Positive Metastatic Breast Cancer Patients Research Articles

Abstract PO5-04-01: Inetetamab combined with sirolimus and chemotherapy in HER2 positive metastatic breast cancer patients with abnormal activation of Pi3k/Akt/mTOR pathway after trastuzumab treatment

Abstract Objective: To explore the efficacy and safety of Inetetamab Combined with Sirolimus and Chemotherapy in the treatment of human epidermal factor receptor 2 (HER2) positive metastatic breast cancer patients with abnormal activation of Pi3k/Akt/mTOR (PAM) pathway after Trastuzumab treatment. Methods: This prospective multicenter clinical study enrolled HER2 positive metastatic breast cancer patients at the National Cancer Center in China from July 2021 to September 2022 with PAM pathway mutation confirmed by histology or peripheral blood genetic testing. The inclusion criteria included: 1) Recurrent and metastatic breast cancer patients with HER2 positive confirmed histologically; 2) ECOG PS score ≤ 2; 3) Patients who have progressed after previous treatment with Trastuzumab; 4) Patients with PAM pathway mutation confirmed by Genetic testing; 5) Patients' cardiac, pulmonary, liver and bone marrow functions are basically normal; 6) Patients voluntarily sign informed consent forms. The exclusion criteria included: 1) Patients previously treated with mTOR inhibitors or pyrotinib; 2) Patients who have used chronic Corticosteroid for more than 3 months or used Immunosuppressive drug within 4 weeks; 3) Patients with symptomatic central nervous system metastasis; 4) Patients with left ventricular Ejection fraction< 50% or clinical manifestations of obvious arrhythmia, myocardial ischemia, severe atrioventricular block, cardiac insufficiency, and severe valvular disease; 5)Diagnosed with other malignant tumors within five years. Patients were randomly divided into trial group and control group. The patients in the trial group received inetetamab combined with sirolimus and chemotherapy, and control group received pyrotinib and chemotherapy. RECISTv1.1 standard was used to evaluate the efficacy of patients. Descriptive statistics are used to summarize clinical pathological features. Kaplan Meier method was used to draw survival curves. Log-rank test was used to compare progression free survival (PFS) differences between groups. P< 0.05 was considered statistically significant. Results:A total of 59 HER2 positive metastatic breast cancer patients with abnormal activation of PAM pathway were included, of which 37 patients received the treatment of inetetamab group and 22 patients received the treatment of pyrotinib group. The median follow-up time was 5.33 months. In the intent-to-treat set, the median PFS of patients in the inetetamab group was 4.64 months, which was shorter than 5.69 months in the pyrotinib group. There was no statistically significant difference between the two groups (P=0.51; HR=0.41; 95% CI, 0.12-1.45). The objective response rate (ORR) of the inetetamab group and the pyrotinib group were 28.1% and 29.4%, respectively. In terms of safety, the incidence of adverse events (AE) in the experimental group was (33/37) 89.1%, while (16/21) 76.2% in the control group. The incidence of AE above level 3 in the inetetamab group and pyrotinib group were (13/37) 35.1% and (3/21) 14.3%, respectively. Conclusion:For the metastatic HER2 positive breast cancer patients with abnormal activation of PAM pathway treated with trastuzumab previously, the combination of inetetamab with sirolimus and chemotherapy regimen are equivalent to the combination of pyrotinib and chemotherapy regimen. It demonstrated that combination of inetetamab with sirolimus and chemotherapy could be one of the treatment options for the metastatic HER2 positive breast cancer patients. Citation Format: Qiao Li, Dan Lv, Xiaoying Sun, Mengyuan Wang, Li Cai, Feng Liu, Chenghui Li, Jiuda Zhao, Jing Sun, Yehui Shi, Fei Ma. Inetetamab combined with sirolimus and chemotherapy in HER2 positive metastatic breast cancer patients with abnormal activation of Pi3k/Akt/mTOR pathway after trastuzumab treatment [abstract]. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO5-04-01.

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 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.

The clinical impact of plasma estrogen receptor-1 mutation in patients with metastatic breast cancer: A meta-analysis.

The relevance of the discovered plasma ESR1 mutations in positive metastatic breast cancer (BC) patients who had progressing disease after aromatase inhibitor (AI)-based therapy is still being debated. We conducted this meta-analysis to explore the prognostic and predictive role of plasma ESR1 mutations in patients with progressive BC who have previously received AI therapy. We searched for relevant studies in the PubMed, Embase and Cochrane Library databases to be included in the meta-analysis. This study was performed to compute combined hazard ratios (HRs) with 95% confidence intervals (95% CIs) for the progression-free survival (PFS) rate and overall survival (OS) rate. Subgroup and sensitivity analyses were also performed. The heterogeneity between studies was evaluated using the I2 statistic. In this meta-analysis, a total of 1,844 patients with metastatic BC and positive for estrogen receptors (ERs) were enrolled from 8 articles. The analysis revealed that patients with circulating ESR1 mutations had significantly worse PFS (HR: 1.34; 95% CI: 1.17-1.55; p < 0.001) and OS (HR: 1.59; 95% CI: 1.31-1.92; p < 0.001) compared to wild-type ESR1 patients. Subgroup analysis showed that the types of plasma ESR1 mutations were associated with differences in the prognosis of metastatic BC. The D538G mutation showed a statistically significant lower PFS (p = 0.03), while the Y537S mutation was not significantly correlated with PFS (p = 0.354). According to the findings of this meta-analysis, the assessment for plasma ESR1 mutations may provide prognostic and clinical guidance regarding subsequent endocrine therapy decisions for ER-positive, metastatic BC patients who had received prior therapy with AIs.

