Receptor-mediated pharmacodynamics of prednisolone in the rat

Effect of postoperative systemic prednisolone on short-term and long-term outcomes in chronic rhinosinusitis with nasal polyps: A multi-centered randomized clinical trial

Introduction

The objective of this study was to determine whether postoperative additive systemic steroid administration in chronic rhinosinusitis with nasal polyps (CRSwNP) impacted selected endoscopic, subjective and objective outcome measures.

Methods

This was a prospective, randomized, double-blind, placebo-controlled, noninferiority multicenter trial of n=106 patients with CRSwNP. All patients underwent primary functional endoscopic sinus surgery (FESS) followed by topical nasal steroids. Patients were randomized to a systemic steroid or placebo for 1 month. Patients were followed up for 2 years over 9 time points. The primary outcome measures were the differences between groups with respect to the nasal polyp score (NPS) and sinonasal quality of life (SNQoL). Secondary outcome measures included interactions with respect to the Lund-Kennedy score (LKS), sinonasal symptoms, general quality of life (GQoL), 16-item odor identification test scores, recurrence rates, need for revision surgery and mucus biomarker levels.

Results

106 patients were randomized to either the placebo or the systemic steroid group (n=53 per group). Postoperative systemic steroids were not superior to placebo with respect to all primary (p= 0.077) and secondary outcome measures (p>0.05 for all). Reported adverse events were similar between the two groups.

Conclusion

In conclusion, the addition of postoperative systemic steroids after primary FESS did not confer a benefit over topical steroid nasal spray alone with respect to NPS, SNQOL, LKS, GQOL, sinonasal symptoms, smell scores, recurrence rates, the need for revision surgery or biomarkers over a short-term follow-up of up to 9 months and a long-term follow-up of up to 24 months in CRSwNP patients. Functional endoscopic surgery did, however, show a strong effect on all outcome measures, which remained relatively stable up to the endpoint at 2 years.

Semi-mechanistic PK/PD model to assess pulmonary targeting of beclomethasone dipropionate and its active metabolite

PURPOSE

The objective of this study was to describe the pulmonary targeting of beclomethasone dipropionate (BDP) and its active metabolite beclomethasone 17-monopropionate (BMP) in rats using a semi-mechanistic PK/PD model.

METHODS

Rat plasma and tissue concentrations of BDP and BMP, and tissue receptor occupancies of BMP after systemic and pulmonary delivery of BDP and BMP were integrated in a newly developed semi-mechanistic PK/PD model.

RESULTS

After IV administration of BDP, 95.4% of BDP was converted to BMP, while after pulmonary delivery of BDP, 46.6% of deposited BDP was absorbed as BMP. The developed semi-mechanistic PK model described plasma and tissue concentrations of BDP and BMP as well as receptor occupancies sufficiently well. The model incorporated dissolution, metabolic activation, and drug absorption processes to describe the local fate of BDP and BMP after systemic and pulmonary delivery. Dissolution rate constants of BDP and BMP were estimated to be 0.47/h and 2.01/h, respectively, and the permeabilities in central lung were estimated to be 15.0 and 2.9 × 10⁶ cm/s for BDP and BMP, respectively. The EC₅₀ of the binding of BMP to to the receptor was estimated to be 0.0017 ng/ml. Overall, receptor occupancies in the lung were more pronounced than those in the systemic circulation after pulmonary delivery of BDP or BMP. Simulations using the developed semi-mechanistic PK/PD model demonstrated that a slow dissolution rate and low permeability can improve pulmonary targeting.

CONCLUSIONS

A semi-mechanistic model was developed to describe the fate of an inhaled glucocorticoid pro-drug and its active metabolite in lung and the systemic circulation, both after pulmonary and systemic administration , thereby facilitating the understanding of the complex interplay between drug, prodrug and pharmacodynamic properties for quantifying the degree pulmonary targeting.

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.

Seeking Nonspecific Binding: Assessing the Reliability of Tissue Dilutions for Calculating Fraction Unbound

It has become commonplace (270+ article citations to date) to measure the fraction unbound (FrUn) of drugs in tissue homogenates and diluted plasma and then use a Correction Factor Equation (CFE) to extrapolate to the undiluted state. The CFE is based on assumptions of nonspecific binding with experimental use of very low drug concentrations. There are several possible determinants of apparent nonspecific binding as measured by methods such as equilibrium dialysis: true macromolecule binding and lipid partitioning along with receptor, enzyme, and transporter interactions. Theoretical calculations based on nonlinear protein binding indicate that the CFE will be most reliable to obtain FrUn when added drug concentration is small, binding constants are weak, protein concentrations are relatively high, and tissue dilution is minimal. When lipid partitioning is the sole factor determining apparent tissue binding, the CFE should be perfectly accurate. Use of very low drug concentrations, however, makes it more likely that specific binding to receptors and other targets may occur, and thus FrUn may reflect some binding to such components. Inclusion of trapped blood can clearly cause minor to marked discrepancies from purely tissue binding alone, which can be corrected. Furthermore, assessment of the occurrence of ionization/pH shifts, drug instability, and tissue metabolism may be necessary. Caution is needed in the use and interpretation of results from tissue dilution studies and other assessments of nonspecific binding, particularly for very strongly bound drugs with very small FrUn values and in tissues with metabolic enzymes, receptors, and trapped blood. SIGNIFICANCE STATEMENT: The use of tissue, plasma, and cell preparations to help obtain fraction unbound and tissue-to-plasma partition coefficients in pharmacokinetics has grown commonplace, especially for brain. This report examines theoretical, physiological, and experimental issues that need consideration before trusting such measurements and calculations.

Modeling Pathway Dynamics of the Skeletal Muscle Response to Intravenous Methylprednisolone (MPL) Administration in Rats: Dosing and Tissue Effects

A model-based approach for the assessment of pathway dynamics is explored to characterize metabolic and signaling pathway activity changes characteristic of the dosing-dependent differences in response to methylprednisolone in muscle. To consistently compare dosing-induced changes we extend the principles of pharmacokinetics and pharmacodynamics and introduce a novel representation of pathway-level dynamic models of activity regulation. We hypothesize the emergence of dosing-dependent regulatory interactions is critical to understanding the mechanistic implications of MPL dosing in muscle. Our results indicate that key pathways, including amino acid and lipid metabolism, signal transduction, endocrine regulation, regulation of cellular functions including growth, death, motility, transport, protein degradation, and catabolism are dependent on dosing, exhibiting diverse dynamics depending on whether the drug is administered acutely of continuously. Therefore, the dynamics of drug presentation offer the possibility for the emergence of dosing-dependent models of regulation. Finally, we compared acute and chronic MPL response in muscle with liver. The comparison revealed systematic response differences between the two tissues, notably that muscle appears more prone to adapt to MPL.

