Anti-proliferative activity of the mTOR inhibitor AP23573 in combination with cytotoxic and targeted agents

Abstract

3887 Background: AP23573 is a novel non-prodrug analog of rapamycin that inhibits mTOR signalling in tumors, leading to cell cycle arrest, tumor cell shrinkage and inhibition of angiogenesis. We have previously demonstrated that the compound potently inhibits the proliferation of human-derived tumor cell lines in vitro, and that these effects are additive with those of camptothecin, cisplatin and 5-fluorouracil. AP23573 is now being evaluated in phase 1 clinical trials in cancer patients. Methods: To guide future clinical studies, we used cellular proliferation assays to evaluate the effects of combinations of AP23573 with the cytotoxic agents docetaxel, doxorubicin, cisplatin and topotecan, as well as the targeted anti-tumor agents including trastuzumab, imatinib, and an EGFR inhibitor on a range of human-derived tumor cell lines in vitro. Drugs were added simultaneously to cells and incubated for 72 hours prior to assay. Data were analyzed for additivity by both the fractional product and median effect methods of analysis. Results: Analysis using the fractional product method demonstrated that the effects of AP23573 were additive with those of the cytotoxic agents docetaxel, doxorubicin, cisplatin and topotecan on breast (MCF-7), ovarian (SKOV3) and endometrial (AN3CA) tumor cell lines. AP23573 also showed additive anti-tumor effects with trastuzumab and an EGFR inhibitor on the breast tumor cell lines BT-474 and SKBR-3 respectively, and with imatinib on BCR-ABL-positive chronic myelogenous leukemia cell line K562. Analysis using the median effect method indicated that some of the combinations may be synergistic over certain concentration ranges. Equivalent results were obtained whether cells were assayed using a cell viability/ metabolic activity assay or a direct assay for proliferation (BrdU incorporation). Future studies will include those in which drugs are added in sequence rather than simultaneously, and in vivo evaluation of drug combinations in xenograft models. Conclusions: These studies demonstrate that AP23573 has the potential to be successfully combined with a broad variety of cytotoxic or targeted agents to treat a variety of cancers.