In vitro assessment of antitumor activities of the PI3K/mTOR inhibitor GSK2126458

Background: Up-regulation of the PI3K/mTOR (phosphatidylinositol-3' kinase/mammalian target of rapamycin) signaling is common in carcinoma. Consistently, targeting these molecules has been shown to halt the growth of many tumors. The main purpose of this study was to develop surrogate biomarkers of the antitumor activity of PI3K/mTOR inhibitors.

Methods: Fragments from eight tumors were collected immediately after resection in ice-cold RPMI gassed with 95% O2 :5% CO2. Viability was determined by measuring tumor cellular respiration (mitochondrial O2 consumption). The specimens were incubated at 37°C with and without 50 nM GSK2126458 (a highly potent and selective inhibitor of PI3K/mTOR) for 90 min. The tissue was then processed for histology, measurement of intracellular caspase-3 activity (using the caspase-3 substrate N-acetyl-asp-glu-val-asp-7-amino-4-methylcoumarin), and immunohistochemical detection of the apoptotic biomarkers caspase-3, cytochrome C, and annexin A2.

Results: GSK2126458 induced morphologic changes in four tumors (two invasive ductal carcinomas, one invasive lobular carcinoma, and one ovarian dysgerminoma), intracellular caspase-3 activity in three tumors (two invasive ductal carcinomas and one poorly differentiated signet ring adenocarcinoma of gastric origin), and immunohistochemical evidence of apoptosis in at least four tumors (three invasive ductal carcinomas and one adenocarcinoma of gastric origin). Two tumors (ovarian serous carcinoma and moderately differentiated adenocarcinoma of colorectal origin) demonstrated no treatment effect.

Conclusion: These preliminary results demonstrate the feasibility of using in vitro biomarkers for detecting antitumor activities of the rapidly emerging PI3K/mTOR inhibitors.