Involvement of Carrier-Mediated Transport in the Retinal Uptake of Clonidine at the Inner Blood–Retinal Barrier

Interaction of systemic drugs causing ocular toxicity with organic cation transporter: an artificial intelligence prediction

Chronic disease patients (cancer, arthritis, cardiovascular diseases) undergo long-term systemic drug treatment. Membrane transporters in ocular barriers could falsely recognize these drugs and allow their trafficking into the eye from systemic circulation. Hence, despite their pharmacological activity, these drugs accumulate and cause toxicity at the non-target site, such as the eye. Since around 40% of clinically used drugs are organic cation in nature, it is essential to understand the role of organic cation transporter (OCT1) in ocular barriers to facilitate the entry of systemic drugs into the eye. We applied machine learning techniques and computer simulation models (molecular dynamics and metadynamics) in the current study to predict the potential OCT1 substrates. Artificial intelligence models were developed using a training dataset of a known substrates and non-substrates of OCT1 and predicted the potential OCT1 substrates from various systemic drugs causing ocular toxicity. Computer simulation studies was performed by developing the OCT1 homology model. Molecular dynamic simulations equilibrated the docked protein-ligand complex. And metadynamics revealed the movement of substrates across the transporter with minimum free energy near the binding pocket. The machine learning model showed an accuracy of about 80% and predicted the potential substrates for OCT1 among systemic drugs causing ocular toxicity - not known earlier, such as cyclophosphamide, bupivacaine, bortezomib, sulphanilamide, tosufloxacin, topiramate, and many more. However, further invitro and invivo studies are required to confirm these predictions.Communicated by Ramaswamy H. Sarma.

Characterization of LysoTracker Red uptake by in vitro model cells of the outer blood-retinal barrier: Implication of lysosomal trapping with cytoplasmic vacuolation and cytotoxicity

Lysosomal trapping, a physicochemical process in which lipophilic cationic compounds are sequestered in lysosomes, can affect drug disposition and cytotoxicity. To better understand lysosomal trapping at the outer blood-retinal barrier (BRB), we investigated the distribution of LysoTracker Red (LTR), a probe compound for lysosomal trapping, in conditionally immortalized rat retinal pigment epithelial (RPE-J) cells. LTR uptake by RPE-J cells was dependent on temperature and attenuated by ammonium chloride and protonophore, which decreased the pH gradient between the lysosome and cytoplasm, suggesting lysosomal trapping of LTR in RPE-J cells. The involvement of lysosomal trapping in response to cationic drugs, including neuroprotectants such as desipramine and memantine, was also suggested by an inhibition study of LTR uptake. Chloroquine, which is known to show ocular toxicity, induced cytoplasmic vacuolization in RPE-J cells with a half-maximal effective concentration of 1.35 μM. This value was 59 times lower than the median lethal concentration (= 79.1 μM) of chloroquine, suggesting that vacuolization was not a direct trigger of cell death. These results are helpful for understanding the lysosomal trapping of cationic drugs, which is associated with drug disposition and cytotoxicity in the outer BRB.

[Role of the Blood-Retinal Barrier Transporters: Antiaging in Retina]

Since the retina continuously receives light to enable vision, reactive oxygen species (ROS) are easily generated in neural retina. The oxidative stress induced by ROS may be involved in the onset and progression of blinding aging diseases such as age-related macular degeneration, diabetic retinopathy, and glaucoma. Although supply of antioxidants to the retina is important to maintain the redox homeostasis in neural retina, the blood-retinal barrier (BRB) is created by complex tight-junctions of retinal capillary endothelial cells and retinal pigment epithelial cells to prevent the free diffusion of substances. The BRB is equipped with several membrane transporters to supply nutrients and essential molecules including antioxidants and drugs which exhibit antiaging effect to the retina from the circulating blood. In this review, the transporter-mediated retinal distribution of key endogenous compounds and drugs, such as vitamin C, l-cystine and gabapentin, is introduced for antiaging of the retina.

