Biodistribution and imaging of 131I labeled anti-neuropilin-1 monoclonal antibody in malignant gliomas xenografts

Objective To synthesize 131I labeled anti-neuropilin-1 monoclonal antibody A6 (131I-A6) and evaluate its biodistribution and imaging in malignant glioma xenografts. Methods (1) A6 was labeled with 131I by Iodogen method under the optimum labeling conditions, then the labeling efficiency, radiochemical purity and stability were measured in vitro. (2) In vitro bioactivity, cellular uptake and receptor affinity of 131I-A6 with U87MG cells were measured. (3) The nude mice bearing human U87MG cells were randomly divided into 5 groups with 5 in each group. The nude mice were sacrificed by cervical dislocation and dissected at 24, 48, 72, 96, and 120 h, respectively, after intravenous injection of 1.2 MBq 131I-A6. The biodistribution of the agent was measured as %ID/g, and the ratios of tumor/blood (T/B) and tumor/muscle (T/M) were calculated. (4) SPECT/CT imaging was performed in 6 mice including 3 in the competitive inhibition control group at 24, 48, 72, 96, and 120 h post injection. Two-sample t test was used for data analysis. Results (1) The labeling yield of 131I-A6 was (95.46±3.34)%, and the radiochemical purity was more than 95%. At 96 h of incubation in PBS, the radiochemical purity was more than 85%. (2) 131I-A6 had rapid accumulation in U87MG cells and reached the peak of (15.80±1.30)% at 1 h. When the probe was incubated with large excesses of non-radioactive A6, the uptake level of 131I-A6 in U87MG cells was significantly inhibited (t=2.862, P<0.05). Kd of 131I-A6 binding to NRP-1 was (1.67±0.14) nmol/L in U87MG cells. (3) Biodistribution study showed that the uptake in blood, liver and tumor was (8.00±1.42), (7.68±1.56) and (6.00±1.24) %ID/g at 24 h, respectively. The uptake in muscle, brain and bone was lower. The T/B and T/M were 0.78±0.10 and 3.20±0.30 at 24 h, and they reached the highest level of 1.87±0.50 and 7.13±0.24 at 120 h. (4) The SPECT imaging showed that the tumors could be visualized at 24 h and delineated more clearly at 120 h post injection of 131I-A6. Conclusions 131I-A6 can be easily synthesized by Iodogen method with high radiochemical purity. The specific tumor uptake of 131I-A6, which correlates with NRP-1 expression in gliomas, suggests that it may be a new promising tumor targeting radiotracer. Key words: Glioma; Neuropilin-1; Isotope labeling; Iodine radioisotopes; Radionuclide imaging; Mice, nude