Receptor-positive Breast Cancer Research Articles

Abstract OT1-18-01: A randomized controlled phase III study of bevacizumab and paclitaxel in combination with atezolizumab as a treatment for patients with locally advanced or metastatic hormone receptor-positive HER2 negative breast cancer: JCOG1919E/AMBITION study

Abstract Background: Hormone receptor (HR)-positive HER2-negative breast cancers have a lower tumor mutation burden than other types of cancer and triple-negative breast cancer, resulting in relatively low immunogenicity. It is also known that estrogen suppressively regulates immunity through regulatory T cells in the tumor microenvironment. Therefore, the development of immunotherapy in HR-positive and HER2-negative breast cancer is a challenging issues. Anti-VEGF therapy has been shown to improve antigen presentation, local migration of cytotoxic T lymphocytes, and suppressive tumor microenvironment in preclinical and clinical studies of various cancers. Anti-VEGF therapy can be hypothesized to modulate immunotherapy. Methods: JCOG1919E (AMBITION) is a randomized, multicenter, open-label, phase III trial to evaluate efficacy and safety of bevacizumab and paclitaxel in combination with atezolizumab (BEV+PTX+ATZ) comparing to bevacizumab and paclitaxel (BEV+PTX) in patients with HR-positive HER2 negative locally advanced or metastatic breast cancer. The key eligibility criteria are as follows: 1) ECOG PS 0-2, 2) ER and/or PgR positive and HER2-negative breast cancer, 3) endocrine resistance or life-threatening disease, 4) measurable disease, 5) centrally confirmed programmed death-ligand 1 (PD-L1) status on tumor-infiltrating immune cells, 6) no prior chemotherapy for locally advanced or metastatic breast cancer, 7) adequate organ function. Patients will be randomized (staratified by recurrence/de novo, liver metastasis, or PD-L1 status) in a 1:1 ratio, to BEV+PTX arm or BEV+PTX+ATZ arm. Patients in arm A receive PTX (90 mg/m2/day, days 1, 8 and 15, iv) and BEV (10 mg/kg/day, days 1 and 15, iv) on a 28-day cycle. Patients in arm B receive PTX and BEV plus ATZ (840 mg/body/day, days 1 and 15, iv). Treatment in both arms is continued until disease progression or unacceptable toxicity. The primary endpoint is progression-free survival (PFS) assessed by investigators. The secondary endpoints include overall survival, PFS assessed by blinded independent central review, response rate, adverse events, PFS (PD-L1 positive), and OS (PD-L1 positive). We assumed a median PFS of 16 months in BEV+PTX+ATZ arm and 11 months in BEV+PTX arm. The planned sample size was calculated as a total of 280 patients (140 patients per arm) with a one-sided alpha of 2.5%, power of 80%, an accrual period of 2.5 years, and a follow-up period of 2 years. Translational research also will be conducted accompanying with this trial. Patient accrual was started in January 29, 2021 and 37 patients were enrolled by June 18, 2021. Safety of patients in this study is being monitored by IDMC. For further information on this trial, visit ClinicalTrials.gov (NCT04732598). Citation Format: Fumikata Hara, Makiko Ono, Shigehisa Kitano, Shigehira Saji, Akihiko Shimomura, Takashi Yamanaka, Takayuki Kadoya, Mitsuya Ito, Hiroko Bando, Hiroyuki Yasojima, Keita Sasaki, Tomoko Kataoka, Tadahiko Shien, Kan Yonemori, Hiroji Iwata. A randomized controlled phase III study of bevacizumab and paclitaxel in combination with atezolizumab as a treatment for patients with locally advanced or metastatic hormone receptor-positive HER2 negative breast cancer: JCOG1919E/AMBITION study [abstract]. In: Proceedings of the 2021 San Antonio Breast Cancer Symposium; 2021 Dec 7-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2022;82(4 Suppl):Abstract nr OT1-18-01.

Abstract P4-02-15: Thromboembolic events (TEE) in patients with HER2-negative, hormone receptor-positive metastatic breast cancer treated with Ribociclib combined with Letrozole or Fulvestrant. Real-world data

Abstract Background: Breast cancer is the most common cancer diagnosed among women worldwide. More than 70% of breast cancers are hormone receptor (HR)-positive, for which endocrine therapy (ET) is the preferred initial treatment especially so for those with low burden disease. Majority of patients with HR-positive metastatic breast cancer (mBC) acquire resistance to endocrine therapy. CDK4/6 inhibitors modulate this resistance and are integral treatment for patients with advanced HR-positive, HER2-negative breast cancer. Since their approval, CDK4/6 inhibitors are widely used in clinical practice. Thromboembolic events (TEE) were not a major issue among patients treated on clinical trials. However, conflicting data starts to emerge describing higher than expected rates of both arterial and venous thrombosis in patients treated with CDK4/6 inhibitors. In this study, we use real world data to study TEE rates in patients treated with ribociclib in real-world settings.Methods: The study is a retrospective analysis of individual patients’ data. All consecutive patients with mBC treated with ribociclib combined with aromatase inhibitors or fulvestrant were reviewed. Data were collected from patients’ electronic medical records and from radiology department archives. All episodes of radiology confirmed arterial or venous thrombosis were recorded. TEE was considered ribociclib related if diagnosed while patients on the drug or withing 4 weeks after the last dose.Results: During the study period, a total of 305 patients were enrolled. Median age (range) was 49 (22-87) years. All patients had metastatic disease and 168 (55.1%) were de novo metastasis. Most (n=241, 79.0%) of the patients had visceral metastasis at time of ribociclib therapy; only 64 (20.9%) had bone-only disease. Ribociclib was used in combination with letrozole in first-line setting in 195 (63.9%) and with fulvestrant or letrozole in 110 (35.9%) patients who failed one or more lines of ET or chemotherapy. Median duration (range) of ribociclib therapy was 7 (1-45) months. Episodes of venous thromboembolism were confirmed in only 5 (1.6%) patients; 3 were PE with or without DVT and 2 were lower extremity DVT and all were symptomatic. Another patient had arterial thrombosis (lower limb ischemia). During the chart review, 7 (2.3%) more patients had a confirmed diagnosis of VTE prior to ribociclib and another 4 (1.3%) patients had a confirmed episode after (range: 2-7 months) stopping it. All patients were treated with low-molecular heparin (LMWH) with no complications. Given the small number of patients with TEE, no clinical or pathological predictors could be used to identify subgroups of patients at higher risk for TEE while on ribociclib.Conclusions: In real-world settings, and contrary to recent reports, breast cancer patients treated with ribociclib with AI or fulvestrant are not at higher risk for arterial or venous thromboembolic events. Patients had similar rates prior to initiation, and after stopping ribociclib. Citation Format: Hikmat Abdel-Razeq, Faris Tamimi, Sarah Edaily, Baha' Sharaf, Mahmoud Abunasser, Osama Salama, Rashid Abdel-Razeq, Hazem Abdulelah, Rayan Bater. Thromboembolic events (TEE) in patients with HER2-negative, hormone receptor-positive metastatic breast cancer treated with Ribociclib combined with Letrozole or Fulvestrant. Real-world data [abstract]. In: Proceedings of the 2021 San Antonio Breast Cancer Symposium; 2021 Dec 7-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2022;82(4 Suppl):Abstract nr P4-02-15.

Abstract P5-16-11: Ipatasertib (ipat) in combination with palbociclib (palbo) and fulvestrant (fulv) in patients (pts) with hormone receptor-positive (HR+) HER2-negative advanced breast cancer (aBC)

