Differential effects of the immunosuppressive agents cyclosporin A, tacrolimus and sirolimus on drug transport by multidrug resistance proteins

Impact of Donor Statin Treatment on Graft-versus-Host Disease after Allogeneic Hematopoietic Cell Transplantation

Some retrospective studies have suggested that long-term donor statin use may protect against graft-versus-host disease (GVHD) in patients receiving cyclosporine (CSP)-based immunosuppression after allogeneic hematopoietic cell transplantation (HCT), but prospective studies of short-term treatment of donors with statin have shown conflicting results. We conducted 2 consecutive prospective clinical trials to assess whether donor statin treatment was associated with protection against severe acute GVHD (aGVHD). In a single-arm phase II trial (study 1), we evaluated whether short-term statin treatment of HLA-matched related donors for 14 days before HCT prevented grade III-IV aGVHD. In a prospective observational cohort study (study 2), we evaluated whether longer-term (>14 days) donor statin use was required for GVHD-protective effects. Study 1 was terminated after 6 of the 35 recipients (17%) developed grade III-IV GVHD. For study 2, we identified 135 patients whose unrelated donors had received long-term treatment with statins up to the time of HCT and 4942 patients whose donors had not received long-term statin treatment. The adjusted odds ratio for grade III-IV aGVHD (statin versus no statin) was .83 (95% confidence interval [CI], .46 to 1.50; P = .54). Multivariable analysis showed no statistically significant differences between the 2 groups in the risk of grade II-IV aGVHD, chronic GVHD, nonrelapse mortality, recurrent malignancy, or overall mortality. Among patients receiving CSP-based immunosuppression, including 35 with donors receiving long-term statin treatment and 973 with donors who did not receive statins, the adjusted odds ratio of grade III-IV aGVHD was .30 (95% CI, .07 to 1.35; P = .12). In study 1, short-term statin treatment of donors was ineffective in preventing grade III-IV GVHD. In study 2, in the prespecified subgroup of recipients given CSP-based immunosuppression, nondefinitive evidence suggested that donor statin use was associated with a reduced risk of severe aGVHD.

Perspectives on drug repurposing to overcome cancer multidrug resistance mediated by ABCB1 and ABCG2

The overexpression of the human ATP-binding cassette (ABC) transporters in cancer cells is a common mechanism involved in developing multidrug resistance (MDR). Unfortunately, there are currently no approved drugs specifically designed to treat multidrug-resistant cancers, making MDR a significant obstacle to successful chemotherapy. Despite over two decades of research, developing transporter-specific inhibitors for clinical use has proven to be a challenging endeavor. As an alternative approach, drug repurposing has gained traction as a more practical method to discover clinically effective modulators of drug transporters. This involves exploring new indications for already-approved drugs, bypassing the lengthy process of developing novel synthetic inhibitors. In this context, we will discuss the mechanisms of ABC drug transporters ABCB1 and ABCG2, their roles in cancer MDR, and the inhibitors that have been evaluated for their potential to reverse MDR mediated by these drug transporters. Our focus will be on providing an up-to-date report on approved drugs tested for their inhibitory activities against these drug efflux pumps. Lastly, we will explore the challenges and prospects of repurposing already approved medications for clinical use to overcome chemoresistance in patients with high tumor expression of ABCB1 and/or ABCG2.

Efforts in Bioprospecting Research: A Survey of Novel Anticancer Phytochemicals Reported in the Last Decade

Bioprospecting natural products to find prominent agents for medical application is an area of scientific endeavor that has produced many clinically used bioactive compounds, including anticancer agents. These compounds come from plants, microorganisms, and marine life. They are so-called secondary metabolites that are important for a species to survive in the hostile environment of its respective ecosystem. The kingdom of Plantae has been an important source of traditional medicine in the past and is also enormously used today as an exquisite reservoir for detecting novel bioactive compounds that are potent against hard-to-treat maladies such as cancer. Cancer therapies, especially chemotherapies, are fraught with many factors that are difficult to manage, such as drug resistance, adverse side effects, less selectivity, complexity, etc. Here, we report the results of an exploration of the databases of PubMed, Science Direct, and Google Scholar for bioactive anticancer phytochemicals published between 2010 and 2020. Our report is restricted to new compounds with strong-to-moderate bioactivity potential for which mass spectroscopic structural data are available. Each of the phytochemicals reported in this review was assigned to chemical classes with peculiar anticancer properties. In our survey, we found anticancer phytochemicals that are reported to have selective toxicity against cancer cells, to sensitize MDR cancer cells, and to have multitarget effects in several signaling pathways. Surprisingly, many of these compounds have limited follow-up studies. Detailed investigations into the synthesis of more functional derivatives, chemical genetics, and the clinical relevance of these compounds are required to achieve safer chemotherapy.

The History of the ABC Proteins in Human Trypanosomiasis Pathogens

Human trypanosomiasis affects nearly eight million people worldwide, causing great economic and social impact, mainly in endemic areas. T. cruzi and T. brucei are protozoan parasites that present efficient mechanisms of immune system evasion, leading to disease chronification. Currently, there is no vaccine, and chemotherapy is effective only in the absence of severe clinical manifestations. Nevertheless, resistant phenotypes to chemotherapy have been described in protozoan parasites, associated with cross-resistance to other chemically unrelated drugs. Multidrug resistance is multifactorial, involving: (i) drug entry, (ii) activation, (iii) metabolism and (iv) efflux pathways. In this context, ABC transporters, initially discovered in resistant tumor cells, have drawn attention in protozoan parasites, owing to their ability to decrease drug accumulation, thus mitigating their toxic effects. The discovery of these transporters in the Trypanosomatidae family started in the 1990s; however, few members were described and functionally characterized. This review contains a brief history of the main ABC transporters involved in resistance that propelled their investigation in Trypanosoma species, the main efflux modulators, as well as ABC genes described in T. cruzi and T. brucei according to the nomenclature HUGO. We hope to convey the importance that ABC transporters play in parasite physiology and chemotherapy resistance.

A curated binary pattern multitarget dataset of focused ATP-binding cassette transporter inhibitors

Multitarget datasets that correlate bioactivity landscapes of small-molecules toward different related or unrelated pharmacological targets are crucial for novel drug design and discovery. ATP-binding cassette (ABC) transporters are critical membrane-bound transport proteins that impact drug and metabolite distribution in human disease as well as disease diagnosis and therapy. Molecular-structural patterns are of the highest importance for the drug discovery process as demonstrated by the novel drug discovery tool ‘computer-aided pattern analysis’ (‘C@PA’). Here, we report a multitarget dataset of 1,167 ABC transporter inhibitors analyzed for 604 molecular substructures in a statistical binary pattern distribution scheme. This binary pattern multitarget dataset (ABC_BPMDS) can be utilized for various areas. These areas include the intended design of (i) polypharmacological agents, (ii) highly potent and selective ABC transporter-targeting agents, but also (iii) agents that avoid clearance by the focused ABC transporters [e.g., at the blood-brain barrier (BBB)]. The information provided will not only facilitate novel drug prediction and discovery of ABC transporter-targeting agents, but also drug design in general in terms of pharmacokinetics and pharmacodynamics.

