Role of l-Type Amino Acid Transporter 1 at the Inner Blood-Retinal Barrier in the Blood-to-Retina Transport of Gabapentin

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.

Branched-Chain Amino Acids Metabolism and Their Roles in Retinopathy: From Relevance to Mechanism

Retinopathy is one of the leading causes of irreversible blindness and vision loss worldwide. Imbalanced nutrients play important roles in the pathogenesis and pathophysiology of retinal diseases. Branched-Chain Amino Acids (BCAAs), as essential amino acids, perform a variety of biological functions, including protein synthesis, glucose metabolism, lipid metabolism, inflammation, and oxidative stress in metabolic tissues of diabetes and aging-related diseases. Recently, it has been shown that BCAAs are highly related to neuroprotection, oxidative stress, inflammatory and glutamate toxicity in the retina of retinopathy. Therefore, this review summarizes the alterations of BCAA levels in retinopathy, especially diabetic retinopathy and aging-related macular disease, and the genetics, functions, and mechanisms of BCAAs in the retina as well as other metabolic tissues for reference. All of these efforts aim to provide fundamental knowledge of BCAAs for further discoveries and research on retina health based on the sensing and signaling of essential amino acids.

Development of Sesamol Carbamate-L-Phenylalanine Prodrug Targeting L-Type Amino Acid Transporter1 (LAT1) as a Potential Antiproliferative Agent against Melanoma

Sesamol is a compound reported to have anti-melanogenesis and anti-melanoma actions. Sesamol, however, has low intracellular drug concentration and fast excretion, which can limit its benefits in the clinic. To overcome this drawback and increase intracellular delivery of sesamol into the target melanoma, research has focused on L-type amino acid transporter 1 (LAT1)-mediated prodrug delivery into melanoma cells. The sesamol prodrug was designed by conjugating sesamol with L-phenylalanine at the para position with a carbamate bond. LAT1 targeting was evaluated vis-à-vis a competitive [¹⁴C]-leucine uptake inhibition. The sesamol prodrug has a higher [¹⁴C]-leucine uptake inhibition than sesamol in human LAT1-transfected HEK293 cells. Moreover, the sesamol prodrug was taken up by LAT1-mediated transport into SK-MEL-2 cells more effectively than sesamol. The sesamol prodrug underwent complete hydrolysis, releasing the active sesamol at 72 h, which significantly exerted its cytotoxicity (IC₅₀ of 29.3 µM) against SK-MEL-cells more than sesamol alone. Taken together, the strategy for LAT1-mediated prodrug delivery has utility for the selective uptake of sesamol, thereby increasing its intracellular concentration and antiproliferation activity, targeting melanoma SK-MEL-2 cells that overexpress the LAT1 protein. The sesamol prodrug thus warrants further evaluation in an in vivo model.

Route of intracellular uptake and cytotoxicity of sesamol, sesamin, and sesamolin in human melanoma SK-MEL-2 cells

The intracellular uptake concentration determines drug absorption, drug activity, and toxicity. Sesamol, sesamin, and sesamolin are promising bioactive components from Sesame indicum L. Their respective intracellular uptake pathway and cytotoxicity were evaluated using melanoma and non-cancerous cells. Quantitative structure-activity relationship (QSAR) models were built to identify the molecular features affecting drug uptake in cells. The respective intracellular uptake pathway for sesamol vs. sesamin and sesamolin was carrier-mediated vs. passive transport. Topological polar surface area (PSA) and 2D autocorrections increase the intracellular concentration (C/M ratio) of these compounds. Sesamol has the lowest C/M ratio compared to sesamin and sesamolin, but only sesamol inhibits the cell viability of melanoma and provides an inhibition concentration at 50% (IC₅₀) against melanoma cells. The slightly aqueous solubility of sesamin and sesamolin, therefore, limits testing of their cytotoxicity. In conclusion, sesamol has the potential to inhibit melanoma cell growth, but requires improvement of the C/M ratio to increase its physicochemical properties. Thus, in order to investigate the cytotoxicity of sesamin and sesamolin against melanoma cells a solubility enhancer is needed.

