Advances in the management of metastatic lobular breast cancer: Current evidence and emerging treatments.

Invasive lobular carcinoma (ILC) comprises ∼10%-15% of breast cancers and is characterized by loss of the cell-adhesion molecule E-cadherin (encoded by CDH1), discohesive growth, predominant estrogen receptor (ER) positivity, low-to-intermediate proliferation, and atypical metastatic spread to bone and gastrointestinal/peritoneal sites. Diagnostic assessment is often challenging owing to diffuse infiltration, frequently yielding non-measurable disease per response evaluation criteria in solid tumors (RECIST). Molecularly, ILC is enriched for phosphoinositide 3-kinase (PI3K) activation and harbors emerging vulnerabilities-such as ROS1 synthetic lethality in CDH1-deficient tumors and fibroblast growth factor receptor 1 (FGFR1)/bromodomain and extra-terminal (BET) dependencies-now under study. Because metastatic ILC remains underrepresented in trials, systemic therapy often mirrors invasive ductal carcinoma (IDC). This short communication synthesizes current evidence to distinguish shared from plausibly lobular-specific signals; highlights near-term opportunities-including antibody-drug conjugates (ADCs), oral selective ER degraders (SERDs), and selective use of immunotherapy in an immune-enriched subset with higher tumor-infiltrating lymphocytes (TILs) and PD-L1; and outlines trial-design adaptations-such as incorporating 18F-fluoroestradiol PET (FES-PET)-to improve representation and interpretability in metastatic ILC research.