Summary The effects of combretastatin A4 prodrug on perfusion and the levels of 31p metabolites in an implanted murine tumour were investigated for 3 h after drug treatment using nuclear magnetic resonance imaging (MRI) and spectroscopy (MRS). The area of regions of low signal intensity in spin-echo images of tumours increased slightly after treatment with the drug. These regions of low signal intensity corresponded to necrosis seen in histological sections, whereas the expanding regions surrounding them corresponded to haemorrhage. Tumour perfusion was assessed before and 160 min after drug treatment using dynamic MRI measurements of gadolinium diethylenetriaminepentaacetate (GdDTPA) uptake and washout. Perfusion decreased significantly in central regions of the tumour after treatment. This was attributed to disruption of the vasculature and was consistent with the haemorrhage seen in histological sections. The mean apparent diffusion coefficient of water within the tumour did not change, indicating that there was no expansion of necrotic regions during the 3 h after drug treatment. Localized 31P-MRS showed that there was decline in cellular energy status in the tumour after treatment with the drug. The concentrations of nucleoside triphosphates within the tumour fell, the inorganic phosphate concentration increased and there was a significant decrease in tumour pH for 80 min after drug treatment. The rapid, selective and extensive damage caused to these tumours by combretastatin A4 prodrug has highlighted the potential of the agent as a novel cancer chemotherapeutic agent. We have shown that the response of tumours to treatment with the drug may be monitored non-invasively using MRI and MRS experiments that are appropriate for use in a clinical setting.Keywords: combretastatin A4; tumour perfusion; magnetic resonance imaging; 31P-spectroscopy; GdDTPA; haemorrhage Combretastatin A4 is a tubulin-binding compound originally isolated from Combretum caffrum, a Southern African shrub (Pettit et al, 1989). Solubility in aqueous solutions is poor but is improved by conversion to disodium combretastatin A4 3-0-phosphate (combretastatin A4 prodrug) from which the phosphate group is cleaved by endogenous non-specific phosphatases under physiological conditions (Pettit et al, 1995). The two compounds were shown to have similar activity against model murine, rat and human tumours (Dark et al, 1997). The drug has been shown to have a direct and selective effect on tumour vasculature. In an isolated tumour model, infusion of the drug was shown to result in vascular shutdown within 20 min. Experiments in vitro have demonstrated that the drug has antiproliferative/cytotoxic effects against proliferating endothelial cells. However, it is thought unlikely that these effects of the drug on vascular endothelial cells could, on their own, explain the rapid disruption of tumour blood flow observed in vivo (Dark et al, 1997). In contrast with colchicine and other tubulin-binding agents that have been investigated as agents for disrup...

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