Hormone Receptor-positive Metastatic Breast Research Articles

Prognostic and Predictive Significance of HER2-low Expression in Metastatic Hormone Receptor Positive Breast Cancer Patients Receiving CDK4-6 Inhibitor Therapy

Objective: This study aimed to analyze the predictive and prognostic value of HER2-low expression in hormone receptor (HR) positive human epidermal growth factor receptor-2 (HER2) negative metastatic breast cancer patients receiving cyclin-dependent kinase-4/6 inhibitor (CDK4/6i) therapy. Methods: This retrospective study included patients who received CDK4/6i plus endocrine therapy (ET). The pathological and clinical characteristics and survival times of the patients were compared and analyzed. Results: Our study included 122 patients. There were HER2-zero 88(72%) and HER2-low 34 (28%) patients. The median progression free survival (mPFS) of all patients who received CDK4/6i+ET was 21 (95% confidence interval (CI),18.5–23.5) months, while mPFS was not reached in the HER2-zero group, and mPFS in the HER2-low group was 12 (95%CI, 6.8–17.1) months (p=0.001). The mPFS was shorter in patients with primary endocrine resistance (6 vs. 21 months, p=0.001). There was a change in the HER2-low status of 26(45%) patients with recurrence compared to the first biopsy. In the HER2-zero and HER2-low groups, 22(25%) and 24(71%) patients, respectively, progressed with CDK4/6i+ET (p=0.001). Estrogen receptor (ER) levels less than and greater than 50% resulted different mPFS (6 and 21 months, respectively) (p=0.025). Median PFS differed based on CDK4/6i+ET combination, treatment line, and best treatment response (all p=0.001). In multivariate analysis, HER2 status(p=0.018), chemotherapy status(p=0.006), best response status with CDK4/6i (p=0.001) for PFS, and best response status with CDK4/6i therapy (p=0.007) for OS were significant. Conclusions: In patients with HR+HER- metastatic breast cancer receiving CDK4/6i therapy, the duration of mPFS was lower in the HER2-low group than that in the HER2-zero group. HER2-low expression is a predictive biomarker of response to CDK4/6 inhibitor therapy.

Abstract PO4-04-10: Selection criteria for second-line treatments after Cyclin-dependent kinase 4/6 inhibitors failure in Hormone Receptor positive metastatic breast cancer patients: the preliminary experience of the HERMIONE-13 trial

Abstract BACKGROUND: Cyclin-dependent kinase 4/6 inhibitors (CDK4/6i) represent the standard of care for hormone receptor-positive (HR+)/Human Epidermal Growth Factor 2 negative (HER2-) metastatic breast cancer (MBC); no consensus actually exists for second-line (2L) treatments. The HERMIONE-13 trial evaluates the adopted therapeutic options after CDK4/6i failure and the potential factors that influence these choices. METHODS: HERMIONE-13 is a retrospective and prospective multicentric observational trial involving 15 Italian Centers. Information about therapies after disease progression to CDK4/6i from January 2016 until December 2020 was collected for the present analysis, aiming at evaluating which are the clinical criteria guiding the choice of 2L therapy after CDK4/6i failure. RESULTS: 114 pts have been enrolled: 14 pts (12.3%) received neoadjuvant treatment, 80 (70.2%) adjuvant therapy, of whom 46 (57.5%) chemotherapy (CT)+ endocrine therapy (ET) and 33 (41.3%) ET alone. Among the 79 pts treated with adjuvant ET, 44 pts (55.7%) showed primary or secondary endocrine resistance. 39 pts (34.2%) received first-line (1L) CDK 4/6i in combination with Fulvestrant (F), the remaining with aromatase inhibitors (AIs). Median duration of 1L CDK4-6i was 10.4 months (interquartile range (IQR) 4.7-17.4). Median age at 2L start was 59.5 years (IQR 51-68). As 2L therapy, 79 pts (69.3%) received CT±targeted therapy (TT) and 35 (30.7%) ET±TT. Visceral metastases at the beginning of 2L were documented in 46 pts (40.4%). Comorbidities were present in 48 (42.9%) of the pts, mainly cardiac diseases (25, 22.3%); 71 pts (63.4%) showed a 1L-related adverse events (AEs) (NA: 2 pts), mainly hematological toxicity (60, 53.6%). Among 79 pts treated with adjuvant ET, 60 (76%) were addressed to 2L CT±TT, while 19 (24%) to an ET-based therapy. A longer duration of 1L was observed in pts treated with 2L ET±TT (12.5 months, 7.2-22.2) in comparison with those treated with CT±TT (8.7 months, 4.2-14.9). The majority of pts treated with 1L F+CDK 4/6i underwent 2L CT-based treatment, as summarized in Table 1. Analyzing the population treated with 2L CT±TT, we observed that median age was 58 years (49-67), slightly lower than that of pts treated with ET±TT (63, 54-69). Among pts with visceral metastases, 30 pts (65.2%) were addressed to 2L CT±TT and 16 pts (34.8%) to ET±TT; CT±TT was also the favorite choice for the majority of pts with concomitant disease (33, 68.8%) and with previous CDK4/6i-related toxicity (47, 66.2%). Among age, visceral metastases, comorbidities, adjuvant treatment, duration of 1L therapy, use of 1L F and 1L related AEs, univariate analysis revealed that previous use of adjuvant ET (p=0.021), 1L F (p< 0.001), duration of 1L treatment (p=0.043) and age at 2L start (p=0.053) are all factors influencing the choice of 2L therapy. At the multivariate logistic regression model, only the use of F in association with CDK4/6i as 1L treatment and age at 2L therapy were associated with the choice between ET±TT vs CT± TT, with a backward selection, as summarized in table 2. CONCLUSION: The preliminary results of our study suggest that previous F in combination with CDK4/6i is a statistically confirmed predictor of choice for subsequent CT, whereas the increase in age is significantly associated with the choice for ET. Further research is needed to investigate the optimal sequencing of treatments following CDK4/6i and to determine predictive factors of response to 2L therapies. Table 1 2L choices in 1L F+CDK 4/6i pts. Table 2 Statistical model with the significant variables. Citation Format: Marina Elena Cazzaniga, Emanuela Rossi, Antonella Turla, Massimo Ambroggi, Alice Baggi, Rossana Berardi, Fulvio Borella, Serena Capici, Federica Cicchiello, Luigi Coltelli, Ugo De Giorgi, Antonella Ferro, Ornella Garrone, Monica Giordano, Elisabetta Landucci, Marco Mazzotta, Gabriella Moretti, Raffaella Palumbo, Francesca Fulvia Pepe, Patrizia Vici, Fable Zustovich, Viola Cogliati. Selection criteria for second-line treatments after Cyclin-dependent kinase 4/6 inhibitors failure in Hormone Receptor positive metastatic breast cancer patients: the preliminary experience of the HERMIONE-13 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 PO4-04-10.

