Evaluation of Novel N¹-Methyl-2-phenylindol-3-ylglyoxylamides as a New Chemotype of 18 kDa Translocator Protein-Selective Ligand Suitable for the Development of Positron Emission Tomography Radioligands

A novel series of N¹-methyl-(2-phenylindol-3-yl)glyoxylamides, 19−31, designed in accordance with our previously reported pharmacophore/topological model, showed high affinity for the 18 kDa translocator protein (TSPO) and paved the way for developing a new radiolabeled probe. Thus ligand 31, N,N-di-n-propyl-(N¹-methyl-2-(4′-nitrophenyl)indol-3-yl)glyoxylamide, featuring the best combination of affinity and lipophilicity, was labeled with carbon-11 for evaluation with positron emission tomography (PET) in monkey. After intravenous injection, [¹¹C]31 entered brain to give a high proportion of TSPO-specific binding. These findings augur well for the future application of [¹¹C]31 in humans. Consequently, the binding of 31 to human TSPO was tested on samples of brain membranes from deceased subjects who through ethically approved in vitro study had previously been established to be high-affinity binders (HABs), mixed-affinity binders (MABs), or low-affinity binders (LABs) for the known TSPO ligand, PBR28 (2). 31 showed high affinity for HABs, MABs, and LABs. In conclusion, [¹¹C]31 represents a promising new chemotype for developing novel TSPO radioligands as biomarkers of neuroinflammation.