Interspecies variation in liver weight, hepatic blood flow, and antipyrine intrinsic clearance: extrapolation of data to benzodiazepines and phenytoin

Another string to your bow: machine learning prediction of the pharmacokinetic properties of small molecules

INTRODUCTION

Prediction of pharmacokinetic (PK) properties is crucial for drug discovery and development. Machine-learning (ML) models, which use statistical pattern recognition to learn correlations between input features (such as chemical structures) and target variables (such as PK parameters), are being increasingly used for this purpose. To embed ML models for PK prediction into workflows and to guide future development, a solid understanding of their applicability, advantages, limitations, and synergies with other approaches is necessary.

AREAS COVERED

This narrative review discusses the design and application of ML models to predict PK parameters of small molecules, especially in light of established approaches including in vitro-in vivo extrapolation (IVIVE) and physiologically based pharmacokinetic (PBPK) models. The authors illustrate scenarios in which the three approaches are used and emphasize how they enhance and complement each other. In particular, they highlight achievements, the state of the art and potentials of applying machine learning for PK prediction through a comphrehensive literature review.

EXPERT OPINION

ML models, when carefully crafted, regularly updated, and appropriately used, empower users to prioritize molecules with favorable PK properties. Informed practitioners can leverage these models to improve the efficiency of drug discovery and development process.

Risk assessment of N-nitrosamines in food

EFSA was asked for a scientific opinion on the risks to public health related to the presence of N-nitrosamines (N-NAs) in food. The risk assessment was confined to those 10 carcinogenic N-NAs occurring in food (TCNAs), i.e. NDMA, NMEA, NDEA, NDPA, NDBA, NMA, NSAR, NMOR, NPIP and NPYR. N-NAs are genotoxic and induce liver tumours in rodents. The in vivo data available to derive potency factors are limited, and therefore, equal potency of TCNAs was assumed. The lower confidence limit of the benchmark dose at 10% (BMDL10) was 10 μg/kg body weight (bw) per day, derived from the incidence of rat liver tumours (benign and malignant) induced by NDEA and used in a margin of exposure (MOE) approach. Analytical results on the occurrence of N-NAs were extracted from the EFSA occurrence database (n = 2,817) and the literature (n = 4,003). Occurrence data were available for five food categories across TCNAs. Dietary exposure was assessed for two scenarios, excluding (scenario 1) and including (scenario 2) cooked unprocessed meat and fish. TCNAs exposure ranged from 0 to 208.9 ng/kg bw per day across surveys, age groups and scenarios. 'Meat and meat products' is the main food category contributing to TCNA exposure. MOEs ranged from 3,337 to 48 at the P95 exposure excluding some infant surveys with P95 exposure equal to zero. Two major uncertainties were (i) the high number of left censored data and (ii) the lack of data on important food categories. The CONTAM Panel concluded that the MOE for TCNAs at the P95 exposure is highly likely (98-100% certain) to be less than 10,000 for all age groups, which raises a health concern.

Establishing allometric relationships between microsomal protein and cytochrome P450 content with body weight in vertebrate species

Data from in vitro studies are routinely used to estimate in vivo hepatic clearance of chemicals and this information is needed to parameterise physiologically based kinetic models. Such clearance data can be obtained from laboratory experiments using liver microsomes, hepatocytes, precision-cut liver slices or recombinant enzymes. Irrespective of the selected test system, scaling factors are required to convert the in vitro measured intrinsic clearance to a whole liver intrinsic clearance. Scaling factors such as the hepatic microsomal protein per gram of liver and/or the amount of cytochrome P450 per hepatocyte provide a means to calculate the whole liver intrinsic clearance. Here, a database from the peer-reviewed literature has been developed and provides quantitative metrics on microsomal protein (MP) and cytochrome P450 contents in vertebrate orders namely amphibians, mammals, birds, fish and reptiles. This database allows to address allometric relationships between body weight and MP content, and body weight and cytochrome P450 content. A total of 85 and 74 vertebrate species were included to assess the relationships between log10 body weight versus log10 MP, and between log10 body weight and log10 cytochrome P450 content, respectively. The resulting slopes range from 0.76 to 1.45 in a range of vertebrate species. Such data-driven allometric relationships can be used to estimate the MP content necessary for in vitro to in vivo extrapolation of in vitro clearance data. Future work includes applications of these relationships for different vertebrate taxa using quantitative in vitro to in vivo extrapolation models coupled to physiologically based kinetic models using chemicals of relevance as case studies including pesticides, contaminants and feed additives.

Surgical Models of Liver Regeneration in Pigs: A Practical Review of the Literature for Researchers

The remarkable capacity of regeneration of the liver is well known, although the involved mechanisms are far from being understood. Furthermore, limits concerning the residual functional mass of the liver remain critical in both fields of hepatic resection and transplantation. The aim of the present study was to review the surgical experiments regarding liver regeneration in pigs to promote experimental methodological standardization. The Pubmed, Medline, Scopus, and Cochrane Library databases were searched. Studies evaluating liver regeneration through surgical experiments performed on pigs were included. A total of 139 titles were screened, and 41 articles were included in the study, with 689 pigs in total. A total of 29 studies (71% of all) had a survival design, with an average study duration of 13 days. Overall, 36 studies (88%) considered partial hepatectomy, of which four were an associating liver partition and portal vein ligation for staged hepatectomy (ALPPS). Remnant liver volume ranged from 10% to 60%. Only 2 studies considered a hepatotoxic pre-treatment, while 25 studies evaluated additional liver procedures, such as stem cell application, ischemia/reperfusion injury, portal vein modulation, liver scaffold application, bio-artificial, and pharmacological liver treatment. Only nine authors analysed how cytokines and growth factors changed in response to liver resection. The most used imaging system to evaluate liver volume was CT-scan volumetry, even if performed only by nine authors. The pig represents one of the best animal models for the study of liver regeneration. However, it remains a mostly unexplored field due to the lack of experiments reproducing the chronic pathological aspects of the liver and the heterogeneity of existing studies.

