Kaufmann, Scott H.

TPS259 Background: Despite currently approved therapies, most patients with metastatic colorectal cancer (mCRC) progress through all approved treatments and eventually succumb to their disease. Thus, novel therapies for chemotherapy-refractory mCRC are desperately needed. Inhibition of the DNA unwinding enzyme TOP1 is a well-characterized and clinically successful treatment strategy for mCRC. CBX-12 is a novel peptide-drug conjugate comprised of an exatecan warhead linked to a pH-Low Insertion Peptide (pHLIP). The principle behind this targeting mechanism is based on the Warburg effect, in which tumors establish a low pH extracellular environment. The pHLIP moiety of CBX-12 preferentially forms a unidirectional, transmembrane alpha helix in a low-pH extracellular environment. Thus, CBX-12 delivers its exatecan cytotoxic warhead preferentially into tumor cells. In mouse xenograft models, CBX-12 inhibited the HCT116 CRC xenograft model more potently than an equimolar dose of free exatecan. The phase 1 study of CBX-12 reported cytopenias as the predominant AEs and confirmed responses in several solid tumors including mCRC. Methods: This is a single-arm, phase 2 trial, with a Simon 2-stage design assessing CBX-12 for the treatment of chemotherapy-refractory mCRC. The primary objective is efficacy determined by RECIST v1.1 response rate. Secondary and exploratory objectives include evaluating the PK and PD as well as the progression-free survival (PFS) on CBX-12. Key inclusion criteria include: age ≥18 years, ECOG performance status ≤1, histologically or cytologically confirmed mCRC that is MSS/pMMR based on CLIA-approved testing, and progression on 2 or more lines of chemotherapy. This design requires 17 patients for stage 1, with termination if 1 or fewer objective responses are seen out of 17. If 2 or more patients out of 17 responds, then an additional 18 patients will be enrolled for stage 2. If 5 out of the total of 35 patients (14%) respond, then the trial would be declared successful. Correlative studies will include tumor plus peripheral blood PK to evaluate effectiveness of CBX-12 tumor targeting, ctDNA, and several tumor PD markers including TOP1-DNA trapping, SLFN11 expression, DNA damage, and apoptosis. The trial opened to enrollment on 8/21/25. Clinical trial information: NCT06730100 .

Abstract Background Missense mutations in IDH1 are well-established therapeutic targets in acute myeloid leukemia (AML). In contrast, the synonymous IDH1 variant p.Gly105Gly (rs11554137), present in 5–10% of AML cases, has been considered a benign polymorphism due to its high population frequency (5.7% overall, 0.7–10.5%) and lack of protein-altering effect. However, prior studies have associated this variant with inferior outcomes (Wagner, J Clin Oncol, 2010), though findings have been inconsistent. Few studies have examined its prognostic significance in the context of treatment intensity or allogeneic stem cell transplantation (SCT). Methods With IRB approval, clinical and genomic data were extracted from the Mayo Clinic AML database for patients diagnosed between Jan 1, 2018, and Jan 31, 2025. Patients were stratified by IDH1 Gly105Gly (rs11554137) status via NGS on bone marrow. Outcomes included co-mutation profiles, treatment response, measurable residual disease (MRD) by multiparametric flow cytometry, and survival. Multivariable Cox and logistic regression models for relapse-free survival (RFS) and overall survival (OS) were adjusted for age at diagnosis, ELN 2022 risk category, and SCT conditioning regimen. Results Among 379 AML patients, 36 (9.5%) harbored the IDH1 Gly105Gly (rs11554137) variant (cases) (median VAF 49%, IQR 48–49.2). In the full cohort, median age was 68 years (IQR 59–74), M:F ratio was 1.5; 75% had de novo AML, and median follow-up was 52 months. Among cases, common cytogenetic abnormalities included complex karyotype (28%), del(7q) (14%), and trisomy 8 (6%), while frequently mutated genes included DNMT3A (19%), IDH2 (17%), and BCOR (14%). Of 345 treated patients, 43% received intensive, and 56% received low-intensity