dPEG®ylation of 124I-labeled mCC49Fab'-NEM improves tumor imaging in mice

Objectives TAG-72 (Tumor Associated Glycoprotein) is highly expressed in human colorectal adenocarcinomas, and anti-TAG-72 antibodies like CC49 are promising in diagnosis and treatment. We investigated the effect of site specific PEGylation with discrete, molecular dPEG® entities at an exposed cysteine thiol on (mouse) mCC49Fab'-NEM, conjugating Mal-dPEG®12-tris(dPEG®24COOH)3 (Mal-dPEG®-A), Mal-dPEG®12-tris(dPEG®12COOH)3 (Mal-dPEG®-B), or Mal-dPEG®12-tris(m-dPEG®24)3 (Mal-dPEG®-C), and radiolabeling with 124I.

Methods The in vitro binding ability was determined by a CC49 competitive binding assay to TAG-72 plates, and in vivo efficiency was investigated via pharmacokinetics, PET/CT scanning, and biodistribution in nude mice bearing LS174T human colon carcinoma xenografts.

Results Mal-dPEG®-A reduced the in vitro binding affinity of mCC49Fab'-NEM by 30% while significantly lengthening in vivo blood retention of mCC49Fab'-NEM (47.5 vs 28.1%/ID at 1h, 25.1 vs 8.4 %/ID at 5 h post injection). It also increased tumor:background (T:B) in PET images of LS174T tumors by 213.8%: 603.1+/-153.8 vs 282.2+/-76.0 MBq/mL/μCi-dose, and increased tumor concentration at 72 h by 130% measured by counted excised tissues: 5.09 +/-0.83 vs 3.83 +/- 1.50 %ID/g, p 0.05. In contrast, Mal-dPEG®-B or -C reduced in vitro TAG72 binding affinity of mCC49Fab'-NEM (by 70%), and decreased PET T:B, reducing tumor concentration by 49% (1.97+/-0.29, p 0.05) and 63% (1.42+/- 0.35 %ID/g, p 0.05), respectively.

Conclusions We conclude that subtle changes in charge and PEG chain length in MAL-dPEG®ylation molecules can improve or diminish tumor concentration and background clearance of mCC49Fab'-NEM in TAG-72 expressing mice tumor xenografts.

Research Support Quanta Biodesign, Stefanie Spielman Foundation, The Ohio State University