ATP-binding cassette transporter G2 mediates the efflux of phototoxins on the luminal membrane of retinal capillary endothelial cells

Dissimilar Effect of P-Glycoprotein and Breast Cancer Resistance Protein Inhibition on the Distribution of Erlotinib to the Retina and Brain in Humans and Mice

P-glycoprotein (P-gp) and breast cancer resistance protein (BCRP) are two ATP-binding cassette efflux transporters that are coexpressed at the human blood-brain barrier (BBB) and blood-retina barrier (BRB). While pharmacological inhibition of P-gp and/or BCRP results in increased brain distribution of dual P-gp/BCRP substrate drugs, such as the tyrosine kinase inhibitor erlotinib, the effect of P-gp and/or BCRP inhibition on the retinal distribution of such drugs has hardly been investigated. In this study, we used positron emission tomography (PET) imaging to assess the effect of transporter inhibition on the distribution of [11C]erlotinib to the human retina and brain. Twenty two healthy volunteers underwent two PET scans after intravenous (i.v.) injection of a microdose (<5 μg) of [11C]erlotinib, a baseline scan, and a second scan either with concurrent i.v. infusion of tariquidar to inhibit P-gp (n = 5) or after oral intake of single ascending doses of erlotinib (300 mg, 650 mg, or 1000 mg, n = 17) to saturate erlotinib transport. In addition, transport of [3H]erlotinib to the retina and brain was assessed in mice by in situ carotid perfusion under various drug transporter inhibition settings. In comparison to the baseline PET scan, coadministration of tariquidar or erlotinib led to a significant decrease of [11C]erlotinib total volume of distribution (VT) in the human retina by -25 ± 8% (p ≤ 0.05) and -41 ± 16% (p ≤ 0.001), respectively. In contrast, erlotinib intake led to a significant increase in [11C]erlotinib VT in the human brain (+20 ± 16%, p ≤ 0.001), while administration of tariquidar did not result in any significant changes. In situ carotid perfusion experiments showed that both P-gp and BCRP significantly limit the distribution of erlotinib to the mouse retina and brain but revealed a similar discordant effect at the mouse BRB and BBB following co-perfusion with tariquidar and erlotinib as in humans. Co-perfusion with prototypical inhibitors of solute carrier transporters did not reveal a significant contribution of organic cation transporters (e.g., OCTs and OCTNs) and organic anion-transporting polypeptides (e.g., OATP2B1) to the retinal and cerebral distribution of erlotinib. In conclusion, we observed a dissimilar effect after P-gp and/or BCRP inhibition on the retinal and cerebral distribution of [11C]erlotinib. The exact mechanism for this discrepancy remains unclear but may be related to the function of an unidentified erlotinib uptake carrier sensitive to tariquidar inhibition at the BRB. Our study highlights the great potential of PET to study drug distribution to the human retina and to assess the functional impact of membrane transporters on ocular drug distribution.

Selective drug delivery to the retinal cells: Biological barriers and avenues

Retinal drug delivery is a challenging, but important task, because most retinal diseases are still without any proper therapy. Drug delivery to the retina is hampered by the anatomical and physiological barriers resulting in minimal bioavailability after topical ocular and systemic administrations. Intravitreal injections are current method-of-choice in retinal delivery, but these injections show short duration of action for small molecules and low target bioavailability for many protein, gene based drugs and nanomedicines. State-of-art delivery systems are based on prolonged retention, controlled drug release and physical features (e.g. size and charge). However, drug delivery to the retina is not cell-specific and these approaches do not facilitate intracellular delivery of modern biological drugs (e.g. intracellular proteins, RNA based medicines, gene editing). In this focused review we highlight biological factors and mechanisms that form the basis for the selective retinal drug delivery systems in the future. Therefore, we are presenting current knowledge related to retinal membrane transporters, receptors and targeting ligands in relation to nanomedicines, conjugates, extracellular vesicles, and melanin binding. These issues are discussed in the light of retinal structure and cell types as well as future prospects in the field. Unlike in some other fields of targeted drug delivery (e.g. cancer research), selective delivery technologies have been rarely studied, even though cell targeted delivery may be even more feasible after local administration into the eye.

Newly-established in vitro inner BRB spheroids to elucidate retinal Ang2-linked substance transfer

Conjugation of angiopep-2 (Ang2) with drugs/compounds is known to increase plasma membrane permeability across endothelial barriers. The inner blood-retinal barrier (BRB) regulates retinal drug distribution and is formed by retinal capillary endothelial cells, supported by Müller cells and retinal pericytes. To elucidate the potential of Ang2 conjugation in promoting retinal drug distribution after peripheral administration across the inner BRB, an in vivo administration study and in vitro transport experiments using newly developed multicellular inner BRB spheroids were performed. After intravenous administration of Ang2-linked green fluorescence protein (GFP-Ang2) in mice, GFP-derived signals were observed in the neural retina. In contrast, GFP-derived signals were not observed after intravenous GFP administration, suggesting the promotion of the retinal distribution of substances by Ang2 conjugation. To overcome the limitations of in vitro studies using cells cultured on dishes, inner BRB spheroids were established using conditionally immortalized rat retinal capillary endothelial cells, Müller cells, and retinal pericytes. Immunocytochemistry of marker molecules suggests that the central part of the spheroids is occupied by Müller cells, and encapsulated by retinal pericytes and capillary endothelial cells. Studies on the expression and functions of tight junctions suggest that tight junctions are formed on the surface of the inner BRB spheroids by retinal capillary endothelial cells. The functional expression of drug transporters, such as P-glycoprotein, was observed in the spheroids, implying that the inner side of the spheroids reflects the retinal side of the inner BRB. In the inner BRB spheroids, energy-dependent accumulation of GFP-Ang2 and Ang2-linked 5(6)-carboxyfluorescein (FAM-Ang2) was observed. Moreover, an endocytic inhibition study revealed that clathrin-dependent endocytosis/transcytosis was involved in the transcellular transport of Ang2-conjugated drugs/compounds across the inner BRB. Consequently, it is suggested that the Ang2 linkage is useful for promoting retinal drug distribution via clathrin-dependent transcytosis at the inner BRB.

