Kinetic Analysis of Transport and Opioid Receptor Binding of [ 3 H](−)-Cyclofoxy in Rat Brain in vivo: Implications for Human Studies

[³H]Cyclofoxy (CF: 17-cyclopropylmethyl-3,14-dihydroxy-4,5-α-epoxy-6-β-fluoromorphinan) is an opioid antagonist with affinity to both mu and kappa subtypes that was synthesized for quantitative evaluation of opioid receptor binding in vivo. Two sets of experiments in rats were analyzed. The first involved determining the metabolite-corrected blood concentration and tissue distribution of CF in brain 1 to 60 min after i.v. bolus injection. The second involved measuring brain washout for 15 to 120 s following intracarotid artery injection of CF. A physiologically based model (Sawada et al., 1990a) and a classical compartmental pharmacokinetic model (Wong et al., 1986a) were compared. The models included different assumptions for transport across the blood-brain barrier (BBB); estimates of nonspecific tissue binding and specific binding to a single opiate receptor site were found tobe essentially the same with both models. The nonspecific binding equilibrium constant varied modestly in different brain structures (K_eq = 3–9), whereas the binding potential (BP) varied over a much broader range (BP = 0.6–32). In vivo estimates of the opioid receptor dissociation constant were similar for different brain structures (K_D = 2.1–5.2 nM), whereas the apparent receptor density (B_max) varied between 1 (cerebellum) and 78 (thalamus) pmol/g of brain. The receptor dissociation rate constants in cerebrum (k₄ = 0.08–0.16 min⁻¹; k_off = 0.16–0.23 min⁻¹) and brain vascular permeability (PS = 1.3–3.4 ml/min/g) are sufficiently high to achieve equilibrium conditions within a reasonable period of time. Graphical analysis (Patlak and Blasberg, 1985) of the data is inappropriate due to the high tissue-loss rate constant (k_b = 0.03–0.07 min⁻¹) for CF in brain. From these findings, CF should be a very useful opioid receptor ligand for the estimation of the receptor binding parameters in human subjects using [¹⁸F]CF and positron emission tomography.