text: Identification of predictive biomarkers of efficacy for tumor types that are sensitive to mTOR inhibitors remains a major issue.
  Possible predictive biomarkers for tumor response to mTOR inhibitors, as have been described in glioblastoma, breast and prostate cancer cells, may be the differential expression of mTOR pathway proteins, PTEN, AKT, and S6. Thus, this data is based on preclinical assays, based on in vitro cultured tumor cell lines, which suggest that the effects of mTOR inhibitors may be more pronounced in cancers displaying loss of PTEN functions or PIK3CA mutations. However, the use of PTEN, PIK3CA mutations, and AKT–phospho status for predicting rapalog sensitivity has not been fully validated in clinic. To date, attempts to identify biomarkers of rapalog response have been unsuccessful.