Impact of p95HER2 Expression on Clinical Outcomes in HER2-Positive Breast Cancers: A Literature Review

Dr. Lesiba Terrance Khalo. (2025). Impact of p95HER2 Expression on Clinical Outcomes in HER2-Positive Breast Cancers: A Literature Review. MAR Oncology and Hematology. (2025) 5:10 Impact of p95HER2 Expression on Clinical Outcomes in HER2-Positive Breast Cancers: A Literature Review Dr. Lesiba Terrance Khalo * *Correspondence to: Dr. Lesiba Terrance Khalo, Military Hospital, Pretoria, Gauteng Province, South Africa. Copyright. © 2025 Dr. Lesiba Terrance Khalo This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Received: 08 Oct 2025 Published: 17 Oct 2025 MAR Oncology and Hematology (2025) 5:10 Literature Review Dr. Lesiba Terrance Khalo, MAR Oncology and Hematology (2025) 5:10. Page 2 of 12 Dr. Lesiba Terrance Khalo. (2025). Impact of p95HER2 Expression on Clinical Outcomes in HER2-Positive Breast Cancers: A Literature Review. MAR Oncology and Hematology. (2025) 5:10 Introduction Human epidermal growth factor receptor 2 (HER2), is one of a family of 4 membrane tyrosine kinases (TKs), which was first discovered 25 years ago, and shown to be overexpressed in a human breast cancer cell (5) with its prevalence shown to be important in pathogenesis and disease progression in breast cancer patients (6). The HER2 pathway was described in systems biology terms as a complex biological network composed of 3 layers: 1) An input layer of membrane receptors, alongside their ligands, whose function is to activate signal extracellular cascades, 2) A core system processing layer of protein kinases (PKs) who’s roles are signal transmission to the nucleus, followed by 3) An output layer of transcription factors, which function by regulating genes that effect various cellular functions, such as proliferation, migration, apoptosis as well as angiogenesis (1). The input layer is composed of 4 membrane receptors/kinases (HER1–4) and their many ligands, including endothelin growth factor (EGF), transforming growth factor alpha (TGF-α), and heregulin. However, in breast cancer, HER2 is the dominant kinase receptor, overexpressed in 20% of cases (6). Upon ligand binding, the HER receptors undergo conformational changes that enable homoor heterodimerization, followed by transphosphorylation of the receptors. HER2 does not have ligandbinding capabilities, but its open conformation allows for dimerization. When overexpressed, the activated form heterodimerizes with other HER members that have ligand-binding capabilities or undergoes homo-dimerization (1). This dimerization leads to the activation of the PI3K/AKT antiapoptosis pathway (Figure 1), thereby stimulating the activation of transcription factors and exhibiting a proliferative, migratory, angiogenic, and differentiation cancer phenotypes. The estrogen receptor (ER) and the HER2 signaling pathways are the dominant drivers of cell proliferation and survival in most (85%) breast cancers (1). Dr. Lesiba Terrance Khalo, MAR Oncology and Hematology (2025) 5:10. Page 3 of 12 Dr. Lesiba Terrance Khalo. (2025). Impact of p95HER2 Expression on Clinical Outcomes in HER2-Positive Breast Cancers: A Literature Review. MAR Oncology and Hematology. (2025) 5:10 Figure 1: HER-2 signaling pathway (24). Some HER2 positive breast cancers have been shown to express the truncated form of HER2, known as p95HER2, which lacks the extracellular domain. This lack of the extracellular domain has been attributed to resistance to drugs such as trastuzumab, as it works through domain-binding mechanisms (7, 8). This study focuses on the expression of p95HER2, a truncated form of HER2, and its potential impact on breast cancer patients receiving trastuzumab. The objective is to determine whether high levels of p95HER2 in these patients are associated with a worse disease prognosis compared to patients without these mutations. The writer will conduct a comprehensive literature review to explore this issue in the context of South Africa. Hypothesis Elevated expression of P95HER2 in HER2-positive breast cancers is associated with poorer clinical outcomes, including progression-free survival (PFS) and overall survival (OS), due to its role in mediating resistance to