Small-Molecule Compounds Exhibiting Target-Mediated Drug Disposition (TMDD): A Minireview

Characterizing the Nonlinear Pharmacokinetics and Pharmacodynamics of BI 187004, an 11β-Hydroxysteroid Dehydrogenase Type 1 Inhibitor, in Humans by a Target-Mediated Drug Disposition Model

BI 187004, a selective small-molecule inhibitor of 11β-hydroxysteroid dehydrogenase-1 (11β-HSD1), displayed complex nonlinear pharmacokinetics (PK) in humans. Following nine single oral doses, BI 187004 exhibited nonlinear PK at low doses and linear PK at higher doses. Notably, substantial hepatic 11β-HSD1 inhibition (50%) was detected in a very low-dose group, achieving a consistent 70% hepatic enzyme inhibition in subsequent ascending doses without any dose-dependent effects. The unusual PK and PD profiles of BI 187004 suggest the presence of pharmacological target-mediated drug disposition (TMDD), arising from the saturable binding of BI 187004 compound to its high-affinity and low-capacity target 11β-HSD1. The non-intuitive dose, exposure, and response relationship for BI 187004 pose a significant challenge in rational dose selection. This study aimed to construct a TMDD model to explain the complex nonlinear PK behavior and underscore the importance of recognizing TMDD in this small-molecule compound. Among the various models explored, the best model was a two-compartment TMDD model with three transit absorption components. The final model provides insights into 11β-HSD1 binding-related parameters for BI 187004, including the total amount of 11β-HSD1 in the liver (estimated to be 8000 nmol), the second order association rate constant (estimated to be 0.102 nM-1h-1), and the first-order dissociation rate constant (estimated to be 0.11 h-1). Our final population PK model successfully characterized the intricate nonlinear PK of BI 187004 across a wide dose range. This modeling work serves as a valuable reference for the rational selection of the dose regimens for BI 187004's future clinical trials.

Population Target-Mediated Pharmacokinetic/Pharmacodynamic Modeling to Evaluate SPI-62 Exposure and Hepatic 11β-Hydroxysteroid Dehydrogenase Type 1 (HSD-1) Inhibition in Healthy Adults

INTRODUCTION

SPI-62 is a small-molecule 11β-hydroxysteroid dehydrogenase type 1 (HSD-1) inhibitor exhibiting complicated nonlinear pharmacokinetics (PK) in human. Previously, we developed a target-mediated drug disposition (TMDD) model to characterize the substantial nonlinear PK of SPI-62.

OBJECTIVE

The aim of the current analysis was to perform population PK/PD analysis to further link SPI-62 exposure (i.e., PK) with its response (i.e., inhibition of hepatic HSD-1 activity) to gain a quantitative understanding of the SPI-62 dose-exposure-response relationship.

METHODS

PK and PD data from the first-in-human (FIH) clinical trials, including single ascending dose (SAD) and multiple ascending dose (MAD) studies, were used for model development. During the model development process, the final model selection was based on biological and physiological plausibility, goodness-of-fit plots, stability of parameter estimates, and objective function value. The nonlinear-mixed effect modeling (NONMEM) software was used for both the implementation of the PK/PD model and model simulation. SPI-62 plasma levels and hepatic HSD-1 inhibition over time following various dose regimens were simulated.

RESULTS

The final model was a two-compartment TMDD model component for SPI-62 and an inhibitory Imax model component for hepatic HSD-1 activity. The TMDD-hepatic PD model that we established adequately characterized all remarkable PK and PD behaviors of SPI-62, such as extremely low plasma exposures following the first low doses, nonlinear PK turned into linear PK after repeated low doses, and substantial and long-lasting hepatic HSD-1 inhibition following low doses. SPI-62 was estimated to bind to the target with a second-order association rate constant (Kon) of 8.43 nM-1 h-1 and first-order dissociation rate constant (Koff) value of 0.229 h-1, indicating that SPI-62 binds rapidly to, and dissociates slowly from, its pharmacological target. The estimated target capacity (Rtot) of 5460 nmol corresponds to approximately 2.2 mg of SPI-62, which comports well with the dose range in which PK nonlinearity is prominent. Model simulation results reveal that a 6 mg once-daily regimen can lead to long-lasting and substantial hepatic HSD-1 inhibition.

CONCLUSIONS

A population TMDD-PD model that explains SPI-62 nonlinear PK and hepatic HSD-1 inhibition following different dose regimens in healthy adults was successfully established. Our simulation results provide a solid foundation for model-informed development of SPI-62.

