HER2-directed Therapy Research Articles

SDPS-46 CENTRAL NERVOUS SYSTEM (CNS)-RELATED MORTALITY IN PATIENTS WITH HER2-POSITIVE METASTATIC BREAST CANCER

Abstract BACKGROUND Approximately one third of patients with HER2-positive (+) metastatic breast cancer (MBC) develop brain metastases (BrM). In light of discordant control of extracranial metastases (ECM) versus BrM for standard-of-care antibody-based therapies for these patients, and the frequent use of radiation and surgical therapies particularly for patients with CNS-centric disease, we sought to determine the proportion of HER2+ MBC patients with BrM who suffered neurologic death in the CNS-only and intracranial-plus-extracranial metastasis settings, and the clinical risk factors of such. METHODS Clinicopathologic data for HER2+ MBC patients with BrM treated at an NCI-designated Comprehensive Cancer Center between August 2010 and April 2022 were collected, including disease burden at cancer presentation, the timing and pattern of BrM and ECM, and neurologic complications. CNS-related death was defined as any mortality caused by BrM, leptomeningeal progression, or BrM-associated complication. RESULTS 275 patients were included. 58/275 (21%) presented with CNS as only and first site of metastasis; the remaining (217/275, 79%) developed BrM synchronously with or subsequent to ECM. 125/275 (45%) had de novo MBC, and nearly all patients were treated with CNS therapies (254/275 [92%] received at least one radiation treatment and 88/275 [32%] underwent BrM resection). After a median follow-up of 2.5 years, 193/275 (70%) patients died, of whom 105 (54%) of CNS-related cause. The median OS was 3 years (95% CI=1.47-5.14) and 2 years (95% CI=1.74-2.34) for patients with CNS-only versus BrM concurrently/subsequent to ECM, respectively (p=0.059). The cumulative incidence of CNS-related death at 5 years was 40% (95%CI=33-46%). CONCLUSIONS Almost half of patients with HER2+ BrM suffered neurologic death. Patients with CNS-only disease had better OS than patients with synchronous/preceding ECM, suggesting that overall burden of disease affects mortality. With high rates of durable extracranial control with HER2-directed therapies, new strategies to prevent and eradicate CNS disease are urgent.

Treatment Algorithm for Patients With Gastric Adenocarcinoma: Austrian Consensus on Systemic Therapy - An Update.

Over the last decade, therapeutic options for patients with gastric cancer have improved significantly. However, despite these recent advances, mortality is still substantial. Surgery and chemotherapy represent the cornerstones of patient management. Immune checkpoint inhibitors as well as targeted treatments such as HER2-directed therapies and antiangiogenic agents contribute to improved patient prognosis. Herein, we present the updated version of an Austrian consensus on the systemic treatment of patients with gastric adenocarcinoma and adenocarcinoma of the lower gastroesophageal junction, including those with human epidermal growth receptor 2 (HER2) overexpression, microsatellite instability, programmed death-ligand 1 (PD-L1)-positive disease, and claudin 18.2 positivity. The consensus considers the curative setting as well as first-line and later-line systemic treatment options in advanced disease. For HER2-positive disease, HER2 testing is discussed in addition to a review of first-line and later-line therapies. Potential future therapies are also listed, with a focus on targeted [e.g., fibroblast growth factor receptor 2 (FGRF2)-directed] treatments that might provide a further step forward in the management of patients with gastric cancer.

GASTric Cancer HER2 Re-Assessment Study 2 (GASTHER2): HER2 Re-assessment for Initially HER2-Negative Advanced Gastric Cancer Patients after Progression on First-Line Treatment.

Heterogeneous human epidermal growth factor receptor 2 (HER2) overexpression in gastric cancer may lead to a misdiagnosis of HER2 status. Accurate assessment of HER2 status is essential for optimal treatment as novel HER2-directed agents are being investigated in various clinical settings. We evaluated the usefulness of HER2 re-assessment following progression on first-line treatment in initially HER2-negative advanced gastric cancer (AGC) patients. We enrolled 177 patients with baseline HER2-negative AGC and performed HER2 re-assessment after progression on first-line treatment from February 2012 to June 2016 at Asan Medical Center, Seoul, Korea. The re-assessed HER2 status was analyzed with baseline HER2 status and clinical characteristics. The median age was 54 years (range, 24 to 80 years), and 123 patients (69.5%) were men. Seven patients (4.0%) were HER2-positive on the re-assessment. Patients with baseline HER2 negativity confirmed by a single test (n=100) had a higher HER2-positive re-assessment rate compared to those who had repeated baseline testing (n=77) (5.0% vs. 2.6%). Among the patients with single baseline HER2 testing, the rate was higher in patients with baseline HER2 immunohistochemistry (IHC) 1+ compared to those with IHC 0 (13.4% vs. 3.6%). Overall, 4.0% of patients with baseline HER2-negative AGC were HER2-positive on re-assessment, and the HER2-positive re-assessment rate was higher among patients who had a single test at baseline. HER2 re assessment may be considered for initially HER2-negative patients to determine their eligibility for HER2-directed therapy, particularly if their HER2 negativity was determined by a single test, especially if they had a single baseline HER2 IHC 1+ test.

Clinical outcomes of de novo metastatic HER2-positive inflammatory breast cancer

Inflammatory breast cancer (IBC) is a rare, aggressive form of breast cancer that presents as de novo metastatic disease in 20–30% of cases, with one-third of cases demonstrating HER2-positivity. There has been limited investigation into locoregional therapy utilization following HER2-directed systemic therapy for these patients, and their locoregional progression or recurrence (LRPR) and survival outcomes. Patients with de novo HER2-positive metastatic IBC (mIBC) were identified from an IRB-approved IBC registry at Dana-Farber Cancer Institute. Clinical, pathology, and treatment data were abstracted. Rates of LRPR, progression-free survival (PFS), overall survival (OS), and pathologic complete response (pCR) were determined. Seventy-eight patients diagnosed between 1998 and 2019 were identified. First-line systemic therapy comprised chemotherapy for most patients (97.4%) and HER2-directed therapy for all patients (trastuzumab [47.4%]; trastuzumab+pertuzumab [51.3%]; or trastuzumab emtansine [1.3%]). At a median follow-up of 2.7 years, the median PFS was 1.0 year, and the median OS was 4.6 years. The 1- and 2-year cumulative incidence of LRPR was 20.7% and 29.0%, respectively. Mastectomy was performed after systemic therapy in 41/78 patients (52.6%); 10 had a pCR (24.4%) and all were alive at last follow-up (1.3–8.9 years after surgery). Among 56 patients who were alive and LRPR-free at one year, 10 developed LRPR (surgery group = 1; no-surgery group = 9). In conclusion, patients with de novo HER2-positive mIBC who undergo surgery have favorable outcomes. More than half of patients received systemic and local therapy with good locoregional control and prolonged survival, suggesting a potential role for local therapy.

