Physiologically Based Pharmacokinetic Modeling Involving Nonlinear Plasma and Tissue Binding: Application to Prednisolone and Prednisone in Rats.

The pharmacokinetics (PK) of prednisolone (PNL) exhibit nonlinearity related to plasma protein binding, tissue binding, metabolic interconversion with prednisone (PN), and renal elimination. Blood and 11 tissues were collected from male Wistar rats after steady-state (SS) infusion and after subcutaneous boluses of 50 mg/kg of PNL. Concentrations of PNL and PN were measured by liquid chromatography-tandem mass spectrometry. Plasma and tissue profiles were described using a complex physiologically based pharmacokinetics (PBPK) model. Concentrations of PN and PNL were in rapid equilibrium in plasma and tissues. The tissue partition coefficients (K p ) of PNL calculated from most subcutaneously dosed tissue and plasma areas were similar to SS infusion and in silico values. The blood-to-plasma ratio of PNL was 0.71 with similar red blood cell and unbound-plasma concentrations. Plasma protein binding (60%-90%) was related to corticosteroid-binding globulin (CBG) saturation. Tissue distribution was nonlinear. The equilibrium dissociation constant (K d ) of PNL shared by all tissues was 3.01 ng/ml, with the highest binding in muscle, followed by liver, heart, intestine, and bone and the lowest binding in skin, spleen, fat, kidney, lung, and brain. Fat and bone distribution assumed access only to interstitial space. Brain PNL concentrations (K p = 0.05) were low owing to presumed P-glycoprotein-mediated efflux. Clearances of CBG-free PNL were 1789 from liver and 191.2 ml/h from kidney. The PN/PNL ratio was nonlinear for plasma, spleen, heart, intestine, bone, fat, and linear for the remaining tissues. Our PBPK model with multiple complexities well described the PK profiles of PNL and PN in blood, plasma, and diverse tissues. SIGNIFICANCE STATEMENT: Because steroids, such as prednisolone and prednisone, have similar and complex pharmacokinetics properties in various species, receptors in most tissues, and multiple therapeutic and adverse actions, this physiologically based pharmacokinetics (PBPK) model may provide greater insights into the pharmacodynamic complexities of corticosteroids. The complex properties of these compounds require innovative PBPK modeling approaches that may be instructive for other therapeutic agents.

Physiologically Based Pharmacokinetics of Dexamethasone in Rats.

Blood and multitissue concentration-time profiles for dexamethasone (DEX), a synthetic corticosteroid, were measured in male rats after subcutaneous bolus and infusion dosing. A physiologically based pharmacokinetics (PBPK) model was applied for 12 measured tissues. Tissue partition coefficients (K p ) and metabolic clearance were assessed from infusion studies. Blood cell to plasma partitioning (0.664) and plasma free fraction (0.175) for DEX were found to be moderate. DEX was extensively partitioned into liver (K p = 6.76), whereas the calculated K p values of most tissues ranged between 0.1 and 1.5. Despite the moderate lipophilicity of DEX (log P = 1.8), adipose exhibited very limited distribution (K p = 0.17). Presumably due to P-glycoprotein-mediated efflux, DEX concentrations were very low in brain compared with its expected high permeability. Infusion studies yielded K p values from male and female rats at steady state that were similar. In silico K p values calculated for different tissues by using GastroPlus software were similar to in vivo values except for adipose and liver. Glucocorticoid receptors are found in diverse tissues, and these PBPK modeling results may help provide exposure profiles driving pharmacodynamic effects of DEX. SIGNIFICANCE STATEMENT: Our physiologically based pharmacokinetics model describes the experimentally determined tissue and plasma dexamethasone (DEX) pharmacokinetics (PK) profiles in rats reasonably well. This model can serve for further investigation of DEX tissue distribution in rats as the PK driving force for PD effects in different tissues. No major sex differences were found for DEX tissue distribution. Knowledge gained in this study may be translatable to higher-order species including humans.

Interactions of Tofacitinib and Dexamethasone on Lymphocyte Proliferation.

