Targeting HER2 heterogeneity in breast cancer

Omics Technologies Improving Breast Cancer Research and Diagnostics

Breast cancer (BC) has yielded approximately 2.26 million new cases and has caused nearly 685,000 deaths worldwide in the last two years, making it the most common diagnosed cancer type in the world. BC is an intricate ecosystem formed by both the tumor microenvironment and malignant cells, and its heterogeneity impacts the response to treatment. Biomedical research has entered the era of massive omics data thanks to the high-throughput sequencing revolution, quick progress and widespread adoption. These technologies-liquid biopsy, transcriptomics, epigenomics, proteomics, metabolomics, pharmaco-omics and artificial intelligence imaging-could help researchers and clinicians to better understand the formation and evolution of BC. This review focuses on the findings of recent multi-omics-based research that has been applied to BC research, with an introduction to every omics technique and their applications for the different BC phenotypes, biomarkers, target therapies, diagnosis, treatment and prognosis, to provide a comprehensive overview of the possibilities of BC research.

The emergence of targeted therapy for HER2-low triple-negative breast cancer: a review of fam-trastuzumab deruxtecan

INTRODUCTION

Metastatic triple-negative breast cancer (TNBC) is an aggressive sub-type of breast cancer. Despite recent advances, metastatic TNBC remains difficult to treat with limited targeted treatment options. Fam-trastuzumab deruxtecan (T-DXd), is a novel antibody-drug conjugate (ADC) targeting human epidermal growth factor receptor 2 (HER2) and is composed of a unique linker bound to the topoisomerase I inhibitor DXd. T-DXd has significant anti-tumor activity in patients with HER2-low TNBC.

AREAS COVERED

This review reports on the mechanism, pre-clinical/clinical studies, efficacy, and tolerability of T-DXd. A literature search was conducted via PubMed using keywords such as 'fam-trastuzumab deruxtecan,' 'Enhertu,' and 'HER2-low cancers.'

EXPERT OPINION

The Phase III Destiny-Breast04 Trial showed benefit in progression-free and overall survival in patients with HER2-low metastatic breast cancers treated with T-DXd compared to treatment of physician's choice chemotherapy. T-DXd is the first pharmaceutical to effectively target a HER2-low population with clinically meaningful efficacy in patients with HER2-low TNBC. Compared to chemotherapy, T-DXd has a similar safety profile, with the additional need for close monitoring for interstitial lung disease. Given the clinical activity of T-DXd in TNBC, it is likely there will be continued efforts to refine HER2-low diagnostics and to develop additional ADCs with other protein targets.

Increased plasma lipids in triple-negative breast cancer and impairment in HDL functionality in advanced stages of tumors

The association between plasma lipids and breast cancer (BC) has been extensively explored although results are still conflicting especially regarding the relationship with high-density lipoprotein cholesterol (HDLc) levels. HDL mediates cholesterol and oxysterol removal from cells limiting sterols necessary for tumor growth, inflammation, and metastasis and this may not be reflected by measuring HDLc. We addressed recently diagnosed, treatment-naïve BC women (n = 163), classified according to molecular types of tumors and clinical stages of the disease, in comparison to control women (CTR; n = 150) regarding plasma lipids and lipoproteins, HDL functionality and composition in lipids, oxysterols, and apo A-I. HDL was isolated by plasma discontinuous density gradient ultracentrifugation. Lipids (total cholesterol, TC; triglycerides, TG; and phospholipids, PL) were determined by enzymatic assays, apo A-I by immunoturbidimetry, and oxysterols (27, 25, and 24-hydroxycholesterol), by gas chromatography coupled with mass spectrometry. HDL-mediated cell cholesterol removal was determined in macrophages previously overloaded with cholesterol and ¹⁴C-cholesterol. Lipid profile was similar between CTR and BC groups after adjustment per age. In the BC group, lower concentrations of TC (84%), TG (93%), PL (89%), and 27-hydroxicholesterol (61%) were observed in HDL, although the lipoprotein ability in removing cell cholesterol was similar to HDL from CRT. Triple-negative (TN) BC cases presented higher levels of TC, TG, apoB, and non-HDLc when compared to other molecular types. Impaired HDL functionality was observed in more advanced BC cases (stages III and IV), as cholesterol efflux was around 28% lower as compared to stages I and II. The altered lipid profile in TN cases may contribute to channeling lipids to tumor development in a hystotype with a more aggressive clinical history. Moreover, findings reinforce the dissociation between plasma levels of HDLc and HDL functionality in determining BC outcomes.