[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.

Participation of Monocarboxylate Transporter 8, But Not P-Glycoprotein, in Carrier-Mediated Cerebral Elimination of Phenytoin across the Blood-Brain Barrier

PURPOSE

In this study, we investigated in detail the transport of phenytoin across the blood-brain barrier (BBB) to identify the transporter(s) involved in BBB-mediated phenytoin efflux from the brain.

METHODS

We evaluated the brain-to-blood efflux transport of phenytoin in vivo by determining the brain efflux index (BEI) and uptake in brain slices. We additionally conducted brain perfusion experiments and BEI studies in P-glycoprotein (P-gp)-deficient mice. In addition, we determined the mRNA expression of monocarboxylate transporter (MCT) in isolated brain capillaries and performed phenytoin uptake studies in MCT-expressing Xenopus oocytes.

RESULTS

[¹⁴C]Phenytoin brain efflux was time-dependent with a half-life of 17 min in rats and 31 min in mice. Intracerebral pre-administration of unlabeled phenytoin attenuated BBB-mediated phenytoin efflux transport, suggesting carrier-mediated phenytoin efflux transport across the BBB. Pre-administration of P-gp substrates in rats and genetic P-gp deficiency in mice did not affect BBB-mediated phenytoin efflux transport. In contrast, pre-administration of MCT8 inhibitors attenuated phenytoin efflux. Moreover, rat MCT8-expressing Xenopus oocytes exhibited [¹⁴C]phenytoin uptake, which was inhibited by unlabeled phenytoin.

CONCLUSION

Our data suggest that MCT8 at the BBB participates in phenytoin efflux transport from the brain to the blood.

[Membrane Transporters and Their Regulatory Mechanisms at the Brain and Retinal Barriers to Establish Therapies for Refractory Central Nervous System Diseases]

The central nervous system (CNS) is segregated from the circulating blood and peripheral tissues by endothelial and epithelial barriers. To overcome refractory CNS diseases, it is important to understand the membrane transport systems of drugs and the endogenous compounds that relate to the pathogenesis of CNS diseases at these barriers. The endothelial barrier in the brain is the blood-brain barrier (BBB). Our studies clarified the efflux transport of prostaglandin E₂ (PGE₂), a modulator of neural excitation and inflammatory responses, across the BBB via plasma membrane transporters such as organic anion transporter 3 (Oat3) and multidrug resistance-associated protein 4 (Mrp4). This efflux transport was attenuated by peripheral inflammation or cerebral treatment with neuroexcitatory l-glutamate, suggesting that BBB-mediated PGE₂ elimination was altered under several pathological conditions. We also examined excitatory amino acid transporter (EAAT) 1 and 3 as l-glutamate efflux transporters of the inner blood-retinal barrier (BRB) and blood-cerebrospinal barrier. It was considered that these efflux membrane transporters participated in the homeostasis of neuroexcitatory and neuroinflammatory responses in the brain and retina. Moreover, we identified connexin 43 (Cx43) hemichannels as a new membrane transport system that is activated under pathological conditions and recognizes several monocarboxylate drugs, such as valproate. As it is expected that the action of these membrane transporters across the CNS barriers is of great importance in understanding the pathology of various neuroexcitatory diseases, our studies should contribute to the establishment of therapeutic strategies for refractory CNS diseases.

Monocarboxylate transporter 12 as a guanidinoacetate efflux transporter in renal proximal tubular epithelial cells

