Clinicogenomic characterization of inflammatory breast cancer.

Background: Inflammatory breast cancer (IBC) is a rare and poorly characterized type of breast cancer with an aggressive clinical presentation. The biological mechanisms driving the IBC phenotype are relatively undefined-partially due to a lack of comprehensive, large-scale genomic studies and limited clinical cohorts. Patients and Methods: A retrospective analysis of 2457 patients with metastatic breast cancer who underwent targeted tumor-only DNA-sequencing was performed at Dana-Farber Cancer Institute. Clinicopathologic, single nucleotide variant (SNV), copy number variant (CNV) and tumor mutational burden (TMB) comparisons were made between clinically confirmed IBC cases within a dedicated IBC center versus non-IBC cases. Results: Clinicopathologic differences between IBC and non-IBC cases were consistent with prior reports-including IBC being associated with younger age at diagnosis, higher grade, and enrichment with hormone receptor (HR)-negative and HER2-positive tumors. The most frequent somatic alterations in IBC involved TP53 (72%), ERBB2 (32%), PIK3CA (24%), CCND1 (12%), MYC (9%), FGFR1 (8%) and GATA3 (8%). A multivariate logistic regression analysis revealed a significant enrichment in TP53 SNVs in IBC; particularly in HER2-positive and HR-positive disease which was associated with worse outcomes. Tumor mutational burden (TMB) did not differ substantially between IBC and non-IBC cases and a pathway analysis revealed an enrichment in NOTCH pathway alterations in HER2-positive disease. Conclusion: Taken together, this study provides a comprehensive, clinically informed landscape of somatic alterations in a large cohort of patients with IBC. Our data support higher frequency of TP53 mutations and a potential enrichment in NOTCH pathway activation-but overall; a lack of major genomic differences. These results both reinforce the importance of TP53 alterations in IBC pathogenesis as well as their influence on clinical outcomes; but also suggest additional analyses beyond somatic DNA-level changes are warranted.

Prognosis and treatment outcomes for patients with stage IA triple-negative breast cancer.

To evaluate the role of chemotherapy in stage IA triple-negative breast cancer, we conducted a retrospective population-based study including 8601 patients. The use of chemotherapy significantly increased from 2010 to 2019 in patients with T1b and T1c tumors (p = 0.001 and p < 0.001, respectively). Receipt of chemotherapy was associated with improved breast cancer-specific survival (BCSS, adjusted hazard ratio = 0.70; p = 0.006), particularly in patients with T1c tumors (5-year BCSS 94.5% vs. 91.2%).

Evolution of HER2 expression between pre-treatment biopsy and residual disease after neoadjuvant therapy for breast cancer.

INTRODUCTION: We have previously found that HER2 expression is dynamic, and can change from the primary breast tumor to matched recurrences. With this work, we aimed to assess the dynamics of HER2 during neoadjuvant treatment.(NAT). METHODS: We reviewed HER2 expression in pre- and post-treatment samples from consecutive patients with early-stage breast cancer that received NAT and underwent surgery at Dana-Farber Brigham Cancer Center between 01/2016-08/2022. The primary outcome was evolution of HER2 expression from pre- to post-NAT specimens in patients with residual disease. RESULTS: Among 1613 patients receiving NAT, 1080 had residual disease at surgery. A total of 319 patients (29.5%) experienced a change in HER2 expression (HER2 0 vs. HER2-low vs. HER2-positive) from the pre-treatment sample to residual disease, with roughly equal distribution between decreased (50.5%) and increased HER2 expression (49.5%). Similar rates of change in HER2 expression were observed with anthracycline-based (31.8%) or taxane/platinum-based regimens (32.4%). Patients with HER2-0 or HER2-low tumors at diagnosis were likelier to experience a change in HER2 expression post-NAT compared to HER2-positive (32.3% vs. 21.3%, p < 0.001). Changes in HER2 expression post-NAT were prognostic among patients with HER2-positive tumors at diagnosis (3-year recurrence-free survival for change vs. no change: 71.6% vs. 89.6%, p = 0.006) but not among those with HER2-negative tumors at diagnosis (3-year recurrence-free survival for change vs. no change: 79.3% vs. 81.1%, p = 0.31). CONCLUSIONS: Nearly 30% of patients with early-stage breast cancer showed a change in HER2 expression after NAT. Changes in HER2 expression post-NAT were only prognostic in the setting of HER2-positive tumors becoming HER2-negative at surgery.