Development, efficacy and side effects of antibody‑drug conjugates for cancer therapy (Review)

Antibody-drug conjugates (ADCs) are anticancer drugs that combine cytotoxic small-molecule drugs (payloads) with monoclonal antibodies through a chemical linker and that transfer toxic payloads to tumor cells expressing target antigens. All ADCs are based on human IgG. In 2009, the Food and Drug Administration (FDA) approved gemtuzumab ozogamicin as the initial first-generation ADC. Since then, at least 100 ADC-related projects have been initiated, and 14 ADCs are currently being tested in clinical trials. The limited success of gemtuzumab ozogamicin has led to the development of optimization strategies for the next generation of drugs. Subsequently, experts have improved the first-generation ADCs and have developed second-generation ADCs such as ado-trastuzumab emtansine. Second-generation ADCs have higher specific antigen levels, more stable linkers and longer half-lives and show great potential to transform cancer treatment models. Since the first two generations of ADCs have served as a good foundation, the development of ADCs is accelerating, and third-generation ADCs, represented by trastuzumab deruxtecan, are ready for wide application. Third-generation ADCs are characterized by strong pharmacokinetics and high pharmaceutical activity, and their drug-to-antibody ratio mainly ranges from 2 to 4. In the past decade, the research prospects of ADCs have broadened, and an increasing number of specific antigen targets and mechanisms of cytotoxic drug release have been discovered and studied. To date, seven ADCs have been approved by the FDA for lymphoma, and three have been approved to treat breast cancer. The present review explores the function and development of ADCs and their clinical use in cancer treatment.

Detailed curriculum vitae of HER2-targeted therapy

With the booming development of precision medicine, molecular targeted therapy has been widely used in clinical oncology treatment due to a smaller number of side effects and its superior accuracy compared to that of traditional strategies. Among them, human epidermal growth factor receptor 2 (HER2)-targeted therapy has attracted considerable attention and has been used in the clinical treatment of breast and gastric cancer. Despite excellent clinical effects, HER2-targeted therapy remains in its infancy due to its resulting inherent and acquired resistance. Here, a comprehensive overview of HER2 in numerous cancers is presented, including its biological role, involved signaling pathways, and the status of HER2-targeted therapy.

Endocrine Disruptors in Food, Estrobolome and Breast Cancer

The microbiota is now recognized as one of the major players in human health and diseases, including cancer. Regarding breast cancer (BC), a clear link between microbiota and oncogenesis still needs to be confirmed. Yet, part of the bacterial gene mass inside the gut, constituting the so called "estrobolome", influences sexual hormonal balance and, since the increased exposure to estrogens is associated with an increased risk, may impact on the onset, progression, and treatment of hormonal dependent cancers (which account for more than 70% of all BCs). The hormonal dependent BCs are also affected by environmental and dietary endocrine disruptors and phytoestrogens which interact with microbiota in a bidirectional way: on the one side disruptors can alter the composition and functions of the estrobolome, ad on the other the gut microbiota influences the metabolism of endocrine active food components. This review highlights the current evidence about the complex interplay between endocrine disruptors, phytoestrogens, microbiome, and BC, within the frames of a new "oncobiotic" perspective.

Comparison of clinicopathological characteristics and response to neoadjuvant systemic therapy in patients with HER2-low-positive and HER2-zero breast cancer

Previous studies have observed that human epidermal growth factor receptor 2 (HER2)-low-positive patients and HER2-zero patients have different prognoses. This study was conducted to investigate whether there are differences in clinicopathological characteristics and the response to neoadjuvant systemic therapy (NST) defined as systemic treatment prior to surgery between HER2-low-positive patients and HER2-zero patients. We retrospectively analyzed the data of patients with HER2-negative breast cancer who received NST at the First Affiliated Hospital of Nanjing Medical University from 2014 to 2021. There were 238 patients with HER2-low-positive status and 198 patients with HER2-zero status. The proportion of hormone receptor (HR)-positive patients in the HER2-low-positive group was significantly higher than that in the HER2-zero group (82.8% vs 52.0%, p < 0.001). The HER2-low-positive group had more patients with low Ki67 expression (23.9% vs 16.2%, p = 0.045), higher mastectomy rate (94.5% vs 88.9%, p = 0.031), and larger pathological tumor size (21.6 vs 17.8 mm, p = 0.028) than the HER2-zero group. However, no significant differences were found in pathologic complete response (pCR) rates between the two groups. We draw a conclusion that patients with HER2-low status and HER2-zero status were not found to have different pCR rates after NST, irrespective of HR status. However, differences were observed in some clinicopathological characteristics between the two groups.