Measurement(s)	Influx • Efflux • Tracer • Transport velocity
Technology Type(s)	Fluorometry • Radioactivity • Plate reader • Flow cytometer • Tracer distribution
Factor Type(s)	half-maximal inhibition concentration
Sample Characteristic - Organism	Homo sapiens
Sample Characteristic - Environment	cell culture
Sample Characteristic - Location	Kingdom of Norway • Germany • Australia • Latvia

Population pharmacokinetics of tacrolimus in Chinese adult liver transplant patients

Tacrolimus is widely used in organ transplantation to prevent rejection. However, the narrow therapeutic window and the large inter-and intra-individual variability in the pharmacokinetics (PK) of tacrolimus make it difficult for individualization of dosing. This study aimed at developing a population pharmacokinetic model for estimating the oral clearance of tacrolimus in Chinese liver transplant patients, and identifying factors that contribute to the PK variability of tacrolimus. Data of 151 liver transplant patients who received tacrolimus were analyzed in this study. The population PK model was analyzed and the covariates including population demographic and biochemical characteristics, drug combination, and genetic polymorphism were explored using non-linear mixed-effects modeling approach. A single-compartment population PK model was developed, and the final model was CL/F = (14.6-2.38 × cytochrome P450 (CYP) 3A5-3.72 × WZC+1.04 × (POD/9)+2.48 × COR) × Exp(η_i ), where CYP3A5 was 1 for CYP3A5*3/*3, Wuzhi Capsule (WZC) was 1 when patients took tacrolimus combined with WZC, otherwise it was 0, corticosteroids (COR) was 1 when patients take tacrolimus combined with COR, otherwise, it was 0, POD was the post-operative day. Visual inspection and bootstrap indicated that the final model was stable and robust. In this study, we developed the first tacrolimus population PK model in Chinese adult liver transplant patients. We first determined the influence of WZC on tacrolimus in these people, which could provide useful PK information for the drug combination of tacrolimus and WZC. We also revealed the influence of genetic polymorphism of CYP3A5, POD, and a combination of COR on tacrolimus PK. Therefore, these significant factors should be taken into consideration in optimizing dosage regimens.

Insights into an Immunotherapeutic Approach to Combat Multidrug Resistance in Hepatocellular Carcinoma

Hepatocellular carcinoma (HCC) has emerged as one of the most lethal cancers worldwide because of its high refractoriness and multi-drug resistance to existing chemotherapies, which leads to poor patient survival. Novel pharmacological strategies to tackle HCC are based on oral multi-kinase inhibitors like sorafenib; however, the clinical use of the drug is restricted due to the limited survival rate and significant side effects, suggesting the existence of a primary or/and acquired drug-resistance mechanism. Because of this hurdle, HCC patients are forced through incomplete therapy. Although multiple approaches have been employed in parallel to overcome multidrug resistance (MDR), the results are varying with insignificant outcomes. In the past decade, cancer immunotherapy has emerged as a breakthrough approach and has played a critical role in HCC treatment. The liver is the main immune organ of the lymphatic system. Researchers utilize immunotherapy because immune evasion is considered a major reason for rapid HCC progression. Moreover, the immune response can be augmented and sustained, thus preventing cancer relapse over the post-treatment period. In this review, we provide detailed insights into the immunotherapeutic approaches to combat MDR by focusing on HCC, together with challenges in clinical translation.

Scaffold fragmentation and substructure hopping reveal potential, robustness, and limits of computer-aided pattern analysis (C@PA)

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Exploratory changes in substructure patterns are well tolerated by C@PA.

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Extended positive substructures support prediction capability.

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Increased biological hit rate of 40% for multitarget pan-ABC transporter inhibition.

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Contribution to major understanding of pattern analysis and multitarget activity.

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Pan-ABC transporter inhibitors as tool for elucidation of multitarget binding site.

Computer-aided pattern analysis (C@PA) was recently presented as a powerful tool to predict multitarget ABC transporter inhibitors. The backbone of this computational methodology was the statistical analysis of frequently occurring molecular features amongst a fixed set of reported small-molecules that had been evaluated toward ABCB1, ABCC1, and ABCG2. As a result, negative and positive patterns were elucidated, and secondary positive substructures could be suggested that complemented the multitarget fingerprints. Elevating C@PA to a non-statistical and exploratory level, the concluded secondary positive patterns were extended with potential positive substructures to improve C@PA’s prediction capabilities and to explore its robustness. A small-set compound library of known ABCC1 inhibitors with a known hit rate for triple ABCB1, ABCC1, and ABCG2 inhibition was taken to virtually screen for the extended positive patterns. In total, 846 potential broad-spectrum ABCB1, ABCC1, and ABCG2 inhibitors resulted, from which 10 have been purchased and biologically evaluated. Our approach revealed 4 novel multitarget ABCB1, ABCC1, and ABCG2 inhibitors with a biological hit rate of 40%, but with a slightly lower inhibitory power than derived from the original C@PA. This is the very first report about discovering novel broad-spectrum inhibitors against the most prominent ABC transporters by improving C@PA.

Oxypurinol pharmacokinetics and pharmacodynamics in healthy volunteers: Influence of BCRP Q141K polymorphism and patient characteristics

The missense variant, breast cancer resistance protein (BCRP) p.Q141K, which encodes a reduced function BCRP, has been linked to poor response to allopurinol. Using a multifaceted approach, we aimed to characterize the relationship(s) between BCRP p.Q141K, the pharmacokinetics (PK) and pharmacodynamics (PD) of oxypurinol (the active metabolite of allopurinol), and serum uric acid (SUA) levels. A prospective clinical study (NCT02956278) was conducted in which healthy volunteers were given a single oral dose of 300 mg allopurinol followed by intensive blood sampling. Data were analyzed using noncompartmental analysis and population PK/PD modeling. Additionally, electronic health records were analyzed to investigate whether clinical inhibitors of BCRP phenocopied the effects of the p.Q141K variant with respect to SUA. Subjects homozygous for p.Q141K had a longer half‐life (34.2 ± 12.2 h vs. 19.1 ± 1.42 h) of oxypurinol. The PK/PD model showed that women had a 24.8% lower volume of distribution. Baseline SUA was affected by p.Q141K genotype and renal function; that is, it changed by 48.8% for every 1 mg/dl difference in serum creatinine. Real‐world data analyses showed that patients prescribed clinical inhibitors of BCRP have higher SUA levels than those that have not been prescribed inhibitors of BCRP, consistent with the idea that BCRP inhibitors phenocopy the effects of p.Q141K on uric acid levels. This study identified important covariates of oxypurinol PK/PD that could affect its efficacy for the treatment of gout as well as a potential side effect of BCRP inhibitors on increasing uric acid levels, which has not been described previously.