A 20-Year Research Overview: Quantitative Prediction of Hepatic Clearance Using the In Vitro-In Vivo Extrapolation Approach Based on Physiologically Based Pharmacokinetic Modeling and Extended Clearance Concept

Understanding the extended clearance concept and establishing a physiologically based pharmacokinetic (PBPK) model are crucial for investigating the impact of changes in transporter and metabolizing enzyme abundance/functions on drug pharmacokinetics in blood and tissues. This mini-review provides an overview of the extended clearance concept and a PBPK model that includes transporter-mediated uptake processes in the liver. In general, complete in vitro and in vivo extrapolation (IVIVE) poses challenges due to missing factors that bridge the gap between in vitro and in vivo systems. By considering key in vitro parameters, we can capture in vivo pharmacokinetics, a strategy known as the top-down or middle-out approach. We present the latest progress, theory, and practice of the Cluster Gauss-Newton method, which is used for middle-out analyses. As examples of poor IVIVE, we discuss "albumin-mediated hepatic uptake" and "time-dependent inhibition" of OATP1Bs. The hepatic uptake of highly plasma-bound drugs is more efficient than what can be accounted for by their unbound concentration alone. This phenomenon is referred to as "albumin-mediated" hepatic uptake. IVIVE was improved by measuring hepatic uptake clearance in vitro in the presence of physiologic albumin concentrations. Lastly, we demonstrate the application of Cluster Gauss-Newton method-based analysis to the target-mediated drug disposition of bosentan. Incorporating saturable target binding and OATP1B-mediated hepatic uptake into the PBPK model enables the consideration of nonlinear kinetics across a wide dose range and the prediction of receptor occupancy over time. SIGNIFICANCE STATEMENT: There have been multiple instances where researchers' endeavors to unravel the underlying mechanism of poor in vitro-in vivo extrapolation have led to the discovery of previously undisclosed truths. These include 1) albumin-mediated hepatic uptake, 2) the target-mediated drug disposition in small molecules, and 3) the existence of a trans-inhibition mechanism by inhibitors for OATP1B-mediated hepatic uptake of drugs. Consequently, poor in vitro-in vivo extrapolation and the subsequent inquisitiveness of scientists may serve as a pivotal gateway to uncover hidden mechanisms.

Predicting In Vivo Target Occupancy (TO) Profiles via Physiologically Based Pharmacokinetic-TO Modeling of Warfarin Pharmacokinetics in Blood: Importance of Low Dose Data and Prediction of Stereoselective Target Interactions

Warfarin is well recognized for its high-affinity and capacity-limited binding to the pharmacological target and undergoes target-mediated drug disposition. Here, we developed a physiologically based pharmacokinetic (PBPK) model that incorporated saturable target binding and other reported hepatic disposition components of warfarin. The PBPK model parameters were optimized by fitting to the reported blood pharmacokinetic (PK) profiles of warfarin with no stereoisomeric separation after oral dosing of racemic warfarin (0.1, 2, 5, or 10 mg) using the Cluster Gauss-Newton method (CGNM). The CGNM-based analysis yielded multiple "accepted" sets for six optimized parameters, which were then used to simulate the warfarin blood PK and in vivo target occupancy (TO) profiles. When further analyses examined the impact of dose selection on uncertainty in parameter estimation by the PBPK modeling, the PK data from 0.1 mg dose (well below target saturation) was important in practically identifying the target binding-related parameters in vivo. When stereoselective differences were incorporated for both hepatic disposition and target interactions, our PBPK modeling predicted that R-warfarin (of slower clearance and lower target affinity than S-warfarin) contributes to TO prolongation after oral dosing of racemic warfarin. Our results extend the validity of the approach by which the PBPK-TO modeling of blood PK profiles can yield TO prediction in vivo (applicable to the drugs with targets of high affinity and abundance and limited distribution volume via nontarget interactions). Our findings support that model-informed dose selection and PBPK-TO modeling may aid in TO and efficacy assessment in preclinical and clinical phase 1 studies. SIGNIFICANCE STATEMENT: The current physiologically based pharmacokinetic modeling incorporated the reported hepatic disposition components and target binding of warfarin and analyzed the blood pharmacokinetic (PK) profiles from varying warfarin doses, practically identifying target binding-related parameters in vivo. By implementing the stereoselective differences between R- and S-warfarin, our analysis predicted the role of R-warfarin in prolonging overall target occupancy. Our results extend the validity of analyzing blood PK profiles to predict target occupancy in vivo, which may guide efficacy assessment.

