Preclinical Evaluation of a Novel (99m)Tc-Labeled CB86 for Rheumatoid Arthritis Imaging

[Image: see text] Early diagnosis and therapy are crucial to control disease progression optimally and achieve a good prognosis in rheumatoid arthritis (RA). Previous study showed that a technetium-99m ((99m)Tc)-labeled TSPO ligand ((99m)Tc-CB256 [2-(8-(2-(bis(pyridin-2-yl)methyl)amino)acetamido)-2-(4-chlorophenyl)H-imidazo[1,2-a]pyridin-3-yl)-N,N-dipropylacetamide] composed of a translocator protein (TSPO) ligand CB86 [[2-(4-chlorophenyl)-8-amino-imidazo[1,2-a]-pyridin-3-yl]-N,N-di-n-propylacetamide] and di-(2-picolyl)amine, a bifunctional chelate agent, was used to image a TSPO-rich cancer cell in vitro; however, few (99m)Tc-CB256 in vivo evaluation has been reported so far probably due to the cytotoxicity of CB256 (ca. 75 times more than analogous CB86). Herein, we describe a novel TSPO targeting radiopharmaceutical consisting of CB86 and diethylenetriaminepentaacetic acid (DTPA), a conventional bifunctional chelating ligand in clinical trials used to prepare (99m)Tc-labeled CB86, and its evaluation as a (99m)Tc-single-photon emission computed tomography (SPECT) probe. The radiosynthesis and characterization of (99m)Tc-DPTA-CB86 including hydrophilicity and stability tests were determined. Additionally, the binding affinity and specificity of (99m)Tc-DTPA-CB86 to TSPO were evaluated using RAW264.7 macrophage cells. Biodistribution and (99m)Tc-SPECT studies were conducted on rheumatoid arthritis (RA) rat models after the injection of (99m)Tc-DTPA-CB86 with or without co-injection of unlabeled DTPA-CB86. The radiosynthesis of (99m)Tc-DTPA-CB86 was completed successfully with the labeling yields and radiochemical purity of 95.86 ± 2.45 and 97.45 ± 0.69%, respectively. The probe displayed good stability in vitro and binding specificity to RAW264.7 macrophage cells. In the biodistribution studies, (99m)Tc-DTPA-CB86 exhibited rapid inflammatory ankle accumulation. At 180 min after administration, (99m)Tc-DTPA-CB86 uptakes of the left inflammatory ankle were 2.35 ± 0.10 percentage of the injected radioactivity per gram of tissue (% ID/g), significantly higher than those of the normal tissues. (99m)Tc-SPECT imaging studies revealed that (99m)Tc-DTPA-CB86 could clearly identify the left inflammatory ankle with good contrast at 30–180 min after injection. Therefore, (99m)Tc-DTPA-CB86 may be a promising probe for arthritis (99m)Tc-SPECT imaging.