Gadolinium-conjugated CB86: a novel TSPO-targeting MRI contrast agent for imaging of rheumatoid arthritis

TSPO is up-regulated in activated macrophages, and serves as an attractive target for macrophages molecular imaging and therapy. MRI may be an ideal technique in the clinical management of RA due to its excellent spatial resolution. In the present study, a novel TSPO-targeting MRI contrast agent was developed by conjugating a novel TSPO ligand CB86 with gadolinium chelate to visualise inflamed regions in RA mice model. A novel TSPO ligand CB86 was linked to DTPAA, followed by chelation with gadolinium to obtain MRI targeted contrast agent CB86-DTPA-Gd. CB86-DTPA-Gd was characterised by MRI relaxivity, cell cytotoxicity, cell specificity and in vitro stability analysis. The distribution and MRI intensity was evaluated in RA rat model. Synthesis of CB86-DTPA-Gd was completed successfully with MRI relaxivity of 11.05/mM/sec (9.4 T, 25 °C). CB86-DTPA-Gd exhibited a good stability in human serum, high RAW264.7 cells specificity and no cytotoxicity in RAW264.7 cells. The biodistribution and MRI studies showed that the accumulation and signal intensity of CB86-DTPA-Gd in the right RA ankles was higher and stronger than those of Gd‑DTPA. This study demonstrates that CB86-DTPA-Gd can identify phagocytic active macrophages in the synovial joints, and has potential as a promising targeting MRI contrast agent for imaging of peripheral inflammation.