Species difference in glucuronidation formation kinetics with a selective mTOR inhibitor

Area of Science:

• Pharmacology
• Drug Metabolism
• Biochemistry

Background:

• The mammalian target of rapamycin (mTOR) is a key protein kinase implicated in age-related diseases and cancer.
• mTOR inhibitors are promising therapeutic agents, necessitating a thorough understanding of their metabolic profiles.
• Compound 1, a potent ATP-competitive mTOR inhibitor, was selected for metabolic investigation.

Purpose of the Study:

• To elucidate the metabolic elimination pathways of compound 1 in vivo using bile duct-cannulated rats.
• To compare the in vitro metabolism of compound 1 in liver microsomes across various species, including rat, dog, human, and cynomolgus monkey.
• To assess the species selectivity of N-glucuronidation and its potential implications for drug development and animal modeling.

Main Methods:

• Bile duct-cannulation in rats to study in vivo drug elimination.
• In vitro incubation of compound 1 with liver microsomes from rat, dog, human, and cynomolgus monkey.
• Identification and quantification of N-glucuronide metabolites using analytical techniques.
• Kinetic analysis of metabolite formation to determine reaction kinetics.

Main Results:

• Compound 1 undergoes extensive N-glucuronidation in rats, forming metabolites on the pyrazole moiety (N1, N2) and the primary amine.
• All three N-glucuronide metabolites were found in rat, dog, and human liver microsomes, but primary amine glucuronidation was absent in monkeys.
• Species-selective N-glucuronidation was observed, with N2 favored in rat, dog, and human, and N1 favored in monkeys. Monkey N-glucuronidation exhibited sigmoidal kinetics, distinguishing it from other species.
• These findings suggest that monkeys may not be ideal models for predicting human N-glucuronidation of UGT1A9 or UGT1A1 substrates.

Conclusions:

• N-glucuronidation is a significant metabolic pathway for compound 1, with notable species-specific differences.
• The observed metabolic profile, particularly in higher species like monkeys and humans, could lead to high clearance and impact systemic exposure.
• Medicinal chemistry strategies may be beneficial to reduce the metabolic liability of compound 1, thereby improving its pharmacokinetic properties for in vivo studies.