Combretastatin A-4-phosphate effectively increases tumor retention of the therapeutic antibody, 131I-A5B7, even at doses that are sub-optimal for vascular shut-down

Radioimmunotherapy using 131I-A5B7, an anti-CEA antibody, in combination with the vascular disrupting agent, combretastatin A4-phosphate (CA-4-P, 200 mg/kg), has produced tumor cures in SW1222 colorectal xenografts. CA-4-P causes acute tumor blood vessel shutdown, which can be monitored in clinical trials using dynamic contrast enhanced magnetic resonance imaging (DCE-MRI). The purpose of this study was to determine the magnitude of the anti-vascular effect of CA-4-P in the SW1222 tumor, at 200 mg/kg and at lower, more clinically relevant doses, using conventional assays; relate effects to changes in DCE-MRI parameters and determine the corresponding effects on tumor retention of 131I-A5B7. The tumor vascular effects of 30, 100 and 200 mg/kg CA-4-P were determined, at 4- and 24-h post-treatment, using DCE-MRI, uptake of Hoechst 33342 for tumor vascular volume and conventional histology for necrosis. The effect of CA-4-P on tumor and normal tissue 131I-A5B7 retention was also determined. A significant reduction in tumor DCE-MRI kinetic parameters, the initial area under the contrast agent concentration time curve (IAUGC) and the transfer constant (Ktrans), was demonstrated at 4 h after CA-4-P, for all dose levels. These effects persisted for at least 24 h for the 200 mg/kg group but not for lower doses. A similar pattern was seen for vascular volume and necrosis. Despite this dose response, all three dose levels increased tumor retention of radio labeled antibody to a similar degree. These results demonstrate that moderate tumor blood flow reduction following antibody administration is sufficient to improve tumor antibody retention. This is encouraging for the combination of CA-4-P and 131I-A5B7 in clinical trials.