Comparison of toxicokinetic properties of eleven analogues of Bisphenol A in pig after intravenous and oral administrations

Due to the restrictions of its use, Bisphenol A (BPA) has been replaced by many structurally related bisphenols (BPs) in consumer products. The endocrine disrupting potential similar to that of BPA has been described for several bisphenols, there is therefore an urgent need of toxicokinetic (TK) data for these emerging BPs in order to evaluate if their internal exposure could increase the risk of endocrine disruption. We investigated TK behaviors of eleven BPA substitutes (BPS, BPAF, BPB, BPF, BPM, BPZ, 3-3BPA, BP4-4, BPAP, BPP, and BPFL) by intravenous and oral administrations of mixtures of them to piglets and serial collection of blood over 72 h and urine over 24 h, to evaluate their disposition. Data were analyzed using nonlinear mixed-effects modeling and a comparison was made with TK predicted by the generic model HTTK package. The low urinary excretion of some BPs, in particular BPM, BPP and BPFL, is an important aspect to consider in predicting human exposure based on urine biomonitoring. Despite their structural similarities, for the same oral dose, all BPA analogues investigated showed a higher systemic exposure (area under the plasma concentration-time curve (AUC) of the unconjugated Bisphenol) than BPA (2 to 4 fold for 3-3BPA, BPAF, BPB and BPZ, 7-20 fold for BP4-4, BPAP, BPP, BPFL, BPF and BPM and 150 fold for BPS) due mainly to a considerable variation of oral bioavailability (proportion of BP administered by oral route that attains the systemic circulation unchanged). Given similarities in the digestive tract between pigs and humans, our TK data suggest that replacing BPA with some of its alternatives, particularly BPS, will likely lead to higher internal exposure to potential endocrine disruptive compounds. These findings are crucial for evaluating the risk of human exposure to these emerging BPs.

Comparison of hydromorphone and butorphanol for management of pain in equine patients undergoing elective arthroscopy: a randomized clinical trial

OBJECTIVE

To compare the effects of hydromorphone and butorphanol in horses undergoing arthroscopy and describe the pharmacokinetics of hydromorphone in anesthetized horses.

STUDY DESIGN

Randomized controlled clinical trial.

ANIMALS

A total of 40 adult horses admitted for elective arthroscopy.

METHODS

Horses were randomly assigned to be administered intravenous hydromorphone (0.04 mg kg-1; group TxH; n = 19) or butorphanol (0.02 mg kg-1; group TxB; n = 21) prior to surgery as part of a standardized anesthetic protocol. Pain was scored by two observers unaware of group assignment using the Equine Utrecht University Scale for Facial Assessment of Pain (EQUUS-FAP) and a composite pain scale (CPS) prior to surgery (baseline), 2 hours (P2) and 4 hours (P4) following recovery from anesthesia. Blood samples were collected at various time points for determination of plasma hydromorphone concentration using liquid chromatography-tandem mass spectrometry. Data were analyzed with a mixed-effect model.

RESULTS

Median (range) baseline EQUUS-FAP was 1.2 (0.0-4.0) with no effect of group, time points or interaction. Baseline CPS was similar between groups. Group TxH baseline CPS was 2.5 (0.0-10.0), increased at P2 [4.5 (0-10.0); p = 0.046] and returned to baseline values at P4 [3.0 (0.0-11.0)]. Group TxB baseline CPS was 2.0 (0.0-8.0), increased at P2 [3.5 (0.0-11.0); p = 0.009] and P4 [5.0 (0.0-11.0); p < 0.001]. Pharmacokinetic terminal half-life was 774 ± 82.3 minutes, area under the curve was 1362 ± 314 ng minutes mL-1, clearance was 30.7 ± 7.23 mL minute-1 kg-1 and volume of distribution at steady state was 884 ± 740 mL kg-1.

CONCLUSIONS

Hydromorphone, but not butorphanol, decreased CPS back to baseline at P4 after recovery.

CLINICAL RELEVANCE

Hydromorphone may provide superior postoperative analgesia compared with butorphanol in horses undergoing arthroscopy.

Toxicokinetics of U-47700, tramadol, and their main metabolites in pigs following intravenous administration: is a multiple species allometric scaling approach useful for the extrapolation of toxicokinetic parameters to humans?

New synthetic opioids (NSOs) pose a public health concern since their emergence on the illicit drug market and are gaining increasing importance in forensic toxicology. Like many other new psychoactive substances, NSOs are consumed without any preclinical safety data or any knowledge on toxicokinetic (TK) data. Due to ethical reasons, controlled human TK studies cannot be performed for the assessment of these relevant data. As an alternative animal experimental approach, six pigs per drug received a single intravenous dose of 100 µg/kg body weight (BW) of U-47700 or 1000 µg/kg BW of tramadol to evaluate whether this species is suitable to assess the TK of NSOs. The drugs were determined in serum and whole blood using a fully validated method based on solid-phase extraction and LC–MS/MS. The concentration–time profiles and a population (pop) TK analysis revealed that a three-compartment model best described the TK data of both opioids. Central volumes of distribution were 0.94 L/kg for U-47700 and 1.25 L/kg for tramadol and central (metabolic) clearances were estimated at 1.57 L/h/kg and 1.85 L/h/kg for U-47700 and tramadol, respectively. The final popTK model parameters for pigs were upscaled via allometric scaling techniques. In comparison to published human data, concentration–time profiles for tramadol could successfully be predicted with single species allometric scaling. Furthermore, possible profiles for U-47700 in humans were simulated. The findings of this study indicate that unlike a multiple species scaling approach, pigs in conjunction with TK modeling are a suitable tool for the assessment of TK data of NSOs and the prediction of human TK data.

Effects of membrane transport activity and cell metabolism on the unbound drug concentrations in the skeletal muscle and liver of drugs: A microdialysis study in rats

The unbound concentrations of 14 commercial drugs, including five non-efflux/uptake transporter substrates-Class I, five efflux transporter substrates-class II and four influx transporter substrates-Class III, were simultaneously measured in rat liver, muscle, and blood via microanalysis. Kpuu,liver and Kpuu,muscle were calculated to evaluate the membrane transport activity and cell metabolism on the unbound drug concentrations in the skeletal muscle and liver. For Class I compounds, represented by antipyrine, unbound concentrations among liver, muscle and blood are symmetrically distributed when compound hepatic clearance is low. And when compound hepatic clearance is high, unbound concentrations among liver, muscle and blood are asymmetrically distributed, such as Propranolol. For Class II and III compounds, overall, the unbound concentrations among liver, muscle, and blood are asymmetrically distributed due to a combination of hepatic metabolism and efflux and/or influx transporter activity.

