In Vitro Hepatic Metabolism Explains Higher Clearance of Voriconazole in Children versus Adults: Role of CYP2C19 and Flavin-Containing Monooxygenase 3

Abstract

ABSTRACT:Voriconazole is a broad spectrum antifungal agent for treating life-threatening fungal infections. Its clearance is approximately 3-fold higher in children compared with adults. Voriconazole is cleared predominantly via hepatic metabolism in adults, mainly by CYP3A4, CYP2C19, and flavin-containing monooxygenase 3 (FMO3). In vitro metabolism of voriconazole by liver microsomes prepared from pediatric and adult tissues (n ‫؍‬ 6/group) mirrored the in vivo clearance differences in children versus adults, and it showed that the oxidative metabolism was significantly faster in children compared with adults as indicated by the in vitro half-life (T 1/2 ) of 33.8 ؎ 15.3 versus 72.6 ؎ 23.7 min, respectively. The K m for voriconazole metabolism to N-oxide, the major metabolite formed in humans, by liver microsomes from children and adults was similar (11 ؎ 5.2 versus 9.3 ؎ 3.6 M, respectively). In contrast, apparent V max was approximately 3-fold higher in children compared with adults (120.5 ؎ 99.9 versus 40 ؎ 13.9 pmol/min/mg). The calculated in vivo clearance from in vitro data was found to be approximately 80% of the observed plasma clearance values in both populations. Metabolism studies in which CYP3A4, CYP2C19, or FMO was selectively inhibited provided evidence that contribution of CYP2C19 and FMO toward voriconazole N-oxidation was much greater in children than in adults, whereas CYP3A4 played a larger role in adults. Although expression of CYP2C19 and FMO3 is not significantly different in children versus adults, these enzymes seem to contribute to higher metabolic clearance of voriconazole in children versus adults.

Voriconazole plasma levels in children are highly variable

“…These observations suggest that hepatic metabolism in children differs from that in adults, which was recently explored and confirmed by Yanni et al [10]. In this study, the in vitro metabolism of voriconazole by liver microsomes from 6 children between 2 and 10 years of age was compared with that of adults, to explore the role of the metabolizing hepatic enzymes CYP2C19, CYP3A4 and flavincontaining monooxygenase 3 (FMO3).…”

Quantification of Flavin-containing Monooxygenases 1, 3, and 5 in Human Liver Microsomes by UPLC-MRM-Based Targeted Quantitative Proteomics and Its Application to the Study of Ontogeny

“…In contrast, FMO3 expression, the major adult isozyme, turns on after birth and increases over time, reaching an adult level in the early teenage years (Koukouritaki et al, 2002). This differential enzyme expression has garnered much attention, specifically in terms of adjusting the dosage of FMO substrate drugs for infants and children (Yokoi, 2009;Yanni et al, 2010). FMO5 mRNA expression in adult liver exceeds that of FMO3 (Cashman and Zhang, 2006); however, earlier reports have suggested the opposite (Cashman, 1995(Cashman, , 2000.…”

Physiologically Based Pharmacokinetic Predictions of Tramadol Exposure Throughout Pediatric Life: an Analysis of the Different Clearance Contributors with Emphasis on CYP2D6 Maturation

“…As has been described above, the clearance prediction for this PBPK model is based on three hepatic components (CYP2D6, CYP3A4, and CYP2B6) and one renal component (GFR). Possible explanations for an underprediction are (i) incorrect maturation function for either of the implemented CYP enzymes, (ii) other metabolic pathways contributing to tramadol's metabolism in pediatric life (FMO, CYP3A7) (12,38), or (iii) neglecting other minor pathways in tramadol metabolism (10). Therefore, the total clearance of tramadol was decomposed in its constituents and compared to in vivo observations, when possible.…”

“…These assumptions should always be checked since they are key in the mechanistic prediction of pediatric PK from adult data. Violation of these assumptions may result in clearance under-or overprediction if the clearance pathways differ between children and adults (12), if enzyme processes become saturated or enzyme affinities decrease (e.g., due to the presence of free fatty acids or bilirubin), and if liver/kidney perfusion is altered (e.g., in the case of cirrhosis). Neglecting important transporter involvements might substantially over-or underpredict the clearance, depending on the specific transport mechanism at hand (13).…”