Efficacy of anlotinib in Chinese patients with metastatic breast cancer: A retrospective observational study.

Anlotinib, a multitarget tyrosine kinase inhibitor, can inhibit tumour angiogenesis proliferation, metastasis, promote vascular normalization, increase T cell and NK cell activity and infiltration, remodel tumour microenvironment and synergistic immune enhancement. Our study aimes to evaluate the efficacy of anlotinib in the treatment of advanced metastatic breast cancer (MBC) after multiple lines of therapy. Patients included were treated with anlotinib for advanced MBC in the Affiliated Cancer Hospital of Zhengzhou University from 1 January 2019 to 30 June 2023. The objective remission rate, disease-free progression survival and adverse reactions were analysed. We compared and analysed the efficacy of anlotinib in the treatment of advanced metastatic breast cancer, which showed that ORR was 23.6% and DCR was 69.1%. The DCR of monotherapy was 66.7% and that of combination therapy was 69.6% in MBC patients. The combination therapy, combined with chemotherapy had the best effect (79.3%), combined with immunotherapy came second. In addition, the DCR (88.9%) was higher in MBC patients having received prior antiangiogenic therapy. According to the Kaplan-Meier (K-M) survival estimate analysis, the mPFS was 4.17 months (95% CI, 1.758-6.582 months) in Her-2 positive MBC patients, and 7.83 months (95% CI, 2.416-9.104) in Her-2 negative MBC patients. The mPFS was 5.76 months (95% CI, 3.231-8.298 m) in HR positive MBC patients, 7.83 months (95% CI, 3.182-12.478 m) in TNBC patients. Fatigue (20.0%), hypertension (21.8%) and liver dysfunction (18.2%) were common adverse reactions, followed by bone marrow suppression (16.4%), anorexia (14.5%), hypothyroidism (14.5%) and diarrhoea (14.5%). Altogether, Anlotinib monotherapy or combination therapy provides a viable third (or above)-line therapeutic strategy in patients with metastatic breast cancer. The adverse reactions of anlotinib are well tolerated and controllable.

478P Germline BRCA1/2 pathogenetic variants (gBRCA1/2 PV) affect outcome of hormone (HR)-positive HER2-negative metastatic breast cancer (MBC) patients (pts) treated with cyclin-dependent kinase 4/6 inhibitor (CDK 4/6i) plus endocrine therapy (ET): The BREAK study

478P Germline BRCA1/2 pathogenetic variants (gBRCA1/2 PV) affect outcome of hormone (HR)-positive HER2-negative metastatic breast cancer (MBC) patients (pts) treated with cyclin-dependent kinase 4/6 inhibitor (CDK 4/6i) plus endocrine therapy (ET): The BREAK study

Safety and efficacy study of oral metronomic capecitabine combined with pyrotinib in HER2-positive metastatic breast cancer: A phase II trial

PurposeTo analyze the safety and efficacy of orally administered metronomic capecitabine plus pyrotinib in HER2 positive metastatic breast cancer (MBC) patients, we conducted a prospective phase II study with a single-arm design. MethodsHER2 positive patients received oral metronomic capecitabine 500 mg three times a day and pyrotinib 400 mg per day. The primary endpoint was progression-free survival (PFS). Other endpoints included objective response rate (ORR), overall survival (OS), clinical benefit rate (CBR) and safety. ResultsThe study included 50 patients with MBC that was HER2-positive, while 1 patient was excluded due to nonstandard medication. The median PFS and OS was 11.9 months (95%CI 8.8–14.6) and 29.3 months (95%CI 24.4–34.8) respectively. ORR was 34.7%, and CBR was 81.6% with 2 CR (4.1%), 15 PR (30.6%) and 23 SD (46.9%). The mPFS in first- or second-line treatment was 12.2 months. The most frequent treatment-related adverse events included hand-foot syndrome, diarrhea, vomiting and nausea. Grade 3 adverse events occurred in 15(30.6%) patients, including hand-foot syndrome (12.2%), diarrhea (12.2%), vomiting (4.1%), and nausea (2.0%). 1 grade 4 adverse event of diarrhea (2.0%) was observed. ConclusionThe combination of metronomic capecitabine and pyrotinib is a promising regimen with competitive efficacy and improved tolerability in HER2 positive metastatic breast cancer patients.

