For decades, HER2-positive breast cancer was associated with poor outcomes and higher mortality rates than other breast cancer subtypes. However, the advent of Trastuzumab (Herceptin) has significantly changed the treatment paradigm of patients afflicted with HER2-positive breast cancer. The discovery of newer HER2-targeted therapies, such as Pertuzumab (Perjeta), has further added to the armamentarium of treating HER2-positive breast cancers. This review highlights recent advancements in the treatment of HER2-positive diseases, including the newer HER2-targeted therapies and immunotherapies in clinical trials, which have paved (and will further update) the way for clinical practice, and become part of the standard of care in the neoadjuvant, adjuvant or metastatic setting.

Introduction: Fulvestrant is currently the only selective estrogen receptor degrader (SERD) that is approved for clinical use in estrogen receptor (ER) positive advanced breast cancer (ABC). The drug is approved as single-agent therapy in the first and second-line setting of metastatic ER-positive breast cancer. Areas covered: In this review, the authors review the preclinical studies that were pivotal in the development of fulvestrant, the pharmacologic properties of the drug, and the key clinical trials that resulted in its approval for clinical use. The authors discuss mechanisms of endocrine resistance and potential targets for endocrine refractory disease while highlighting ongoing studies that assess fulvestrant use with novel agents. Expert opinion: While fulvestrant has limited use in the first-line setting in advanced breast cancer, it is most frequently used in the second line after progression with aromatase inhibitors. The combination of fulvestrant with CDK4/6 inhibitors has shown a clear benefit over monotherapy in patients who progress on prior endocrine therapy. Further study is necessary to assess if patient outcomes can be enhanced by optimizing the sequence of endocrine therapies, targeting resistance pathways with novel agents, and development of new agents in the SERD class.

Purpose: Breast cancers with ESR1 mutations are resistant to antiestrogen therapy. In this study, we aimed to investigate the association of ESR1 mutations with resistance to CDK4/6 inhibitors (CDK4/6i) using real-world data analysis and experimental validation. Experimental Design: A total of 3,958 patients with estrogen receptor–positive metastatic breast cancer with DNA sequencing data were analyzed. Breast tumor DNA and ctDNA were sequenced using the Tempus xT tumor assay and Tempus xF liquid biopsy, respectively. Patients were stratified into either treated with CDK4/6i (tumor tissue: 1,070; ctDNA: 1,885) or CDK4/6i naïve (tumor tissue: 750; ctDNA: 253). Engineered MCF7 cells carrying ESR1Y537S or ESR1D538G knock-in mutations were used to study antitumor efficacy of the CDK4/6i palbociclib in vitro and in vivo. Results: In both xF and xT assays, ESR1 mutations were the only somatic alterations significantly more frequent in patients who received CDK4/6i compared with those who did not. Knock-in of ESR1Y537S or ESR1D538G in MCF7 cells resulted in upregulation of cell cycle–related gene signatures upon treatment with CDK4/6i ± antiestrogen compared with cells with nonmutant ESR1. MCF7 xenografts harboring ESR1Y537S and ESR1D538G mutations established in nude mice were resistant to palbociclib. Conclusions: We report herein real-world and preclinical evidence that ESR1 mutations, particularly Y537S and D538G, can drive resistance to CDK4/6i.

Purpose: We report herein a phase Ib trial to determine the safety, tolerability, and antitumor activity of erdafitinib, a pan-FGFR tyrosine kinase inhibitor, with fulvestrant and palbociclib in patients with hormone receptor–positive/HER2-negative metastatic breast cancers (NCT03238196). Patients and Methods: Thirteen patients were enrolled on the escalation phase in a traditional 3 + 3 trial design to determine the maximum tolerated dose (MTD). Subsequently, 22 patients were treated at the established MTD during the expansion phase. All patients had received prior treatment with cyclin-dependent kinase-4/6 inhibitors and endocrine therapy, and 29 showed FGFR pathway alterations in their tumors. Results: The MTD of erdafitinib was 6 mg taken orally once daily when combined with palbociclib and fulvestrant. The triple combination showed clinically manageable tolerability. Most common adverse events were neutropenia, likely attributable to palbociclib, and oral mucositis and hyperphosphatemia, attributable to erdafitinib. Three patients showed a partial response, one of them lasting more than 2.5 years, despite lacking detectable FGFR1 to FGFR4 somatic alterations. FGFR1 amplification was not associated with response to FGFR inhibition, but high FGFR1 protein expression, measured by IHC, correlated with longer progression-free survival within the FGFR1-amplified cohort. There was no correlation between FGFR1 copy number and FGFR1 protein levels in specimens from metastatic sites, potentially highlighting the need for a more recent metastatic tumor biopsy for biomarker evaluation. Conclusions: The trial endpoint was met establishing the MTD of erdafitinib at 6 mg. Whereas the triplet regimen may pose tolerability challenges, alterative doublets with selective FGFR1 inhibitors in patients with FGFR1-dependent tumors, possibly administered in sequence, are worthy of further investigation.

The role of the tumor immune microenvironment (TIME) in modulating responses to antiestrogen therapy in hormone receptor–positive (HR ⁺ ) breast cancers remains unclear. We analyzed pre- and on-treatment biopsies from patients with HR ⁺ breast cancer treated with letrozole to induce estrogen deprivation (ED). Stromal tumor–infiltrating lymphocytes, assessed by H&E staining, and immune-related gene sets, including IFN-γ signaling genes, measured by RNA-Seq, were increased in ED-resistant tumors. Cyclic immunofluorescence and spatial transcriptomics revealed an abundance of CD8 ⁺ T cells and enhanced antigen processing and immune gene signatures in ED-resistant tumors. In this group, the expression of CXCL9 , CXCL10 , and CXCL11 — chemokine genes involved in CD8 ⁺ T cell recruitment — and the CXCR3 receptor were upregulated both before and after letrozole treatment. CXCL11 levels were higher in conditioned media from HR ⁺ breast cancer cells cocultured with CD8 ⁺ T cells. Both recombinant CXCL11 and coculture with CD8 ⁺ T cells promoted MCF7 and T47D cell growth in estrogen-free conditions. Finally, deletion combined with silencing of the CXCL11 receptors CXCR3 and CXCR7 in MCF7 cells impaired proliferation in response to exogenous CXCL11 and to coculture with CD8 ⁺ T cells in estrogen-free conditions. These findings suggest that CD8 ⁺ T cell–associated CXCL11 in the TIME modulated the response of HR ⁺ breast cancer cells to estrogen suppression.