Mechanistic insights from comparing intrinsic clearance values between human liver microsomes and hepatocytes to guide drug design

The Critical Role of Passive Permeability in Designing Successful Drugs

Passive permeability is a key property in drug disposition and delivery. It is critical for gastrointestinal absorption, brain penetration, renal reabsorption, defining clearance mechanisms and drug-drug interactions. Passive diffusion rate is translatable across tissues and animal species, while the extent of absorption is dependent on drug properties, as well as in vivo physiology/pathophysiology. Design principles have been developed to guide medicinal chemistry to enhance absorption, which combine the balance of aqueous solubility, permeability and the sometimes unfavorable compound characteristic demanded by the target. Permeability assays have been implemented that enable rapid development of structure-permeability relationships for absorption improvement. Future advances in assay development to reduce nonspecific binding and improve mass balance will enable more accurately measurement of passive permeability. Design principles that integrate potency, selectivity, passive permeability and other ADMET properties facilitate rapid advancement of successful drug candidates to patients.

In vitro evaluation of the metabolic stability of nine fragrance chemicals in trout and human hepatocytes

In vitro metabolic stability of nine fragrance chemicals: p-tolyl acetate, cashmeran, ethylene brassylate, celestolide, galaxolide, traseolide, ambretone, tonalide and pentadecanolide, was evaluated in trout and human hepatocytes. The compounds were incubated with trout hepatocytes at 12°C and human hepatocytes at 37°C. Quantification of compound disappearance with time was performed using gas chromatography/mass spectrometry. in vivo hepatic intrinsic clearance values were calculated from the in vitro data. Significant metabolism was observed with trout hepatocytes for five of the nine fragrance chemicals, while all nine were metabolized significantly with human hepatocytes. Previously published models were used to examine expected bioaccumulation and persistence in whole organisms. Calculated half-lives due to metabolism of the nine chemicals are significantly shorter for humans than trout: <1 hour and <1 day, respectively. For all chemicals with demonstrated hepatic metabolism, the models indicate a lack of accumulation. For those where metabolism was demonstrated in trout, calculated bioconcentration factors would not be classified as bioaccumulative under prevailing regulatory systems.

Privileged scaffold-based design to identify a novel drug-like 5-HT7 receptor-preferring agonist to target Fragile X syndrome

Recent preclinical studies have shown that activation of the serotonin 5-HT₇ receptor has the potential to treat neurodevelopmental disorders such as Fragile X syndrome, a rare disease characterized by autistic features. With the aim to provide the scientific community with diversified drug-like 5-HT₇ receptor-preferring agonists, we designed a set of new long-chain arylpiperazines by exploiting structural fragments present in clinically approved drugs or in preclinical candidates (privileged scaffolds). The new compounds were synthesized, tested for their affinity at 5-HT₇ and 5-HT_1A receptors, and screened for their in vitro stability to microsomal degradation and toxicity. Selected compounds were characterized as 5-HT₇ receptor-preferring ligands, endowed with high metabolic stability and low toxicity. Compound 7g emerged as a drug-like 5-HT₇ receptor-preferring agonist capable to rescue synaptic plasticity and attenuate stereotyped behavior in a mouse model of Fragile X syndrome.

Novel Cassette Assay To Quantify the Outer Membrane Permeability of Five β-Lactams Simultaneously in Carbapenem-Resistant Klebsiella pneumoniae and Enterobacter cloacae

Poor penetration through the outer membrane (OM) of Gram-negative bacteria is a major barrier of antibiotic development. While β-lactam antibiotics are commonly used against Klebsiella pneumoniae and Enterobacter cloacae, there are limited data on OM permeability especially in K. pneumoniae Here, we developed a novel cassette assay, which can simultaneously quantify the OM permeability to five β-lactams in carbapenem-resistant K. pneumoniae and E. cloacae Both clinical isolates harbored a blaKPC-2 and several other β-lactamases. The OM permeability of each antibiotic was studied separately ("discrete assay") and simultaneously ("cassette assay") by determining the degradation of extracellular β-lactam concentrations via multiplex liquid chromatography-tandem mass spectrometry analyses. Our K. pneumoniae isolate was polymyxin resistant, whereas the E. cloacae was polymyxin susceptible. Imipenem penetrated the OM at least 7-fold faster than meropenem for both isolates. Imipenem penetrated E. cloacae at least 258-fold faster and K. pneumoniae 150-fold faster compared to aztreonam, cefepime, and ceftazidime. For our β-lactams, OM permeability was substantially higher in the E. cloacae compared to the K. pneumoniae isolate (except for aztreonam). This correlated with a higher OmpC porin production in E. cloacae, as determined by proteomics. The cassette and discrete assays showed comparable results, suggesting limited or no competition during influx through OM porins. This cassette assay allowed us, for the first time, to efficiently quantify the OM permeability of multiple β-lactams in carbapenem-resistant K. pneumoniae and E. cloacae Characterizing the OM permeability presents a critical contribution to combating the antimicrobial resistance crisis and enables us to rationally optimize the use of β-lactam antibiotics.IMPORTANCE Antimicrobial resistance is causing a global human health crisis and is affecting all antibiotic classes. While β-lactams have been commonly used against susceptible isolates of Klebsiella pneumoniae and Enterobacter cloacae, carbapenem-resistant isolates are spreading worldwide and pose substantial clinical challenges. Rapid penetration of β-lactams leads to high drug concentrations at their periplasmic target sites, allowing β-lactams to more completely inactivate their target receptors. Despite this, there are limited tangible data on the permeability of β-lactams through the outer membranes of many Gram-negative pathogens. This study presents a novel, cassette assay, which can simultaneously characterize the permeability of five β-lactams in multidrug-resistant clinical isolates. We show that carbapenems, and especially imipenem, penetrate the outer membrane of K. pneumoniae and E. cloacae substantially faster than noncarbapenem β-lactams. The ability to efficiently characterize the outer membrane permeability is critical to optimize the use of β-lactams and combat carbapenem-resistant isolates.

