What's the difference between HER2-positive and HER2-low?

HER2-positive = IHC 3+ or HER2/CEP17 ISH ratio ≥2.0. HER2-low = IHC 1+ or 2+/ISH-. Historically lumped with HER2-negative; now T-DXd is approved for HER2-low metastatic disease (DESTINY-Breast04). The IHC 0 vs 1+ distinction is critical and pathologist-dependent.

Why is T-DXd preferred over T-DM1 in second-line metastatic HER2+?

DESTINY-Breast03 showed T-DXd had ~28-month median PFS vs ~7 months for T-DM1 in HER2+ metastatic breast cancer previously treated with trastuzumab and taxane. T-DXd is now standard 2L. The trade-off is the ILD/pneumonitis risk, which requires active monitoring.

When does tucatinib come into play?

Tucatinib + trastuzumab + capecitabine (HER2CLIMB) is approved for HER2+ metastatic breast cancer with active brain metastases. Its small-molecule TKI nature allows CNS penetration that the antibody therapies cannot match.

Variant cheat sheet · breast cancer

HER2-positive breast cancer (ERBB2 amplification)

Free clinical reference. Educational use only — not a substitute for professional medical advice.

TL;DR

HER2-positive breast cancer is AMP/ASCO/CAP Tier I-A for trastuzumab + pertuzumab + taxane (CLEOPATRA, first-line metastatic) and T-DXd (trastuzumab deruxtecan) as second-line (DESTINY-Breast03). The HER2-low category (IHC 1+ or 2+/ISH-) now has its own indication for T-DXd (DESTINY-Breast04). Diagnosis: IHC 3+ or HER2/CEP17 ratio ≥2.0 by ISH.

Biology

HER2 (ERBB2) is a member of the EGFR family of receptor tyrosine kinases. Amplification drives constitutive dimerization with HER3 / EGFR and downstream MAPK + PI3K/AKT signaling. Overexpression is a strong proliferative driver and a vulnerability that monoclonal antibodies (trastuzumab, pertuzumab) and antibody-drug conjugates (T-DM1, T-DXd) exploit.

Epidemiology

HER2 amplification occurs in roughly 15-20% of breast cancers. HER2-low (IHC 1+ or 2+/ISH-) represents an additional ~50% of historically HER2-negative tumors and is now a treatable category.

Detection

IHC: scored 0, 1+, 2+, 3+. IHC 3+ = HER2-positive. IHC 0 vs 1+ distinction is critical for HER2-low eligibility.
FISH/ISH: HER2/CEP17 ratio ≥2.0 = HER2-positive. Reflex from IHC 2+.
Comprehensive NGS panels (FoundationOne CDx, Tempus xT, Caris) also report ERBB2 amplification status.

FDA-approved targeted therapies

Drug	Indication	Line	Tier
Trastuzumab + pertuzumab + taxane	HER2+ metastatic breast 1L (CLEOPATRA)	1L	I-A
Trastuzumab deruxtecan (T-DXd)	HER2+ metastatic, 2L+ (DESTINY-Breast03); HER2-low 2L+ (DESTINY-Breast04)	2L+	I-A
T-DM1 (ado-trastuzumab emtansine)	HER2+ metastatic, post-trastuzumab	2L+	I-A
Tucatinib + trastuzumab + capecitabine	HER2+ metastatic with brain mets (HER2CLIMB)	2L+	I-A
Trastuzumab (neoadjuvant/adjuvant)	HER2+ early-stage	Early-stage standard of care	I-A

Tier = AMP/ASCO/CAP 2017 (Li MM et al., J Mol Diagn 2017). Drug names link to openFDA labels where available. Synthetic data; consult primary sources before treatment.

Resistance pathways

PI3K/AKT/PTEN pathway activation (PIK3CA mutations, PTEN loss).
HER2 truncation (p95 HER2).
HER3 upregulation.
For T-DXd specifically: interstitial lung disease/pneumonitis is a key dose-limiting and treatment-limiting toxicity.

Frequently asked questions

What's the difference between HER2-positive and HER2-low?: HER2-positive = IHC 3+ or HER2/CEP17 ISH ratio ≥2.0. HER2-low = IHC 1+ or 2+/ISH-. Historically lumped with HER2-negative; now T-DXd is approved for HER2-low metastatic disease (DESTINY-Breast04). The IHC 0 vs 1+ distinction is critical and pathologist-dependent.
Why is T-DXd preferred over T-DM1 in second-line metastatic HER2+?: DESTINY-Breast03 showed T-DXd had ~28-month median PFS vs ~7 months for T-DM1 in HER2+ metastatic breast cancer previously treated with trastuzumab and taxane. T-DXd is now standard 2L. The trade-off is the ILD/pneumonitis risk, which requires active monitoring.
When does tucatinib come into play?: Tucatinib + trastuzumab + capecitabine (HER2CLIMB) is approved for HER2+ metastatic breast cancer with active brain metastases. Its small-molecule TKI nature allows CNS penetration that the antibody therapies cannot match.