Longitudinal Imaging Studies of Tumor Microenvironment in Mice Treated with the mTOR Inhibitor Rapamycin

Radiation Biology Branch, Center for Cancer Research, National Cancer Institute, National Institutes of Health, Bethesda, Maryland, United States of America.
PLoS ONE (Impact Factor: 3.23). 11/2012; 7(11):e49456. DOI: 10.1371/journal.pone.0049456
Source: PubMed


Rapamycin is an allosteric inhibitor of mammalian target of rapamycin, and inhibits tumor growth and angiogenesis. Recent studies suggested a possibility that rapamycin renormalizes aberrant tumor vasculature and improves tumor oxygenation. The longitudinal effects of rapamycin on angiogenesis and tumor oxygenation were evaluated in murine squamous cell carcinoma (SCCVII) by electron paramagnetic resonance imaging (EPRI) and magnetic resonance imaging (MRI) to identify an optimal time after rapamycin treatment for enhanced tumor radioresponse. Rapamycin treatment was initiated on SCCVII solid tumors 8 days after implantation (500-750 mm(3)) and measurements of tumor pO(2) and blood volume were conducted from day 8 to 14 by EPRI/MRI. Microvessel density was evaluated over the same time period by immunohistochemical analysis. Tumor blood volume as measured by MRI significantly decreased 2 days after rapamycin treatment. Tumor pO(2) levels modestly but significantly increased 2 days after rapamycin treatment; whereas, it decreased in non-treated control tumors. Furthermore, the fraction of hypoxic area (pixels with pO(2)<10 mm Hg) in the tumor region decreased 2 days after rapamycin treatments. Immunohistochemical analysis of tumor microvessel density and pericyte coverage revealed that microvessel density decreased 2 days after rapamycin treatment, but pericyte coverage did not change, similar to what was seen with anti-angiogenic agents such as sunitinib which cause vascular renormalization. Collectively, EPRI/MRI co-imaging can provide non-invasive evidence of rapamycin-induced vascular renormalization and resultant transient increase in tumor oxygenation. Improved oxygenation by rapamycin treatment provides a temporal window for anti-cancer therapies to realize enhanced response to radiotherapy.

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Available from: J. Silvio Gutkind
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    • "Our lab has developed imaging capabilities to serially map tumor oxygen in vivo and changes in tumor pO2 distribution in response to treatment using electron paramagnetic resonance imaging (EPRI) [22]–[24]. Studies from our lab and others using such imaging techniques have found that tumors display both spatial and temporal heterogeneities in pO2 status. Additional studies have shown that these tumors can be rendered significantly more hypoxic for a period of up to 5 hours by a bolus administration of pyruvate [25]; administration of pyruvate prior to radiotherapy in an SCCVII xenograft model was found to impose radioresistance. "
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    ABSTRACT: BackgroundTH-302 is a hypoxia-activated prodrug (HAP) of bromo isophosphoramide mustard that is selectively activated within hypoxic regions in solid tumors. Our recent study showed that intravenously administered bolus pyruvate can transiently induce hypoxia in tumors. We investigated the mechanism underlying the induction of transient hypoxia and the combination use of pyruvate to potentiate the anti-tumor effect of TH-302.Methodology/ResultsThe hypoxia-dependent cytotoxicity of TH-302 was evaluated by a viability assay in murine SCCVII and human HT29 cells. Modulation in cellular oxygen consumption and in vivo tumor oxygenation by the pyruvate treatment was monitored by extracellular flux analysis and electron paramagnetic resonance (EPR) oxygen imaging, respectively. The enhancement of the anti-tumor effect of TH-302 by pyruvate treatment was evaluated by monitoring the growth suppression of the tumor xenografts inoculated subcutaneously in mice. TH-302 preferentially inhibited the growth of both SCCVII and HT29 cells under hypoxic conditions (0.1% O2), with minimal effect under aerobic conditions (21% O2). Basal oxygen consumption rates increased after the pyruvate treatment in SCCVII cells in a concentration-dependent manner, suggesting that pyruvate enhances the mitochondrial respiration to consume excess cellular oxygen. In vivo EPR oxygen imaging showed that the intravenous administration of pyruvate globally induced the transient hypoxia 30 min after the injection in SCCVII and HT29 tumors at the size of 500–1500 mm3. Pretreatment of SCCVII tumor bearing mice with pyruvate 30 min prior to TH-302 administration, initiated with small tumors (∼550 mm3), significantly delayed tumor growth.Conclusions/SignificanceOur in vitro and in vivo studies showed that pyruvate induces transient hypoxia by enhancing mitochondrial oxygen consumption in tumor cells. TH-302 therapy can be potentiated by pyruvate pretreatment if started at the appropriate tumor size and oxygen concentration.
    Full-text · Article · Sep 2014 · PLoS ONE
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    • "Saito et al32 provided noninvasive evidence of RPM-induced vascular renormalization and the resultant transient increase in tumor oxygenation. The improved oxygenation from RPM treatment provides a temporal window for anticancer therapies to enhance radiotherapy response. "
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