The goal of this research project is to define the structure and radioactive label for ligands that will permit quantitative measurement of the peripheral benzodiazepine receptor (PBR ) in living brain by external imaging with positron (PET) or single photon (SPECT) emission tomography. A radio-tracer binding specifically to PBR would be of great utility in diagnosis, monitoring treatment, and etiological research of neuropsychiatric disorders such as brain damage, neurodegenerative diseases, anxiety and stress disorders. The PBR is thought to be associated with many biological functions, including the regulation of cellular proliferation, immunomodulation, porphyrin transport, heme biosynthesis, anion transport, regulation of steroidogenesis, and apoptosis. For example, increases in PBR have been observed in brain tissue from patients with HIV/AIDS, Alzheimer's disease, Huntington's disease, multiple sclerosis, and gliosis. A PET study in patients with AIDS showed increased cortical and subcortical PBR receptor binding, supporting the role of glial cell activation in HIV patients with dementia. This study highlights the limitations of current radioligands used and points out the need to further optimize quantitation of PBR. We found that certain indole glyoxylamide derivatives showed nanomolar affinity to PBR. However, brain uptake of two of them labeled with 123I was low in nonhuman primate. Key questions posed by these results are: a) Can PBR affinity and selectivity be increased while reducing lipophilicity with different heterocyclic substituents? And b) are the homogenate binding results applicable to in vivo uptake and distribution? In this application we propose to test the following hypotheses: 1) N,N-dialkyl indolylglyoxylamides with appropriately functionalized heterocyclic substituents on the 2-position of indole will favor binding to PBR relative to CBR; and 2) introduction of these heteroaromatic rings will alter the lipophilicity of our compounds so as to have lower nonspecific binding and higher brain uptake. We will synthesize a series of N,N-dialkyl-2-heterocyclic substituted indolylglyoxylamides. The candidates with binding affinities below 10 nM for PBR and selectivity vis-a-vis CBR above 100 will be radiolabeled with F-18 or I-123 and their lipophilicity (log D) will be measured. In vivo binding will be measured in nonhuman primates by imaging regional brain distribution of the radio-tracer under control conditions and with specific pharmacological challenges. Correlation of log D, in vitro affinity, and in vivo selectivity will be evaluated. At the conclusion of this project period, we expect to have identified a radioligand suitable for imaging the PBR in NeuroAIDS, dementia and other diseases.
