Pharmacokinetic-pharmacodynamic relationship of the cardiovascular effects of adenosine A1 receptor agonist N6-cyclopentyladenosine in the rat

The purpose of the investigation was to develop a pharmacokinetic-pharmacodynamic model for the characterization of the cardiovascular effects of the adenosine A1 receptor agonist N6-cyclopentyladenosine (CPA) in individual normotensive rats. After the i.v. administration of 200 micrograms/kg (0.60 mumol/kg) of CPA, the time course of heart rate and arterial blood pressure was monitored in conjunction with serial blood sampling. Potential interference of the concentration-cardiovascular effect relationship by the development of acute tolerance or the formation of (inter)active metabolites was investigated by infusion of CPA with different rates and by determination of blood concentrations both by high-performance liquid chromatography and radioreceptor assay. In the individual rats the concentration-hemodynamic effect relationships were satisfactorily modeled according to the sigmoidal Emax pharmacodynamic model. For the negative chronotropic effect, the pharmacodynamic parameters proved to be independent of the infusion rate, indicating the absence of development of acute tolerance during the experiment. Potency (EC50) and intrinsic efficacy (Emax) were 2.7 +/- 0.5 ng/ml and -209 +/- 10 bpm, respectively (mean +/- S.E., n = 17). The concentrations of CPA as determined by the radioreceptor assay were identical to those determined by high-performance liquid chromatography, thereby excluding the formation of (inter)active metabolites. It is concluded that on the basis of this integrated pharmacokinetic-pharmacodynamic model, with the negative chronotropic effect as a pharmacodynamic endpoint, estimates of the potency and the intrinsic efficacy of adenosine A1 receptor agonists in vivo can be obtained after the administration of a single dose.