Substrate-dependent inhibition of organic anion transporting polypeptide 1B1: comparative analysis with prototypical probe substrates estradiol-17β-glucuronide, estrone-3-sulfate, and sulfobromophthalein

Organic anion transporting polypeptides: Pharmacology, toxicology, structure, and transport mechanisms

Organic anion transporting polypeptides (OATPs) are membrane proteins that mediate the uptake of a wide range of substrates across the plasma membrane of various cells and tissues. They are classified into 6 subfamilies, OATP1 through OATP6. Humans contain 12 OATPs encoded by 11 solute carrier of organic anion transporting polypeptide (SLCO) genes: OATP1A2, OATP1B1, OATP1B3, the splice variant OATP1B3-1B7, OATP1C1, OATP2A1, OATP2B1, OATP3A1, OATP4A1, OATP4C1, OATP5A1, and OATP6A1. Most of these proteins are expressed in epithelial cells, where they mediate the uptake of structurally unrelated organic anions, cations, and even neutral compounds into the cytoplasm. The best-characterized members are OATP1B1 and OATP1B3, which have an important role in drug metabolism by mediating drug uptake into the liver and are involved in drug-drug interactions. In this review, we aimed to (1) provide a historical perspective on the identification of OATPs and their nomenclature and discuss their phylogenic relationships and molecular characteristics; (2) review the current knowledge of the broad substrate specificity and their role in drug disposition and drug-drug interactions, with a special emphasis on human hepatic OATPs; (3) summarize the different experimental systems that are used to study the function of OATPs and discuss their advantages and disadvantages; (4) review the available experimental 3-dimensional structures and examine how they can help elucidate the transport mechanisms of OATPs; and (5) finally, summarize the current knowledge of the regulation of OATP expression, discuss clinically important single-nucleotide polymorphisms, and outline challenges of physiologically based pharmacokinetic modeling and in vitro to in vivo extrapolation. SIGNIFICANCE STATEMENT: Organic anion transporting polypeptides (OATPs) are a family of 12 uptake transporters in the solute carrier superfamily. Several members, particularly the liver-expressed OATP1B1 and OATP1B3, are important drug transporters. They mediate the uptake of several endobiotics and xenobiotics, including statins and numerous other drugs, into hepatocytes, and their inhibition by other drugs or reduced expression due to single-nucleotide polymorphisms can lead to adverse drug effects. Their recently solved 3-dimensional structures should help to elucidate their transport mechanisms and broad substrate specificities.

Effect of Organic Anion Transporting Polypeptide 1B1 on Plasma Concentration Dynamics of Clozapine in Patients with Treatment-Resistant Schizophrenia

The involvement of drug-metabolizing enzymes and transporters in plasma clozapine (CLZ) dynamics has not been well examined in Japanese patients with treatment-resistant schizophrenia (TRS). Therefore, this clinical study investigated the relationship between single nucleotide polymorphisms (SNPs) of various pharmacokinetic factors (drug-metabolizing enzymes and transporters) and dynamic changes in CLZ. Additionally, we aimed to determine whether CLZ acts as a substrate for pharmacokinetic factors using in vitro assays and molecular docking calculations. We found that 6 out of 10 patients with TRS and with multiple organic anion transporting polypeptide (OATP) variants (OATP1B1: *1b, *15; OATP1B3: 334T>G, 699G>A; and OATP2B1: *3, 935G>A, 601G>A, 76_84del) seemed to be highly exposed to CLZ and/or N-desmethyl CLZ. A CLZ uptake study using OATP-expressing HEK293 cells showed that CLZ was a substrate of OATP1B1 with Km and Vmax values of 38.9 µM and 2752 pmol/mg protein/10 min, respectively. The results of molecular docking calculations supported the differences in CLZ uptake among OATP molecules and the weak inhibitory effect of cyclosporine A, which is a strong inhibitor of OATPs, on CLZ uptake via OATP1B1. This is the first study to show that CLZ is an OATP1B1 substrate and that the presence of SNPs in OATPs potentially alters CLZ pharmacokinetic parameters.

Interaction of myricetin, ampelopsin (dihydromyricetin), and their sulfate metabolites with serum albumin, cytochrome P450 (CYP2C9, 2C19, and 3A4) enzymes, and organic anion-transporting polypeptides (OATP1B1 and OATP2B1)

Myricetin (MYR) and ampelopsin (AMP, or dihydromyricetin) are flavonoid aglycones found in certain plants and dietary supplements. During the presystemic biotransformation of flavonoids, mainly sulfate and glucuronide derivatives are produced, which are the dominant metabolites in the circulation. In this study, we tested the interactions of MYR, myricetin-3'-O-sulfate (M3'S), AMP, and ampelopsin-4'-O-sulfate (A4'S) with human serum albumin (HSA), cytochrome P450 enzymes (CYPs), and organic anion-transporting polypeptides (OATPs) using in vitro models, including the recently developed method for measuring flavonoid levels in living cells. M3'S and MYR bound to albumin with high affinity, and they showed moderate displacing effects versus the Site I marker warfarin. MYR, M3'S, AMP, and A4'S exerted no or only minor inhibitory effects on CYP2C9, CYP2C19, and CYP3A4 enzymes. M3'S and MYR caused considerable inhibitory actions on OATP1B1 at low micromolar concentrations (IC50 = 1.7 and 6.4 μM, respectively), while even their nanomolar levels resulted in strong inhibitory effects on OATP2B1 (IC50 = 0.3 and 0.4 μM, respectively). In addition, M3'S proved to be a substrate of OATP1B1 and OATP2B1. These results suggest that MYR-containing dietary supplements may affect the OATP-mediated transport of certain drugs, and OATPs are involved in the tissue uptake of M3'S.

Transporter-mediated drug-drug interactions: regulatory guidelines, in vitro and in vivo methodologies and translation, special populations, and the blood-brain barrier

This review, part of a special issue on drug-drug interactions (DDIs) spearheaded by the International Society for the Study of Xenobiotics (ISSX) New Investigators, explores the critical role of drug transporters in absorption, disposition, and clearance in the context of DDIs. Over the past two decades, significant advances have been made in understanding the clinical relevance of these transporters. Current knowledge on key uptake and efflux transporters that affect drug disposition and development is summarized. Regulatory guidelines from the FDA, EMA, and PMDA that inform the evaluation of potential transporter-mediated DDIs are discussed in detail. Methodologies for preclinical and clinical testing to assess potential DDIs are reviewed, with an emphasis on the utility of physiologically based pharmacokinetic (PBPK) modeling. This includes the application of relative abundance and expression factors to predict human pharmacokinetics (PK) using preclinical data, integrating the latest regulatory guidelines. Considerations for assessing transporter-mediated DDIs in special populations, including pediatric, hepatic, and renal impairment groups, are provided. Additionally, the impact of transporters at the blood-brain barrier (BBB) on the disposition of CNS-related drugs is explored. Enhancing the understanding of drug transporters and their role in drug disposition and toxicity can improve efficacy and reduce adverse effects. Continued research is essential to bridge remaining gaps in knowledge, particularly in comparison with cytochrome P450 (CYP) enzymes.

An update on drug-drug interactions in older adults living with human immunodeficiency virus (HIV)

INTRODUCTION

People with HIV are living longer due to advances in antiretroviral therapy. With improved life expectancy comes an increased lifetime risk of comorbid conditions - such as cardiovascular disease and cancer - and polypharmacy. Older adults, particularly those living with HIV, are more vulnerable to drug interactions and adverse effects, resulting in negative health outcomes.

AREA COVERED

Antiretrovirals are involved in many potential drug interactions with medications used to treat common comorbidities and geriatric conditions in an aging population of people with HIV. We review the mechanisms and management of significant drug-drug interactions involving antiretroviral medications and non-antiretroviral medications commonly used among older people living with HIV. The management of these interactions may require dose adjustments, medication switches to alternatives, enhanced monitoring, and considerations of patient- and disease-specific factors.

EXPERT OPINION

Clinicians managing comorbid conditions among older people with HIV must be particularly vigilant to side effect profiles, drug-drug interactions, pill burden, and cost when optimizing treatment. To support healthier aging among people living with HIV, there is a growing need for antiretroviral stewardship, multidisciplinary care models, and advances that promote insight into the correlations between an individual, their conditions, and their medications.

Beyond the basics: A deep dive into parameter estimation for advanced PBPK and QSP models

Physiologically-based pharmacokinetic (PBPK) models and quantitative systems pharmacology (QSP) models have contributed to drug development strategies. The parameters of these models are commonly estimated by capturing observed values using the nonlinear least-squares method. Software packages for PBPK and QSP modeling provide a range of parameter estimation algorithms. To choose the most appropriate method, modelers need to understand the basic concept of each approach. This review provides a general introduction to the key points of parameter estimation with a focus on the PBPK and QSP models, and the respective parameter estimation algorithms. The latter part assesses the performance of five parameter estimation algorithms - the quasi-Newton method, Nelder-Mead method, genetic algorithm, particle swarm optimization, and Cluster Gauss-Newton method - using three examples of PBPK and QSP modeling. The assessment revealed that some parameter estimation results were significantly influenced by the initial values. Moreover, the choice of algorithms demonstrating good estimation results heavily depends on factors such as model structure and the parameters to be estimated. To obtain credible parameter estimation results, it is advisable to conduct multiple rounds of parameter estimation under different conditions, employing various estimation algorithms.