regimens, including hypomethylating agent (HMA) alone (n=13), HMA+venetoclax (n=159), and targeted therapy (n=7). Among cases, complete remission (CR) rates were higher with intensive vs. low-intensity therapy (70% vs. 17%, p=0.03). MRD positivity post induction was more common in cases (66% vs. 26%, p=0.01), suggesting suboptimal leukemic clearance. Among 103 patients who underwent allogeneic SCT in CR1 (47% received myeloablative conditioning), morphologic relapse occurred in 64.3% of cases compared to 23.6% of controls (p = 0.004). Cases experienced significantly inferior median RFS (8 months [95% CI 5–NA] vs. not reached for controls, p <0.001), indicating more durable post-SCT remission in the control group. In a multivariable Cox proportional hazards model adjusting for age, ELN risk, conditioning intensity, MRD status, and complex karyotype, the IDH1 Gly105Gly variant remained independently associated with shorter RFS (HR 3.97, 95% CI: 1.74–9.05, p = 0.001). Complex karyotype was also an independent predictor of relapse (HR 3.35, 95% CI: 1.28–8.73, p = 0.014). Consistent with this, logistic regression revealed a significantly increased odds of post-SCT relapse among cases (OR 7.95, 95% CI: 2.19–35.27, p=0.001), independent of clinical covariates. Relapsed cases were significantly more likely to harbor FLT3-ITD mutations than relapsed controls (p=0.041). Pathway-level analysis revealed increased frequency of mutations in signal transduction genes (FLT3-ITD, CSF3R, NF1) (p=0.002). All CNS relapses among cases occurred in patients harboring at least one of these mutations. Despite the higher relapse rate, there were no significant differences in OS (14.4 vs. 11.3 months, p=0.36) between cases and controls. The IDH1 Gly105Gly variant was not independently associated with OS (HR 1.26, p=0.30). In contrast, age (p<0.0001) and adverse ELN risk (p<0.0001) were significant predictors of inferior OS. These findings suggest that the prognostic impact of IDH1 Gly105Gly is most pronounced in the post-SCT setting. Conclusion The synonymous IDH1 Gly105Gly variant (rs11554137) is an independent predictor of post-SCT relapse in AML, potentially mediated by co-occurring mutations in signal transduction pathways. Despite no observed impact on OS, likely due to effective salvage therapies, the variant's link to lower CR rates with low-intensity therapy and higher MRD burden suggests treatment intensity influences its clinical relevance. These findings support reevaluating the prognostic role of synonymous variants in AML and call for larger studies and mechanistic investigations into splicing, transcript stability, and translation.

Abstract BRCA2 mutations contribute to the pathogenesis and treatment sensitivity of a subset of ovarian, breast, prostate, and pancreatic cancers. When these cancers become therapy resistant, secondary mutations that restore the BRCA2 open reading frame are found in half the cases, but other causes of resistance remain incompletely understood. Here, we identified translational readthrough of a premature termination codon (PTC) as a cause of resistance to poly(ADP-ribose) polymerase inhibitors (PARPis) and cisplatin in cells derived from the BRCA2-mutated ovarian cancer line PEO1 by PARPi selection. Despite persistence of the signature 4965C > G (p.Y1655X) BRCA2 mutation, low-level expression of full-length BRCA2 protein was detectable in these cells by immunoblotting and tandem mass spectrometry. Either BRCA2 knockdown or gene interruption 5′ or 3′ to the PTC restored treatment sensitivity, implicating BRCA2 in the resistance. Reporter assays demonstrated UAG-selective readthrough in the resistant clones but not parental cells. Moreover, custom searching of global proteomic data indicated readthrough of stop codons, particularly UAGs, in additional proteins in the resistant clones. Finally, multi-omic analysis identified multiple changes in the nonsense-mediated decay and termination machineries that favor readthrough. Accordingly, the present results identify PTC readthrough as a potential mechanism of drug resistance in cells with BRCA2 nonsense mutations.

