Atypical kinetic profiles in drug metabolism reactions

Interaction and Transport of Benzalkonium Chlorides by the Organic Cation and Multidrug and Toxin Extrusion Transporters

Humans are chronically exposed to benzalkonium chlorides (BACs) from environmental sources. The U.S. Food and Drug Administration (FDA) has recently called for additional BAC safety data, as these compounds are cytotoxic and have great potential for biochemical interactions. Biodistribution studies revealed that BACs extensively distribute to many tissues and accumulate at high levels, especially in the kidneys, but the underlying mechanisms are unclear. In this study, we characterized the interactions of BACs of varying alkyl chain length (C8 to C14) with the human organic cation transporters (hOCT1-3) and multidrug and toxin extrusion proteins (hMATE1/2K) with the goal to identify transporters that could be involved in BAC disposition. Using transporter-expressing cell lines, we showed that all BACs are inhibitors of hOCT1-3 and hMATE1/2K (IC50 ranging 0.83-25.8 μM). Further, the short-chain BACs (C8 and C10) were identified as substrates of these transporters. Interestingly, although BAC C8 displayed typical Michaelis-Menten kinetics, C10 demonstrated a more complex substrate-inhibition profile. Transwell studies with transfected Madin-Darby canine kidney cells revealed that intracellular accumulation of basally applied BAC C8 and C10 was substantially higher (8.2- and 3.7-fold, respectively) in hOCT2/hMATE1 double-transfected cells in comparison with vector-transfected cells, supporting a role of these transporters in mediating renal accumulation of these compounds in vivo. Together, our results suggest that BACs interact with hOCT1-3 and hMATE1/2K as both inhibitors and substrates and that these transporters may play important roles in tissue-specific accumulation and potential toxicity of short-chain BACs. Our findings have important implications for understanding human exposure and susceptibility to BACs due to environmental exposure. SIGNIFICANCE STATEMENT: Humans are systemically exposed to benzalkonium chlorides (BACs). These compounds broadly distribute through tissues, and their safety has been questioned by the FDA. Our results demonstrate that hOCT2 and hMATE1 contribute to the renal accumulation of BAC C8 and C10 and that hOCT1 and hOCT3 may be involved in the tissue distribution of these compounds. These findings can improve our understanding of BAC disposition and toxicology in humans, as their accumulation could lead to biochemical interactions and deleterious effects.

Kinetic Characterization of Estradiol Glucuronidation by Liver Microsomes and Expressed UGT Enzymes: The Effects of Organic Solvents

BACKGROUND AND OBJECTIVE

In vitro glucuronidation of 17β-estradiol (estradiol) is often performed to assess the role of uridine 5'-diphospho-glucuronosyltransferase 1A1 (UGT1A1) in xenobiotic/drug metabolism. The objective of this study was to determine the effects of four commonly used organic solvents [i.e., dimethyl sulfoxide (DMSO), methanol, ethanol, and acetonitrile] on the glucuronidation kinetics of estradiol, which can be glucuronidated at C3 and C17 positions.

METHODS

The impacts of organic solvents on estradiol glucuronidation were determined by using expressed UGT enzymes and liver microsomes from both human and animals.

RESULTS

In human liver microsomes (HLM), methanol, ethanol, and acetonitrile significantly altered estradiol glucuronidation kinetics with increased Vmax (up to 2.6-fold) and CLmax (up to 2.8-fold) values. Altered estradiol glucuronidation in HLM was deduced to be attributed to the enhanced metabolic activities of UGT1A1 and UGT2B7, whose activities differ at the two glucuronidation positions. The effects of organic solvents on estradiol glucuronidation were glucuronidation position-, isozyme-, and solvent-specific. Furthermore, both ethanol and acetonitrile have a greater tendency to modify the glucuronidation activity of estradiol in animal liver microsomes.

CONCLUSION

Organic solvents such as methanol, ethanol, and acetonitrile showed great potential in adjusting the glucuronidation of estradiol. DMSO is the most suitable solvent due to its minimal influence on estradiol glucuronidation. Researchers should be cautious in selecting appropriate solvents to get accurate results when assessing the metabolism of a new chemical entity.

Inhibitory effects of the main metabolites of Apatinib on CYP450 isozymes in human and rat liver microsomes

PURPOSE

The inhibitory effect of Apatinib on cytochrome P450 (CYP450) enzymes has been studied. However, it is unknown whether the inhibition is related to the major metabolites, M1-1, M1-2 and M1-6.

METHODS

A 5-in-1 cocktail system composed of CYP2B6/Cyp2b1, CYP2C9/Cyp2c11, CYP2E1/Cyp2e1, CYP2D6/Cyp2d1 and CYP3A/Cyp3a2 was used in this study. Firstly, the effects of APA and its main metabolites on the activities of HLMs, RLMs and recombinant isoforms were examined. The reaction mixture included HLMs, RLMs or recombinant isoforms (CYP3A4.1, CYP2D6.1, CYP2D6.10 or CYP2C9.1), analyte (APA, M1-1, M1-2 or M1-6), probe substrates. The reactions were pre-incubated for 5 min at 37 °C, followed by the addition of NAPDH to initiate the reactions, which continued for 40 min. Secondly, IC50 experiments were conducted to determine if the inhibitions were reversible. The reaction mixture of the "+ NADPH Group" included HLMs or RLMs, 0 to 100 of μM M1-1 or M1-2, probe substrates. The reactions were pre-incubated for 5 min at 37 °C, and then NAPDH was added to initiate reactions, which proceeded for 40 min. The reaction mixture of the "- NADPH Group" included HLMs or RLMs, probe substrates, NAPDH. The reactions were pre-incubated for 30 min at 37 °C, and then 0 to 100 μM of M1-1 or M1-2 was added to initiate the reactions, which proceeded for 40 min. Finally, the reversible inhibition of M1-1 and M1-2 on isozymes was determined. The reaction mixture included HLMs or RLMs, 0 to 10 μM of M1-1 or M1-2, probe substrates with concentrations ranging from 0.25Km to 2Km.

RESULTS

Under the influence of M1-6, the activity of CYP2B6, 2C9, 2E1 and 3A4/5 was increased to 193.92%, 210.82%, 235.67% and 380.12% respectively; the activity of CYP2D6 was reduced to 92.61%. The inhibitory effects of M1-1 on CYP3A4/5 in HLMs and on Cyp2d1 in RLMs, as well as the effect of M1-2 on CYP3A in HLMs, were determined to be noncompetitive inhibition, with the Ki values equal to 1.340 μM, 1.151 μM and 1.829 μM, respectively. The inhibitory effect of M1-1 on CYP2B6 and CYP2D6 in HLMs, as well as the effect of M1-2 on CYP2C9 and CYP2D6 in HLMs, were determined to be competitive inhibition, with the Ki values equal to 12.280 μM, 2.046 μM, 0.560 μM and 4.377 μM, respectively. The inhibitory effects of M1-1 on CYP2C9 in HLMs and M1-2 on Cyp2d1 in RLMs were determined to be mixed-type, with the Ki values equal to 0.998 μM and 0.884 μM. The parameters could not be obtained due to the atypical kinetics of CYP2E1 in HLMs under the impact of M1-2.

CONCLUSIONS

M1-1 and M1-2 exhibited inhibition for several CYP450 isozymes, especially CYP2B6, 2C9, 2D6 and 3A4/5. This observation may uncover potential drug-drug interactions and provide valuable insights for the clinical application of APA.

High-throughput assay to simultaneously evaluate activation of CYP3A and the direct and time-dependent inhibition of CYP3A, CYP2C9, and CYP2D6 using liquid chromatography-tandem mass spectrometry

In the early stages of drug discovery, adequate evaluation of the potential drug-drug interactions (DDIs) of drug candidates is important. Several CYP3A activators are known to lead to underestimation of DDIs. These compounds affect midazolam 1'-hydroxylation but not midazolam 4-hydroxylation.We used both metabolic reactions of midazolam to evaluate the activation and inhibition of CYP3A activators simultaneously. For our CYP inhibition assay using cocktail probe substrates, simultaneous liquid chromatography-tandem mass spectrometry monitoring of 1'-hydroxymidazolam and 4-hydroxymidazolam for CYP3A was established in addition to monitoring of 4-hydroxydiclofenac and 1'-hydroxybufuralol for CYP2C9 and CYP2D6.The results of our cocktail inhibition assay were well correlated with those of a single inhibition assay, as were the estimated inhibition parameters for typical CYP3A inhibitors. In our assay, a proprietary compound that activated midazolam 1'-hydroxylation and tended to inhibit 4-hydroxylation was evaluated along with known CYP3A activators. All compounds were well characterised by comparison of the results of midazolam 1'- and 4-hydroxylation.In conclusion, our CYP cocktail inhibition assay can detect CYP3A activation and assess the direct and time-dependent inhibition potentials for CYP3A, CYP2C9, and CYP2D6. This method is expected to be very efficient in the early stages of drug discovery.

