Evaluation of radiolabelled peripheral benzodiazepine receptor ligand in the central nervous system inflammation of experimental autoimmune encephalomyelitis

Abstract

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 Freund's 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.