Combination of eribulin with trastuzumab and pertuzumab for HER2 positive advanced or recurrent breast cancer: JBCRGM03.

e13031 Background: Pertuzumab (P) was an effective first-line treatment for HER2 positive metastatic breast cancer (MBC) in the CLEOPATRA study, improving both overall and progression-free survival (PFS). However, it is challenging to provide intravenous docetaxel to MBC patients over long term at a dosage of 75 mg/m2 every three weeks. Eribulin (E) is a well tolerated cytotoxic agent. In a multicenter, open-label phase II research (UMIN000012232), we have shown the effectiveness and safety of E combined with trastuzumab (T) with P as first- and second-line treatment for metastatic or advanced BC. Methods: The study included HER2 positive MBC patients with no or one previous treatment, including advanced or recurrent chemotherapy. All patients were administered with T and taxane as perioperative or first-line chemotherapy. E (1.4 mg/m2) along with T (8 mg/kg loading dosage &gt; 6 mg/kg), and P (840 mg loading dose &gt; 420 mg) was administered intravenously on days 1 and 8 of a 21-day cycle, once every three weeks. The primary end point was PFS and the secondary endpoints were the response rate (RR) based on RECIST ver1.1. Overall patient survival effectiveness following P administration, E compliance, and success of follow-up therapies were observed. Results: A total of 50 patients were enrolled, with 49 of them qualifying for the safety analysis. However, the full analysis set (FAS) consisted of 46 individuals. The average patient age was 56 years old (range 23–70), and 9 (18%) and 40 (82%) patients received care in first- and second-line settings, respectively. The median PFS for patients who received main therapy was 20.5 months, whereas the median PFS for patients who received subsequent treatment was 8.6 months. Thirty-five patients (76.1%) were able to receive concomitant E until the end of protocol treatment. The overall RR was 56.5% among the 46 patients in the FAS. Nine patients (19.5%) responded completely, whereas 17 patients (37.0%) responded partially. There were 13 cases of stable disease (28.2%), six cases of progressing disease (13.0%), and one case was not evaluable (1.2%). The median survival of the primary treatment population was not reached; however, the median survival of the secondary treatment population was 42.4 months. Twenty three patients received T-DM1 as a follow-up treatment, 13 patients received TP plus chemotherapy, and five patients received additional therapies. The average duration of treatment with T-DM1 as a consequent therapy was approximately four months, whereas the average duration of treatment with HP plus chemotherapy was 11 months. Conclusions: For HER2positive MBC, E in combination with T plus P was well tolerated and may be an alternative to docetaxel-based combination therapy. Results from the JBCRG M06 trial research, which compares taxanes combination with T plus P with E combined with T plus P in first-line therapy, are still pending. Clinical trial information: UMIN000012232 .

Differences in Outcomes Between Human Epidermal Growth Factor Receptor 2 (HER2)-Positive Metastatic Breast Cancer Patients in Upper Middle-Income versus High-Income Countries: A Retrospective Study from 4 Oncology Centers.

BACKGROUND Therapeutic options for human epidermal growth factor 2 (HER2)-positive metastatic breast cancer (mBC) are developing rapidly. This study aimed to determine the differences in the survival outcomes of patients with HER2-positive mBC in relation to access to anti-HER therapy at 3 oncology centers in upper-middle-income countries (UMICs) and 1 oncology center in a high-income country (HIC). MATERIAL AND METHODS We retrospectively identified 42 patients from Croatia (HIC), 71 patients from Serbia (UMIC), and 57 from Bosnia and Herzegovina (UMIC) diagnosed with HER2-positive mBC who were treated between January 2015 and December 2020. The pathohistological features of the tumors were obtained from the pathological findings, which were made according to standard procedures for each center. Patients were treated depending on the availability of therapy, which differed for centers in different countries. We evaluated disease-free survival, progression-free survival, and overall survival (OS) based on the availability of first- and second-line anti-HER2 therapy in UMICs vs HIC. RESULTS OS in first-line therapy was better in patients treated with dual HER2 blockade than in patients treated without dual HER2 blockade, P<0.001. OS in second-line therapy was significantly better in patients treated with trastuzumab emtansine than in patients treated with other reported regimens, P=0.004. CONCLUSIONS Results of our study showed superior survival among patients who were treated with dual first-line HER2 therapy as well as second-line trastuzumab emtansine therapy than in those patients in other centers where these drugs were not available. Raising awareness about this could help improve the situation.

Prognostic Factors Influencing Progression-Free Survival in HER2-Positive Metastatic Breast Cancer Patients Who Were Treated With A Combination of Lapatinib and Capecitabine.

The aim was to assess the prognostic variables in human epidermal growth factor receptor 2 (HER2)-positive metastatic breast cancer patients receiving lapatinib plus capecitabine. Retrospective data on HER2-positive metastatic breast cancer patients who received lapatinib and capecitabine were analyzed. Survival outcome was obtained with Cox regression analysis and the Kaplan-Meier method. The study included 102 patients. Forty-four (43.1%) patients had de novo metastatic disease. The most frequent metastatic sites were, in order, bone (61.8%), brain (57.8%), liver (35.3%), and lung (34.3%). All of the patients had previously received chemotherapy based on trastuzumab. With combined lapatinib and capecitabine, complete response was observed in 7.8%, partial response in 30.4%, and stable disease in 24.5%. Progression-free survival was 8 (95% confidence interval, 5.1-10.8) months. In multivariable analysis, endocrine therapy (p = 0.02), de novo metastatic disease (p = 0.02), and age (p = 0.02) were prognostic factors for progression-free survival. However, the number of chemotherapy cycles with trastuzumab, palliative radiotherapy, history of breast surgery, and the number of metastatic sites were not significant in this respect. These results have demonstrated the effectiveness of lapatinib plus capecitabine in metastatic HER2-positive breast cancer patients. Furthermore, unfavorable prognostic factors for progression-free survival were shown to be hormone-negative tumor, de novo metastatic disease, and young age.