A Review of CYP-Mediated Drug Interactions: Mechanisms and In Vitro Drug-Drug Interaction Assessment

Drug metabolism is a major determinant of drug concentrations in the body. Drug-drug interactions (DDIs) caused by the co-administration of multiple drugs can lead to alteration in the exposure of the victim drug, raising safety or effectiveness concerns. Assessment of the DDI potential starts with in vitro experiments to determine kinetic parameters and identify risks associated with the use of comedication that can inform future clinical studies. The diverse range of experimental models and techniques has significantly contributed to the examination of potential DDIs. Cytochrome P450 (CYP) enzymes are responsible for the biotransformation of many drugs on the market, making them frequently implicated in drug metabolism and DDIs. Consequently, there has been a growing focus on the assessment of DDI risk for CYPs. This review article provides mechanistic insights underlying CYP inhibition/induction and an overview of the in vitro assessment of CYP-mediated DDIs.

The molecular basis of dapsone activation of CYP2C9-catalyzed nonsteroidal anti-inflammatory drug oxidation

Positive heterotropic cooperativity, or "activation," results in an instantaneous increase in enzyme activity in the absence of an increase in protein expression. Thus, cytochrome P450 (CYP) enzyme activation presents as a potential drug-drug interaction mechanism. It has been demonstrated previously that dapsone activates the CYP2C9-catalyzed oxidation of a number of nonsteroidal anti-inflammatory drugs in vitro. Here, we conducted molecular dynamics simulations (MDS) together with enzyme kinetic investigations and site-directed mutagenesis to elucidate the molecular basis of the activation of CYP2C9-catalyzed S-flurbiprofen 4'-hydroxylation and S-naproxen O-demethylation by dapsone. Supplementation of incubations of recombinant CYP2C9 with dapsone increased the catalytic efficiency of flurbiprofen and naproxen oxidation by 2.3- and 16.5-fold, respectively. MDS demonstrated that activation arises predominantly from aromatic interactions between the substrate, dapsone, and the phenyl rings of Phe114 and Phe476 within a common binding domain of the CYP2C9 active site, rather than involvement of a distinct effector site. Mutagenesis of Phe114 and Phe476 abrogated flurbiprofen and naproxen oxidation, and MDS and kinetic studies with the CYP2C9 mutants further identified a pivotal role of Phe476 in dapsone activation. MDS additionally showed that aromatic stacking interactions between two molecules of naproxen are necessary for binding in a catalytically favorable orientation. In contrast to flurbiprofen and naproxen, dapsone did not activate the 4'-hydroxylation of diclofenac, suggesting that the CYP2C9 active site favors cooperative binding of nonsteroidal anti-inflammatory drugs with a planar or near-planar geometry. More generally, the work confirms the utility of MDS for investigating ligand binding in CYP enzymes.

In Vitro Enzyme Kinetics and NMR-Based Product Elucidation for Glutathione S-Conjugation of the Anticancer Unsymmetrical Bisacridine C-2028 in Liver Microsomes and Cytosol: Major Role of Glutathione S-Transferase M1-1 Isoenzyme

This work is the next step in studying the interplay between C-2028 (anticancer-active unsymmetrical bisacridine developed in our group) and the glutathione S-transferase/glutathione (GST/GSH) system. Here, we analyzed the concentration- and pH-dependent GSH conjugation of C-2028 in rat liver microsomes and cytosol. We also applied three recombinant human GST isoenzymes, which altered expression was found in various tumors. The formation of GSH S-conjugate of C-2028 in liver subfractions followed Michaelis-Menten kinetics. We found that C-2028 was conjugated with GSH preferentially by GSTM1-1, revealing a sigmoidal kinetic model. Using a colorimetric assay (MTT test), we initially assessed the cellular GST/GSH-dependent biotransformation of C-2028 in relation to cytotoxicity against Du-145 human prostate cancer cells in the presence or absence of the modulator of GSH biosynthesis. Pretreatment of cells with buthionine sulfoximine resulted in a cytotoxicity decrease, suggesting a possible GSH-mediated bioactivation process. Altogether, our results confirmed the importance of GSH conjugation in C-2028 metabolism, which humans must consider when planning a treatment strategy. Finally, nuclear magnetic resonance spectroscopy elucidated the structure of the GSH-derived product of C-2028. Hence, synthesizing the compound standard necessary for further advanced biological and bioanalytical investigations will be achievable.

Partial Reversible Inhibition of Enzymes and Its Metabolic and Pharmaco-Toxicological Implications

Partial reversible inhibition of enzymes, also called hyperbolic inhibition, is an uncommon mechanism of reversible inhibition, resulting from a productive enzyme-inhibitor complex. This type of inhibition can involve competitive, mixed, non-competitive and uncompetitive inhibitors. While full reversible inhibitors show linear plots for reciprocal enzyme initial velocity versus inhibitor concentration, partial inhibitors produce hyperbolic plots. Similarly, dose-response curves show residual fractional activity of enzymes at high doses. This article reviews the theory and methods of analysis and discusses the significance of this type of reversible enzyme inhibition in metabolic processes, and its implications in pharmacology and toxicology.

How does multiple substrate binding lead to substrate inhibition of CYP2D6 metabolizing dextromethorphan? A theoretical study

CYP2D6 is one of the most important metalloenzymes involved in the biodegradation of many drug molecules in the human body. It has been found that multiple substrate binding can lead to substrate inhibition of CYP2D6 metabolizing dextromethorphan (DM), but the corresponding theoretical mechanism is rarely reported. Therefore, we chose DM as the probe and performed molecular dynamics simulations and quantum mechanical calculations on CYP2D6-DM systems to investigate the mechanism of how the multiple substrate binding leads to the substrate inhibition of CYP2D6 metabolizing substrates. According to our results, three gate residues (Arg221, Val374, and Phe483) for the catalytic pocket are determined. We also found that the multiple substrate binding can lead to substrate inhibition by reducing the stability of CYP2D6 binding DM and increasing the reactive activation energy of the rate-determining step. Our findings would help to understand the substrate inhibition of CYP2D6 metabolizing the DM and enrich the knowledge of the drug-drug interactions for the cytochrome P450 superfamily.

Impact of intestinal microbiota on metabolic toxicity and potential detoxification of amygdalin

Amygdalin (Amy) is metabolized into cyanide in vivo, which may lead to fatal poisoning after oral administration. The defense mechanisms against toxic cyanide have not yet been adequately studied. In this study, comparative toxicokinetics study of Amy was performed in normal and pseudo germ-free rats. The efficiency of cyanide release was significant higher in normal group when given a single oral dose of 440 mg/kg (50% median lethal dose). Thiocyanate, the detoxification metabolite, was firstly detected in feces, caecum, and intestinal microbiota incubation enzymic system. The results suggest intestinal microbiota is involved in bidirectional regulation of toxicity and detoxification of Amy. We further identified the species related to cyanogenesis of Amy with metagenomic sequencing, such as Bifidobacterium pseudolongum, Marvinbryantia formatexigens, and Bacteroides fragilis. Functional analysis of microbiota reveals the detoxification potential of intestinal microbiota for cyanide. Sulfurtransferase superfamily, such as rhodanese, considered as main detoxification enzymes for cyanide, are largely found in Coriobacteriaceae bacterium, Butyricicoccus porcorum, Akkermansia muciniphila, etc. Besides, cyanoamino acid metabolism pathway dominated by Escherichia coli may contribute to the detoxification metabolism of cyanide. In summary, intestinal microbiota may be the first line of defense against the toxicity induced by Amy.

Biotransformation of phenytoin in the electrochemically-driven CYP2C19 system

The possibility of the detection of atypical kinetic profiles of drug biotransformation using electrochemical systems based on immobilized cytochromes P450 with phenytoin hydroxylation by cytochrome P450 2C19 (CYP2C19) as an example was evaluated for the first time. For this purpose, we developed an electrochemical system, where one of the electrodes was modified by didodecyldimethylammonium bromide (DDAB) and was used as an electron donor for reduction of heme iron ion of the immobilized CYP2C19 and initiation of the catalytic reaction, while the second electrode was not modified and served for an electrochemical quantitation of 4-hydroxyphenytoin, which is a metabolite of antiepileptic drug phenytoin, by its oxidation peak. It was revealed that the dependence of the rate of 4-hydroxyphenytoin formation on phenytoin concentration is described by the equation for two enzymes or two binding sites indicating the existing of high- and low-affinity forms of the enzyme. The atypical kinetics and the kinetic parameters of CYP2C19-mediated phenytoin hydroxylation in the electrochemical system correlate to the same characteristics obtained by other authors in an alternative enzymatic system. Our results demonstrate the possibility of electrochemical systems based on cytochromes P450 to be applied for the detection of atypical kinetic profiles of drug metabolism.