Immunotherapy for Early-Stage Triple Negative Breast Cancer: Is Earlier Better?

PURPOSE OF REVIEW: In this narrative review, we discuss the optimal timing of immune checkpoint inhibitors (ICI) in early triple negative breast cancer (TNBC), the landscape of predictive biomarkers for the use of immunotherapy, and the mounting literature suggesting a benefit for an early use of ICI. RECENT FINDINGS: TNBC is associated with a poor prognosis relative to other breast cancer subtypes, and until recently, the treatment of TNBC was limited to cytotoxic chemotherapy. In 2021, the immune-checkpoint inhibitor, pembrolizumab, was approved in combination with neoadjuvant chemotherapy for patients with high-risk early stage TNBC. This approval changed the treatment paradigm of early TNBC concomitantly raised several challenges in clinical practice, pertaining to patient selection, toxicity management, and post-neoadjuvant treatment, among others. The introduction of neoadjuvant chemoimmunotherapy has transformed the treatment landscape for early TNBC. However, several challenges, including patient selection, toxicity management, and the identification of predictive biomarkers, need to be addressed. Future research should focus on refining the timing and duration of immunotherapy, optimizing the chemotherapy partner, and exploring novel predictive biomarkers of response or toxicity.

Defining the Biology of Estrogen Receptor-Low-Positive Breast Cancer.

BACKGROUND: We sought to better define estrogen receptor-low-positive (ER-low+) breast cancer biology and determine the utility of the Oncotype DX Breast Recurrence Score® (RS) assay in this population. METHODS: Patients with information regarding percentage ER positivity and PAM50 subtype were identified in The Cancer Genome Atlas (TCGA) and subtype distribution was determined. Next, patients with ER-low+ (ER 1-10%), HER2- breast cancer undergoing upfront surgery with known RS result were identified in the National Cancer Database (NCDB) and our institutional Dana-Farber Brigham Cancer Center (DF/BCC) database; RS distribution was examined. Finally, patients with ER-low+, HER2- breast cancer treated at DF/BCC from 2011 to 2020 without prior RS results and in whom tissue was available to perform the assay were identified. RS results, treatment, recurrence and breast cancer-specific survival (BCSS) were determined. RESULTS: Of 1033 patients in TCGA, ER percentage and PAM50 subtype were available for 342 (33.1%) patients. Forty-six (13.5%) had ER-low+/HER2- tumors, among whom 82.6% were basal and 4.3% were luminal A. Among 3423 patients with ER-low+/HER2- disease in the NCDB, RS results were available for 689 (20.1%) patients; 67% had an RS ≥26. In our institutional database, only two patients with ER-low+/HER2- disease and an RS were identified, both with RS ≥26. Among 37 patients in our institutional cohort without prior RS, 35 (97.4%) had an RS ≥26, determined with testing. After a median follow-up of 40 months (range 3-106), three patients, all treated with chemotherapy, recurred. Three-year BCSS was 97.0% (95% confidence interval 96.9-97.1%). CONCLUSIONS: Most ER-low+/HER2- breast cancers are basal-like, with RS ≥26 suggesting these tumors are similar to triple-negative disease.

Comprehensive genomic characterization of HER2-low and HER2-0 breast cancer.

The molecular underpinnings of HER2-low and HER2-0 (IHC 0) breast tumors remain poorly defined. Using genomic findings from 1039 patients with HER2-negative metastatic breast cancer undergoing next-generation sequencing from 7/2013-12/2020, we compare results between HER2-low (n = 487, 47%) and HER2-0 tumors (n = 552, 53%). A significantly higher number of ERBB2 alleles (median copy count: 2.05) are observed among HER2-low tumors compared to HER2-0 (median copy count: 1.79; P = 2.36e-6), with HER2-0 tumors harboring a higher rate of ERBB2 hemideletions (31.1% vs. 14.5%). No other genomic alteration reaches significance after accounting for multiple hypothesis testing, and no significant differences in tumor mutational burden are observed between HER2-low and HER2-0 tumors (median: 7.26 mutations/megabase vs. 7.60 mutations/megabase, p = 0.24). Here, we show that the genomic landscape of HER2-low and HER2-0 tumors does not differ significantly, apart from a higher ERBB2 copy count among HER2-low tumors, and a higher rate of ERBB2 hemideletions in HER2-0 tumors.