Investigating the glucuronidation and sulfation pathways contribution and disposition kinetics of Bisphenol S and its metabolites using LC-MS/MS-based nonenzymatic hydrolysis method

Despite showing serious health consequences and widespread exposure, the toxicokinetic information required to evaluate the health risks of BPS is insufficient. Thus, we aim to describe the comprehensive toxicokinetics of BPS and its glucuronide (BPS-G) and sulfate (BPS-S) metabolites in rats. Simultaneous quantification of BPS and its metabolites (authentic standards) was accomplished using UPLC-MS/MS method. BPS displayed rapid absorption, extensive metabolism and fast elimination after oral administration. Following intravenous administration, BPS exhibited CL (8.8 L/h/kg) higher than the rat hepatic blood flow rate suggesting the likelihood of extrahepatic clearance. The CL value differed from those reported previously (sheep and piglets) and the probable reason could be attributed to dose- and/or interspecies differences. BPS was extensively metabolized and excreted primarily through urine as BPS-G (∼56%). BPS and BPS-S exhibited a high protein binding capacity in comparison to BPS-G. In in vitro metabolic stability study, BPS was predominantly metabolized through glucuronidation. The predicted in vivo hepatic clearance of BPS suggested it to be a high and intermediate clearance chemical in rats and humans, respectively. The significant interspecies difference observed in the clearance of BPS between rats and humans indicated that toxicokinetics of BPS should be considered for health risk assessment in humans.

Nicotine Population Pharmacokinetics in Healthy Smokers After Intravenous, Oral, Buccal and Transdermal Administration

Background

In 4 decades, numerous nicotine replacement therapy products have been developed. Population pharmacokinetic models can support exposure–response modeling and inform nicotine replacement therapy product development, but only limited model-based cross-study population pharmacokinetic analyses for nicotine replacement therapy products have been published.

Objectives

The aim of this retrospective analysis was to assess the population pharmacokinetics of nicotine across intravenous, oral, transdermal and oromucosal (mouth spray, chewing gum, lozenge and inhaler) routes and formulations in healthy smoking subjects.

Methods

Data on 930 unique subjects (46,016 observations) from 29 single- and repeated-dose studies with multiple formulations across intravenous, oral, transdermal and oromucosal routes of administration were included. Data from intravenous and extravascular routes of administration were modelled separately for run efficiency reasons. For developing extravascular models, clearance and disposition parameters and their inter-individual variabilities were fixed to the estimates for intravenously delivered nicotine. Detectable pre-dose nicotine concentrations were modelled as a hypothetical nicotine bolus into the central compartment at the start of wash-out. Modelling repeated-dose oral and buccal administrations required a time-dependent increase in clearance or decrease in bioavailability to describe the data adequately.

Results

Disposition of intravenous nicotine was best described by a three-compartment model with initial and terminal half-lives of 7 min and 4.5 h, respectively, and the absorption of single oral doses was best described with a first-order absorption rate constant of 1.55 h⁻¹. The data of buccal formulations were modelled with parallel oromucosal absorption and gastrointestinal absorption of a part of the dose that is swallowed. For transdermal nicotine, parallel zero- and first-order release from the patch and a transit-compartment absorption model best described the data. Key pharmacokinetic parameters were reliably estimated, with typical values for clearance (67 L/h for a 70-kg subject), volume of distribution (4.3 L/kg), oral bioavailability (40%) and transdermal bioavailability (76%) within expected ranges. The estimated fraction of the dose swallowed for buccal formulations ranged from 55% (gum) to 69% (lozenge).

Conclusions

Robust population pharmacokinetic models were developed for five nicotine replacement therapy product types and for intravenous and oral nicotine. These population pharmacokinetic models are used in exposure–response analyses and simulation-based nicotine replacement therapy product design.

Electronic supplementary material

The online version of this article (10.1007/s40262-020-00960-5) contains supplementary material, which is available to authorized users.

Exposure of Rats to Multiple Oral Doses of Dichloroacetate Results in Upregulation of Hepatic Glutathione Transferases and NAD(P)H Dehydrogenase [Quinone] 1

Dichloroacetate (DCA) is an investigational drug that is used in the treatment of various congenital and acquired disorders of energy metabolism. Although DCA is generally well tolerated, some patients experience peripheral neuropathy, a side effect more common in adults than children. Repetitive DCA dosing causes downregulation of its metabolizing enzyme, glutathione transferase zeta 1 (GSTZ1), which is also critical in the detoxification of maleylacetoacetate and maleylacetone. GSTZ1 (-/-) knockout mice show upregulation of glutathione transferases (GSTs) and antioxidant enzymes as well as an increase in the ratio of oxidized glutathione (GSSG) to reduced glutathione (GSH), suggesting GSTZ1 deficiency causes oxidative stress. We hypothesized that DCA-mediated depletion of GSTZ1 causes oxidative stress and used the rat to examine induction of GSTs and antioxidant enzymes after repeated DCA exposure. We determined the expression of alpha, mu, pi, and omega class GSTs, NAD(P)H dehydrogenase [quinone] 1 (NQO1), gamma-glutamylcysteine ligase complex (GCLC), and glutathione synthetase (GSS). GSH and GSSG levels were measured by liquid chromatography-tandem mass spectrometry. Enzyme activity was measured in hepatic cytosol using 1-chloro-2,4-dinitrobenzene, 1,2-dichloro-4-nitrobenzene, and 2,6-dichloroindophenol as substrates. In comparison with acetate-treated controls, DCA dosing increased the relative expression of GSTA1/A2 irrespective of rodent age, whereas only adults displayed higher levels of GSTM1 and GSTO1. NQO1 expression and activity were higher in juveniles after DCA dosing. GSH concentrations were increased by DCA in adults, but the GSH:GSSG ratio was not changed. Levels of GCLC and GSS were higher and lower, respectively, in adults treated with DCA. We conclude that DCA-mediated depletion of GSTZ1 causes oxidative stress and promotes the induction of antioxidant enzymes that may vary between age groups. SIGNIFICANCE STATEMENT: Treatment with the investigational drug, dichloroacetate (DCA), results in loss of glutathione transferase zeta 1 (GSTZ1) and subsequent increases in body burden of the electrophilic tyrosine metabolites, maleylacetoacetate and maleylacetone. Loss of GSTZ1 in genetically modified mice is associated with induction of glutathione transferases and alteration of the ratio of oxidized to reduced glutathione. Therefore, we determined whether pharmacological depletion of GSTZ1 through repeat administration of DCA produced similar changes in the liver, which could affect responses to other drugs and toxicants.

