A phase Ib study of pertuzumab, a recombinant humanised antibody to HER2, and docetaxel in patients with advanced solid tumours

Amiloride Sensitizes Prostate Cancer Cells to the Reversible Tyrosine Kinase Inhibitor Lapatinib by Modulating ERBB3 Subcellular Localization

Neoadjuvant therapy (NAT) has been studied in clinically localized prostate cancer (PCa) to improve the outcomes from radical prostatectomy (RP) by 'debulking' of high-risk PCa; however, using androgen deprivation at this point risks castration resistant PCa (CRPC) clonal proliferation with potentially profound side effects such as fatigue, loss of libido, hot flashes, loss of muscle mass, and weight gain. Our goal is to identify alternative NAT that reduce hormone sensitive PCa (HSPC) without affecting androgen receptor (AR) transcriptional activity. PCa is associated with increased expression and activation of the epidermal growth factor receptor (EGFR) family, including HER2 and ErbB3. Dimerization between these receptors is required for activation of downstream targets involved in tumor progression. The FDA-approved HER2 inhibitor lapatinib has been tested in PCa but was ineffective due to continued activation of ErbB3. We now demonstrate that this is due to ErbB3 being localized to the nucleus in HSPC and thus protected from lapatinib which affect membrane localized HER2/ErbB3 dimers. Here, we show that the well-established, well-tolerated diuretic amiloride hydrochloride dose dependently prevented ErbB3 nuclear localization via formation of plasma membrane localized HER2/ErbB3 dimers. This in turn allowed lapatinib inactivation of these dimers via inhibition of its target HER2, which dephosphorylated downstream survival and proliferation regulators AKT and ERK1/2. Amiloride combined with lapatinib significantly increased apoptosis but did not affect AR transcriptional activity. Thus, our data indicate that a combination of amiloride and lapatinib could target HSPC tumors without problems associated with androgen deprivation therapy in localized PCa.

Monoclonal Antibodies and Antibody-drug Conjugates as Emerging Therapeutics for Breast Cancer Treatment

When breast cells divide and multiply out of control, it is called breast cancer. Symptoms include lump formation in the breast, a change in the texture or color of the breast, or a discharge from the nipple. Local or systemic therapy is frequently used to treat breast cancer. Surgical and radiation procedures limited to the affected area are examples of local management. There has been significant worldwide progress in the development of monoclonal antibodies (mAbs) since 1986, when the first therapeutic mAb, Orthoclone OKT3, became commercially available. mAbs can resist the expansion of cancer cells by inducing the destruction of cellular membranes, blocking immune system inhibitors, and preventing the formation of new blood vessels. mAbs can also target growth factor receptors. Understanding the molecular pathways involved in tumor growth and its microenvironment is crucial for developing effective targeted cancer therapeutics. Due to their unique properties, mAbs have a wide range of clinical applications. Antibody-drug conjugates (ADCs) are drugs that improve the therapeutic index by combining an antigen-specific antibody with a payload. This review focuses on the therapeutic applications, mechanistic insights, characteristics, safety aspects, and adverse events of mAbs like trastuzumab, bevacizumab, pertuzumab, ertumaxomab, and atezolizumab in breast cancer treatment. The creation of novel technologies utilizing modified antibodies, such as fragments, conjugates, and multi-specific antibodies, must be a central focus of future studies. This review will help scientists working on developing mAbs to treat cancers more effectively.

Application of population physiologically based pharmacokinetic modelling to optimize target expression and clearance mechanisms of therapeutic monoclonal antibodies

AIMS

To use population physiologically based pharmacokinetic (PopPBPK) modelling to optimize target expression, kinetics and clearance of HER1/2 directed therapeutic monoclonal antibodies (mAbs). Thus, to propose a general workflow of PopPBPK modelling and its application in clinical pharmacology.

METHODS

Full PBPK model of pertuzumab (PTZ) was developed in patient population using Simcyp V21R1 incorporating mechanistic targeted-mediated drug disposition process by fitting known clinical PK and sparse receptor proteomics data to optimize target expression and kinetics of HER2 receptor. Trastuzumab (TTZ) PBPK modelling was used to validate the optimized HER2 target. Additionally, the simulator was also used to develop a full PBPK model for the HER1-directed mAb cetuximab (CTX) to assess the underlying targeted-mediated drug disposition-independent elimination mechanisms.

RESULTS

HER2 final parameterisation coming from the PBPK modelling of PTZ was successfully cross validated through PBPK modelling of TTZ with average fold error (AFE), absolute AFE and percent prediction error values for area under the concentration-time curve (AUC) and maximum plasma concentration (Cmax ) of 1.13, 1.16 and 16, and 1.01, 1.07 and 7, respectively. CTX PBPK model performance was validated after the incorporation of an additional systemic clearance of 0.033 L/h as AFE and absolute AFE showed an acceptable predictive power of AUC and Cmax with percent prediction error of 13% for AUC and 10% for Cmax .

CONCLUSIONS

Optimisation of both system and drug related parameters were performed through PBPK modelling to improve model performance of therapeutic mAbs (PTZ, TTZ and CTX). General workflow was proposed to develop and apply PopPBPK to support clinical development of mAbs targeting same receptor.

Treatment of small (T1mic, T1a, and T1b) node-negative HER2+ breast cancer - a review of current evidence for and against the use of anti-HER2 treatment regimens

INTRODUCTION

Since the advent of anti-HER2 therapies, evidence surrounding adjuvant treatment of small (T1mic, T1a, and T1b), node-negative, HER2-positive breast cancer (HER2+BC) has remained limited. Practices vary widely between institutions with little known regarding the added benefit of systemic therapy, including cytotoxic chemotherapy and HER2-directed treatments. Our group has set out to perform an extensive review of available literature on this topic.

AREAS COVERED

In this review, we examined HER2 biology, anti-HER therapies, outcome definitions, and available prospective and retrospective data surrounding the use of adjuvant therapy in those with small, node-negative, HER2+BC. For outcomes, we primarily explored breast cancer-specific survival (BCSS), invasive disease-free survival (iDFS), and overall survival (OS). We also investigated the incidence of adverse events with a particular focus on symptomatic and asymptomatic declines in left ventricular ejection fraction.

EXPERT OPINION

Retrospective data will likely be the main driver for future treatment decisions. Given what we know, high risk T1b and T1c subgroups derive measurable added benefit from HER2-guided combination therapies but it's not clear whether these benefits outweigh known risks associated with this combination therapy. For tumors ≤0.5cm (T1mic and T1a), treatment remains highly controversial with limited evidence available through retrospective analysis that suggest over-treatment may be occurring.