Combination Therapies to Improve the Efficacy of Immunotherapy in Triple-negative Breast Cancer.

Immune checkpoint inhibition combined with chemotherapy is currently approved as first-line treatment for patients with advanced PD-L1-positive triple-negative breast cancer (TNBC). However, a significant proportion of metastatic TNBC is PD-L1-negative and, in this population, chemotherapy alone largely remains the standard-of-care and novel therapeutic strategies are needed to improve clinical outcomes. Here, we describe a triple combination of anti-PD-L1 immune checkpoint blockade, epigenetic modulation thorough bromodomain and extra-terminal (BET) bromodomain inhibition (BBDI), and chemotherapy with paclitaxel that effectively inhibits both primary and metastatic tumor growth in two different syngeneic murine models of TNBC. Detailed cellular and molecular profiling of tumors from single and combination treatment arms revealed increased T- and B-cell infiltration and macrophage reprogramming from MHCIIlow to a MHCIIhigh phenotype in mice treated with triple combination. Triple combination also had a major impact on gene expression and chromatin profiles shifting cells to a more immunogenic and senescent state. Our results provide strong preclinical evidence to justify clinical testing of BBDI, paclitaxel, and immune checkpoint blockade combination.

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.

Adjuvant Olaparib for Germline BRCA Carriers With HER2-Negative Early Breast Cancer: Evidence and Controversies.

In the OlympiA study, 1 year of adjuvant olaparib significantly extended invasive disease-free survival and overall survival. The benefit was consistent across subgroups, and this regimen is now recommended after chemotherapy for germline BRCA1/2 mutation (gBRCA1/2m) carriers with high-risk, HER2-negative early breast cancer. However, the integration of olaparib in the landscape of agents currently available in the post(neo)adjuvant setting-ie, pembrolizumab, abemaciclib, and capecitabine-is challenging, as there are no data suggesting how to select, sequence, and/or combine these therapeutic approaches. Furthermore, it is unclear how to best identify additional patients who could benefit from adjuvant olaparib beyond the original OlympiA criteria. Since it is unlikely that new clinical trials will answer these questions, recommendations for clinical practice can be made through indirect evidence. In this article, we review available data that could help guide treatment decisions for gBRCA1/2m carriers with high-risk, early-stage breast cancer.

PD-1 blockade and CDK4/6 inhibition augment nonoverlapping features of T cell activation in cancer.

We performed single-cell RNA-sequencing and T cell receptor clonotype tracking of breast and ovarian cancer patients treated with the CDK4/6 inhibitor ribociclib and PD-1 blockade. We highlight evidence of two orthogonal treatment-associated phenomena: expansion of T cell effector populations and promotion of T cell memory formation. Augmentation of the antitumor memory pool by ribociclib boosts the efficacy of subsequent PD-1 blockade in mouse models of melanoma and breast cancer, pointing toward sequential therapy as a potentially safe and synergistic strategy in patients.

Multiomics in primary and metastatic breast tumors from the AURORA US network finds microenvironment and epigenetic drivers of metastasis.

The AURORA US Metastasis Project was established with the goal to identify molecular features associated with metastasis. We assayed 55 females with metastatic breast cancer (51 primary cancers and 102 metastases) by RNA sequencing, tumor/germline DNA exome and low-pass whole-genome sequencing and global DNA methylation microarrays. Expression subtype changes were observed in ~30% of samples and were coincident with DNA clonality shifts, especially involving HER2. Downregulation of estrogen receptor (ER)-mediated cell-cell adhesion genes through DNA methylation mechanisms was observed in metastases. Microenvironment differences varied according to tumor subtype; the ER+/luminal subtype had lower fibroblast and endothelial content, while triple-negative breast cancer/basal metastases showed a decrease in B and T cells. In 17% of metastases, DNA hypermethylation and/or focal deletions were identified near HLA-A and were associated with reduced expression and lower immune cell infiltrates, especially in brain and liver metastases. These findings could have implications for treating individuals with metastatic breast cancer with immune- and HER2-targeting therapies.

