An Investigation into the Prediction of In Vivo Clearance for a Range of Flavin-containing Monooxygenase Substrates

Abstract

Flavin-containing monooxygenases (FMO) are metabolic enzymes mediating the oxygenation of nucleophilic atoms such as nitrogen, sulfur, phosphorus, and selenium. These enzymes share similar properties to the cytochrome P450 system but can be differentiated through heat inactivation and selective substrate inhibition by methimazole. This study investigated 10 compounds with varying degrees of FMO involvement to determine the nature of the correlation between human in vitro and in vivo unbound intrinsic clearance. To confirm and quantify the extent of FMO involvement six of the compounds were investigated in human liver microsomal (HLM) in vitro assays using heat inactivation and methimazole substrate inhibition. Under these conditions FMO contribution varied from 21% (imipramine) to 96% (itopride). Human hepatocyte and HLM intrinsic clearance (CL) data were scaled using standard methods to determine the predicted unbound intrinsic clearance (predicted CL) for each compound. This was compared with observed unbound intrinsic clearance (observed CL) values back calculated from human pharmacokinetic studies. A good correlation was observed between the predicted and observed CL using hepatocytes ( = 0.69), with 8 of the 10 compounds investigated within or close to a factor of 2. For HLM the in vitro-in vivo correlation was maintained ( = 0.84) but the accuracy was reduced with only 3 out of 10 compounds falling within, or close to, twofold. This study demonstrates that human hepatocytes and HLM can be used with standard scaling approaches to predict the human in vivo clearance for FMO substrates.

Metabolism and Disposition of a Novel Selective α7 Neuronal Acetylcholine Receptor Agonist ABT-126 in Humans: Characterization of the Major Roles for Flavin-Containing Monooxygenases and UDP-Glucuronosyl Transferase 1A4 and 2B10 in Catalysis

“…Unfortunately, quantitative contribution of FMO1 and FMO3 to M1 formation and overall ABT-126 clearance is uncertain because of the lack of scaling factors or well established methods to extrapolate in vitro data to clinical observations. Recent progress using hepatocytes and heat deactivated or chemically inhibited HLMs is encouraging (Jones et al, 2017). Further studies are warranted to determine the inter-system extrapolation factor values for cDNA-expressed FMO1 and FMO3 to extrapolate activity from these recombinant systems to the activity present in HLMs or HKMs.…”

Utility of Chimeric Mice with Humanized Liver for Predicting Human Pharmacokinetics in Drug Discovery: Comparison with <i>in Vitro</i>–<i>in Vivo</i> Extrapolation and Allometric Scaling

“…They used hepatic and intestinal cytosol for SULT metabolism. Nishimuta et al 31) evaluated the prediction of human CL for eight CES 1 substrates using hepatocytes and S9. The mean value of predicted CL int,in vivo based on hepatocyte CL int, in vitro showed only 20% of the observed value.…”

An Investigation into the Prediction of the Plasma Concentration-Time Profile and Its Interindividual Variability for a Range of Flavin-Containing Monooxygenase Substrates Using a Physiologically Based Pharmacokinetic Modeling Approach

“…The PBPK model input parameters used are listed in Table 2. The in vitro parameters such as CL int , plasma protein binding (unbound fraction in plasma), and fraction unbound in hepatocyte incubation were experimentally measured (Jones et al, 2017); in some instances, the PBPK platform's prediction toolbox was employed to predict the drug-related properties such as the effective human permeability (P eff ) value using the mechanistic permeability model or the polar surface area/hydrogen bond donor model if permeability data were not available. The first-order absorption rate constant and fraction absorbed were estimated from clinical data or predicted based on the P eff value within the Simcyp simulator.…”

“…Intrinsic Clearance Measurement Using Hepatocytes and Human Liver Microsomes. The measured in vitro data are detailed in Supplemental Table 1, and for methodology refer to Jones et al (2017). The assignment of FMO metabolic intrinsic clearance and methodology employed to assess drug elimination are given Supplemental Table 2.…”

“…There is a lack of validated methods for extrapolating in vitro hepatic intrinsic clearance (CL int ) for FMO to in vitro/in vivo extrapolation (IVIVE) clearance (CL). Our earlier paper (Jones et al, 2017) investigated a set of 10 literature compounds with varying degrees of FMO involvement. The compounds were profiled in in vitro assays, which defined the extent of FMO involvement based on heat inactivation and inhibition by the selective substrate methimazole.…”