OXIDATIVE-METABOLISM OF A REXINOID AND RAPID PHASE-II METABOLITE IDENTIFICATION BY MASS-SPECTROMETRY

LGD1069 (Targretin), a retinoid ''X'' receptor-selective ligand, or re xinoid, is in clinical trials for treating cancer. Biologically-active oxidized LGD1069 metabolites have been observed in patient plasma sam ples, making corresponding structural characterizations necessary. For mation of multiple metabolite isomers in vivo has created technical ch allenges in metabolite structural analysis; however, mass spectrometry (MS) was able to pinpoint two sites of Phase I metabolism. A carbon-1 3 trideuterated analog was used as an isotopic marker to probe Phase I I metabolism of LGD1069. Rats were orally gavaged with an equimolar mi xture of LGD1069 and [(CH3)-C-13-H-2]LGD1069, then anesthetized prior to bile-duct cannulation. Bile was collected for 7 hr, extracted, and concentrated, Recovered metabolites were analyzed by narrow-bore, grad ient liquid chromatography (LC) with negative ion, electrospray ioniza tion MS detection. When resultant total ion chromatograms were interro gated for mass spectra exhibiting isotope clusters separated by 4 dalt ons, 13 such clusters corresponding to Phase II LGD1069 metabolites of nine different molecular weights were detected. Acyl-glucuronide and taurine conjugates of both parent compound and hydroxy-LGD1069 were ob served. The sulfate and taurine conjugates of oxo-LGD1069 were also id entified, as were 6,7-dihydroxy-LGD1069 taurine, LGD1069 ether glucuro nide, and a secondary conjugate (taurine) of the latter. Identities of selected conjugates were confirmed by MS/MS. The results of this stud y demonstrate that when combined with traditional GC/MS and MS/MS data , the isotope cluster technique can provide powerful selectivity in id entifying numerous Phase II drug metabolites during a single LC/MS ana lysis.