Abstract Background: Ipat is a potent oral AKT inhibitor that has been studied in multiple clinical trials, primarily in breast and prostate cancers. Combining fulv and AKT inhibition demonstrated efficacy in pts with HR+ aBC regardless of PI3K/AKT pathway alterations [Jones, Lancet Oncol 2020]. IPATunity150 (NCT04060862) was designed as a phase III trial with an open-label phase Ib portion adding ipat to palbo plus fulv in biomarker-unselected HR+ HER2-negative aBC. The biological rationale was to prevent or delay resistance to CDK4/6 inhibition plus endocrine therapy (ET). AKT1 alterations and PTEN loss have been implicated in resistance to CDK4/6 inhibitors [Wander, Cancer Discov 2020; Costa, Cancer Discov 2020]. We report results from the open-label phase Ib portion. Patients and Methods: The primary objective was to assess safety and pharmacokinetics (PK) of ipat in combination with palbo and fulv; several efficacy parameters were also analyzed. Pts with measurable disease who had not previously received a CDK4/6 inhibitor and had experienced relapse during adjuvant ET were treated with ipat at a dose of 300 mg/d, d1-21 q28d, plus standard-of-care doses of palbo (125 mg/d, d1-21 q28d) plus fulv (500 mg q28d with a loading dose in cycle 1). The selected ipat dose was lower than the 400 mg typically used in other studies because of the anticipated drug-drug interaction (DDI) when combining ipat (a sensitive CYP3A4 substrate and mild-to-moderate CYP3A inhibitor) with palbo (a weak time-dependent CYP3A4 inhibitor and CYP3A substrate). Results: Of the 20 pts treated, 20% were Asian, 65% had primary endocrine resistance (relapse ≤2 years after starting adjuvant ET), 80% had received prior (neo)adjuvant chemotherapy, and 60% had liver and/or lung metastases. At the data cutoff (19 Mar 2021; median follow-up 6.1 months), median treatment duration was 5.1, 5.9, and 5.3 months for ipat, palbo, and fulv, respectively. Treatment was ongoing in 13 pts. Grade 3/4 adverse events (AEs) occurred in 80% of pts (no grade 5). The most common AEs were diarrhea (80% any grade, 10% grade 3, no grade 4) and neutropenia (75% any grade, 45% grade 3, 20% grade 4). Other notable grade ≥3 AEs were grade 3 liver function test elevations in 10%. There were no cases of febrile neutropenia and only 1 case of pneumonitis (grade 1). AEs led to at least one dose reduction of ipat in 6 pts (30%; diarrhea n=3 [with vomiting in 1 pt], neutropenia n=3 [with fatigue in 1 pt]) and of palbo in 9 pts (45%; all for neutropenia). One pt (5%) discontinued ipat and palbo permanently due to ongoing neutropenia after protocol-defined dose reductions. As expected, a DDI led to increased ipat exposure (AUC0-24,ss ~60% and Cmax ~40%) when ipat and palbo were combined. Based on population PK analysis, palbo AUC0-24,ss was ~30% higher than reported from the PALOMA 1 and 2 trials, which was expected and consistent with previously reported physiologically based PK modeling of palbo exposure when administered with moderate CYP3A inhibitors [Yu, J Clin Pharmacol 2017]. All 20 pts had at least one post-baseline tumor assessment. Best overall response rate was 45% (95% CI: 23-68%), including confirmed responses in 7 pts (35%; 5% complete response, 30% partial response) at the clinical cutoff date. Median duration of response was 9.6 months (95% CI: 7.1-not estimable). An additional 10 pts (50%) had stable disease. Progression-free survival results were immature (events in 7 pts). There was no obvious association between efficacy and mutations in PIK3CA/AKT1 as tested in ctDNA. Conclusion: The triplet combination of ipat, fulv, and palbo had an acceptable safety profile generally consistent with that of the individual study drugs; ipat exposure was increased through a predicted DDI. Updated results will be presented. Citation Format: Mafalda Oliveira, Aditya Bardia, Sung-Bae Kim, Naoki Niikura, Cristina Hernando, Gustavo Werutsky, Yoland Antill, Pedro Liedke, Catherine Oakman, Eriko Tokunaga, Seth Wander, Vanessa Krause, Toshinari Yamashita, Frauke Schimmoller, Jacob Rotmensch, Heidi Savage, Rucha Sane, Nicholas Turner. Ipatasertib (ipat) in combination with palbociclib (palbo) and fulvestrant (fulv) in patients (pts) with hormone receptor-positive (HR+) HER2-negative advanced breast cancer (aBC) [abstract]. In: Proceedings of the 2021 San Antonio Breast Cancer Symposium; 2021 Dec 7-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2022;82(4 Suppl):Abstract nr P5-16-11.

Abstract P5-07-05: Deciphering the inferior prognosis of young women with estrogen receptor-positive early-stage breast cancer through full transcriptome analysis: A FLEX database sub-study

Abstract Introduction: Young women with estrogen receptor (ER)-positive early-stage breast cancer (EBC) frequently present with larger, higher-grade, more aggressive tumors, with lower ER expression than older women. Post-hoc analyses within the MINDACT and TAILORx cohorts reported that women aged 40-50 with ER+ EBC exhibited a chemotherapy benefit independent of genomic risk compared to women aged > 50, possibly due to chemotherapy-induced ovarian suppression (CIOS). However, women < 40 years, who seldom develop CIOS, are under-represented in clinical trials. Therefore, it is important to distinguish the tumor biological profile within this younger age group. To understand the biological basis underlying why younger women have poorer outcomes than older women, this study aimed to identify genes that distinguish tumors in younger women from older women. Methods: EBC patients enrolled in the FLEX study (NCT03053193) undergo standard of care MammaPrint (MP) and BluePrint (BP) tests, and consent to clinically annotated whole transcriptome data collection. MP categorizes tumors as High Risk (HR) or Low Risk (LR) of recurrence. Together, MP and BP classify molecular subtype as Luminal A, Luminal B, HER2, or Basal. Whole transcriptome gene expression differences were compared among ER+ tumor specimens from three age groups: < 40 years old (n=283), 40-54 years old (n=1535), and ≥ 55 years old (n=4355). Differentially expressed genes (DEGs) were identified from 100 randomized iterations for each comparison; an equal number of LR and HR samples were analyzed in each group. DEGs found in > 20 iterations indicated gene stability within the age group. DEGs were also identified within each age group after correcting for BP subtype, and between pre-menopausal (n=1314) and post-menopausal (n=4859) women. Gene expression data were quantile normalized using R package ‘limma’. DEGs with an adjusted p<0.05 and fold change ≥ 2 were considered significant. Results: Overall, 76.0% of women < 40 years, 53.6% of women aged 40-54, and 48.5% of women ≥ 55 years had MP HR tumors. In addition, women < 40 years had higher frequencies of BP Basal and HER2 tumors (20.5% and 9.2%, respectively) compared with women ≥ 55 years (8.0% and 2.4%, respectively; p<0.0001). In line with unsupervised hierarchical clustering and previous studies, tumors from patients aged 40-54 exhibited limited DEGs in comparison to women ≥ 55 years. In contrast, most gene expression differences were primarily observed between women < 40 and ≥ 55 years (11 DEGs; 10 downregulated and 1 upregulated). We identified limited DEGs within BP Luminal A (n=23), Luminal B (n=10), and Basal (n=7) tumors from women aged < 40 relative to women ≥ 55 years. No DEGs were found in HER2-type tumors. Within Luminal A and Basal tumors, upregulated genes in women < 40 years are important for proliferation and immune responses. Luminal B tumors in women aged < 40 have increased DEGs involved in ER and HER2 signaling. Within all subtypes, DEGs supporting metabolic functions were downregulated in tumors of women < 40 years compared with women ≥ 55 years. Few DEGs were observed between pre-menopausal women and post-menopausal women, though none with > 2-fold change. Conclusion: Tumors from women aged 40-54 had few DEGs, suggesting observed chemotherapy benefit represents differences in host biology rather than intrinsic tumor biology as compared with tumors from women < 40 or ≥ 55, where detected DEGs are associated with proliferation, receptor signaling, and metabolism. Fewer DEGs were observed by menopausal status than age, indicating age is a more relevant cutoff. Future studies will aim to further characterize this high risk young patient population. Citation Format: Shubhada Dhage, Mina Gendy, Nina D'Abreo, Ruth Oratz, Sami G. Diab, VK Gadi, Cathy Graham, Midas Kuilman, Shiyu Wang, Patricia Dauer, Andrea Menicucci, William Audeh, Douglas K. Marks. Deciphering the inferior prognosis of young women with estrogen receptor-positive early-stage breast cancer through full transcriptome analysis: A FLEX database sub-study [abstract]. In: Proceedings of the 2021 San Antonio Breast Cancer Symposium; 2021 Dec 7-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2022;82(4 Suppl):Abstract nr P5-07-05.

Abstract P4-05-02: Impact of hormone receptor status on clinicopathological characteristics and outcomes among HER2-positive metastatic breast cancer patients in the ESME database

Abstract Introduction. Evidence suggests that human epidermal growth factor receptor 2-positive (HER2+) metastatic breast cancer (MBC) patients have different clinical characteristics and outcomes according to the hormone receptor (HR) status. We aimed to evaluate the impact of HR status in this population extracted from a large real-world database. Methods. We performed a retrospective analysis of HER2+ MBC patients (pts) included in the real world ESME MBC database (NCT03275311) between 2008 and 2017. Descriptive statistics, the Kaplan-Meier method and Cox proportional hazards models were used to report characteristics, outcomes and prognostic factors. Results. Of 4,145 HER2+ MBC pts eligible for our analysis, 1,449 (35%) had HR-negative (HR-), while 2,696 (65%) had HR-positive (HR+) tumors. Pts with HR- tumors had metastases earlier (median 9.3 vs 28.0 mo from primary cancer), had more often de novo MBC (47.6% vs 38.1%), grade III tumors (50.9% vs 33.6%), visceral metastases (70.9% vs 60.8%) and less often bone only disease (8.4% vs 21.1%) compared to those with HR+ tumors, all p<0.001. Pts with HR+ MBC received less frequently an anti-HER2+ targeted therapy during first line of treatment than pts with HR- tumors (74.9% vs 90.8%, p<0.001). Pts with HR- MBC had significantly worse outcomes than those with HER2+/HR+ MBC, with median overall survival (OS) 42.0 mo [95% CI 38.8-45.2] vs 55.9 mo [95% CI 53.7-59.4], p<0.001, and median progression-free survival (PFS) 9.8 mo [95%CI 9.2-11] vs 12.2 mo [95%CI 11.5-12.9], p=0.012, respectively. The independent prognostic value of HR status was confirmed in a multivariable analysis for OS (HR- versus HR+, hazard ratio=1.25 [1.13-1.39], p<0.001) but not for PFS (hazard ratio=1.03 [95%CI 0.95-1.13], p=0.8) (Table 1 for OS). The multivariable analysis also included tumor grade, age at and time to MBC diagnosis, presence of visceral metastases, number of metastatic sites and performance status as prognostic factors. In the HR+ population, ER and PR statuses were not prognostic. Conclusions. These data confirm the remaining strong and independent adverse prognostic effect of negative hormone receptors among pts with HER2+ MBC treated in the past 12 years. Cox multivariable model for overall survival in patients with HER2-positive MBCCategoriesNHazard ratio95% CIp valueTumor gradeGrade I/II17711<0.001Grade III15741.13[1.03-1.25]NA5811.15[1.01-1.31]ER/PR statusER+/PR+14571<0.001ER+/PR-9711.03[0.92-1.16]ER-/PR+1021.03[0.77-1.36]ER-/PR-13961.25[1.13-1.39]Age at MBC diagnosis<55 years17561<0.001≥55 years21701.26[1.16-1.38]Time to MBC diagnosis<6 months16351<0.0016-24 months5082.53[2.22-2.89]24-60 months8721.75[1.56-1.96]≥60 months9111.19[1.06-1.35]Visceral metastasesNo14061<0.001Yes25201.54[1.38-1.71]No of metastatic sites<330911<0.001≥38351.65[1.48-1.84]ECOG PSPS 010101<0.001PS 18021.62[1.4-1.87]PS 2-43923.34[2.83-3.95]NA17222.34[2.07-2.64] Citation Format: Luc Cabel, Matthieu Carton, Veronique Dieras, Thierry Petit, Severine Guiu, Corinne Veyret, Anthony Goncalves, Lionel Uwer, Paule Augereau, Jean-Marc Ferrero, Christelle Levy, Florence Dalenc, Isabelle Desmoulins, Marie Ange Mouret-Reynier, Marc Debled, Thomas Bachelot, Jean-Christophe Eymard, Barbara Pistilli, Jean Sebastien Frenel, Michael Chevrot, Audrey Mailliez, Marcela Carausu. Impact of hormone receptor status on clinicopathological characteristics and outcomes among HER2-positive metastatic breast cancer patients in the ESME database [abstract]. In: Proceedings of the 2021 San Antonio Breast Cancer Symposium; 2021 Dec 7-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2022;82(4 Suppl):Abstract nr P4-05-02.