Transitioning from Basic toward Systems Pharmacodynamic Models: Lessons from Corticosteroids

Technology in bioanalysis, -omics, and computation have evolved over the past half century to allow for comprehensive assessments of the molecular to whole body pharmacology of diverse corticosteroids. Such studies have advanced pharmacokinetic and pharmacodynamic (PK/PD) concepts and models that often generalize across various classes of drugs. These models encompass the "pillars" of pharmacology, namely PK and target drug exposure, the mass-law interactions of drugs with receptors/targets, and the consequent turnover and homeostatic control of genes, biomarkers, physiologic responses, and disease symptoms. Pharmacokinetic methodology utilizes noncompartmental, compartmental, reversible, physiologic [full physiologically based pharmacokinetic (PBPK) and minimal PBPK], and target-mediated drug disposition models using a growing array of pharmacometric considerations and software. Basic PK/PD models have emerged (simple direct, biophase, slow receptor binding, indirect response, irreversible, turnover with inactivation, and transduction models) that place emphasis on parsimony, are mechanistic in nature, and serve as highly useful "top-down" methods of quantitating the actions of diverse drugs. These are often components of more complex quantitative systems pharmacology (QSP) models that explain the array of responses to various drugs, including corticosteroids. Progressively deeper mechanistic appreciation of PBPK, drug-target interactions, and systems physiology from the molecular (genomic, proteomic, metabolomic) to cellular to whole body levels provides the foundation for enhanced PK/PD to comprehensive QSP models. Our research based on cell, animal, clinical, and theoretical studies with corticosteroids have provided ideas and quantitative methods that have broadly advanced the fields of PK/PD and QSP modeling and illustrates the transition toward a global, systems understanding of actions of diverse drugs. SIGNIFICANCE STATEMENT: Over the past half century, pharmacokinetics (PK) and pharmacokinetics/pharmacodynamics (PK/PD) have evolved to provide an array of mechanism-based models that help quantitate the disposition and actions of most drugs. We describe how many basic PK and PK/PD model components were identified and often applied to the diverse properties of corticosteroids (CS). The CS have complications in disposition and a wide array of simple receptor-to complex gene-mediated actions in multiple organs. Continued assessments of such complexities have offered opportunities to develop models ranging from simple PK to enhanced PK/PD to quantitative systems pharmacology (QSP) that help explain therapeutic and adverse CS effects. Concurrent development of state-of-the-art PK, PK/PD, and QSP models are described alongside experimental studies that revealed diverse CS actions.

Quantitative Assessment of Pulmonary Targeting of Inhaled Corticosteroids Using Ex Vivo Receptor Binding Studies

The goal of locally acting inhaled corticosteroids is to achieve distinct pulmonary effects with reduced systemic side effects. The present work using an ex vivo receptor binding model in rats was interested in assessing pulmonary targeting for several commercially available corticosteroids by monitoring receptor occupancies in the lung and systemic organs (liver, kidney, spleen, and brain) after intravenous (IV) injection or intratracheal (IT) instillation of a dry powder administration at a dose of 100 μg/kg. Pulmonary targeting, defined as the difference in cumulative receptor occupancies (AUC_E) between the lung and kidney after pulmonary delivery, differed across the investigated corticosteroids (ΔAUC_E range, 33 ± 46 to 143 ± 52% *h) with the highest degree found for corticosteroids with high systemic clearance and pronounced lipophilicity (presumably allowing a long pulmonary residence time). Additionally, this study demonstrated differences in the receptor occupancies across systemic organs. Using kidney receptor occupancies as the comparator, liver receptor occupancies were reduced (ΔAUC_E range: - 157 ± 43 to 178 ± 42% *h) after IV and IT administration for corticosteroids with high intrinsic clearance, while they were increased for corticosteroid prodrugs due to hepatic activation. Spleen receptor occupancies were increased after IT (ΔAUC_E range: 33 ± 35 to 135 ± 28% *h), but not after IV administration. This was especially true for slowly dissolving drugs. Reduced brain uptake was also observed for ciclesonide (CIC) and des-ciclesonide (desCIC), two compounds previously not investigated. In summary, ex vivo receptor binding studies represent a powerful tool to assess the fate of ICSs.

Pathway-Based Analysis of the Liver Response to Intravenous Methylprednisolone Administration in Rats: Acute Versus Chronic Dosing

Pharmacological time-series data, from comparative dosing studies, are critical to characterizing drug effects. Reconciling the data from multiple studies is inevitably difficult; multiple in vivo high-throughput -omics studies are necessary to capture the global and temporal effects of the drug, but these experiments, though analogous, differ in (microarray or other) platforms, time-scales, and dosing regimens and thus cannot be directly combined or compared. This investigation addresses this reconciliation issue with a meta-analysis technique aimed at assessing the intrinsic activity at the pathway level. The purpose of this is to characterize the dosing effects of methylprednisolone (MPL), a widely used anti-inflammatory and immunosuppressive corticosteroid (CS), within the liver. A multivariate decomposition approach is applied to analyze acute and chronic MPL dosing in male adrenalectomized rats and characterize the dosing-dependent differences in the dynamic response of MPL-responsive signaling and metabolic pathways. We demonstrate how to deconstruct signaling and metabolic pathways into their constituent pathway activities, activities which are scored for intrinsic pathway activity. Dosing-induced changes in the dynamics of pathway activities are compared using a model-based assessment of pathway dynamics, extending the principles of pharmacokinetics/pharmacodynamics (PKPD) to describe pathway activities. The model-based approach enabled us to hypothesize on the likely emergence (or disappearance) of indirect dosing-dependent regulatory interactions, pointing to likely mechanistic implications of dosing of MPL transcriptional regulation. Both acute and chronic MPL administration induced a strong core of activity within pathway families including the following: lipid metabolism, amino acid metabolism, carbohydrate metabolism, metabolism of cofactors and vitamins, regulation of essential organelles, and xenobiotic metabolism pathway families. Pathway activities alter between acute and chronic dosing, indicating that MPL response is dosing dependent. Furthermore, because multiple pathway activities are dominant within a single pathway, we observe that pathways cannot be defined by a single response. Instead, pathways are defined by multiple, complex, and temporally related activities corresponding to different subgroups of genes within each pathway.

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/EBPβ) 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.

Highly multiplexed and reproducible ion-current-based strategy for large-scale quantitative proteomics and the application to protein expression dynamics induced by methylprednisolone in 60 rats

A proteome-level time-series study of drug effects (i.e., pharmacodynamics) is critical for understanding mechanisms of action and systems pharmacology, but is challenging, because of the requirement of a proteomics method for reliable quantification of many biological samples. Here, we describe a highly reproducible strategy, enabling a global, large-scale investigation of the expression dynamics of corticosteroid-regulated proteins in livers from adrenalectomized rats over 11 time points after drug dosing (0.5–66 h, N = 5/point). The analytical advances include (i) exhaustive tissue extraction with a Polytron/sonication procedure in a detergent cocktail buffer, and a cleanup/digestion procedure providing very consistent protein yields (relative standard deviation (RSD%) of 2.7%–6.4%) and peptide recoveries (4.1–9.0%) across the 60 animals; (ii) an ultrahigh-pressure nano-LC setup with substantially improved temperature stabilization, pump-noise suppression, and programmed interface cleaning, enabling excellent reproducibility for continuous analyses of numerous samples; (iii) separation on a 100-cm-long column (2-μm particles) with high reproducibility for days to enable both in-depth profiling and accurate peptide ion-current match; and (iv) well-controlled ion-current-based quantification. To obtain high-quality quantitative data necessary to describe the 11 time-points protein expression temporal profiles, strict criteria were used to define “quantifiable proteins”. A total of 323 drug-responsive proteins were revealed with confidence, and the time profiles of these proteins provided new insights into the diverse temporal changes of biological cascades associated with hepatic metabolism, response to hormone stimuli, gluconeogenesis, inflammatory responses, and protein translation processes. Most profile changes persisted well after the drug was eliminated. The developed strategy can also be broadly applied in preclinical and clinical research, where the analysis of numerous biological replicates is crucial.

