Visualizing the acute effects of vascular-targeted therapy in vivo using intravital microscopy and magnetic resonance imaging: correlation with endothelial apoptosis, cytokine induction, and treatment outcome.

The acute effects of the vascular-disrupting agent 5,6-dimethylxanthenone-4-acetic acid (DMXAA) were investigated in vivo using intravital microscopy (IVM) and magnetic resonance imaging (MRI). Changes in vascular permeability and blood flow of syngeneic CT-26 murine colon adenocarcinomas were assessed at 4 and 24 hours after DMXAA treatment (30 mg/kg, i.p.) and correlated with induction of tumor necrosis factor-alpha (TNF-alpha), endothelial damage [CD31/terminal deoxynucleotidyl transferase (TdT)], and treatment outcome. Intravital imaging revealed a marked increase in vascular permeability 4 hours after treatment, consistent with increases in intratumoral mRNA and protein levels of TNF-alpha. Parallel contrast-enhanced MRI studies showed a approximately 4-fold increase in longitudinal relaxation rates (DeltaR(1)), indicative of increased contrast agent accumulation within the tumor. Dual immunostained tumor sections (CD31/TdT) revealed evidence of endothelial apoptosis at this time point. Twenty-four hours after treatment, extensive hemorrhage and complete disruption of vascular architecture were observed with IVM, along with a significant reduction in DeltaR(1); and virtual absence of CD31 immunostaining. DMXAA-induced tumor vascular damage resulted in significant long-term (60-day) cures compared to untreated controls. Multimodality imaging approaches are useful in visualizing the effects of antivascular therapy in vivo. Such approaches allow cross validation and correlation of findings with underlying molecular changes contributing to treatment outcome.