trastuzumab therapy. Dr. Lesiba Terrance Khalo, MAR Oncology and Hematology (2025) 5:10. Page 4 of 12 Dr. Lesiba Terrance Khalo. (2025). Impact of p95HER2 Expression on Clinical Outcomes in HER2-Positive Breast Cancers: A Literature Review. MAR Oncology and Hematology. (2025) 5:10 Project objectives A comprehensive literature review delineating the impact of p95HER2 expression on clinical outcomes in HER2-positive breast cancers has was undertaken, to assess the correlation between p95HER2 expression levels and clinical outcomes such as overall survival (OS), progression-free survival (PFS), recurrence-free survival (RFS), pathologic complete response (pCR) as well as examining the therapeutic implications of p95HER2 expression on the efficacy of HER2 targeted therapies in trastuzumab. Methodology We conducted comprehensive literature review; 1) To gain a better understanding on the topic, by identifying recent literature reviews on the aforementioned, followed by selection of relevant original papers, 2) The search strategy and selection criteria to narrow down the search, 3) Appropriate methods for selecting relevant and up-to-date papers were undertaken; such include, but are not limited to the search language, prominent authors, papers with more citations, as well as credible peer-reviewed journals with higher impact factors, 4) The search was not limited to peer-reviewed papers and books, but extended to published conference proceedings and thesis. Below, in Figure 2 is a graphical depiction of the review strategy and workflow. Figure 2: The graphical depiction of the methodology workflow. A selection of 16 peer reviewed articles specifically looking at clinical data depicting the impact of p95HER2 expression, and its association with disease severity were carefully selected for this review. Dr. Lesiba Terrance Khalo, MAR Oncology and Hematology (2025) 5:10. Page 5 of 12 Dr. Lesiba Terrance Khalo. (2025). Impact of p95HER2 Expression on Clinical Outcomes in HER2-Positive Breast Cancers: A Literature Review. MAR Oncology and Hematology. (2025) 5:10 Scholarly databases and search engines, such as PubMed, Scopus, Google Scholar, Elsevier, and ScienceDirect, were used to develop a comprehensive search strategy. The researcher used the following keywords in the search: p95HER2, HER2-positive breast cancer, clinical outcomes, trastuzumab, treatment response, and resistance. The Medical Subject Headings (MeSH) keywords and Boolean operators were used to narrow the search. Studies published 10-15 years ago, to date, were included. Both clinical studies and original research articles were included in the criteria. Studies that included the use of the following scientific techniques —immunohistochemistry, FISH, and sequencing of HER2positive tissue/cells — were included. Preference was given to authors whose research was primarily conducted within the South African context, particularly in clinical studies. Pre-clinical (animal) studies were excluded from the search, alongside articles that were older than 15 years Results Following a comprehensive literature search using the above databases (PubMed, Scopus, Google Scholar, Elsevier, ScienceDirect), a vast body of literature was retrieved. Followed by a manual pick of original articles, thus excluding literature reviews. The focus was on clinical studies, whether retrospective or Phase 1, 2, or 3 clinical trials. Here, we picked very few articles that specifically touched on p95HER2-positive breast cancer patients on trastuzumab treatment. Below is an account of the selected articles that depict the correlation between p95HER2 expression in breast cancer patients and clinical outcomes, specifically PFS and OS (Table 1). Other endpoint clinical outcomes in this literature, such as recurrence-free survival (RFS), disease-free survival (DFS), and pathological complete response (pCR), were also included. Table 1: Studies that depict the correlation between p95HER2 expression in breast cancer patients and clinical outcomes, specifically the PFS and OS, as well as second-line treatment studies. Author Study type (clinical) Patient sample type (Number of participants) Major findings Reference Goh et al., 2024 Retrospective FFPE (59) Prevalence of p95HER2 in premenopausal women, with worsening DFS. 14 Rigakos et al., 2021 Retrospective FFPE (114) An aggressive cancer phenotype was associated with high expression of p95HER2, as per high CNVs, as well as expression of Ki67. 