Kinetic modeling of the plasma pharmacokinetic profiles of ADAMTS13 fragment and its Fc-fusion counterpart in mice.

Introduction: Fusion of the fragment crystallizable (Fc) to protein therapeutics is commonly used to extend the circulation time by enhancing neonatal Fc-receptor (FcRn)-mediated endosomal recycling and slowing renal clearance. This study applied kinetic modeling to gain insights into the cellular processing contributing to the observed pharmacokinetic (PK) differences between the novel recombinant ADAMTS13 fragment (MDTCS) and its Fc-fusion protein (MDTCS-Fc). Methods: For MDTCS and MDTCS-Fc, their plasma PK profiles were obtained at two dose levels following intravenous administration of the respective proteins to mice. The plasma PK profiles of MDTCS were fitted to a kinetic model with three unknown protein-dependent parameters representing the fraction recycled (FR) and the rate constants for endocytosis (kup, for the uptake into the endosomes) and for the transfer from the plasma to the interstitial fluid (kpi). For MDTCS-Fc, the model was modified to include an additional parameter for binding to FcRn. Parameter optimization was done using the Cluster Gauss-Newton Method (CGNM), an algorithm that identifies multiple sets of approximate solutions ("accepted" parameter sets) to nonlinear least-squares problems. Results: As expected, the kinetic modeling results yielded the FR of MDTCS-Fc to be 2.8-fold greater than that of MDTCS (0.8497 and 0.3061, respectively). In addition, MDTCS-Fc was predicted to undergo endocytosis (the uptake into the endosomes) at a slower rate than MDTCS. Sensitivity analyses identified the association rate constant (kon) between MDTCS-Fc and FcRn as a potentially important factor influencing the plasma half-life in vivo. Discussion: Our analyses suggested that Fc fusion to MDTCS leads to changes in not only the FR but also the uptake into the endosomes, impacting the systemic plasma PK profiles. These findings may be used to develop recombinant protein therapeutics with extended circulation time.

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.

Size optimization of carfilzomib nanocrystals for systemic delivery to solid tumors.

Carfilzomib (CFZ) is a second-generation proteasome inhibitor effective in blood cancer therapy. However, CFZ has shown limited efficacy in solid tumor therapy due to the short half-life and poor tumor distribution. Albumin-coated nanocrystal (NC) formulation was shown to improve the circulation stability of CFZ, but its antitumor efficacy remained suboptimal. We hypothesize that NC size reduction is critical to the formulation safety and efficacy as the small size would decrease the distribution in the reticuloendothelial system (RES) and selectively increase the uptake by tumor cells. We controlled the size of CFZ-NCs by varying the production parameters in the crystallization-in-medium method and compared the size-reduced CFZ-NCs (z-average of 168 nm, NC168) with a larger counterpart (z-average of 325 nm, NC325) as well as the commercial CFZ formulation (CFZ-CD). Both CFZ-NCs showed similar or higher cytotoxicity than CFZ-CD against breast cancer cells. NC168 showed greater uptake by cancer cells, less uptake by macrophages and lower immune cell toxicity than NC325 or CFZ-CD. NC168, but not NC325, showed a similar safety profile to CFZ-CD in vivo. The biodistribution and antitumor efficacy of CFZ-NCs in mice were also size-dependent. NC168 showed greater antitumor efficacy and tumor accumulation but lower RES accumulation than NC325 in 4T1 breast cancer model. These results support that NC formulation with an optimal particle size can improve the therapeutic efficacy of CFZ in solid tumors.

Developing a Loop-Mediated Isothermal Amplification Assay for the Rapid Detection of Seven Respiratory Viruses including SARS-CoV-2.

