The prognosis for metastatic breast cancer (MBC) remains poor, and treatment options are limited for patients with heavily pretreated disease. Although antiangiogenic agents may provide clinical benefit in this setting, real‐world evidence regarding their efficacy, safety, and optimal combination strategies remains limited. This study evaluated antiangiogenic therapy combined with chemotherapy in heavily pretreated MBC, with exploratory analyses across clinically relevant subgroups.

This multicenter retrospective study enrolled 348 patients with heavily pretreated MBC who received antiangiogenic therapy plus chemotherapy or chemotherapy alone. A 1:1 propensity score‐matched cohort of 174 pairs was generated for the primary comparative analysis. Chemotherapy backbones were categorized as microfilament–microtubule inhibitor (MMI)‐based or non‐MMI‐based regimens. The primary endpoint was progression‐free survival (PFS), and secondary endpoints included objective response rate (ORR) and safety. Exploratory subgroup analyses were conducted according to antiangiogenic agent, chemotherapy backbone, prior lines of therapy, and dosing strategy.

In the propensity score‐matched cohort, antiangiogenic therapy combined with chemotherapy was associated with significantly longer progression‐free survival than chemotherapy alone (mPFS, 122 days [95% CI, 104–151] vs. 90 days [95% CI, 83–113]; HR, 0.62; 95% CI, 0.49–0.78; p < 0.001). The incidence of treatment‐related adverse events was similar between the antiangiogenic therapy plus chemotherapy and chemotherapy‐alone groups (72.4% vs. 76.4%), with most events being grade 1–2. Exploratory analyses suggested that PFS outcomes varied according to antiangiogenic agent and chemotherapy backbone, with more favorable estimates observed for bevacizumab‐ or apatinib‐based combinations and for MMI‐based chemotherapy backbones. In exploratory descriptive dose‐stratified analyses, PFS estimates varied across dose groups, and these findings should not be interpreted as evidence of comparable efficacy between dose levels.

Antiangiogenic therapy combined with chemotherapy was associated with longer progression‐free survival and a manageable safety profile in patients with heavily pretreated metastatic breast cancer. Exploratory analyses suggested that treatment outcomes may vary according to antiangiogenic agent, chemotherapy backbone, and prior treatment exposure. Dose–stratified analyses were descriptive and should not be interpreted as evidence of equivalence or non‐inferiority between dose levels. Given the retrospective observational design, these findings should be considered hypothesis‐generating and warrant prospective validation.

The detection of estrogen receptor 1 (ESR1) ligand‐binding domain mutations in circulating tumor DNA (ctDNA) is crucial for guiding therapy in estrogen receptor‐positive metastatic breast cancer. However, widespread clinical adoption of approaches for monitoring drug resistance and guiding treatment decisions is hindered by limitations of current methods regarding sensitivity, cost, and multiplexing capability.

The application of switch‐blocker technology, which has been patented for detecting ESR1 hotspot mutations (Y537S/C, D538G, E380Q, and L536H/P), suppresses the amplification of wild‐type alleles while allowing specific amplification of low‐frequency mutant alleles. We used a switch‐blocker to inhibit the amplification of a DNA target approximately 10 base pairs in length (e.g., the switch‐blocker covering codon 536 of ESR1 targets various variants at positions 536, 537, and 538). Targeted enrichment was achieved by quantitative polymerase chain reaction, followed by pyrosequencing to confirm mutation components. Next‐generation sequencing and Sanger sequencing served as supplementary methods for the verification of results.

The ESR1‐targeted DNA assay was validated for feasibility on plasmid circular templates and ctDNA linear templates. In tests using gradient‐diluted ESR1 plasmid templates, the proportion of L536H mutant copies increased from 0.0015% to 16.89% after targeted amplification, while the proportion of E380Q mutant copies increased from 0.0015% to 1.35%. In ctDNA samples previously analyzed by next‐generation sequencing, the switch‐blocker considerably enriched other mutant copies within the coverage range of the switch element.

This switch‐blocker–enhanced pyrosequencing assay presents a targeted, multiplexed, and accessible approach for detecting ESR1 hotspot mutations in liquid biopsies. This assay has potential for dynamic monitoring of therapeutic resistance, facilitating timely treatment decisions in advanced breast cancer management.