Abstract Cancer can recur when a subset of tumor cells, termed persister cells, survive therapy and re-enter the cell cycle. The cellular lineages that give rise to persister cells and the mechanisms that confer the persister state remain poorly understood. Through single-cell multiomic profiling (snRNA-seq and snATAC-seq) on a cohort of (1) non-malignant fallopian tube, (2) treatment-naïve, and (3) neoadjuvant chemotherapy (NACT)-treated high-grade serous ovarian cancer (HGSOC) patient samples, we identified an epigenetic signature that defines the chemotherapy-tolerant persister state. The changes in chromatin accessibility characterizing the signature were identified in residual NACT tumors but are also present in treatment-naive samples from patients who later developed resistance. Furthermore, this epigenetic signature independently predicted chemotherapy response in patient-derived xenograft models of HGSOC and in a separate patient cohort. Cells enriched in the persister state arose from multiple lineages and displayed activation of oncogenic pathways, including altered stress responses, epithelial to mesenchymal transition, and changes to the cell cycle promoting quiescence. Finally, we identified a subset of genes that are epigenetically primed for expression before treatment and are upregulated after treatment. These findings suggest that an intrinsic epigenetic program primes tumor cells towards chemotherapy tolerance and reveal new vulnerabilities that could be exploited to delay or prevent cancer recurrence. Citation Format: Mihai G. Dumbrava, Wazim M. Ismail, Leticia Sandoval, Amelia Mazzone, Syed Mohammed Musheer Aalam, Megan L. Ritting, Xiaonan Hou, Yiwen Xie, Shariska Harrington, Scott H. Kaufmann, Nagarajan Kannan, S. John Weroha, Alexandre Gaspar-Maia. Single cell multiomic analysis of high-grade serous ovarian carcinoma reveals an intrinsic epigenetic program that primes chemotherapy tolerance in persister cells [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 PR008.

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.

Abstract Introduction: Despite treatment advances, ovarian cancer (OC) remains the deadliest gynecological cancer, with a ∼70% mortality rate. OCs initially respond to platinum chemotherapy but often develop resistance. While PARP inhibitors improve relapse-free survival in responsive cases, they do not extend overall survival. This underscores the need for effective combination therapies beyond BRCA status. The lysosomal protein cathepsin L (CTSL) can traffic to the nucleus, where it can regulate DNA repair and serve as a chromatin modifier. We found that nucleoside analog clofarabine (CLF) induces CTSL nuclear trafficking in OC cells. Here we tested the hypothesis that accumulation nuclear CTSL (nCTSL) is a biomarker of sensitivity to DNA damaging therapy in OCs for improved therapeutic options. Methods: To determine the role of CLF induced nuclear CTSL in DNA damage and its synergy with PARP and CRM1 inhibitors in PARP inhibitor–sensitive and –resistant ovarian cancer (OC) cell lines, ex vivo cultures of ascites-derived patient OC cells (OVA-3D spheroids), primary ovarian tumors, and patient-derived xenograft (PDX) models and validated by colony formation assays. Nuclear CTSL localization was determined in subcellular fractionated lysates by western blots and immunofluorescence cytometry (IFC). Tumor regression was determined in vivo using ex vivo cultures of PDX derived cells. Results: In ovarian cancer (OC) cell lines, CLF monotherapy induced nuclear CTSL (nCTSL) in a subset of models classified as CLF-responsive (CLF-r), sensitizing them to the PARP inhibitors olaparib and rucaparib. In CLF-non-responsive (CLF-nr) models, nCTSL induction and therapeutic synergy were observed only with the CLF+olaparib combination. Synergy was detected in 47% of CLF-r and 24% of CLF-nr patient-derived ascites samples (OVA). Mechanistically, CLF induced nuclear import of CTSL via KPNB1 in CLF-r cells, while this import was triggered by the CLF+olaparib combination in CLF-nr cells. CLF also downregulated the nuclear export protein CRM1 (XPO1) in both groups. Knockdown of CTSL in either CLF-r or CLF-nr cells abrogated sensitivity to the combination. Notably, in 29% of OVA samples classified as CLF-resistant (CLF-Res), CLF failed to suppress CRM1 expression. In these cases, CRM1 inhibition with KPT8602 restored CLF+PARP inhibitor synergy. In vivo, CLF-r and CLF-nr PDX models treated with the combination exhibited