Abstract SLC34A2 is overexpressed in 61% of ovarian cancer and 18% of uterine cancer tumors. SLC34A2 overexpression is believed to be driven by the aberrant activity of the essential PAX8 transcription factor in these tumors. Overexpression of SLC34A2, a phosphate importer, sensitizes the cells to the genetic depletion of XPR1, the sole phosphate exporter. Importantly, pharmacologic inhibition of XPR1 by XRBD, a peptide molecule that binds the extracellular domain 3 (ECL3) of XPR1 to antagonize its function, phenocopies the lethal impact of XPR1 knockdown on SLC34A2 overexpressing cancer cells. This observation supported the hypothesis that an antibody that binds to XPR1 will antagonize its function and selectively kill cancer cells that overexpress SLC34A2. There are no XPR1 therapeutics reported in clinical development indicating a first-in-class opportunity for an XPR1 therapeutic antibody. The goal of this program was to identify an antagonist antibody that inhibits XPR1 phosphate export function as a novel therapy for ovarian and uterine cancers patients with tumors that overexpress SLC34A2 (SLC34A2-high). We utilized XRBD to understand XPR1 biology in SLC34A2- high and low expressing cancer cell line models. Mechanistically, in SLC34A2-high expressing cell lines, genetic or pharmacologic inhibition of XPR1-dependent phosphate efflux led to the toxic accumulation of intracellular phosphate. XPR1 inhibition in these cell lines also resulted in increased cellular nucleotide triphosphate levels. Interestingly, we found altered mitochondrial gene expression and metabolism upon XPR1 inhibition and/or loss. Further, XPR1 inhibition induced gamma H2AX suggesting increased DNA damage. This finding was also supported by synergistic growth inhibition and cytotoxicity when XPR1 inhibitor was combined with cisplatin. On our path to therapeutic XPR1 antibody discovery, we identified antibodies that bound to overexpressed and endogenous XPR1, however they did not induce expected pharmacodynamic responses or efficacy against SLC34A2 -high expressing cancer cell lines. These results indicate that the binding of these antibodies to XPR1 does not recapitulate the antagonistic activity of XRBD against the viability of SLC34A2 overexpressing cancer cells. Citation Format: Mansi Babbar, Dion Daniels, Katie Vowell, Rajesh Sundaresan, Shuzhen Wu, Feng Wang, James Lin, Jenny Laraio, Justin Munro, Joseph Kozole, John Kreeger, Geeta Sharma, Bryan Joosse, Florence Patel, Joseph Tomlinson, Katherine Welbeck, Richard Priest, Jennifer Percival-Alwyn, Trevor Wattam, Gurjinder Heer, Iain Moal, Sreenivas Nannapaneni, Benjamin Schwartz, Biju Mangatt, Anthony Mazurek. XPR1 and SLC34A2 (NaPi2B)- A tale of targeting the phosphate homeostasis in cancer cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 2976.