Abstract B25: Preclinical evaluation of radiolabeled MUC16 antibodies for PET imaging of epithelial ovarian carcinoma.

The CA-125 antigen is the most widely used serum biomarker to monitor epithelial ovarian carcinoma (EOC) and for the differential diagnosis of pelvic masses. A repeating glycopeptide, CA-125 is an epitope of the larger transmembrane mucin protein, MUC16, which is overexpressed in EOC. MUC16 has an average molecular weight of 3-5 million Da and includes a putative cleavage site proximal to a tandem repeat region. To date, the majority of antibodies against MUC16 have targeted the tandem repeat region of the protein, which is shed into circulation following cleavage. Using synthetic MUC16 peptides, high affinity antibodies (9C9, 19C11, 7B12) were raised against the portion of MUC16 retained by the cell. Imaging modalities currently used for EOC are generally viewed as inadequate and as such, the goal of this study is to evaluate these three antibodies to determine the best candidate to be used as a PET imaging agent for EOC. These antibodies were conjugated with desferrioxamine-isothiocyanate and radiolabelled with 89 Zr. Using a MUC16-positive cell line (SKOV3-MUC16 c114 ), in vitro studies were performed to confirm immunoreactivity and binding affinities of the radiolabeled antibodies. In addition, in vitro internalization studies were conducted which demonstrated similar uptake among the tracers. Subcutaneous EOC tumors were established in athymic, nude mice for the in vivo characterization of the MUC16 antibodies. Firstly, biodistribution data for 89 Zr-9C9 revealed an accumulation of the tracer in the tumor tissue at 24 hours post-injection of 9.41 ± 1.98 %ID/g. Kidney uptake of this tracer, although washing out over time, was extremely high at 24 hours (37.02 ± 1.35 %ID/g). The liver and spleen uptake of the tracer at 24 hours post-injection were 9.74 ± 0.76 and 4.76 ± 0.55 %ID/g, respectively. For 89 Zr-19C11, despite in vitro data that demonstrated active internalization, biodistribution data for 89 Zr-19C11 revealed minimal tumor uptake in vivo 24 hours post-injection (3.12 ± 0.68 %ID/g). Kidney uptake of this tracer was 4.84 ± 0.98 %ID/g and liver and spleen uptake were high 24 hours post-injection (34.73 ± 4.39 and 9.25 ± 0.90 %ID/g, respectively) indicating potential aggregation or stability issues in vivo . Unlike the other tracers, serum stability data of 89 Zr-19C11 demonstrated a purity of 74.43 ± 1.51% after 7 days, an uncharacteristically low stability for a 89 Zr-labeled antibody (typically greater than 90%). For 89 Zr-7B12, biodistribution data revealed modest tumor uptake 24 hours post-injection (8.54 ± 0.95 %ID/g), kidney uptake of 7.19 ± 0.75 %ID/g, and liver and spleen uptake of 6.24 ± 0.83 and 7.22 ± 1.26 %ID/g, respectively. These in vitro and in vivo studies point to 89 Zr-7B12 as the leading candidate thus far. Further studies, including imaging, will be conducted at longer time points to evaluate in vivo tumor uptake of 89 Zr-7B12 in addition to blocking studies to demonstrate the specificity of the tracer. Research funded, in part, by the Center to Reduce Cancer Health Disparities of the National Cancer Institute (3R01CA176671-02S1). Citation Format: Brandon E. Nemieboka, Dharmarao Thapi, Ariana Bitton, Jason S. Lewis. Preclinical evaluation of radiolabeled MUC16 antibodies for PET imaging of epithelial ovarian carcinoma. [abstract]. In: Proceedings of the AACR Special Conference on Advances in Ovarian Cancer Research: Exploiting Vulnerabilities; Oct 17-20, 2015; Orlando, FL. Philadelphia (PA): AACR; Clin Cancer Res 2016;22(2 Suppl):Abstract nr B25.