Mechanism-Based Pharmacokinetic-Pharmacodynamic Modeling of the Effects of N-Cyclopentyladenosine Analogs on Heart Rate in Rat: Estimation of in Vivo Operational Affinity and Efficacy at Adenosine A1 Receptors

We have developed a pharmacokinetic-pharmacodynamic strategy based on the operational model of agonism to obtain estimates of apparent affinity and efficacy of N-cyclopentyladenosine (CPA) analogs for the adenosine A1 receptor-mediated in vivo effect on heart rate in the rat. All analogs investigated produced a significant decrease of the heart rate after intravenous infusion. Individual concentration-effect curves were fitted to the operational model of agonism with the values of Emax and n constrained to the intrinsic activity (273 bpm) and Hill slope (1.18), respectively, obtained with the agonist that displayed the highest intrinsic activity, 59-deoxy-CPA. In all cases, the model converged and estimates of apparent affinity and efficacy were obtained for each agonist. Affinity estimates correlated well with pKi values for the adenosine A1 receptor in rat brain homogenates. In addition, a highly significant correlation was found between the estimates of the in vivo efficacy parameter and the GTP shift (the ratio between Ki in the presence and absence of GTP). In conclusion, the operational model of agonism can provide meaningful measures of agonist affinity and efficacy at adenosine A1 receptors in vivo. The model should be of use in the development of partial adenosine A1 receptor agonists. Adenosine is believed to exert its physiological effects through interactions with at least four receptor subtypes: the A1, A2a, A2b and A3 receptors (see Fredholm et al., 1994). In the heart, stimulation of adenosine A1 receptors produces negative dromo-, chronoand inotropic effects (Olsson and Pearson, 1990) and adenosine itself has been used for the treatment of arrhythmias (Roden, 1996). On the other hand, the pronounced cardiodepressant effects have been a major impediment to the research into the potential of adenosine A1 receptor agonists as drugs in other therapeutic areas, such as diseases of the central nervous system and lipid metabolism. Until recently, all known adenosine A1 agonists appeared to behave as full agonists in every experimental system. Because greater organ selectivity can be expected of low-efficacy agonists as opposed to high-efficacy agonists (see Kenakin, 1993a), it has been proposed to design partial agonists for the adenosine A1 receptor as novel pharmacological tools with less cardiac side effects (IJzerman et al., 1994, 1996). In the search for partial agonists, several series of adenosine derivatives have been synthesized leading to the identification of analogs of CPA for which the ratio between apparent affinity in the presence and absence of GTP for the adenosine A1 receptor in radioligand binding studies is lower than for CPA (Van der Wenden et al., 1995; Roelen et al., 1996). Because this ratio, generally referred to as “GTP shift,” is considered to be a measure of efficacy (see Cohen et al., 1996; IJzerman et al., 1996; Kenakin, 1993b, 1996), it was concluded that these ligands are partial agonists for the adenosine A1 receptor. Accordingly, the CPA analogs were tested for their in vivo effect on heart rate in the normotensive rat. By applying an integrated simultaneous pharmacokinetic-pharmacodynamic modeling approach, it was possible to describe the concentration-effect relationships by the three-parameter Hill equation. It was found that ligands with a reduced in vitro GTP shift displayed a lower in vivo intrinsic activity than CPA (Mathôt et al., 1995a; Van Schaick et al., 1997). Furthermore, the in vivo potency (EC50) was correlated with the affinity (Ki) for the adenosine A1 receptor obtained from radioligand binding studies. These results suggest that the in vivo pharmacodynamics of adenReceived for publication January 27, 1997. ABBREVIATIONS: CHA, N-cyclohexyladenosine; CPA, N-cyclopentyladenosine; 29dCPA, 29-deoxy-CPA; 39dCPA, 39-deoxy-CPA; 59dCPA, 59-deoxy-CPA; DPCPX, 1,3-dipropyl-8-cyclopentylxanthine; 8MCPA, 8-(methylamino)-CPA; 8ECPA, 8-(ethylamino)-CPA; 8PCPA, 8-(propylamino)-CPA; 8BCPA, 8-(butylamino)-CPA; 8CPCPA, 8-(cyclopentylamino)-CPA; R-PIA, R(2)-N-(2-phenylisopropyl)adenosine 0022-3565/97/2832-0809$03.00/0 THE JOURNAL OF PHARMACOLOGY AND EXPERIMENTAL THERAPEUTICS Vol. 283, No. 2 Copyright © 1997 by The American Society for Pharmacology and Experimental Therapeutics Printed in U.S.A. JPET 283:809–816, 1997 809 at A PE T Jornals on A ril 9, 2017 jpet.asjournals.org D ow nladed from osine A1 receptor agonists may be explained in mechanistic terms of the drug-receptor interaction, such as affinity and efficacy. However, analysis of concentration-effect data with the empirical Hill equation only provides limited insights in this matter because the potency of an agonist is determined by both affinity and efficacy. Furthermore, the intrinsic activity is a function of both compound (intrinsic efficacy) and system (receptor density and the function relating receptor occupancy to pharmacological effect) characteristics. Therefore, in the present study we have investigated to what extent a model of agonism which explicitly includes parameters for affinity and efficacy can explain differences between the in vivo effects of adenosine A1 receptor ligands and whether it is possible to obtain meaningful estimates of apparent affinity and efficacy in vivo. It is important to answer these questions, because this will provide quantitative information about the adenosine A1 receptor agonists and their physiological targets in vivo that was previously unattainable. Furthermore, it will provide a theoretical framework which can serve as a link between in vitro and in vivo studies in future research. Accordingly, we have developed a novel, mechanism-based pharmacokinetic-pharmacodynamic strategy based on the operational model of agonism (Black and Leff, 1983) and reanalyzed datasets of the in vivo effects on heart rate in rats of CPA (Mathôt et al., 1994), the deoxyribose CPA analogs 29dCPA, 39dCPA and 59dCPA (Mathôt et al., 1995a), the C8-amino-substituted analogs 8MCPA, 8ECPA, 8PCPA, 8BCPA and 8CPCPA (Van Schaick et al., 1997) and R-PIA (Mathôt et al., 1995b).