Prediction of tumor hypoxia using paired images of tumor proliferation and glucose metabolism in rodent breast cancers

Objectives Imaging tumor hypoxia is very important not only to predict prognosis but also to select the appropriate therapy. Many “hypoxia” imaging PET tracers have limitations of low signal, blood flow dependence or imaging non-hypoxic elements. Previous in vitro studies have shown high glucose metabolism but low proliferation in hypoxic human tumor cells (Clavo AC et al. JNM 1996 Mar; 37(3):502-6). The goal of this study is to estimate hypoxic areas in vivo using images of tumor proliferation and glucose metabolism.

Methods Rats bearing syngeneic mammary tumor (RMT) in their interscapular subcutaneous region were prepared. A well-established hypoxia marker, pimonidazole (60mg/kg) was injected intraperitoneally six hours before sacrifice. A mixture of 18FDG (1mCi) and 14C-thymidine (5μCi) was injected intravenously two hours before sacrifice. Immunofluorescent staining for anti-pimonidazole antibody using a confocal microscope on frozen sections was performed after quantitative digital autoradiography for 18F and 14C. The images of the distribution of the three agents were quantitatively assessed for density on tumor sections using ImageJ. Correlation among the tracers was analyzed. Furthermore, the data was classified into groups according to 18FDG and 14C-thymidine density, and 3D graphs were created to analyze the correlations among the three tracers.

Results Pimonidazole and thymidine showed a predominantly negative correlation (r=-0.58, P<0.000001). Glucose and thymidine uptake showed positive correlation up to a middle level of thymidine uptake, afterward glucose uptake started to decrease relative to thymidine uptake. The intratumoral cell population with low thymidine and high glucose uptake showed the highest pimonidazole accumulation in the 3D graph comparing the three tracers.

Conclusions The combination of proliferation and glucose metabolism images can indirectly, but accurately, predict tumor hypoxia. Areas of preserved glycolysis with low proliferation appear to be mainly hypoxic