The 2-aminoethyl diphenylborinate-based fluorescent method identifies quercetin and luteolin metabolites as substrates of Organic anion transporting polypeptides, OATP1B1 and OATP2B1

Organic anion transporting polypeptides (OATPs), OATP1B1 and OATP2B1 are membrane proteins mediating the cellular uptake of chemically diverse organic compounds. OATP1B1 is exclusively expressed in hepatocytes and plays a key role in hepatic detoxification. The ubiquitously expressed OATP2B1 promotes the intestinal absorption of orally administered drugs. Flavonoids are widely found in foods and beverages, and many of them can inhibit OATP function, resulting in food-drug interactions. In our previous work, we have shown that not only luteolin (LUT) and quercetin (Q), but also some of their metabolites can inhibit OATP1B1 and OATP2B1 activity. However, data about the potential direct transport of these flavonoids by OATPs have been incomplete. Hence, in the current study, we developed a simple, fluorescence-based method for the measurement of intracellular flavonoid levels. The method applies a cell-permeable small molecule (2-aminoethyl diphenylborinate, 2-APB), that, upon forming a complex with flavonoids, results in their fluorescence enhancement. This way the direct uptake of LUT and Q, and also their metabolites' could be investigated both by confocal microscopy and in a fluorescence plate reader in living cells. With this approach we identified quercetin-3'-O-sulfate, luteolin-3'-O-glucuronide, luteolin-7-O-glucuronide and luteolin-3'-O-sulfate as substrates of both OATP1B1 and OATP2B1. Our results highlight that OATP1B1 and OATP2B1 can be key participants in the transmembrane movement of LUT and Q conjugates with otherwise low cell permeability. In addition, the novel method developed in this study can be a good completion to existing fluorescence-based assays to investigate OATP function.

Assessing Trans-Inhibition of OATP1B1 and OATP1B3 by Calcineurin and/or PPIase Inhibitors and Global Identification of OATP1B1/3-Associated Proteins

Organic anion transporting polypeptide (OATP) 1B1 and OATP1B3 are key determinants of drug-drug interactions (DDIs). Various drugs including the calcineurin inhibitor (CNI) cyclosporine A (CsA) exert preincubation-induced trans-inhibitory effects upon OATP1B1 and/or OATP1B3 (abbreviated as OATP1B1/3) by unknown mechanism(s). OATP1B1/3 are phosphoproteins; calcineurin, which dephosphorylates and regulates numerous phosphoproteins, has not previously been investigated in the context of preincubation-induced trans-inhibition of OATP1B1/3. Herein, we compare the trans-inhibitory effects exerted on OATP1B1 and OATP1B3 by CsA, the non-analogous CNI tacrolimus, and the non-CNI CsA analogue SCY-635 in transporter-overexpressing human embryonic kidney (HEK) 293 stable cell lines. Preincubation (10-60 min) with tacrolimus (1-10 µM) rapidly and significantly reduces OATP1B1- and OATP1B3-mediated transport up to 0.18 ± 0.03- and 0.20 ± 0.02-fold compared to the control, respectively. Both CsA and SCY-635 can trans-inhibit OATP1B1, with the inhibitory effects progressively increasing over a 60 min preincubation time. At each equivalent preincubation time, CsA has greater trans-inhibitory effects toward OATP1B1 than SCY-635. Preincubation with SCY-635 for 60 min yielded IC50 of 2.2 ± 1.4 µM against OATP1B1, which is ~18 fold greater than that of CsA (0.12 ± 0.04 µM). Furthermore, a proteomics-based screening for protein interactors was used to examine possible proteins and processes contributing to OATP1B1/3 regulation and preincubation-induced inhibition by CNIs and other drugs. A total of 861 and 357 proteins were identified as specifically associated with OATP1B1 and OATP1B3, respectively, including various protein kinases, ubiquitin-related enzymes, the tacrolimus (FK506)-binding proteins FKBP5 and FKBP8, and several known regulatory targets of calcineurin. The current study reports several novel findings that expand our understanding of impaired OATP1B1/3 function; these include preincubation-induced trans-inhibition of OATP1B1/3 by the CNI tacrolimus, greater preincubation-induced inhibition by CsA compared to its non-CNI analogue SCY-635, and association of OATP1B1/3 with various proteins relevant to established and candidate OATP1B1/3 regulatory processes.

Utilization of OATP1B Biomarker Coproporphyrin-I to Guide Drug-Drug Interaction Risk Assessment: Evaluation by the Pharmaceutical Industry

Drug-drug interactions (DDIs) involving hepatic organic anion transporting polypeptides 1B1/1B3 (OATP1B) can be substantial, however, challenges remain for predicting interaction risk. Emerging evidence suggests that endogenous biomarkers, particularly coproporphyrin-I (CP-I), can be used to assess in vivo OATP1B activity. The present work under the International Consortium for Innovation and Quality in Pharmaceutical Development was aimed primarily at assessing CP-I as a biomarker for informing OATP1B DDI risk. Literature and unpublished CP-I data along with pertinent in vitro and clinical DDI information were collected to identify DDIs primarily involving OATP1B inhibition and assess the relationship between OATP1B substrate drug and CP-I exposure changes. Static models to predict changes in exposure of CP-I, as a selective OATP1B substrate, were also evaluated. Significant correlations were observed between CP-I area under the curve ratio (AUCR) or maximum concentration ratio (Cmax R) and AUCR of substrate drugs. In general, the CP-I Cmax R was equal to or greater than the CP-I AUCR. CP-I Cmax R < 1.25 was associated with absence of OATP1B-mediated DDIs (AUCR < 1.25) with no false negative predictions. CP-I Cmax R < 2 was associated with weak OATP1B-mediated DDIs (AUCR < 2). A correlation was identified between CP-I exposure changes and OATP1B1 static DDI predictions. Recommendations for collecting and interpreting CP-I data are discussed, including a decision tree for guiding DDI risk assessment. In conclusion, measurement of CP-I is recommended to inform OATP1B inhibition potential. The current analysis identified changes in CP-I exposure that may be used to prioritize, delay, or replace clinical DDI studies.

Mechanistic Static Model based Prediction of Transporter Substrate Drug-Drug Interactions Utilizing Atorvastatin and Rifampicin

OBJECTIVE

An in vitro relative activity factor (RAF) technique combined with mechanistic static modeling was examined to predict drug-drug interaction (DDI) magnitude and analyze contributions of different clearance pathways in complex DDIs involving transporter substrates. Atorvastatin and rifampicin were used as a model substrate and inhibitor pair.

METHODS

In vitro studies were conducted with transfected HEK293 cells, hepatocytes and human liver microsomes. Prediction success was defined as predictions being within twofold of observations.

RESULTS

The RAF method successfully translated atorvastatin uptake from transfected cells to hepatocytes, demonstrating its ability to quantify transporter contributions to uptake. Successful translation of atorvastatin's in vivo intrinsic hepatic clearance (CLint,h,in vivo) from hepatocytes to liver was only achieved through consideration of albumin facilitated uptake or through application of empirical scaling factors to transporter-mediated clearances. Transporter protein expression differences between hepatocytes and liver did not affect CLint,h,in vivo predictions. By integrating cis and trans inhibition of OATP1B1/OATP1B3, atorvastatin-rifampicin (single dose) DDI magnitude could be accurately predicted (predictions within 0.77-1.0 fold of observations). Simulations indicated that concurrent inhibition of both OATP1B1 and OATP1B3 caused approximately 80% of atorvastatin exposure increases (AUCR) in the presence of rifampicin. Inhibiting biliary elimination, hepatic metabolism, OATP2B1, NTCP, and basolateral efflux are predicted to have minimal to no effect on AUCR.

CONCLUSIONS

This study demonstrates the effective application of a RAF-based translation method combined with mechanistic static modeling for transporter substrate DDI predictions and subsequent mechanistic interpretation.

