Abstract: Objective To establish a two-step pretargeting approach to lymphoma radioimmunoimaging in mice using
biotinynaled CD45 monoclonal antibody (McAb) and 188Re-avidin in a tumor-bearing mouse model. Methods Six Nod-Scid
mice bearing lymphoma cell xenograft were randomized to receive either an intravenous injection of 50 μg/200 μL biotinyled
CD45 McAb followed 24 h later by an intraperitoneal injection of 3.7 MBq (50 μg/100 μL) 188Re-avidin (two-step pretargeting
group), or a single intravenous injection of 3.7 MBq (100 μg/100 μL) 188Re-CD45 McAb (control group). SPECT was performed
at 0.5, 1, 6 and 23 h post-injection to characterize 188Re isotope biodistribution. At 24 h pos-injection, the mice were sacrificed for
measurement of radioactivity uptake in the tumor and normal tissues and calculation of the tumor-to-non-tumor (T/NT) ratios.
Results SPECT showed that the two-step pretargeting method resulted in a low radioactivity in the blood pool during the
imaging and a concentrated radioactivity in the liver and spleen. The transplanted tumor began to be displayed at 1 h
post-injection and was clearly displayed at 1-6 h; the images were clear even at 23 h. With the two-step pretargeting method,
the radioactive uptake at 24 h post-injection were (1.34±0.52)%, (6.77±2.32)%, and (2.81±1.25)% in the tumor, kidney and liver,
respectively, with low radioactivity levels in other organs and high tumor/blood and tumor/muscle ratios (4.28±0.82 and 8.00±
0.88, respectively). In the control group, SPECT revealed intense radioactivity in the liver, spleen, and kidneys with obscure
display of the tumor; at 20 h, the radioactivity in the blood pool remained high but that in the tumor was low, and the tumor/
blood and tumor/muscle ratios at 24 h were only 0.58±0.06 and 3.21±0.24, respectively. Conclusion Compared with 188Re-CD45
McAb, the two-step pretargeting approach exhibits a good specificity in targeting lymphoma with an increased T/NT ratio in
mice and allows early tumor display at 1 h post-injection.