ABCB1 and ABCG2 Together Limit the Distribution of ABCB1/ABCG2 Substrates to the Human Retina and the ABCG2 Single Nucleotide Polymorphism Q141K (c.421C> A) May Lead to Increased Drug Exposure

The widely expressed and poly-specific ABC transporters breast cancer resistance protein (ABCG2) and P-glycoprotein (ABCB1) are co-localized at the blood-brain barrier (BBB) and have shown to limit the brain distribution of several clinically used ABCB1/ABCG2 substrate drugs. It is currently not known to which extent these transporters, which are also expressed at the blood-retinal barrier (BRB), may limit drug distribution to the human eye and whether the ABCG2 reduced-function single-nucleotide polymorphism (SNP) Q141K (c.421C > A) has an impact on retinal drug distribution. Ten healthy male volunteers (five subjects with the c.421CC and c.421CA genotype, respectively) underwent two consecutive positron emission tomography (PET) scans after intravenous injection of the model ABCB1/ABCG2 substrate [¹¹C]tariquidar. The second PET scan was performed with concurrent intravenous infusion of unlabelled tariquidar to inhibit ABCB1 in order to specifically reveal ABCG2 function.In response to ABCB1 inhibition with unlabelled tariquidar, ABCG2 c.421C > A genotype carriers showed significant increases (as compared to the baseline scan) in retinal radiotracer influx K₁ (+62 ± 57%, p = 0.043) and volume of distribution V_T (+86 ± 131%, p = 0.043), but no significant changes were observed in subjects with the c.421C > C genotype. Our results provide the first evidence that ABCB1 and ABCG2 may together limit the distribution of systemically administered ABCB1/ABCG2 substrate drugs to the human retina. Functional redundancy between ABCB1 and ABCG2 appears to be compromised in carriers of the c.421C > A SNP who may therefore be more susceptible to transporter-mediated drug-drug interactions at the BRB than non-carriers.

Roles of Drug Transporters in Blood-Retinal Barrier

Blood-retinal barrier (BRB) includes inner BRB (iBRB) and outer BRB (oBRB), which are formed by retinal capillary endothelial (RCEC) cells and by retinal pigment epithelial (RPE) cells in collaboration with Bruch's membrane and the choriocapillaris, respectively. Functions of the BRB are to regulate fluids and molecular movement between the ocular vascular beds and retinal tissues and to prevent leakage of macromolecules and other potentially harmful agents into the retina, keeping the microenvironment of the retina and retinal neurons. These functions are mainly attributed to absent fenestrations of RCECs, tight junctions, expression of a great diversity of transporters, and coverage of pericytes and glial cells. BRB existence also becomes a reason that systemic administration for some drugs is not suitable for the treatment of retinal diseases. Some diseases (such as diabetes and ischemia-reperfusion) impair BRB function via altering tight junctions, RCEC death, and transporter expression. This chapter will illustrate function of BRB, expressions and functions of these transporters, and their clinical significances.

Population variability in animal health: Influence on dose-exposure-response relationships: Part I: Drug metabolism and transporter systems

There is an increasing effort to understand the many sources of population variability that can influence drug absorption, metabolism, disposition, and clearance in veterinary species. This growing interest reflects the recognition that this diversity can influence dose-exposure-response relationships and can affect the drug residues present in the edible tissues of food-producing animals. To appreciate the pharmacokinetic diversity that may exist across a population of potential drug product recipients, both endogenous and exogenous variables need to be considered. The American Academy of Veterinary Pharmacology and Therapeutics hosted a 1-day session during the 2017 Biennial meeting to explore the sources of population variability recognized to impact veterinary medicine. The following review highlights the information shared during that session. In Part I of this workshop report, we consider sources of population variability associated with drug metabolism and membrane transport. Part II of this report highlights the use of modeling and simulation to support an appreciation of the variability in dose-exposure-response relationships.

Recent advances in drug and nutrient transport across the blood-retinal barrier

INTRODUCTION

The blood-retinal barrier (BRB) is the barrier separating the blood and neural retina, and transport systems for low-weight molecules at the BRB are expected to be useful for developing drugs for the treatment of ocular neural disorders and maintaining a healthy retina. Areas covered: This review discusses blood-to-retina and retina-to-blood transport of drugs and nutrients at the BRB. In particular, P-gp (ABCB1/MDR1) has low impact on the transport of cationic drugs at the BRB, suggesting a significant role of novel organic cation transporters in influx and efflux transport of lipophilic cationic drugs between blood and the retina. The transport of pravastatin at the BRB involves transporters including organic anion transporting polypeptide 1a4 (Oatp1a4). Recent studies have shown the involvement of solute carrier transporters in the blood-to-retina transport of nutrients including riboflavin, L-ornithine, β-alanine, and L-histidine, implying that dipeptide transport at the BRB is minimal. Expert opinion: Novel organic cation transport systems and the elimination-dominant transport of pravastatin at the BRB are expected to be useful in systemic drug delivery to the neural retina without CNS side effects. The mechanism of nutrient transport at the BRB is expected to provide a new strategy for delivery of nutrient-mimetic drugs.

