Brain metastasis as the first and only metastatic relapse site portends worse survival in patients with advanced HER2 + breast cancer

Current systemic therapy guidelines for patients with HER2 + breast cancer brain metastases (BCBrM) diverge based on the status of extracranial disease (ECD). An in-depth understanding of the impact of ECD on outcomes in HER2 + BCBrM has never been performed. Our study explores the implications of ECD status on intracranial progression-free survival (iPFS) and overall survival (OS) after first incidence of HER2 + BCBrM and radiation. A retrospective analysis was performed of 151 patients diagnosed with initial HER2 + BCBrM who received radiation therapy to the central nervous system (CNS) at Duke between 2008 and 2021. The primary endpoint was iPFS defined as the time from first CNS radiation treatment to intracranial progression or death. OS was defined as the time from first CNS radiation or first metastatic disease to death. Systemic staging scans within 30 days of initial BCBrM defined ECD status as progressive, stable/responding or none (isolated brain relapse). In this cohort, > 70% of patients had controlled ECD with either isolated brain relapse (27%) or stable/responding ECD (44%). OS from initial metastatic disease to death was markedly worse for patients with isolated intracranial relapse (median = 28.4 m) compared to those with progressive or stable/responding ECD (48.8 m and 71.5 m, respectively, p = 0.0028). OS from first CNS radiation to death was significantly worse for patients with progressive ECD (16.9 m) versus stable/responding (36.6 m) or isolated intracranial relapse (28.4 m, p = 0.007). iPFS did not differ statistically based on ECD status. Receipt of systemic therapy after first BCBrM significantly improved iPFS (HR 0.45, 95% CI: 0.25–0.81, p = 0.008) and OS (HR: 0.43 (95% CI: 0.23–0.81); p = 0.001). OS in patients with HER2 + isolated BCBrM was inferior to those with concurrent progressive or stable/responding ECD. Studies investigating initiation of brain-penetrable HER2-targeted therapies earlier in the disease course of isolated HER2 + intracranial relapse patients are warranted.


Introduction
Breast cancer (BC) is one of the most common causes of brain metastasis from solid tumors [1][2][3][4]. The incidence of breast cancer brain metastases (BCBrM) varies based on tumor biomarker subtype, occurring in up to 30% of patients in the advanced setting [3,5]. Amongst human epidermal growth factor receptor 2 positive (HER2+) BC, the incidence of BCBrM has increased to nearly 50% of patients with metastatic breast cancer (MBC) [6]. The average time to development of BCBrM in HER2+ MBC is 32 months, and risk factors include estrogen receptor negativity, older age, and number of sites of metastatic disease [3,7].
Despite major advances in the treatment of HER2+ BC, BCBrM remain a substantial challenge for patients and clinicians. The median survival of HER2+ BCBrM is only 18 months post-diagnosis [3,8,9]. Consensus guidelines for systemic therapy after an initial HER2+ BCBrM depend on the status of extracranial disease (ECD) [10]. In patients with isolated brain relapse or those with stable or responding ECD, continuing current systemic therapy after local therapy (radiation ± surgical resection) is recommended per 2022 ASCO guidelines [11]. If the patient's concurrent ECD is progressive, transition to the next line of systemic therapy per standard algorithms is recommended [11].
Recent studies have demonstrated the role of systemic therapy after local therapy in improving overall survival (OS) of patients with new BCBrM [1,3,[12][13][14]. Specifically, the benefit of systemic therapy post-local treatment was demonstrated in patients with isolated intracranial metastasis, in the absence of any concurrent ECD [8]. New therapies in advanced HER2+ BCBrM have improved progression-free and OS in patients with stable, progressive, or untreated BCBrM [9]. Given these new therapies, there is great interest in developing clinical studies with highly CNS penetrable agents for primary and secondary BCBrM prevention in HER2+ MBC. As we begin to focus on intracranial progression-free survival (iPFS) as an endpoint in clinical trial design, we lack historical data to use as a benchmark to examine success with new agents. Another challenge is understanding the heterogeneity of the HER2+ BCBrM population and whether iPFS and OS change based on ECD status.
The present study aims to characterize the ECD status and outcomes in patients with newly diagnosed HER2+ BCBrM after their initial radiation treatment. We also aim to understand the impact of ECD on iPFS and OS with intention to understand the needs of potentially diverse populations.