Abstract PO4-12-10: Sacituzumab Govitecan Toxicity Outcomes In Arab Patients with Metastatic Breast Cancer

Abstract Background: The Arab population is often underrepresented in clinical trials, which limits our knowledge of drug safety and efficacy in this population due to genetic variability that can affect drug metabolism and adverse events (AEs). Sacituzumab govitecan (SG) is approved for metastatic triple negative breast cancer (mTNBC), and recently hormone receptor positive metastatic breast cancer (HR+ MBC). Here we report toxicity outcomes and RDI in Arab population. Methods: A multi-center retrospective study of patients with mTNBC and HR+ MBC treated with SG from January 2021 until May 2023 was conducted. Demographics and clinical variables including site of metastases, prior lines of therapy, relative dose intensity (RDI), adverse events (AEs) were collected. For continuous variables, Mann-U Whitney and Kruskal Wallis tests were used to compare mean, median, standard deviation, and range. For categorical variables, Fisher's exact tests and Pearson Chi-square tests were used. SAS v9.4 was used to perform statistical analysis at a significance level of 0.05. Results: A total of 35 patients was enrolled. The median age at diagnosis of metastatic breast cancer was 39 years interquartile range (IQR) (35, 44). The median age at starting SG was 44.5 years IQR (38,50). The majority of patients (65%) had an Eastern Cooperative Oncology Group performance status (ECOG PS) of 1, (21%) of the patients had an ECOG PS of 0, and (15%) had an ECOG PS of 2. 48.6% received 3 or more lines of prior therapy in the metastatic setting while 51.4% received less than 3 lines of prior therapy in the metastatic setting. 77.1% of patients had 3 or > organs involved with metastatic disease, while 22.9% of patients had < 3 organ involved. The most common metastatic sites were the lung (54.3%), liver (62.9%), bone (68.6%), and brain (45.7%). The median RDI of SG was 100% for a sample of 34 individuals, with one missing value IQR (94%,100%). The rate of overall AEs was (77.1%), with the most common AEs being; neutropenia (44.4%), diarrhea (37.0%), anemia (22.2%), thrombocytopenia (22.2%), nausea (14.8%), and vomiting (7.4%). Garde 3 AEs rate was 34.3%. The most common grade 3 adverse events were diarrhea (11.4%), fatigue (5.7%), and alopecia (2.9%). The rate of treatment interruption and dose reduction due to AEs was 31.4%. Only one patient (2.9%) had to discontinue SG treatment due to adverse events. Conclusion: The study suggests that SG is safe and well-tolerated in young Arab patient population. The sample consisted of relatively young patients, which may reflect a reluctance to prescribe SG to older Arab patients. Caution should be exercised when generalizing the findings due to limitations of the study. Further studies with larger sample sizes are needed to validate these findings and to explore the efficacy of SG in Arab patients with mTNBC and HR+ MBC. Citation Format: Maymouna Altarturi, Kausar Suleman, Adhar AlSayed, Humaid Al-Shamsi, Aydah Alawadhi, Bassam Basuliamn, Sabah Alaklabi. Sacituzumab Govitecan Toxicity Outcomes In Arab 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 PO4-12-10.

Abstract PO1-04-14: PLK1 Inhibitor Onvansertib Extends the Response and Overcomes Resistance to Paclitaxel in Palbociclib-resistant HR+ Breast Cancer Patient-derived Xenografts

Abstract Paclitaxel is a first-line chemotherapy for hormone receptor positive (HR+) metastatic breast cancer patients after progression on a combination of endocrine and CDK4/6 inhibitor therapy. Response rate to paclitaxel ranges between 20-40%, and most patients progress due to intrinsic or acquired resistance, leaving them with limited treatment choices. Therapeutic strategies to overcome paclitaxel resistance and extend its clinical benefit are urgently needed. Paclitaxel-resistant tumors have increased expression of polo-like kinase 1 (PLK1), a serine-threonine-protein kinase that regulates mitosis and cell cycle progression. PLK1 has also been shown to mediate resistance to CDK4/6 inhibitor palbociclib in HR+ breast cancer. Onvansertib is an orally bioavailable, highly potent, and selective PLK1 inhibitor in clinical development. Onvansertib exhibited synergy in vitro and showed potent anti-tumor activity in vivo in combination with paclitaxel in ovarian cancer and triple negative breast cancer (TNBC) preclinical models. A phase 1b/2 clinical trial is ongoing to evaluate the safety and efficacy of onvansertib plus paclitaxel in advanced TNBC (NCT05383196). Here we investigated whether the potential of onvansertib and paclitaxel combination could be extended to HR+ breast cancer. Co-treatment with onvansertib and paclitaxel synergistically inhibited the viability of HR+ breast cancer cell lines (MCF-7, T-47D, EFM-19, CAMA-1, ZR-75-1). Compared to the single agents, the combination induced increased apoptosis. The combination was further tested in vivo in patient-derived xenograft (PDX) models. To this end, six HR+ breast cancer PDXs with intrinsic or acquired resistance to palbociclib were established from primary breast tumors (n=1) or metastatic bone biopsies (n=5). The PDX tumors were engrafted subcutaneously in nude mice and the effect of onvansertib (45 mg/kg, oral, 5 days a week) and paclitaxel (15-25mg/kg, IP, once a week) was investigated as monotherapy and in combination. The combination of onvansertib and paclitaxel was tolerated with minimal toxicity and showed enhanced anti-tumor activity compared to either agent alone in all 6 PDX models. Paclitaxel and onvansertib monotherapies had little to no anti-tumor activity in 4 out of 6 PDX models (HBCx124palboR25, HBCx-139 palboR5, HBCx-202 and HBCx-131). Importantly, the combination of onvansertib and paclitaxel induced strong anti-tumor activity in these 4 models. Tumor regression was observed in the 2 PDX models with acquired resistance to palbociclib, HBCx-124palboR25 and HBCx-139palboR5, with 62% (5/8 mice) and 54% (6/11 mice) complete response rates, respectively. The combination showed tumor stasis in HBCx-131 and induced tumor regression in HBCx-202, established from patient resistant to palbociclib. In 2 of the 6 PDXs, paclitaxel exhibited anti-tumor activity, inducing tumor regression in HBCx-137paboR26 and tumor stasis in HBCx-86. In comparison, the combination of onvansertib and paclitaxel had greater activity, inducing tumor regression in both models with a higher rate of complete response than the monotherapy. Complete response rates for paclitaxel versus the combination were 62% and 100% in the HBCx-137palboR26 model and 0% and 67% in the HBCx-86 model. Notably, the antitumor activity of the onvansertib and paclitaxel combination was very durable, showing a robust delay in tumor relapse after stopping therapy. Together, our data strongly support that combining paclitaxel with the PLK1 inhibitor onvansertib extends its benefit and overcomes paclitaxel resistance, and represents a promising therapeutic strategy for HR+ breast cancer patients after progression on a combination of endocrine and CDK4/6 inhibitor therapy. Citation Format: Maya Ridinger, Pierre Painsec, Sreeja Sreekumar, Dalia Gonzalez, Davis Klein, Laura Sourd, Elodie Montaudon, Paul Cottu, Tod Smeal, Elisabetta Marangoni. PLK1 Inhibitor Onvansertib Extends the Response and Overcomes Resistance to Paclitaxel in Palbociclib-resistant HR+ Breast Cancer Patient-derived Xenografts [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 PO1-04-14.