C@PA: Computer-Aided Pattern Analysis to Predict Multitarget ABC Transporter Inhibitors

Based on literature reports of the last two decades, a computer-aided pattern analysis (C@PA) was implemented for the discovery of novel multitarget ABCB1 (P-gp), ABCC1 (MRP1), and ABCG2 (BCRP) inhibitors. C@PA included basic scaffold identification, substructure search and statistical distribution, as well as novel scaffold extraction to screen a large virtual compound library. Over 45,000 putative and novel broad-spectrum ABC transporter inhibitors were identified, from which 23 were purchased for biological evaluation. Our investigations revealed five novel lead molecules as triple ABCB1, ABCC1, and ABCG2 inhibitors. C@PA is the very first successful computational approach for the discovery of promiscuous ABC transporter inhibitors.

Rational drug design of 6-substituted 4-anilino-2-phenylpyrimidines for exploration of novel ABCG2 binding site

In the search for novel, highly potent, and nontoxic adjuvant chemotherapeutics to resolve the major issue of ABC transporter-mediated multidrug resistance (MDR), pyrimidines were discovered as a promising compound class of modern ABCG2 inhibitors. As ABCG2-mediated MDR is a major obstacle in leukemia, pancreatic carcinoma, and breast cancer chemotherapy, adjuvant chemotherapeutics are highly desired for future clinical oncology. Very recently, docking studies of one of the most potent reversers of ABCG2-mediated MDR were reported and revealed a putative second binding pocket of ABCG2. Based on this (sub)pocket, a series of 16 differently 6-substituted 4-anilino-2-phenylpyrimidines was designed and synthesized to explore the potential increase in inhibitory activity of these ABCG2 inhibitors. The compounds were assessed for their influence on the ABCG2-mediated pheophorbide A transport, as well as the ABCB1- and ABCC1-mediated transport of calcein AM. They were additionally evaluated in MDR reversal assays to determine their half-maximal reversal concentration (EC₅₀). The 6-substitution did not only show increased toxicity against ABCG2-overexpressing cells in combination with SN-38 but also a negative influence on cell viability in general. Nevertheless, several candidates had EC₅₀ values in the low double-digit nanomolar concentration range, qualifying them as some of the most potent reversers of ABCG2-mediated MDR. In addition, five novel multitarget ABCB1, ABCC1, and ABCG2 inhibitors were discovered, four of them exerting their inhibitory power against the three stated transporters at least in the single-digit micromolar concentration range.

Superior Pyrimidine Derivatives as Selective ABCG2 Inhibitors and Broad-Spectrum ABCB1, ABCC1, and ABCG2 Antagonists

In the search for highly effective modulators addressing ABCG2-mediated MDR, 23 pyrimidines were synthesized and biologically assessed. Seven derivatives with (a) nitrogen- and/or halogen-containing residue(s) had extraordinary potencies against ABCG2 (IC₅₀ < 150 nM). The compounds competitively inhibited ABCG2-mediated Hoechst 33342 transport but were not substrates of ABCG2. The most potent MDR reverser, compound 19, concentration-dependently increased SN-38-mediated cancer cell death at 11 nM (EC₅₀), time-dependently doubled SN-38 toxicity in a period of 7 days at 10 nM, and half-maximally accelerated cell death combined with SN-38 at 17 nM. No induction of ABCG2 was observed. Furthermore, 11 pyrimidines were revealed as triple ABCB1/ABCC1/ABCG2 inhibitors. Five possessed IC₅₀ values below 10 μM against each transporter, classifying them as some of the 50 most potent multitarget ABC transporter inhibitors. The most promising representative, compound 37, reversed ABCB1-, ABCC1-, and ABCG2-mediated MDR, making it one of the three most potent ABC transporter inhibitors and reversers of ABC transporters-mediated MDR.

Role of FK506 Binding Protein on Tacrolimus Distribution in Red Blood Cells

PURPOSE

Tacrolimus is distributed mainly in red blood cells (RBCs) after transfer into blood. This study aimed to evaluate the effect of FK506-binding proteins (FKBPs) on RBC distribution of tacrolimus in a physiological environment.

METHODS

Human RBCs were isolated from fresh blood samples from healthy volunteers. The effect of FKBPs on each process of the RBC distribution of tacrolimus was evaluated in vitro. Effect of intracellular FKBPs was assessed by inhibition experiment with rapamycin, which competitively inhibits the binding of tacrolimus to FKBPs. Effect of extracellular FKBPs was examined by pre-exposure of RBCs to FKBP and preincubation of tacrolimus with FKBP.

RESULTS

Pretreatment with rapamycin significantly reduced the rate of tacrolimus distribution in RBCs in a concentration-dependent manner. Pre-exposure of RBCs to FKBP12 followed by exposure to tacrolimus significantly decreased tacrolimus distribution in RBCs in a concentration-dependent manner. In addition, preincubation of tacrolimus with FKBP12 significantly reduced the rate of tacrolimus distribution in RBCs.

CONCLUSIONS

FKBP played an important role in the distribution of tacrolimus in RBCs. The effect of intracellular and extracellular FKBPs on RBC distribution of tacrolimus in circulating blood was substantial. FKBP was shown as a potential biomarker for predicting the pharmacokinetics and pharmacodynamics of tacrolimus.

A dual PI3 kinase/mTOR inhibitor BEZ235 reverses doxorubicin resistance in ABCB1 overexpressing ovarian and pancreatic cancer cell lines

BACKGROUND

Multi-drug resistance (MDR) develops because cancer cells evade toxicity of several structurally unrelated drugs. Besides other mechanisms, MDR is linked to the overexpression of ATP Binding Cassette (ABC), transporters, among which ABCB1 is the best characterized one. Since overactivation of PI3K/Akt/mTOR plays a pivotal role in the growth of human cancers, we hypothesized whether dual PI3K and mTOR inhibitor, BEZ235 (BEZ, dactolisib) reverses resistance to doxorubicin (DOX).

METHODS

Ovarian (A2780) and pancreatic (MiaPaca2) cancer cells were used to generate DOX-resistant clones by overexpressing ABCB1 or stepwise treatment of DOX. Intracellular accumulation of DOX was measured by flow cytometry after treatment with BEZ.

RESULTS

BEZ treatment caused an increase in intracellular levels of DOX which was almost identical to the naïve parental cell lines. BEZ was found to be a weak substrate for ABCB1 as demonstrated by minimal increase in ATPase activity. BEZ treatment caused a dose-dependent decrease in cell viability in combination with DOX, which was associated with an increase in cleaved PARP expression in the drug resistant clones.

CONCLUSIONS

These results suggest that BEZ is a non-substrate inhibitor of ABCB1 and is able to effectively re-sensitize cells overexpressing ABCB1 to the effects of DOX.

GENERAL SIGNIFICANCE

Dual PI3 Kinase/mTOR inhibitor, BEZ, has the potential to reverse MDR in cancer patients.