15 Dr. Lesiba Terrance Khalo, MAR Oncology and Hematology (2025) 5:10. Page 6 of 12 Dr. Lesiba Terrance Khalo. (2025). Impact of p95HER2 Expression on Clinical Outcomes in HER2-Positive Breast Cancers: A Literature Review. MAR Oncology and Hematology. (2025) 5:10 Lipton et al., 2013 Retrospective FFPE (89) Both HER3 and p95HER2 expressing patients exhibited poor clinical outcomes. 16 Chumsri et al., 2018 Retrospective FFPE (91) The ratio of p95HER2/HER2 was significantly associated with worsening PFS. 17 Duchnowska et al., 2017 Retrospective FFPE (189) Second-line Lapatinib treated patients with high p95HER2 expressions may have the best clinical outcomes while on treatment. 20 Han et al., 2012 Retrospective FFPE (52) With combination therapy of Lapatinib plus Capecitabine, there was no significant association between p95 isoform and PFS/OS. 21 Cao et al., 2023 Single-arm, multicentre phase 2 trial Clinical trial (100 patients) With a combination of pyrotinib and capecitabine, the median PFS increased from 8 to 11 months post treatment. 22 Several studies have shown that the increased expression of a truncated version of HER2, p95HER2, is a critical determinant of poor prognosis and therapeutic response in HER2-positive breast cancer. It has also been shown that the median survival rate of these patients can be as low as 8 months. To better understand these underlying resistance mechanisms, conventionally, 2D tumor cell culture models have been used. However, these models, due to their monolayer nature, cannot fully recapitulate the tumor micro-environment, thereby warranting the development of more biologically complex, yet clinically relevant models of disease. Scaffold (matrix)-free self-assembled aggregates of cancer cells (spheroids), particularly known as organoids, are suitable in vitro models of disease (9-12). They have therefore been extensively used in various settings to study tumor progression and mechanisms of resistance (12). This literature review aims to assess the correlation between p95HER2 expression levels and clinical outcomes, including progression-free survival (PFS) and overall survival (OS). In addition, it examined the therapeutic implications of p95HER2 expression on the efficacy of HER2-targeted therapies in trastuzumab. Trastuzumab (Herceptin) and pertuzumab are two of the HER-2 receptor-binding humanized monoclonal antibodies that have been approved by the FDA as therapeutic agents for HER2-positive breast cancers (13). However, some breast cancers have been shown to express an aberrant form of HER2, missing the extracellular domain. This truncated isoform of HER2 is commonly referred to as p95HER2, which has been attributed to resistance to drugs such as trastuzumab. Below is an account of this resistance mechanism, alongside the resulting clinical outcomes in patients harboring such mutations. Dr. Lesiba Terrance Khalo, MAR Oncology and Hematology (2025) 5:10. Page 7 of 12 Dr. Lesiba Terrance Khalo. (2025). Impact of p95HER2 Expression on Clinical Outcomes in HER2-Positive Breast Cancers: A Literature Review. MAR Oncology and Hematology. (2025) 5:10 A recent study by Goh et al. (2024) showed a higher recurrence and metastasis rate in patients with p95HER2positive tumors compared to those with no p95HER2 expression. This was a retrospective study wherein formalin-fixed paraffin-embedded (FFPE) tissue sections were stained with the p95HER2 antibody to evaluate expression. This study was designed to include samples from 30 non-relapsed breast cancer patients, alongside 29 primary trastuzumab-resistant patient samples. In their study, the group demonstrated that p95HER2 positivity was associated with worse disease-free survival (DFS) in patients undergoing trastuzumab treatment (14). Similarly, a study by Rigakos et al. (2021) showed an association between the P95-isoform and resistance to trastuzumab. Here, the authors found that high p95-isoform expressing patients had a greater risk of death, and