Pharmacokinetics and pharmacodynamics of beta blockers in heart failure

Although beta-blockers have been used for nearly three decades in the management of heart failure, only recent randomized clinical trials have demonstrated substantial benefit in reducing morbidity and mortality. Carvedilol, metoprolol succinate and bisprolol have evidence supporting their use in heart failure while other beta blockers either lack evidence supporting their use or have not been shown to be useful in heart failure. The only currently approved beta-blockers in the U. S. for heart failure are metoprolol succinate and carvedilol. Beta-blockers differ in their pharmacokinetic and pharmacodynamic properties. It should not be assumed that potential benefit in heart failure is a class effect since differences in the half-life, volume of distribution, protein binding, and route of elimination may give rise to differences in duration of beta blockade and potential drug interactions. Furthermore, pharmacodynamic differences exist because of selectivity for beta(1), beta(2) or alpha(1) adrenoreceptor blockade among the beta-blockers. Receptor kinetics also differ among the beta-blockers and this may influence the extent and duration of beta and alpha blockade across the category. Carvedilol is an inherently long-acting beta-blocker while the duration of beta blockade for metoprolol is dependent on the salt and formulation, which is used. Metoprolol tartrate is a short-acting form of metoprolol while metoprolol succinate is a longer acting salt and the commercially available product is designed as a once daily formulation. A recently published trial, the Carvedilol or Metoprolol European Trial (COMET) tested carvedilol given twice daily versus metoprolol tartrate given twice daily in patients with chronic heart failure. Although carvedilol reduced all cause mortality when compared with metoprolol tartrate, extrapolation to similar findings with metoprolol succinate are not possible since the pharmacokinetic and pharmacodynamic effects of these two formulations are different. Furthermore, the dosing of metoprolol tartrate in COMET may have been inadequate based on prior studies. Additional studies are needed to compare carvedilol directly to metoprolol succinate before concluding inequivalency exists for these two beta-blockers in heart failure.