Summary

Early diagnosis and therapy are crucial to control disease progression optimally and achieve a good prognosis in rheumatoid arthritis (RA). Moreover, therapeutic intervention should start as soon as the diagnosis has been established, with the aim of stopping inflammation before irreversible damage is caused, but unfortunately current diagnostic methods are still not very sensitive and specific to RA.

Early hallmark of RA is the increased number of activated macrophages in the synovium with strong increase of their translocator protein (TSPO) level.

In previous studies a 99mtechnetium-labeled TSPO ligand (99mTc-CB256) 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 (75 times more than analogous CB86). Here, a novel TSPO targeting radiopharmaceutical consisting of CB86 and diethylenetriaminepentaacetic acid (DTPA) is described.

Cytotoxicity, 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 RA rat models after the injection of 99mTc-DTPA-CB86 with or without co-injection of unlabeled DTPA-CB86.

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.

Results from nanoScan SPECT/CT

The RA rats (n = 4 for each group) were injected with 99mTc-DTPA-CB86 (0.37 MBq, 100 μL) with or without co-injection of unlabeled DTPA-CB86 (300 μg) through the tail vein. At 30, 90, and 180 min after injection, they were anesthetized with 2% isoflurane and placed on the SPECT bed. SPECT acquiring parameters were as follows: a 140 keV energy peak for 99mTc, window width of 20%, a matrix of 256 × 256 and time frame 30 s. Whole-body static images (200 000 counts) were acquired with a matrix of 218 × 218, and a zoom of 2.0. CT data were acquired using an X-ray voltage biased to 50 kVp with a 670 μA current, with #projections 720°. Regions of interest (ROI) were drawn over the left inflammatory ankle and normal muscle, and then the ratios of the left inflammatory ankle to muscle were calculated.

• 99mTc-DTPA-CB86 accumulated in the left inflammatory ankles at 30 min and then showed a gradual increase of uptake. During 90−180 min after injection, the left inflammatory ankles were clearly visible, with good inflammatory to background contrast.
• When co-injected with unlabeled DTPA-CB86 (300 μg), the left inflammatory ankles were barely visible on SPECT images at 30−180 min after injection.
• Regions of interest (ROI) analysis of SPECT showed a high ratio of the left inflammatory ankle to muscle for RA rats injected unblocking dose compared to with 300 μg blocking dose at 30−180 min postinjection (P < 0.05).
• Evaluation of the probe in these RA rats demonstrated that 99mTc-DTPA-CB86 may be a promising agent for TSPO SPECT imaging.

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