increased DNA damage, reduced tumor burden, and extended survival. Conclusion: These findings identify nuclear CTSL (nCTSL) induction as a potential biomarker of response to DNA-damaging therapies. The combination of CLF and olaparib represents a promising strategy to overcome drug resistance in ovarian cancer by promoting CTSL nuclear localization and enhancing DNA damage. Citation Format: Prabhu Thirusangu, Ling Jin, Upasana Ray, Anya Zhao, Julie Staub, Xinyan Wu, Xiaonan Hou, Jamison L. VanBlaricom, Syed Mohamed Musheer Aalam, Sun-Hee Lee, Ann Oberg, Nagarajan Kannan, John Weroha, Jeremy Chien, Scott H. Kaufmann, Jamie N. Bakkum-Gamez, Viji Shridhar. Role of drug induced nuclear CTSL (nCTSL) in DNA damage response in cancer- therapeutic implications [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 A050.

Abstract Purpose: Our goal was to determine if the HRDefine biomarker combining methylation and mutation assessment of key homologous recombination repair genes identifies BRCA wildtype (BRCAwt) ovarian cancer (OC) with outcomes similar to BRCA mutated (BRCAm) OC within the context of a frontline PARP inhibitor trial for advanced ovarian cancer. Methods: We performed an exploratory biomarker analysis of blood and neoplastic samples from the VELIA trial, a phase 3 study evaluating the PARP inhibitor veliparib as frontline maintenance therapy in OC. We developed a highly sensitive droplet digital PCR assay for quantitative BRCA1 and RAD51C promoter methylation and assessed germline/somatic mutations using the BROCA next generation sequencing platform. Cases were grouped into: BRCAm, HRDefine positive (BRCAwt HRDefine+), and HRDefine negative (BRCAwt HRDefine-). Carcinomas were considered HRDefine+ if quantitative BRCA1 or RAD51C promoter methylation was >/= 60% or if a germline or somatic pathogenic variant (PV) was identified in the following non-BRCA HRR repair genes: RAD51C, RAD51D, PALB2, BARD1. For this analysis, the control group included the two study arms with placebo maintenance compared to the investigational arm of veliparib with chemotherapy and continued as maintenance (veliparib throughout group). HRDefine was compared to the Myriad genomic instability score (GIS) used as HRD assessment in the trial. Progression free survival (PFS) was compared using log rank tests. Results: Among 1140 VELIA participants, biomarker analysis could be performed for 487 cases. Of these, 255 (52%) were BRCAm, 60 (12%) were BRCAwt HRDefine+, and 172 (35%) were BRCAwt HRDefine-. BRCAwt HRDefine+ was driven by promoter methylation (57%), germline (40%), or somatic (3%) PVs. Median PFS (months, all arms combined) was 26 months (BRCAm), 27 months (BRCAwt HRDefine+), and 15 months (BRCAwt HRDefine-), with corresponding hazard ratios (HR) of 0.50 (95% CI: 0.39-0.65) and 0.50 (0.34-0.73) versus HRDefine-. All HRDefine+ molecular subgroups were associated with longer PFS, including RAD51C/D mutations (HR 0.44, 0.25-0.77) and promoter methylation (HR 0.60, 0.36-0.97). HRDefine positivity was significantly associated with improved PFS compared to HRDefine-/GIS low (n=60 vs n=111, median PFS 27 versus 14 months, HR 0.44, 0.30-0.65), while the HRDefine- cases with high GIS demonstrated a trend toward an intermediate PFS (n=56, median 18 months, HR 0.67, 0.45-1.01). When compared by treatment arm, median PFS was significantly longer in the veliparib throughout arm compared to control only for the BRCAm group (32 months vs 23 months, HR 0.61, 0.42-0.87), though the analysis was underpowered for the BRCAwt HRDefine+ sub-group (30 months vs 25 months, HR 0.79, 0.4-1.57). Conclusions: HRDefine identifies BRCAwt OC associated with a PFS that is like that of BRCAm cases and may improve stratification for PARP inhibitor therapy beyond current HRD assays. GIS may have some, albeit reduced, prognostic implication that is independent of HRDefine. Citation Format: Isabel V. Rodriguez, Marc Radke, Ronit Katz, Persephone Miller, Paula Ladd, Nithisha Khasnavis, Jennifer Ochoa, Melanie Dillon, Heather Lankes, Peter Ansell, Carol Aghajanian, Gini Fleming, Michael Birrer, Scott Kaufmann, Charles Leath, Kathleen Moore, Theresa Werner, Linda Van Le, Robert L. Coleman, Elizabeth M. Swisher. Application of HRDefine: a biomarker for identifying root causes of homologous recombination deficiency to a phase 3, randomized, placebo-controlled trial of veliparib in front-line treatment of advanced ovarian cancer (VELIA/GOG-3005) [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 A044.