Sulfation and Its Effect on the Bioactivity of Magnolol, the Main Active Ingredient of Magnolia Officinalis

Magnolol, the main active ingredient of Magnolia officinalis, has been reported to display anti-inflammatory activity. Sulfation plays an important role in the metabolism of magnolol. The magnolol sulfated metabolite was identified by the ultra-performance liquid chromatography to quadrupole time-of-flight mass spectrometry (UPLC-Q-TOF-MS) and a proton nuclear magnetic resonance (1H-NMR). The magnolol sulfation activity of seven major recombinant sulfotransferases (SULTs) isoforms (SULT1A1*1, SULT1A1*2, SULT1A2, SULT1A3, SULT1B1, SULT1E1, and SULT2A1) was analyzed. The metabolic profile of magnolol was investigated in liver S9 fractions from human (HLS9), rat (RLS9), and mouse (MLS9). The anti-inflammatory effects of magnolol and its sulfated metabolite were evaluated in RAW264.7 cells stimulated by lipopolysaccharide (LPS). Magnolol was metabolized into a mono-sulfated metabolite by SULTs. Of the seven recombinant SULT isoforms examined, SULT1B1 exhibited the highest magnolol sulfation activity. In liver S9 fractions from different species, the CLint value of magnolol sulfation in HLS9 (0.96 µL/min/mg) was similar to that in RLS9 (0.99 µL/min/mg) but significantly higher than that in MLS9 (0.30 µL/min/mg). Magnolol and its sulfated metabolite both significantly downregulated the production of inflammatory mediators (IL-1β, IL-6 and TNF-α) stimulated by LPS (p < 0.001). These results indicated that SULT1B1 was the major enzyme responsible for the sulfation of magnolol and that the magnolol sulfated metabolite exhibited potential anti-inflammatory effects.

Cytochromes P450 2C8 and 3A Catalyze the Metabolic Activation of the Tyrosine Kinase Inhibitor Masitinib

Masitinib is a small molecule tyrosine kinase inhibitor under investigation for the treatment of amyotrophic lateral sclerosis, mastocytosis, and COVID-19. Hepatotoxicity has been reported in some patients while taking masitinib. The liver injury is thought to involve hepatic metabolism of masitinib by cytochrome P450 (P450) enzymes to form chemically reactive, potentially toxic metabolites. The goal of the current investigation was to determine the P450 enzymes involved in the metabolic activation of masitinib in vitro. In initial studies, masitinib (30 μM) was incubated with pooled human liver microsomes in the presence of NADPH and potassium cyanide to trap reactive iminium ion metabolites as cyano adducts. Masitinib metabolites and cyano adducts were analyzed using reversed-phase liquid chromatography-tandem mass spectrometry. The primary active metabolite, N-desmethyl masitinib (M485), and several oxygenated metabolites were detected along with four reactive metabolite cyano adducts (MCN510, MCN524, MCN526, and MCN538). To determine which P450 enzymes were involved in metabolite formation, reaction phenotyping experiments were conducted by incubation of masitinib (2 μM) with a panel of recombinant human P450 enzymes and by incubation of masitinib with human liver microsomes in the presence of P450-selective chemical inhibitors. In addition, enzyme kinetic assays were conducted to determine the relative kinetic parameters (apparent K_m and V_max) of masitinib metabolism and cyano adduct formation. Integrated analysis of the results from these experiments indicates that masitinib metabolic activation is catalyzed primarily by P450 3A4 and 2C8, with minor contributions from P450 3A5 and 2D6. These findings provide further insight into the pathways involved in the generation of reactive, potentially toxic metabolites of masitinib. Future studies are needed to evaluate the impact of masitinib metabolism on the toxicity of the drug in vivo.

Comparison of Experimental Strategies to Study l-Type Amino Acid Transporter 1 (LAT1) Utilization by Ligands

l-Type amino acid transporter 1 (LAT1), expressed abundantly in the brain and placenta and overexpressed in several cancer cell types, has gained a lot of interest in drug research and development, as it can be utilized for brain-targeted drug delivery, as well as inhibiting the essential amino acid supply to cancer cells. The structure of LAT1 is today very well-known and the interactions of ligands at the binding site of LAT1 can be modeled and explained. However, less is known of LAT1′s life cycle within the cells. Moreover, the functionality of LAT1 can be measured by several different methods, which may vary between the laboratories and make the comparison of the results challenging. In the present study, the usefulness of indirect cis-inhibition methods and direct cellular uptake methods and their variations to interpret the interactions of LAT1-ligands were evaluated. Moreover, this study also highlights the importance of understanding the intracellular kinetics of LAT1-ligands, and how they can affect the regular function of LAT1 in critical tissues, such as the brain. Hence, it is discussed herein how the selected methodology influences the outcome and created knowledge of LAT1-utilizing compounds.

Complex Cytochrome P450 Kinetics Due to Multisubstrate Binding and Sequential Metabolism. Part 1. Theoretical Considerations

Complexities in P450-mediated metabolism kinetics include multisubstrate binding, multiple-product formation, and sequential metabolism. Saturation curves and intrinsic clearances were simulated for single-substrate and multisubstrate models using derived velocity equations and numerical solutions of ordinary differential equations (ODEs). Multisubstrate models focused on sigmoidal kinetics because of their dramatic impact on clearance predictions. These models were combined with multiple-product formation and sequential metabolism, and simulations were performed with random error. Use of single-substrate models to characterize multisubstrate data can result in inaccurate kinetic parameters and poor clearance predictions. Comparing results for use of standard velocity equations with ODEs clearly shows that ODEs are more versatile and provide better parameter estimates. It would be difficult to derive concentration-velocity relationships for complex models, but these relationships can be easily modeled using numerical methods and ODEs. SIGNIFICANCE STATEMENT: The impact of multisubstrate binding, multiple-product formation, and sequential metabolism on the P450 kinetics was investigated. Numerical methods are capable of characterizing complicated P450 kinetics.

Emerging investigator series: human CYP2A6 catalyzes the oxidation of 6:2 fluorotelomer alcohol

The biotransformation of 6:2 fluorotelomer alcohol (6:2 FTOH) results in the production of bioactive and persistent metabolites, including perfluorinated carboxylic acids (PFCAs). While the products of 6:2 FTOH metabolism have been elucidated in several animal models, the responsible cytochrome P450 (CYP) isoform(s) have not been reported. Here, we characterized the in vitro oxidation of 6:2 FTOH using human liver microsomes and recombinant human CYPs. Six major xenobiotic metabolizing CYPs were screened for their capacity to catalyze 6:2 FTOH oxidation using chemical inhibitors selective towards CYP isoforms. Of the CYP isoforms investigated, CYP2A6 was the only enzyme capable of catalyzing 6:2 FTOH in human liver microsomes, with K_M and V_max values of 4076 ng mL^-1 and 69 ng mL^-1 min^-1, respectively. We further probed the metabolic mechanism by plotting the 6:2 FTOH kinetic profile and extrapolating data to several possible kinetic models. 6:2 FTOH oxidation followed the typical one-site Michaelis-Menten kinetic model. This study also reports that 6:2 FTOH loss is associated with active CYP2A6 by incubating microsomes with the selective CYP2A6 inhibitor tranylcypromine, which bound competitively to the enzyme as determined by an increased K_M (8796 ng mL^-1) but unchanged V_max value. Collectively, these findings provide a mechanistic perspective on the potential importance of CYP2A6 in the metabolic activation and phase I elimination of 6:2 FTOH and indirect human exposure to PFCAs.

The detoxification effect of cytochrome P450 3A4 on gelsemine-induced toxicity

Gelsemine (GA), the principal alkaloid in Gelsemium elegans Benth, exhibits potent and specific antinociception in chronic pain without the induction of apparent tolerance. However, GA also exerts neurotoxicity and hepatotoxicity when overdosed, and potential detoxification pathways are urgently needed. Cytochrome P450 enzymes (CYPs) are important phase I enzymes involved in the detoxification of xenobiotic compounds. The study aimed to investigate the role of CYPs-mediated metabolism in GA-induced toxicity. Microsomes, chemical special inhibitors and human recombinant CYPs indicated that GA was mainly metabolized by CYP3A4/5. The major metabolite of GA was isolated and identified as 4-N-demethyl-GA by high-resolution mass spectrometry and nuclear magnetic resonance technology. The CYP3A4 inhibitor ketoconazole significantly inhibited the metabolism of GA. This drastically increased GA toxicity which is caused by increasing the level of malondialdehyde and decreasing the level of the superoxide dismutase in mice. In contrast, the CYP3A4 inducer dexamethasone significantly increased GA metabolism and markedly decreased GA toxicity in mice. Notably, in CYP3A4-humanized mice, the toxicity of GA was significantly reduced compared to normal mice. These findings demonstrated that CYP3A4-mediated metabolism is a robust detoxification pathway for GA-induced toxicity.

