E. A. Van Schaick

Selectivity of action of 8‐alkylamino analogues of N6‐cyclopentyladenosine in vivo: haemodynamic versus anti‐lipolytic responses in rats

A1 adenosine receptor agonists with reduced intrinsic activity may be therapeutically useful as result of an increased selectivity of action. In this study the tissue selectivity of three 8‐alkylamino substituted analogues of N6‐cyclopentyladenosine (CPA) was investigated for haemodynamic and anti‐lipolytic effects using an integrated pharmacokinetic‐pharmacodynamic approach. Chronically instrumented male Wistar rats received intravenous infusions of 4.0 mg kg−1 8‐methylaminoCPA (8MCPA), 12.0 mg kg−1 8‐ethylaminoCPA (8ECPA), 20.0 mg kg−1 8‐butylaminoCPA (8BCPA) or vehicle during 15 min. During experimentation, serial arterial blood samples were drawn for the determination of agonist concentrations and plasma non‐esterified fatty acid (NEFA) levels. Blood pressure and heart rate were monitored continuously. In addition to the CPA analogues, each rat received a rapid bolus infusion of CPA to determine the maximal effects of the full agonist. The concentration‐time profiles of the CPA analogues could be described by a bi‐exponential function. Values for clearance, volume of distribution at steady state and elimination half‐life were 44±5, 48±6 and 39±2 ml min−1 kg−1, 0.97±0.09, 0.84±0.10 and 1.05±0.07 1 kg−1 and 25±2, 28±2 and 40±2 min for 8MCPA, 8ECPA and 8BCPA, respectively (mean±s.e.mean, n=6–8). Different models were used to derive the concentration‐effect relationships for heart rate and NEFA, yielding estimates of potency (EC50) and instrinsic activity (Emax) for both effects of the compounds in vivo. On heart rate the compounds acted as partial agonists, with Emax values of −173±14, −131±11 and −71±6 beats min−1 for 8MCPA, 8ECPA and 8BCPA, respectively. These Emax values were significantly lower than the maximal effect of CPA (−208±8 beats min−1). With regard to the anti‐lipolytic effect all three compounds were full agonists and lowered NEFA levels to the same extent as CPA (69%). The estimated Emax values were 63±5, 63±4 and 68±2%, respectively. Furthermore, the compounds were more potent in causing anti‐lipolytic than cardiovascular effects. The EC50 values for the NEFA and heart rate lowering effects were 37±15, 68±22 and 659±108 ng ml−1 and 164±22, 341±76 and 975±190 ng ml−1 for 8MCPA, 8ECPA and 8BCPA, respectively (mean±s.e.mean, n=6–8). This study demonstrates that partial agonists for the A1 adenosine receptor have increased selectivity of action in vivo. The 8‐alkylamino analogues of CPA may be useful anti‐lipolytics with less pronounced haemodynamic side effects.

High-performance liquid chromatography of the adenosine A1 agonist N6-cyclopentyladenosine and the A1 antagonist 8-cyclopentyltheophylline and its application in a pharmacokinetic study in rats

This report describes a rapid and sensitive analysis for the simultaneous detection of the adenosine A1 receptor ligands N6-cyclopentyladenosine (CPA) and 8-cyclopentyltheophylline (CPT) in rat blood. The method involved alkaline extraction of the compounds and internal standard N6-cyclohexyladenosine (CHA) with ethyl acetate, followed by isocratic reversed-phase high-performance liquid chromatography on a 3-microns MicroSphere C18 column with UV detection at 269 nm. The mobile phase consisted of a mixture of 10 mM acetate buffer (pH 4.0)-methanol-acetonitrile (56:40:4, v/v/v) with a flow-rate of 0.50 ml/min. The total run time was ca. 19 min. For CPA and CPT extraction yields were greater than 77 and 66% in the concentration range of 0.010-0.75 microgram/ml and 0.025-15 micrograms/ml, respectively, with intra- and inter-assay variations less than 9%. In 100 microliter blood samples the corresponding limits of detection were 3.3 and 6.2 ng/ml (signal-to-noise ratio = 3). CPA was found to be degraded in rat blood in vitro with a half-life of 24 min at 37 degrees C. The utility of the analytical method was established by analyzing blood samples from rats which had received an intravenous administration of 200 micrograms/kg CPA or 12 mg/kg CPT. Due to its rapidity and sensitivity this method is concluded to be particularly useful in pharmacokinetic studies with CPA and CPT.

Pharmacokinetic‐pharmacodynamic modelling of the anti‐lipolytic and anti‐ketotic effects of the adenosine A1‐receptor agonist N6‐(p‐sulphophenyl)adenosine in rats

1 The purpose of this study was to develop and validate an integrated pharmacokinetic‐pharmacodynamic model for the anti‐lipolytic effects of the adenosine A1‐receptor agonist N6‐(p‐sulphophenyl)adenosine (SPA). Tissue selectivity of SPA was investigated by quantification of haemodynamic and anti‐lipolytic effects in individual animals. 2 After intravenous infusion of SPA to conscious normotensive Wistar rats, arterial blood samples were drawn for determination of blood SPA concentrations, plasma non‐esterified fatty acid (NEFA) and β‐hydroxybutyrate levels. Blood pressure and heart rate were monitored continuously. 3 The relationship between the SPA concentrations and the NEFA lowering effect was described by the indirect suppression model. Administration of SPA at different rates and doses (60 μg kg−1 in 5 min and 15 min, and 120 μg kg−1 in 60 min) led to uniform pharmacodynamic parameter estimates. The averaged parameters (mean±s.e., n=19) were Emax: −80±2% (% change from baseline), EC50: 22±2 ng ml−1, and Hill factor: 2.2±0.2. 4 In another group, given 400 μg kg−1 SPA in 15 min, pharmacodynamic parameters for both heart rate and anti‐lipolytic effect were derived within the same animal. The reduction in heart rate was directly related to blood concentration on the basis of the sigmoidal Emax model. SPA inhibited lipolysis at concentrations lower than those required for an effect on heart rate. The EC50 values (mean±s.e., n=6) were 131±31 ng ml−1 and 20±3 ng ml−1 for heart rate and NEFA lowering effect, respectively. 5 In conclusion, the relationship between blood SPA concentrations and anti‐lipolytic effect was adequately described by the indirect suppression model. For SPA a 6 fold difference in potency was observed between the effects on heart rate and NEFAs, indicating some degree of tissue selectivity in vivo.

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 μg/kg N6-(p-sulphophenyl) adenosine (SPA), 200 μg/kg N6-cyclopentyladenosine (CPA) or 600 μg/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.