Abstract P4-06-03: Clinical-risk assessment of ER positive, Her2 negative breast cancer patients: Correlation between the average modified magee score and mammaprint

Abstract Background: MammaPrint (MP) is an FDA approved early breast cancer prognostic genomic assay that examines the expression levels of 70 genes and classifies clinically high-risk patients with early-stage breast cancer into low or high risk of distant metastasis. Current ASCO guidelines do not support using MP testing in clinically low-risk patients. Historically, assessing a patient’s clinical risk for breast cancer recurrence has relied on a subjective evaluation of histologic factors, immunohistochemical features, and clinical characteristics. Genomic assay testing provides more objective prognostic and predictive information than those more traditional methods; however, several studies have shown that statistical models using standard histologic variables can give valuable prognostic information, and might be used as an alternative approach to genomic assays for clinical risk-assessment and risk-stratification, particularly in lower risk breast cancer patients, potentially resulting in significant cost savings for health care systems. One of these statistical models, the average Modified Magee equation, has been shown to be prognostic for a low risk of recurrence in ER positive Her2 negative breast cancer patients who have an average Modified Magee score (aMMs) of ≤ 12. The aMMs has been shown to correlate well with the Oncotype DX recurrence score; however, there is limited data on the correlation between the aMMs and MP. In this study, we evaluated the concordance between the aMMs and the MP index (MPI). Methods: 222 consecutive patients with a new diagnosis of hormone receptor positive Her-2 negative breast cancer at the University of Rochester between April 2020 and December 2020 were sent for MP testing. The aMMs was determined for each patient based on the estrogen receptor status, progesterone receptor status, HER-2 status, Ki-67 proliferation index, Nottingham score, and tumor size. Based on the previous literature, patients were risk-stratified by the aMMs into three groups: 1) low risk (aMMs ≤ 12; n = 75); 2) low - intermediate risk (aMMs >12 and ≤ 18; n = 96); and, 3) high risk (aMMs >18, n = 51). Based on the previous literature, patients were risk-stratified by the MPI into two groups: 1) low risk (MPI 0 to +1); and, 2) high risk (MPI -1 to 0). The correlation between the aMMs and the MPI was evaluated using the Pearson correlation coefficient. Results: The aMMs showed a negative correlation (r= -0.55) with the MPI, consistent with a lower risk aMMs more likely being associated with a lower risk MPI. 92% of patients with an aMMs ≤ 12 (very low risk) were classified as low risk by MP. 72% of patients with an aMMs > 12 and ≤ 18 (low - intermediate risk) were classified as low risk by MP. 47% of patients with an aMMs > 18 (high risk) were classified as high risk by MP (Table 1). All patients with an aMMs ≥ 24.7 (n=8) were classified as high risk by MP. Conclusion: Our study suggests that hormone receptor positive, Her-2 negative breast cancer patients with a lower risk aMMs are likely to have a lower MPI, suggesting that these patients have a decreased risk for breast cancer recurrence. As the aMMs approaches higher risk, the concordance with the MPI decreases; however 100% of patients with an aMMs ≥ 24.7 were classified as high risk by MP. The aMMs can be helpful in clinical risk-assessment prior to making a decision about sending a patient specimen out for MP testing. Additional studies are warranted. Table 1.Risk of recurrence using the average Modified Magee score (aMMs) and the MammaPrint (MP) IndexaMMs Risk-stratification categoryMP Low risk n (%)MP High risk n (%)Very low (aMMs ≤ 12)69 (92%)6 (8%)Low - Intermediate (aMMs > 12 and ≤ 18)69 (72%)27 (28%)High (aMMs >18)27 (53%)24 (47%) Citation Format: Hani Katerji, Huina Zhang, David Hicks, Bradley M Turner. Clinical-risk assessment of ER positive, Her2 negative breast cancer patients: Correlation between the average modified magee score and mammaprint [abstract]. In: Proceedings of the 2021 San Antonio Breast Cancer Symposium; 2021 Dec 7-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2022;82(4 Suppl):Abstract nr P4-06-03.

Abstract P3-14-03: The DNA damage repair landscape in Black women with breast cancer

Abstract Black women have 42% higher mortality rate from estrogen receptor positive (ER+) breast cancer than white women. Molecular mechanisms underlying worse outcome in black women are understudied. Recently, downregulation of specific DNA damage repair (DDR) genes was shown to causally induce resistance to standard care endocrine therapy by dysregulating cell cycle regulation, and to associate with poor outcome in white women with ER+ breast cancer. However, frequency and patterns of DDR dysregulation in Black women with ER+ breast cancer and impact on survival outcomes remains untested. By assessing RNA expression of 104 DDR genes across three tumor, and two normal breast, datasets, here, we map for the first time, global patterns of DDR gene expression regulation specific to Black women, both in tumors and in the normal breast. We identify a specific subset of 8 candidate DDR genes that are dysregulated at the RNA level in tumors from Black women. Of note, a novel DDR regulation signature where genes from the homologous recombination pathway are upregulated and genes from SSBR pathways (mainly base excision repair) are coincidently downregulated is almost uniquely detectable in tumors from Black women (8% incidence relative to 1% in tumors from white women, p=0.01). Moreover, this coincident DDR signature associates with dysregulated cell cycle gene expression (p<0.001). In accordance, patients whose tumors demonstrate this coincident DDR signature also have significantly worse survival outcomes across datasets (hazard ratio of 9.5, p<0.001). Overall, these results constitute the first systematic analysis of differences in DDR gene expression regulation between Black and white women and identify a specific DDR signature associated with poor outcome in tumors from Black women. These results provide new grounds for refining biomarker profiles and improving precision medicine for underserved populations. Citation Format: Aloran Mazumder, Athena Jimenez, Rachel E Ellsworth, Sophia George, Stephen J Freedland, Matthew N Bainbridge, Svasti Haricharan. The DNA damage repair landscape in Black women with breast cancer [abstract]. In: Proceedings of the 2021 San Antonio Breast Cancer Symposium; 2021 Dec 7-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2022;82(4 Suppl):Abstract nr P3-14-03.

Abstract ES6-2: CDK4/6 for ER+

Abstract Hormone receptor positive (HR+) breast cancer is the most common subtype of breast cancer. A reasonable proportion of HR+ tumors recur during or following adjuvant endocrine therapy, and late recurrences (beyond 10 years from diagnosis) are not uncommon. The CDK4/6 inhibitors palbociclib, ribociclib and abemaciclib were approved in combination with endocrine therapy for the treatment of first and second-line metastatic HR+ breast cancer, based on impressive improvements in progression-free survival and overall survival. Therefore, transitioning these drugs to the adjuvant setting in high-risk patients made sense, to further prevent short and long-term metastatic recurrence of HR+ breast cancer. To date, 3 of the 4 phase III clinical trials with CDK4/6 inhibitors in the adjuvant setting have been reported, with mixed results. Penelope-B and PALLAS, both exploring the use of palbociclib for 1 and 2 years, respectively, along with endocrine therapy in patients with high-risk clinical characteristics failed to demonstrate a benefit in 4-years and 2-years disease-free survival (DFS), despite an early signal of benefit in the Penelope-B trial at 2-years. The Monarch-E trial, using abemaciclib for 2 years along with endocrine therapy for patients with very high anatomical and biologic recurrence risk, has shown a 3% higher 2-years DFS than endocrine therapy alone, but follow-up is quite short (19 months), and it is unclear if it will be maintained over time. The NATALEE trial, using ribociclib for 3 years with endocrine therapy is expected to yield results in 2025. These data highlight several questions that remained unanswered. Are the CDK4/6 inhibitors different from each other despite an identical mechanism of action? Does duration of treatment matter? Does disease biology matter? Discontinuation rates, albeit different between these trials, seems to be an unlikely answer for the discrepant results. Fortunately, all these adjuvant trials have collected a rich biorepository of tissue and blood, which allows the investigators to better ellucidate unique biomarkers that may predict individual benefit for each of the approved CDK4/6 inhibitors, that will hopefully guide optimal patient selection and treatment combinations. Till then, longer follow-up on the Monarch-E and NATALEE trials are necessary to explore if CDK4/6 inhibitors should or not be incorporated to endocrine therapy in the adjuvant setting for patients at high-risk of metastatic recurrence. Citation Format: IA Mayer. CDK4/6 for ER+ [abstract]. In: Proceedings of the 2021 San Antonio Breast Cancer Symposium; 2021 Dec 7-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2022;82(4 Suppl):Abstract nr ES6-2.