PURPOSES: Lymphocyte proliferation is a major factor determining the magnitude of the immune response. Both dexamethasone (DEX) and tofacitinib (TOF) exert marked immunosuppressive effects and are mainstay drugs in the treatment of rheumatoid arthritis (RA). This study was aimed to explore the single and combined anti-proliferative action of DEX and TOF on lymphocytes and their sex differences. METHODS: The single-drug effects and dual-drug interactions of TOF and DEX were assessed on the in vitro concanavalin A-stimulated proliferation of lymphocytes isolated from male and female rat and human peripheral blood. RESULTS: DEX was more potent than TOF across species and sex. DEX showed greater inhibition on rat lymphocytes compared to those from humans, which was reflected in both Imax and IC50. The antiproliferative action of TOF was comparable in rats and humans with exception of a higher IC50 in male rats. Both sex- and species-related differences were detected in DEX/TOF interactions with synergistic effects in male lymphocytes, and additive and antagonistic effect for females in humans and rats. CONCLUSION: TOF has a promising steroid-sparing potential with the beneficial effects of the combination therapy more likely in males than females.

Modeling Combined Anti-Inflammatory Effects of Dexamethasone and Tofacitinib in Arthritic Rats.

Tofacitinib (TOF), a Janus kinase (JAK) inhibitor, which was approved in 2012, has been recommended for the treatment of clinically active rheumatoid arthritis (RA). Dexamethasone (DEX), a potent corticosteroid, is also used in RA therapy but with limited usefulness due to dose- and time-dependent adverse effects. This pilot study examines the single and combined effects of DEX and TOF in order to explore the steroid-sparing potential of TOF. Collagen-induced arthritic (CIA) rats were subcutaneously (SC) dosed with vehicle, 1.5 mg/kg TOF, 5 mg/kg TOF, 0.225 mg/kg DEX, or a combination of 1.5 mg/kg TOF and 0.225 mg/kg DEX. Paw sizes were measured as an index of disease and drug efficacy and dynamically depicted using a logistic function for natural paw growth, a turnover model for disease progression, an indirect response model for inhibitory effects of TOF and DEX and a non-competitive interaction model for the combined effect of DEX and TOF. TOF alone exerted only a slight inhibitory effect on RA paw edema compared to DEX, which reduced edema by 40%. In combination, TOF and DEX had additive effects with an interaction factor of 0.76. Using model simulations, a single SC dose of TOF does not have a visible steroid-sparing potential, although BID oral dosing has such potential. The current study suggests an additive effect of TOF and DEX and simulations indicate that further exploration of TOF and DEX administration timing may produce desirable drug efficacy with lower DEX doses.

Modeling Corticosteroid Pharmacokinetics and Pharmacodynamics, Part II: Sex Differences in Methylprednisolone Pharmacokinetics and Corticosterone Suppression.

Methylprednisolone (MPL), a corticosteroid of intermediate potency, remains an important immunomodulatory agent for autoimmune diseases. Although sex differences in corticosteroid pharmacokinetics/pharmacodynamics (PK/PD) have been documented in humans, comprehensive preclinical assessments of such differences have not been conducted. Limited in vitro evidence indicates possible sex differences in corticosteroid PK and PD. Therefore, it is hypothesized that comparative PK/PD assessments of MPL disposition and selected PD actions in both sexes will provide insights into factors controlling sex differences in steroid responses. This report focused on the plasma and tissue pharmacokinetics of MPL and its adrenal suppressive effects. Because time-dependent (estrous) regulation of sex hormones in females can influence drug responses, female rats were studied in the proestrus (high estradiol/progesterone) and estrus (low estradiol/progesterone) phases of the reproductive cycle. Cohorts of male and female rats were given a 50 mg/kg bolus dose of MPL intramuscularly. Plasma and liver concentrations of MPL as well as plasma corticosterone concentrations were assayed using high-performance liquid chromatography. An enhanced minimal physiologically-based PK/PD model was developed to characterize MPL kinetics and corticosterone dynamics. The clearance of MPL was ∼3-fold higher in males compared with females, regardless of estrous phase, likely attributable to sex-specific hepatic metabolism in males. Strong inhibitory effects on adrenal suppression were observed in all animals. These temporal steroid profiles in plasma and tissues will be used to drive receptor/gene-mediated PD effects of MPL in both sexes, as described in a companion article (Part III). SIGNIFICANCE STATEMENT: Sex is a relevant factor influencing the pharmacokinetics (PK) and pharmacodynamics (PD) of drugs. Few preclinical PK/PD studies, however, include sex as a variable. Sex differences in the PK and adrenal suppressive effects of the synthetic corticosteroid, methylprednisolone, were assessed in male and female rats as a function of the 4-day rodent reproductive cycle. Drug exposure was 3-fold higher in females, regardless of estrous stage, compared with males. An extended minimal physiologically-based PK/PD model utilizing in vitro and in vivo measurements was developed and applied. These studies provide a framework to account for sex-dependent variability in drug and endogenous agonist (corticosterone) exposures, serving as a prelude to more intricate assessments of sex-related variability in receptor/gene-mediated PD corticosteroid actions.

