Abstract
The anticancer prodrug ifosfamide (IFA) contains a chiral phosphorous atom and is administered clinically as a racemic mixture ofR and S enantiomers. Animal model studies and clinical data indicate enantioselective differences in cytochrome P-450 (CYP) metabolism, pharmacokinetics, and therapeutic efficacy between the two enantiomers; however, the metabolism of individual IFA enantiomers has not been fully characterized. The role of CYP enzymes in the stereoselective metabolism of R-IFA andS-IFA was investigated by monitoring the formation of both 4-hydroxy (activated) and N-dechloroethyl (DCl) (inactive, neurotoxic) metabolites. In the 4-hydroxylation reaction, cDNA-expressed CYPs 3A4 and 3A5 preferentially metabolizedR-IFA, whereas CYP2B6 was more active towardS-IFA. Enantioselective IFA 4-hydroxylation (R > S) was observed with six of eight human liver samples. In the N-dechloroethylation reaction, CYPs 3A4 and 2B6 both catalyzed a significantly higher intrinsic metabolic clearance (Vmax/Km) ofS-IFA compared with R-IFA. Striking P-450 form specificity in the formation of individual DCl metabolites was evident. CYPs 3A4 and 3A5 preferentially produced (R)N2-DCl-IFA and (R)N3-DCl-IFA (derived from R-IFA and S-IFA, respectively), whereas CYP2B6 correspondingly formed (S)N3-DCl-IFA and (S)N2-DCl-IFA. In human liver microsomes, the CYP3A-specific inhibitor troleandomycin suppressed (R)N2- and (R)N3-DCl-IFA formation by ≥80%, whereas (S)N2- and (S)N3-DCl-IFA formation were selectively inhibited (≥85%) by a CYP2B6-specific monoclonal antibody. The overall extent of IFAN-dechloroethylation varied with the CYP3A4 and CYP2B6 content of each liver, but was significantly lower forR-IFA (32 ± 13%) than for S-IFA (62 ± 17%, n = 8; p < .001) in all livers examined. R-IFA thus has more favorable liver metabolic properties than S-IFA with respect to less extensive N-dechloroethylation and more rapid 4-hydroxylation, indicating that R-IFA may have a distinct clinical advantage over racemic IFA.
Footnotes
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Send reprint requests to: Dr. David J. Waxman, Department of Biology, Boston University, 5 Cummington St., Boston, MA. E-mail: djw{at}bio.bu.edu
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This work was supported in part by National Institutes of Health Grant CA49248 (to D.J.W.).
- Abbreviations used are::
- CYP
- cytochrome P-450
- IFA
- ifosfamide
- rac-IFA
- racemic IFA
- CPA
- cyclophosphamide
- DCl
- dechloroethyl
- R2-DCl-IFA and S2-DCl-IFA
- N2-dechloroethylated metabolites of R-IFA andS-IFA, respectively
- S3-DCl-IFA and R3-DCl-IFA
- N3-dechloroethylated metabolites ofR-IFA and S-IFA, respectively
- HLS
- human liver microsomal sample
- OR
- NADPH P-450 oxidoreductase
- mAb-2B6
- monoclonal antibody to CYP2B6
- TAO
- troleandomycin
- 7-EFC
- 7-ethoxy-4-trifluoromethyl coumarin
- Received May 4, 1999.
- Accepted August 2, 1999.
- The American Society for Pharmacology and Experimental Therapeutics
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