Cytochrome P450-Catalyzed Metabolism of Cannabidiol to the Active Metabolite 7-Hydroxy-Cannabidiol

Cannabidiol (CBD) is a naturally occurring nonpsychotoxic phytocannabinoid that has gained increasing attention as a popular consumer product and for its use in Food and Drug Administration-approved Epidiolex (CBD oral solution) for the treatment of Lennox-Gastaut syndrome and Dravet syndrome. CBD was previously reported to be metabolized primarily by CYP2C19 and CYP3A4, with minor contributions from UDP-glucuronosyltransferases. 7-Hydroxy-CBD (7-OH-CBD) is the primary active metabolite with equipotent activity compared with CBD. Given the polymorphic nature of CYP2C19, we hypothesized that variable CYP2C19 expression may lead to interindividual differences in CBD metabolism to 7-OH-CBD. The objectives of this study were to further characterize the roles of cytochrome P450 enzymes in CBD metabolism, specifically to the active metabolite 7-OH-CBD, and to investigate the impact of CYP2C19 polymorphism on CBD metabolism in genotyped human liver microsomes. The results from reaction phenotyping experiments with recombinant cytochrome P450 enzymes and cytochrome P450-selective chemical inhibitors indicated that both CYP2C19 and CYP2C9 are capable of CBD metabolism to 7-OH-CBD. CYP3A played a major role in CBD metabolic clearance via oxidation at sites other than the 7-position. In genotyped human liver microsomes, 7-OH-CBD formation was positively correlated with CYP2C19 activity but was not associated with CYP2C19 genotype. In a subset of single-donor human liver microsomes with moderate to low CYP2C19 activity, CYP2C9 inhibition significantly reduced 7-OH-CBD formation, suggesting that CYP2C9 may play a greater role in CBD 7-hydroxylation than previously thought. Collectively, these data indicate that both CYP2C19 and CYP2C9 are important contributors in CBD metabolism to the active metabolite 7-OH-CBD. SIGNIFICANCE STATEMENT: This study demonstrates that both CYP2C19 and CYP2C9 are involved in CBD metabolism to the active metabolite 7-OH-CBD and that CYP3A4 is a major contributor to CBD metabolism through pathways other than 7-hydroxylation. 7-OH-CBD formation was associated with human liver microsomal CYP2C19 activity, but not CYP2C19 genotype, and CYP2C9 was found to contribute significantly to 7-OH-CBD generation. These findings have implications for patients taking CBD who may be at risk for clinically important cytochrome P450-mediated drug interactions.

Development of a microplate method for the determination of hepatic UDP-glucuronosyltransferase activity in rainbow trout (Oncorhynchus mykiss) and Nile tilapia (Oreochromis niloticus)

Hepatic glucuronidation represents an important phase II biotransformation reaction in both mammals and fish. The kinetic characteristics of uridine 5'-diphosphate (UDP) glucuronosyltransferases (UDPGTs) in rainbow trout liver microsomes were examined using p-nitrophenol (p-NP) as an aglycone and UDP-glucuronic acid (UDPGA) as a glucuronyl donor according to an existing protocol. The kinetic data obtained with varying concentrations of p-NP best fit the Hill equation and UDPGT activity was successfully induced following an i.p. injection of β-naphthoflavone (β-NF). The assay was subsequently adapted to a microplate method for determination of UDPGT activity in microsomal samples obtained from rainbow trout as well as Nile tilapia. In contrast to rainbow trout, UDPGT activity of Nile tilapia was best described by Michaelis-Menten kinetics. Based on the linearity of p-NP glucuronide formation, a p-NP concentration of 0.60 mM and a UDPGA concentration of 6.89 mM were determined to be suitable for assaying UDPGT activity in samples from rainbow trout and Nile tilapia. The microplate method offers several advantages over the historical assay; most notably it enables the observation of successive kinetics which ensures that enzyme activity is calculated in the most linear (initial) rate of the reaction. It also provides practical advantages in terms of ease-of-use and efficiency. This may be relevant to researchers investigating exposure of wild or farmed fish to environmental or feed-borne contaminants which are substrates of UDPGTs.

Neurosteroids: Structure-Uptake Relationships and Computational Modeling of Organic Anion Transporting Polypeptides (OATP)1A2

In this study, we investigated the delivery of synthetic neurosteroids into MCF-7 human breast adenocarcinoma cells via Organic Anionic Transporting Polypeptides (OATPs) (pH 7.4 and 5.5) to identify the structural components required for OATP-mediated cellular uptake and to get insight into brain drug delivery. Then, we identified structure-uptake relationships using in-house developed OATP1A2 homology model to predict binding sites and modes for the ligands. These binding modes were studied by molecular dynamics simulations to rationalize the experimental results. Our results show that carboxylic acid needs to be at least at 3 carbon-carbon bonds distance from amide bond at the C-3 position of the androstane skeleton and have an amino group to avoid efflux transport. Replacement of hydroxyl group at C-3 with any of the 3, 4, and 5-carbon chained terminal carboxylic groups improved the affinity. We attribute this to polar interactions between carboxylic acid and side-chains of Lys33 and Arg556. The additional amine group showed interactions with Glu172 and Glu200. Based on transporter capacities and efficacies, it could be speculated that the functionalization of acetyl group at the C-17 position of the steroidal skeleton might be explored further to enable OAT1A2-mediated delivery of neurosteroids into the cells and also across the blood-brain barrier.

Fundamentals of Enzyme Kinetics: Michaelis-Menten and Non-Michaelis-Type (Atypical) Enzyme Kinetics

This chapter will provide a general introduction to the kinetics of enzyme-catalyzed reactions, including a general discussion of catalysts, reaction rates, and binding constants. This section will be followed by a discussion of various types of enzyme kinetics observed in drug metabolism reactions. A large number of enzymatic reactions can be adequately described by Michaelis-Menten kinetics. The Michaelis-Menten equation represents a rectangular hyperbola, with a y-asymptote at the V_max value. However, in other cases, more complex kinetic models are required to explain the observed data. Atypical kinetic profiles are believed to arise from the simultaneous binding of multiple molecules within the active site of the enzyme (Tracy and Hummel, Drug Metab Rev 36:231-242, 2004). Several cytochromes P450 (CYPs) have large active sites that enable binding of multiple molecules (Yano et al., J Biol Chem 279:38091-38094, 2004; Wester et al., J Biol Chem 279:35630-35637, 2004). Thus, atypical kinetics are not uncommon in in vitro drug metabolism studies.

Case Study 1: Practical Considerations with Experimental Design and Interpretation

This chapter deals with practical considerations on key issues such as choosing an enzyme source, determining linear conditions, and choosing appropriate substrate and organic solvent concentrations. Practical solutions for working with limited resources and carrying out inhibition experiments are also addressed. Thus, after reading this chapter, the novice reader should have a better idea of how to design, develop, and interpret basic experiments using drug metabolism enzymes.

Numerical Methods for Modeling Enzyme Kinetics

Differential equations are used to describe time-dependent changes in enzyme kinetics and pharmacokinetics. Analytical and numerical methods can be used to solve differential equations. This chapter describes the use of numerical methods in solving differential equations and its applications in characterizing the complexities observed in enzyme kinetics. A discussion is included on the use of numerical methods to overcome limitations of explicit equations in the analysis of metabolism kinetics, reversible inhibition kinetics, and inactivation kinetics. The chapter describes the advantages of using numerical methods when Michaelis-Menten assumptions do not hold.

Case Study 4: Application of Basic Enzyme Kinetics to Metabolism Studies-Real-Life Examples

An appreciation of enzyme kinetic principles can be applied in a number of drug metabolism applications. The concept for this chapter arose from a simple discussion on selecting appropriate time points to most efficiently assess metabolite profiles in a human Phase 1a clinical study (Subheading 4). By considering enzyme kinetics, a logical approach to the issue was derived. The dialog was an important learning opportunity for the participants in the discussion, and we have endeavored to capture this experience with other questions related to determination of K_m and V_max parameters, a consideration of the value of hepatocytes vs. liver microsomes, and enzyme inhibition parameters.