Abstract PO1-04-13: CARDIAC-STAR: Prevalence of Cardiovascular Comorbidities in Hormone Receptor Positive Human Epidermal Growth Factor Negative metaStatic breasT cAnceR

Abstract BACKGROUND: Baseline cardiovascular (CV) comorbidities can impact treatment decisions for patients (pts) with metastatic breast cancer (mBC). There is limited data on the prevalence of CV comorbidities at the time of hormone receptor positive human epidermal growth factor negative (HR+/HER2-) mBC diagnosis. The primary objective of this study is to describe the prevalence of pre-existing CV comorbidities in women and men with newly diagnosed HR+/HER2- mBC. A secondary objective is to describe the first documented cancer treatment for patients with or without CV comorbidities. METHODS: Women and men ≥18 years of age with newly diagnosed HR+/HER2- mBC from 01/2016 to 12/2021 were included in this retrospective, observational study using the Merative™ Marketscan® Commercial and Medicare Databases which is nationally representative. Evidence of mBC was defined as having two or more claims containing one or more diagnosis code for BC at least 30 days apart and at least two secondary malignancy diagnosis codes. Previously published claims-based algorithms were used to identify the HR+/HER2- molecular subtype. The index date is defined as the date of first mBC diagnosis. Patients had a 1-year pre-index period for observation of CV comorbidities and a 6-month post-index period for first documented cancer treatmen RESULTS: We identified 5,452 pts with HR+/HER2- mBC, 99% of whom were female, with a median age of 58 years (yrs) (IQR 13), and 83% having a health plan from an employer; i.e., are still employed. At mBC diagnosis, 3,335 pts (61.2%) had at least one pre-existing CV comorbidity (see Table). The most reported CV comorbidities were hypertension (50.7%), hyperlipidemia (33.7%), and diabetes (19.2%). The median age of pts with or without a CV comorbidity was 61 and 52 yrs, respectively; 16.2% with CV comorbidities were ≥75 yrs. Additional data, including first documented treatment, will be presented at the meeting. CONCLUSIONS: The prevalence of CV comorbidities in pts with HR+/HER2- mBC is often underrecognized. In this analysis, more than half of pts had at least one CV comorbidity by mBC diagnosis. CV comorbidities may impact treatment decisions in the HR+/HER2- mBC population, particularly in older pts. Table Citation Format: Susan Dent, Avirup Guha, Heather Moore, Rachael McCaleb, Irene Arias, Stella Stergiopoulos, Benjamin Li, Doris Makari, Michael Fradley. CARDIAC-STAR: Prevalence of Cardiovascular Comorbidities in Hormone Receptor Positive Human Epidermal Growth Factor 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 PO1-04-13.

Abstract PO4-19-09: Comparison of clinical efficacy between letrozole + ribociclib vs. Fulvestrant + letrozole + ribociclib in Hormone receptor positive, HER2 negative metastatic breast cancer – a randomized, phase 2 study (KCSG BR22-20)

Abstract Background The current standard first-line treatment in advanced or metastatic hormone receptor (HR) positive breast cancer is combination of aromatase inhibitor (AI) and cyclin D kinase-4/6 (CDK4/6) inhibitor. Emergence of ESR1 mutation is a key mechanism of resistance during first-line AI + CDK4/6 inhibitor treatment. Selective estrogen receptor downregulator (SERD) such as elacestrant or fulvestrant is suggested as second-line treatment, but the progression-free survival (PFS) outcome of second-line treatment is reported up to 3.78 months in patients who harbour ESR1 mutation. Recently, early switch of AI + CDK4/6 inhibitor to fulvestrant + CDK4/6 inhibitor at the point of rising ESR1 mutation showed prolongation of PFS up to 6 months, suggesting there may be a new strategy to overcome ESR1 mutation by modifying endocrine treatment. SWOG S0226 suggested that upfront fulvestrant + AI is feasible and associated with prolongation of PFS compared to AI alone. After CDK4/6 inhibitor has been introduced to standard treatment, there were no trials evaluating the role of AI + fulvestrant with CDK4/6 inhibitor. In this trial, we investigated triplet combination of AI + fulvestrant + CDK4/6 inhibitor as a treatment to overcome resistant mechanism such as development of ESR1 mutation when compared to current standard treatment of AI + CDK4/6 inhibitor as the first line treatment in HR positive advanced or metastatic breast cancer. Methods This study is a randomized, multicenter, open-label phase II trial comparing AI plus ribociclib versus AI, fulvestrant plus ribociclib in HR-positive advanced or metastatic breast cancer. De novo patients or patients with treatment-free interval (TFI) over 12 months after completion of adjuvant AI can be enrolled. If patients received adjuvant tamoxifen, patients with TFI less than 12 months can be also enrolled for the study. Prior endocrine treatment or cytotoxic chemotherapy at metastatic setting was not allowed. In cases of premenopausal women, gonadotropin releasing hormone agonist (GnRHa) is administered every 4 weeks with treatment. A total of 202 patients will be enrolled for 30 months, and 30 months of follow-up is planned after last-patient enrolment. Patients are randomized to 1:1, stratified based on the presence of visceral metastasis and previous administration of adjuvant AI. Study treatment of the control arm consists of letrozole 2.5mg daily with ribociclib 600mg (Day 1-Day 21 daily by month, 1 week off) until progression. In the experimental arm fulvestrant 500mg intramuscular injection is added (Day 1, Day 15, Day 29 and every 4 weeks thereafter) until progression. For exploratory analysis of ESR1 mutations, circulating tumor DNA (ctDNA) is collected at baseline, and then every 3 months thereafter. Primary endpoint is to evaluate the difference of 24-months PFS rate. Secondary endpoint included PFS, overall survival (OS), emergence and frequency of ESR1 mutation, overall response rate (ORR) and clinical benefit rate (CBR). The analysis is planned after completion of 30 months of follow-up, which provides approximately 90% power to detect superiority 24 months-PFS rate assumed by Kaplan-Meier curve of letrozole + fulvestrant + ribociclib versus letrozole + ribociclib using a log-rank test, assuming a hazard ratio of 0.64 at a two-sided alpha of 0.2. Citation Format: Jieun Lee, Su-Jin Koh, Jung Hye Kwon, Gun Min Kim, Jee Hung Kim, Ho Young Kim, Hee-Jun Kim, Keon-Uk Park, Kyong Hwa Park, Yeon Hee Park, Mi Sun Ahn, Hee Kyung Ahn, Gyeong-Won Lee, Kyoung Eun Lee, Kyung-Hun Lee, Moon Hee Lee, Sung Sook Lee, Jung Lim Lee, Kyung Hae Jung, Joo Young Jung, In Hae Park. Comparison of clinical efficacy between letrozole + ribociclib vs. Fulvestrant + letrozole + ribociclib in Hormone receptor positive, HER2 negative metastatic breast cancer – a randomized, phase 2 study (KCSG BR22-20) [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 PO4-19-09.

