Flavin‐Containing Monooxygenase 3 Polymorphic Variants Significantly Affect Clearance of Tamoxifen and Clomiphene

Abstract

Human flavin-containing monooxygenase 3 (hFMO3) is a drug-metabolising enzyme that oxygenates many drugs and xenobiotics in the liver. This enzyme is also known to exhibit single nucleotide polymorphisms (SNPs) that can alter the rates of monooxygenation of therapeutic agents. The purpose of this study was to investigate the effect of the three common polymorphic variants of hFMO3 (V257M, E158K and E308G) on the metabolism and clearance of three structurally similar compounds: tamoxifen (breast cancer medication), clomiphene (infertility medication) and GSK5182 (antidiabetic lead molecule). For GSK5182, none of the three variants showed any significant differences in its metabolism when compared to the wild-type enzyme. In the case of clomiphene, two of the variants, V257M and E308G, exhibited a significant increase in all the kinetic parameters measured with nearly two times faster clearance. Finally, for tamoxifen, a mixed behaviour was observed; E158K variant showed a significantly higher clearance compared to the wild type, whereas V257M mutation had the opposite effect. Overall, the data obtained demonstrate that there is no direct correlation between the SNPs and the metabolism of these three hFMO3 substrates. The metabolic capacity is both variant-dependent and substrate-dependent and therefore when testing new drugs or administering already approved therapies, these differences should be taken into consideration.

Production of drug metabolites by human FMO3 in Escherichia coli

“…In this case, the human FMO3 shows no selectivity in its activity. The identification of the N-oxide products of clomiphene have been previously reported by our group [16].…”

“…N-oxide metabolites of GSK5182 and tozasertib are not commercially available however, these metabolites have been previously identified and confirmed by mass spectrometry analysis by our group [16,18].…”

“…4. There are currently no commercially available standards and therefore the N-oxide product identification was performed by LC-MS [16]. An overall conversion of 98.4% was obtained for this drug which corresponds to the maximum theoretical conversion yield ( Fig.…”

“…In all cases an isocratic elution was performed for the HPLC analyses. This meant that only the ratios of the two different mobile phases were modified for each of the 4 drugs; clomiphene (34% A and 66% B, monitored at 290 nm) [16], dasatinib (22% A and 78% B, monitored at 320 nm), GSK5182 (25% A and 75% B, monitored at 280 nm), and tozasertib (20% A and 80% B, monitored at 250 nm). Tamoxifen which was used for in the organic solvent as well as the truncated human FMO3 experiments, was also separated from its N-oxide product using the same mobile phases (40% A and 60% B and monitored at 276 nm).…”

“…GSK5182, a highly selective inverse agonist of estrogen-related receptor γ, is currently under preclinical investigation as a new anti-diabetic agent for type 2 diabetes mellitus [15]. N-demethylation and hydroxylation of this drug are carried out by cytochrome P450s whereas FMO3 contributes to the GSK5182 N-oxidation [16]. Tozasertib (VX-680, MK0457), a pan-aurora kinase inhibitor, is under evaluation as an anticancer drug.…”