Abstract PARP inhibitors (PARPis) are now a standard of care for high-grade serous ovarian cancer (HGSOC), but resistance to these drugs poses a significant concern, necessitating the exploration of novel treatment strategies. Here we investigated the mechanism underlying enhanced PARPi activity in HGSOC cells following treatment with brigatinib—a second-generation anaplastic lymphoma kinase (ALK) inhibitor approved for treating lung cancer. Our previous work has shown that brigatinib is a dual inhibitor of tyrosine kinases FAK and EPHA2 with subsequent additive reduction in downstream PI3K/AKT and MAPK/ERK cell survival signaling. Suppression of these pathways conferred PARPi sensitivity to HGSOC cell lines and patient-derived xenograft (PDX) models, particularly in those with high FAK and EPHA2. To dissect the mechanism linking the FAK/EPHA2 – AKT/ERK pathway to PARPi synergy, we focused on proteomics approaches which revealed that FRA1—an Activator Protein 1 (AP-1) transcription factor known to be downstream of AKT/ERK—and its targets were decreased by brigatinib treatment. In subsequent phosphoproteomics approaches and cell-based confirmatory studies, a phosphorylation crucial for FRA1 stability was found to be disrupted, leading to reduction in total FRA1 protein levels and synergy with PARPis. To explore how FRA1 mediates response to PARPis, immunoblotting analysis revealed that PARPis increased the abundance of FRA1 in HGSOC cells at early timepoints, indicating potential involvement in an adaptive survival response. Follow-up pharmacologic and gene silencing techniques demonstrated that brigatinib still reduces FRA1 in PARPi-treated cells, and FRA1 depletion induces apoptosis when combined with PARPi to levels comparable to brigatinib treatment. Overall, our studies unveil a beneficial ALK-independent effect of brigatinib that may offer new avenues for considering its repurposing as a HGSOC therapy through disruption of a PARPi-induced adaptive survival mechanism. Citation Format: Julie R. Duffield, Xiaonan Hou, Benjamin W. Wilson, Anjali Prasad, Iman K. McKeon-Makki, Amelia M. Huehls, Xinyan Wu, Cristina Correia, Melissa C. Larson, Fergus J. Couch, Ann L. Oberg, Scott H. Kaufmann, Larry M. Karnitz, Arun Kanakkanthara. A PARP-inhibitor-induced early adaptive survival response is tackled through depletion of FRA1 by brigatinib in high-grade serous ovarian carcinoma [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 A024.

