Comparison of lorazepam [7-chloro-5-(2-chlorophenyl)-1,3-dihydro-3-hydroxy-2H-1,4-benzodiazepin-2-one] occupancy of rat brain gamma-aminobutyric acid(A) receptors measured using in vivo [3H]flumazenil (8-fluoro 5,6-dihydro-5-methyl-6-oxo-4H-imidazo[1,5-a][1,4]benzodiazepine-3-carboxylic acid ethyl ester) binding and [11C]flumazenil micro-positron emission tomography.

The occupancy by lorazepam of the benzodiazepine binding site of rat brain GABA(A) receptors was compared when measured using either in vivo binding of [(3)H]flumazenil (8-fluoro 5,6-dihydro-5-methyl-6-oxo-4H-imidazo[1,5-a][1,4]benzodiazepine-3-carboxylic acid ethyl ester) in terminal studies or [(11)C]flumazenil binding in anesthetized animals assessed using a small animal positron emission tomography (PET) scanner (micro-PET). In addition, as a bridging study, lorazepam occupancy was measured using [(3)H]flumazenil in vivo binding in rats anesthetized and dosed under micro-PET conditions. Plasma lorazepam concentrations were also determined, and for each occupancy method, the concentration required to produce 50% occupancy (EC(50)) was calculated because this parameter is independent of the route of lorazepam administration. For the in vivo binding assay, lorazepam was dosed orally (0.1-10 mg/kg), whereas for the micro-PET study, lorazepam was given via the i.v. route as a low dose (0.75 mg/kg bolus) and then a high dose (0.5 mg/kg bolus then 0.2 mg/ml infusion). The lorazepam plasma EC(50) in the [(11)C]flumazenil micro-PET study was 96 ng/ml [95% confidence intervals (CIs) = 74-124 ng/ml], which was very similar to the [(3)H]flumazenil micro-PET simulation study (94 ng/ml; 95% CI = 63-139 ng/ml), which in turn was comparable with the [(3)H]flumazenil in vivo binding study (134 ng/ml; 95% CI = 119-151 ng/ml). These data clearly show that despite the differences in dosing (i.v. in anesthetized versus orally in conscious rats) and detection (in vivo dynamic PET images versus ex vivo measurements in filtered and washed brain homogenates), [(11)C]flumazenil micro-PET produces results similar to [(3)H]flumazenil in vivo binding.

3'-deoxy-3'-[18F]fluorothymidine as a new marker for monitoring tumor response to antiproliferative therapy in vivo with positron emission tomography.

3'-Deoxy-3'-[(18)F]fluorothymidine ([(18)F]FLT) has been proposed as a new marker for imaging tumor proliferation by positron emission tomography (PET). The uptake of [(18)F]FLT is regulated by cytosolic S-phase-specific thymidine kinase 1 (TK1). In this article, we have investigated the use of [(18)F]FLT to monitor the response of tumors to antiproliferative treatment in vivo. C3H/Hej mice bearing the radiation-induced fibrosarcoma 1 tumor were treated with 5-fluorouracil (5-FU; 165 mg/kg i.p.). Changes in tumor volume and biodistribution of [(18)F]FLT and 2-[(18)F]fluoro-2-deoxy-D-glucose ([(18)F]FDG) were measured in three groups of mice (n = 8-12/group): (a) untreated controls; (b) 24 h after 5-FU; and (c) 48 h after 5-FU. In addition, dynamic [(18)F]FLT-PET imaging was performed on a small animal scanner for 60 min. The metabolism of [(18)F]FLT in tumor, plasma, liver, and urine was determined chromatographically. Proliferation was determined by staining histological sections for proliferating cell nuclear antigen (PCNA). Tumor levels of TK1 protein and cofactor (ATP) were determined by Western blotting and bioluminescence, respectively. Tumor [(18)F]FLT uptake decreased after 5-FU treatment (47.8 +/- 7.0 and 27.1 +/- 3.7% for groups b and c, respectively, compared with group a; P < 0.001). The drug-induced reduction in tumor [(18)F]FLT uptake was significantly more pronounced than that of [(18)F]FDG. The PET image data confirmed lower tumor [(18)F]FLT retention in group c compared with group a, despite a trend toward higher radiotracer delivery for group c. Other than phosphorylation in tumors, [(18)F]FLT was found to be metabolically stable in vivo. The decrease in tumor [(18)F]FLT uptake correlated with the PCNA-labeling index (r = 0.71, P = 0.031) and tumor volume changes after 5-FU treatment (r = 0.58, P = 0.001). In this model system, the decrease in [(18)F]FLT uptake could be explained by changes in catalytic activity but not translation of TK1 protein. Compared with group a, TK1 levels were lower in group b (78.2 +/- 5.2%) but higher in group c (141.3 +/- 9.1%, P < 0.001). In contrast, a stepwise decrease in ATP levels was observed from group a to b to c (P < 0.001). In conclusion, we have demonstrated the ability to measure tumor response to antiproliferative treatment with [(18)F]FLT and PET. In our model system, the radiotracer uptake was correlated with PCNA-labeling index. The decrease in [(18)F]FLT uptake after 5-FU was more pronounced than that of [(18)F]FDG. [(18)F]FLT is, therefore, a promising marker for monitoring antiproliferative drug activity in oncology that warrants additional testing.