Background and Objectives: The coronavirus disease (COVID-19), caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), continues to be a pandemic even in 2022. As the initial symptoms of COVID-19 overlap with those of infections from other respiratory viruses, an accurate and rapid diagnosis of COVID-19 is essential for administering appropriate treatment to patients. Currently, the most widely used method for detecting respiratory viruses is based on real-time polymerase chain reaction (PCR) and includes reverse-transcription real-time quantitative PCR (RT-qPCR). However, RT-qPCR assays require sophisticated facilities and are time-consuming. This study aimed to develop a real-time quantitative loop-mediated isothermal amplification (RT-qLAMP) assay and compare its analytical performance with RT-qPCR. Materials and Methods: A total of 315 nasopharyngeal swabs from patients with symptoms of respiratory infections were included in this study. A primary screening of the specimens was performed using RT-qPCR. RNA/DNA from standard strains for respiratory viruses and heat-inactivated preparations of standard strains for SARS-CoV-2 were used to evaluate the accuracy and target specificity of the RT-qLAMP assay. Results: We successfully developed an RT-qLAMP assay for seven respiratory viruses: respiratory syncytial virus (RSV) A, RSV B, adenovirus, influenza (Flu) A (H1N1 and H3N2), Flu B, and SARS-CoV-2. RT-qLAMP was performed in a final reaction volume of 9.6 µL. No cross-reactivity was observed. Compared with the RT-PCR results, the sensitivity and specificity of the RT-qLAMP assay were 95.1% and 100%, respectively. The agreement between the two methods was 97.1%. The median amplification time to RT-qLAMP positivity was 22:34 min (range: 6:80-47:98 min). Conclusions: The RT-qLAMP assay requires a small number of reagents and samples and is performed with an isothermal reaction. This study established a fast, simple, and sensitive test that can be applied to point-of-care testing devices to facilitate the detection of respiratory viruses, including SARS-CoV-2.

Successful Treatment of Thrombotic Thrombocytopenic Purpura in a Patient with Mixed Connective Tissue Disease.

BACKGROUND: Although the survival rate of thrombocytopenic purpura (TTP) has increased significantly due to the introduction of therapeutic plasma exchange (TPE). TTP in patients with mixed connective tissue disease (MCTD) has a very high mortality rate and a very small number of reported cases. In TTP, daily TPE is administered until a treatment response is achieved; however, in practice, TPE is often not performed for such long durations. METHODS: We report a case of TTP with MCTD in a female patient. She had developed thrombocytopenia and hemolytic anemia 9 months after delivery. She had status epilepticus and lapsed into a coma. RESULTS: The patient was successfully treated with extended sessions of TPE with corticosteroids and rituximab. CONCLUSIONS: Although the TTP regimen has not yet been established and remains controversial, this report demonstrates the importance of continuing daily TPE until achieving a treatment response.

Practical Considerations for Clinical Laboratories in Top-down Approach for Assessing the Measurement Uncertainty of Clinical Chemistry Analytes.

Background: The top-down (TD) approach using internal quality control (IQC) data is regarded a practical method for estimating measurement uncertainty (MU) in clinical laboratories. We estimated the MU of 14 clinical chemistry analytes using the TD approach and evaluated the effect of lot changes on the MU. Methods: MU values were estimated using subgrouping by reagent lot changes or using the data as a whole, and both methods were compared. Reagent lot change was simulated using randomly generated data, and the mean values and MU for two IQC datasets (different QC material lots) were compared using statistical methods. Results: All MU values calculated using subgrouping were lower than the total values; however, the average differences were minimal. The simulation showed that the greater the increase in the extent of the average shift, the larger the difference in MU. In IQC data comparison, the mean values and MU exhibited statistically significant differences for most analytes. The MU calculation methods gave rise to minimal differences, suggesting that IQC data in clinical laboratories show no significant shift. However, the simulation results demonstrated that notable differences in the MU can arise from significant variations in IQC results before and after a reagent lot change. Additionally, IQC material lots should be treated separately when IQC data are collected for MU estimation. Conclusions: Lot changes in IQC data are a key factor affecting MU estimation and should not be overlooked during MU estimation.

Experimental and Modeling Evidence Supporting the Trans-Inhibition Mechanism for Preincubation Time-Dependent, Long-Lasting Inhibition of Organic Anion Transporting Polypeptide 1B1 by Cyclosporine A.

