title: Modulation of the cytochrome P450-mediated metabolism of ifosfamide by ketoconazole and rifampin.
abstract: BACKGROUND
  The autoinducible metabolic transformation of the anticancer agent ifosfamide involves activation through 4-hydroxyifosfamide to the ultimate cytotoxic ifosforamide mustard and deactivation to 2- and 3-dechloroethylifosfamide with concomitant release of the neurotoxic chloroacetaldehyde. Activation is mediated by cytochrome P450 (CYP) 3A4 and deactivation by CYP3A4 and CYP2B6. The aim of this study was to investigate modulation of the CYP-mediated metabolism of ifosfamide with ketoconazole, a potent inhibitor of CYP3A4, and rifampin (INN, rifampicin), an inducer of CYP3A4/CYP2B6.
  
  
  METHODS
  In a double-randomized, 2-way crossover study a total of 16 patients received ifosfamide 3 g/m(2) per 24 hours intravenously, either alone or in combination with 200 mg ketoconazole twice daily (1 day before treatment and 3 days of concomitant administration) or 300 mg rifampin twice daily (3 days before treatment and 3 days of concomitant administration). Plasma pharmacokinetics and urinary excretion of ifosfamide, 2- and 3-dechloroethylifosfamide, and 4-hydroxyifosfamide were assessed in both courses. Data analysis was performed with a population pharmacokinetic model with a description of autoinduction of ifosfamide.
  
  
  RESULTS
  Rifampin increased the clearance of ifosfamide at the start of therapy at 102%. The fraction of ifosfamide metabolized to the dechloroethylated metabolites was increased, whereas exposure to the metabolites was decreased as a result of increased elimination. The fraction metabolized and the exposure to 4-hydroxyifosfamide were not significantly influenced. Ketoconazole did not affect the fraction metabolized or the exposure to the dechloroethylated metabolites, whereas both parameters were reduced with 4-hydroxyifosfamide.
  
  
  CONCLUSIONS
  Coadministration of ifosfamide with ketoconazole or rifampin did not produce changes in the pharmacokinetics of the parent or metabolites that may result in an increased benefit of ifosfamide therapy.
authors.affiliation: Department of Pharmacy and Pharmacology, The Netherlands Cancer Institute, Amsterdam. Thomas.Kerbusch@biof.uu.se
authors.forename: T
authors.initials: T
authors.lastname: Kerbusch
authors.forename: R L
authors.initials: RL
authors.lastname: Jansen
authors.forename: R A
authors.initials: RA
authors.lastname: Mathôt
authors.forename: A D
authors.initials: AD
authors.lastname: Huitema
authors.forename: M
authors.initials: M
authors.lastname: Jansen
authors.forename: R E
authors.initials: RE
authors.lastname: van Rijswijk
authors.forename: J H
authors.initials: JH
authors.lastname: Beijnen
mesh_terms: D000328:Adult; D000368:Aged; D000904:Antibiotics, Antitubercular / Q000494:pharmacology; D000935:Antifungal Agents / Q000494:pharmacology; D018906:Antineoplastic Agents, Alkylating / Q000008:administration & dosage / Q000097:blood / Q000493:pharmacokinetics*; D019540:Area Under Curve; D001189:Aryl Hydrocarbon Hydroxylases; D001499:Bayes Theorem; D018592:Cross-Over Studies; D065702:Cytochrome P-450 CYP2B6; D051544:Cytochrome P-450 CYP3A; D003577:Cytochrome P-450 Enzyme System / Q000096:biosynthesis / Q000187:drug effects* / Q000378:metabolism*; D004334:Drug Administration Schedule; D004790:Enzyme Induction / Q000187:drug effects; D004791:Enzyme Inhibitors / Q000008:administration & dosage / Q000494:pharmacology*; D005260:Female; D006801:Humans; D007069:Ifosfamide / Q000008:administration & dosage / Q000097:blood / Q000493:pharmacokinetics*; D007654:Ketoconazole / Q000008:administration & dosage / Q000494:pharmacology*; D008297:Male; D008875:Middle Aged; D006899:Mixed Function Oxygenases / Q000187:drug effects / Q000378:metabolism; D010089:Oxidoreductases, N-Demethylating / Q000187:drug effects / Q000378:metabolism; D012293:Rifampin / Q000008:administration & dosage / Q000494:pharmacology*; D013997:Time Factors
chemical_list: D000904:Antibiotics, Antitubercular; D000935:Antifungal Agents; D018906:Antineoplastic Agents, Alkylating; D004791:Enzyme Inhibitors; D003577:Cytochrome P-450 Enzyme System; D006899:Mixed Function Oxygenases; D001189:Aryl Hydrocarbon Hydroxylases; C585599:CYP2B6 protein, human; C104464:CYP3A protein, human; D065702:Cytochrome P-450 CYP2B6; D051544:Cytochrome P-450 CYP3A; C510163:CYP3A4 protein, human; D010089:Oxidoreductases, N-Demethylating; D007654:Ketoconazole; D007069:Ifosfamide; D012293:Rifampin