Pharmacokinetics of methylprednisolone after intravenous and intramuscular administration in rats

Methylprednisolone (MPL) pharmacokinetics was examined in adrenalectomized (ADX) and normal rats to assess the feasibility of intramuscular (i.m.) dosing for use in pharmacodynamic studies. Several study phases were pursued. Parallel group studies were performed in normal and ADX rats given 50 mg/kg MPL (i.v. or i.m.) and blood samples were collected up to 6 h. Data from studies where normal rats were dosed with 50 mg/kg MPL i.m. and killed over either 6 or 96 h were combined to determine muscle site and plasma MPL concentrations. Lastly, ADX rats were dosed with 50 mg/kg MPL i.m. and killed over 18 h to assess hepatic tyrosine aminotransferase (TAT) dynamics. MPL exhibited bi-exponential kinetics after i.v. dosing with a terminal slope of 2.1 h(-1). The i.m. drug was absorbed slowly with two first-order absorption rate constants, 1.26 and 0.219 h(-1) indicating flip-flop kinetics with overall 50% bioavailability. The kinetics of MPL at the injection site exhibited slow, dual absorption rates. Although i.m. MPL showed lower bioavailability compared with other corticosteroids in rats, TAT dynamics revealed similar i.m. and i.v. response profiles. The more convenient intramuscular dosing can replace the i.v. route without causing marked differences in pharmacodynamics.

Modeling receptor/gene-mediated effects of corticosteroids on hepatic tyrosine aminotransferase dynamics in rats: dual regulation by endogenous and exogenous corticosteroids

Receptor/gene-mediated effects of corticosteroids on hepatic tyrosine aminotransferase (TAT) were evaluated in normal rats. A group of normal male Wistar rats were injected with 50 mg/kg methylprednisolone (MPL) intramuscularly at the nadir of their plasma corticosterone (CST) rhythm (early light cycle) and sacrificed at various time points up to 96 h post-treatment. Blood and livers were collected to measure plasma MPL, CST, hepatic glucocorticoid receptor (GR) mRNA, cytosolic GR density, TAT mRNA, and TAT activity. The pharmacokinetics of MPL showed bi-exponential disposition with two first-order absorption components from the injection site and bioavailability was 21%. Plasma CST was reduced after MPL dosing, but resumed its daily circadian pattern within 36 h. Cytosolic receptor density was significantly suppressed (90%) and returned to baseline by 72 h resuming its biphasic pattern. Hepatic GR mRNA follows a circadian pattern which was disrupted by MPL and did not return during the study. MPL caused significant down-regulation (50%) in GR mRNA which was followed by a delayed rebound phase (60-70 h). Hepatic TAT mRNA and activity showed up-regulation as a consequence of MPL, and returned to their circadian baseline within 72 and 24 h of treatment. A mechanistic receptor/gene-mediated pharmacokinetic/pharmacodynamic model was able to satisfactorily describe the complex interplay of exogenous and endogenous corticosteroid effects on hepatic GR mRNA, cytosolic free GR, TAT mRNA, and TAT activity in normal rats.

Assessing the dynamics of nuclear glucocorticoid-receptor complex: adding flexibility to gene expression modeling

A retrospective analysis was performed to modify our fourth-generation pharmacodynamic model for glucocorticoid receptor (GR) dynamics with incorporation of more physiological features. This modified model was developed by integrating previously reported free cytosolic GR and GR mRNA data following single (10, 50 mg/kg) and dual (50 mg/kg at 0 and 24 hr) intravenous doses of methylprednisolone (MPL) in adrenalectomized (ADX) male Wistar rats with several in vitro studies describing real-time kinetics of the transfer of rat steroid-receptor complex from the cell cytosol to the nucleus. Additionally, free hepatic cytosolic GR and its mRNA data from a chronic infusion dosing study of MPL (0.1 and 0.3 mg/kg/hr) in male ADX Wistar rats were used to verify the predictability of the model. Incorporation of information regarding in vitro receptor kinetics allowed us to describe the receptor-mediated pharmacogenomic effects of MPL for a larger variety of genes in rat liver from microarray studies. These included early responsive gene like CCAAT/enhancer binding protein-beta (CEBP-beta), a transcription factor, as well as the later responsive gene for tyrosine aminotransferase (TAT), a classical biomarker of glucocorticoid (GC) genomic effects. This more mechanistic model of GR dynamics can be applied to characterize profiles for a greater number of genes in liver.

Pharmacokinetic-pharmacodynamic modelling: history and perspectives

A major goal in clinical pharmacology is the quantitative prediction of drug effects. The field of pharmacokinetic-pharmacodynamic (PK/PD) modelling has made many advances from the basic concept of the dose-response relationship to extended mechanism-based models. The purpose of this article is to review, from a historical perspective, the progression of the modelling of the concentration-response relationship from the first classic models developed in the mid-1960s to some of the more sophisticated current approaches. The emphasis is on general models describing key PD relationships, such as: simple models relating drug dose or concentration in plasma to effect, biophase distribution models and in particular effect compartment models, models for indirect mechanism of action that involve primarily the modulation of endogenous factors, models for cell trafficking and transduction systems. We show the evolution of tolerance and time-variant models, non- and semi-parametric models, and briefly discuss population PK/PD modelling, together with some example of more recent and complex pharmacodynamic models for control system and nonlinear HIV-1 dynamics. We also discuss some future possible directions for PK/PD modelling, report equations for general classes of novel semi-parametric models, as well as describing two new classes, additive or set-point, of regulatory, additive feedback models in their direct and indirect action variants.

Integrated QSPR--pharmacodynamic model of genomic effects of several corticosteroids

The results from a quantitative structure-property relationship (QSPR) model was integrated into a fifth-generation pharmacokinetic/pharmacodynamic (PK/PD) model of corticosteroid receptor/gene-mediated effects. The proposed model was developed using previously reported tyrosine aminotransferase (TAT) activity data following a 50 mg/kg intravenous dose of methylprednisolone in male adrenalectomized (ADX) rats. Induced TAT activity is a classical measure of corticosteroid genomic effects and the typical time course shows an initial lag-time, a slow rise to peak response, and a gradual return toward baseline values. The TAT activity profiles were subsequently predicted for two additional steroids (dexamethasone and hydrocortisone), which were confirmed experimentally. Two groups of male ADX Wistar rats (n = 18 each) were given either 0.1 mg/kg dexamethasone or 50 mg/kg hydrocortisone by penile vein injections. Plasma drug concentrations and liver TAT activity were measured at various time points. Baseline TAT activity was significantly lower in this study as compared to previous reports. Model simulations well captured the pharmacodynamic data once initial conditions were corrected for observed baseline values. Additional TAT profiles reported in the literature for prednisolone were also reasonably predicted using the final model. This study serves as a demonstration of how in vitro pharmacologic data and QSPR modeling results may be incorporated into existing mechanistic PK/PD models to anticipate the effects of other chemically related compounds.

Fifth-generation model for corticosteroid pharmacodynamics: application to steady-state receptor down-regulation and enzyme induction patterns during seven-day continuous infusion of methylprednisolone in rats

A fifth-generation model for receptor/gene-mediated corticosteroid effects was proposed based on results from a 50 mg/kg i.v. bolus dose of methylprednisolone (MPL) in male adrenalectomized rats, and confirmed using data from other acute dosage regimens. Steady-state equations for receptor down-regulation and tyrosine aminotransferase (TAT) enzyme induction patterns were derived. Five groups of male Wistar rats (n = 5/group) were subcutaneously implanted with Alzet mini-pumps primed to release saline or 0.05, 0.1, 0.2, and 0.3 mg/kg/hr of MPL for 7 days. Rats were sacrificed at the end of the infusion. Plasma MPL concentrations, blood lymphocyte counts, and hepatic cytosolic free receptor density, receptor mRNA, TAT mRNA, and TAT enzyme levels were quantitated. The pronounced steroid effects were evidenced by marked losses in body weights and changes in organ weights. All four treatments caused a dose-dependent reduction in hepatic receptor levels, which correlated with the induction of TAT mRNA and TAT enzyme levels. The 7 day receptor mRNA and free receptor density correlated well with the model predicted steady-state levels. However, the extent of enzyme induction was markedly higher than that predicted by the model suggesting that the usual receptor/gene-mediated effects observed upon single/intermittent dosing of MPL may be countered by alterations in other aspects of the system. A mean IC50 of 6.1 ng/mL was estimated for the immunosuppressive effects of methylprednisolone on blood lymphocytes. The extent and duration of steroid exposure play a critical role in mediating steroid effects and advanced PK/PD models provide unique insights into controlling factors.