Expression, activity and pharmacokinetic impact of ocular transporters

The eye is protected by several tissues that limit the permeability and entry of potentially harmful substances, but also hamper the delivery of drugs in the treatment of ocular diseases. Active transport across the ocular barriers may affect drug distribution, but the impact of drug transporters on ocular drug delivery is not well known. We have collected and critically reviewed the literature for ocular expression and activity of known drug transporters. The review concentrates on drug transporters that have been functionally characterized in ocular tissues or primary cells and on transporters for which there is available expression data at the protein level. Species differences are highlighted, since these may explain observed inconsistencies in the influence of specific transporters on drug disposition. There is variable evidence about the pharmacokinetic role of transporters in ocular tissues. The strongest evidence for the role of active transport is available for the blood-retinal barrier. We explored the role of active transport in the cornea and blood retinal barrier with pharmacokinetic simulations. The simulations show that the active transport is important only in the case of specific parameter combinations.

Pharmacokinetic aspects of retinal drug delivery

Drug delivery to the posterior eye segment is an important challenge in ophthalmology, because many diseases affect the retina and choroid leading to impaired vision or blindness. Currently, intravitreal injections are the method of choice to administer drugs to the retina, but this approach is applicable only in selected cases (e.g. anti-VEGF antibodies and soluble receptors). There are two basic approaches that can be adopted to improve retinal drug delivery: prolonged and/or retina targeted delivery of intravitreal drugs and use of other routes of drug administration, such as periocular, suprachoroidal, sub-retinal, systemic, or topical. Properties of the administration route, drug and delivery system determine the efficacy and safety of these approaches. Pharmacokinetic and pharmacodynamic factors determine the required dosing rates and doses that are needed for drug action. In addition, tolerability factors limit the use of many materials in ocular drug delivery. This review article provides a critical discussion of retinal drug delivery, particularly from the pharmacokinetic point of view. This article does not include an extensive review of drug delivery technologies, because they have already been reviewed several times recently. Instead, we aim to provide a systematic and quantitative view on the pharmacokinetic factors in drug delivery to the posterior eye segment. This review is based on the literature and unpublished data from the authors' laboratory.

Topotecan Delivery to the Optic Nerve after Ophthalmic Artery Chemosurgery

Extraocular retinoblastoma is a major challenge worldwide, especially in developing countries. Current treatment involves the administration of systemic chemotherapy combined with radiation, but there is a clear need for improvement of chemotherapy bioavailability in the optic nerve. Our aim was to study the ophthalmic artery chemosurgery (OAC) local route for drug delivery assessing ocular and optic nerve exposure to chemotherapy and to compare it to exposure after intravenous infusion (IV) of the same dose in an animal model. Topotecan was used as a prototype drug that is active in retinoblastoma and based on the extensive knowledge of its pharmacokinetics in preclinical and clinical settings. Five Landrace pigs received 4mg of topotecan via OAC as performed in retinoblastoma patients. At the end of the infusion, the eyes were enucleated, the optic nerve and retina were dissected, and the vitreous and plasma were separated. After recovery and a wash-out period, the animals received a 30-min IV infusion of topotecan (4 mg). The remaining eye was enucleated and tissues and fluids were separated. All samples were stored until quantitation using HPLC. A significantly higher concentration of topotecan in the optic nerve, vitreous, and retina was obtained in eyes after OAC compared to IV infusion (p<0.05). The median (range) ratio between topotecan concentration attained after OAC to IV infusion in the optic nerve, retina and vitreous was 84(54-668), 143(49-200) and 246(56-687), respectively. However, topotecan systemic exposure after OAC and IV infusion remained comparable (p>0.05). The median optic nerve-to-plasma ratio after OAC and IV was 44 and 0.35, respectively. Topotecan OAC delivery attained an 80-fold higher concentration in the optic nerve compared to the systemic infusion of the same dose with similar plasma concentrations in a swine model. Patients with retinoblastoma extension into the optic nerve may benefit from OAC for tumor burden by increased chemotherapy bioavailability in the optic nerve without increasing systemic exposure or toxicity.

Blood-brain and retinal barriers show dissimilar ABC transporter impacts and concealed effect of P-glycoprotein on a novel verapamil influx carrier

BACKGROUND AND PURPOSE

The respective impact and interplay between ABC (P-glycoprotein/P-gp/Abcb1a, BCRP/ABCG2, MRP/ABCC) and SLC transporter functions at the blood-brain barrier (BBB) and blood-retinal barriers (BRB) are incompletely understood.

EXPERIMENTAL APPROACH

We measured the initial cerebral and retinal distribution of selected ABC substrates by in situ carotid perfusion using P-gp/Bcrp knockout mice and chemical ABC/SLC modulation strategies. P-gp, Bcrp, Mrp1 and Mrp4 were studied by confocal retina imaging.