Real world evidence for continuing HER2-directed therapy for metastatic breast cancer.

e18623 Background: Continuing HER2-directed therapy in metastatic breast cancer (MBC) beyond first line has been demonstrated to improve progression free survival (PFS) and overall survival (OS) in many randomized studies. Due to cost, many publicly funded health care systems restrict public funding to 2 lines of HER2 directed therapy (HER2Rx) in MBC. The purpose of this study is to assess the proportion of patients with HER2 positive MBC who were eligible to receive systemic therapy beyond second-line and successfully accessed continued HER2Rx despite lack of public funding. We also analyzed how the ability to access continued HER2Rx impacts survival. Methods: The BC Cancer Breast Cancer Outcomes Unit collects clinical, pathological and outcome data on patients diagnosed within BC. In addition, all publically funded anti-neoplastic therapy is recorded in the BC Cancer pharmacy database. These 2 databases were cross-referenced to identify patients who received any HER2Rx for MBC between 2013 and 2018, in the era where trastuzumab plus pertuzumab and TDM-1 were standard, publically available options. The number of lines of therapy received, treatment choice, and fitness to continue therapy beyond 2 lines were analyzed by manual chart review. Survival data was analyzed. Results: 571 patients met inclusion criteria. Median follow up was 47.6 months. Overall, 267 (47%) patients were eligible to continue therapy beyond second-line. Of these, 210 (37% of the whole cohort) accessed continued HER2-directed therapy, while 57 (10%) were eligible but unable to access continued HER2Rx. Of the remaining 304 patients in the total cohort, 110 patients (19%) had stable disease on continued first- or second-line therapy and 194 patients (34%) had deteriorating status precluding further treatment beyond second-line. The median lines of therapy in the entire study population was 3 (range 1-12). Median number of cycles of HER2-directed therapy received beyond second-line for those eligible to continue was 18 cycles. Median OS for those who continued HER2Rx was 59.7 months compared to 31.4 months for those who were eligible but unable to access continued HER2Rx; HR 0.42 (95% CI 0.31-0.58, p < 0.001). The vast majority of patients who continued to access HER2Rx did so through clinical trial or patient assistance programs, very rarely through private health insurance or out-of-pocket expenditures. Conclusions: Nearly half of patients with HER2 positive MBC were eligible to receive more than 2 lines of HER2 directed therapy. The majority of eligible patients do ultimately secure access despite prohibitive public funding policies. Receiving further HER2Rx was associated with a significant and clinically meaningful OS benefit of 28 months over those who go on to receive non-HER2 directed therapy. This survival advantage provides real-world evidence in support of the urgent need for more permissive public funding of HER2 directed therapy for MBC.

Neoadjuvant HER2-targeted therapy ± immunotherapy with pembrolizumab (neoHIP): An open label randomized phase II trial.

TPS634 Background: Immune checkpoint inhibition (ICI) is synergistic with HER2-directed therapy in pre-clinical models. Clinically, pembrolizumab (K)-mediated ICI plus HER2-directed therapy with trastuzumab (H) was safe and demonstrated modest activity in H-resistant HER2-positive (HER2+) metastatic breast cancer. Because ICI may confer more robust activity when administered earlier in the course of disease, H and K administered in the curative-intent, treatment-naive setting may allow for de-escalation of cytotoxics; confer life-long, tumor-specific immunity; and ultimately, improve cure rates. Moreover, the synergy of H and K with paclitaxel (T) may overcome the need for dual HER2-blockade with H plus pertuzumab (P). In this randomized, multicenter, phase II, open-label trial the efficacy and safety of neoadjuvant THP vs THP-K vs TH-K are explored. Methods: Patients (pts) ≥18y with previously untreated, stage II-III, HER2+ breast cancer are being randomized and stratified by clinical nodal status (positive vs. negative) and hormone receptor status (positive vs. negative). In arm A, pts received T at 80mg/m2 weekly for 12 weeks, H at 8mg/Kg (loading dose) and then 6mg/Kg every 3 weeks x 3 doses, P at 840 mg (loading dose) and then 420mg/Kg every 3 weeks x 3 doses (THP). In arm B, pts received THP plus K at 200mg every 3 weeks x 4 doses (THP-K). In arm C, pts received TH-K. After enrollment of 22 pts to arm C, a prespecified interim efficacy analysis was conducted, and enrollment to this arm was subsequently terminated. Enrollment to the other arms continues with 32/58 pts enrolled to arm A and 33/58 pts enrolled to arm B as of 2/14/2023. Definitive surgery is 3-6 weeks after the last dose. After surgery, pts are treated per the treating physician’s discretion per local clinical standard. The primary end point is pathologic complete response (pCR) rate in the breast and axilla (ypT0/Tis ypN0). Secondary end points include pCR rate by ypT0ypN0 and ypT0/Tis, residual cancer burden index, event free survival, breast conserving surgery rate, safety and overall survival. Exploratory correlative studies will characterize potential immune biomarkers predictive of efficacy and/or toxicity. Clinical trial information: NCT03747120 .

Association of HER2/CEP17 ratio with pCR after HER2-directed neoadjuvant treatments in the phase III NeoALTTO trial.

600 Background: HER2-directed therapies are approved for the treatment of patients with HER2-positive invasive breast cancer as defined by HER2 protein overexpression, or HER2 gene amplification based on HER2/chromosome enumeration probe (CEP17) ratio ≥ 2.2. Above this accepted HER2 status determination, however, it is still unknown whether the efficacy of HER2-directed therapy in early breast cancer increases with increasing HER2/CEP17 ratios. Therefore, the purpose of the presented work is to evaluate whether quantitative assessment of the HER2/CEP17 ratio predicts pathological complete response (pCR) and event-free survival (EFS) of patients treated with neo-adjuvant HER2-based regimen in the prospective phase III NeoALTTO trial. Methods: 455 women with HER2-positive early breast cancer, who had received neo-adjuvant trastuzumab and/or lapatinib for 6 weeks and then together with 12 cycles of weekly paclitaxel were included in this analysis. The HER2/CEP17 ratio in the primary tumor samples was correlated with pCR and survival outcome. Results: The Median HER2/CEP17 ratio in NeoALTTO was 5.1 (range: 1.1 – 100.0), and ratios were not associated with age, hormone receptor status, or any of the other clinicopathological variables analyzed. The log HER2/CEP17 ratio significantly predicted for pCR in both uni-variate (OR: 1.83; 95% CI: 1.11 - 3.01, p = 0.0176) and multivariate analysis (OR: 1.79; 95% CI: 1.07 - 2.99, p = 0.0257). Higher HER2/CEP17 ratios were, however, not associated with improved EFS (adjusted HR = 0.79; p = 0.3537). A pCR prediction model which included HER2/CEP17 ratio, treatment arm, and hormone receptor status improved the predictive strength of treatment arm alone from a ROC AUC value of 0.60 to 0.69. Conclusions: In patients treated with HER2-based neoadjuvant therapy, quantitative analysis of the readily available pre-treatment HER2/CEP17 ratio by FISH is predictive of pCR but not EFS.

