Abstract A51: Chemotherapy rescues hypoxic tumor cells and induces reoxygenation and repopulation - an effect that is inhibited by the hypoxia-activated pro-drug TH-302
Authors: Saggar Jasdeep K. , Tannock Ian F. 
Published: May 2015

Abstract
Introduction: Hypoxia is associated with tumor treatment failure. Tumor cells closest to blood vessels are well nourished and rapidly proliferating but hypoxic cells located farther away are slowly-proliferating. Chemotherapy targets rapidly proliferating cells and therefore may spare hypoxic cells because of poor drug distribution to them. Intervals between administrations of chemotherapy allow normal tissues to recover but might also allow re-oxygenation and resumed proliferation of formerly hypoxic cells due to a better supply of nutrients to them. Tumor cell repopulation following radiation therapy is well-documented; but repopulation following chemotherapy and its effect on hypoxia has not been studied. Hypoxia activated pro-drugs (i.e. TH-302) may inhibit tumor repopulation by killing hypoxic cells. Here we evaluate repopulation following chemotherapy and the effect of TH-302 on changes in hypoxia over time in human tumor xenografts. Methods: Human breast (MCF7) tumor bearing mice were administered two specific markers of hypoxia (pimonidazole [pimo] and EF5). Hypoxia-labeled tumor cells were recognized in tumor sections (in relation to DioC7+ve functional tumor blood vessels) using immunohistochemistry. Proliferating cells were identified by an antibody to Ki67. Mice were treated with pimo and: saline, doxorubicin, TH-302 or TH-302 + doxorubicin; EF5 was given after a variable interval of 24, 48, 72, 96 or 120 hours and tumors collected. Changes in the hypoxia, proliferation and oxygen status of formerly hypoxic (pimo+ve) cells were quantified by their Ki67 status and uptake of EF5 as a function of time. Results: Following doxorubicin treatment, the proportion of hypoxic cells in the entire tumor decreased from 1.5% (pimo+ve) prior to injection to 0.7% (EF5+ve) at 24 hours. The percentage of pimo+ve formerly hypoxic cells that are no longer hypoxic (i.e. EF5 – ve) at 24 hours was 90% after doxorubicin compared to 27% in controls, indicating rescue of previous hypoxic cells that would have died in the absence of treatment. Proliferation of these pimo+ve cells that were cycling (Ki67+ve) increased from 7% to 15.0% at 24 hours and then slowly decreased. TH-302 + doxorubicin combination inhibited repopulation by reducing Ki-67 to 1%. Conclusions: Chemotherapy (doxorubicin) treatment paradoxically contributes to tumor treatment failure (tumor cell repopulation) at all time points investigated by causing formerly hypoxic cells to become oxygenated and proliferate above those levels found in the untreated control tumors. TH-302 treatment led to increased DNA damage and inhibited hypoxic cell proliferation and tumor repopulation, thereby increasing chemotherapeutic outcome.
Supported by a research grant from the Canadian Institutes for Health Research.
Citation Format: Jasdeep K. Saggar, Ian F. Tannock. Chemotherapy rescues hypoxic tumor cells and induces reoxygenation and repopulation - an effect that is inhibited by the hypoxia-activated pro-drug TH-302. [abstract]. In: Abstracts: AACR Special Conference on Cellular Heterogeneity in the Tumor Microenvironment; 2014 Feb 26-Mar 1; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2015;75(1 Suppl):Abstract nr A51. doi:10.1158/1538-7445.CHTME14-A51