KEY RESULTS

Chemical or physical disruption of P-gp increased [(3) H]-verapamil transport by ~10-fold at the BBB and ~1.5-fold at the BRB. [(3) H]-Verapamil transport involved influx-mediated by an organic cation clonidine-sensitive/diphenhydramine-sensitive proton antiporter at both barriers; this effect was unmasked when P-gp was partially or fully inhibited/disrupted at the BBB. Studies of [(3) H]-mitoxantrone and [(3) H]-zidovudine transport suggested, respectively, that Bcrp efflux was less involved at the BRB than BBB, whereas Mrps were significantly and similarly involved at both barriers. Confocal imaging showed that P-gp and Bcrp were expressed in intra-retinal vessels (inner BRB/iBRB) but absent from the blood/basal membrane of cells of the retinal pigment epithelium (outer BRB/oBRB/RPE) where, in contrast, Mrp1 and Mrp4 were localized.

CONCLUSIONS AND IMPLICATIONS

P-gp, Bcrp, Mrp1 and Mrp4 are differentially expressed at the outer and inner BRB, resulting in an altered ability to limit substrate distribution at the retina as compared with the BBB. [(3) H]-Verapamil distribution is not P-gp-specific and involves a proton antiporter at both the BBB and BRB. However, this transport is concealed by P-gp at the BBB, but not at the BRB, where P-gp activity is reduced.

Prediction of Ocular Drug Distribution from Systemic Blood Circulation

Systemically circulating drugs may distribute to ocular tissues across the blood-ocular barriers. Ocular distribution is utilized in the treatment of ocular diseases with systemic medications, but ocular delivery of systemic drugs and xenobiotics may also lead to adverse ocular effects. Ocular distribution after systemic drug administration has not been predicted or modeled. In this study, distribution clearance between vitreous and plasma was obtained from a previous QSPR model for clearance of intravitreal drugs. These values were used in a pharmacokinetic simulation model to describe entry of unbound drug from plasma to vitreous. The simulation models predicted ocular distribution of 10 systemic drugs in rabbit eyes within 1.96 mean fold error and the distribution of cefepime from plasma to vitreous in humans. This is the first attempt to predict ocular distribution of systemic drugs. Reliable predictions were obtained using systemic concentrations of unbound drug, computational value of ocular distribution clearance, and a simple pharmacokinetic model. This approach can be used in drug discovery to estimate ocular drug exposure at an early stage.

Insights into Chemoresistance of Prostate Cancer

Prostate cancer (PCa) remains the most prevalent malignancy among males in the western world. Though hormonal therapies through chemical or surgical castration have been proposed many years ago, heretofore, such mainstay for the treatment on advanced PCa has not fundamentally changed. These therapeutic responses are temporary and most cases will eventually undergo PCa recurrence and metastasis, or even progress to castration-resistant prostate cancer (CRPC) due to persistent development of drug resistance. Prostate cancer stem cells (PCSCs) are a small population of cells, which possess unlimited self-renewal capacities, and can regenerate tumorigenic progenies, and play an essential role in PCa therapy resistance, metastasis and recurrence. Nowadays advanced progresses have been made in understanding of PCSC properties, roles of androgen receptor signaling and ATP-binding cassette sub-family G member 2 (ABCG2), as well as roles of genomic non-coding microRNAs and key signaling pathways, which have led to the development of novel therapies which are active against chemoresistant PCa and CRPC. Based on these progresses, this review is dedicated to address mechanisms underlying PCa chemoresistance, unveil crosstalks among pivotal signaling pathways, explore novel biotherapeutic agents, and elaborate functional properties and specific roles of chemoresistant PCSCs, which may act as a promising target for novel therapies against chemoresistant PCa.

Role of the breast cancer resistance protein (BCRP/ABCG2) in drug transport--an update

The human breast cancer resistance protein (BCRP, gene symbol ABCG2) is an ATP-binding cassette (ABC) efflux transporter. It was so named because it was initially cloned from a multidrug-resistant breast cancer cell line where it was found to confer resistance to chemotherapeutic agents such as mitoxantrone and topotecan. Since its discovery in 1998, the substrates of BCRP have been rapidly expanding to include not only therapeutic agents but also physiological substances such as estrone-3-sulfate, 17β-estradiol 17-(β-D-glucuronide) and uric acid. Likewise, at least hundreds of BCRP inhibitors have been identified. Among normal human tissues, BCRP is highly expressed on the apical membranes of the placental syncytiotrophoblasts, the intestinal epithelium, the liver hepatocytes, the endothelial cells of brain microvessels, and the renal proximal tubular cells, contributing to the absorption, distribution, and elimination of drugs and endogenous compounds as well as tissue protection against xenobiotic exposure. As a result, BCRP has now been recognized by the FDA to be one of the key drug transporters involved in clinically relevant drug disposition. We published a highly-accessed review article on BCRP in 2005, and much progress has been made since then. In this review, we provide an update of current knowledge on basic biochemistry and pharmacological functions of BCRP as well as its relevance to drug resistance and drug disposition.

Tyrosine kinase inhibitors enhance ciprofloxacin-induced phototoxicity by inhibiting ABCG2

The tyrosine kinase inhibitor (TKI) class of anticancer agents inhibits ABCG2-mediated drug efflux. ABCG2 is an important component of the blood-retinal barrier, where it limits retinal exposure to phototoxic compounds such as fluoroquinolone antibiotics. Patients treated with TKIs would be expected to be at greater risk for retinal phototoxicity. Using an in vitro system, our results indicate that the TKIs gefitinib and imatinib abrogate the ability of ABCG2 to protect cells against ciprofloxacin-induced phototoxicity. We conclude that the concurrent administration of ABCG2 inhibitors with photoreactive fluoroquinolone antibiotics may result in retinal damage.