Real-world treatment patterns in patients with HER2+ unresectable or metastatic breast cancer: Interim results from HER2 REAL Asia cohort.

1047 Background: HER2 REAL (NCT04857619) is the first multi-country, retrospective study exploring the treatment practices and outcomes in patients (pts) with HER2+ locally advanced, unresectable (u)/metastatic (m) breast cancer (BC) from routine clinical care in the APAC and LATAM. Methods: Adult HER2+ u/mBC pts diagnosed since reimbursement or wider access of trastuzumab emtansine (T-DM1) or 01 Jan 2017, whichever was earlier with ≥12 months (mo) of follow-up data from index date (u/mBC diagnosis) and treated with ≥1-line of therapy (LOT) were enrolled per medical chart review from 6 countries. Here we present interim descriptive analyses (cut-off 30 June 2022) on demographics, clinical characteristics, and treatment patterns. Results: Of the 763 enrolled pts, 370 with median (range) age of 55 (20–81) yrs from Hong Kong, Korea, Singapore, and Taiwan were eligible for interim analyses; 368 (99.5%) were female with 210 (57.1%) postmenopausal. A total of 210 (57.1%) pts with reported data had a median time of 2 (0–18) yrs from initial BC diagnosis to u/mBC relapse. At index date, majority had ductal carcinoma (227/254 [89.4%]), visceral (255/355 [71.8%]), and non-visceral (258/355 [72.3%]) metastases; 80/355 (22.5%) had CNS metastases. Family history of BC was reported by 45/370 (12.2%) pts completing the questionnaire; 209/370 (56.5%) had hormone receptor-positive BC. Treatments reported in the first to fifth line were quite variable. HER2-directed therapy was received by 323/367 (88.0%) in LOT1 (mainly trastuzumab [TRA] and pertuzumab [PTZ]-based) and 298/343 (86.9%) in LOT2 (mainly T-DM1). Median duration of LOT1 and LOT2 were 7.8 (0–130) and 5.0 (0–100) mo, respectively, that decreased to 2.3 (0–30) mo in LOT5. The prime reason for treatment discontinuation was disease progression (LOT1: 294 [80.1%]; LOT2: 273 [79.6%]; LOT3: 187 [68.5%]). Serious adverse effects from treatments were rare (LOT1: 7 [1.9%]; LOT2: 10 [2.9%]; LOT3: 5 [1.8%]). Conclusions: The interim analyses show heterogeneous treatment patterns in real-world among pts with HER2+ u/mBC progressing on HER2-directed therapies possibly due to access disparity in these 4 Asian countries. Clinical trial information: NCT04857619 . [Table: see text]

De novo metastatic Her2+ breast cancer in a diverse population of women under age 40.

e13045 Background: Breast cancer (BC) is the most common malignancy among women. Her2 mutation has historically been associated with more aggressive disease and worse prognosis, however this paradigm is changing with Her2-directed therapies. Her2 positive BC presents with de novo metastatic disease more often than other subtypes. Young women are more likely to be diagnosed with Her2 positive BC, however previous studies have not had adequate representation of Black/African American women. We evaluated a diverse group of young women under the age of 40 to determine if Her2+ disease was more likely to present with de novo metastatic disease. Methods: We performed a retrospective chart review and collected data on 746 women diagnosed with BC under the age of 40 from 2012 to 2022. We collected age at diagnosis, Her2 receptor status, self-identified race, and stage at diagnosis. 187 of these patients were Her2 positive, and 276 patients were Black. We stratified patients as Black versus White, and as stage IV at diagnosis versus stages I to III at diagnosis. We performed Fisher's exact test to determine if Her2 positive BC was more or less likely to present as de novo metastatic disease compared to Her2 negative BC. Results: In our population of 746 young women under the age of 40 presenting to our health system in a 10-year period, 48 (6.4%) presented with de novo metastatic BC, and 19 (39.6%) of these were Her2 positive. 36% of our study population was Black, which is representative of the demographics of our region. We performed Fisher's exact test, and found that Her2 positive BC was more likely to present as de novo metastatic disease compared to Her2 negative BC. This was statistically significant (p = 0.024). When divided for race, this was not statistically significant for White patients alone (p = 0.052) or Black patients alone (p = 0.234). Conclusions: Our data shows that Her2 positive BC in women under the age of 40 is statistically more likely to present with de novo metastatic disease. Previous research in this area has not been inclusive of Black patients. Current guidelines only recommend staging imaging among patients with systemic symptoms. Based on this data, consideration should be made for staging scans for young women presenting with new HER2+ breast cancer diagnoses. There is limited data available for breast cancer subtype incidence and prognosis among Black patients, and this study underscores the need for more inclusive study populations.[Table: see text]

Clinical and molecular characterization of HER2-low metastatic colorectal cancer.

