L-type amino acid transporter 1 utilizing prodrugs of ferulic acid revealed structural features supporting the design of prodrugs for brain delivery

Improving drug delivery to the brain: the prodrug approach

INTRODUCTION

The prodrug approach has been thought to be a simple solution to improve brain drug delivery for decades. Nevertheless, it still comes as a surprise that there is relatively little success in the field. The best example anti-parkinsonian drug levodopa has been serendipitously discovered to be a transporter-utilizing brain-delivered prodrug rather than a rationally developed one.

AREAS COVERED

The lack of success can mainly be explained by the insufficient understanding of the role of membrane proteins that can facilitate drug delivery at dynamic barriers, such as the blood-brain barrier (BBB), but also by the sparse knowledge of prodrug bioconverting enzymes in the brain. This review summarizes the current status of the prodrug attempts that have been developed in the past to improve brain drug delivery.

EXPERT OPINION

With the expandingly improved analytical and computational technologies, it is anticipated that enhanced brain drug delivery will be eventually achieved for most of the central nervous system (CNS) acting drugs. However, this requires that carrier-mediated (pro)drug delivery methods are implemented in the very early phases of the drug development processes and not as a last step to survive a problematic investigational drug candidate.

Designing drugs optimized for both blood-brain barrier permeation and intra-cerebral partition

INTRODUCTION

With the increasing incidence and prevalence of neurological disorders globally, there is a paramount need for new pharmacotherapies. BBB effectively protects the brain but raises a profound challenge to drug permeation, with less than 2% of most drugs reaching the CNS.

AREAS COVERED

This article reviews aspects of the most recent design strategies, providing insights into ideas and concepts in CNS drug discovery. An overview of the products available on the market is given and why clinical trials are continuously failing is discussed.

EXPERT OPINION

Among the available CNS drugs, small molecules account for most successful CNS therapeutics due to their ability to penetrate the BBB through passive or carrier-mediated mechanisms. The development of new CNS drugs is very difficult. To date, there is a lack of effective drugs for alleviating or even reversing the progression of brain diseases. Particularly, the use of artificial intelligence strategies, together with more appropriate animal models, may enable the design of molecules with appropriate permeation, to elicit a biological response from the neurotherapeutic target.

Design strategies and recent development of bioactive modulators for glutamine transporters

Glutamine transporters are integral to the metabolism of glutamine in both healthy tissues and cancerous cells, playing a pivotal role in maintaining amino acid balance, synthesizing biomolecules, and regulating redox equilibrium. Their critical functions in cellular metabolism make them promising targets for oncological therapies. Recent years have witnessed substantial progress in the field of glutamine transporters, marked by breakthroughs in understanding of their protein structures and the discovery of novel inhibitors, prodrugs, and radiotracers. This review provides a comprehensive update on the latest advancements in modulators targeting the glutamine transporter, with special attention given to LAT1 and ASCT2. It also discusses innovative approaches in drug design aimed at these transporters.

Amino acid derivative of probenecid potentiates apoptosis-inducing effects of vinblastine by increasing oxidative stress in a cancer cell-specific manner

Many chemotherapeutic drugs suffer from multidrug resistance (MDR). Efflux transporters, namely ATP-binding cassettes (ABCs), that pump the drugs out of the cancer cells comprise one major reason behind MDR. Therefore, ABC inhibitors have been under development for ages, but unfortunately, without clinical success. In the present study, an l-type amino acid transporter 1 (LAT1)-utilizing derivative of probenecid (PRB) was developed as a cancer cell-targeted efflux inhibitor for P-glycoprotein (P-gp), breast cancer resistant protein (BCRP) and/or several multidrug resistant proteins (MRPs), and its ability to increase vinblastine (VBL) cellular accumulation and apoptosis-inducing effects were explored. The novel amino acid derivative of PRB (2) increased the VBL exposure in triple-negative human breast cancer cells (MDA-MB-231) and human glioma cells (U-87MG) by 10-68 -times and 2-5-times, respectively, but not in estrogen receptor-positive human breast cancer cells (MCF-7). However, the combination therapy had greater cytotoxic effects in MCF-7 compared to MDA-MB-231 cells due to the increased oxidative stress recorded in MCF-7 cells. The metabolomic study also revealed that compound 2, together with VBL, decreased the transport of those amino acids essential for the biosynthesis of endogenous anti-oxidant glutathione (GSH). Moreover, the metabolic differences between the outcomes of the studied breast cancer cell lines were explained by the distinct expression profiles of solute carriers (SLCs) that can be concomitantly inhibited. Therefore, attacking several SLCs simultaneously to change the nutrient environment of cancer cells can serve as an adjuvant therapy to other chemotherapeutics, offering an alternative to ABC inhibitors.

Exploring Chinese herbal medicine for ischemic stroke: insights into microglia and signaling pathways

Ischemic stroke is a prevalent clinical condition affecting the central nervous system, characterized by a high mortality and disability rate. Its incidence is progressively rising, particularly among younger individuals, posing a significant threat to human well-being. The activation and polarization of microglia, leading to pro-inflammatory and anti-inflammatory responses, are widely recognized as pivotal factors in the pathogenesis of cerebral ischemia and reperfusion injury. Traditional Chinese herbal medicines (TCHMs) boasts a rich historical background, notable efficacy, and minimal adverse effects. It exerts its effects by modulating microglia activation and polarization, suppressing inflammatory responses, and ameliorating nerve injury through the mediation of microglia and various associated pathways (such as NF-κB signaling pathway, Toll-like signaling pathway, Notch signaling pathway, AMPK signaling pathway, MAPK signaling pathway, among others). Consequently, this article focuses on microglia as a therapeutic target, reviewing relevant pathway of literature on TCHMs to mitigate neuroinflammation and mediate IS injury, while also exploring research on drug delivery of TCHMs. The ultimate goal is to provide new insights that can contribute to the clinical management of IS using TCHMs.