Abstract PD15-03: Overlapping molecular features (proliferation, immune signatures andTP53mutations) associated with palbociclib resistance inER+HER2- primary breast cancer

Abstract Background: Cyclin-dependent kinase 4/6 (CDK4/6) inhibitors in combination with anti-hormone therapy are highly effective treatments for oestrogen receptor positive (ER+) and HER2 negative (HER2-) advanced breast cancer. Pre-clinical and clinical studies have reported mechanisms of resistance to CDK4/6 inhibitors to include interferon signalling, high CCNE1 expression and loss of RB1 expression. The PALLET phase II randomized neoadjuvant trial of letrozole (LET) ± palbociclib (PALBO) in postmenopausal ER+HER2- primary breast cancer showed that suppression of proliferation as measured by Ki67 was significantly greater with addition of PALBO to LET but did not result in all patients achieving complete cell-cycle arrest, indicating intrinsic resistance in some patients. We report phenotypes/genotypes associated with that resistance. Methods: In the PALLET trial, 307 patients were randomized to LET+PALBO (n=204) or LET (n=103) for 14wks. For the first 2wks of LET+PALBO patients were randomised to LET, PALBO or LET+PALBO. RNA-seq of baseline samples from consented patients was performed on fresh frozen biopsies for 224 patients (LET-only n=77; LET+PALBO n=147); whole exome sequencing was performed on 188 tumors and matched blood samples (LET-only n=61; LET+PALBO n=127). After 14wks of treatment, those patients with Ki67% < 2.7% were classified as having complete cell-cycle arrest (CCCA). Differentially expressed genes (DEGs) were identified between patients sensitive (CCCA) and resistant (non-CCCA) to treatments with or without PALBO at 14wks by DESeq2. Mutect2 and VarScan was used to identify somatic mutations and CNVkit was used to identify copy number alterations in whole exome sequencing (WES). Results: In LET+PALBO treated patients, higher expression of E2F targets, interferon gamma response and mTORC1 signalling genes were observed in baseline gene expression of non-CCCA patients at 14wks (FDR<0.05, GSEA). Similar results were also observed if using 2wk Ki67 data. In LET-only non-CCCA patients, higher expression of mTORC1 signalling and lower expression of oestrogen response genes (FDR<0.05, GSEA) were observed. Additional analysis of baseline gene expression for non-CCCA at 14wks LET+PALBO patients showed higher expression of immune checkpoint inhibition associated genes including IFNG, IDO1, PD-L1 (FDR<0.05, DESeq2), higher expression of genes expressed only in immune cells and two gene signatures related to interferon signalling and immune checkpoint blockade (FDR<0.05, GSEA). Somatic mutation analysis showed a trend for enrichment of mutations in TP53 for both LET-only and LET+PALBO non-CCCA patients (p=0.02 and p=0.06, respectively, Fisher-exact) and significant enrichment of MAP3K1 mutations in LET-only CCCA patients (p<0.05, Fisher-exact). TP53 mutations were also associated with higher proliferation, mTORC1 and immune related signatures (all p<0.01, Mann-Whitney). Change at 14wks Ki67 was significantly different (p=0.02, Mann-Whitney) between TP53wt and TP53MUT for LET-only patients (median WT = -92%, MUT = -66%, p=0.02, Mann-Whitney ) but not for LET+PALBO (median WT = -99% MUT = -95%, p=0.13, Mann-Whitney). No copy number alterations were significantly enriched in LET+PALBO non-CCCA patients. Conclusion: We observe, confirming previous studies, an association of CDK4/6 inhibitor resistance, high expression of CCNE1 and genes related to interferon gamma signalling. We show that there is an overlap between resistance mechanisms and TP53 mutations. However, ER+HER2- patients with TP53 mutations may still benefit from PALBO adding to suppression of proliferation compared to LET-only treatment. Citation Format: Eugene F Schuster, Hui Xiao, Maggie Cheang, Elena Lopez-Knowles, Lucy Kilburn, Viktoriya Korchina, Sejal Salvi, Samuel A Jacobs, Melanie Finnigan, David A Wheeler, Shannon Puhalla, Donna Muzny, Harsha Doddapaneni, Katherine Pogue-Geile, Yuan Liu, Judith Bliss, Stephen Johnston, Mitch Dowsett, Mothaffar Rimawi, On behalf of the PALLET Trialists. Overlapping molecular features (proliferation, immune signatures andTP53mutations) associated with palbociclib resistance inER+HER2- primary breast cancer [abstract]. In: Proceedings of the 2021 San Antonio Breast Cancer Symposium; 2021 Dec 7-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2022;82(4 Suppl):Abstract nr PD15-03.

Abstract PD15-02: HER2-enriched subtype and novel molecular subgroups drive aromatase inhibitor resistance and an increased risk of relapse in early ER+/HER2+ breast cancer

Abstract Background: Approximately 10% of estrogen receptor positive (ER+) early breast cancers (EBC) are human epidermal growth factor positive/overexpressed (HER2+). Resistance to aromatase inhibitors (AIs) is poorly understood among ER+HER2+ BC. We aimed to identify biomarkers of response and resistance using the HER2+ tumours in the POETIC trial. Methods: POETIC was a phase III trial of post-menopausal women with ER+ EBC randomized 2:1 to 2-weeks of peri-surgical AI vs control, followed by standard-of-care. Baseline formalin-fixed paraffin-embedded samples from 342 ER+HER2+ BC (237 treated/105 controls) were gene expression profiled using Breast cancer 360™ panel (NanoString) covering intrinsic subtypes and 40 key biological signatures. New molecular subgroups were identified using consensus clustering (CC).Proliferation rate was estimated as percentage of cancer-cells staining for Ki67. Ki67 response was defined by percentage-changes from baseline to surgery: poor (PR; reduction <50%), intermediate (IR; 50-75%) and good (GR; >75%). Residual Ki67 at 2 weeks timepoint (Ki672w) was defined as High (H) (≥ 10%) vs Low (L) (<10%). Association of molecular features with Ki67 response was assessed using logistic regression models and with time to recurrence (TTR) using multivariable Cox regression models adjusted for post-surgery clinicopathological variables: grade, tumour size, nodal status and age. Results: Ki672w was measured for all treated samples (n = 227, 48% H, 52% L) and a fraction of controls (n = 50, 96% H, 4% L). At baseline, 45% of tumours were HER2-enriched (HER2-E), 53% Luminal A/B and 2% Basal-Like. HER2-E associated with high Ki672W (84% H, 16% L) compared to non-HER2-E (29% H, 71% L, p<0.001). Amongst the 40 BC360 signatures, 9 were significantly associated with Ki67 response (Table). High expression of endocrine-related and apoptosis signatures associated with GR while HRD and TP53 with PR. These features were also related to Ki672W, with additional signatures involved in immune-checkpoint component and immune-enrichment (IM) (IDO1, PD-L1, IFN-gamma and T-reg) associated with high Ki672W.. Using CC, 4 molecular subgroups predicting differential response to AI were identified. Cluster (C) 1 was characterized by overexpression of IM features and low ER signaling (42%); C2 low IM but highest ERBB2 expression (15%); C3 ESR1 high and PgR low (4%) and C4 high endocrine signaling and lowest ERBB2 expression (39%). Distribution of Intrinsic subtypes varied across these molecular subgroups with the majority of C1 being HER2-E (62%) and Luminal B (27%); C2 HER2-E (74%); C3 Luminal B (64%) and C4 Luminal B (55%) and Luminal A (30%). HER2-E had significantly poorer TTR (HR 2.14; 95%CI1.11-4.17; p=0.0224) compared to Luminal tumours, remaining significant (HR 2.55, 95%CI1.14-5.69; p=0.0222) in the multivariable analysis. C2 (HR 4.67, 95%CI 1.90-11.51; p<0.001) and C4 (HR 2.42, 95%CI1.05-4.5.58; p=0.0385) were independent predictors of shorter TTR compared to C1, adding significant value beyond intrinsic subtypes (Likelihood ratio test, p=0.00429). Conclusions: Our results confirm that HER2-E, TP53 mutation, HRD and IM features are the main components driving poor early response to AI in ER+/HER2+ EBC. Novel molecular subgroups identify additional non-HER-E tumours not responding to AI with an increased risk of relapse. The appropriate additional treatment warrants further investigation. List of significant gene expression signatures associated with response as defined by reduction of Ki67. Odds ratio (OR) were determined by ordinal logistic regression analyses. OR: <1 indicating associated with good response and > 1.0 associated with worse response.Odds Ratiop-valuep-adjusted (False Discovery Rate)ESR10.65< 0.001< 0.001Estrogen Receptor Signaling0.44< 0.001< 0.001ERBB21.52< 0.001< 0.001TP53 mutant signature2.65< 0.001< 0.001FOXA10.42< 0.001< 0.001Apoptosis0.26< 0.0010.00228Homologous Recombination Deficiency (HRD)2.33< 0.0010.00349PgR0.820.001480.00737Hypoxia2.180.001660.00737 Citation Format: Milana A Bergamino Sirvén, Elena López-Knowles, Gabriele Morani, Holly Tovey, Lucy Kilburn, Chris Holcombe, Anthony Skene, Ian Smith, John Robertson, Gene Schuster, Judith M Bliss, Mitch Dowsett, Maggie CU Cheang, POETIC investigators. HER2-enriched subtype and novel molecular subgroups drive aromatase inhibitor resistance and an increased risk of relapse in early ER+/HER2+ breast cancer [abstract]. In: Proceedings of the 2021 San Antonio Breast Cancer Symposium; 2021 Dec 7-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2022;82(4 Suppl):Abstract nr PD15-02.

