Unravelling the complex drug-drug interactions of the cardiovascular drugs, verapamil and digoxin, with P-glycoprotein

Probing the Mechanisms Underlying the Transport of the Vinca Alkaloids by P-glycoprotein

The efficacy of many cancer drugs is hindered by P-glycoprotein (Pgp), a cellular pump that removes drugs from cells. To improve chemotherapy, drugs capable of evading Pgp must be developed. Despite similarities in structure, vinca alkaloids (VAs) show disparate Pgp-mediated efflux ratios. ATPase activity and binding affinity studies show at least two binding sites for the VAs: high- and low-affinity sites that stimulate and inhibit the ATPase activity rate, respectively. The affinity for ATP from the ATPase kinetics curve for vinblastine (VBL) at the high-affinity site was 2- and 9-fold higher than vinorelbine (VRL) and vincristine (VCR), respectively. Conversely, VBL had the highest Km (ATP) for the low-affinity site. The dissociation constants (KDs) determined by protein fluorescence quenching were in the order VBL < VRL< VCR. The order of the KDs was reversed at higher substrate concentrations. Acrylamide quenching of protein fluorescence indicate that the VAs, either at 10 µM or 150 µM, predominantly maintain Pgp in an open-outward conformation. When 3.2 mM AMPPNP was present, 10 µM of either VBL, VRL, or VCR cause Pgp to shift to an open-outward conformation, while 150 µM of the VAs shifted the conformation of Pgp to an intermediate orientation, between opened inward and open-outward. However, the conformational shift induced by saturating AMPPNP and VCR condition was less than either VBL or VRL in the presence of AMPPNP. At 150 µM, atomic force microscopy (AFM) revealed that the VAs shift Pgp population to a predominantly open-inward conformation. Additionally, STDD NMR studies revealed comparable groups in VBL, VRL, and VCR are in contact with the protein during binding. Our results, when coupled with VAs-microtubule structure-activity relationship studies, could lay the foundation for developing next-generation VAs that are effective as anti-tumor agents. A model that illustrates the intricate process of Pgp-mediated transport of the VAs is presented.

Drug-Induced Conformational Dynamics of P-Glycoprotein Underlies the Transport of Camptothecin Analogs

P-glycoprotein (Pgp) plays a pivotal role in drug bioavailability and multi-drug resistance development. Understanding the protein's activity and designing effective drugs require insight into the mechanisms underlying Pgp-mediated transport of xenobiotics. In this study, we investigated the drug-induced conformational changes in Pgp and adopted a conformationally-gated model to elucidate the Pgp-mediated transport of camptothecin analogs (CPTs). While Pgp displays a wide range of conformations, we simplified it into three model states: 'open-inward', 'open-outward', and 'intermediate'. Utilizing acrylamide quenching of Pgp fluorescence as a tool to examine the protein's tertiary structure, we observed that topotecan (TPT), SN-38, and irinotecan (IRT) induced distinct conformational shifts in the protein. TPT caused a substantial shift akin to AMPPNP, suggesting ATP-independent 'open-outward' conformation. IRT and SN-38 had relatively moderate effects on the conformation of Pgp. Experimental atomic force microscopy (AFM) imaging supports these findings. Further, the rate of ATPase hydrolysis was correlated with ligand-induced Pgp conformational changes. We hypothesize that the separation between the nucleotide-binding domains (NBDs) creates a conformational barrier for substrate transport. Substrates that reduce the conformational barrier, like TPT, are better transported. The affinity for ATP extracted from Pgp-mediated ATP hydrolysis kinetics curves for TPT was about 2-fold and 3-fold higher than SN-38 and IRT, respectively. On the contrary, the dissociation constants (KD) determined by fluorescence quenching for these drugs were not significantly different. Saturation transfer double difference (STDD) NMR of TPT and IRT with Pgp revealed that similar functional groups of the CPTs are accountable for Pgp-CPTs interactions. Efforts aimed at modifying these functional groups, guided by available structure-activity relationship data for CPTs and DNA-Topoisomerase-I complexes, could pave the way for the development of more potent next-generation CPTs.

Advantages and potential limitations of applying AFM kymograph analysis to pharmaceutically relevant membrane proteins in lipid bilayers

