(124)I-HuCC49deltaC(H)2 for TAG-72 antigen-directed positron emission tomography (PET) imaging of LS174T colon adenocarcinoma tumor implants in xenograft mice: preliminary results

BACKGROUND: (18)F-fluorodeoxyglucose positron emission tomography ((18)F-FDG-PET) is widely used in diagnostic cancer imaging. However, the use of (18)F-FDG in PET-based imaging is limited by its specificity and sensitivity. In contrast, anti-TAG (tumor associated glycoprotein)-72 monoclonal antibodies are highly specific for binding to a variety of adenocarcinomas, including colorectal cancer. The aim of this preliminary study was to evaluate a complimentary determining region (CDR)-grafted humanized C(H)2-domain-deleted anti-TAG-72 monoclonal antibody (HuCC49deltaC(H)2), radiolabeled with iodine-124 ((124)I), as an antigen-directed and cancer-specific targeting agent for PET-based imaging. METHODS: HuCC49deltaC(H)2 was radiolabeled with (124)I. Subcutaneous tumor implants of LS174T colon adenocarcinoma cells, which express TAG-72 antigen, were grown on athymic Nu/Nu nude mice as the xenograft model. Intravascular (i.v.) and intraperitoneal (i.p.) administration of (124)I-HuCC49deltaC(H)2 was then evaluated in this xenograft mouse model at various time points from approximately 1 hour to 24 hours after injection using microPET imaging. This was compared to i.v. injection of (18)F-FDG in the same xenograft mouse model using microPET imaging at 50 minutes after injection. RESULTS: At approximately 1 hour after i.v. injection, (124)I-HuCC49deltaC(H)2 was distributed within the systemic circulation, while at approximately 1 hour after i.p. injection, (124)I-HuCC49deltaC(H)2 was distributed within the peritoneal cavity. At time points from 18 hours to 24 hours after i.v. and i.p. injection, (124)I-HuCC49deltaC(H)2 demonstrated a significantly increased level of specific localization to LS174T tumor implants (p = 0.001) when compared to the 1 hour images. In contrast, approximately 50 minutes after i.v. injection, (18)F-FDG failed to demonstrate any increased level of specific localization to a LS174T tumor implant, but showed the propensity toward more nonspecific uptake within the heart, Harderian glands of the bony orbits of the eyes, brown fat of the posterior neck, kidneys, and bladder. CONCLUSIONS: On microPET imaging, (124)I-HuCC49deltaC(H)2 demonstrates an increased level of specific localization to tumor implants of LS174T colon adenocarcinoma cells in the xenograft mouse model on delayed imaging, while (18)F-FDG failed to demonstrate this. The antigen-directed and cancer-specific (124)I-radiolabled anti-TAG-72 monoclonal antibody conjugate, (124)I-HuCC49deltaC(H)2, holds future potential for use in human clinical trials for preoperative, intraoperative, and postoperative PET-based imaging strategies, including fused-modality PET-based imaging platforms.