Modeling Corticosteroid Pharmacokinetics and Pharmacodynamics, Part I: Determination and Prediction of Dexamethasone and Methylprednisolone Tissue Binding in the Rat.

The plasma and tissue binding properties of two corticosteroids, dexamethasone (DEX) and methylprednisolone (MPL), were assessed in the rat in anticipation of developing physiologically based pharmacokinetic and pharmacokinetic/pharmacodynamic models. The tissue-to-plasma partition coefficients (K P) of DEX and MPL were measured in liver, muscle, and lung in vivo after steady-state infusion and bolus injection in rats. Since K P is often governed by reversible binding to macromolecules in blood and tissue, an attempt was made to assess K P values of DEX and MPL by in vitro binding studies using rat tissue homogenates and to compare these estimates to those obtained from in vivo kinetics after dosing. The K P values of both steroids were also calculated in rat tissues using mechanistic tissue composition-based equations. The plasma binding of DEX and MPL was linear with moderate binding (60.5% and 82.5%) in male and female rats. In vivo estimates of steroid uptake appeared linear across the tested concentrations and K P was highest in liver and lowest in muscle for both steroids. Assessment of hepatic binding of MPL in vitro was severely affected by drug loss at 37°C in male liver homogenates, whereas DEX was stable in both male and female liver homogenates. With the exception of MPL in liver, in vitro-derived K P estimates reasonably agreed with in vivo values. The mechanistic equations modestly underpredicted K P for both drugs. Tissue metabolism, saturable tissue binding, and active uptake are possible factors that can complicate assessments of in vivo tissue binding of steroids when using tissue homogenates. SIGNIFICANCE STATEMENT: Assuming the free hormone hypothesis, the ratio of the unbound drug fraction in plasma and in tissues defines the tissue-to-plasma partition coefficient (K P), an important parameter in physiologically based pharmacokinetic modeling that determines total drug concentrations within tissues and the steady-state volume of distribution. This study assessed the plasma and tissue binding properties of the synthetic corticosteroids, dexamethasone and methylprednisolone, in rats using ultrafiltration and tissue homogenate techniques. In vitro-in vivo and in silico-in vivo extrapolation of K P was assessed for both drugs in liver, muscle, and lung. Although the extrapolation was fairly successful across the tissues, in vitro homogenate studies severely underpredicted the K P of methylprednisolone in liver, partly attributable to the extensive hepatic metabolism.

Modeling Corticosteroid Pharmacokinetics and Pharmacodynamics, Part III: Estrous Cycle and Estrogen Receptor-Dependent Antagonism of Glucocorticoid-Induced Leucine Zipper (GILZ) Enhancement by Corticosteroids.

