Reoxygenation after Evofosfamide Treatment in Pancreatic Ductal Adenocarcinoma Xenografts is due to Decreased Oxygen Consumption and not Increased Oxygen Supply
Evofosfamide is designed to release a cytotoxic bromo-isophosphoramide (Br-IPM) moiety in a hypoxic microenvironment. This drug therefore preferentially attacks hypoxic regions in tumors where other standard anti-cancer treatments such as chemotherapy and radiation therapy are often ineffective. Various combination therapies with evofosfamide have been proposed and tested in preclinical and clinical settings. However, the treatment effect of evofosfamide monotherapy on tumor hypoxia has not been fully understood, partly due to the lack of quantitative methods to assess tumor pO2 in vivo. Here, we use quantitative pO2 imaging by EPR to evaluate the change in tumor hypoxia in response to evofosfamide treatment using two pancreatic ductal adenocarcinom a xenograft models; MIA Paca-2 tumors responding to evofosfamide and Su.86.86 tumors which do not respond. EPR imaging showed oxygenation improved globally after evofosfamide treatment in hypoxic MIA Paca-2 tumors, in agreement with the ex vivo results obtained from hypoxia staining by pimonidazole and in apparent contrast to the decrease in Ktrans observed in DCE MRI. This suggests reoxygenation after treatment is due to decreased oxygen demand rather than improved prefusion. Following the change in pO2 after treatment may therefore yield a way of monitoring treatment response. The observation that evofosfamide not only kills the hypoxic region of the tumor but also improves oxygenation in the residual tumor regions provides a rationale for combination therapies using radiation and anti-proliferatives post evofosfamide for improved outcomes.