Abstract 6448: Prevalence and spectrum of PIK3CA mutations in Indian breast cancer patients

Abstract Introduction: Breast carcinomas are the most commonly encountered cancers in women globally. Due to advancements in cancer research the death rate in breast cancer has reduced by 41% since 1989; however, the downward trend has recently slowed emphasizing on new breast carcinomas treatment discovery. Breast carcinomas are molecularly categorized by the expression of specific hormone receptors, as well as the overexpression of human epidermal growth factor receptor 2 (HER2). Approximately 80% of breast cancers have been termed HER2-negative, but researchers now recognize that 55% of all breast cancers may actually contain low levels of HER2, levels that are still targetable by HER2-directed therapies such as Trastuzumab Deruxtecan. The phosphoinositide 3-kinase (PI3K) pathway is the most frequently altered pathway in HR+/HER2- breast carcinomas and is associated with tumor development, disease progression, and endocrine resistance. Methodology: The study entailed histological verification of breast cancer, and thence the extraction of DNA from formalin fixed, paraffin embedded (FFPE) tissue samples. A tumor content of 20% was the cut-off for performing the PIK3CA mutation analysis. The DNA was subjected to mutation analysis using a FDA-approved PIK3CA RGQ Real Time PCR based assay that has ability to detect 11 mutations in the PIK3CA gene encompasses exons 7, 9, and 20. Results: A total of 696 breast cancer patient samples (with more than 20% neoplastic tissue content) were screened for PIK3CA mutations. Of these, 275 (39.5%) (Male: Female ratio = 1:26.5; Age range = 30-80 years) harbored a mutation of the PIK3CA gene. Females were commonly affected; however, the males were also found to have PIK3CA mutation. The most common affected age group is from 41 to50 years. The positivity rate exon wise were as follows: Exon 7 (1.45%), Exon 9 (39.27%), and Exon 20 (44.27%). There was presence of co-mutations amongst different exons, which comprised of 4%. HR+/HER2- (50%) followed by HR-/Her2- (3.27%) and HR+/HER2+ (1.82%). The HER2- cases (Score 1 and Score 2) by IHC were considered as low positive cases (14%). Conclusion: This is one of the largest study to report frequency of PIK3CA mutations in the Indian population. PIK3CA mutations are commonly encountered in hormone receptor positive and HER2 Negative metastatic breast cancer and have shown promising clinical outcome leading to better prognosis. It provides insights into genotyping each breast cancer and exploits the usage of PI3K/AKT inhibitors together with other combination of drugs for effective target treatment strategies. Keywords: Breast carcinomas, PIK3CA, HR/HER2, Targeted Therapeutic Citation Format: Rahul Katara, Tina Bhardwaj, Monika Rawat, Deepak K. Sharma, Vipin Kumar, Shivani Sharma, Vaishali Dagar, Pawan Kumar, Sambit K. Mohanty. Prevalence and spectrum of PIK3CA mutations in Indian breast cancer patients [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2024; Part 1 (Regular Abstracts); 2024 Apr 5-10; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2024;84(6_Suppl):Abstract nr 6448.

Radiomic predicts early response to CDK4/6 inhibitors in hormone receptor positive metastatic breast cancer

The combination of Cyclin-dependent kinase 4/6 inhibitors (CDK4/6i) and endocrine therapy (ET) is the standard of care for hormone receptor-positive (HR + ), human epidermal growth factor receptor 2-negative (HER2-) metastatic breast cancer (MBC). Currently, there are no robust biomarkers that can predict response to CDK4/6i, and it is not clear which patients benefit from this therapy. Since MBC patients with liver metastases have a poorer prognosis, developing predictive biomarkers that could identify patients likely to respond to CDK4/6i is clinically important. Here we show the ability of imaging texture biomarkers before and a few cycles after CDK4/6i therapy, to predict early response and overall survival (OS) on 73 MBC patients with known liver metastases who received palbociclib plus ET from two sites. The delta radiomic model was associated with OS in validation set (HR: 2.4; 95% CI, 1.06–5.6; P = 0.035; C-index = 0.77). Compared to RECIST response, delta radiomic features predicted response with area under the curve (AUC) = 0.72, 95% confidence interval (CI) 0.67–0.88. Our study revealed that radiomics features can predict a lack of response earlier than standard anatomic/RECIST 1.1 assessment and warrants further study and clinical validation.

Ribociclib-induced pulmonary infiltrates in a patient with breast cancer.

Breast cancer is the most common cancer in women worldwide. One of the treatment choices used in hormone receptor positive and HER2 expression negative metastatic breast cancer is cyclin dependent kinase 4/6 (CDK) inhibitors. There are differences between the effects and adverse events of all CDK4/6 inhibitors. In this report, we describe a 48-year-old female patient with breast cancer who developed pulmonary infiltrates secondary to drug in the second year of follow-up with Ribociclib therapy. The patient who used ribociclib for metastatic breast cancer was admitted to hospital for routine controls. The patient, in the eighth month of ribociclib treatment, had no active complaints. In the patient's imaging, pulmonary infiltrates were detected in the lower lobe of the right lung. The drug was continued in the patient who was asymptomatic and had no infectious signs. Pneumonitis, interstitial lung disease, tuberculosis, COVID-19 and other viral agents were excluded in the patient. A minimal increase in pulmonary infiltrates was observed in the follow-up. However, the patient who does not have any complaints is currently continuing ribociclib at 600 mg/day. The pulmonary infiltrates that developed in the patient increased with continued use of ribociclib, suggesting that this was due to ribociclib. The Naranjo Adverse Drug Reaction Probability Scale also indicates probable relationship.

Comparing the prognostic and predictive utility of serum thymidine kinase 1 and CA 15-3 in patients with hormone receptor positive metastatic breast cancer starting first-line endocrine therapy in SWOG S0226.

1076 Background: Serum levels of thymidine kinase 1 activity (TKa), a fundamental enzyme in DNA synthesis and cellular proliferation, are prognostic of benefit from endocrine therapy (ET) in patients (pts) with hormone receptor positive (HR+) metastatic breast cancer (MBC) who participated in SWOG S0226, a prospective randomized trial comparing anastrozole vs. anastrozole and fulvestrant given in 4 week cycles. The assay for TKa (DiviTum TKa) was FDA approved in July 2022 for monitoring of postmenopausal HR+ MBC pts. The assay for circulating MUC1, CA 15-3, is routinely utilized in monitoring of pts with MBC. We compared the prognostic and predictive utilities of CA 15-3 and TKa in pts from S0226. Methods: TKa was measured in 1725 sera from 432 pts while CA 15-3 was measured in 1298 sera from 326 pts at baseline (BL), cycles 2, 3, 4 and 7. Prespecified cutoff levels of ≥ 250 DuA and ≥ 30 U/mL were considered high for TKa and CA 15-3 respectively. Progression-free survival (PFS) and overall survival (OS) were analyzed by Kaplan-Meier and Cox regression stratified by prior adjuvant tamoxifen use. To examine markers over time, a landmarked Cox regression analysis was done starting at initiation of Cycle 4. BL and recent (Cycle 3 if available, otherwise Cycle 2) markers were assessed for simultaneous prediction of subsequent PFS and OS. Results: BL TKa was elevated in 190/432 (44%) and CA 15-3 was elevated in 254/326 (78%). Agreement between assays was 53%. Pts with high versus low BL TKa had significantly worse PFS (median 11.6 vs. 17.2 months, HR = 1.68, 95% confidence interval (CI) 1.37-2.06) and OS (median 34 vs. 58 months, HR = 2.16, 95% CI 1.73-2.70) whereas pts with high versus normal BL CA 15-3 had no significant difference in PFS (median 13.6 vs. 16.1 months, HR = 1.23, 95% CI 0.93-1.62) but worse OS (median 45 vs. 66 months, HR = 1.82, 95% CI 1.30-2.53). A multivariable Cox model with both markers shows only TKa as being prognostic for PFS (TKa HR = 1.61, 95% CI 1.28-2.03; CA 15-3 HR = 1.21, 95% CI 0.91-1.59), but both prognostic for OS (TKa HR = 2.09, 95% CI 1.61-2.71; CA 15-3 HR = 1.70, 95% CI 1.22-2.38). In the landmarked multivariable analysis, positive TKa at BL was a strong predictor of PFS (HR = 1.65, 95% CI 1.28-2.14), but recent TKa was not (HR = 1.25, 95% CI 0.93-1.68). In contrast, positive CA15-3 at BL was not a predictor of PFS (HR = 0.73, 95% CI 0.46-1.17), but recent CA15-3 was (HR = 1.97, 95% CI 1.26-3.06). Conclusions: BL TKa is highly prognostic in pts with HR+ MBC initiating first-line systemic ET, with low BL TKa conferring superior prognosis. In contrast, CA 15-3 is only modestly prognostic at BL, but is prognostic after 3 cycles of treatment. These hypothesis-generating data suggest further study is needed to determine how these biomarkers should be employed in a complementary manner to monitor response to systemic therapy in HR+ MBC. Clinical trial information: NCT00075764 .

