Evaluation of a radiolabelled peripheral benzodiazepine receptor ligand in the central nervous system inflammation of experimental autoimmune encephalomyelitis: a possible probe for imaging multiple sclerosis

Purpose Peripheral benzodiazepine receptors (PBRs) are upregulated on macrophages and activated microglia, and radioligands for the PBRs can be used to detect in vivo neuroinflammatory changes in a variety of neurological insults, including multiple sclerosis. Substituted 2-phenyl imidazopyridine-3-acetamides with high affinity and selectivity for PBRs have been prepared that are suitable for radiolabelling with a number of positron emission tomography and single-photon emission computed tomography (SPECT) isotopes. In this investigation, the newly developed high-affinity PBR ligand 6-chloro-2-(4-iodophenyl)-3-(N,N-diethyl)imidazo[1,2-a]pyridine-3-acetamide, or CLINDE, was radiolabelled with ¹²³I and its biodistribution in the central nervous system (CNS) of rats with experimental autoimmune encephalomyelitis (EAE) evaluated.Methods EAE was induced in male Lewis rats by injection of an emulsion of myelin basic protein and incomplete Freunds adjuvant containing Mycobacterium butyricum. Biodistribution studies with ¹²³I-CLINDE were undertaken on EAE rats exhibiting different clinical disease severity and compared with results in controls. Disease severity was confirmed by histopathology in the spinal cord of rats. The relationship between inflammatory lesions and PBR ligand binding was investigated using ex vivo autoradiography and immunohistochemistry on rats with various clinical scores.Results ¹²³I-CLINDE uptake was enhanced in the CNS of all rats exhibiting EAE when compared to controls. Binding reflected the ascending nature of EAE inflammation, with lumbar/sacral cord > thoracic cord > cervical cord > medulla. The amount of ligand binding also reflected the clinical severity of disease. Ex vivo autoradiography and immunohistochemistry revealed a good spatial correspondence between radioligand signal and foci of inflammation and in particular ED-1⁺ cells representing macrophages and microglia.Conclusion These results demonstrate the ability of ¹²³I-CLINDE to measure in vivo inflammatory events represented by increased density of PBRs and suggest that ¹²³I-CLINDE warrants further investigation as a potential SPECT marker for imaging of CNS inflammation.     

In vivo imaging of microglial activation using a peripheral benzodiazepine receptor ligand: [11C]PK-11195 and animal PET following ethanol injury in rat striatum

OBJECTIVE: To investigate whether [(11)C]PK-11195, a specific peripheral benzodiazepine receptors (PBRs) ligand for positron emission tomography (PET), can show activated microglia in a rat brain injury model. METHODS: On day 1, ethanol was injected into the rat's right striatum (ST) using a stereotaxic operative procedure. On day 3, head magnetic resonance imaging (MRI) scans for surgically treated rats were performed to evaluate ethanol injury morphologically. On day 4, dynamic PET scans (17 injured rats and 7 non-injured controls) were performed for 60 min with an animal PET scanner under chloral hydrate anesthesia following a bolus injection of [(11)C]PK-11195 through tail vein. Because PBRs are present throughout the brain, there is no suitable receptor-free reference region. The reference tissue model may not be applicable because of low target to background ratio for low affinity of [(11)C]PK-11195 to PBRs. We evaluated the PBRs binding with regions of interest (ROIs)-based approach to estimate total distribution volume (V). We used an integral from 0 min to 60 min (V (60)) as an estimate of V. On the coronal PET image, ROIs were placed on bilateral ST. Differences in right/left ST V (60) ratios between lesioned and unlesioned control rats were compared using unpaired t tests. Immunohistochemical staining was performed for confirming the presence of activated microglia following decapitation on the PET experiment day. RESULTS: The right/left ST V (60) ratios in lesioned rats (1.07 +/- 0.08) were significantly higher than those in unlesioned control rats (1.00 +/- 0.06, P < 0.05). On immunohistochemical staining, activated microglia were exclusively observed in the injured right ST but not in the noninjured left ST of the injury rats and the bilateral ST of the non-injured control rats. CONCLUSIONS: These results suggest that [(11)C]PK-11195 PET imaging would be a useful tool for evaluating microglial activation in a rat brain injury model.     

