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).