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Indications

Metoprolol belongs to the class of compounds known as β-adrenergic antagonists. The β-adrenergic antagonist is a lipophilic compound with a molecular weight of approximately 267.3 g/mol. The chemical structure of metoprolol consists of substituted phenylpropanolamine, providing the necessary structural features for selective β-1 adrenergic receptor blockade.[1] As one of the first selective β blockers, metoprolol mainly affects the β-1 receptors in the heart and less affects the β-2 receptors in the lungs and blood vessels.[2] Metoprolol was patented in 1970 and approved for medical use in 1978. Metoprolol is now a generic medication under various brand names, such as Lopressor, Toprol-XL, and Betaloc.[3]

Metoprolol was developed as part of a series of compounds that were designed to have fewer side effects than non-selective β-blockers (such as propranolol), which also block β-2 receptors and can cause bronchoconstriction, hypoglycemia, and peripheral vasoconstriction. Metoprolol was also designed to have low membrane-stabilizing activity, which means that it does not interfere with the electrical activity of the cardiac cells and does not cause arrhythmia. Metoprolol was tested in various animal models and clinical trials before being approved for human use. One of the most important clinical trials that demonstrated the efficacy and safety of metoprolol was the Metoprolol Randomised Intervention Trial in Congestive Heart Failure (MERIT-HF), conducted between 1997 and 1999. This multicenter, double-blind, placebo-controlled trial enrolled 3991 patients with chronic heart failure who were randomized to receive either metoprolol succinate (a long-acting formulation) or placebo in addition to standard therapy. The primary endpoint was all-cause mortality.[4] The results showed that metoprolol succinate reduced the risk of death by 34% compared with placebo over a mean follow-up of 1 year. Metoprolol succinate also reduced the risk of hospitalization for worsening heart failure by 19% and improved the patient’s symptoms and quality of life. The drug was well tolerated and did not cause any serious adverse events.

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Other randomized trials in the 1980s showed a mortality benefit for β-blockers in acute myocardial infarction (AMI).[5][6][7] The Metoprolol Atherosclerosis Prevention in Hypertensives (MAPHY) trial specifically showed the benefit of metoprolol over diuretics regarding sudden cardiac death and AMI.[8] Results from a large randomized trial of over 50,000 patients in the 1990s showed metoprolol reduced mortality and re-infarction when used chronically after myocardial infarction.[9] β-blockers have demonstrated prognostic benefits and reduced mortality in the treatment of chronic heart failure. The Carvedilol or Metoprolol European trial (COMET) enrolled patients with stable heart failure and compared each drug head-to-head. Carvedilol was associated with a statistically significantly lower risk of all-cause death.[10] However, recent trials have had conflicting evidence on whether selective β-1 blockers such as metoprolol have any benefit over other β-blockers like carvedilol.[11]

Food and Drug Administration-Approved Indications

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According to the Food and Drug Administration, FDA, metoprolol has been approved for the following indications:

Off-Label Uses

Overall, conflicting evidence exists regarding selecting a particular β-blocker optimally for medical intervention.[11] The role of β-blockers as initial therapy for hypertension, particularly in the absence of compelling indications, is unclear.[19][20]

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