Effects of Multiple Doses of Dichloroacetate on GSTZ1 Expression and Activity in Liver and Extrahepatic Tissues of Young and Adult Rats

Glutathione transferase zeta 1 (GSTZ1), expressed in liver and several extrahepatic tissues, catalyzes dechlorination of dichloroacetate (DCA) to glyoxylate. DCA inactivates GSTZ1, leading to autoinhibition of its metabolism. DCA is an investigational drug for treating several congenital and acquired disorders of mitochondrial energy metabolism, including cancer. The main adverse effect of DCA, reversible peripheral neuropathy, is more common in adults treated long-term than in children, who metabolize DCA more quickly after multiple doses. One dose of DCA to Sprague Dawley rats reduced GSTZ1 expression and activity more in liver than in extrahepatic tissues; however, the effects of multiple doses of DCA that mimic its therapeutic use have not been studied. Here, we examined the expression and activity of GSTZ1 in cytosol and mitochondria of liver, kidney, heart, and brain 24 hours after completion of 8-day oral dosing of 100 mg/kg per day sodium DCA to juvenile and adult Sprague Dawley rats. Activity was measured with DCA and with 1,2-epoxy-3-(4-nitrophenoxy)propane (EPNPP), reported to be a GSTZ1-selective substrate. In DCA-treated rats, liver retained higher expression and activity of GSTZ1 with DCA than other tissues, irrespective of rodent age. DCA-treated juvenile rats retained more GSTZ1 activity with DCA than adults. Consistent with this finding, there was less measurable DCA in tissues of juvenile than adult rats. DCA-treated rats retained activity with EPNPP, despite losing over 98% of GSTZ1 protein. These data provide insight into the differences between children and adults in DCA elimination under a therapeutic regimen and confirm that the liver contributes more to DCA metabolism than other tissues. SIGNIFICANCE STATEMENT: Dichloroacetate (DCA) is one of few drugs exhibiting higher clearance from children than adults, after repeated doses, for reasons that are unclear. We hypothesized that juveniles retain more glutathione transferase zeta 1 (GSTZ1) than adults in tissues after multiple DCA doses and found this was the case for liver and kidney, with rat as a model to assess GSTZ1 protein expression and activity with DCA. Although 1,2-epoxy-3-(4-nitrophenoxy)propane was reported to be a selective GSTZ1 substrate, its activity was not reduced in concert with GSTZ1 protein.

Toxicokinetics and toxicodynamics of the fentanyl homologs cyclopropanoyl-1-benzyl-4´-fluoro-4-anilinopiperidine and furanoyl-1-benzyl-4-anilinopiperidine

The two fentanyl homologs cyclopropanoyl-1-benzyl-4´-fluoro-4-anilinopiperidine (4F-Cy-BAP) and furanoyl-1-benzyl-4-anilinopiperidine (Fu-BAP) have recently been seized as new psychoactive substances (NPS) on the drugs of abuse market. As their toxicokinetic and toxicodynamic characteristics are completely unknown, this study focused on elucidating their in vitro metabolic stability in pooled human liver S9 fraction (pHLS9), their qualitative in vitro (pHLS9), and in vivo (zebrafish larvae) metabolism, and their in vitro isozyme mapping using recombinant expressed isoenzymes. Their maximum-tolerated concentration (MTC) in zebrafish larvae was studied from 0.01 to 100 µM. Their µ-opioid receptor (MOR) activity was analyzed in engineered human embryonic kidney (HEK) 293 T cells. In total, seven phase I and one phase II metabolites of 4F-Cy-BAP and 15 phase I and four phase II metabolites of Fu-BAP were tentatively identified by means of liquid chromatography high-resolution tandem mass spectrometry, with the majority detected in zebrafish larvae. N-Dealkylation, N-deacylation, hydroxylation, and N-oxidation were the most abundant metabolic reactions and the corresponding metabolites are expected to be promising analytical targets for toxicological analysis. Isozyme mapping revealed the main involvement of CYP3A4 in the phase I metabolism of 4F-Cy-BAP and in terms of Fu-BAP additionally CYP2D6. Therefore, drug-drug interactions by CYP3A4 inhibition may cause elevated drug levels and unwanted adverse effects. MTC experiments revealed malformations and changes in the behavior of larvae after exposure to 100 µM Fu-BAP. Both substances were only able to produce a weak activation of MOR and although toxic effects based on MOR activation seem unlikely, activity at other receptors cannot be excluded.

In vitro-in vivo extrapolation of metabolic clearance using human liver microsomes: factors showing variability and their normalization

In vitro-in vivo extrapolation (IVIVE) using human liver microsomes has been widely used to predict metabolic clearance, but some of the factors used in the process of prediction show variability for the same compound: notably, microsomal intrinsic clearance values corrected by the unbound fraction (CL_{int, u}), physiological parameters used for scale-up, and the source of in vivo clearance data.The purpose of this study was to assess the correlation between in vitro and in vivo CL_int with a focus on factors showing variability using four cytochrome P450 (CYP)3A substrates.We surveyed in vivo clearance values in literature and also determined the microsomal CL_{int, u} values. A scaling factor (SF_direct) was defined as in vivo CL_int divided by the microsomal CL_{int, u}, which ranged from 1190 to 2310 (mg protein per kg body weight). The application of a mean SF_direct of 1600 (mg protein per kg body weight) and further normalization by the microsomal CL_{int, u} values of midazolam, the most commonly used substrate, resulted in improved prediction accuracy for CL_{int, u} values from various microsomal batches.The results suggest the normalization of variability might be useful for predicting the in vivo CL_int.

Toxicokinetic Studies and Analytical Toxicology of the New Synthetic Opioids Cyclopentanoyl-Fentanyl and Tetrahydrofuranoyl-Fentanyl

The growing number of new synthetic opioids (NSO) on the new psychoactive substances (NPS) market bears new challenges in toxicology. As their toxicodynamics and particularly their toxicokinetics are usually unknown, impact on human health is not yet fully understood. Detection of the 2 NSO cyclopentanoyl-fentanyl (CP-F) and tetrahydrofuranoyl-fentanyl (THF-F) was first reported in 2016. Both were involved in several fatal intoxication cases, but no detailed information about their toxicological characteristics is available so far. The main purpose of this study was therefore to investigate the in vitro toxicokinetics and in vivo analytical toxicology of CP-F and THF-F by means of liquid chromatography high-resolution tandem mass spectrometry (LC-HRMS/MS). These studies included metabolic stability, phase I and II metabolism, isozyme mapping, plasma protein binding and detectability in LC-HRMS/MS standard urine screening approaches (SUSA) using rat urine samples. In total, 12 phase I metabolites of CP-F and 13 of THF-F were identified, among them 9 metabolites described for the first time. Overall, N-dealkylations, hydroxylations and dihydroxylations were the main metabolic reactions. The cytochrome P450 (CYP) isozymes mainly involved were CYP2D6 and CYP3A4, leading to elevated drug levels and intoxications in CYP2D6 poor metabolizers. CP-F showed a high plasma protein binding of 99%, which may increase the risk of toxicity by simultaneous intake of other highly bound drugs. Detectability studies showed that neither the parent compounds nor their metabolites were detectable in rat urine using LC-HRMS/MS SUSA. However, a more sophisticated analytical strategy was successfully applied and should be used for analytical confirmation of an intake of CP-F and/or THF-F.