Targeting Glial Cells by Organic Anion-Transporting Polypeptide 1C1 (OATP1C1)-Utilizing l-Thyroxine-Derived Prodrugs

OATP1C1 (organic anion-transporting polypeptide 1C1) transports thyroid hormones, particularly thyroxine (T4), into human astrocytes. In this study, we investigated the potential of utilizing OATP1C1 to improve the delivery of anti-inflammatory drugs into glial cells. We designed and synthesized eight novel prodrugs by incorporating T4 and 3,5-diiodo-l-tyrosine (DIT) as promoieties to selected anti-inflammatory drugs. The prodrug uptake in OATP1C1-expressing human U-87MG glioma cells demonstrated higher accumulation with T4 promoiety compared to those with DIT promoiety or the parent drugs themselves. In silico models of OATP1C1 suggested dynamic binding for the prodrugs, wherein the pose changed from vertical to horizontal. The predicted binding energies correlated with the transport profiles, with T4 derivatives exhibiting higher binding energies when compared to prodrugs with a DIT promoiety. Interestingly, the prodrugs also showed utilization of oatp1a4/1a5/1a6 in mouse primary astrocytes, which was further supported by docking studies and a great potential for improved brain drug delivery.

LAT1 (SLC7A5) catalyzes copper(histidinate) transport switching from antiport to uniport mechanism

LAT1 (SLC7A5) is one of the most studied membrane transporters due to its relevance to physiology in supplying essential amino acids to brain and fetus, and to pathology being linked to nervous or embryo alterations; moreover, LAT1 over-expression is always associated with cancer development. Thus, LAT1 is exploited as a pro-drug vehicle and as a target for anti-cancer therapy. We here report the identification of a new substrate with pathophysiological implications, i.e., Cu-histidinate, and an unconventional uniport mechanism exploited for the Cu-histidinate transport. Crystals of the monomeric species Cu(His)2 were obtained in our experimental conditions and the actual transport of the complex was evaluated by a combined strategy of bioinformatics, site-directed mutagenesis, radiolabeled transport, and mass spectrometry analysis. The LAT1-mediated transport of Cu(His)2 may have profound implications for both the treatment of copper dysmetabolism diseases, such as the rare Menkes disease, and of cancer as an alternative to platinum-based therapies.

Dimeric ferulic acid conjugate as a prodrug for brain targeting after nasal administration of loaded solid lipid microparticles

OBJECTIVE

Ferulic acid (Fer) displays antioxidant/anti-inflammatory properties useful against neurodegenerative diseases. To increase Fer uptake and its central nervous system residence time, a dimeric prodrug, optimizing the Fer loading on nasally administrable solid lipid microparticles (SLMs), was developed.

METHODS

The prodrug was synthesized as Fer dimeric conjugate methylated on the carboxylic moiety. Prodrug antioxidant/anti-inflammatory properties and ability to release Fer in physiologic environments were evaluated. Tristearin or stearic acid SLMs were obtained by hot emulsion technique. In vivo pharmacokinetics were quantified by HPLC.

RESULTS

The prodrug was able to release Fer in physiologic environments (whole blood and brain homogenates) and induce in vitro antioxidant/anti-inflammatory effects. Its half-life in rats was 18.0 ± 1.9 min. Stearic acid SLMs, exhibiting the highest prodrug loading and dissolution rate, were selected for nasal administration to rats (1 mg/kg dose), allowing to obtain high prodrug bioavailability and prolonged residence in the cerebrospinal fluid, showing AUC (Area Under Concentration) values (108.5 ± 3.9 μg∙mL-1∙min) up to 30 times over those of Fer free drug, after its intravenous/nasal administration (3.3 ± 0.3/5.16 ± 0.20 μg∙mL-1∙min, respectively) at the same dose. Chitosan presence further improved the prodrug brain uptake.

CONCLUSIONS

Nasal administration of prodrug-loaded SLMs can be proposed as a noninvasive approach for neurodegenerative disease therapy.

Conjugation, Prodrug, and Co-Administration Strategies in Support of Nanotechnologies to Improve the Therapeutic Efficacy of Phytochemicals in the Central Nervous System

Phytochemicals, produced as secondary plant metabolites, have shown interesting potential therapeutic activities against neurodegenerative diseases and cancer. Unfortunately, poor bioavailability and rapid metabolic processes compromise their therapeutic use, and several strategies are currently proposed for overcoming these issues. The present review summarises strategies for enhancing the central nervous system's phytochemical efficacy. Particular attention has been paid to the use of phytochemicals in combination with other drugs (co-administrations) or administration of phytochemicals as prodrugs or conjugates, particularly when these approaches are supported by nanotechnologies exploiting conjugation strategies with appropriate targeting molecules. These aspects are described for polyphenols and essential oil components, which can improve their loading as prodrugs in nanocarriers, or be part of nanocarriers designed for targeted co-delivery to achieve synergistic anti-glioma or anti-neurodegenerative effects. The use of in vitro models, able to simulate the blood-brain barrier, neurodegeneration or glioma, and useful for optimizing innovative formulations before their in vivo administration via intravenous, oral, or nasal routes, is also summarised. Among the described compounds, quercetin, curcumin, resveratrol, ferulic acid, geraniol, and cinnamaldehyde can be efficaciously formulated to attain brain-targeting characteristics, and may therefore be therapeutically useful against glioma or neurodegenerative diseases.