Ketamine analogues: Comparative toxicokinetic in vitro-in vivo extrapolation and quantification of 2-fluorodeschloroketamine in forensic blood and hair samples

Recently, the new psychoactive substance (NPS) ketamine analogue 2-fluoro-deschloroketamine (2FDCK) was observed in driving-under-the-influence-of-drugs whole blood samples and in a forensic hair investigation case in Denmark. The molecular structure variations among the NPS subgroups may alter the metabolic fate and drug potency, thereby posing a threat for drug users. This study reports quantification of 2FDCK in whole blood samples and forensic hair and compares the following toxicokinetic parameters: unbound fraction (f_u) and in vitro-in vivo-extrapolation (IVIVE) of hepatic clearance for ketamine, norketamine, 2FDCK, methoxetamine and deschloroketamine. The f_u was investigated with ultrafiltration, while clearance studies were conducted at 1 μM with pooled human liver microsomes. Samples were analysed by liquid chromatography and mass spectrometry. For the first time, 2FDCK was determined in a concentration range between 0.005 and 0.48 mg/kg in blood samples. Segmental hair analysis demonstrated 2FDCK at concentrations from 0.007 to 0.034 ng/mg throughout the three investigated segments. Toxicokinetic comparison of the five compounds gave a f_u between 0.54 and 0.84, with ketamine being the most bound and deschloroketamine being the least bound, in accordance with the logP of the compounds. Conversely, a negative correlation was observed between the molecular weight of the halogen in the ortho-position and IVIVE hepatic clearance. The IVIVE of hepatic clearance, CL_{parallel-tube}, gave values from 18.1 to 5.44 mL/min/kg for ketamine and methoxetamine, respectively. The deschloroketamine IVIVE was disregarded due to low drug elimination under the experimental conditions used. This study provides a basis for toxicokinetic understanding of ketamine analogues.

Physiologically based kinetic modelling-facilitated reverse dosimetry to predict in vivo red blood cell acetylcholinesterase inhibition following exposure to chlorpyrifos in the Caucasian and Chinese population

Organophosphates have a long history of use as insecticides over the world. The aim of the present study was to investigate the interethnic differences in kinetics, biomarker formation, and in vivo red blood cell acetylcholinesterase inhibition of chlorpyrifos (CPF) in the Chinese and the Caucasian population. To this purpose, physiologically based kinetic models for CPF in both the Chinese and Caucasian population were developed, and used to study time- and dose-dependent interethnic variation in urinary biomarkers and to convert concentration-response curves for red blood cell acetylcholinesterase inhibition to in vivo dose-response curves in these 2 populations by reverse dosimetry. The results obtained revealed a marked interethnic difference in toxicokinetics of CPF, with lower urinary biomarker levels at similar dose levels and slower CPF bioactivation and faster chlorpyrifos-oxon detoxification in the Chinese compared with the Caucasian population, resulting in 5- to 6-fold higher CPF sensitivity of the Caucasian than the Chinese population. These differences might be related to variation in the frequency of single-nucleotide polymorphisms for the major biotransformation enzymes involved. To conclude, the interethnic variation in kinetics of CPF may affect both its biomarker-based exposure assessment and its toxicity and risk assessment and physiologically based kinetic modeling facilitates the characterization and quantification of these interethnic variations.

Impact of Intracellular Concentrations on Metabolic Drug-Drug Interaction Studies

Accurate prediction of drug-drug interactions (DDI) is a challenging task in drug discovery and development. It requires determination of enzyme inhibition in vitro which is highly system-dependent for many compounds. The aim of this study was to investigate whether the determination of intracellular unbound concentrations in primary human hepatocytes can be used to bridge discrepancies between results obtained using human liver microsomes and hepatocytes. Specifically, we investigated if Kp_uu could reconcile differences in CYP enzyme inhibition values (K_i or IC₅₀). Firstly, our methodology for determination of Kp_uu was optimized for human hepatocytes, using four well-studied reference compounds. Secondly, the methodology was applied to a series of structurally related CYP2C9 inhibitors from a Roche discovery project. Lastly, the Kp_uu values of three commonly used CYP3A4 inhibitors—ketoconazole, itraconazole, and posaconazole—were determined and compared to compound-specific hepatic enrichment factors obtained from physiologically based modeling of clinical DDI studies with these three compounds. Kp_uu obtained in suspended human hepatocytes gave good predictions of system-dependent differences in vitro. The Kp_uu was also in fair agreement with the compound-specific hepatic enrichment factors in DDI models and can therefore be used to improve estimations of enrichment factors in physiologically based pharmacokinetic modeling.

In Vitro Metabolism and Hepatic Intrinsic Clearance of the Synthetic Cannabinoid Receptor Agonist JWH-122 and Its Four ω-Halogenated Analogues

The number of new psychoactive substances (NPS) emerging on the illicit drug market has increased over the last decade. Halogenation of existing illicit drugs is a particular trend, with the purpose of both circumventing the law and altering the toxicodynamic and toxicokinetic profiles of the compounds. This study investigates the in vitro impact of JWH-122 ω-halogenation (fluoro, chloro, bromo and iodo) on the metabolism, apparent intrinsic hepatic clearance and analytical targets for detecting drug consumption. Metabolite profiling was conducted with pooled human liver microsomes, suspended rat hepatocytes and pooled human hepatocytes. The in vitro half-life was also determined in pooled human hepatocytes. All samples were analysed by liquid chromatography/high-resolution mass spectrometry. All compounds, except for JWH-122, showed high formation rates of phase I metabolites, predominantly ω-COOH and methylnaphthyl hydroxylation metabolites. Phase II metabolites were ω-O-glucuronides, methylnaphthyl O-glucuronides and ω-glutathione conjugates. The relative ion intensity of the glutathione conjugates increased with the ω-halogen size, with I-JWH-122 having the highest intensity. Stability studies gave a low half-life and a high intrinsic hepatic clearance for JWH-122 (1305 mL/min/kg) and MAM-2201 (1408 mL/min/kg). Cl-, Br- and I-JWH-122 showed increasing half-life with increasing ω-halogen size, with intrinsic clearance values of 235-502 mL/min/kg. The recommended analytical targets for consumption of JWH-122 or ω-halogenated JWH-122 analogues are the ω-COOH metabolites for unspecific profiling and the methylnaphthyl hydroxylated metabolites to distinguish the compounds. Furthermore, ω-halogenation with larger halogens appears to increase the intrinsic hepatic stability, thereby prolonging exposure and possibly the duration of action.

