Study to Compare Cardiovascular Side Effects of Teysuno Versus Capecitabine

Capecitabine is a chemotherapy drug used to treat many types of cancer including bowel and stomach cancer. Unfortunately a side effect of this drug is that it causes heart problems including heart attacks. An alternative drug, called teysuno is used extensively in other countries instead of capecitabine and appears to have less of a bad effect on the heart whilst still killing cancer cells. This study will investigate the effect of these two drugs on the heart and blood vessels and will be the first of its kind in humans.