Real-world efficacy and safety of pyrotinib in patients with HER2-positive metastatic breast cancer: A prospective real-world study.

Background: Pyrotinib, a novel irreversible EGFR/HER2 dual tyrosine kinase inhibitor, shows encouraging anticancer activity and acceptable tolerability in multiple phase II and phase III randomized clinical trials, but the real-world data of pyrotinib, especially the outcomes in HER2-positive metastatic breast cancer, have been rarely reported. Here, we evaluated the treatment outcomes of pyrotinib in real-world practice in patients with HER2-positive metastatic breast cancer (MBC). Methods: This was a prospective, real-world, observational cohort study. Through the Breast Cancer Information Management System, HER-2 positive MBC patients treated with pyrotinib between 2017/06 and 2020/09 were included. Provider-reported objective response rate, progression-free survival (PFS), and overall survival (OS) were considered in the assessment of treatment outcomes. Tumor responses to pyrotinib treatment were calculated using RECIST 1.1. Adverse events were evaluated using clinical records. Results: The trial involved 113 individuals who were receiving pyrotinib treatment, with an average age of 51 years. Complete response, partial response and stable disease were observed in 9 (8.0%), 66 (58.4%), and 17 (15.0%) patients, respectively, while progressive disease was recorded in 20 (17.7%) patients. After a median follow-up of 17.2 months, the median PFS was 14.1. The most common adverse events of any grade were diarrhea (87.6%), vomiting (31.9%), and palmar-plantar erythrodysesthesia (26.6%). Among the patients with brain metastases, the median PFS and OS were 15.2 and 19.8 months, respectively. In addition, pyrotinib has similar efficacy in various subtypes of HER2-positive MBC patients, as shown by the lack of a significant difference of PFS and OS among pyrotinib-treated patients with or without brain metastases, or patients using pyrotinib as first-line, second-line, third-line or beyond therapies. Conclusion: Our real-world results demonstrated equivalent clinical efficacy in HER-2 positive MBC patients compared to phase II and phase III clinical trials with pyrotinib, and promising outcomes in patients with brain metastases.

Abstract P1-11-04: Assessing the clinico-pathological characteristics of HER2 positive metastatic breast cancer patients experiencing radiologic complete response in a nationwide cohort

Abstract Background: Up to 6% of patients (pts) with HER2 positive (pos) metastatic breast cancer (MBC) experience a radiologic complete response (rCR) to a first line of therapy, but these results mostly derive from dated and/or limited cohorts. Aim of this study was to define the clinico-pathological characteristics of HER2 positive (pos) MBC pts experiencing a rCR. Methods: Pts were selected from the database of the GIM14 study (NCT02284581) and classified according to the best radiologic response obtained to the first line chemotherapy (CT) and upon time-to-treatment-failure (TTF). rCR was defined as complete response (CR) with a TTF &amp;gt; 3 months. The association across variables was tested through logistic regression and their prognostic impact in terms of overall survival (OS) was estimated using the Kaplan-Meier method and compared using the log-rank test. Results: Of the 3,423 pts included in the GIM14 study, 814 had HER2 pos MBC. After exclusion of pts treated with first line endocrine therapy and/or with TTF &amp;lt; 3 months, 656 pts were included in the present analysis, of which 96 (14.6%) experienced a rCR. Instead, the best response was a partial response for 295 pts (45.0%), stable disease for 221 pts (33.7%), and progression for 44 pts (6.7%). Most pts (59.8%) presented de novo MBC; 379 pts (57.8%) had visceral metastases (mets), 609 pts (92.8%) did not have central nervous system (CNS) involvement and 318 pts (48.5%) had only 1 site of distant mets. Also, 445 pts (67.9%) had hormone receptor (HR) pos disease, a HER2 3+ score at immunohistochemistry (IHC) was present in 59.8% of cases versus 40.2% with HER2 2+ at IHC and in situ hybridization (ISH) + disease. Taxanes were the main CT backbone (489 pts, 74.5%), 341 pts (52.0%) had received a Trastuzumab-Pertuzumab doublet. At multivariable analysis, higher odds of experiencing a rCR were reported for presence of non-visceral mets (OR 1.87, 95%CI 1.10-3.17), low number of metastatic sites (OR 2.42, 95%CI 0.80-7.33 for 1 site only) and HER2 3+ score at IHC (OR 1.80, 95%CI 1.09-2.98). Disease-free interval (DFI) was associated to rCR at univariable but not at multivariable analysis. HR status, CT backbone and type of anti-HER2 regimen were not associated with rCR neither at univariable nor at multivariable analysis. Median follow-up was 76.2 months. Amongst pts with TTF&amp;gt;12 months, those with rCR had a significantly higher OS compared to those not experiencing a rCR (median OS 133 and 90 months, respectively; p=0.0191). OS rates in pts with TTF ≥ 12 months were 97.8% at 2-year follow-up and 59.4% at 5-year follow-up. Instead, in pts with TTF ≥ 60 months, OS rates were 76.7% at 10-year follow-up. Amongst the 96 pts experiencing a CR, 38 had a rCR with TTF between 12 and 60 months, while 22 pts had a rCR with a TTF ≥ 60 months. The remaining pts had a CR with a TTF &amp;lt; 12 months. Pts with HR negative (neg) disease were found to be more likely to experience a rCR with a with TTF between 12 and 60 months, whilst pts with HR pos disease had a higher probability to experience a rCR with a TTF ≥ 60 months (p=0.0074). Pts with HER2 3+ score at IHC had a higher probability to achieve a rCR with a TTF ≥ 12 months compared to pts with HER2 2+ score at IHC and ISH + (p=0.0216). Age at diagnosis, menopausal status, DFI, number and site of mets, CT backbone and anti-HER2 therapy did not influence the duration of the rCR obtained. Conclusions: This study characterized a real-world cohort of HER2 positive MBC patients experiencing radiologic complete response to a first line treatment. Based on these results a clinical trial focused on liquid biopsy-based minimal residual disease is being designed. Novel anti-HER2 agents are gaining momentum as ever increasingly effective treatments and future de-escalation strategies after complete response will represent a growing need. Citation Format: Linda Cucciniello, Eva Blondeaux, Claudia Bighin, Simona Gasparro, Stefania Russo, Arianna Dri, Palma Pugliese, Andrea Fontana, Giuseppe Naso, Antonella Ferzi, Ferdinando Riccardi, Valentina Sini, Luca Boni, Alessandra Fabi, Filippo Montemurro, Michelino De Laurentiis, Grazia Arpino, Lucia Del Mastro, Lorenzo Gerratana, Fabio Puglisi. Assessing the clinico-pathological characteristics of HER2 positive metastatic breast cancer patients experiencing radiologic complete response in a nationwide cohort [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 P1-11-04.

