Abstract 2286: XmAb30819, an XmAb®2+1 ENPP3 x CD3 bispecific antibody for RCC, demonstrates safety and efficacy in in vivo preclinical studies

Bispecific T cell engagers simultaneously bind CD3 on T cells and tumor-associated antigens to promote T cell-mediated killing of tumor cells. While targeting lineage-restricted antigens such as CD19 or CD20 have found clinical success in hematopoietic cancers, targeting solid tumors requires high expressing tumor associated antigens (TAAs) with minimal normal tissue expression. One underexplored TAA is ectonucleotide pyrophosphatase/phosphodiesterase 3 (ENPP3) for renal cell carcinoma (RCC). An activation marker for basophils, ENPP3 is a type II transmembrane protein that can promiscuously hydrolyze a large variety of nucleotide-containing molecules. Although its specific function in the tumor microenvironment is unknown, bulk RNAseq data from the TCGA describes broad overexpression of ENPP3 in clear cell and papillary RCC and less frequently in hepatocellular carcinoma (HCC). This pattern of expression has been confirmed by IHC and makes ENPP3 a potential target for a CD3 bispecific approach. Building upon the XmAb® heterodimeric Fc platform, we generated bispecific antibodies in an XmAb 2+1 Fab2-scFv-Fc format that are bivalent for ENPP3 and monovalent for CD3. Reducing the affinity of both specificities encouraged avid binding and strong potency on high ENPP3 expressing cell lines while minimizing reactivity on low expressing cell lines, matching ENPP3's differential expression pattern. In vitro T cell-dependent cellular cytotoxicity (TDCC) assays on the relevant cell lines confirmed selective potency on high versus low expressing cells. In vivo potency was evaluated by xenograft tumor models in human PBMC-engrafted NSG mice. In a KU812 tumor model, the lead candidate, XmAb30819, exhibited complete responses both as a single agent and in combination with checkpoint blockade. When tested against RXF393, an RCC cell line that overexpresses ENPP3 upon tumor engraftment, XmAb30819 demonstrated complete responses at all tested doses. Tolerability in non-human primates was evaluated in an escalating dose model and then confirmed in an expansion cohort. Dose-dependent T cell margination was observed, and doses that were active in the mouse tumor studies were well-tolerated. Preclinical studies have demonstrated the safety and efficacy of XmAb30819, an XmAb 2+1 ENPP3 x CD3 bispecific antibody for RCC. Citation Format: Alex Nisthal, Sung-Hyung Lee, Yoon Kyung Kim, Christine Bonzon, Rumana Rashid, Kendra N. Avery, Liz Bogaert, Connie Ardila, Irene W. Leung, Nicole Rodriguez, Umesh S. Muchhal, Gregory L. Moore, Seung Y. Chu, John R. Desjarlais. XmAb30819, an XmAb®2+1 ENPP3 x CD3 bispecific antibody for RCC, demonstrates safety and efficacy in in vivo preclinical studies [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 2286.