A Novel Unified Approach to Predict Human Hepatic Clearance for Both Enzyme- and Transporter-Mediated Mechanisms Using Suspended Human Hepatocytes

The accurate prediction of human pharmacokinetics is critically important in modern drug discovery since it drives both pharmacological and toxicological effects. Although significant progress has been made in predicting drug disposition by hepatic drug-metabolizing enzymes, predicting transporter-mediated clearance is still highly uncertain. Furthermore, different approaches are often used to predict clearance with and without transporter involvement, hence the major clearance pathway for a compound must first be determined to know which approach to use. As a result of these challenges, a novel unified method has been developed using cryopreserved suspended human hepatocytes to predict human hepatic clearance for both enzyme- and transporter-mediated mechanisms. This method hypothesizes that, once in vitro metabolic stability is scaled by partition coefficients between hepatocytes and buffer with 4% bovine serum albumin, in vivo clearance can be better predicted. With this method, good in vitro-in vivo correlation of human hepatic clearance has been obtained for a set of 32 structurally diverse compounds, including such transporters as organic anion-transporting polypeptide substrates. The clearance predictions for most compounds are within 3-fold of observed values. This is the first time that multiple compounds result in good in vitro-in vivo extrapolation using an entirely "bottom-up" approach without any empirical scaling factor when transporter-mediated clearance is involved. Potential exceptions are compounds with significant biliary and/or extra-hepatic clearance. The method offers an alternative approach to more accurately predict human hepatic clearance when multiple complex mechanisms are involved.

In Vitro-In Vivo Extrapolation and Hepatic Clearance-Dependent Underprediction

Accurately predicting the hepatic clearance of compounds using in vitro to in vivo extrapolation (IVIVE) is crucial within the pharmaceutical industry. However, several groups have recently highlighted the serious error in the process. Although empirical or regression-based scaling factors may be used to mitigate the common underprediction, they provide unsatisfying solutions because the reasoning behind the underlying error has yet to be determined. One previously noted trend was intrinsic clearance-dependent underprediction, highlighting the limitations of current in vitro systems. When applying these generated in vitro intrinsic clearance values during drug development and making first-in-human dose predictions for new chemical entities though, hepatic clearance is the parameter that must be estimated using a model of hepatic disposition, such as the well-stirred model. Here, we examine error across hepatic clearance ranges and find a similar hepatic clearance-dependent trend, with high clearance compounds not predicted to be so, demonstrating another gap in the field.

6-Chloro-5-[4-(1-Hydroxycyclobutyl)Phenyl]-1H-Indole-3-Carboxylic Acid is a Highly Selective Substrate for Glucuronidation by UGT1A1, Relative to β-Estradiol

6-Chloro-5-[4-(1-hydroxycyclobutyl)phenyl]-1H-indole-3-carboxylic acid (PF-06409577) is a direct activator of the human β1-containing adenosine monophosphate-activated protein kinase (ΑMPK) isoforms. The clearance mechanism of PF-06409577 in animals and humans involves uridine diphosphoglucuronosyl transferase (UGT)-mediated glucuronidation to an acyl glucuronide metabolite of PF-06409577 [(2S,3S,4S,5R,6S)-6-((6-chloro-5-(4-(1-hydroxycyclobutyl)phenyl)-1H-indole-3-carbonyl)oxy)-3,4,5-trihydroxytetrahydro-2H-pyran-2-carboxylic acid (M1)], which retains selective activation of human β1-containing AMPK isoforms. This paper describes a detailed characterization of the human UGT isoform(s) responsible for glucuronidation of PF-06409577 to M1. Studies using a panel of 13 human recombinant UGT (hrUGT) enzymes indicated that PF-06409577 was converted to M1 in a highly selective fashion by UGT1A1, which was further verified in human liver microsomes treated with specific chemical inhibitors, and in different UGT1A1 expressers. Conversion of PF-06409577 to M1 by UGT1A1 occurred in a relatively selective fashion, compared with β-estradiol (ES), a conventional probe substrate of UGT1A1. The Michaelis-Menten constant (K M) and V max values describing the formation of M1 from PF-06409577 in hrUGT1A1 and microsomal preparations from human intestine, liver, and kidney ranged from 131 to 212 μM (K M) and 107-3834 pmol/min per milligram (V max) in the presence of 2% bovine serum albumin. Relative activity factors (RAF) were determined for UGT1A1 using PF-06409577 and ES to enable estimation of intrinsic clearance from various tissues. RAF values from PF-06409577 and ES were generally comparable with the exception of intestinal microsomes, where ES overestimated the RAF of UGT1A1 due to glucuronidation by intestinal UGT1A8 and UGT1A10. Our results suggest the potential utility of PF-06409477 as a selective probe UGT1A1 substrate for UGT reaction phenotyping and inhibition studies in preclinical discovery/development.

Organic Anion Transporter 2-Mediated Hepatic Uptake Contributes to the Clearance of High-Permeability-Low-Molecular-Weight Acid and Zwitterion Drugs: Evaluation Using 25 Drugs

High-permeability-low-molecular-weight acids/zwitterions [i.e., extended clearance classification system class 1A (ECCS 1A) drugs] are considered to be cleared by metabolism with a minimal role of membrane transporters in their hepatic clearance. However, a marked disconnect in the in vitro-in vivo (IVIV) translation of hepatic clearance is often noted for these drugs. Metabolic rates measured using human liver microsomes and primary hepatocytes tend to underpredict. Here, we evaluated the role of organic anion transporter 2 (OAT2)-mediated hepatic uptake in the clearance of ECCS 1A drugs. For a set of 25 ECCS 1A drugs, in vitro transport activity was assessed using transporter-transfected cells and primary human hepatocytes. All but two drugs showed substrate affinity to OAT2, whereas four (bromfenac, entacapone, fluorescein, and nateglinide) also showed OATP1B1 activity in transfected cells. Most of these drugs (21 of 25) showed active uptake by plated human hepatocytes, with rifamycin SV (pan-transporter inhibitor) reducing the uptake by about 25%-95%. Metabolic turnover was estimated for 19 drugs after a few showed no measurable substrate depletion in liver microsomal incubations. IVIV extrapolation using in vitro data was evaluated to project human hepatic clearance of OAT2-alone substrates considering 1) uptake transport only, 2) metabolism only, and 3) transporter-enzyme interplay (extended clearance model). The transporter-enzyme interplay approach achieved improved prediction accuracy (average fold error = 1.9 and bias = 0.93) compared with the other two approaches. In conclusion, this study provides functional evidence for the role of OAT2-mediated hepatic uptake in determining the pharmacokinetics of several clinically important ECCS 1A drugs.