that this isoform was correlated to increased expression levels of proliferation marker (ki-67) as well as gain of HER2 copy number variations (CNVs), thus indicative of a more aggressive cancer phenotype (15). In their quest to further elucidate the “trastuzumab escape” biomarkers, a study by Lipton et al. (2013 designed a dual antibody proximity-based assay for the precise quantitation of HER3 total protein (H3T). Here, 89 FFPE sections of metastatic breast patients previously on trastuzumab treatment were assessed for clinical outcomes such as OS and PFS using a VeraTag H3T assay. The data was analyzed via Kaplan–Meier and decision tree analysis. This study also showed that patients on the treatment exhibited poor clinical outcomes (16). Here, patients with high expression levels of H3T showed a lower PFS compared to those with low expression levels. However, there were no significant differences in OS between the highand low-expressing samples. Furthermore, to validate the results, IHC, ELISA, and FACS were also performed, and the results correlated (16). Due to the varying data regarding the correlation between p95HER2 expression and poor clinical outcomes, Chumsri et al. (2018 sought to evaluate the ratio of p95HER2/HER2 in breast cancer patients. Here, two Phase II clinical trials were conducted to evaluate the correlation between quantitative levels of HER2, p95HER2, and p95HER2/HER2, and clinical outcomes following trastuzumab-based chemotherapy in metastatic HER2positive breast cancer patients (17). The study concluded that worsening PFS and OS are significantly correlated with p95HER2/HER2 ratio (17). Furthermore, the studies described mechanisms underlying the correlation between the p96-isoform and poor clinical outcomes were limited in the clinical setting. Most similar studies are in vitro (9-12). Recent clinical studies have indicated that there may be specific co-expressed biomarkers correlating to the pathological and clinical parameters of poor prognosis in HER2-positive breast cancer (18), as well as specific mutations (19). A study by Hergueta-Redondo et al. (2016 showed that the HER2 coamplified and co-expressed Gasdermin Dr. Lesiba Terrance Khalo, MAR Oncology and Hematology (2025) 5:10. Page 8 of 12 Dr. Lesiba Terrance Khalo. (2025). Impact of p95HER2 Expression on Clinical Outcomes in HER2-Positive Breast Cancers: A Literature Review. MAR Oncology and Hematology. (2025) 5:10 (encoded by the gene GSDMB is a potential critical determinant of poor prognosis and therapeutic response in HER2-positive breast cancer (23). To elucidate the genome alterations in this cancer phenotype, a recent study by Ma et al. (2024 performed a gene-targeted genome sequencing in HER2+ breast cancer patients. In this study, approximately 650 mutations were reported. Two mutations of note were those in the tumor suppressor genes NF1 and ATM, which were associated with trastuzumab resistance (19). Specifically, the NFI mutation was associated with a higher risk of early patient relapses (19). This is the first study to identify the unique cancer-related gene mutation profile, thus laying a foundation for prospective studies aimed at further elucidating resistance mechanisms to trastuzumab. Due to the nature of resistance to treatment in patients with positive p95HER2 expression, second-line treatment options have been made available to these patients to increase their PFS and OS rates. In their retrospective study, Duchnowska et al. (2017) quantified the expression levels of HER2, HER3, and p95HER2 proteins, thus allowing for a more in-depth analysis of their potential predictive relevance in determining patients’ PFS and OS. This study showed that patients treated with Lapatinib (a HER1 and HER2 tyrosine kinase inhibitor (TKI)) who have high p95HER2 expression may have the best clinical outcomes while on treatment (20). The Lapatinib Expanded Access Program (a combinatory treatment regimen of Lapatinib and capecitabine) utilized VeraTag assays to quantify the expression levels of HER2, HER3, and p95HER2 proteins from FFPE tissue samples from breast cancer patients who showed resistance to trastuzumab. Here, there was no significant correlation between clinical