Study design and participants
This study was approved for exemption from IRB review as it used only de-identified patient information in accordance with federal regulations [15]. Using retrospective analysis, 486 patients were identified that were treated with either stereotactic radiosurgery (SRS) or whole brain radiation therapy (WBRT) for BCBrM at Duke University between March 2008 and October 2021. 26 patients were excluded for lack of biomarker status, non-breast cancer primary, presence of leptomeningeal disease without intraparenchymal disease, or for being a duplicate record. Of the remaining 460 records, 151 patients had HER2+ disease and radiation treatment for a first HER2+ BCBrM (Table 1).
Retrospective descriptive data were extracted from these 151 HER2+ patient charts, including age at first metastatic disease, age at initial diagnosis by biopsy, age at first metastatic diagnosis by biopsy (or imaging, if biopsy unavailable), race, ethnicity, and estrogen receptor (ER)/progesterone receptor (PR) and HER2 status per ASCO CAP guidelines [16]. HER2-positivity was defined by most recent tissue biopsy with descending preference given to brain biopsy, primary breast biopsy, or another metastatic biopsy. Date of first brain metastasis was collected based on first imaging showing BCBrM. MRI was given preference over CT. Status of ECD at time of initial BCBrM diagnosis and second intracranial progression was defined as none (isolated brain relapse), responding, stable, progressive, or unknown. The most recent staging scan had to be within 30 days of first BCBrM diagnosis. ECD status was classified based on RECIST1.1 criteria by investigator assessment. All radiation therapy records were collected for treatment of the patient's first and second intracranial progressions. Second intracranial progression was defined by investigator-assessed RANO-BM criteria [17]. Date of death or last known contact was collected based on chart review. Cause of death was assessed based on death certificates or most recent notes. The research was approved for exemption by the Duke University Institutional Review Board (IRB Pro00105398).

Statistical analysis
Patient characteristics are summarized using median and range for continuous variables, and categorical descriptors are summarized using frequencies and percentages. Kaplan-Meier methods were used to estimate OS and iPFS. OS was evaluated as time from first CNS radiation or first metastatic disease to death and censored at last follow-up visit. First metastatic disease was defined as the first presentation of ECD, intracranial disease, or both presenting at the same time. Presentation with isolated intracranial relapse resulted in the same date of initial metastatic disease and initial BCBrM. iPFS was evaluated from the date of first CNS radiation to the subsequent documentation of intracranial progression or death and is censored at date of last follow-up for those alive without progressive disease. Curves were compared using the log-rank test. Proportional hazards regression was used for multivariate analyses.
Analyses were conducted using SAS software (Version 9.4; SAS Institute Inc., Cary, NC) and plots were created in the R language and environment for statistical computing (R Foundation for Statistical Computing, Vienna, Austria) or Prism (GraphPad Software, San Diego, California, USA).

Demographics and patient characteristics
151 patients with confirmed HER2+ breast cancer and a first BCBrM with subsequent brain radiation treatment at Duke University were included. Clinical characteristics are presented in Table 1. Median age at first metastatic disease was 50 years old (range: 24-75). Two-thirds of patients' tumors were ER-positive (ER+).
Of the patients in this cohort, the ECD status of 25 patients was unknown. Of the 126 patients with known ECD status, 73% had concurrent ECD when initially diagnosed with HER2+ BCBrM. Twenty-seven percent (27%) of patients presented with isolated brain relapse or no evidence of ECD. Most patients with known ECD status had systemic disease control or isolated brain relapse at time of diagnosis: 44% had stable/responding ECD, 29% had progressive ECD, and 27% had isolated brain relapse (Fig. 1). All patients without ECD presented with isolated brain relapse as first metastatic event. A majority of patients (58%) developed their first HER2+ BCBrMs during adjuvant, first, or secondline metastatic therapy. About half of patients (54%) were receiving trastuzumab-based therapy at time of BCBrM diagnosis. Eighty-five (85%) of patients were on any HER2directed therapy at the time of developing BCBrM.
CNS radiation treatment modality for first HER2+ BCBrM was 56% SRS and 44% WBRT. After radiation, 90% of patients were placed on systemic therapy. Reasons for not placing patients on systemic therapy varied and included disease progression prior to initiation of therapy, contraindications to systemic therapy, and absence of guideline-directed treatment for those with isolated intracranial relapse. Twenty-three percent (23%) of patients received HER2-targeted therapy with trastuzumab and pertuzumabbased therapy, 19% received trastuzumab-based therapy without pertuzumab, 19% received lapatinib combination therapy, and 15% received ado-trastuzumab emtansine (T-DM1) ( Table 1). 3) months for patients with isolated brain relapse ( Table 2 and Fig. 2), (p = 0.14). In the univariate analysis (Table 3), only use of systemic therapy after first radiation was associated with a significantly prolonged iPFS hazard ratio (HR) (0.45, 95% CI: 0.25-0.81, p = 0.008). ER positivity, age at diagnosis, and multi-focality of BCBrM were not significantly associated with iPFS.