Survival Outcomes in Patients With Hormone Receptor–Positive Metastatic Breast Cancer With Low or No ERBB2 Expression Treated With Targeted Therapies Plus Endocrine Therapy

Approximately 45% to 60% of hormone receptor (HR)-positive metastatic breast cancer (mBC) shows a low-level expression of ERBB2. Low ERBB2 expression is defined as ERBB2 immunohistochemical expression of 1+ or 2+ with a negative ERBB2 amplification by in situ hybridization. The efficacy of the antibody-drug conjugate trastuzumab deruxtecan in low-ERBB2, HR-positive mBC has been practice changing. However, there are conflicting data on the prognostic value of low ERBB2 expression in HR-positive mBC and whether low ERBB2 expression is a separate entity. To examine whether outcomes differ by immunohistochemical analysis for patients with HR-positive mBC with low ERBB2 expression vs those without ERBB2 expression when treated with targeted therapy (TT) plus endocrine therapy (ET). This single-institution cohort study used prospectively collected electronic data from the MD Anderson Cancer Center for patients with a diagnosis of HR-positive mBC treated with ET in combination with a TT (cyclin-dependent kinase 4/6 inhibitors [CDK4/6is], everolimus, or alpelisib) between January 1, 2010, and December 31, 2021. HR-positive mBC with either low or no ERBB2 expression. The main outcomes were median progression-free survival and overall survival. Data on demographic characteristics, estrogen and progesterone receptor status, ERBB2 status, histologic subtype, menopausal status, treatment duration, and survival status were collected. A total of 1585 women (median [range] age, 51 [24-92] years) were included in the study. Of these women, 1013 (63.9%) had mBC with low ERBB2 expression and 572 (36.1%) had mBC with no ERBB2 expression. A total of 1084 (68.4%) were treated with a CDK4/6i (912 patients were treated in the first line and 172 were treated in the second line); 475 (30.0%) received everolimus and 26 (1.6%) received alpelisib. In the patients who received a first-line CDK4/6i, 618 (67.8%) received an aromatase inhibitor as their ET backbone and 265 (29.1%) received fulvestrant. With a median follow-up time of 17.9 months (range, 1-111 months), progression-free survival and overall survival were not statistically different between the patients with low and no ERBB2 expression treated with TT plus ET. In this cohort study of patients with HR-positive mBC treated with TT plus ET, low ERBB2 expression did not have a significant association with prognosis.

Abstract 6744: Cell-free DNA detection of alterations in the MAPK pathway in metastatic hormone receptor positive breast cancer: A multi-institutional analysis of incidence and clinical outcomes

Abstract Background Alterations in the MAPK pathway are known mechanisms for tumorigenesis in multiple solid tumors. While not major drivers in early breast cancers, activating MAPK pathway alterations have been invoked as potential resistance mechanisms in advanced hormone receptor positive (HR+) breast cancer. While MAPK pathway mutations are believed to be relatively rare in breast cancer, the accessibility of cell-free DNA (cfDNA) analysis allows for evaluation of their prevalence, co-occurring mutations, and associated clinical outcomes. In this study, we evaluated the incidence of MAPK pathway alterations and impact on clinical outcomes among patients with metastatic breast cancer (MBC). Methods Plasma was collected in patients with HR+ MBC at the Massachusetts General Hospital and Washington University in St. Louis, and cfDNA was analyzed via the Guardant 360 assay, a 74-gene next generation sequencing panel. The impact of MAPK pathway alterations on progression-free survival (PFS) and overall survival (OS) was analyzed using multivariable Cox regression analysis, adjusting for age, number of prior therapies, visceral metastases, de novo metastases, and PIK3CA alterations. PFS and OS were evaluated in the overall study population, as well as in subgroups that received endocrine therapy + CDK4/6 inhibitor, endocrine monotherapy and chemotherapy. Results Out of 647 HR+ MBC patients, 103 (16%) had non-synonymous mutations in the MAPK pathway detected in cfDNA. Median age was similar (61.9 and 60.7) in MAPK-altered and non-altered patients, respectively. Both groups had received a median of 2 prior lines of therapy (p=0.08). MAPK pathway alterations included NF1 (n = 45, 7.0%), KRAS (n = 22, 3.4%), BRAF (n = 22, 3.4%), MAPK1 (n = 8, 1.2%), MAP2K1 (n = 6, 0.9%), NRAS (n = 5, 0.8%), RAF1 (n = 5, 0.8%), HRAS (n = 4, 0.6%), ARAF (n = 4, 0.6%), MAP2K2 (n = 4, 0.6%), RIT1 (n = 3, 0.5%), and MAPK3 (n = 2, 0.3%). Mutant allele fractions ranged from 0.03 to 26. Co-alterations in PIK3CA occurred in 49% (n = 51), TP53 in 41% (n = 42), and ESR1 in 27% (n = 28). In multivariable analysis, patients with MAPK-altered HR+ MBC had significantly poorer median PFS, 7.6 months vs 11.5 months (HR: 1.6; p = 0.005; 95% CI: 1.2-2.2). There was no statistically significant impact on outcomes when stratifying by treatment type. Conclusions MAPK pathway alterations are associated with a significantly poorer PFS among patients with HR+ MBC. Further research is needed to independently validate these observations and evaluate the impact of genotype-directed therapy targeting MAPK-altered, HR+ MBC. Citation Format: Arielle J. Medford, Andrzej Niemierko, Whitney L. Hensing, Andrew A. Davis, Katherine Clifton, Jennifer C. Keenan, Lesli Kiedrowski, Ami N. Shah, Lorenzo Gerratana, Massimo Cristofanilli, Aditya Bardia. Cell-free DNA detection of alterations in the MAPK pathway in metastatic hormone receptor positive breast cancer: A multi-institutional analysis of incidence and clinical outcomes [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 6744.