An examination of protein binding and protein-facilitated uptake relating to in vitro-in vivo extrapolation

As explained by the free drug theory, the unbound fraction of drug has long been thought to drive the efficacy of a molecule. Thus, the fraction unbound term, or f_u, appears in equations for fundamental pharmacokinetic parameters such as clearance, and is used when attempting in vitro to in vivo extrapolation (IVIVE). In recent years though, it has been noted that IVIVE does not always yield accurate predictions, and that some highly protein bound ligands have more efficient uptake than can be explained by their unbound fractions. This review explores the evolution of f_u terms included when implementing IVIVE, the concept of protein-facilitated uptake, and the mechanisms that have been proposed to account for facilitated uptake.

Sugar Acetonides are a Superior Motif for Addressing the Large, Solvent-Exposed Ribose-33 Pocket of tRNA-Guanine Transglycosylase

The intestinal disease shigellosis caused by Shigella bacteria affects over 120 million people annually. There is an urgent demand for new drugs as resistance against common antibiotics emerges. Bacterial tRNA-guanine transglycosylase (TGT) is a druggable target and controls the pathogenicity of Shigella flexneri. We report the synthesis of sugar-functionalized lin-benzoguanines addressing the ribose-33 pocket of TGT from Zymomonas mobilis. Ligand binding was analyzed by isothermal titration calorimetry and X-ray crystallography. Pocket occupancy was optimized by variation of size and protective groups of the sugars. The participation of a polycyclic water-cluster in the recognition of the sugar moiety was revealed. Acetonide-protected ribo- and psicofuranosyl derivatives are highly potent, benefiting from structural rigidity, good solubility, and metabolic stability. We conclude that sugar acetonides have a significant but not yet broadly recognized value in drug development.

NF-κB activation in astrocytes drives a stage-specific beneficial neuroimmunological response in ALS

Astrocytes are involved in non‐cell‐autonomous pathogenic cascades in amyotrophic lateral sclerosis (ALS); however, their role is still debated. We show that astrocytic NF‐κB activation drives microglial proliferation and leukocyte infiltration in the SOD1 (G93A) ALS model. This response prolongs the presymptomatic phase, delaying muscle denervation and decreasing disease burden, but turns detrimental in the symptomatic phase, accelerating disease progression. The transition corresponds to a shift in the microglial phenotype showing two effects that can be dissociated by temporally controlling NF‐κB activation. While NF‐κB activation in astrocytes induced a Wnt‐dependent microglial proliferation in the presymptomatic phase with neuroprotective effects on motoneurons, in later stage, astrocyte NF‐κB‐dependent microglial activation caused an accelerated disease progression. Notably, suppression of the early microglial response by CB₂R agonists had acute detrimental effects. These data identify astrocytes as important regulators of microglia expansion and immune response. Therefore, stage‐dependent microglia modulation may be an effective therapeutic strategy in ALS.

N-aryl-piperidine-4-carboxamides as a novel class of potent inhibitors of MALT1 proteolytic activity

Starting from a weak screening hit, potent and selective inhibitors of the MALT1 protease function were elaborated. Advanced compounds displayed high potency in biochemical and cellular assays. Compounds showed activity in a mechanistic Jurkat T cell activation assay as well as in the B-cell lymphoma line OCI-Ly3, which suggests potential use of MALT1 inhibitors in the treatment of autoimmune diseases as well as B-cell lymphomas with a dysregulated NF-κB pathway. Initially, rat pharmacokinetic properties of this compound series were dominated by very high clearance which could be linked to amide cleavage. Using a rat hepatocyte assay a good in vitro-in vivo correlation could be established which led to the identification of compounds with improved PK properties.

Sandwich-Cultured Hepatocytes for Mechanistic Understanding of Hepatic Disposition of Parent Drugs and Metabolites by Transporter-Enzyme Interplay

Functional interplay between transporters and drug-metabolizing enzymes is currently one of the hottest topics in the field of drug metabolism and pharmacokinetics. Uptake transporter-enzyme interplay is important to determine intrinsic hepatic clearance based on the extended clearance concept. Enzyme and efflux transporter interplay, which includes both sinusoidal (basolateral) and canalicular efflux transporters, determines the fate of metabolites formed in the liver. As sandwich-cultured hepatocytes (SCHs) maintain metabolic activities and form a canalicular network, the whole interplay between uptake and efflux transporters and drug-metabolizing enzymes can be investigated simultaneously. In this article, we review the utility and applicability of SCHs for mechanistic understanding of hepatic disposition of both parent drugs and metabolites. In addition, the utility of SCHs for mimicking species-specific disposition of parent drugs and metabolites in vivo is described. We also review application of SCHs for clinically relevant prediction of drug-drug interactions caused by drugs and metabolites. The usefulness of mathematical modeling of hepatic disposition of parent drugs and metabolites in SCHs is described to allow a quantitative understanding of an event in vitro and to develop a more advanced model to predict in vivo disposition.

Rational approach to highly potent and selective apoptosis signal-regulating kinase 1 (ASK1) inhibitors

Many diseases are believed to be driven by pathological levels of reactive oxygen species (ROS) and oxidative stress has long been recognized as a driver for inflammatory disorders. Apoptosis signal-regulating kinase 1 (ASK1) has been reported to be activated by intracellular ROS and its inhibition leads to a down regulation of p38-and JNK-dependent signaling. Consequently, ASK1 inhibitors may have the potential to treat clinically important inflammatory pathologies including renal, pulmonary and liver diseases. Analysis of the ASK1 ATP-binding site suggested that Gln756, an amino acid that rarely occurs at the GK+2 position, offered opportunities for achieving kinase selectivity for ASK1 which was applied to the design of a parallel medicinal chemistry library that afforded inhibitors of ASK1 with nanomolar potency and excellent kinome selectivity. A focused optimization strategy utilizing structure-based design resulted in the identification of ASK1 inhibitors with low nanomolar potency in a cellular assay, high selectivity when tested against kinase and broad pharmacology screening panels, and attractive physicochemical properties. The compounds we describe are attractive tool compounds to inform the therapeutic potential of ASK1 inhibition.