Improved l-Type amino acid transporter 1 (LAT1)-mediated delivery of anti-inflammatory drugs into astrocytes and microglia with reduced prostaglandin production

Non-steroidal anti-inflammatory drugs (NSAIDs) can have protective effects in the brain by inhibition of cyclooxygenases (COX). However, the delivery into the brain across the blood-brain barrier (BBB) and particularly into the brain parenchymal cells is hindered. Therefore, in the present study, we developed four l-type amino acid transporter 1 (LAT1)-utilizing prodrugs of flurbiprofen, ibuprofen, naproxen, and ketoprofen, since LAT1 is expressed on both, the BBB endothelial cells as well as parenchymal cells. The cellular uptake and utilization of LAT1 by novel prodrugs were studied in mouse cortical primary astrocytes and immortalized microglia (BV2), and the release of the parent NSAID in several tissue and cell homogenates. Finally, the effects of the studied prodrugs on prostaglandin E₂ (PGE₂) production and cell viability were explored. The gained results showed that all four prodrugs were carried into their target cells via LAT1. They also released their parent NSAIDs via carboxylesterases (CES) and most likely also other un-identified enzymes, which need to be carefully considered when administrating these compounds orally or intravenously. Most importantly, all the studied prodrugs reduced the PGE₂ production in astrocytes and microglia after lipopolysaccharide (LPS)-induced inflammation by 29-94% and without affecting the cell viability with the studied concentration (20 µM).

Targeting of Perforin Inhibitor into the Brain Parenchyma Via a Prodrug Approach Can Decrease Oxidative Stress and Neuroinflammation and Improve Cell Survival

The cytolytic protein perforin has a crucial role in infections and tumor surveillance. Recently, it has also been associated with many brain diseases, such as neurodegenerative diseases and stroke. Therefore, inhibitors of perforin have attracted interest as novel drug candidates. We have previously reported that converting a perforin inhibitor into an L-type amino acid transporter 1 (LAT1)-utilizing prodrug can improve the compound's brain drug delivery not only across the blood-brain barrier (BBB) but also into the brain parenchymal cells: neurons, astrocytes, and microglia. The present study evaluated whether the increased uptake into mouse primary cortical astrocytes and subsequently improvements in the cellular bioavailability of this brain-targeted perforin inhibitor prodrug could enhance its pharmacological effects, such as inhibition of production of caspase-3/-7, lipid peroxidation products and prostaglandin E2 (PGE2) in the lipopolysaccharide (LPS)-induced neuroinflammation mouse model. It was demonstrated that increased brain and cellular drug delivery could improve the ability of perforin inhibitors to elicit their pharmacological effects in the brain at nano- to picomolar levels. Furthermore, the prodrug displayed multifunctional properties since it also inhibited the activity of several key enzymes related to Alzheimer's disease (AD), such as the β-site amyloid precursor protein (APP) cleaving enzyme 1 (BACE1), acetylcholinesterase (AChE), and most probably also cyclooxygenases (COX) at micromolar concentrations. Therefore, this prodrug is a potential drug candidate for preventing Aβ-accumulation and ACh-depletion in addition to combatting neuroinflammation, oxidative stress, and neural apoptosis within the brain. Graphical abstract.

Metabolic mechanism and anti-inflammation effects of sinomenine and its major metabolites N-demethylsinomenine and sinomenine-N-oxide

AIMS

Sinomenine (SIN) is clinically used as an anti-rheumatic drug. However, the metabolic and pharmacological mechanisms of SIN combined with its metabolites are unclear. This study aims to explore the cyclic metabolic mechanism of SIN, the anti-inflammation effects of SIN and its major metabolites (N-demethylsinomenine (DS) and sinomenine-N-oxide (SNO)), and the oxidation property of SNO.

MATERIALS AND METHODS

SIN was administrated to rats via gavage. Qishe pills (a SIN-containing drug) were orally administrated to humans. The bio-samples were collected to identify SIN's metabolites. Enzymatic and non-enzymatic incubations were used to reveal SIN's metabolic mechanism. Impacts of SIN, SNO and DS on the inflammation-related cytokine's levels and nuclear translocation of NF-κB were evaluated in LPS-induced Raw264.7 cells. ROS induced by SNO (10 μM) was also assessed.

KEY FINDINGS

CYP3A4 and ROS predominantly mediated the formation of SNO, and CYP3A4 and CYP2C19 primarily mediated the formation of DS. Noteworthily, SNO underwent N-oxide reduction both enzymatically, by xanthine oxidase (XOD), and non-enzymatically, by ferrous ion and heme moiety. The levels of IL-6 and TNF-α and nuclear translocation of NF-κB were ameliorated after pretreatment of SIN in LPS-induced Raw264.7 cells, while limited attenuations were observed after pretreatment of DS (SNO) even at 200 μM. In contrast, SNO induced ROS production.

SIGNIFICANCE

This study elucidated that SIN underwent both enzymatic and non-enzymatic cyclic metabolism and worked as the predominant anti-inflammation compound, while SNO induced ROS production, suggesting more studies of SIN combined with SNO and DS are necessary in case of DDI and potential toxicities.

Effect of Polyethylene Glycol-Induced Molecular Crowding on the Enzymatic Activity and Thermal Stability of β-Galactosidase from Kluyveromyces lactis

Here, we report the effect of polyethylene glycol (PEG₆₀₀₀)-induced molecular crowding (MC) on the catalytic activity and thermal stability of Kluyveromyces lactis β-galactosidase (β-Gal). The β-Gal-catalyzed hydrolysis of o-nitrophenyl-β-d-galactopyranoside followed a Michaelian kinetics at [PEG₆₀₀₀] ≤ 25% w/v and positive cooperativity at higher concentrations (35% w/v PEG₆₀₀₀). Compared with dilute solutions, in the MC media, β-Gal exhibited stronger thermal stability, as shown by the increase in the residual activity recovered after preincubation at high temperatures (e.g., 45 °C) and by the slower inactivation kinetics. Considering the effects of water thermodynamic activity on the reaction kinetics and protein structure and the effect of the exclusion volume on protein conformation, we suggest that changes in the protein oligomerization state and hydration could be the responsible for the behavior observed at the highest MC levels assayed. These results could be relevant and should be taken into account in industrial food processes applying β-Gal from K. lactis.

Glucuronidation and its effect on the bioactivity of amentoflavone, a biflavonoid from Ginkgo biloba leaves

OBJECTIVES

Ginkgo biloba leaves contain amentoflavone (AMF), a dietary flavonoid that possesses antioxidant and anticancer activity. Flavonoids are extensively subjected to glucuronidation. This study aimed to determine the metabolic profile of AMF and the effect of glucuronidation on AMF bioactivity.

METHODS

A pharmacokinetic study was conducted to determine the plasma concentrations of AMF and its metabolites. The metabolic profile of AMF was elucidated using different species of microsomes. The antioxidant activity of AMF metabolites was determined using DPPH/ABTS radical and nitric oxide assays. The anticancer activity of AMF metabolites was evaluated in U87MG/U251 cells.

KEY FINDINGS

Pharmacokinetic studies indicated that the oral bioavailability of AMF was 0.06 ± 0.04%, and the area under the curve of the glucuronidated AMF metabolites (410.938 ± 62.219 ng/ml h) was significantly higher than that of AMF (194.509 ± 16.915 ng/ml h). UGT1A1 and UGT1A3 greatly metabolized AMF. No significant difference was observed in the antioxidant activity between AMF and its metabolites. The anticancer activity of AMF metabolites significantly decreased.

CONCLUSIONS

A low AMF bioavailability was due to extensive glucuronidation, which was mediated by UGT1A1 and UGT1A3. Glucuronidated AMF metabolites had the same antioxidant but had a lower anticancer activity than that of AMF.

Astrocyte-Targeted Transporter-Utilizing Derivatives of Ferulic Acid Can Have Multifunctional Effects Ameliorating Inflammation and Oxidative Stress in the Brain

Ferulic acid (FA) is a natural phenolic antioxidant, which can exert also several other beneficial effects to combat neuroinflammation and neurodegenerative diseases, such as Alzheimer's disease. One of these properties is the inhibition of several enzymes and factors, such as β-site amyloid precursor protein (APP) cleaving enzyme 1 (BACE1), cyclooxygenases (COXs), lipoxygenases (LOXs), mammalian (or mechanistic) target for rapamycin (mTOR), and transcription factor NF-κB. We have previously synthesized three L-type amino acid transporter 1- (LAT1-) utilizing FA-derivatives with the aim to develop brain-targeted prodrugs of FA. In the present study, the cellular uptake and bioavailability of these FA-derivatives were evaluated in mouse primary astrocytic cell cultures together with their inhibitory effects towards BACE1, COX/LOX, mTOR, NF-κB, acetylcholinesterase (AChE), and oxidative stress. According to the results, all three FA-derivatives were taken up 200–600 times more effectively at 10 μM concentration into the astrocytes than FA, with one derivative having a high intracellular bioavailability (K_p,uu), particularly at low concentrations. Moreover, all of the derivatives were able to inhibit BACE1, COX/LOX, AChE, and oxidative stress measured as decreased cellular lipid peroxidation. Furthermore, one of the derivatives modified the total mTOR amount. Therefore, these derivatives have the potential to act as multifunctional compounds preventing β-amyloid accumulation as well as combating inflammation and reducing oxidative stress in the brain. Thus, this study shows that converting a parent drug into a transporter-utilizing derivative not only may increase its brain and cellular uptake, and bioavailability but can also broaden the spectrum of pharmacological effects elicited by the derivative.