Individual Components of Polymyxin B Modeled via Population Pharmacokinetics to Design Humanized Dosage Regimens for a Bloodstream and Lung Infection Model in Immune-Competent Mice

Polymyxin B is a "last-line-of-defense" antibiotic approved in the 1960s. However, the population pharmacokinetics (PK) of its four main components has not been reported in infected mice. We aimed to determine the PK of polymyxin B1, B1-Ile, B2, and B3 in a murine bloodstream and lung infection model of Acinetobacter baumannii and develop humanized dosage regimens. A linear 1-compartment model, plus an epithelial lining fluid (ELF) compartment for the lung model, best described the PK. Clearance and volume of distribution were similar among the four components. The bioavailability fractions were 72.6% for polymyxin B1, 12.0% for B1-Ile, 11.5% for B2, and 3.81% for B3 for the lung model and were similar for the bloodstream model. While the volume of distribution was comparable between both models (17.3 mL for the lung and ~27 mL for the bloodstream model), clearance was considerably smaller for the lung (2.85 mL/h) compared to that of the bloodstream model (5.59 mL/h). The total drug exposure (AUC) in ELF was high due to the saturable binding of polymyxin B presumably to bacterial lipopolysaccharides. However, the modeled unbound AUC in ELF was ~16.7% compared to the total drug AUC in plasma. The long elimination half-life (~4 h) of polymyxin B enabled humanized dosage regimens with every 12 h dosing in mice. Daily doses that optimally matched the range of drug concentrations observed in patients were 21 mg/kg for the bloodstream and 13 mg/kg for the lung model. These dosage regimens and population PK models support translational studies for polymyxin B at clinically relevant drug exposures.

A Pharmacometrics Model to Characterize a New Type of Target-Mediated Drug Disposition (TMDD) - Nonlinear Pharmacokinetics of Small-Molecule PF-07059013 Mediated By Its High-capacity Pharmacological Target Hemoglobin With Positive Cooperative Binding

In general, small-molecule target-mediated drug disposition (TMDD) is caused by the interaction of a drug with its high-affinity, low-capacity pharmacological target. In the current work, we developed a pharmacometrics model to characterize a new type of TMDD, where the nonlinear pharmacokinetics (PK) is mediated by a high-capacity pharmacological target with cooperative binding instead of target saturation. The model drug we used was PF-07059013, a noncovalent hemoglobin modulator that demonstrated promising preclinical efficacy to treat sickle cell disease (SCD), and showed complex nonlinear PK in mice with the fraction of unbound drug in blood (fu_b) decreased with an increase in PF-07059013 concentrations/doses due to the positive cooperative binding of PF-07059013 to hemoglobin. Among the various models we evaluated, the best one is a semi-mechanistic model where only drug molecules not bound to hemoglobin were allowed for elimination, with the nonlinear pharmacokinetics being captured by incorporating cooperative binding for drug molecules bound to hemoglobin. Our final model provided valuable insight on target binding-related parameters, such as the Hill coefficient γ (estimated to be 1.6), binding constant K_H (estimated to be 1450 µM), and the amount of total hemoglobin R_tot (estimated to be 2.13 µmol). As the dose selection of a compound with positive cooperative binding is tricky and challenging due to the nonproportional and steep response, our model may be valuable in facilitating the rational dose regimen selection for future preclinical animal and clinical trials for PF-07059013 and other compounds whose nonlinear pharmacokinetics are caused by similar mechanisms.

A Physiological-Based Pharmacokinetic Model Embedded with a Target-Mediated Drug Disposition Mechanism Can Characterize Single-Dose Warfarin Pharmacokinetic Profiles in Subjects with Various CYP2C9 Genotypes under Different Cotreatments

Warfarin, a commonly prescribed oral anticoagulant medication, is highly effective in treating deep vein thrombosis and pulmonary embolism. However, the clinical dosing of warfarin is complicated by high interindividual variability in drug exposure and response and its narrow therapeutic index. CYP2C9 genetic polymorphism and drug-drug interactions (DDIs) are substantial contributors to this high variability of warfarin pharmacokinetics (PK), among numerous factors. Building a physiology-based pharmacokinetic (PBPK) model for warfarin is not only critical for a mechanistic characterization of warfarin PK but also useful for investigating the complicated dose-exposure relationship of warfarin. Thus, the objective of this study was to develop a PBPK model for warfarin that integrates information regarding CYP2C9 genetic polymorphisms and their impact on DDIs. Generic PBPK models for both S- and R-warfarin, the two enantiomers of warfarin, were constructed in R with the mrgsolve package. As expected, a generic PBPK model structure did not adequately characterize the warfarin PK profile collected up to 15 days following the administration of a single oral dose of warfarin, especially for S-warfarin. However, following the integration of an empirical target-mediated drug disposition (TMDD) component, the PBPK-TMDD model well characterized the PK profiles collected for both S- and R-warfarin in subjects with different CYP2C9 genotypes. Following the integration of enzyme inhibition and induction effects, the PBPK-TMDD model also characterized the PK profiles of both S- and R-warfarin in various DDI settings. The developed mathematic framework may be useful in building algorithms to better inform the clinical dosing of warfarin. SIGNIFICANCE STATEMENT: The present study found that a traditional physiology-based pharmacokinetic (PBPK) model cannot sufficiently characterize the pharmacokinetic profiles of warfarin enantiomers when warfarin is administered as a single dose, but a PBPK model with a target-mediated drug disposition mechanism can. After incorporating CYP2C9 genotypes and drug-drug interaction information, the developed model is anticipated to facilitate the understanding of warfarin disposition in subjects with different CYP2C9 genotypes in the absence and presence of both cytochrome P450 inhibitors and cytochrome P450 inducers.

