PARP Inhibitors Research Articles

Antibody Drug Conjugate Sacituzumab Govitecan Enables A Sequential TOP1/PARP Inhibitor Cancer Therapy Strategy in Breast Cancer Patients.

The Antibody-Drug Conjugate (ADC) Sacituzumab govitecan (SG) comprises the topoisomerase 1 (TOP1) inhibitor SN-38, coupled to a monoclonal antibody targeting trophoblast cell surface antigen 2 (TROP-2). Poly (ADP-ribose) polymerase (PARP) inhibition may synergize with TOP1 inhibitors and SG, but previous studies combining systemic PARP and TOP1 inhibitors failed due to dose-limiting myelosuppression. Here, we assess proof-of-mechanism and clinical feasibility for SG and talazoparib employing an innovative sequential dosing schedule. In vitro models tested pharmacodynamic endpoints, and in a phase 1b clinical trial (NCT04039230) 30 patients with metastatic Triple-Negative Breast Cancer (mTNBC) received SG and talazoparib using a concurrent (N=7) or sequential (N=23) schedule. Outcome measures included safety, tolerability, preliminary efficacy and establishment of a recommended phase 2 dose (RP2D). We hypothesized that tumor-selective delivery of TOP1i via SG would reduce non-tumor toxicity and create a temporal window, enabling sequential dosing of SG and PARP inhibition. In vitro, sequential SG followed by talazoparib delayed TOP1 cleavage complex clearance, increased DNA damage and promoted apoptosis. In the clinical trial, sequential SG/talazoparib successfully met primary objectives and demonstrated median PFS of 7.6 months without Dose-Limiting Toxicities (DLTs), while concurrent dosing yielded 2.3 months PFS and multiple DLTs including severe myelosuppression. While SG dosed concurrently with talazoparib is not tolerated clinically due to an insufficient therapeutic window, sequential dosing of SG then talazoparib proved a viable strategy. These findings support further clinical development of the combination and suggest that ADC-based therapy may facilitate novel, mechanism-based dosing strategies.

Comparing niraparib versus platinum-taxane doublet chemotherapy as neoadjuvant treatment in patients with newly diagnosed homologous recombination–deficient stage III/IV ovarian cancer: study protocol for cohort C of the open-label, phase 2, randomized controlled multicenter OPAL trial

BackgroundMaintenance therapy with niraparib, a poly(ADP-ribose) polymerase inhibitor, has been shown to extend progression-free survival in patients with newly diagnosed advanced ovarian cancer who responded to first-line platinum-based chemotherapy, regardless of biomarker status. However, there are limited data on niraparib’s efficacy and safety in the neoadjuvant setting. The objective of Cohort C of the OPAL trial (OPAL-C) is to evaluate the efficacy, safety, and tolerability of neoadjuvant niraparib treatment compared with neoadjuvant platinum-taxane doublet chemotherapy in patients with newly diagnosed stage III/IV ovarian cancer with confirmed homologous recombination–deficient tumors.MethodsOPAL is an ongoing global, multicenter, randomized, open-label, phase 2 trial. In OPAL-C, patients will be randomized 1:1 to receive three 21-day cycles of either neoadjuvant niraparib or platinum-taxane doublet neoadjuvant chemotherapy per standard of care. Patients with a complete or partial response per Response Evaluation Criteria in Solid Tumors version 1.1 (RECIST v1.1) will then undergo interval debulking surgery; patients with stable disease may proceed to interval debulking surgery or alternative therapy at the investigator’s discretion. Patients with disease progression will exit the study treatment and proceed to alternative therapy at the investigator’s discretion. After interval debulking surgery, all patients will receive up to three 21-day cycles of platinum-taxane doublet chemotherapy followed by niraparib maintenance therapy for up to 36 months. Adult patients with newly diagnosed stage III/IV ovarian cancer eligible to receive neoadjuvant platinum-taxane doublet chemotherapy followed by interval debulking surgery may be enrolled. Patients must have tumors that are homologous recombination–deficient. The primary endpoint is the pre–interval debulking surgery unconfirmed overall response rate, defined as the investigator-assessed percentage of patients with unconfirmed complete or partial response on study treatment before interval debulking surgery per RECIST v1.1.DiscussionOPAL-C explores the use of niraparib in the neoadjuvant setting as an alternative to neoadjuvant platinum-taxane doublet chemotherapy to improve postsurgical residual disease outcomes for patients with ovarian cancer with homologous recombination–deficient tumors. Positive findings from this approach could significantly impact preoperative ovarian cancer therapy, particularly for patients who are ineligible for primary debulking surgery.Trial registrationClinicalTrials.gov NCT03574779. Registered on February 28, 2022.

Effect of food on the pharmacokinetics and safety profiles of a new PARP inhibitor fuzuloparib capsules in healthy volunteers.

This study assessed effect of food on pharmacokinetics (PK) and safety of fuzuloparib capsules. A randomized, open-label, two-cycle, two-sequence, crossover clinical trial was conducted. 20 subjects were randomly assigned to 2 groups at a 1:1 ratio. The first group subjects were orally administered 150mg fuzuloparib capsules under fasting condition in first dosing cycle. The same dose of fuzuloparib capsules were taken under postprandial state after a 7-day washout period. The second group was reversed. 3ml whole blood was collected at each blood collection point until 72h post dose. PK parameters were calculated. Furthermore, safety assessment was performed. The time to maximum concentration (Tmax) was prolonged to 3h and maximum concentration (Cmax) decreased by 18.6% on high-fat diets. 90% confidence intervals (CIs) of geometric mean ratios (GMRs) for Cmax, area under the concentration-time curve from time zero to time t (AUC0-t), and area under the concentration-time curve extrapolated to infinity (AUC0-∞) after high-fat meal were 71.6-92.6%, 81.7-102.7% and 81.6-102.5%, respectively. All treatment-emergent adverse events (TEAEs) were grade 1; No serious adverse events (SAEs), serious unexpected suspected adverse reaction (SUSAR) or deaths were reported. Food decreased the absorption rate and slowed time to peak exposure of fuzuloparib capsules, without impact on absorption extent. Dosing with food was found to be safe for fuzuloparib capsules in this study. This study was registered with chinadrugtrials.org.cn (identifier: CTR20221498).

Abstract PO4-06-06: Clinical Implications of Body Mass Index (BMI) in BRCA-associated Advanced Breast Cancer Patients Treated With PARP (Poly ADP-Ribose Polymerase) Inhibitors

Abstract Background: PARP inhibitors (PARPi) have demonstrated promising results in patients with advanced as well as early breast cancer carrying germline BRCA1/2 pathogenic variants (PV). Although there is increasing attention to the effect of BMI on the outcomes of cancer patients, there is limited data on its effect on BRCA-associated advanced breast cancer patients receiving PARPi therapy. In this report, we evaluate the effect of BMI on the outcomes of advanced breast cancer patients with a BRCA PV and who received PARPi therapy. Methods: Patients with BRCA 1/2 germline PVs who presented to the University of Texas MD Anderson Cancer Center Breast Medical Oncology clinics and were treated with PARPi between 2008-2022 as part of their treatment for metastatic or recurrent disease were included in the analysis. The patients were divided into four groups according to BMI level as follows: underweight (< 18.5 kg/m2), normal weight (18.5-24.9 kg/m2), overweight (25-29.9 kg/m2), and obese (≥30 kg/m2). Descriptive statistics, the Kaplan-Meier method, the Cox proportional hazards regression model, and Fisher’s exact test were used to report patient characteristics and outcomes. Progression-free survival (PFS) was calculated from the start of the PARPi therapy to the time of progression or death. Overall survival (OS) was calculated from the start of the PARPi therapy to the time of death of any cause. Results: One hundred and seven patients were treated with PARPi. The median BMI was 25.9 kg/m2 (14.8-61.2). In patients who received ≥2 cycles of PARPi, the overall response rate was 100% in the underweight group (n=4, 3.8%), 65.8% in the normal weight group (n=43, 41%), 64% in the overweight group (n=30, 28.6%), and 58.3% in the obese group (n=28, 26.7%). The median PFS was 5.4 months, 9.2 months, 9 months, and 8.6 months in underweight, normal weight, overweight, and obese patients, respectively. While the obese patients had a better outcome in terms of PFS (p = 0.0288), there was no significant difference observed in OS between BMI groups. Conclusion: In this analysis, we showed that obese patients with BRCA-associated advanced breast cancer had a better PFS. Future studies analyzing body composition parameters in BRCA-associated breast cancer patients treated with PARPi therapy could shed more light on the impact of obesity on outcomes. Citation Format: Fatma Akkoc Mustafayev, Mark Munsell, Rachel Layman, Clinton Yam, Angelica Gutierrez Barrera, Banu Arun. Clinical Implications of Body Mass Index (BMI) in BRCA-associated Advanced Breast Cancer Patients Treated With PARP (Poly ADP-Ribose Polymerase) Inhibitors [abstract]. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO4-06-06.