Identification and Characterization of Efflux Transporters That Modulate the Subtoxic Disposition of Diclofenac and Its Metabolites

In the present work, in vivo transporter knockout (KO) mouse models were used to characterize the disposition of diclofenac (DCF) and its primary metabolites following a single subtoxic dose in mice lacking breast cancer resistance protein (Bcrp) or multidrug resistance-associated protein (Mrp)3. The results indicate that Bcrp acts as a canalicular efflux mediator for DCF, as wild-type (WT) mice had biliary excretion values that were 2.2- to 2.6-fold greater than Bcrp KO mice, although DCF plasma levels were not affected. The loss of Bcrp resulted in a 1.8- to 3.2-fold increase of diclofenac acyl glucuronide (DCF-AG) plasma concentrations in KO animals compared with WT mice, while the biliary excretion of DCF-AG increased 1.4-fold in WT versus KO mice. Furthermore, Mrp3 was found to mediate the basolateral transport of DCF-AG, but not DCF or 4'-hydroxy diclofenac. WT mice had DCF-AG plasma concentrations 7.0- to 8.6-fold higher than Mrp3 KO animals; however, there were no changes in biliary excretion of DCF-AG. Vesicular transport experiments with human MRP3 demonstrated that MRP3 is able to transport DCF-AG via low- and high-affinity binding sites. The low-affinity MRP3 transport had a V _max and K _m of 170 pmol/min/mg and 98.2 µM, respectively, while the high-affinity V _max and K _m parameters were estimated to be 71.9 pmol/min/mg and 1.78 µM, respectively. In summary, we offer evidence that the disposition of DCF-AG can be affected by both Bcrp and Mrp3, and these findings may be applicable to humans.

Impact of Lipid Partitioning on the Design, Analysis, and Interpretation of Microsomal Time-Dependent Inactivation

Nonspecific drug partitioning into microsomal membranes must be considered for in vitro-in vivo correlations. This work evaluated the effect of including lipid partitioning in the analysis of complex TDI kinetics with numerical methods. The covariance between lipid partitioning and multiple inhibitor binding was evaluated. Simulations were performed to test the impact of lipid partitioning on the interpretation of TDI kinetics, and experimental TDI datasets for paroxetine (PAR) and itraconazole (ITZ) were modeled. For most kinetic schemes, modeling lipid partitioning results in statistically better fits. For MM-IL simulations (KI,u = 0.1 µM, kinact = 0.1 minute-1), concurrent modeling of lipid partitioning for an fumic range (0.01, 0.1, and 0.5) resulted in better fits compared with post hoc correction (AICc: -526 vs. -496, -579 vs. -499, and -636 vs. -579, respectively). Similar results were obtained with EII-IL. Lipid partitioning may be misinterpreted as double binding, leading to incorrect parameter estimates. For the MM-IL datasets, when fumic = 0.02, MM-IL, and EII model fits were indistinguishable (δAICc = 3). For less partitioned datasets (fumic = 0.1 or 0.5), the inclusion of partitioning resulted in better models. The inclusion of lipid partitioning can lead to markedly different estimates of KI,u and kinact A reasonable alternate experimental design is nondilution TDI assays, with post hoc fumic incorporation. The best fit models for PAR (MIC-M-IL) and ITZ (MIC-EII-M-IL and MIC-EII-M-Seq-IL) were consistent with their reported mechanism and kinetics. Overall, experimental fumic values should be concurrently incorporated into TDI models with complex kinetics, when dilution protocols are used.

Characterization of Porcine Hepatic and Intestinal Drug Metabolizing CYP450: Comparison with Human Orthologues from A Quantitative, Activity and Selectivity Perspective

Over the past two decades, the pig has gained attention as a potential model for human drug metabolism. Cytochrome P450 enzymes (CYP450), a superfamily of biotransformation enzymes, are pivotal in drug metabolism. Porcine CYP450 has been demonstrated to convert typical substrates of human CYP450. Nevertheless, knowledge and insight into porcine CYP450 quantity and substrate selectivity is scant, especially regarding intestinal CYP450. The current study aimed to map the quantities of hepatic and intestinal CYP450 in the conventional pig by using a proteomic approach. Moreover, the selectivity of the six most common used probe substrates (phenacetin, coumarin, midazolam, tolbutamide, dextromethorphan, and chlorzoxazone) for drug metabolizing enzyme subfamilies (CYP1A, CYP2A, CYP3A, CYP2C, CYP2D and CYP2E respectively), was investigated. Hepatic relative quantities were 4% (CYP1A), 31% (CYP2A), 14% (CYP3A), 10% (CYP2C), 28% (CYP2D) and 13% (CYP2E), whereas for the intestine only duodenal CYP450 could be determined with 88% for CYP3A and 12% for CYP2C. Furthermore, the results indicate that coumarin (CYP2A), midazolam (CYP3A), tolbutamide (CYP2C), and dextromethorphan (CYP2D) are as selective for porcine as for human CYP450. However, phenacetin (CYP1A2) and chlorzoxazone (CYP2E1) are less selective for the specific enzyme, despite similarities in selectivity towards the different enzymes involved compared to humans.

Mechanism of the efflux transport of demethoxycurcumin-O-glucuronides in HeLa cells stably transfected with UDP-glucuronosyltransferase 1A1

Demethoxycurcumin (DMC) is a safe and natural food-coloring additive, as well as an agent with several therapeutic properties. However, extensive glucuronidation in vivo has resulted in its poor bioavailability. In this study, we aimed to investigate the formation of DMC-O-glucuronides by uridine 5'-diphospho-glucuronosyltransferase 1A1 (UGT1A1) and its transport by breast cancer resistance protein (BCRP) and multidrug resistance-associated proteins (MRPs) in HeLa cells stably transfected with UGT1A1 (named HeLa1A1 cells). The chemical inhibitors Ko143 (a selective BCRP inhibitor) and MK571 (a pan-MRP inhibitor) both induced an obvious decrease in the excretion rate of DMC-O-glucuronides and a significant increase in intracellular DMC-O-glucuronide concentrations. Furthermore, BCRP knock-down resulted in a marked reduction in the level of excreted DMC-O-glucuronides (maximal 55.6%), whereas MRP1 and MRP4 silencing significantly decreased the levels of excreted DMC-O-glucuronides (a maximum of 42.9% for MRP1 and a maximum of 29.9% for MRP3), respectively. In contrast, neither the levels of excreted DMC-O-glucuronides nor the accumulation of DMC-O-glucuronides were significantly altered in the MRP4 knock-down HeLa cells. The BCRP, MRP1 and MRP3 transporters were identified as the most important contributors to the excretion of DMC-O-glucuronides. These results may significantly contribute to improving our understanding of mechanisms underlying the cellular disposition of DMC via UGT-mediated metabolism.

Kinetics of Cyclophosphamide Metabolism in Humans, Dogs, Cats, and Mice and Relationship to Cytotoxic Activity and Pharmacokinetics

Cyclophosphamide (CP), a prodrug that is enzymatically converted to the cytotoxic 4-hydroxycyclophosphamide (4OHCP) by hepatic enzymes, is commonly used in both human and veterinary medicine to treat cancers and modulate the immune system. We investigated the metabolism of CP in humans, dogs, cats, and mice using liver microsomes; apparent K M, V max, and intrinsic clearance (V max/K M) parameters were estimated. The interspecies and intraspecies variations in kinetics were vast. Dog microsomes were, on average, 55-fold more efficient than human microsomes, 2.8-fold more efficient than cat microsomes, and 1.2-fold more efficient than mouse microsomes at catalyzing CP bioactivation. These differences translated to cell-based systems. Breast cancer cells exposed to 4OHCP via CP bioactivation by microsomes resulted in a stratification of cytotoxicity that was dependent on the species of microsomes measured by IC50: dog (31.65 μM), mouse (44.95 μM), cat (272.6 μM), and human (1857 μM). The contributions of cytochrome P450s, specifically, CYP2B, CYP2C, and CYP3A, to CP bioactivation were examined: CYP3A inhibition resulted in no change in 4OHCP formation; CYP2B inhibition slightly reduced 4OHCP in humans, cats, and mice; and CYP2C inhibition drastically reduced 4OHCP formation in each species. Semiphysiologic modeling of CP metabolism using scaled metabolic parameters resulted in simulated data that closely matched published pharmacokinetic profiles, determined by noncompartmental analysis. The results highlight differential CP metabolism delineated by species and demonstrate the importance of metabolism on CP clearance.

