title: Metabolic activation of a 1,3-disubstituted piperazine derivative: evidence for a novel ring contraction to an imidazoline.
abstract: MB243 (a 1,3-disubstituted piperazine) is a new, potent, and selective melanocortin receptor subtype-4 agonist with potential application in the treatment of obesity and/or erectile dysfunction. MB243 was observed to covalently bind extensively to liver microsomal proteins from rats and humans. In the presence of glutathione, two thioether adducts were detected in liver microsomal incubations by radiochromatography and LC/MS/MS analysis. These adducts were also formed when bile duct-cannulated rats were dosed with MB243. The two adducts were isolated, and their structures were determined by accurate mass MS/MS and NMR analyses. The proposed structures resulted from a novel contraction of the piperazine ring to yield a substituted imidazoline. A mechanism is proposed, which involves an initial six electron oxidation of the piperazine ring to form a reactive intermediate, which is trapped by glutathione. Hydrolysis of the glutamic acid residue followed by internal aminolysis by the cysteine amino group resulted in opening of the piperazine ring, which is followed by ring closure to an imidazoline. The resulting cysteinyl-glycine conjugate underwent subsequent hydrolysis of the glycine residue. Understanding of the mechanism of bioactivation led to the design of MB243 analogues that exhibited reduced covalent protein binding.
authors.affiliation: Department of Drug Metabolism, Merck Research Laboratories, P.O. Box 2000, Rahway, New Jersey 07065, USA. george_doss@merck.com
authors.forename: George A
authors.initials: GA
authors.lastname: Doss
authors.forename: Randall R
authors.initials: RR
authors.lastname: Miller
authors.forename: Zhoupeng
authors.initials: Z
authors.lastname: Zhang
authors.forename: Yohannes
authors.initials: Y
authors.lastname: Teffera
authors.forename: Ravi P
authors.initials: RP
authors.lastname: Nargund
authors.forename: Brenda
authors.initials: B
authors.lastname: Palucki
authors.forename: Min K
authors.initials: MK
authors.lastname: Park
authors.forename: Yui S
authors.initials: YS
authors.lastname: Tang
authors.forename: David C
authors.initials: DC
authors.lastname: Evans
authors.forename: Thomas A
authors.initials: TA
authors.lastname: Baillie
authors.forename: Ralph A
authors.initials: RA
authors.lastname: Stearns
mesh_terms: D000818:Animals; D001646:Bile / Q000187:drug effects; D001711:Biotransformation; D003500:Cyclization; D005978:Glutathione / Q000187:drug effects; D006801:Humans; D048288:Imidazolines / Q000138:chemical synthesis* / Q000378:metabolism*; D008297:Male; D008862:Microsomes, Liver / Q000187:drug effects / Q000378:metabolism; D015394:Molecular Structure; D009249:NADP / Q000187:drug effects; D010879:Piperazines / Q000008:administration & dosage / Q000138:chemical synthesis / Q000493:pharmacokinetics*; D011485:Protein Binding; D051381:Rats; D017207:Rats, Sprague-Dawley
chemical_list: D048288:Imidazolines; D010879:Piperazines; D009249:NADP; D005978:Glutathione