Pharmacokinetics and Pharmacodynamics of Seven Opioids in P-Glycoprotein-Competent Mice: Assessment of Unbound Brain EC_50,uand Correlation of in Vitro, Preclinical, and Clinical Data

Abstract

This study was conducted to assess the utility of unbound brain EC 50 (EC 50,u ) as a measure of in vivo potency for centrally active drugs. Seven -opioid agonists (alfentanil, fentanyl, loperamide, methadone, meperidine, morphine, and sufentanil) were selected as model central nervous system drugs because they elicit a readily measurable central effect (antinociception) and their clinical pharmacokinetics/pharmacodynamics are well understood. Mice received an equipotent subcutaneous dose of one of the model opioids. The time course of antinociception and the serum and brain concentrations were determined. A pharmacokinetic/pharmacodynamic model was used to estimate relevant parameters. In vitro measures of opioid binding affinity (K i ) and functional activity [EC 50 for agonist stimulated guanosine 5Ј-O-(3-[35 S]thio)triphosphate binding] and relevant clinical parameters were obtained to construct in vitro-to-preclinical and preclinical-to-clinical correlations. The strongest in vitro-to-in vivo correlation was observed between K i and unbound brain EC 50,u (r 2 ϳ 0.8). A strong correlation between mouse serum and human plasma EC 50 was observed (r 2 ϭ 0.949); the correlation was improved when corrected for protein binding (r 2 ϭ 0.995). Clinical equipotent i.v. dose was only moderately related to K i . However, estimates of ED 50 and EC 50 (total serum, unbound serum, total brain, and unbound brain) were significant predictors of clinical equipotent i.v. dose; the best correlation was observed for brain EC 50,u (r 2 ϭ 0.982). For each opioid, brain equilibration half-life in mice was almost identical to the plasma effect-site equilibration half-life measured clinically. These results indicate that the mouse is a good model for opioid human brain disposition and clinical pharmacology and that superior in vitro-to-preclinical and preclinicalto-clinical correlations can be achieved with relevant unbound concentrations.In drug discovery, in vitro assays and preclinical animal studies are widely used to assess compound potency and to identify compound(s) that may have a desirable clinical response. Several options for assessing compound potency are available, including in vitro (receptor binding or functional assays) and in vivo (animal studies to determine dose-response or concentration-response relationships) protocols. In vitro binding and functional assays, by nature, are designed to estimate the intrinsic affinity or potency at the receptor of interest, whereas in vivo experiments take into account the full spectrum of pharmacokinetic and pharmacodynamic processes that ultimately determine biological response.Ideally, in vitro potency would translate to or predict in vivo potency. Often, this is the case, and significant correlations between in vivo ED 50 or EC 50 and in vitro potency have been established for a variety of therapeutic targets (Leysen et al., 1983;Visser et al., 2003). However, when there is no correlation between in vivo and in vitro potency measures, the validity of the in vitro as...

Unbound Brain Concentration Determines Receptor Occupancy: A Correlation of Drug Concentration and Brain Serotonin and Dopamine Reuptake Transporter Occupancy for Eighteen Compounds in Rats

“…For P-gp substrates, the unbound plasma concentration is expected to be higher than the unbound brain concentration Liu et al, 2008). This result has also been recently demonstrated by Kalvass et al (2007b). They showed that the unbound plasma EC 50 for antinociceptive activity in mice of loperamide, a P-gp substrate, was more than 50-fold of its in vitro K I , whereas its unbound brain EC 50 was similar to its K I .…”

“…Therefore, total brain concentration does not represent the drug concentration at the biophase. Although the scientific rationale for the free drug hypothesis is very strong, there are limited data in the literature to actually prove that the unbound brain concentration, not the total brain tissue concentration, determines CNS activity (Cox et al, 1998;Kalvass et al, 2007b;Watson et al, 2009). Because of the lack of extensive well documented data in the literature and the lack of understanding of novel drug targets in most drug discovery and development programs, sometimes it is debatable whether the unbound brain concentration or the total brain concentration determines in vivo activity.…”

Preferential Delivery of an Opioid Antagonist to the Fetal Brain in Pregnant Mice

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