Our previous report examined the pharmacokinetics (PK) of methylprednisolone (MPL) and adrenal suppression after a 50 mg/kg IM bolus in male and female rats, and we described in detail the development of a minimal physiologically based pharmacokinetic/pharmacodynamic (mPBPK/PD) model. In continuation of such assessments, we investigated sex differences in genomic MPL responses (PD). Message expression of the glucocorticoid-induced leucine zipper (GILZ) was chosen as a multitissue biomarker of glucocorticoid receptor (GR)-mediated drug response. Potential time-dependent interplay between sex hormone and glucocorticoid signaling in vivo was assessed by comparing the enhancement of GILZ by MPL in the uterus [high estrogen receptor (ER) density] and in liver (lower ER density) from male and female rats dosed within the proestrus (high estradiol/progesterone) and estrus (low estradiol/progesterone) phases of the rodent estrous cycle. An expanded-systems PD model of MPL considering circadian rhythms, multireceptor (ER and GR) control, and estrous variations delineated the determinants controlling receptor/gene-mediated steroid responses. Hepatic GILZ response was ∼3-fold greater in females, regardless of estrous stage, compared with males, driven predominantly by increased MPL exposure in females and a negligible influence of estrogen interaction. In contrast, GILZ response in the uterus during proestrus in females was 60% of that observed in estrus-phased females, despite no PK or receptor differences, providing in vivo support to the hypothesis of estrogen-mediated antagonism of glucocorticoid signaling. The developed model offers a mechanistic platform to assess the determinants of sex and tissue specificity in corticosteroid actions and, in turn, reveals a unique PD drug-hormone interaction occurring in vivo. SIGNIFICANCE STATEMENT: Mechanisms relating to sex-based pharmacodynamic variability in genomic responses to corticosteroids have been unclear. Using combined experimental and systems pharmacology modeling approaches, sex differences in both pharmacokinetic and pharmacodynamic mechanisms controlling the enhancement of a sensitive corticosteroid-regulated biomarker, the glucocorticoid-induced leucine zipper (GILZ), were clarified in vivo. The multiscale minimal physiologically based pharmacokinetics/pharmacodynamic model successfully captured the experimental observations and quantitatively discerned the roles of the rodent estrous cycle (hormonal variation) and tissue specificity in mediating the antagonistic coregulation of GILZ gene synthesis. These findings collectively support the hypothesis that estrogens antagonize pharmacodynamic signaling of genomic corticosteroid actions in vivo in a time- and estrogen receptor-dependent manner.

Modeling Sex Differences in Anti-inflammatory Effects of Dexamethasone in Arthritic Rats.

PURPOSE: Collagen-induced arthritic (CIA) rats are used commonly for preclinical pharmacologic research into rheumatoid arthritis (RA). Dexamethasone (DEX), a potent corticosteroid (CS), remains an important component in combination therapy for RA. Although sex differences in RA and CS pharmacokinetics/pharmacodynamics (PK/PD) have been documented in humans, there has been no such comprehensive evaluation of sex differences in CIA rats. METHODS: Paw size measurements were obtained for males and females from four groups of animals: healthy controls, non-drug treated arthritic animals, and both 0.225 and 2.25 mg/kg DEX-treated arthritic animals. A turnover model for disease progression, minimal PBPK model for drug concentrations, and inhibitory indirect response model were applied using population PK/PD modeling. RESULTS: The clearances of DEX were 43% greater in males, but other PK parameters were similar. The temporal profiles of paw swelling exhibited earlier progression, peak edema times, and disease remission in females. DEX suppressed paw edema well in both males and females with similar capacity (Imax) values (=1.0), but DEX potency was less in females with higher IC50 values (0.101 versus 0.015 ng/mL). CONCLUSIONS: The pharmacology of DEX was well characterized in CIA rats. This study addresses knowledge gaps about sex differences and can be a guide for more mechanistic assessment of sex, drug, and disease differences in RA.

Modeling Corticosteroid Pharmacogenomics and Proteomics in Rat Liver.

