A pre-prodrug of 5-fluorouracil (5-FU).  After oral uptake, capecitabine is first metabolized to 5'-deoxy-5-fluorocytidine (5'-DFCR), which occurs mainly in the liver by carboxyl-esterase.  The metabolite is converted to 5'-deoxy-5-fluoro-uridine (5'-DFUR) by cytidine deaminase in liver and tumor tissue.  Then 5'-DFUR is converted to 5-FU intracellularly by thymidine phosphorylase, which is often expressed in tumor tissue.