Pharmacodynamics and pharmacogenomics of methylprednisolone during 7-day infusions in rats

An array of adverse steroid effects was examined on a whole body, tissue, and molecular level. Groups of male adrenalectomized Wistar rats were subcutaneously implanted with Alzet mini-pumps giving zero-order release rates of 0, 0.1, and 0.3 mg/kg/h methylprednisolone for 7 days. The rats were sacrificed at various times during the 7-day infusion period. A two-compartment model with a zero order input could adequately describe the kinetics of methylprednisolone upon infusion. Blood lymphocyte counts dropped to a minimum by 6 h and were well characterized by the cell trafficking model. The time course of changes in body and organ (liver, spleen, thymus, gastrocnemius muscle, and lungs) weights was described using indirect response models. Markers of gene-mediated steroid effects included hepatic cytosolic free receptor density, receptor mRNA, tyrosine aminotransferase (TAT) mRNA, and TAT levels. Our fifth-generation model of acute corticosteroid pharmacodynamics was used to predict the time course of receptor/gene-mediated effects. An excellent agreement between the expected and observed receptor dynamics suggested that receptor events and mRNA autoregulation are not altered upon 7-day methylprednisolone dosing. However, the model indicated a decoupling between the receptor and TAT dynamics with this infusion. The strong tolerance seen in TAT mRNA induction could be partly accounted for by receptor down-regulation. An amplification of translation of TAT mRNA to TAT and/or a reduction in the enzyme degradation rate could account for the observed exaggerated TAT activity. Our results exemplify the importance of biological signal transduction variables in controlling receptor/gene-mediated steroid responses during chronic dosing.

Dose-dependence and repeated-dose studies for receptor/gene-mediated pharmacodynamics of methylprednisolone on glucocorticoid receptor down-regulation and tyrosine aminotransferase induction in rat liver

Dose-dependent and repeated-dose effects of methylprednisolone (MPL) on down-regulation of glucocorticoid receptor messenger RNA (GR mRNA) and GR density, as well as tyrosine aminotransferase (TAT) mRNA and TAT induction by receptor/gene-mediated mechanisms in rat liver were examined. A previously developed pharmacokinetic/pharmacodynamic (PK/PD) model was used to design these studies which sought to challenge the model. Three groups of male adrenalectomized Wistar rats received MPL by i.v. injection: low-dose (10 mg/kg at Time 0), high-dose (50 mg/kg at Time 0), and dual-dose (50 mg/kg at Time 0 and 24 hr). Plasma concentrations of MPL, and hepatic content of free GR, GR mRNA, TAT mRNA, and TAT activity were determined. The P-Pharm program was applied for population analysis of MPL PK revealing low interindividual variation in CL and Vc values (3-14%). Two indirect response models were applied to test two competing hypotheses for GR mRNA dynamics. Indirect Pharmacodynamic Response Model I (Model A) where the complex in the nucleus decreases the transcription rate of GR mRNA better described GR mRNA/GR down-regulation. Levels of TAT mRNA began to increase at 1-2 hr, reached a maximum at 5-6 hr, and declined to the baseline at 12-14 hr after MPL dosing. The induction of TAT activity followed a similar pattern with a delay of about 1-2 hr. The low-dose group had 50-60% of the TAT mRNA and TAT induction compared to the high-dose group. Since the GR density returned to about 70% of the baseline level before the second 50 mg/kg dose at 24 hr, tolerance was found for TAT mRNA/TAT induction where only 50-60% of the initial responses were produced. Our fourth-generation model describes the dose dependence and tolerance effects of TAT mRNA/TAT induction by MPL involving multiple-step signal transduction controlled by the steroid regimen, free GR density, and GR occupancy. This model may provide the foundation for studying other induced proteins or enzymes mediated by the similar receptor/nuclear events.

Fourth-generation model for corticosteroid pharmacodynamics: a model for methylprednisolone effects on receptor/gene-mediated glucocorticoid receptor down-regulation and tyrosine aminotransferase induction in rat liver

A fourth-generation pharmacokinetic/pharmacodynamic (PK/PD) model for receptor/genemediated effects of corticosteroids was developed. Male adrenalectomized Wistar rats received a 50 mg/kg i.v. bolus dose of methylprednisolone (MPL). Plasma concentrations of MPL, hepatic glucocorticoid receptor (GR) messenger RNA (mRNA) and GR density, tyrosine amino-transferase (TAT) mRNA, and TAT activity in liver were determined at various time points up to 72 hr after MPL dosing. Down-regulation of GR mRNA and GR density were observed: GR mRNA level declined to 45-50% of the baseline in 8-10 hr, and slowly returned to predose level in about 3 days; GR density fell to 0 soon after dosing and returned to the baseline in two phases. The first phase, occurring in the first 10 hr, entailed recovery from 0 to 30%. The second phase was parallel to the GR mRNA recovery phase. Two indirect response models were applied for GR mRNA dynamics regulated by activated steroid-receptor complex. A full PK/PD model for GR mRNA/GR down-regulation was proposed, including GR recycling theory. TAT mRNA began to increase at about 1.5 hr, reached the maximum at about 5.5 hr, and declined to the baseline at about 14 hr after MPL dosing. TAT induction followed a similar pattern with a delay of about 1-2 hr. A transcription compartment was applied as one of the cascade events leading to TAT mRNA and TAT induction. Pharmacodynamic parameters were obtained by fitting seven differential equations piecewise using the maximum likelihood method in the ADAPT II program. This model can describe GR down-regulation and the precursor/product relationship between TAT mRNA and TAT in receptor/gene-mediated corticosteroid effects.

Pharmacokinetics and receptor-mediated pharmacodynamics of corticosteroids

The corticosteroids, such as prednisolone and methylprednisolone, provide diverse antiinflammatory and immunosuppressive effects which typically show responses with slow onset and prolonged duration. This report summarizes modeling efforts which are successful in describing such steroid effects. Clinical effects with such a pattern, including adrenal suppression and altered trafficking of basophils and helper T-cells, can be related to plasma drug concentrations by models containing an inhibition function and differential equations for controlling input and disposition of the response variable. Some responses have circadian-controlled inputs which add time-dependent complexities to the models. Kinetic/dynamic data for several corticosteroid effects yield IC50 values which agree well with receptor KD values. A relationship of linear AUC of effect versus log AUC of steroid in plasma is found with these models over a large range of doses. Gene-mediated effects of corticosteroids are initiated by receptor-binding which causes a cascade effect altering DNA transcription, RNA, mRNA and proteins or enzymes accounting for drug effects. Models for such behavior have been developed in animals for hepatic tyrosine aminotransferase (TAT) enzyme activity. Studies with methylprednisolone formulated in liposomes show tissue sequestration of steroid, prolonged receptor-binding and extended inhibition of splenocyte proliferation. The data and models usually show good correspondence of the AUC of receptor occupancy with the AUC of pharmacologic response.