Abstract PO3-18-12: Results from a first-in-human study of DAN-222, a novel high-capacity drug conjugate in metastatic breast cancer as monotherapy and in combination with a PARPi [NCT05261269]

Abstract Background: Topoisomerase I inhibitors (Topo I) have been shown to be highly effective across a broad range of tumors including breast cancer. Poly (ADP-ribose) polymerase inhibitors (PARPi) have been shown to be effective in BRCAm breast cancers. The combination of Topo I plus PARPi is known to be synergistic beyond the BRCAm subtype but overlapping myelotoxicities have prohibited successful clinical combinations. DAN-222 is a novel therapeutic HDC (High-capacity drug conjugate) whose payload is the topoisomerase 1 inhibitor camptothecin (CPT). Our previously published preclinical data demonstrates synergy of DAN-222 plus a PARPi regardless of BRCA or HRD status. Furthermore, DAN-222 demonstrates reduced bone marrow exposure enabling clinical combinations. Herein we present data for the first-in-human study of DAN-222 as mono- and combination therapy with a PARPi. The expectation is clinical benefit with this complimentary combination for patients with HRD+ and HRD- tumors, and not restricted to BRCAm, while minimizing overlapping myelotoxicities. Methods: A phase 1 study of DAN-222 as monotherapy and in combination with niraparib is being conducted in heavily pretreated patients with metastatic breast cancer. The objectives are to evaluate the safety, tolerability, and pharmacokinetics of DAN-222, and initial clinical activity based on RECIST v1.1 criteria to determine the recommended phase 2 dose. PK will be characterized for the total conjugated and the unconjugated CPT. Adults (≥18 years) with HER2-negative metastatic breast cancer that progressed on standard therapies were enrolled, without restriction on BRCA or HRD status. Results: As of June 2023, 30 patients had enrolled in dose escalation of monotherapy (2-16 mg/m2) or combination therapy with niraparib (100 mg). The median age was 61.5 years old (range, 36-75). The median number of prior treatment lines was 6. All patients were PARPi naïve and BRCAwt. In the monotherapy cohorts, the hematological adverse events (AEs) were neutropenia (28%; grade [Gr] ≥3: 17%), thrombocytopenia (22%; Gr ≥3: 6%) anemia (17%; Gr ≥3: 0%), and lymphopenia (11%; Gr ≥3: 0%), the non-hematologic adverse events Gr ≥3 was one case of cystitis. In the combination cohorts, the hematological AEs were anemia (25%; Gr ≥3: 17%), thrombocytopenia (17%; Gr ≥3: 8%), neutropenia (8%; grade Gr ≥3: 8%), and lymphopenia 8%; Gr ≥3: 0%) and there was no Gr ≥3 non-hematologic adverse events. The exposure of DAN-222 was linear and increased in a dose-proportional manner. The mean half-life ranged from 25.2 to 33.5 hours across all cohorts. Clinical activity during dose escalation was demonstrated with stable disease in 28% of patients with monotherapy and 67% of patients with combination therapy across all dose levels. Conclusions: No DLTs were observed in monotherapy or combination therapy in the dose escalation phase of this study. This study demonstrates the feasibility for DAN-222 to be combined with a PARPi at clinically relevant doses. The reduced myelotoxicity of DAN-222 will also enable combination with other therapies to treat patients with other types of cancer. The promising antitumor activity seen with DAN-222 in combination with PARPi in BRCAwt, supports continued clinical development. Citation Format: Sara Hurvitz, Erika Hamilton, Timothy Pluard, Aki Morikawa, Hatem Soliman, Kay Yeung, Adam Brufsky, Amy Tiersten, Wajeeha Razaq, Kristen Foye, Timothy Hagerty. Results from a first-in-human study of DAN-222, a novel high-capacity drug conjugate in metastatic breast cancer as monotherapy and in combination with a PARPi [NCT05261269] [abstract]. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO3-18-12.

Abstract PO1-16-03: GDP-mannose enhances the efficacy of PARP inhibitors and immunotherapy in triple-negative breast cancer

Abstract Background: Triple-negative breast cancer (TNBC) patients with high HRD scores benefit from poly (ADP-ribose) polymerase (PARP) inhibitors, while those with low HRD scores still lack therapeutic options. Homologous recombination deficiency (HRD) induction is an effective strategy to broaden the indications of PARP inhibitors. Metabolic reprogramming is a critical feature of TNBC. In this study, we aimed to explore novel metabolic biomarkers for HRD and new strategy for sensitizing PARP inhibitors. Methods: We utilized TNBC metabolomics to systematically evaluate metabolites that were correlated with HRD. A crucial metabolite, GDP-mannose (GDP-M), that impedes homologous recombination repair (HR) and sensitizes PARP inhibitors was identified and functionally validated. We further explored the detailed mechanism and proposed a potential treatment strategy utilizing GDP-M for TNBC in preclinical models. Results: Systematic metabolomic analysis revealed that GDP-mannose (GDP-M) was significantly enriched in basal-like tumors with HRD. GDP-M promoted cisplatin-induced DNA double-strand breaks by inducing HRD. Mechanistically, the low expression of the upstream enzyme GMPPA led to the endogenous upregulation of GDP-M, which further promoted the ubiquitin-mediated degradation of BRCA2 to inhibit HR. GMPPA expression and GDP-M could serve as predictive biomarkers for HRD and the response to PARP inhibitors. Therapeutically, we validated that the combination of GDP-M and PARP inhibitors synergistically inhibit tumor growth in multiple preclinical models. Moreover, GDP-M supplementation plus PARP inhibition activated STING-dependent antitumor immunity and further augmented the efficacy of anti-PD-1 antibodies. Conclusions: GDP-M promotes the degradation of BRCA2 and thus enhances the sensitivity of PARP inhibitors in TNBC. The combination of GDP-M, PARP inhibitors and anti-PD-1 immunotherapy may be a potential treatment strategy for HRD-low TNBC. Citation Format: Yi Xiao, Jiahan Ding, Yi-Zhou Jiang, Zhi-Ming Shao, Genhong Di, Fan Yang, Xiaoqing Song. GDP-mannose enhances the efficacy of PARP inhibitors and immunotherapy in triple-negative breast cancer [abstract]. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO1-16-03.

Abstract PO4-19-08: Randomized phase II study of talazoparib vs talazoparib plus atezolizumab for patients with premenopausal HR+/HER2- metastatic breast cancer harboring homologous recombination deficiency scar (Young-PALETTA, KCSG BR21-09)

Abstract Background Cyclin D kinase-4/6(CDK4/6) inhibitor combined with endocrine treatment have significantly increased progression free survival(PFS) and overall survival(OS) of patients with hormone receptor(HR)-positive/HER2-negative metastatic breast cancer, and currently is a standard 1st line endocrine treatment option. Homologous recombination deficiency(HRD) including germline(g) BRCA1/2 pathogenic mutation is known to be prevalent among young premenopausal patients. Recently several retrospective data suggested that patients with germline BRCA1/2 pathogenic mutation or HRD have poorer outcome with CDK4/6 inhibitor treatment, however there is no prospective evidence. Poly ADP-ribose polymerase(PARP) inhibitors have shown significant benefit over standard chemotherapy in patients with HER2-neg metastatic breast cancer and gBRCA1/2 pathogenic mutation in two Phase 3 trials. Since preclinical data suggested that PARP inhibitors play a role in anti-tumor immune modulation, immunotherapy combined with PARP inhibitors were explored and showed promising antitumor efficacy in several single arm trials. In this trial, we investigate talazoparib and atezolizumab combination compared with talazoparib monotherapy in premenopausal women with HR+/HER2- metastatic breast cancer with HRD-scar after failure with 1st line palbociclib and endocrine treatment combination. Methods This study is a multicenter, randomized, open-label phase II trial comparing talazoparib versus talazoparib with atezolizumab in premenopausal women with HR+/HER2- metastatic breast cancer harboring HRD scar whose disease have progressed with endocrine and palbociclib combination as the 1st-line endocrine treatment. Prior one line of cytotoxic chemotherapy other than platinum for metastatic setting is allowed, but prior palliative endocrine treatment is not allowed. Eligible patients will be pre-screened with tumor tissue biopsy and peripheral blood sampling before endocrine and palbociclib treatment commencement. HRD status is determined through targeted sequencing(SOLIDaccuTestTM by NGeneBio) of tumor tissue and germline HRD genes including BRCA1/2 variant test. Whole genome sequencing and RNA sequencing are performed for biomarker analyses. Patients harboring HRD scar will receive first line endocrine treatment with palbociclib plus aromatase inhibitor with ovarian suppression. After progression with 1L endocrine treatment, patients will be randomized to either talazoparib alone or talazoparib plus atezolizumab combination. Germline BRCA1/2 pathogenic mutation is a stratification factor. Primary endpoint is a progression free survival after randomization(PFS2). Secondary objectives are a composite of PFS1 and PFS2, PFS1, overall survival, patient reported outcome, and biomarkers for treatment response. A total of 178 subjects are required to detect hazard ratio of 0.62 (median PFS2 of at least 10.5 months with talazoparib and atezolizumab combination vs 6.5 months with talazoparib alone), to achieve 80% power at a two-sided a significance level at 10%, considering drop-out rate of 5%. The study period is expected to be 72 months overall(36 months for accrual and randomization and 24 months of follow-up for primary endpoint PFS2). Citation Format: Hee Kyung Ahn, Jieun Lee, Jee Hung Kim, Kyung-Hun Lee, Jee Hyun Kim, Min Hwan Kim, Minsuk Kwon, Han Jo Kim, Kyung Hae Jung, Yeon Hee Park. Randomized phase II study of talazoparib vs talazoparib plus atezolizumab for patients with premenopausal HR+/HER2- metastatic breast cancer harboring homologous recombination deficiency scar (Young-PALETTA, KCSG BR21-09) [abstract]. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO4-19-08.

