Abstract PR-7: Overcoming chemoresistance in solid cancers by targeting quiescent tumor cells

Abstract

Abstract Overview: Quiescent tumor cells in solid cancers contribute significantly to treatment failure of conventional anti-proliferative based therapies. The hypothesis of this study is that transiently increasing the proliferative fraction of a tumor will increase response to chemotherapy and result in a net benefit to overall survival. Initial work was carried out using an in vitro engineered-tissue assay to develop and test strategies to induce proliferation. Using this assay a combination of growth factor stimulation and O2 supplementation was found to override tumor-derived quiescence and enhance response to a panel of chemotherapy drugs. Background: The microenvironment in solid tumors limits the efficacy of anti-proliferative therapies by generating a quiescent tumor cell subpopulation through diffusion-limited supply of nutrients supplied by the blood. Recent findings in our laboratory have shown that the distribution of proliferating cells in solid tumors can be modified if the limiting factors controlling it are understood. Quiescence in tissues grown from two human colorectal cancer cell lines was shown to be driven by oxygen and growth factor deprivation. The combined supplementation of these two factors was found to reverse tumor microenvironment derived quiescence. Methods: An in vitro engineered-tissue model grown from HCT116 & HT29 cells was used to study the interrelation of key factors involved in limiting tumor cell proliferation. The model was able to replicate gradients in diffusible factors and reproduce quiescence seen in solid tumors. Findings were validated in tumor xenografts using a technique in which proliferation was mapped in relation to tumor vasculature. Strategies to stimulate proliferation in combination with treatment were then tested against a panel of drugs with cell survival as the endpoint (doxorubicin, paclitaxel, vinorelbine, gemcitabine & SN38). Results: Stimulation of the IGF-1 receptor in combination with O2 supplementation in the engineered-tissue model was able to initiate proliferation in quiescent areas with maximal induction occurring after 16–24 hours, tissue proliferation increased 3±0.4-fold (HCT116) and 4±1-fold (HT29) as measured using a BrdUrd endpoint. Chemotherapy given at the time of maximal induction induced a 3 to 6-fold increase in cell kill when compared to tissue grown under normal physiological conditions. Findings showed that combined stimulation always yielded a greater effect than individual stimulation via IGF-1R or oxygen alone. Cell kill was also greater when chemotherapy was given 16 hours following initiation of stimulation compared to when given simultaneously. Typical results: 1µM gemcitabine with no induction SF=0.71±0.08, with IGF-1R stimulation SF=0.25±0.03, with O2 supplementation SF=0.47±0.04, with combined induction 12 hours prior SF=0.10±0.03, with combined induction at time SF=0.43±0.03 (mean ± SD, n=8). Conclusions: Findings from this study indicate that transient stimulation of quiescent tumor cells represents a promising target for improving the activity of most standard chemotherapy regimes. The in vitro engineered-tissue model was found to be a powerful tool to study diffusion limited supply of molecules in solid tumors. Initial results using the engineered-tissue model have been validated in tumor xenografts and further work to translate these findings is underway. Citation Information: Mol Cancer Ther 2009;8(12 Suppl):PR-7.