e15573 Background: HER2 is a well-established biomarker and target in multiple tumor types, including metastatic colorectal cancer (mCRC). Despite the lack of HER2 amplification, certain HER2-low tumors were recently found to respond to novel HER2-directed antibody-drug conjugates, warranting an exploration of the clinical and molecular features of these entities. Yet, the real-world prevalence and the clinicopathologic characteristics of HER2-low mCRC are largely unknown. Here, we aim to describe the features of HER2-low mCRC and compare them to HER2-0 colorectal tumors. Methods: We retrospectively reviewed clinicopathologic data from consecutive patients with mCRC undergoing HER2 testing (immunohistochemistry [IHC] and in-situ hybridization [ISH], when appropriate) between 2017 and 2021 at Hadassah Medical Center, Israel. Tumors were defined HER2-positive (HER2+) if scored IHC 3 + or IHC 2+/ISH positive; HER2-low if scored IHC 2+/ISH negative or IHC 1+; HER2-0 if scored IHC 0. Genomic alterations were determined through a targeted next-generation sequencing (NGS) panel. Clinicopathologic and genomic data were compared between the cohorts with the Mann-Whitney or Fisher's exact test, and survival using Log-rank test. Results: A total of 100 consecutive mCRC patients were found eligible for our study, including 4 (4%) with HER2+, 30 (30%) with HER2-low, and 66 (66%) with HER2-0 disease. In the HER2-low group, five tumors were HER2 +2 / ISH negative, and 25 were scored HER2 +1. The fraction of female patients was 40% in HER2-low group, and 42% in the HER2-0 cohort (p > 0.99). The prevalence of mismatch repair deficient (MMR-D) tumors was also similar, with one MMR-D tumor in the HER2-low group, compared to 3 tumors in the HER2-0 (p > 0.99). 60% of the primary tumors in both groups originated in the left colon and rectum. Also, the prevalence of mucinous histology was comparable between the HER2-low and the HER2-neg cohorts (7.4% vs. 4.8%, p = 0.64). Among patients receiving local somatic NGS testing (n = 61), the fractions of patients with KRAS, BRAF, and TP53 mutations were largely comparable between the cohorts (Table 1). With a median follow up of 37 months, the median overall survival was 41 months for the HER2-low group and 51 months for the HER2-neg group (p = 0.15). Conclusions: In our cohort of patients with HER2-negative mCRC, the clinicopathologic and genomic characteristics of HER2-low and HER2-0 tumors did not differ significantly. However, our data suggest that HER2-low tumors represent a significant portion of mCRC, supporting further development of novel HER2-directed therapies for this population. Additionally, our data also support the notion that HER2-low, as currently defined, is not a distinct molecular subtype of mCRC. [Table: see text]

Anti-HER2 antibody inetetamab plus camrelizumab and utidelone for pretreated HER2-positive metastatic breast cancer: Final results from the phase 2 ICU trial.

1042 Background: We previously reported (Yan, ASCO 2022) promising efficacy and acceptable safety of anti-HER2 antibody inetetamab in combination with camrelizumab and utidelone regimen in preliminary 20 HER2-positive metastatic breast cancer (MBC) patients who progressed after at least two lines of HER2-directed therapies with trastuzumab and TKIs. Inetetamab is a novel anti-HER2 antibody. Camrelizumab is an anti-programmed death receptor 1 [PD-1] antibody. Utidelone, a genetically engineered epothilone analogue, has shown promising efficacy in heavily pretreated MBC. Here we report the final ICU study results. Methods: In the ICU study (NCT04681287), patients received intravenous camrelizumab (200 mg once every 3 weeks), inetetamab (initial dose of 8 mg/kg, then 6 mg/kg, once every 3 weeks), and utidelone (30 mg/m2, days 1-5, every 3 weeks) until disease progression or intolerable toxicity. The primary endpoint was 3-month progression-free survival (PFS) rate according to the Response Evaluation Criteria in Solid Tumors, version 1.1. Secondary endpoints included objective response rate (ORR), PFS and safety. Simon’s two-stage design was adopted for this study. All AEs were graded according to the grading criteria in the Common Terminology Criteria for the Evaluation of Adverse Events, 5th edition. Results: From Apr 23 2021 to Sept 1 2022, a total of 46 HER2-positive MBC patients were enrolled with median age of 52 (range 43-57). As of Jan 18, 2023, the median follow-up was 11.37 months. There were 12 patients (26.09%) with ER+/PR+, 7 patients (15.22%) with ER+/PR-, one patient (2.17%) with ER-/PR+, 26 patients (56.52%) with ER-/PR-. The median number of previous systemic therapies for advanced disease was three. Previous HER2-targeted therapies included trastuzumab (46 [100%]), TKIs (46 [100%], of which pyrotinib 39 [84.78%] and lapatinib 12 [26.09%]), pertuzumab (13 [28.26%]), and margetuximab (3 [6.52%]), T-DM1 or other HER2-ADC (7 [15.22%]). The 3-month PFS rate in 46 patients was 71.84%. The confirmed ORR was 28% (13/46), include one (2.17%) patient achieved complete response. The median PFS was 5.59 months. The most common treatment-related adverse events (TRAEs) were peripheral neuropathy (40 [86.96%]), capillary proliferation (27 [58.7%]), and alopecia (17 [36.96%]). Grade ≥3 TRAEs included rash (3 [6.52%]), peripheral neuropathy (1 [2.17%]) and AST increase (1 [2.17%]). Moreover, there were no grade 4 or above TRAEs. No related treatment discontinuation or deaths occurred. Conclusions: Final efficacy and safety results were consistent with previous ICU study preliminary analyses. The ICU study showed a favorable benefit-risk profile and is an important option for Chinese patients with HER2-positive MBC after at least two lines of HER2-directed therapies with trastuzumab and TKIs. Clinical trial information: NCT04681287 .

Abstract 4639: Single-cell immunoprofiling and spatial analysis of hormone receptor subtypes in HER2+ and HER2low breast tumors using multiplexed immunofluorescence