Abstract PO1-13-11: ZNF689 promotes homologous recombination via NBS1 ubiquitination and its loss confers sensitivity to PARP inhibition in triple-negative breast cancer

Abstract Background: Triple-negative breast cancer (TNBC) poses significant clinical challenges due to the lack of targeted therapies and poor prognosis. Homologous recombination (HR) deficiency is a defining characteristic of TNBC and renders tumors susceptible to Poly (ADP-ribose) polymerase (PARP) inhibitors. However, not all TNBC patients respond to PARP inhibitors, highlighting the need to identify novel biomarkers to predict therapeutic response and improve treatment strategies. Methods: In this study, we investigated the role of zinc finger protein 689 (ZNF689) in DNA damage response (DDR) and its impact on PARP inhibitor sensitivity in TNBC. Using a comprehensive set of molecular and cellular techniques, including immunoprecipitation, in vitro and in vivo ubiquitination assays, protein expression analysis, and functional assays using TNBC cell lines, patient-derived organoids (PDOs), patient-derived xenograft (PDX) models, and cell line xenograft models, we elucidated the mechanistic role of ZNF689 in HR and PARP inhibitor response. Results: Our findings revealed ZNF689 as a novel player in DDR. ZNF689 is activated by ATM-mediated phosphorylation and recruited to DNA damage sites in an ATM-dependent manner. At the DNA double-strand break sites, ZNF689 collaborates directly with NBS1 to facilitate K63-linked ubiquitination of NBS1 via the E3 ligase SKP2. Furthermore, ZNF689-mediated NBS1 ubiquitination stabilizes the MRE11-RAD50-NBS1 (MRN) complex and feedbacks activation of ATM, therefore accelerating HR repair. Loss of ZNF689 results in HR deficiency, leading to increased sensitivity of TNBC cells to PARP inhibitors in both in vitro and in vivo models. Additionally, the combination of PARP inhibitors and paclitaxel demonstrated potential for improved efficacy in treating ZNF689-deficient TNBC. In our clinical analysis, we observed a positive correlation between ZNF689 expression and the levels of NBS1 and phosphorylated ATM in TNBC patient samples. Importantly, low ZNF689 expression was associated with a favorable response to PARP inhibitors in TNBC patients. Conclusions: Our study highlights the significant role of ZNF689 in HR and its impact on the response to PARP inhibitors in TNBC. These findings suggest ZNF689 as a promising therapeutic target for enhancing the effectiveness of PARP inhibitors in TNBC patients. Working Modle Citation Format: Li-Ping Ge, Zi-Yu Wang, Xi Jin, Yi-Zhou Jiang, Zhi-Ming Shao. ZNF689 promotes homologous recombination via NBS1 ubiquitination and its loss confers sensitivity to PARP inhibition in triple-negative breast cancer [abstract]. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO1-13-11.

Abstract PS04-02: BRCA testing and PARP inhibitor utilization in real-world HER2-negative metastatic breast cancer

Abstract Background: PARP inhibitors (PARPi) have demonstrated progression-free survival benefits vs. chemotherapy in patients (pts) with metastatic breast cancer (mBC) and germline BRCA mutations (gBRCAm). NCCN guidelines recommend that all pts with mBC are offered gBRCAm testing to aid in systemic treatment decisions with PARPi. Understanding real-world gBRCAm testing patterns and PARPi use would be beneficial for improving clinical outcomes of pts with gBRCAm mBC. Methods: This retrospective cohort study identified pts ≥ 18 years old with HER2-negative mBC, diagnosed from 2014 to 2022 (US Flatiron Electronic Health Record database). gBRCAm testing patterns in mBC over time were examined by BC subtype (hormone receptor-positive [HR+] or triple-negative breast cancer [TNBC]) and disease stage at diagnosis [de novo or recurrent]). Timing of gBRCAm testing and subsequent initiation of first-line or later PARPi in pts with gBRCAm HR+ and TNBC mBC was assessed in pts diagnosed after 2018 when PARPi were approved in mBC. Prevalence of gBRCAm was calculated overall and by mBC subtype. Demographic and clinical characteristics were described by gBRCAm testing and PARPi initiation (yes/no). Real-world overall survival (OS) by PARPi use since mBC diagnosis was estimated using the Kaplan-Meier method and a log-rank test. Results: Of 15,006 total pts, 4654 (31.0%) had a gBRCAm test. Compared with pts who were not tested, gBRCAm-tested pts were younger at initial mBC diagnosis (median age: 53.5 vs. 62.0 years), had a shorter median disease-free interval before mBC diagnosis (821 vs. 959 days), and were less likely to have de novo mBC (28.0% vs. 31.2%). gBRCAm testing prevalence was 6.1% in the HR+ group (n=11,945) and 8.9% in the TNBC group (n=2430). gBRCAm testing rates mostly increased from 2014 to 2022 and were lowest in HR+ tumor subtypes; < 2/3 of all pts had a gBRCAm test from 2020 onwards (Table). Most pts with recurrent mBC (HR+: 68.3%; TNBC: 76.1%) were gBRCAm tested before diagnosis, whereas pts with de novo mBC were tested at a later timepoint ( > 60 days after mBC diagnosis – HR+: 48.3%; TNBC: 38.4%). Of all pts with gBRCAm diagnosed from 2018 onwards (n=187), 44.4% initiated PARPi. A higher proportion of pts with gBRCAm in the TNBC group initiated PARPi (52.9%) than in the HR+ group (42.4%). Pts with gBRCAm in the TNBC group received PARPi earlier than pts with gBRCAm in the HR+ group (mean days from mBC diagnosis to first PARPi use: 173 and 411 days, respectively). Fewer pts with de novo mBC (35.7%) initiated PARPi than pts with recurrent mBC (50.8%). With a median follow-up of 15.3 months, pts with gBRCAm initiated on PARPi had numerically longer median OS (32.3 months [95% CI: 24.2–47.4]) than PARPi non-initiators (21.4 months [95% CI:18.4–43.5]). OS results should be regarded with caution due to the small sample size (n=187). Conclusions: Despite NCCN guideline gBRCAm-testing recommendations, < 2/3 of pts with HER2-negative mBC in our study were gBRCAm tested from 2020 onwards. gBRCAm testing was lower in pts with HR+ disease and was performed at a later timepoint for pts with de novo disease. Wider and timelier implementation of gBRCAm testing to identify pts that could benefit from PARPi is warranted. Fewer pts with gBRCAm who had HR+ tumors or de novo disease received PARPi than pts with TNBC. Our data suggest that pts who initiated PARPi had numerically longer OS than PARPi non-initiators; further research into OS trends and PARPi use in gBRCAm mBC would be of interest. Editorial acknowledgment: Medical writing assistance was provided by Leigh-Ann Booth, Ph.D. from BOLDSCIENCE Inc., funded by AstraZeneca and Merck Sharp & Dohme LLC, a subsidiary of Merck & Co., Inc., Rahway, NJ, US Legal entity responsible for the study: AstraZeneca. Funding: This study was supported by AstraZeneca and Merck Sharp & Dohme LLC, a subsidiary of Merck & Co., Inc., Rahway, NJ, US, who are codeveloping olaparib. Table. gBRCAm testing rates over time in pts with HER2-negative mBC HER2-negative mBC is defined as IHC 0, or 1-positive, or IHC2-positive/ISH-negative. *Olaparib approved in mBC; †Olaparib approved in eBC. eBC, early breast cancer; gBRCAm, germline breast cancer gene mutation; HER2, human epidermal growth factor receptor 2; HR, hormone receptor; IHC, immunohistochemistry; ISH, in situ hybridization; mBC, metastatic breast cancer; pt, patient; TNBC, triple-negative breast cancer. Citation Format: Siddhartha Yadav, Sam Hillman, Linlin Luo, Weiyan Li, Jagadeswara Earla, Xiaoqing Xu. BRCA testing and PARP inhibitor utilization in real-world HER2-negative metastatic breast cancer [abstract]. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PS04-02.

