Quantitative estimation of myocardial fibrosis based on receptor occupancy for beta2-adrenergic receptor agonists in rats.

To develop beta2-adrenergic receptor (AR) agonists with higher selectivity, it is essential to evaluate the cardiac side effects which are the most serious side effects of this class of drugs. We studied receptor occupancy of beta1-ARs in rats as a possible cause for the side effect of beta2-AR agonists, namely myocardial fibrosis. Myocardial fibrosis in rats was observed on Day 7 after the administration of salbutamol and terbutaline, both of which are selective beta2-AR agonists, at higher dose levels. To evaluate receptor occupancy, plasma concentrations of (R)-salbutamol and (R)-terbutaline, plasma protein binding and the EC50 for chronotropic effects in rats were determined. Based on the plasma concentrations, the plasma protein binding and EC50, receptor occupancy-time profiles were constructed. The relationship between the receptor occupancy-time profile under the curve, the AUCphi, and the degree of myocardial fibrosis was evaluated with a multiple correlation analysis. Myocardial fibrosis was significantly correlated (r2 > 0.78) to the AUCphi with the threshold above approximately 50%, but not to plasma concentrations. These results indicate that the receptor occupancy theory is also useful for the evaluation of the chronotropic side effects of beta2-AR agonists.