Weroha, S. John

Abstract Despite therapeutic advances, ovarian cancer (OC) recurrence remains common and outcomes for advanced disease are poor, highlighting the need for novel therapeutic targets. To this end, we identified CTPS1 as a highly expressed and essential gene for the survival of many OC cell lines, including models resistant to chemotherapy and PARP inhibitors. CTPS1 is one of two enzymes, alongside CTPS2, responsible for the de novo synthesis of cytidine triphosphate (CTP), a critical nucleotide required for DNA and phospholipid biosynthesis. We discovered that CTPS1 is the predominant isoform expressed in OC cells, with significantly higher levels of expression in advanced and resistant disease compared to normal tissue, benign lesions and treatment naïve cancers. Intriguingly, 25% of all OC samples were found to completely lack CTPS2 protein expression suggesting their near complete reliance on CTPS1 for survival. Using CTPS1 siRNAs and genetically engineered OC cells with a degradation tag (dTAG) fused to the C-terminus of endogenous CTPS1 alleles, we showed that CTPS1 knockdown or proteasomal degradation induces an S-phase cell cycle arrest followed by apoptosis. Leveraging a first-in-class, highly selective, and orally bioavailable CTPS1 inhibitor (STP938) developed by Step Pharma, we identified nanomolar-range IC50 values across a panel of over 20 OC cell lines and validated its anti-neoplastic activity in ex vivo patient-derived xenograft (PDX) models. STP938 synergized with standard-of-care chemotherapy and PARP inhibitors, even in resistant models, further supporting its clinical potential. To elucidate STP938’s mechanism of action, we performed multi-omic profiling (bulk RNAseq, phospho- and total proteomics) on sensitive and isogenic resistant OC lines, as well as single-cell RNAseq of an OC PDX tumor following in vivo treatment with vehicle or STP938, revealed dysregulation of DNA replication, cell cycle, and metabolic pathways. As expected, STP938 rapidly depleted CTP levels, which remain suppressed for 72 hours, suggesting insufficient salvage pathway compensation which ultimately compromised cancer cell viability. As CTP is also a direct precursor for activated intermediates in phospholipid biosynthesis. Mass-spectrometry based lipidomics and metabolomics were employed to interrogate the impact of STP938 in these processes where numerous changes were found. To uncover mechanisms of resistance to STP938 and reveal therapeutic vulnerabilities, we conducted genome-wide CRISPR knockout screens in multiple treatment naïve and STP938 resistant models and analysis of results are ongoing. In collaboration with Step Pharma, a Phase 1a/b clinical trial of STP938 is currently enrolling patients with advanced solid tumors, including an expansion cohort for CTPS2 null OC patients (NCT06297525). These laboratory discoveries and ongoing clinical efforts aim to enable broader uptake of this novel therapeutic approach to improve long-term outcomes in patients with CTPS1-dependent disease. Citation Format: Xiyin Wang, Michael J. Emch, Lauren A. Voll, Rebecca Epp, Esther P. Rodman, Noa Odell, Nicole A. Pearson, Xiaonan Hou, Matthew P. Goetz, Scott H. Kaufmann, S. John Weroha, Philip Beer, John R. Hawse. Exploiting CTPS1 dependency for the treatment of ovarian cancerPR005: Exploiting CTPS1 dependency for the treatment of ovarian cancer [abstract]. In: Proceedings of the AACR Special Conference in Cancer Research: Advances in Ovarian Cancer Research; 2025 Sep 19-21; Denver, CO. Philadelphia (PA): AACR; Cancer Res 2025;85(18_Suppl):Abstract nr PR005.

Importance Ovarian cancer is the eighth most common cause of cancer and cancer death in women worldwide. In 2022, ovarian cancer was diagnosed in approximately 324 398 individuals, and 206 839 died of ovarian cancer worldwide. In 2025, it is estimated that 20 890 US women will be diagnosed with ovarian cancer and 12 730 patients will die of ovarian cancer. Observations Approximately 90% of ovarian cancers are epithelial malignancies, of which 70% to 80% are high-grade serous ovarian cancers. Less common epithelial subtypes include endometrioid, clear cell, low-grade serous, mucinous, and carcinosarcoma. The median age at diagnosis of ovarian cancer is 63 years. Risk factors include older age, family history of breast or ovarian cancer, endometriosis, and nulliparity. Hereditary factors are associated with 25% of cases, predominantly linked to BRCA1 / 2 gene variants. At diagnosis, approximately 95% of patients experience nonspecific symptoms, such as abdominal pain, bloating, and urinary urgency and frequency, and about 80% have advanced-stage disease (stage III-IV), including extrapelvic disease, ascites, and abdominal masses. Diagnostic and staging evaluation includes pelvic ultrasound; computed tomography of the chest, abdomen, and pelvis; and serum tumor markers such as carbohydrate antigen 125, carbohydrate antigen 19-9, and carcinoembryonic antigen. First-line treatment for early-stage ovarian cancer, defined as limited to the ovary or fallopian tube (stage I) or confined to the pelvis (stage II), is surgery (hysterectomy, bilateral salpingo-oophorectomy, omentectomy, and lymphadenectomy), followed by adjuvant chemotherapy (carboplatin and paclitaxel). With treatment, early-stage ovarian cancer has a 5-year overall survival of 70% to 95%. Advanced-stage ovarian cancer may be treated with primary cytoreductive surgery (removal of all visible cancer in the abdominal cavity) and adjuvant chemotherapy (carboplatin and paclitaxel) or with neoadjuvant chemotherapy followed by cytoreductive surgery and adjuvant chemotherapy. Most patients with advanced-stage ovarian cancer receive maintenance therapy with bevacizumab (a monoclonal antibody that blocks angiogenesis) and/or poly–adenosine diphosphate ribose polymerase (PARP) inhibitors. With treatment, the 5-year overall survival rate for advanced-stage ovarian cancer is 10% to 40%. However, individuals with BRCA -related gene variants have a 5-year overall survival rate of approximately 70% with PARP inhibitor treatment. Despite an initial remission rate of 80%, approximately 75% of patients with advanced-stage disease have ovarian cancer relapse within 2 years. Conclusions and Relevance Approximately 21 000 women are diagnosed with ovarian cancer annually in the US, and approximately 80% have advanced-stage ovarian cancer at diagnosis. First-line treatment of early-stage ovarian cancer is surgery and adjuvant platinum-based chemotherapy. Treatment of advanced-stage ovarian cancer includes cytoreductive surgery, platinum-based chemotherapy, and targeted maintenance therapies such as bevacizumab and/or PARP inhibitors.