Rat Tumor Response to the Vascular-Disrupting Agent 5,6-Dimethylxanthenone-4-Acetic Acid as Measured by Dynamic Contrast-Enhanced Magnetic Resonance Imaging, Plasma 5-Hydroxyindoleacetic Acid Levels, and Tumor Necrosis 1

The dose-dependent effects of 5,6-dimethylxanthenone-4-acetic acid (DMXAA) on rat GH3 prolactinomas were investigated in vivo. Dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) was used to assess tumor blood flow/permeability pretreatment and 24 hours posttreatment with 0, 100, 200, or 350 mg/kg DMXAA. DCE-MRI data were analyzed using Ktrans and the integrated area under the gadolinium time curve (IAUGC) as response biomarkers. Highperformance liquid chromatography (HPLC) was used to determine the plasma concentration of the serotonin metabolite 5-hydroxyindoleacetic acid (5-HIAA) following treatment to provide an index of increased vessel permeability and vascular damage. Finally, tumor necrosis was assessed by grading hematoxylin and eosin-stained sections cut from the same tumors investigated by MRI. Both tumor Ktrans and IAUGC were significantly reduced 24 hours posttreatment with 350 mg/kg DMXAA only, with no evidence of dose response. HPLC demonstrated a significant increase in plasma 5-HIAA 24 hours posttreatment with 200 and 350 mg/kg DMXAA. Histologic analysis revealed some evidence of tumor necrosis following treatment with 100 or 200 mg/kg DMXAA, reaching significance with 350 mg/kg DMXAA. The absence of any reduction in Ktrans or IAUGC following treatment with 200 mg/kg, despite a significant increase in 5-HIAA, raises concerns about the utility of established DCE-MRI biomarkers to assess tumor response to DMXAA.