The final step of conversion of capecitabine into 5-FU is catalyzed by thymidine phosphorylase (TP), an enzyme which has been shown to be over expressed in colorectal tumors. This selective tumor activation has been demonstrated to lead to higher concentrations of 5-FU in tumor cells as compared to normal tissue, suggesting tumor targeting by the oral drug. Capecitabine was initially approved by the FDA for treatment of refractory breast carcinoma and has shown activity in metastatic colorectal carcinoma. Capecitabine is an attractive agent for use in combination chemotherapy due to its non-overlapping toxicity profile with many agents and lack of hematologic toxicity.