 1.2 Rationale 1.2.1 Capecitabine for NSCLC Capecitabine is an oral prodrug of 5-Fu.It is absorbed through the intestine and converted to 5-deoxy-S-fluorocytidine 5-DFCR by carboxylesterase and then to 5-deoxy-S-fluorouridine 5-DFUR by cytidine deaminase, both steps taking place in the liver. Finally,it is converted to the only active metabolite, FU, by thymidine phosphorylaseTP. This occurs in both tumor and normal tissues; however, the TP is found at higher concentrations in some tumor tissue compared with normal healthy tissue.The expression of this enzymes may influence the effect of the capecitabine. Han et al examined the TP expression in tumor tissue samples from NSCLC patients who enrolled in a previous phase II study of capecitabinedocetaxel chemotherapy and found that the patients with high tumour cell thymidine phosphorylase expression show a better response to capecitabine based chemotherapy . 