Preclinical Evaluation of a Novel ^99mTc-Labeled CB86 for Rheumatoid Arthritis Imaging

Abstract

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 (99mTc)-labeled TSPO ligand (99mTc-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 99mTc-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 99mTc-labeled CB86, and its evaluation as a 99mTc-single-photon emission computed tomography (SPECT) probe. The radiosynthesis and characterization of 99mTc-DPTA-CB86 including hydrophilicity and stability tests were determined. Additionally, the binding affinity and specificity of 99mTc-DTPA-CB86 to TSPO were evaluated using RAW264.7 macrophage cells. Biodistribution and 99mTc-SPECT studies were conducted on rheumatoid arthritis (RA) rat models after the injection of 99mTc-DTPA-CB86 with or without co-injection of unlabeled DTPA-CB86. The radiosynthesis of 99mTc-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, 99mTc-DTPA-CB86 exhibited rapid inflammatory ankle accumulation. At 180 min after administration, 99mTc-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. 99mTc-SPECT imaging studies revealed that 99mTc-DTPA-CB86 could clearly identify the left inflammatory ankle with good contrast at 30–180 min after injection. Therefore, 99mTc-DTPA-CB86 may be a promising probe for arthritis 99mTc-SPECT imaging.

A Review of Advances in Molecular Imaging of Rheumatoid Arthritis: From In Vitro to Clinic Applications Using Radiolabeled Targeting Vectors with Technetium-99m

“…[ 99m Tc]Tc-DTPA-CB86 rapidly accumulated in the inflamed ankle ( Figure 8 ). After 180 min of administration, [ 99m Tc]Tc-DTPA-CB86 uptake in the inflamed ankle was 2.35 ± 0.10%, which was significantly increased than that in normal tissues using SPECT [ 116 ].…”

“… SPECT/CT imaging of [ 99m Tc]Tc-DTPA-CB86 in RA model rats coinjected with 0 μg (unblock, ( A )) or 300 μg (block, ( B )) of DTPA-CB86 at various intervals after injection. Reprinted from open source (CC BY) [ 116 ]. …”