Preincubation Time-Dependent, Long-Lasting Inhibition of Drug Transporters and Impact on the Prediction of Drug-Drug Interactions

Transporter-mediated drug-drug interaction (DDI) is of clinical concern, and the quantitative prediction of DDIs is an indispensable part of drug development. Cell-based inhibition assays, in which a representative probe substrate and a potential inhibitor are coincubated, are routinely performed to assess the inhibitory potential of new molecular entities on drug transporters. However, the inhibitory effect of cyclosporine A (CsA) on organic anion transporting polypeptide (OATP) 1B1 is substantially potentiated with CsA preincubation, and this effect is both long-lasting and dependent on the preincubation time. This phenomenon has also been reported with transporters other than OATP1Bs, but it is considered more prevalent among OATP1Bs and organic cation transporters. Regulatory agencies have also noted this preincubation effect and have recommended that pharmaceutical companies consider inhibitor preincubation when performing in vitro OATP1B1 and OATP1B3 inhibition studies. Although the underlying mechanisms responsible for the preincubation effect are not fully understood, a trans-inhibition mechanism was recently demonstrated for OATP1B1 inhibition by CsA, in which CsA inhibited OATP1B1 not only extracellularly (cis-inhibition) but also intracellularly (trans-inhibition). Furthermore, the trans-inhibition potency of CsA was much greater than that of cis-inhibition, suggesting that trans-inhibition might be a key driver of clinical DDIs of CsA with OATP1B substrate drugs. Although confidence in transporter-mediated DDI prediction is generally considered to be low, the predictability might be further improved by incorporating the trans-inhibition mechanism into static and dynamic models for preincubation-dependent inhibitors of OATP1Bs and perhaps other transporters. SIGNIFICANCE STATEMENT: Preincubation time-dependent, long-lasting inhibition has been observed for OATP1B1 and other solute carrier transporters in vitro. Recently, a trans-inhibition mechanism for the preincubation effect of CsA on OATP1B1 inhibition was identified, with the trans-inhibition potency being greater than that of cis-inhibition. The concept of trans-inhibition may allow us to further understand the mechanism of transporter-mediated DDIs not only for OATP1B1 but also for other transporters and to improve the accuracy and confidence of DDI predictions.

Targeting a xenobiotic transporter to ameliorate vincristine-induced sensory neuropathy

Vincristine is a widely used chemotherapeutic drug for the treatment of multiple malignant diseases that causes a dose-limiting peripheral neurotoxicity. There is no clinically effective preventative treatment for vincristine-induced sensory peripheral neurotoxicity (VIPN), and mechanistic details of this side effect remain poorly understood. We hypothesized that VIPN is dependent on transporter-mediated vincristine accumulation in dorsal root ganglion neurons. Using a xenobiotic transporter screen, we identified OATP1B3 as a neuronal transporter regulating the uptake of vincristine. In addition, genetic or pharmacological inhibition of the murine orthologue transporter OATP1B2 protected mice from various hallmarks of VIPN - including mechanical allodynia, thermal hyperalgesia, and changes in digital maximal action potential amplitudes and neuronal morphology - without negatively affecting plasma levels or antitumor effects of vincristine. Finally, we identified α-tocopherol from an untargeted metabolomics analysis as a circulating endogenous biomarker of neuronal OATP1B2 function, and it could serve as a companion diagnostic to guide dose selection of OATP1B-type transport modulators given in combination with vincristine to prevent VIPN. Collectively, our findings shed light on the fundamental basis of VIPN and provide a rationale for the clinical development of transporter inhibitors to prevent this debilitating side effect.

Plasma Protein-Mediated Uptake and Contradictions to the Free Drug Hypothesis: A Critical Review

According to the free drug hypothesis (FDH), only free, unbound drug is available to interact with biological targets. This hypothesis is the fundamental principle that continues to explain the vast majority of all pharmacokinetic and pharmacodynamic processes. Under the FDH, the free drug concentration at the target site is considered the driver of pharmacodynamic activity and pharmacokinetic processes. However, deviations from the FDH are observed in hepatic uptake and clearance predictions, where observed unbound intrinsic hepatic clearance (CL_int,u) is larger than expected. Such deviations are commonly observed when plasma proteins are present and form the basis of the so-called plasma protein-mediated uptake effect (PMUE). This review will discuss the basis of plasma protein binding as it pertains to hepatic clearance based on the FDH, as well as several hypotheses that may explain the underlying mechanisms of PMUE. Notably, some, but not all, potential mechanisms remained aligned with the FDH. Finally, we will outline possible experimental strategies to elucidate PMUE mechanisms. Understanding the mechanisms of PMUE and its potential contribution to clearance underprediction is vital to improving the drug development process.

Differential Preincubation Effects of Nicardipine on OATP1B1- and OATP1B3-Mediated Transport in the Presence and Absence of Protein: Implications in Assessing OATP1B1- and OATP1B3-Mediated Drug-Drug Interactions

Impaired transport activity of hepatic OATP1B1 and OATP1B3 due to drug-drug interactions (DDIs) often leads to increased systemic exposure to substrate drugs (e.g., lipid-lowering statins). Since dyslipidemia and hypertension frequently coexist, statins are often concurrently used with antihypertensives, including calcium channel blockers (CCBs). OATP1B1/1B3-related DDIs in humans have been reported for several CCBs. To date, the OATP1B1/1B3-mediated DDI potential of CCB nicardipine has not been assessed. The current study was designed to assess the OATP1B1- and OATP1B3-mediated DDI potential of nicardipine using the R-value model, following the US-FDA guidance. IC50 values of nicardipine against OATP1B1 and OATP1B3 were determined in transporter-overexpressing human embryonic kidney 293 cells using [3H]-estradiol 17β-D-glucuronide and [3H]-cholecystokinin-8 as substrates, respectively, with or without nicardipine-preincubation in protein-free Hanks' Balanced Salt Solution (HBSS) or in fetal bovine serum (FBS)-containing culture medium. Preincubation with nicardipine for 30 min in protein-free HBSS buffer produced lower IC50 and higher R-values for both OATP1B1 and OATP1B3 compared to in FBS-containing medium, yielding IC50 values of 0.98 and 1.63 µM and R-values of 1.4 and 1.3 for OATP1B1 and OATP1B3, respectively. The R-values were higher than the US-FDA cut-off value of 1.1, supporting that nicardipine has the potential to cause OATP1B1/3-mediated DDIs. Current studies provide insight into the consideration of optimal preincubation conditions when assessing the OATP1B1/3-mediated DDIs in vitro.

Studying the right transporter at the right time: an in vitro strategy for assessing drug-drug interaction risk during drug discovery and development

INTRODUCTION

Transporters are significant in dictating drug pharmacokinetics, thus inhibition of transporter function can alter drug concentrations resulting in drug-drug interactions (DDIs). Because they can impact drug toxicity, transporter DDIs are a regulatory concern for which prediction of clinical effect from in vitro data is critical to understanding risk.

AREA COVERED

The authors propose in vitro strategies to assist mitigating/removing transporter DDI risk during development by frontloading specific studies, or managing patient risk in the clinic. An overview of clinically relevant drug transporters and observed DDIs are provided, alongside presentation of key considerations/recommendations for in vitro study design evaluating drugs as inhibitors or substrates. Guidance on identifying critical co-medications, clinically relevant disposition pathways and using mechanistic static equations for quantitative prediction of DDI is compiled.

EXPERT OPINION

The strategies provided will facilitate project teams to study the right transporter at the right time to minimise development risks associated with DDIs. To truly alleviate or manage clinical risk, the industry will benefit from moving away from current qualitative basic static equation approaches to transporter DDI hazard assessment towards adopting the use of mechanistic models to enable quantitative DDI prediction, thereby contextualising risk to ascertain whether a transporter DDI is simply pharmacokinetic or clinically significant requiring intervention.

In vitro evaluation suggests fenfluramine and norfenfluramine are unlikely to act as perpetrators of drug interactions

Studies support the safety and efficacy of fenfluramine (FFA) as an antiseizure medication (ASM) in Dravet syndrome, Lennox-Gastaut syndrome, or CDKL5 deficiency disorder, all pharmacoresistant developmental and epileptic encephalopathies. However, drug-drug interactions with FFA in multi-ASM regimens have not been fully investigated. We characterized the perpetrator potential of FFA and its active metabolite, norfenfluramine (nFFA), in vitro by assessing cytochrome P450 (CYP450) inhibition in human liver microsomes, CYP450 induction in cultured human hepatocytes, and drug transporter inhibition potential in permeability or cellular uptake assays. Mean plasma unbound fraction was ~50% for both FFA and nFFA, with no apparent concentration dependence. FFA and nFFA were direct in vitro inhibitors of CYP2D6 (IC₅₀ , 4.7 and 16 µM, respectively) but did not substantially inhibit CYP1A2, CYP2B6, CYP2C8, CYP2C9, CYP2C19, or CYP3A4/5. No time- or metabolism-dependent CYP450 inhibition occurred. FFA and nFFA did not induce CYP1A2; both induced CYP2B6 (up to 2.8-fold and up to 2.0-fold, respectively) and CYP3A4 (1.9- to 3.0-fold and 3.6- to 4.8-fold, respectively). Mechanistic static pharmacokinetic models predicted that neither CYP450 inhibition nor induction was likely to be clinically relevant at doses typically used for seizure reduction (ratio of area under curve [AUCR] for inhibition <1.25; AUCR for induction >0.8). Transporters OCT2 and MATE1 were inhibited by FFA (IC₅₀ , 19.8 and 9.0 μM) and nFFA (IC₅₀ , 5.2 and 4.6 μM) at concentrations higher than clinically achievable; remaining transporters were not inhibited. Results suggest that FFA and nFFA are unlikely drug-drug interaction perpetrators at clinically relevant doses of FFA (0.2-0.7 mg/kg/day).