Abstract P5-02-51: Single-cell transcriptome reveals distinct peripheral blood immune landscapes associated with sensitivity to anti-HER2 treatment in HER2-positive metastatic breast cancer patients

Abstract Background: Different immune cell states reflect distinct tumor microenvironment and led to various clinical outcomes for cancer patients. However, very few studies examined the contribution of peripheral blood (PB) immune landscapes to the treatment response due to the limited applications. This study aimed to explore the circulating immune cell landscapes associated the sensitivity to cytotoxic chemotherapy with trastuzumab in HER2 positive metastatic breast cancer patients. Methods: Whole blood were drawn at baseline and after 2 cycles of trastuzumab plus cytotoxic chemotherapy from six patients (3 responders and 3 non-responders). Approximately 3,500 to 10,000 peripheral blood mononuclear cells per patients were profiled using single-cell RNA sequencing (scRNA-seq). scRNA-seq data were further processed and analyzed using Seurat package version 3.1. Cell populations were clustered using the Louvain algorithm and subsequently annotated using known marker genes. Differential abundance in cell population was quantified using MiloR and differentially expressed genes were detected using MAST between responders and non-responders. Results: After removing low quality cells, a total of 65,295 cells were clustered into 18 clusters. CD8 Effector T, CD4 Naïve T, CD4 Effector T, Cytotoxic NK, Naïve B, Plasma B and Monocytes were significantly enriched in responders compared to non-responders. Especially, CD8 Effector T, NK, Plasma B and Classical Monocytes showed distinct patterns that those cells were enriched in pre-treatment than post-treatment of responder but not in non-responder. From the differentially expressed gene analysis, cytotoxic or costimulatory marker genes (GZMK, GZMA, GNLY, CCL5, NKG7, PRF1) were enriched in responders. While, exhausted or coinhibitory marker genes (DNAJB1, LGALS9, HAVCR2) were enriched in non-responders. Gene set enrichment analysis revealed four pathways associated with T cell, B cell receptor signaling, NK cell mediated cytotoxicity and Cytokine-cytokine receptor interaction which showed differences between responders and non-responders following chemotherapy. Finally, validation with flow cytometry using independent cohort showed that constant expression manner in HAVCR2, LGALS9 and LGALS3 genes. Conclusions: Single-cell transcriptome analysis identified distinct PB immune landscapes associated with treatment response in HER2-positive metastatic breast cancer patients. Differential abundance and unique gene expression programs of immune cell populations could serve as potential predictive biomarkers for anti-HER2 therapy. Citation Format: Jongwon Lee, Jungmin Choi, In Hae Park. Single-cell transcriptome reveals distinct peripheral blood immune landscapes associated with sensitivity to anti-HER2 treatment in HER2-positive metastatic breast cancer patients [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 P5-02-51.

Abstract P5-14-04: Genomic characterization of hormone receptor-positive advanced breast cancer with high tumor mutational burden: fresh-frozen tissue genomic analysis from MUTATION-1 study (KCSG BR17-04)