Bile canaliculi remodeling activates YAP via the actin cytoskeleton during liver regeneration

The mechanisms of organ size control remain poorly understood. A key question is how cells collectively sense the overall status of a tissue. We addressed this problem focusing on mouse liver regeneration. Using digital tissue reconstruction and quantitative image analysis, we found that the apical surface of hepatocytes forming the bile canalicular network expands concomitant with an increase in F-actin and phospho-myosin, to compensate an overload of bile acids. These changes are sensed by the Hippo transcriptional co-activator YAP, which localizes to apical F-actin-rich regions and translocates to the nucleus in dependence of the integrity of the actin cytoskeleton. This mechanism tolerates moderate bile acid fluctuations under tissue homeostasis, but activates YAP in response to sustained bile acid overload. Using an integrated biophysical-biochemical model of bile pressure and Hippo signaling, we explained this behavior by the existence of a mechano-sensory mechanism that activates YAP in a switch-like manner. We propose that the apical surface of hepatocytes acts as a self-regulatory mechano-sensory system that responds to critical levels of bile acids as readout of tissue status.

Pharmacokinetics of maropitant citrate in New Zealand White rabbits (Oryctolagus cuniculus)

OBJECTIVE

To determine the pharmacokinetics and adverse effects of maropitant citrate after IV and SC administration to New Zealand White rabbits (Oryctolagus cuniculus).

ANIMALS

11 sexually intact (3 males and 8 females) adult rabbits.

PROCEDURES

Each rabbit received maropitant citrate (1 mg/kg) IV or SC. Blood samples were collected at 9 (SC) or 10 (IV) time points over 48 hours. After a 2-week washout period, rabbits received maropitant by the alternate administration route. Pharmacokinetic parameters were calculated. Body weight, food and water consumption, injection site, mentation, and urine and fecal output were monitored.

RESULTS

Mean ± SD maximum concentration after SC administration was 14.4 ± 10.9 ng/mL and was detected at 1.25 ± 0.89 hours. Terminal half-life after IV and SC administration was 10.4 ± 1.6 hours and 13.1 ± 2.44 hours, respectively. Bioavailability after SC administration was 58.9 ± 13.3%. Plasma concentration at 24 hours was 2.87 ± 1.69 ng/mL after IV administration and 3.4 ± 1.2 ng/mL after SC administration. Four rabbits developed local dermal reactions at the injection site after SC injection. Increased fecal production was detected on the day of treatment and 1 day after treatment.

CONCLUSIONS AND CLINICAL RELEVANCE

Plasma concentrations of rabbits 24 hours after SC and IV administration of maropitant citrate (1 mg/kg) were similar to those of dogs at 24 hours. Reactions at the SC injection site were the most common adverse effect detected. Increased fecal output may suggest an effect on gastrointestinal motility. Additional pharmacodynamic and multidose studies are needed.

Pharmacokinetics and pharmacodynamics of hydromorphone after intravenous and intramuscular administration in horses

OBJECTIVE

To compare the pharmacokinetics and pharmacodynamics of hydromorphone in horses after intravenous (IV) and intramuscular (IM) administration.

STUDY DESIGN

Randomized, masked, crossover design.

ANIMALS

A total of six adult horses weighing [mean ± standard deviation (SD))] 447 ± 61 kg.

METHODS

Horses were administered three treatments with a 7 day washout. Treatments were hydromorphone 0.04 mg kg^⁻1 IV with saline administered IM (H-IV), hydromorphone 0.04 mg kg^⁻1 IM with saline IV (H-IM), or saline IV and IM (P). Blood was collected for hydromorphone plasma concentration at multiple time points for 24 hours after treatments. Pharmacodynamic data were collected for 24 hours after treatments. Variables included thermal nociceptive threshold, heart rate (HR), respiratory frequency (f_R), rectal temperature, and fecal weight. Data were analyzed using mixed-effects linear models. A p value of less than 0.05 was considered statistically significant.

RESULTS

The mean ± SD hydromorphone terminal half-life (t_1/2), clearance and volume of distribution of H-IV were 19 ± 8 minutes, 79 ± 12.9 mL minute^⁻1 kg^⁻1 and 1125 ± 309 mL kg^⁻1. The t_1/2 was 26.7 ± 9.25 minutes for H-IM. Area under the curve was 518 ± 87.5 and 1128 ± 810 minute ng mL^⁻1 for H-IV and H-IM, respectively. The IM bioavailability was 217%. The overall thermal thresholds for both H-IV and H-IM were significantly greater than P (p < 0.0001 for both) and baseline (p = 0.006). There was no difference in thermal threshold between H-IV and H-IM. No difference was found in physical examination variables among groups or in comparison to baseline. Fecal weight was significantly less than P for H-IV and H-IM (p = 0.02).

CONCLUSIONS AND CLINICAL RELEVANCE

IM hydromorphone has high bioavailability and provides a similar degree of antinociception to IV administration. IM hydromorphone in horses provides a similar degree and duration of antinociception to IV administration.

Toxicokinetics and analytical toxicology of the abused opioid U-48800 - in vitro metabolism, metabolic stability, isozyme mapping, and plasma protein binding

Due to the risk of new synthetic opioids (NSOs) for human health, the knowledge of their toxicokinetic characteristics is important for clinical and forensic toxicology. U-48800 is an NSO structurally non-related to classical opioids such as morphine or fentanyl and offered for abuse. As toxicokinetic data of U-48800 is not currently available, the aims of this study were to identify the in vitro metabolites of U-48800 in pooled human liver S9 fraction (pS9), to map the isozymes involved in the initial metabolic steps, and to determine further toxicokinetic data such as metabolic stability, including the in vitro half-life (t_1/2 ), and the intrinsic (CL_int ) and hepatic clearance (CL_h ). Furthermore, drug detectability studies in rat urine should be done using hyphenated mass spectrometry. In total, 13 phase I metabolites and one phase II metabolite were identified. N-Dealkylation, hydroxylation, and their combinations were the predominant metabolic reactions. The isozymes CYP2C19 and CYP3A4 were mainly involved in these initial steps. CYP2C19 poor metabolizers may suffer from an increased U-48800 toxicity. The in vitro t_1/2 and CL_int could be rated as moderate, compared to structural related compounds. After administration of an assumed consumer dose to rats, the unchanged parent compound was found only in very low abundance but three metabolites were detected additionally. Due to species differences, metabolites found in rats might be different from those in humans. However, phase I metabolites found in rat urine, the parent compound, and additionally the N-demethyl metabolite should be used as main targets in toxicological urine screening approaches.