Drug Permeability: From the Blood-Brain Barrier to the Peripheral Nerve Barriers

Drug delivery into the peripheral nerves and nerve roots has important implications for effective local anesthesia and treatment of peripheral neuropathies and chronic neuropathic pain. Similar to drugs that need to cross the blood-brain barrier (BBB) and blood-spinal cord barrier (BSCB) to gain access to the central nervous system (CNS), drugs must cross the peripheral nerve barriers (PNB), formed by the perineurium and blood-nerve barrier (BNB) to modulate peripheral axons. Despite significant progress made to develop effective strategies to enhance BBB permeability in therapeutic drug design, efforts to enhance drug permeability and retention in peripheral nerves and nerve roots are relatively understudied. Guided by knowledge describing structural, molecular and functional similarities between restrictive neural barriers in the CNS and peripheral nervous system (PNS), we hypothesize that certain CNS drug delivery strategies are adaptable for peripheral nerve drug delivery. In this review, we describe the molecular, structural and functional similarities and differences between the BBB and PNB, summarize and compare existing CNS and peripheral nerve drug delivery strategies, and discuss the potential application of selected CNS delivery strategies to improve efficacious drug entry for peripheral nerve disorders.

Transporter-Mediated Drug Delivery

Transmembrane transport of small organic and inorganic molecules is one of the cornerstones of cellular metabolism. Among transmembrane transporters, solute carrier (SLC) proteins form the largest, albeit very diverse, superfamily with over 400 members. It was recognized early on that xenobiotics can directly interact with SLCs and that this interaction can fundamentally determine their efficacy, including bioavailability and intertissue distribution. Apart from the well-established prodrug strategy, the chemical ligation of transporter substrates to nanoparticles of various chemical compositions has recently been used as a means to enhance their targeting and absorption. In this review, we summarize efforts in drug design exploiting interactions with specific SLC transporters to optimize their therapeutic effects. Furthermore, we describe current and future challenges as well as new directions for the advanced development of therapeutics that target SLC transporters.

Sodium-Dependent Neutral Amino Acid Transporter 2 Can Serve as a Tertiary Carrier for l-Type Amino Acid Transporter 1-Utilizing Prodrugs

Membrane transporters are the key determinants of the homeostasis of endogenous compounds in the cells and their exposure to drugs. However, the substrate specificities of distinct transporters can overlap. In the present study, the interactions of l-type amino acid transporter 1 (LAT1)-utilizing prodrugs with sodium-coupled neutral amino acid transporter 2 (SNAT2) were explored. The results showed that the cellular uptake of LAT1-utilizing prodrugs into a human breast cancer cell line, MCF-7 cells, was mediated via SNATs as the uptake was increased at higher pH (8.5), decreased in the absence of sodium, and inhibited in the presence of unselective SNAT-inhibitor, (α-(methylamino)isobutyric acid, MeAIB). Moreover, docking the compounds to a SNAT2 homology model (inward-open conformation) and further molecular dynamics simulations and the subsequent trajectory and principal component analyses confirmed the chemical features supporting the interactions of the studied compounds with SNAT2, which was found to be the main SNAT expressed in MCF-7 cells.

Structural Features Affecting the Interactions and Transportability of LAT1-Targeted Phenylalanine Drug Conjugates

L-type amino acid transporter 1 (LAT1) transfers essential amino acids across cell membranes. Owing to its predominant expression in the blood-brain barrier and tumor cells, LAT1 has been exploited for drug delivery and targeting to the central nervous system (CNS) and various cancers. Although the interactions of amino acids and their mimicking compounds with LAT1 have been extensively investigated, the specific structural features for an optimal drug scaffold have not yet been determined. Here, we evaluated a series of LAT1-targeted drug-phenylalanine conjugates (ligands) by determining their uptake rates by in vitro studies and investigating their interaction with LAT1 via induced-fit docking. Combining the experimental and computational data, we concluded that although LAT1 can accommodate various types of structures, smaller compounds are preferred. As the ligand size increased, its flexibility became more crucial in determining the compound's transportability and interactions. Compounds with linear or planar structures exhibited reduced uptake; those with rigid lipophilic structures lacked interactions and likely utilized other transport mechanisms for cellular entry. Introducing polar groups between aromatic structures enhanced interactions. Interestingly, compounds with a carbamate bond in the aromatic ring's para-position displayed very good transport efficiencies for the larger compounds. Compared to the ester bond, the corresponding amide bond had superior hydrogen bond acceptor properties and increased interactions. A reverse amide bond was less favorable than a direct amide bond for interactions with LAT1. The present information can be applied broadly to design appropriate CNS or antineoplastic drug candidates with a prodrug strategy and to discover novel LAT1 inhibitors used either as direct or adjuvant cancer therapy.