Cyclosporine A (CsA) and rifampin are potent inhibitors of organic anion transporting polypeptide (OATP) 1B1 and are widely used to assess the risk for drug-drug interactions. CsA displays preincubation time-dependent, long-lasting inhibition of OATP1B1 in vitro and in rats in vivo, and a proposed mechanism is the trans-inhibition by which CsA inhibits OATP1B1 from the inside of cells. The current study aimed to experimentally validate the proposed mechanism using human embryonic kidney 293 cells stably expressing OATP1B1. The uptake of CsA reached a plateau following an approximate 60-minute incubation, with the cell-to-buffer concentration ratio of 3930, reflective of the high-affinity, high-capacity intracellular binding of CsA. The time course of CsA uptake was analyzed to estimate the kinetic parameters for permeability clearance and intracellular binding. When the OATP1B1-mediated uptake of [3H]estradiol-17ß-glucuronide was measured following preincubation with CsA for 5 to 120 minutes, apparent Ki values became lower with longer preincubation. Our kinetic modeling incorporated the two reversible inhibition constants [Ki,trans and Ki,cis for the inhibition from inside (trans-inhibition) and outside (cis-inhibition) of cells, respectively] and estimated Ki,trans value of CsA was smaller by 48-fold than the estimated Ki,cis value. Rifampin also displayed preincubation time-dependent inhibition of OATP1B1, albeit the extent of enhancement was only twofold. The current study provides experimental evidence for the preincubation time-dependent shift of apparent Ki values and a mechanistic basis for physiologically based pharmacokinetic modeling that incorporates permeability clearance, extensive intracellular binding, and asymmetry of Ki values between the inside and outside of cells. SIGNIFICANCE STATEMENT: In vitro data and kinetic modeling support that preincubation time-dependent, long-lasting inhibition of OATP1B1 by CsA can be explained by the extensive intracellular binding and reversible OATP1B1 inhibition intracellularly (trans-inhibition) as well as extracellularly (cis-inhibition). For inhibitors to display time-dependency, the following factors were found important: time to reach a steady-state cellular concentration, trans-inhibition potency relative to cis-inhibition, and the degree of cellular inhibitor accumulation. This study would aid in the accurate prediction of drug-drug interactions mediated by OATP1B1 inhibition.

Consideration of albumin-mediated hepatic uptake for highly protein-bound anionic drugs: Bridging the gap of hepatic uptake clearance between in vitro and in vivo.

The accuracy in predicting in vivo hepatic clearance of drugs from in vitro data (often termed as in vitro-to-in vivo extrapolation, IVIVE) has improved in part by applying the extended-clearance concept that considers the interplay between hepatic metabolism and uptake/efflux processes. However, the IVIVE-based prediction performs poorly in predicting the hepatic uptake clearance of highly albumin-bound anionic drugs. Their hepatic uptake clearances tend to be much higher than expected based on the free-drug theory. Such observation can be attributable to a phenomenon called albumin-mediated hepatic uptake, for which various models have been thus far proposed. Our group has been applying a facilitated-dissociation model, which assumes the enhanced dissociation of the drug-albumin complex upon interaction with the cell surface. By considering the albumin-mediated hepatic uptake (using the facilitated-dissociation model or alternative kinetic models), a number of investigations demonstrated the improvement in the prediction accuracy for the hepatic clearance of highly protein-bound anionic drugs that are substrates for hepatic uptake transporters. This review summarizes the reported kinetic analyses of the albumin-mediated hepatic uptake of highly albumin-bound drugs concerning the IVIVE and the clinical and physiological relevance.

Macrocyclic Immunoproteasome Inhibitors as a Potential Therapy for Alzheimer's Disease.

Previously, we reported that immunoproteasome (iP)-targeting linear peptide epoxyketones improve cognitive function in mouse models of Alzheimer's disease (AD) in a manner independent of amyloid ß. However, these compounds' clinical prospect for AD is limited due to potential issues, such as poor brain penetration and metabolic instability. Here, we report the development of iP-selective macrocyclic peptide epoxyketones prepared by a ring-closing metathesis reaction between two terminal alkenes attached at the P2 and P3/P4 positions of linear counterparts. We show that a lead macrocyclic compound DB-60 (20) effectively inhibits the catalytic activity of iP in ABCB1-overexpressing cells (IC50: 105 nM) and has metabolic stability superior to its linear counterpart. DB-60 (20) also lowered the serum levels of IL-1α and ameliorated cognitive deficits in Tg2576 mice. The results collectively suggest that macrocyclic peptide epoxyketones have improved CNS drug properties than their linear counterparts and offer promising potential as an AD drug candidate.

