3.6 Evaluation of IP150-based
radio-conjugates in the CDX/PDX melanoma model
Four in vivo CDX tumor models were established to validate the ability
of 89Zr-IP150 to target MUC18 by imaging. IHC staining
confirmed that the A375, HMVII and SK-Mel-28 CDXs were MUC18 (+), while
the A549 CDX was MUC18 (-) (Figure 3B). Moreover,89Zr-IP150 had a high PET signal in the MUC18(+) A375,
HMVII, and SK-Mel-28 tumors and a low signal in the MUC18(-) A549 tumor
(Figure 3A). In the A375 model (small tumor), the time-dependent
accumulation of 89Zr-IP150 in the tumor was visualized
by static PET/CT at 2, 24, 48, 72, 96, 120, and 168 hours p.i. The tumor
was visible at 24 hours p.i., and a high contrast image
(tumor/background) was available at 48 p.i.; nearly no radiotracer was
detected in normal tissues except the liver, and no significant residue
or accumulation of the 89Zr conjugate was detected in
the joints. According to image semiquantitative uptake calculations, the
tumor-specific signal increased mainly from 2 to 24 h p.i., with an
SUVmax ranging from 0.69±0.03 to 2.56±0.12, and the tumor uptake
continued to increase to 5.67±0.29 at 168 p.i. (Supplementary Figure
2A). Furthermore, the specificity of 89Zr-IP150 to
MUC18 binding was confirmed by intravenous preinjection (24 hours before
injection of 89Zr-IP150) with 1 mg of unlabeled IP150
mAb (Figure 4A). The SUVmax of the tumor at 2 hours was 0.72±0.05, which
was not significantly different from that of the nonblock group
(p =0.54), but the accumulation of the tracer at the tumor
(Supplementary Figure 2A and 2B) was significantly different
(p <0.05) at 24 hours (2.56±0.12 vs. 0.39±0.02, 3.16±0.12
vs. 1.68±0.14, 4.22±0.14 vs. 1.84±0.13, 4.38±0.10 vs. 1.83±0.05,
4,87±0.14 vs. 1.87±0.13, and 5.57±0.29 vs. 1.95±0.08). In the HMVII
model (medium tumor), 89Zr-IP150 also showed visible
tumor uptake at 24, 48, 72 and 120 hours and peaked uptake at 96 hours,
with an SUVmax of 7.11±0.08 (Supplementary Figure 2C and 2D). Similarly,
tumor uptake was also significantly decreased (p <0.05) in
the block group at 24 hours (2.99±0.02 vs. 0.96±0.06, 4.99±0.12 vs.
0.09±0.04, 5.44±0.09 vs. 1.32±0.02, 7.11±0.08 vs. 1.34±0.06, 3.69±0.04
vs. 1.49±0.05, and 3.63±0.06 vs. 1.52±0.09). Moreover,89Zr-IP150 was also a good indicator of the location
of SK-Mel-28 (large tumor) (Supplementary Figure 2E). For the A549 model
(MUC18-), almost no radioactive uptake was observed at the tumor site,
with a low SUVmax of 0.72±0.04 (Figure 3A and 4A).