From Structure-Activity Relationships on Thiazole Derivatives to the In Vivo Evaluation of a New Radiotracer for Cannabinoid Subtype 2 PET Imaging

Upregulation of the cannabinoid type 2 receptors (CB₂R) unveils inflammation processes of pathological disorders, such as cancer, pain, or neurodegenerative diseases. Among others, CB₂R agonist A-836339 has been labeled with carbon-11 for PET imaging of the CB₂R and displayed promising results in a mouse model of Alzheimer's disease. The aim of the present work was to develop fluorinated analogs of A-836339 for labeling with fluorine-18 to design a new PET tracer for CB₂R imaging. Seven fluorinated analogs of A-836339 were synthesized in two to three steps and their binding affinities and selectivities for both the human and the mouse CB₂R were measured as well as their early ADME profiles. Among them, compound 2f (K_ihCB2R = 0.1 nM, K_ihCB1R/K_ihCB2R = 300) displayed high affinity and selectivity for CB₂R but also promising lipophilicity, kinetic solubility, and membrane permeation properties and was further selected for in vitro metabolism studies. Incubation of 2f with human or rat liver microsomes followed by LC/MS analysis revealed the presence of six different metabolites mainly resulting from oxidation reactions. A tosylated precursor of 2f was synthesized in two steps and radiolabeled with fluorine-18 to afford [¹⁸F]2f in 15 ± 5% radiochemical yield and a molar activity of 110 ± 30 GBq/μmol. Autoradiographies of rat spleen and biodistribution studies in healthy rats including pretreatments with either CB₂R or CB₁R-specific compounds suggested that [¹⁸F]2f is a specific tracer for the CB₂R in vivo. We have therefore demonstrated here that [¹⁸F]2f is a promising novel tracer for imaging CB₂R in vivo using PET. Further investigation in animal models of inflammation will follow.

An Accelerator Mass Spectrometry-Enabled Microtracer Study to Evaluate the First-Pass Effect on the Absorption of YH4808

¹⁴ C-labeled YH4808, a novel potassium-competitive acid blocker, was intravenously administered as a microtracer at 80 μg (11.8 kBq or 320 nCi) concomitantly with the nonradiolabeled oral drug at 200 mg to determine the absolute bioavailability and to assess the effect of pharmacogenomics on the oral absorption of YH4808. The absolute bioavailability was low and highly variable (mean, 10.1%; range, 2.3-19.3%), and M3 and M8, active metabolites of YH4808, were formed 22.6- and 38.5-fold higher after oral administration than intravenous administration, respectively. The product of the fraction of an oral YH4808 dose entering the gut wall and the fraction of YH4808 passing on to the portal circulation was larger in subjects carrying the variants of the CHST3, SLC15A1, and SULT1B1 genes. A combined LC+AMS is a useful tool to construct a rich and highly informative pharmacokinetic knowledge core in early clinical drug development at a reasonable cost.

Cannabinoid CB2 receptor ligand profiling reveals biased signalling and off-target activity

The cannabinoid CB2 receptor (CB2R) represents a promising therapeutic target for various forms of tissue injury and inflammatory diseases. Although numerous compounds have been developed and widely used to target CB2R, their selectivity, molecular mode of action and pharmacokinetic properties have been poorly characterized. Here we report the most extensive characterization of the molecular pharmacology of the most widely used CB2R ligands to date. In a collaborative effort between multiple academic and industry laboratories, we identify marked differences in the ability of certain agonists to activate distinct signalling pathways and to cause off-target effects. We reach a consensus that HU910, HU308 and JWH133 are the recommended selective CB2R agonists to study the role of CB2R in biological and disease processes. We believe that our unique approach would be highly suitable for the characterization of other therapeutic targets in drug discovery research.

Quantitative Assessment of Population Variability in Hepatic Drug Metabolism Using a Perfused Three-Dimensional Human Liver Microphysiological System

In this work, we first describe the population variability in hepatic drug metabolism using cryopreserved hepatocytes from five different donors cultured in a perfused three-dimensional human liver microphysiological system, and then show how the resulting data can be integrated with a modeling and simulation framework to accomplish in vitro-in vivo translation. For each donor, metabolic depletion profiles of six compounds (phenacetin, diclofenac, lidocaine, ibuprofen, propranolol, and prednisolone) were measured, along with metabolite formation, mRNA levels of 90 metabolism-related genes, and markers of functional viability [lactate dehydrogenase (LDH) release, albumin, and urea production]. Drug depletion data were analyzed with mixed-effects modeling. Substantial interdonor variability was observed with respect to gene expression levels, drug metabolism, and other measured hepatocyte functions. Specifically, interdonor variability in intrinsic metabolic clearance ranged from 24.1% for phenacetin to 66.8% for propranolol (expressed as coefficient of variation). Albumin, urea, LDH, and cytochrome P450 mRNA levels were identified as significant predictors of in vitro metabolic clearance. Predicted clearance values from the liver microphysiological system were correlated with the observed in vivo values. A population physiologically based pharmacokinetic model was developed for lidocaine to illustrate the translation of the in vitro output to the observed pharmacokinetic variability in vivo. Stochastic simulations with this model successfully predicted the observed clinical concentration-time profiles and the associated population variability. This is the first study of population variability in drug metabolism in the context of a microphysiological system and has important implications for the use of these systems during the drug development process.

Small structural changes of the imidazopyridine diacylglycerol acyltransferase 2 (DGAT2) inhibitors produce an improved safety profile

Small molecule DGAT2 inhibitors have shown promise for the treatment of metabolic diseases in preclinical models. Herein, we report the first toxicological evaluation of imidazopyridine-based DGAT2 inhibitors and show that the arteriopathy associated with imidazopyridine 1 can be mitigated with small structural modifications, and is thus not mechanism related.