Oral Systemic Bioavailability of Bisphenol A and Bisphenol S in Pigs

BACKGROUND

Given its hormonal activity, bisphenol S (BPS) as a substitute for bisphenol A (BPA) could actually increase the risk of endocrine disruption if its toxicokinetic (TK) properties, namely its oral availability and systemic persistency, were higher than those of BPA.

OBJECTIVES

The TK behavior of BPA and BPS was investigated by administering the two compounds by intravenous and oral routes in piglet, a known valid model for investigating oral TK.

METHODS

Experiments were conducted in piglets to evaluate the kinetics of BPA, BPS, and their glucuronoconjugated metabolites in plasma and urine after intravenous administration of BPA, BPS, and BPS glucuronide (BPSG) and gavage administration of BPA and BPS. A population semiphysiologically based TK model describing the disposition of BPA and BPS and their glucuronides was built from these data to estimate the key TK parameters that drive the internal exposure to active compounds.

RESULTS

The data indicated that almost all the BPS oral dose was absorbed and transported into the liver where only 41% of BPS was glucuronidated, leading to a systemic bioavailability of 57.4%. In contrast, only 77% of the oral dose of BPA was absorbed and underwent an extensive first-pass glucuronidation either in the gut (44%) or in the liver (53%), thus accounting for the low systemic bioavailability of BPA (0.50%). Due to the higher systemic availability of BPS, in comparison with BPA, and its lower plasma clearance (3.5 times lower), the oral BPS systemic exposure was on average about 250 times higher than for BPA for an equal oral molar dose of the two compounds.

CONCLUSION

Given the similar digestive tracts of pigs and humans, our results suggest that replacing BPA with BPS will likely lead to increased internal exposure to an endocrine-active compound that would be of concern for human health. https://doi.org/10.1289/EHP4599.

Interpretation of Non-Clinical Data for Prediction of Human Pharmacokinetic Parameters: In Vitro-In Vivo Extrapolation and Allometric Scaling

Extrapolation of pharmacokinetic (PK) parameters from in vitro or in vivo animal to human is one of the main tasks in the drug development process. Translational approaches provide evidence for go or no-go decision-making during drug discovery and the development process, and the prediction of human PKs prior to the first-in-human clinical trials. In vitro-in vivo extrapolation and allometric scaling are the choice of method for projection to human situations. Although these methods are useful tools for the estimation of PK parameters, it is a challenge to apply these methods since underlying biochemical, mathematical, physiological, and background knowledge of PKs are required. In addition, it is difficult to select an appropriate methodology depending on the data available. Therefore, this review covers the principles of PK parameters pertaining to the clearance, volume of distribution, elimination half-life, absorption rate constant, and prediction method from the original idea to recently developed models in order to introduce optimal models for the prediction of PK parameters.

Novel hits for acetylcholinesterase inhibition derived by docking-based screening on ZINC database

The inhibition of the enzyme acetylcholinesterase (AChE) increases the levels of the neurotransmitter acetylcholine and symptomatically improves the affected cognitive function. In the present study, we searched for novel AChE inhibitors by docking-based virtual screening of the standard lead-like set of ZINC database containing more than 6 million small molecules using GOLD software. The top 10 best-scored hits were tested in vitro for AChE affinity, neurotoxicity, GIT and BBB permeability. The main pharmacokinetic parameters like volume of distribution, free fraction in plasma, total clearance, and half-life were predicted by previously derived models. Nine of the compounds bind to the enzyme with affinities from 0.517 to 0.735 µM, eight of them are non-toxic. All hits permeate GIT and BBB and bind extensively to plasma proteins. Most of them are low-clearance compounds. In total, seven of the 10 hits are promising for further lead optimisation. These are structures with ZINC IDs: 00220177, 44455618, 66142300, 71804814, 72065926, 96007907, and 97159977.

The pharmacokinetics and pharmacodynamics of intravenous hydromorphone in horses

OBJECTIVE

Describe the pharmacokinetics and pharmacodynamics of intravenous hydromorphone in healthy horses.

STUDY DESIGN

Masked, randomized, cross-over, Latin square design.

ANIMALS

A group of eight healthy adult horses METHODS: Horses were administered each of four treatments with an 8 day washout. Treatments groups included intravenous hydromorphone 0.02 mg kg^-1 (LD), 0.04 mg kg^-1 (MD), 0.08 mg kg^-1 (HD) and saline (P). Blood samples for hydromorphone analysis were obtained for 24 hours after treatment. Plasma hydromorphone was quantified and pharmacokinetic parameters were determined using non-compartmental analysis. Pharmacodynamic data collected for 24 hours after treatment included thermal nociceptive threshold, heart rate (HR), respiratory rate (f_R) and rectal temperature, and analyzed using mixed-effects linear models.

RESULTS

Mean (± standard deviation) hydromorphone terminal half-life (t_1/2), systemic clearance and apparent volume of distribution at steady state (Vd_ss) were 18.1 ± 18.6, 34.0 ± 12.8, and 41.3 ± 32.5 minutes, 66.6 ± 5.3, 550.0 ± 76.4, and 92.7 ± 13.9 mL kg^-1 minute^-1, and 1118 ± 369, 1460 ± 325 and 2242 ± 950 mL kg^-1 for treatments LD, MD and HD, respectively. Thermal threshold increased significantly compared to baseline for all treatments for up to 12 hours. HR was elevated above baseline in treatments LD, MD and HD, extending to 30, 15 and 105 minutes after treatment, respectively. Respiratory rate was elevated above baseline in treatments MD and HD from 30 to 195 minutes and from 45 to 480 minutes after treatment, respectively. Temperature was elevated above baseline in treatment HD until 255 minutes after treatment.

CONCLUSIONS

Hydromorphone exhibited a short t_1/2, rapid clearance and large Vd_ss in horses. It also provided a dose-dependent increase in thermal threshold with associated increases in HR, f_R and rectal temperature.

CLINICAL RELEVANCE

Hydromorphone 0.04 mg kg^-1 provided clinically relevant thermal antinociception with minimal adverse effects.