Differential dynamics of receptor down-regulation and tyrosine aminotransferase induction following glucocorticoid treatment

Autoregulation of glucocorticoid receptor (GR) concentration in vivo may be an important determinant of steroid sensitivity. The dynamics of GR regulation were assessed and compared to regulation of tyrosine aminotransferase (TAT) expression in liver tissue taken from rats treated with a single 50 mg/kg i.v. dose of methylprednisolone. Plasma methylprednisolone concentrations were determined by HPLC analysis. Receptor and TAT message levels were determined by quantitative Northern hybridization. Methylprednisolone plasma kinetics showed a half-life of 0.6 h. Receptor occupancy occurred rapidly and cytosolic GR reappeared over 2-12 h. TAT activity rose between 2 and 6 h and then dissipated. Reduction in receptor mRNA levels occurred very rapidly, being detectable by 30 min following steroid administration. A down-regulated steady-state in GR message expression was reached by 2 h post-injection, and was maintained throughout the 18 h examined in this study. Comparison of methylprednisolone kinetics demonstrated that down-regulation was maintained long after drug was eliminated. In contrast, TAT message induction occurred with a sharp peak; maximal induction occurred between 5-6 h and return to baseline at approx. 8-10 h post-induction. This study shows that unlike TAT induction, GR message repression in vivo does not require continual presence of hormone.

Effect of corticosteroid binding globulin on the pharmacokinetics of prednisolone in rats

PURPOSE

The effect of exogenous corticosteroid binding globulin (CBG) on the pharmacokinetics of intravenous prednisolone was determined in rats to test the "free hormone hypothesis."

METHODS

A dose of CBG to yield 95% binding with 1000 ng/ml of prednisolone in vitro in rat plasma or saline was administered before dosing 2 mg/kg of prednisolone hemisuccinate or methylprednisolone intravenously. Drug concentrations in plasma samples were assayed by HPLC.

RESULTS

Single administration of CBG decreased apparent prednisolone clearance by 56% (155 to 66 ml/min/kg) and reduced apparent VSS by 35% (4.1 to 2.7 L/kg) (p < 0.001). Methylprednisolone pharmacokinetics, studied as a negative control because the drug does not bind to CBG, did not change.

CONCLUSIONS

The corticosteroid bound to CBG does not appear to be available for removal by clearance organs.

Third-generation model for corticosteroid pharmacodynamics: roles of glucocorticoid receptor mRNA and tyrosine aminotransferase mRNA in rat liver

A third-generation pharmacokinetic/pharmacodynamic model was proposed for receptor/gene-mediated corticosteroid effects. The roles of the messenger RNA (mRNA) for the glucocorticoid receptor (GR) in hepatic GR down-regulation and the mRNA for hepatic tyrosine aminotransferase (TAT) induction by methylprednisolone (MPL) were examined. Male adrenalectomized Wistar rats received 50 mg/kg MPL iv. Blood and liver samples were collected at various time points for a period of 18 hr. Plasma concentrations of MPL, free hepatic cytosolic GR densities, GR mRNA, TAT mRNA, and TAT activities in liver were determined. Plasma MPL profile was biexponential with a terminal t1/2 of 0.57 hr. Free hepatic GR density rapidly disappeared from cytoplasm after the MPL dose and then slowly returned to about 60% of starting level after 16 hr. Meanwhile, GR mRNA level fell to 45% of baseline within 2 hr postdosing, and remained at that level for at least 18 hr. The GR down-regulation of GR mRNA and protein turnover rate were modeled. The TAT mRNA began to increase at about 2 hr, reached a maximum at about 5 hr, and declined to baseline by 14 hr. TAT induction followed a similar pattern, except the induction was delayed about 0.5 hr. Pharmacodynamic parameters were obtained by fitting seven differential equations in a piecewise fashion. The cascade of corticosteroid steps were modeled by a series of inductions for steroid-receptor-DNA complex, two intermediate transit compartments, TAT mRNA, and TAT activity. Results indicate that GR mRNA and TAT mRNA are major controlling factors for the receptor/gene-mediated effects of corticosteroids.

Assessment of glucocorticoid lung targeting by ex-vivo receptor binding studies in rats

Triamcinolone acetonide (TA, 22 micrograms) was given to rats by intravenous (i.v.) injection or intratracheal (IT) instillation. Free glucocorticoid receptors were monitored over time in liver and lung using an ex-vivo receptor binding technique. After i.v. administration of a TA solution, the reduction of free receptors over time was very similar in lung and liver (AUCLung = 280 +/- 47% h; AUCLiver = 320 +/- 76% h). Intratracheal instillation of the same solution produced time profiles which mirrored those of i.v. injection (AUCLung = 260 +/- 41% h; AUCLiver = 330 +/- 50% h). The lack of lung targeting was also reflected in the failure to show any significant difference in the pulmonary targeting factor T (AUCLung/AUCLiver) between i.v. (T = 0.84 +/- 0.18) and IT (T = 0.78 +/- 0.03) administration. In contrast, a certain degree of lung specificity was observed after IT instillation of a glucocorticoid suspension (22 micrograms; AUCLung = 160 +/- 135% h; AUCLiver = 65 +/- 91% h, T = 2.3 +/- 0.5) as indicated by significant differences in T between i.v. injection and IT instillation (p = 0.038). The method presented provides a means of simultaneously assessing pulmonary and systemic effects after different forms and routes of administration and might be of value in further studying multiple aspects of inhalation glucocorticoid therapy.

Plasma concentrations and therapeutic efficacy of phenylbutazone and flunixin meglumine in the horse: pharmacokinetic/pharmacodynamic modelling

The purpose of the present study was to establish in the horse the relationship between plasma concentration profiles of phenylbutazone (PBZ) and flunixin meglumine (FM) and their pharmacological effects in order to build a predictive pharmacokinetic/pharmacodynamic (PK/PD) model. In five horses, an experimental arthritis was induced by injecting Freund's adjuvant into a carpal joint. PBZ (4 mg/kg) and FM (1 mg/kg) were injected by the intravenous route as a single intravenous dose in two different trials. Five pharmacodynamic end-points were regularly measured after test article injection using standardized procedures: local skin temperature, stride length, the rest angle flexion and the maximal carpal flexion of the injured leg and circumference of the inflamed joint. Plasma drug concentrations and pharmacodynamic data were analysed according to an integrated PK/PD model; for the stride length, the PBZ EC50, i.e. the plasma concentration for which half the maximum effect could be obtained, was 3.6 +/- 2.2 micrograms/ml and the maximum potential effect was 10.7 +/- 9.4% above the control value. For FM, the corresponding values were 0.93 +/- 0.35 micrograms/ml and 16.3 +/- 4.6%. EC50 values for rest angle flexion and local skin temperature were similar to that obtained for stride length. Maximal carpal flexion was an unreliable end-point, and circumference of the joint did not display significant response to the drugs. Using these experimental parameters, a dose-effect relationship was simulated for both drugs; it was shown for PBZ that the model predicts an absence of effect for a 1 mg/kg dose and a maximum effect at about 2 mg/kg; at higher PBZ doses, the maximum effect was not modified, but its duration was increased from 8 h with a 2 mg/kg dose to about 24 h with an 8 mg/kg dose. For FM the model predicts that a dose of 0.5 mg/kg will be without significant effect, whereas a 1 mg/kg dose allows a nearly maximal effect with a return to the control value after a delay of 16 h. A 2 mg/kg dose allows the effect to be maintained for 24 h. It is concluded that PK/PD is a tool of potential value for the preclinical screening of a dosage regimen.

Effects of indomethacin on the pharmacokinetics and pharmacodynamics of prednisolone in rats

The effect of indomethacin on the disposition of prednisolone and the induction of tyrosine aminotransferase (TAT) was examined in male Sprague-Dawley rats. Rats were pretreated with either indomethacin (5 mg/kg, intraperitoneally) or phosphate buffered saline (control) twice daily for 6 days followed by a single dose of prednisolone. Blood samples were collected after prednisolone administration. In separate animals, hepatic TAT activity (pharmacologic effect) was measured 4 h after the prednisolone dose. In addition, the effect of indomethacin on the in vitro protein binding of prednisolone was examined in pooled rat and human plasma. The clearance and apparent volume of distribution of prednisolone in the control and indomethacin-treated animals were similar, averaging 4.71 versus 4.05 L/h/kg and 1.37 versus 1.33 L/kg, respectively. The elimination half-life was 0.48 h in both groups. Indomethacin also did not affect the protein binding of prednisolone in rat or human plasma. However, indomethacin pretreatment increased the hepatic TAT activity induced by prednisolone. These studies indicate that indomethacin may affect the pharmacological effects of prednisolone without influencing its pharmacokinetics.

