Reactive Oxygen Species-Responsive Liposomes via Boronate-Caged Phosphatidylethanolamine

pH/H2O2 Cascade-Responsive Nanoparticles of Lipid-Like Prodrugs through Dynamic-Covalent and Coordination Interactions for Chemotherapy

Traditional lipid nanoparticles (LNPs) suffer from low drug loading capacity (DLC), weak stability, and lack of responsiveness. Conventional approaches to address these issues involve the synthesis of lipid-prodrug by incorporating responsive covalent linkers. However, such approaches often result in suboptimal sensitivity for drug release and undermine therapeutic effectiveness. Herein, the study reports a fundamentally different concept for designing lipid-like prodrugs through boron-nitrogen (B-N) coordination and dynamic covalent interaction. The 5-fluorouracil-based lipid-like prodrugs, featuring a borate ester consisting of a glycerophosphoryl choline head and a boronic acid-modified 5Fu/dodecanamine complex tail, are used to prepare pH/H2O2 cascade-responsive LNPs (5Fu-LNPs). The 5Fu-LNPs exhibit enhanced DLC and stability in a neutral physiological environment due to the B-N coordination and enhanced hydrophobicity. In tumors, acidic pH triggers the dissociation of B-N coordination to release prodrugs, which further responds to low H2O2 concentrations to release drugs, showcasing a potent pH/H2O2-cascade-responsive property. Importantly, 5Fu-LNPs demonstrate greater antitumor efficiency and lower toxicity compared to the commercial 5Fu. These results highlight 5Fu-LNPs as a safer and more effective alternative to chemotherapy. This work presents a unique LNP fabrication strategy that can overcome the limitations of conventional LNPs and broaden the range of intelligent nanomaterial preparation techniques.

Recapitulating the Lateral Organization of Membrane Receptors at the Nanoscale

Many cell membrane functions emerge from the lateral presentation of membrane receptors. The link between the nanoscale organization of the receptors and ligand binding remains, however, mostly unclear. In this work, we applied surface molecular imprinting and utilized the phase behavior of lipid bilayers to create platforms that recapitulate the lateral organization of membrane receptors at the nanoscale. We used liposomes decorated with amphiphilic boronic acids that commonly serve as synthetic saccharide receptors and generated three lateral modes of receptor presentation─random distribution, nanoclustering, and receptor crowding─and studied their interaction with saccharides. In comparison to liposomes with randomly dispersed receptors, surface-imprinted liposomes resulted in more than a 5-fold increase in avidity. Quantifying the binding affinity and cooperativity proved that the boost was mediated by the formation of the nanoclusters rather than a local increase in the receptor concentration. In contrast, receptor crowding, despite the presence of increased local receptor concentrations, prevented multivalent oligosaccharide binding due to steric effects. The findings demonstrate the significance of nanometric aspects of receptor presentation and generation of multivalent ligands including artificial lectins for the sensitive and specific detection of glycans.

Development of GTP-responsive liposomes by exchanging the metal-DPA binding site in a synthetic lipid switch

We report stimuli-responsive liposomes that selectively release encapsulated contents upon treatment with guanosine triphosphate (GTP) over a wide variety of phosphorylated metabolites, validated by fluorescence-based leakage assays. Significant changes in liposome self-assembly properties were also observed. Our results showcase the potential of this platform for triggered release applications.

Clot-Targeted Antithrombotic Liposomal Nanomedicine Containing High Content of H2O2-Activatable Hybrid Prodrugs

Liposomes have been extensively explored as drug carriers, but their clinical translation has been hampered by their low drug-loading content and premature leakage of drug payloads. It was reasoned that vesicle-forming prodrugs could be incorporated into the lipid bilayer at a high molar fraction and therefore serve as a therapeutic agent as well as a structural component in liposomal nanomedicine. Boronated retinoic acid (BORA) was developed as a prodrug, which can self-assemble with common lipids to form liposomes at a high molar fraction (40%) and release all-trans retinoic acid (atRA) and hydroxybenzyl alcohol (HBA) simultaneously, in response to hydrogen peroxide (H2O2). Here, we report fucoidan-coated BORA-incorporated liposomes (f-BORALP) as clot-targeted antithrombotic liposomal nanomedicine with H2O2-triggered multiple therapeutic actions. In the mouse model of carotid arterial thrombosis, f-BORALP preferentially accumulated in the injured blood vessel and significantly suppressed thrombus formation, demonstrating their potential as targeted antithrombotic nanomedicine. This study also provides valuable insight into the development of vesicle-forming and self-immolative prodrugs to exploit the benefits of liposomal drug delivery.