In vitro evaluation of fenfluramine and norfenfluramine as victims of drug interactions

Fenfluramine (FFA) has potent antiseizure activity in severe, pharmacoresistant childhood‐onset developmental and epileptic encephalopathies (e.g., Dravet syndrome). To assess risk of drug interaction affecting pharmacokinetics of FFA and its major metabolite, norfenfluramine (nFFA), we conducted in vitro metabolite characterization, reaction phenotyping, and drug transporter−mediated cellular uptake studies. FFA showed low in vitro clearance in human liver S9 fractions and in intestinal S9 fractions in all three species tested (t_1/2 > 120 min). Two metabolites (nFFA and an N‐oxide or a hydroxylamine) were detected in human liver microsomes versus six in dog and seven in rat liver microsomes; no metabolite was unique to humans. Selective CYP inhibitor studies showed FFA metabolism partially inhibited by quinidine (CYP2D6, 48%), phencyclidine (CYP2B6, 42%), and furafylline (CYP1A2, 32%) and, to a lesser extent (<15%), by tienilic acid (CYP2C9), esomeprazole (CYP2C19), and troleandomycin (CYP3A4/5). Incubation of nFFA with rCYP1A2, rCYP2B6, rCYP2C19, and rCYP2D6 resulted in 10%−20% metabolism and no clear inhibition of nFFA metabolism by any CYP‐selective inhibitor. Reaction phenotyping showed metabolism of FFA by recombinant human cytochrome P450 (rCYP) enzymes rCYP2B6 (10%–21% disappearance for 1 and 10 µM FFA, respectively), rCYP1A2 (22%−23%), rCYP2C19 (49%−50%), and rCYP2D6 (59%−97%). Neither FFA nor nFFA was a drug transporter substrate. Results show FFA metabolism to nFFA occurs through multiple pathways of elimination. FFA dose adjustments may be needed when administered with strong inhibitors or inducers of multiple enzymes involved in FFA metabolism (e.g., stiripentol).

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-17β-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.

Identification of the organic anion transporting polypeptides responsible for the hepatic uptake of the major metabolite of epyrifenacil, S-3100-CA, in mice

Epyrifenacil is a novel herbicide that acts as an inhibitor of protoporphyrinogen oxidase (PPO) and produces hepatotoxicity in rodents by inhibiting PPO. Our previous research revealed that the causal substance of hepatotoxicity is S-3100-CA, a major metabolite of epyrifenacil, and that human hepatocyte uptake of S-3100-CA was significantly lower than rodent one, suggesting less relevant to hepatotoxicity in humans. To clarify the species difference in the uptake of S-3100-CA, we focused on organic anion transporting polypeptides (OATPs) and carried out an uptake assay using human, rat, and mouse OATP hepatic isoforms-expressing 293FT cells. As a result, all the examined OATPs were found to contribute to the S-3100-CA uptake, suggesting that the species difference was not due to the differences in selectivity toward OATP isoforms. When [14 C]epyrifenacil was administered to mice, the liver concentration of S-3100-CA was higher in males than in females. Furthermore, when [14 C]epyrifenacil was administered with OATP inhibitors, the liver/plasma ratio of S-3100-CA was significantly decreased by rifampicin, an Oatp1a1/Oatp1a4 inhibitor in mice, but not by digoxin, an Oatp1a4-specific inhibitor. This result indicates that Oatp1a1, the predominant transporter in male mice, is the main contributor to the hepatic transport of S-3100-CA, and consequently to the gender difference. Moreover, we conclude that the species difference in the hepatic uptake of S-3100-CA observed in our previous research is not due to differences in the selectivity toward OATP isoforms but rather to the significantly higher expression of OATPs which mediate uptake of S-3100-CA in rodents than in humans.

8-acetoxy-trisulfopyrene as the first activatable fluorogenic probe for add-and-read assessment of Organic anion-transporting polypeptides, OATP1B1, OATP1B3, and OATP2B1

Organic anion-transporting polypeptides, OATP1B1, OATP1B3, and OATP2B1 are multispecific membrane proteins mediating the hepatocellular uptake of structurally diverse endo- and exogenous compounds, including various kinds of drugs. Co-administration of OATP1B/2B1 substrates may lead to altered pharmacokinetics or even toxicity. Therefore, the study of the interaction with these OATPs is essential in drug development and is recommended by international regulatory agencies, the FDA, EMA, and PMDA. In general, radiolabeled indicators are used to measure drug interactions of OATPs, and, lately, fluorescent probes are also gaining wider application in OATP tests. However, all of the currently available methods (either radioactive or fluorescence-based) comprise multiple steps, including the removal of the indicator in the end of the experiment. Hence, they are not ideally suited for high-throughput screening. In the current study, in order to find an indicator allowing real-time assessment of hepatic OATP function, we searched for an activatable fluorogenic OATP substrate. Here, we show that 8-acetoxypyrene-1,3,6-trisulfonate (Ace), a fluorogenic derivative of the hepatic OATP substrate pyranine (8-hydroxypyrene-1,3,6-trisulfonate) enters the cells via OATP1B1/3 or OATP2B1 function. In living cells, Ace is then converted into highly fluorescent pyranine, allowing "no-wash" measurement of OATP function and drug interactions. Furthermore, we demonstrate that Ace can be used in an indirect assay termed as competitive counterflow suitable to distinguish between transported substrates and inhibitors of OATP1B1. The fluorescence-based methods described here are unique and open the way toward high-throughput screening of interactions between new molecular entities and OATPs.

Sex-specific expression mechanism of hepatic estrogen inactivating enzyme and transporters in diabetic women

Circulating estrogens levels significantly decrease in menopause and levels off in postmenopausal women. Accordingly, the liver represses levels of enzymes and membrane transporters, thereby decreasing capability of inactivating and excreting estrogens. Women increasingly develop type 2 diabetes during or after menopause. Estrogens are known to promote liver diseases in these women. Here, we have found that the estrogen inactivating sulfotransferase (SULT1E1) and an ATP-binding cassette subfamily G member 2 (ABCG2), a gene encoding breast cancer resistance protein that exports sulfated estrogens, increased their expression levels in diabetic women but not men. For the sulfotransferase gene, phosphorylated nuclear receptors ERα and RORα, at Ser212 and Ser100, respectively, bind their response elements to activate the SULT1E1 promoter in women. This coordinated increase in estrogen inactivation and excretion, and the phosphorylated nuclear receptor-mediated gene activation could be a defense mechanism against toxicities of estrogens through inactivation and excretion in the livers of women.

In Vitro Liver Toxicity Testing of Chemicals: A Pragmatic Approach

The liver is among the most frequently targeted organs by noxious chemicals of diverse nature. Liver toxicity testing using laboratory animals not only raises serious ethical questions, but is also rather poorly predictive of human safety towards chemicals. Increasing attention is, therefore, being paid to the development of non-animal and human-based testing schemes, which rely to a great extent on in vitro methodology. The present paper proposes a rationalized tiered in vitro testing strategy to detect liver toxicity triggered by chemicals, in which the first tier is focused on assessing general cytotoxicity, while the second tier is aimed at identifying liver-specific toxicity as such. A state-of-the-art overview is provided of the most commonly used in vitro assays that can be used in both tiers. Advantages and disadvantages of each assay as well as overall practical considerations are discussed.

Drugs in COVID-19 Clinical Trials: Predicting Transporter-Mediated Drug-Drug Interactions Using In Vitro Assays and Real-World Data

Numerous drugs are currently under accelerated clinical investigation for the treatment of coronavirus disease 2019 (COVID‐19); however, well‐established safety and efficacy data for these drugs are limited. The goal of this study was to predict the potential of 25 small molecule drugs in clinical trials for COVID‐19 to cause clinically relevant drug‐drug interactions (DDIs), which could lead to potential adverse drug reactions (ADRs) with the use of concomitant medications. We focused on 11 transporters, which are targets for DDIs. In vitro potency studies in membrane vesicles or HEK293 cells expressing the transporters coupled with DDI risk assessment methods revealed that 20 of the 25 drugs met the criteria from regulatory authorities to trigger consideration of a DDI clinical trial. Analyses of real‐world data from electronic health records, including a database representing nearly 120,000 patients with COVID‐19, were consistent with several of the drugs causing transporter‐mediated DDIs (e.g., sildenafil, chloroquine, and hydroxychloroquine). This study suggests that patients with COVID‐19, who are often older and on various concomitant medications, should be carefully monitored for ADRs. Future clinical studies are needed to determine whether the drugs that are predicted to inhibit transporters at clinically relevant concentrations, actually result in DDIs.

