Interaction of the efflux transporters ABCB1 and ABCG2 with imatinib, nilotinib, and dasatinib

Dorsomorphin attenuates ABCG2-mediated multidrug resistance in colorectal cancer

Colorectal cancer is a common malignant tumor with high mortality, for which chemotherapy resistance is one of the main reasons. The high expression of ABCG2 in the cancer cells and expulsion of anticancer drugs directly cause multidrug resistance (MDR). Therefore, the development of new ABCG2 inhibitors that block the active causes of MDR may provide a strategy for the treatment of colorectal cancer. In this study, we find that dorsomorphin (also known as compound C or BML-275) potently inhibits the transporter activity of ABCG2, thereby preserving the chemotherapeutic agents mitoxantrone and doxorubicin to antagonize MDR in ABCG2-overexpressing colorectal cancer cells. Additionally, dorsomorphin does not alter ABCG2 protein expression. The results of molecular docking studies show that dorsomorphin is bound stably to the ABCG2-binding pocket, suggesting that dorsomorphin is a potent ABCG2 inhibitor that attenuates ABCG2-mediated MDR in colorectal cancer.

Overexpression of ABCC1 and ABCG2 confers resistance to talazoparib, a poly (ADP-Ribose) polymerase inhibitor

AIMS

The overexpression of ABC transporters on cancer cell membranes is one of the most common causes of multidrug resistance (MDR). This study investigates the impact of ABCC1 and ABCG2 on the resistance to talazoparib (BMN-673), a potent poly (ADP-ribose) polymerase (PARP) inhibitor, in ovarian cancer treatment.

METHODS

The cell viability test was used to indicate the effect of talazoparib in different cell lines. Computational molecular docking analysis was conducted to simulate the interaction between talazoparib and ABCC1 or ABCG2. The mechanism of talazoparib resistance was investigated by constructing talazoparib-resistant subline A2780/T4 from A2780 through drug selection with gradually increasing talazoparib concentration.

RESULTS

Talazoparib cytotoxicity decreased in drug-selected or gene-transfected cell lines overexpressing ABCC1 or ABCG2 but can be restored by ABCC1 or ABCG2 inhibitors. Talazoparib competitively inhibited substrate drug efflux activity of ABCC1 or ABCG2. Upregulated ABCC1 and ABCG2 protein expression on the plasma membrane of A2780/T4 cells enhances resistance to other substrate drugs, which could be overcome by the knockout of either gene. In vivo experiments confirmed the retention of drug-resistant characteristics in tumor xenograft mouse models.

CONCLUSIONS

The therapeutic efficacy of talazoparib in cancer may be compromised by its susceptibility to MDR, which is attributed to its interactions with the ABCC1 or ABCG2 transporters. The overexpression of these transporters can potentially diminish the therapeutic impact of talazoparib in cancer treatment.

Physiologically Based Pharmacokinetic Modeling of Nilotinib for Drug-Drug Interactions, Pediatric Patients, and Pregnancy and Lactation

Nilotinib is a second-generation BCR-ABL tyrosine kinase inhibitor for the treatment of Philadelphia chromosome-positive chronic myeloid leukemia in both adult and pediatric patients. The pharmacokinetics (PK) of nilotinib in specific populations such as pregnant and lactating people remain poorly understood. Therefore, the objectives of the current study were to develop a physiologically based pharmacokinetic (PBPK) model to predict nilotinib PK in virtual drug-drug interaction (DDI) studies, as well as in pediatric, pregnant, and lactating populations. The nilotinib PBPK model was built in PK-Sim, which is part of the free and open-source software Open Systems Pharmacology. The observed clinical data for the validation of the nilotinib models were obtained from the literature. The model reasonably predicted nilotinib concentrations in the adult population; the DDIs between nilotinib and rifampin or ketoconazole in the adult population; and the PK in the pediatric, pregnant, and lactating populations, although in the latter 2 populations plasma concentrations were slightly underestimated. The ratio of predicted versus observed PK parameters for the adult model ranged from 0.71 to 1.11 for area under the concentration-time curve and 0.55 to 0.95 for maximum concentration. For the DDI, the predicted area under the concentration-time curve ratio and maximum concentration ratio fell within the Guest criterion. The current study demonstrated the utility of using PBPK modeling to understand the mechanistic basis of PK differences between adults and specific populations, such as pediatrics, and pregnant and lactating individuals, indicating that this technology can potentially inform or optimize dosing conditions in specific populations.

Current Progress on the Influence Human Genetics Has on the Efficacy of Tyrosine Kinase Inhibitors Used to Treat Chronic Myeloid Leukemia

The use of tyrosine kinase inhibitors (TKIs) has become the mainstay of treatment in patients suffering from chronic myeloid leukemia (CML), an adult leukemia caused by a reciprocal translocation between chromosomes 9 and 22, which creates an oncogene resulting in a myeloproliferative neoplasm. These drugs function by inhibiting the ATP-binding site on the fusion oncoprotein and subsequently halting proliferative activity. The goal of this work is to investigate the current state of research into genetic factors that influence the efficacy of four FDA-approved TKIs used to treat CML. This overview attempts to identify genetic criteria that could be considered when choosing one drug over the others and to identify where more research is needed. Our results suggest that the usual liver enzymes impacting patient response may not be a major factor affecting the efficacy of imatinib, nilotinib, and bosutinib, and yet, that is where most of the past research has focused. More research is warranted on the impact that human polymorphisms of the CYP enzymes have on dasatinib. The impact of polymorphisms in UGT1A1 should be investigated thoroughly in other TKIs, not only nilotinib. The role of influx and efflux transporters has been inconsistent thus far, possibly due to failures to account for the multiple proteins that can transport TKIs and the impact that tumors have on transporter expression. Because physicians cannot currently use a patient's genetic profile to better target their treatment with TKIs, it is critical that more research be conducted on auxiliary pathways or off-target binding effects to generate new leads for further study. Hopefully, new avenues of research will help explain treatment failures and improve patient outcomes.

Structure-ATPase Activity Relationship of Rhodamine Derivatives as Potent Inhibitors of P-Glycoprotein CmABCB1

Understanding the transport and inhibition mechanisms of substrates by P-glycoprotein (P-gp) is one of the important approaches in addressing multidrug resistance (MDR). In this study, we evaluated a variety of rhodamine derivatives as potential P-gp inhibitors targeting CmABCB1, a P-gp homologue, with a focus on their ATPase activity. Notably, a Q-rhodamine derivative with an o,o'-dimethoxybenzyl ester moiety (RhQ-DMB) demonstrated superior affinity and inhibitory activity, which was further confirmed by a drug susceptibility assay in yeast strains expressing CmABCB1. Results from a tryptophan fluorescence quenching experiment using a CmABCB1 mutant suggested that RhQ-DMB effectively enters and binds to the inner chamber of CmABCB1. These findings underscore the promising potential of RhQ-DMB as a tool for future studies aimed at elucidating the substrate-bound state of CmABCB1.