Importance of Target-Mediated Drug Disposition (TMDD) of Small-Molecule Compounds and Its Impact on Drug Development-Example of the Class Effect of HSD-1 Inhibitors

With more potent drug candidates being developed, the incidence of target-mediated drug disposition (TMDD) in small-molecule compounds has significantly increased in the past decade. Moreover, TMDD appears to apply to some small-molecule compound classes. The main purpose of the current review is to increase the awareness of TMDD in a series of small-molecule inhibitors of 11β-hydroxysteroid dehydrogenase type 1 (HSD-1) using ABT-384, SPI-62, MK-0916, BMS-823778, and BI-187004 as case examples. Although developed independently by different pharmaceutical companies, these HSD-1 inhibitors demonstrated strikingly similar nonlinear pharmacokinetic behaviors when wide dose ranges were evaluated in first-in-human (FIH) single ascending dose (SAD) and multiple ascending dose (MAD) studies. Recognizing TMDD in small-molecule compounds is important, as the information can be leveraged to select the appropriate dose regimen, improve clinical trial design, as well as predict pharmacological target occupancy. In this review, we summarize the general pharmacokinetic features that facilitate the recognition of small-molecule TMDD, provide case examples of specific HSD-1 inhibitors, highlight the importance of recognizing TMDD of small-molecule compounds during clinical development, and comment on the importance of utilizing pharmacometric modeling to facilitate the quantitative understanding of small-molecule compounds exhibiting TMDD.

Impact of target-mediated drug disposition on hetrombopag pharmacokinetics and pharmacodynamics in Chinese healthy subjects and patients with chronic idiopathic thrombocytopenic purpura

AIMS

The pharmacokinetics (PK) of hetrombopag were found to be nonlinear across evaluated dose ranges. The aim of this study was to develop a mechanism-based population pharmacokinetic/pharmacodynamic (PopPK/PD) model and to provide a reasonable expected therapeutic dose for a future confirmatory clinical study of hetrombopag.

METHODS

Nonlinear mixed-effects modelling was performed using pooled 2168 hetrombopag concentrations and 1526 platelet counts from 72 healthy subjects and 32 chronic idiopathic thrombocytopenic purpura (ITP) patients from two phase I studies and one phase II study. The final model was evaluated via goodness-of-fit plots, visual predictive check and nonparametric bootstrap. Simulations from the validated PopPK/PD model were used to devise an expected therapeutic dose for later confirmatory clinical study.

RESULTS

The pharmacokinetic data of hetrombopag were well described by a modified target-mediated drug disposition (TMDD) model with dual sequential first-order absorption. Mean parameter estimates (interindividual variability) were CL/F 7.66 L/h (63.5%), V_c /F 30.0 L (77.2%) and K_deg 0.693/h (87.1%). The pharmacodynamic profile was well described by a five-compartment lifespan model with four-transit and one-platelet compartments. Simulation results suggested that chronic ITP patients following 10 mg once-daily hetrombopag would able to achieve an ideal platelet count level (50-200 × 10⁹ /L).

CONCLUSION

TMDD was the primary reason leading to nonlinear PK profile of hetrombopag. Our PK/PD modelling and simulation results support 10 mg once-daily as the recommended therapeutic dose for chronic ITP patients in subsequent confirmatory clinical study of hetrombopag.