Noscapine-Amino Acid Conjugates Suppress the Progression of Cancer Cells

Lung cancer is the leading cause of cancer deaths globally; 1 in 16 people are diagnosed with lung cancer in their lifetime. Microtubules, a critical cytoskeletal assembly, have an essential role in cell division. Interference with the microtubule assembly leads to genetic instability during mitosis and cancer cell death. Currently, available antimitotic drugs such as vincas and taxanes are limited due to side effects such as alopecia, myelosuppression, and drug resistance. Noscapine, an opium alkaloid, is a tubulin-binding agent and can alter the microtubule assembly, causing cancer cell death. Amino acids are fundamental building blocks for protein synthesis, making them essential for the biosynthesis of cancer cells. However, the ability of amino acids in drug transportation has yet to be exploited in developing noscapine analogues as a potential drug candidate for cancer. Hence, in the present study, we have explored the ninth position of noscapine by introducing a hydroxymethylene group using the Blanc reaction and further coupled it with a series of amino acids to construct five target conjugates in good yields. The synthesized amino acid conjugate molecules were biologically evaluated against the A549 lung cancer cell line, among which the noscapine-tryptophan conjugate showed IC50 = 32 μM, as compared to noscapine alone (IC50 = 73 μM). Morphological changes in cancer cells, cell cycle arrest in the G1 phase, and ethidium bromide/acridine orange staining indicated promising anticancer properties. Molecular docking confirmed strong binding to tubulin, with a score of -41.47 kJ/mol with all 3D coordinates and significant involvement of molecular forces, including the hydrogen bonds and hydrophobic interactions. Molecular dynamics simulations demonstrated a stable binding of noscapine-tryptophan conjugate for a prolonged time (100 ns) with the involvement of free energy through the reaction coordinates analyses, solving the bioavailability of parent noscapine to the body.

Aminopeptidase B can bioconvert L-type amino acid transporter 1 (LAT1)-utilizing amide prodrugs in the brain

A prodrug approach is a powerful method to temporarily change the physicochemical and thus, pharmacokinetic properties of drugs. However, in site-selective targeted prodrug delivery, tissue or cell-specific bioconverting enzyme is needed to be utilized to release the active parent drug at a particular location. Unfortunately, ubiquitously expressed enzymes, such as phosphatases and carboxylesterases are well used in phosphate and ester prodrug applications, but less is known about enzymes selectively expressed, e.g., in the brain and enzymes that can hydrolyze more stable prodrug bonds, such as amides and carbamates. In the present study, L-type amino acid transporter 1 (LAT1)-utilizing amide prodrugs bioconverting enzyme was identified by gradually exploring the environment and possible determinants, such as pH and metal ions, that affect amide prodrug hydrolysis. Based on inducement by cobalt ions and slightly elevated pH (8.5) as well as localization in plasma, liver, and particularly in the brain, aminopeptidase B was proposed to be responsible for the bioconversion of the majority of the studied amino acid amide prodrugs. However, this enzyme hydrolyzed only those prodrugs that contained an aromatic promoiety (L-Phe), while leaving the aliphatic promoeities (L-Lys) and the smallest prodrug (with L-Phe promoiety) intact. Moreover, the parent drugs’ structure (flexibility and the number of aromatic rings) largely affected the bioconversion rate. It was also noticed in this study, that there were species differences in the bioconversion rate by aminopeptidase B (rodents > human), although the in vitro–in vivo correlation of the studied prodrugs was relatively accurate.

L-type Amino Acid Transporter 1 Utilizing Ferulic Acid Derivatives Show Increased Drug Delivery in the Mouse Pancreas Along with Decreased Lipid Peroxidation and Prostaglandin Production

Oxidative stress and pathological changes of Alzheimer’s disease (AD) overlap with metabolic diseases, such as diabetes mellitus (DM). Therefore, tackling oxidative stress with antioxidants is a compelling drug target against multiple chronic diseases simultaneously. Ferulic acid (FA), a natural antioxidant, has previously been studied as a therapeutic agent against both AD and DM. However, FA suffers from poor bioavailability and delivery. As a solution, we have previously reported about L-type amino acid transporter 1 (LAT1)-utilizing derivatives with increased brain delivery and efficacy. In the present study, we evaluated the pharmacokinetics and antioxidative efficacy of the two derivatives in peripheral mouse tissues. Furthermore, we quantified the LAT1 expression in studied tissues with a targeted proteomics method to verify the transporter expression in mouse tissues. Additionally, the safety of the derivatives was assessed by exploring their effects on hemostasis in human plasma, erythrocytes, and endothelial cells. We found that both derivatives accumulated substantially in the pancreas, with over a 100-times higher area under curve compared to the FA. Supporting the pharmacokinetics, the LAT1 was highly expressed in the mouse pancreas. Treating mice with the LAT1-utilizing derivative of FA lowered malondialdehyde and prostaglandin E₂ production in the pancreas, highlighting its antioxidative efficacy. Additionally, the LAT1-utilizing derivatives were found to be hemocompatible in human plasma and endothelial cells. Since antioxidative derivative 1 was substantially delivered into the pancreas along the previously studied brain, the derivative can be considered as a safe dual-targeting drug candidate in both the pancreas and the brain.

Effects of Microencapsulated Ferulic Acid or Its Prodrug Methyl Ferulate on Neuroinflammation Induced by Muramyl Dipeptide

Ferulic acid (Fer) is known for its antioxidant and anti-inflammatory activities, which are possibly useful against neurodegenerative diseases. Despite the ability of Fer to permeate the brain, its fast elimination from the body does not allow its therapeutic use to be optimized. The present study proposes the preparation and characterization of tristearin- or stearic acid-based solid lipid microparticles (SLMs) as sustained delivery and targeting systems for Fer. The microparticles were produced by conventional hot emulsion techniques. The synthesis of the methyl ester of Fer (Fer-Me) allowed its encapsulation in the SLMs to increase. Fer-Me was hydrolyzed to Fer in rat whole blood and liver homogenate, evidencing its prodrug behavior. Furthermore, Fer-Me displayed antioxidant and anti-inflammatory properties. The amount of encapsulated Fer-Me was 0.719 ± 0.005% or 1.507 ± 0.014% in tristearin or stearic acid SLMs, respectively. The tristearin SLMs were able to control the prodrug release, while the stearic acid SLMs induced a significant increase of its dissolution rate in water. Jointly, the present results suggest that the tristearin SLMs loaded with Fer-Me could be a potential formulation against peripheral neuropathic pain; conversely, the stearic acid SLMs could be useful for Fer-Me uptake in the brain after nasal administration of the formulation.