Transporter-Mediated Cellular Distribution of Tyrosine Kinase Inhibitors as a Potential Resistance Mechanism in Chronic Myeloid Leukemia

Chronic myeloid leukemia (CML) is a hematologic neoplasm characterized by the expression of the BCR::ABL1 oncoprotein, a constitutively active tyrosine kinase, resulting in uncontrolled growth and proliferation of cells in the myeloid lineage. Targeted therapy using tyrosine kinase inhibitors (TKIs) such as imatinib, nilotinib, dasatinib, bosutinib, ponatinib and asciminib has drastically improved the life expectancy of CML patients. However, treatment resistance occurs in 10-20% of CML patients, which is a multifactorial problem that is only partially clarified by the presence of TKI inactivating BCR::ABL1 mutations. It may also be a consequence of a reduction in cytosolic TKI concentrations in the target cells due to transporter-mediated cellular distribution. This review focuses on drug-transporting proteins in stem cells and progenitor cells involved in the distribution of TKIs approved for the treatment of CML. Special attention will be given to ATP-binding cassette transporters expressed in lysosomes, which may facilitate the extracytosolic sequestration of these compounds.

Adverse outcomes for chronic myeloid leukemia patients with splenomegaly and low in vivo kinase inhibition on imatinib

Variability in the molecular response to frontline tyrosine kinase inhibitor (TKI) therapy in chronic myeloid leukemia may be partially driven by differences in the level of kinase inhibition induced. We measured in vivo BCR::ABL1 kinase inhibition (IVKI) in circulating mononuclear cells after 7 days of therapy. In 173 patients on imatinib 600 mg/day, 23% had low IVKI (<11% reduction in kinase activity from baseline); this was associated with higher rates of early molecular response (EMR) failure; lower rates of major molecular response (MMR), and MR4.5 by 36 months, compared to high IVKI patients. Low IVKI was more common (39%) in patients with large spleens (≥10 cm by palpation). Notably 55% of patients with large spleens and low IVKI experienced EMR failure whereas the EMR failure rate in patients with large spleens and high IVKI was only 12% (p = 0.014). Furthermore, patients with large spleen and low IVKI had a higher incidence of blast crisis, inferior MMR, MR4.5, and event-free survival compared to patients with large spleen and high IVKI and remaining patients. In nilotinib-treated patients (n = 73), only 4% had low IVKI. The combination of low IVKI and large spleen is associated with markedly inferior outcomes and interventions in this setting warrant further studies.

The acquisition order of leukemic drug resistance mutations is directed by the selective fitness associated with each resistance mechanism

In Chronic Myeloid Leukemia, the transition from drug sensitive to drug resistant disease is poorly understood. Here, we used exploratory sequencing of gene transcripts to determine the mechanisms of drug resistance in a dasatinib resistant cell line model. Importantly, cell samples were collected sequentially during drug exposure and dose escalation, revealing several resistance mechanisms which fluctuated over time. BCR::ABL1 overexpression, BCR::ABL1 kinase domain mutation, and overexpression of the small molecule transporter ABCG2, were identified as dasatinib resistance mechanisms. The acquisition of mutations followed an order corresponding with the increase in selective fitness associated with each resistance mechanism. Additionally, it was demonstrated that ABCG2 overexpression confers partial ponatinib resistance. The results of this study have broad applicability and help direct effective therapeutic drug usage and dosing regimens and may be useful for clinicians to select the most efficacious therapy at the most beneficial time.

Knowledge graph aids comprehensive explanation of drug and chemical toxicity

In computational toxicology, prediction of complex endpoints has always been challenging, as they often involve multiple distinct mechanisms. State-of-the-art models are either limited by low accuracy, or lack of interpretability due to their black-box nature. Here, we introduce AIDTox, an interpretable deep learning model which incorporates curated knowledge of chemical-gene connections, gene-pathway annotations, and pathway hierarchy. AIDTox accurately predicts cytotoxicity outcomes in HepG2 and HEK293 cells. It also provides comprehensive explanations of cytotoxicity covering multiple aspects of drug activity, including target interaction, metabolism, and elimination. In summary, AIDTox provides a computational framework for unveiling cellular mechanisms for complex toxicity endpoints.

Impact of ABCG2 and ABCB1 Polymorphisms on Imatinib Plasmatic Exposure: An Original Work and Meta-Analysis

Adequate imatinib plasma levels are necessary to guarantee an efficacious and safe treatment in gastrointestinal stromal tumor (GIST) and chronic myeloid leukemia (CML) patients. Imatinib is a substrate of the drug transporters ATP-binding cassette subfamily B member 1 (ABCB1) and ATP-binding cassette subfamily G member 2 (ABCG2) that can affect its plasma concentration. In the present study, the association between three genetic polymorphisms in ABCB1 (rs1045642, rs2032582, rs1128503) and one in ABCG2 (rs2231142) and the imatinib plasma trough concentration (C_trough) was investigated in 33 GIST patients enrolled in a prospective clinical trial. The results of the study were meta-analyzed with those of other seven studies (including a total of 649 patients) selected from the literature through a systematic review process. The ABCG2 c.421C>A genotype demonstrated, in our cohort of patients, a borderline association with imatinib plasma trough levels that became significant in the meta-analysis. Specifically, homozygous carriers of the ABCG2 c.421 A allele showed higher imatinib plasma C_trough with respect to the CC/CA carriers (C_trough, 1463.2 ng/mL AA, vs. 1196.6 ng/mL CC + AC, p = 0.04) in 293 patients eligible for the evaluation of this polymorphism in the meta-analysis. The results remained significant under the additive model. No significant association could be described between ABCB1 polymorphisms and imatinib C_trough, neither in our cohort nor in the meta-analysis. In conclusion, our results and the available literature studies sustain an association between ABCG2 c.421C>A and imatinib plasma C_trough in GIST and CML patients.

A Structure-Based View on ABC-Transporter Linked to Multidrug Resistance

The discovery of the first ATP-binding cassette (ABC) transporter, whose overexpression in cancer cells is responsible for exporting anticancer drugs out of tumor cells, initiated enormous efforts to overcome tumor cell multidrug resistance (MDR) by inhibition of ABC-transporter. Because of its many physiological functions, diverse studies have been conducted on the mechanism, function and regulation of this important group of transmembrane transport proteins. In this review, we will focus on the structural aspects of this transporter superfamily. Since the resolution revolution of electron microscope, experimentally solved structures increased rapidly. A summary of the structures available and an overview of recent structure-based studies are provided. More specifically, the artificial intelligence (AI)-based predictions from AlphaFold-2 will be discussed.