Synthesis and Characterization of Bodipy-FL-Cyclosporine A as a Substrate for Multidrug Resistance-Linked P-Glycoprotein (ABCB1)

Fluorescent conjugates of drugs can be used to study cellular functions and pharmacology. These compounds interact with proteins as substrates or inhibitors, helping in the development of unique fluorescence-based methods to study in vivo localization and molecular mechanisms. P-glycoprotein (P-gp, ABCB1) is an ATP-binding cassette (ABC) transporter that effluxes most anticancer drugs from cells, contributing to the development of drug resistance. To study the transport function of P-gp, we synthesized a Bodipy-labeled fluorescent conjugate of cyclosporine A (BD-CsA). After synthesis and characterization of its chemical purity, BD-CsA was compared with the commonly used 7-nitrobenz-2-oxa-1,3-diazol-4-yl (NBD)-CsA probe. In flow cytometry assays, the fluorescence intensity of BD-CsA was almost 10 times greater than that of NBD-CsA, enabling us to use significantly lower concentrations of BD-CsA to achieve the same fluorescence levels. We found that BD-CsA is recognized as a transport substrate by both human and mouse P-gp. The rate of efflux of BD-CsA by human P-gp is comparable to that of NBD-CsA. The transport of BD-CsA was inhibited by tariquidar, with similar IC₅₀ values to those for NBD-CsA. BD-CsA and NBD-CsA both partially inhibited the ATPase activity of P-gp with similar IC₅₀ values. In silico docking of BD-CsA and NBD-CsA to the human P-gp structure indicates that they both bind in the drug-binding pocket with similar docking scores and possibly interact with similar residues. Thus, we demonstrate that BD-CsA is a sensitive fluorescent substrate of P-gp that can be used to efficiently study the transporter's localization and function in vitro and in vivo. SIGNIFICANCE STATEMENT: The goal of this study was to develop an effective probe to study drug transport by P-glycoprotein (P-gp). Fluorophore-conjugated substrates are useful to study the P-gp transport mechanism, structural characteristics, and development of its inhibitors. Cyclosporine A (CsA), a cyclic peptide comprising 11 amino acids, is a known substrate of P-gp. P-gp affects CsA pharmacokinetics and interactions with other coadministered drugs, especially during transplant surgeries and treatment of autoimmune disorders, when CsA is given as an immunosuppressive agent. We synthesized and characterized Bodipy-FL-CsA as an avid fluorescent substrate that can be used to study the function of P-gp both in vitro and in vivo. We demonstrate that Bodipy-FL-conjugation does not affect the properties of CsA as a P-gp substrate.

Identification of genetic variants associated with tacrolimus metabolism in kidney transplant recipients by extreme phenotype sampling and next generation sequencing

An extreme phenotype sampling (EPS) model with targeted next-generation sequencing (NGS) identified genetic variants associated with tacrolimus (Tac) metabolism in subjects from the Deterioration of Kidney Allograft Function (DeKAF) Genomics cohort which included 1,442 European Americans (EA) and 345 African Americans (AA). This study included 48 subjects separated into 4 groups of 12 (AA high, AA low, EA high, EA low). Groups were selected by the extreme phenotype of dose-normalized Tac trough concentrations after adjusting for common genetic variants and clinical factors. NGS spanned >3 Mb of 28 genes and identified 18,661 genetic variants (3,961 previously unknown). A group of 125 deleterious variants, by SIFT analysis, were associated with Tac troughs in EAs (burden test, p=0.008), CYB5R2 was associated with Tac troughs in AAs (SKAT, p=0.00079). In CYB5R2, rs61733057 (increased allele frequency in AAs) was predicted to disrupt protein function by SIFT and PolyPhen2 analysis. The variants merit further validation.

FK506 Attenuates the MRP1-Mediated Chemoresistant Phenotype in Glioblastoma Stem-Like Cells

Poor response to current treatments for glioblastoma has been attributed to the presence of glioblastoma stem-like cells (GSCs). GSCs are able to expel antitumor drugs to the extracellular medium using the multidrug resistance-associated protein 1 (MRP1) transporter. Tacrolimus (FK506) has been identified as an MRP1 regulator in differentiated glioblastoma (GBM) cells (non-GSCs); however, the effect of FK506 on GSCs is currently unknown. The objective of the following research is to evaluate the effect of FK506 on the MRP1-related chemo-resistant phenotype of GSCs. For this, U87MG and C6 glioma cell lines were used to generate non-GSCs and GSCs. mRNA and MRP1-positive cells were evaluated by RT-qPCR and flow cytometry, respectively. A Carboxyfluorescein Diacetate (CFDA)-retention assay was performed to evaluate the MRP1 activity. Apoptosis and MTT assays were employed to evaluate the cytotoxic effects of FK506 plus Vincristine (MRP1 substrate). GSC-derived subcutaneous tumors were generated to evaluate the in vivo effect of FK506/Vincristine treatment. No differences in transcript levels and positive cells for MRP1 were observed in FK506-treated cells. Lesser cell viability, increased apoptosis, and CFDA-retention in the FK506/Vincristine-treated cells were observed. In vivo, the FK506/Vincristine treatment decreased the tumor size as well as ki67, Glial Fibrillary Acidic Protein (GFAP), and nestin expression. We conclude that FK506 confers a chemo-sensitive phenotype to MRP1-drug substrate in GSCs.

Effective Drug Delivery in Diffuse Intrinsic Pontine Glioma: A Theoretical Model to Identify Potential Candidates

Despite decades of clinical trials for diffuse intrinsic pontine glioma (DIPG), patient survival does not exceed 10% at two years post-diagnosis. Lack of benefit from systemic chemotherapy may be attributed to an intact bloodbrain barrier (BBB). We aim to develop a theoretical model including relevant physicochemical properties in order to review whether applied chemotherapeutics are suitable for passive diffusion through an intact BBB or whether local administration via convection-enhanced delivery (CED) may increase their therapeutic potential. Physicochemical properties (lipophilicity, molecular weight, and charge in physiological environment) of anticancer drugs historically and currently administered to DIPG patients, that affect passive diffusion over the BBB, were included in the model. Subsequently, the likelihood of BBB passage of these drugs was ascertained, as well as their potential for intratumoral administration via CED. As only non-molecularly charged, lipophilic, and relatively small sized drugs are likely to passively diffuse through the BBB, out of 51 drugs modeled, only 8 (15%)-carmustine, lomustine, erlotinib, vismodegib, lenalomide, thalidomide, vorinostat, and mebendazole-are theoretically qualified for systemic administration in DIPG. Local administration via CED might create more therapeutic options, excluding only positively charged drugs and drugs that are either prodrugs and/or only available as oral formulation. A wide variety of drugs have been administered systemically to DIPG patients. Our model shows that only few are likely to penetrate the BBB via passive diffusion, which may partly explain the lack of efficacy. Drug distribution via CED is less dependent on physicochemical properties and may increase the therapeutic options for DIPG.

Longitudinal Pharmacokinetics of Everolimus When Combined With Low-level of Tacrolimus in Elderly Renal Transplant Recipients

BACKGROUND

Although the proportion of elderly patients among renal transplant recipients has increased, pharmacokinetic (PK) studies of immunosuppressants rarely include older patients.

METHODS

We studied 12-hour everolimus (EVL) PK in 16 elderly renal transplant recipients (all whites; 10 men; mean age, 64 ± 2 years (61-71 years), in 4 separate timepoints (at 7, 30, 60, and 150 days) after EVL introduction, corresponding to a mean postrenal transplantation day: PK1 (43 ± 4 days), PK2 (65 ± 7 days), PK3 (106 ± 17 days), and PK4 (206 ± 40 days). Patients received EVL (target trough level (Ctrough, 3-8 ng/mL), prednisone, and tacrolimus (TCL) (target Ctrough, 2-5 ng/mL).