Increased/Targeted Brain (Pro)Drug Delivery via Utilization of Solute Carriers (SLCs)

Membrane transporters have a crucial role in compounds’ brain drug delivery. They allow not only the penetration of a wide variety of different compounds to cross the endothelial cells of the blood–brain barrier (BBB), but also the accumulation of them into the brain parenchymal cells. Solute carriers (SLCs), with nearly 500 family members, are the largest group of membrane transporters. Unfortunately, not all SLCs are fully characterized and used in rational drug design. However, if the structural features for transporter interactions (binding and translocation) are known, a prodrug approach can be utilized to temporarily change the pharmacokinetics and brain delivery properties of almost any compound. In this review, main transporter subtypes that are participating in brain drug disposition or have been used to improve brain drug delivery across the BBB via the prodrug approach, are introduced. Moreover, the ability of selected transporters to be utilized in intrabrain drug delivery is discussed. Thus, this comprehensive review will give insights into the methods, such as computational drug design, that should be utilized more effectively to understand the detailed transport mechanisms. Moreover, factors, such as transporter expression modulation pathways in diseases that should be taken into account in rational (pro)drug development, are considered to achieve successful clinical applications in the future.

Targeting Transporters for Drug Delivery to the Brain: Can We Do Better?

Limited drug delivery to the brain is one of the major reasons for high failure rates of central nervous system (CNS) drug candidates. The blood–brain barrier (BBB) with its tight junctions, membrane transporters, receptors and metabolizing enzymes is a main player in drug delivery to the brain, restricting the entrance of the drugs and other xenobiotics. Current knowledge about the uptake transporters expressed at the BBB and brain parenchymal cells has been used for delivery of CNS drugs to the brain via targeting transporters. Although many transporter-utilizing (pro)drugs and nanocarriers have been developed to improve the uptake of drugs to the brain, their success rate of translation from preclinical development to humans is negligible. In the present review, we provide a systematic summary of the current progress in development of transporter-utilizing (pro)drugs and nanocarriers for delivery of drugs to the brain. In addition, we applied CNS pharmacokinetic concepts for evaluation of the limitations and gaps in investigation of the developed transporter-utilizing (pro)drugs and nanocarriers. Finally, we give recommendations for a rational development of transporter-utilizing drug delivery systems targeting the brain based on CNS pharmacokinetic principles.

Pharmacoproteomics of Brain Barrier Transporters and Substrate Design for the Brain Targeted Drug Delivery

One of the major reasons why central nervous system (CNS)-drug development has been challenging in the past, is the barriers that prevent substances entering from the blood circulation into the brain. These barriers include the blood-brain barrier (BBB), blood-spinal cord barrier (BSCB), blood-cerebrospinal fluid barrier (BCSFB), and blood-arachnoid barrier (BAB), and they differ from each other in their transporter protein expression and function as well as among the species. The quantitative expression profiles of the transporters in the CNS-barriers have been recently revealed, and in this review, it is described how they affect the pharmacokinetics of compounds and how these expression differences can be taken into account in the prediction of brain drug disposition in humans, an approach called pharmacoproteomics. In recent years, also structural biology and computational resources have progressed remarkably, enabling a detailed understanding of the dynamic processes of transporters. Molecular dynamics simulations (MDS) are currently used commonly to reveal the conformational changes of the transporters and to find the interactions between the substrates and the protein during the binding, translocation in the transporter cavity, and release of the substrate on the other side of the membrane. The computational advancements have also aided in the rational design of transporter-utilizing compounds, including prodrugs that can be actively transported without losing potency towards the pharmacological target. In this review, the state-of-art of these approaches will be also discussed to give insights into the transporter-mediated drug delivery to the CNS.

Comparison of Experimental Strategies to Study l-Type Amino Acid Transporter 1 (LAT1) Utilization by Ligands

l-Type amino acid transporter 1 (LAT1), expressed abundantly in the brain and placenta and overexpressed in several cancer cell types, has gained a lot of interest in drug research and development, as it can be utilized for brain-targeted drug delivery, as well as inhibiting the essential amino acid supply to cancer cells. The structure of LAT1 is today very well-known and the interactions of ligands at the binding site of LAT1 can be modeled and explained. However, less is known of LAT1′s life cycle within the cells. Moreover, the functionality of LAT1 can be measured by several different methods, which may vary between the laboratories and make the comparison of the results challenging. In the present study, the usefulness of indirect cis-inhibition methods and direct cellular uptake methods and their variations to interpret the interactions of LAT1-ligands were evaluated. Moreover, this study also highlights the importance of understanding the intracellular kinetics of LAT1-ligands, and how they can affect the regular function of LAT1 in critical tissues, such as the brain. Hence, it is discussed herein how the selected methodology influences the outcome and created knowledge of LAT1-utilizing compounds.

Novel strategies to improve tumour therapy by targeting the proteins MCT1, MCT4 and LAT1

Poor selectivity, potential systemic toxicity and drug resistance are the main challenges associated with chemotherapeutic drugs. MCT1 and MCT4 and LAT1 play vital roles in tumour metabolism and growth by taking up nutrients and are thus potential targets for tumour therapy. An increasing number of studies have shown the feasibility of including these transporters as components of tumour-targeting therapy. Here, we summarize the recent progress in MCT1-, MCT4-and LAT1-based therapeutic strategies. First, protein structures, expression, relationships with cancer, and substrate characteristics are introduced. Then, different drug targeting and delivery strategies using these proteins have been reviewed, including designing protein inhibitors, prodrugs and nanoparticles. Finally, a dual targeted strategy is discussed because these proteins exert a synergistic effect on tumour proliferation. This article concentrates on tumour treatments targeting MCT1, MCT4 and LAT1 and delivery techniques for improving the antitumour effect. These innovative tactics represent current state-of-the-art developments in transporter-based antitumour drugs.

