SPECT quantification of [¹²³I]iomazenil binding to benzodiazepine receptors in nonhuman primates: II. Equilibrium analysis of constant infusion experiments and correlation with in vitro parameters

In vivo benzodiazepine receptor equilibrium dissociation constant, K_D, and maximum number of binding sites, B_max, were measured by single photon emission computerized tomography (SPECT) in three baboons. Animals were injected with a bolus followed by a constant i.v. infusion of the high affinity benzodiazepine ligand [¹²³I]iomazenil. Plasma steady-state concentration and receptor-ligand equilibrium were reached within 2 and 3 h, respectively, and were sustained for the duration (4-9 h) of the experiments (n = 15). At the end of the experiments, a receptor saturating dose of flumazenil (0.2 mg/kg) was injected to measure nondisplaceable activity. Experiments were carried out at various levels of specific activity, and Scatchard analysis was performed for derivation of the K_D (0.59 ± 0.09 nM) and B_max (from 126 nM in the occipital region to 68 nM in the striatum). Two animals were killed and [¹²⁵I]iomazenil B_max and K_D were measured at 22 and 37°C on occipital homogenate membranes. In vitro values of B_max (114 ± 33 nM) and 37°C K_D (0.66 ± 0.16 nM) were in good agreement with in vivo values measured by SPECT. This study demonstrates that SPECT can be used to quantify central neuroreceptors density and affinity.