Corticosteroids (CS) regulate the expression of numerous genes at the mRNA and protein levels. The time course of CS pharmacogenomics and proteomics were examined in livers obtained from adrenalectomized rats given a 50-mg/kg bolus dose of methylprednisolone. Microarrays and mass spectrometry-based proteomics were employed to quantify hepatic transcript and protein dynamics. One-hundred, sixty-three differentially expressed mRNA and their corresponding proteins (163 genes) were clustered into two dominant groups. The temporal profiles of most proteins were delayed compared with their mRNA, attributable to synthesis delays and slower degradation kinetics. On the basis of our fifth-generation model of CS, mathematical models were developed to simultaneously describe the emergent time patterns for an array of steroid-responsive mRNA and proteins. The majority of genes showed time-dependent increases in mRNA and protein expression before returning to baseline. A model assuming direct, steroid-mediated stimulation of mRNA synthesis was applied. Some mRNAs and their proteins displayed down-regulation following CS. A model assuming receptor-mediated inhibition of mRNA synthesis was used. More complex patterns were observed for other genes (e.g., biphasic behaviors and opposite directionality in mRNA and protein). Models assuming either stimulation or inhibition of mRNA synthesis coupled with dual secondarily induced regulatory mechanisms affecting mRNA or protein turnover were derived. These findings indicate that CS-regulated gene expression manifested at the mRNA and protein levels are controlled via mechanisms affecting key turnover processes. Our quantitative models of CS pharmacogenomics were expanded from mRNA to proteins and provide extended hypotheses for understanding the direct, secondary, and downstream mechanisms of CS actions.

Modeling circadian variability of core-clock and clock-controlled genes in four tissues of the rat.

Circadian clocks, present in almost all cells of the body, are entrained to rhythmic changes in the environment (e.g. light/dark cycles). Genes responsible for this timekeeping are named core-clock genes, which through transcriptional feedback interactions mediated by transcription factor binding to Ebox/RRE/Dbox elements can generate oscillatory activity of their expression. By regulating the transcription of other clock-controlled genes (CCGs) circadian information is transmitted to tissue and organ levels. Recent studies have indicated that there is a considerable variability of clock-controlled gene expression between tissues both with respect to the circadian genes that are regulated and to their phase lags. In this work, a mathematical model was adapted to explore the dynamics of core-clock and clock-controlled genes measured in four tissues of the rat namely liver, muscle, adipose, and lung. The model efficiently described the synchronous rhythmicity of core-clock genes and further predicted that their phases are mainly regulated by Per2 and Cry1 transcriptional delays and Rev-Erba and Cry1 degradation rates. Similarly, after mining databases for potential Ebox/RRE/Dbox elements in the promoter region of clock-controlled genes, the phase variabilities of the same genes between different tissues were described. The analysis suggests that inter-tissue circadian variability of the same clock-controlled genes is an inherent component of homeostatic function and may arise due to different transcription factor activities on Ebox/RRE/Dbox elements.

Daily variation of gene expression in diverse rat tissues.

Circadian information is maintained in mammalian tissues by a cell-autonomous network of transcriptional feedback loops that have evolved to optimally regulate tissue-specific functions. An analysis of daily gene expression in different tissues, as well as an evaluation of inter-tissue circadian variability, is crucial for a systems-level understanding of this transcriptional circuitry. Affymetrix gene chip measurements of liver, muscle, adipose, and lung tissues were obtained from a rich time series light/dark experiment, involving 54 normal rats sacrificed at 18 time points within the 24-hr cycle. Our analysis revealed a high degree of circadian regulation with a variable distribution of phases among the four tissues. Interestingly, only a small number of common genes maintain circadian activity in all tissues, with many of them consisting of "core-clock" components with synchronous rhythms. Our results suggest that inter-tissue circadian variability is a critical component of homeostatic body function and is mediated by diverse signaling pathways that ultimately lead to highly tissue-specific transcription regulation.

Receptor/gene/protein-mediated signaling connects methylprednisolone exposure to metabolic and immune-related pharmacodynamic actions in liver.

A multiscale pharmacodynamic model was developed to characterize the receptor-mediated, transcriptomic, and proteomic determinants of corticosteroid (CS) effects on clinically relevant hepatic processes following a single dose of methylprednisolone (MPL) given to adrenalectomized (ADX) rats. The enhancement of tyrosine aminotransferase (TAT) mRNA, protein, and enzyme activity were simultaneously described. Mechanisms related to the effects of MPL on glucose homeostasis, including the regulation of CCAAT-enhancer binding protein-beta (C/EBPß) and phosphoenolpyruvate carboxykinase (PEPCK) as well as insulin dynamics were evaluated. The MPL-induced suppression of circulating lymphocytes was modeled by coupling its effect on cell trafficking with pharmacogenomic effects on cell apoptosis via the hepatic (STAT3-regulated) acute phase response. Transcriptomic and proteomic time-course profiles measured in steroid-treated rat liver were utilized to model the dynamics of mechanistically relevant gene products, which were linked to associated systemic end-points. While time-courses of TAT mRNA, protein, and activity were well described by transcription-mediated changes, additional post-transcriptional processes were included to explain the lack of correlation between PEPCK mRNA and protein. The immune response model quantitatively discerned the relative roles of cell trafficking versus gene-mediated lymphocyte apoptosis by MPL. This systems pharmacodynamic model provides insights into the contributions of selected molecular events occurring in liver and explores mechanistic hypotheses for the multi-factorial control of clinically relevant pharmacodynamic outcomes.