Abstract RF01-05: Functional assessment of RAD51 foci and replication fork dynamics in PARPi resistant BRCA1/2 mutated breast cancer

Abstract Background: Although platinum salts (Pt) or Poly (ADP-Ribose) Polymerase inhibitors (PARPi) are effective in treating homologous recombination defective (HRD) breast cancer, resistance often emerges, especially in advanced disease. Predicting response and relapse is complex, even in patients with germline BRCA1/2 mutations (gBRCA1/2m). Clinically approved HRD detection methods are limited to identification of pathogenic mutations in HR genes or mutational signatures in the genome of tumors caused by HRD. In PDX models derived from HRD breast cancer, the restoration of nuclear RAD51 foci formation, a key feature of functional HR, can predict resistance to HRD-targeted treatment.1 In addition, the restoration of replication fork stability, despite PARPi or induction of Pt adducts that induce replication fork arrest and collapse, confers resistance in pre-clinical models2; to date this resistance mechanism has not been clinically validated. Here we analyse RAD51 foci in FFPE samples and DNA replication fork dynamics, using DNA fibre combing assays, in patient derived organoids (PDO) relating these to clinical response to illustrate how such assays might predict clinical HRD-targeted therapy resistance in metastatic breast cancer (MBC). Patients and Methods: We used immunofluorescent detection of RAD51 foci as a marker for HR proficiency (HRP) in tumors from 29 patients with gBRCA1/2m with MBC treated with HRD-targeted treatment (n=6 PARPi, n=2 Pt and n=21 both agents in sequence). All patients developed resistance that was either de novo or acquired. RAD51 and BRCA1 foci were scored in a minimum of 50 geminin (a marker of S/G2 phase) positive tumor cells; cells with ≥5 foci were classified as positive, and tumours where ≤10% or >10% of cells were positive were considered HRD or HRP, respectively. DNA replication fork dynamics were assessed in with or without PARPi using thymidine analogue labelling. DNA fibre analysis was performed in PDOs developed from fresh tumor sampling of HRD-targeted treatment sensitive or resistant tumors, to determine the relationship between replication fork dynamics, stability and PARPi sensitivity. Results: RAD51 analysis was performed on 9 treatment naïve samples (n=9 patients), 8 samples obtained after HRD-targeted treatment resistance (n=8 patients) and 27 samples obtained pre and post HRD-targeted treatment resistance (n=12 patients). Functional HRD by RAD51 in treatment naive samples was seen in 100% (n=17) of patients with acquired resistance and 66% of patients with de novo resistance. All patients, whether with de novo or acquired resistance, exhibited high RAD51 scores in post-resistance tumour samples, suggesting restoration of HR function is the dominant mechanism of PARPi resistance. As such, RAD51 analysis shows potential as a biomarker of clinical PARPi resistance. Replication fork stability fibre was analysed after exposure to potent PARP1 trapping inhibitors or an ATRi control in 3 PDOs (2 gBRCA1m, 1 BRCAwt). As controls we used an isogenic 2D cell line with and without a CRISPR engineered BRCA1 reversion mutation (SUM149 parental BRCA1m and SUM149BS*1 revertant) or with a PARP1 mutation (SUM149 TR2 clone) that prevents PARP1 being trapped on DNA. These experiments indicate that replication fork dynamics can be assessed in “patient derived” models of breast cancer and that PARPi sensitivity was associated with PARPi induced replication fork instability (median ratio IdU:CldU in resistant lines: 0.98, sensitive lines: 0.60). Conclusion: We show that HRP restoration and RAD51 foci in advanced BRCA1/2m breast cancers is the dominant form resistance to HRD-targeted treatment. We also demonstrate for the first time that analysis of DNA replication fork dynamics can be carried out in breast cancer PDOs and could be further explored as a functional predictive biomarker of PARPi resistance. 1.Pellegrino et al, 2022 (PMID: 35425960) 2.Chaudhuri et al, 2016 (PMID: 27443740) Citation Format: Elizabeth Harvey-Jones, Maya Raghunandan, Luisa Robbez-Masson, Thanussuyah Alaguthurai, Alba Llop-Guevara, Jennifer Trendell, Olga Rodriguez, Marta Guzman, Daniel Weekes, Christelle deRenty, Eleanor Knight, Rebecca Marlow, Christopher Starling, Ruifang Liu, Nivedita Ravindran, Ioannis Roxanis, Natalia Lukashchuk, Violeta Serra, Stephen Pettitt, Christopher Lord, Andrew NJ Tutt. Functional assessment of RAD51 foci and replication fork dynamics in PARPi resistant BRCA1/2 mutated breast cancer [abstract]. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr RF01-05.

Abstract PO4-19-06: Phase II study of a PARP inhibitor, talazoparib, in HER2- metastatic breast cancer with a somatic BRCA1/2 mutation present in cell-free DNA or tumor tissue genotyping

Abstract Background: PARP inhibitors improve both progression free survival (PFS) and patient quality of life in germline BRCA1/2 mutant metastatic breast cancer (MBC), which accounts for 5-10% of breast cancer, leading to their approval in this setting. We have previously shown that a subset of patients with MBC who are not germline BRCA1/2 carriers may harbor pathogenic somatic BRCA1/2 mutations that are identified by cell-free DNA and/or tumor tissue genotyping. In our prior work, we developed a circulating tumor cell culture from a patient with MBC harboring a pathogenic somatic BRCA1 mutation, and demonstrated that a PARP inhibitor induced cell growth inhibition similar to germline BRCA1/2 mutant cultures. Thus, we hypothesize that a PARP inhibitor may be effective in treating somatic BRCA1/2 mutant MBC. In this clinical trial, we are studying the efficacy of a PARP inhibitor in somatic BRCA1/2 mutant MBC. Our work may help expand the clinical application of PARP inhibitors in MBC. Trial Design: This phase II investigator-initiated clinical trial is enrolling 30 patients with MBC who have a pathogenic somatic BRCA1/2 mutation identified by a CLIA certified cell-free DNA and/or tumor tissue genotyping assay. Patients are treated with the PARP inhibitor talazoparib 1 mg/day until progression of disease. Imaging (CT chest, abdomen and pelvis, and bone scan) occurs for disease assessment at baseline and every 3 months. Cell-free DNA is collected at baseline and then monthly to evaluate changes in the genomic environment. Patients also undergo the Cancer Risk B (CR-B) assay, a novel flow variant assay to identify double-strand break repair mutations in circulating blood cells, at baseline. Eligibility Criteria: Patients must have MBC with a pathogenic somatic BRCA1/2 mutation identified in a cell-free DNA and/or tumor tissue genotyping assay, with pathogenicity confirmed by a genetics counselor using validated genomic databases such as ClinVar. Patients may have triple-negative (receipt of at least 1 prior chemotherapy) or hormone receptor positive/HER2- MBC (receipt of at least 1 prior hormone therapy). Patients must not be known germline BRCA1/2 carriers. Any number of prior therapies including a prior platinum (in absence of progressive disease on a platinum) are allowed, but patients should not have received a prior PARP inhibitor. Patients must have adequate organ function and performance status. Specific Aims: The primary aim is to determine PFS (RECIST 1.1). Secondary aims include determining the objective response rate and toxicity (NCI CTCAE v 5.0). Exploratory aims include evaluating serial changes in BRCA1/2 mutant allelic frequency in cell-free DNA, understanding the impact of BRCA1/2 reversion mutations in cell-free DNA, comparing pre- and post-treatment cell-free DNA results to understand changes in the genomic environment, studying the CR-B assay positivity rate, and correlating these biomarker analyses with treatment response. Statistical Methods: This study has 81% power to demonstrate that the 12-week PFS is 53% or higher. In contrast, there is a 4% (alpha) probability of concluding that the 12-week PFS is ≥ 53% if the true 12-week PFS is 30% or lower. If 14 or more of 30 total patients achieve PFS > 12 weeks, the null hypothesis (12-week PFS ≤ 30%) will be rejected. Present accrual and target accrual: This study (NCT03990896) is open at Massachusetts General Hospital, MD Anderson, University of California San Francisco, Emory, Northwestern, and Vanderbilt. As of 7/2023, 14/30 patients are enrolled. Funding: This study is funded by a Pfizer ASPIRE award and Conquer Cancer Foundation of ASCO–Breast Cancer Research Foundation- Career Development Award. Contact information: Neelima Vidula, MD, Massachusetts General Hospital, nvidula@mgh.harvard.edu. Citation Format: Neelima Vidula, Senthil Damodaran, Manali Bhave, Hope Rugo, Ami N. Shah, Erica Blouch, Nathan Royce Ruffle-Deignan, Ogadinma Ogbenna, Lisa E. Flaum, Massimo Cristofanilli, Joseph Sparano, Harry Ostrer, Vandana Abramson, Nora Horick, Aditya Bardia. Phase II study of a PARP inhibitor, talazoparib, in HER2- metastatic breast cancer with a somatic BRCA1/2 mutation present in cell-free DNA or tumor tissue genotyping [abstract]. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO4-19-06.