Determination of Unbound Partition Coefficient and in Vitro-in Vivo Extrapolation for SLC13A Transporter-Mediated Uptake

Unbound partition coefficient (Kpuu) is important to an understanding of the asymmetric free drug distribution of a compound between cells and medium in vitro, as well as between tissue and plasma in vivo, especially for transporter-mediated processes. Kpuu was determined for a set of compounds from the SLC13A family that are inhibitors and substrates of transporters in hepatocytes and transporter-transfected cell lines. Enantioselectivity was observed, with (R)-enantiomers achieving much higher Kpuu (>4) than the (S)-enantiomers (<1) in human hepatocytes and SLC13A5-transfected human embryonic 293 cells. The intracellular free drug concentration correlated directly with in vitro pharmacological activity rather than the nominal concentration in the assay because of the high Kpuu mediated by SLC13A5 transporter uptake. Delivery of the diacid PF-06649298 directly or via hydrolysis of the ethyl ester prodrug PF-06757303 resulted in quite different Kpuu values in human hepatocytes (Kpuu of 3 for diacid versus 59 for prodrug), which was successfully modeled on the basis of passive diffusion, active uptake, and conversion rate from ester to diacid using a compartmental model. Kpuu values changed with drug concentrations; lower values were observed at higher concentrations possibly owing to a saturation of transporters. Michaelis-Menten constant (Km) of SLC13A5 was estimated to be 24 μM for PF-06649298 in human hepatocytes. In vitro Kpuu obtained from rat suspension hepatocytes supplemented with 4% fatty acid free bovine serum albumin showed good correlation with in vivo Kpuu of liver-to-plasma, illustrating the potential of this approach to predict in vivo Kpuu from in vitro systems.

Etoxazole is Metabolized Enantioselectively in Liver Microsomes of Rat and Human in Vitro

Acaricide etoxazole belongs to the ovicides/miticides diphenyloxazole class, affecting adults to lay sterile eggs by inhibiting chitin biosynthesis possibly. The reverse-phase HPLC-MS/MS method was used to determine the etoxazole enantiomers. The enantioselective degradation behavior of rac-etoxazole in liver microsomes of rat and human in vitro with NADPH was dramatically different. The t1/2 of (R)-etoxazole was 15.23 min in rat liver microsomes and 30.54 min in human liver microsomes, while 21.73 and 23.50 min were obtained for (S)-etoxazole, respectively. The Vmax of (R)-etoxazole was almost 5-fold of (S)-etoxazole in liver microsomes of rat in vitro. However, the Vmax of (S)-etoxazole was almost 2-fold of (R)-etoxazole in liver microsomes of human in vitro. The CLint of etoxazole was also shown the enantioselectivity on the contrary in liver microsomes of rat and human. These results indicated that the metabolism of two etoxazole enantiomers was selective in liver microsomes of rat and human in vitro, and enantioselectivity in the two kinds of liver microsomes was in the difference in degradation performance. The reason might be related to the composition and content involved in the enzyme system.

Applicability of the Rayleigh equation for enantioselective metabolism of chiral xenobiotics by microsomes, hepatocytes and in-vivo retention in rabbit tissues

In this study we propose a new approach for analyzing the enantioselective biodegradation of some antidepressant drugs mediated by human and rat liver microsomes by using the Rayleigh equation to describe the enantiomeric enrichment−conversion dependencies. Analysis of reported degradation data of additional six pesticides, an alpha blocker and a flame retardant by microsomes or hepatocytes in vitro reaffirmed the universality of the approach. In all the in vitro studied cases that involved enantioselective degradation, a Rayleigh dependence of the enantiomeric enrichment was observed. Published data regarding in vivo retention of myclobutanil in liver, kidney, muscle and brain tissues of rabbits following injection of the racemate were remodeled showing prevalence of the Rayleigh law for the chiral enrichment of the fungicide in the various tissues. This approach will revolutionize data organization in metabolic pathway research of target xenobiotics by either liver microsomes, hepatocytes or their organ-specific in vivo retention. The fact that the enantiomeric enrichment as a function of the conversion can be described by a single quantifier, will pave the road for the use of structure activity predictors of the enantiomeric enrichment and for mechanistic discrimination based on parametric dependence of the quantifier.

Enantioselective Metabolism of Flufiprole in Rat and Human Liver Microsomes

The enantioselective metabolism of flufiprole in rat and human liver microsomes in vitro was investigated in this study. The separation and determination were performed using a liquid chromatography system equipped with a triple-quadrupole mass spectrometer and a Lux Cellulose-2 chiral column. The enantioselective metabolism of rac-flufiprole was dramatically different in rat and human liver microsomes in the presence of the β-nicotinamide adenine dinucleotide phosphate regenerating system. The half-lives (t1/2) of flufiprole in rat and human liver microsomes were 7.22 and 21.00 min, respectively, for R-(+)-flufiprole, whereas the values were 11.75 and 17.75 min, respectively, for S-(-)-flufiprole. In addition, the Vmax of R-(+)-flufiprole was about 3-fold that of S-(-)-flufiprole in rat liver microsomes, whereas its value in the case of S-(-)-flufiprole was about 2-fold that of R-(+)-flufiprole in human liver microsomes. The CLint of rac-flufiprole also showed opposite enantioselectivy in rat and human liver microsomes. The different compositions and contents of metabolizing enzyme in the two liver microsomes might be the reasons for the difference in the metabolic behavior of the two enantiomers.

QSARs for estimating intrinsic hepatic clearance of organic chemicals in humans

Quantitative structure-activity relationships (QSARs) were developed to predict the in vitro clearance (CLINT) of xenobiotics metabolised in human hepatocytes (118 compounds) and microsomes (115 compounds). Clearance values were gathered from the scientific literature and multiple linear models were built and validated selecting at most 6 predictors from a pool of over 2000 potential molecular descriptors. For the hepatocytes QSAR, the explained variance (Radj(2)) was 67% and the predictive ability (Rext(2)) was 62%. For the microsomes QSAR, Radj(2) was 50% and Rext(2) 30%. For both liver assays, the most important descriptor relates to electronic properties of the compound. Functional groups of fragments were useful to identify specific compounds that have a deviating reaction rate compared to the others, such as polychlorobiphenyls (PCBs) and organic amides which were poorly metabolised by hepatocytes and microsomes, respectively. For hepatocytes, clearance was predominantly determined by electronic characteristics, while size and shape characteristics were less important and partitioning properties were absent. This may suggest that uptake across the membrane and enzyme binding are not rate-limiting steps. Particularly for hepatocytes the QSAR statistics are encouraging, allowing application of the outcomes in in vitro to in vivo extrapolation.