Abstract OT2-01-02: First in human phase 1 dose escalation and expansion study of the safety and pharmacokinetics of the oral CDK7 inhibitor XL102 as a single-agent and in combination therapy in patients with inoperable locally advanced or metastatic solid tumors, including breast cancer

Abstract Background: XL102 is an orally bioavailable, selective, and covalent small-molecule inhibitor of cyclin-dependent kinase 7 (CDK7). CDK7 is a serine/threonine kinase which controls cell cycle progression via the phosphorylation of CDKs (1, 2, 4 & 6) and regulates transcription through phosphorylation of RNA polymerase II. CDK7 overexpression has been reported in multiple tumor types including hormone receptor positive (HR+) breast cancer (BC) and triple negative breast cancer (TNBC). XL102 induced cell death in a number of cancer cell lines and caused tumor regression in murine xenograft models of multiple tumor types including HR+ BC and TNBC. Here, we present the study design of an ongoing phase 1 trial in solid tumors which includes cohorts with advanced HR+ BC, TNBC, epithelial ovarian cancer (EOC), and metastatic castration-resistant prostate cancer (mCRPC). Trial Design and Statistical Methods: This phase 1, open-label trial will assess the safety, tolerability, pharmacokinetics, anti-tumor activity and effect on biomarkers of XL102 administered orally alone and in combination regimens in selected tumor types (NCT04726332). The study consists of dose-escalation stages and disease-specific cohort expansion stages. In the dose-escalation stages (modified interval 3+3 design), a maximum tolerated (MTD) and/or recommended XL102 dose (RD) for use alone and in combination therapy with standard dose fulvestrant (HR+BC) or abiraterone/prednisone (mCRPC) will be established in independent strata. Upon reaching the MTD/RD, the expansion stage will enroll according to Simon’s Two-Stage Minimax design, assuming a power of 80% and one-sided α of 15%, for single agent XL102 (HR+BC, TNBC, EOC, and mCRPC) and XL102 in combination therapy (HR+BC and mCRPC). Eligibility criteria: All patients must have adequate organ function and be able to take oral medications. Patients with CNS disease are eligible if treated and stable, provided they meet the other inclusion and exclusion criteria. Patients enrolled in expansion cohorts must have measurable disease. Patients in HR+ BC expansion cohorts must have received ≥ 1 prior endocrine anticancer therapy and 1 prior CDK4/6 inhibitor therapy either sequentially or concurrently and may have received ≤ 2 additional prior lines of systemic anticancer therapy for locally advanced or metastatic disease. Patients in TNBC and EOC expansion cohorts must have received ≥ 1 line but < 3 lines of prior systemic anticancer therapy for locally advanced or metastatic disease or platinum-resistant ovarian cancer (eg, cytotoxic therapy, targeted therapy, immunotherapy), respectively. Patients in EOC expansion cohort may include primary peritoneal cancer and fallopian tube cancer who have platinum-resistant disease following treatment with a platinum-containing chemotherapy. Patients with ovarian borderline epithelial tumors and platinum-refractory disease are ineligible. Study objectives: The primary objective of the expansion stage is the objective response rate of XL102 alone and in combination therapy as assessed by the investigator per RECIST 1.1. The secondary objectives are to evaluate safety, tolerability, and pharmacokinetics. Exploratory objectives include duration of response, progression-free survival, overall survival, and correlation of tumor and blood biomarkers with response. Accrual: The study began enrolling patients in February 2021 and is ongoing. Total enrollment estimated to be up to 298 subjects. Citation Format: Amita Patnaik, Minal Barve, Manali Bhave, Vivek Subbiah, Sumandeep Atwal, Keerti Sharma, Christian Scheffold, Cynthia Wetmore, Geoffrey Shapiro. First in human phase 1 dose escalation and expansion study of the safety and pharmacokinetics of the oral CDK7 inhibitor XL102 as a single-agent and in combination therapy in patients with inoperable locally advanced or metastatic solid tumors, including breast cancer [abstract]. In: Proceedings of the 2021 San Antonio Breast Cancer Symposium; 2021 Dec 7-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2022;82(4 Suppl):Abstract nr OT2-01-02.

Abstract P5-08-07: The complete estrogen receptor antagonist OP-1250 can combine with HER2 inhibition to inhibit estrogen receptor-driven cellular proliferation and shrink xenograft tumors in ER+/HER2+ breast cancer models

Abstract The human epidermal growth factor receptor 2 (HER2, also known as ERBB2) oncogene is overexpressed in approximately 25% of breast cancer tumors and is associated with a high rate of brain metastasis. Patients with brain metastases have a poor prognosis and limited treatment options available. Since about half of HER2+ tumors are also hormone receptor positive, and cross talk has been shown to occur between these two signaling pathways, it is plausible that these tumors could benefit from the addition of hormonal therapy on top of HER2 inhibition. OP-1250 is a complete estrogen receptor antagonist (CERAN) and selective estrogen receptor degrader (SERD), and has been shown to effectively inhibit estrogen receptor positive (ER+) breast cancer growth in both cell-based assays and xenograft studies. OP-1250 has favorable pharmacokinetic properties and demonstrated brain penetrance in non-clinical models. While extensively characterized in ER+/HER2- breast cancer models, OP-1250 had not previously been tested in a HER2+ context. Here we report OP-1250 inhibited estrogen receptor-driven proliferation and effectively degraded the estrogen receptor in multiple ER+/HER2+ cell lines. OP-1250 combined effectively with HER2 inhibitors tucatinib and trastuzumab, demonstrating that inhibition of both HER2 and estrogen receptor signaling was more effective in inhibiting breast cancer cell proliferation than inhibiting either pathway alone. The addition of OP-1250 to a HER2 inhibitor also resulted in greater tumor shrinkage in multiple ER+/HER2+ xenograft models. Our data suggests a potential clinical benefit in the addition of OP-1250 on top of HER2 inhibition for the treatment of ER+/HER2+ breast cancer and provides strong rationale for a clinical study evaluating the combination of OP-1250 and HER2 targeted agents, which is planned for 2022. Citation Format: Alison D Parisian, Leslie Hodges-Gallagher, Richard Sun, Susanna Barratt, Gopinath S Palanisamy, Julia Lawrence, Pamela Klein, David C Myles, Cyrus L Harmon, Peter J Kushner. The complete estrogen receptor antagonist OP-1250 can combine with HER2 inhibition to inhibit estrogen receptor-driven cellular proliferation and shrink xenograft tumors in ER+/HER2+ breast cancer models [abstract]. In: Proceedings of the 2021 San Antonio Breast Cancer Symposium; 2021 Dec 7-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2022;82(4 Suppl):Abstract nr P5-08-07.

Abstract OT1-08-01: Concurrent versus sequential chemo-endocrine therapy in er positive and her2 negative non-metastatic breast cancer- an open-label, phase III, randomized controlled trial