Nodal Positivity in Early-Stage Triple-Negative Breast Cancer: Implications for Preoperative Immunotherapy.

BACKGROUND: Adding pembrolizumab to preoperative chemotherapy improves event-free survival in patients with early-stage triple-negative breast cancer (TNBC). However, owing to potential toxicities, the risk-benefit ratio of pembrolizumab must be considered. There is consensus that the addition of immunotherapy should be recommended in node-positive patients. This study is undertaken to determine nodal positivity rates in patients with TNBC presenting with cT1-2N0 disease undergoing upfront surgery and to evaluate the utility of axillary ultrasound and biopsy in the setting of a negative clinical examination. PATIENTS AND METHODS: Patients with cT1-2N0 TNBC undergoing upfront surgery were identified from our institutional database (January 2016-February 2021; n = 343) and from the National Cancer Database (NCDB) (n = 46,015). Pathologic nodal status was determined. A second cohort of patients with cT1-T2 TNBC with a negative clinical examination was defined in our institutional database (n = 499), and utilization of axillary ultrasound was examined. RESULTS: For patients undergoing upfront surgery, pathologically positive nodes were found in 14.6% patients of our institutional cohort: 9.4% cT1a/b, 14.9% cT1c, and 20.8% cT2 tumors. In the NCDB cohort, 13.7% patients were node positive: 4.9% cT1a/b, 11.4% cT1c, and 19.7% cT2 tumors. For patients with a normal clinical examination undergoing axillary ultrasound, 7.5% of cT1c and 8.7% of cT2 had suspicious nodes biopsied and confirmed positive for metastasis. CONCLUSIONS: Pathologic node-positive disease is found in > 10 and 20% patients with cT1cN0 and cT2N0 TNBC, respectively. Axillary ultrasound can be used to identify patients presenting with a normal clinical examination for whom preoperative pembrolizumab should be considered.

Emerging Targeted Therapies for Early Breast Cancer.

Breast cancer is the most common malignancy and the second leading cause of cancer-related mortality in the United States (US). Most patients are diagnosed with early-stage disease; however, there is still a need to prevent recurrences that often present as incurable metastatic disease. The treatment landscape of early-stage breast cancer is evolving rapidly. The immune checkpoint inhibitor pembrolizumab is approved in combination with neoadjuvant chemotherapy for the treatment of high-risk triple-negative breast cancer (TNBC). The cyclin-dependent kinase (CDK) 4 and 6 inhibitor abemaciclib is approved for adjuvant treatment of patients with high-risk hormone receptor (HR)-positive disease. While adjuvant olaparib has shown significant improvement in outcomes for patients with pathogenic/likely pathogenic BRCA1/2 mutations and high-risk human epidermal growth factor receptor 2 (HER2)-negative breast cancer, and is approved in this setting. For the HER2-positive subtype, the post-neoadjuvant therapy can be tailored based on the response to neoadjuvant chemotherapy and HER2-targeted agents. In this narrative review, we summarize the most recent approvals for early-stage breast cancer as well as frequently encountered clinical challenges utilizing these medications.

Understanding resistance to immune checkpoint inhibitors in advanced breast cancer.

INTRODUCTION: The addition of immune checkpoint inhibitors (ICIs) to frontline chemotherapy has improved survival for patients with advanced triple-negative breast cancer (TNBC) expressing programmed death-ligand 1 (PD-L1). Nonetheless, most patients develop resistance, with outcomes remaining poor for this population. Moreover, unsatisfactory activity has been observed with ICIs in PD-L1-negative TNBC and in other breast cancer (BC) subtypes, warranting a deeper understanding of resistance to ICIs in BC. AREAS COVERED: We discuss the immune landscape of distinct BC subtypes, review the clinical activity of immunotherapy in BC, and highlight strategies under development to overcome resistance to ICIs. EXPERT OPINION: Activity and resistance to ICIs in BC are strongly related to the intrinsic immunophenotype of the tumor tissue. Several promising biomarkers reflecting the immunological state of BC are emerging, with only PD-L1 expression currently adopted into clinical practice. However, limitations make of PD-L1 a sub-optimal biomarker for patient selection, which require efforts to integrate this marker with other immunological features. Concomitantly, a wide variety of drug combinations designed to overcome immune-resistance are being evaluated, with some encouraging signals observed in early-phase trials. Combination strategies tailored to patient and tumor immunophenotype may allow to overcome resistance and fully exploit the potential of ICIs.