DR5 mAb-conjugated, DTIC-loaded immuno-nanoparticles effectively and specifically kill malignant melanoma cells in vivo

We combined chemo- and immunotherapies by constructing dual therapeutic function immuno-nanoparticles (NPs) consisting of death receptor 5 monoclonal antibody (DR5 mAb)-conjugated nanoparticles loaded with dacarbazine (DTIC) (DTIC-NPs-DR5 mAb). We determined the in vivo targeting specificity of DTIC-NPs-DR5 mAb by evaluating distribution in tumor-bearing nude mice using a real-time imaging system. Therapeutic efficacy was assessed in terms of its effect on tumor volume, survival time, histomorphology, microvessel density (MVD), and apoptotic index (AI). Systemic toxicity was evaluated by measuring white blood cells (WBC) counts, alanine aminotransferase (ALT) levels, and creatinine clearance (CR).In vivo and ex vivo imaging indicates that DR5 mAb modification enhanced the accumulation of NPs within the xenograft tumor. DTIC-NPs-DR5 mAb inhibited tumor growth more effectively than DTIC or DR5 mAb alone, indicating that combining DTIC and DR5 mAb through pharmaceutical engineering achieves a better therapeutic effect. Moreover, the toxicity of DTIC-NPs-DR5 mAb was much lower than that of DTIC, implying that DR5 mAb targeting reduces nonspecific uptake of DTIC into normal tissue and thus decreases toxic side effects. These results demonstrate that DTIC-NPs-DR5 mAb is a safe and effective nanoparticle formulation with the potential to improve the efficacy and specificity of melanoma treatment.

Pertuzumab for the treatment of breast cancer: a safety review

INTRODUCTION

Approximately twenty to thirty percent of newly diagnosed breast cancers are human epidermal growth factor receptor 2 (HER2) positive. The use of trastuzumab, and more recently pertuzumab, has significantly improved the progression free survival (PFS) and overall survival (OS) in this patient population. However, pertuzumab has side effects that can impact treatment tolerability and quality of life.

AREAS COVERED

This review describes the safety and tolerability of pertuzumab, a monoclonal antibody targeted at HER2 approved by the United States Food and Drug Administration (FDA) for use in the neoadjuvant and first line metastatic settings.

EXPERT OPINION

The combination of trastuzumab, pertuzumab, and chemotherapy is approved in the neoadjuvant and first line metastatic settings and should be strongly considered by providers. Further studies are needed to look at side effect prevention, novel pertuzumab containing regimens, and re-treating patients with pertuzumab.

Pertuzumab in gastrointestinal cancer

INTRODUCTION

Human epidermal growth factor receptor 2 (HER2) and HER3 are altered in multiple tumor types, including gastrointestinal cancer. The HER2/HER3 dimer is crucial for HER2-mediated signaling in HER2-positive tumors. HER2-targeting agents, including trastuzumab, lapatinib, trastuzumab emtansine, and pertuzumab, have been approved for the treatment of HER2-positive breast cancer, with trastuzumab also approved for the treatment of HER2-positive gastric cancer. Pertuzumab, a recombinant humanized immunoglobulin (Ig) G1 monoclonal antibody targeting HER-2, binds to the dimerization domain (extracellular domain II) of HER2, which leads to blocking of ligand-induced HER2 heterodimerization. It is under investigation in gastrointestinal cancers, including HER2-positive gastric cancer.

AREA COVERED

In this review, the authors summarize the biology of HER2/HER3 and its alterations in gastrointestinal cancers. The authors focus specifically on the current status of development of pertuzumab in gastrointestinal cancers.

EXPERT OPINION

The HER2/HER3 alteration in gastrointestinal cancers is quite interesting. In HER2-positive gastric cancer, the dual blockade of HER2 and HER3 using trastuzumab and pertuzumab is being tested in an international phase III trial, the JACOB study. This strategy may benefit HER2-positive gastric cancer patients more as in the case of HER2-positive breast cancer. In other gastrointestinal cancers, including biliary tract cancer, esophageal cancer, pancreatic cancer, and colorectal cancer, there is huge room for the development of pertuzumab.

Inter-patient variability in docetaxel pharmacokinetics: A review

Docetaxel is a frequently used chemotherapeutic agent in the treatment of solid cancers. Because of the large inter-individual variability (IIV) in the pharmacokinetics (PK) of docetaxel, it is challenging to determine the optimal dose in individual patients in order to achieve optimal efficacy and acceptable toxicity. Despite the established correlation between systemic docetaxel exposure and efficacy, the precise factors influencing docetaxel PK are not yet completely understood. This review article highlights currently known factors that influence docetaxel PK, and focusses on those that are clinically relevant. For example, liver impairment should be taken into account when calculating docetaxel dosages as this may decrease docetaxel clearance. In addition, drug-drug interactions may be of distinct clinical importance when using docetaxel. Particularly, drugs strongly inhibiting CYP3A4 such as ketoconazole should not be concurrently administered without dose modification, as they may decrease the clearance of docetaxel. Gender, castration status, and menopausal status might be of importance as potential factors influencing docetaxel PK. The role of pharmacogenetics in predicting docetaxel PK is still limited, since no polymorphisms of clinical importance have yet been established.

A critical role for HER3 in HER2-amplified and non-amplified breast cancers: function of a kinase-dead RTK

ERBB3/HER3 is the most intriguing RTK by virtue of its ability to transduce multiple cytosolic signals for the proliferation and growth of tumor cells in spite of being a "kinase dead" receptor that binds to its true ligand, heregulin. Although other members of the HER3 family like EGFR and HER2 have long been recognized to be associated with breast tumorigenesis and studied because of their predictive and prognostic value, the significance of HER3 as an irrefutable component of HER family signalosome is a relatively new development. The recent understanding of signals originating from the oncogenic partnership of HER3 with HER2 in the context of HER2 amplification/overexpression showed the critical clinical value for the treatment of HER2+BC. The downstream signaling cascade (included but not limited to the PI3K signaling) associated with signals originating from HER2:HER3 dimers play a vital role in the tumorigenesis, drug-resistance and tumor progression of HER2+BC. The upregulation of HER3 activity provides an alternate "escape route" via which tumor cells bypass either the inhibition of the HER family RTKs or the inhibition of the downstream PI3K-AKT-mTOR signaling pathway. By understanding the signaling that provides this "escape route" for these tumor cells treated with a targeted therapy (HER2 inhibitors or inhibitors of downstream PI3K-AKT-mTOR signaling pathway), we are just beginning to appreciate the prognostic value of HER3 in breast cancer. In this review, we will discuss the relevance of HER3 signaling in the context of, (1) downstream oncogenic signals and (2) therapeutic options in HER2 amplified BC.