Abstract Background: Breast cancer is characterized by distinct molecular subtypes based on expression of estrogen and progesterone hormone receptors (ER and PR), and epidermal growth factor receptor 2 (HER2). To investigate presence and location of distinct immune cell populations on single cell level we utilized the multiplexed immunofluorescence (mIF) platform MultiOmyx to investigate the tumor immune microenvironment (TME) in HER2+ breast cancer. We have built on growing data implicating distinct immunophenotypes in the breast cancer TME with breast cancer outcomes by profiling 1) prevalence; 2 location (e.g. intratumoral, stromal); 3) phenotype (e.g. activated, exhausted) of infiltrating immune cells. Methods: We optimized a custom 26-marker MultiOmyx panel interrogating HER2 IF expression, HER2 signaling, and immune markers. This mIF platform leverages serial IF image capture to allow concurrent profiling of all 26 markers on a pathologic section at single cell resolution. We applied the 26-marker panel to a tissue microarray of 208 unique patients with matched tumor/normal tissue cores (1-4 cores/patient; total 333 tumor and 307 normal cores). HER2-positive was defined via ASCO/CAP guidelines; HER2-low was defined as HER2 immunohistochemistry (IHC) 1+/2+ but HER2 in-situ hybridization (ISH) negative. Results: The 208 unique tumors profiled included 88.9% (185/208) HER2-positive and 11.1% (23/208) HER2-low; 62.5% (130/208) hormone receptor (HR) positive and 37.5% (78/208) HR negative. Median follow-up from diagnosis was 143 months. Among HER2-positive patients, 98.9% (n=183/185) received HER2-directed therapy in the (neo)adjuvant or metastatic setting or were diagnosed prior to FDA approval of trastuzumab for early stage disease. In sum, from 1166 regions of interest in 640 total cores, a total of 1,076,700 single cells were profiled via the 26-marker panel. Tumors categorized based on HR and HER2 reflected distinct immunophenotype. Relative to HR+HER2low, HR-HER2low tumors had a significant increase in the density of T cells (CD3) and tumor-associated macrophages (TAMs) (CD68). The increase in T cells was observed for T helper (CD3+CD4+), T regulatory (CD3+CD4+FoxP3+), as well as T cytotoxic cells (CD3+CD8+). Observed increases in pro-tumorigenic M2 TAM density (CD68+CD163+) were observed for both HR negative subtypes when compared to HR+HER2+ and HR+HER2low groups, indicating a negative correlation between M2 TAM infiltration and ER/PR status. HER2low tumors had significantly lower tumor PDL1+ than HER2+ tumors. Additionally, 18 of the 208 patients with equal distribution across the hormone-receptor subtypes underwent whole slide multiplexed analysis to perform a deeper spatial analysis. Conclusions: In a large, clinically annotated cohort of breast cancers distinct immunophenotypes are evident among HR and HER2+ subsets. Citation Format: Anna Juncker-Jensen, David Tallman, Harry Nunns, Heather Lefebvre, Karen Yamamoto, Katharine A. Collier, Mark Vater, Ava Strahan, Ava Willoughby, Olivia Bouchard, Madison Kingsbury, Mathew Cherian, Ashley C. Pariser, Preeti K. Sudheendra, Bhuvaneswari Ramaswamy, Margaret Gatti-Mays, Ainura Kyshtoobayeva, Zaibo Li, Daniel G. Stover. Single-cell immunoprofiling and spatial analysis of hormone receptor subtypes in HER2+ and HER2low breast tumors using multiplexed immunofluorescence. [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 4639.

Incidence and prognostic impact of HER2-positivity loss after dual HER2-directed neoadjuvant therapy for HER2+ breast cancer.

Loss of HER2 "positivity" can occur in patients with residual disease after neoadjuvant treatment, but the incidence of HER2-positivity loss after neoadjuvant dual HER2-targeted treatment plus chemotherapy, the current standard-of-care for most early stage HER2-positive breast cancers, is not well described. Previous studies that report the HER2 discordance rate after neoadjuvant treatment also do not include the novel HER2-low category. In this retrospective study, we determine the incidence and prognostic impact of HER2-positivity loss, including the evolution to HER2-low disease, after neoadjuvant dual HER2-targeted therapy with chemotherapy. Clinicopathologic data for patients with stage I-III HER2+ breast cancer diagnosed between 2015 and 2019 were reviewed in this single institution retrospective study. Patients who received dual HER2-targeted treatment with chemotherapy were included, and HER2 status before and after neoadjuvant therapy was interrogated. A total of 163 female patients were included in the analysis with a median age of 50 years. A pathologic complete response (pCR as defined by ypT0/is) was achieved in 102 (62.5%) of 163 evaluable patients. Among the 61 patients with residual disease after neoadjuvant therapy, 36 (59.0%) had HER2-positive and 25 (41.0%) had HER2-negative residual disease. Of the 25 patients with HER2-negative residual disease, 22 (88%) of patients were classified as HER2-low. After a median follow-up of 3.3 years, patients who retained HER2-positivity after neoadjuvant treatment had a 3-year IDFS rate of 91% (95% CI, 91%-100%), while patients who lost HER2-positivity had a 3-year IDFS rate of 82% (95% CI, 67%-100%). Almost half of patients with residual disease following neoadjuvant dual HER2-targeted therapy plus chemotherapy lost HER2-positivity. The loss of HER2-positivity may not confer negative prognostic impact, although the results were limited by short follow-up time. Further research on the HER2 status after neoadjuvant treatment may help guide treatment decisions in the adjuvant setting.

Long-term outcomes of dual vs single HER2-directed neoadjuvant therapy in NSABP B-41.

The primary aim of this randomized neoadjuvant trial in operable, HER2-positive breast cancer, was to determine the efficacy on pathologic complete response (pCR) of substituting lapatinib (L) for trastuzumab (T) or adding L to T, in combination with weekly paclitaxel (WP) following AC. Results on pCR were previously reported. Here, we report data on planned secondary endpoints, recurrence-free interval (RFI) post-surgery, and overall survival (OS). All patients received standard AC q3 weeks × 4 cycles followed by WP (80mg/m2) on days 1, 8, and 15, q28 days × 4 cycles. Concurrently with WP, patients received either T (4mg/kg load, then 2mg/kg) weekly until surgery, L (1250mg) daily until surgery, or weekly T plus L (750mg) daily until surgery. Following surgery, all patients received T to complete 52 weeks of HER2-targeted therapy. 522 of 529 randomized patients had follow-up. Median follow-up was 5.1years. RFI at 4.5years was 87.2%, 79.4% (p = 0.34; HR = 1.37; 95% CI 0.80, 2.34), and 89.4% (p = 0.37; HR = 0.70; 0.37, 1.32) for arms T, L, and TL, respectively. The corresponding five-year OS was 94.8%, 89.1% (p = 0.34; HR = 1.46; 0.68, 3.11), and 95.8% (p = 0.25; HR = 0.58; 0.22, 1.51), respectively. Patients with pCR had a much better prognosis, especially in the ER-negative cohort: RFI (HR = 0.23, p < 0.001) and OS (HR = 0.28, p < 0.001). Although pCR, RFI, and OS were numerically better with the dual combination and less with L, the differences were not statistically significant. However, achievement of pCR again correlated with improved outcomes, especially remarkable in the ER-negative subset. NCT00486668.

