4-Hydroxymephenytoin D3 is the deuterium labeled 4-Hydroxymephenytoin. 4-Hydroxymephenytoin is a metabolism of an antiepileptic drug mephenytoin, which is used as a CYP2C19 substrate[1][2].

Stable heavy isotopes of hydrogen, carbon, and other elements have been incorporated into drug molecules, largely as tracers for quantitation during the drug development process. Deuteration has gained attention because of its potential to affect the pharmacokinetic and metabolic profiles of drugs[1].

[1]. Russak EM, et al. Impact of Deuterium Substitution on the Pharmacokinetics of Pharmaceuticals. Ann Pharmacother. 2019;53(2):211-216.
[2]. Tanaka M, et al. Simple and selective assay of 4-hydroxymephenytoin in human urine using solid-phase extraction and high-performance liquid chromatography with electrochemical detection and its preliminary application to phenotyping test. J Chromatogr B Biomed Appl. 1996 Feb 9;676(1):87-94.
[3]. Stewart NA, et al. A UPLC-MS/MS assay of the "Pittsburgh cocktail": six CYP probe-drug/metabolites from human plasma and urine using stable isotope dilution. Analyst. 2011 Feb 7;136(3):605-12.