ERBB and P-glycoprotein inhibitors break resistance in relapsed neuroblastoma models through P-glycoprotein

Chemotherapy resistance is a persistent clinical problem in relapsed high-risk neuroblastomas. We tested a panel of 15 drugs for sensitization of neuroblastoma cells to the conventional chemotherapeutic vincristine, identifying tariquidar, an inhibitor of the transmembrane pump P-glycoprotein (P-gp/ABCB1), and the ERBB family inhibitor afatinib as the top resistance breakers. Both compounds were efficient in sensitizing neuroblastoma cells to vincristine in trypan blue exclusion assays and in inducing apoptotic cell death. The evaluation of ERBB signaling revealed no functional inhibition, that is, dephosphorylation of the downstream pathways upon afatinib treatment but direct off-target interference with P-gp function. Depletion of ABCB1, but not ERRB4, sensitized cells to vincristine treatment. P-gp inhibition substantially broke vincristine resistance in vitro and in vivo (zebrafish embryo xenograft). The analysis of gene expression datasets of more than 50 different neuroblastoma cell lines (primary and relapsed) and more than 160 neuroblastoma patient samples from the pediatric precision medicine platform INFORM (Individualized Therapy For Relapsed Malignancies in Childhood) confirmed a pivotal role of P-gp specifically in neuroblastoma resistance at relapse, while the ERBB family appears to play a minor part.

Role of Drug Transporters in Elucidating Inter-Individual Variability in Pediatric Chemotherapy-Related Toxicities and Response

Pediatric cancer treatment has evolved significantly in recent decades. The implementation of risk stratification strategies and the selection of evidence-based chemotherapy combinations have improved survival outcomes. However, there is large interindividual variability in terms of chemotherapy-related toxicities and, sometimes, the response among this population. This variability is partly attributed to the functional variability of drug-metabolizing enzymes (DME) and drug transporters (DTS) involved in the process of absorption, distribution, metabolism and excretion (ADME). The DTS, being ubiquitous, affects drug disposition across membranes and has relevance in determining chemotherapy response in pediatric cancer patients. Among the factors affecting DTS function, ontogeny or maturation is important in the pediatric population. In this narrative review, we describe the role of drug uptake/efflux transporters in defining pediatric chemotherapy-treatment-related toxicities and responses. Developmental differences in DTS and the consequent implications are also briefly discussed for the most commonly used chemotherapeutic drugs in the pediatric population.

PBK/TOPK inhibitor OTS964 resistance is mediated by ABCB1-dependent transport function in cancer: in vitro and in vivo study

ABCB1 overexpression significantly desensitized both drug-selected and gene-transfected cells, which overexpress ABCB1, to OTS964 and that this drug resistance can be antagonized by verapamil, a known ABCB1 inhibitor. Consistently, a similar trend was observed in tumor-bearing mice.

OTS964 stimulated ATPase activity of ABCB1 and upregulated expression levels of ABCB1, resulting in induced resistance to other ABCB1 substrate-drugs, such as paclitaxel.

OTS964 received a comparable affinity score and can dock into the substrate-binding site of human ABCB1 protein.

A new porphyrin as selective substrate-based inhibitor of breast cancer resistance protein (BCRP/ABCG2)

The ABCG2 transporter plays a pivotal role in multidrug resistance, however, no clinical trial using specific ABCG2 inhibitors have been successful. Although ABC transporters actively extrude a wide variety of substrates, photodynamic therapeutic agents with porphyrinic scaffolds are exclusively transported by ABCG2. In this work, we describe for the first time a porphyrin derivative (4B) inhibitor of ABCG2 and capable to overcome multidrug resistance in vitro. The inhibition was time-dependent and 4B was not itself transported by ABCG2. Independently of the substrate, the porphyrin 4B showed an IC₅₀ value of 1.6 μM and a mixed type of inhibition. This compound inhibited the ATPase activity and increased the binding of the conformational-sensitive antibody 5D3. A thermostability assay confirmed allosteric protein changes triggered by the porphyrin. Long-timescale molecular dynamics simulations revealed a different behavior between the ABCG2 porphyrinic substrate pheophorbide a and the porphyrin 4B. Pheophorbide a was able to bind in three different protein sites but 4B showed one binding conformation with a strong ionic interaction with GLU446. The inhibition was selective toward ABCG2, since no inhibition was observed for P-glycoprotein and MRP1. Finally, this compound successfully chemosensitized cells that overexpress ABCG2. These findings reinforce that substrates may be a privileged source of chemical scaffolds for identification of new inhibitors of multidrug resistance-linked ABC transporters.

Resistance to Tyrosine Kinase Inhibitors in Chronic Myeloid Leukemia-From Molecular Mechanisms to Clinical Relevance

Simple Summary

Chronic myeloid leukemia (CML) is a myeloproliferative neoplasia associated with a molecular alteration, the fusion gene BCR-ABL1, that encodes the tyrosine kinase oncoprotein BCR-ABL1. This led to the development of tyrosine kinase inhibitors (TKI), with Imatinib being the first TKI approved. Although the vast majority of CML patients respond to Imatinib, resistance to this targeted therapy contributes to therapeutic failure and relapse. Here we review the molecular mechanisms and other factors (e.g., patient adherence) involved in TKI resistance, the methodologies to access these mechanisms, and the possible therapeutic approaches to circumvent TKI resistance in CML.

Abstract

Resistance to targeted therapies is a complex and multifactorial process that culminates in the selection of a cancer clone with the ability to evade treatment. Chronic myeloid leukemia (CML) was the first malignancy recognized to be associated with a genetic alteration, the t(9;22)(q34;q11). This translocation originates the BCR-ABL1 fusion gene, encoding the cytoplasmic chimeric BCR-ABL1 protein that displays an abnormally high tyrosine kinase activity. Although the vast majority of patients with CML respond to Imatinib, a tyrosine kinase inhibitor (TKI), resistance might occur either de novo or during treatment. In CML, the TKI resistance mechanisms are usually subdivided into BCR-ABL1-dependent and independent mechanisms. Furthermore, patients’ compliance/adherence to therapy is critical to CML management. Techniques with enhanced sensitivity like NGS and dPCR, the use of artificial intelligence (AI) techniques, and the development of mathematical modeling and computational prediction methods could reveal the underlying mechanisms of drug resistance and facilitate the design of more effective treatment strategies for improving drug efficacy in CML patients. Here we review the molecular mechanisms and other factors involved in resistance to TKIs in CML and the new methodologies to access these mechanisms, and the therapeutic approaches to circumvent TKI resistance.

Prospective Drug Candidates as Human Multidrug Transporter ABCG2 Inhibitors: an In Silico Drug Discovery Study

Breast cancer resistance protein (ABCG2) is a human ATP-binding cassette (ABC) that plays a paramount role in multidrug resistance (MDR) in cancer therapy. The discovery of ABCG2 inhibitors could assist in designing unprecedented therapeutic strategies for cancer treatment. There is as yet no approved drug targeting ABCG2, although a large number of drug candidates have been clinically investigated. In this work, binding affinities of 181 drug candidates in clinical-trial or investigational stages as ABCG2 inhibitors were inspected using in silico techniques. Based on available experimental data, the performance of AutoDock4.2.6 software was first validated to predict the inhibitor-ABCG2 binding mode and affinity. Combined molecular docking calculations and molecular dynamics (MD) simulations, followed by molecular mechanics-generalized Born surface area (MM-GBSA) binding energy calculations, were then performed to filter out the studied drug candidates. From the estimated docking scores and MM-GBSA binding energies, six auspicious drug candidates-namely, pibrentasvir, venetoclax, ledipasvir, avatrombopag, cobicistat, and revefenacin-exhibited auspicious binding energies with value < -70.0 kcal/mol. Interestingly, pibrentasvir, venetoclax, and ledipasvir were observed to show even higher binding affinities with the ABCG2 transporter with binding energies of < -80.0 kcal/mol over long MD simulations of 100 ns. The stabilities of these three promising candidates in complex with ABCG2 transporter were demonstrated by their energetics and structural analyses throughout the 100 ns MD simulations. The current study throws new light on pibrentasvir, venetoclax, and ledipasvir as curative options for multidrug resistant cancers by inhibiting ABCG2 transporter.