The association between HER2-low expression and prognosis of breast cancer: a systematic review and meta-analysis

Background

The use of antibody-drug conjugates for the treatment of advanced-stage human epidermal growth factor receptor 2 (HER2)-low expression in breast cancer (BC) has shown prominent curative effects, which has led to increased academic interest. However, the role of HER2-low expression in the prognosis of BC remains controversial.

Methods

We conducted a systematic search of the PubMed, Embase, and Cochrane library databases and several oncology conferences until 20 September 2022. We used fixed- and random-effects models to calculate odds ratio (OR) or hazard ratio (HR) with 95% confidence interval (CI) for overall survival (OS), disease-free survival (DFS), progression-free survival (PFS), and pathological complete response (pCR) rates.

Results

Overall, 26 studies encompassing 677,248 patients were included in the meta-analysis. Patients with HER2-low BC showed significantly better OS than those with HER2-zero BC in the overall population (HR = 0.90; 95% CI: 0.85-0.97) and hormone receptor-positive population (HR = 0.98; 95% CI: 0.96-0.99), whereas no significant difference was observed in the OS of the hormone receptor-negative population (p > 0.05). In addition, there was no significant difference in the DFS of the overall and hormone receptor-negative population (p > 0.05), but better DFS than those with HER2-zero BC in the hormone receptor-negative population (HR = 0.96; 95% CI: 0.94-0.99). There was also no significant difference in the PFS of the overall population, hormone receptor-positive, and hormone receptor-negative population (p > 0.05). Patients with HER2-low BC had a lower pCR rate after neoadjuvant treatment than those with HER2-zero BC.

Conclusions

Compared to patients with HER2-zero BC, those with HER2-low BC had better OS in the overall population and hormone receptor-positive population, DFS in hormone receptor-positive population and lower pCR in the overall population. The biological differences between HER2-low and HER2-zero BCs, particularly in hormone receptor-positive patients, and the relationship between HER2-low expression status and prognosis need to be explored further.

Resistance to antibody-drug conjugates in breast cancer: mechanisms and solutions

Antibody-drug conjugates (ADCs) are a rapidly developing therapeutic approach in cancer treatment that has shown remarkable activity in breast cancer. Currently, there are two ADCs approved for the treatment of human epidermal growth factor receptor 2-positive breast cancer, one for triple-negative breast cancer, and multiple investigational ADCs in clinical trials. However, drug resistance has been noticed in clinical use, especially in trastuzumab emtansine. Here, the mechanisms of ADC resistance are summarized into four categories: antibody-mediated resistance, impaired drug trafficking, disrupted lysosomal function, and payload-related resistance. To overcome or prevent resistance to ADCs, innovative development strategies and combination therapy options are being investigated. Analyzing predictive biomarkers for optimal therapy selection may also help to prevent drug resistance.

The HER2-low revolution in breast oncology: steps forward and emerging challenges