Abstract 5468: Which are the clinicopathological characteristics useful to define the metastatic breast cancer patients that will respond to CDK4/6 inhibitors and hormone therapy? An Italian real-world experience

Abstract The development of CDK4/6 inhibitors has changed the therapeutic management of hormone receptor positive (HR+) metastatic breast cancer (mBC) by targeting the cell cycle machinery and overcoming endocrine resistance. However, a high proportion of patients will present disease progression due to the resistance of cancer cells to CDK4/6 inhibitors. Loss of retinoblastoma function, dysregulation of several signaling pathways and overexpression of CDK6, CDK7 and cyclin E have been described as main actors in the development of resistance to CDK4/6 inhibitors. Despite these findings on the role of new emerging biomarkers, we wondered if the clinicopathological characteristics could be useful to identify the patients that will respond to CDK4/6 inhibitors by the analysis of a retrospective case series of patients with HR+ mBC treated with hormone therapy plus CDK4/6 inhibitors (ribociclib, palbociclib, abemaciclib) at IRCCS Istituto Romagnolo per lo Studio dei Tumori (IRST) "Dino Amadori" (Meldola, Italy). 177 mBC patients 66 of whom were treated with CD4/6 inhibitors plus letrozole and 111 treated with CDK4/6 inhibitors and fulvestrant were enrolled in the study. The median age was 63 years (range 38-92), of whom 103 younger than 65 years. 152 were in postmenopausal status. Among the clinical characteristics, neutropenia was developed in 80 patients. ki67 was low (<20%) in 85 patients. 21 (23.3%) patients had bone metastases, 36 visceral metastases (23.3%) and 33 (36.7%) in other organs. The multivariable Cox PH model showed that having received a prior adjuvant treatment and the number of metastases were the only factors associated with the progression-free survival (PFS). Furthermore, when introducing the presence of neutropenia as a potential predictor of PFS in the Cox PH model, it resulted to be a risk predictor of progression. To better describe the mechanism linking PFS and neutropenia a multistate model was developed. Low body surface area and older age were associated with an increased risk of developing neutropenia. High Ki67, the presence of visceral metastases and the absence of a prior adjuvant chemotherapy were prognostic factors of progression/death. As expected, among neutropenic patients, those with multiple previous lines of treatment were at higher risk of disease progression/death. Finally, the neutropenia status was associated with a more than double risk of progression/death with respect to patients without neutropenia (HR=2.311; p=0.025). Given that we identified a set of factors associated with the probability to develop neutropenia and considering that neutropenia itself is associated with an increased risk of progression, these baseline characteristics should be taken into account in order to reduce the occurrence of both neutropenia and disease progression. Citation Format: Sara Bravaccini, Andrea Roncadori, William Balzi, Giovanni Martinelli, Maria Teresa Montella, Michela Palleschi, Roberta Maltoni. Which are the clinicopathological characteristics useful to define the metastatic breast cancer patients that will respond to CDK4/6 inhibitors and hormone therapy? An Italian real-world experience. [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 5468.

Filling the Gap after CDK4/6 Inhibitors: Novel Endocrine and Biologic Treatment Options for Metastatic Hormone Receptor Positive Breast Cancer.

The rise of cyclin-dependent kinase (CDK)4/6 inhibitors has rapidly reshaped treatment algorithms for hormone receptor (HR)-positive metastatic breast cancer, with endocrine treatment (ET) plus a CDK4/6-inhibitor currently representing the standard of care in the first line setting. However, treatment selection for those patients experiencing progression while on ET + CDK4/6-inhibitors remains challenging due to the suboptimal activity or significant toxicities of the currently available options. There is also a paucity of data regarding the efficacy of older regimens, such as everolimus + exemestane, post-CDK4/6 inhibition. In this setting of high unmet need, several clinical trials of novel drugs have recently reported encouraging results: the addition of the AKT-inhibitor capivasertib to fulvestrant demonstrated a significant improvement in progression-free survival (PFS); the oral selective estrogen receptor degrader (SERD) elacestrant prolonged PFS compared to traditional ET in a phase 3 trial, particularly among patients with detectable ESR1 mutations; finally, PARP inhibitors are available treatment options for patients with pathogenic BRCA1/2 germline mutations. Overall, a plethora of novel endocrine and biologic treatment options are finally filling the gap between first-line ET and later line chemotherapy. In this review article, we recapitulate the activity of these novel treatment options and their potential role in future treatment algorithms.

A phase I trial of palbociclib and bosutinib with fulvestrant in patients with metastatic hormone receptor positive and HER2 negative (HR+ HER2-) breast cancer refractory to an aromatase inhibitor and a CDK4/6 inhibitor

Standard treatment for metastatic hormone positive (HR+) breast cancer includes a combination of a CDK4/6 inhibitor and antiestrogen therapy. Despite durable responses, eventual endocrine resistance results in disease progression. The Src/Abl pathway has been shown to mediate endocrine resistance in breast cancer, thus providing a promising target for novel therapies. Bosutinib is a tyrosine kinase inhibitor that targets the Src/Abl pathway, which has been studied in hematologic malignancies. Preclinical data suggests that the addition of bosutinib to a CDK4/6 inhibitor and antiestrogen therapy has the potential to reverse endocrine resistance.This is a phase I, single arm, open-label clinical trial in which we evaluate the combination of palbociclib and fulvestrant with bosutinib in metastatic HR+ breast cancer. Patients with confirmed advanced HR+/HER2- breast cancer who have received no more than three lines of chemotherapy and have progressed on at least one aromatase inhibitor and one CDK4/6 inhibitor will be enrolled. Participants will be given a combination of palbociclib, fulvestrant and bosutinib over 28-day cycles. The primary objective of this study is to assess the safety and tolerability of bosutinib in combination with palbociclib and fulvestrant in the study population. Secondary objectives are to 1) determine the anti-tumor effect of this therapeutic combination by assessing overall response rate (ORR) and clinical benefit rate (CBR) after 6 months of treatment, 2) to determine the clinical pharmacology parameters of bosutinib in this regimen, and 3) to build a tissue repository at Georgetown Lombardi Comprehensive Cancer Center for further translational study.

Aromatase inhibition plus/minus Src inhibitor saracatinib (AZD0530) in advanced breast cancer therapy (ARISTACAT): a randomised phase II study

PurposeThe development of oestrogen resistance is a major challenge in managing hormone-sensitive metastatic breast cancer. Saracatinib (AZD0530), an oral Src kinase inhibitor, prevents oestrogen resistance in animal models and reduces osteoclast activity. We aimed to evaluate the efficacy of saracatinib addition to aromatase inhibitors (AI) in patients with hormone receptor-positive metastatic breast cancer.MethodsThis phase II multicentre double-blinded randomised trial allocated post-menopausal women to AI with either saracatinib or placebo (1:1 ratio). Patients were stratified into an “AI-sensitive/naïve” group who received anastrozole and “prior-AI” group who received exemestane. Primary endpoint was progression-free survival (PFS). Secondary endpoints included overall survival (OS), objective response rate (ORR) and toxicity.Results140 patients were randomised from 20 UK centres to saracatinib/AI (n = 69) or placebo/AI (n = 71). Saracatinib was not associated with an improved PFS (3.7 months v. 5.6 months placebo/AI) and did not reduce likelihood of bony progression. There was no benefit in OS or ORR. Effects were consistent in “AI-sensitive/naive” and “prior-AI” sub-groups. Saracatinib was well tolerated with dose reductions in 16% and the main side effects were gastrointestinal, hypophosphatemia and rash.ConclusionSaracatinib did not improve outcomes in post-menopausal women with metastatic breast cancer. There was no observed beneficial effect on bone metastases.CRUKE/11/023, ISRCTN23804370.