Abstract Background-Traditionally, endocrine therapy follows chemotherapy for hormone receptor (HR) positive breast cancer (BC) in the adjuvant setting. Concurrent chemo-hormonal therapy fell into disrepute due to previous studies showing a probable, small detriment in survival. (Albain et al, Lancet, 2009; Bedognetti et al, JNCI, 2009; Pico et al, Ann Oncol, 2004) However, the early studies are fraught with biases. The biological rationale was that endocrine therapy is cytostatic, pushing cancer cells into G0 phase which was feared to reduce the efficacy of chemotherapy which is known to act best on rapidly dividing cells. Thus, the two mechanisms of actions were thought to be antagonistic. However, this fear may be irrational and evidence at molecular level is contradictory. The current sequential strategy delays the start of an equally effective treatment modality in HR+ BC patients. Recent studies have shown that concurrent chemo-endocrine therapy is safe in metastatic HR+ setting. (Sledge et al, J Clin Oncol, 2000) One also needs to rethink neoadjuvant strategies in these patients to improve the current dismal pathological complete response (pCR) rates in this subset. (Cortazar et al, Lancet, 2014) Concurrent treatment seems to have improved pCR rate. (Sagiu K et al, Acta Med Okayama, 2015) Here, we describe a phase III, randomized study aimed to evaluate the efficacy of concurrent chemo-endocrine therapy in the adjuvant and neoadjuvant setting. (CTRI/2018/09/015643) Aims and Objectives- The primary objective is to assess the improvement in disease-free survival by administering concurrent versus sequential, (neo) adjuvant chemo-endocrine therapy in HR+, HER2 negative, non-metastatic BC. The secondary objectives are to assess improvement in overall survival, pCR rates, breast conservation rates. Biomarkers of response to treatment and tumour dormancy in HR+ BC will be studied in a subset of patients.Patients and methods-This is a prospective, open label study where non-metastatic, biopsy proven, HR+, HER2- BC patients who warrant chemotherapy in the adjuvant or neoadjuvant setting are considered. The study started accrual in December 2018 and 285 patients are randomized till date. Pregnant or lactating women and those with inflammatory breast carcinoma are excluded. After obtaining informed consent, patients are randomized to receive concurrent versus sequential chemo-endocrine treatment. Patients will receive all chemo and endocrine therapy as per the institutional standard protocols otherwise. Stratification criteria are neoadjuvant vs adjuvant therapy, pre vs postmenopausal status, node positive vs node negative status, 1st gen (anthracyclines alone) versus 2nd gen (including taxanes) chemotherapy. A two-sided log-rank test with an overall sample size of 2316 (2432 with a 5% dropout rate) (1158 in control and 1158 in treatment group) achieves 80% power at a 0.05 significance level to detect a hazard ratio of 0.80 when the proportion surviving in the control group is 0.70. An interim with respect to safety is planned in the protocol. Expected results and conclusion-Ours is a large, prospective trial evaluating the efficacy of concurrent chemo-endocrine therapy in HR+ BC patients. This has the potential to improve pCR rates in the neoadjuvant setting and change the standard of care in the management of HR+ BC patients. The biomarker analysis will help us further understand the biology of HR+ BC and shed light upon tumour dormancy signatures. Citation Format: Shalaka Joshi, Sridevi Murali-Nanavati, Vaibhav Vanmali, Rohini Hawaldar, Mujahid Gafur Shaikh, Mohammad Sadique Ansari, Sushmita Rath, Jaya Ghosh, Seema Gulia, Jyoti Bajpayi, Nita Nair, Vani Parmar, Purvi Thakkar, Garvit Chitkara, Tanuja Shet, Sangeeta Desai, Ayushi Sahay, Sudeep Gupta, Rajendra Badwe. Concurrent versus sequential chemo-endocrine therapy in er positive and her2 negative non-metastatic breast cancer- an open-label, phase III, randomized controlled trial [abstract]. In: Proceedings of the 2021 San Antonio Breast Cancer Symposium; 2021 Dec 7-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2022;82(4 Suppl):Abstract nr OT1-08-01.

Abstract P4-02-07: Lasofoxifene as a potential treatment for aromatase inhibitor resistant ER positive breast cancer

Abstract Postmenopausal patients with estrogen receptor positive (ERα+) primary invasive breast cancer are typically treated with aromatase inhibitors (AIs) as first line adjuvant therapy. Patients who become resistant to AIs are treated with fulvestrant and/or CDK4/6 inhibitors, such as palbociclib, as second line therapy. Lasofoxifene, a selective estrogen receptor modulator (SERM), was developed for the treatment of vaginal atrophy and osteoporosis. Our previous studies, performed in an MCF-7 xenograph metastatic breast cancer mouse model with activating ERα mutations, showed that lasofoxifene, both alone and in combination with palbociclib, was more effective than fulvestrant at inhibiting tumor growth and metastasis to the liver, lung, bone and brain. In the current study, we compared the efficacy of lasofoxifene +/- palbociclib to fulvestrant +/- palbociclib combinations in a letrozole-induced, AI-resistant breast tumor model (MCF-7 LTLT cells) that does not express ERα activating mutations. Luciferase-GFP tagged LTLT cells were injected into the mammary ducts of NSG mice (MIND model) and tumor progression was monitored by live luminescence imaging of primary tumors, as well as ex vivo imaging and histochemical analysis of metastatic sites at study endpoint. Primary tumor area was also measured at study endpoint. Lasofoxifene +/- palbociclib was significantly more effective than fulvestrant +/- palbociclib at inhibiting primary tumor growth. In addition, all treatments, except fulvestrant, inhibited bone metastasis versus vehicle. These data show that lasofoxifene is more effective than fulvestrant in this tumor model and suggest that lasofoxifene has potential as an effective therapy for AI resistant breast cancers that do not express ERα activating mutations. Citation Format: Muriel Laine, Marianne E Greene, Tiffany Leng, Justyna D Kurleto, Sophia Li, Barry Komm, Geoffrey L Greene. Lasofoxifene as a potential treatment for aromatase inhibitor resistant ER positive breast cancer [abstract]. In: Proceedings of the 2021 San Antonio Breast Cancer Symposium; 2021 Dec 7-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2022;82(4 Suppl):Abstract nr P4-02-07.

Abstract P3-16-03: Clinical and pharmacoeconomic analyses of CDK4/6 inhibitors use in stage IV breast cancer females in the state of Qatar: A comparative retrospective observational study with cost effectiveness and cost utility analyses

Abstract Introduction: Cyclin-dependent kinase 4 and 6 (CDK4/6) inhibitors are indicated in the first-line treatment of hormonal receptor positive and HER-2 negative (HR+/HER2- negative) metastatic breast cancer (MBC). Although phase III randomized controlled trials (RCTs) proved their clinical efficacy, they can increase healthcare expenditure. To date, there are no observational studies to validate the clinical findings of the existed RCTs. In addition, only a few pharmacoeconomic evaluations were published regarding the two common CDK4/6 inhibiting agents, palbociclib and ribociclib to evaluate their financial burden. Objectives: To evaluate the clinical efficacy of palbociclib and ribociclib in the local settings in Qatar. Moreover, to conduct a thorough pharmacoeconomic analysis for the two medications and compare them in terms of their cost-effectiveness and cost-utility. Methodology: A retrospective observational study on HR+/HER-2 negative stage IV breast cancer patients receiving palbociclib or ribociclib in Qatar was conducted, followed by a comparative pharmacoeconomic analysis. The primary outcomes were progression-free survival (PFS) and overall-survival (OS). Safety profiles of both medications were also evaluated. Clinical data of all MBC in Qatar were collected from the National Center for Cancer Care and Research (NCCCR) from January 2017 to December 2019 using Cerner ® system. To evaluate the long-term costs-effectiveness, a 10-year within-cycle corrected Markov’s model was developed. The Markov’s model consisted of three main health states: progression-free (PFS), progressed disease (PD), and death. Incremental cost-effectiveness ratio (ICER) and incremental cost-utility ratio (ICUR) were generated and compared to a three gross-domestic-product (3 GDP) per capita. Model costs were obtained from the department of accounting and finance in the NCCCR, transition probabilities were calculated from individual patient data obtained in the clinical phase, and utilities were summarized from the published literature. Deterministic and probabilistic sensitivity analyses were carried out. Modeling was conducted via TreeAge Pro ® software. Results: There was no statistically significant difference between palbociclib and ribociclib in PFS; median PFS time was 17.85 versus 13.55 months respectively; p> 0.05. Similarly, there was no statistically significant difference in OS (29.82 versus 31.72 months; p>0.05). For the pharmacoeconomic analysis, ribociclib dominated palbociclib in yielding an ICER of - 83,090.88 QAR per life-year gained, and ICUR of -31,868.06 QAR per quality-adjusted life-year gained. The results remained robust in all cases of the deterministic sensitivity analyses suggesting ribociclib is more cost-effective than palbociclib. Taking all combined uncertainties into account, the overall confidence in the base-case conclusion was around 60%. Conclusions: Both treatment strategies have similar efficacy and safety profiles. Nonetheless, ribociclib is a more cost-effective option than palbociclib based on the base-case results and based on the addressed uncertainties related to the model inputs. Citation Format: Nour Hisham Al-Ziftawi, Shereen Elazzazy, Mohammed Fasihul Alam, Asrul Akmal Shafie, Anas Hamad, Salha Bujassoum, Mohamed Izham Ibrahim. Clinical and pharmacoeconomic analyses of CDK4/6 inhibitors use in stage IV breast cancer females in the state of Qatar: A comparative retrospective observational study with cost effectiveness and cost utility analyses [abstract]. In: Proceedings of the 2021 San Antonio Breast Cancer Symposium; 2021 Dec 7-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2022;82(4 Suppl):Abstract nr P3-16-03.