Machine Learning Approaches to Predict Hepatotoxicity Risk in Patients Receiving Nilotinib

Background: Although nilotinib hepatotoxicity can cause severe clinical conditions and may alter treatment plans, risk factors affecting nilotinib-induced hepatotoxicity have not been investigated. This study aimed to elucidate the factors affecting nilotinib-induced hepatotoxicity. Methods: This retrospective cohort study was performed on patients using nilotinib from July of 2015 to June of 2020. We estimated the odds ratio and adjusted odds ratio from univariate and multivariate analyses, respectively. Several machine learning models were developed to predict risk factors of hepatotoxicity occurrence. The area under the curve (AUC) was analyzed to assess clinical performance. Results: Among 353 patients, the rate of patients with grade I or higher hepatotoxicity after nilotinib administration was 40.8%. Male patients and patients who received nilotinib at a dose of ≥300 mg had a 2.3-fold and a 3.5-fold increased risk for hepatotoxicity compared to female patients and compared with those who received <300 mg, respectively. H2 blocker use decreased hepatotoxicity by 11.6-fold. The area under the curve (AUC) values of machine learning methods ranged between 0.61–0.65 in this study. Conclusion: This study suggests that the use of H2 blockers was a reduced risk of nilotinib-induced hepatotoxicity, whereas male gender and a high dose were associated with increased hepatotoxicity.

Anlotinib suppresses metastasis and multidrug resistance via dual blockade of MET/ABCB1 in colorectal carcinoma cells

Anlotinib, a highly selective multi-targeted tyrosine kinase inhibitor (TKI) has therapeutic effects on non-small-cell lung cancer (NSCLC). In this study, the anti-tumor activity and molecular mechanism of anlotinib in metastatic colorectal cancer (mCRC) was explored. The anti-angiogenesis, anti-metastasis, anti-proliferative, and anti-multidrug resistance efficacy of anlotinib were analyzed by using in vitro and in vivo models of human CRC cells. The results indicated that anlotinib boosted chemo-sensitivity of CRC cells, and restrained its proliferation. Besides the suppression of the MET signaling pathway, anlotinib also inhibited invasion and migration of CRC cells. Furthermore, anlotinib prevented VEGF-induced angiogenesis, N-cadherin (CDH2)-induced cell migration, and reversed ATP-binding cassette subfamily B member 1 (ABCB1) -mediated CRC multidrug resistance in CRC. The CRC liver metastasis and subcutaneously implanted xenograft model testified that anlotinib could inhibit proliferation and liver metastasis in CRC cells. Such an observation suggested that a combination of anlotinib with anti-cancer drugs could attenuate angiogenesis, metastasis, proliferative, and multidrug resistance, which constitutes a novel treatment strategy for CRC patients with metastasis.

Gonçalves C, P. Martins E, Miranda-Lorenzo I, T. Cerqueira M, Longatto-Filho A, A. Pinto A, L. Reis R, Sousa N, Heeschen C, M. Costa B. Intracellular Autofluorescence as a New Biomarker for Cancer Stem Cells in Glioblastoma

The identification of cancer stem cells (CSCs), which are implicated in tumor initiation, progression, therapy resistance, and relapse, is of great biological and clinical relevance. In glioblastoma (GBM), this is still a challenge, as no single marker is able to universally identify populations of GBM cancer stem cells (GSCs). Indeed, there is still controversy on whether biomarker-expressing cells fulfill the functional criteria of bona fide GSCs, despite being widely used. Here, we describe a novel subpopulation of autofluorescent (Fluo+) cells in GBM that bear all the functional characteristics of GSCs, including higher capacity to grow as neurospheres, long-term self-renewal ability, increased expression of stem cell markers, and enhanced in vivo tumorigenicity. Mechanistically, the autofluorescent phenotype is largely due to the intracellular accumulation of riboflavin, mediated by the ABC transporter ABCG2. In summary, our work identifies an intrinsic cellular autofluorescent phenotype enriched in GBM cells with functional stem cells features that can be used as a novel, simple and reliable biomarker to target these highly malignant tumors, with implications for GBM biological and clinical research.

OTS964, a TOPK Inhibitor, Is Susceptible to ABCG2-Mediated Drug Resistance

OTS964 is a potent T-LAK cell-originated protein kinase (TOPK) inhibitor. Herein, we investigated the interaction of OTS964 and multidrug resistance (MDR)-associated ATP-binding cassette sub-family G member 2 (ABCG2). The cell viability assay indicated that the effect of OTS964 is limited in cancer drug-resistant and transfected cells overexpressing ABCG2. We found that the known ABCG2 transporter inhibitor has the ability to sensitize ABCG2-overexpressing cells to OTS964. In mechanism-based studies, OTS964 shows inhibitory effect on the efflux function mediated by ABCG2, and in turn, affects the pharmacokinetic profile of other ABCG2 substrate-drugs. Furthermore, OTS964 upregulates ABCG2 protein expression, resulting in enhanced resistance to ABCG2 substrate-drugs. The ATPase assay demonstrated that OTS964 stimulates ATPase activity of ABCG2 in a concentration-dependent manner. The computational molecular docking analysis combined with results from ATPase assay suggested that OTS964 interacts with drug-binding pocket of ABCG2 and has substrate-like behaviors. Thus, OTS964 is an MDR-susceptible agent due to its interactions with ABCG2, and overexpression of ABCG2 transporter may attenuate its therapeutic effect in cancer cells.

Mechanisms of the Cardiac Myocyte-Damaging Effects of Dasatinib

The anticancer drug dasatinib (Sprycel) is a BCR-ABL1-targeted tyrosine kinase inhibitor used in treating chronic myelogenous leukemia that has been shown in clinical trials to display cardiovascular toxicities. While dasatinib potently inhibits BCR-ABL1, it is not a highly selective kinase inhibitor and may have off-target effects. A neonatal rat cardiac myocyte model was used to investigate potential mechanisms by which dasatinib damaged myocytes. The anthracycline cardioprotective drug dexrazoxane was shown to be ineffective in preventing dasatinib-induced myocyte damage. Dasatinib treatment increased doxorubicin accumulation in myocytes and doxorubicin-induced myocyte damage, likely through its ability to bind to one or more ABC-type efflux transporters. Dasatinib induced myocyte damage either after a brief treatment that mimicked the clinical situation, or more potently after continuous treatment. Dasatinib slightly induced apoptosis in myocytes as evidenced by increases in caspase-3/7 activity. Dasatinib treatment reduced pERK levels in myocytes most likely through inhibition of RAF, which dasatinib strongly inhibits. Thus, inhibition of the RAF/MEK/ERK pro-survival pathway in the heart may be, in part, a mechanism by which dasatinib induces cardiovascular toxicity.