Differential sensitivity in cell lines to photodynamic therapy in combination with ABCG2 inhibition

BACKGROUND

ABCG2 is an ATP-binding cassette transporter protein which has a role in the regulation of endogenous protoporphyrin IX (PpIX) levels.

OBJECTIVE

To understand the influence of ABCG2 on porphyrin-based photodynamic therapy (PDT) and fluorescence diagnosis (FD), we examined the role of endogenous ABCG2 in four human cell lines from the epidermis (HaCaT keratinocytes), oesophagus (OE19 adenocarcinoma), brain (SH-SY5Y neuroblastoma) and bladder (HT1197 carcinoma).

METHODS

Cells were incubated with ALA or MAL in the presence or absence of the ABCG2 activity inhibitor Ko-143. Porphyrin accumulation was detected by spectrofluorimetric analysis and high performance liquid chromatography (HPLC) with porphyrin localisation observed by confocal laser scanning microscopy. PDT efficacy was assessed 24h post irradiation (1.5J/cm(2) red light) by the neutral red (NR) assay.

RESULTS

We show cell-specific differences when Ko-143 was co-incubated with ALA or, in particular with, MAL. Enhanced PDT-induced cell kill was shown in HaCaT, OE19 and HT1197 cells, but not SH-SY5Y cells and could be explained by porphyrin accumulation and expression of ABCG2. We have also found that despite high levels of intracellular PpIX, the OE19 cells were protected from phototoxic cell death by PpIX compartmentalisation. This could be reversed by Ko-143.

CONCLUSION

The results from this study show a possible cause of reduced sensitivity to ALA/MAL-PDT, with a potential solution to overcome this effect in certain tissue types. The potential to improve PDT with Ko-143 remains promising.

Gene expression profiling of transporters in the solute carrier and ATP-binding cassette superfamilies in human eye substructures

The barrier epithelia of the cornea and retina control drug and nutrient access to various compartments of the human eye. While ocular transporters are likely to play a critical role in homeostasis and drug delivery, little is known about their expression, localization and function. In this study, the mRNA expression levels of 445 transporters, metabolic enzymes, transcription factors and nuclear receptors were profiled in five regions of the human eye: cornea, iris, ciliary body, choroid and retina. Through RNA expression profiling and immunohistochemistry, several transporters were identified as putative targets for drug transport in ocular tissues. Our analysis identified SLC22A7 (OAT2), a carrier for the antiviral drug acyclovir, in the corneal epithelium, in addition to ABCG2 (BCRP), an important xenobiotic efflux pump, in retinal nerve fibers and the retinal pigment epithelium. Collectively, our results provide an understanding of the transporters that serve to maintain ocular homeostasis and which may be potential targets for drug delivery to deep compartments of the eye.

Role of breast cancer resistance protein (BCRP/ABCG2) in cancer drug resistance

Since cloning of the ATP-binding cassette (ABC) family member breast cancer resistance protein (BCRP/ABCG2) and its characterization as a multidrug resistance efflux transporter in 1998, BCRP has been the subject of more than two thousand scholarly articles. In normal tissues, BCRP functions as a defense mechanism against toxins and xenobiotics, with expression in the gut, bile canaliculi, placenta, blood-testis and blood-brain barriers facilitating excretion and limiting absorption of potentially toxic substrate molecules, including many cancer chemotherapeutic drugs. BCRP also plays a key role in heme and folate homeostasis, which may help normal cells survive under conditions of hypoxia. BCRP expression appears to be a characteristic of certain normal tissue stem cells termed "side population cells," which are identified on flow cytometric analysis by their ability to exclude Hoechst 33342, a BCRP substrate fluorescent dye. Hence, BCRP expression may contribute to the natural resistance and longevity of these normal stem cells. Malignant tissues can exploit the properties of BCRP to survive hypoxia and to evade exposure to chemotherapeutic drugs. Evidence is mounting that many cancers display subpopulations of stem cells that are responsible for tumor self-renewal. Such stem cells frequently manifest the "side population" phenotype characterized by expression of BCRP and other ABC transporters. Along with other factors, these transporters may contribute to the inherent resistance of these neoplasms and their failure to be cured.

Strategies for therapy of retinal diseases using systemic drug delivery: relevance of transporters at the blood-retinal barrier

INTRODUCTION

There is an increasing need for managing rapidly progressing retinal diseases because of the potential loss of vision. Although systemic drug administration is one possible route for treating retinal diseases, retinal transfer of therapeutic drugs from the circulating blood is strictly regulated by the blood-retinal barrier (BRB).

AREAS COVERED

This review discusses the constraints and challenges of drug delivery to the retina. In addition, this article discusses the properties of drugs and the conditions of the BRB that affect drug permeability. The reader will gain insights into the strategies for developing therapeutic drugs that are able to cross the BRB for treating retinal diseases. Further, the reader will gain insights into the role of BRB physiology including barrier functions, and the effect of influx and efflux transporters on retinal drug delivery.

EXPERT OPINION

When designing and selecting optimal drug candidates, it's important to consider the fact that they should be recognized by influx transporters and that efflux transporters at the BRB should be avoided. Although lipophilic cationic drugs are known to be transported to the brain across the blood-brain barrier, verapamil transport to the retina is substantially higher than to the brain. Therefore, lipophilic cationic drugs do have a great ability to increase influx transport across the BRB.