A Target-Mediated Drug Disposition Model to Explain Nonlinear Pharmacokinetics of the 11β-Hydroxysteroid Dehydrogenase Type 1 Inhibitor SPI-62 in Healthy Adults

SPI‐62 is a selective and potent small‐molecule inhibitor of 11β‐hydroxysteroid dehydrogenase type 1 (HSD‐1). SPI‐62 has demonstrated substantial and complex nonlinear pharmacokinetics (PK) in humans that is characterized by unusually low plasma exposure at low doses, dose‐dependent volume of distribution, nonlinear PK following the first dose, and dose‐proportional PK at steady state, as well as unusually high accumulation ratios at low doses. The most likely explanation for the observed nonlinearity of SPI‐62 is the saturable binding of SPI‐62 to its pharmacological target HSD‐1, a phenomenon known as target‐mediated drug disposition (TMDD). Because of the nonlinear and complex PK of SPI‐62, the relationship among SPI‐62 dose, exposure, and response is no longer intuitive and consequently dose selection can be challenging. To facilitate dose selection and clinical trial design, in the current study population PK analysis was performed to characterize SPI‐62 dose‐exposure relationship in humans quantitatively. SPI‐62 PK was best characterized by a 2‐compartment TMDD model with 3 transit absorption compartments. The model was successfully established to explain the substantial and unusual nonlinear PK of SPI‐62 in humans, and it provided adequate fitting for both single‐ and multiple‐dose data. Our modeling work has provided a strong foundation for dose selection in future SPI‐62 clinical trials.

Drug discovery for enzymes

Enzymes are essential, physiological catalysts involved in all processes of life, including metabolism, cellular signaling and motility, as well as cell growth and division. They are attractive drug targets because of the presence of defined substrate-binding pockets, which can be exploited as binding sites for pharmaceutical enzyme inhibitors. Understanding the reaction mechanisms of enzymes and the molecular mode of action of enzyme inhibitors is indispensable for the discovery and development of potent, efficacious, and safe novel drugs. The combination of classical concepts of enzymology with new experimental and data analysis methods opens new routes for drug discovery.

The exploration of aza-quinolines as hematopoietic prostaglandin D synthase (H-PGDS) inhibitors with low brain exposure

GlaxoSmithKline and Astex Pharmaceuticals recently disclosed the discovery of the potent H-PGDS inhibitor GSK2894631A 1a (IC₅₀ = 9.9 nM) as part of a fragment-based drug discovery collaboration with Astex Pharmaceuticals. This molecule exhibited good murine pharmacokinetics, allowing it to be utilized to explore H-PGDS pharmacology in vivo. Yet, with prolonged dosing at higher concentrations, 1a induced CNS toxicity. Looking to attenuate brain penetration in this series, aza-quinolines, were prepared with the intent of increasing polar surface area. Nitrogen substitutions at the 6- and 8-positions of the quinoline were discovered to be tolerated by the enzyme. Subsequent structure activity studies in these aza-quinoline scaffolds led to the identification of 1,8-naphthyridine 1y (IC₅₀ = 9.4 nM) as a potent peripherally restricted H-PGDS inhibitor. Compound 1y is efficacious in four in vivo inflammatory models and exhibits no CNS toxicity.

Target-Mediated Drug Disposition-A Class Effect of Soluble Epoxide Hydrolase Inhibitors

Pharmacological target-mediated drug disposition (TMDD) represents a special source of nonlinear pharmacokinetics, and its occurrence in large-molecule compounds has been well recognized because numerous protein drugs have been reported to have TMDD due to specific binding to their pharmacological targets. Although TMDD can also happen in small-molecule compounds, it has been largely overlooked. In this mini-review, we summarize the occurrence of TMDD that we discovered recently in a series of small-molecule soluble epoxide hydrolase (sEH) inhibitors. Our journey started with an accidental discovery of target-mediated kinetics of 1-(1-propanoylpiperidin-4-yl)-3-[4-(trifluoromethoxy)phenyl]urea (TPPU), a potent sEH inhibitor, in a pilot clinical study. To confirm what we observed in humans, we conducted a series of mechanism experiments in animals, including pharmacokinetic experiments using sEH knockout mice as well as in vivo displacement experiments with co-administration of another potent sEH inhibitor. Our mechanism studies confirmed that the TMDD of TPPU is due to its pharmacological target sEH. We further expanded our evaluation to various other sEH inhibitors and found that TMDD is a class effect of this group of small-molecule sEH inhibitors. In addition to summarizing the occurrence of TMDD in sEH inhibitors, in this mini-review we also highlighted the importance of recognizing TMDD of small-molecule compounds and its impact in clinical development as well as using pharmacometric modeling in facilitating quantitative understanding of TMDD.