Abstract PO3-20-02: Olaparib front line therapy with endocrine therapy in locally advanced unresectable or in metastatic breast cancer: OFELIA TRIAL

Abstract INTRODUCTION: Currently, patients with recurrent or metastatic hormone-sensitive breast cancer are treated with endocrine therapy (ET) in combination with cyclin inhibitors, as it has been shown to improve both overall survival and progression-free survival. However, it has been observed that patients with gBRCA1 or gBRCA2 mutations, who receive treatment with endocrine therapy and CDK4/6 inhibitors, have lower OS and lower PFS compared to those without mutations. These findings could be due to the presence of higher cyclin E mRNA levels, which encode the protein that stimulates CDK2 activity provoking a resistance to CDK4/6 inhibitors. Having this said, the use of PARP inhibitors a potential first-line therapeutic option in combination with endocrine therapy could be an option to be re evaluated. JUSTIFICATION: PARP inhibitors (iPARP) are recommended in international guidelines as second-line palliative treatment and also as adjuvant management in the case of Olaparib. The use of iPARP is approved only as a second-line treatment in HER2 hormone-sensitive breast cancer, according to the results of the EMBRACA and OlympiAD clinical trials. However, these two clinical trials were not designed to evaluate its effectiveness as first-line palliative treatment. Given that patients with gBRCA mutations have lower overall survival when administered cyclin inhibitors (iCDK 4/6) + Endocrine therapy (ET) compared to those without mutations, it is necessary to reevaluate the use of iPARP as first-line palliative treatment in conjunction with ET, with the expectation of achieving a similar objective response rate and progression-free survival. OBJECTIVE: To Determine the ORR in patients with recurrent or metastatic hormone-sensitive HER2-negative breast cancer managed with PARP inhibitors + Endocrine therapy. HYPOTHESIS: The use of PARP inhibitors in combination with endocrine therapy will have an objective response rate of 60% or higher in both early stages and advanced disease. METHODOLOGY: Phase 2 open-label, multicenter clinical trial. Study Population: Cohort : All pre- or postmenopausal patients with a diagnosis of recurrent or metastatic hormone-sensitive HER2-negative breast cancer with gBRCA1 and/or gBRCA2 mutation. Sample: The inclusion of patients will be based on meeting the inclusion criteria. We plan to achieve an ORR of at least 60% with an acceptable minimum of 40%. Assuming a one-sided alpha level of 0.05 and statistical power of 80%, a two-stage Simon design was used to calculate the required sample size. In the first stage, a minimum of 23 patients is assumed, of which 14 patients need to achieve the desired response rate to proceed to the second stage. In the second stage, an additional 28 patients will be added to obtain a total sample size of 51 patients. The objective response rate will be evaluated using RECIST v1.1 criteria at 8 weeks after initiating treatment, with continued assessment during the first year, followed by assessments every 12 weeks during the second and third year, and then every 24 weeks until progression or toxicity. EXPECTED RESULTS: PARP inhibitors + Endocrine therapy will have a similar ORR as reported with cyclin inhibitors + Endocrine therapy, as well as a benefit in progression-free survival. Citation Format: Carlos Arturo González Núñez, Paula Cabrera-Galeana, Juan Enrique Bargalló Rocha, Rafael Vazquez-Romo, Sergio Aguilar-Villanueva, Alexandra Garcilazo, Areli Velazquez-Martinez, Andrea Maliachi, Ruben Rodriguez. Olaparib front line therapy with endocrine therapy in locally advanced unresectable or in metastatic breast cancer: OFELIA TRIAL [abstract]. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO3-20-02.

Abstract PO4-23-08: Estrogen therapy induces receptor-dependent DNA damage enhanced by PARP inhibition in ER+ breast cancer

Abstract Although clinical evidence indicates that treatment with estrogens elicits anti-cancer effects in ~30% of patients with advanced endocrine-resistant ER+ breast cancer, the underlying mechanism of action is unclear and this treatment remains under-utilized. ER+ breast cancer cells with acquired resistance to long-term estrogen deprivation (LTED) were used to model resistance to aromatase inhibitors (AIs) seen clinically. Genome-wide CRISPR/Cas9 knockout screening and transcriptomic profiling of LTED cells highlighted DNA Damage Response as a required pathway for therapeutic response to the estrogen 17b-estradiol. Tumor specimens from 2 patients with advanced ER+ breast cancer showed increased DNA damage response upon treatment with 17b-estradiol therapy compared to baseline. LTED cells treated with 17b-estradiol exhibited replication-dependent markers of DNA damage and the DNA damage response prior to apoptosis, while parental cells did not. Such DNA damage was partially driven by the formation of DNA:RNA hybrids (R-loops), and RNase H1-mediated prevention of R-loop formation abrogated 17b-estradiol-induced DNA damage. Pharmacological suppression of the DNA damage response via poly(ADP-ribose) polymerase (PARP) inhibition with olaparib enhanced 17b-estradiol-induced DNA damage. PARP inhibition synergized with 17b-estradiol to suppress growth and prevent tumor recurrence in both BRCA1/2-mutant and BRCA1/2-wild-type cell line and patient-derived xenograft models. We therefore conclude that 17b-estradiol-induced ER activity drives DNA damage and growth inhibition in endocrine-resistant breast cancer cells, and inhibition of the DNA damage response using drugs such as PARP inhibitors can exacerbate transcriptional stress and enhance therapeutic response to 17b-estradiol. This concept is being explored clinically in the PHOEBE trial testing the combination of 17b-estradiol and olaparib (NCT05900895). These findings also warrant further study of other DNA damage response inhibitors in advanced ER+ breast cancer. Moreover, these data indicate that PARP inhibitors may have applications beyond homologous recombination-deficient tumors. Citation Format: Nicole Traphagen, Gary Schwartz, Steven Tau, Alyssa Roberts, Amanda Jiang, Sarah Hosford, Jonathan Marotti, Abigail Goen, Bianca Romo, Anneka Johnson, Emily Duffy, Eugene Demidenko, Paul Heverly, Yaron Mosesson, Shannon Soucy, Fred Kolling, Todd Miller. Estrogen therapy induces receptor-dependent DNA damage enhanced by PARP inhibition in ER+ breast cancer [abstract]. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO4-23-08.

Abstract PO5-20-03: Phase I/II study of stereotactic radiosurgery with concurrent olaparib followed by adjuvant durvalumab and physician’s choice systemic therapy in subjects with breast cancer brain metastases

Abstract BACKGROUND: Despite progress in the treatment of brain metastasis for HER2+ breast cancer, outcomes for patients with HER2-negative breast cancer brain metastases remain poor. Current standard of care consists of local therapies, including surgery and radiotherapy, followed by systemic therapy. Preclinical studies show inhibitors of poly(ADP-ribose) polymerase (PARP) are effective in combination with radiation therapy as a DNA damage response inhibitor. Triple-negative breast cancer (TNBC) has higher rates of homologous recombination deficiency compared to other breast cancer subtypes, and together with HER2-negative, BRCA-mutated breast cancer would be particularly sensitive to PARP inhibition. PARP inhibition has also demonstrated promising efficacy combined with immunotherapy in patients with germline BRCA-mutant and metastatic TNBC in clinical trials (MEDIOLA, TOPACIO). In addition, immunotherapy with stereotactic radiosurgery (SRS) is associated with favorable intracranial control and survival in patients with brain metastases. We hypothesize that this biologically-driven combination will enhance local control of SRS-treated brain metastases through synergy with PARP inhibition, while controlling micrometastatic disease in the brain and extracranial sites via potentiation of the immune response. METHODS: We are conducting a multi-institution, Phase I/II trial of SRS plus olaparib, followed by durvalumab (with physician’s choice systemic therapy), for patients with TNBC (any BRCA status) or HER2-negative with BRCA-mutated (germline or somatic) breast cancer brain metastases [NCT04711824]. A total of 41 patients are planned for enrollment at 8 sites. The primary objectives are to evaluate safety and tolerability (Phase I) and determine intracranial disease control at 6 months (Phase II) of this treatment combination. Secondary objectives include determining clinical activity via intracranial and global progression-free survival, overall survival, and intracranial and extracranial response rate. Exploratory objectives will assess potential biomarkers of treatment response, including changes in circulating tumor cells and DNA in blood and cerebrospinal fluid, germline and tumor mutations in DNA repair pathway genes, and PD-L1 expression, as well as quality of life and patient-reported outcomes. A surgical sub-study (n=5) will evaluate olaparib concentration/distribution in resected brain metastases. As of June 2023, cohort 2 of phase I has been completed without dose-limiting toxicity. Citation Format: Colette Shen, Yara Abdou, Linda Chen, Xianming Tan, Gaorav Gupta, Mina Lobbous, Filipa Lynce, Erica Stringer-Reasor, Carey Anders. Phase I/II study of stereotactic radiosurgery with concurrent olaparib followed by adjuvant durvalumab and physician’s choice systemic therapy in subjects with breast cancer brain metastases [abstract]. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO5-20-03.