Relationship between Brain Tissue Changes and Blood Biomarkers of Cyclophilin A, Heme Oxygenase-1, and Inositol-Requiring Enzyme 1 in Patients with Alzheimer's Disease.

Cyclophilin A (CypA), heme oxygenase-1 (HO-1), and inositol-requiring enzyme 1 (IRE1) are believed to be associated with Alzheimer's disease (AD). In this study, we investigated the association between gray matter volume (GMV) changes and blood levels of CypA, HO-1, and IRE1 in cognitively normal (CN) subjects and those with amnestic mild cognitive impairment (aMCI) and AD. Forty-five elderly CN, 34 aMCI, and 39 AD subjects were enrolled in this study. The results of voxel-based multiple regression analysis showed that blood levels of CypA, HO-1, and IRE1 were correlated with GMV on brain magnetic resonance imaging (MRI) in the entire population (p = 0.0005). The three serum protein levels were correlated with GMV of signature AD regions in the population as a whole. CypA values increased with increasing GMV in the occipital gyrus (r = 0.387, p < 0.0001) and posterior cingulate (r = 0.196, p = 0.034). HO-1 values increased with increasing GMV at the uncus (r = 0.307, p = 0.0008), lateral globus pallidus and putamen (r = 0.287, p = 0.002), and hippocampus (r = 0.197, p = 0.034). IRE1 values decreased with increasing GMV at the uncus (r = -0.239, p = 0.010) and lateral globus pallidus and putamen (r = -0.335, p = 0.0002). Associations between the three serum protein levels and regional GMV indicate that the blood levels of these biomarkers may reflect the pathological mechanism of AD in the brain.

Revisiting Nonlinear Bosentan Pharmacokinetics by Physiologically Based Pharmacokinetic Modeling: Target Binding, Albeit Not a Major Contributor to Nonlinearity, Can Offer Prediction of Target Occupancy.

Bosentan is a high-affinity antagonist of endothelin receptors and one of the earliest examples for target-mediated drug disposition [a type of nonlinear pharmacokinetics (PKs) caused by saturable target binding]. The previous physiologically based PK (PBPK) modeling indicated that the nonlinear PKs of bosentan was explainable by considering saturable hepatic uptake. However, it remained unexamined to what extent the saturable target binding contributes to the nonlinear PKs of bosentan. Here, we developed a PBPK model incorporating saturable target binding and hepatic uptake and analyzed the clinical bosentan PK data using the cluster Gauss-Newton method (CGNM). The PBPK model without target binding fell short in capturing the bosentan concentrations below 100 nM, based on the PK profiles and the goodness-of-fit plot. Both global and local identifiability analyses (using the CGNM and Fisher information matrix, respectively) informed that the target binding parameters were identifiable only if the observations from the lowest dose (10 mg) were included. By analyzing blood PK profiles alone, the PBPK model with target binding yielded practically identifiable target binding parameters and predicted the maximum target occupancies of 0.6-0.8 at clinical bosentan doses. Our results indicate that target binding, albeit not a major contributor to the nonlinear bosentan PKs, may offer a prediction of target occupancy from blood PK profiles alone and potential guidance on achieving optimal efficacy outcomes, under the condition when the high-affinity drug target is responsible for the efficacy of interest and when the dose ranges cover varying degrees of target binding. SIGNIFICANCE STATEMENT: By incorporating saturable target binding, our physiologically based pharmacokinetic (PBPK) model predicted in vivo target occupancy of bosentan based only on the blood concentration-time profiles obtained from a wide range of doses. Our analysis highlights the potential utility of PBPK models that incorporate target binding in predicting target occupancy in vivo.

A Simple Decision Tree Suited for Identification of Early Oral Drug Candidates With Likely Pharmacokinetic Nonlinearity by Intestinal CYP3A Saturation.