Incorporating Pharmacological Target-Mediated Drug Disposition (TMDD) in a Whole-Body Physiologically Based Pharmacokinetic (PBPK) Model of Linagliptin in Rat and Scale-up to Human

Linagliptin demonstrates substantial nonlinear pharmacokinetics due to its saturable binding to its pharmacological target dipeptidyl peptide 4 (DPP-4), a phenomenon known as target-mediated drug disposition (TMDD). In the current study, we established a novel whole-body physiologically-based pharmacokinetic (PBPK)-TMDD model for linagliptin. This comprehensive model contains plasma and 14 tissue compartments, among which TMDD binding process was incorporated in 9 of them, namely the plasma, kidney, liver, spleen, lung, skin, salivary gland, thymus, and reproductive organs. Our final model adequately captured the concentration-time profiles of linagliptin in both plasma and various tissues in both wildtype rats and DPP4-deficient rats following different doses. The association rate constant (k_on) in plasma and tissues were estimated to be 0.943 and 0.00680 nM^-1 h^-1, respectively, and dissociation rate constant (k_off), in plasma and tissues were estimated to be 0.0698 and 0.00880 h^-1, respectively. The binding affinity of linagliptin to DPP-4 (Kd) was predicted to be higher in plasma (0.0740 nM) than that in tissue (1.29 nM). When scaled up to a human, this model captured the substantial and complex nonlinear pharmacokinetic behavior of linagliptin in human adults that is characterized by less-than dose-proportional increase in plasma exposure, dose-dependent clearance and volume of distribution, as well as long terminal half-life with minimal accumulation after repeated doses. Our modeling work is not only novel but also of high significance as the whole-body PBPK-TMDD model platform developed using linagliptin as the model compound could be applied to other small-molecule compounds exhibiting TMDD to facilitate their optimal dose selection. Graphical abstract.

Phase 0/microdosing approaches: time for mainstream application in drug development?

Phase 0 approaches - which include microdosing - evaluate subtherapeutic exposures of new drugs in first-in-human studies known as exploratory clinical trials. Recent progress extends phase 0 benefits beyond assessment of pharmacokinetics to include understanding of mechanism of action and pharmacodynamics. Phase 0 approaches have the potential to improve preclinical candidate selection and enable safer, cheaper, quicker and more informed developmental decisions. Here, we discuss phase 0 methods and applications, highlight their advantages over traditional strategies and address concerns related to extrapolation and developmental timelines. Although challenges remain, we propose that phase 0 approaches be at least considered for application in most drug development scenarios.

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.

A Translational Physiologically Based Pharmacokinetics/Pharmacodynamics Framework of Target-Mediated Disposition, Target Inhibition and Drug-Drug Interactions of Bortezomib

Bortezomib is a potent 20S proteasome inhibitor approved for the treatment of multiple myeloma and mantle cell lymphoma. Despite the extensive clinical use of bortezomib, the mechanism of the complex time-dependent pharmacokinetics of bortezomib has not been fully investigated in context of its pharmacodynamics (PD) and drug-drug interaction (DDI) profiles. Here, we aimed to develop a mechanistic physiologically based (PB) PK/PD model to project PK, blood target inhibition and DDI of bortezomib in patients. A minimal PBPK/PD model consisting of six compartments was constructed using a bottom-up approach with pre-clinical data and human physiological parameters. Specifically, the target-mediated drug disposition (TMDD) of bortezomib in red blood cells (RBC), which determines target inhibition in blood, was characterized by incorporating the proteasome binding affinity of bortezomib and the proteasome concentration in RBC. The hepatic clearance and fraction metabolized by different CYP isoforms were estimated from in vitro metabolism and phenotyping experiments. The established model adequately characterized the multi-exponential and time-dependent plasma pharmacokinetics, target binding and blood proteasome inhibition of bortezomib. Further, the model was able to accurately predict the impact of a strong CYP3A inducer (rifampicin) and inhibitor (ketoconazole) on bortezomib exposure. In conclusion, the mechanistic PBPK/PD model successfully described the complex pharmacokinetics, target inhibition and DDIs of bortezomib in patients. This study illustrates the importance of incorporating target biology, drug-target interactions and in vitro clearance parameters into mechanistic PBPK/PD models and the utility of such models for pharmacokinetic, pharmacodynamic and DDI predictions.

Simultaneous Target-Mediated Drug Disposition Model for Two Small-Molecule Compounds Competing for Their Pharmacological Target: Soluble Epoxide Hydrolase

