Abstract 3801A: Reoxygenation and repopulation of hypoxic cells in a solid tumor after chemotherapy: a cause of treatment failure.
Authors: Saggar Jasdeep K. , Tannock Ian F. 
Published: August 2014

Background: Hypoxia occurs in solid tumors: chronic diffusion-limited hypoxia occurs distal from functional blood vessels, while acute hypoxia may occur because of interruptions in blood flow. Well-nourished tumor cells close to blood vessels tend to be rapidly proliferating but hypoxic cells located farther away are slowly-proliferating. Chemotherapy may spare hypoxic cells because of poor drug distribution to them and because most drugs are selectively toxic to proliferating cells. Intervals between administration of chemotherapy allow for recovery of normal tissues (e.g. repopulation of bone marrow) but might allow re-oxygenation and resumed proliferation of formerly hypoxic cells due to better supply of nutrients to them, as is observed during radiotherapy. Here we evaluate these processes in human tumor xenografts Methods: Two specific markers of hypoxic cells (pimonidazole [pimo] and EF5) were injected into mice bearing MCF7 tumor xenografts, and tumor cells labeled with one or both markers were recognized in tumor sections (in relation to DioC7+ve functional tumor blood vessels) using appropriate fluorescence-labeled antibodies and imunohistochemistry. Proliferating cells were identified by an antibody to Ki67. Mice were treated with pimo and then either doxorubicin or saline one hour later; EF5 was given after a variable interval of 24, 48, 72, 96 or 120 hours; mice were killed two hours after the second injection. Changes in the location, proliferation and oxygen status of formerly hypoxic (pimo+ve) cells were quantified by their distance from hypoxia, Ki67 status and uptake of EF5 as a function of time. Results: Following treatment with doxorubicin, 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 docxorubicin 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. Conclusions: Formerly hypoxic cells in MCF7 xenografts undergo re-oxygenation and increase their proliferation following treatment with doxorubicin. Originally hypoxic cells (that may have died in the absence of treatment) can move closer to blood vessels, re-oxygenate and repopulate a tumor. Treatment directed to killing hypoxic cells (e.g. with hypoxia activated pro-drugs) has potential to improve the outcome of chemotherapy.

Supported by a research grant from the Canadian Institutes for Health Research.

Citation Format: Jasdeep K. Saggar, Ian F. Tannock. Reoxygenation and repopulation of hypoxic cells in a solid tumor after chemotherapy: a cause of treatment failure. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 3801A. doi:10.1158/1538-7445.AM2013-3801A