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

Kinetic methods were used to obtain regional estimates of benzodiazepine receptor concentration ( B max ) and equilibrium dissociation constant ( K d ) from high and low specific activity (SA) [ 11 C]flumazenil ([ 11 C] Ro 15-1788) positron emission tomography studies of five normal volunteers. The high and low SA data were simultaneously fit to linear and nonlinear three-compartment models, respectively. An additional inhibition study (pretreatment with 0.15 mg/kg of flumazenil) was performed on one of the volunteers, which resulted in an average gray matter K 1 / k 2 estimate of 0.68 ± 0.08 ml/ml (linear three-compartment model, nine brain regions). The free fraction of flumazenil in plasma ( f 1 ) was determined for each study (high SA f 1 : 0.50 ± 0.03; low SA f 1 : 0.48 ± 0.05). The free fraction in brain ( f 2 ) was calculated using the inhibition K 1 / k 2 ratio and each volunteer's mean f 1 value ( f 2 across volunteers = 0.72 ± 0.03 ml/ml). Three methods (Methods I–III) were examined. Method I determined five kinetic parameters simultaneously [ K 1 , k 2 , k 3 (= k on f 2 B max ), k 4 , and k on f 2 /SA] with no a priori constraints. An average k on value of 0.030 ± 0.003 n M −1 min −1 was estimated for receptor-rich regions using Method I. In Methods II and III, the k on f 2 /SA parameter was specifically constrained using the Method I value of k on and the volunteer's values of f 2 and low SA (Ci/μmol). Four parameters were determined simultaneously using Method II. In Method III, K 1 / k 2 was fixed to the inhibition value and only three parameters were estimated. Method I provided the most variable results and convergence problems for regions with low receptor binding. Method II provided results that were less variable but very similar to the Method I results, without convergence problems. However, the K 1 / k 2 ratios obtained by Method II ranged from 1.07 in the occipital cortex to 0.61 in the thalamus. Fixing the K 1 / k 2 ratio in Method III provided a method that was physiologically consistent with the fixed value of f 2 and resulted in parameters with considerably lower variability. The average B max values obtained using Method III were 100 ± 25 n M in the occipital cortex, 64 ±18 n M in the cerebellum, and 38 ± 5.5 n M in the thalamus; the average K d was 8.9 ± 1.0 n M (five brain regions).