Recent Advances in Biological Activity, New Formulations and Prodrugs of Ferulic Acid

Trans-ferulic acid (FA) is a derivative of 4-hydroxycinnamic acid, which is found in many food products, fruits and beverages. It has scientifically proven antioxidant, anti-inflammatory and antibacterial properties. However, its low ability to permeate through biological barriers (e.g., the blood-brain barrier, BBB), its low bioavailability and its fast elimination from the gastrointestinal tract after oral administration limit its clinical use, e.g., for the treatment of neurodegenerative diseases, such as Alzheimer's disease. Therefore, new nanotechnological approaches are developed in order to regulate intracellular transport of ferulic acid. The objective of this review is to summarize the last decade's research on biological properties of ferulic acid and innovative ways of its delivery, supporting pharmacological therapy.

Strategies for Structural Modification of Small Molecules to Improve Blood-Brain Barrier Penetration: A Recent Perspective

In the development of central nervous system (CNS) drugs, the blood-brain barrier (BBB) restricts many drugs from entering the brain to exert therapeutic effects. Although many novel delivery methods of large molecule drugs have been designed to assist transport, small molecule drugs account for the vast majority of the CNS drugs used clinically. From this perspective, we review studies from the past five years that have sought to modify small molecules to increase brain exposure. Medicinal chemists make it easier for small molecules to cross the BBB by improving diffusion, reducing efflux, and activating carrier transporters. On the basis of their excellent work, we summarize strategies for structural modification of small molecules to improve BBB penetration. These strategies are expected to provide a reference for the future development of small molecule CNS drugs.

Prodrug strategy for enhanced therapy of central nervous system disease

Central nervous system (CNS) disease is one of the most notorious arch-criminals of human health across the world. Although considerable efforts have been devoted to promote the development of CNS drugs, ideal therapeutical effects are yet far from enough. The blood-brain barrier remains a major player that impedes the full potential of CNS therapeutical agents as it blocks the entry of CNS drugs into the brain. The past few decades have witnessed the upspring of prodrug strategies as a promising method to accelerate CNS drug development. The prodrug strategy with the ability to overcome the formidable blood-brain barrier enhances the delivery to the brain and hence improves the effects of the CNS therapeutics. In this Feature Article, we summarize the reported barriers and strategies for CNS therapeutics and spotlight prodrug design strategies to improve the efficiency of crossing the blood-brain barrier.

Improved l-Type amino acid transporter 1 (LAT1)-mediated delivery of anti-inflammatory drugs into astrocytes and microglia with reduced prostaglandin production

Non-steroidal anti-inflammatory drugs (NSAIDs) can have protective effects in the brain by inhibition of cyclooxygenases (COX). However, the delivery into the brain across the blood-brain barrier (BBB) and particularly into the brain parenchymal cells is hindered. Therefore, in the present study, we developed four l-type amino acid transporter 1 (LAT1)-utilizing prodrugs of flurbiprofen, ibuprofen, naproxen, and ketoprofen, since LAT1 is expressed on both, the BBB endothelial cells as well as parenchymal cells. The cellular uptake and utilization of LAT1 by novel prodrugs were studied in mouse cortical primary astrocytes and immortalized microglia (BV2), and the release of the parent NSAID in several tissue and cell homogenates. Finally, the effects of the studied prodrugs on prostaglandin E₂ (PGE₂) production and cell viability were explored. The gained results showed that all four prodrugs were carried into their target cells via LAT1. They also released their parent NSAIDs via carboxylesterases (CES) and most likely also other un-identified enzymes, which need to be carefully considered when administrating these compounds orally or intravenously. Most importantly, all the studied prodrugs reduced the PGE₂ production in astrocytes and microglia after lipopolysaccharide (LPS)-induced inflammation by 29-94% and without affecting the cell viability with the studied concentration (20 µM).

L-Type Amino Acid Transporter 1 Enables the Efficient Brain Delivery of Small-Sized Prodrug across the Blood-Brain Barrier and into Human and Mouse Brain Parenchymal Cells

Membrane transporters have long been utilized to improve the oral, hepatic, and renal (re)absorption. In the brain, however, the transporter-mediated drug delivery has not yet been fully achieved due to the complexity of the blood-brain barrier (BBB). Because L-type amino acid transporter 1 (LAT1) is a good candidate to improve the brain delivery, we developed here four novel LAT1-utilizing prodrugs of four nonsteroidal anti-inflammatory drugs. As a result, all the prodrugs were able to cross the BBB and localize into the brain cells. The brain uptake of salicylic acid (SA) was improved five times, not only across the mouse BBB but also into the cultured mouse and human brain cells. The naproxen prodrug was also transported efficiently into the mouse brain achieving less peripheral exposure, but the brain release of naproxen from the prodrug was not improved. Contrarily, the high plasma protein binding of the flurbiprofen prodrug and the premature bioconversion of the ibuprofen prodrug in the mouse blood hindered the efficient brain delivery. Thus, the structure of the parent drug affects the successful brain delivery of the LAT1-utilizing prodrugs, and the small-sized LAT1-utilizing prodrug of SA constituted a successful model to specifically deliver its parent drug across the mouse BBB and into the cultured mouse and human brain cells.