Abstract P4-01-38: A single arm phase II trial of Palbociclib in combination with Tamoxifen as first line therapy for metastatic hormone receptor positive breast cancer

Abstract Background: Palbociclib is a CDK4/6 inhibitor used to treat metastatic hormone receptor-positive (HR+) breast cancer (MBC) in combination with endocrine therapy. Tamoxifen is an effective treatment for HR+ MBC, with different toxicity profile compared with aromatase inhibitors (AI) and fulvestrant. Preclinical data demonstrated synergy for the combination of tamoxifen and palbociclib, being effective in a model of acquired tamoxifen resistance. Methods: We conducted a non-randomized, open-label, single-arm, multicenter, phase II trial of palbociclib in combination with tamoxifen in patients with HR+/HER2 - advanced BC, with no prior therapy for MBC. Ovarian suppression was recommended for pre-menopausal women. Primary objective was progression free survival, PFS. Secondary objectives: objective response rate, ORR (CR or PR) based on RECIST 1.1 or MDA Criteria (for patients with bone only disease); safety and tolerability (using CTCAE v4); clinical benefit rate, CBR (CR, PR or SD lasting min 24 weeks); 2-year overall survival. Correlative objectives: proteomic analysis of plasma exosomes to identify mechanisms of primary and secondary resistance to tamoxifen/palbociclib. Results: Between 6/30/2016 and 7/02/2019, we enrolled 49 patients (47 evaluable): 23 pts with de-novo metastatic disease and 24 pts with recurrent BC (12 pts were on adjuvant treatment with AI at time of recurrence and 12 pts on surveillance). As of 6/30/2022 data cut-off, 2 pts were still on treatment. This is an updated analysis with median follow-up time of 24 months (range 8-51). Median age was 60 (range 39-82). The median PFS was 19.8 months with 95% CI (8-41) for pts with de-novo MBC and 7 months (2-12) for pts with recurrent BC. The ORR was 30% overall, 39% for pts with de novo MBC, 21% for pts with recurrent BC. CBR was 64% overall, 77% for pts with de novo MBC and 50% for pts with recurrent BC. CBR was 65% for white pts and 55% for AA pts. Best response per RECIST1.1: 14 pts (34%) had PR, 18 pts (44%) had SD, 9 pts (22%) had PD. All 6 pts with bone only disease had SD. The most common drug related grade ≥ 3 AE was neutropenia (51%), transient and manageable by dose modifications, no cases of febrile neutropenia. Four patients (8%) developed thromboembolic events (grades 2, 3, 4) and discontinued treatment. One patient died while on treatment from PD. More than 800 exosome proteins were detected in the samples analyzed so far. Exosomal protein networks in pretreatment samples predicted treatment response with 89% sensitivity and 95% specificity in unsupervised clustering. Data on correlative objectives will be presented in a separate abstract. Conclusions: The combination of palbociclib and tamoxifen showed tolerable, expected safety profile. This may be an alternative approach for selected patients in first line treatment of HR+ MBC, especially those who present with de-novo metastatic disease and are intolerant to AI; although this small study indicates a lower PFS. Table 1: Baseline Demographics Citation Format: Oana C. Danciu, Kent Hoskins, Jennifer Weiss, Cristina I. Truica, Anne Blaes, Deimante Tamkus, Jatin Rana, Tandra Pavankumar, Lauren Green, Yu Gao, Menggang Yu, Qianqian Zhao, Deborah Toppmeyer, Ruth O’Regan, Kari B. Wisinski. A single arm phase II trial of Palbociclib in combination with Tamoxifen as first line therapy for metastatic hormone receptor positive breast cancer [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr P4-01-38.

Abstract OT2-01-06: Phase II trial of palbociclib plus endocrine therapy followed by combination of pembrolizumab, palbociclib and endocrine therapy in patients with hormone receptor positive metastatic breast cancer

Abstract Background: The combination of CDK4/6 inhibitors (CDK4/6i) and endocrine therapy (ET) is standard-of-care for patients with hormone receptor positive (HR+) HER2- metastatic breast cancer (MBC). Immune modulatory effects of CDK4/6i are well documented preclinically but poorly understood in the clinical setting. Our previous study combining letrozole, palbociclib and pembrolizumab in patients with HR+ MBC (NCT02778685) showed a promising complete response rate of 31%. Dynamic changes in peripheral blood mononuclear cell (PBMC) subpopulations indicated that palbociclib may increase CD8+ TEMRA (terminally differentiated effector memory cells) and CD4+ TEM (effector memory cells) and enable immune activation. The current cohort 3 was designed to study the immune modulatory effect of palbociclib as an immune-priming agent with a biomarker enriched design. Methods: Women with ECOG 0-1, HR+ HER2- MBC, RECIST 1.1 measurable disease, no prior therapy for MBC were enrolled. Patients with endocrine therapy, including aromatase inhibitor +/- ovarian suppression or fulvestrant, were eligible. A palbociclib + ET lead-in design was used, starting on day -28 followed by combination therapy with pembrolizumab added on C1D1. Peripheral blood and tumor biopsy at baseline and on-treatment were collected to allow in-depth analysis of biomarkers predicting response to the combination. The primary endpoint was to evaluate if the palbociclib potentiated immune responses as a “priming” agent through PBMC analysis and on treatment tumor biopsy. Secondary endpoints included other immune cell subsets and changes that follow the combination with pembrolizumab. With 25 patients, assuming a standard deviation of 0.51 in the relative change in classic monocytes in PBMCs, there is 90% power to detect a relative change of log(C1D1/baseline) of 34.5% with a type I error (two-sided) of 0.05. Results: Between August 2020 and April 2022, 16 patients were enrolled in cohort 3. Currently 11 patients have adverse event (AEs) and 16 patients have response data. Median age was 57 years (39-72). 8/11 (73%) were non-Hispanic white, 1/11 (9%) Hispanic, 1/11 (9%) Asian, and 1/11 (9%) African American. 87% patients had grade 3 AEs, and 30% had grade 4 AEs. Grade 3 AEs were 9/11 (82%) neutropenia, 5/11 (45%) leukopenia, 1/11 (9%) elevated LFTs, and 1/11 (9%) each lymphopenia, hot flashes, febrile neutropenia, and pneumonitis. Grade 4 AEs were 1/11 (9%) lymphopenia. 8/16 (50%) patients achieved a partial response (PR), 5/16 (31%) had stable disease (SD), and 1/16 (6%) had progression of disease (PD) by RECIST 1.1. Additionally, 2/16 (13%) patients were too early to determine best overall response. Response rate (CR+PR) was 50%. PBMCs and tumor microenvironment profiling are ongoing. Conclusion: The combination of palbociclib, pembrolizumab and ET is well tolerated, and a response rate of 50% was identified in HR+ MBC patients who received this combination as front-line therapy. Dynamic changes in peripheral blood mononuclear cells and tumor microenvironment profiling are ongoing. Citation Format: Yuan Yuan, Colt A. Egelston, Weihua Guo, Susan E. Yost, Paul H. Frankel, Christopher Ruel, Mireya Murga, Aileen Tang, Norma Martinez, Daniel Schmolze, Daphne Stewart, James Waisman, Kelly Yap, Joanne Mortimer, Niki Tank. Phase II trial of palbociclib plus endocrine therapy followed by combination of pembrolizumab, palbociclib and endocrine therapy in patients with hormone receptor positive metastatic breast cancer [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr OT2-01-06.