Prednisolone pharmacodynamics: leukocyte trafficking in the rat

Leukocytes circulate throughout the body patrolling for foreign antigens and facilitating immune responses. Corticosteroids exert their immunosuppressive actions, in part, by inhibiting the normal trafficking of these cells. The rat was used to investigate corticosteroid-induced changes in circulating total lymphocytes, CD4+ cells, and granulocytes. Prednisolone doses of 5, 25, and 50 mg/kg or saline were given i.v. Blood was taken over 24 hr for analysis of cell subsets by flow cytometry. Steroid exposure was assessed by assaying plasma prednisolone by HPLC. Response profiles were complicated, possibly by opposing effects on the recirculation of cells to blood. This prospect was investigated using pharmacodynamic cell trafficking models. Steroid-like effects in saline treated animals that may be due to stress or other factors limited data interpretation. As an animal model to characterize cell trafficking actions, the rat is an imperfect model.

Comparison of four basic models of indirect pharmacodynamic responses

Four basic models for characterizing indirect pharmacodynamic responses after drug administration have been developed and compared. The models are based on drug effects (inhibition or stimulation) on the factors controlling either the input or the dissipation of drug response. Pharmacokinetic parameters of methylprednisolone were used to generate plasma concentration and response-time profiles using computer simulations. It was found that the responses produced showed a slow onset and a slow return to baseline. The time of maximal response was dependent on the model and dose. In each case, hysteresis plots showed that drug concentrations preceded the response. When the responses were fitted with pharmacodynamic models based on distribution to a hypothetical effect compartment, the resulting parameters were dose-dependent and inferred biological implausibility. Indirect response models must be treated as distinct from conventional pharmacodynamic models which assume direct action of drugs. The assumptions, equations, and data patterns for the four basic indirect response models provide a starting point for evaluation of pharmacologic effects where the site of action precedes or follows the measured response variable.

Glucocorticoid-recognizing and -effector sites in rat liver plasma membrane. Kinetics of corticosterone uptake by isolated membrane vesicles--II. Comparative influx and efflux

To elucidate the initial step in the interaction between glucocorticoids (GC) and the hepatocyte, we examined at 22 degrees C further kinetic properties of active corticosterone (B) transport mediated by a putative, plasma membrane-inserted carrier for GC (GCC) as previously reported [Alléra and Wildt, J. Steroid Biochem. Molec. Biol. 42 (1992) 737-756]. We used a purified, well-characterized, osmotically active vesicle fraction of plasma membrane (PM), free of ATP, isolated from rat liver and a method developed by us to describe transport processes mathematically: (1) uptake (U) of 7 nM B into the vesicles (influx, I) occurred very rapidly whereby T1/2 = 8.3 s, the time (S) required for half maximum transport; the influx velocity (dU/dS = V) decreased degressively with time following second-order kinetics characterized by an initial transport V (VT0) of 177.7 fmol/mg membrane protein/s. (2) VToI of B-influx rose with temperature biphasically (P less than 0.025): activation energy above and below 15 degrees C (at PM phase transition) amounted to 9.5 and 26.5 kJ/mol. Neither at 45 nor at 60 degrees C did transport take place, revealing the high thermolability of GCC. (3) Efflux (E) of 6.5 nM B, i.e. transport out of the vesicles preincubated with the steroid, showed that influx had resulted in a 19.6-fold intravesicular hormone accumulation, indicating active ("uphill") transport. (4) The efflux velocity (dE/dS = V) exhibited almost the same kinetic quality as that of influx: it decreased following mainly second-order kinetics whereby T1/2 = 8.0 s. However, its whole time-course was much slower and the VT0 of efflux (VToE) was 6.3 lower than VToI. (5) Using physics and thermodynamics, we deduced that the affinity (AF) between B and GCC is proportional to the square of VT0. (6) Thus, because AF approximately (1/6.3)2, AF of the B-GCC interaction after completion of influx was calculated to be 40 times lower (Kd = 708 nM; delta G degrees = -34.9 kJ/mol) than at outset of influx, whereby delta G degrees = -44.0 kJ/mol. Concluding from these and previous findings, we present a new hypothesis on B transport into the hepatocyte: There is no difference (P greater than 0.3) in free enthalpy between transcortin (CBG) and the intracellular GC receptor interacting with B (delta G degrees = -40.1 and -40.4 kJ/mol). The GCC, however, is characterized by its ability to switch from a high- to lower-affinity when interacting with B (and vice versa due to metabolic energy input).(ABSTRACT TRUNCATED AT 400 WORDS)

Receptor-mediated methylprednisolone pharmacodynamics in rats: Steroid-induced receptor down-regulation

Several approaches to receptor down-regulation were examined to extend previous receptor/gene-mediated pharmacokinetic/dynamic models of corticosteroids. Down-regulation of the glucocorticoid receptor was considered as an instantaneous event or as a gradual steroid-receptor-mediated process. Concentrations of plasma methylprednisolone, free hepatic cytosolic receptors, and the activity of hepatic tyrosine aminotransferase (TAT) enzyme were measured for 16 hr following administration of 0, 10, and 50 mg/kg methylprednisolone sodium succinate to 93 adrenalectomized rats. Receptor down-regulation was best described by a fractional decrement in the rate of return of free cytosolic glucocorticoid receptor. Predicted values for free receptor, bound receptor, nuclear bound receptor, and transfer compartments were in accord with the expected rank order values based on the high and low steroid doses. Model parameter estimates were independent of dose and described the rapid depletion of free cytosolic receptor, late-phase return of cytosolic receptor to a new baseline level that was 20-40% lower than control, and the TAT induction/dissipation pattern following steroid dosing. The microscopic association and dissociation constants describing the steroid-receptor interaction were 0.23 L/nmole per hr (k(on)) and 4.74 hr-1 (k(off)) for methylprednisolone compared to previously obtained values of 0.20 L/nmole per hr and 15.7 hr-1 for the related steroid prednisolone. The time course of TAT induction was similar to that observed previously for prednisolone. Efficiency of TAT induction was more closely related to steroid receptor occupancy than plasma methylprednisolone concentrations due to receptor saturability and receptor recycling.