Guanidinoacetate (GAA), which is a precursor of creatine, is mainly biosynthesized in the renal proximal tubular epithelial cells (RPTECs). Plasma concentration of GAA has been reported to be reduced in patients with monocarboxylate transporter 12 (MCT12) mutation (p.Q215X). However, the mechanism underlying GAA release from the RPTECs remains unclear. Therefore, to elucidate the role of MCT12 in renal GAA release, MCT12-mediated GAA transport was evaluated using the human and rat MCT12-expressing Xenopus laevis oocytes and primary-cultured rat RPTECs. [¹⁴C]GAA uptake by the human and rat MCT12-expressing oocytes was significantly higher than that by the water-injected oocytes. Rat MCT12-mediated uptake of [¹⁴C]GAA by the oocytes was found to be sodium ion (Na⁺)-independent and exhibited saturable kinetics with a Michaelis-Menten constant of 3.38 mM. Transport activities of rat MCT12 tend to increase along with increasing of extracellular pH. In addition, the efflux transport of [¹⁴C]GAA from the human and rat MCT12-expressing oocytes was significantly higher than that from the water-injected oocytes. These results suggest that both the influx and efflux transport of GAA is mediated by MCT12. In the primary-cultured rat RPTECs, [¹⁴C]GAA efflux transport was significantly reduced by the transfection of MCT12-specific siRNAs, suggesting that MCT12 participates in GAA efflux transport in rat RPTECs. Therefore, it suggests that MCT12 is involved in GAA release from RPTECs to the circulating blood, since MCT12 is known to be localized on the basal membrane of RPTECs.

Putative roles of SLC7A5 (LAT1) transporter in pain

Large amino acid transporter 1 (LAT1), also known as SLC7A5, is an essential amino acid transporter that forms a heterodimeric complex with the glycoprotein cell-surface antigen heavy chain (4F2hc (CD98, SLC3A2)). Within nociceptive pathways, LAT1 is expressed in the dorsal root ganglia and spinal cord. Although LAT1 expression is upregulated following spinal cord injury, little is known about LAT1 in neuropathic pain. To date, only circumstantial evidence supports LAT1/4F2hc's role in pain. Notably, LAT1's expression and regulation link it to key cell types and pathways implicated in pain. Transcriptional regulation of LAT1 expression occurs via the Wnt/frizzled/β-catenin signal transduction pathway, which has been shown to be involved in chronic pain. The LAT1/4F2hc complex may also be involved in pain pathways related to T- and B-cells. LAT1's expression induces activation of the mammalian target of rapamycin (mTOR) signaling axis, which is involved in inflammation and neuropathic pain. Similarly, hypoxia and cancer induce activation of hypoxia-inducible factor 2 alpha, promoting not only LAT1's expression but also mTORC1's activation. Perhaps the strongest evidence linking LAT1 to pain is its interactions with key voltage-gated ion channels connected to nociception, namely the voltage-gated potassium channels Kv1.1 and Kv1.2 and the voltage-gated sodium channel Nav1.7. Through functional regulation of these channels, LAT1 may play a role in governing the excitatory to inhibitory ratio which is altered in chronic neuropathic pain states. Remarkably, the most direct role for LAT1 in pain is to mediate the influx of gabapentin and pregabalin, two first-line neuropathic pain drugs, that indirectly inhibit high voltage-activated calcium channel auxiliary subunit α2δ-1. In this review, we discuss the expression, regulation, relevant signaling pathways, and protein interactions of LAT1 that may link it to the development and/or maintenance of pain. We hypothesize that LAT1 expressed in nociceptive pathways may be a viable new target in pain.

L-Type amino acid transporter 1 as a target for drug delivery

Our growing understanding of membrane transporters and their substrate specificity has opened a new avenue in the field of targeted drug delivery. The L-type amino acid transporter 1 (LAT1) has been one of the most extensively investigated transporters for delivering drugs across biological barriers. The transporter is predominantly expressed in cerebral cortex, blood-brain barrier, blood-retina barrier, testis, placenta, bone marrow and several types of cancer. Its physiological function is to mediate Na⁺ and pH independent exchange of essential amino acids: leucine, phenylalanine, etc. Several drugs and prodrugs designed as LAT1 substrates have been developed to improve targeted delivery into the brain and cancer cells. Thus, the anti-parkinsonian drug, L-Dopa, the anti-cancer drug, melphalan and the anti-epileptic drug gabapentin, all used in clinical practice, utilize LAT1 to reach their target site. These examples provide supporting evidence for the utility of the LAT1-mediated targeted delivery of the (pro)drug. This review comprehensively summarizes recent advances in LAT1-mediated targeted drug delivery. In addition, the use of LAT1 is critically evaluated and limitations of the approach are discussed.

