Cancers with estrogen receptor (ER) expression of 50% or less, while technically HR+, often behave biologically like basal or triple-negative tumors. These cancers are not primarily endocrine-driven and show a significant benefit from the addition of immune checkpoint inhibitors, challenging traditional subtype classifications.

Hormone receptor-positive (HR+) HER2+ breast cancers often show lower rates of pathologic complete response (pCR) to pre-surgical therapy. This is due to their slower-growing biology, not treatment ineffectiveness. Clinicians should recognize this nuance and not assume a worse prognosis based on pCR alone in this subtype.

Positive data from both DESTINY-Breast09 (TDXD-based) and PATINA (CDK4/6i maintenance) create a new dilemma. With similar PFS outcomes, the first-line choice for metastatic HER2+/HR+ patients now hinges on toxicity profiles and patient preference rather than a single efficacy winner.

Even within recent major clinical trials like HER2CLIMB-05, less than half of eligible hormone receptor-positive patients received endocrine therapy. This highlights a critical and widespread gap in clinical practice, as this treatment adds significant benefit.

NGS testing is revealing that acquired HER2 kinase domain mutations, not amplifications, are an emerging resistance mechanism in ER+ lobular breast cancer. This creates a targetable population for HER2 TKIs like neratinib or tucatinib, offering a new line of targeted therapy.

The PATINA trial revealed palbociclib, not known for CNS activity, unexpectedly lowered the incidence of CNS progression in triple-positive metastatic breast cancer (13.8% vs 20.4%). This suggests a potential new benefit for this drug in a patient population at high risk for brain metastases.

When treating elderly patients (e.g., age 80+) with metastatic breast cancer, clinicians may prioritize quality of life over marginal overall survival gains seen in clinical trials. This justifies using a better-tolerated CDK4/6 inhibitor like palbociclib, even though ribociclib has demonstrated a statistical survival benefit, especially when patients have comorbidities or a preference for fewer side effects.

To mitigate long-term toxicity from TDXD, oncologists are proposing an "induction/maintenance" approach. Patients receive TDXD for an initial period to achieve maximal response, then switch to a less toxic maintenance regimen for a "chemotherapy holiday," improving quality of life.

In addition to improving systemic progression-free survival, maintenance therapies with palbociclib (PATINA trial) and tucatinib (HER2CLIM-05 trial) both demonstrated a benefit in reducing central nervous system (CNS) progression, a critical secondary benefit for HER2+ patients.