ANTIANGINAL DRUGS

ANTIANGINAL DRUGS

Angina pectoris is a clinical manifestation described as a sudden, sharp retrosternal pain. An acute anginal attack is thought to occur because of an imbalance between myocardial oxygen supply and demand owing to the inability of coronary blood flow to increase in proportion to increases in myocardial oxygen requirements. This is generally the result of severe coronary artery atherosclerosis. Angina pectoris may also occur as a result of vasospasm of large epicardial coronary vessels or one of their major branches. In addition, angina in certain patients may result from a combination of coronary vasoconstriction, platelet aggregation, plaque rupture, and an increase in myocardial oxygen demand.

Antianginal drugs may relieve attacks of acute myocardial ischemia by increasing myocardial oxygen supply or by decreasing myocardial oxygen demand or both. Three groups of pharmacological agents have been shown to be effective in reducing the frequency, severity, or both of primary or secondary angina. These agents include the nitrates, β-adrenoceptor antagonists, and calcium entry blockers.

The major therapeutic objectives in the treatment of angina are aimed at terminating or preventing an acute attack and increasing the patient’s exercise capacity. These objectives can be achieved by reducing overall myocardial oxygen demand or by increasing oxygen supply to ischemic areas.

Organic Nitrates

The prototype of these agents is nitroglycerin. Other common organic nitrates are isosorbide mononitrate, isosorbide dinitrate.

The mechanism of action of nitroglycerin and other organic nitrates is thought to involve an interaction with nitrate receptors that are present in vascular smooth muscle. This interaction lead to the increase in intracellular cyclic guanosine monophosphate (cGMP) formation. The increase in cGMP results in vascular smooth muscle relaxation. The effects of nitrates is seen majorly on venous dilation leading to decreased venous return to the heart, thereby reducing myocardial oxygen demand. At higher doses, nitrates cause arterial dilation which manifests as decreased peripheral resistance and reduced cardiac workload. In summary, nitrates reduce the preload (myocardial tension during filling) and afterload.

NITROGLYCERIN

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β-Adrenoceptor Blocking Agents

β-Adrenoceptor blockade is a rational approach to the treatment of angina pectoris, since an increase in sympathetic nervous system activity is a common feature in acute anginal attacks. Based on their ability to reduce oxygen demand, all β-blockers tested so far have also been shown to be effective in the treatment of secondary angina.

Mechanism of Action

The myocardial response to exercise includes an increase in heart rate and myocardial contractility. These effects are mediated in part by the sympathetic nervous system. Propranolol and other β-adrenoceptor blockers antagonize the actions of catecholamines on the heart and thereby attenuate the myocardial response to stress or exercise. Arterial blood pressure (afterload) is also reduced by propranolol. Thus, propranolol may exert a part of its beneficial effects in secondary angina by decreasing three of the major determinants of myocardial oxygen demand, that is, heart rate, contractility, and systolic wall tension. Finally, there is evidence that β- blockers can inhibit platelet aggregation.

PROPRANOLOL (Discussed earlier)

ATENOLOL (Discussed earlier)

Calcium Entry or Calcium Channel Blockers

The calcium entry blockers or calcium channel blockers are a group of orally active drugs that have been approved for use in the treatment of vasospastic and effort- induced angina. These compounds block calcium channels in vascular smooth muscle and the heart, block platelet aggregation, and are particularly effective in the prophylaxis of coronary vasospasm or variant angina. In addition, these compounds are used in the chronic treatment of secondary angina. Two members of this group, verapamil and diltiazem are widely used in cases of angina.

VERAPAMIL

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DILTIAZEM

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