Serum-dependent export of protoporphyrin IX by ATP-binding cassette transporter G2 in T24 cells

Accumulation of protoporphyrin IX (PpIX) in cancer cells is a basis of 5-aminolevulinic acid (ALA)-induced photodymanic therapy. We studied factors that affect PpIX accumulation in human urothelial carcinoma cell line T24, with particular emphasis on ATP-binding cassette transporter G2 (ABCG2) and serum in the medium. When the medium had no fetal bovine serum (FBS), ALA induced PpIX accumulation in a time- and ALA concentration-dependent manner. Inhibition of heme-synthesizing enzyme, ferrochelatase, by nitric oxide donor (Noc18) or deferoxamine resulted in a substantial increase in the cellular PpIX accumulation, whereas ABCG2 inhibition by fumitremorgin C or verapamil induced a slight PpIX increase. When the medium was added with FBS, cellular accumulation of PpIX stopped at a lower level with an increase of PpIX in the medium, which suggested PpIX efflux. ABCG2 inhibitors restored the cellular PpIX level to that of FBS(-) samples, whereas ferrochelatase inhibitors had little effects. Bovine serum albumin showed similar effects to FBS. Fluorescence microscopic observation revealed that inhibitors of ABC transporter affected the intracellular distribution of PpIX. These results indicated that ABCG2-mediated PpIX efflux was a major factor that prevented PpIX accumulation in cancer cells in the presence of serum. Inhibition of ABCG2 transporter system could be a new target for the improvement of photodynamic therapy.

Tumor-associated retinal astrocytes promote retinoblastoma cell proliferation through production of IGFBP-5

Retinoblastomas consist of cone-like neoplastic cells and diverse non-neoplastic cells whose roles in tumorigenesis have not been defined. Here, we investigated the glial cells that constitute 2% to 3% of the cells in retinoblastoma tumors, including their origin, their relationship to a potential retinoblastoma stem cell population, and their effects on tumor cell proliferation. Retinoblastoma glia consistently expressed the retinal astrocyte marker Pax2 but inconsistently expressed the Müller cell and occasional astrocyte marker CRALBP. Many of the glia expressed the stem cell-associated Sox2 but nevertheless were non-neoplastic as they coexpressed Rb and/or retained two RB1 alleles. Conversely, the glia were distinct from the non-neoplastic cells that strongly expressed the stem cell-associated ABCG2. Adherent Pax2(+),Sox2(+),Rb(+) glia readily grew from explanted retinoblastomas and produced soluble factors that enhanced the proliferation of cocultured retinoblastoma cells. This effect was emulated by normal retinal glia and appeared to be mediated by insulin-like growth factor binding protein-5 (IGFBP-5), as it was mimicked by recombinant IGFBP-5 and mitigated by neutralizing IGFBP-5 antibody. As glia-derived IGFBP-5 was earlier found to promote photoreceptor survival, our findings indicate that retinal astrocytes enhance the proliferation of cone-like retinoblastoma cells by deploying a factor that also provides trophic support to the tumor cells' non-neoplastic counterparts. These observations suggest that a tissue-specific microenvironmental feature cooperates with oncogenic mutations in a cancer cell of origin to promote retinoblastoma tumorigenesis.

Xenobiotic, bile acid, and cholesterol transporters: function and regulation

Transporters influence the disposition of chemicals within the body by participating in absorption, distribution, and elimination. Transporters of the solute carrier family (SLC) comprise a variety of proteins, including organic cation transporters (OCT) 1 to 3, organic cation/carnitine transporters (OCTN) 1 to 3, organic anion transporters (OAT) 1 to 7, various organic anion transporting polypeptide isoforms, sodium taurocholate cotransporting polypeptide, apical sodium-dependent bile acid transporter, peptide transporters (PEPT) 1 and 2, concentrative nucleoside transporters (CNT) 1 to 3, equilibrative nucleoside transporter (ENT) 1 to 3, and multidrug and toxin extrusion transporters (MATE) 1 and 2, which mediate the uptake (except MATEs) of organic anions and cations as well as peptides and nucleosides. Efflux transporters of the ATP-binding cassette superfamily, such as ATP-binding cassette transporter A1 (ABCA1), multidrug resistance proteins (MDR) 1 and 2, bile salt export pump, multidrug resistance-associated proteins (MRP) 1 to 9, breast cancer resistance protein, and ATP-binding cassette subfamily G members 5 and 8, are responsible for the unidirectional export of endogenous and exogenous substances. Other efflux transporters [ATPase copper-transporting beta polypeptide (ATP7B) and ATPase class I type 8B member 1 (ATP8B1) as well as organic solute transporters (OST) alpha and beta] also play major roles in the transport of some endogenous chemicals across biological membranes. This review article provides a comprehensive overview of these transporters (both rodent and human) with regard to tissue distribution, subcellular localization, and substrate preferences. Because uptake and efflux transporters are expressed in multiple cell types, the roles of transporters in a variety of tissues, including the liver, kidneys, intestine, brain, heart, placenta, mammary glands, immune cells, and testes are discussed. Attention is also placed upon a variety of regulatory factors that influence transporter expression and function, including transcriptional activation and post-translational modifications as well as subcellular trafficking. Sex differences, ontogeny, and pharmacological and toxicological regulation of transporters are also addressed. Transporters are important transmembrane proteins that mediate the cellular entry and exit of a wide range of substrates throughout the body and thereby play important roles in human physiology, pharmacology, pathology, and toxicology.