A novel monoclonal antibody induces cancer cell apoptosis and enhances the activity of chemotherapeutic drugs

A novel monoclonal antibody (mAb), known as AC10364, was identified from an antibody library generated by immunization of mice with human carcinoma cells. The mAb recognized proteins in lysates from multiple carcinoma cell lines. Cell cytotoxicity assays showed that AC10364 significantly inhibited cell growth and induced apoptosis in multiple carcinoma cell lines, including Bel/fu, KATO-III and A2780. Compared with mAb AC10364 or chemotherapeutic drugs alone, the combination of mAb AC10364 with chemotherapeutic drugs demonstrated enhanced growth inhibitory effects on carcinoma cells. These results suggest that mAb AC10364 is a promising candidate for cancer therapy.

Recent advances in the development of anti-HER2 antibodies and antibody-drug conjugates

Human epidermal growth factor receptor 2 (HER2)-targeted therapies have revolutionized the treatment of HER2-positive breast cancer, both in the metastatic and early stage settings. While trastuzumab and lapatinib had been the mainstays of treatment in combination with chemotherapy, innate and acquired resistance to these therapies occur. More recently, two additional HER2-directed therapies have been approved for HER2-positive breast cancer. Pertuzumab is a humanized monoclonal antibody that binds to the extracellular portion of the receptor on a domain distinct from the binding site of trastuzumab. The addition of pertuzumab to trastuzumab results in synergistic tumor cell inhibition and has been shown to significantly improve clinical outcomes for patients with HER2-positive metastatic breast cancer (MBC) compared to trastuzumab plus chemotherapy alone. In addition, ado-trastuzumab emtansine (T-DM1), a novel antibody-drug conjugate linking trastuzumab with the cytotoxic maytansinoid, DM1, is an effective treatment for HER2-positive breast cancer that has progressed on other HER2-directed therapies. Both pertuzumab and T-DM1 are relatively well tolerated. This review presents the mechanisms of action as well as phase I, II and III clinical data describing the safety and efficacy of pertuzumab and T-DM1 for HER2-positive breast cancer.

Pertuzumab in combination with trastuzumab and docetaxel for HER2-positive metastatic breast cancer

Overexpression of HER2 - found in approximately 15-20% of all breast cancers - is a negative prognostic factor. Although trastuzumab significantly improves the prognosis of HER2-positive breast cancer, half of the patients with metastatic breast cancer experience disease progression within 1 year. Pertuzumab is a novel HER2-targeted humanized monoclonal antibody that binds to the dimerization domain of HER2 and acts synergically with trastuzumab in inhibiting tumor progression. The CLEOPATRA trial demonstrated that adding pertuzumab to trastuzumab plus docetaxel significantly prolonged progression-free survival and overall survival without increasing severe adverse events. Conclusively, pertuzumab was approved by the US FDA in June 2012 for use in combination with trastuzumab and docetaxel for the treatment of patients with HER2-positive metastatic breast cancer. Furthermore, various clinical trials to evaluate the efficacy and safety of pertuzumab combined with other cytotoxic agents are ongoing at present. Thus, pertuzumab has been becoming important for the treatment of patients with HER2-positive metastatic breast cancer.

[Pertuzumab (Perjeta®) approval in HER2-positive metastatic breast cancers]

Fifteen to 20% of breast cancers display HER2 amplification. Many therapeutic successes have been obtained for this subtype in the last decade since trastuzumab approval for metastatic and localized diseases. Pertuzumab, a new anti-HER2 antibody, has been approved in 2013 by the European Medicine Agency. This drug can be used in combination with trastuzumab and docetaxel for the first line treatment of metastatic or locally recurrent non resecable HER2-positive breast cancers not previously treated by chemotherapy or HER2-inhibitors in the metastatic setting. This approval has been done after the CLEOPATRA trial results. This was a randomized, double-blind, multicentre, phase III trial evaluating the standard treatment (trastuzumab plus docetaxel) associated to pertuzumab or placebo. The authors have reported a statistically significant and clinically relevant benefit for the pertuzumab-based treatment. Median progression-free survival was 18.4 for the pertuzumab arm versus 12.5 months for the control group (p<0.001). They also observed benefits concerning the secondary endpoints: overall response rate and overall survival. Patients receiving pertuzumab presented more frequent diarrhea and febrile neutropenia but no increase in cardiac events. This drug has already been evaluated in the neoadjuvant setting with a FDA approval recently obtained. Its use in the adjuvant setting is under evaluation.

A phase IIa dose-finding and safety study of first-line pertuzumab in combination with trastuzumab, capecitabine and cisplatin in patients with HER2-positive advanced gastric cancer

BACKGROUND

Pertuzumab plus trastuzumab provides a more comprehensive blockade of HER2 signalling than trastuzumab alone. Therefore, we conducted a phase IIa study of the pharmacokinetics and safety of pertuzumab plus trastuzumab and chemotherapy in advanced gastric cancer (aGC).

METHODS

Patients received pertuzumab 840 mg for cycle 1 and 420 mg q3w for cycles 2-6 (Arm A) or pertuzumab 840 mg q3w for six cycles (Arm B). Trastuzumab, cisplatin and capecitabine were also given for six cycles, then trastuzumab q3w until disease progression or unmanageable toxicity. The co-primary endpoints were day 43 pertuzumab serum trough concentration (Cmin) and safety.

RESULTS

Thirty patients were randomised. Mean pertuzumab Cmin at day 43 was 40.0 μg ml(-1) (s.d.: 17.3) in Arm A and 62.7 μg ml(-1) (29.1) in Arm B. Mean day 43 Cmin in Arm A was ~37% lower than that seen in metastatic breast cancer. The safety profiles were similar between arms and treatment was well tolerated. Partial responses were achieved by 86% and 55% of patients in Arms A and B, respectively.

CONCLUSIONS

On the basis of the pharmacokinetic and safety data, the 840 mg q3w pertuzumab dose has been selected for a phase III study of pertuzumab, trastuzumab and chemotherapy in HER2-positive aGC.

