Abstract 4376: Characterization and targeting of TAF15, a radiation-inducible target in multiple cancer types

Proceedings: AACR 106th Annual Meeting 2015; April 18-22, 2015; Philadelphia, PA Currently, development of anti-cancer therapies is limited to antigens that are either overexpressed or that are specific to cancer. This limits both the number of available targets and their tumor selectivity. Using phage display technology, we identified a number of proteins in multiple cancer types that are induced by sub-lethal doses of ionizing radiation (IR), including TATA-box-binding protein associated factor 15 (TAF-15), a key transcriptional regulator. TAF15 presents a novel synergistic molecular target in cancer for both therapeutic and imaging purposes. In the first part of this study, we characterized TAF15 as a radiation-inducible molecular target both in vitro and in vivo using various analytical approaches. Surface expression analysis of TAF15 via flow cytometry showed that TAF15 is not only expressed on the surface of brain, lung and breast cancer, but is also induced by IR reproducibly on the surface of breast cancer and endothelial cells, and not in normal cells. Using a heterotopic tumor model in athymic nude mice, near-infrared (NIR) imaging analysis results showed that TAF15 can be successfully targeted in several different irradiated human tumor xenografts using both an anti-TAF15 antibody (Ab) and our in-house anti-TAF15 peptide. More importantly, we demonstrated that TAF15 can also be targeted in an orthotopic primary tumor model; NIR results showed high affinity binding of an anti-TAF15 Ab to irradiated primary breast tumors established in the mammary fat pads of nude mice as compared to isotype-matched controls. Furthermore, we have successfully genetically modified the knob domain of an adenovirus (Ad) type 5 fiber protein to contain our lead anti-TAF15 peptide and showed that this novel Ad-TAF15 targeting peptide binds strongly to irradiated breast tumor xenografts in a recent pilot study using bioluminescence imaging (BLI), a result that has major therapeutic and tumor imaging implications. We are currently evaluating the microscopic biodistribution of TAF15 in the primary tumor microenvironment using immunofluorescence and whether cell surface binding of TAF15 can activate an immune response, either via antibody-dependent cell mediated cytotoxicity (ADCC) or antibody dependent cell mediated phagocytosis (ADCP). In the second part of this study, we initiated anti-TAF15 monoclonal antibody (mAb) production against 3 different TAF15 epitopes using hybridoma technology. After screening for binding specificity to TAF15 binding via Western and dot blot analysis, we identified several lead anti-TAF15 antibody clones for further sub-cloning. We are currently evaluating the binding affinity to TAF15, in vivo efficacy and cancer cell specificity of these lead clones. Future studies include humanization of these candidate mAbs and evaluation of their cancer specificity and anti-cancer cytotoxicity with the goal of introducing the most efficacious and selective anti-TAF15 mAbs into clinical trials. Note: This abstract was not presented at the meeting. Citation Format: Lincoln Muhoro, Heping Yan, Sergey Kaliberov, Jerry Jaboin, David Curiel, Dennis Hallahan. Characterization and targeting of TAF15, a radiation-inducible target in multiple cancer types. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 4376. doi:10.1158/1538-7445.AM2015-4376