Exploring the systemic delivery of a poorly water-soluble model drug to the retina using PLGA nanoparticles

During the drug development process, many pharmacologically active compounds are discarded because of poor water solubility, but nanoparticle-based formulations are increasingly proposed as a solution for this problem. We therefore studied the distribution of nanoparticulate carriers and the delivery of their poorly water-soluble cargo to a structure of the central nervous system, the retina, under naive and pathological conditions. The lipophilic fluorescent dye coumarin 6 (Cou6) was encapsulated into poly(lactic-co-glycolic acid) PLGA nanoparticles (NPs). After intravenous administration in rats, we analyzed the distribution of cargo Cou6 and of the NP carrier covalently labeled with Cy5.5 in healthy animals and animals with optic nerve crush (ONC). In vivo real-time retina imaging revealed that Cou6 was rapidly released from PLGA NPs and penetrated the inner blood-retina barrier (BRB) within 15 min and PLGA NPs were gradually eliminated from the retinal blood circulation. Ex vivo microscopy of retinal flat mounts indicated that the Cou6 accumulated predominantly in the extracellular space and to a lesser extent in neurons. While the distribution of Cou6 in healthy animals and post ONC was comparable at early time point post-operation, the elimination of the NPs from the vessels was faster on day 7 post ONC. These results demonstrate the importance of considering different kinetics of nano-carrier and poorly water-soluble cargo, emphasizing the critical role of their parenchymal distribution, i.e. cellular/extracellular, and function of different physiological and pathological conditions.

Comprehensive Evidence of Carrier-Mediated Distribution of Amantadine to the Retina across the Blood-Retinal Barrier in Rats

Amantadine, a drug used for the blockage of NMDA receptors, is well-known to exhibit neuroprotective effects. Accordingly, assessment of amantadine transport at retinal barriers could result in the application of amantadine for retinal diseases such as glaucoma. The objective of this study was to elucidate the retinal distribution of amantadine across the inner and outer blood–retinal barrier (BRB). In vivo blood-to-retina [³H]amantadine transport was investigated by using the rat retinal uptake index method, which was significantly reduced by unlabeled amantadine. This result indicated the involvement of carrier-mediated processes in the retinal distribution of amantadine. In addition, in vitro model cells of the inner and outer BRB (TR-iBRB2 and RPE-J cells) exhibited saturable kinetics (K_m in TR-iBRB2 cells, 79.4 µM; K_m in RPE-J cells, 90.5 and 9830 µM). The inhibition of [³H]amantadine uptake by cationic drugs/compounds indicated a minor contribution of transport systems that accept cationic drugs (e.g., verapamil), as well as solute carrier (SLC) organic cation transporters. Collectively, these outcomes suggest that carrier-mediated transport systems, which differ from reported transporters and mechanisms, play a crucial role in the retinal distribution of amantadine across the inner/outer BRB.

Optical glucose biosensor built-in disposable strips and wearable electronic devices

On-demand screening, real-time monitoring and rapid diagnosis of ubiquitous diseases, such as diabetes, at early stages are indispensable in personalised treatment. Emerging impacts of nano/microscale materials on optical and portable biosensor strips and devices have become increasingly important in the remarkable development of sensitive visualisation (i.e. visible inspection by the human eye) assays, low-cost analyses and personalised home testing of patients with diabetes. With the increasing public attention regarding the self-monitoring of diabetes, the development of visual readout, easy-to-use and wearable biosensors has gained considerable interest. Our comprehensive review bridges the practical assessment gap between optical bio-visualisation assays, disposable test strips, sensor array designs and full integration into flexible skin-based or contact lens devices with the on-site wireless signal transmission of glucose detection in physiological fluids. To date, the fully modulated integration of nano/microscale optical biosensors into wearable electronic devices, such as smartphones, is critical to prolong periods of indoor and outdoor clinical diagnostics. Focus should be given to the improvements of invasive, wireless and portable sensing technologies to improve the applicability and reliability of screen display, continuous monitoring, dynamic data visualisation, online acquisition and self and in-home healthcare management of patients with diabetes.