The European Medicines Agency Review of Pertuzumab for the treatment of adult patients with HER2-positive metastatic or locally recurrent unresectable breast cancer: summary of the scientific assessment of the committee for medicinal products for human use

Pertuzumab is a recombinant humanized monoclonal antibody that specifically targets the extracellular dimerization domain (subdomain II) of HER2. Based on the positive opinion from the European Medicines Agency (EMA) on March 4, 2013, a marketing authorization valid throughout the European Union (EU) was issued for pertuzumab (Perjeta) for use in combination with trastuzumab and docetaxel for the treatment of adult patients with HER2-positive metastatic or locally recurrent unresectable breast cancer who have not received previous anti-HER2 therapy or chemotherapy for their metastatic disease. The demonstration of clinical benefit for pertuzumab was based on a single, phase III, randomized, double-blind, placebo-controlled trial comparing the efficacy and safety of pertuzumab plus trastuzumab plus docetaxel versus placebo plus trastuzumab plus docetaxel in previously untreated patients with locally advanced or metastatic HER2-positive breast cancer. In the primary analysis, median progression-free survival was 18.5 months in the pertuzumab group compared with 12.4 months in the placebo group (hazard ratio [HR]: 0.62; 95% confidence interval [CI]: 0.51-0.75; p < .0001). For the secondary endpoints, overall survival (HR: 0.66; 95% CI: 0.52-0.84; p = .0008) and objective response rate (80.2% vs. 69.3%) were also favored in the pertuzumab group. Toxicity was similar between groups except for higher incidence of diarrhea, rash, mucosal inflammation, dry skin, and neutropenia for pertuzumab compared with placebo. This paper summarizes the scientific review of the application leading to approval in the EU. The detailed scientific assessment report and product information, including the summary of product characteristics, are available on the EMA website (http://www.ema.europa.eu).

Pertuzumab for the treatment of breast cancer

HER2-targeted therapies have revolutionized the outcome of patients with HER2-positive breast cancer. Pertuzumab is the first in a new class of monoclonal antibodies that target the extracellular dimerization domain of HER2 receptors, also known as HER dimerization inhibitors. The development of pertuzumab and preclinical and clinical data in breast cancer are reviewed. Regulatory affairs related to pertuzumab and the recent accelerated approval granted by the FDA for the treatment of breast cancer in the neoadjuvant setting are also covered. This process opens doors for further approvals which could considerably shorten the time between initial drug development and availability.

Pertuzumab in human epidermal growth-factor receptor 2-positive breast cancer: clinical and economic considerations

In the absence of specific therapy, the 15%–20% of breast cancers demonstrating human epidermal growth-factor receptor 2 (HER2) protein overexpression and/or gene amplification are characterized by a more aggressive phenotype and poorer prognosis compared to their HER2-negative counterparts. Trastuzumab (Herceptin), the first anti-HER2-targeted therapy, has been associated with improved survival outcomes in HER2-positive breast cancer. However, many patients with early stage disease continue to relapse, and metastatic disease remains incurable. In order to further improve these outcomes, several novel HER2-targeted agents have recently been developed. Pertuzumab (Perjeta), a monoclonal antibody against the HER2 dimerization domain, has also been associated with improved patient outcomes in clinical trials, and has recently been approved in combination with chemotherapy and trastuzumab for neoadjuvant therapy of early stage, HER2-positive breast cancer and first-line treatment of metastatic disease. This review briefly summarizes pertuzumab’s clinical development as well as the published evidence supporting its use, and highlights some of the currently unanswered questions that will influence pertuzumab’s incorporation into clinical practice.

Pertuzumab in Combination with Trastuzumab and Chemotherapy in the Treatment of HER2-Positive Metastatic Breast Cancer: Safety, Efficacy, and Progression Free Survival

Tyrosine kinase inhibitors have revolutionized the oncology community and were pioneered by the use in HER2-targeted therapies. Improved outcomes were seen with the advent of trastuzumab, leading investigators to develop newer agents to target the HER2 pathway such as the novel monoclonal antibody pertuzumab. In this paper, we describe the attributes of pertuzumab including: mechanism of action, pharmacokinetics and metabolism, safety/cardiotoxicity, drug interactions, efficacy, and role in HER2-positive breast cancer management. Newly reviewed here versus previously published reviews on pertuzumab oriented therapy are data of pertuzumab monotherapy as it is used in combination with other anti-HER2 agents derived from preclinical research and ongoing clinical trials.

Pertuzumab: a review of its use for first-line combination treatment of HER2-positive metastatic breast cancer

Pertuzumab (Perjeta®) is a humanized anti-HER2 monoclonal antibody that binds to the extracellular dimerization subdomain of the HER2 receptor and reduces HER2 intracellular signalling by preventing HER2 from forming heterodimers with other HER receptors. Inhibition of HER2 signalling results in a reduction of tumour cell proliferation, invasiveness and survival. Pertuzumab and trastuzumab bind to different sites on the HER2 receptor and have complementary antitumour activities; they act synergistically in inhibiting the growth of HER2-overexpressing breast cancer cell lines in vitro. The efficacy of intravenous pertuzumab (840 mg loading dose, then 420 mg every 3 weeks) in combination with trastuzumab plus docetaxel in the first-line treatment of HER2-positive metastatic breast cancer was demonstrated in the randomized, double-blind, placebo-controlled, multinational, phase III CLEOPATRA trial. Pertuzumab in combination with trastuzumab and docetaxel significantly increased independently assessed median progression-free survival (primary endpoint), objective response rate and overall survival compared with placebo in combination with trastuzumab and docetaxel. Pertuzumab had an acceptable tolerability profile when added to trastuzumab and docetaxel in the pivotal CLEOPATRA trial. Thus, pertuzumab is a valuable addition to the growing list of anti-HER2 targeted therapies for breast cancer.

Pertuzumab for the treatment of human epidermal growth factor receptor type 2-positive metastatic breast cancer

Purpose

The pharmacology, pharmacokinetics, clinical efficacy, safety, and administration of pertuzumab in patients with metastatic human epidermal growth factor receptor type 2 (HER2)-positive breast cancer are reviewed.

Summary

Disease progression in HER2-positive breast cancer is often due to resistance to or a lack of efficacy of trastuzumab-based anti-HER2 therapy. Pertuzumab is the first humanized monoclonal antibody in a new class of drugs, the HER dimerization inhibitors, approved by the Food and Drug Administration for the first-line treatment of patients with metastatic HER2-positive breast cancer who have not received prior anti-HER2 therapy or chemotherapy for metastatic disease. Since pertuzumab binds to a different epitope than trastuzumab, combination therapy with pertuzumab and trastuzumab results in a more complete blockade of HER2 signaling than trastuzumab monotherapy. The efficacy of adding pertuzumab to trastuzumab–docetaxel dual therapy was demonstrated in a pivotal randomized multicenter Phase III trial, which showed a significant benefit in terms of progression-free survival, with improved overall survival, in favor of the triple therapy as an initial regimen in treatment-naive patients with metastatic HER2-positive breast cancer. The combination of pertuzumab and trastuzumab has been found to have a tolerable toxicity profile. As clinical trials of pertuzumab for adjuvant, neoadjuvant, and metastatic-disease treatment continue, its role in the treatment of HER2-positive breast cancer will continue to evolve.