Review of the Correlation of LAT1 With Diseases: Mechanism and Treatment

LAT1 is a member of the system L transporter family. The main role of the LAT1 is to transport specific amino acids through cell membranes to provide nutrients to cells and participate in several metabolic pathways. It also contributes to the transport of hormones and some drugs, which are essential for the development and treatment of some diseases. In recent years, many studies have shown that LAT1 is related to cancer, obesity, diabetes, and other diseases. However, the specific mechanism underlying the influence of LAT1 on such conditions remains unclear. Through the increasing number of studies on LAT1, we have obtained a preliminary understanding on the function of LAT1 in diseases. These studies also provide a theoretical basis for finding treatments for LAT1-related diseases, such as cancer. This review summarizes the function and mechanism of LAT1 in different diseases and the treatment of LAT1-related diseases. It also provides support for the development of novel and reliable disease treatments.

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.

Revisiting the blood-brain barrier: A hard nut to crack in the transportation of drug molecules

Barriers are the hallmark of a healthy physiology, blood-brain barrier (BBB) being a tough nut to crack for most of the antigens and chemical substances. The presence of tight junctions plays a remarkable role in defending the brain from antigenic and pathogenic attacks. BBB constitutes a diverse assemblage of multiple physical and chemical barriers that judiciously restrict the flux of blood solutes into and out of the brain. Restrictions through the paracellular pathway and the tight junctions between intercellular clefts, together create well regulated metabolic and transport barricades, critical to brain pathophysiology. The brain being impermeable to many essential metabolites and nutrients regulates transportation via specialized transport systems across the endothelial abluminal and luminal membranes. The epithelial cells enveloping capillaries of the choroid plexus regulates the transport of complement, growth factors, hormones, microelements, peptides and trace elements into ventricles. Nerve terminals, microglia, and pericytes associated with the endothelium support barrier induction and function, ensuring an optimally stable ionic microenvironment that facilitates neurotransmission, orchestrated by multiple ion channels (Na⁺, K⁺ Mg²⁺, Ca²⁺) and transporters. Brain pathology which can develop due to genetic mutations or secondary to other cerebrovascular, neurodegenerative diseases can cause aberration in the microvasculature of CNS which is the uniqueness of BBB. This can also alter BBB permeation and result in BBB breakdown and other neurodegenerative disorders such as Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis, and multiple sclerosis. The concluding section outlines contemporary trends in drug discovery, focusing on molecular determinants of BBB permeation and novel drug-delivery systems, such as dendrimers, liposomes, nanoparticles, nanogels, etc.

L-Type Amino Acid Transporter 1-Utilizing Prodrugs of Ketoprofen Can Efficiently Reduce Brain Prostaglandin Levels

In order to efficiently combat neuroinflammation, it is essential to deliver the anti-inflammatory drugs to their target sites in the brain. Pro-drugs utilizing the L-type amino acid transporter 1 (LAT1) can be transported across the blood-brain barrier (BBB) and the cellular barriers of the brain’s parenchymal cells. In this study, we evaluated, for the first time, the efficacy of LAT1-utilizing prodrugs of ketoprofen (KPF) on cyclooxygenase (COX) enzymes in vitro and prostaglandin E2 production in vivo by using an enzymatic assay and liquid chromatography- tandem mass spectrometry method, respectively. Aliphatic amino acid-conjugated pro-drugs inhibited the peroxidase activity of COX in vitro in their intact form (85% inhibition, IC50 ≈ 1.1 µM and 79%, IC50 ≈ 2.3 µM), which was comparable to KPF (90%, IC50 ≈ 0.9). Thus, these compounds acted more as KPF derivatives rather than pro-drugs. In turn, aromatic amino acid-conjugated pro-drugs behaved differently. The ester pro-drug inhibited the COX peroxidase activity in vitro (90%, IC50 ≈ 0.6 µM) due to its bioconversion to KPF, whereas the amide pro-drug was inactive toward COX enzymes in vitro. However, the amide pro-drug released KPF in the mouse brain in sufficient and effective amounts measured as reduced PGE2 levels.

Targeted tumour theranostics in mice via carbon quantum dots structurally mimicking large amino acids

Strategies for selectively imaging and delivering drugs to tumours typically leverage differentially upregulated surface molecules on cancer cells. Here, we show that intravenously injected carbon quantum dots, functionalized with multiple paired α-carboxyl and amino groups that bind to the large neutral amino acid transporter 1 (which is expressed in most tumours), selectively accumulate in human tumour xenografts in mice and in an orthotopic mouse model of human glioma. The functionalized quantum dots, which structurally mimic large amino acids and can be loaded with aromatic drugs through π–π stacking interactions, enabled—in the absence of detectable toxicity—near-infrared fluorescence and photoacoustic imaging of the tumours and a reduction in tumour burden after the targeted delivery of chemotherapeutics to the tumours. The versatility of functionalization and high tumour selectivity of the quantum dots make them broadly suitable for tumour-specific imaging and drug delivery.

Intravenously injected functionalized carbon quantum dots that bind to the large neutral amino acid transporter 1 and that structurally mimic large amino acids selectively accumulate in human tumours in mice, facilitating targeted theranostics.

Amino acid transporters: Emerging roles in drug delivery for tumor-targeting therapy

Amino acid transporters, which play a vital role in transporting amino acids for the biosynthesis of mammalian cells, are highly expressed in types of tumors. Increasing studies have shown the feasibility of amino acid transporters as a component of tumor-targeting therapy. In this review, we focus on tumor-related amino acid transporters and their potential use in tumor-targeting therapy. Firstly, the expression characteristics of amino acid transporters in cancer and their relationship with tumor growth are reviewed. Secondly, the recognition requirements are discussed, focusing on the "acid-base" properties, conformational isomerism and structural analogues. Finally, recent developments in amino acid transporter-targeting drug delivery strategies are highlighted, including prodrugs and nanocarriers, with special attention to the latest findings of molecular mechanisms and targeting efficiency of transporter-mediated endocytosis. We aim to offer related clues that might lead to valuable tumor-targeting strategies by the utilization of amino acid transporters.