Membrane proteins play critical roles in disease and in the disposition of many pharmaceuticals. A prime example is P-glycoprotein (Pgp) which moves a diverse range of drugs across membranes and out of the cell before a therapeutic payload can be delivered. Conventional structural biology methods have provided a valuable framework for comprehending the complex conformational changes underlying Pgp function, which also includes ATPase activity, but the lack of real-time information hinders understanding. Atomic force microscopy (AFM) is a single-molecule technique that is well-suited for studying active membrane proteins in bilayers and is poised to advance the field beyond static snapshots. After verifying Pgp activity in surface-support bilayers, we used kymograph analysis in conjunction with AFM imaging and simulations to study structural transitions at the 100 ms timescale. Though kymographs are frequently employed to boost temporal resolution, the limitations of the method have not been well characterized, especially for sparse non-crystalline distributions of pharmaceutically relevant membrane proteins like Pgp. Common experimental challenges are analyzed, including protein orientation, instrument noise, and drift. Surprisingly, a lateral drift of 75% of the protein dimension leads to only a 12% probability of erroneous state transition detection; average dwell time error achieves a maximum value of 6%. Rotational drift of proteins like Pgp, with azimuthally-dependent maximum heights, can lead to artifactual transitions. Torsional constraints can alleviate this potential pitfall. Confidence in detected transitions can be increased by adding conformation-altering ligands such as non-hydrolysable analogs. Overall, the data indicate that AFM kymographs are a viable method to access conformational dynamics for Pgp, but generalizations of the method should be made with caution.

Four Decades of Cytochrome P450 2B Research: From Protein Adducts to Protein Structures and Beyond

This article features selected findings from the senior author and colleagues dating back to 1978 and covering approximately three-fourths of the 60 years since the discovery of cytochrome P450. Considering the vast number of P450 enzymes in this amazing superfamily and their importance for so many fields of science and medicine, including drug design and development, drug therapy, environmental health, and biotechnology, a comprehensive review of even a single topic is daunting. To make a meaningful contribution to the 50th anniversary of Drug Metabolism and Disposition, we trace the development of the research in a single P450 laboratory through the eyes of seven individuals with different backgrounds, perspectives, and subsequent career trajectories. All co-authors are united in their fascination for the structural basis of mammalian P450 substrate and inhibitor selectivity and using such information to improve drug design and therapy. An underlying theme is how technological advances enable scientific discoveries that were impossible and even inconceivable to prior generations. The work performed spans the continuum from: 1) purification of P450 enzymes from animal tissues to purification of expressed human P450 enzymes and their site-directed mutants from bacteria; 2) inhibition, metabolism, and spectral studies to isothermal titration calorimetry, deuterium exchange mass spectrometry, and NMR; 3) homology models based on bacterial P450 X-ray crystal structures to rabbit and human P450 structures in complex with a wide variety of ligands. Our hope is that humanizing the scientific endeavor will encourage new generations of scientists to make fundamental new discoveries in the P450 field. SIGNIFICANCE STATEMENT: The manuscript summarizes four decades of work from Dr. James Halpert's laboratory, whose investigations have shaped the cytochrome P450 field, and provides insightful perspectives of the co-authors. This work will also inspire future drug metabolism scientists to make critical new discoveries in the cytochrome P450 field.

Colchicine and macrolides: a cohort study of the risk of adverse outcomes associated with concomitant exposure

Colchicine is increasingly used as the number of potential indications expands. However, it also has a narrow therapeutic index that is associated with bothersome to severe side effects. When concomitantly use with medications inhibiting its metabolism, higher plasma levels will result and increase likelihood of colchicine toxicity. We conducted a cohort study using electronic health records comparing encounters with colchicine plus a macrolide and colchicine with an antibiotic non-macrolide. We assessed the relationship between the two groups using adjusted multivariate logistic regression models and the risk of rhabdomyolysis, pancytopenia, muscular weakness, heart failure, acute hepatic failure and death. 12670 patients on colchicine plus an antibiotic non-macrolide were compared to 2199 patients exposed to colchicine plus a macrolide. Patients exposed to colchicine and a macrolide were majority men (n = 1329, 60.4%) and white (n = 1485, 67.5%) in their late sixties (mean age in years 68.4, SD 15.6). Heart failure was more frequent in the colchicine plus a macrolide cohort (n = 402, 18.3%) vs the colchicine non-macrolide one (n = 1153, 9.1%) (p < 0.0001) and also had a higher mortality rate [(85 (3.87%) vs 289 (2.28%), p < 0.0001 macrolides vs non-macrolides cohorts, respectively]. When the sample was limited to individuals exposed to either clarithromycin or erythromycin and colchicine, the adjusted OR for acute hepatic failure was 2.47 (95% CI 1.04–5.91) and 2.06 for death (95% CI 1.07–3.97). There is a significant increase in the risk of hepatic failure and mortality when colchicine is concomitantly administered with a macrolide. Colchicine should not be used concomitantly with these antibiotics or should be temporarily discontinued to avoid toxic levels of colchicine.