RESULTS

Mean TCL-Ctrough was 7.2 ± 3.8, 4.9 ± 2.2, 4.9 ± 2.2, and 4.5 ± 1.2 ng/mL at PK1, PK2, PK3, and PK4, respectively. There were no differences among timepoints for mean EVL daily dose (data shown as PK3) (3.5 ± 1.3 mg/d), Ctrough (4.7 ± 2.5 ng/mL), AUC0-12h (106 ± 51 ng/h per mL), Caverage (8.8 ± 4.2 ng/mL), Cmax (19.2 ± 9.7 ng/mL), apparent Half-life (11.7 ± 4.2 hours), estimated total body clearance (0.39 ± 0.27 L/h), or fluctuation (166 ± 65%). Also, none of those PK parameters differed statistically when adjusted for body weight. EVL-Ctrough showed a very high correlation (r = 0.849) with AUC0-12h.

CONCLUSIONS

Our data indicate that elderly renal transplant recipients starting EVL 1 month after transplantation along with a steady-state TCL level, present stable EVL-PK parameters without significant changes in dose or exposure during the first 6 months after renal transplantation.

Hypoxia as a target for drug combination therapy of liver cancer

Hepatocellular carcinoma (HCC) is the third most frequent cause of cancer deaths worldwide. The standard of care for intermediate HCC is transarterial chemoembolization, which combines tumour embolization with locoregional delivery of the chemotherapeutic doxorubicin. Embolization therapies induce hypoxia, leading to the escape and proliferation of hypoxia-adapted cancer cells. The transcription factor that orchestrates responses to hypoxia is hypoxia-inducible factor 1 (HIF-1). The aim of this work is to show that targeting HIF-1 with combined drug therapy presents an opportunity for improving outcomes for HCC treatment. HepG2 cells were cultured under normoxic and hypoxic conditions exposed to doxorubicin, rapamycin and combinations thereof, and analyzed for viability and the expression of hypoxia-induced HIF-1α in response to these treatments. A pilot study was carried out to evaluate the antitumour effects of these drug combinations delivered from drug-eluting beads in vivo using an ectopic xenograft murine model of HCC. A therapeutic doxorubicin concentration that inhibits the viability of normoxic and hypoxic HepG2 cells and above which hypoxic cells are chemoresistant was identified, together with the lowest effective dose of rapamycin against normoxic and hypoxic HepG2 cells. It was shown that combinations of rapamycin and doxorubicin are more effective than doxorubicin alone. Western Blotting indicated that both doxorubicin and rapamycin inhibit hypoxia-induced accumulation of HIF-1α. Combination treatments were more effective in vivo than either treatment alone. mTOR inhibition can improve outcomes of doxorubicin treatment in HCC.

Renal Drug Transporters and Drug Interactions

Transporters in proximal renal tubules contribute to the disposition of numerous drugs. Furthermore, the molecular mechanisms of tubular secretion have been progressively elucidated during the past decades. Organic anions tend to be secreted by the transport proteins OAT1, OAT3 and OATP4C1 on the basolateral side of tubular cells, and multidrug resistance protein (MRP) 2, MRP4, OATP1A2 and breast cancer resistance protein (BCRP) on the apical side. Organic cations are secreted by organic cation transporter (OCT) 2 on the basolateral side, and multidrug and toxic compound extrusion (MATE) proteins MATE1, MATE2/2-K, P-glycoprotein, organic cation and carnitine transporter (OCTN) 1 and OCTN2 on the apical side. Significant drug-drug interactions (DDIs) may affect any of these transporters, altering the clearance and, consequently, the efficacy and/or toxicity of substrate drugs. Interactions at the level of basolateral transporters typically decrease the clearance of the victim drug, causing higher systemic exposure. Interactions at the apical level can also lower drug clearance, but may be associated with higher renal toxicity, due to intracellular accumulation. Whereas the importance of glomerular filtration in drug disposition is largely appreciated among clinicians, DDIs involving renal transporters are less well recognized. This review summarizes current knowledge on the roles, quantitative importance and clinical relevance of these transporters in drug therapy. It proposes an approach based on substrate-inhibitor associations for predicting potential tubular-based DDIs and preventing their adverse consequences. We provide a comprehensive list of known drug interactions with renally-expressed transporters. While many of these interactions have limited clinical consequences, some involving high-risk drugs (e.g. methotrexate) definitely deserve the attention of prescribers.

High-content screening of clinically tested anticancer drugs identifies novel inhibitors of human MRP1 (ABCC1)

Multidrug resistance protein 1 (MRP1/ABCC1), an integral transmembrane efflux transporter, belongs to the ATP-binding cassette (ABC) protein superfamily. MRP1 governs the absorption and disposition of a wide variety of endogenous and xenobiotic substrates including various drugs across organs and physiological barriers. Additionally, its overexpression has been implicated in multidrug resistance in chemotherapy of multiple cancers. Here, we describe the development of a high content imaging-based screening assay for MRP1 activity. This live cell-based automated microscopy assay is very robust and allows simultaneous detection of cell permeable, non-toxic and potent inhibitors. The validity of the assay was demonstrated by profiling a library of 386 anti-cancer compounds, which are under clinical trials, for interactions with MRP1. The assay identified 12 potent inhibitors including two known MRP1 inhibitors, cyclosporine A and rapamycin. On the other hand, MRP1-inhibitory activity of tipifarnib, AZD1208, deforolimus, everolimus, temsirolimus, HS-173, YM201636, ESI-09, TAK-733, and CX-6258 has not been previously reported. Inhibition of MRP1 activity was further validated using flow cytometry and confocal microscopy for the respective detection of calcein and doxorubicin in MRP1-overexpressing cells. Among the identified compounds, tipifarnib, AZD1208, rapamycin, deforolimus, everolimus, TAK-733, and temsirolimus resensitized MRP1-overexpressing H69AR cells towards vincristine, a cytotoxic chemotherapeutic agent, by 2-6-fold. Using purified HEK293 membrane vesicles overexpressing MRP1, MRP2, MRP3, and MRP4, we also demonstrated that the identified compounds exert differential and selective response on the uptake of estradiol glucuronide, an endogenous MRP substrate. In summary, we demonstrated the effectiveness of the high content imaging-based high-throughput assay for profiling compound interaction with MRP1.