Conclusion

Pertuzumab, a novel HER2 dimerization inhibitor, has been shown to be effective in the treatment of metastatic HER2-positive breast cancer when used in combination with trastuzumab and docetaxel and is recommended for first-line therapy.

Use of pertuzumab for the treatment of HER2-positive metastatic breast cancer

INTRODUCTION

Targeting the human epidermal growth factor receptor (HER) family of tyrosine kinase receptors has proven to be effective as a therapeutic strategy for HER type 2 (HER2)-positive breast cancer. Since resistance to trastuzumab occurs relatively frequently, particularly in the metastatic setting, novel anti-HER2 targeted therapies with complementary and/or synergistic mechanisms of action have been under development. Pertuzumab, a HER2-targeted monoclonal antibody that prevents HER2 dimerisation, is the first of a class of promising targeted agents for the treatment of HER2-positive breast cancer.

METHODS

A review of the biomedical literature published prior to February 2013 was conducted in English using PubMed. ClinicalTrials.gov was searched for appropriate clinical trials. The search terms used included breast neoplasm, pertuzumab, dimerisation, and HER2-positive. Abstracts of studies presented at the ASCO and ESMO Annual Meetings, and San Antonio Breast Cancer Symposium were also included.

RESULTS

Pertuzumab represents a novel anti-HER2 targeted therapy for HER2-positive breast cancers. In this article, we describe the mechanism of action of pertuzumab, as well as its drug development process and preclinical testing results. Based on the results of ancillary studies, dual inhibition using pertuzumab and trastuzumab was shown to be effective for the management of HER2-positive metastatic breast cancers pre-treated with trastuzumab-based therapy. For the first-line setting, the combination of both pertuzumab and trastuzumab with docetaxel (CLEOPATRA trial; clinical evaluation of pertuzumab and trastuzumab) has changed the paradigm of patient management.

CONCLUSION

Pertuzumab provided a more comprehensive inhibition of HER2-driven signalling pathways. When administered together with trastuzumab, pertuzumab represent a significant advancement for the treatment of HER2-positive metastatic breast cancer patients.

Pertuzumab : evolving therapeutic strategies in the management of HER2-overexpressing breast cancer

INTRODUCTION

HER2 overexpression or amplification is present in approximately one-fifth of breast cancers and historically was associated with aggressive disease and poorer prognosis. The introduction of the humanized monoclonal antibody trastuzumab dramatically improved disease-free survival (DFS) and overall survival (OS) in this subgroup. As the majority of patients with metastatic disease ultimately develop resistance to trastuzumab, a need exists for more effective targeted therapies. Pertuzumab is an anti-HER2/neu-targeted therapy in the late stages of clinical development. The combination of pertuzumab, trastuzumab and docetaxel has been found to have an OS benefit in patients with HER2 positive metastatic breast cancer (MBC) when used in the first-line setting. This reflects a new standard of care, and pertuzumab was recently approved for this indication by the Food and Drug Administration (FDA). The efficacy of pertuzumab and trastuzumab in conjunction with chemotherapy is currently being evaluated in the adjuvant setting.

AREAS COVERED

This article provides an overview of preclinical investigations in addition to reviewing pertinent Phase I, Phase II and Phase III clinical trials.

EXPERT OPINION

Pertuzumab, in combination with the humanized monoclonal antibody trastuzumab, and docetaxel is a standard of care for patients with previously untreated metastatic breast cancer based on the CLEOPATRA study showing a survival benefit. There is no increase in cardiac toxicity with the combined HER2-targeted therapy. Future issues will address appropriate sequencing and combination with other anti-HER2-targeted therapies and/or chemotherapy.

Pertuzumab for the treatment of patients with previously untreated HER2-positive metastatic breast cancer

Pertuzumab is a humanized monoclonal antibody directed at the dimerization domain of the receptor tyrosine-protein kinase erbB-2 (HER2) receptor. It possesses a unique and complimentary mechanism of action compared to trastuzumab, which has historically been the cornerstone of therapy for HER2-amplified breast cancer. Clinical trials demonstrate improved outcomes, with minimal increases in toxicity with the addition of pertuzumab to trastuzumab in patients with HER2-positive metastatic breast cancer, indicating the advantage of dual HER2 receptor blockade. Pertuzumab is approved as first-line therapy in combination with trastuzumab and docetaxel for HER2-positive metastatic breast cancer, with future opportunities to investigate its efficacy in other stages of breast cancer, as well as in the treatment of other malignancies.

Pertuzumab: new hope for patients with HER2-positive breast cancer

BACKGROUND

Human epidermal growth factor receptor 2 (HER2) overexpression is detected in approximately 15% to 20% of all breast cancers (BCs). A revolutionary change in the prognosis of this subgroup of patients has occurred since trastuzumab therapy was introduced into daily clinical practice. However, because trastuzumab resistance is common, new molecules with complementary and/or synergistic mechanisms of action have been developed. Pertuzumab is a new anti-HER2 humanized monoclonal antibody that prevents the formation of HER2 dimers.

MATERIAL AND METHODS

A computer-based literature search was carried out using PubMed (keywords: breast neoplasm, dimerization, HER-2, pertuzumab); data reported at international meetings are included.

RESULTS

This paper describes pertuzumab's mechanism of action, safety, and role in HER2-positive BCs. It also explores the role of pertuzumab as a single agent or combined with trastuzumab by reviewing data from preclinical research to ongoing clinical trials. Recently published trials, particularly the CLEOPATRA study, highlight the efficacy, tolerability, and increase in disease-free survival associated with this novel agent when combined with trastuzumab.

CONCLUSION

The pertuzumab and trastuzumab anti-HER2 dual blockade is likely to represent a substantial advance for patients with HER2-positive BCs and a new milestone on the way to personalized medicine.

