Therapeutic effects of the antivascular agent combretastatin A-1 phosphate on orthotopic and subcutaneous colorectal tumours in nude mice.

3016 Many common solid tumours possess an extremely heterogeneous pathophysiology, and successful treatment requires a combination of therapeutic strategies. To optimise combined therapies, we need detailed microscopic information on regional tumour response to treatment. The combretastatins are a family of compounds that interact with tubulin at or near the colchicine binding site, producing vascular shut-down and resulting in central haemorrhagic necrosis of up to 95% of the tumour. We have previously studied the effects of combretastatin A4 phosphate (CA-4-P) in combination with antibody-targeted therapies, which has culminated in an on-going clinical trial. The current study investigates the therapeutic efficacy of combretastatin A-1 phosphate (CA-1-P), as a potential candidate for combined studies. CA-1-P was administered intraperitoneally at 40mg/kg to nude mice bearing either subcutaneous or orthotopic (metastatic deposits in the liver) SW1222 colorectal xenografts. Tumours were removed at selected time points, and a range of tumour parameters were studied concomitantly by quantitative fluorescence microscopy in order to demonstrate changes related to vascular shut-down (eg. blood vessels, hypoxia, perfusion). Subcutaneous tumours were studied at 40 min, 1, 3, 6, 24 and 48h post CA-1-P. Untreated tumours were mostly viable, with perfused blood vessels throughout and only small areas of hypoxia. Tumour perfusion had ceased in all but the tumour periphery by 40 min post CA-1-P, widespread hypoxia had developed by 1 h, and the centre of the tumour was becoming necrotic by 6 h after the drug. By 24h, only a thin outer rim of tumour cells survived. These effects were similar to those achieved with CA-4-P, but results were more reproducible and more dramatic. Orthotopic tumours were grown by intrasplenic injection of SW1222 tumour cells, and the effects of CA1P studied at 1 and 24h post treatment. Results were similar to those seen in the s.c. model. Central perfusion had ceased in most of the tumour deposits by 1h after treatment, but with no effect on normal liver. By 24h there was no resumption of perfusion within the tumour, and many of the deposits showed extensive central necrosis. In preliminary therapy experiments groups of 6 mice were given either no treatment, or CA-1-P (40mg/kg) i.p. on days 1, 8 and 21. CA-1-P alone significantly inhibited tumour growth compared with controls, which was not found when CA-4-P was used as a single agent. The potential for this novel antivascular agent to enhance antibody-targeted therapies is therefore promising, and the relevant experiments are currently underway. Supported by CR UK, EU FP6 (STROMA Project), NTRAC. CA1P supplied by Oxigene.