Transport-Metabolism Interplay of Atazanavir in Rat Hepatocytes

The aim of this study was to explore the mechanisms governing the intra- to extracellular unbound concentration ratio (Kpu,u) for the HIV protease inhibitor atazanavir (ATV) in rat hepatocytes. We had previously proposed a new method to determine Kpu,u by using the unbound Km values from metabolism studies with suspended rat hepatocytes and rat liver microsomes. Following that method, we determined that the value of ATV Kpu,u was 0.32, indicating that ATV hepatocellular clearance is uptake rate-limited. This hypothesis was supported by the linear correlation between Kpu,u and active uptake clearance (P = 0.04; R(2)=0.82) in the presence of increasing concentrations of the uptake transport inhibitor losartan. Moreover, in contrast to an expected increase of Kpu,u upon inhibition of ATV metabolism, a decrease of Kpu,u was observed, suggesting an increased impact of sinusoidal efflux. In summary, involvement of active uptake transport does not guarantee high intracellular accumulation; however, it has a key role in regulating intracellular drug concentrations and drug metabolism. These findings will help improve future in vitro-to-in vivo extrapolations and likewise physiologically based pharmacokinetic models.

Pharmacokinetics and Disposition of the Thiouracil Derivative PF-06282999, an Orally Bioavailable, Irreversible Inactivator of Myeloperoxidase Enzyme, Across Animals and Humans

The thiouracil derivative PF-06282999 [2-(6-(5-chloro-2-methoxyphenyl)-4-oxo-2-thioxo-3,4-dihydropyrimidin-1(2H)-yl)acetamide] is an irreversible inactivator of myeloperoxidase and is currently in clinical trials for the potential treatment of cardiovascular diseases. Concerns over idiosyncratic toxicity arising from bioactivation of the thiouracil motif to reactive species in the liver have been largely mitigated through the physicochemical (molecular weight, lipophilicity, and topological polar surface area) characteristics of PF-06282999, which generally favor elimination via nonmetabolic routes. To test this hypothesis, pharmacokinetics and disposition studies were initiated with PF-06282999 using animals and in vitro assays, with the ultimate goal of predicting human pharmacokinetics and elimination mechanisms. Consistent with its physicochemical properties, PF-06282999 was resistant to metabolic turnover from liver microsomes and hepatocytes from animals and humans and was devoid of cytochrome P450 inhibition. In vitro transport studies suggested moderate intestinal permeability and minimal transporter-mediated hepatobiliary disposition. PF-06282999 demonstrated moderate plasma protein binding across all of the species. Pharmacokinetics in preclinical species characterized by low to moderate plasma clearances, good oral bioavailability at 3- to 5-mg/kg doses, and renal clearance as the projected major clearance mechanism in humans. Human pharmacokinetic predictions using single-species scaling of dog and/or monkey pharmacokinetics were consistent with the parameters observed in the first-in-human study, conducted in healthy volunteers at a dose range of 20-200 mg PF-06282999. In summary, disposition characteristics of PF-06282999 were relatively similar across preclinical species and humans, with renal excretion of the unchanged parent emerging as the principal clearance mechanism in humans, which was anticipated based on its physicochemical properties and supported by preclinical studies.

In Vitro Kinetic Characterization of Axitinib Metabolism

N-Methyl-2-[3-((E)-2-pyridin-2-yl-vinyl)-1H-indazol-6-ylsulfanyl]-benzamide (axitinib) is an oral inhibitor of vascular endothelial growth factor receptors 1-3, which is approved for the treatment of advanced renal cell cancer. Human [(14)C]-labeled clinical studies indicate axitinib's primary route of clearance is metabolism. The aims of the in vitro experiments presented herein were to identify and characterize the enzymes involved in axitinib metabolic clearance. In vitro biotransformation studies of axitinib identified a number of metabolites including an axitinib sulfoxide, several less abundant oxidative metabolites, and glucuronide conjugates. The most abundant NADPH- and UDPGA-dependent metabolites, axitinib sulfoxide (M12) and axitinib N-glucuronide (M7) were selected for phenotyping and kinetic study. Phenotyping experiments with human liver microsomes (HLMs) using chemical inhibitors and recombinant human cytochrome P450s demonstrated axitinib was predominately metabolized by CYP3A4/5, with minor contributions from CYP2C19 and CYP1A2. The apparent substrate concentration at half-maximal velocity (Km) and Vmax values for the formation of axitinib sulfoxide by CYP3A4 or CYP3A5 were 4.0 or 1.9 µM and 9.6 or 1.4 pmol·min(-1)·pmol(-1), respectively. Using a CYP3A4-specific inhibitor (Cyp3cide) in liver microsomes expressing CYP3A5, 66% of the axitinib intrinsic clearance was attributable to CYP3A4 and 15% to CYP3A5. Axitinib N-glucuronidation was primarily catalyzed by UDP-glucuronosyltransferase (UGT) UGT1A1, which was verified by chemical inhibitors and UGT1A1 null expressers, with lesser contributions from UGTs 1A3, 1A9, and 1A4. The Km and Vmax values describing the formation of the N-glucuronide in HLM or rUGT1A1 were 2.7 µM or 0.75 µM and 8.9 or 8.3 pmol·min(-1)·mg(-1), respectively. In summary, CYP3A4 is the major enzyme involved in axitinib clearance with lesser contributions from CYP3A5, CYP2C19, CYP1A2, and UGT1A1.

Discovery and characterization of novel inhibitors of the sodium-coupled citrate transporter (NaCT or SLC13A5)

Citrate is a key regulatory metabolic intermediate as it facilitates the integration of the glycolysis and lipid synthesis pathways. Inhibition of hepatic extracellular citrate uptake, by blocking the sodium-coupled citrate transporter (NaCT or SLC13A5), has been suggested as a potential therapeutic approach to treat metabolic disorders. NaCT transports citrate from the blood into the cell coupled to the transport of sodium ions. The studies herein report the identification and characterization of a novel small dicarboxylate molecule (compound 2) capable of selectively and potently inhibiting citrate transport through NaCT, both in vitro and in vivo. Binding and transport experiments indicate that 2 specifically binds NaCT in a competitive and stereosensitive manner, and is recognized as a substrate for transport by NaCT. The favorable pharmacokinetic properties of 2 permitted in vivo experiments to evaluate the effect of inhibiting hepatic citrate uptake on metabolic endpoints.