Abstract Background The hormone receptor (HR)-positive metastatic breast cancer (MBC) patients show a diverse range of tumor mutational burden (TMB), but its biological and clinical implication has been largely unrevealed. Here we report genomic landscape of 117 HR+ MBC patients who were included in the pre-screening tissue genomic analysis of MUTATION-1 study (SABCS 2021; Abs P1-19-03) according to TMB of tumors. Patients and method The MUTATION-1 study (NCT03608865) enrolled HR-positive MBC patients who received prior ≥ 1 line of systemic therapy, and performed prescreening with whole exome sequencing (WES) and RNA-seq of fresh-frozen tissue of metastatic or recurred tumors. Patients who met upper 30% of TMB were eligible for treatment phase and received durvalumab plus tremelimumab. This study analyzed 117 prescreening tissues of MUTATION-1 study patients for mutation and transcriptomic landscape analysis. (WES, n=117; RNA-seq, n=107) Results The 117 patients showed diverse TMB (range 0~21.7 mut/Mb, median 2.0 mut/Mb) and genomic alterations. The most frequently mutated gene included PIK3CA (29.1%), TP53 (27.4%), ESR1 (23.9%), GATA3 (19.7%), and MAP3K1 (12.0%). There was no association between patient survival and TMB. We estimated single base substitution (SBS) mutational signature of patients with SigMA algorithm. The patients were classified according to their dominant mutational signatures: APOBEC (25.6%), HRD (41.0%), clockwise (28.2%), SBS8, and SBS17. The APOBEC patients showed higher TMB (median 3.47 mut/Mb) and higher mutation prevalence in PIK3CA (63.3% vs. 29.1%), ARID1A (16.7% vs. 6.0%), and NF1 (16.7% vs. 6.8%) compared with other patients. The high TMB positively correlated with time from MBC diagnosis to biopsy. Tumors with TMB ≥ 5 mut/Mb were exclusively found in patients diagnosed as MBC ≥ 36 months before the timing of biopsy. In the matched RNA-seq analysis, TMB was higher in luminal B and HER2-enriched intrinsic subtype patients than basal or luminal A subtype. The high TMB (≥ 3.16 mut/Mb, cutoff used for treatment phase patient selection) patients showed upregulation of G2/M checkpoint, MYC, E2F1, and MTORC1 signature compared to low TMB patients. In the tumor microenvironment analysis by CIBESORT, PIK3CA mutant patients showed lower score of cytotoxic T cell than others. Conclusions The high TMB in HR+ breast cancer was associated with longer time duration from MBC diagnosis to biopsy, high APOBEC signature, and cell cycle/MYC signature gene upregulation. Further therapeutic targeting of high TMB patients is warranted based on their genomic and immunologic characteristics. Citation Format: Min Hwan Kim, Yohan Yang, Eunyoung Kim, Yong Wha Moon, Gun Min Kim, Seul-Gi Kim, Yee Soo Chae, Jieun Lee, Jae Ho Jeong, Kyung-Hun Lee, Han Jo Kim, Joo Young Jung, Su-Jin Koh, Kyoung Eun Lee, Hee-Jun Kim, Kyong Hwa Park, Seungtaek Lim, Yeon Hee Park, Sangwoo Kim, Joo Hyuk Sohn. Genomic characterization of hormone receptor-positive advanced breast cancer with high tumor mutational burden: fresh-frozen tissue genomic analysis from MUTATION-1 study (KCSG BR17-04) [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 P5-14-04.

Abstract P3-05-26: Association of muscle mass and density with outcomes in patients with ER positive metastatic breast cancer: correlative analysis of ECOG-ACRIN 2112

Abstract Introduction: Observational data investigating the relationship between body habitus and survival or toxicity in breast cancer has been largely centered in the curative setting and focused on weight- based metrics, with variable and inconsistent results. Muscle is a large, active endocrine organ that affects physical function, drug metabolism, inflammation, and quality of life, but is not adequately measured by body weight alone. Very few studies have evaluated muscle measures in metastatic breast cancer (MBC) and have been focused on patients receiving cytotoxic chemotherapy. Here, we evaluate the impact of muscle mass and muscle density measured on CT scan on outcomes in patients with MBC receiving endocrine- based therapy. Methods: Baseline CT scans done at the time of study enrollment were centrally collected from participants in ECOG-ACRIN E2112, a randomized phase III study of exemestane with or without entinostat in MBC, which ultimately did not impact survival. A transverse cut at the L3 level was extracted and processed using semi-automated SliceOmatic software (Tomovision) by two independent investigators to obtain total body skeletal muscle mass and muscle attenuation. Low muscle mass was defined as skeletal muscle index (SMI, lean muscle area/height, cm2/m2) less than 41 and low skeletal muscle attenuation (SMA) was defined as average muscle density less than 41 HU, or less than 33 HU if the patient is overweight or obese by BMI. Chi-square tests were used to determine the association between SMI and SMA and other clinical characteristics, including body weight, race, and performance status. Multivariable Cox proportional hazard models were used to determine the association between low SMI or low SMA and overall survival (OS), progression free survival (PFS), and patient- reported outcomes. Results: Of the 608 patients randomized in E2112, 546 had analyzable CT scans and follow up data available. 45% (n=246) of participants had obesity by BMI (≥30); 39% (n=212) had low SMI and 56% (n=305) had low SMA. Obese patients were more likely to have higher SMI (p&amp;lt; 0.001); however, 9.5% (n=52) of the study population had both obesity and low SMI. Low SMA was associated with higher rate of obesity and worse performance status (p&amp;lt; 0.001), consistent with muscle quality being a predictor of functional status. Low SMI was not associated with survival outcomes (OS HR 1.04 95%CI 0.83-1.30, PFS HR 1.12 95% CI 0.92-1.36), nor was low SMA (OS HR 1.02 95%CI 0.81-1.28; PFS HR 1.02 95%CI 0.84-1.23). In addition, BMI was not related to survival outcomes. Conclusions: Low muscle mass and low muscle density are prevalent in estrogen receptor positive MBC patients. Muscle measures correlated with obesity and performance status; however, neither low SMI nor low SMA were associated with worse prognosis in this population. Further work is needed to refine body composition measurements and select optimal cutoffs and meaningful endpoints in specific breast cancer populations, particularly in those living with metastatic disease. Citation Format: Tarah J. Ballinger, Gloria Xue, Helga S. Marques, Constantine Gatsonis, Richard Hoffman, Kathy D. Miller, Fengmin Zhao, Joseph Sparano, Roisin Connolly. Association of muscle mass and density with outcomes in patients with ER positive metastatic breast cancer: correlative analysis of ECOG-ACRIN 2112 [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 P3-05-26.

