The aim of this paper was to examine if there were clinical studies supporting a beneficial effect of an even plasma concentration over 24 h for the most frequently prescribed beta 1-blockers in clinical practice, metoprolol CR/ZOK and atenolol. There are several studies showing that metoprolol CR/ZOK has a more even plasma concentration compared with atenolol and conventional metoprolol tablets when administered once daily. There are also studies showing that metoprolol CR/ZOK produces a more even beta 1-blockade over 24 h. This is determined by expressing the percentage reduction in exercise heart rate in relation to placebo at intervals throughout the 24-h period and comparing the results with those of atenolol and conventional metoprolol tablets. Clinical studies indicate that the low peak plasma concentration produced by metoprolol CR/ZOK leads to more beta 1 selectivity than atenolol and conventional metoprolol tablets. The loss of beta 1 selectivity at peak plasma concentrations may cause unwanted side-effects. The peak plasma concentration of atenolol coincides with increased general and leg fatigue, problems less evident when patients are on metoprolol CR/ZOK. The frequency and severity of other central nervous system related side-effects are comparable with metoprolol CR/ZOK and atenolol. In conclusion, there are several clinical studies supporting a beneficial effect of the even plasma concentration over 24 h achieved with metoprolol CR/ZOK.
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Agewall et al. (1997) studied this question.