Applicability of second-generation upcyte® human hepatocytes for use in CYP inhibition and induction studies

Human upcyte® hepatocytes are proliferating hepatocytes that retain many characteristics of primary human hepatocytes. We conducted a comprehensive evaluation of the application of second-generation upcyte® hepatocytes from four donors for inhibition and induction assays using a selection of reference inhibitors and inducers. CYP1A2, CYP2B6, CYP2C9, and CYP3A4 were reproducibly inhibited in a concentration-dependent manner and the calculated IC50 values for each compound correctly classified them as potent inhibitors. Upcyte® hepatocytes were responsive to prototypical CYP1A2, CYP2B6, CYP2C9, and CYP3A4 inducers, confirming that they have functional AhR-, CAR-, and PXR-mediated CYP regulation. A panel of 11 inducers classified as potent, moderate or noninducers of CYP3A4 and CYP2B6 were tested. There was a good fit of data from upcyte® hepatocytes to three different predictive models for CYP3A4 induction, namely the Relative Induction Score (RIS), AUCu/F2, and C max,u/Ind50. In addition, PXR (rifampicin) and CAR-selective (carbamazepine and phenytoin) inducers of CYP3A4 and CYP2B6 induction, respectively, were demonstrated. In conclusion, these data support the use of second-generation upcyte® hepatocytes for CYP inhibition and induction assays. Under the culture conditions used, these cells expressed CYP activities that were equivalent to or higher than those measured in primary human hepatocyte cultures, which could be inhibited or induced by prototypical CYP inhibitors and inducers, respectively. Moreover, they can be used to predict in vivo CYP3A4 induction potential using three prediction models. Bulk availability of cells from multiple donors makes upcyte® hepatocytes suitable for DDI screening, as well as more in-depth mechanistic investigations.

Verapamil hepatic clearance in four preclinical rat models: towards activity-based scaling

The current study was designed to cross-validate rat liver microsomes (RLM), suspended rat hepatocytes (SRH) and the isolated perfused rat liver (IPRL) model against in vivo pharmacokinetic data, using verapamil as a model drug. Michaelis-Menten constants (Km), for the metabolic disappearance kinetics of verapamil in RLM and SRH (freshly isolated and cryopreserved), were determined and corrected for non-specific binding. The 'unbound' Km determined with RLM (2.8 µM) was divided by the 'unbound' Km determined with fresh and cryopreserved SRH (3.9 µM and 2.1 µM, respectively) to calculate the ratio of intracellular to extracellular unbound concentration (Kpu,u). Kpu,u was significantly different between freshly isolated (0.71) and cryopreserved (1.31) SRH, but intracellular capacity for verapamil metabolism was maintained after cryopreservation (200 vs. 191 µl/min/million cells). Direct comparison of intrinsic clearance values (Clint) in RLM versus SRH, yielded an activity-based scaling factor (SF) of 0.28-0.30 mg microsomal protein/million cells (MPPMC). Merging the IPRL-derived Clint with the MPPMC and SRH data, resulted in scaling factors for MPPGL (80 and 43 mg microsomal protein/g liver) and HPGL (269 and 153 million cells/g liver), respectively. Likewise, the hepatic blood flow (61 ml/min/kg b.wt) was calculated using IPRL Clint and the in vivo Cl. The scaling factors determined here are consistent with previously reported CYP450-content based scaling factors. Overall, the results show that integrated interpretation of data obtained with multiple preclinical tools (i.e. RLM, SRH, IPRL) can contribute to more reliable estimates for scaling factors and ultimately to improved in vivo clearance predictions based on in vitro experimentation.

Rationalizing underprediction of drug clearance from enzyme and transporter kinetic data: from in vitro tools to mechanistic modeling

Over the years, there has been an increase in the number and quality of available in vitro tools for the assessment of clearance. Complexity of data analysis and modelling of corresponding in vitro data has increased in an analogous manner, in particular for the simultaneous characterization of transporter and metabolism kinetics, together with intracellular binding and passive diffusion. In the current chapter, the impact of different factors on the in vitro-in vivo extrapolation of clearance will be addressed in a stepwise manner, from the selection of the most adequate in vitro system and experimental design/condition to the corresponding modelling of data generated. The application of static or physiologically based pharmacokinetic models in the prediction of clearance will be discussed, highlighting limitations and current challenges of some of the approaches. Particular focus will be on the ability of in vitro and in silico predictive tools to overcome the trend of clearance underprediction. Improvements made as a result of inclusion of extrahepatic metabolism and consideration of transporter-metabolism interplay across different organs will be discussed.

Permeability comparison between hepatocyte and low efflux MDCKII cell monolayer

Determination of passive permeability is not only important for predicting oral absorption and brain penetration, but also for accurately predicting hepatic clearance. High throughput (HT) measurement of passive permeability across hepatocyte cell membrane is technically more challenging than using monolayer cell-based permeability assays. In this study, we evaluated if the HT Madin-Darby canine kidney II-low efflux (MDCKII-LE) cell monolayer permeability assay can be used as a surrogate to predict the passive permeability of hepatocytes. Apparent passive permeability of MDCKII-LE is well correlated to passive diffusion clearance of human and rat hepatocytes, suggesting that the HT MDCKII-LE assay can be used as a surrogate to estimate the passive permeability of hepatocytes. In addition, lipophilicity (Log D determined at pH 7.4) was also found to be well correlated with both MDCKII-LE and hepatocyte permeability for most compounds, hence it may serve as another permeability surrogate.

Metabolism-directed structure optimization of benzimidazole-based Francisella tularensis enoyl-reductase (FabI) inhibitors

1. FabI is a potential antibiotic target against Francisella tularensis, which has been classified as a Category A biowarfare agent of high risk to public health. Our previous work demonstrated that N-benzyl benzimidazole compounds possess promising FabI inhibitory activity, but their druggability properties, including metabolic stability, are unknown. 2. The objective of this study was to characterize structure-metabolism relationships of a series of N-benzyl benzimidazole compounds to guide chemical optimization for better metabolic stability. To this end, metabolic stability data were obtained for 22 initial lead compounds using mouse hepatic microsomes. 3. Metabolic hotspots on the benzimidazole core structure as well as the benzyl ring were identified and verified by metabolite identification studies of four model compounds. Interestingly, the proposed structure-metabolism relationships did not apply to nine newly synthesized cyclopentane or oxacyclopentane derivatives of N-benzyl benzimidazole. 4. Subsequently, in silico quantitative structure-property relationship models were developed. Four molecular descriptors representing molecular polarity/polarisability, symmetry and size were identified to best explain variability in metabolic stability of different compounds. Multi-linear and non-linear regression models based on the selected molecular descriptors were developed and validated. 5. The structure-metabolism relationships for N-benzyl benzimidazole compounds should help optimization of N-benzyl benzimidazole compounds for better pharmacokinetic behavior.