ABSTRACT
Metoprolol is a widely used cardioselective beta-blocker. However, like all other beta-blockers it is also a racemic mixture of R- and S- isomers. The beta 1 blocking activity (cardioselectivity) of metoprolol resides in S-isomer while R-isomer exhibits beta 2 blocking activity. As both these isomers have different pharmacological properties, racemic metoprolol can be considered a combination of two different drugs in a fixed 1:1 ratio. The needless administration of the non beta-blocking R-enantiomer that makes up 50% of racemate actually puts the patient at an increased risk of side-effects, drug interactions and loss of cardioselectivity with up-titration of dosing. Clinical experience with chirally pure S-metoprolol at half the dose of racemate has shown it to be as effective as racemate in the treatment of patients with hypertension and angina. S-metoprolol has been shown to be effective and well-tolerated in patients with coexisting diabetes, COPD, and hyperlipidaemia. This confirms higher cardioselectivity of S-metoprolol in clinical settings. Less interaction potential of S-metoprolol compared to R-isomer further makes it a sensible choice in patients taking CYP2D6 inhibitors or in patients with heart failure or hepatic insufficiency. This article reviews differing properties of two isomers of metoprolol with focus on clinical experience with S-metoprolol.