Targeted treatments of HER2-positive metastatic breast cancer: trastuzumab and beyond

SUMMARY HER2 positivity defines a molecular subtype of breast cancer with aggressive biological behavior. HER2 has been clinically validated as a prominent therapeutic target in breast cancer, and an abundance of data from clinical trials proving the efficacy of trastuzumab, a humanized anti-HER2 monoclonal antibody, has changed the natural history of this disease. Despite positive results from many trials, resistance to anti-HER2 agents inevitably occurs in both the metastatic and adjuvant settings. This clinical reality has led to the development of various other targeted agents designed to block the HER2 receptor. Similarly, attempts to elucidate the molecular mechanisms of resistance to the already established anti-HER2 agents have opened new therapeutic avenues with numerous promising agents. This review presents a comprehensive overview of the data available on anti-HER2-targeted agents other than trastuzumab, and describes both challenges and directions for the future.

A phase Ib, dose-finding study of erlotinib in combination with a fixed dose of pertuzumab in patients with advanced non-small-cell lung cancer

BACKGROUND

Pertuzumab, a dimerization inhibitor of human epidermal growth factor receptor 2 (HER2), has demonstrated pharmacodynamic activity, with stable disease in non-small-cell lung cancer. Combining erlotinib and pertuzumab may enhance antitumor activity. This study aimed to establish the recommended dosing of the erlotinib and pertuzumab combination; assess safety, preliminary efficacy, and pharmacokinetics; and analyze biomarkers.

PATIENTS AND METHODS

Fifteen patients with stage IIIb/IV non-small-cell lung cancer who failed chemotherapy were recruited. The patients received erlotinib (days -8 to -1), then combination therapy (21-day cycles for 6 cycles). Pertuzumab was given intravenous at 840 mg, then 420 mg once every three weeks, with erlotinib given daily (100 or 150 mg).

RESULTS

No dose-limiting toxicities were observed. Adverse events were generally grade 1/2 and manageable. The objective response rate was 20% (3/15 patients; 2 responders had mutant HER1, 1 responder had wild-type HER1), median overall progression-free survival was 9.3 weeks. High HER1, HER2, and HER3 messenger RNA expression correlated with increased progression-free survival. Combination therapy did not affect erlotinib's pharmacokinetics; however, pertuzumab mean exposures (maximum concentration, 231 mg/L; area under the concentration-time curve from 0 to 21 days, 1780 mg*d/L) were slightly higher than in previous studies.

CONCLUSIONS

Combination therapy was well tolerated in patients with good performance status, with encouraging efficacy. A loading dose of pertuzumab 840 mg followed by 420 mg once every three weeks plus daily erlotinib 150 mg appears to be the most appropriate regimen for this combination.

Pooled analysis of cardiac safety in patients with cancer treated with pertuzumab

BACKGROUND

Pertuzumab, a human epidermal growth factor receptor (HER) 2 dimerization inhibitor, has demonstrated promising efficacy in combination with trastuzumab in patients with metastatic breast cancer. As HER signaling pathways are not only involved in oncogenesis, but also in myocardial homeostasis, an analysis of cardiac safety data was undertaken in a large group of patients treated with pertuzumab.

PATIENTS AND METHODS

A complete database of patients treated with full-dose pertuzumab was used to describe the incidence of asymptomatic left ventricular systolic dysfunction (LVSD) and symptomatic heart failure (HF).

RESULTS

Information for 598 unique patients was available for the current analysis. Of the patients treated with pertuzumab alone (n = 331) or pertuzumab in combination with a non-anthracycline-containing cytotoxic (n = 175) or trastuzumab (n = 93), 23 (6.9%), 6 (3.4%), and 6 (6.5%), respectively, developed asymptomatic LVSD and 1 (0.3%), 2 (1.1%), and 1 (1.1%), respectively, displayed symptomatic HF. None of the 15 patients receiving both pertuzumab and erlotinib demonstrated LVSD.

CONCLUSIONS

Patients treated with pertuzumab experienced relatively low levels of asymptomatic LVSD or symptomatic HF. There was no notable increase in cardiac side-effects when pertuzumab was given in combination with other anticancer agents.

Secretory phospholipase A2-IIa is involved in prostate cancer progression and may potentially serve as a biomarker for prostate cancer

The majority of prostate cancers are indolent, whereas a significant portion of patients will require systemic treatment during the course of their disease. To date, only high Gleason scores are best associated with a poor prognosis in prostate cancer. No validated serum biomarker has been identified with prognostic power. Previous studies showed that secretory phospholipase A2-IIa (sPLA2-IIa) is overexpressed in almost all human prostate cancer specimens and its elevated levels are correlated with high tumor grade. Here, we found that sPLA2-IIa is overexpressed in androgen-independent prostate cancer LNCaP-AI cells relative to their androgen-dependent LNCaP cell counterparts. LNCaP-AI cells also secrete significantly higher levels of sPLA2-IIa. Blocking sPLA2-IIa function compromises androgen-independent cell growth. Inhibition of the ligand-induced signaling output of the HER network, by blocking PI3K-Akt signaling and the nuclear factor-kappaB (NF-κB)-mediated pathway, compromises both sPLA2-IIa protein expression and secretion, as a result of downregulation of sPLA2-IIa promoter activity. More importantly, we demonstrated elevated serum sPLA2-IIa levels in prostate cancer patients. High serum sPLA2-IIa levels are associated significantly with high Gleason score and advanced disease stage. Increased sPLA2-IIa expression was confirmed in prostate cancer cells, but not in normal epithelium and stroma by immunohistochemistry analysis. We showed that elevated signaling of the HER/HER2-PI3K-Akt-NF-κB pathway contributes to sPLA2-IIa overexpression and secretion by prostate cancer cells. Given that sPLA2-IIa overexpression is associated with prostate development and progression, serum sPLA2-IIa may serve as a prognostic biomarker for prostate cancer and a potential surrogate prostate biomarker indicative of tumor burden.

NCT01110291: induction of CYP3A activity and lowered exposure to docetaxel in patients with primary breast cancer

PURPOSE

To study the CYP3A activity before and after docetaxel administration. Furthermore, it was investigated whether peroral midazolam could predict docetaxel exposure and adverse events.

METHODS

Twenty patients with primary high risk breast cancer were given docetaxel as a 1-h infusion 80 mg/m(2) in a 21-day cycle in 3 cycles followed by 3 cycles of cyclophosphamide, epirubicin and fluorouracil. CYP3A activity was assessed a day before and a day after docetaxel by 7.5 mg oral midazolam. All patients were given peroral dexamethasone a total dose of 45 mg, of which 15 mg was given before docetaxel infusion and 30 mg before the latter assessment of CYP3A activity. All except one patient were given 11-19 mg of intravenous dexamethasone before docetaxel infusion.

