Receptor binding characterization of the benzodiazepine radioligand 125I-Ro16-0154: potential probe for SPECT brain imaging

The binding of an iodinated benzodiazepine (BZ) radioligand has been characterized, particularly in regard to its potential use as a neuroreceptor brain imaging agent with SPECT (Single Photon Emission Computed Tomography). Ro16-0154 is an iodine-containing BZ antagonist and a close analog of Ro15-1788. In tissue homogenates prepared from human and monkey brain, the binding of 125I-labeled Ro16-0154 was saturable, of high affinity (Kd = 0.5 nM at 37 degrees C), and had high ratios of specific to non-specific binding (approximately 40:1). Physiological concentrations of NaCl (150 mM) enhanced specific binding approximately 15% compared to buffer without this salt. Kinetic studies of association and dissociation demonstrated a temperature dependent decrease in affinity with increasing temperature. Drug displacement studies confirmed that 125I-Ro16-0154 binds to the "central" type BZ receptor: binding is virtually identical to that of 3H-Ro15-1788 except that 125I-Ro16-0154 shows an almost 10 fold higher affinity at 37 degrees C. These in vitro results suggest that 123I-labeled Ro16-0154 shows promise as a selective, high affinity SPECT probe of the brain's BZ receptor.