For high-grade, PR-negative ER+ breast cancers with very high Ki-67, oncologists may use intrinsic subtyping assays. If a 'basal-like' subtype is identified, they consider treating the patient with a triple-negative breast cancer regimen (like KEYNOTE-522) instead of standard ER+ therapy, a non-standard but biology-driven approach.

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.

Clinicians are moving beyond strict immunohistochemistry cutoffs for treatment decisions. Tumors with low estrogen receptor expression (ER-low, <10%) are often considered not to be primarily estrogen-driven and are treated with immunotherapy-based regimens standard for triple-negative disease, reflecting a shift toward biologically-informed therapy.

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.

The KEYNOTE-756 and Checkmate 7FL trials found high pathological complete response (pCR) rates with neoadjuvant immunotherapy in ER-low (1-10%) breast cancers. This suggests this unique subgroup, often excluded from triple-negative trials but behaving similarly, may benefit significantly from immunotherapy, though it is not yet standard of care.

Contrary to the belief that HR+ breast cancer primarily carries a late recurrence risk, data shows high-risk, node-positive patients can be extremely aggressive early on. With recurrence rates up to 29.1% within five years, this subgroup can perform as poorly, or even worse, than triple-negative breast cancer, highlighting the need for intensive adjuvant therapy.

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.

While the HER2 pathway is the dominant driver for achieving an initial tumor response in HER2+/HR+ disease, targeting the ER pathway is critical for maintaining that response and achieving long-term benefit. This dual-targeting benefit becomes more apparent over extended periods, as seen in the Affinity trial.

The standard HER2 tests were developed to identify HER2-positive tumors, not to precisely quantify low levels of expression. This creates a diagnostic challenge for identifying patients eligible for HER2-low targeted ADCs, requiring closer collaboration with pathology to interpret results that may be near the threshold, such as HER2-zero but with some minimal staining.