FGFR1 Amplification Mediates Endocrine Resistance but Retains TORC Sensitivity in Metastatic Hormone Receptor-Positive (HR+) Breast Cancer

Abstract

Purpose: While FGFR1 amplification has been described in breast cancer, the optimal treatment approach for FGFR1-amplified (FGFR1⁺) metastatic breast cancer (MBC) remains undefined.Experimental Design: We evaluated clinical response to endocrine and targeted therapies in a cohort of patients with hormone receptor-positive (HR⁺)/HER2^- MBC and validated the functional role of FGFR1-amplification in mediating response/resistance to hormone therapy in vitro.

Results: In the clinical cohort (N = 110), we identified that patients with FGFR1⁺ tumors were more likely to have progesterone receptor (PR)-negative disease (47% vs. 20%; P = 0.005), coexisting TP53 mutations (41% vs. 21%; P = 0.05), and exhibited shorter time to progression with endocrine therapy alone and in combination with CDK4/6 inhibitor, but not with a mTOR inhibitor (everolimus), adjusting for key prognostic variables in multivariate analysis. Furthermore, mTOR-based therapy resulted in a sustained radiological and molecular response in an index case of FGFR1⁺ HR⁺/HER2^- MBC. In preclinical models, estrogen receptor-positive (ER⁺)/FGFR1-amplified CAMA1 human breast cancer cells were only partially sensitive to fulvestrant, palbociclib, and alpelisib, but highly sensitive to everolimus. In addition, transduction of an FGFR1 expression vector into ER⁺ T47D cells induced resistance to fulvestrant that could be overcome by added TORC1 inhibition, but not PI3K or CDK4/6 inhibition.

Conclusions: Collectively, these findings suggest that while FGFR1 amplification confers broad resistance to ER, PI3K, and CDK4/6 inhibitors, mTOR inhibitors might have a unique therapeutic role in the treatment of patients with ER⁺/FGFR1⁺ MBC.

Figure 1.. Clinical and genomic characteristics associated with FGFR1 amplification in patients with HR+/HER2− Metastatic Breast Cancer (MBC)

1A) Clinical characteristics of patients with metastatic HR+/HER2− breast cancer, harboring FGFR1 amplification (FGFR1+) versus non-amplified patient (FGFR1-). PR = Progesterone Receptor. 1B) Prevalence of tumor genomic alterations in FGFR+ vs. FGFR1− patients based on SNAPSHOT profiling.

Figure 2.. Time to Progression with Various Therapies Among Patients with Metastatic HR+/HER2− Breast cancer, Stratified by FGFR1 Amplification

1A) Time to Progression (TTP) on first line Endocrine-based Therapy in patients with metastatic HR+/HER2− breast cancer, stratified by FGFR1 amplification status. 1B) TTP on first exposure to Chemotherapy among patients with metastatic HR+/HER2− breast cancer, stratified by FGFR1 amplification status. 1C) TTP on first line Endocrine Therapy in combination with CDK 4/6 inhibitor among patients with metastatic HR+/HER2− breast cancer, stratified by FGFR1 amplification status. 1D) TTP on first exposure to Endocrine Therapy in combination with mTOR inhibitor among patients with metastatic HR+/HER2− breast cancer, stratified by FGFR1 amplification status.

Figure 3.. Radiological and molecular response with mTOR based therapy in an index patient with FGFR1 amplified metastatic breast cancer

2A) Restaging CT scan obtained before and during treatment with endocrine therapy in combination with CDK 4/6 inhibitor and mTOR inhibitor, in an index patient with FGFR1 amplified metastatic breast cancer. Target lesions in liver are indicated with yellow arrow. 2B) Changes in the FGFR1 copy number based on cfDNAbefore and during treatment with endocrine therapy in combination with CDK 4/6 inhibitor and mTOR inhibitor, in an index patient with FGFR1-amplified metastatic breast cancer. 2C) FGFR1 FISH of primary tumor tissue. The FGFR1 probe is in red, with control probe (centromere 8) in green.

Figure 4.

4A-B) Growth inhibition of ER+/FGFR1 amplified CAMA-1 breast cancer cells by various inhibitors. Representative images (A) and quantification (B) of integrated intensity are shown (**, P < 0.01 vs. control, t test). 4C) Pharmacodynamic impact on cell-signaling in ER+/FGFR1 amplified CAMA-1 breast cancer cells by various inhibitors. Immunoblot analysis with the indicated antibodies of lysates treated for 6 hours with vehicle, fulvestrant, everolimus, alpelisib, selumetinib, lucitanib, paclitaxel and the combination in 10% DMEM-FBS plus 2ng/mL FGF2. 4D) Growth inhibition of ER+ T47D cells with a FGFR1 expression vector by various inhibitors. Representative images (A) and quantification (B) of integrated intensity are shown (**, P < 0.01 vs. control, t test). 4E) Pharmacodynamic impact on cell-signaling in ER+ T47D cells with a FGFR1 expression vector by various inhibitors. Immunoblot analysis with the indicated antibodies of lysates treated for 6 hours with vehicle, fulvestrant, everolimus, alpelisib, selumetinib, lucitanib, paclitaxel and the combination in 10% DMEM-FBS plus 2ng/mL FGF2. PR mRNA levels from T47D cells with/without a FGFR1 expression vector were analyzed by qRT-PCR.