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From SNPedia

Capecitabine, sold under the brand name Xeloda among others, is a chemotherapy medication used to treat breast cancer, gastric cancer and colorectal cancer. For breast cancer it is often used together with docetaxel.Wikipedia

Capecitabine is a prodrug that is selectively tumour-activated to its cytotoxic moiety, fluorouracil, by thymidine phosphorylase, an enzyme found in higher concentrations in many tumors compared to normal tissues or plasma.Drugbank

Most fluorouracil is metabolized by dihydropyrimidine dehydrogenase (DPD), encoded by the DPYD gene. A small proportion of patients develop extreme toxicity after administration of a fluoropyrimidine due to partial or complete DPD deficiency and hence a strongly reduced capacity to degrade 5-FU (also known as Folfox). 5-fluorouracil treatment may even result in a lethal outcome in patients with complete DPD deficiency. Mutations in the TPMT gene also increase risk for 5-FU toxicity.

PGx Table for Capecitabine (Xeloda)
Genotype/Genoset Diplotype Gene Drug Use/Category/Class Medical Field Summary Descriptive Sentence
gs313 Normal metabolizer DPYD Chemotherapeutic Oncology Normal Response Expected You do not appear to carry any DPYD mutations linked to adverse side effects from capecitabine use.
gs312 Intermediate Metabolizer DPYD Chemotherapeutic Oncology Consider Half-Dose and/or Pharmacokinetic Studies You carry a nonfunctional DPYD allele that increases your risk for a toxicity upon capecitabine administration; consider starting at half-dose and/or doing pharmacokinetic studies.
gs306 Poor Metabolizer DPYD Chemotherapeutic Oncology Avoid 5-FU; use an alternative drug You may be at very high risk for severe toxicity from capecitabine administration; all major drug regulatory agencies recommend you select a non-fluoropyrimidine alternative chemotherapeutic.