Dermal Lymphatic Invasion, Survival, and Time to Recurrence or Progression in Inflammatory Breast Cancer.

OBJECTIVES: Dermal lymphatic invasion (DLI) with tumor emboli is a common pathologic characteristic of inflammatory breast cancer (IBC), although its presence is not required for diagnosis. We examined whether documented DLI on skin biopsy was associated with survival and time to recurrence or progression in IBC. MATERIALS AND METHODS: A total of 340 women enrolled in the IBC Registry at Dana-Farber Cancer Institute between 1997 and 2019 were included in this study. Kaplan-Meier curves and multivariable Cox proportional hazards models were used to estimate hazard ratios and 95% confidence intervals for associations of DLI and overall survival, time to locoregional recurrence/progression, and distant metastasis by stage at presentation. RESULTS: DLI was detected in 215 (63.2%) of IBC cases overall. At disease presentation, IBC with DLI had a higher prevalence of de novo metastases (37.7% vs. 26.4%), breast skin ulceration (6.1% vs. 2.4%), and lymphovascular invasion within the breast parenchyma (52.9% vs. 25.5%) and a lower prevalence of palpable breast mass (48.2% vs. 70.6%) than IBC without DLI. Over a median follow-up of 2.0 years, 147 deaths occurred. DLI was not associated with survival or recurrence in multivariable models (all P ≥0.10). For example, among women with stage III disease, hazard ratios (95% confidence intervals) for DLI presence was 1.29 (0.77-2.15) for overall survival, 1.29 (0.56-3.00) for locoregional recurrence, and 1.71 (0.97-3.02) for distant metastasis. CONCLUSION: Although the extent of tumor emboli in dermal lymphatics may be associated with biological features of IBC, DLI was not an independent prognostic marker of clinical outcomes in this study.

Utility of the 21-Gene Recurrence Score in Node-Positive Breast Cancer.

The 21-gene Recurrence Score (RS) assay has been validated as both a prognostic and predictive tool in node-negative (pN0), estrogen receptor-positive (ER+), HER2-negative (HER2-) breast cancer. A large body of evidence supports the clinical utility of the RS in the node positive (pN+) population as well. Retrospective analyses of archived tissue from multiple clinical trials have found the RS to be prognostic in both endocrine therapy (ET)-treated and chemotherapy-treated patient with pN+ disease. Distribution of RS results in pN+ patients have also been consistent with those of pN0 populations. Data from the SWOG 8814 trial and large population-based registries further support the prognostic and potential predictive value of the RS. Specifically, patients with 1 to 3 positive nodes and RS less than 18 derived negligible benefit from adjuvant chemotherapy in these studies. In the prospective West German Study Group PlanB and ADAPT trials, pN+ patients with RS less than 11 and RS 25 or less, respectively, who were treated with ET alone experienced excellent outcomes. Finally, 5-year results of the RxPONDER clinical trial randomizing patients with 1 to 3 positive nodes and RS 25 or less to ET alone vs ET plus chemotherapy confirmed an absence of chemotherapy benefit in postmenopausal patients. Clinical practice guidelines support use of the RS in the pN+, ER+/HER2- population, and many institutions have adopted the RS to guide clinical decisionmaking, resulting in a net reduction of adjuvant chemotherapy use. This review highlights the existing data supporting the prognostic and predictive ability of the RS in pN+ disease, current practice patterns related to RS use in this population, and emerging applications.

Genomic Characterization of de novo Metastatic Breast Cancer.