Abstract PO2-09-03: Twenty-one gene recurrence scores in individuals with breast cancer associated with PALB2 germline pathogenic variants

Abstract Background: The 21-gene recurrence score assay (Oncotype DX, Genomic Health Inc., CA) is prognostic, quantifies predicted benefit of adjuvant chemotherapy, and informs clinical decisions for patients with early-stage, hormone receptor-positive (HR+), human epidermal growth factor receptor 2-negative (HER2-) breast cancer (BC). Differences in Oncotype recurrence scores (RS) in individuals with and without BRCA1 and BRCA2 (BRCA1/2) pathogenic variants (PV) have been demonstrated, but the results of this assay in patients with BC and germline PALB2 PV have not previously been described. This is of particular relevance as PALB2 BC has demonstrated biologic features similar to BRCA1/2 BC such as poly(adenosine diphosphate ribose) polymerase inhibitor (PARPi) sensitivity. Methods: Patients with early-stage, HR+, HER2-, invasive breast cancer and a germline PALB2 PV were ascertained from academic cancer center databases and the Prospective Registry of MultiPlex Testing (PROMPT), an online research registry for individuals who have had multigene panel testing for inherited cancer susceptibility. Study participants (pts) were categorized by age < 50 and ≥50. Oncotype DX RS were retrospectively reviewed and categorized as those that would and would not warrant consideration of chemotherapy (≥16 and < 16, respectively, for pts < 50; ≥26 and < 26, respectively, for pts ≥50). Yates corrected Chi-square tests were used to compare RS distributions between carriers and a previously published genetically unselected reference population. Results: Oncotype DX reports were reviewed for 20 pts with BC and germline PALB2 PV. Median RS was 19 (range 6-41) overall. Four patients were diagnosed under the age of 50 (range 33-48); median RS for these individuals was 19 (range 12-41). Sixteen patients were diagnosed at age 50 or greater (median 59, range 50-70); median RS for these individuals was also 19 (range 6-31). Compared to the genetically unselected reference population, Oncotype DX RS in pts with PALB2-associated BC demonstrated a numerical but not statistically significant shift in distribution towards categories in which chemotherapy would be considered. Among pts under the age of 50, 75% had Oncotype DX RS for which chemotherapy would be considered and 25% had Oncotype DX RS for which endocrine therapy alone would likely be recommended (p=0.726 compared to reference). Among pts aged 50 and older, 25% had Oncotype DX RS for which chemotherapy would be considered and 75% had Oncotype DX RS for which endocrine therapy alone would likely be recommended (p=0.579 compared to reference). Conclusions: Compared to data from the published genetically unselected commercial database, BC associated with PALB2 PV may have higher Oncotype DX RS reflective of intrinsically less favorable biology and greater therapeutic sensitivity. Larger datasets are forthcoming to confirm presence of significant differences in Oncotype DX RS distributions between BC with and without PALB2 PV. Table 1. Oncotype DX RS distribution by age among pts with gPALB2 PV and genetically unselected reference *Jakubowski et al., J Surg Oncol 2020. PMID 3249731 Citation Format: Payal Shah, Siddhartha Yadav, Jamie Brower, Jada Hamilton, Erica Simmons, Judy Garber, Kenneth Offit, Mark Robson, Fergus Couch, Susan Domchek. Twenty-one gene recurrence scores in individuals with breast cancer associated with PALB2 germline pathogenic variants [abstract]. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO2-09-03.

Abstract PS07-07: The RXR agonist, IRX4204, delays the formation of Brca1 mutant mammary tumors via modulation of the anti-tumor immune response

Abstract Background: Women with germline BRCA1 mutations are at an increased risk for developing breast cancer in their lifetime, often at a young age with more aggressive tumors. At present, prophylactic bilateral mastectomy is the most effective strategy for reducing breast cancer risk. However, this invasive procedure is irreversible and associated with potential complications. We and others have found that PARP inhibitors can delay Brca1-mutant tumor formation in mice and could be beneficial for the prevention of breast cancer. However, currently available PARP inhibitors are associated with modest toxicities that may not be acceptable to women without cancer. Thus, there remains an urgent need for the development of safe and effective therapies for the prevention of breast cancer. Here, we present data demonstrating the activity of IRX4204, a minimally toxic and highly specific agonist of the nuclear retinoid X receptor (RXR), to delay the formation of mammary tumors in a Brca1-deficient mouse model. This inhibitory effect on tumor growth is due, in part, to the role of IRX4204 in stimulating the anti-tumor immune response. Methods: We used the established MMTV-Cre, conditional Brca1 gene knockout, p53 heterozygous loss mouse model (BRCA1co/co; MMTV-Cre+/+; p53+/-) and selected for mutant female pups using PCR genotyping. At 16 weeks of age, all mice were separated into 4 treatment groups (n=10 per group): (1) sesame oil control; (2) the novel RXR agonist, IRX-4204 (10mg/kg); (3) high-dose IRX4204 (20 mg/kg); and (4) the RXR agonist, 9-cis-UAB-30 (5 mg/kg). All treatments were given by oral gavage five days per week, and mice were observed daily for tumor formation and toxicity. At the study endpoint, tumors and normal mammary glands were collected for additional analyses. Immunohistochemical staining was used to quantify CD8a, Ki-67 and cleaved caspase 3 expression in Brca1-deficient tumors. Oil Red O staining was used to measure changes in lipid accumulation in Brca1-deficient cell lines treated with IRX4204. qPCR was used to quantify the changes in gene expression of lipid metabolism-associated genes upon treatment with IRX4204 in vitro. Results: Vehicle-treated Brca1-deficient mice had a median time-to-tumor formation (TTF) of 211 days, with 100% developing tumors by 330 days. Mice treated with UAB 5 mg/kg had an improved median TTF of 261 days, whereas mice treated with IRX4204 10mg/kg or 20mg/kg had a median TTF of 347 and 304 days, respectively (p < 0.01). In addition, 60% of mice treated with IRX4204 10 mg/kg remained tumor-free at 330 days. IRX4204-treated tumors showed an increased infiltration of CD8-positive T-cells over vehicle-treated tumors (p < 0.05). Treatment of Brca1-deficient cell lines with IRX4204 in vitro resulted in a significant increase in lipid accumulation accompanied by a 2-fold increase of Srebf1 expression (a key transcription factor that regulates lipid homeostasis) within 24 hours of treatment (p < 0.05). Conclusion: These data demonstrate a novel use of the RXR agonist, IRX4204, to delay the formation of Brca1-deficient mammary tumors. We have found that IRX4204 treatment modulates the tumor immune response through increased infiltration of cytotoxic CD8-positive T-cells in Brca1-deficient mammary tumors in vivo. We have also determined that IRX4204 modulates lipid metabolism in breast cancer cell lines in vitro. It is known that lipid-derived antigens can stimulate T-cell activity. Our findings suggest that RXR agonists may alter lipid antigen production to activate an anti-tumor response. Additional immune and lipidomic studies are on-going. This work was supported by NCI-PREVENT grant (to PB and AM HHSN26100008) and CFP Foundation (Odyssey Fellowship to CM). Citation Format: Cassandra Moyer, Darian Coleman, Jamal Hill, Lana A. Vornik, Michelle Savage, Martin Sanders, Sei Shizuko, Altaf Mohammed, Powel Brown, Abhijit Mazumdar. The RXR agonist, IRX4204, delays the formation of Brca1 mutant mammary tumors via modulation of the anti-tumor immune response [abstract]. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PS07-07.

