Evaluation of [123I]CLINDE and SPECT as a Marker of Inflammation in Subjects With Parkinson Disease or Alzheimer Disease and in Healthy Subjects

When microglia become activated they express peripheral benzodiazepine receptors (PBR) or binding sites on their mitochondrial membrane. PBRs are functionally and structurally distinct from central benzodiazepine receptors associated with y-aminiobutric acid (GABA)-regulated chloride channels. PBRs are found in abundance in peripheral organs and hematologic cells, but are present at only very low levels in the normal central nervous system (Banati, 2002). CLINDE is a phenylimidazopyridine and appears to bind selectively to the PBR. In the absence of excessive blood in the CNS an increase CLINDE binding to PBR is a potential marker of microglial activation in the CNS. The increase in CLINDE binding may be an indicator of the transition of microglia from a resting to an activated state. When labeled with 123-I and used as a SPECT radiotracer, CLINDE may serve as an in vivo marker of microglial activation in Alzheimer disease and Parkinson disease.

The 123-I radioactive tag offers distinct advantages for large-scale clinical imaging studies of anti-inflammatory targeted treatments as a marker of microglial activation and efficacy of therapeutic intervention. The half-life (13.1 h) of 123-I permits imaging in multiple subjects in a single research-dedicated imaging center, with multiple research subjects per day. This minimizes variability introduced in multi-center quantitative imaging trials where different cameras, image processing methods, and QA procedures all conspire to increase the variance imaging biomarkers. Using this model, our group pioneered a method to evaluate the loss of dopamine function in Parkinson's disease using a radioactive drug 123-I β-CIT which binds directly to dopamine nerve terminals.

The adaptation of imaging agents like 123-I CLINDE as a biomarker of microglial activation in neurodegenerative diseases requires human validation studies. Expanding upon our previous work with b-amyloid ligands (123I-IMPY, 123-I MNI-187) for AD and dopamine transporter ligands (123-I B-CIT, Altropane) for PD, we desire to develop and characterize 123-I CLINDE as a potential marker for microglial activation in association with neuronal damage that may be applicable to multiple neurodegenerative diseases. Ultimately a marker of microglial activation could be used for large-scale quantitative brain imaging trials in AD or PD, specifically to investigate the agent as an objective biomarker in treatments aimed at reducing inflammatory changes in these conditions. The significance of this work lies in applying state-of-art quantitative neuroimaging tools to develop a relevant biomarker in individuals with neurodegenerative diseases with the intention of using this efficiently in large clinical imaging trials.