Digoxin Derivatives Sensitize a Saccharomyces cerevisiae Mutant Strain to Fluconazole by Inhibiting Pdr5p

The poor outcome of treatments for fungal infections is a consequence of the increasing incidence of resistance to antifungal agents, mainly due to the overexpression of efflux pumps. To surpass this mechanism of resistance, a substance able to inhibit these pumps could be administered in association with antifungals. Saccharomyces cerevisiae possesses an efflux pump (Pdr5p) homologue to those found in pathogenic yeast. Digoxin is a natural product that inhibits Na⁺, K⁺-ATPase. The aim of this study was to evaluate whether digoxin and its derivatives (i.e., DGB, digoxin benzylidene) can inhibit Pdr5p, reversing the resistance to fluconazole in yeasts. An S. cerevisiae mutant strain that overexpresses Pdr5p was used in the assays. The effects of the compounds on yeast growth, efflux activity, and Pdr5p ATPase activity were measured. All derivatives enhanced the antifungal activity of fluconazole against S. cerevisiae, in comparison to fluconazole alone, with FICI values ranging from 0.031 to 0.500. DGB 1 and DGB 3 presented combined effects with fluconazole against a Candida albicans strain, with fractional inhibitory concentration index (FICI) values of 0.625 and 0.281, respectively The compounds also inhibited the efflux of rhodamine 6G and Pdr5p ATPase activity, with IC₅₀ values ranging from 0.41 μM to 3.72 μM. The results suggest that digoxin derivatives impair Pdr5p activity. Considering the homology between Pdr5p and efflux pumps from pathogenic fungi, these compounds are potential candidates to be used in association with fluconazole to treat resistant fungal infections.

Structure-based alteration of tryptophan residues of the multidrug transporter CmABCB1 to assess substrate binding using fluorescence spectroscopy

ABCB1, also known as P-glycoprotein, is an essential component of many physiological barriers and extrudes a variety of hydrophobic chemicals out of the cell. Structures of ABCB1 provided insights into the structural changes that occur upon ATP binding and the characteristic architecture of the substrate binding site. Yet, the structure-function relationship between substrate binding and transporting still remains largely obscured because there is no robust method for accurately measuring substrate binding constants. The methods currently used cannot identify whether the bound substrates are located in the inner chamber of the molecule in the transmembrane region or not because of the low spatial resolution. Here, we report a system for measuring the affinity of substrate binding to the Cyanidioschyzon merolae ABCB1 (CmABCB1) using site-specific tryptophan (Trp) fluorescence quenching. We designed a CmABCB1 mutant with an extrinsic Trp residue introduced into the inner chamber. Trp fluorescence was quenched by three substrates and one inhibitor, including rhodamine 6G, in a saturable fashion, allowing for accurate estimation of the dissociation constant (KD ) for each molecule. The KD for rhodamine 6G is similar to that determined using a reciprocal fluorescence quenching assay using rhodamine 6G fluorescence, suggesting that Trp fluorescence of the mutant was quenched by the interaction between the extrinsic Trp and substrates bound in the inner chamber. Structural comparison of the ABCB1 structures suggests that the system presented in this study could be ideal method of choice to determine the substrate binding affinities of compounds bound to the chamber of mammalian ABCB1.

NMR Characterization of the Interactions Between Glycosaminoglycans and Proteins

Glycosaminoglycans (GAGs) constitute a considerable fraction of the glycoconjugates found on cellular membranes and in the extracellular matrix of virtually all mammalian tissues. The essential role of GAG-protein interactions in the regulation of physiological processes has been recognized for decades. However, the underlying molecular basis of these interactions has only emerged since 1990s. The binding specificity of GAGs is encoded in their primary structures, but ultimately depends on how their functional groups are presented to a protein in the three-dimensional space. This review focuses on the application of NMR spectroscopy on the characterization of the GAG-protein interactions. Examples of interpretation of the complex mechanism and characterization of structural motifs involved in the GAG-protein interactions are given. Selected families of GAG-binding proteins investigated using NMR are also described.

Pharmacokinetic interactions of esaxerenone with amlodipine and digoxin in healthy Japanese subjects

Background

To investigate the effects of coadministration of esaxerenone with amlodipine on the pharmacokinetics (PK) of each drug, and of esaxerenone on the PK of digoxin.

Methods

In three open-label, single-sequence, crossover studies, healthy Japanese males received single oral doses of esaxerenone 2.5 mg (Days 1, 15), with amlodipine 10 mg/day (Days 8–18) (Study 1, N = 24); single doses of amlodipine 2.5 mg (Days 1, 21), with esaxerenone 5 mg/day (Days 8–25) (Study 2; N = 20); or digoxin 0.25 mg/day (Days 1–15) with esaxerenone 5 mg/day (Days 11–15) (Study 3; N = 20). PK parameters and safety were assessed.

