<p><b>OBJECTIVE</b>To establish a two-step pretargeting approach to
lymphoma radioimmunoimaging in
mice using biotinynaled CD45
monoclonal antibody (McAb) and (188)Re-
avidin in a
tumor-bearing
mouse model.</p><p><b>
METHODS</b>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) (188)Re-
avidin (two-step pretargeting group), or a single
intravenous injection of 3.7 MBq (100 µg/100 µL) (188)Re-CD45 McAb (
control group).
SPECT was performed at 0.5, 1, 6 and 23 h post-
injection to characterize (188)Re
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.</p><p><b>RESULTS</b>
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.</p><p><b>CONCLUSION</b>Compared with (188)Re-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.</p>