Uptake Study in Lysosome-Enriched Fraction: Critical Involvement of Lysosomal Trapping in Quinacrine Uptake but Not Fluorescence-Labeled Verapamil Transport at Blood-Retinal Barrier

Lysosomal trapping at the blood-retinal barrier (BRB) was investigated through quinacrine and fluorescence-labeled verapamil (EFV) uptake. Quinacrine uptake by conditionally immortalized rat retinal capillary endothelial (TR-iBRB2) cells suggested saturable and non-saturable transport processes in the inner BRB. The reduction of quinacrine uptake by bafilomycin A1 suggested quinacrine distribution to the acidic intracellular compartments of the inner BRB, and this notion was also supported in confocal microscopy. In the study using the lysosome-enriched fraction of TR-iBRB2 cells, quinacrine uptake was inhibited by bafilomycin A1, suggesting the lysosomal trapping of quinacrine in the inner BRB. Pyrilamine, clonidine, and nicotine had no effect on quinacrine uptake, suggesting the minor role of lysosomal trapping in their transport across the inner BRB. Bafilomycin A1 had no effect on EFV uptake, and lysosomal trapping driven by the acidic interior pH was suggested as a minor mechanism for EFV transport in the inner BRB. The minor contribution of lysosomal trapping was supported by the difference in inhibitory profiles between EFV and quinacrine uptakes. Similar findings were observed in the outer BRB study with the fraction of conditionally immortalized rat retinal pigment epithelial (RPE-J) cells. These results suggest the usefulness of lysosome-enriched fractions in studying lysosomal trapping at the BRB.

Cellular uptake properties of lamotrigine in human placental cell lines: Investigation of involvement of organic cation transporters (SLC22A1-5)

Lamotrigine (LTG) is an important antiepileptic drug for the treatment of seizures in pregnant women with epilepsy. However, it is not known if the transport of LTG into placental cells occurs via a carrier-mediated pathway. The aim of this study was to investigate the uptake properties of LTG into placental cell lines (BeWo and JEG-3), and to determine the involvement of organic cation transporters (OCTs, SLC22A1-3) and organic cation/carnitine transporter (OCTNs, SLC22A4-5) in the uptake process. The uptake of LTG at 37 °C was higher than that at 4 °C. OCT1 and OCTNs were detected in both cell lines. The uptake of LTG was not greatly affected by the extracellular pH, Na⁺-free conditions, or the presence of l-carnitine, suggesting that OCTNs were not involved. Although several potent inhibitors of OCTs (chloroquine, imipramine, quinidine, and verapamil) inhibited LTG uptake, other typical inhibitors had no effect. In addition, siRNA targeted to OCT1 had no significant effect on LTG uptake. The mRNA expression in human term placenta followed the order OCTN2 > OCT3 > OCTN1 > OCT1 ≈ OCT2. These observations suggested that LTG uptake into placental cells was carrier-mediated, but that OCTs and OCTNs were not responsible for the placental transport process.

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.

Blood-to-Retina Transport of Imperatorin Involves the Carrier-Mediated Transporter System at the Inner Blood-Retinal Barrier

This study investigated the mechanism of transporting imperatorin across the inner blood-retinal barrier (iBRB). The carotid artery single injection method was used to calculate the retinal uptake index (RUI) of [³H]imperatorin in vivo, whereas the retinal capillary endothelial cell lines were used for the in vitro uptake and mRNA expression assays. RUI value of [³H]imperatorin was greater than that of the reference compound ([¹⁴C]n-butanol). [³H]Imperatorin significantly reduced the RUI in the presence of neuroprotective organic cationic drugs at 10 mM. However, tetraethylammonium and p-aminohippuric acid showed no significant effects. [³H]Imperatorin uptake by TR-iBRB2 cells was time-, pH-, energy-, and concentration-dependent with a K_m value of 679 ± 130 μM. In addition, the uptake study showed insensitivity to sodium and membrane potential. Various organic cations including pyrilamine, nicotine, and clonidine significantly reduced the uptake of [³H]imperatorin, whereas organic anions and monocarboxylic acids did not. Furthermore, the mRNA expression level dropped markedly with rOCTN1, rOCTN2, rPMAT, and rMATE1 small interfering RNAs in the transfection study. Moreover, [³H]imperatorin uptake remained neutral with small interfering RNA transfections. Our results indicate that imperatorin transport across the iBRB involves carrier-mediated transporter system.

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.

Blood-to-Retina Transport of Fluorescence-Labeled Verapamil at the Blood-Retinal Barrier

PURPOSE

To investigate the blood-to-retina verapamil transport at the blood-retinal barrier (BRB).

METHODS

EverFluor FL Verapamil (EFV) was adopted as the fluorescent probe of verapamil, and its transport across the BRB was investigated with common carotid artery infusion in rats. EFV transport at the inner and outer BRB was investigated with TR-iBRB2 cells and RPE-J cells, respectively.