Physiological Roles of ERM Proteins and Transcriptional Regulators in Supporting Membrane Expression of Efflux Transporters as Factors of Drug Resistance in Cancer

One factor contributing to the malignancy of cancer cells is the acquisition of drug resistance during chemotherapy via increased expression of efflux transporters, such as P-glycoprotein (P-gp), multidrug resistance-associated proteins (MRPs), and breast cancer resistance protein (BCRP). These transporters operate at the cell membrane, and are anchored in place by the scaffold proteins ezrin (Ezr), radixin (Rdx), and moesin (Msn) (ERM proteins), which regulate their functional activity. The identity of the regulatory scaffold protein(s) differs depending upon the transporter, and also upon the tissue in which it is expressed, even for the same transporter. Another factor contributing to malignancy is metastatic ability. Epithelial-mesenchymal transition (EMT) is the first step in the conversion of primary epithelial cells into mesenchymal cells that can be transported to other organs via the blood. The SNAI family of transcriptional regulators triggers EMT, and SNAI expression is used is an indicator of malignancy. Furthermore, EMT has been suggested to be involved in drug resistance, since drug excretion from cancer cells is promoted during EMT. We showed recently that ERM proteins are induced by a member of the SNAI family, Snail. Here, we first review recent progress in research on the relationship between efflux transporters and scaffold proteins, including the question of tissue specificity. In the second part, we review the relationship between ERM scaffold proteins and the transcriptional regulatory factors that induce their expression.

The Hidden Pathogenesis of CML: Is BCR-ABL1 the First Event?

PURPOSE OF REVIEW

Identification of the BCR-ABL1 fusion oncogene in patients diagnosed with chronic myeloid leukemia (CML) led to the development of targeted therapy responsible for the dramatic survival benefits observed in the past two decades. However, despite these revolutionary findings, there remains marked disparity in patient outcomes. Why do some patients present de novo while others evolve to the more aggressive stages of CML? Why can select patients successfully discontinue therapy as part of a treatment-free remission attempt whereas others fail to meet specific molecular milestones?

RECENT FINDINGS

BCR-ABL1 kinase mutations are only identified in approximately 50% of patients with poor responses and disease progression, suggesting the presence of alternative resistance mechanisms. Numerous institutions have identified the presence of additional genomic events in addition to BCR-ABL1 with the increasing availability of next-generation sequencing. We explore the potential pathways and events that may cooperate with BCR-ABL1 to answer these questions but also challenge the fundamental tenet that BCR-ABL1 is always the sole event initiating CML.

Relationship between trough level of tyrosine kinase inhibitor (imatinib and nilotinib) and BCR-ABL ratios in an Indonesian chronic-phase chronic myeloid leukemia (CML) population

Objectives Among Chronic Myeloid Leukemia (CML) patients treated with Tyrosine Kinase Inhibitor (TKI-imatinib-nilotinib), some showed a suboptimal response. Based on pharmacokinetic studies, TKI trough level ( C m i n ∞ ${C}_{min}\hat{\infty }$ ) is associated with clinical outcomes, reflected by the BCR-ABL ratio. However, the interindividual pharmacokinetic variability of imatinib and nilotinib is found to be moderate-high. This study aims to analyze the relationship between TKI C m i n ∞   ${C}_{min}\hat{\infty }$ and BCL-ABL ratio in chronic-phase CML patients. Methods Cross-sectional study to CML chronic-phase patients treated with imatinib 400 mg daily or nilotinib 400 or 800 mg daily for ≥12 months. The exclusion criteria were therapy discontinuation within 29 days (imatinib) or 8 days (nilotinib) before the sampling day. Blood samples were drawn 1 h before the next dose. Imatinib-nilotinib C m i n ∞ ${C}_{min}\hat{\infty }$ and BCR-ABL ratio were measured using HPLC and RT-qPCR. The relationship was analyzed using bivariate correlation Spearman's rho test. Results Twenty-three imatinib and 11 nilotinib patients met the inclusion criteria. The mean imatinib and nilotinib C m i n ∞ ${C}_{min}\hat{\infty }$ were 1,065.46 ± 765.71 and 1,445 ± 1,010.35 ng/mL respectively. There were large interindividual variations in both groups (71.87% vs. 69.88%). Half of the patients in each group were found to reach C m i n ∞ ${C}_{min}\hat{\infty }$ target (≥1.000 ng/mL, imatinib; ≥800 ng/mL nilotinib), but only 12 (35,29%) of them result in BCR-ABL ratio ≤0.1%. C m i n ∞   ${C}_{min}\hat{\infty }$ imatinib was found to be significantly associated with BCR-ABL ratio. But, not with the nilotinib group. Conclusions There were high interindividual variations of imatinib and nilotinib correlated with BCR-ABL ratio, but no correlation in nilotinib.

Erdafitinib Antagonizes ABCB1-Mediated Multidrug Resistance in Cancer Cells

ABCB1 overexpression is known to contribute to multidrug resistance (MDR) in cancers. Therefore, it is critical to find effective drugs to target ABCB1 and overcome MDR. Erdafitinib is a tyrosine kinase inhibitor (TKI) of fibroblast growth factor receptor (FGFR) that is approved by the FDA to treat urothelial carcinoma. Previous studies have demonstrated that some TKIs exhibit MDR reversal effect. In this work, we examined whether erdafitinib could reverse MDR mediated by ABCB1. The results of reversal experiments showed that erdafitinib remarkably reversed ABCB1-mediated MDR without affecting ABCG2-mediated MDR. The results of immunofluorescence and Western blot analysis demonstrated that erdafitinib did not affect the expression of ABCB1 or its cellular localization. Further study revealed that erdafitinib inhibited ABCB1 efflux function leading to increasing intracellular drug accumulation, thereby reversing MDR. Furthermore, ATPase assay indicated that erdafitinib activated the ABCB1 ATPase activity. Docking study suggested that erdafitinib interacted with ABCB1 on the drug-binding sites. In summary, this study demonstrated that erdafitinib can reverse MDR mediated by ABCB1, suggesting that combination of erdafitinib and ABCB1-substrate conventional chemotherapeutic drugs could potentially be used to overcome MDR mediated by ABCB1.

ABCG2 transports anticancer drugs via a closed-to-open switch

ABCG2 is an ABC transporter that extrudes a variety of compounds from cells, and presents an obstacle in treating chemotherapy-resistant cancers. Despite recent structural insights, no anticancer drug bound to ABCG2 has been resolved, and the mechanisms of multidrug transport remain obscure. Such a gap of knowledge limits the development of novel compounds that block or evade this critical molecular pump. Here we present single-particle cryo-EM studies of ABCG2 in the apo state, and bound to the three structurally distinct chemotherapeutics. Without the binding of conformation-selective antibody fragments or inhibitors, the resting ABCG2 adopts a closed conformation. Our cryo-EM, biochemical, and functional analyses reveal the binding mode of three chemotherapeutic compounds, demonstrate how these molecules open the closed conformation of the transporter, and establish that imatinib is particularly effective in stabilizing the inward facing conformation of ABCG2. Together these studies reveal the previously unrecognized conformational cycle of ABCG2.