RESULTS

CYP3A activity was clearly induced when assessed a day after docetaxel administration as shown by lower midazolam AUC (P < 0.0001) and higher AUC ratio (1-OH-midazolam/midazolam, P = 0.018). The mean docetaxel AUC was about a half of that previously reported in the literature. Incidence of febrile neutropenia was smaller (15%) than reported in literature with comparable docetaxel doses and seemed to associate with slower metabolism. No correlation between pharmacokinetics of midazolam and docetaxel was found at baseline.

CONCLUSIONS

We show here a markedly reduced docetaxel exposure followed by CYP3A induction by, most likely, dexamethasone. Peroral midazolam seemed not to predict docetaxel exposure. Slow CYP3A-mediated metabolism might predispose patients to adverse events of docetaxel.

Open-label, phase II, multicenter, randomized study of the efficacy and safety of two dose levels of Pertuzumab, a human epidermal growth factor receptor 2 dimerization inhibitor, in patients with human epidermal growth factor receptor 2-negative metastatic breast cancer

PURPOSE

Pertuzumab is a humanized monoclonal antibody inhibiting human epidermal growth factor receptor 2 (HER2) dimerization. The aim of this phase II trial was to assess the antitumor activity and safety profile of pertuzumab monotherapy in patients with HER2-negative metastatic breast cancer. The utility of biomarkers detected in paraffin-embedded tissue as predictors of response was also explored.

PATIENTS AND METHODS

This was an international, multicenter, open-label, randomized phase II study. Patients (n = 79) with centrally confirmed HER2-negative metastatic breast cancer were randomly assigned to receive pertuzumab once every 3 weeks with a loading dose of 840 mg followed thereafter by either 420 mg (arm A) or 1,050 mg (arm B). Patients were stratified by country and prior taxane therapy.

RESULTS

Of 79 patients who were randomly assigned, 78 were included in the intent-to-treat population. In arm A (n = 41), two patients had partial responses, and 18 patients (44%) experienced stable disease (SD) lasting > or = 12 weeks. In arm B (n = 37), SD was observed in 14 patients (38%). Overall, six of 78 patients responded or had SD > or = 6 months. Pertuzumab was generally well tolerated, and most adverse events were mild to moderate. Decline in left ventricular ejection fraction of > or = 10% and/or to less than 50% was observed in eight patients, with one case of congestive heart failure in arm A. Pharmacokinetic data supported a fixed dose of pertuzumab once every 3 weeks.

CONCLUSION

The limited efficacy observed in this study, generally SD of relatively short duration, suggested little benefit of further investigation of single-agent pertuzumab in unselected patients with HER2-negative disease.

Antibodies to watch in 2010

Monoclonal antibodies (mAbs) are a burgeoning class of therapeutics, with more than 25 approved in countries worldwide. Novel molecules are entering clinical study at a rate of nearly 40 per year, and the commercial pipeline includes approximately 240 mAb therapeutics in clinical studies that have not yet progressed to regulatory approval or been approved. Of particular interest are the 26 mAbs that are currently at Phase 3, when safety and efficacy data critical to approval is established. Phase 3 study lengths are typically two to four years, so results for some studies might be announced in 2010, but data from others might not be presented until 2014. This overview of the 26 candidates provides a brief description of the background and the on-going Phase 3 studies of each mAb. Additional mAbs that have progressed to regulatory review or been approved may also be in Phase 3 studies, but these, as well as Fc fusion proteins, have been excluded. Due to the large body of primary literature about the 26 candidates, only selected references are given, with a focus on recent publications and articles that were relevant to Phase 3 studies. Current as of October 2009, the results presented here will serve as a baseline against which future progress can be measured.

Inhibition of functional HER family members increases the sensitivity to docetaxel in human ovarian cancer cell lines

Human epidermal growth factor (HER) family-targeted therapy combined with standard cytotoxic agents might improve the treatment of ovarian cancer. Human ovarian cancer cell lines OVCAR-3, IGROV-1, and SKOV-3 with differential EGFR, HER2, and HER3 expression levels were used to study whether EGFR-directed (cetuximab) or HER2-directed (trastuzumab, pertuzumab) monoclonal antibodies inhibited cell growth and abrogated activated receptor signaling routes. Possible increase of antiproliferative effects and further activation of caspase-3 as a read-out for apoptosis were analyzed when monoclonal antibodies were combined with docetaxel. Cetuximab alone inhibited cell growth in OVCAR-3 and IGROV-1, which was more pronounced when combined with pertuzumab in OVCAR-3. SKOV-3 cell growth was not significantly affected by any of the antibodies. Cetuximab increased the 50% growth-inhibiting effects of docetaxel in OVCAR-3 and IGROV-1, but not in SKOV-3. Coaddition of pertuzumab to cetuximab plus docetaxel in OVCAR-3 and IGROV-1, and, to a lesser extent trastuzumab in OVCAR-3, inhibited cell growth even further. Caspase-3 activation by docetaxel was enhanced after addition of cetuximab in OVCAR-3 and after addition of cetuximab plus pertuzumab in IGROV-1 and SKOV-3. Functional EGFR-signaling, HER2-signaling, and HER3-signaling routes as shown from abrogation of EGF-stimulated and heregulin-stimulated phosphorylated ERK1/2 by cetuximab, trastuzumab, and pertuzumab, respectively, were shown in OVCAR-3 and IGROV-1, but hardly in SKOV-3. Pertuzumab was able to abrogate phosphorylated HER2 by EGF and heregulin, except in SKOV-3. In conclusion, a combination of docetaxel with inhibitors of HER family members, such as cetuximab plus pertuzumab, may be considered for a clinical trial in ovarian carcinomas with functional receptors.

Signalling pathways in prostate carcinogenesis: potentials for molecular-targeted therapy

Prostate cancer represents a major health issue and its incidence is rising globally. In developed countries, prostate cancer is the most frequently diagnosed cancer and the second most common cause of death from cancer in men. Androgen deprivation reduces tumour activity in approx. 80% of patients with advanced disease, but most tumours relapse within 2 years to an incurable hormone-resistant state. Even for patients with early disease at the time of diagnosis, a proportion of patients will unfortunately develop relapsed disease following radical therapy. Treatment options for patients with hormone-resistant prostate cancer are very limited and, even with toxic therapy, such as docetaxel, the life expectancy is only improved by a median of 2 months. Advances in molecular oncology have identified key signalling pathways that are considered to be driving events in prostate carcinogenesis. The activation of multiple signalling pathways increases further the possibility of cross-talk among 'linear' signalling cascades. Hence signalling networks that may incorporate distinct pathways in prostate cancer, particularly in hormone-resistant disease, are increasingly appreciated in drug development programmes. With the development of potent small-molecule inhibitors capable of specifically suppressing the activities of individual 'linear' cascades, it may be that, by combining these agents as guided by the molecular signature of prostate cancer, a more efficient therapeutic regime may be developed. Therefore the present review focuses on evidence of abnormal signalling in prostate cancer and the potential of these targets in drug development, and incorporates key findings of relevant clinical trials to date.