Role of intestinal efflux transporters in the intestinal absorption of methotrexate in rats

The role of intestinal efflux transporters such as P-glycoprotein (P-gp), breast cancer resistance protein (BCRP) and multidrug resistance-associated proteins (MRPs) in intestinal absorption of methotrexate was examined in rats. In everted intestine, the mucosal efflux of methotrexate after application to serosal side was higher in jejunum than ileum, and the efflux in jejunum was suppressed by pantoprazole, a BCRP inhibitor, and probenecid, an MRP inhibitor, but not by verapamil, a P-gp inhibitor. The mucosal methotrexate efflux in ileum was suppressed by pantoprazole, but not by other inhibitors. On the other hand, the serosal efflux of methotrexate after application to mucosal side was greater in ileum than jejunum, and was suppressed by probenecid. In in-vivo rat studies, the intestinal absorption of methotrexate was significantly higher when methotrexate was administered to ileum than jejunum. Pantoprazole increased methotrexate absorption from jejunum and ileum. Probenecid increased the absorption of methotrexate from jejunum but decreased the absorption from ileum, as evaluated by peak plasma methotrexate levels. In conclusion, BCRP and MRPs are involved in the regional difference in absorption of methotrexate along the intestine, depending on their expression sites.

Inner blood-retinal barrier transporters: role of retinal drug delivery

The inner blood-retinal barrier (inner BRB) forms complex tight junctions of retinal capillary endothelial cells to prevent the free diffusion of substances between the circulating blood and the neural retina. Thus, understanding of the inner BRB transport mechanisms could provide a basis for the development of strategies for drug delivery to the retina. Recent progress in inner BRB research has revealed that retinal endothelial cells express a variety of unique transporters which play a role in the influx transport of essential molecules and the efflux transport of xenobiotics. In this review we focus on the transport mechanism at the inner BRB in relation to its importance in influencing the inner BRB permeability of drugs.

Efflux protein expression in human retinal pigment epithelium cell lines

PURPOSE

The objective of this study was to characterize efflux proteins (P-glycoprotein (P-gp), multidrug resistance proteins (MRP1-6) and breast cancer resistance protein (BCRP)) of retinal pigment epithelium (RPE) cell lines.

METHODS

Expression of efflux proteins in two secondary (ARPE-19, D407) and two primary (HRPEpiC and bovine) RPE cell lines was measured by quantitative RT-PCR and western blotting. Furthermore, activity of MRP1 and MRP5 of ARPE-19 cell line was assessed with calcein-AM and carboxydichlorofluorescein (CDCF) probes.

RESULTS

Similar efflux protein profile was shared between ARPE-19 and primary RPE cells, whereas D407 cell line was notably different. D407 cells expressed MRP2 and BCRP, which were absent in other cell lines and furthermore higher MRP3 transcript expression was found. MRP1, MRP4 and MRP5 were identified from all human RPE cell lines and MRP6 was not expressed in any cell lines. The pattern of efflux protein expression did not change when ARPE-19 cells were differentiated on filters. The calcein-AM and CDCF efflux tests provided evidence supporting MRP1 and MRP5 activity in ARPE-19 cells.

CONCLUSIONS

MRP1, MRP4 and MRP5 are the main efflux transporters in RPE cell lines. There are differences in efflux protein expression between RPE cell lines.

Physiological and pharmacological roles of ABCG2 (BCRP): recent findings in Abcg2 knockout mice

The multidrug transporter ABCG2 (BCRP/MXR/ABCP) can actively extrude a broad range of endogenous and exogenous substrates across biological membranes. ABCG2 limits oral availability and mediates hepatobiliary and renal excretion of its substrates, and thus influences the pharmacokinetics of many drugs. Recent work, relying mainly on the use of Abcg2(-/-) mice, has revealed important contributions of ABCG2 to the blood-brain, blood-testis and blood-fetal barriers. Together, these functions indicate a primary biological role of ABCG2 in protecting the organism from a range of xenobiotics. In addition, several other physiological functions of ABCG2 have been observed, including extrusion of porphyrins and/or porphyrin conjugates from hematopoietic cells, liver and harderian gland, as well as secretion of vitamin B(2) (riboflavin) and possibly other vitamins (biotin, vitamin K) into breast milk. However, the physiological significance of these processes has been difficult to establish, indicating that there is still a lot to learn about this intriguing protein.

Characterization of the mechanism of zidovudine uptake by rat conditionally immortalized syncytiotrophoblast cell line TR-TBT

PURPOSE

To characterize the uptake mechanism of zidovudine (AZT), a nucleoside reverse transcriptase inhibitor, in syncytiotrophoblast cells using the TR-TBT 18d-1 cell line previously established by our group.

MATERIALS AND METHODS

The effects of several transporter inhibitors on the initial and steady-state apical uptake of AZT by TR-TBT 18d-1 were characterized, in order to identify the transporter(s) involved.