Time Course of Aldehyde Oxidase and Why It Is Nonlinear

Many promising drug candidates metabolized by aldehyde oxidase (AOX) fail during clinical trial owing to underestimation of their clearance. AOX is species-specific, which makes traditional allometric studies a poor choice for estimating human clearance. Other studies have suggested using half-life calculated by measuring substrate depletion to measure clearance. In this study, we proposed using numerical fitting to enzymatic pathways other than Michaelis-Menten (MM) to avoid missing the initial high turnover rate of product formation. Here, product formation over a 240-minute time course of six AOX substrates-O⁶-benzylguanine, N-(2-dimethylamino)ethyl)acridine-4-carboxamide, zaleplon, phthalazine, BIBX1382 [N8-(3-Chloro-4-fluorophenyl)-N2-(1-methyl-4-piperidinyl)-pyrimido[5,4-d]pyrimidine-2,8-diamine dihydrochloride], and zoniporide-have been provided to illustrate enzyme deactivation over time to help better understand why MM kinetics sometimes leads to underestimation of rate constants. Based on the data provided in this article, the total velocity for substrates becomes slower than the initial velocity by 3.1-, 6.5-, 2.9-, 32.2-, 2.7-, and 0.2-fold, respectively, in human expressed purified enzyme, whereas the K _m remains constant. Also, our studies on the role of reactive oxygen species (ROS), such as superoxide and hydrogen peroxide, show that ROS did not significantly alter the change in enzyme activity over time. Providing a new electron acceptor, 5-nitroquinoline, did, however, alter the change in rate over time for mumerous compounds. The data also illustrate the difficulties in using substrate disappearance to estimate intrinsic clearance.

Optimization of Experimental Conditions of Automated Glucuronidation Assays in Human Liver Microsomes Using a Cocktail Approach and Ultra-High Performance Liquid Chromatography-Tandem Mass Spectrometry

UDP-glucuronosyltransferase (UGT)-mediated metabolism is possibly the most important conjugation reaction for marketed drugs. However, there are currently no generally accepted standard incubation conditions for UGT microsomal assays, and substantial differences in experimental design and methodology between laboratories hinder cross-study comparison of in vitro activities. This study aimed to define optimal experimental conditions to determine glucuronidation activity of multiple UGT isoforms simultaneously using human liver microsomes. Hepatic glucuronidation activities of UGT1A1, UGT1A3, UGT1A4, UGT1A6, UGT1A9, UGT2B4, UGT2B7, UGT2B10, UGT2B15, and UGT2B17 were determined using cocktail incubations of 10 UGT probe substrates. Buffer components and cosubstrates were assessed over a range of concentrations including magnesium chloride (MgCl₂; 0-10 mM) and uridine 5'-diphosphoglucuronic acid (UDPGA; 1-25 mM) with either Tris-HCl or potassium phosphate buffer (100 mM, pH 7.4). Greater microsomal glucuronidation activity by different hepatic UGT isoforms was obtained using 10 mM MgCl₂ and 5 mM UDPGA with 100 mM Tris-HCl buffer. The influence of bovine serum albumin (BSA; 0.1%-2% w/v) on glucuronidation activity was also assessed. Enzyme- and substrate-dependent effects of BSA were observed, resulting in decreased total activity of UGT1A1, UGT1A3, and UGT2B17 and increased total UGT1A9 and UGT2B7 activity. The inclusion of BSA did not significantly reduce the between-subject variability of UGT activity. Future in vitro UGT profiling studies under the proposed optimized experimental conditions would allow high-quality positive control data to be generated across laboratories, with effective control of a high degree of between-donor variability for UGT activity and for chemical optimization toward lower-clearance drug molecules in a pharmaceutical drug discovery setting.

Interplay of Efflux Transporters with Glucuronidation and Its Impact on Subcellular Aglycone and Glucuronide Disposition: A Case Study with Kaempferol

Glucuronidation is a major process of drug metabolism and elimination that generally governs drug efficacy and toxicity. Publications have demonstrated that efflux transporters control intracellular glucuronidation metabolism. However, it is still unclear whether and how efflux transporters interact with UDP-glucuronosyltransferases (UGTs) in subcellular organelles. In this study, kaempferol, a model fluorescent flavonoid, was used to investigate the interplay of glucuronidation with transport at the subcellular level. Human recombinant UGTs and microsomes were utilized to characterize the in vitro glucuronidation kinetics of kaempferol. The inhibition of UGTs and efflux transporters on the subcellular disposition of kaempferol were determined visually and quantitatively in Caco-2/TC7 cells. The knockout of transporters on the subcellular accumulation of kaempferol in liver and intestine were evaluated visually. ROS and Nrf2 were assayed to evaluate the pharmacological activities of kaempferol. The results showed that UGT1A9 is the primary enzyme responsible for kaempferol glucuronidation. Visual and quantitative data showed that the UGT1A9 inhibitor carvacrol caused a significant rise in subcellular aglycone and reduction in subcellular glucuronides of kaempferol. The inhibition and knockout of transporters, such as P-glycoprotein (P-gp), breast cancer resistance protein (BCRP), and multidrug resistance-associated proteins (MRPs), exhibited a marked increase in subcellular kaempferol and decrease in its subcellular glucuronides. Correspondingly, inhibition of UGT1A9 and transporters led to increased kaempferol and, consequently, a significantly enhanced ROS scavenging efficiency and nuclear translocation of Nrf2. In conclusion, the interplay of efflux transporters (P-gp, BCRP, and MRPs) and UGTs govern the subcellular exposure and corresponding pharmacological activity of kaempferol.

Inhibition of Sodium-Hydrogen Antiport by Antibodies to NHA1 in Brush Border Membrane Vesicles from Whole Aedes aegypti Larvae

The present research report describes Na⁺/H⁺ antiport by brush border membrane vesicles isolated from whole larvae of Aedes aegypti (AeBBMVw). Our hypothesis is that acid quenching of acridine orange by AeBBMVw is predominantly mediated by Na⁺/H⁺ antiport via the NHA1 component of the AeBBMVw in the absence of amino acids and ATP. AeNHA1 is a Na⁺/H⁺ antiporter that has been postulated to exchange Na⁺ and H⁺ across the apical plasma membrane in posterior midgut of A. aegypti larvae. Its principal function is to recycle the H⁺ and Na⁺ that are transported during amino acid uptake, e.g., phenylalanine. This uptake is mediated, in part, by a voltage-driven, Na⁺-coupled, nutrient amino acid transporter (AeNAT8). The voltage is generated by an H⁺ V-ATPase. All three components, V-ATPase, antiporter, and nutrient amino acid transporter (VAN), are present in brush border membrane vesicles isolated from whole larvae of A. aegypti. By omitting ATP and amino acids, Na⁺/H⁺ antiport was measured by fluorescence quenching of acridine orange (AO) caused by acidification of either the internal vesicle medium (Na⁺_in > Na⁺_out) or the external fluid-membrane interface (Na⁺_in < Na⁺_out). Vesicles with 100 micromolar Na⁺ inside and 10 micromolar Na⁺ outside or with 0.01 micromolar Na⁺ inside and 100 micromolar Na⁺ outside quenched fluorescence of AO by as much as 30%. Acidification did not occur in the absence of AeBBMVw. Preincubation of AeBBMVw with antibodies to NHA1 inhibit Na⁺/H⁺ antiport dependent fluorescence quenching, indicating that AeNHA1 has a significant role in Na⁺/H⁺ exchange.