Results

Study 1: esaxerenone peak plasma concentration (C_max) and time to C_max were unaltered by amlodipine coadministration, but mean half-life was slightly prolonged from 18.5 to 20.9 h. Geometric least-squares mean (GLSM) ratios for C_max, area under the plasma concentration–time curve (AUC) from zero to last measurable concentration and from zero to infinity for esaxerenone + amlodipine versus esaxerenone were 0.958, 1.154, and 1.173, respectively. Study 2: corresponding GLSM ratios for amlodipine + esaxerenone versus amlodipine were 1.099, 1.185, and 1.214. Study 3: esaxerenone did not markedly alter digoxin PK. GLSM ratios for C_max, trough plasma concentration, and AUC during a dosing interval for digoxin versus esaxerenone + digoxin were 1.130, 1.088, and 1.072, respectively.

Conclusions

No drug–drug interactions are expected during combination therapy with esaxerenone and either amlodipine or digoxin, based on a lack of any clinically relevant PK changes.

Trial registration

Studies 1 and 2: JapicCTI-163379 (registered on 20 September 2016); Study 3: JapicCTI-163443 (registered on 24 November 2016).

A Mechanistic, Enantioselective, Physiologically Based Pharmacokinetic Model of Verapamil and Norverapamil, Built and Evaluated for Drug-Drug Interaction Studies

The calcium channel blocker and antiarrhythmic agent verapamil is recommended by the FDA for drug–drug interaction (DDI) studies as a moderate clinical CYP3A4 index inhibitor and as a clinical Pgp inhibitor. The purpose of the presented work was to develop a mechanistic whole-body physiologically based pharmacokinetic (PBPK) model to investigate and predict DDIs with verapamil. The model was established in PK-Sim^®, using 45 clinical studies (dosing range 0.1–250 mg), including literature as well as unpublished Boehringer Ingelheim data. The verapamil R- and S-enantiomers and their main metabolites R- and S-norverapamil are represented in the model. The processes implemented to describe the pharmacokinetics of verapamil and norverapamil include enantioselective plasma protein binding, enantioselective metabolism by CYP3A4, non-stereospecific Pgp transport, and passive glomerular filtration. To describe the auto-inhibitory and DDI potential, mechanism-based inactivation of CYP3A4 and non-competitive inhibition of Pgp by the verapamil and norverapamil enantiomers were incorporated based on in vitro literature. The resulting DDI performance was demonstrated by prediction of DDIs with midazolam, digoxin, rifampicin, and cimetidine, with 21/22 predicted DDI AUC ratios or C_trough ratios within 1.5-fold of the observed values. The thoroughly built and qualified model will be freely available in the Open Systems Pharmacology model repository to support model-informed drug discovery and development.

Beta-blocker Use in Moderate and Severe Chronic Obstructive Pulmonary Disease

Introduction:

The most appropriate choice of pharmacological treatment of heart rhythm disorders occurring in patients with chronic obstructive pulmonary disease (COPD) and cardiovascular comorbidity is often a topic of debate between pulmonologists and cardiologists in clinical practice, although numerous studies and clinical trials have demonstrated evidence to support the use of selective beta-blockers (BBs) in these patients.

Aim:

To examine the difference in the number of exacerbations in patients treated with a combination of verapamil and digoxin or BB alone in patients with different COPD stages.

Patients and methods:

The study included 68 patients (n = 68) diagnosed with COPD who were followed-up during a 12-month period, and the number of exacerbations were analyzed. The patients were divided into two groups according to the stage of COPD: GOLD II (moderate), and GOLD III (severe), and in each group a subdivision was established in relation to the use of either a combination of verapamil and digoxin or the use of BBs alone in pharmacological treatment. The inclusion criteria for patients were defined as following: a) established diagnosis of COPD according to present or deteriorated relevant clinical symptoms and signs, b) the ejection fraction (EF) of a left ventricle (LV) >35%, and c) spirometric cut-points classified as GOLD II (FEV1 / FVC <0.7, FEV1 predicted 50-80%), or GOLD III (FEV1/FVC <0.7, FEV1 predicted 30-50%) stage of the COPD. The exclusion criteria were EF of LV <35% and a lethal outcome during a follow-up period (2 patients were encountered). Exacerbation was defined as functional deterioration of the COPD symptoms verified by spirometric functional testing, frequency of hospitalizations according to GOLD stage assignment or verified clinical symptoms deterioration.

Results:

Regardless the pharmacological treatment, there is a statistically significant increase in the number of COPD exacerbations, in a 12-month period follow-up, in the GOLD III group (severe) compared to the GOLD II group (moderate). In the group of patients taking verapamil and digoxin, a two-tailed t-test was used to analyze the results between the GOLD II and GOLD III stage groups, p = 0.01, and 2. In the group of patients taking BBs, a two-tailed t-test was also used to analyze the results between the GOLD II and GOLD III stage groups, p = 0.003). Within the COPD GOLD II stage group, there appears to be no statistically significant difference in the number of exacerbations between the patients taking verapamil and digoxin (n = 24) and the patients taking BBs alone (n = 15), although, in patients taking BBs alone, there appears to be a trend towards a decrease in the exacerbations compared to the number of exacerbations in patients taking verapamil and digoxin (p = 0.007). Within the COPD GOLD III stage group, there is no difference in the number of exacerbations between the patients taking verapamil and digoxin (n = 20), and the patients taking BBs alone (n = 9), as analyzed by a two-tailed t-test, p = 0.577.

