Studies of tumor microenvironment and metabolic activity in HER-2/neu overexpressing breast cancer xenografts by MicroPET® and MRI Free

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Background: HER-2/neu overexpression in breast cancers is correlated with resistance to treatments and poor prognosis. Recently, we showed that HER-2/neu overexpression in ER^- MDA-435/LCC6 human breast cancer xenografts (LCC6^HER-2) results in significantly increased tumor viability and levels of viable hypoxic tumor cells, compared to isogenic control tumors (LCC6^Vector) despite similar tumor growth rates and vascularization (vessel density and percentage of CD31 staining). Since oxygenation of tumor cells is linked to tumor tissue perfusion and metabolism, these parameters were evaluated in situ to explain the observed biological differences between the LCC6^HER-2 and LCC6^Vector tumors. Methods: LCC6^HER-2 and LCC6^Vector tumors were grown s.c. (^∼300 mm²) in T/B deficient Rag2M mice. Animals were injected with ¹⁸F-flurodeoxyglucose (FDG), and tumors scanned for 30-60 min with small animal Positron Emission Tomography (MicroPET®). The next day perfusion in the same tumors was evaluated with MRI. After MicroPET® and MRI scans tumors were harvested, disaggregated and analyzed for the content of viable hypoxic cells (EF5). Results: Analysis of MicroPET® images indicated that FDG uptake normalized per unit area of metabolically active tumor tissue (counts per second/cm²), in the plane with the greatest activity, was 2.1-fold higher in LCC6^HER-2 compared to LCC6^Vector tumors (p<0.05). Preliminary MRI data showed that perfusion was also elevated in LCC6^HER-2 tumors (2.2-fold). LCC6^HER-2 and LCC6^Vector tumors grew at the same rates, but the former were more hypoxic than the latter (23.0% vs. 16.2% of total viable tumor cells, respectively). Conclusions: PET and MRI were successfully used to evaluate the in vivo environmental and metabolic status of human breast cancer xenografts in Rag2M mice. The observed characteristics of the LCC6^HER-2 tumors suggest that HER-2/neu expression in vivo is associated with metabolic processes that involve higher glucose and oxygen consumption resulting in more hypoxic tumor phenotype.