Mechanistic Multi-Tissue Modeling of Glucocorticoid-Induced Leucine Zipper Regulation: Integrating Circadian Gene Expression with Receptor-Mediated Corticosteroid Pharmacodynamics.

The glucocorticoid-induced leucine zipper (GILZ) is an important mediator of anti-inflammatory corticosteroid action. The pharmacokinetic/pharmacodynamic/pharmacogenomic effects of acute and chronic methylprednisolone (MPL) dosing on the tissue-specific dynamics of GILZ expression were examined in rats. A mechanism-based model was developed to investigate and integrate the role of MPL and circadian rhythms on the transcriptional enhancement of GILZ in multiple tissues. Animals received a single 50-mg/kg intramuscular bolus or a 7-day 0.3-mg/kg/h subcutaneous infusion of MPL and were euthanized at several time points. An additional group of rats were euthanized at several times and served as 24-hour light/dark (circadian) controls. Plasma MPL and corticosterone concentrations were measured by high-performance liquid chromatography. The expression of GILZ and glucocorticoid receptor (GR) mRNA was quantified in tissues using quantitative real-time reverse-transcription polymerase chain reaction. The pharmacokinetics of MPL were described using a two-compartment model. Mild-to-robust circadian oscillations in GR and GILZ mRNA expression were characterized in muscle, lung, and adipose tissues and modeled using Fourier harmonic functions. Acute MPL dosing caused significant down-regulation (40%-80%) in GR mRNA and enhancement of GILZ mRNA expression (500%-1080%) in the tissues examined. While GILZ returned to its rhythmic baseline following acute dosing, a new steady-state was observed upon enhancement by chronic dosing. The model captured the complex dynamics in all tissues for both dosing regimens. The model quantitatively integrates physiologic mechanisms, such as circadian processes and GR tolerance phenomena, which control the tissue-specific regulation of GILZ by corticosteroids. These studies characterize GILZ as a pharmacodynamic marker of corticosteroid actions in several tissues.

Effects of High-Fat Feeding on Skeletal Muscle Gene Expression in Diabetic Goto-Kakizaki Rats.

In the present report, we examined the responses of diabetic Goto-Kakizaki (GK) rats and control Wistar-Kyoto (WKY) rats fed either a standard chow or high-fat diet (HFD) from weaning to 20 weeks of age. This comparison included gene expression profiling of skeletal muscle using Affymetrix gene array chips. The expression profiling is interpreted within the context of a wide array of physiological measurements. Genes whose expressions are different between the 2 strains regardless of diet, as well as genes that differ between strains only with HFD, were identified. In addition, genes that were regulated by diet in 1 or both strains were identified. The results suggest that both strains respond to HFD by an increased capacity to oxidize lipid fuels in the musculature but that this adaptation occurs more rapidly in WKY rats. The results also demonstrated an impaired cytokine signalling and heightened inflammatory status in the GK rats.

Modeling Combined Immunosuppressive and Anti-inflammatory Effects of Dexamethasone and Naproxen in Rats Predicts the Steroid-Sparing Potential of Naproxen.

