HER2-family signalling mechanisms, clinical implications and targeting in breast cancer

Approximately 20 % of human breast cancers (BC) overexpress HER2 protein, and HER2-positivity is associated with a worse prognosis. Although HER2-targeted therapies have significantly improved outcomes for HER2-positive BC patients, resistance to trastuzumab-based therapy remains a clinical problem. In order to better understand resistance to HER2-targeted therapies in HER2-positive BC, it is necessary to examine HER family signalling as a whole. An extensive literature search was carried out to critically assess the current knowledge of HER family signalling in HER2-positive BC and response to HER2-targeted therapy. Known mechanisms of trastuzumab resistance include reduced receptor-antibody binding (MUC4, p95HER2), increased signalling through alternative HER family receptor tyrosine kinases (RTK), altered intracellular signalling involving loss of PTEN, reduced p27kip1, or increased PI3K/AKT activity and altered signalling via non-HER family RTKs such as IGF1R. Emerging strategies to circumvent resistance to HER2-targeted therapies in HER2-positive BC include co-targeting HER2/PI3K, pan-HER family inhibition, and novel therapies such as T-DM1. There is evidence that immunity plays a key role in the efficacy of HER-targeted therapy, and efforts are being made to exploit the immune system in order to improve the efficacy of current anti-HER therapies. With our rapidly expanding understanding of HER2 signalling mechanisms along with the repertoire of HER family and other targeted therapies, it is likely that the near future holds further dramatic improvements to the prognosis of women with HER2-positive BC.

Endpoints, patient selection, and biomarkers in the design of clinical trials for cancer vaccines

Therapeutic cancer vaccines are an emerging and potentially effective treatment modality. Cancer vaccines are usually very well tolerated, with minimal toxicity compared with chemotherapy. Unlike conventional cytotoxic therapies, immunotherapy does not result in immediate tumor shrinkage but may alter growth rate and thus prolong survival. Multiple randomized controlled trials of various immunotherapeutic agents have shown a delayed separation in Kaplan-Meier survival curves, with no evidence of clinical benefit within the first 6-12 months of vaccine treatment. Overall survival benefit is seen in patients with lower disease burden who are not expected to die within those initial 6-12 months. The concept of improved overall survival without marked initial tumor reduction represents a significant shift from the current paradigms established by standard cytotoxic therapies. Future clinical studies of therapeutic vaccines should enroll patients with either lower tumor burden, more indolent disease or both, and must seek to identify early markers of clinical benefit that may correlate with survival. Until then, improved overall survival is the only clear, discriminatory endpoint for therapeutic vaccines as monotherapies.

The ubiquitin-like protein, ISG15, is a novel tumor-associated antigen for cancer immunotherapy

The recent announcement of the first FDA-approved therapeutic vaccine for prostate cancer, Sipuleucel-T, is a watershed moment for the field of tumor immunotherapy. However, while Sipuleucel-T provides a powerful tool to clinicians for the most prevalent form of cancer in men, there remains an unmet need for a similar therapeutic strategy against breast cancer, the most prevalent cancer in women. While current breast cancer vaccines in development target several antigens, the most prevalent is the tumor-associated antigen, HER2. Initial results with HER2 vaccines appear promising in terms of efficacy; however, the lack of HER2 overexpression by a majority of breast tumors and the safety concerns associated with current HER2-targeted immunotherapy suggest that additional therapeutic strategies would be beneficial. Recently, several studies have identified ISG15 as a molecule highly expressed in numerous malignancies. ISG15 is a small ubiquitin-like protein regulated by type-I interferon and classically associated with viral defense. Elevated ISG15 expression in breast cancer is especially well documented and is independent of HER2, progesterone receptor, and estrogen receptor status. Additionally, high ISG15 expression in breast cancer correlates with an unfavorable prognosis and poor responses to traditional treatment strategies such as chemotherapy and radiation. To overcome these challenges, we employ a novel strategy to specifically target tumor-associated ISG15 expression with immunotherapy. We demonstrate that vaccination against ISG15 results in significant CD8-mediated reductions in both primary and metastatic mammary tumor burden. These results validate ISG15 as a tumor-associated antigen for cancer immunotherapy.

Identification of immunologic biomarkers associated with clinical response after immune-based therapy for cancer

The identification of immunologic biomarkers associated with clinical response after immune intervention for cancer is an area of intensive investigation. The field would benefit from a more systemic and directed approach for biomarker identification and evaluation. Lessons can be learned from other fields, such as cancer diagnostics, as to how to develop response-associated biomarkers. Studies in both human in vitro models as well as murine models of cancer can significantly inform and streamline the choice of candidates. Adoptive T-cell therapy is an interesting model for exploring potential immunologic surrogates that may predict clinical response. Most likely the clinical effectiveness of immune-based treatments will be predicted by panels of markers rather than single assays of a specific immune effector cell.