CAPSY 500 Tablets CAPSY 500 contains Capecitabine, an orally administered prodrug that is enzymatically converted inside the body into its active form, 5-fluorouracil (5-FU). This conversion occurs preferentially within tumor tissues due to higher concentrations of the activating enzyme thymidine phosphorylase in cancerous cells. Once active, it interferes with cellular DNA and RNA synthesis, inhibiting tumor growth and causing cell death in colorectal, gastric, and breast cancers.