Approximately half of breast cancers (BCs), historically categorized as human epidermal growth factor receptor 2 (HER2)-negative, have low expression of HER2 defined as an immunohistochemical (IHC) score of 1+ or 2+ with negative in situ hybridization. Retrospective evidence suggest that HER2-low BC does not represent a distinct subtype from a biological and prognostic perspective. Nonetheless, it currently constitutes an essential biomarker to guide treatment selection and its introduction has led to reconsidering the binary classification of HER2 status according to which only patients with HER2-positive BC were thought to derive benefit from anti-HER2 therapies. Trastuzumab deruxtecan has recently been approved by the U.S. Food and Drug Administration for the treatment of patients with HER2-low metastatic BC based on the results of the DESTINY-Breast04 phase III trial, and other antibody-drug conjugates (ADCs) targeting HER2 are showing promising results. Treatment paradigms for both triple-negative and hormone receptor-positive BCs exhibiting HER2-low expression are thus rapidly evolving. Given its therapeutic implications, it is essential to accurately recognize the level of HER2 expression, and the development of more sensitive and reliable methods for HER2 testing and scoring is warranted, especially since the minimum threshold of HER2 expression required for T-DXd efficacy is currently under investigation. Given the signs of activity of T-DXd even in patients with HER2-0 (IHC 0) disease, an evolution in the way we define HER2-low is anticipated. Considering the expansion of the therapeutic armamentarium for BC patients, with several ADCs approaching the clinic, research efforts are needed to clarify whether the expression level of targets can enrich for responders to a given ADC as well as to understand mechanisms of resistance with the goal of optimizing the sequencing of ADCs.

Photoelectrochemical immunosensor for HER2 detection based on BiVO4-Bi2S3 heterojunction as photoactive material and CdS as signal probe

A sandwiched photoelectrochemical (PEC) sensor was developed for sensitive detection of human epidermal growth factor receptor 2 (HER2) based on BiVO₄-Bi₂S₃ heterojunction as the photoelectric material accompanied with magnetic nanoparticles for enrichment of HER2 and CdS for signal amplification. The in situ generation of Bi₂S₃ on the surface of BiVO₄ forming a BiVO₄-Bi₂S₃ heterojunction is more conducive to the transit of electron-hole pairs. Antibody-modified MNs are utilized to capture and separate HER2 from samples. After forming a sandwich immune structure, under illumination, the photocurrent shows an increasing trend with the increment of HER2 concentration. The PEC immunosensor displays a good linear concentration range between 1.00 and 1.00 × 10³ pg·mL^-1 and a low limit of detection down to 0.680 pg·mL^-1 (S/N = 3) for HER2 under a bias voltage of 0.1 V (vs. Ag/AgCl electrode). Furthermore, the sensor was successfully applied to detect HER2 in serum samples with recoveries that ranged between 96.1 and 114% with RSDs between 1.3 and 5.9%.

Antibody-Drug Conjugates: A New Therapeutic Approach for Triple-Negative Breast Cancer

Triple-negative breast cancer (TNBC) is an aggressive breast cancer subset associated with a worse prognosis and poor response to conventional chemotherapy. Despite recent advances in drug discovery, its management is still a challenge for clinicians, illuminating the unmet need to develop novel treatment approaches. Antibody-drug conjugates (ADC) are innovative oncology drugs that combine the specificity of monoclonal antibodies and the high efficacy of anticancer payloads, to deliver cytotoxic drugs selectively to cancer cells. Various ADCs were investigated for TNBC and have provided a promise for this aggressive women's cancer including the FDA-approved sacituzumab govitecan. In this chapter, we reviewed different ADCs studied for TNBC including their mechanisms of action, efficacy, and tolerability. Moreover, we have also discussed their therapeutic potential based on combinatorial approaches with other targeted therapies in early and metastatic TNBC.

The Role of Theragnostics in Breast Cancer: A Systematic Review of the Last 12 Years

BACKGROUND

Breast cancer is the most common malignancy in women, with high morbidity and mortality. Molecular alterations in breast cancer involve the expression or upregulation of various molecular targets that can be used for diagnostic nuclear medicine imaging and radiopharmaceutical treatment. Theragnostics is based on the binding of radionuclides to molecular targets. These radionuclides can induce a cytotoxic effect on the specific tumor cell (target) or its vicinity, thus allowing a personalized approach to patients with effective treatment and comparably small side effects.

AIM

This review aims to describe the most promising molecular targets currently under investigation for theragnostics and precision oncology in breast cancer.

METHODS

A comprehensive literature search of studies on theragnostics in breast cancer was performed in the PubMed, PMC, Scopus, Google Scholar, Embase, Web of Science, and Cochrane library databases, between 2010 and 2022, using the following terms: breast neoplasm*, breast, breast cancer*, theragnostic*, theranostic*, radioligand therap*, RLT, MET, FLT, FMISO, FES, estradiol, trastuzumab, PD-L1, PSMA, FAPI, FACBC, fluciclovine, FAZA, GRPR, DOTATOC, DOTATATE, CXC4, endoglin, gastrin, mucin1, and syndecan1.