Conclusion:

Use of selective BBs in the treatment of cardiovascular comorbidity in patients with COPD represents a far better choice of pharmacological approach in the treatment of patients diagnosed with COPD GOLD II (moderate) stage.

The effects of anthracycline drugs on the conformational distribution of mouse P-glycoprotein explains their transport rate differences

P-glycoprotein (Pgp) is an ATP-dependent efflux transporter and plays a major role in anti-cancer drug resistance by pumping a chemically diverse range of cytotoxic drugs from cancerous tumors. Despite numerous studies with the transporter, the molecular features that drive anti-cancer drug efflux are not well understood. Even subtle differences in the anti-cancer drug molecular structure can lead to dramatic differences in their transport rates. To unmask these structural differences, this study focused on two closely-related anthracycline drugs, daunorubicin (DNR), and doxorubicin (DOX), with mouse Pgp. While only differing by a single hydroxyl functional group, DNR has a 4 to 5-fold higher transport rate than DOX. They both non-competitively inhibited Pgp-mediated ATP hydrolysis below basal levels. The K_m of Pgp-mediated ATP hydrolysis extracted from the kinetics curves was lower for DOX than DNR. However, the dissociation constants (K_Ds) for these drugs determined by fluorescence quenching were virtually identical. Acrylamide quenching of Pgp tryptophan fluorescence to probe the tertiary structure of Pgp suggested that DNR shifts Pgp to a "closed" conformation, while DOX shifts Pgp to an "intermediate" conformation. The effects of these drugs on the Pgp conformational distributions in a lipid bilayer were also examined by atomic force microscopy (AFM). Analysis of AFM images revealed that DNR and DOX cause distinct and significant shifts in the conformational distribution of Pgp. The results were combined to build a conformational distribution model for anthracycline transport by Pgp.

Drug Transport at the Brain and Endothelial Dysfunction in Preeclampsia: Implications and Perspectives

Transport of drugs across biological barriers has been a subject of study for decades. The discovery and characterization of proteins that confer the barrier properties of endothelia and epithelia, including tight junction proteins and membrane transporters belonging to the ATP-binding cassette (ABC) and Solute Carrier (SLC) families, represented a significant step forward into understanding the mechanisms that govern drug disposition. Subsequently, numerous studies, including both pre-clinical approaches and clinical investigations, have been carried out to determine the influence of physiological and pathological states on drug disposition. Importantly, there has been increasing interest in gaining a better understanding of drug disposition during pregnancy, since epidemiological and clinical studies have demonstrated that the use of medications by pregnant women is significant and this condition embodies a series of significant anatomical and physiological modifications, particularly at excretory organs and barrier sites (e.g., placenta, breast) expressing transporter proteins which influence pharmacokinetics. Currently, most of the research in this field has focused on the expression profiling of transporter proteins in trophoblasts and endothelial cells of the placenta, regulation of drug-resistance mechanisms in disease states and pharmacokinetic studies. However, little attention has been placed on the influence that the cerebrovascular dysfunction present in pregnancy-related disorders, such as preeclampsia, might exert on drug disposition in the mother’s brain. This issue is particularly important since recent findings have demonstrated that preeclamptic women suffer from long-term alterations in the integrity of the blood-brain barrier (BBB). In this review we aim to analyze the available evidence regarding the influence of pregnancy on the expression of transporters and TJ proteins in brain endothelial cells, as well the mechanisms that govern the pathophysiological alterations in the BBB of women who experience preeclampsia. Future research efforts should be focused not only on achieving a better understanding of the influence of preeclampsia-associated endothelial dysfunction on drug disposition, but also in optimizing the pharmacological treatments of women suffering pregnancy-related disorders, its comorbidities and to develop new therapies aiming to restore the integrity of the BBB.