Abstract P5-02-08: Biomarker associated with response to CDK4/6 inhibitors in metastatic hormone receptor positive breast cancer

Abstract Background: In spite of widespread use and known mechanism of action, predictive biomarkers for the use of CDK4/6 inhibitors in conjunction with endocrine therapy have yet to emerge. Here a cohort of patients treated with standard-of-care combination regimens was utilized to explore features of disease and determinants of progression-free survival (PFS). Patients and Methods: In this cohort of 235 patients, >90% of patients were treated with Palbociclib in combination with either an aromatase inhibitor (AI) or fulvestrant (FUL). The PFS mirrored that observed in randomized clinical trials. A total of 151 patient tumor tissues were used for targeted gene expression analyses with the HTG-Oncology Biomarker Panel. The association of disease state and gene expression analyses were interrogated for disease evolution and association with PFS. Results: HER2 immunohistochemistry (HER20, Her21+, Her22+) was not associated with PFS in full cohort, or AI and FUL subgroup analyses. The lack of progesterone receptor (PR+, PR-), was associated with shorter PFS in the full patient cohort (p=0.012) and selectively in patients treated with AI (p=0.005), but not FUL. Gene expression-based subtyping indicated that the majority of patients, as expected, had luminal breast cancer; however, the predominant subtypes changed with treatment and disease evolution. Primary tissue from tumor resection was dominated by luminal A subtype, which diminished in the context of metastatic disease, and was rare in post-progression specimens. The luminal B, HER2, and basal subtypes exhibited shorter PFS in CDK4/6 inhibitor combinations (AI, p=0.01; FUL, p=0.03). Existing clinically developed breast cancer signatures (e.g. breast cancer index) had variable associations with PFS; however, high expression of gene signatures associated with cell cycle were broadly associated with short PFS. Concordantly, utilizing unbiased analyses, gene expression programs linked to cell cycle were associated with short PFS, while interferon response processes were associated with longer PFS. Algorithms that incorporated standard pathological and clinical variables with the gene expression data were developed that exhibited potent predictive power. Conclusions: Tumor evolution occurs on treatment with CDK4/6 inhibitors; however, analyses of pretreatment biopsies can inform the duration of PFS. These data support discrete biological processes associated with sensitivity/resistance. Predictive algorithms could be developed to inform features of treatment decision which will require prospective validation which is ongoing. Citation Format: Agnieszka Witkiewicz. Biomarker associated with response to CDK4/6 inhibitors in metastatic hormone receptor positive breast cancer [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr P5-02-08.

Abstract P5-02-07: Cell-free DNA detection of GATA3 mutations in metastatic hormone receptor positive breast cancer: a retrospective, observational multi-institutional analysis

Abstract Background GATA3 mutations (GATA3mut) have been reported in 10-20% of hormone receptor positive (HR+) breast cancers. It has been shown that targeting GATA3mut HR+ breast cancer with MDM2 inhibitors invokes synthetic lethality. MDM2 is an E3 ubiquitin ligase that targets p53 for degradation, and research suggests that restoring p53 by blocking MDM2 may be effective in treating GATA3mut HR+ breast cancer. One potential mechanism of this efficacy has been shown to be through the PI3K-AKT pathway. We thus sought to characterize the GATA3mut landscape in a multi-institutional cell-free DNA (cfDNA) analysis and to determine the association between GATA3mut and TP53 mutations, as well as alterations in the PI3K-AKT pathway and the impact of GATA3 on survival. Methods We analyzed cfDNA data collected at the Massachusetts General Hospital and at Washington University in St Louis via Guardant360, a next generation sequencing assay that analyzed up to 74 genes during the study period. The association of GATA3mut and co-mutations as well as number of prior therapies was estimated using Pearson’s chi-squared test for categorical variables, two-sample Wilcoxon rank-sum test for continues variables, and multivariable logistic regression. The impact of GATA3mut and GATA3 wildtype (WT) on progression-free survival (PFS) and overall survival (OS) was analyzed using multivariable Cox regression analysis, adjusting for age, number of prior therapies, visceral metastases, and de novo metastases. PFS and OS were evaluated in the overall study population, as well as in subgroups of patients that received endocrine monotherapy and chemotherapy. Results Out of 647 patients with HR+ MBC, 10% (n = 68) had non-synonymous GATA3 mutations. Among these 68 GATA3mut patients, 37% (n = 25) were mutations in exon 5, all but two of which were in the second zinc finger, and 62% (n = 42) were in exon 6. 62% (n = 42) were frameshift mutations, 20% (n = 14) were indels, and 18% (n = 12) were point mutations. Median mutant allele fraction (MAF) of GATAmut was 0.95% (range 0.03 – 30.5%). There was no statistically significant association of GATA3mut with the number of prior therapies, PR status, or the presence of ESR1, TP53, or PI3K-AKT pathway mutations. In the GATA3mut population, TP53 co-mutations (n = 21) were found with a median MAF of 0.6%. PI3K-AKT pathway alterations occurred in 47% (n=32) of GATA3mut patients (PIK3CA n = 27; AKT n = 2; PTEN n = 3). In the combined cohort, there was no significant difference in PFS or OS after adjusting for visceral metastases, de novo disease, number of prior therapies, and age. In a cohort of 80 patients that received endocrine monotherapy (GATA3 WT n = 74, GATA3mut n = 6), GATA3mut were associated with borderline worse PFS (HR 2.6; p = 0.061) and worse OS (HR 4.5; p = 0.009). There was no statistically significant difference in PFS or OS in a subgroup that received chemotherapy. Conclusions GATA3 mutations can be identified via cfDNA in patients with HR+ MBC. Co-mutations in TP53 occurred at overall low MAF. Further research is needed to characterize the functional impact of these low level TP53 co-mutations and develop therapeutic strategies to target GATA3 mutant MBC. Citation Format: Arielle J. Medford, Marko Velimirovic, Andrzej Niemierko, Whitney L. Hensing, Andrew A. Davis, Katherine K. Clifton, Jennifer C. Keenan, Charles S. Dai, Lesli A. Kiedrowski, Ami N. Shah, Lorenzo Gerratana, Laura M. Spring, Leif Ellisen, Robert C. Doebele, Massimo Cristofanilli, Aditya Bardia. Cell-free DNA detection of GATA3 mutations in metastatic hormone receptor positive breast cancer: a retrospective, observational multi-institutional analysis [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr P5-02-07.