S (Sensitive) — cells are sensitive to trastuzumab:
Low viability (eg <70%)

High growth reduction (ΔGR > 1.2)
E.g. BT-474, SK-BR-3

R (Resistant) — cells are resistant:
High viability (>80%), growth is preserved (ΔGR ≈ 1 or <1)
Often these are either initially resistant lines or “.R” — resistant derivatives.

Cells were classified as sensitive (S) or resistant (R) by testing cell proliferation in the presence and absence of 15 µg/ml trastuzumab for 7 days. Response to trastuzumab was quantified by calculating the fold change in the growth rate (∆GR) of the treated cells relative to the non-treated cells.

[Generation, characterization, and maintenance of trastuzumab-resistant HER2+ breast cancer cell lines]

HER2 Testing in Breast Cancer - 2023 Guideline Update
The 2023 “Human Epidermal Growth Factor Receptor 2 (HER2) Breast Testing Guideline Update” reaffirms the 2018 “HER2 Breast Testing Guideline Focused Update.”
This reaffirmation was propelled by results of the 2022 DESTINY-Breast04 trial, which prompted the United States Food and Drug Administration (FDA) to expand the approval of the HER2 antibody-drug conjugate, trastuzumab deruxtecan, from metastatic breast cancer patients with HER2 protein over-expression/amplification to also include metastatic patients with HER2 IHC 1+ or 2+/ISH negative results. This guideline update does not support the use of a HER2-Low interpretive category because the DESTINY-Breast04 trial did not include patients with HER2 IHC 0 results, there is no evidence to support that IHC 1+ or 2+/ISH negative results are predictive of trastuzumab deruxtecan treatment response when compared to IHC 0 results. Therefore, the FDA expansion of approval to this group was only based on clinical trial eligibly criteria rather than a new predictive indication for HER2 testing.
The guideline update is published as an early online release in the Archives of Pathology & Laboratory Medicine.

Herceptin contains the active substance trastuzumab (anti-p185, rhuMab HER2), which is a humanised monoclonal antibody that binds to the HER2 protein. Laboratory testing can determine ERBB2 status and aid in the prediction of response to HER2-directed therapy. Clinical trials have demonstrated that trastuzumab can significantly increase disease-free survival, and overall survival, Comprehensive biomarker analysis of long-term response to trastuzumab in patients with HER2-positive. study demonstrated that the addition of trastuzumab, a humanised monoclonal HER2-targeting antibody, to chemotherapy significantly improved the survival of patients with HER2-positive mGC/mGEJC in comparison to chemotherapy alone Even though this was a tremendous success, the overall survival remained poor with a median of 13.8 months However, several case reports and few smaller studies have reported that a subgroup of HER2-positive mGC/mGEJC patients showed prolonged survival for several years under maintenance therapy with trastuzumab alone To date, a biomarker that reliably identifies this long-term responder to trastuzumab treatment remains elusive.
A high level of HER2 gene amplification was proposed as improved predictor of sensitivity to trastuzumab treatment, but the studies defined different thresholds Furthermore, a tumour mutational burden (TMB) of more than 10 mutations per Mb was indicated to be a potential biomarker for trastuzumab efficacy ]. Moreover, initial resistance to trastuzumab or impaired treatment response was associated with the presence of co-amplifications and mutations of oncogenes or genes related to the HER2 signalling pathway such as EGFR and KRAS In addition, intratumoural heterogeneity of HER2 protein expression, frequently found in GC and GEJC, was associated with poor prognosis
In 2021, immunotherapy offered new treatment options for mGC/mGEJC patients and the combination of nivolumab/pembrolizumab and chemotherapy was approved as first-line therapy for programmed cell death ligand 1 (PD-L1) expressing tumours. Since then, PD-L1 expression is routinely assessed in clinical diagnostics in addition to the HER2 status []. Recent studies like the KEYNOTE-811 explored the efficacy of anti-PD-L1 treatment in HER2-positive mGC/mGEJC showing that the combination of pembrolizumab and trastuzumab with chemotherapy leads to improved response rates compared to treatment with chemotherapy and trastuzumab alone ]. In addition, the combined expression of PD-L1 and HER2 was found to be a positive prognostic factor for survival in GC Findings from a preliminary biomarker analysis of the DESTINY-Gastric03 trial showed a large overlap between HER2 and PD-L1 expression in 85% of the patients GC or GEJC In the KEYNOTE-811 study, similar frequencies for double positivity (HER2+/PD-L1+) have been reported
In this retrospective study, we divided HER2-positive mGC/mGEJC patients into two groups based on progression-free survival (PFS) under trastuzumab-based treatment and performed an explorative analysis of biomarkers. We applied targeted next-generation sequencing (NGS) using a 409 gene panel, Affymetrix gene expression analysis and immunohistochemistry to evaluate biomarkers and investigated their prognostic impact in this cohort.. In one study, response rates and time to progression were maintained from first to beyond second line treatment. Response rates were 42.6% in first line trastuzumab therapy, 25.9% in second line, and 30% in beyond second line respectively. Median time to progression was 6 months in all treatment lines

response rates and time to progression were maintained from first to beyond second line treatment. Response rates were 42.6% in first line trastuzumab therapy, 25.9% in second line, and 30% in beyond second line respectively. A number of biomarkers have been evaluated in their potential to predict response to trastuzumab-based therapies.

a patient will respond to trastuzumab, a targeted therapy for HER2-positive breast cancer, doctors use a test called HER2 testing. This test helps identify tumors that overexpress or amplify the HER2 protein, which is the target of trastuzumab. If a tumor is found to be HER2-positive, then trastuzumab is more likely to be an effective treatment.

EC-DL=epirubicin plus cyclophosphamide x 4 cycles followed by docetaxel plus lapatinib;
EC-DT=epirubicin plus cyclophosphamide x 4 cycles followed by docetaxel plus trastuzumab;
pCR=pathological complete response.