Current Situation of Diagnosis and Treatment of HER2-Positive Metastatic Breast Cancer Patients in China: A Nationwide Cross-Sectional Survey of Doctors

The Advanced Breast Cancer Alliance conducted a nationwide investigation to understand the current situation of the diagnosis and treatment of human epidermal growth factor receptor 2 (HER2)-positive metastatic breast cancer (MBC) patients. In 2019, electronic questionnaires including basic information about respondents, characteristics of patients, and the present status of diagnosis and treatment were sent to 495 doctors from 203 medical centers covering 28 provinces. The factors that influenced treatment plans included the disease process, the performance status, and the economic status of patients. Regimens and response to neoadjuvant/adjuvant chemotherapy were important factors in the decision of the first-line treatment. Overall, 54% of doctors retained trastuzumab and replaced chemotherapy drugs in second-line treatment regimens for patients with progression-free survival (PFS) ≥ 6 months in the first-line setting, while 52% of participants chose pyrotinib plus capecitabine for patients with PFS < 6 months. Economic factors played an important role in doctors' decision-making and the varying treatment options for respondents in first-tier, second-tier, and other cities. This large-scale survey regarding the diagnosis and treatment of HER2-positive MBC patients revealed that clinical decisions made by Chinese doctors followed the guidelines, but their choices were constrained by economic factors.

Model-based prediction of effective target exposure for MEN1611 in combination with trastuzumab in HER2-positive advanced or metastatic breast cancer patients.

MEN1611 is a novel orally bioavailable PI3K inhibitor currently in clinical development for patients with HER2-positive (HER2+) PI3KCA mutated advanced/metastatic breast cancer (BC) in combination with trastuzumab (TZB). In this work, a translational model-based approach to determine the minimum target exposure of MEN1611 in combination with TZB was applied. First, pharmacokinetic (PK) models for MEN1611 and TZB in mice were developed. Then, in vivo tumor growth inhibition (TGI) data from seven combination studies in mice xenograft models representative of the human HER2+ BC non-responsive to TZB (alterations of the PI3K/AkT/mTOR pathway) were analyzed using a PK-pharmacodynamic (PD) TGI model for co-administration of MEN1611 and TZB. The established PK-PD relationship was used to quantify the minimum effective MEN1611 concentration, as a function of TZB concentration, needed for tumor eradication in xenograft mice. Finally, a range of minimum effective exposures for MEN1611 were extrapolated to patients with BC, considering the typical steady-state TZB plasma levels in patients with BC following three alternative regimens (i.v. 4 mg/kg loading dose +2mg/kg q1w, i.v. 8 mg/kg loading dose +6mg/kg q3w or s.c. 600 mg q3w). A threshold of about 2000 ng·h/ml for MEN1611 exposure associated with a high likelihood of effective antitumor activity in a large majority of patients was identified for the 3-weekly and the weekly i.v. schedule for TZB. A slightly lower exposure (i.e., 25% lower) was found for the 3-weekly s.c. schedule. This important outcome confirmed the adequacy of the therapeutic dose administered in the ongoing phase 1b B-PRECISE-01 study in patients with HER2+ PI3KCA mutated advanced/metastatic BC.

Pyrotinib for HER2-positive metastatic breast cancer: a systematic review and meta-analysis.

Human epidermal growth factor receptor 2 (HER2)-positive metastatic breast cancer patients continue to progress despite multiple anti-HER2-targeted treatments. A number of studies have found that Pyrotinib, a small-molecule pan-ErbB receptor tyrosine kinase inhibitor (TKI), is effective in treating patients with HER2-positive metastatic breast cancer. This systematic review and meta-analysis aimed to evaluate the efficacy and safety of Pyrotinib in the treatment of HER2-positive metastatic breast cancer. PubMed, Embase, Web of Science, and Cochrane Library databases were searched until February 2022. Research on HER2-positive metastatic breast cancer being treated with Pyrotinib in any line of therapy was included, both prospective and retrospective. Statistical pooling and meta-analysis of data from the included studies were performed to explore the efficacy and safety of Pyrotinib in HER2-positive metastatic breast cancer. In this meta-analysis, 23 studies were included. The overall objective response rate was 0.49 (95% CI: 0.40, 0.58) for Pyrotinib in HER2-positive metastatic breast cancer and 0.52 (95% CI: 0.32, 0.71) in those with brain metastases. The objective response rate of Pyrotinib was superior to that of other second-line therapeutics in comparison (RR =1.38, 95% CI: 1.25, 1.52), but was relatively inferior to trastuzumab emtansine (T-DM1) (RR =0.82, 95% CI: 0.36, 1.85). The combined median progression-free survivals (PFSs) for Pyrotinib in metastatic breast cancer and those with brain metastases were 8.2 (95% CI: 6.8, 9.5) months and 8.9 (95% CI: 6.2, 11.7) months, respectively. The most common adverse reaction was diarrhea with an all-grade incidence of 0.84 (95% CI: 0.74, 0.92), followed by nausea and vomiting of 0.52 (95% CI: 0.36, 0.68). In any line of treatment for HER2-positive metastatic breast cancer, the Pyrotinib-containing regimens demonstrated considerable tumor response, disease control, and survival with manageable adverse effects.

