Metoprolol Succinate is NOT a Single S-Enantiomer
Metoprolol succinate is a racemic mixture containing both R- and S-enantiomers in equal proportions, not a pure S-form preparation. This is clearly stated in the FDA drug label, which describes metoprolol succinate as containing "(±)" (the symbol for a racemic mixture) in its chemical nomenclature 1.
Key Pharmacological Distinctions Between Enantiomers
The two enantiomers have markedly different pharmacological activities:
- S-metoprolol is approximately 500-fold more potent at β1-adrenoceptors compared to R-metoprolol 2
- S-metoprolol has a β1-adrenoceptor affinity of 7.73 versus 5.00 for R-metoprolol 2
- The β1-blocking potency differs dramatically: -log ED50 of 7.04 for S-metoprolol versus 4.65 for R-metoprolol 2
- Despite this difference, both enantiomers show similar cardiodepressive effects at high doses 2
Clinical Formulations
Both major metoprolol formulations are racemic:
- Metoprolol tartrate (immediate-release): racemic mixture 3, 4
- Metoprolol succinate (extended-release): racemic mixture 1
The succinate formulation provides controlled release through a multiple-unit pellet system, resulting in lower peak plasma levels (one-fourth to one-half) compared to tartrate, but maintains equivalent β1-blockade over 24 hours 1.
Metabolism and Enantiomeric Differences
Metoprolol undergoes stereoselective metabolism by CYP2D6 1, 5:
- Plasma levels of R-metoprolol are typically lower than S-metoprolol after oral administration 4
- CYP2D6 poor metabolizers (approximately 8% of Caucasians) experience several-fold increases in metoprolol levels, which decreases cardioselectivity 1
- Urinary excretion patterns show substantial inter-individual variability in enantiomer ratios, with S/R ratios exceeding 1 in most patients but less than 1 in approximately 40% 6
Clinical Implications
The racemic nature of metoprolol succinate means patients receive both enantiomers, though the S-form provides the predominant β1-blocking effect. The guidelines consistently reference metoprolol succinate (not specifying enantiomeric form) for heart failure 7, 8, 9, 10, supraventricular tachycardia 11, 12, and hypertension 13, with target doses of 200 mg daily for heart failure and up to 400 mg daily for other indications 14, 11.