Metoprolol vs Atenolol

## Overview

Metoprolol and atenolol are both cardioselective beta-1 adrenergic receptor antagonists widely used for hypertension, angina, and heart failure. Key differences include their lipophilicity, CNS penetration, metabolism, and outcome data in heart failure.

## Mechanism of Action

Both competitively block beta-1 adrenoceptors, reducing heart rate, myocardial contractility, and blood pressure. At standard doses, both are relatively selective for beta-1 over beta-2 receptors, minimizing bronchospasm risk. This selectivity diminishes at high doses.

## Pharmacokinetics

Metoprolol is highly lipophilic, extensively absorbed (bioavailability ~50% immediate-release due to first-pass metabolism), and metabolized by CYP2D6. Genetic polymorphisms in CYP2D6 significantly alter metoprolol plasma levels — poor metabolizers can have 5-fold higher exposure. Metoprolol crosses the blood-brain barrier readily. Half-life 3–7 h (immediate-release) or 12–24 h (extended-release XL).

Atenolol is hydrophilic, absorbed ~50%, minimally metabolized (80–90% excreted unchanged by kidneys), and has a half-life of 6–9 h. It does not cross the blood-brain barrier significantly.

## Pharmacodynamics and Clinical Evidence

Both provide effective antihypertensive and antianginal effects. However, a critical pharmacological distinction exists: the MERIT-HF trial demonstrated that metoprolol succinate XL significantly reduces mortality in heart failure with reduced ejection fraction (HFrEF), establishing it as one of three beta-blockers with proven mortality benefit in HF (alongside carvedilol and bisoprolol). Atenolol lacks this evidence for HFrEF.

The ASCOT-BPLA trial raised concerns that atenolol was inferior to amlodipine-based therapy for cardiovascular prevention, partly due to less effective 24-h BP control and potentially unfavorable metabolic effects. Metoprolol XL, not atenolol, is guideline-recommended for HF.

## Renal Impairment

Atenolol requires dose reduction in renal impairment (CrCl <35 mL/min) as it is renally eliminated. Metoprolol does not require renal dose adjustment.

## CNS Effects

Metoprolol's lipophilicity allows CNS penetration, which may contribute to side effects such as fatigue, depression, and sleep disturbances. Atenolol causes fewer CNS-related adverse effects.

## Key Takeaways

- Both cardioselective beta-1 blockers with similar antihypertensive efficacy
- Metoprolol XL: proven mortality benefit in HFrEF (MERIT-HF); atenolol lacks this
- Atenolol: renally cleared, dose reduce in CKD; metoprolol: CYP2D6 metabolized
- Atenolol causes fewer CNS side effects due to hydrophilicity