Association of extracranial disease status and overall survival
OS from first metastatic diagnosis to death was markedly worse for patients with isolated brain relapse ( In the univariate analysis (Table 5a), use of systemic therapy after first radiation was associated with a decreased risk of death (HR: 0.43 (95% CI: 0.23-0.81); p = 0.001), while ER positivity, age at diagnosis, and multi-focality BCBrM were not significantly associated with risk. In the multivariate analysis (Table 5b), use of systemic therapy after first radiation was again associated with a decreased risk of death (HR: 0.43 (95% CI: 0.23-0.82), p = 0.010) when controlling for ER status.
From first BCBrM to death, the univariate analysis (Table 7) demonstrated a markedly decreased risk of death in patients treated with systemic therapy post-first radiation (HR: 0.11 (95% CI: 0.05-0.22), p < 0.001). There was a significantly increased risk of death in patients > 60 years In this series, there were no statistically significant differences in OS from first radiation to death among patients who were treated with trastuzumab-based chemotherapy versus T-DM1 versus HER2-targeted tyrosine kinase inhibitors, such as neratinib, lapatinib, and tucatinib post-radiation (Supplemental Table 1 and Supplemental Fig. 1). Median survival for patients was 32.7 months (95% CI: 28.2-41.9) on trastuzumab-based chemotherapy, 32.4 months (95% CI: 28.0-NR) on TDM-1, and 19.7 months (95% CI: 15.0-40.8) on HER2-targeted tyrosine kinase inhibitors (p = 0.28).
Finally, there were no statistically significant differences in OS from first radiation to death among patients who received SRS versus WBRT as their first radiation (Supplemental Table 2a Overall survival did not differ between unifocal and multifocal brain metastases (Supplemental Table 2b and Supplemental Fig. 2). Median OS for patients with unifocal intracranial metastasis was 28.7 months (95% CI: 19.7-40.5) and 29.0 months (95% CI: 24.4-36.0) for patients with multifocal disease (p = 0.64). Additionally, the median total volume of SRS (11.7 cc) was significantly less than the expected total volume irradiated with WBRT (Supplemental Table 3). Furthermore, total SRS treatment volume greater or less than the median did not appear clinically significant in OS for this cohort. Median OS for patients treated to less than 12 cc (0.2-11.7 cc) was 32.7 months (95% CI: 20.6-52.0) and 41.92 (95% CI: 23.0-NR) for those with treatment volumes greater than the median (12.0-94.6 cc). (Supplemental Table 4).