Abstract PS06-04: The prognostic and predictive impact of circulating tumour DNA (ctDNA) dynamics in patients with metastatic Triple Negative Breast Cancer (TNBC) on olaparib based therapy: Results from Cohort E of the PlasmaMATCH trial

Abstract Background: Early changes in ctDNA levels, ctDNA dynamics, may help identify which patients are responding to therapy earlier than imaging. Few studies have assessed ctDNA dynamics during PARP inhibitor therapy. Here we report paired baseline and early on treatment ctDNA analysis from cohort E of plasmaMATCH, that recruited patients with TNBC to treatment with olaparib (PARP inhibitor) plus ceralasertib (ATR inhibitor). Methods: The plasmaMATCH trial assessed the ability of ctDNA testing to allocate patients to mutation matched treatment cohorts (A-D). Patients with TNBC, and without mutations matching cohorts B-D, were enrolled on cohort E. Samples were collected for ctDNA analysis pre-treatment at cycle 1-day1 (C1D1) and cycle 2-day 1 (C2D1). A minimum of 14 days of treatment in the first cycle was required for inclusion in this analysis. Samples were sequenced using error-corrected gene targeted panels (Guardant360, or GuardantOMNI, Guardant Health). Circulating DNA ratio (CDR) was calculated as the ratio of C2D1 ctDNA level to C1D1, pre-specified using the weighted mean of variant allele fractions (AF) of clonal mutations at C1D1, excluding variants with AF < 0.3%, and variants in genes frequently mutated in clonal haematopoesis (GNAS, JAK2, IDH1, IDH2 and ATM). The optimal cut-point for predicting progression free survival (PFS) was assessed as the cut-point with the highest Harrell’s C-index. Results: Of the 75 patients that were enrolled into cohort E, 53 patients had samples sent for paired C1D1-C2D1 ctDNA sequencing, 2 failed sequencing, and all 51 (68%) patients had detectable ctDNA at C1D1. The ctDNA analysis set was representative of the overall enrolled population. The optimal ctDNA dynamics C-index cut-point for predicting PFS was 0 (undetectable ctDNA at C2D1). Median PFS with ctDNA CDR >0 (detectable ctDNA at C2D1) was 4.3 months (95% CI 2.4-5.8), and with undetectable ctDNA was 12 months (95% CI 8-NA) (HR 4.02, 95% CI 1.22-13.23, p=0.01). Splitting patients by median CDR was not predictive (HR=0.98; 95%CI: 0.52-1.82, p=0.94). Confirmed objective response rate was 85.7% (42.1-99.6) in patients with undetectable ctDNA at C2D1, and with detectable ctDNA was 11.4% (3.8-24.6) (OR 4.02, 95% CI 1.22-13.23, p=0.01). Of the 7 patients with undetectable ctDNA at C2D1, one had a BRCA2 germline mutation, and all other patients were wildtype for BRCA1/2 mutations in tumour and germline. All patients with undetectable ctDNA and BRCA1/2 wildtype had a confirmed response.In cohorts A-D (mutation targeted therapies, in predominantly ER positive cancer), the optimal ctDNA dynamics C-index CDR cut-point was 0.165 (HR 3.44, 95%CI: 2.06-5.75, p< 0.001), with median CDR cut-point also highly predictive (HR=2.14, 95%CI:1.36-3.36, p=0.001). Undetectable ctDNA was also strongly predictive (HR=4.41; 95%CI: 1.97-9.87, p< 0.001) in cohort A-D.In cohort E, a significant association was found between baseline ctDNA and PFS, with an optimal C-index cutpoint of 6.81% (HR=3.02, 95% CI: 1.39-6.56, p=0.001). Median PFS for baseline ctDNA ≤ 6.81% was 10.2 months (95% CI 3.7-17.2), and for baseline ctDNA >6.81% was 4.4 months (95% CI 2.2-5.5). Conclusions: ‘Clearance’ of ctDNA to become undetectable at C2D1 identified sporadic TNBC patients who benefited from olaparib and ceralasertib. Although ‘clearance’ of ctDNA was associated with good outcome on olaparib plus ceralasertib, median CDR was not predictive of treatment benefit. This contrasts the results of ctDNA dynamic assessment of cohort A-D, where median CDR was highly predictive of treatment benefit. ctDNA dynamic assessment may differ between mutation targeted therapies (cohorts A-D) that induce cell-cycle arrest, and PARP inhibitors (cohort E) that inhibit DNA repair mechanisms. Implementing ctDNA dynamics into clinical trials and care may require distinct analysis for different therapies. Citation Format: Iseult Browne, Javier Pascual, Rosalind Cutts, Belinda Kingston, Sarah Hrebien, Lucy Kilburn, Alex Pearson, Laura Moretti, Andrew Wardley, Iain Macpherson, Richard Baird, Rebecca Roylance, Iris Faull, Kimberly C Banks, Isaac Garcia-Murillas, Judith Bliss, Alistair Ring, Nicholas Turner. The prognostic and predictive impact of circulating tumour DNA (ctDNA) dynamics in patients with metastatic Triple Negative Breast Cancer (TNBC) on olaparib based therapy: Results from Cohort E of the PlasmaMATCH trial [abstract]. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PS06-04.

Abstract PO3-09-04: Identification of germline pathogenic variants to guide systemic therapy: Prevalence and spectrum of PVs among Mexican patients with metastatic breast cancer

Abstract BACKGROUND Currently, international genetic cancer risk assessment (GCRA) guidelines recommend genetic testing for the identification of germline pathogenic variants (PVs) in patients with metastatic breast cancer (MBC) to guide systemic therapy. Patients with PVs in BRCA genes are candidates to receive poly(adenosine diphosphate–ribose) polymerase inhibitors (PARPi). The prevalence of PVs in patients with HER2-negative MBC has been reported to be 10%. The proportion of MBC in low- and middle-income countries (LMIC) is higher. However, access to genetic testing in LMIC is limited and contributes to the gap in genetic epidemiology knowledge. We aimed to describe the frequency of germline PVs in breast cancer associated genes and clinic-pathologic characteristics of Mexican women with MBC. METHODS This retrospective study included Mexican women from 7 centers in Mexico, who were diagnosed with MBC and had genetic testing. Pathogenic and likely pathogenic variants were reported. Demographic, histopathological characteristics, family history and access to treatment with PARPi, were obtained from chart review. RESULTS From August 2017 to May 2023, 114 Mexican women with MBC were included, median age at diagnosis was 47 years (range 24-80 y), 42% had de novo metastatic, and 58% recurrent disease. 86.8% had infiltrating ductal carcinoma, 51.3% had HR+HER2- (n=58), 4.4% HR+HER+ (n=5), 5.3% HR-HER2+ (n=6), and 38.6% triple negative tumors (n=44). Sites of metastatic disease were as follows: 61.4% non-regional nodes (n=70), 57% bone (n=65), 40.3% lung (n=46), 30.7% liver (n=35), 15.8% central nervous system (n=18) and 7% peritoneal carcinomatosis (n=8). The frequency of PVs was 22% (25/114); 84% of the PVs were heterozygous, mainly identified in BRCA genes (BRCA1 n=12, BRCA2 n=9), and in 4 patients in other genes (PALB2 n=2, PTEN n=1, MUTYH n=1). Among PARPi candidates, 40% (10/25) received Olaparib, the majority received it on the second or subsequent lines of treatment (7/10) with a median progression free survival of 6 months (3-13 m). BRCA carriers were younger at BC diagnosis (38.5 vs. 49 years, p=0.00694) and had a significant higher proportion of family history of ovarian cancer (p=0.00039), but not for breast (p=0.06), prostate (p=0.71) or pancreatic cancer (p=0.098), compared to non-carriers. CONCLUSION We found a high frequency of PVs in BRCA genes in Mexican women with MBC. Access to the genetic testing and targeted therapy should be improve in the Mexican population in order to decrease disparities in cancer care in the Mexican Population. In addition, genetic testing allows to identify other family members at risk with cascade testing and to recommend them risk reduction strategies. Citation Format: Jose Luis Rodriguez-Olivares, Maria Fernanda Esparza-Orozco, Jose Alfredo Ramirez-Gonzalez, Roberto Ivan Sanchez-Reyes, Enrique Isay Talamantes-Gamez, Gregorio Quintero-Beulo, Gilberto Lopez-Rosas, Karla Alicia Centelles-López, Brizio Moreno-Jaime, Daniela Vazquez-Juarez, Ana Maria Hernandez-Murillo, Ilana de la Puente Tawil, Jeffrey Weitzel, Cynthia Villareal Garza, Yanin Chavarri-Guerra. Identification of germline pathogenic variants to guide systemic therapy: Prevalence and spectrum of PVs among Mexican patients with metastatic breast cancer [abstract]. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO3-09-04.