The conformation and dynamics of P-glycoprotein in a lipid bilayer investigated by atomic force microscopy

The membrane-bound P-glycoprotein (Pgp) transporter plays a major role in human disease and drug disposition because of its ability to efflux a chemically diverse range of drugs through ATP hydrolysis and ligand-induced conformational changes. Deciphering these structural changes is key to understanding the molecular basis of transport and to developing molecules that can modulate efflux. Here, atomic force microscopy (AFM) is used to directly image individual Pgp transporter molecules in a lipid bilayer under physiological pH and ambient temperature. Analysis of the Pgp AFM images revealed "small" and "large" protrusions from the lipid bilayer with significant differences in protrusion height and volume. The geometry of these "small" and "large" protrusions correlated to the predicted extracellular (EC) and cytosolic (C) domains of the Pgp X-ray crystal structure, respectively. To assign these protrusions, simulated AFM images were produced from the Pgp X-ray crystal structures with membrane planes defined by three computational approaches, and a simulated 80 Å AFM cantilever tip. The theoretical AFM images of the EC and C domains had similar heights and volumes to the "small" and "large" protrusions in the experimental AFM images, respectively. The assignment of the protrusions in the AFM images to the EC and C domains was confirmed by changes in protrusion volume by Pgp-specific antibodies. The Pgp domains showed a considerable degree of conformational dynamics in time resolved AFM images. With this information, a model of Pgp conformational dynamics in a lipid bilayer is proposed within the context of the known Pgp X-ray crystal structures.

New insights in the in vitro characterisation and molecular modelling of the P-glycoprotein inhibitory promiscuity

The presence of several binding sites for both substrates and inhibitors is yet a poorly explored thematic concerning the assessment of the drug-drug interactions risk due to interactions of multiple drugs with the human transport protein P-glycoprotein (P-gp or MDR1, gene ABCB1). In this study we measured the inhibitory behaviour of a set of known drugs towards P-gp by using three different probe substrates (digoxin, Hoechst 33,342 and rhodamine 123). A structure-based model was built to unravel the different substrates binding sites and to rationalize the cases where drugs were not inhibiting all the substrates. A separate set of experiments was used to validate the model and confirmed its suitability to either detect the substrate-dependent P-gp inhibition and to anticipate proper substrates for in vitro experiments case by case. The modelling strategy described can be applied for either design safer drugs (P-gp as antitarget) or to target specific sub-site inhibitors towards other drugs (P-gp as target).

A Conformationally Gated Model of Methadone and Loperamide Transport by P-Glycoprotein

P-glycoprotein (Pgp) is a multidrug resistance transporter that limits the penetration of a wide range of neurotherapeutics into the brain including opioids. The diphenylpropylamine opioids methadone and loperamide are structurally similar, but loperamide has about a 4-fold higher Pgp-mediated transport rate. In addition to these differences, they showed significant differences in their effects on Pgp-mediated adenosine triphosphate (ATP) hydrolysis. The activation of Pgp-mediated ATP hydrolysis by methadone was monophasic, whereas loperamide activation of ATP hydrolysis was biphasic implying methadone has a single binding site and loperamide has 2 binding sites on Pgp. Quenching of tryptophan fluorescence with these drugs and digoxin showed competition between the opioids and that loperamide does not compete for the digoxin-binding site. Acrylamide quenching of tryptophan fluorescence to probe Pgp conformational changes revealed that methadone- and loperamide-induced conformational changes were distinct. These results were used to develop a model for Pgp-mediated transport of methadone and loperamide where opioid binding and conformational changes are used to explain the differences in the opioid transport rates between methadone and loperamide.

Boosted coupling of ATP hydrolysis to substrate transport upon cooperative estradiol-17-β-D-glucuronide binding in a Drosophila ATP binding cassette type-C transporter

ATP binding cassette type-C (ABCC) transporters move molecules across cell membranes upon hydrolysis of ATP; however, their coupling of ATP hydrolysis to substrate transport remains elusive. Drosophila multidrug resistance-associated protein (DMRP) is the functional ortholog of human long ABCC transporters, with similar substrate and inhibitor specificity, but higher activity. Exploiting its high activity, we kinetically dissected the catalytic mechanism of DMRP by using E₂-d-glucuronide (E₂G), the physiologic substrate of human ABCC. We examined the DMRP-mediated interdependence of ATP and E₂G in biochemical assays. We observed E₂G-dependent ATPase activity to be biphasic at subsaturating ATP concentrations, which implies at least 2 E₂G binding sites on DMRP. Furthermore, transport measurements indicated strong nonreciprocal cooperativity between ATP and E₂G. In addition to confirming these findings, our kinetic modeling with the Complex Pathway Simulator indicated a 10-fold decrease in the E₂G-mediated activation of ATP hydrolysis upon saturation of the second E₂G binding site. Surprisingly, the binding of the second E₂G allowed for substrate transport with a constant rate, which tightly coupled ATP hydrolysis to transport. In summary, we show that the second E₂G binding-similar to human ABCC2-allosterically stimulates transport activity of DMRP. Our data suggest that this is achieved by a significant increase in the coupling of ATP hydrolysis to transport.-Karasik, A., Ledwitch, K. V., Arányi, T., Váradi, A., Roberts, A., Szeri, F. Boosted coupling of ATP hydrolysis to substrate transport upon cooperative estradiol-17-β-D-glucuronide binding in a Drosophila ATP binding cassette type-C transporter.