Drug Delivery Strategies to Overcome the Blood-Brain Barrier (BBB)

The brain capillary endothelium serves both as an exchange site for gases and solutes between blood and brain and as a protective fence against neurotoxic compounds from the blood. While this "blood-brain barrier" (BBB) function protects the fragile environment in the brain, it also poses a tremendous challenge for the delivery of drug compounds to the brain parenchyma. Paracellular brain uptake of drug compounds is limited by the physical tightness of the endothelium, which is tightly sealed with junction complexes. Transcellular uptake of lipophilic drug compounds is limited by the activity of active efflux pumps in the luminal membrane. As a result, the majority of registered CNS drug compounds are small lipophilic compounds which are not efflux transporter substrates. Small molecule CNS drug development therefore focuses on identifying compounds with CNS target affinity and modifies these in order to optimize lipophilicity and decrease efflux pump interactions. Since efflux pump activity is limiting drug uptake, it has been investigated whether coadministration of drug compounds with efflux pump inhibitors could increase drug uptake. While the concept works to some extent, a lot of challenges have been encountered in terms of obtaining efficient inhibition while avoiding adverse effects.Some CNS drug compounds enter the brain via nutrient transport proteins, an example is the levodopa, a prodrug of Dopamine, which crosses the BBB via the large neutral amino acid transporter LAT1. While carrier-mediated transport of drug compounds may seem attractive, the development of drugs targeting transporters is very challenging, since the compounds should have a good fit to the binding site, while still maintaining their CNS target affinity.Receptor-mediated transport of drug compounds, especially biotherapeutics, conjugated to a receptor-binding ligand has shown some promise, although the amounts transported are rather low. This also holds true for drug-conjugation to cell-penetrating peptides. Due to the low uptake of biotherapeutics, barrier-breaching approaches such as mannitol injections and focused ultrasound have been employed with some success to patient groups with no other treatment options.

Astrocyte-Targeted Transporter-Utilizing Derivatives of Ferulic Acid Can Have Multifunctional Effects Ameliorating Inflammation and Oxidative Stress in the Brain

Ferulic acid (FA) is a natural phenolic antioxidant, which can exert also several other beneficial effects to combat neuroinflammation and neurodegenerative diseases, such as Alzheimer's disease. One of these properties is the inhibition of several enzymes and factors, such as β-site amyloid precursor protein (APP) cleaving enzyme 1 (BACE1), cyclooxygenases (COXs), lipoxygenases (LOXs), mammalian (or mechanistic) target for rapamycin (mTOR), and transcription factor NF-κB. We have previously synthesized three L-type amino acid transporter 1- (LAT1-) utilizing FA-derivatives with the aim to develop brain-targeted prodrugs of FA. In the present study, the cellular uptake and bioavailability of these FA-derivatives were evaluated in mouse primary astrocytic cell cultures together with their inhibitory effects towards BACE1, COX/LOX, mTOR, NF-κB, acetylcholinesterase (AChE), and oxidative stress. According to the results, all three FA-derivatives were taken up 200–600 times more effectively at 10 μM concentration into the astrocytes than FA, with one derivative having a high intracellular bioavailability (K_p,uu), particularly at low concentrations. Moreover, all of the derivatives were able to inhibit BACE1, COX/LOX, AChE, and oxidative stress measured as decreased cellular lipid peroxidation. Furthermore, one of the derivatives modified the total mTOR amount. Therefore, these derivatives have the potential to act as multifunctional compounds preventing β-amyloid accumulation as well as combating inflammation and reducing oxidative stress in the brain. Thus, this study shows that converting a parent drug into a transporter-utilizing derivative not only may increase its brain and cellular uptake, and bioavailability but can also broaden the spectrum of pharmacological effects elicited by the derivative.

L-Type Amino Acid Transporter 1 (LAT1/Lat1)-Utilizing Prodrugs Can Improve the Delivery of Drugs into Neurons, Astrocytes and Microglia

l-Type Amino Acid Transporter 1 (LAT1/Lat1) is responsible for carrying large, neutral l-amino acids as well as several drugs and prodrugs across the blood-brain barrier (BBB). However, the BBB is not the only barrier that hinders drugs acting effectively within the brain; the brain parenchymal cell membranes represent a secondary barrier for the drugs with intracellular target sites. In this study, expression and function of Lat1 was quantified in mouse primary neuron, astrocyte and immortalized microglia (BV2) cultures. Moreover, ability of Lat1 to carry prodrugs inside these brain cells was evaluated. The results showed that Lat1 was localized at the similar level in all studied cells (3.07 ± 0.92–3.77 ± 0.91 fmol/µg protein). The transporter was also functional in all three cell types, astrocytes having the highest transport capacity and affinity for the LAT1/Lat1-substrate, [¹⁴C]-l-leucine, followed by neurons and microglia. The designed prodrugs (1-6) were able to utilize Lat1 for their cellular uptake and it was mainly much higher than the one of their parent drugs. Interestingly, improved cellular uptake was also achieved in cells representing Alzheimer’s Disease phenotype. Therefore, improved delivery and intra-brain targeting of drugs can be attained by utilizing LAT1/Lat1 and prodrug approach.