Abstract P1-11-11: HER2-directed therapy in early breast cancer – improvement over 20 years

Abstract Introduction: HER2-positive breast cancer is considered aggressive, but due to the development of targeted drugs in the past 20 years, a substantial improvement of therapy results can be postulated. Proven evidence is provided by data of randomized clinical trials. The present study aimed at demonstrating the improvement of therapy results over time using single-center real world data. Material and Methods: In our center, we prospectively built a consecutive tumor and data base from 2000 to 2020 and identified patients with early HER2-positive breast cancer (n=396). The cohort was divided in four groups by year of diagnosis according to the changing therapy concepts: A) 2000-2004 (no HER2-directed therapy, n=83), B) 2005-2011 (trastuzumab, n=96), C) 2012-2017 (trastuzumab/pertuzumab, n=135), D) 2018-2020 (T-DM1 in patients with non-pCR), n=55). HER2 was measured by IHC and ISH corresponding to the ASCO-CAP guidelines. HER2-directed therapy was indicated according to the respective national guidelines (AGO). The median follow-up was 58 months in the entire cohort (1-266), group A 84 months (10-266), group B 94 months (2-201), group C 56 months (1-118), group D 32 months (8-53). The primary endpoint was overall survival (OS), and secondary endpoint was iDFS. Kaplan-Meier estimates were calculated and multivariate analyses were performed using SPSS 28 (IBM, Armonk, NY, USA). Results: Overall, 57% of the 396 patients had an age of 50 years or more, 85% had a NST histology, 54% of the tumors were larger than 2cm, 45% were node-positive; 34% were steroid hormone receptor negative. HER2-directed therapy was delivered to 7 of 83 pts in group A (8.4%), to 62 of 96 pts in group B (64.6%), to 119 of 135 in group C (88.15%), and to 50 of 55 pts in group D (90.91%), resp.. Overall, in the first 3 years we observed 19 deaths and the probability for OS was 89.7 % for group A, 92.2 % for group B, 96.4 % for group C and 100% for group D, resp.; iDFS was 77.4% in A, 86.8% in B, 94.9% in C and 95.2% in D, resp.. After adjustment for nodal status, grading and steroid hormone receptor status, we calculated the effect size of the incremental improvement of treatment for OS and iDFS as HRs referred to group A (Table 1). Conclusion: With this prospectively established single center cohort, we are able to confirm a significant improvement of the treatment results in patients with HER2-positive early breast cancer over the last 20 years applying individualized HER2-directed therapies. Table 1. Multivariate analyses of OS and iDFS adjusted for nodal status, grading and hormone receptor status. Citation Format: Christoph Thomssen, Julia Engel, Kristin Reinhardt, Hans-Georg Strauß, Regina Große, Susanne Barrot, Marcus Bauer, Lisa van Uden, Sandy Kaufhold, Kathleen Schüler, Vanessa Wieder, Eva J. Kantelhardt, Martina Vetter. HER2-directed therapy in early breast cancer – improvement over 20 years [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 P1-11-11.

Abstract MS1-3: Treatment of ER+/HER2+ breast cancer

Abstract Co-expression of hormone receptors (HR) and human epidermal growth factor receptor 2 (HER2) occurs in 10% of all breast cancers, and due to cross-talk mechanisms that influence cell signalling the co-expression of these receptors modulates response of these cancers to both HER2-directed and endocrine therapies. Prior to the advent of HER2 targeted therapies, patients with HR+/HER2+ breast cancer had a significantly worse prognosis compared with HR+/HER2- breast tumours, primarily related to HER2 signalling driving endocrine resistance. Likewise, prior clinical studies in the neoadjuvant setting showed that patients with HR+/HER2+ primary breast cancer had a lower pathological complete response rate to HER2-targeted therapy plus chemotherapy that those with HR-/HER2+ tumours, suggesting a different biology. As such, strategies that combine receptor blockade against HER2 and HR have been developed in metastatic breast cancer (MBC), confirming that combined blockade is better than endocrine therapy (ET) alone, and then that dual HER2 blockade + ET was superior to single HER2 therapy + ET. More recently combinations of a CDK 4/6 inhibitor with ET and HER2 therapy has shown significant clinical efficacy and survival benefit in heavily pre-treated patients, confirming the efficacy for non-chemotherapy targeted combinations in HR+/HER2+ MBC. In the early breast cancer (EBC) setting, most HER2 directed therapy is given in combination with chemotherapy in the neoadjuvant setting, while studies of combined ET and HER2 targeted strategies without chemotherapy work slower with lower pCR rates. As such, the real benefit for combined targeted therapy may be in the adjuvant setting where ET and HER2 therapy are routinely given together following completion of chemotherapy. Furthermore, extended adjuvant ET/HER2 therapy beyond completion of antibody based treatment may also provide extra benefit, and in higher risk patients longer treatment may prove an important consideration. Citation Format: Stephen Johnston. Treatment of ER+/HER2+ 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 MS1-3.

Abstract P4-03-19: Expected real-world impact of escalation and de-escalation of Her2-directed therapy in breast cancer patients receiving neoadjvuant therapy

Abstract Background: For patients with palpable, early stage, Her2+ breast cancer, neoadjuvant therapy (NAT) is the preferred approach to curative treatment. While Trastuzumab (T) is standardly included in NAT protocols globally, there is variable ability to escalate or potentially de-escalate therapy due to perceived cost/benefit of additional Her2 targeting therapies, such as Pertuzumab (P) in the early stage setting. The impact of neoadjuvant Pertuzumab (NeoP) does have potentially significant impact on the need for therapeutic interventions in the setting of recurrence. It is challenging to estimate the real world impact financially on escalated therapy in the neoadjuvant setting and likewise the long-term effects of cost of recurrent therapy. We wanted to assess and estimate the impact of NeoP addition to T in British Columbian patients with Her2+ early breast cancer, and estimate incremental impact on short and long-term costs to our cancer system. Methods: Utilizing quality assurance database prospectively updated at BC Cancer Vancouver, we extracted cases of patients with Her2+ breast cancer who received NAT between 2012 – 2021 to provide real world estimations of prevalence of cases and estimated annual costs. Descriptive statistics were utilized to calculate rate of pCR in patients receiving neoadjuvant her2 directed therapy. Expected rates of improvement in pCR with the addition of NeoP were extracted from meta-analyses of Her2 directed therapy in the neoadjuvant setting. The publicly available national pCODR Ecomonic Guidance Reports were utilized to identify cost per cycle per patient of Pertuzumab, Trastuzumab, TDM-1 and chemotherapy in the Canadian healthcare system. The additional cost of NeoP per centre, per year, was compared to the current cost of non-Pertuzumab containing neoadjvuant regimen. Based on recurrence data collected from the database, the expected impact of NeoP on rate of recurrence and subsequent need for Her2 directed therapy in the metastatic setting was calculated. Results: 347 patients with HER2+ breast cancer received neoadjuvant T + chemotherapy at the BC Cancer Vancouver Centre between May 2012 and May 2022. 137 patients (39.5%) achieved pCR and 210 (60.5%) did not achieve pCR. Of those who did not achieve pCR, 12% recurred and 6% died, compared to only 5% and 1% respectively in the group that did achieve pCR. Based on a risk ratio of 1.57, we calculated the expected rate of pCR in our real world cohort would have increased from 39.5% to 62.3% with the availability of NeoP routinely. For each year of data in our cohort, an average of 40 patients with Her2 positive disease were treated with neoadjvant therapy. Using modeling based on our real world cohort we predicted an added cost of $669,920 to provide neoadjuvant pertuzuamb at one Canadian Cancer centre per year. Based on extraction of data from current state, where adjvuant TDM-1 is provided for 13 cycles in the setting of no PCR, the adjuvant TDM-1 costs were estimated to be $1, 729, 151 without NeoP and $1,067377 with NeoP. The cost difference for addition of NeoP in the treatment of early Her2+ breast cancer at one Canadian Centre per year was calculated as $8,146 increase in cost. In patients who had recurrence in our cohort we observed a shorter median survival than expected with most patients succumbing to their recurrence within a 2-3 year period. We expanded our modeling to assess the incremental impact on estimated cost of treatment for those who may recur. We estimated a cost saving of $85,054 per centre, per year, related to decrease in risk of recurrence of Her2+ breast cancer in those who receive NeoP. Conclusion: While the additional cost per centre of adding NeoP was $8146, the subsequent decrease in expected rate of recurrence resulted in a cost savings of $85,054. Citation Format: Andrew Mollenthiel, Aidan Morris, Corke Lauren, Christine Simmons. Expected real-world impact of escalation and de-escalation of Her2-directed therapy in breast cancer patients receiving neoadjvuant therapy. [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 P4-03-19.