To identify oral drugs that likely display nonlinear pharmacokinetics due to saturable metabolism by intestinal CYP3A, our previous report using CYP3A substrate drugs proposed an approach using thresholds for the linear index number (LIN3A = dose/Km; Km, Michaelis-Menten constant for CYP3A) and the intestinal availability (FaFg). Here, we aimed to extend the validity of the previous approach using both CYP3A substrate and non-substrate drugs and to devise a decision tree suited for early drug candidates using in vitro metabolic intrinsic clearance (CLint, vitro) instead of FaFg. Out of 152 oral drugs (including 136 drugs approved in Japan, US or both), type I nonlinearity (in which systemic drug exposure increases in a more than dose-proportional manner) was noted with 82 drugs (54%), among which 58 drugs were identified as CYP3A substrates based on public information. Based on practical feasibility, 41 drugs were selected from CYP3A substrates and subjected to in-house metabolic assessment. The results were used to determine the thresholds for CLint, vitro (0.45 µL/min/pmol CYP3A4) and LIN3A (1.0 L). For four drugs incorrectly predicted, potential mechanisms were looked up. Overall, our proposed decision tree may aid in the identification of early drug candidates with intestinal CYP3A-derived type I nonlinearity.

Transient, Tunable Expression of NTCP and BSEP in MDCKII Cells for Kinetic Delineation of the Rate-Determining Process and Inhibitory Effects of Rifampicin in Hepatobiliary Transport of Taurocholate.

In predicting the hepatic elimination of compounds, the extended clearance concept has proven useful. Yet, its experimental proof was scarce partly due to the lack of models with the controlled expression of transporters. Here, the uptake and efflux transporters [NTCP (SLC10A1) and BSEP (ABCB11), respectively] were doubly and transiently expressed in MDCKII cells by electroporation-based transfection (with the BSEP plasmid amount varied and with the NTCP plasmid fixed), achieving the activity levels of NTCP and BSEP comparable to those of sandwich cultured human hepatocytes. The biliary excretion clearance for taurocholate increased proportionally to the BSEP expression level. Under the same conditions, the basal-to-apical transcellular clearance of taurocholate displayed an initial increase, and a subsequent plateau, indicating that the basolateral uptake of taurocholate became rate-limiting. The doubly transfected MDCKII cells were also used to kinetically analyze the inhibitory effects of rifampicin on BSEP and NTCP. The obtained results showed a bell-shaped profile for cell-to-medium concentration ratios over a range of rifampicin concentrations, which were quantitatively captured by kinetic modeling based on the extended clearance concept. The present study highlights the utility of the transient, tunable transporter expression system in delineating the rate-determining process and providing mechanistic insights into intracellular substrate accumulation.

Improved Prediction of the Drug-Drug Interactions of Pemafibrate Caused by Cyclosporine A and Rifampicin via PBPK Modeling: Consideration of the Albumin-Mediated Hepatic Uptake of Pemafibrate and Inhibition Constants With Preincubation Against OATP1B.

Pemafibrate (PMF) is highly albumin-bound (>99.8%) and a substrate for hepatic uptake transporters (OATP1B) and CYP enzymes. Here, we developed a PBPK model of PMF to capture drug-drug interactions (DDI) incurred by cyclosporine (CsA) and rifampicin (RIF), the two OATP1B inhibitors. Initial PBPK modeling of PMF utilized in vitro hepatic uptake clearance (PSinf) obtained in the absence of albumin, but failed in capturing the blood PMF pharmacokinetic (PK) profiles. Based on the results that in vitro PSinf of unbound PMF was enhanced in the presence of albumin, we applied the albumin-facilitated dissociation model and the resulting PSinf parameters improved the prediction of the blood PMF PK profiles. In refining our PBPK model toward improved prediction of the observed DDI data (PMF co-administered with single dosing of CsA or RIF; PMF following multiple RIF dosing), we adjusted the previously obtained in vivo OATP1B inhibition constants (Ki,OATP1B) of CsA or RIF for pitavastatin by correcting for substrate-dependency. We also incorporated the induction of OATP1B and CYP enzymes after multiple RIF dosing. Sensitivity analysis informed that the higher gastrointestinal absorption rate constant could further improve capturing the observed DDI data, suggesting the possible inhibition of intestinal ABC transporter(s) by CsA or RIF.

Evaluation of Hepatic Uptake of OATP1B Substrates by Short Term-Cultured Plated Human Hepatocytes: Comparison With Isolated Suspended Hepatocytes.