Abstract P5-16-10: RLY-2608: The first allosteric mutant- and isoform-selective inhibitor of PI3Kα, is efficacious as a single agent and drives regressions in combination with standard of care therapies in PIK3CA mutant breast cancer models

Abstract Inhibition of CDK4/6 combined with the estrogen receptor (ER) degrader fulvestrant significantly improves progression free survival and overall survival in advanced hormone receptor positive (HR+) breast cancer patients and is now the standard of care (SOC) in this disease. Up to 40% of HR+ breast cancers harbor PIK3CA mutations leading to activation of phosphoinositide 3-kinase alpha (PI3Kα), which has been associated with resistance to CDK4/6 inhibitors and fulvestrant. Therefore, PI3Kα inhibitor combinations with CDK4/6 inhibitors and/or fulvestrant are of high interest in HR+, PIK3CA mutant breast cancer. The therapeutic index of active site (orthosteric) inhibitors of PI3Kα has been limited by the dual issues of no clinically meaningful selectivity for mutant versus wild-type (WT) PI3Kα and off-isoform inhibitory activity. Alpelisib, the only approved orthosteric PI3Kα inhibitor, is emblematic of the class with toxicity related to inhibition of wild type PI3Kα and other PI3K isoforms resulting in sub-optimal inhibition of mutant PI3Kα, frequent discontinuation, and challenges in combining with CDK4/6 inhibitors. To overcome these limitations, we designed RLY-2608, the first allosteric, mutant, and isoform-selective inhibitor of PI3Kα. We solved the full-length cryo-EM structure of PI3Kα, performed long time-scale molecular dynamic simulations to elucidate conformational differences between WT and mutant PI3Kα, and leveraged these insights to enable the design of RLY-2608. RLY-2608 does not compete with orthosteric inhibitors for binding and associates 8x faster with mutant PI3Kα relative to WT PI3Kα. In biochemical assays, RLY-2608 inhibits kinase domain (H1047R) and helical domain (E542K, and E545K) mutant PI3Kα activity, demonstrating <10nM potency with 8-12x selectivity relative to WT. RLY-2608 is > 1000-fold selective over the β, δ, and γ PI3K isoforms in biochemical assays and demonstrates exquisite selectivity across a panel of 322 kinases, with no other kinases showing >50% inhibition. We performed in vitro combinations in two HR+ PIK3CA mutant cell lines (MCF7: E545K; T47D: H1047R) and observed synergy between RLY-2608 and fulvestrant or CDK4/6 inhibitors. In vivo, we tested combinations of RLY-2608 with fulvestrant and/or the CDK4/6 inhibitor abemaciclib in the MCF7 xenograft model. Oral administration of RLY-2608 in combination with fulvestrant led to improved efficacy compared to either agent alone in a dose-dependent manner, with regressions observed in the combination arms at all doses. Furthermore, the triple combination of RLY-2608, fulvestrant, and abemaciclib resulted in superior efficacy compared to either the RLY-2608 + fulvestrant or RLY-2608 + abemaciclib doublets, with deep regressions observed in the triple combination arm. In addition, in vivo combination efficacy with fulvestrant and CDK4/6 inhibitors (palbociclib or abemaciclib) was assessed in patient-derived xenografts harboring the PIK3CA H1047R or E545K mutation along with a second site PIK3CA minor mutation. In these studies, combination benefit was observed with doses of RLY-2608 significantly lower than the dose required for maximum efficacy as a single agent. RLY-2608 synergizes in vitro with both anti-estrogen and CDK4/6 inhibitors in cell models of HR+/PIK3CA mutant breast cancer. RLY-2608 can be combined with fulvestrant and CDK4/6 inhibitors in vivo with tumor regressions observed in both cell- and patient-derived xenograft models. The pre-clinical profile of RLY-2608 supports the clinical development of RLY-2608 both in single agent and combination clinical trials in patients with PIK3CA mutant tumors, including HR+/PIK3CA mutant breast cancer. Citation Format: Ermira Pazolli, Randy Kipp, Alessandro Boezio, Hakan Gunaydin, Amanda Iskandar, Matthew Zubrowski, Bret Williams, Kelley Shortsleeves, Alexandre Larivee, Tom McLean, Klaus Michelsen, Hongtao Zeng, Jonathan LaRochelle, Joe Manna, Lucian DiPietro, Andre Lescarbeau, Mary Mader, Bindu Bennet, Jeremy Wilbur, Qi Wang, Levi Pierce, Iain Martin, James Watters, Pascal Fortin, Donald Bergstrom. RLY-2608: The first allosteric mutant- and isoform-selective inhibitor of PI3Kα, is efficacious as a single agent and drives regressions in combination with standard of care therapies in PIK3CA mutant breast cancer models [abstract]. In: Proceedings of the 2021 San Antonio Breast Cancer Symposium; 2021 Dec 7-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2022;82(4 Suppl):Abstract nr P5-16-10.

Abstract P5-05-03: Adipocyte associated methionine domain containing (AAMDC): A nuclear envelope protein with predictive and prognostic potential in luminal breast cancer

Abstract Background Estrogen receptor (ER) positive breast cancers (BCs) are less likely to relapse than other types, due to a lower propensity to disseminate and to sensitivity to anti-estrogen drugs in the adjuvant setting. However, combined analysis of DNA mutation and mRNA expression profiles in BC identifies a number of Integrative Clusters (IntClus), one of which, IntClus2, is characterized by ER positivity but high relpase and mortality. A hallmark of IntClus2 tumours is an amplification of chromosome 11, at the center of which lies the gene for the protein AAMDC. Pre-clinical research shows this protein to drive proliferation, migration, colony formation, switching between glucose and lipid metabolism, folate metabolism and anti-estrogen resistance. We postulated that resultant clinical behaviours could include; high relapse and mortality, anti-estrogen resistance, an interaction with the protective effect on relapse from statins and sensitivity to fluorouracil (FU)-based therapies. Methods The retrospective study cohort comprised 1766 ER positive early BC patients treated 1994-2014. Patient and tumour demographic, statin prescription and treatment data were collated. Primary tumour tissue was available with AAMDC expression data on 419 patients. On noting nuclear (N), cytoplasmic (C) and nuclear envelope (NE) expression, separate intensity scores were attributed to each localization. Intensity score cut-points were based on providing statistically useable sub-group sizes for comparison. Results No significant correlations between BC biology and patterns of AAMDC expression were seen. Cytoplasmic AAMDC weakly linked to some aggressive features, including high grade, luminal B sub-type and progesterone receptor negativity, although less lymph node spread. In contrast, nuclear AAMDC associated with some less aggressive features, being less in high grade and luminal B cancers. Nuclear envelope AAMDC was rarer in the HER2 positive sub-type without other correlates. A trend to higher distant relapse was seen for NE +ive BCs (20 v 14%, p=0.09). Overall mortality was statistically higher for NE expression (54 v 44% mortality, p=0.04) and cytoplasmic expression (56 v 47% mortality, p=0.04) and a trend to lower mortality for nuclear expression (46 v 54%, p=0.06). Looking at combinatorial expression, BC deaths were significantly lower in the nuclear +ive/NE -ive phenotype (4.7%) than the nuclear -ive/NE -ive (19.5%, p=0.013), nuclear -ive/NE +ive (17.6%, p=0.017) or nuclear +ive/NE +ive phenotypes (19.3%, p=0.011).Considering the effects of statins on early BC outcome, overall our patients on statins had notably lower BC return rates (HR 0.31, p=0.004). Considering interactions with AAMDC, in high-risk NE +ive cancers statin protection was high (HR for relapse 0.33, p=0.032), compared to no protection in already low risk NE -ive tumours (HR 0.90, p=0.41). The hypothesis that AAMDC confers sensitivity to FU-based agents was confirmed with a correlation observed for longer responses to FU-based chemotherapy for increasing NE AAMDC expression (R=0.334), whereas no link was seen for other agents or other AAMDC expression sub-locations. The mean times on FU-based therapy for low, moderate and high NE expression were 2.0, 5.7 and 12.2 months respectively (p=0.02). This work indicates that AAMDC expression can BC affect outcome, identifying nuclear envelope located AAMDC as both associating with higher mortality and correlating with superior protection from statins as well as response to FU-based chemotherapies. This could guide metastatic chemotherapy selection for affected patients, implies NE positive BCs might preferentially benefit from adjuvant statins and suggests that AAMDC and NE-associated AAMDC partners may be future therapeutic targets. Citation Format: Indunil Weerasena, Lisa Spalding, Benjamin F Dessauvagie, Emily Golden, Eleanor Woodward, Pilar Blancafort, Andrew Redfern. Adipocyte associated methionine domain containing (AAMDC): A nuclear envelope protein with predictive and prognostic potential in luminal breast cancer [abstract]. In: Proceedings of the 2021 San Antonio Breast Cancer Symposium; 2021 Dec 7-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2022;82(4 Suppl):Abstract nr P5-05-03.

Abstract P5-05-01: Metabolite profiling and RNA-seq identifies novel metabolomic-genomic biomarker and therapeutic options for rapidly proliferating breast cancers

Abstract Metabolic dysregulation is one of the distinctive features of breast cancer, yet remains under-characterized in different subtypes of breast cancer. In this study, we performed full metabolome profiling and RNA-seq gene expression analyses on patient samples comprised of Triple Negative Breast Cancer (TNBC) and Estrogen Receptor (ER) positive breast cancers, as well as TNBC patient-derived xenografts (PDX). We identified two major metabolic groups using hierarchical clustering analysis of global metabolite levels: a Nucleotide/Carbohydrate-Enriched group and a Lipid/Fatty Acid-Enriched group. The Nucleotide/Carbohydrate-Enriched group is populated by the majority of TNBCs and all TNBC PDX samples, and shows high levels of energy consumption and nucleotide biosynthesis related metabolites, while the Lipid/Fatty Acid-Enriched group contained all ER+ cancers (and normal breast tissues) and showed high levels of lipids and fatty acids. Using these two metabolite-defined groups, we compared metabolic signatures to RNA-seq expression data and identified gene expression signatures that correlated with each metabolic group. This novel integrated signature identified classic proliferation-associated genes as highly expressed in the Nucleotide/Carbohydrate-Enriched metabolomics group. We next sought to therapeutically target the Nucleotide/Carbohydrate-Enriched group through targeting of nucleotide metabolism using a pyrimidine biosynthesis inhibitor (DHODH inhibitor, Brequinar), and/or a glutaminase inhibitor (CB-839), and observed tumor volume reductions and improved survival times using the TNBC PDX models. Notably,. when the DHODH inhibitor Brequinar was tested on four different immune-competent TNBC mouse models, it showed significant single agent activity, and in 3/4 cases had better activity than carboplatin/paclitaxel combination, and with less toxicity. Our study reveals a new molecularly targeted therapy for rapidly proliferating TNBCs, guided by a novel set of metabolic-genomic biomarkers, which might be translated quickly as DHODH inhibitors are already FDA approved for use in other diseases. Citation Format: Cherise R Glodowski, Chengheng Liao, Cheng Fan, Juan Liu, Kevin R Mott, Akash Kaushik, Hieu Vu, Jason W Locasale, Samuel K McBrayer, Ralph J DeBerardinis, Charles M Perou, Qing Zhang. Metabolite profiling and RNA-seq identifies novel metabolomic-genomic biomarker and therapeutic options for rapidly proliferating breast cancers [abstract]. In: Proceedings of the 2021 San Antonio Breast Cancer Symposium; 2021 Dec 7-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2022;82(4 Suppl):Abstract nr P5-05-01.