RESULTS

Fifty-three studies were included in the systematic review and summarized in six clinical sections: 1) human epidermal growth factor receptor 2 (HER2); 2) somatostatin receptors (SSTRS); 3) prostate-specific membrane antigen radiotracers (PSMA); 4) fibroblast activation protein-α targeted radiotracers; 5) gastrin-releasing peptide receptor-targeted radiotracers; 6) other radiotracers for theragnostics.

CONCLUSION

The theragnostic approach will progressively allow better patient selection, and improve the prediction of response and toxicity, avoiding unnecessary and costly treatment.

HER2-Low Status Is Not Accurate in Breast Cancer Core Needle Biopsy Samples: An Analysis of 5610 Consecutive Patients

Background: HER2-Low status is found in approximately half of breast cancer patients and shows potential benefits from novel antibody−drug conjugates (ADCs). Data on the accuracy of HER2-Low status between core needle biopsy (CNB) and surgical excision specimen (SES) samples are lacking. We aimed to investigate the accuracy of HER2-Low status diagnosis between CNB and SES samples. Methods: Consecutive early-stage breast cancer patients who underwent surgery from January 2009 to March 2022 with paired CNB and SES samples were retrospectively reviewed. HER2-Low was defined as IHC 1+ or IHC2+ and FISH-negative. Concordance rates were analyzed by the Kappa test. Further clinicopathological characteristics were compared among different HER2 status and their changes. Results: A total of 5610 patients were included, of whom 3209 (57.2%) and 3320 (59.2%) had HER2-Low status in CNB and SES samples, respectively. The concordance rate of HER2 status in the whole population was 82.37% (Kappa = 0.684, p < 0.001), and was 76.87% in the HER2-Negative patients (Kappa = 0.372, p < 0.001). Among 1066 HER2-0 cases by CNB, 530 patients were classified as HER2-Low tumors. On the contrary, in 3209 patients with HER2-Low tumor by CNB, 387 were scored as HER2-0 on the SES samples. ER-negative or Ki67 high expression tumor by CNB had a high concordance rate of HER2-Low status. Conclusions: A relatively low concordance rate was found when evaluating HER2-Low status between CNB and SES samples in HER2-Negative breast cancer patients, indicating the necessity of retesting HER2 low status at surgery, which may guide further therapy in the era of anti-HER2 ADCs.

Emerging Role of ERBB2 in Targeted Therapy for Metastatic Colorectal Cancer: Signaling Pathways to Therapeutic Strategies

Despite recent improvements in the comprehensive therapy of malignancy, metastatic colorectal cancer (mCRC) continues to have a poor prognosis. Notably, 5% of mCRC cases harbor Erb-B2 receptor tyrosine kinase 2 (ERBB2) alterations. ERBB2, commonly referred to as human epidermal growth factor receptor 2, is a member of the human epidermal growth factor receptor family of protein tyrosine kinases. In addition to being a recognized therapeutic target in the treatment of gastric and breast malignancies, it is considered crucial in the management of CRC. In this review, we describe the molecular biology of ERBB2 from the perspective of biomarkers for mCRC-targeted therapy, including receptor structures, signaling pathways, gene alterations, and their detection methods. We also discuss the relationship between ERBB2 aberrations and the underlying mechanisms of resistance to anti-EGFR therapy and immunotherapy tolerance in these patients with a focus on novel targeted therapeutics and ongoing clinical trials. This may aid the development of a new standard of care in patients with ERBB2-positive mCRC.

An Overview of Clinical Development of Agents for Metastatic or Advanced Breast Cancer Without ERBB2 Amplification (HER2-Low)

Importance

Erb-b2 receptor tyrosine kinase 2 (ERBB2; formerly HER2 [human epidermal growth factor receptor 2]) is an important prognostic and predictive factor in breast cancer. Anti-ERBB2 therapies have improved outcomes in ERBB2-positive breast cancer. However, based on current definitions, tumors with low ERBB2 expression are included in the ERBB2-negative subtype, and therefore, are ineligible for anti-ERBB2 therapies; patients with ERBB2-low (immunohistochemistry [IHC] 1 positive [+] or IHC 2+/in situ hybridization [ISH] negative [-]) tumors account for up to approximately 50% of breast cancer cases. Although the prognostic role of ERBB2-low needs to be defined, ERBB2 offers a potential therapeutic target in these patients.