Glucocorticoid-recognizing and -effector sites in rat liver plasma membrane. Kinetics of corticosterone uptake by isolated membrane vesicles--I. Binding and transport

To gain insight into the mechanisms governing cellular uptake of glucocorticoids, we studied the binding and membrane transport of corticosterone (B) on a highly purified plasma membrane fraction from rat liver that was homogenized using a gentle, isotonic procedure. The fraction was mostly in the form of right-side out and osmotically active vesicles that were free of intracellular glucocorticoid receptors (GCR), transcortin (CBG) and ATP. Our uptake and binding studies carried out at 22 degrees C with [3H]B in physiological concentrations resulted in the following findings: (1) unlabeled B competed with [3H]B for uptake by the membrane vesicles; half-maximal competition of specific uptake was achieved with a 10- to 11-fold molar excess of unlabeled B. (2) [3H]B uptake was a saturable process of unusual kinetics (multiple sigmoidity); modified Scatchard plots revealed three significantly different apparent Kd-values of 1.3, 4.7 and 17.3 nM, corresponding to free B in the blood of non-stressed rats (4-16 nM). (3) Osmotic shrinkage of the vesicles led to a linear decrease in specific uptake, while non-specific uptake was independent of vesicle volume. Passive diffusion of [3H]B took place in leaky, but not in intact, vesicles. Reversible binding to, and mediated transport through, the membrane were interdependent parts of a strongly linked process. B was accumulated inside the vesicle up a concentration gradient by an active transport that followed first-order kinetics (Kt:3.9 nM); for its statistically reliable mathematical formulation and kinetic analysis, a replot was developed that revealed that relative accumulation increased with decreasing external hormone concentration. (4) Comparative binding studies disclosed that the apparent Kd-values (86.5 +/- 7.3 and 77.0 +/- 14.3 nM, respectively) of the [3H]B interactions with CBG and GCR did not differ (P greater than 0.3). These findings permit the conclusion that a plasma membrane-inserted carrier for B, effectively operating at physiological concentrations in the blood, is involved in a functional and regulatory manner in the biological action of glucocorticoids.

Anti-inflammatory drugs in the treatment of allergic disease

The treatment of asthma is undergoing significant change with an emphasis on anti-inflammatory therapy. While glucocorticoids are the most potent anti-inflammatory agent, certain patients fail to respond. These patients may be candidates for alternative anti-inflammatory therapy, such as troleandomycin, methotrexate, gold, hydroxychloroquine, or dapsone. In addition, the application of immunomodulator therapy, such as intravenous gamma globulin or cyclosporine, may be useful.

Bioavailability and nonlinear disposition of methylprednisolone and methylprednisone in the rat

Bioavailability of low (10 mg/kg) and high (50 mg/kg) doses of methylprednisolone was determined after oral administration of the free alcohol of methylprednisolone and iv administration of methylprednisolone sodium succinate. Plasma concentrations of methylprednisolone and methylprednisone (reversible metabolite) were measured by HPLC. Methylprednisolone systemic availability (F) was 49-57% after iv administration and approximately 35% after oral administration. Solubilization of steroids with PEG:ethanol had no effect on their disposition. Apparent systemic clearance (CL) of methylprednisolone was 21 mL/min (low dose), approximately twice the liver blood flow. Dose-dependent changes in steady-state volume of distribution (Vdss) and central volume of distribution (Vdc), volumes, and apparent CL were observed. The methylprednisolone-to-methylprednisone AUC ratio decreased with dose due to saturation of methylprednisone formation clearance (CL12), but this is a minor metabolic pathway. The mean residence time (MRT) increased threefold with dose. Graphical estimates of the Michaelis-Menten capacity (Vmax) and affinity (Km) constants were in reasonable agreement with CL values for the low-dose experimental data. Low systemic availability of iv methylprednisolone sodium succinate was in part due to sequential first-pass hepatic metabolism of the methylprednisolone formed. Methylprednisolone disposition is complex in the rat due to extensive first-pass effects, nonlinear elimination, nonlinear distribution, and reversible metabolism.

Disposition of methylprednisolone and its sodium succinate prodrug in vivo and in perfused liver of rats: nonlinear and sequential first-pass elimination

The disposition of methylprednisolone (MP) and its prodrug succinate ester, methylprednisolone sodium succinate (MS), were examined both in vivo and in situ (perfused livers) in rats. In vivo studies included iv and oral dosing of 10 or 50 mg/kg of MP in both forms, while liver perfusion involved initial perfusate concentrations of 5 and 25 micrograms/mL of either compound. Steroid concentrations were measured by HPLC. In the intact rat, clearance (CL) values of both compounds were high, twice the hepatic plasma flow, and decreased by one-half after the high dose, indicating nonlinear kinetics. The volumes of distribution of MS and MP were essentially constant with dose. Incomplete availability of MP from iv MS (52-55%) and from the oral dose (10%) was found. Sequential first-pass metabolism was investigated in situ. Extensive hepatic extraction of MP (84%) occurred at the low dose, but decreased to 48% at the high dose, supporting in vivo observations of high CL and nonlinearity. Extraction of MS was also high (83%), but MP availability was slight (8%). The MS and MP data were fitted to a sequential first-pass model yielding an average fraction of MS metabolized-to-MP value of 0.22. The prodrug MS and the active metabolite MP thus demonstrate both systemic and hepatic nonlinearity in rats, and the low availability of MP from iv MS was due, in part, to sequential first-pass elimination. This factor is more extensive in rats than in other species.

Pharmacodynamics of methylprednisolone phosphate after single intravenous administration to healthy volunteers

The pharmacokinetics and pharmacodynamics of methylprednisolone were investigated after intravenous administration of methylprednisolone phosphate to healthy subjects at seven different doses (16 to 1000 mg). Forty different pharmacodynamic parameters were followed for 1 week. The pharmacodynamic data were analyzed as a function of time as well as cumulative effects in form of the areas under the effect-time curves. Statistically significant dose-dependent effects of methylprednisolone were observed for 15 pharmacodynamic parameters. Highly significant (P less than or equal to 0.0001) effects were increases in glucose levels, number of white blood cells, and segmented granulocytes as well as a decrease in the number of lymphocytes. For these pharmacodynamic effects an integrated pharmacokinetic/pharmacodynamic model was derived that translates the methylprednisolone plasma concentration-time profiles into effect-time profiles. This model allows prediction of pharmacodynamic effects for any single dose in the range studied at any time point.

Nonlinear pharmacokinetics and interconversion of prednisolone and prednisone in rats

The pharmacokinetics of prednisolone and prednisone were examined in 32 rats at four intravenous doses (5, 10, 25, and 50 mg/kg). Each rat was given one dose of either intravenous prednisolone or prednisone, and plasma concentrations of both compounds were measured by HPLC. Mammillary moment analysis showed the apparent clearance and volume of distribution of prednisolone and the apparent clearance of prednisone to be dose-dependent. Further diagnostic analysis using a linear interconversion model revealed modest interconversion between the two steroids and at least two saturable clearance processes: the conversion of prednisolone to prednisone and the irreversible elimination of prednisone. A comprehensive model which incorporates the nonlinear clearances of prednisolone and prednisone plus the additional feature of nonlinear tissue distribution of prednisolone was then constructed. Four differential equations describing the rate of change of each steroid in each compartment were used to numerically fit by nonlinear least squares analysis all plasma concentration-time profiles simultaneously. The final estimates from the full model only partly agreed with the results from the two moment analyses but confirmed the general structure of the model. The nonlinear tissue distribution of prednisolone was reinforced by assay of muscle tissue. This study demonstrates the utility of the model-building process where simpler models yield insights into more elaborate schemes with complex nonlinear features.

Receptor-mediated prednisolone pharmacodynamics in rats: model verification using a dose-sparing regimen

Our receptor/gene-mediated model of corticosteroid action was tested and extended by examining the pharmacokinetics/dynamics of multiple low doses vs. a single higher dose of intravenously administered prednisolone in adrenalectomized male Wistar rats. Low-dose rats received 3 bolus doses (5 mg/kg) of prednisolone at 0, 0.5 and 1.0 hr. High-dose animals were given a single 25 mg/kg dose of prednisolone. Both regimens were expected to produce equivalent net responses based on model predictions. Control rats were not dosed. The profiles of free hepatic cytosolic glucocorticoid receptors and the hepatic tyrosine aminotransferase (TAT) enzyme were examined. Plasma prednisolone concentrations showed bi-exponential decline for both doses using pooled animal data. Clearance of total plasma prednisolone was 4.16 and 3.21 L/hr per kg in low- and high-dose groups. Volume of distribution at steady state (approximately 1.50 L/kg) and central volume (approximately 0.6 L/kg) were similar for both groups. Receptor levels from 5-16 hr stabilized at 64% of the 0-hr control value. Receptor and TAT profiles were essentially superimposable for both dosing groups. Our previous model was used to simultaneously describe prednisolone plasma concentrations, hepatic receptors, and TAT activity. The ability of total plasma prednisolone (Cp), corticosteroid binding globulin (CBG)-free plasma prednisolone (CCBG), and free plasma prednisolone (CF) to describe the kinetics/dynamics were examined. The CF values produced optimum fitting of all receptor data. The similarity of the two dosing groups supports the view that appropriately timed doses of a steroid can be used in an optimally efficacious manner by first filling all receptor sites and then replacing steroid as receptors are expected to recycle from nuclear/DNA binding sites as the steroid is eliminated.