Discussion
In the present study, we describe the outcomes of 151 breast cancer patients newly diagnosed with HER2+ BCBrM who received radiation at a single institution. For the first time to our knowledge, we describe the impact of ECD status in a large cohort at the time of first HER2+ BCBrM. In this patient cohort, most patients had ECD that was absent or controlled, indicating that therapeutic efficacy between intracranial and extracranial disease diverges in modern times. Most patients developed first BCBrM during early lines of therapy. Trastuzumab remains the most commonly used HER2-targeting agent in neoadjuvant, adjuvant, and first-line metastatic HER2+ breast cancer. Trastuzumab has very limited intracranial activity in an intact blood-brain barrier, allowing metastatic escape into the CNS. Our findings demonstrate that patients with isolated brain relapse as their first metastatic event have inferior OS from metastatic diagnosis to death compared to those that develop BCBrM later in their metastatic disease course with concurrent ECD (Fig. 3). The current first-line systemic therapy for advanced HER2+ BC remains a taxane, pertuzumab and trastuzumab regardless of BCBrM status [20]. The intracranial activity of trastuzumab and pertuzumab is minimal compared to newer generation HER2-targeted therapies [21]. Tucatinib is a highly HER2-selective small-molecule oral tyrosine kinase inhibitor. When used in combination with trastuzumab and capecitabine in the HER2CLIMB clinical trial, patients with baseline active, stable or progressive BCBrM had improved iPFS and OS with the addition of tucatinib [9]. Given the poor OS in patients with isolated brain relapse, we believe clinical trials utilizing combinations with brain-penetrable HER2-targeted therapies such as tucatinib in the first-line setting are warranted. Clinical trials utilizing tucatinib added to T-DM1 or trastuzumab/pertuzumab after isolated brain relapse for secondary BCBrM prevention are underway (NCT05323955).
OS from BCBrM to death also differed significantly depending on ECD status (Fig. 4). Patients with concurrent    not large enough to make conclusions on optimal systemic therapy following first BCBrM; however, receipt of HER2directed therapy is strongly recommended. This study has several limitations. This study was only performed at one institution, Duke University, which inherently limits our population of patients. However, with over 150 HER2+ patients, this study constitutes one of the largest retrospective analyses of HER2+ intracranial metastases. Another limitation is the retrospective nature of this study. All patients with HER2+ BCBrM treated with any radiation at Duke between 2008 and 2021 were included, but there may still be confounding factors uncontrollable in retrospective series. Furthermore, only clinically significant brain metastases leading to MRI diagnosis were included as is standard practice, and implications of asymptomatic brain metastases are unknown. Also, few patients in this cohort were treated with newer HER2-targeted agents such as trastuzumab deruxtecan (T-DXd) or tucatinib-containing regimens which both prolong survival in HER2+ BCBrM patients. Finally, it is unknown when these patients may have developed asymptomatic BCBrM as MRI screening is not standard of care for breast cancer treatment, which could have an impact on the demonstrated survival in this study.
In conclusion, patients with isolated HER2+ brain relapse had shorter OS compared to those developing BCBrM later in their treatment course with either controlled or progressive ECD. Systemic HER2-directed therapy post-radiation improved both iPFS and OS and is strongly recommended. Clinical trials exploring highly brain-penetrable HER2-targeting agents after initial first brain relapse are warranted in this population.
Funding Funding for this project was provided to Laura Noteware by the Stead Scholarship Program of the Duke University Department of Medicine. Scott Floyd is supported by grants from the NIH (NIH 5R01-NS100866-05, NIH U01TR0033715-01, and NIH 5R38-CA245204-02) and the American Cancer Society (13394-RSG-19-030-01-DMC). Additional research funding from PUMA, Lilly, Merck, Seattle Genetics, Nektar, Tesaro, G1-Therapeutics, ZION, Novartis, and Pfizer were provided to Carey Anders. Sarah Sammons receives institutional funding from Astra Zeneca, Abbvie, Bristol Myers Squibb, Eli Lilly, SEAGEN, and Sermonix. The authors declare that no additional funds, grants, or other support were received during the preparation of this manuscript.

Data availability
The datasets generated and analyzed during the current study are not publicly available as they contain identifying information but available from the corresponding author on reasonable request.

Declarations
Conflict of interest Carey Anders has been a compensated consultant for Genentech, Eisai, IPSEN, Seattle Genetics, Astra Zeneca, Novartis, Immunomedics, Elucida, and Athenex. Dr. Anders also receives royalties from UpToDate, Jones and Bartlett. Sarah Sammons is a compensated consultant at Foundation Medicine, Astra Zeneca, Daichii Sankyo, Eli Lilly, Pfizer, Sermonix, and Novartis. The additional authors have no relevant financial or non-financial interests to disclose.