Abstract 4298: Comparison of dual labeling strategies for NIRF/PET hybrid imaging

Objectives: Positron Emission Tomography (PET) plays a pivotal role in the surgical care of cancer patients pre- and post-operatively, however, no functional imaging approaches are currently available for real-time surgical guidance. Recently, antibodies dual-labeled with radioactive and near-infrared fluorescent (NIRF) labels have emerged as potential agents to extend whole-body nuclear imaging capabilities into the operating room, but structural limitations in conventional labeling reagents necessitate a random, two-step labeling process which can reduce bioactivity and batch reproducibility. Here, we evaluate the utility of a site-specific multimodal chelator (MMC) that combines PET/NIRF labeling into a single moiety for enhanced production of immunoconjugates for hybrid imaging. Methods: An anti-epithelial cell adhesion molecule (EpCAM)-specific antibody, mAb7, was treated with varying amounts of the reducing agent dithiothreitol (DTT) to partially reduce the interchain disulfides and permit conjugation with MMC-maleimide. The number of MMC moieties per mAb were quantified by isotopic dilution using non-radioactive Cu and immunoreactivity was assessed by ELISA. 64Cu labeling was performed and samples were purified by size exclusion prior to HPLC analysis. Stability of the immunoconjugates was evaluated by HPLC. Multimodal PET/NIRF imaging was performed in an orthotopic prostate tumor model to assess in vivo targeting. Results: mAbs treated with increasing amounts of DTT had higher conjugation ratios, with values ranging from 1-8 MMC moieties per mAb. ELISAs showed no differences in binding potency for the MMC-immunoconjugates. Radiochemical yields were high for all samples (>80%) and increased as a function of conjugation ratio. Interestingly, initial 64Cu labeling of the immunoconjugate with a MMC/mAb ratio of 4 had a corresponding HPLC trace showing >95% radiochemical purity, however, labeling 8 weeks later revealed formation of a second, prominent radioactive peak. Conversely, the HPLC profile of the immunoconjugate with a MMC/mAb ratio of 2 was consistent throughout the study, suggesting a possible correlation between the extent of partial reduction and protein stability. This observation was supported by in vivo imaging findings as lesions were better visualized in mice receiving 64Cu-MMC-mAb7 with a MMC/mAb ratio of 2. Conclusions: The results indicate that site-specific conjugation of MMC is effective for dual labeling antibodies. However, partial reduction conditions must be optimized in order to obtain a multimodal immunoconjugate with suitable imaging properties and stability. Citation Format: Sukhen C. Ghosh, Barrett R. Harvey, Holly Robinson, Kenneth L. Pinkston, Nathaniel Wilganowski, Banghe Zhu, Eva M. Sevick-Muraca, Ali Azhdarinia. Comparison of dual labeling strategies for NIRF/PET hybrid imaging. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 4298. doi:10.1158/1538-7445.AM2014-4298