Calculation of agonist efficacy, apparent affinity, and receptor population changes after administration of insurmountable antagonists: comparison of different analytical approaches.

The determination of the affinity and efficacy of an agonist in functional (as opposed to radioligand binding) experiments is necessary to explain its potency. Using modeled dose-response data both in their ideal form and with an added average deviation as well as previously published experimental data, a variety of analytical approaches were compared which differed in goodness-of-fit, ease of handling, and range of successful application. A nonlinear curve-fitting algorithm that analyzed several dose-response curves simultaneously and fitted them to an extended version of the operational model of Black and Leff (1983) (Proc R Soc Lond B 220:141-162) was demonstrated to be superior to the other approaches using the above criteria. However, judging from the limitations of the analytical approaches, claims of efficacy or affinity differences between agonists that are based on less than 10-fold numerical differences in the same behavioral paradigm should be viewed with skepticism. It was also found that simple inspection of dose-response curves obtained before and after administration of an insurmountable antagonist give estimates of fair accuracy under most circumstances.

Receptor reserve and affinity of mu opioid agonists in mouse antinociception: correlation with receptor binding.

In order to quantitate the extent to which opioid agonist potencies obtained in behavioral assays are determined by the apparent in vivo affinity and efficacy of the agonist, the antinociceptive effects of the mu opioid agonists morphine, fentanyl, etonitazene, and NIH 10741 were assessed before and after administration of the insurmountable mu opioid antagonist clocinnamox (CCAM) in a 55 degrees C warm-water tail withdrawal test in Swiss albino mice. Under control conditions, all four mu opioid agonists produced a full antinociceptive response with the following ED50 values (in mg/kg): morphine, 12; fentanyl, 0.47; etonitazene, 0.039; NIH 10741, 0.0051. Analysis of CCAM's effects according to Black and Leff gave the following agonist efficacy or tau values: Morphine, 4; fentanyl 15, etonitazene, 7; and NIH 10741, 59. The respective KA values were (in mg/kg): morphine, 29; fentanyl, 7.3; etonitazene, 0.22; and NIH 10741, 0.30. The major determinant of the experimentally observed ED50 values seemed to be the apparent in vivo affinity of the respective agonist and not its efficacy. KA values (expressed as mol/kg) correlated with the Ki values (in mol/l) obtained with [3H]DAMGO radioligand binding (r = 0.96 for pKA vs. pKi), although being on average 11,000-fold higher. Values for q, the available receptor fraction as determined in the behavioral experiments, correlated strongly (r = 0.96) with the q values determined by ex vivo [3H]DAMGO- and [3H]naltrexone equilibrium binding (i.e., Bmax,clocinnamox/Bmax,control), the relationship approaching unity.