The deuterium isotope effect for the alpha-hydroxylation of tamoxifen by rat liver microsomes accounts for the reduced genotoxicity of [D5-ethyl]tamoxifen.
This study describes the application of on line HPLC-electrospray ionization MS in the structural determination of the metabolites formed following incubation with rat liver microsomes of an equimolar mixture of the anticancer drug tamoxifen and its [D5-ethyl]-analogue. The ratio of ca 3:1 between unlabelled and D4-labelled alpha-hydroxytamoxifen, indicating a large isotope effect for this metabolic process, accounted for the previously observed lower yield of DNA adducts formed in the livers of rats treated with D5-tamoxifen compared with unlabelled drug. The loss of one deuterium atom on metabolism discriminated hydroxyethylated metabolites from others and enabled two further such metabolites to be detected, namely alpha-hydroxytamoxifen N-oxide and alpha-hydroxy-N-desmethyltamoxifen of which the latter is novel. Furthermore, the use of [alpha-D2-ethyl]- and [beta-D3-ethyl] tamoxifens discriminated alpha- from beta-hydroxylated metabolites and proved that the metabolites described here were alpha-hydroxylated. In contrast to the alpha-hydroxylated metabolites, the other metabolites identified, namely tamoxifen N-oxide, N-desmethyltamoxifen, 4-hydroxytamoxifen and their deuterated counterparts were not depleted in the deuterated components. The use of on line HPLC-electrospray ionization MS combined with isotopic labelling is a powerful technique for probing the structures of metabolites, and, applied to tamoxifen, has provided further evidence that alpha-hydroxylation is an important pathway for the conversion of the drug into a DNA-reactive metabolite.
Jarman
M
M
CRC Centre for Cancer Therapeutics, Institute of Cancer Research, Sutton, Surrey, UK.
Poon
G K
GK
Rowlands
M G
MG
Grimshaw
R M
RM
Horton
M N
MN
Potter
G A
GA
McCague
R
R
D000818:Animals; D002851:Chromatography, High Pressure Liquid / Q000379:methods; D003903:Deuterium / Q000494:pharmacology*; D006900:Hydroxylation; D008658:Inactivation, Metabolic; D013058:Mass Spectrometry / Q000379:methods; D008862:Microsomes, Liver / Q000378:metabolism*; D010084:Oxidation-Reduction; D051381:Rats; D012680:Sensitivity and Specificity; D013629:Tamoxifen / Q000378:metabolism* / Q000493:pharmacokinetics* / Q000633:toxicity
D013629:Tamoxifen; D003903:Deuterium