Dexamethasone (DEX), a widely prescribed corticosteroid, has long been the cornerstone of the treatment of inflammation and immunologic dysfunctions in rheumatoid arthritis. Corticosteroids are frequently used in combination with other antirheumatic agents such as nonsteroidal anti-inflammatory drugs (NSAIDs) and disease-modifying antirheumatic drugs to mitigate disease symptoms and minimize unwanted effects. We explored the steroid dose-sparing potential of the NSAID naproxen (NPX) with in vitro and in vivo studies. The single and joint suppressive effects of DEX and NPX on the in vitro mitogen-induced proliferation of T lymphocytes in blood and their anti-inflammatory actions on paw edema were investigated in female and male Lewis rats with collagen-induced arthritis (CIA). As expected, DEX was far more potent than NPX in these systems. Mathematical models incorporating an interaction term ψ were applied to quantitatively assess the nature and intensity of pharmacodynamic interactions between DEX and NPX. Modest synergistic effects of the two drugs were found in suppressing the mitogenic response of T lymphocytes. A pharmacokinetic/pharmacodynamic/disease progression model integrating dual drug inhibition quantitatively described the pharmacokinetics, time-course of single and joint anti-inflammatory effects (paw edema), and sex differences in CIA rats, and indicated additive effects of DEX and NPX. Further model simulations demonstrated the promising steroid-sparing potential of NPX in CIA rats, with the beneficial effects of the combination therapy more likely in males than females.

Characterization and Interspecies Scaling of rhTNF-α Pharmacokinetics with Minimal Physiologically Based Pharmacokinetic Models.

Tumor necrosis factor-α (TNF-α) is a soluble cytokine and target of specific monoclonal antibodies (mAbs) and other biologic agents used in the treatment of inflammatory diseases. These biologics exert their pharmacological effects through binding and neutralizing TNF-α, and thus they prevent TNF-α from interacting with its cell surface receptors. The magnitude of the pharmacological effects is governed not only by the pharmacokinetics (PK) of mAbs, but also by the kinetic fate of TNF-α We have examined the pharmacokinetics of recombinant human TNF-α (rhTNF-α) in rats at low doses and quantitatively characterized its pharmacokinetic features with a minimal physiologically based pharmacokinetic model. Our experimental and literature-digitalized PK data of rhTNF-α in rats across a wide range of doses were applied to global model fitting. rhTNF-α exhibits permeability rate-limited tissue distribution and its elimination is comprised of a saturable clearance pathway mediated by tumor necrosis factor receptor binding and disposition and renal filtration. The resulting model integrated with classic allometry was further used for interspecies PK scaling and resulted in model predictions that agreed well with experimental measurements in monkeys. In addition, a semimechanistic model was proposed and applied to explore the absorption kinetics of rhTNF-α following s.c. and other routes of administration. The model suggests substantial presystemic degradation of rhTNF-α for s.c. and i.m. routes and considerable lymph uptake contributing to the overall systemic absorption through the stomach wall and gastrointestinal wall routes of dosing. This report provides comprehensive modeling and key insights into the complexities of absorption and disposition of a major cytokine.

Interrelationships between Infliximab and Recombinant Tumor Necrosis Factor-α in Plasma Using Minimal Physiologically Based Pharmacokinetic Models.

The soluble cytokine tumor necrosis factor-α (TNF-α) is an important target for many therapeutic proteins used in the treatment of rheumatoid arthritis. Biologics targeting TNF-α exert their pharmacologic effects through binding and neutralizing this cytokine and preventing it from binding to its cell surface receptors. The magnitude of their pharmacologic effects directly corresponds to the extent and duration of free TNF-α suppression. However, endogenous TNF-α is of low abundance, so it is quite challenging to assess the free TNF-α suppression experimentally. Here we have applied an experimental approach to bypass this difficulty by giving recombinant human TNF-α (rhTNF-α) to rats by s.c. infusion. This boosted TNF-α concentration enabled quantification of TNF-α in plasma. Free rhTNF-α concentrations were measured after separation from the infliximab-rhTNF-α complex using Dynabeads Protein A. The interrelationship of infliximab and TNF-α was assessed with minimal physiologically based pharmacokinetic models for TNF-α and infliximab with a target-mediated drug disposition component. Knowledge of TNF-α pharmacokinetics allows reliable prediction of the free TNF-α suppression with either free or total TNF-α concentration profiles. The experimental and modeling approaches in our study may aid in the development of next-generation TNF-α inhibitors with improved therapeutic effects.

