Christine A. Parachoniak, Andrew Rankin, Bernadette Gaffney, Ryan Hartmaier, Dan Spritz, Rachel L. Erlich, Vincent A. Miller, Deborah Morosini, Phil Stephens, Jeffrey S. Ross, John Keech, Juliann Chmielecki

Metastatic triple negative breast cancer comprises 12-17% of breast cancers and carries a poor prognosis relative to other breast cancer subtypes. Treatment options in this disease are largely limited to systemic chemotherapy. A majority of clinical studies assessing efficacy of targeted therapeutics (e.g. the mTOR inhibitor everolimus) in advanced breast cancer patients have not utilized predictive genomic biomarker-based selection and have reported only modest improvement in clinical outcome relative to standard of care. However, recent reports have highlighted significant clinical responses of breast malignancies harboring alterations in genes involved in the PI3K/AKT/mTOR signaling pathway to mTOR-inhibitor-involving regimens, underscoring the potential clinical benefit of treating subsets of breast cancer patients with molecularly matched targeted therapies. As the paradigm of cancer treatment shifts from chemotherapeutic regimens to more personalized approaches, the identification of additional reliable biomarkers is essential for identifying patients likely to derive maximum benefit from targeted therapies. Herein, we report a near-complete and ongoing 14-month response to everolimus therapy of a heavily pre-treated patient with biphenotypic, metastatic breast cancer. Genomic profiling of the metastatic triple negative liver specimen identified a single reportable point mutation, STK11 F354L that appears to have undergone loss of heterozygosity. No other alterations within the PI3K/mTOR pathway were observed. Published functional biochemical data on this variant are conflicting and germline data, albeit with unclear zygosity status, is suggestive of a benign polymorphism role. Together with the preclinical data, this case suggests further investigation of this variant is warranted to better understand its role as a potential biomarker for mTOR inhibitor sensitivity in the appropriate clinical context.