Drug-drug interactions between immunosuppressants and antidiabetic drugs in the treatment of post-transplant diabetes mellitus

Post-transplant diabetes mellitus is a frequent complication of solid organ transplantation that generally requires treatment with lifestyle interventions and antidiabetic medication. A number of demonstrated and potential pharmacokinetic drug-drug interactions (DDIs) exist between commonly used immunosuppressants and antidiabetic drugs, which are comprehensively summarized in this review. Cyclosporine (CsA) itself inhibits the cytochrome P450 (CYP) 3A4 enzyme and a variety of drug transporters. As a result, it increases exposure to repaglinide and sitagliptin, will likely increase the exposure to nateglinide, glyburide, saxagliptin, vildagliptin and alogliptin, and could theoretically increase the exposure to gliquidone and several sodium-glucose transporter (SGLT)-2 inhibitors. Currently available data, although limited, suggest that these increases are modest and, particularly with regard to gliptins and SGLT-2 inhibitors, unlikely to result in hypoglycemia. The interaction with repaglinide is more pronounced but does not preclude concomitant use if repaglinide dose is gradually titrated. Mycophenolate mofetil and azathioprine do not engage in DDIs with any antidiabetic drug. Although calcineurin inhibitors (CNIs) and mammalian target of rapamycin inhibitors (mTORi) are intrinsically prone to DDIs, their disposition is not influenced by metformin, pioglitazone, sulfonylureas (except possibly glyburide) or insulin. An effect of gliptins on the disposition of CNIs and mTORi is unlikely, but has not been definitively ruled out. Based on their disposition profiles, glyburide and canagliflozin could affect CNI and mTORi disposition although this requires further study. Finally, delayed gastric emptying as a result of glucagon-like peptide-1 agonists seems to have a limited, but not necessarily negligible effect on CNI disposition.

Cell permeability beyond the rule of 5

Drug discovery for difficult targets that have large and flat binding sites is often better suited to compounds beyond the "rule of 5" (bRo5). However, such compounds carry higher pharmacokinetic risks, such as low solubility and permeability, and increased efflux and metabolism. Interestingly, recent drug approvals and studies suggest that cell permeable and orally bioavailable drugs can be discovered far into bRo5 space. Tactics such as reduction or shielding of polarity by N-methylation, bulky side chains and intramolecular hydrogen bonds may be used to increase cell permeability in this space, but often results in decreased solubility. Conformationally flexible compounds can, however, combine high permeability and solubility, properties that are keys for cell permeability and intestinal absorption. Recent developments in computational conformational analysis will aid design of such compounds and hence prediction of cell permeability. Transporter mediated efflux occurs for most investigated drugs in bRo5 space, however it is commonly overcome by high local intestinal concentrations on oral administration. In contrast, there is little data to support significant impact of transporter-mediated intestinal absorption in bRo5 space. Current knowledge of compound properties that govern transporter effects of bRo5 drugs is limited and requires further fundamental and comprehensive studies.

The role of ABC transporters in ovarian cancer progression and chemoresistance

Over 80% of ovarian cancer patients develop chemoresistance which results in a lethal course of the disease. A well-established cause of chemoresistance involves the family of ATP-binding cassette transporters, or ABC transporters that transport a wide range of substrates including metabolic products, nutrients, lipids, and drugs across extra- and intra-cellular membranes. Expressions of various ABC transporters, shown to reduce the intracellular accumulation of chemotherapy drugs, are increased following chemotherapy and impact on ovarian cancer survival. Although clinical trials to date using ABC transporter inhibitors have been disappointing, ABC transporter inhibition remains an attractive potential adjuvant to chemotherapy. A greater understanding of their physiological functions and role in ovarian cancer chemoresistance will be important for the development of more effective targeted therapies. This article will review the role of the ABC transporter family in ovarian cancer progression and chemoresistance as well as the clinical attempts used to date to reverse chemoresistance.

Control of spasticity in a multiple sclerosis model using central nervous system-excluded CB1 cannabinoid receptor agonists

The purpose of this study was the generation of central nervous system (CNS)-excluded cannabinoid receptor agonists to test the hypothesis that inhibition of spasticity, due to CNS autoimmunity, could be controlled by affecting neurotransmission within the periphery. Procedures included identification of chemicals and modeling to predict the mode of exclusion; induction and control of spasticity in the ABH mouse model of multiple sclerosis; conditional deletion of CB1 receptor in peripheral nerves; side-effect profiling to demonstrate the mechanism of CNS-exclusion via drug pumps; genome-wide association study in N2(129×ABH) backcross to map polymorphic cannabinoid drug pump; and sequencing and detection of cannabinoid drug-pump activity in human brain endothelial cell lines. Three drugs (CT3, SAB378 and SAD448) were identified that control spasticity via action on the peripheral nerve CB1 receptor. These were peripherally restricted via drug pumps that limit the CNS side effects (hypothermia) of cannabinoids to increase the therapeutic window. A cannabinoid drug pump is polymorphic and functionally lacking in many laboratory (C57BL/6, 129, CD-1) mice used for transgenesis, pharmacology, and toxicology studies. This phenotype was mapped and controlled by 1-3 genetic loci. ABCC1 within a cluster showing linkage is a cannabinoid CNS-drug pump. Global and conditional CB1 receptor-knockout mice were used as controls. In summary, CNS-excluded CB1 receptor agonists are a novel class of therapeutic agent for spasticity.

Genetic differences in Native Americans and tacrolimus dosing after kidney transplantation

Tacrolimus pharmacokinetics vary due to single nucleotide polymorphisms (SNPs) in metabolizing enzymes and membrane transporters that alter drug elimination. Clinically we observed that Native Americans require lower dosages of tacrolimus to attain trough levels similar to Caucasians. We previously demonstrated that Native Americans have decreased oral clearance of tacrolimus, suggesting that Native Americans may have more variant SNPs and, therefore, altered tacrolimus pharmacokinetic parameters. We conducted 12-hour pharmacokinetic studies on 24 adult Native American kidney transplant recipients on stable doses of tacrolimus for at least 1 month posttransplantation. Twenty-four Caucasian kidney transplant recipients were compared as controls. SNPs encoding the genes for the enzymes (CYP3A4, CYP3A5) and transporters (ABCB1, BCRP, and MRP1) were typed using TaqMan. The mean daily tacrolimus dose in the Native Americans was 0.03 ± 0.02 compared with the Caucasians 0.5 ± 0.3 (mg/kg/d; P = .002), with no significant differences in trough levels, (6.7 ± 3.1 vs 7.4 ± 2.1 ng/dL; P = .4). Many Native Americans, but not Caucasians, demonstrated the 3/*3 - C3435T CC and the *3/*3 -G2677T GG genotype combination previously associated with low tacrolimus dosing. Native Americans required significantly lower tacrolimus doses than Caucasians to achieve similar tacrolimus trough levels, in part due to lower tacrolimus clearance from decreased drug metabolism and excretion.

Effect of the ATP-binding cassette transporter ABCG2 on pharmacokinetics: experimental findings and clinical implications

INTRODUCTION

The ATP-binding cassette transporter ABCG2 can actively extrude a broad range of endogenous and exogenous substrates across biological membranes. Thereby, ABCG2 limits oral drug bioavailability, mediates hepatobiliary and renal excretion and participates functionally in the blood-brain barrier.

AREAS COVERED

The paper provides a review of the clinical evidence of the role of ABCG2 in the bioavailability and brain disposition of drugs. It also sheds light on the value of experimental/preclinical data in predicting the role of ABCG2 in pharmacokinetics in humans.

