PT  - JOURNAL ARTICLE
AU  - Xiaoliang Zhuo
AU  - Xiaohua Stella Huang
AU  - Andrew P. Degnan
AU  - Lawrence B. Snyder
AU  - Fukang Yang
AU  - Hong Huang
AU  - Yue-Zhong Shu
AU  - Benjamin M. Johnson
TI  - Identification of Glutathione Conjugates of Acetylene-Containing Positive Allosteric Modulators of Metabotropic Glutamate Receptor Subtype 5
AID  - 10.1124/dmd.114.061879
DP  - 2015 Jan 29
TA  - Drug Metabolism and Disposition
PG  - dmd.114.061879
4099  - http://dmd.aspetjournals.org/content/early/2015/01/29/dmd.114.061879.short
4100  - http://dmd.aspetjournals.org/content/early/2015/01/29/dmd.114.061879.full
AB  - A recent medicinal chemistry campaign to identify positive allosteric modulators (PAMs) of metabotropic glutamate receptor subtype 5 (mGluR5) led to the discovery of potent compounds featuring an oxazolidinone structural core flanked by biaryl acetylene and haloaryl moieties. However, biotransformation studies of some of these mGluR5 PAMs demonstrated the formation of glutathione (GSH) conjugates. The conjugates in question were formed independently of NADPH as the main products in liver microsomes and liver cytosol (rat and human) and exhibited masses that were 307 u greater than their respective substrates, indicating the involvement of a reductive step in the formation of these metabolites. To further characterize the relevant metabolic sequences, GSH conjugates of (4R,5R)-5-(3-fluorophenyl)-4-(5-(pyrazin-2-ylethynyl)pyridin-3-yl)oxazolidin-2-one and (4R,5R)-5-(4-fluorophenyl)-4-(6-((3-fluoropyridin-2-yl)ethynyl)pyridin-2-yl)oxazolidin-2-one were biosynthesized and isolated. Subsequent analysis by NMR showed that GSH had reacted with the acetylene carbon atoms of these mGluR5 PAMs, suggesting a conjugate addition mechanism and implicating cytosolic and microsomal glutathione S-transferases (GSTs) in catalysis. Interestingly, five closely related mGluR5 PAMs were not similarly prone to the formation of GSH conjugates in vitro. These compounds also featured acetylenes, but were flanked by either phenyl or cyclohexyl rings, indicating that the formation of GSH conjugates was influenced by proximal functional groups that modulated the electron density of the triple bond and/or differences in enzyme-substrate specificity. These results informed an ongoing drug-discovery effort to identify mGluR5 PAMs with drug-like properties and a low risk of reactivity with endogenous thiols.