RESULTS

The signal of EFV was detected in the retinal tissue during the weak signal of cell impermeable compound. In TR-iBRB2 cells, the localization of EFV differed from that of LysoTracker^® Red, a lysosomotropic agent, and was not altered by acute treatment with NH₄Cl. In RPE-J cells, the punctate distribution of EFV was partially observed, and this was reduced by acute treatment with NH₄Cl. EFV uptake by TR-iBRB2 cells was temperature-dependent and membrane potential- and pH-independent, and was significantly reduced by NH₄Cl treatment during no significant effect obtained by different extracellular pH and V-ATPase inhibitor. The EFV uptake by TR-iBRB2 cells was inhibited by cationic drugs, and inhibited by verapamil in a concentration-dependent manner with an IC₅₀ of 98.0 μM.

CONCLUSIONS

Our findings provide visual evidence to support the significance of carrier-mediated transport in the blood-to-retina verapamil transport at the BRB.

[Carrier-mediated Transport of Cationic Drugs across the Blood-Tissue Barrier]

Studies of neurological dysfunction have revealed the neuroprotective effect of several cationic drugs, suggesting their usefulness in the treatment of neurological diseases. In the brain and retina, blood-tissue barriers such as blood-brain barrier (BBB) and blood-retinal barrier (BRB) are formed to restrict nonspecific solute transport between the circulating blood and neural tissues. Therefore study of cationic drug transport at these barriers is essential to achieve systemic delivery of neuroprotective agents into the neural tissues. In the retina, severe diseases such as diabetic retinopathy and macular degeneration can cause neurological dysfunction that dramatically affects patients' QOL. The BRB is formed by retinal capillary endothelial cells (inner BRB) and retinal pigment epithelial cells (outer BRB). Blood-to-retina transport of cationic drugs was investigated at the inner BRB, which is known to nourish two thirds of the retina. Blood-to-retinal transport of verapamil suggested that the barrier function of the BRB differs from that of the BBB. Moreover, carrier-mediated transport of verapamil and pyrilamine revealed the involvement of novel organic cation transporters at the inner BRB. The identified transport systems for cationic drugs are sensitive to several cationic neuroprotective and anti-angiogenic agents such as clonidine and propranolol, and the involvement of novel transporters was also suggested in their blood-to-retina transport across the inner BRB.

A polyspecific drug/proton antiporter mediates diphenhydramine and clonidine transport at the mouse blood-retinal barrier

BACKGROUND AND PURPOSE

Transporters at the blood-retinal barrier (BRB), as at the blood-brain barrier (BBB), regulate the distribution of compounds into the neural parenchyma. However, the expression of BRB transporters and their quantitative impact in vivo are still poorly understood.

EXPERIMENTAL APPROACH

Clonidine and diphenhydramine are substrates of a novel BBB drug/proton-antiporter. We evaluated their transport at the BRB by in situ carotid perfusion in wild-type or knocked-out mice for Oct1-3 (Slc22a1-3).

KEY RESULTS

At pharmacological exposure levels, carrier-mediated BRB influx was 2 and 12 times greater than the passive diffusion rate for clonidine and diphenhydramine, respectively. Functional identification demonstrated the involvement of a high-capacity potassium- and sodium-independent proton-antiporter that shared the features of the previously characterized clonidine, diphenhydramine and cocaine BBB transporter. The functional characterization suggests that SLC transporters Oct1-3, Mate1 (Slc47a1) and Octn1-2 (Slc22a4-5) are not involved. Melanin/retinal toxic drugs like antimalarials (amodiaquine, quinine), quinidine and tricyclic antidepressants (imipramine) acted as inhibitors of this proton-antiporter. The endogenous indole derivative tryptamine inhibited the transporter, unlike 5-HT (serotonin), dopamine or L-DOPA. Trans-stimulation experiments with [(3) H]-clonidine at the BRB indicated that diphenhydramine, nicotine, oxycodone, naloxone, tramadol, 3,4-methylenedioxyamphetamine (MDMA, ecstasy), heroin, methadone and verapamil are common substrates.

CONCLUSIONS AND IMPLICATIONS

A proton-antiporter is physiologically involved in the transport of clonidine and diphenhydramine and is quantitatively more important than their passive diffusion flux at the mouse BRB. The features of this molecularly unidentified transporter highlight its importance in regulating drug delivery at the retina and suggest that it has the capacity to handle several drugs.