Expression of phosphorylated mTOR and its clinical significances in small cell lung cancer

The mammalian target of rapamycin (mTOR) is a serine/threonine kinase regulating the cell cycle and protein synthesis, and is an attractive molecule for novel molecular targeting therapy in various cancers, including non-small cell lung cancer (NSCLC). In contrast with NSCLC, mTOR expression has not been fully investigated in SCLC. In this study, we evaluated the correlations between mTOR expression and clinical characteristics in SCLC. Immunohistochemical staining for phosphorylated mTOR (p-mTOR) was performed and histoscores were calculated on 115 SCLC tissue specimens. Based on the distribution of the data, a histoscore of 60 was used as a cutoff to dichotomize SCLCs into low versus high expression groups. Extended-stage SCLCs showed significantly lower p-mTOR expression than those of a limited-stage (P = 0.008). Lymph node metastasis was more frequently detected in the low than high expression group (P = 0.074). The high p-mTOR expression group had a weak tendency toward prolonged overall survival, but the difference was not statistically significant (P = 0.170). We found that there is a significant difference in p-mTOR expression between different clinical stages in SCLC. This result indicates that p-mTOR might play a more pivotal role in the biologic behavior of early SCLCs than advanced ones and the effectiveness of mTOR inhibitors might vary according to the extent of disease.