Molecular imaging of CEACAM6 using antibody probes of different sizes

Objectives Carcinocinoembryonic antigen-related cell adhesion molecule 6 (CEACAM6) is overexpressed in a number of human malignancies, especially in pancreatic cancer. In this study, we labeled anti-CEACAM6 antibodies (Abs) of different sizes with ⁶⁴Cu to image the CEACAM6 expression in xenograft pancreatic tumors and biodistribution of antibodies in vivo.

Methods Three antibodies specifically against CEACAM6 including a single-domain antibody 2A3 (16 kDa), a heavy chain antibody 2A3-mFc (80 kDa) and a full length antibody 9A6 (150 kDa) were used. The antibodies were conjugated with FITC and Cy5.5 for in vitro and ex vivo immunostaining and DOTA for further ⁶⁴Cu labeling. Xenograft tumor models were developed BxPC3 human pancreatic cancer cell line that has high level of CEACAM6 expression. For PET imaging, the mice were intravenously injected with ⁶⁴Cu-DOTA-Abs and static scans were acquired at 4, 18, 24 and 48 hr post-injection (p.i.).

Results High expression of CEACAM6 on BxPC3 cells was confirmed by high fluorescence intensity on cell surface with FITC-Ab staining. Ex vivo immunostaining on tumor sections at 24 hr after FITC and Cy5.5-Abs demonstrated specific tumor targeting of both 2A3-mFc and 9A6. ⁶⁴Cu-DOTA-2A3 showed low tumor uptake and fast clearance on PET images. BxPC3 tumors were clearly visualized with ⁶⁴Cu-DOTA-2A3mFc and ⁶⁴Cu-DOTA-9A6 PET imaging. At 48 hr post injection of ⁶⁴Cu-DOTA-2A3mFc and ⁶⁴Cu-DOTA-9A6, the tumor uptake was 80.3 ± 15.9 and 121.4 ± 20.8 % ID/g, respectively. Compared with the full length antibody 9A6, the heavy chain antibody 2A3-mFc showed higher tumor uptake, lower liver uptake and shorter circulation half-life. The results from optical imaging with Cy5.5-Abs were consistent with those of PET imaging.

Conclusions The heavy chain antibody 2A3-mFc is superior to the single domain antibody and the full length antibody with regard to tumor detection and biodistribution. It has great potential to be developed for CEACAM6-targeted pancreatic cancer imaging and therapy