Real-world data of cardiotoxicity during long-term therapy with trastuzumab in human epidermal growth factor receptor-2-positive metastatic breast cancer

Introduction/Objective. This study aims to investigate the cardiotoxicity of long-term therapy with trastuzumab in patients with HER-2-positive metastatic breast cancer. Methods. A total of 48 patients with metastatic HER-2-positive breast cancer were analyzed. The patients received long-term trastuzumab (time of application was longer than 20 months). The analyzed characteristics of the patients were the following: age, initial stage of the disease, application of anti-HER-2 therapy and anthracyclines in the adjuvant setting, the number and type of applied systemic therapies concomitant with trastuzumab in the metastatic setting. Cardiac toxicity was assessed using left ventricular ejection fraction (LVEF) values at three time-points: at the beginning, in the middle, and at the end of treatment period for each patient separately. Results. In 17 (35.4%) patients the trastuzumab treatment was temporary discontinued. The average time of trastuzumab therapy interval was 52.2 ? 23.5 months. The mean LVEF values were 66.73 ? 7.02%, 64.62 ? 5.7%, and 63.44 ? 6.1%, respectively. The mean values of LVEF differed significantly in the three observed time-points (F = 4.9 p = 0.009). Post hoc pairwise comparison, using Bonferroni correction, confirmed significantly lower mean LVEF values at the end point (at the end of treatment) compared with the mean LVEF values at the beginning of anti-HER-2 treatment (p = 0.019), but within the reference range of LVEF ? 50%. Conclusion. The data confirm good safety profile of long-term trastuzumab therapy in HER-2 positive metastatic breast cancer patients considering cardiotoxicity.

Efficacy and safety of margetuximab plus chemotherapy vs. trastuzumab plus chemotherapy in Chinese patients with pretreated HER2-positive advanced metastatic breast cancer: results from a randomized, open-label, multicenter, phase II bridging study.

Trastuzumab is the recommended first-line treatment for human epidermal growth factor receptor 2 (HER2)-positive metastatic breast cancer (MBC) patients in China, but therapeutic resistance to trastuzumab and other early-line treatments is common and late-line treatment options are limited. Derived from the same murine precursor antibody, margetuximab has enhanced anti-tumor activity compared with trastuzumab and may be an effective late-line treatment. However, data regarding the use of margetuximab in pre-treated Chinese patients are scarce. This study aimed to evaluate the efficacy and safety of margetuximab plus chemotherapy vs. trastuzumab plus chemotherapy in Chinese patients, and to determine whether the results are consistent with the clinical benefit of margetuximab observed in the pivotal global phase III study. In this randomized, open-label, multicenter, phase II bridging study, eligible Chinese patients pretreated with ≥2 lines of anti-HER2 therapies were randomized 1:1 by stratified block randomization to margetuximab (15 mg/kg over at least 120 minutes) or trastuzumab (6 mg/kg over at least 30 minutes), each administered intravenously once every 21-day cycle and plus chemotherapy. Disease assessment was conducted once every two treatment cycles (6 weeks ± 7 days). The primary endpoint was progression-free survival (PFS) by blinded independent central review (BICR). Secondary endpoints included overall survival (OS), investigator-assessed PFS, objective response rate (ORR), duration of response (DoR), clinical benefit rate (CBR), and the incidence and severity of treatment-emergent adverse events (TEAEs). Between February 4, 2020 and February 23, 2021, 123 patients were randomized to the margetuximab (n=62) and trastuzumab (n=61) arms. Among them, 15 and 7 patients, respectively, were still on study treatment as of data cut-off (September 3, 2021). Overall, 99.2% were female, median age was 53 years old. All patients were pretreated with trastuzumab, and 83.7% and 25.2%, respectively, were pretreated with tyrosine kinase inhibitors (TKIs) and pertuzumab. Baseline characteristics were numerically balanced between arms. BICR-assessed median PFS (mPFS) was 5.5 months in the margetuximab arm and 4.1 months in the trastuzumab arm, with a hazard ratio (HR) of 0.69 [95% confidence interval (CI): 0.43-1.12], which met the consistency criterion (HR <0.88) for bridging success. Median investigator-assessed PFS was 5.5 months in the margetuximab arm and 4.0 months in the trastuzumab arm (HR =0.63; 95% CI: 0.41-0.96). Median OS (mOS) was not yet reached. Both ORR and CBR were greater in the margetuximab arm (25.5% vs. 12.5%; 32.7% vs. 14.3%). Safety results were numerically comparable between the two arms. Anti-HER2 treatment-related infusion-related reactions (IRRs) were more common in the margetuximab arm than in the trastuzumab arm (12.9% vs. 1.7%). All IRRs could be resolved. Margetuximab was effective and well-tolerated in this study, supporting its clinical use in pretreated HER2-positive MBC patients in China. ClinicalTrials.gov Identifier: NCT04262804.