ABCG2 is a human ABC transporter that actively extrudes a wide variety of compounds from cells but the mechanisms of multidrug transport remain obscure. Here authors present cryo-EM structures of ABCG2 in the apo state, and bound to the three structurally distinct chemotherapeutics and demonstrate how these molecules open the closed conformation of the transporter.

Factors Influencing Imatinib-Induced Hepatotoxicity

Purpose

Although imatinib-induced hepatotoxicity may aggravate the patient’s clinical condition and alter the treatment plan, the underlying mechanism of and factors influencing imatinib-induced hepatotoxicity have rarely been investigated. The purpose of this study was to investigate factors affecting on the incidence of hepatotoxicity within 90 days after starting imatinib treatment and time to onset of imatinib-induced hepatotoxicity.

Materials and Methods

We retrospectively evaluated the records of 177 patients receiving imatinib from October 2012 to September 2017. The analyzed factors included sex, age, body weight, body surface area, underlying disease, and concomitant drugs.

Results

The proportion of patients with hepatotoxicity within 90 days after imatinib administration was 33.9%. Proton pump inhibitors (PPIs) increased the incidence of hepatotoxicity approximately 3.8-fold and doubled the hazard of time to reach hepatotoxicity. Patients with liver disease or hepatitis B virus (HBV) carriers had a more than 8-fold higher risk of hepatotoxicity and a 5.2-fold increased hazard of hepatotoxicity compared to those without liver disease or HBV. Patients with body weight under 55 kg had a 2.2-fold higher risk for occurrence of hepatotoxicity. Patients with an imatinib dose > 400 mg had a 2.3-fold increased hazard of time to reach hepatotoxicity compared to those with an imatinib dose ≤ 400 mg.

Conclusion

The findings of this study suggest that the use of PPIs and presence of liver disease or HBV were associated with imatinib-induced hepatotoxicity. Thus, close liver function monitoring is recommended, especially in patients with liver impairment or using PPIs.

Tepotinib reverses ABCB1-mediated multidrug resistance in cancer cells

Overexpression of ABCB1 transporters plays a crucial role in mediating multidrug resistance (MDR). Therefore, it is important to inhibit ABCB1 activity in order to maintain an effective intracellular level of chemotherapeutic drugs. Tepotinib is a MET tyrosine kinase inhibitor with potential anticancer effect and it is currently in clinical trials. In this study, we investigated whether tepotinib could antagonize ABC transporters-mediated MDR. Our results suggest that tepotinib significantly reversed ABCB1-mediated MDR but not ABCG2- or ABCC1-mediated MDR. Mechanistic studies show that tepotinib significantly reversed ABCB1-mediated MDR by attenuating the efflux activity of ABCB1 transporter. The ATPase assay showed that tepotinib inhibited the ATPase activity of ABCB1 in a concentration-dependent manner. Furthermore, treatment with tepotinib did not change protein expression or subcellular localization of ABCB1. Docking analysis indicated that tepotinib interacted with the drug-binding site of the ABCB1 transporter. Our study provides a potential chemotherapeutic strategy of co-administrating tepotinib with other conventional chemotherapeutic agents to overcome MDR and improve therapeutic effect.

Alterations in WNT Signaling in Leukemias

The WNT/β-catenin signaling pathway plays an important role in the differentiation and proliferation of hematopoietic cells. In recent years, special attention has been paid to the role of impairments in the WNT signaling pathway in pathogenesis of malignant neoplasms of the hematopoietic system. Disorders in the WNT/β-catenin signaling in leukemias identified to date include hypersensitivity to the WNT ligands, epigenetic repression of WNT antagonists, overexpression of WNT ligands, impaired β-catenin degradation in the cytoplasm, and changes in the activity of the TCF/Lef transcription factors. At the molecular level, these impairments involve overexpression of the FZD protein, hypermethylation of the SFRP, DKK, WiF, Sox, and CXXC gene promoters, overexpression of Lef1 and plakoglobin, mutations in GSK3β, and β-catenin phosphorylation by the BCR-ABL kinase. This review is devoted to the systematization of these data.

GeneGini: Assessment via the Gini Coefficient of Reference "Housekeeping" Genes and Diverse Human Transporter Expression Profiles

The expression levels of SLC or ABC membrane transporter transcripts typically differ 100- to 10,000-fold between different tissues. The Gini coefficient characterizes such inequalities and here is used to describe the distribution of the expression of each transporter among different human tissues and cell lines. Many transporters exhibit extremely high Gini coefficients even for common substrates, indicating considerable specialization consistent with divergent evolution. The expression profiles of SLC transporters in different cell lines behave similarly, although Gini coefficients for ABC transporters tend to be larger in cell lines than in tissues, implying selection. Transporter genes are significantly more heterogeneously expressed than the members of most non-transporter gene classes. Transcripts with the stablest expression have a low Gini index and often differ significantly from the "housekeeping" genes commonly used for normalization in transcriptomics/qPCR studies. PCBP1 has a low Gini coefficient, is reasonably expressed, and is an excellent novel reference gene. The approach, referred to as GeneGini, provides rapid and simple characterization of expression-profile distributions and improved normalization of genome-wide expression-profiling data.

The new allosteric inhibitor asciminib is susceptible to resistance mediated by ABCB1 and ABCG2 overexpression in vitro

Asciminib (previously ABL001), which binds the myristate-binding pocket of the Bcr-Abl kinase domain, is in phase I clinical trials as monotherapy and in combination with imatinib, nilotinib and dasatinib for the treatment of patients with refractory CML or Ph+ ALL. Asciminib sensitivity was evaluated in asciminib naïve BCR-ABL1+ cell lines K562 (negligible ABCB1/ABCG2 expression), K562-Dox (ABCB1-overexpressing through doxorubicin exposure) and K562-ABCG2 (ABCG2 overexpression via transduction) with results demonstrating asciminib efflux by both ABCB1 and ABCG2 transporters. K562-Dox and K562-ABCG2 cells demonstrated increased LD50^asciminib vs K562 control cells: 256 and 299 nM respectively vs 24 nM, p < 0.001. Sensitivity was completely restored with specific inhibitors cyclosporine (ABCB1) and Ko143 (ABCG2): K562-Dox LD50^{asciminib+cyclosporine} = 13 nM, K562-ABCG2 LD50^{asciminib+Ko143} = 15 nM (p < 0.001). When asciminib resistance was modelled in vitro, ABCB1 and ABCG2 overexpression was integral in the development of asciminib resistance. In K562 asciminib-resistant cells, ABCG2 expression increased prior to BCR-ABL1 overexpression and remained high (up to 7.6-fold greater levels in resistant vs control cells, p < 0.001). K562-Dox asciminib-resistant cells had increased ABCB1 expression (2.1-fold vs control cells p = 0.0033). KU812 asciminib-resistant cells overexpressed ABCB1 (5.4-fold increase, p < 0.001) and ABCG2 (6-fold increase, p < 0.001) before emergence of a novel myristate-binding pocket mutation (F497L). In all three cell lines, asciminib resistance was reversible upon chemical inhibition of ABCB1, ABCG2 or both (p < 0.001). When K562 asciminib-resistant cells were treated with asciminib in combination with clinically achievable doses of either imatinib or nilotinib, reversal of the resistance phenotype was also observed (p < 0.01). Overexpression of efflux transporters will likely be an important pathway for asciminib resistance in the clinical setting. Given the lack of evidence for ABCG2-mediated transport of nilotinib or imatinib at clinically relevant concentrations, our data provide an additional rationale for using asciminib in combination with either TKI.