EXPERT OPINION

Experimental studies indicate that ABCG2 may limit the oral bioavailability and brain penetration of many drugs. ABCG2 has also been recognized as an important determinant of the disposition of some drugs in humans. For example, loss-of-function variants of ABCG2 affect the pharmacokinetics and pharmacodynamics of rosuvastatin in a clinically significant manner. Moreover, clinically relevant pharmacokinetic drug-drug interactions have been attributed to ABCG2 inhibition. However, examples from human studies are still rare compared with the overwhelming evidence from experimental studies. The large degree of functional redundancy of ABCG2 with other transporters such as P-glycoprotein may explain the rare occurrence of ABCG2-dependent drug-drug interactions in humans. Providing clinicians with consolidated information on the clinically relevant interactions of drugs with ABCG2 remains a matter of future exploration.

PI3-kinase and mTOR inhibitors differently modulate the function of the ABCG2 multidrug transporter

The ATP-binding cassette (ABC) transporter ABCG2 plays an important role in tissue detoxification and confers multidrug resistance to cancer cells. Identification of expressional and functional cellular regulators of this multidrug transporter is therefore intensively pursued. The PI3-kinase/Akt signaling axis has been implicated as a key element in regulating various cellular functions, including the expression and plasma membrane localization of ABCG2. Here we demonstrate that besides inhibiting their respective target kinases, the pharmacological PI3-kinase inhibitor LY294002 and the downstream mTOR kinase inhibitor rapamycin also directly inhibit ABCG2 function. In contrast, wortmannin, another commonly used pharmacological inhibitor of PI3-kinase does not interact with the transporter. We suggest that direct functional modulation of ABCG2 should be taken into consideration when pharmacological agents are applied to dissect the specific role of PI3-kinase/Akt/mTOR signaling in cellular functions.

The ABCG family of membrane-associated transporters: you don't have to be big to be mighty

Along with many other mammalian ATP-binding cassette (ABC) transporters, members of the ABCG group are involved in the regulated transport of hydrophobic compounds across cellular membranes. In humans, five ABCG family members have been identified, encoding proteins ranging from 638 to 678 amino acids in length. All five have been the subject of intensive investigation to better understand their physiological roles, expression patterns, interactions with substrates and inhibitors, and regulation at both the transcript and protein level. The principal substrates for at least four of the ABCG proteins are endogenous and dietary lipids, with ABCG1 implicated in particular in the export of cholesterol, and ABCG5 and G8 forming a functional heterodimer responsible for plant sterol elimination from the body. ABCG2 has a much broader substrate specificity and its ability to transport numerous diverse pharmaceuticals has implications for the absorption, distribution, metabolism, excretion and toxicity (ADMETOx) profile of these compounds. ABCG2 is one of at least three so-called multidrug resistant ABC transporters expressed in humans, and its activity is associated with decreased efficacy of anti-cancer agents in several carcinomas. In addition to its role in cancer, ABCG2 also plays a role in the normal physiological transport of urate and haem, the implications of which are described. We summarize here data on all five human ABCG transporters and provide a current perspective on their roles in human health and disease.

Two drug interaction studies of sirolimus in combination with sorafenib or sunitinib in patients with advanced malignancies

PURPOSE

Sirolimus is the prototypical mTOR inhibitor. Sorafenib and sunitinib are small molecule inhibitors of multiple kinases including VEGF receptor (VEGFR) kinases. These agents have different mechanisms of action, providing a strong rationale for combination.

EXPERIMENTAL DESIGN

Patients with advanced cancer were assigned to receive either sirolimus or the VEGFR inhibitor alone for a 2-week lead-in period, followed by combination therapy. The primary end point of each trial was to determine whether a drug interaction exists between sirolimus and either sorafenib or sunitinib, as defined by a difference in C(max) for each drug alone compared with its C(max) during combination therapy.

RESULTS

The sorafenib and sunitinib trials enrolled 34 and 23 patients, respectively. There were no clinically significant differences in C(max) for any of the drugs alone compared with the C(max) during combination therapy. Toxicity profiles were similar to those expected for each drug alone. One patient with adrenal cortical cancer had a partial response to sirolimus and sunitnib.

CONCLUSIONS

Sirolimus can be safely combined with sorafenib or sunitinib. Our trial design is feasible and informative in screening for potential drug-drug interactions, using a relatively small number of patients and limited pharmacokinetic sampling.

Targeting the autophagy pathway using ectopic expression of Beclin 1 in combination with rapamycin in drug-resistant v-Ha-ras-transformed NIH 3T3 cells

The effectiveness of an apoptosis-targeting therapy may be limited in tumor cells with defects in apoptosis. Recently, considerable attention in the field of cancer therapy has been focused on the mammalian rapamycin target (mTOR), inhibition of which results in autophagic cell death. In our study using multidrug-resistant v-Ha-ras-transformed NIH3T3 (Ras-NIH 3T3/Mdr) cells, we demonstrated that rapamycin-induced cell death may result from 2 different mechanisms. At high rapamycin concentrations (≥ 100 nM), cell death may occur via an autophagy-dependent pathway, whereas at lower concentrations (≤ 10 nM), cell death may occur after G1-phase cell cycle arrest. This effect was accompanied by upregulation of p21(Cip1) and p27(Kip1) expression via an autophagy-independent pathway. We also tested whether inhibition of mTOR with low concentrations of rapamycin and ectopic Beclin-1 expression would further sensitize multidrug resistance (MDR)-positive cancer cells by upregulating autophagy. Rapamycin at low concentrations might be insufficient to initiate autophagosome formation in autophagy but Beclin-1 overexpression triggered additional processes downstream of mTOR during G(1) cell cycle arrest by rapamycin. Our findings suggest that these combination strategies targeting autophagic cell death may yield significant benefits for cancer patients, because lowering rapamycin concentration for cancer treatment minimizes its side effects in patients undergoing chemotherapy.

In vitro and in vivo evidence for the importance of breast cancer resistance protein transporters (BCRP/MXR/ABCP/ABCG2)

The breast cancer resistance protein (BCRP/ABCG2) is a member of the G-subfamiliy of the ATP-binding cassette (ABC)-transporter superfamily. This half-transporter is assumed to function as an important mechanism limiting cellular accumulation of various compounds. In context of its tissue distribution with localization in the sinusoidal membrane of hepatocytes, and in the apical membrane of enterocytes ABCG2 is assumed to function as an important mechanism facilitating hepatobiliary excretion and limiting oral bioavailability, respectively. Indeed functional assessment performing mouse studies with genetic deletion or chemical inhibition of the transporter, or performing pharmacogenetic studies in humans support this assumption. Furthermore the efflux function of ABCG2 has been linked to sanctuary blood tissue barriers as described for placenta and the central nervous system. However, in lactating mammary glands ABCG2 increases the transfer of substrates into milk thereby increasing the exposure to potential noxes of a breastfed newborn. With regard to its broad substrate spectrum including various anticancer drugs and environmental carcinogens the function of ABCG2 has been associated with multidrug resistance and tumor development/progression. In terms of cancer biology current research is focusing on the expression and function of ABCG2 in immature stem cells. Recent findings support the notion that the physiological function of ABCG2 is involved in the elimination of uric acid resulting in higher risk for developing gout in male patients harboring genetic variants. Taken together ABCG2 is implicated in various pathophysiological and pharmacological processes.