The roles of cytochrome P450 and P-glycoprotein in the pharmacokinetics of florfenicol in chickens

The effects of three selective oral inhibitors, fluvoxamine (FLU), ketoconazole (KET), and verapamil (VER), on the pharmacokinetics (PK) of florfenicol (FFC) were investigated in chickens. The chickens were administered orally with saline solution (SAL), FLU (60 mg/kg), KET (25 mg/kg), or VER (9 mg/kg) for 7 consecutive days. Florfenicol was given to the chickens at a single dose of 30 mg/kg orally. Blood samples were collected from each chicken at 0 to 12 h post-administration of FFC. The plasma concentration of FFC was analyzed by high-performance liquid chromatography (HPLC). The AUC of FFC increased and the CL_s of FFC decreased with oral co-administration of KET in chickens, and the C_max of FFC increased with VER. While the AUC, the CL_s and the C_max of FFC were all invariable with FLU. These data suggested that CYP 3A played a key role in the PK of FFC in chickens, however, P-glycoprotein (P-gp) and CYP 1A did not. The results imply that the adverse drug-drug interaction may occur in the use of FFC if the co-administrated drugs are the substrates, inducers or inhibitors of CYP 3A or/and P-gp.

LUCS (Light-Up Cell System), a universal high throughput assay for homeostasis evaluation in live cells

Observations of fluorescent cyanine dye behavior under illumination at 500 nm lead to a novel concept in cell biology allowing the development of a new live cell assay called LUCS, for Light-Up Cell System, measuring homeostasis in live cells. Optimization of the LUCS process resulted in a standardized, straightforward and high throughput assay with applications in toxicity assessment. The mechanisms of the LUCS process were investigated. Electron Paramagnetic Resonance experiments showed that the singlet oxygen and hydroxyl radical are involved downstream of the light effect, presumably leading to deleterious oxidative stress that massively opens access of the dye to its intracellular target. Reversible modulation of LUCS by both verapamil and proton availability indicated that plasma membrane proton/cation antiporters, possibly of the MATE drug efflux transport family, are involved. A mechanistic model is presented. Our data show that intracellular oxidation can be controlled by tuning light energy, opening applications in regulatory purposes, anti-oxidant research, chemotherapy efficacy and dynamic phototherapy strategies.

Drug absorption related nephrotoxicity assessment on an intestine-kidney chip

Drug absorption in the intestine is tightly related to drug-induced nephrotoxicity, which is a relatively common side effect in clinical practice. It highlights a great need to develop predictive models with high accuracy in the early stage during new drug discovery and development. Herein, we presented a novel intestine-kidney chip, which recapitulated drug absorption in the intestine and its resultant drug toxicity on the kidney. This work aims to provide an integrated tool for accurate assessment of drug absorption-related nephrotoxicity in vitro. A microfluidic device with multi-interfaces was designed, which facilitated the co-culture of the intestinal and glomerular endothelial cells in compartmentalized micro-chambers. Thus, drug absorption and following nephrotoxicity could be explored in a single assay based on the formation of the intact intestine function on the chip. Specifically, we adopt digoxin (DIG) as a model drug combined with colestyramine (COL) or Verapamil (VER), which significantly influence DIG absorption in the intestine. Different degrees of nephrotoxicity under drug combinations were further observed on the chip, including cell apoptosis, cell viability, and lactate dehydrogenase leakage. These features were consistent with the variance of DIG absorption by the intestinal cells. In agreement with clinical observations, our data demonstrated that DIG-induced nephrotoxicity was enhanced combined with VER but weakened with COL. All of these findings suggest that the established microdevice might provide a useful and cost-effective platform in vitro for testing drug absorption and nephrotoxicity in preclinical trials during new drug development.

Insights Into the Molecular Mechanism of Triptan Transport by P-glycoprotein

The P-glycoprotein (Pgp) transporter reduces the penetration of a chemically diverse range of neurotherapeutics at the blood-brain barrier, but the molecular features of drugs and drug-Pgp interactions that drive transport remain to be clarified. In particular, the triptan neurotherapeutics, eletriptan (ETT) and sumatriptan (STT), were identified to have a >10-fold difference in transport rates despite being from the same drug class. Consistent with these transport differences, ETT activated Pgp-mediated ATP hydrolysis ∼2-fold, whereas STT slightly inhibited Pgp-mediated ATP hydrolysis by ∼10%. The interactions between them were also noncompetitive, suggesting that they occupy different binding sites on the transporter. Despite these differences, protein fluorescence spectroscopy revealed that the drugs have similar affinity to the transporter. NMR with Pgp and the drugs showed that they have distinct interactions with the transporter. Tertiary conformational changes probed by acrylamide quenching of Pgp tryptophan fluorescence with the drugs and a nonhydrolyzable ATP analog implied that the STT-bound Pgp must undergo larger conformational changes to hydrolyze ATP than ETT-bound Pgp. These results and previous transport studies were used to build a conformationally driven model for triptan transport with Pgp where STT presents a higher conformational barrier for ATP hydrolysis and transport than ETT.

