Time course of action of three adenosine A1 receptor agonists with differing lipophilicity in rats: comparison of pharmacokinetic, haemodynamic and EEG effects.

In this study we investigated the relationship between the pharmacokinetics and the cardiovascular and electroencephalogram (EEG) effects of three adenosine agonists with differing lipophilicity. Conscious normotensive rats received either 600 microg/kg N6-(p-sulphophenyl) adenosine (SPA), 200 microg/kg N6-cyclopentyladenosine (CPA) or 600 microg/kg 1-deaza-2-chloro-N6-cyclopentyladenosine (DCCA) in a 5-min intravenous infusion. Changes in haemodynamics and EEG were monitored in conjunction with arterial blood sampling to determine blood concentrations of the compounds. The three adenosine agonists showed large differences in pharmacokinetic properties, resulting in terminal half-lives of 66 +/- 10, 8.2 +/- 0.4 and 24 +/- 1 min (mean +/- SEM) for SPA, CPA, and DCCA respectively. SPA had a significantly lower blood clearance relative to CPA and DCCA, whereas DCCA had the largest volume of distribution and degree of plasma protein binding. The relationship between concentration and heart rate could be described adequately by the sigmoidal Emax model. For SPA, CPA, and DCCA the EC50 values based on free drug concentrations were 423 +/- 92, 1.8 +/- 0.4 and 9.5 +/- 1.1 nM respectively. These in vivo values correlated closely with the affinity of the compounds for the adenosine A1 receptor as determined in radioligand binding studies, with corresponding Ki values of 1410 +/- 220, 4.7 +/- 0.6 and 102 +/- 74 nM (mean +/- SEM) respectively. In the EEG, only CPA produced a small decrease in the amplitude of beta waves. This study demonstrates that the three adenosine analogues have large differences in pharmacokinetics, which complicates comparison of their cardiovascular and central responses simply on the basis of dose. The application of an integrated PK/PD approach permits estimates of potency and activity which are independent of underlying dose and pharmacokinetics.