Understanding Physiology in the Continuum: Integration of Information from Multiple -Omics Levels.

In this paper, we discuss approaches for integrating biological information reflecting diverse physiologic levels. In particular, we explore statistical and model-based methods for integrating transcriptomic, proteomic and metabolomics data. Our case studies reflect responses to a systemic inflammatory stimulus and in response to an anti-inflammatory treatment. Our paper serves partly as a review of existing methods and partly as a means to demonstrate, using case studies related to human endotoxemia and response to methylprednisolone (MPL) treatment, how specific questions may require specific methods, thus emphasizing the non-uniqueness of the approaches. Finally, we explore novel ways for integrating -omics information with PKPD models, toward the development of more integrated pharmacology models.

Effect of Disease-Related Changes in Plasma Albumin on the Pharmacokinetics of Naproxen in Male and Female Arthritic Rats.

Naproxen (NPX) is used in the treatment of rheumatoid arthritis (RA) for alleviation of pain and inflammation. In view of the extensive albumin binding of NPX, this study investigates whether chronic inflammation and sex influence the physiologic albumin concentrations, plasma protein binding, and pharmacokinetics (PK) of NPX. The PK of NPX was evaluated in a rat model of RA [collagen-induced arthritis (CIA) in Lewis rats] and in healthy controls. These PK studies included 1) NPX in female and male CIA rats that received 10, 25, or 50 mg/kg NPX i.p.; and 2) NPX in healthy female and male rats after i.p. dosing of NPX at 50 mg/kg. Plasma albumin concentrations were quantified by enzyme-linked immunosorbent assay, and protein binding was assessed using ultrafiltration. The NPX concentrations in plasma and filtrates were determined by liquid chromatography-tandem mass spectrometry (LC-MS/MS). Plasma concentration-time data of NPX were first assessed by noncompartmental analysis (NCA). Nonlinear PK as indicated by dose-dependent NCA clearances and distribution volumes was observed. A two-compartment model with a first-order absorption process incorporating nonlinear protein binding in plasma and tissues jointly described the PK data of all groups. Saturable albumin binding accounts for the nonlinearity of NPX PK in all rats as well as part of the PK differences in arthritic rats. The CIA rats exhibited reduced albumin concentrations, reduced overall protein binding, and reduced clearances of unbound NPX, consistent with expectations during inflammation. The net effect of chronic inflammation was an elevation of the Cmax and area under the plasma concentration-time curve (AUC) of unbound drug.

Modeling Sex Differences in Pharmacokinetics, Pharmacodynamics, and Disease Progression Effects of Naproxen in Rats with Collagen-Induced Arthritis.

Naproxen (NPX) is a frequently used nonsteroidal anti-inflammatory drug for rheumatoid arthritis (RA). Lack of quantitative information about the drug exposure-response relationship has resulted in empirical dosage regimens for use of NPX in RA. Few studies to date have included sex as a factor, although RA predominates in women. A pharmacokinetic, pharmacodynamic, and disease progression model described the anti-inflammatory effects of NPX in collagen-induced arthritic (CIA) male and female rats. Three groups of rats were included for each sex: healthy animals, CIA controls, and CIA rats given a single 50-mg/kg dose of NPX intraperitoneally. Paw volumes of healthy rats indicated natural growth, and disease status was measured by paw edema. An innovative minimal physiologically based pharmacokinetic (mPBPK) model incorporating nonlinear albumin binding of NPX in both plasma and interstitial fluid (ISF) was applied. Arthritic rats exhibited lower plasma and ISF albumin concentrations and reduced clearances of unbound drug to explain pharmacokinetic profiles. The unbound ISF NPX concentrations predicted by the mPBPK model were used as the driving force for pharmacological effects of NPX. A logistic growth function accounting for natural paw growth and an indirect response model for paw edema and drug effects (inhibition of kin) was applied. Female rats showed a higher incidence of CIA, earlier disease onset, and more severe symptoms. NPX had stronger effects in males, owing to higher unbound ISF NPX concentrations and lower IC50 values. The model described the pharmacokinetics, unbound NPX in ISF, time course of anti-inflammatory effects, and sex differences in CIA rats.