Role of L-Type Amino Acid Transporter 1 (LAT1) for the Selective Cytotoxicity of Sesamol in Human Melanoma Cells

Sesamol is effective against melanoma cells with less damage to normal cells. The underlying selective cytotoxicity of sesamol in melanoma vs. non-cancerous cells is undefined. Melanoma cells differ from normal cells by over-expression of the L-type amino acid transporter 1 (LAT1). We sought to clarify the transport mechanism on selective cytotoxicity of sesamol in melanoma cells. A human melanoma cell line (SK-MEL-2) and African monkey epithelial cell line (Vero) were used to study the cellular uptake and cytotoxicity of sesamol. The intracellular concentration of sesamol was quantified by UV-HPLC. The cytotoxicity was determined by neutral red uptake assay. Sesamol showed a higher distribution volume and uptake clearance in SK-MEL-2 than Vero cells. Sesamol was distributed by both carrier-mediated and passive transport by having greater carrier-mediated transport into SK-MEL-2 cells than Vero cells. Higher mRNA expression and function of LAT1 over LAT2 were evident in SK-MEL-2 cells compared to Vero cells. Sesamol uptake and sesamol cytotoxicity were inhibited by the LAT1 inhibitor, suggesting LAT1 had a role in sesamol transport and its bioactivity in melanoma. The LAT1-mediated transport of sesamol is indicative of how it engages cytotoxicity in melanoma cells with promising therapeutic benefits.

Pharmacogenetics-based population pharmacokinetic analysis of gabapentin in patients with chronic pain: Effect of OCT2 and OCTN1 gene polymorphisms

Gabapentin (GAB) is eliminated unchanged in urine, and organic cation transporters (OCT2 and OCTN1) have been shown to play a role in GAB renal excretion. This prospective clinical study aimed to evaluate the genetic polymorphisms effect on GAB pharmacokinetic (PK) variability using a population pharmacokinetic approach. Data were collected from 53 patients with chronic pain receiving multiple doses of GAB. Patients were genotyped for SLC22A2 c.808G>T and SLC22A4 c.1507C>T polymorphisms. Both polymorphisms' distribution followed the Hardy-Weinberg equilibrium. An one-compartment model with first-order absorption and linear elimination best described the data. The absorption rate constant, volume of distribution, and clearance estimated were 0.44 h^-1 , 86 L, and 17.3 × (estimated glomerular filtration ratio/89.58)^1.04  L/h, respectively. The genetic polymorphism SLC22A4 c.1507C>T did not have a significant influence on GAB absorption, distribution or elimination. Due to the low minor allelic frequency of SLC22A2 c.808G>T, further studies require higher number of participants to confirm its effect on GAB renal elimination. In conclusion, GAB clinical pharmacokinetics are strongly influenced by renal function and absorption process, but not by the OCTN1 (SLC22A4 c.1507C>T) polymorphism.

Reevaluating the Substrate Specificity of the L-Type Amino Acid Transporter (LAT1)

The L-type amino acid transporter 1 (LAT1, SLC7A5) transports essential amino acids across the blood-brain barrier (BBB) and into cancer cells. To utilize LAT1 for drug delivery, potent amino acid promoieties are desired, as prodrugs must compete with millimolar concentrations of endogenous amino acids. To better understand ligand-transporter interactions that could improve potency, we developed structural LAT1 models to guide the design of substituted analogues of phenylalanine and histidine. Furthermore, we evaluated the structure-activity relationship (SAR) for both enantiomers of naturally occurring LAT1 substrates. Analogues were tested in cis-inhibition and trans-stimulation cell assays to determine potency and uptake rate. Surprisingly, LAT1 can transport amino acid-like substrates with wide-ranging polarities including those containing ionizable substituents. Additionally, the rate of LAT1 transport was generally nonstereoselective even though enantiomers likely exhibit different binding modes. Our findings have broad implications to the development of new treatments for brain disorders and cancer.