Objective To prepare ¹²⁵I-CD90 monoclonal antibody (mAb) and evaluate its potential as a molecular imaging agent to trace mesenchymal stem cells (MSCs).

Methods ¹²⁵I-CD90 mAb was prepared by chloramine T method, and labeling rate was tested. (1) In in vitro experiment, the radioactive count of the supernatant and precipitate was measured after incubating ¹²⁵I-CD90 mAb with MSCs. Binding fraction was calculated at six time points. (2) In in vivo experiment, tumor-bearing BALB/c mice were constructed and divided into four groups (a, b, c and d) by completely random design. MSCs and normal saline were administered by intraperitoneal injection in groups a and b, respectively. MSCs and normal saline were given by intratumoral injection in groups c and d, respectively. ¹²⁵I-CD90 mAb (3.7 MBq/0.2 mL) was injected via caudal vein in each mouse, and Micro-SPECT/CT analysis was performed. The radioactive uptake values (percentage activity of injection dose per gram of tissue (%ID/g)) in tumor and main organs was calculated at four different time points. The difference between two groups was analyzed by two independent sample t test.

Results The labeling rate and radiochemical purity of ¹²⁵I-CD90 mAb were 54.4% and 98.79%, respectively. (1) After incubation for 10 min, 30 min, 1 h, 2 h, 6 h and 8 h, the binding ratios of ¹²⁵I-CD90 mAb in MSCs were 0.86%, 1.73%, 1.88%, 5.67%, 12.20% and 10.69%, respectively. The highest binding ratio was detected at 6 h of incubation. (2) Tumors of tumor-bearing mice (150−200 mm³) were used. Micro-SPECT/CT images showed varying degrees of ¹²⁵I-CD90 mAb distribution in the tumor and main organs of MSCs tumor-bearing mice after 6 h, 1 d, 2 d and 3 d of incubation. The radioactive uptake values in the tumor were (3.66±1.69), (2.35±1.30), (1.36±0.95) and (1.33±0.84)%ID/g in group a, which were higher than those ((2.93±1.74), (1.92±1.15), (1.12±0.78) and (1.03±0.72)%ID/g) in group b, at 6 h, 1 d, 2 d and 3 d, respectively, but the differences were not statistically significant (t=0.35−0.52, all P>0.05). The radioactive uptake values in the tumor were (5.75±1.30), (3.75±0.77), (2.70±0.44) and (1.88±0.48)%ID/g in group c, which were higher than those ((3.17±0.75), (2.03±0.54), (1.44±0.39) and (1.38±0.27)%ID/g) in group d, at 6 h, 1 d, 2 d and 3 d, respectively, and all the differences were statistically significant (t=1.59−3.70, all P<0.05).

Conclusion The prepared ¹²⁵I-CD90 mAb exhibits good binding ability to MSCs and has the potential as a nuclide probe to trace MSCs.