Preclinical Pharmacokinetics of C118P, a Novel Prodrug of Microtubules Inhibitor and Its Metabolite C118 in Mice, Rats, and Dogs

C118P, a phosphate prodrug of C118, which is a novel microtubule protein inhibitor, is currently under Phase I clinical development in China for treating ovarian cancer and lung cancer. The preclinical pharmacokinetics of prodrug C118P and its metabolite C118 were extensively characterized in vivo in mice, rats, and dogs and in vitro to support the further development of C118P. The preclinical tissue distribution and excretion were investigated in rats. Plasma protein binding in mice, rat, and human, and hepatic microsomal metabolic stability in mice, rat, dog, monkey, and human, were also evaluated. The (AUC_0-inf) and C_30s of C118P at 50 mg/kg in rats and 6 mg/kg in dogs, and the C_2min of C118 at 6 mg/kg in dogs increased less than the dosage increase, suggested nonlinear pharmacokinetic occurred at high dose. As a prodrug, C118P can be quickly hydrolyzed into C118 after an intravenous administration. The unbound C118 in plasma is slightly higher than C118P. C118P can hardly penetrate the tissue, while C118 can distribute widely into tissues. In tumor-bearing nude mice, the concentration of C118 is high in lung, ovary, and tumor, with an extended half-life in tumor. C118P is a promising candidate prodrug for further clinical development.

Investigation of Interactive Activity of Electro-Acupuncture on Pharmacokinetics of Sildenafil and Their Synergistic Effect on Penile Blood Flow in Rats

Erectile dysfunction (ED) is a disorder found in males throughout the world, which negatively affects relationships with partners with advancing age. Hence, in this study, we tested a combined novel treatment of electro-acupuncture (EA) and sildenafil citrate against ED. In addition to EA therapy, the sildenafil citrate, a phosphodiesterase 5 inhibitor, is a widely recognized drug that has achieved considerable success in the treatment of ED. However, the combined effect of both the EA and sildenafil has not yet been investigated. Hence, we aimed to examine the effect of EA on the pharmacokinetics and pharmacodynamics of sildenafil in rat plasma. The pharmacokinetic parameters were determined using ultra performance liquid chromatography (UPLC) after EA and sildenafil administration (10 mg/Kg). Following this, the pharmacodynamics was studied via blood flow pattern using developing Doppler images of the lower body and penis. The pharmacokinetic studies demonstrated that sildenafil significantly increases by administration of low-frequency EA. Further, the pharmacodynamic studies using Doppler imaging revealed an elevated blood flow in rat penis compared with lower body during combined treatment of sildenafil and low-frequency EA. These data indicate a synergistic therapeutic effect of EA and sildenafil for the treatment of ED.

Comparison of predicted intrinsic hepatic clearance of 30 pharmaceuticals in canine and feline liver microsomes

1. Known cytochrome P450 (CYP) substrates in humans are used in veterinary medicine, with limited knowledge of the similarity or variation in CYP metabolism. Comparison of canine and feline CYP metabolism via liver microsomes report that human CYP probes and inhibitors demonstrate differing rates of intrinsic clearance (CL_int). 2. The purpose of this study was to utilize a high-throughput liver microsome substrate depletion assay, combined with microsomal and plasma protein binding to compare the predicted hepatic clearance (CL_hep) of thirty therapeutic agents used off-label in canines and felines, using both the well-stirred and parallel tube models. 3. In canine liver microsomes, 3/30 substrates did not have quantifiable CL_int, while midazolam and amitriptyline CL_int was too rapid for accurate determination. A CL_hep was calculated for 29/30 substrates in feline microsomes. Overall, canine CL_hep was faster compared to the feline, with fold differences ranging from 2-20-fold. 4. A comparison between the well-stirred and parallel tube model indicates that the parallel tube model reports a slighter higher CL_hep in both species. 5. The differences in CYP metabolism between canine and feline highlight the need for additional research into CYP expression and specificity.

Boi-ogi-to (TJ-20), a Kampo Formula, Suppresses the Inflammatory Bone Destruction and the Expression of Cytokines in the Synovia of Ankle Joints of Adjuvant Arthritic Rats

TJ-20 is a formula consisting of 6 herbs that has been used in the clinical treatment of rheumatoid arthritis (RA) in China and Japan for centuries. However, scientific evidence of the effects of TJ-20 has not been established. In the present study, we focused on the therapeutic effects and investigated the main function of TJ-20 on adjuvant arthritis (AA), an animal model of RA, which was induced with complete Freund's adjuvant (CFA). TJ-20 was administered orally at 600 mg/kg once a day from 0, 7, and 10 days to 8 weeks after the CFA treatment. TJ-20 significantly ameliorated inflammatory progression and bone destruction in AA in a time-dependent manner. Furthermore, TJ-20 significantly reduced the increased changes in a number of macrophages and helper T cells. Moreover, TJ-20 suppressed the expression of TNF-α whereas it augmented the expression of IL-10 and attenuated Th1 cells responses in the synovia of the ankle joint. Therefore, TJ-20 regulated the expression of proinflammatory and anti-inflammatory cytokines in macrophages and Th1/Th2 balance in the synovia of ankle joints in AA rats. These results suggest the positive anti-inflammatory effect of TJ-20 and provide a scientific basis for the clinical use of TJ-20 for RA.

Pharmacokinetics cannot explain the increased effective dose requirement for morphine and midazolam in rats during their extended administration alone or in combination

Objectives

Chronic administration of morphine and midazolam, alone or in combination, can induce tolerance to their effects. Data showed that co-administration of morphine and midazolam increased effective dose requirement of morphine, exceeding that observed with morphine alone.

Methods

To elucidate the pharmacokinetic component to the tolerance, we administered midazolam (2 mg/kg) and morphine (10 mg/kg) alone or their combination daily to rats for 12 days followed by a pharmacokinetic study on day 13. On the study day, each animal received a single bolus dose of 5 mg/kg morphine, and 2 mg/kg of midazolam 30 s later. Multiple blood samples were obtained for 6 h. Plasma drug concentrations were assayed by mass spectrometry optimized for small samples.

Key findings

Mean morphine clearance was as follows: 22.2, 27.2, 26.0 and 23.4 l/h per kg in the saline–saline, saline–midazolam, saline–morphine and midazolam–morphine groups, respectively. Corresponding midazolam clearances were 32.8, 23.0, 22.2 and 31.1 l/h per kg. ANOVA indicated no significant differences among the four groups in the clearances, half-lives, and volumes of distribution. Morphine and midazolam clearances were significantly correlated (R2 = 0.48, P &lt; 0.001).

Conclusions

This animal model suggests that altered pharmacokinetics cannot explain tolerance evidenced as increased dose requirement for morphine or midazolam, when administered alone or combination, for extended periods.