The GD2-specific 14G2a monoclonal antibody induces apoptosis and enhances cytotoxicity of chemotherapeutic drugs in IMR-32 human neuroblastoma cells

Neuroblastoma (NB) is the most common extracranial solid tumor of childhood. The majority of children suffers from high risk neuroblastoma and has disseminated disease at the time of diagnosis. Despite recent advances in chemotherapy, the prognoses for children with high risk NB remain poor. Therefore, new treatment modalities are urgently needed. GD2 ganglioside is an antigen that is highly expressed on NB cells with only limited distribution on healthy tissues. Consequently, it appears to be an ideal target for both active and passive immunotherapy. The immunological effector mechanisms mediated by anti-GD2 monoclonal antibodies (mAbs) have been already well characterized. However, a growing number of reports suggest that GD2-specific antibodies may exhibit anti-proliferative effects without the immune system involvement. Here, we have shown that anti-GD2 14G2a mAb is capable of decreasing survival of IMR-32 human neuroblastoma cells in a dose-dependent manner. Death induced by this antibody exhibited several characteristics typical for apoptosis such as increased number of Annexin V- and propidium iodide-positive cells, cleavage of caspase 3 and prominent rise in caspase activity. The use of a pan caspase inhibitor Z-VAD-fmk suggested that the killing potential of this mAb is partially caspase-dependent. 14G2a mAb was rapidly endocytosed upon antigen binding. Employment of chloroquine, an inhibitor of lysosomal degradation, did not rescue IMR-32 cells from antibody-induced cell death suggesting lack of ceramide involvement in the observed effect. Most importantly, our studies showed that at particular drug concentrations 14G2a mAb exerts a synergistic effect with doxorubicin and topotecan, as well as an additive effect with carboplatin in killing IMR-32 cells in vitro. Our results provide guidance regarding how to best combine GD2-specific 14G2a antibody with existing cancer therapeutic agents to improve available treatment modalities for neuroblastoma.

Treatment of mammary carcinomas in HER-2 transgenic mice through combination of genetic vaccine and an agonist of Toll-like receptor 9

PURPOSE

Oligodeoxynucleotides containing unmethylated CpG dinucleotides induce innate and adaptive immunity through Toll-like receptor 9 (TLR9). In the present study, we have examined the ability of a novel agonist of TLR9, called immunomodulatory oligonucleotide (IMO), to enhance effects of a HER-2/neu plasmid DNA electroporation/adenovirus (DNA-EP/Ad) vaccine.

EXPERIMENTAL DESIGN

BALB/NeuT mice were treated with DNA-EP vaccine alone, IMO alone, or the combination of two agents starting at week 13, when all mice showed mammary neoplasia. Tumor growth and survival were documented. Antibody and CD8+ T-cell responses were determined. Peptide microarray analysis of sera was carried out to identify immunoreactive epitopes. Additionally, microCT and microPET imaging was carried out in an advanced-stage tumor model starting treatment at week 17 in BALB/NeuT mice.

RESULTS

The combination of DNA-EP and IMO resulted in significant tumor regression or delay to tumor progression. 2-Deoxy-2-[18F]fluoro-D-glucose microPET and microCT imaging of mice showed reduced tumor size in the DNA-EP/IMO combination treatment group. Mice treated with the combination produced greater antibody titers with IgG2a isotype switch and antibody-dependent cellular cytotoxicity activity than did mice treated with DNA-EP vaccine. An immunogenic B-cell linear epitope, r70, within the HER-2 dimerization domain was identified through microarray analysis. Heterologous DNA-EP/Ad vaccination combined with IMO increased mice survival.

CONCLUSION

The combination of HER-2/neu genetic vaccine and novel agonist of TLR9 had potent antitumor activity associated with antibody isotype switch and antibody-dependent cellular cytotoxicity activities. These results support possible clinical trials of the combination of DNA-EP/Ad-based cancer vaccines and IMO.

Antibodies and Tyrosine Kinase Inhibitors in Breast Cancer Therapies

In recent years, new classes of molecules have been established as opportunities for the treatment of breast cancer. The approval of trastuzumab, the antibody against Her2/neu, in the late 1990s was followed by the approval of the antiangiogenic antibody bevacizumab in 2007. Progress in the understanding of the molecular mechanisms of carcinogenesis and tumour growth led to the development of new molecules, mostly kinase inhibitors. A few of these new molecues gained approval in several countries; clinical trials aiming at further approvals are ongoing. This short review covers the actual state-of-the-art and possible future developments in the targeted therapy of breast cancer.

A splice variant of HER2 corresponding to Herstatin is expressed in the noncancerous breast and in breast carcinomas

Herstatin (HST) is an alternatively spliced HER2 product with growth-inhibitory properties in experimental cancer systems. The role of HST in adult human tissues and disease remains unexplored. Here, we investigated HST expression at the mRNA and protein (immunohistochemistry [IHC]) level in parallel with parameters reflecting HER activation in 187 breast carcinomas and matched noncancerous breast tissues (NCBT). Noncancerous breast tissues demonstrated the highest HST/HER2 transcript ratios corresponding to a few positive epithelial and stromal cells by IHC. Although HST/HER2 transcript ratios in tumors were inversely associated with HER2 IHC grading (P = .0048 for HER2 IHC-1+ and P = .0006 for HER2 IHC-2+ vs HER2-negative tumors), relative HST expression within the same tumor/NCBT system remained constant. HST/HER2 ratios did not predict the presence of HST protein, which was found in 46 (25%) of 187 tumors. A subgroup of HER2 IHC-3+ tumors exhibited high HST/HER2 transcript ratios, strong HST protein positivity, and cytoplasmic phospho-Akt/PKB and p21(CIP1/WAF1) localization. In conclusion, HST may act as a paracrine factor in the adult breast. Because HST is described as an endogenous pan-HER inhibitor, the presence of this protein in breast carcinomas may portent the inefficiency of exogenous efforts to block HER2 dimerization, whereas its absence may justify such interventions.