Hydrolysis of capecitabine to 5'-deoxy-5-fluorocytidine by human carboxylesterases and inhibition by loperamide

J Pharmacol Exp Ther. 2005 Jun;313(3):1011-6. doi: 10.1124/jpet.104.081265. Epub 2005 Feb 1.


Capecitabine is an oral prodrug of 5-fluorouracil that is indicated for the treatment of breast and colorectal cancers. A three-step in vivo-targeted activation process requiring carboxylesterases, cytidine deaminase, and thymidine phosphorylase converts capecitabine to 5-fluorouracil. Carboxylesterases hydrolyze capecitabine's carbamate side chain to form 5'-deoxy-5-fluorocytidine (5'-DFCR). This study examines the steady-state kinetics of recombinant human carboxylesterase isozymes carboxylesterase (CES) 1A1, CES2, and CES3 for hydrolysis of capecitabine with a liquid chromatography/mass spectroscopy assay. Additionally, a spectrophotometric screening assay was utilized to identify drugs that may inhibit carboxylesterase activation of capecitabine. CES1A1 and CES2 hydrolyze capecitabine to a similar extent, with catalytic efficiencies of 14.7 and 12.9 min(-1) mM(-1), respectively. Little catalytic activity is detected for CES3 with capecitabine. Northern blot analysis indicates that relative expression in intestinal tissue is CES2 > CES1A1 > CES3. Hence, intestinal activation of capecitabine may contribute to its efficacy in colon cancer and toxic diarrhea associated with the agent. Loperamide is a strong inhibitor of CES2, with a K(i) of 1.5 muM, but it only weakly inhibits CES1A1 (IC(50) = 0.44 mM). Inhibition of CES2 in the gastrointestinal tract by loperamide may reduce local formation of 5'-DFCR. Both CES1A1 and CES2 are responsible for the activation of capecitabine, whereas CES3 plays little role in 5'-DFCR formation.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • Antimetabolites, Antineoplastic / metabolism*
  • Capecitabine
  • Carboxylesterase / antagonists & inhibitors
  • Carboxylesterase / genetics
  • Carboxylesterase / metabolism*
  • Deoxycytidine / analogs & derivatives*
  • Deoxycytidine / metabolism*
  • Fluorouracil / analogs & derivatives
  • Gastrointestinal Tract / enzymology
  • Humans
  • Hydrolysis
  • Isoenzymes / antagonists & inhibitors
  • Isoenzymes / genetics
  • Isoenzymes / metabolism*
  • Loperamide / pharmacology*
  • Prodrugs / metabolism*


  • Antimetabolites, Antineoplastic
  • Isoenzymes
  • Prodrugs
  • Deoxycytidine
  • Capecitabine
  • Loperamide
  • 5'-deoxy-5-fluorocytidine
  • Carboxylesterase
  • Fluorouracil