Observations

Most breast cancer tumors have some ERBB2 expression, with ERBB2-low being more common in hormone receptor-positive than in hormone receptor-negative breast cancer. Although an early clinical study failed to demonstrate benefit of adjuvant trastuzumab for ERBB2-low disease, several novel anti-ERBB2 therapies have shown efficacy in ERBB2-low breast cancer, including the antibody-drug conjugate trastuzumab deruxtecan in a phase 3 trial, and trastuzumab duocarmazine and the bispecific antibody zenocutuzumab in early-phase studies. Although reports are conflicting, some differences in biology and patient outcomes have been found between ERBB2-low and ERBB2 IHC-0 breast cancer. Currently, no established guidelines exist for scoring ERBB2-low expression in breast cancer because the focus has been on binary classification as ERBB2-positive or ERBB2-negative. Additional interpretive cutoffs may be needed to select patients for treatment with effective agents in ERBB2-low breast cancer, along with standardized laboratory quality assurance programs to ensure consistent patient identification for eligibility for ERBB2-low targeting agents.

Conclusions and Relevance

This review suggests that ERBB2-low may be a distinct, clinically relevant breast cancer entity warranting reassessment of traditional diagnostic and therapeutic paradigms. Ongoing clinical trials and further investigations may provide optimized strategies for diagnosing and treating ERBB2-low breast cancer, including reproducible, consistent definitions to identify patients in this diagnostic category and demonstration of benefits of emerging therapies.

Improving HER2 testing reproducibility in HER2-low breast cancer

HER2 is a pillar biomarker in breast cancer, and it is assessed by immunohistochemistry (IHC) using a three-tier scoring system and reflex in situ hybridization (ISH) for IHC score 2+. Novel HER2-directed antibody-drug conjugates have demonstrated significant antitumor activity in breast cancers with low levels of HER2 expression, i.e. IHC score 1+ or ISH-negative IHC score 2+. Both primary and acquired resistance to anti-HER2 therapies remains a challenge in the treatment of breast cancers according to the HER2 positivity continuum. Thus, the ability to precisely discriminate among HER2-zero, HER2-low, and HER2-positive breast cancers is no longer a mere academic exercise. HER2 testing criteria, guidelines, and quality controls are re-gaining momentum for this new clinical need. Therefore, all preanalytical and analytical variables that might trouble the sensitivity and reproducibility of this test should be carefully considered to address all possible issues and open all possible therapeutic opportunities for breast cancer patients.

Lentiviral vector–based xenograft tumors as candidate reference materials for detection of HER2-low breast cancer

The human epidermal growth factor receptor 2 (HER2) is an important biomarker that plays a pivotal role in therapeutic decision-making for patients with breast cancer (BC). Patients with HER2-low BC can benefit from new HER2 targeted therapy. For ensuring the accurate and reproducible detection of HER2-low cancer, reliable reference materials are required for monitoring the sensitivity and specificity of detection assays. Herein, a lentiviral vector was used to transduce the HER2 gene into MDA-MB-231 cells that exhibited low HER2 density, and the cells were characterized by droplet digital PCR to accurately determine the copy number variation. Then, the formalin-fixed paraffin-embedded (FFPE) samples from xenografts were prepared and evaluated for suitability as candidate reference materials by immunohistochemistry (IHC) and fluorescence in situ hybridization (FISH). The FFPE reference materials were selected on the basis of IHC score of 2+ and negative FISH result to meet the requirement for HER2-low BC detection. Furthermore, the FFPE reference materials exhibited typical histological structures that resembled the clinical BC specimens. These novel FFPE reference materials displayed the high stability and homogeneity, and they were produced in high quantity. In summary, we generated high-quality reference materials for internal quality control and proficiency testing in HER2-low detection.