outcomes and p95 expression levels in their cohort of HER2-positive, trastuzumab-refractory metastatic breast cancer patients (21). On the contrary, a recent investigator-initiated, single-arm, phase 2 trial conducted in China at 16 sites revealed that both combinations of pyrotinib and capecitabine can be a treatment option for HER2-positive advanced breast cancer patients exhibiting trastuzumab resistance (22). In this study, the primary endpoint was the investigator-initiated PFS. This study showed that the median PFS increased from 8 to 11 months post-treatment (22). Discussion This literature review aimed to assess the correlation between p95HER2 expression levels and clinical outcomes, including progression-free survival (PFS) and overall survival (OS). In addition, we examined the therapeutic implications of p95HER2 expression on the efficacy of HER2-targeted therapies in trastuzumab. Dr. Lesiba Terrance Khalo, MAR Oncology and Hematology (2025) 5:10. Page 9 of 12 Dr. Lesiba Terrance Khalo. (2025). Impact of p95HER2 Expression on Clinical Outcomes in HER2-Positive Breast Cancers: A Literature Review. MAR Oncology and Hematology. (2025) 5:10 Several studies have shown that the increased expression of p95HER2 is a critical determinant of poor prognosis and therapeutic response in HER2-positive breast cancer, with the median survival rate of these patients as low as 8 months. Studies have shown a higher recurrence and metastasis rate in patients with p95HER2-positive tumors. The research discovered that these patients had a greater risk of death. The p95HER2 expression correlated with increased expression levels of proliferation markers, such as Ki-67, as well as the gain of HER2 copy number variations (CNVs), thus indicating a more aggressive cancer phenotype, which led to lower PFS as well as OS rates in patients with high expression levels of p95HER2 in a clinical setting. While our understanding of the mechanisms underlying the correlation between the p95-isoform and poor clinical outcomes is growing, there is still much to be done. Most research in this area has been conducted in vitro, and the clinical setting presents its own limitations. However, recent clinical studies have indicated that there may be specific co-expressed biomarkers and gene mutations that correlate with the pathological and clinical parameters of poor prognosis in HER2-positive breast cancer. For instance, a study by HerguetaRedondo et al. (2016) identified GSDMB as a co-expressed biomarker. This gene product is a member of a family of gasdermins that regulate apoptosis in epithelial cells and contribute to disease progression. Through Rab7 activation, GSDMB is involved in autophagy and plays a pivotal role in modulating autophagosome maturation. The process, in turn, facilitates cancer cell resistance through the mechanism of autophagy. Ma et al. (2024) also identified two mutations in the tumor suppressor genes NF1 and ATM, showing that these were associated with trastuzumab resistance. Both studies highlight the need for further research to fully understand the mechanisms underlying the correlation between p95HER2 expression levels and poor clinical outcomes. This work is urgent and of utmost importance in the fight against p95HER2-positive breast cancer. Due to the nature of resistance to treatment in patients with p95HER2 expression, second-line treatment regimens, as well as combination chemotherapies, have been made available to these patients to increase their PFS and OS rates. Drugs that target other HER isoforms, in place of HER2, have been used in the clinical setting—for instance, Lapatinib (a HER1 and HER3 inhibitor was used as a secondline drug. Furthermore, in a study by Han et al. (2012), Lapatinib, a small-molecule dual tyrosine-kinase inhibitor was used in combination therapies, where it was paired with capecitabine, which is activated to 5'-fluorouracil (5FU) and inhibits thymidine synthesis. However, there have been contradictory clinical outcomes, with variations in patient PFS. Some studies have shown an increase in PFS following their second-line combination chemotherapies (22), while others have shown no significant increase (21). These variations were attributed