Effects of Probenecid on Hepatic and Renal Disposition of Hexadecanedioate, an Endogenous Substrate of Organic Anion Transporting Polypeptide 1B in Rats

The aim of the present study was to investigate changes in plasma concentrations and tissue distribution of endogenous substrates of organic anion transporting polypeptide (OATP) 1B, hexadecanedioate (HDA), octadecanedioate (ODA), tetradecanedioate (TDA), and coproporphyrin-III, induced by its weak inhibitor, probenecid (PBD), in rats. PBD increased the plasma concentrations of these four compounds regardless of bile duct cannulation, whereas liver-to-plasma (K_p,liver) and kidney-to-plasma concentration ratios of HDA and TDA were reduced. Similar effects of PBD on plasma concentrations and K_p,liver of HDA, ODA, and TDA were observed in kidney-ligated rats, suggesting a minor contribution of renal disposition to the overall distribution of these three compounds. Tissue uptake clearance of deuterium-labeled HDA (d-HDA) in liver was 16-fold higher than that in kidney, and was reduced by 80% by PBD. This was compatible with inhibition by PBD of d-HDA uptake in isolated rat hepatocytes. Such inhibitory effects of PBD were also observed in the human OATP1B1-mediated uptake of d-HDA. Overall, the disposition of HDA is mainly determined by hepatic OATP-mediated uptake, which is inhibited by PBD. HDA might, thus, be a biomarker for OATPs minimally affected by urinary and biliary elimination in rats.

Differential in vitro interactions of the Janus kinase inhibitor ruxolitinib with human SLC drug transporters

Interactions of the Janus kinase (JAK) inhibitor ruxolitinib with solute carriers (SLCs) remain incompletely characterised. The present study was therefore designed to investigate this issue.The interactions of ruxolitinib with SLCs were analysed using transporter-overexpressing human embryonic kidney HEK293 cells. Substrate accumulation was detected by spectrofluorimetry, liquid chromatography coupled to tandem mass spectrometry or scintillation counting.Ruxolitinib was found to potently inhibit the activities of organic anion transporter 3 (OAT3), organic cation transporter 2 (OCT2), multidrug and toxin extrusion 1 (MATE1) and MATE2-K (half maximal inhibitory concentration (IC₅₀) < 10 µM). It blocked OAT1, OAT4, OATP1B1, OATP1B3, OATP2B1 and OCT3, but in a weaker manner (IC₅₀ > 10 µM), whereas OCT1 was not impacted. No time-dependent inhibition was highlighted. When applying the US Food and Drug Administration (FDA) criteria for transporters-related drug-drug interaction risk, OCT2 and MATE2-K, unlike MATE1 and OAT3, were predicted to be in vivo inhibited by ruxolitinib. Cellular uptake studies additionally indicated that ruxolitinib is a substrate for MATE1 and MATE2-K, but not for OAT3 and OCT2.Ruxolitinib in vitro blocked activities of most of SLC transporters. Only OCT2 and MATE-2K may be however clinically inhibited by the JAK inhibitor, with the caution for OCT2 that in vitro inhibition data were generated with an FDA-non recommended fluorescent substrate. Ruxolitinib MATEs-mediated transport may additionally deserve attention for its possible pharmacological consequences in MATE-positive cells.

Assessing OATP1B1- and OATP1B3-Mediated Drug-Drug Interaction Potential of Vemurafenib Using R-Value and Physiologically-Based Pharmacokinetic Models

Organic anion transporting polypeptides (OATP) 1B1 and OATP1B3 are important determinants of transporter-mediated drug-drug interactions (DDIs). Current studies assessed the OATP1B1 and OATP1B3-mediated DDI potential of vemurafenib, a kinase inhibitor drug with high protein binding and low aqueous solubility, using R-value and physiologically-based pharmacokinetic (PBPK) models. The total half-maximal inhibitory concentration (IC50,total) values of vemurafenib against OATP1B1 and OATP1B3 were determined in 100% human plasma in transporter-overexpressing human embryonic kidney 293 stable cell lines. The unbound fraction of vemurafenib in human plasma before (fu,plasma) and after addition into the uptake assay plate (fu,plasma,inc) were determined by rapid equilibrium dialysis. There was no statistically significant difference between fu,plasma and fu,plasma,inc. Vemurafenib IC50,total values against OATP1B1 and OATP1B3 are 175 ± 82 and 231 ± 26 μM, respectively. The R-values [R = 1 + fu,plasma × Iin,max/(fu,plasma,inc × IC50,total)] were then simplified as R = 1+Iin,max/IC50,total, and were 1.76 and 1.57 for OATP1B1 and OATP1B3, respectively. The simulated pravastatin AUC ratio was 1.28 when a single dose of pravastatin (40 mg) was co-administered with vemurafenib (960 mg, twice daily) at steady-state, compared to pravastatin alone. Both R-value and PBPK models predict that vemurafenib has the potential to cause OATP1B1- and OATP1B3-mediated DDIs.

6-Hydroxyindole is an endogenous long-lasting OATP1B1 inhibitor elevated in renal failure patients

The hepatic uptake transporter organic anion transporting polypeptide (OATP) 1B1 is inhibited by some uremic toxins; however, direct inhibition can only partially explain the delayed systemic elimination of substrate drugs in renal failure patients. This study aimed to examine the long-lasting inhibition of OATP1B1 by uremic toxins and their metabolites. Preincubation of HEK293/OATP1B1 cells with 21 uremic toxins resulted in almost no change in the uptake of a typical substrate [³H]estrone-3-sulfate (E₁S), although some directly inhibited [³H]E₁S uptake. In contrast, preincubation with an indole metabolite, 6-hydroxyindole, reduced [³H]E₁S uptake, even after the inhibitor was washed out before [³H]E₁S incubation. Such long-lasting inhibition by 6-hydroxyindole was time-dependent and recovered after a 3-h incubation without 6-hydroxyindole. Preincubation with 6-hydroxyindole increased the Km for [³H]E₁S uptake with minimal change in Vmax. This was compatible with no change in the cell-surface expression of OATP1B1, as assessed by a biotinylation assay. Preincubation with 6-hydroxyindole reduced [³H]E₁S uptake in human hepatocytes without changes in OATP1B1 mRNA. Plasma concentration of 6-hydroxyindole in renal failure patients increased as renal function decreased, but might be insufficient to exhibit potent OATP1B1 inhibition. In conclusion, 6-hydroxyindole is an endogenous long-lasting OATP1B1 inhibitor with elevated plasma concentrations in renal failure patients.

Effects of Chrysin and Its Major Conjugated Metabolites Chrysin-7-Sulfate and Chrysin-7-Glucuronide on Cytochrome P450 Enzymes and on OATP, P-gp, BCRP, and MRP2 Transporters

Chrysin is an abundant flavonoid in nature, and it is also contained by several dietary supplements. Chrysin is highly biotransformed in the body, during which conjugated metabolites chrysin-7-sulfate and chrysin-7-glucuronide are formed. These conjugates appear at considerably higher concentrations in the circulation than the parent compound. Based on previous studies, chrysin can interact with biotransformation enzymes and transporters; however, the interactions of its metabolites have been barely examined. In this in vitro study, the effects of chrysin, chrysin-7-sulfate, and chrysin-7-glucuronide on cytochrome P450 enzymes (2C9, 2C19, 3A4, and 2D6) as well as on organic anion-transporting polypeptides (OATPs; 1A2, 1B1, 1B3, and 2B1) and ATP binding cassette [P-glycoprotein, multidrug resistance-associated protein 2, and breast cancer resistance protein (BCRP)] transporters were investigated. Our observations revealed that chrysin conjugates are strong inhibitors of certain biotransformation enzymes (e.g., CYP2C9) and transporters (e.g., OATP1B1, OATP1B3, OATP2B1, and BCRP) examined. Therefore, the simultaneous administration of chrysin-containing dietary supplements with medications needs to be carefully considered due to the possible development of pharmacokinetic interactions. SIGNIFICANCE STATEMENT: Chrysin-7-sulfate and chrysin-7-glucuronide are the major metabolites of flavonoid chrysin. In this study, we examined the effects of chrysin and its conjugates on cytochrome P450 enzymes and on organic anion-transporting polypeptides and ATP binding cassette transporters (P-glycoprotein, breast cancer resistance protein, and multidrug resistance-associated protein 2). Our results demonstrate that chrysin and/or its conjugates can significantly inhibit some of these proteins. Since chrysin is also contained by dietary supplements, high intake of chrysin may interrupt the transport and/or the biotransformation of drugs.