Preclinical pharmacokinetics and ADME characterization of a novel anticancer chalcone, cardamonin

Cardamonin (CRD), a chalconoid obtained from several medicinal plants of Zingiberaceae family, had shown promising potential in cancer prevention and therapy. For further development and better pharmacological elucidation, we performed a series of in vitro and in vivo studies to characterize its preclinical pharmacokinetics. The study samples were analyzed using validated liquid chromatography-tandem mass spectrometry (LC-MS/MS) and high performance liquid chromatography-ultra violet (HPLC-UV) methods. CRD is partially soluble (<10 μM) and possess high permeability (>0.2 × 10-4 cm/sec). It is moderately bound to plasma proteins (<50%). It shows partitioning in red blood cell (RBC) compartment with the partition coefficient between RBCs and plasma (K_RBC/P ) of 0.95 at 0 min to 1.39 at 60 min, indicating significant but slow RBC uptake. In mice, CRD is poorly absorbed after oral administration with 18% oral bioavailability. It possesses high clearance, short mean residence time, and high volume of distribution in mice. It exhibited multiple peak phenomena both after oral and intravenous administration and is excreted both as conjugated and unchanged CRD in bile. It is majorly excreted in faeces and negligibly in urine. The preclinical absorption, distribution, metabolism, and excretion data are expected to succour the future clinical investigations of CRD as a promising anticancer agent. Copyright © 2016 John Wiley & Sons, Ltd.

Obesogenic diets have deleterious effects on fat deposits irrespective of the nature of dietary carbohydrates in a Yucatan minipig model

The effects of digestible carbohydrates, fructose in particular, on the development of metabolic disturbances remain controversial. We explored the effects of prolonged consumption of high-fat diets differing in their carbohydrate source on fat deposits in the adult Yucatan minipig. Eighteen minipigs underwent computed tomographic imaging and blood sampling before and after 8 weeks of three isocaloric high-fat diets with different carbohydrate sources (20% by weight for starch in the control diet, glucose or fructose, n=6 per diet). Body adiposity, liver volume, and fat content were estimated from computed tomographic images (n=18). Liver volume and lipid content were also measured post mortem (n=12). We hypothesized that the quantity and the spatial distribution of fat deposits in the adipose tissue or in the liver would be altered by the nature of the carbohydrate present in the obesogenic diet. After 8 weeks of dietary exposure, body weight (from 26±4 to 58±3 kg), total body adiposity (from 38±1 to 47±1%; P<.0001), liver volume (from 1156±31 to 1486±66 mL; P<.0001), plasma insulin (from 10±1 to 14±2 mIU/L; P=.001), triacylglycerol (from 318±37 to 466±33 mg/L; P=.005), and free-fatty acids (from 196±60 to 396±59 μmol/L; P=.0001) increased irrespective of the carbohydrate type. Similarly, the carbohydrate type did not induce changes in the spatial repartition of the adipose tissue. Divergent results were obtained for fat deposits in the liver depending on the investigation method. In conclusion, obesogenic diets alter adipose tissue fat deposits and the metabolic profile independently of the nature of dietary carbohydrates.

Dynamic contrast-enhanced ultrasound of slaughterhouse porcine livers in machine perfusion

The aim of this study was to enable investigations into novel imaging and surgical techniques by developing a readily accessible, versatile liver machine perfusion system. Slaughterhouse pig livers were used, and dynamic contrast-enhanced ultrasound was introduced to optimize the procurement process and provide real-time perfusion monitoring. The system comprised a single pump, oxygenator, bubble trap and two flowmeters for pressure-controlled perfusion of the vessels using an off-the-shelf perfusate at room temperature. Successful livers exhibited homogeneous perfusion in both the portal vein and hepatic artery with dynamic contrast-enhanced ultrasound, which correlated with stable oxygen uptake, bile production and hepatic resistance and normal histology at the end of 3 h of perfusion. Dynamic contrast-enhanced ultrasound revealed perfusion abnormalities invisible to the naked eye, thereby providing context to the otherwise systemic biochemical/hemodynamic measurements and focal biopsy findings. The model developed here is a simple, cost-effective approach for stable ex vivo whole-organ machine perfusion.

In vitro hepatic microsomal metabolism of meloxicam in koalas (Phascolarctos cinereus), brushtail possums (Trichosurus vulpecula), ringtail possums (Pseudocheirus peregrinus), rats (Rattus norvegicus) and dogs (Canis lupus familiaris)

Quantitative and qualitative aspects of in vitro metabolism of the non-steroidal anti-inflammatory drug meloxicam, mediated via hepatic microsomes of specialized foliage (Eucalyptus) eating marsupials (koalas and ringtail possums), a generalized foliage eating marsupial (brushtail possum), rats, and dogs, are described. Using a substrate depletion method, intrinsic hepatic clearance (in vitro Clint) was determined. Significantly, rates of oxidative transformation of meloxicam, likely mediated via cytochromes P450 (CYP), were higher in marsupials compared to rats or dogs. The rank order of apparent in vitro Clint was brushtail possums (n=3) (mean: 394μL/min/mg protein), >koalas (n=6) (50), >ringtail possums (n=2) (36) (with no significant difference between koalas and ringtail possums), >pooled rats (3.2)>pooled dogs (in which the rate of depletion, as calculated by the ratio of the substrate remaining was <20% and too slow to determine). During the depletion of meloxicam, at a first-order rate constant, 5-hydroxymethyl metabolite (M1) was identified in the brushtail possums and the rat as the major metabolite. However, multiple hydroxyl metabolites were observed in the koala (M1, M2, and M3) and the ringtail possum (M1 and M3) indicating that these specialized foliage-eating marsupials have diverse oxidation capacity to metabolize meloxicam. Using a well-stirred model, the apparent in vitro Clint of meloxicam for koalas and the rat was further scaled to compare with published in vivo Cl. The closest in vivo Cl prediction from in vitro data of koalas was demonstrated with scaled hepatic Cl(total) (average fold error=1.9) excluding unbound fractions in the blood and microsome values; whereas for rats, the in-vitro scaled hepatic Cl fu(blood, mic), corrected with unbound fractions in the blood and microsome values, provided the best prediction (fold error=1.86). This study indicates that eutherians such as rats or dogs serve as inadequate models for dosage extrapolation of this drug to marsupials due to differences in hepatic turnover rate. Furthermore, as in vivo Cl is one of the pharmacokinetic indexes for determining therapeutic drug dosages, this study demonstrates the utility of in vitro to in vivo scaling as an alternative prediction method of drug Cl in koalas.