Improving toxicology testing protocols using computer simulations

Computer simulation can be used to integrate existing toxicity information within a biologically realistic framework. Simulation models calculate relevant measures of target tissue dose based on physiological, biochemical and physicochemical properties and readily support the dose, route, species and interchemical extrapolations necessary for human risk assessment. Because these models require very specific information, much of which can be obtained in vitro, they are much less dependent on extensive animal experiments than conventional risk assessment methods. With continuing development, simulation modeling will become an invaluable tool for improving experimental designs, for interpreting animal toxicity tests, and for estimating the importance of the animal toxicity observations for people.

Pharmacokinetics and pharmacodynamic modeling of direct suppression effects of methylprednisolone on serum cortisol and blood histamine in human subjects

Pharmacodynamic models for "directly suppressive" effects of methylprednisolone are based on the premise that receptor interactions of steroids are followed by immediate suppression of either the circadian secretion of cortisol or the constant rate recirculation of histamine-containing basophils that persists until inhibitory concentrations of methylprednisolone disappear. Methylprednisolone doses of 0, 10, 20, and 40 mg were given as the 21-succinate sodium salt in a balanced crossover study to six normal men. Plasma steroid concentrations and blood histamine were measured simultaneously. Both forms of methylprednisolone exhibited linear kinetic parameters. One dynamic model quantitates the baseline circadian pattern and the decline and return of cortisol with similar parameter estimates for all three dose levels. A similar model describes the monoexponential decline and the log-linear return to steady-state baseline of blood histamine. Similar inhibitory concentration values for both effects approximated the equilibrium dissociation constant of in vitro steroid receptor binding. The new models are more physiologically appropriate for these steroid effects than three other models that are commonly employed in pharmacodynamics. Steroid effects generally appear to be receptor mediated with either nongene immediate responses or gene-mediated delayed effects. These models allow quantitation of the rapid effects of steroids with simple equations and common fitted parameters for all steroid dose levels.

Comparative pharmacokinetics of coumarin anticoagulants. XLIX: Nonlinear tissue distribution of S-warfarin in rats

The serum protein binding of the oral anticoagulant drug warfarin varies widely among rats and largely accounts for corresponding variations in the total serum clearance of the drug. The hepatic uptake of warfarin is concentration dependent despite the concentration independence of the free fraction of warfarin in serum over a wide concentration range. This investigation was designed to determine the distribution of the S enantiomer of warfarin in rats as a function of warfarin concentration, free fraction in serum, dose, and time. Two groups of rats, one with relatively low (0.0043) and the other with relatively high (0.0105) average serum free fraction values, were selected from a large number of adult male Sprague-Dawley rats. All animals received an iv injection of S-warfarin, either 0.25 or 1.0 mg/kg, and were sacrificed at intervals over a period of 10 d. Concentrations of S-warfarin in serum, liver, kidneys, muscle, and fat were determined by HPLC. The tissue:serum concentration ratio (T:S) of the drug was highly concentration dependent, but was independent of dose, time, and (except for fat) free fraction in serum. The T:S for fat was higher in animals with the larger serum free fraction values. The T:S of S-warfarin for the liver was greater than 10 at low concentrations and reached a limiting value of 0.25 at relatively high concentrations of the drug. In general, the T:S versus concentration profiles of S-warfarin are consistent with the presence of two classes of binding sites in the tissues, one with very high affinity and low capacity, the other with lower affinity and apparently unlimited capacity under the experimental conditions.(ABSTRACT TRUNCATED AT 250 WORDS)

Second generation model for prednisolone pharmacodynamics in the rat

An improved model describing receptor/gene-mediated pharmacodynamics of prednisolone is presented which consists of seven differential equations. Data for plasma prednisolone concentrations, free hepatic glucocorticoid receptors, and hepatic tyrosine aminotransferase activity (TAT) following low (5 mg/kg) and high (50 mg/kg) doses of prednisolone are used to quantitate the kinetics and dynamics of this synthetic steroid in the rat. In contrast to the earlier model, the newer model provides for a coupling and simultaneous fitting of receptor and TAT data and is able to describe the recycling of receptors between cytosol and nucleus and the return of cytosolic receptors to baseline following glucocorticoid elimination. A numerical technique to determine the efficiency of TAT induction based on area under the curve calculations is presented, which supports the hypothesis that nonlinear dose-response effects are due to dose and time-dependent receptor depletion in the cytosol. Simulations are presented to examine the major determinants of corticosteroid effects and to compare the effects of single- and multiple-dose regimens in maximizing drug effects.

Analysis of methylprednisolone, methylprednisone and corticosterone for assessment of methylprednisolone disposition in the rat

A sensitive, specific and precise high-performance liquid chromatographic assay for the simultaneous determination of methylprednisolone, methylprednisone and corticosterone using betamethasone as the internal standard is reported. Rat serum (0.5 ml) is extracted with methylene chloride, washed with sodium hydroxide, then water and the extract is injected onto a microparticulate silica gel column with ultraviolet detection at 254 nm. Calculated limits of quantitation are less than 10 ng/ml and the intra-day coefficient of variation is less than 5% for each steroid. This assay has been applied to preliminary studies of methylprednisolone disposition in the rat. The plasma concentration-time profile for each steroid was determined following intravenous administration of methylprednisolone (10 mg/kg). Peak serum methylprednisone concentrations of ca. 250 ng/ml occurred within 5 min of methylprednisolone administration and the average area under the curve ratio (methylprednisolone/methylprednisone) was 9.3. These findings demonstrate that methylprednisone is a metabolite of methylprednisolone in the rat and suggest that the metabolic back-conversion of methylprednisone to methylprednisolone may be less than in other species.

Dose-dependent pharmacokinetics of prednisolone in normal and adrenalectomized rats

The pharmacokinetics of prednisolone after 5- and 50-mg/kg doses given as the sodium succinate salt was examined in normal and adrenalectomized rats. Prednisolone, prednisone, and corticosterone concentrations in plasma were determined by HPLC and free prednisolone measured by equilibrium dialysis. Prednisolone sodium succinate was rapidly and completely hydrolyzed to prednisolone as indicated by the absence of the ester from plasma within 5 min after intravenous injection. Prednisolone was rapidly metabolized to prednisone, while corticosterone concentrations in normal rats declined rapidly and were undetectable by 1 hr. Adrenalectomy had no effect on the disposition and protein binding of prednisolone. Dose, however, had a marked effect on prednisolone pharmacokinetics, with mean plasma clearance decreasing from 6.18 to 3.07 L/h per kg and mean steady-state volume of distribution decreasing from 2.14 to 1.05 L/kg from the lower to higher steroid dose. Half-life (0.50 hr) and mean residence time (0.35 hr) were unaffected by dose. Prednisolone plasma protein binding was nonlinear due to saturation of transcortin binding. Changes in pharmacokinetic parameters were not related to the nonlinear plasma binding, but were more likely caused by saturation of elimination pathways and tissue binding sites.