1-(1-propanoylpiperidin-4-yl)-3-[4-(trifluoromethoxy)phenyl]urea (TPPU) and 1-(4-trifluoro-methoxy-phenyl)-3-(1-cyclopropanecarbonyl-piperidin-4-yl)-urea (TCPU) are potent inhibitors of soluble epoxide hydrolase (sEH) that have much better efficacy in relieving nociceptive response than the Food and Drug Administration-approved drug gabapentin in a rodent model of diabetic neuropathy. Experiments conducted in sEH knockout mice or with coadministration of a potent sEH displacer demonstrated that the pharmacokinetics of TPPU and TCPU were influenced by the specific binding to their pharmacologic target sEH, a phenomenon known as target-mediated drug disposition (TMDD). To quantitatively characterize the complex pharmacokinetics of TPPU and TCPU and gain better understanding on their target occupancy, population pharmacokinetics analysis using a nonlinear mixed-effect modeling approach was performed in the current study. The final model was a novel simultaneous TMDD interaction model, in which TPPU and TCPU compete for sEH, with TCPU binding to an additional unknown target pool with larger capacity that we refer to as a refractory pool. The total amount of sEH enzyme in mice was predicted to be 16.2 nmol, which is consistent with the experimental value of 10 nmol. The dissociate rate constants of TPPU and TCPU were predicted to be 2.24 and 2.67 hours^-1, respectively, which is close to the values obtained from in vitro experiments. Our simulation result predicted that 90% of the sEH will be occupied shortly after a low dose of 0.3 mg/kg TPPU administration, with ≥40% of sEH remaining to be bound with TPPU for at least 7 days. Further efficacy experiments are warranted to confirm the predicted target occupancy. SIGNIFICANCE STATEMENT: Although target-mediated drug disposition (TMDD) models have been well documented, most of them were established in a single compound scenario. Our novel model represents the first TMDD interaction model for two small-molecule compounds competing for the same pharmacological target.

Dose Correction for a Michaelis-Menten Approximation of a Target-Mediated Drug Disposition Model with a Multiple Intravenous Dosing Regimens

This study aimed to develop a method for implementing dose correction in a Michaelis-Menten (M-M) approximation of a target-mediated drug disposition (TMDD) model with multiple intravenous (IV) bolus administrations. We derived the formula of a correction factor (Fcorr) for each dose in a multiple IV bolus dosing regimens for M-M model. Fcorr depends on the residual free drug amount prior IV bolus dosing event and dose amount. We conducted a stochastic simulation and estimation (SSE) exercise based on therapeutic antibody PK parameters to evaluate the effect of Fcorr on parameter estimation. Previously published clinical PK data of recombinant human erythropoietin (rHuEPO) from four clinical trials in healthy subjects receiving multiple IV bolus doses were analyzed by both M-M model with and without dose correction (MMC and MMNC) as well as the rapid-binding/quasi-steady-state (RB/QSS) TMDD models. Our results showed that MMNC introduced bias to fixed-effect parameter estimates and overestimated random-effect variables. Compared with MMC, MMNC was not able to adequately characterize the nonlinearity in the PK data of antibody and rHuEPO. The MMC-based simulation demonstrated that thricely weekly 10 IU/kg rHuEPO dosing regimen yielded Fcorr = 0.5. This result suggested that the lower-than-expected exposure for rHuEPO at low dose is due to target binding. An M-M approximation of the TMDD model should include a dose correction to avoid model misfitting and potential bias in the estimated PK parameters.

Pharmacokinetics of the Monocarboxylate Transporter 1 Inhibitor AZD3965 in Mice: Potential Enterohepatic Circulation and Target-Mediated Disposition

PURPOSE

To evaluate the pharmacokinetics (PK) of the monocarboxylate transporter 1 (MCT1) inhibitor AZD3965 in mice after IV and oral administration and to develop mechanistic PK models to assess the potential enterohepatic circulation (EHC) and target-mediated drug disposition (TMDD) of AZD3965.

METHODS

Female BALB/c mice were administered AZD3965 by IV injection (10, 50 and 100 mg/kg) or oral gavage (100 mg/kg). Plasma samples were analyzed using LC/MS/MS, and PK parameters determined by compartmental and non-compartmental analyses.

RESULTS

AZD3965 exhibited a large volume of distribution and rapid oral absorption, with a high oral bioavailability. Prominent reentry peaks were observed after both oral and IV administration, suggesting potential EHC of AZD3965 or of a potential glucuronide conjugate. The dose-dependent studies indicated greater than proportional increases in exposure, an increase in the terminal half-life, and decrease in clearance and volume of distribution with increasing IV doses, indicating nonlinear pharmacokinetics and potential TMDD of AZD3965. Mechanistic compartmental models were developed to characterize the complex pharmacokinetics of AZD3965.

CONCLUSIONS

The current study represents the first comprehensive report of the pharmacokinetics of AZD3965 in mice, indicating the potential contribution of EHC and TMDD in the disposition of AZD3965.

Target-Mediated Population Pharmacokinetic Modeling of Endothelin Receptor Antagonists

PURPOSE

Bosentan, clazosentan, and tezosentan are three small-molecule endothelin receptor antagonists (ERAs), displacing endothelin-1 (ET-1) from its binding site. A target-mediated drug disposition (TMDD) pharmacokinetic (PK) model described the non-linearity in the PK of bosentan caused by its high receptor binding affinity with time-dependent varying receptor expression or reappearance. The aim of this analysis was to investigate the presence of TMDD for clazosentan and tezosentan and to corroborate the hypothesis of a diurnal receptor synthesis.