Pharmacological evaluation of [<Superscript>123</Superscript>I]-CLINDE: a radioiodinated imidazopyridine-3-acetamide for the study of peripheral benzodiazepine binding sites (PBBS)

Purpose The study aims to evaluate the iodinated imidazopyridine, N′,N′-diethyl-6-Chloro-(4′-[¹²³I]iodophenyl)imidazo[1,2-a]pyridine-3-acetamide ([¹²³I]-CLINDE) as a tracer for the study of peripheral benzodiazepine binding sites (PBBS). Materials and methods In vitro studies were performed using membrane homogenates and sections from kidney, adrenals, and brain cortex of Sprague–Dawley (SD) rats and incubated with [¹²³I]-CLINDE. For in vivo studies, the rats were injected with [¹²³I]-CLINDE. In competition studies, PBBS-specific drugs PK11195 and Ro 5-4864 and the CBR specific drug Flumazenil were injected before the radiotracer. Results In vitro binding studies in adrenal, kidney, and cortex mitochondrial membranes indicated that [¹²³I]-CLINDE binds with high affinity to PBBS, K _d = 12.6, 0.20, and 3.84 nM, respectively. The density of binding sites was 163, 5.3, and 0.34 pmol/mg protein, respectively. In vivo biodistribution indicated high uptake in adrenals (5.4), heart (1.5), lungs (1.5), kidney (1.5) %ID/g at 6 h p.i. In the central nervous system (CNS), the olfactory bulbs displayed the highest uptake; up to six times the activity in blood. Pre-administration of unlabeled CLINDE, PK11195 and Ro 5–4864 (1 mg/kg) reduced the uptake of [¹²³I]-CLINDE by 70–55% in olfactory bulbs. In the kidney and heart, a reduction of 60–80% ID/g was observed, while an increase was observed in the adrenals requiring 10 mg/kg for significant displacement. Flumazenil had no effect on uptake in peripheral organs and brain. Metabolite analysis indicated >90% of the radioactivity in the above tissues was intact [¹²³I]-CLINDE. Conclusion [¹²³I]-CLINDE displays high and selective uptake for the PBBS and warrants further development as a probe for imaging PBBS using single photon emission computed tomography (SPECT).     

Different sensitivities to competitive inhibition of benzodiazepine receptor binding of11C-iomazenil and11C-flumazenil in rhesus monkey brain

The in vivo binding kinetics of 11C-iomazenil were compared with those of 11C-flumazenil binding in rhesus monkey brain. The monkey was anesthetized with ketamine and intravenously injected with either 11C-iomazenil or 11C-flumazenil in combination with the coadministration of different doses of non-radioactive flumazenil (0, 5 and 20 microg/kg). The regional distribution of 11C-iomazenil in the brain was similar to that of 11C-flumazenil, but the sensitivity of 11C-iomazenil binding to competitive inhibition by non-radioactive flumazenil was much less than that of 11C-flumazenil binding. A significant reduction in 11C-flumazenil binding in the cerebral cortex was observed with 20 microg/kg of flumazenil, whereas a relatively smaller inhibition of 11C-iomazenil binding in the same region was observed with the same dose of flumazenil. These results suggest that 11C-flumazenil may be a superior radiotracer for estimating benzodiazepine receptor occupancy in the intact brain.     