ABCC6 plays a significant role in the transport of nilotinib and dasatinib, and contributes to TKI resistance in vitro, in both cell lines and primary patient mononuclear cells

ATP Binding Cassette family efflux proteins ABCB1 and ABCG2 have previously been demonstrated to interact with Tyrosine Kinase Inhibitors (TKIs); however, evidence for the interaction of other potentially relevant drug transporters with TKIs is lacking. Through Taqman transporter array technology we assessed the impact of nilotinib on mRNA expression of ABC transporters, with ABCC6 identified as a transporter of interest. Additionally, increased expression of ABCC6 mRNA was observed during in vitro development of nilotinib resistance in BCR-ABL1-expressing cell lines. K562 cells exposed to gradually increasing concentrations of nilotinib (to 2 μM) expressed up to 57-fold higher levels of ABCC6 mRNA when compared with control cells (p = 0.002). Analogous results were observed in nilotinib resistant K562-Dox cells (up to 33-fold higher levels of ABCC6, p = 0.002). IC50 experiments were conducted on patient mononuclear cells in the absence and presence of three ABCC6 inhibitors: indomethacin, probenecid and pantoprazole. Results demonstrated that all three inhibitors significantly reduced nilotinib IC50 (p<0.001) indicating ABCC6 is likely involved in nilotinib transport. Cell line data confirmed these findings. Similar results were obtained for dasatinib, but not imatinib. Combined, these studies suggest that nilotinib and dasatinib are likely substrates of ABCC6 and to our knowledge, this is the first report of ABCC6 involvement in TKI transport. In addition, ABCC6 overexpression may also contribute to nilotinib and dasatinib resistance in vitro. With nilotinib and dasatinib now front line therapy options in the treatment of CML, concomitant administration of ABCC6 inhibitors may present an attractive option to enhance TKI efficacy.

A computational study of the inhibition mechanisms of P-glycoprotein mediated paclitaxel efflux by kinase inhibitors

Background

Drug resistance mediated by P-glycoprotein (P-gp) renders many cancer therapies ineffective. One P-gp substrate is the widely used chemotherapy drug paclitaxel. Co-administration of paclitaxel and another drug that inhibits P-gp may enhance the therapeutic effectiveness of paclitaxel by preventing its efflux from tumor cells.

Results

Here we present a computational approach that combines docking studies with mass action kinetic modeling to investigate how kinase inhibitors may inhibit P-gp mediated paclitaxel efflux. The results show that the inhibition can be attributed to competition between paclitaxel and a tyrosine kinase inhibitor (TKI) for the substrate binding domain (SBD) as well as competition between the kinase inhibitor and ATP for the nuclear (ATP) binding domain (NBD). The relative scales of these two competitions are TKI dependent and determined by the relative affinities of paclitaxel and TKIs to the SBD and NBD of P-gp, and their membrane partition coefficients. Additional simulations suggested that there is no single strategy to further improve the ability of TKIs to inhibit paclitaxel efflux and the most efficient way likely depends on the properties of the TKIs.

Conclusions

The developed model fits existing experimental results well and thus detailed analyses of isolated parameters provide insight into the mechanisms of rather important drug efflux. It can be used to guide how to design better TKIs or develop feasible drug combination strategies for targeting P-gp induced drug resistance.

Electronic supplementary material

The online version of this article (10.1186/s12918-017-0498-x) contains supplementary material, which is available to authorized users.

Overcoming ABC transporter-mediated multidrug resistance: The dual role of tyrosine kinase inhibitors as multitargeting agents

Resistance to conventional and target specific antitumor drugs still remains one of the major cause of treatment failure and patience death. This condition often involves ATP-binding cassette (ABC) transporters that, by pumping the drugs outside from cancer cells, attenuate the potency of chemotherapeutics and negatively impact on the fate of anticancer therapy. In recent years, several tyrosine kinase inhibitors (TKIs) (e.g., imatinib, nilotinib, dasatinib, ponatinib, gefitinib, erlotinib, lapatinib, vandetanib, sunitinib, sorafenib) have been reported to interact with ABC transporters (e.g., ABCB1, ABCC1, ABCG2, ABCC10). This finding disclosed a very complex scenario in which TKIs may behave as substrates or inhibitors depending on the expression of specific pumps, drug concentration, affinity for transporters and types of co-administered agents. In this context, in-depth investigation on TKI chemosensitizing functions might provide a strong rationale for combining TKIs and conventional therapeutics in specific malignancies. The reposition of TKIs as antagonists of ABC transporters opens a new way towards anticancer therapy and clinical strategies aimed at counteracting drug resistance. This review will focus on some paradigmatic examples of the complex and not yet fully elucidated interaction between clinical available TKIs (e.g. BCR-ABL, EGFR, VEGFR inhibitors) with the main ABC transporters implicated in multidrug resistance.

Mechanisms of Resistance to ABL Kinase Inhibition in Chronic Myeloid Leukemia and the Development of Next Generation ABL Kinase Inhibitors

Chronic myeloid leukemia is increasingly viewed as a chronic illness; most patients have a life expectancy close to that of the general population. Despite progress made using BCR-ABL1 tyrosine kinase inhibitors (TKIs), drug resistance via BCR-ABL1-dependent and BCR-ABL1-independent mechanisms continues to be an issue. BCR-ABL1-dependent resistance is primarily mediated through oncoprotein kinase domain mutations and usually results in overt resistance to TKIs. However, BCR-ABL1-independent resistance in the setting of effective BCR-ABL1 inhibition is recognized as a major contributor to minimal residual disease. Efforts to eradicate persistent leukemic stem cells have focused on combination therapy.

