Kinetic Analysis of 3-Quinuclidinyl 4-[¹²⁵I]Iodobenzilate Transport and Specific Binding to Muscarinic Acetylcholine Receptor in Rat Brain in vivo: Implications for Human Studies

Abstract

Summary: Radioiodinated R-and S-Quinuclidinyl deriv atives of RS-benzilate (R-and S-1 2 5IQNB) have been syn thesized for quantitative evaluation of muscarinic acetyl choline receptor binding in vivo. Two sets of experiments were performed in rats. The first involved determining the metabolite-corrected blood concentration and tissue distribution of tracer R-IQNB (active enantiomer) and S IQNB (inactive enantiomer) in brain 1 min to 26 h after intravenous injection. The second involved the measure ment of brain tissue washout over a 2-min period after loading the brain by an intracarotid artery injection of the ligands. Various pharmacokinetic models were tested, which included transport across the blood-brain barrier (BBB), nonspecific binding, low-affinity binding, and high-affinity binding. Our analysis demonstrated that the assumptions of rapid equilibrium across the BBB and rapid nonspecific binding are incorrect and result in erro neous estimates of the forward rate constant for binding at the high-affinity receptor sites (k3). The estimated val ues for influx across the BBB (K1), the steady-state ac cumulation rate in cerebrum (K), and the dissociation rate constant at the high-affinity site (k4) of R-IQNB were in dependent of the specific compartmental model used to analyze these data (K1 = 0.23 mllmin/g, K = 0.13 mil min/g, and k4 = 0.0019 min -I for caudate). In contrast, the estimated values of k3 and the efflux rate constant (k 2 ) 781varied over a 10-fold range between different compart mental models (k3 = 2.3-22 min -I and k2 = 1.6-16 min-I in caudate), but their ratios were constant (k31k2 = 1.4). Our analysis demonstrates that the estimates of k3 (and derived values such as the binding potential) are model dependent, that the rate of R-IQNB accumulation in ce rebrum depends on transport across the BBB as well as the rate of binding, and that uptake in cerebrum is essen tially irreversible during the first 360 min after intrave nous administration. Graphical analysis was consistent with compartmental analysis of the data and indicated that steady-state uptake of R-IQNB in cerebrum is estab lished within 1-5 min after intravenous injection. We pro pose a new approach to the analysis of R-IQNB time activity data that yields reliable quantitative estimates of k3, k4, and the nonspecific binding equilibrium constant (Keq) by either compartmental or graphical analysis. The approach is based on determining the free unbound frac tion of radiolabeled ligand in blood and an estimate of K1• The analysis can be applied to time-activity data obtained in a clinical setting using either positron emission tomog raphy or single photon emission computed tomography and has general application for other highly lipophilic ligands. Key Words: Blood-brain barrier-Muscarinic acetylcholine receptor binding-Quinuclidinyl benzilate.

Effects of Blood Flow on [11C]Raclopride Binding in the Brain: Model Simulations and Kinetic Analysis of PET Data

“…The variability in the measuredJp values is another rea son why the DV ratio is a more stable measure for comparing experiments with raclopride than the DV corrected for Jp (DVifp) ' Other drugs have been shown to be sensitive to changes in pH. Sawada et al (1990) found that free quinuclidinyl-[1 2 5I]iodobenzilate was very sensitive to changes in pH in the range 6.0-7.0 due to an increased ionization in the presence of metabolic acidosis. They speculated that the increased ioniza tion of the free fraction would result in a decrease in the transport constant across the blood-brain bar rier.…”

Regional Brain Measurement of Bmax and KD with the Opiate Antagonist Cyclofoxy: Equilibrium Studies in the Conscious Rat

“…, 1989;Kawai et aI. , 1990a.b;Sawada et al, 1990b). This assumption seems rea sonable considering the anatomical location of opi ate receptors on the surface of the cell membrane (Pert et al , 1975) and the negligible amounts of pro tein in brain ECF.…”

Measurement of Benzodiazepine Receptor Number and Affinity in Humans Using Tracer Kinetic Modeling, Positron Emission Tomography, and [11C]Flumazenil

“…The maximal receptor occu pancy achieved at high SA was < 1 % and at low SA � 75% when averaged across the five volunteers. The volumes of distribution of free, nonspecifically bound, and receptor-bound drug were determined using Aw, fl' K I , k2' k3' and k4 in the relationships described by Sawada et al (1990) and Koeppe et al (1991). In the occipital cortex, the volumes of dis tribution of free and nonspecifically bound drug were, respectively, � 6 .…”