Hepatic uptake clearance has been measured in suspended human hepatocytes (SHH). Plated human hepatocytes (PHH) after short-term culturing are increasingly employed to study hepatic transport driven mainly by its higher throughput. To know pros/cons of both systems, the hepatic uptake clearances of several organic anion transporting polypeptide 1B substrates were compared between PHH and SHH by determining the initial uptake velocities or through dynamic model-based analyses. For cerivastatin, pitavastatin and rosuvastatin, initial uptake clearances (PSinf) obtained using PHH were comparable to those using SHH, while cell-to-medium concentration (C/M) ratios were 2.7- to 5.4-fold higher. For pravastatin and dehydropravastatin, hydrophilic compounds with low uptake/cellular binding, their PSinf and C/M ratio in PHH were 1.8- to 3.2-fold lower than those in SHH. These hydrophilic substrates are more prone to wash-off during the uptake study using PHH, which may explain the apparently lower uptake than SHH. The C/M ratios obtained using PHH were stable over an extended time, making PHH suitable to estimate the C/M ratios and hepatocyte-to-medium unbound concentration ratios (Kp,uu). In conclusion, PHH is useful in evaluating hepatic uptake/efflux clearances and Kp,uu of OATP1B substrates in a high-throughput manner, however, a caution is warranted for hydrophilic drugs with low uptake/cellular binding.

Cell-to-Medium Concentration Ratio Overshoot in the Uptake of Statins by Human Hepatocytes in Suspension, but Not in Monolayer: Kinetic Analysis Suggesting a Partial Loss of Functional OATP1Bs.

Suspended human hepatocytes (SHH) have long been used in assessing hepatic drug uptake, while plated human hepatocytes in short-term monolayer culture (PHH) have gained use in recent years. This study aimed to cross-evaluate SHH and PHH in measuring the hepatic uptake mediated by organic anion transporting polypeptide 1Bs (OATP1Bs). We compared the time courses of cell-to-medium (C/M) concentration ratios and initial uptake clearance values of the OATP1B substrates (pitavastatin, rosuvastatin, cerivastatin, pravastatin, dehydropravastatin, and SC-62807) between SHH and PHH. For all compounds except cerivastatin, the C/M ratios in SHH displayed an apparent overshoot (an initial increase followed by a decrease) during the 180-min uptake experiment, but not in PHH. Based on the literature evidence suggesting the possible internalization of OATP1Bs in primary hepatocytes, separate experiments measured the drug uptake after varying lengths of pre-incubation in the drug-free medium. The initial uptake clearances of pitavastatin and rosuvastatin declined in SHH beyond an apparent threshold time of 20-min drug-free pre-incubation, but not in PHH. Kinetic modeling quantitatively captured the decline in the active uptake clearance in SHH, and more than half of the active uptake clearances of pitavastatin and rosuvastatin were prone to loss during the 180-min uptake experiment. These results suggested a partial, time-delayed loss of the functional OATP1Bs in SHH upon prolonged incubation. Our results indicate that PHH is more appropriate for experiments where a prolonged incubation is required, such as estimation of unbound hepatocyte-to-medium concentration ratio (Kp,uu) at the steady-state.

Post-translational regulation of the major drug transporters in the families of organic anion transporters and organic anion-transporting polypeptides.

The organic anion transporters (OATs) and organic anion-transporting polypeptides (OATPs) belong to the solute carrier (SLC) transporter superfamily and play important roles in handling various endogenous and exogenous compounds of anionic charge. The OATs and OATPs are often implicated in drug therapy by impacting the pharmacokinetics of clinically important drugs and, thereby, drug exposure in the target organs or cells. Various mechanisms (e.g. genetic, environmental, and disease-related factors, drug-drug interactions, and food-drug interactions) can lead to variations in the expression and activity of the anion drug-transporting proteins of OATs and OATPs, possibly impacting the therapeutic outcomes. Previous investigations mainly focused on the regulation at the transcriptional level and drug-drug interactions as competing substrates or inhibitors. Recently, evidence has accumulated that cellular trafficking, post-translational modification, and degradation mechanisms serve as another important layer for the mechanisms underlying the variations in the OATs and OATPs. This review will provide a brief overview of the major OATs and OATPs implicated in drug therapy and summarize recent progress in our understanding of the post-translational modifications, in particular ubiquitination and degradation pathways of the individual OATs and OATPs implicated in drug therapy.