The National Cancer Institute ALMANAC: A Comprehensive Screening Resource for the Detection of Anticancer Drug Pairs with Enhanced Therapeutic Activity

To date, over 100 small-molecule oncology drugs have been approved by the FDA. Because of the inherent heterogeneity of tumors, these small molecules are often administered in combination to prevent emergence of resistant cell subpopulations. Therefore, new combination strategies to overcome drug resistance in patients with advanced cancer are needed. In this study, we performed a systematic evaluation of the therapeutic activity of over 5,000 pairs of FDA-approved cancer drugs against a panel of 60 well-characterized human tumor cell lines (NCI-60) to uncover combinations with greater than additive growth-inhibitory activity. Screening results were compiled into a database, termed the NCI-ALMANAC (A Large Matrix of Anti-Neoplastic Agent Combinations), publicly available at https://dtp.cancer.gov/ncialmanac Subsequent in vivo experiments in mouse xenograft models of human cancer confirmed combinations with greater than single-agent efficacy. Concomitant detection of mechanistic biomarkers for these combinations in vivo supported the initiation of two phase I clinical trials at the NCI to evaluate clofarabine with bortezomib and nilotinib with paclitaxel in patients with advanced cancer. Consequently, the hypothesis-generating NCI-ALMANAC web-based resource has demonstrated value in identifying promising combinations of approved drugs with potent anticancer activity for further mechanistic study and translation to clinical trials. Cancer Res; 77(13); 3564-76. ©2017 AACR.

Clinical Implications of P-Glycoprotein Modulation in Drug-Drug Interactions

Drug-drug interactions (DDIs) occur commonly and may lead to severe adverse drug reactions if not handled appropriately. Considerable information to support clinical decision making regarding potential DDIs is available in the literature and through various systems providing electronic decision support for healthcare providers. The challenge for the prescribing physician lies in sorting out the evidence and identifying those drugs for which potential interactions are likely to become clinically manifest. P-glycoprotein (P-gp) is a drug transporting protein that is found in the plasma membranes in cells of barrier and elimination organs, and plays a role in drug absorption and excretion. Increasingly, P-gp has been acknowledged as an important player in potential DDIs and a growing body of information on the role of this transporter in DDIs has become available from research and from the drug approval process. This has led to a clear need for a comprehensive review of P-gp-mediated DDIs with a focus on highlighting the drugs that are likely to lead to clinically relevant DDIs. The objective of this review is to provide information for identifying and interpreting evidence of P-gp-mediated DDIs and to suggest a classification for individual drugs based on both in vitro and in vivo evidence (substrates, inhibitors and inducers). Further, various ways of handling potential DDIs in clinical practice are described and exemplified in relation to drugs interfering with P-gp.

Effect of bosutinib on the absorption of dabigatran etexilate mesylate, a P-glycoprotein substrate, in healthy subjects

PURPOSE

Bosutinib, a dual Src and Abl tyrosine kinase inhibitor for the treatment of chronic myeloid leukemia, demonstrated concentration-dependent inhibitory effects on P-glycoprotein (P-gp)-mediated digoxin efflux in vitro, suggesting that bosutinib may inhibit P-gp substrates. The effect of bosutinib on dabigatran etexilate mesylate (EM) absorption, a P-gp substrate, was evaluated.

METHODS

In this open-label, randomized, single-dose, one-cohort, two-sequence, two-period crossover study, healthy, fed subjects received dabigatran EM (150 mg × 1 orally) alone or 1 h after receiving bosutinib tablets (100 mg × 5 orally).

RESULTS

Dabigatran EM monotherapy and concurrent administration of dabigatran EM with bosutinib resulted in similar values for concentration time curves from time zero extrapolated to infinity (AUC_inf), but slightly lower maximum plasma concentration (C _max) values (AUC_inf, 1182 and 1186 ng·h/mL, respectively; C _max, 129.8 and 114.1 ng/mL). The time to maximum concentration for dabigatran was 2.99 and 3.99 h for combination therapy. The ratio of the adjusted geometric means (test/reference) of dabigatran AUC_inf and C _max (90 % confidence interval) were 101.4 % (89.6-114.9 %) and 89.7 % (77.8-103.4 %), respectively, following administration of dabigatran EM with bosutinib (test) relative to dabigatran EM administered alone (reference). Six subjects receiving combination treatment reported a total of seven adverse events (AEs) versus none for subjects receiving monotherapy alone. All AEs were mild to moderate and considered treatment related.

CONCLUSION

These data demonstrate that single doses of bosutinib do not affect dabigatran exposure, suggesting that bosutinib is not a clinical inhibitor of P-gp.

TRIAL REGISTRATION

ClinicalTrials.gov NCT02102633. https://clinicaltrials.gov/ct2/show/NCT02102633?term=NCT02102633&rank=1.

Impact of Membrane Drug Transporters on Resistance to Small-Molecule Tyrosine Kinase Inhibitors

Small-molecule inhibitors of tyrosine kinases (TKIs) are the mainstay of treatment for many malignancies and represent novel treatment options for other diseases such as idiopathic pulmonary fibrosis. Twenty-five TKIs are currently FDA-approved and >130 are being evaluated in clinical trials. Increasing evidence suggests that drug exposure of TKIs may significantly contribute to drug resistance, independently from somatic variation of TKI target genes. Membrane transport proteins may limit the amount of TKI reaching the target cells. This review highlights current knowledge on the basic and clinical pharmacology of membrane transporters involved in TKI disposition and their contribution to drug efficacy and adverse drug effects. In addition to non-genetic and epigenetic factors, genetic variants, particularly rare ones, in transporter genes are promising novel factors to explain interindividual variability in the response to TKI therapy.

A Cell-Based Assay for Measuring Endogenous BcrAbl Kinase Activity and Inhibitor Resistance

Kinase enzymes are an important class of drug targets, particularly in cancer. Cell-based kinase assays are needed to understand how potential kinase inhibitors act on their targets in a physiologically relevant context. Current cell-based kinase assays rely on antibody-based detection of endogenous substrates, inaccurate disease models, or indirect measurements of drug action. Here we expand on previous work from our lab to introduce a 96-well plate compatible approach for measuring cell-based kinase activity in disease-relevant human chronic myeloid leukemia cell lines using an exogenously added, multi-functional peptide substrate. Our cellular models natively express the BcrAbl oncogene and are either sensitive or have acquired resistance to well-characterized BcrAbl tyrosine kinase inhibitors. This approach measures IC₅₀ values comparable to established methods of assessing drug potency, and its robustness indicates that it can be employed in drug discovery applications. This medium-throughput assay could bridge the gap between single target focused, high-throughput in vitro assays and lower-throughput cell-based follow-up experiments.

ABCB1 Overexpression Is a Key Initiator of Resistance to Tyrosine Kinase Inhibitors in CML Cell Lines

The tyrosine kinase inhibitor (TKI) imatinib has resulted in excellent responses in the majority of Chronic Myeloid Leukaemia (CML) patients; however, resistance is observed in 20–30% of patients. More recently, resistance to the second generation TKIs, nilotinib and dasatinib, has also been observed albeit at a lower incidence. ABCB1 has previously been implicated in TKI export and its overexpression linked to TKI resistance. In this study the dynamics of nilotinib resistance was studied in CML cell lines with particular focus on ABCB1 expression levels during development of resistance. Results revealed ABCB1 overexpression is likely an important initiator of nilotinib resistance in vitro. ABCB1 overexpression was also observed in cell lines as an intermediate step during development of resistance to imatinib and dasatinib in vitro. We conclude that ABCB1 overexpression may provide an initial platform to facilitate development of additional mechanisms for resistance to TKIs. This provides a rationale for investigating this phenomenon in patients undergoing TKI therapy.