7-tert-Butyldimethylsilyl-10-hydroxycamptothecin (AR-67; also known as DB-67) is a novel lipophilic camptothecin analog in early-phase anticancer clinical trials. In support of these studies, we evaluated the metabolism of AR-67 in vitro and identified potential metabolites in patient samples. The lactone form of AR-67 was found to be preferentially metabolized over AR-67 carboxylate in human microsomes. Subsequently, the lactone form was tested as a substrate in a panel of CYP450 and UDP-glucuronosyltransferase (UGT) enzymes known to metabolize the majority of clinically approved molecules. AR-67 was metabolized by CYP3A5, CYP3A4, CYP1A1, and CYP1A2, in order of activity. Extrahepatic UGT1A8 and UGT1A7 possessed at least 6-fold higher metabolizing activity than UGT1A1 and other UGT enzymes tested. CYP1A1 and UGT1A7 displayed Michaelis-Menten kinetics, whereas CYP3A4, CYP3A5, and UGT1A8 displayed kinetics consistent with substrate inhibition. Chromatographic analysis of representative patient plasma and urine samples demonstrated the presence of AR-67 glucuronides and oxidized products in the urine but only in very minimal amounts. We conclude that limited in vivo metabolism of AR-67 by UGT1A1 may partly explain the absence of AR-67 glucuronides in plasma and hypothesize that UGT1A8- and CYP3A-mediated biotransformation within the gastrointestinal epithelium may provide protective mechanisms against AR-67 gastrointestinal toxicity.
Footnotes
This work was supported by the National Institutes of Health National Cancer Institute [Grant CA123867]; The Shumate Foundation; and Arno Therapeutics, Inc.
Article, publication date, and citation information can be found at http://dmd.aspetjournals.org.
doi:10.1124/dmd.110.037390.
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ABBREVIATIONS:
- AR-67
- 7-tert-butyldimethylsilyl-10-hydroxycamptothecin, DB-67
- CPT
- camptothecin
- AUC
- area under the curve
- CPT-11
- irinotecan
- SN-38
- 7-ethyl-10-hydroxycamptothecin
- UGT
- UDP-glucuronosyltransferase
- P450
- cytochrome P450
- SPE
- solid-phase extraction
- DMSO
- dimethyl sulfoxide
- QC
- quality control
- HPLC
- high-pressure liquid chromatography
- MS
- mass spectrometry
- APCI
- atmospheric pressure chemical ionization
- ESI
- electrospray ionization
- HLM
- human liver microsome(s)
- HIM
- human intestinal microsome(s)
- MS/MS
- tandem mass spectrometry
- GI
- gastrointestinal
- OAT
- organic anionic transporting polypeptide
- ABC
- ATP-binding cassette
- BCRP
- breast cancer resistance protein.
- Received November 23, 2010.
- Accepted December 28, 2010.
- Copyright © 2011 by The American Society for Pharmacology and Experimental Therapeutics
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