title: Structural constraints affect the metabolism of 7-ethyl-10-[4-(1-piperidino)-1-piperidino]carbonyloxycamptothecin (CPT-11) by carboxylesterases.
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.
authors.affiliation: Johns Hopkins University School of Medicine, Baltimore, Maryland, USA.
authors.forename: R M
authors.initials: RM
authors.lastname: Wadkins
authors.forename: C L
authors.initials: CL
authors.lastname: Morton
authors.forename: J K
authors.initials: JK
authors.lastname: Weeks
authors.forename: L
authors.initials: L
authors.lastname: Oliver
authors.forename: M
authors.initials: M
authors.lastname: Wierdl
authors.forename: M K
authors.initials: MK
authors.lastname: Danks
authors.forename: P M
authors.initials: PM
authors.lastname: Potter
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D000972:Antineoplastic Agents, Phytogenic / Q000378:metabolism*; D019556:COS Cells; D002166:Camptothecin / Q000031:analogs & derivatives / Q000737:chemistry / Q000378:metabolism*; D002265:Carboxylic Ester Hydrolases / Q000737:chemistry / Q000378:metabolism*; D002384:Catalysis; D004950:Esterases / Q000378:metabolism; D004952:Esters / Q000737:chemistry / Q000378:metabolism; D006801:Humans; D000077146:Irinotecan; D007700:Kinetics; D008958:Models, Molecular; D008969:Molecular Sequence Data; D011355:Prodrugs / Q000378:metabolism*; D011487:Protein Conformation; D011817:Rabbits; D011993:Recombinant Fusion Proteins / Q000378:metabolism; D017386:Sequence Homology, Amino Acid
chemical_list: D000972:Antineoplastic Agents, Phytogenic; D004952:Esters; D011355:Prodrugs; D011993:Recombinant Fusion Proteins; D000077146:Irinotecan; D004950:Esterases; D002265:Carboxylic Ester Hydrolases; D002166:Camptothecin