Abstract PO2-17-06: Real world treatment patterns and clinical outcomes by germline BRCA (gBRCA) mutation status in patients with HER2-negative early breast cancer in the US community oncology setting: a retrospective observational chart-review study

Abstract Background: Women with early-stage breast cancer (eBC) who have inherited BRCA1 or BRCA2 mutations (gBRCAm) are typically diagnosed at a younger age and often require more intensive treatment. In the randomized, double-blind OlympiA trial of HER2-negative (HER2–) gBRCAm patients, olaparib, a targeted poly (ADP-ribose) polymerase inhibitor (PARPi), significantly improved invasive disease free survival (IDFS), distant disease free survival (DDFS) and overall survival (OS) and was subsequently approved for adjuvant treatment of gBRCAm HER2– high-risk eBC patients. This study examined the real-world patient characteristics, treatment patterns and clinical outcomes by gBRCA status among patients with HER2– eBC in a US community oncology setting prior to olaparib US approval for treatment of eBC. Methods: This retrospective observational study used chart review data from The US Oncology Network's iKnowMed database to examine adult patients diagnosed with HER2– eBC (stage I-III) initiating systemic neoadjuvant or adjuvant therapy with chemotherapy and/or endocrine therapy between January 1, 2012 and December 31, 2018. Patients with valid gBRCAm test results (all gBRCAm and randomly selected BRCAwt patients) were included and followed through December 31, 2021. Patients were excluded if they had HER2+ tumors and progressed to stage IV within 6 months after initiation of neoadjuvant therapy or were diagnosed with another primary cancer. Descriptive analyses assessed patient characteristics. Kaplan Meier methods and multivariate Cox proportional hazard models (CPHM) were used to evaluate IDFS, DDFS and OS by gBRCA status and by HR+/HER2– or TNBC eBC subsets. Results: Among 298 BC patients meeting initial criteria, 42% had gBRCAm (n=124, 43% hormone receptor positive [HR+], 56% triple-negative BC [TNBC], 1% unknown), and 58% had gBRCAwt (n=174, 74% HR+, 22% TNBC). Median (interquartile) age at surgery was numerically lower in gBRCAm (45 [36,55] years) than gBRCAwt (48 [41,55] years) patients. A numerically higher proportion of gBRCAm patients received neoadjuvant therapy compared with gBRCAwt (Neoadj: gBRCAm 37.1% vs gBRCAwt 12.6%) but the reverse was true for adjuvant (Adj: gBRCAm 42.7% vs gBRCAwt 65.5%) and neoadjuvant + adjuvant use was numerically similar (Neoadj+Adj: gBRCAm 20.1% vs gBRCAwt 21.8%). The majority of TNBC patients received neoadjuvant therapy while the majority of HR+ patients received adjuvant therapy (Neoadj: TNBC 51.4% vs HR+ 6.6%, Adj: TNBC 29.0% vs HR+ 71.4%, Neoadj+Adj: TNBC 19.6% vs HR+ 22.0%). Median IDFS, DDFS, and OS in both study cohorts were not reached. Median follow-up from surgery was < 5 years (overall 59.0 months; gBRCAm 50.6 months; gBRCAwt 61.3 months). At 60 months, in gBRCAm and gBRCAwt respectively, estimated IDFS were 85.5% and 90.9%, estimated DDFS 88.1% and 92.2%, and estimated survival 91.9% and 95.2%. At 60 months, in HR+/HER2– and TNBC respectively, estimated IDFS were 92.4% and 81.2%, estimated DDFS 93.9% and 85.7%, and estimated survival 96.3% and 90.9%. CPHM results showed that risks of invasive disease (HR 1.74; 95% CI 0.83-3.64; p=0.15), distant disease (HR 1.24; 95% CI 0.50-3.07; p=0.65) and death (HR 1.79; 95% CI 0.52-6.13; p=0.353) were similar between gBRCAm and gBRCAwt patients. Between HR+/HER2– and TNBC patients, risks of invasive disease (HR=0.63; 95% CI 0.21-1.89; p=0.41), distant disease (HR=0.42; 95% CI 0.11-1.67; p=0.22) and death (HR 0.48; 95% CI 0.09-2.70; p=0.41) were similar. Conclusions: This real-world chart review study of data through 2021 (before PARPi approval for eBC) with limited follow up of 5 years found that gBRCAm patients had a numerically shorter survival (IDFS, DDFS and OS) than gBRCAwt, but this difference was not statistically significantly different. Given that the follow-up period was too short to draw conclusions, further studies examining clinical outcomes over a longer follow-up period after approval of PARPi may be of interest. Citation Format: Jay Andersen, Jagadeswara Earla, Nicole Fulcher, Juliet Ndukum, Nicholas J Robert, Mark Robson, Weiyan Li, Jaime Mejia. Real world treatment patterns and clinical outcomes by germline BRCA (gBRCA) mutation status in patients with HER2-negative early breast cancer in the US community oncology setting: a retrospective observational chart-review study [abstract]. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO2-17-06.

Abstract PO1-06-09: Phase 2 Trial with Safety Run-In of Gedatolisib Plus Talazoparib in Advanced Triple Negative or BRCA1/2 Positive, HER2 Negative Breast CancersBig Ten Cancer Research Consortium BTCRC-BRE18-337

Abstract Up to 2/3 of triple negative breast cancers (TNBC) have acquired defects in homologous recombination (HR) DNA repair, yet poly (ADP-ribose) polymerase inhibitor (PARPi) monotherapy has been largely ineffective in the absence of a germline BRCA 1/2 mutation (gBRCA1/2). Phosphoinositide-3-kinase (PI3K)/mTOR pathway alterations are also common in breast cancers. Preclinical data suggest that PI3K/mTOR inhibition may disrupt normal function of the HR complex and increase dependency on PARP enzymes for HR DNA repair. Thus, combining a PI3K/mTOR inhibitor with a PARPi may result in a synergistic anti-neoplastic effect. The run-in portion of this study evaluated the safety of weekly IV gedatolisib (PI3K/mTORi) and continuous daily talazoparib to determine the maximum tolerated dose (MTD) and recommended phase 2 dose (RP2D). A 3+3 design for dose escalation explored two dose levels. The phase II study began once the MTD and R2PD were confirmed. Eligibility required age ≥ 18, 1-2 prior lines of therapy (protocol later amended to allow up to 3 lines), and advanced TNBC or advanced HER2-negative BC with gBRCA1/2 mutation. The sample size for the phase II study was determined based on a 1-sided binomial test under the null and alternative ORR of 5% vs 20% with a type I error rate of 0.1 and power level of 80%. The primary endpoint was overall response rate (ORR) in TNBC without known gBRCA1/2 with secondary endpoints of progression-free survival (PFS) and overall survival (OS). Patients with a gBRCA1/2 mutation were not included in the primary endpoint analysis. Correlative studies are planned to evaluate HR deficiency mutations, PIK3CA mutations, and other exploratory genomics. A total of 33 patients were enrolled: 14 in the safety run-in phase of the trial and 19 in the phase II study (17 TNBC, 2 with gBRCA2 mutation). In the safety run-in, 6 patients were enrolled at dose level 1 (0.75 mg talazoparib po daily and 180 mg gedatolisib IV weekly) and 8 at dose level 2 (1 mg talazoparib po daily and 180 mg gedatolisib IV weekly). 42% of the cohort developed hyperglycemia, which was mostly grade 1. There was 1 DLT of grade 3 neutropenia at dose level 1. There were 3 patients who experienced grade 4 AEs, thrombocytopenia (2) and lymphopenia (1) which were outside of the DLT window. The MTD was 1 mg of talazoparib daily and 180 mg of gedatolisib weekly, and this was selected as the RP2D. A protocol amendment was made during the phase II portion to allow for a 3 week on/1 week off schedule for gedatolisib due to emerging data showing a more favorable safety profile and enhanced antitumor activity with this dosing schedule. In the 17 patients with TNBC and no gBRCA mutation in the phase II cohort, the ORR was 12%. Best response was partial response (PR) in 2 patients (12%), stable disease (SD) in 7 patients (41%), and progressive disease (PD) in 8 patients (47%). The clinical benefit rate (ORR+SD) at 16 weeks was 23.5%. The most common adverse events (AEs) in all 33 patients were anemia (70%), fatigue (67%), oral mucositis (64%), nausea (60%), neutropenia (45%) and anorexia (45%). Of these, most were grade 1-2 other than anemia (35% grade 3), neutropenia (20% grade 3), fatigue (18% grade 3), and oral mucositis (10% grade 3). There were no grade 4 AEs in the phase II study. Median PFS was approximately 3 months and median OS was approximately 6.4 months. Although this study did not meet its primary endpoint, there were 2 TNBC patients without a gBRCA1/2 mutation who achieved a partial response to this non-chemotherapy regimen. Future biomarker testing may help elucidate these findings and possible predictors of response. Citation Format: Sneha Phadke, Kari Wisinski, Oana Danciu, Ami Shah, Menggang Yu, Yi Chen, Kathy Miller, Mark Burkard. Phase 2 Trial with Safety Run-In of Gedatolisib Plus Talazoparib in Advanced Triple Negative or BRCA1/2 Positive, HER2 Negative Breast CancersBig Ten Cancer Research Consortium BTCRC-BRE18-337 [abstract]. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO1-06-09.