Efflux excretion of bisdemethoxycurcumin-O-glucuronide in UGT1A1-overexpressing HeLa cells: Identification of breast cancer resistance protein (BCRP) and multidrug resistance-associated proteins 1 (MRP1) as the glucuronide transporters

Bisdemethoxycurcumin (BDMC) was a natural curcuminoid with many bioactivities present in turmeric (Curcuma longa L.). However, the disposition mechanisms of BDMC via uridine 5'-diphospho-glucuronosyltransferase (UGT) metabolism still remain unclear. Therefore, we aimed to determine the potential efflux transporters for the excretion of BDMC-O-glucuronide. Herein, chemical inhibition assays (Ko143, MK571, dipyridamole, and leukotriene C4) and biological inhibition experiments including stable knocked-down of breast cancer resistance protein (BCRP), multidrug resistance-associated proteins (MRPs) transporters were both performed in a HeLa cell line stably overexpressing UGT1A1 established previously. The results indicated that Ko143 (5 and 20 μM) caused a marked reduction in excretion rate (18.4-55.6%) and elevation of intracellular BDMC-O-glucuronide (28.8-48.1%), whereas MK-571 (5 and 20 μM) resulted in a significant decrease in excretion rate (6.2-61.6%) and increase of intracellular BDMC-O-glucuronide (maximal 27.1-32.6%). Furthermore, shRNA-mediated silencing of BCRP transporter led to a marked reduction in the excretion rate (21.1-36.9%) and an obvious elevation of intracellular glucuronide (24.9%). Similar results were observed when MRP1 was partially silenced. In addition, MRP3 and MRP4 silencing both displayed no obvious changes on the excretion rate and intracellular levels of glucuronide. In conclusion, chemical inhibition and gene silencing results both indicated that generated BDMC-O-glucoside were excreted primarily by the BCRP and MRP1 transporters. © 2018 BioFactors, 44(6):558-569, 2018.

The roles of breast cancer resistance protein (BCRP/ABCG2) and multidrug resistance-associated proteins (MRPs/ABCCs) in the excretion of cycloicaritin-3-O-glucoronide in UGT1A1-overexpressing HeLa cells

Cycloicaritin is a bioactive natural phenolic compound from Epimedium species. However, the glucuronidation and excretion which would influence oral bioavailability and pharmacokinetics of cycloicaritin still remain unknown. Here we aimed to establish UGT1A1 stably transfected HeLa cells, and to determine the contributions of BCRP and MRPs transporters to excretion of cycloicaritin-3-O-glucuronide. First, β-estradiol was used to validate the expression of active UGT1A1 protein in engineered HeLa1A1 cells. Furthermore, Ko143 (5 and 20 μM) led to a significant decrease (42.4%-63.8%, p < 0.01) in CICT-3-G excretion and obvious accumulation (19.7%-54.2%, p < 0.05) of intracellular CICT-3-G, while MK571 (5 and 20 μM) caused a significant reduction (46.8%-64.8%, p < 0.05) in the excretion and obvious elevation (50.7%-85.2%, p < 0.01) of intracellular level of CICT-3-G. Furthermore, BCRP knocked-down brought marked reduction in excretion rates of CICT-3-G (26.0%-42.2%, p < 0.01), whereas MRP1 and MRP4-mediated silencing led to significant decrease in the excretion of CICT-3-G (23.8%-35.4%, p < 0.05 for MRP1 and 11.9%-16.0%, p < 0.05 for MRP4). By contrast, neither CICT-3-G excretion nor CICT-3-G accumulation altered in MRP3 knocked-down cells as compared to scramble cells. Taken together, BCRP, MRP1 and MRP4 were identified as the most important contributors for CICT-3-G excretion. Meanwhile, the UGT1A1 modified HeLa cells were a simple and practical tool to study UGT1A1-mediated glucuronidation and to characterize BCRP and MRPs-mediated glucuronide transport at a cellular level.

Steroid bioconjugation to a CYP3A4 allosteric site and its effect on substrate binding and coupling efficiency

Human cytochrome P450 3A4 (CYP3A4) is an important drug metabolizing enzyme involved in a number of drug-drug and food-drug interactions. As such, much effort has been devoted into investigating its mechanism of interaction with ligands. CYP3A4 has one of the highest levels of substrate promiscuity for an enzyme, and can even bind multiple ligands simultaneously. The location and orientation of these ligands depend on the chemical structure and stoichiometry, and are generally poorly understood. In the case of the steroid testosterone, up to three copies of the molecule can associate with the enzyme at once, likely two in the active site and one at a postulated allosteric site. Recently, we demonstrated that steroid bioconjugation at the allosteric site results in an increase in activity of CYP3A4 toward testosterone and 7-benzyloxy-4-trifluoromethylcoumarin oxidation. Here, using the established bioconjugation methodology, we show how steroid bioconjugation at the allosteric site affects the heme spin state, the binding affinity (KS) of CYP3A4 for testosterone, as well as the enzyme coupling efficiency.

In Vitro Stimulation of Multidrug Resistance-Associated Protein 2 Function Is Not Reproduced In Vivo in Rats

Previously we reported that coproporphyrin-I (CP-I) is an optimal probe substrate for multidrug resistance-associated protein 2 (MRP2), and stimulation of MRP2-mediated transport is probe substrate-dependent. In the present investigation, we assessed if the in vitro stimulation is physiologically relevant. Similar to human MRP2 transport, CP-I was transported by rat Mrp2 in a typical Michaelis-Menten kinetics with apparent K_m and V_max values of 15 ± 6 µM and 161 ± 20 pmol/min/mg protein, respectively. In vivo Mrp2 functions were monitored by biliary and renal secretion of CP-I and its isomer CP-III, in bile-duct cannulated rats before and after treatment with mitoxantrone, progesterone, and verapamil. These compounds stimulated Mrp2-mediated CP-I transport in vitro. No significant increase in biliary or renal clearances, as well as in the cumulative amount of CP-I or CP-III eliminated in bile, were detected following treatment with the in vitro stimulators, indicating an in vitro to in vivo disconnect. In presence of 10 µM bilirubin, the in vitro stimulation was suppressed. We concluded that the in vitro stimulation of CP-I transport mediated by Mrp2 is not translatable in vivo, and proposed that the presence of endogenous compounds such as bilirubin in the liver may contribute to the in vitro to in vivo disconnect.

The sulfoconjugation of androstenone and dehydroepiandrosterone by human and porcine sulfotransferase enzymes

Porcine sulfotransferase 2A1 (pSULT2A1) is a key enzyme involved in the testicular and hepatic sulfoconjugation of steroids such as dehydroepiandrosterone (DHEA) and potentially androstenone. This latter steroid is a major cause of boar taint, which is an unpleasant off-odour and off-flavour in pork from male pigs. Sulfotransferase 2B1 (pSULT2B1) may also be important, although no direct evidence exists for its involvement in sulfoconjugation of steroids. The purpose of this study was to investigate the sulfoconjugation activity of human and porcine sulfotransferases towards DHEA and androstenone. pcDNA 3.1 vectors expressing porcine (p) SULT2A1, pSULT2B1, human (h) SULT2A1, hSULT2B1a, and hSULT2B1b enzymes were transfected into human embryonic kidney cells. Transfected cells were then incubated with either androstenone or dehydroepiandrosterone (DHEA) in both time-course and enzyme kinetics studies. The production of sulfonates of androstenone metabolites and DHEA sulfonate increased over time for all enzymes with the exception of pSULT2B1. Enzyme kinetics analysis showed that androstenone and DHEA were poor substrates for the human orthologs, hSULT2B1a and hSULT2B1b. Human and porcine SULT2A1 showed substantially different substrate affinities for androstenone (K_m 5.8 ± 0.6 µM and 74.1 ± 15.9 µM, respectively) and DHEA (K_m 9.4 ± 2.5 µM and 3.3 ± 1.9 µM, respectively). However, these enzymes did show relatively similar sulfonation efficiencies for DHEA (V_max/K_m 50.5 and 72.9 for hSULT2A1 and pSULT2A1, respectively). These results highlight the species differences in sulfonation activity and provide direct evidence, for the first time, suggesting that pSULT2B1 is not involved in sulfonation of either androstenone metabolites or DHEA.

Curing Reaction and Dielectric Properties of Rigid and Elastic Liquid Crystal Epoxy Networks Modified with Nanofillers

The aim of the paper is to compare the progress of the curing reaction and the dielectric properties of liquid crystalline epoxy networks of different elasticity and to investigate how modification with nanofillers influences their properties. The study focuses on a liquid crystalline epoxy monomer with four aromatic rings in the mesogen, cross-linked with 4,4′-diaminodiphenylmethane (DDM) and pimelic acid (PA) to produce rigid and elastic polymer networks. The obtained results are compared to networks modified with nanofillers (diphenyl aluminum phosphate nanorods). The curing process is monitored in situ with broadband dielectric spectroscopy, which is a good tool to view the progress of the reaction. Two stages with different reaction dynamics are observed in the studied cases. Dielectric properties of the products cured with two chosen agents show significant differences in the obtained parameters (activation energy, glass transition, and fragility index) as well as in the dynamics of the α and β relaxations. Although the curing agent is the main factor determining the properties of the product, the nanofiller additive also modifies the values of specific parameters that characterize both the process of the reaction and the properties of the final composites. Particularly, adding the nanofiller raises the glass transition temperature in both the cases. The obtained results are in good agreement with the earlier calorimetric study.