PURPOSE: In contrast to recurrence after initial diagnosis of stage I-III breast cancer [recurrent metastatic breast cancer (rMBC)], de novo metastatic breast cancer (dnMBC) represents a unique setting to elucidate metastatic drivers in the absence of treatment selection. We present the genomic landscape of dnMBC and association with overall survival (OS). EXPERIMENTAL DESIGN: Targeted DNA sequencing (OncoPanel) was prospectively performed on either primary or metastatic tumors from 926 patients (212 dnMBC and 714 rMBC). Single-nucleotide variants, copy-number variations, and tumor mutational burden (TMB) in treatment-naïve dnMBC primary tumors were compared with primary tumors in patients who ultimately developed rMBC, and correlated with OS across all dnMBC. RESULTS: When comparing primary tumors by subtype, MYB amplification was enriched in triple-negative dnMBC versus rMBC (21.1% vs. 0%, P = 0.0005, q = 0.111). Mutations in KMTD2, SETD2, and PIK3CA were more prevalent, and TP53 and BRCA1 less prevalent, in primary HR+/HER2- tumors of dnMBC versus rMBC, though not significant after multiple comparison adjustment. Alterations associated with shorter OS in dnMBC included TP53 (wild-type: 79.7 months; altered: 44.2 months; P = 0.008, q = 0.107), MYC (79.7 vs. 23.3 months; P = 0.0003, q = 0.011), and cell-cycle (122.7 vs. 54.9 months; P = 0.034, q = 0.245) pathway genes. High TMB correlated with better OS in triple-negative dnMBC (P = 0.041). CONCLUSIONS: Genomic differences between treatment-naïve dnMBC and primary tumors of patients who developed rMBC may provide insight into mechanisms underlying metastatic potential and differential therapeutic sensitivity in dnMBC. Alterations associated with poor OS in dnMBC highlight the need for novel approaches to overcome potential intrinsic resistance to current treatments.

Phase 2 study of buparlisib (BKM120), a pan-class I PI3K inhibitor, in patients with metastatic triple-negative breast cancer.

BACKGROUND: Treatment options for triple-negative breast cancer remain limited. Activation of the PI3K pathway via loss of PTEN and/or INPP4B is common. Buparlisib is an orally bioavailable, pan-class I PI3K inhibitor. We evaluated the safety and efficacy of buparlisib in patients with metastatic triple-negative breast cancer. METHODS: This was a single-arm phase 2 study enrolling patients with triple-negative metastatic breast cancer. Patients were treated with buparlisib at a starting dose of 100 mg daily. The primary endpoint was clinical benefit, defined as confirmed complete response (CR), partial response (PR), or stable disease (SD) for ≥ 4 months, per RECIST 1.1. Secondary endpoints included progression-free survival (PFS), overall survival (OS), and toxicity. A subset of patients underwent pre- and on-treatment tumor tissue biopsies for correlative studies. RESULTS: Fifty patients were enrolled. Median number of cycles was 2 (range 1-10). The clinical benefit rate was 12% (6 patients, all SD ≥ 4 months). Median PFS was 1.8 months (95% confidence interval [CI] 1.6-2.3). Median OS was 11.2 months (95% CI 6.2-25). The most frequent adverse events were fatigue (58% all grades, 8% grade 3), nausea (34% all grades, none grade 3), hyperglycemia (34% all grades, 4% grade 3), and anorexia (30% all grades, 2% grade 3). Eighteen percent of patients experienced depression (12% grade 1, 6% grade 2) and anxiety (10% grade 1, 8% grade 2). Alterations in PIK3CA/AKT1/PTEN were present in 6/27 patients with available targeted DNA sequencing (MSK-IMPACT), 3 of whom achieved SD as best overall response though none with clinical benefit ≥ 4 months. Of five patients with paired baseline and on-treatment biopsies, reverse phase protein arrays (RPPA) analysis demonstrated reduction of S6 phosphorylation in 2 of 3 patients who achieved SD, and in none of the patients with progressive disease. CONCLUSIONS: Buparlisib was associated with prolonged SD in a very small subset of patients with triple-negative breast cancer; however, no confirmed objective responses were observed. Downmodulation of key nodes in the PI3K pathway was observed in patients who achieved SD. PI3K pathway inhibition alone may be insufficient as a therapeutic strategy for triple-negative breast cancer. TRIAL REGISTRATION: NCT01790932 . Registered on 13 February 2013; NCT01629615 . Registered on 27 June 2012.