reducing myocardial oxygen demand during ischemia.
- Vascular Peripheral Effect (Decreased $O_2$ Demand). The second site of action is the smooth muscle of resistance vessels. Calcium channel blockade in this area causes smooth muscle relaxation and prominent vasodilation. Dilation of the vascular bed leads to a drop in total peripheral resistance (TPR). Therefore, the heart requires less effort to eject blood into the aorta, resulting in decreased afterload, which further conserves oxygen.
- Coronary Effect (Increased $O_2$ Delivery). The drug acts directly on calcium channels in the coronary vasculature supplying the heart. Their dilation improves myocardial blood flow, thereby increasing oxygen delivery to ischemic areas.
Indications and Adverse Effects
Based on its pharmacodynamic profile, the drug has established widespread clinical utility.
Main Indications:
- Chronic Stable Angina — for the prevention of anginal chest pain triggered by physical or emotional stress.
- Vasospastic (Prinzmetal's) Angina — due to its ability to relieve coronary artery spasm.
- Supraventricular Arrhythmias — utilized as an effective Class IV antiarrhythmic agent.
Adverse Effects: Despite its high efficacy, therapy requires monitoring due to potential adverse reactions. The most significant complications involve the cardiovascular system. Suppression of the conduction system can precipitate atrioventricular (AV) block. Excessive bradycardia (marked slowing of the heart rate) and hypotension (low blood pressure) may also occur. Among extracardiac side effects, patients most frequently report gastrointestinal disturbances such as nausea and vomiting.