Structural Constraints Affect the Metabolism of 7-Ethyl-10-[4-(1-piperidino)-1-piperidino]carbonyloxycamptothecin (CPT-11) by Carboxylesterases

  1. Randy M. Wadkins1,
  2. Christopher L. Morton2,
  3. James K. Weeks2,
  4. LaGora Oliver2,
  5. Monika Wierdl2,
  6. Mary K. Danks2 and
  7. Philip M. Potter2
  1. 1Johns Hopkins University School of Medicine, Baltimore, Maryland (R.M.W.); and 2Department of Molecular Pharmacology, St. Jude Children's Research Hospital, Memphis, Tennessee (C.L.M., J.K.L., L.O., M.W., M.K.D., P.L.P.)
  1. Dr. Philip M. Potter, Department of Molecular Pharmacology, St. Jude Children's Research Hospital, 332 N. Lauderdale, Memphis, TN 38105. E-mail:phil.potter{at}stjude.org

Abstract

7-Ethyl-10-[4-(1-piperidino)-1-piperidino]carbonyloxycamptothecin [CPT-11 (irinotecan)] is a water-soluble camptothecin-derived prodrug that is activated by esterases to yield the potent topoisomerase I poison SN-38. We identified a rabbit liver carboxylesterase (CE) that was very efficient at CPT-11 metabolism; however, a human homolog that was more than 81% identical to this protein activated the drug poorly. Recently, two other human CEs have been isolated that are efficient in the conversion of CPT-11 to SN-38, yet both demonstrate little homology to the rabbit protein. To understand this phenomenon, we have characterized a series of esterases from human and rabbit, including several chimeric proteins, for their ability to metabolize CPT-11. Computer predictive modeling indicated that the ability of each enzyme to activate CPT-11 was dependent on the size of the entrance to the active site. Kinetic studies with a series of nitrophenyl and naphthyl esters confirmed these predictions, indicating that activation of CPT-11 by a CE is constrained by size-limited access of the drug to the active site catalytic amino acid residues.

Footnotes

  • This work was supported in part by National Institutes of Health Grants CA76202, CA79763, the Cancer Center Core Grant CA21765, and the American Lebanese Syrian Associated Charities.

  • Abbreviations:
    CPT-11
    7-ethyl-10-[4-(1-piperidino)-1-piperidino]carbonyloxycamptothecin (irinotecan)
    SN-38
    7-ethyl-10-hydroxycamptothecin
    CE
    carboxylesterase
    hCE human carboxylesterase
    hiCE, human intestinal carboxylesterase
    AChE
    acetylcholinesterase
    NPA
    nitrophenyl acetate
    rCE
    rabbit liver carboxylesterase
    NPB
    nitrophenyl butyrate
    • Received January 29, 2001.
    • Accepted April 25, 2001.
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  1. Molecular Pharmacology August 1, 2001 vol. 60 no. 2 355-362
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