e15125 Background: Endometrial cancer (EC) is the most common gynecologic malignancy, with endometrioid being the most prevalent histology. Approximately 75% of high-grade endometrioid tumors are TP53 wild-type. During studies testing a novel combination of the PARP inhibitor rucaparib and TOP1 poison SN38 in high-risk primary patient serous and serous-like ECs, a subset of non-serous ECs with normal p53 expression showed unexpected synergy. This suggests broad efficacy in EC, warranting in vivo studies to assess tolerability and efficacy. Given potential overlapping toxicity, a novel formulation, pegylated SN38 (PLX038A), was tested. Methods: Primary patient tumors (n=4) were treated with rucaparib (0µM-25 µM) and SN38 (0µM-0.5 µM) in 3D cell culture experiments. RealTime Glo assessed viability after 3-5 days of drug exposure. Combination indices were determined using the Chou-Talalay method. Tolerability and efficacy were assessed in female SCID-beige mice (n=45) injected with intraperitoneal EC PDX tumors. Mice were randomized to controls, PLX038A (15 mmol/µmol IP, every 14 days), rucaparib (150 mg/kg daily gavage), or combination (same dose and schedule) for 8 weeks. Tolerability was assessed by daily animal weight and total condition score (TCS) from 1 (emaciated, non-ambulatory) to 13 (obese). Weight change more than -20% or TCS < 5 defined moribund criteria. Weekly ultrasound measurements assessed tumor response. Results: All four primary patient tumors exhibited synergy, with a combination index less than 1. Moribund criteria were not met in any mice across all cohorts. The average percent weight change and lowest TCS across cohorts were: rucaparib (-8.51%, min/max -19 to +14%; TCS 6.25, range 5-8, n=8), PLX038A (-3.97%, min/max -19 to +5%; TCS 6.8, range 6-9, n=10), the combination (-5.52%, min/max -15 to +1.8%; TCS 7.18, range 6-8, n=11), and control (+1.61%, range -6 to +8%; TCS 6.5, range 6-7, n=4). Scores were similar between groups, but the slightly lower TCS in the rucaparib cohort was associated with higher tumor burden from ineffective therapy. Tumors in the rucaparib monotherapy group grew similarly to controls. In contrast, the combination therapy cohort exhibited significant regression below baseline during treatment. Conclusions: These findings suggest that the combination of rucaparib and SN38 (PLX038A) is effective in EC primary patient tumor organoids and PDXs with wild-type TP53. No significant negative physiological effects on animal TCS and weight change were observed. No drug-related toxicity was observed in mice receiving combination treatment. Mice receiving combination treatment displayed significant tumor regression below baseline compared to both single agents and control. These data support future exploration of this combination therapy in ECs.

Abstract Cancer can recur when a subset of tumor cells, denoted here as persister cells, are able to survive therapy and re-enter the cell cycle. The precise mechanisms that confer the persister state and whether it is characteristic of a subgroup of cells or arises from multiple cellular lineages remain poorly understood. We hypothesize that an epigenetic signature underlies the drug-tolerant persister state, characterized by transcriptional and chromatin accessibility changes that promote survival of residual cancer following chemotherapy. To identify clinically relevant features of persister cells in untreated tumors and residual disease, we performed single-cell multiomic profiling (snRNA+snATAC) on a cohort of non-malignant fallopian tube, treatment-naïve, and neoadjuvant chemotherapy (NACT)-treated high-grade serous ovarian cancer (HGSOC) samples. We identified differences in gene expression and open chromatin between naïve and residual patient tumors following chemotherapy. Although only a small proportion of the differentially expressed genes enriched in residual HGSOC overlapped with established gene sets for chemo-response and patient prognosis, the epigenomic analysis revealed activity of several DNA-binding factors that are both enriched upon chemotherapy and also high in resistant tumors prior to treatment. From this analysis, we identified an epigenetic signature that precedes expression and defines the persister state. This epigenetic signature also correlated with chemotherapy sensitivity and resistance using patient-derived xenograft models of HGSOC. Gene regulatory networks driven by the persister signature are involved in the activation of oncogenic pathways, including changes to the cell cycle promoting quiescence and stress response. Further study of the persister cells identified by this epigenetic signature may increase understanding of the mechanisms underlying persister cell survival and reveal new vulnerabilities that could be exploited to delay or prevent cancer recurrence.