Abstract P5-13-20: Identifying a metabolite signature that correlates with tumor proliferation in early-stage breast cancer patients treated with CDK4/6 inhibitors from matched plasma and serum samples

Abstract Background: Cyclin-dependent kinase 4/6 inhibitors (CKD4/6i) have demonstrated clinical utility extending progression-free survival (PFS) and overall survival (OS) for advanced hormone receptor positive and HER2 negative (HR+/HER2-) breast cancer patients. The efficacy in early-stage breast cancer (eBC) is unclear, with conflicting results from adjuvant CDK4/6i trials on invasive disease-free survival. Thus, there is a critical need to identify biomarkers of response (BoR) to determine which, if any, eBC patients could benefit from this treatment. This BoR could also stratify advanced BC patients for likelihood to respond to CDK4/6i. Metabolism is influenced by both genome and environment, and changes in the metabolome can be correlated with drug responsiveness. Thus, metabolite BoRs may serve to identify eBC patients for which CDK4/6i would offer a therapeutic benefit.Methods: Plasma and serum samples from 50 early-stage ER+/HER2- breast cancer patients, treated with neoadjuvant CDK4/6 inhibitor palbociclib (palbo) and aromatase inhibitor (AI) anastrozole on NeoPalAna trial (ClinicalTrials.gov identifier NCT01723774), were collected from treatment-naïve patients (BL) and 3 consecutive time points: anastrozole,1 mg daily for 4 weeks (C1D1), anastrozole plus palbo,125 mg daily, for 15 days (C1D15), and for 4-5 months before surgery (SURG). Metabolites were extracted from all samples via methanol and chloroform precipitation and quantified using an unbiased, non-destructive, nuclear magnetic resonance (NMR)-based profiling platform (Olaris®, Inc., Waltham, MA). Statistical analysis and machine learning was used to identify differential metabolites and generate predictive models. A separate validation set of samples was collected from a subset of patients (N=6) who received an additional cycle of palbo treatment prior to surgery to assess model accuracy. Results: Non-parametric differential expression analysis of BL/C1D1, BL/C1D15, and C1D1/C1D15 identified 53 ,97, and 90 differential NMR resonances in plasma (p<0.05) and 36, 34, and 25 differential NMR resonances in serum (p<0.05), respectively. Based on the proliferative marker Ki67 levels at C1D15, 37 patients were classified as responders (Ki67≤2.7%) and 6 patients as non-responders (Ki67>2.7%). Analysis of the responder (R) and non-responder (NR) groups identified that 13 plasma and 14 serum resonances (21 unique resonances and 6 overlapping) were differentially expressed (p<0.05) at C1D1. Many of the differential resonances could be mapped back to amino acid metabolites including several branched chain amino acids such as leucine, valine, and isoleucine, and positively charged amino acids such as lysine. A Olaris® BoR score was generated using 5 differential resonances that had an AUC of 0.931 (training set) and 100% accuracy when predicting palbo-response in a blinded test set (N=6).Conclusion: The differential metabolites identified from matching plasma and serum samples suggest that, compared to serum, plasma has a better representation of the metabolic changes associated with palbo treatment-response. While comparing samples from R and NR patients, amino acids were found to be consistently altered in both serum and plasma before palbo treatment. In addition, a BoR model based on select metabolites could precisely stratify palbo-response in a blinded dataset. A larger independent validation cohort is ongoing. Citation Format: Chen Dong, Shana Thomas, Chandrashekhar Honrao, Leonardo O. Rodrigues, Nathalie Tessier, Bo Zhang, Souzan Sanati, Kiran Vij, Brenda J. Ernst, Karen S. Anderson, Mateusz Opyrchal, Foluso Ademuyiwa, Lindsay L. Peterson, Matthew P. Goetz, Donald Northfelt, Elizabeth O'Day, Cynthia Ma. Identifying a metabolite signature that correlates with tumor proliferation in early-stage breast cancer patients treated with CDK4/6 inhibitors from matched plasma and serum samples [abstract]. In: Proceedings of the 2021 San Antonio Breast Cancer Symposium; 2021 Dec 7-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2022;82(4 Suppl):Abstract nr P5-13-20.

Abstract P5-06-09: Rivaroxaban targets the procoagulant tumour microenvironment in vitro and thereby inhibits breast cancer progression

Abstract Introduction: Breast cancer patients have a four-fold increased risk of developing a venous thromboembolism and those that do have a significantly increased risk of mortality despite adjusting for cancer stage. Tissue Factor (TF) is expressed by breast cancer-associated fibroblasts as well as breast cancer epithelial cells, and at significantly higher levels than by normal breast fibroblasts. TF signals via PAR-1 and PAR-2 to induce proliferation, invasion, angiogenesis and metastasis. Rivaroxaban is a licensed oral anticoagulant that inhibits the TF-Factor VIIa-Factor Xa complex and could therefore be repurposed to target the procoagulant tumour microenvironment in breast cancer. We hypothesise that a procoagulant microenvironment will induce breast cancer progression in vitro and that these promoting effects will be inhibited by anticoagulants, including Rivaroxaban. Methods: Lentivirally transduced TF over-expressing fibroblasts (TFF) and their control (CF) or conditioned media (TFFCM and CFCM), were cultured with oestrogen receptor positive (MCF-7), triple negative (MDA-MB-231) and HER2 positive (BT474) breast cancer cells, in the presence or absence of Rivaroxaban or anti-TF antibody 10H10. Proliferation (sulforhodamine-B/EdU assay), migration (scratch/transwell assay) and stem cell activity (mammosphere forming efficiency (MFE) assay) were assessed. The underlying mechanism was analysed with western blotting and quantitative PCR. The TFFCM and CFCM were analysed with cytokine arrays, enzyme-linked immunosorbent assays (ELISA) and mass spectrometry whilst the TFF and CF were analysed using RNA sequencing. Results: 3D co-culture of MCF-7s with TFF as compared to CF promoted cancer cell migration (p=0.04) and stem cell activity (MFE: p<0.0001), with these effects abrogated by Rivaroxaban (migration: p= 0.0341, MFE: p=0.0003) and 10H10 (migration: p=0.01, MFE: p=0.0028). TFFCM promoted proliferation, migration and stem cell activity in MCF-7 cells (p<0.05) compared to CFCM, with these effects abrogated by Rivaroxaban (migration, MFE: p<0.05) and 10H10 (proliferation, migration, MFE: p<0.05). The cancer-promoting effects of TFFCM in MCF-7 cells was associated with a decrease in CXCL8 (p<0.0001) and an increase in VEGFA (p<0.05) when compared to CFCM; changes which were reversed by Rivaroxaban (VEGFA: p<0.03) and 10H10 (CXCL8, VEGFA: p<0.05). TFFCM also promoted stem cell activity in both MDA-MB-231 (p<0.006) and BT474 (p<0.002) cell lines compared to CFCM. This increased MFE in BT474 was abrogated by Rivaroxaban (p=0.02) and 10H10 (p<0.005) and associated with an increase in VEGFA (p<0.001) and CXCL8 (p<0.0001). Cytokine array and ELISA analysis of TFFCM versus CFCM indicated increased levels of IL-10 (p<0.006), GRO-α (p<0.0001) and IL1-α (p<0.02). GRO-α and IL1-α production was inhibited by fibroblast treatment with Rivaroxaban (GRO-α, p<0.0001 and IL1-α, p<0.001) and 10H10 (p<0.001). TFFCM versus CFCM had elevated levels of plasminogen activator inhibitor 1, integrin linked kinase, thrombospondin 4 and α -fetoprotein on mass spectrometry. RNA sequencing analysis of the TFF as compared to the CF revealed upregulation of AEBP1 (adipocyte enhancer binding protein 1) (p<0.01), NKX2-5 (homeobox protein NKX-2.5) (p<0.0001), TRIM58 (tripartite motif containing 58) (p<0.0001) and KRT81 (keratin 81) (p<0.0001), all of which were decreased by fibroblast treatment with Rivaroxaban (NKX2-5: p<0.01, KRT81: p<0.0001). Conclusion: A procoagulant microenvironment promotes proliferation, migration and stem cell activity in breast cancer in vitro, with these effects abrogated by Rivaroxaban. Targeting the procoagulant tumour microenvironment is a promising future cancer treatment. Citation Format: Emma L Blower, John Castle, Angelica Santiago-Gomez, Robert Clarke, Cliona C Kirwan. Rivaroxaban targets the procoagulant tumour microenvironment in vitro and thereby inhibits breast cancer progression [abstract]. In: Proceedings of the 2021 San Antonio Breast Cancer Symposium; 2021 Dec 7-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2022;82(4 Suppl):Abstract nr P5-06-09.