Detection of Weak Organic Anion-Transporting Polypeptide 1B Inhibition by Probenecid with Plasma-Based Coproporphyrin in Humans

Probenecid (PROB) is a clinical probe inhibitor of renal organic anion transporter (OAT) 1 and OAT3 that inhibits in vitro activity of hepatic drug transporters OATP1B1 and OATP1B3. It was hypothesized that PROB could potentially affect the disposition of OATP1B drug substrates. The plasma levels of the OATP1B endogenous biomarker candidates, including coproporphyrin I (CPI), CPIII, hexadecanedioate (HDA), and tetradecanedioate (TDA), were examined in 14 healthy subjects treated with PROB. After oral administration with 1000 mg PROB alone and in combination with furosemide (FSM), AUC (0-24 h) values were 1.39 ± 0.21-fold and 1.57 ± 0.41-fold higher than predose levels for CPI and 1.34 ± 0.16-fold and 1.45 ± 0.57-fold higher for CPIII. Despite increased systemic exposures, no decreases in CPI and CPIII renal clearance were observed (0.97 ± 0.38-fold and 1.16 ± 0.51-fold for CPI, and 1.34 ± 0.53-fold and 1.50 ± 0.69-fold for CPIII, respectively). These results suggest that the increase of CP systemic exposure is caused by OATP1B inhibition. Consistent with this hypothesis, PROB inhibited OATP1B1- and OATP1B3-mediated transport of CPI in a concentration-dependent manner, with IC50 values of 167 ± 42.0 and 76.0 ± 17.2 µM, respectively, in transporter-overexpressing human embryonic kidney cell assay. The inhibition potential was further confirmed by CPI and CPIII hepatocyte uptake experiments. In contrast, administration of PROB alone did not change AUC (0-24 h) of HDA and TDA relative to prestudy levels, although the administration of PROB in combination with FSM increased HDA and TDA levels compared with FSM alone (1.02 ± 0.18-fold and 0.90 ± 0.20-fold vs. 1.71 ± 0.43-fold and 1.62 ± 0.40-fold). Taken together, these findings indicate that PROB displays weak OATP1B inhibitory effects in vivo and that coproporphyrin is a sensitive endogenous probe of OATP1B inhibition. This study provides an explanation for the heretofore unknown mechanism responsible for PROB's interaction with other xenobiotics. SIGNIFICANCE STATEMENT: This study suggested that PROB is a weak clinical inhibitor of OATP1B based on the totality of evidence from the clinical interaction between PROB and CP and the in vitro inhibitory effect of PROB on OATP1B-mediated CP uptake. It demonstrates a new methodology of utilizing endogenous biomarkers to evaluate complex drug-drug interaction, providing explanation for the heretofore unknown mechanism responsible for PROB's inhibition. It provides evidence to strengthen the claim that CP is a sensitive circulating endogenous biomarker of OATP1B inhibition.

Prediction of Cyclosporin-Mediated Drug Interaction Using Physiologically Based Pharmacokinetic Model Characterizing Interplay of Drug Transporters and Enzymes

Uptake transporter organic anion transporting polypeptides (OATPs), efflux transporters (P-gp, BCRP and MRP2) and cytochrome P450 enzymes (CYP450s) are widely expressed in the liver, intestine or kidney. They coordinately work to control drug disposition, termed as "interplay of transporters and enzymes". Cyclosporine A (CsA) is an inhibitor of OATPs, P-gp, MRP2, BCRP and CYP3As. Drug-drug interaction (DDI) of CsA with victim drugs occurs via disordering interplay of transporters and enzymes. We aimed to establish a whole-body physiologically-based pharmacokinetic (PBPK) model which predicts disposition of CsA and nine victim drugs including atorvastatin, cerivastatin, pravastatin, rosuvastatin, fluvastatin, simvastatin, lovastatin, repaglinide and bosentan, as well as drug-drug interactions (DDIs) of CsA with nine victim drugs to investigate the integrated effect of enzymes and transporters in liver, intestinal and kidney on drug disposition. Predictions were compared with observations. Most of the predictions were within 0.5-2.0 folds of observations. Atorvastatin was represented to investigate individual contributions of transporters and CYP3As to atorvastatin disposition and their integrated effect. The contributions to atorvastatin disposition were hepatic OATPs >> hepatic CYP3A > intestinal CYP3As ≈ efflux transporters (P-gp/BCRP/MRP2). The results got the conclusion that the developed PBPK model characterizing the interplay of enzymes and transporters was successfully applied to predict the pharmacokinetics of 10 OATP substrates and DDIs of CsA with 9 victim drugs.

Numerical analysis of apparent decitabine uptake in HCT116 cells: Incorporation of a bidirectional first-order kinetic parameter for ENT1 transport and Michaelis-Menten parameters for subsequent phosphorylation

Decitabine (DAC), a DNA methylation inhibitor, is transported into cancer cells mainly via equilibrative nucleoside transporter 1 (ENT1) and subsequently phosphorylated by deoxycytidine kinase (dCK). We previously reported that apparent DAC uptake into cells may be described using a simple compartment model with clearance for facilitated diffusion (PS) and subsequent phosphorylation (CL_met). In the present study, time course of apparent intracellular [³H]-DAC uptake was analyzed numerically, and PS and CL_met values were calculated using the compartment model in human colon cancer HCT116 cells. PS at 0.1 μM [³H]-DAC was markedly decreased in the presence of 100 μM irinotecan or etoposide, while CL_met was markedly decreased in the presence of 100 μM cytarabine or gemcitabine. CL_met at 0.1-10 μM [³H]-DAC varied in a concentration-dependent manner and was described by Michaelis-Menten parameters K_m,met and V_max,met. In conclusion, DAC uptake mainly via ENT1 may be described by a bidirectional first-order kinetic parameter, while phosphorylation by dCK may be described by Michaelis-Menten parameters.

Variability in In Vitro OATP1B1/1B3 Inhibition Data: Impact of Incubation Conditions on Variability and Subsequent Drug Interaction Predictions

As the research into the organic anion transporting polypeptides (OATPs) continues to grow, it is important to ensure that the data generated are accurate and reproducible. In the in vitro evaluation of OATP1B1/1B3 inhibition, there are many variables that can contribute to variability in the resulting inhibition constants, which can then, in turn, contribute to variable results when clinical predictions (R-values) are performed. Currently, the only experimental condition recommended by the US Food and Drug Administration (FDA) is the inclusion of a pre-incubation period.¹ To identify other potential sources of variability, a descriptive analysis of available in vitro inhibition data was completed. For each of the 21 substrate/inhibitor pairs evaluated, cell type and pre-incubation were found to have the greatest effect on half-maximal inhibitory concentration (IC₅₀ ) variability. Indeed, when only HEK293 cells and co-incubation conditions were included, the observed variability for the entire data set (highest IC₅₀ /lowest) was reduced from 12.4 to 5.2. The choice of probe substrate used in the study also had a significant effect on inhibitor constant variability. Interestingly, despite the broad range of inhibitory constants identified, these two factors showed little effect on the calculated R-values relative to the FDA evaluation cutoff of 1.1 triggering a clinical evaluation for the inhibitors evaluated. However, because of the small data set available, further research is needed to confirm these preliminary results and define best practice for the study of OATPs.

Involvement of organic anion-transporting polypeptides and organic cation transporter in the hepatic uptake of jatrorrhizine

Jatrorrhizine possesses a wide spectrum of pharmacological activities. However, the mechanism underlying hepatic uptake of jatrorrhizine remains unclear.Rat liver slices, isolated rat hepatocytes and human embryonic kidney 293 (HEK293) cells stably expressing human organic anion-transporting polypeptide (OATP) and organic cation transporter (OCT) were used to evaluate the hepatic uptake of jatrorrhizine in this study.Uptake of jatrorrhizine in rat liver slices and isolated rat hepatocytes was significantly inhibited by glycyrrhizic acid (Oatp1b2 inhibitor) and prazosin (Oct1 inhibitor), but not by ibuprofen (Oatp1a1 inhibitor) or digoxin (Oatp1a4 inhibitor). Uptake of jatrorrhizine in OATP1B3 and OCT1-HEK293 cells indicated a saturable process with the K_m of 8.20 ± 1.28 and 4.94 ± 0.55 μM, respectively. However, the transcellular transport of jatrorrhizine in OATP1B1-HEK293 cells was not observed. Rifampicin (OATP inhibitor) for OATP1B3-HEK293 cells and prazosin for OCT1-HEK293 cells could inhibit the uptake of jatrorrhizine with the IC₅₀ of 5.49 ± 1.05 and 2.77 ± 0.72 μM, respectively.The above data indicate that hepatic uptake of jatrorrhizine is involved in both OATP and OCT, which may have important roles in jatrorrhizine liver disposition and potential drug-drug interactions.