Abstract OT3-25-01: Neoadjuvant HER2-targeted Therapy +/- Immunotherapy with Pembrolizumab (neoHIP): An Open Label Randomized Phase II Trial

Abstract Background: Immune checkpoint inhibition (ICI) is synergistic with HER2-directed therapy in pre-clinical models. Clinically, pembrolizumab (K)-mediated ICI plus HER2-directed therapy with trastuzumab (H) was safe and demonstrated modest activity in H-resistant HER2-positive (HER2+) metastatic breast cancer. Because ICI may confer more robust activity when administered earlier in the course of disease, H and K administered in the curative-intent, treatment-naive setting may allow for de-escalation of cytotoxic chemotherapy; confer life-long, tumor-specific immunity; and ultimately, improve cure rates. Moreover, the synergy of H and K with paclitaxel (T) may overcome the need for dual HER2-blockade with H plus pertuzumab (P). In this randomized, multicenter, phase II, open-label, multi-center trial the efficacy and safety of neoadjuvant THP vs THP-K vs TH-K are explored. Methods: 174 patients (pts) ≥18y with previously untreated, stage II-III, HER2+ breast cancer will be randomized and stratified by clinical nodal status (positive vs. negative) and hormone receptor status (positive vs. negative). In arm A, pts receive T at 80mg/m2 weekly for 12 weeks, H at 8mg/Kg (loading dose) and then 6mg/Kg every 3 weeks x 3 doses, P at 840 mg (loading dose) and then 420mg/Kg every 3 weeks x 3 doses (THP). In arm B, pts receive THP plus K at 200mg every 3 weeks x 4 doses (THP-K). In arm C, pts receive TH-K; however, in a preplanned interim analysis, arm C did not meet the pre-defined efficacy threshold and this arm was subsequently closed. Enrollment to arms A and B continue. Definitive surgery is 3-6 weeks after the last dose. After surgery, pts are treated per the treating physician’s discretion including radiotherapy per local clinical standard. Pts whose tumors are hormone-receptor positive will receive hormone therapy per local standard-of-care. The primary end point is pathologic complete response (pCR) rate in the breast and axilla (ypT0/Tis ypN0). Secondary end points include pCR rate by ypT0ypN0 and ypT0/Tis, residual cancer burden index, event free survival, breast conserving surgery rate, safety and overall survival. Exploratory correlative studies will characterize potential immune biomarkers predictive of efficacy and/or toxicity. Funding sources: BCRF, Merck NCT03747120 Citation Format: Heather McArthur, Jorge Henrique Santos Leal, Christina DiLauro Abaya, Sangeetha Reddy, Meredith Carter, Reva Basho, Michelle Phillips, David Chan, Hugo Hool, Dorothy Park, Mary El-Masry, Philomena McAndrew, Swati Sikaria, Laura M. Spring, Aditya Bardia, Mourad Tighiouart, Farnaz Dadmanesh, Armando Giuliano, Stephen Shiao, David B. Page. Neoadjuvant HER2-targeted Therapy +/- Immunotherapy with Pembrolizumab (neoHIP): An Open Label Randomized Phase II Trial [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 OT3-25-01.

Abstract P1-04-06: Characterization of recurrence risk after lumpectomy and radiotherapy in HER2-positive ductal carcinoma in situ of the breast, using 7-gene predictive biosignature: Implications for the NSABP-B43 trial results

Abstract Background: HER2-positive versus HER2-negative ductal carcinoma in situ (DCIS) of the breast has been associated with an increased risk of local recurrence after breast-conserving surgery (BCS) and radiotherapy (RT). In recognition of this, the NASBP-B43 trial was designed to determine if two doses of trastuzumab would improve local control with BCS plus RT in HER2-positive DCIS. The trial demonstrated a non-statistically significant advantage with the addition of trastuzumab in reducing ipsilateral breast recurrence (IBR). The predictive 7-gene DCIS biosignature, DCISionRT with Residual Risk Subtype (PreludeDxTM, Laguna Hills, CA) has been shown to classify DCIS patients into two distinct groups of patients with substantially different rates of IBR following BCS plus RT. Based upon these differences in outcome, we assessed the IBR rate in patients with HER2(+) DCIS treated with BCS and RT who were or were not in the Residual Risk Subtype group defined using DCISionRT, while accounting for the varying clinicopathologic profile of the patients. Materials &amp; Methods: DCISionRT was evaluated in a subset of 178 women with HER2(+) DCIS treated with BCS and RT as part of a multinational cohort of 926 patients from the United States, Sweden, and Australia, who were used in the validation studies for DCISionRT. Central pathology review and biosignature testing were performed at a CLIA-certified lab (Laguna Hills, CA). HER2(+) DCIS was defined as patients with a HER2(3+) immunohistochemistry &amp;gt;10% per ASCO/CAP guidelines. The IBR rate was calculated for the overall group of HER2(+) patients and those in the Residual Risk group. Individual patient outcome and biosignature results were analyzed using Kaplan Meier and Cox Proportional Hazard analyses. Results: The biosignature classified 113 of the 178 (63%) HER2(+) women into the Residual Risk group (DS&amp;gt;2.8 with RRt). Patients in the Residual Risk group had a significantly greater IBR (HR=8.3; 95%CI: 1.1,50, p=.012) over 10-years, with a corresponding 10-year total IBR rate of 16.2% (95%CI: 9.7%, 26.5%) versus 1.6% (95%CI: 0.2%, 10.9%) for all other HER2(+) patients. In univariate analysis, younger patients tended to have higher IBR rate after BCS plus RT, but only Residual Risk was significantly associated with IBR rate after BCS plus RT. Moreover, multivariable analysis demonstrated that the Residual Risk group was eight times more likely to recur after BCS and RT, while clinicopathologic factors (age, grade, tumor size) were not associated with higher IBR rates. Conclusion: The DCISionRT Residual Risk group was predictive for 10-year IBR risk after BCS plus RT in women with HER2(+) DCIS. Approximately 40% of patients with HER2(+) DCIS would be expected to achieve low rates of recurrence with BCS and RT, while about 60% of these women (classified in the Residual Risk group) would have higher recurrence rates and may benefit from further therapy, such as HER2-directed therapies. These findings suggest that if the results of the B43 trial were re-analyzed using the predictive 7-gene biosignature (DCISionRT with Residual Risk Subtype), better clarity could be gained on the true impact of trastuzumab on IBR rates in patients with HER2(+) DCIS and the patients most likely to benefit from this additional therapy. Table 1. Univariate and Multivariable Cox Proportional Hazards Analyses. Citation Format: Frank Vicini, Chirag Shah, Rachel Rabinovitch, Pat Whitworth, Julie A. Margenthaler, Brian J. Czerniecki, DAVID J. DABBS, Sheila Weinmann, Michael Leo, G Bruce Mann, Fredrik Wärnberg, jess Savala, Steven C. Shivers, Karuna Mittal, Troy Bremer. Characterization of recurrence risk after lumpectomy and radiotherapy in HER2-positive ductal carcinoma in situ of the breast, using 7-gene predictive biosignature: Implications for the NSABP-B43 trial results [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 P1-04-06.