Non-conventional and Investigational PET Radiotracers for Breast Cancer: A Systematic Review

Breast cancer is one of the most common malignancies in women, with high morbidity and mortality rates. In breast cancer, the use of novel radiopharmaceuticals in nuclear medicine can improve the accuracy of diagnosis and staging, refine surveillance strategies and accuracy in choosing personalized treatment approaches, including radioligand therapy. Nuclear medicine thus shows great promise for improving the quality of life of breast cancer patients by allowing non-invasive assessment of the diverse and complex biological processes underlying the development of breast cancer and its evolution under therapy. This review aims to describe molecular probes currently in clinical use as well as those under investigation holding great promise for personalized medicine and precision oncology in breast cancer.

HER2 Low, Ultra-low, and Novel Complementary Biomarkers: Expanding the Spectrum of HER2 Positivity in Breast Cancer

HER2 status in breast cancer is assessed to select patients eligible for targeted therapy with anti-HER2 therapies. According to the American Society of Clinical Oncology (ASCO) and College of American Pathologists (CAP), the HER2 test positivity is defined by protein overexpression (score 3+) at immunohistochemistry (IHC) and/or gene amplification at in situ hybridization (ISH). The introduction of novel anti-HER2 compounds, however, is changing this paradigm because some breast cancers with lower levels of protein expression (i.e. score 1+/2+ with no gene amplification) benefited from HER2 antibody-drug conjugates (ADC). Recently, a potential for HER2 targeting in HER2 "ultra-low" (i.e. score 0 with incomplete and faint staining in ≤10% of tumor cells) and MutL-deficient estrogen receptor (estrogen receptor)-positive/HER2-negative breast cancers has been highlighted. All these novel findings are transforming the traditional dichotomy of HER2 status and have dramatically raised the expectations in this field. Still, a more aware HER2 status assessment coupled with the comprehensive characterization of the clinical and molecular features of these tumors is required. Here, we seek to provide an overview of the current state of HER2 targeting in breast cancers beyond the canonical HER2 positivity and to discuss the practical implications for pathologists and oncologists.

The development of peptide-drug conjugates (PDCs) strategies for paclitaxel

INTRODUCTION

Paclitaxel is a powerful and effective anti-tumor drug with wide clinical application. However, there are still some limitations, including poor water solubility, low specificity, and susceptibility to drug resistance. The peptide-drug conjugates (PDCs) represent a rising class of therapeutic drugs, which combines small-molecule chemotherapeutic drugs with highly flexible peptides through a cleavable or non-cleavable linker. When this strategy is applied, the therapeutic effects of paclitaxel can be improved.

AREAS COVERED

In this review, we discuss the application of the PDCs strategy in paclitaxel, including two parts: the tumor targeting peptide-paclitaxel conjugates and the cell penetrating peptide-paclitaxel conjugates.

EXPERT OPINION

Combining drugs with multifunctional peptides covalently is an effective strategy for delivering paclitaxel to tumors. Depending on different functional peptides, conjugates can increase the water solubility of paclitaxel, tumor permeability of paclitaxel, the accumulation of paclitaxel in tumor tissues, and enhance the antitumor effect of paclitaxel. In addition, due to the change of cell entry mechanism, partial conjugates can restore the therapeutic activity of paclitaxel against resistant tumors. Notably, in order to better translate into the clinical field in the future, more research should be conducted to ensure the safety and effectiveness of peptide-paclitaxel conjugates.

A designed cyclic peptide based on Trastuzumab used to construct peptide-drug conjugates for its HER2-targeting ability

Human epidermal growth factor receptor 2 (HER2) has been recognized as an important therapeutic target for its overexpression in many cancers. Trastuzumab is a monoclonal antibody targeting HER2, which has been approved by FDA to treat HER2-positive cancer. In this research, cyclic peptide Cyclo-GCGPep1 was designed based on the binding mode between antibody and HER2 protein in silico, which has been confirmed possessing good affinity with HER2. Cyclo-GCGPep1 was also used to construct peptide-drug conjugates with Camptothecin. Biological evaluations demonstrated that Conjugate 1 has a good antiproliferative activity on SK-BR-3 and NCI-N87 cells. Conjugate 1 retained the pro-apoptotic and Topo I inhibitory ability of Camptothecin. Meanwhile, it has good targeting ability towards HER2-positive cells with the help of Cyclo-GCGPep1. It also has better permeability in the tumor spheroid model than Camptothecin. In summary, the design of cyclic peptide derived from antibody is of significance for the discovery of targeting peptides and Conjugate 1 is expected as a good therapeutic agent for HER2-positive cancers.