Quantitative Contribution of Six Major Transporters to the Hepatic Uptake of Drugs: "SLC-Phenotyping" Using Primary Human Hepatocytes

Hepatic uptake transporters [solute carriers (SLCs)], including organic anion transporting polypeptide (OATP) 1B1, OATP1B3, OATP2B1, sodium-dependent taurocholate cotransporting polypeptide (NTCP), and organic anion (OAT2) and organic cation (OCT1) transporters, play a key role in determining the systemic and liver exposure of chemically diverse drugs. Here, we established a phenotyping approach to quantify the contribution of the six SLCs, and passive diffusion, to the overall uptake using plated human hepatocytes (PHHs). First, selective inhibitor conditions were identified by screening about 20 inhibitors across the six SLCs using single-transfected human embryonic kidney 293 cells. Data implied rifamycin SV (20 µM) inhibits three OATPs, while rifampicin (5 µM) inhibits OATP1B1/1B3 only. Further, hepatitis B virus myristoylated-preS1 peptide (0.1 µM), quinidine (100 µM), and ketoprofen (100-300 µM) are relatively selective against NTCP, OCT1, and OAT2, respectively. Second, using these inhibitory conditions, the fraction transported (ft ) by the individual SLCs was characterized for 20 substrates with PHH. Generally, extended clearance classification system class 1A/3A (e.g., warfarin) and 1B/3B compounds (e.g., statins) showed predominant OAT2 and OATP1B1/1B3 contribution, respectively. OCT1-mediated uptake was prominent for class 2/4 compounds (e.g., metformin). Third, in vitro ft values were corrected using quantitative proteomics data to obtain "scaled ft " Fourth, in vitro-in vivo extrapolation of the scaled OATP1B1/1B3 ft was assessed, leveraging statin clinical drug-drug interaction data with rifampicin as the perpetrator. Finally, we outlined a novel stepwise strategy to implement phenotypic characterization of SLC-mediated hepatic uptake for new molecular entities and drugs in a drug discovery and development setting.

Amino-terminal region of human organic anion transporting polypeptide 1B1 dictates transporter stability and substrate interaction

Organic anion transporting polypeptides (OATPs) are key players of drug absorption, distribution and excretion due to their broad substrate specificity, wide tissue distribution and the involvement in drug-drug interaction. OATP1B1 is specifically localized at the basolateral membrane of human hepatocytes and serves a crucial role in the drug clearance from the body. Previous studies have shown that transmembrane domains (TMs) are essential for proper functions of OATPs. In the present study, site-directed mutagenesis was performed to study the TM1 and amino-terminus of OATP1B1. Two positively charged residues, K41 and K49, as well as a hydrophobic residue I46, in TM1 were identified to be important for the proper function of the transporter. K41A and K49A exhibited altered Km value at the high and low affinity binding sites of estrone-3- sulfate (ES), respectively; while alanine substitution of I46 showed altered Km and Vmax values for both binding components of ES. Additional replacement of K41 revealed that the positively charged property at this position is important for maintaining OATP1B1 protein level and function; while the specific side-group structure of lysine at position 49 is irreplaceable for the transporter activity. Conservative replacement of I46 with leucine also recovered the function of the transporter. In addition, studies of the amino-terminus of OATP1B1 revealed that residues ranging from 19 to 27 are essential for protein stability and substrate interaction. Therefore, the amino-terminal region, which includes TM1 and the amino-terminus of OATP1B1, is important for proper function of the membrane protein.

Multiple circulating saponins from intravenous ShenMai inhibit OATP1Bs in vitro: potential joint precipitants of drug interactions

ShenMai, an intravenous injection prepared from steamed Panax ginseng roots (Hongshen) and Ophiopogon japonicus roots (Maidong), is used as an add-on therapy for coronary artery disease and cancer; saponins are its bioactive constituents. Since many saponins inhibit human organic anion-transporting polypeptides (OATP)1B, this investigation determined the inhibition potencies of circulating ShenMai saponins on the transporters and the joint potential of these compounds for ShenMai-drug interaction. Circulating saponins and their pharmacokinetics were characterized in rats receiving a 30-min infusion of ShenMai at 10 mL/kg. Inhibition of human OATP1B1/1B3 and rat Oatp1b2 by the individual saponins was investigated in vitro; the compounds' joint inhibition was also assessed in vitro and the data was processed using the Chou-Talalay method. Plasma protein binding was assessed by equilibrium dialysis. Altogether, 49 saponins in ShenMai were characterized and graded into: 10-100 μmol/day (compound doses from ShenMai; 7 compounds), 1-10 μmol/day (17 compounds), and <1 μmol/day (25 compounds, including Maidong ophiopogonins). After dosing, circulating saponins were protopanaxadiol-type ginsenosides Rb₁, Rb₂, Rc, Rd, Ra₁, Rg₃, Ra₂, and Ra₃, protopanaxatriol-type ginsenosides Rg₁, Re, Rg₂, and Rf, and ginsenoside Ro. The protopanaxadiol-type ginsenosides exhibited maximum plasma concentrations of 2.1-46.6 μmol/L, plasma unbound fractions of 0.4-1.0% and terminal half-lives of 15.6-28.5 h (ginsenoside Rg₃, 1.9 h), while the other ginsenosides exhibited 0.1-7.7 μmol/L, 20.8-99.2%, and 0.2-0.5 h, respectively. The protopanaxadiol-type ginsenosides, ginsenosides without any sugar attachment at C-20 (except ginsenoside Rf), and ginsenoside Ro inhibited OATP1B3 more potently (IC₅₀, 0.2-3.5 µmol/L) than the other ginsenosides (≥22.6 µmol/L). Inhibition of OATP1B1 by ginsenosides was less potent than OATP1B3 inhibition. Ginsenosides Rb₁, Rb₂, Rc, Rd, Ro, Ra₁, Re, and Rg₂ likely contribute the major part of OATP1B3-mediated ShenMai-drug interaction potential, in an additive and time-related manner.

Novel in Vitro Method Reveals Drugs That Inhibit Organic Solute Transporter Alpha/Beta (OSTα/β)

Drug interactions with the organic solute transporter alpha/beta (OSTα/β) are understudied even though OSTα/β is an important transporter that is expressed in multiple human tissues including the intestine, kidneys, and liver. In this study, an in vitro method to identify novel OSTα/β inhibitors was first developed using OSTα/β-overexpressing Flp-In 293 cells. Incubation conditions were optimized using previously reported OSTα/β inhibitors. A method including a 10 min preincubation step with the test compound was used to screen for OSTα/β inhibition by 77 structurally diverse compounds and fixed-dose combinations. Seven compounds and one fixed-dose combination (100 μM final concentration) inhibited OSTα/β-mediated dehydroepiandrosterone sulfate (DHEAS) uptake by >25%. Concentration-dependent OSTα/β inhibition was evaluated for all putative inhibitors (atorvastatin, ethinylestradiol, fidaxomicin, glycochenodeoxycholate, norgestimate, troglitazone, and troglitazone sulfate). Ethinylestradiol, fidaxomicin, and troglitazone sulfate yielded a clear concentration-inhibition response with IC50 values <200 μM. Among all tested compounds, there was no clear association between physicochemical properties, the severity of hepatotoxicity, and the degree of OSTα/β inhibition. This study utilized a novel in vitro method to identify OSTα/β inhibitors and, for the first time, provided IC50 values for OSTα/β inhibition. These data provide evidence that several drugs, some of which are associated with cholestatic drug-induced liver injury, may impair the function of the OSTα/β transporter.

Pd-Catalyzed microwave-assisted synthesis of phosphonated 13α-estrones as potential OATP2B1, 17β-HSD1 and/or STS inhibitors

Novel 2- or 4-phosphonated 13α-estrone derivatives were synthesized via the Hirao reaction. Bromo regioisomers (2- or 4-) of 13α-estrone and its 3-benzyl or 3-methyl ether were reacted with diethyl phosphite or diphenylphosphine oxide using Pd(PPh₃)₄ as catalyst under microwave irradiation. The influence of the new compounds on the transport function of the organic anion transporting polypeptide OATP2B1 was investigated by measuring Cascade Blue uptake. Derivatives bearing a 3-benzyl ether function displayed substantial submicromolar OATP2B1 inhibitory activity. The inhibitory effects of the compounds on human placental steroid sulfatase (STS) and 17β-hydroxysteroid dehydrogenase type 1 isozyme (17β-HSD1) were investigated by in vitro radiosubstrate incubation methods. None of the test compounds inhibited the STS markedly. The structure-activity relationship evaluation revealed that 2-substituted 3-hydroxy derivatives are able to inhibit the 17β-HSD1 enzyme with submicromolar IC₅₀ values. Dual OATP2B1 and 17β-HSD1 inhibitors have been identified.