Abstract PD18-10: Treatment of HER2-positive (HER2+) hormone-receptor positive (HR+) metastatic breast cancer (mBC) with the novel combination of zanidatamab, palbociclib, and fulvestrant

Abstract Background: HER2+ mBC remains incurable, with a need for new HER2-directed therapies and regimens, including chemotherapy-free options. Zanidatamab (zani) is a novel HER2-targeted bispecific antibody that binds HER2 in a unique trans configuration, driving multiple mechanisms of antitumor activity, including complement-dependent cytotoxicity. A CDK4/6 inhibitor combined with endocrine therapy is an approved treatment for HER2-negative/HR+ mBC and this combination has also demonstrated encouraging antitumor activity when paired with HER2-targeted therapy(ies) in HER2+/HR+ mBC. Here, we report results from ZWI-ZW25-202 (NCT04224272), an ongoing single-arm phase 2 study of zani combined with palbociclib (palbo) and fulvestrant (fulv) in pts with HER2+/HR+ mBC. Methods: Eligibility requirements include: HER2+/HR+ unresectable, locally advanced BC or mBC; ECOG PS of 0 or 1; prior treatment with trastuzumab, pertuzumab and T DM1 (additional prior HER2-targeting agents are permitted); and no prior treatment with CDK4/6 inhibitors. Part 1 of the study evaluated the safety and tolerability of the zani/palbo/fulv combination and determined the recommended doses for use in Part 2, where the antitumor activity of the combination is being evaluated. Endpoints include safety outcomes, progression-free survival at 6 months (PFS6), confirmed objective response rate (cORR) per RECIST v1.1; disease control rate (DCR=complete response [CR] plus partial response [PR] plus stable disease [SD]); duration of response (DOR); PFS; and overall survival. Results: As of 24 Feb 2022, 34 pts (33 HER2+/HR+ per central analysis) with a median age of 52 (range 36-77) have been treated. In the metastatic setting, pts had received a median (range) of 4 (1-10) prior systemic regimens, including 3 (1-8) different prior HER2 targeted therapies, and 1 (0-4) endocrine therapy. Seven pts (20%) had prior T DXd treatment and 7 pts had prior fulv treatment. All pts received zani (20 mg/kg Q2W) and standard doses of palbo and fulv. Eighteen pts (53%) remained on treatment; median duration of zani treatment was 6.9 mo (range 0.5-16.3). A dose-limiting toxicity (DLT) of neutropenia occurred in 1 of 7 DLT-evaluable pts in Part 1. Among all pts (n=34), the most common (&amp;gt;20%) treatment (zani, palbo and/or fulv)-related adverse events (TRAEs) were diarrhea (74%), neutrophil count decreased/neutropenia (62%), stomatitis (41%), asthenia (26%), nausea (24%), and anemia (21%). Grade (Gr) ≥3 TRAEs in 2 or more pts included neutrophil count decreased/neutropenia (50%), anemia (6%), diarrhea (6%), and thrombocytopenia (6%). AEs of special interest were all Gr ≤2 and included 4 pts with cardiac events (LVEF decrease of ≥10% from baseline) and 1 pt with infusion-related reaction. There were no treatment-related serious AEs. Palbo was discontinued for 1 pt due to an AE (AST increase); no pt discontinued zani treatment as a result of AEs. Two deaths occurred: 1 due to disease progression and 1 due to an unrelated AE of pneumonia caused by COVID-19. In 29 pts with measurable disease, the cORR was 34.5% (95% CI: 17.9, 54.3), all responses were cPRs, of which 1 is pending CR confirmation. DOR ranged from 2.3 to 14.9+ mo, with 8 confirmed responses ongoing, and the DCR was 93.1% (95% CI: 77.2, 99.2). Interim median PFS was 11.3 mo (range 0.03-16.7; 95% CI: 5.6, not estimable). PFS6 analysis is planned following the completion of enrollment. Conclusions: Zani in combination with palbo and fulv shows encouraging antitumor activity with durable responses in heavily pretreated pts and a manageable safety profile. This regimen has the potential to be a chemotherapy-free treatment option in pts with HER2+/HR+ mBC. Enrollment in the study is continuing. Citation Format: Santiago Escrivá-de-Romani, Emilio Alba, Álvaro Rodríguez-Lescure, Sara Hurvitz, Juan Miguel Cejalvo, Maria Gión, Cristiano Ferrario, Manuel Ruiz Borrego, Rossanna C. Pezo, Erika Hamilton, Marc Webster, Timothy Pluard, Muralidhar Beeram, Begoña Jiménez Rodríguez, Hannah Linden, Cristina Saura, Adam Omidpanah, Phoebe Harvey, Marie-France Savard. Treatment of HER2-positive (HER2+) hormone-receptor positive (HR+) metastatic breast cancer (mBC) with the novel combination of zanidatamab, palbociclib, and fulvestrant [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 PD18-10.