Function and expression of ATP-binding cassette transporters in cultured human Y79 retinoblastoma cells

The aim of this study was to reveal the expression and function of P-glycoprotein and multidrug resistance-associated proteins (MRP), members of the ATP-binding cassette (ABC) superfamily of drug transporters, in cultured human Y79 retinoblastoma cells. ABC transporter mRNA expression was evaluated by conventional reverse transcription-polymerase chain reaction (RT-PCR) and real-time PCR analyses. Cellular accumulation of rhodamine 123 (P-glycoprotein substrate), calcein (MRP substrate), and doxorubicin (P-glycoprotein/MRP substrate) was analyzed by fluorometry. Conventional RT-PCR analysis showed the expression of multidrug resistance 1 (MDR1), MRP1, MRP2 and lung resistance-related protein (LRP) mRNAs. Real-time RT-PCR analysis revealed that the expression levels of the MDR1 and MRP2 genes in Y79 cells were much lower than those in human intestinal cell line Caco-2, while the expression level of MRP1 was higher than that in Caco-2 cells. The accumulation of rhodamine 123 was not enhanced by verapamil or reversin 205, inhibitors of P-glycoprotein, indicating no function of P-glycoprotein in Y79 cells. The accumulation of calcein was significantly increased by various MRP inhibitors including probenecid, indicating that MRP functions in Y79 cells. The accumulation of doxorubicin was increased in the presence of metabolic inhibitors (10 mM 2-deoxyglucose and 5 mM sodium azide). However, most MRP inhibitors such as probenecid and indomethacin did not affect doxorubicin accumulation, while cyclosporin A and taclorimus significantly increased doxorubicin accumulation. These results suggest that MRP, but not P-glycoprotein, functions in Y79 cells, and that the efflux of doxorubicin from Y79 cells may be due to an ATP-dependent transporter, which has not been identified yet.

Targeting drug transporters - combining in silico and in vitro approaches to predict in vivo

Transporter proteins are expressed throughout the human body in different vital organs. They play an important role to various extents in determining absorption, distribution, metabolism, excretion, and toxicity (ADME/Tox) properties of therapeutic molecules. Over the past decade, numerous drug transporters have been cloned and considerable progress has been made toward understanding the molecular characteristics of individual transporters. In this chapter several in vitro and in silico techniques are described with applications to understand transporter behavior. These include employing new techniques to rapidly identify novel ligands for transporters. Ultimately these methods should lead to a greater overall appreciation of the role of transporters in vivo.

Intracellular protein binding patterns of the anticancer ruthenium drugs KP1019 and KP1339

The ruthenium compound KP1019 has demonstrated promising anticancer activity in a pilot clinical trial. This study aims to evaluate the intracellular uptake/binding patterns of KP1019 and its sodium salt KP1339, which is currently in a phase I-IIa study. Although KP1339 tended to be moderately less cytotoxic than KP1019, IC(50) values in several cancer cell models revealed significant correlation of the cytotoxicity profiles, suggesting similar targets for the two drugs. Accordingly, both drugs activated apoptosis, indicated by caspase activation via comparable pathways. Drug uptake determined by inductively coupled plasma mass spectrometry (ICP-MS) was completed after 1 h, corresponding to full cytotoxicity as early as after 3 h of drug exposure. Surprisingly, the total cellular drug uptake did not correlate with cytotoxicity. However, distinct differences in intracellular distribution patterns suggested that the major targets for the two ruthenium drugs are cytosolic rather than nuclear. Consequently, drug-protein binding in cytosolic fractions of drug-treated cells was analyzed by native size-exclusion chromatography (SEC) coupled online with ICP-MS. Ruthenium-protein binding of KP1019- and KP1339-treated cells distinctly differed from the platinum binding pattern observed after cisplatin treatment. An adapted SEC-SEC-ICP-MS system identified large protein complexes/aggregates above 700 kDa as initial major binding partners in the cytosol, followed by ruthenium redistribution to the soluble protein weight fraction below 40 kDa. Taken together, our data indicate that KP1019 and KP1339 rapidly enter tumor cells, followed by binding to larger protein complexes/organelles. The different protein binding patterns as compared with those for cisplatin suggest specific protein targets and consequently a unique mode of action for the ruthenium drugs investigated.

Identification of functional FKB protein in Echinococcus granulosus: its involvement in the protoscolicidal action of rapamycin derivates and in calcium homeostasis

FK506 (tacrolimus) and polyketide macrolides such as rapamycin and its derivates bind to FK506-binding proteins (FKBPs). These proteins display a peptidyl-prolyl rotamase function that is believed to catalyze protein folding and they are well-validated anti-proliferative drug targets in certain pathogenic microorganisms, and their functions have been characterized in parasitic protozoa. However, much less is known in helminths and trials with rapalogs on cestoda have not yet been reported. Due to a growing need for new treatment options for human cystic echinococcosis, the in vitro efficacy of rapalogs in Echinococcus granulosus was investigated. We determined the effect of ramapycin, FK506 and everolimus against this cestode, demonstrating their protoscolicidal ability. Also, we observed synergic scolicidal actions during combined therapy with rapalogs plus cyclosporine A, proposing dual administration of drugs to improve pharmacological effects in vivo. We have identified an E. granulosus (Eg)-fkb1 gene that encodes Eg-FKBP, an archetypal protein of the FKBP family, which includes all residues implicated in the binding of pharmacological ligands, in the enzymatic activity and in interactions with possible target proteins. Levels of Eg-fkb1 mRNA are over-expressed by acid but not rapalog treatment. We also described the presence of receptor-operated calcium channels in the larval stage, suggesting that exogenous ligands may dissociate the interaction of Eg-FKBP from these intracellular channels, enhancing the activity of the Ca(2+) release and interfering with their normal regulatory functions. As rapamycin sensitivity is the major criterion used to detect targets of rapamycin kinase, we identified and analyzed in silico critical residues of putative homologs in the Echinococcus genome. These preliminary results will allow us to continue subsequent studies that could reveal the precise intracellular functions of Eg-FKBP, providing greater knowledge for further identification of downstream target proteins, a promising target for chemotherapy of cystic echinococcosis.

Targeting mammalian target of rapamycin to both downregulate and disable the P-glycoprotein pump in multidrug-resistant B-cell lymphoma cell lines

Previous studies have shown that rapamycin can inhibit the growth of several different types of human tumor cells in vitro. In certain cases, it can reverse the phenotype of multidrug resistant (MDR) cells. However, there is limited information concerning its effect on P-glycoprotein (P-gp), a pump that is responsible for chemoresistance in many MDR cells. We investigated the effect of rapamycin on both P-gp function and the MDR phenotype in four cell lines. One cell line was also xenografted into SCID mice to determine whether rapamycin would chemosensitize the cells in vivo. Because rapamycin targets the mammalian target of rapamycin (mTOR) pathway, we also used our cells to confirm that rapamycin modified the expression of mTOR and effectively suppressed the phosphorylation of two downstream effector molecules in the mTOR pathway, S6K1, and 4E-BP1. We demonstrated that it inhibited the growth of the three cell lines in vitro and one in vivo showing that it modulated both the expression and function of P-gp and chemosensitized the three cell lines as effectively as verapamil.