Cardiovascular Ion Channel Inhibitor Drug-Drug Interactions with P-glycoprotein

P-glycoprotein (Pgp) is an ATP-binding cassette (ABC) transporter that plays a major role in cardiovascular drug disposition by effluxing a chemically and structurally diverse range of cardiovascular therapeutics. Unfortunately, drug-drug interactions (DDIs) with the transporter have become a major roadblock to effective cardiovascular drug administration because they can cause adverse drug reactions (ADRs) or reduce the efficacy of drugs. Cardiovascular ion channel inhibitors are particularly susceptible to DDIs and ADRs with Pgp because they often have low therapeutic indexes and are commonly coadministered with other drugs that are also Pgp substrates. DDIs from cardiovascular ion channel inhibitors with the transporter occur because of inhibition or induction of the transporter and the transporter's tissue and cellular localization. Inhibiting Pgp can increase absorption and reduce excretion of drugs, leading to elevated drug plasma concentrations and drug toxicity. In contrast, inducing Pgp can have the opposite effect by reducing the drug plasma concentration and its efficacy. A number of in vitro and in vivo studies have already demonstrated DDIs from several cardiovascular ion channel inhibitors with human Pgp and its animal analogs, including verapamil, digoxin, and amiodarone. In this review, Pgp-mediated DDIs and their effects on pharmacokinetics for different categories of cardiovascular ion channel inhibitors are discussed. This information is essential for improving pharmacokinetic predictions of cardiovascular therapeutics, for safer cardiovascular drug administration and for mitigating ADRs emanating from Pgp.

Cooperativity between verapamil and ATP bound to the efflux transporter P-glycoprotein

The P-glycoprotein (Pgp) transporter plays a central role in drug disposition by effluxing a chemically diverse range of drugs from cells through conformational changes and ATP hydrolysis. A number of drugs are known to activate ATP hydrolysis of Pgp, but coupling between ATP and drug binding is not well understood. The cardiovascular drug verapamil is one of the most widely studied Pgp substrates and therefore, represents an ideal drug to investigate the drug-induced ATPase activation of Pgp. As previously noted, verapamil-induced Pgp-mediated ATP hydrolysis kinetics was biphasic at saturating ATP concentrations. However, at subsaturating ATP concentrations, verapamil-induced ATPase activation kinetics became monophasic. To further understand this switch in kinetic behavior, the Pgp-coupled ATPase activity kinetics was checked with a panel of verapamil and ATP concentrations and fit with the substrate inhibition equation and the kinetic fitting software COPASI. The fits suggested that cooperativity between ATP and verapamil switched between low and high verapamil concentration. Fluorescence spectroscopy of Pgp revealed that cooperativity between verapamil and a non-hydrolyzable ATP analog leads to distinct global conformational changes of Pgp. NMR of Pgp reconstituted in liposomes showed that cooperativity between verapamil and the non-hydrolyzable ATP analog modulate each other's interactions. This information was used to produce a conformationally-gated model of drug-induced activation of Pgp-mediated ATP hydrolysis.

Intracellular Unbound Atorvastatin Concentrations in the Presence of Metabolism and Transport

Accurate prediction of drug target activity and rational dosing regimen design require knowledge of drug concentrations at the target. It is important to understand the impact of processes such as membrane permeability, partitioning, and active transport on intracellular drug concentrations. The present study aimed to predict intracellular unbound atorvastatin concentrations and characterize the effect of enzyme-transporter interplay on these concentrations. Single-pass liver perfusion studies were conducted in rats using atorvastatin (ATV, 1 µM) alone at 4°C and at 37°C in presence of rifampin (RIF, 20 µM) and 1-aminobenzotriazole (ABT, 1 mM), separately and in combination. The unbound intracellular ATV concentration was predicted with a five-compartment explicit membrane model using the parameterized diffusional influx clearance, active basolateral uptake clearance, and metabolic clearance. Chemical inhibition of uptake and metabolism at 37°C proved to be better controls relative to studies at 4°C. The predicted unbound intracellular concentration at the end of the 50-minute perfusion in the +ABT , +ABT+RIF, and the ATV-only groups was 6.5 µM, 0.58 µM, and 5.14 µM, respectively. The predicted total liver concentrations and amount recovered in bile were within 0.94-1.3 fold of the observed value in all groups. The fold difference in total liver concentration did not always extrapolate to the fold difference in predicted unbound concentration across groups. Together, these results support the use of compartmental modeling to predict intracellular concentrations in dynamic organ-based systems. These predictions can provide insight into the role of uptake transporters and metabolizing enzymes in determining drug tissue concentrations.