Effects of diazepam on125I-iomazenil-benzodiazepine receptor binding and epileptic seizures in the El mouse

OBJECTIVE: To investigate changes in free benzodiazepine receptor density in response to repeated, long-term administration of diazepam in epilepsy, we assessed 125I-iomazenil (125I-IMZ) binding in a mouse model. METHODS: El mice were divided into two groups of 12 mice each which received either no diazepam (E1(D[-]) group) or 2 mg/kg of diazepam per week (El(D[+]) group). Nine ddY mice were used as a control. Once each week from the age of 5 to 19 weeks, the El mice received stimulation to produce epileptic seizures 20 minutes after receiving intraperitoneal injections. At 20 weeks of age, a total dose of 0.37 MBq of 125I-IMZ was injected in all mice and their brains were rapidly removed 3 hours later. The incidence of epileptic seizures at the age of 19 weeks and the autoradiograms of the brain were compared. RESULTS: The incidence of epileptic seizures in response to weekly stimulation was significantly lower in the E1(D[+]) group than in the E1(D[-]) group (p < 0.001). The percent injected doses of 125I-IMZ per gram of tissue in the cortex, hippocampus and amygdala were significantly lower in the E1(D[+]) group than in the E1(D[-]) group (p < 0.05). CONCLUSION: The results suggest that diazepam binds competitively to 125I-IMZ as an agonist to free benzodiazepine receptor sites in the cortex, hippocampus and amygdala and shows anticonvulsant effect in E1 mice.     

肺TGF-β1水平及血流分布变化与放射性肺损伤的关系

目的观察放射性肺损伤过程中肺血流分布改变和肺泡灌洗液及血清中TGF-β1水平变化.方法将纯种新西兰白兔55只分为对照组、20和30 Gy照射组（照射组再各分5个亚组）,5只/组.照射后不同时间进行肺灌注扫描观察肺血流分布及使用酶联免疫吸附分析法检测肺泡灌洗液和血清中TGF-β1水平,并观察照射后肺组织病理形态学改变.结果照射后左肺血流量占全肺血流量百分率不断下降,肺泡灌洗液和血清中TGF-β1水平逐渐升高,以3～6个月最显著.与放射性肺损伤病理形态学变化趋势相一致.结论肺泡灌洗液及血清中TGF-β1水平可预测放射性肺损伤发展趋势,肺血流量分布变化可间接反映放射性肺损伤病理形态学改变程度.     

Whole-body distribution and metabolism of [N-methyl-11C](R)-1-(2-chlorophenyl)-N-(1-methylpropyl)-3-isoquinolinecarboxamide in humans; an imaging agent for in vivo assessment of peripheral benzodiazepine receptor activity with positron emission tomography

Purpose   ¹¹C-PK11195 is a radiopharmaceutical for in vivo assessment of peripheral benzodiazepine receptor (PBR) activity using PET. We sought to clarify the metabolic fate of ¹¹C-PK11195 in a test–retest setting using radio-HPLC in comparison with radio-TLC, and the whole-body distribution in humans. Materials and methods  In order to evaluate the reproducibility of radio-HPLC metabolite analyses, ten patients with Alzheimer’s disease (AD) underwent two successive ¹¹C-PK11195 examinations on separate days. For comparison of different analytical methods, plasma samples from seven patients were also analysed by radio-TLC. In addition, we evaluated the whole-body distribution of ¹¹C-PK11195 and its uptake in the brain. Results  The level of unmetabolized ¹¹C-PK11195 decreased slowly from 96.3 ± 1.6% (mean±SD) at 5 min to 62.7 ± 8.3% at 40 min after injection. Large individual variation was observed in the amount of plasma ¹¹C-PK11195 radiometabolites. The whole-body distribution of ¹¹C-PK11195 showed the highest radioactivity levels in urinary bladder, adrenal gland, liver, salivary glands, heart, kidneys, and vertebral column. In addition, the hip bone and breast bone were clearly visualized by PET. In patients with AD, ¹¹C-PK11195 uptake in the brain was the highest in the basal ganglia and thalamus, followed by the cortical grey matter regions and the cerebellum. Low ¹¹C-PK11195 uptake was observed in the white matter. Conclusion  Our results indicate that ¹¹C-PK11195 is eliminated both through the renal and hepatobiliary systems. Careful analysis of plasma metabolites is required to determine the accurate arterial input function for quantitative PET measurement.