Effects of Uremic Serum Residue on OATP1B1- and OATP1B3-Mediated Pravastatin Uptake in OATP-Expressing HEK293 Cells and Human Hepatocytes

Patients with end-stage renal disease have increased plasma concentrations of statins, which is a risk factor for rhabdomyolysis, as well as elevated levels of uremic toxins (UTs). We investigated the effects of uremic serum residue and UTs on organic anion-transporting peptide (OATP1B1)- and OATP1B3-mediated pravastatin uptake. We evaluated the effects of normal serum residue with four UTs (hippuric acid, 3-carboxy-4-methyl-5-propyl-2-furan propionate, indole-3-acetic acid, and 3-indoxyl sulfate) and uremic serum residue on pravastatin uptake by OATP1B1- or OATP1B3-expressing HEK293 cells. Furthermore, we assessed the contribution of each transporter using cryopreserved human hepatocytes. Uremic serum residue and UTs significantly inhibited OATP1B1-mediated pravastatin uptake. Uremic serum residue accelerated OATP1B3-mediated pravastatin uptake, while UTs had no effect. There was no difference in pravastatin uptake between uremic- and normal serum residue-treated hepatocytes. The results suggest that the effects of uremic serum on pravastatin hepatic uptake may be considered negligible in end-stage renal disease patients.

Synthesis and structure-activity relationships of 2- and/or 4-halogenated 13β- and 13α-estrone derivatives as enzyme inhibitors of estrogen biosynthesis

Ring A halogenated 13α-, 13β-, and 17-deoxy-13α-estrone derivatives were synthesised with N-halosuccinimides as electrophile triggers. Substitutions occurred at positions C-2 and/or C-4. The potential inhibitory action of the halogenated estrones on human aromatase, steroid sulfatase, or 17β-hydroxysteroid dehydrogenase 1 activity was investigated via in vitro radiosubstrate incubation. Potent submicromolar or low micromolar inhibitors were identified with occasional dual or multiple inhibitory properties. Valuable structure–activity relationships were established from the comparison of the inhibitory data obtained. Kinetic experiments performed with selected compounds revealed competitive reversible inhibition mechanisms against 17β-hydroxysteroid dehydrogenase 1 and competitive irreversible manner in the inhibition of the steroid sulfatase enzyme.

Comparison of uptake transporter functions in hepatocytes in different species to determine the optimal model for evaluating drug transporter activities in humans

A thorough understanding of species-dependent differences in hepatic uptake transporters is critical for predicting human pharmacokinetics (PKs) from preclinical data. In this study, the activities of organic anion transporting polypeptide (OATP/Oatp), organic cation transporter 1 (OCT1/Oct1), and sodium-taurocholate cotransporting polypeptide (NTCP/Ntcp) in cultured rat, dog, monkey and human hepatocytes were compared. The activities of hepatic uptake transporters were evaluated with respect to culture duration, substrate and species-dependent differences in hepatocytes. Longer culture duration reduced hepatic uptake transporter activities across species except for Oatp and Ntcp in rats. Comparable apparent Michaelis-Menten constant (K_m,app) values in hepatocytes were observed across species for atorvastatin, estradiol-17β-glucuronide and metformin. The K_m,app values for rosuvastatin and taurocholate were significantly different across species. Rat hepatocytes exhibited the highest Oatp percentage of uptake transporter-mediated permeation clearance (PS_inf,act) while no difference in %PS_inf,act of probe substrates were observed across species. The in vitro hepatocyte inhibition data in rats, monkeys and humans provided reasonable predictions of in vivo drug-drug interaction (DDIs) between atorvastatin/rosuvastatin and rifampin. These findings suggested that using human hepatocytes with a short culture time is the most robust preclinical model for predicting DDIs for compounds exhibiting active hepatic uptake in humans.

Simultaneous Assessment of Transporter-Mediated Drug-Drug Interactions Using a Probe Drug Cocktail in Cynomolgus Monkey

We aim to establish an in vivo preclinical model to enable simultaneous assessment of inhibition potential of an investigational drug on clinically relevant drug transporters, organic anion-transporting polypeptide (OATP)1B, breast cancer resistance protein (BCRP), P-glycoprotein (P-gp), and organic anion transporter (OAT)3. Pharmacokinetics of substrate cocktail consisting of pitavastatin (OATP1B substrate), rosuvastatin (OATP1B/BCRP/OAT3), sulfasalazine (BCRP), and talinolol (P-gp) were obtained in cynomolgus monkey-alone or in combination with transporter inhibitors. Single-dose rifampicin (30 mg/kg) significantly (P < 0.01) increased the plasma exposure of all four drugs, with a marked effect on pitavastatin and rosuvastatin [area under the plasma concentration-time curve (AUC) ratio ∼21-39]. Elacridar, BCRP/P-gp inhibitor, increased the AUC of sulfasalazine, talinolol, as well as rosuvastatin and pitavastatin. An OAT1/3 inhibitor (probenecid) significantly (P < 0.05) impacted the renal clearance of rosuvastatin (∼8-fold). In vitro, rifampicin (10 µM) inhibited uptake of pitavastatin, rosuvastatin, and sulfasalazine by monkey and human primary hepatocytes. Transport studies using membrane vesicles suggested that all probe substrates, except talinolol, are transported by cynoBCRP, whereas talinolol is a cynoP-gp substrate. Elacridar and rifampicin inhibited both cynoBCRP and cynoP-gp in vitro, indicating potential for in vivo intestinal efflux inhibition. In conclusion, a probe substrate cocktail was validated to simultaneously evaluate perpetrator impact on multiple clinically relevant transporters using the cynomolgus monkey. The results support the use of the cynomolgus monkey as a model that could enable drug-drug interaction risk assessment, before advancing a new molecular entity into clinical development, as well as providing mechanistic insights on transporter-mediated interactions.

Expression analysis of human solute carrier (SLC) family transporters in nasal mucosa and RPMI 2650 cells

With nearly 400 members, the solute-linked carrier (SLC) superfamily is one of the most important gene classes concerning the disposition of drugs and the transport of physiological substrates in the human body. The mapping of related transport proteins is already well advanced for the intestines, kidneys and liver, but it has recently been brought into focus for various respiratory epithelia. The aim of this study was to evaluate the expression of several SLC transporters in differently cultured RPMI 2650 cells, as well as in specimens of the human nasal mucosa. The expression profiles of PEPT2, OATP1A2, OATP4C1, OCT2, OCTN1 and OCTN2 were investigated at the gene and protein levels by performing RT-PCR, western blot analysis and immunohistological staining. Uptake assays using appropriate substrates and inhibitory substances were performed to compare the activity of peptide, organic anion and organic cation transporters, respectively, among the three models. Expression of the six SLC transporters under investigation was confirmed at the mRNA and protein levels in human nasal mucosa ex vivo as well as in RPMI 2650 cells grown under different culture conditions. The functionality was almost equal among all of the models for the PEPT and OCT(N) transporters, while the functional activity of the OATP transporters was more pronounced for both in vitro models than for excised nasal tissue. Despite negligible variations in transporter capacities, the RPMI 2650 cell cultures and freshly isolated human nasal epithelium showed nearly comparable expression patterns for the examined SLC proteins. Therefore, in vitro models based on the RPMI 2650 cell line could provide helpful data during the preclinical investigation of intranasally administered drug formulations and in the development of strategies to target nasal drug transporters for either local or systemic drug delivery.

Quantitative Analysis of Complex Drug-Drug Interactions between Cerivastatin and Metabolism/Transport Inhibitors Using Physiologically Based Pharmacokinetic Modeling

Cerivastatin (CER) was withdrawn from the world market because of lethal rhabdomyolysis. Coadministrations of CER and cyclosporine A (CsA) or gemfibrozil (GEM) have been reported to increase the CER blood concentration. CsA is an inhibitor of organic anion transporting polypeptide (OATP)1B1 and CYP3A4, and GEM and its glucuronide (GEM-glu) inhibit OATP1B1 and CYP2C8. The purpose of this study was to describe the transporter-/enzyme-mediated drug-drug interactions (DDIs) of CER with CsA or GEM based on unified physiologically based pharmacokinetic (PBPK) models and to investigate whether the DDIs can be quantitatively analyzed by a bottom-up approach. Initially, the PBPK models for CER and GEM/GEM-glu were constructed based on the previously reported standard protocols. Next, the drug-dependent parameters were optimized by Cluster Newton Method. Thus, described concentration-time profiles for CER and GEM/GEM-glu agreed well with the clinically observed data. The DDIs were then simulated using the established PBPK models with previously obtained in vitro inhibition constants of CsA or GEM/GEM-glu against the OATP1B1 and cytochrome P450s. DDIs with the inhibitors were underestimated compared with observed data using the geometric means of reported values. To search for better described parameters within the range of in vitro values, sensitivity analyses were performed for DDIs of CER. Using the in vitro parameter sets selected by sensitivity analyses, these DDIs were well reproduced, indicating that the present PBPK models were able to describe adequately the clinical DDIs based on a bottom-up approach. The approaches in this study would be applicable to the prediction of other DDIs involving both transporters and metabolic enzymes.