METHODS

PK data from healthy subjects after intravenous (i.v.) administration of single ascending doses of bosentan, clazosentan, and tezosentan were analyzed. Frequent blood samples for PK measurements were collected. Population analyses, simulations, and evaluations were performed using a non-linear mixed-effects modeling approach.

RESULTS

Two-compartment TMDD models were successfully developed describing the PK of all three ERAs with different receptor-complex internalization properties. The observed multiple peaks in the concentration-time profiles were captured with cosine functions on the receptor synthesis rate mimicking a diurnal receptor expression or reappearance. The results strongly suggest that TMDD is a class effect of ERAs.

CONCLUSION

The developed TMDD PK models are a next step towards understanding the complex PK of ERAs and further support the hypothesis that TMDD is a class effect of ERAs.

Concept of Pharmacologic Target-Mediated Drug Disposition in Large-Molecule and Small-Molecule Compounds

Target-mediated drug disposition (TMDD) is a term to describe a nonlinear pharmacokinetic (PK) phenomenon that is caused by high-affinity binding of a compound to its pharmacologic targets. As the interaction between a drug and its pharmacologic target belongs to the process of pharmacodynamics (PD), TMDD can be viewed as a consequence of "PD affecting PK." Although there are numerous TMDD-related articles in the literature, most of them focus on characterizing TMDD using various mathematical models, which may not be suitable for those readers who have little interest in mathematical modeling and only want to have an understanding of the basic concept. The goal of this review is to serve as a "primer" on TMDD. This review explains (1) how TMDD happens; (2) why large-molecule and small-molecule compounds exhibiting TMDD demonstrate substantially different nonlinear PK behaviors; (3) what nonlinear PK profiles look like in large-molecule and small-molecule compounds exhibiting TMDD, using pegfilgrastim, erythropoietin, ABT-384, and linagliptin as case examples; and (4) how to identify whether the nonlinear PK of a compound is because of TMDD.

The Anthelmintic Effect on Strongyloides venezuelensis Induced by BnSP- 6, a Lys49-phospholipase A2 Homologue from Bothrops pauloensis Venom

Background:

Phospholipases A2 (PLA2) from snake venoms have a broad potential as pharmacological tools on medicine. In this context, strongyloidiasis is a neglected parasitic disease caused by helminths of the genus Strongyloides. Currently, ivermectin is the drug of choice for treatment, however, besides its notable toxicity, therapeutic failures and cases of drug resistance have been reported. BnSP-6, from Bothorps pauloensis snake venom, is a PLA2 with depth biochemical characterization, reporting effects against tumor cells and bacteria.

Objective:

The aim of this study is to demonstrate for the first time the action of the PLA2 on Strongyloides venezuelensis.

Methods:

After 72 hours of treatment with BnSP-6 mortality of the infective larvae was assessed by motility assay. Cell and parasite viability was evaluated by the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT) assay. Furthermore, autophagic vacuoles were labeled with Monodansylcadaverine (MDC) and nuclei of apoptotic cells were labeled with Propidium Iodide (PI). Tissue degeneration of the parasite was highlighted by Transmission Electron Microscopy (TEM).

Results:

The mortality index demonstrated that BnSP-6 abolishes the motility of the parasite. In addition, the MTT assay attested the cytotoxicity of BnSP-6 at lower concentrations when compared with ivermectin, while autophagic and apoptosis processes were confirmed. Moreover, the anthelmintic effect was demonstrated by tissue degeneration observed by TEM. Furthermore, we report that BnSP-6 showed low cytotoxicity on human intestinal cells (Caco-2).

Conclusion:

Altogether, our results shed light on the potential of BNSP-6 as an anthelmintic agent, which can lead to further investigations as a tool for pharmaceutical discoveries.

Target-mediated drug disposition with drug-drug interaction, Part I: single drug case in alternative formulations

Target-mediated drug disposition (TMDD) describes drug binding with high affinity to a target such as a receptor. In application TMDD models are often over-parameterized and quasi-equilibrium (QE) or quasi-steady state (QSS) approximations are essential to reduce the number of parameters. However, implementation of such approximations becomes difficult for TMDD models with drug-drug interaction (DDI) mechanisms. Hence, alternative but equivalent formulations are necessary for QE or QSS approximations. To introduce and develop such formulations, the single drug case is reanalyzed. This work opens the route for straightforward implementation of QE or QSS approximations of DDI TMDD models. The manuscript is the first part to introduce DDI TMDD models with QE or QSS approximations.