Design, synthesis, and neuroprotective effects of dual-brain targeting naproxen prodrug

Prodrugs and their activation mechanisms for brain drug delivery

Prodrugs are masked drugs that first become pharmacologically active after undergoing a structural change in vivo. They are designed to improve physicochemical/biopharmaceutical drug properties and increase site specificity. The prodrug approach is important when developing brain-targeting drugs due to the presence of the brain barriers that seriously limit the brain entry of highly polar, multifunctional drug entities. While several excellent reviews summarize the structural modifications facilitating transport across the brain barriers, a summary of mechanisms used for the activation of the prodrug in the brain is missing. Given the high need for innovative discoveries in brain drug development, we here review the most important tools being developed since 2000 for CNS prodrug activation.

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.

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.

Investigation of functionalized nanoplatforms using branched-ligands with different chain lengths for glioblastoma targeting

Glioblastoma, a common malignancy of the central nervous system, which is the most destructive type of brain cancer. Clinical treatment remains a major challenge due to high infiltrative growth and the presence of the blood brain barrier (BBB). Therefore, advanced nanoplatforms that can efficiently cross the BBB and target to brain tumor are highly desired. Compared with the targeting efficiency of single ligand nanoplatforms, dual targeting nanoplatforms may lead to better and controllable malignant cell selectivity. In this study, based on our previous research of branched ligands, we finally determined to use tri-branched glucose and two-branched biotin as targeting molecules, and in order to explore the synergetic-targeting capabilities and the mutual influence between the length of the two ligands, we designed three kinds of two-branched biotin ligands with different linker, and co-modified with the tri-branched glucose ligands on the surface of liposomes. The results of in vivo and in vitro experiments showed the (Glu₃+Bio₂)-2-Lip can exert the greatest synergistic targeting ability. The application of branched ligands, the dual-targeting design concept, and the exploration of the interaction between the chain lengths of the two ligands have brought new ideas and new methods for the targeted therapy of glioma.

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.

Cysteine Donor-Based Brain-Targeting Prodrug: Opportunities and Challenges

Overcoming blood-brain barrier (BBB) to improve brain bioavailability of therapeutic drug remains an ongoing concern. Prodrug is one of the most reliable approaches for delivering agents with low-level BBB permeability into the brain. The well-known antioxidant capacities of cysteine (Cys) and its vital role in glutathione (GSH) synthesis indicate that Cys-based prodrug could potentiate therapeutic drugs against oxidative stress-related neurodegenerative disorders. Moreover, prodrug with Cys moiety could be recognized by the excitatory amino acid transporter 3 (EAAT3) that is highly expressed at the BBB and transports drug into the brain. In this review, we summarized the strategies of crossing BBB, properties of EAAT3 and its natural substrates, Cys and its donors, and Cys donor-based brain-targeting prodrugs by referring to recent investigations. Moreover, the challenges that we are faced with and future research orientations were also addressed and proposed. It is hoped that present review will provide evidence for the pursuit of novel Cys donor-based brain-targeting prodrug.

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.

Biotin and glucose co-modified multi-targeting liposomes for efficient delivery of chemotherapeutics for the treatment of glioma

Glioma is one of the most common primary intracranial tumor, but the current treatments of glioma are far from satisfying. As the major treatment option for malignant glioma, chemotherapy has its own disadvantages, including low chemotherapeutic agents delivery across blood-brain barrier (BBB) and lack of specificity. Therefore, new approach permitting glioma targeting ability that can allow an efficient therapeutic delivery into the glioma regions is urgently required. Ligand-mediated liposomes have shown great potential for improving the efficiency of glioma treatment. In our study, the multi-targeting liposomes based on glucose and biotin were constructed for the first time. We synthesized two ligands (Glu₃-Chol, Bio₂-Chol), prepared three types of modified liposomes (Glu₃-Lip, Bio₂-Lip and Bio₂ + Glu₃-Lip) and evaluated the glioma-targeting ability of these liposomes which were using paclitaxel (PTX) as the model drug in vitro. Besides, the uptake mechanism of Bio₂ + Glu₃-Lip was investigated. PTX-loaded Bio₂ + Glu₃-Lip (PTX-Bio₂ + Glu₃-Lip) exhibited satisfactory targeting effect in Bend.3 and C6 cells in vitro, in which the cellular uptake of Bio₂ + Glu₃-Lip were 4.04- and 3.49-fold more than that of the uncoated liposomes (Lip). The results suggested the multi-targeting liposomes (Bio₂ + Glu₃-Lip) is a promising formulation for glioma, which was almost consistent with the results of in vivo imaging. In summary, we have designed and fabricated an effective delivery system to treat glioma.