RESULTS

Initial uptake of AZT was sodium-independent and saturable; the K(m) value was about 16 microM. Nitrobenzylthioinosine (NBMPR), probenecid and cimetidine each had little effect on the saturable AZT uptake, indicating that well characterized transporters, such as organic anion transporters (OATs and OATPs), organic cation transporters (OCTs) and equilibrative nucleoside transporters (ENTs), are not involved. However, thymidine and 2'-deoxyuridine strongly inhibited AZT uptake. These results suggest that an unidentified nucleoside uptake transporter is responsible for the uptake of AZT. Cyclosporin A, Ko143 and probenecid had little effect on AZT accumulation by TR-TBT 18d-1 cells, suggesting that transporter-mediated efflux of AZT is not substantial.

CONCLUSION

Our results indicate that saturable AZT uptake into TR-TBT 18d-1 is mediated by a so-far-unidentified transporter.

Regulation of retinal blood flow in health and disease

Optimal retinal neuronal cell function requires an appropriate, tightly regulated environment, provided by cellular barriers, which separate functional compartments, maintain their homeostasis, and control metabolic substrate transport. Correctly regulated hemodynamics and delivery of oxygen and metabolic substrates, as well as intact blood-retinal barriers are necessary requirements for the maintenance of retinal structure and function. Retinal blood flow is autoregulated by the interaction of myogenic and metabolic mechanisms through the release of vasoactive substances by the vascular endothelium and retinal tissue surrounding the arteriolar wall. Autoregulation is achieved by adaptation of the vascular tone of the resistance vessels (arterioles, capillaries) to changes in the perfusion pressure or metabolic needs of the tissue. This adaptation occurs through the interaction of multiple mechanisms affecting the arteriolar smooth muscle cells and capillary pericytes. Mechanical stretch and increases in arteriolar transmural pressure induce the endothelial cells to release contracting factors affecting the tone of arteriolar smooth muscle cells and pericytes. Close interaction between nitric oxide (NO), lactate, arachidonic acid metabolites, released by the neuronal and glial cells during neural activity and energy-generating reactions of the retina strive to optimize blood flow according to the metabolic needs of the tissue. NO, which plays a central role in neurovascular coupling, may exert its effect, by modulating glial cell function involved in such vasomotor responses. During the evolution of ischemic microangiopathies, impairment of structure and function of the retinal neural tissue and endothelium affect the interaction of these metabolic pathways, leading to a disturbed blood flow regulation. The resulting ischemia, tissue hypoxia and alterations in the blood barrier trigger the formation of macular edema and neovascularization. Hypoxia-related VEGF expression correlates with the formation of neovessels. The relief from hypoxia results in arteriolar constriction, decreases the hydrostatic pressure in the capillaries and venules, and relieves endothelial stretching. The reestablished oxygenation of the inner retina downregulates VEGF expression and thus inhibits neovascularization and macular edema. Correct control of the multiple pathways, such as retinal blood flow, tissue oxygenation and metabolic substrate support, aiming at restoring retinal cell metabolic interactions, may be effective in preventing damage occurring during the evolution of ischemic microangiopathies.

Gene expression profiles of ATP-binding cassette transporter A and C subfamilies in mouse retinal vascular endothelial cells

The purpose of this study was to quantify gene expression levels of the ATP-binding cassette (ABC) transporter A and C subfamilies ABCA1-A9, and ABCC1-6/Mrp1-6, C10/Mrp7 in mouse retinal vascular endothelial cells (RVEC) using a combination of a magnetic isolation method for mouse RVEC and real-time quantitative PCR analysis. The transcript level of endothelial cell markers, such as CD31, Tie-2, claudin-5, occludin, ABCB1a/mdr1a, and ABCG2, were more than 20-fold higher than those in the non-RVEC fraction, suggesting that RVEC in the RVEC fraction are concentrated at least 20-fold compared with those of the non-RVEC fraction. In the ABCA1 to A9 families, the transcript level of ABCA3 and A9 in the RVEC fraction was 1.2- and 32-fold higher than that in the non-RVEC fraction. Although ABCA3 was expressed in both the RVEC and non-RVEC fractions, A9 is predominantly expressed in the RVEC fraction. In the ABCC1 to C6 and C10 families, the transcript level of ABCC3, C4, and C6 in the RVEC fraction was 27-, 251-, and 242-fold higher, respectively, than that in the non-RVEC fraction, suggesting that ABCC3, C4, and C6 are predominantly expressed in the RVEC. In conclusion, ABCA3, ABCA9, ABCC3, ABCC4, and ABCC6 mRNAs are predominantly expressed at the inner blood-retina barrier (inner BRB) and appear to play a major role in the efflux transport of their substrates at the inner BRB.

Drug transport in corneal epithelium and blood-retina barrier: emerging role of transporters in ocular pharmacokinetics

Corneal epithelium and blood-retina barrier (i.e. retinal capillaries and retinal pigment epithelium (RPE)) are the key membranes that regulate the access of xenobiotics into the ocular tissues. Corneal epithelium limits drug absorption from the lacrimal fluid into the anterior chamber after eyedrop administration, whereas blood-retina barrier restricts the entry of drugs from systemic circulation to the posterior eye segment. Like in general pharmacokinetics, the role of transporters has been considered to be quite limited as compared to the passive diffusion of drugs across the membranes. As the functional role of transporters is being revealed it has become evident that the transporters are widely important in pharmacokinetics. This review updates the current knowledge about the transporters in the corneal epithelium and blood-retina barrier and demonstrates that the information is far from complete. We also show that quite many ocular drugs are known to interact with transporters, but the studies about the expression and function of those transporters in the eye are still sparse. Therefore, the transporters probably have greater role in ocular pharmacokinetics than we currently realise.