Copper-responsive liposomes for triggered cargo release employing a picolinamide-lipid conjugate

In this work, we report triggered content release from liposomes brought about by copper chelation to a synthetic lipid switch containing a picolinamide headgroup. Fluorescence-based dye-leakage assays showcase release of carboxyfluorescein dye cargo upon copper treatment and control of liposomal release based on copper abundance. Our results additionally show that this platform is selective for copper and is accompanied by significant changes to liposome properties upon treatment with this ion.

Liposomes for Tumor Targeted Therapy: A Review

Liposomes, the most widely studied nano-drug carriers in drug delivery, are sphere-shaped vesicles consisting of one or more phospholipid bilayers. Compared with traditional drug delivery systems, liposomes exhibit prominent properties that include targeted delivery, high biocompatibility, biodegradability, easy functionalization, low toxicity, improvements in the sustained release of the drug it carries and improved therapeutic indices. In the wake of the rapid development of nanotechnology, the studies of liposome composition have become increasingly extensive. The molecular diversity of liposome composition, which includes long-circulating PEGylated liposomes, ligand-functionalized liposomes, stimuli-responsive liposomes, and advanced cell membrane-coated biomimetic nanocarriers, endows their drug delivery with unique physiological functions. This review describes the composition, types and preparation methods of liposomes, and discusses their targeting strategies in cancer therapy.

Sticking the Landing: Enhancing Liposomal Cell Delivery using Reversible Covalent Chemistry and Caged Targeting Groups

Liposomes are highly effective nanocarriers for encapsulating and delivering a wide range of therapeutic cargo. While advancements in liposome design have improved several pharmacological characteristics, an important area that would benefit from further progress involves cellular targeting and entry. In this concept article, we will focus on recent progress utilizing strategies including reversible covalent bonding and caging groups to activate liposomal cell entry. These approaches take advantage of advancements that have been made in complementary fields including molecular sensing and chemical biology and direct this technology toward controlling liposome cell delivery properties. The decoration of liposomes with groups including boronic acids and cyclic disulfides is presented as a means for driving delivery through reaction with functional groups on cell surfaces. Additionally, caging groups can be exploited to activate cell delivery only upon encountering a target stimulus. These approaches provide promising new avenues for controlling cell delivery in the development of next-generation liposomal therapeutic nanocarriers.

Metabolite-Responsive Liposomes Employing Synthetic Lipid Switches Driven by Molecular Recognition Principles

The ability to exert control over lipid properties, including structure, charge, function, and self-assembly characteristics is a powerful tool that can be implemented to achieve a wide range of biomedical applications. Examples in this arena include the development of caged lipids for controlled activation of signaling properties, metabolic labeling strategies for tracking lipid biosynthesis, lipid activity probes for identifying cognate binding partners, approaches for in situ membrane assembly, and liposome triggered release strategies. In this Account, we describe recent advancements in the latter area entailing the development of stimuli-responsive liposomes through programmable changes to lipid self-assembly properties, which can be harnessed to drive the release of encapsulated contents toward applications including drug delivery. We will focus on an emerging paradigm involving liposomal platforms that are sensitized toward chemical agents ranging from metal cations to small organic molecules that exhibit dysregulation in disease states. This has been achieved by developing synthetic lipid switches that are designed to undergo programmed conformational changes upon the recognition of specific target analytes. These structural alterations are leveraged to perturb the packing of lipids within the membrane and thereby drive the release of encapsulated contents.We provide an overview of the inspiration, design, and characterization of liposomes that selectively respond to wide-ranging target analytes. This series of studies began with the development of calcium-responsive liposomes utilizing a lipid switch inspired by sensors including indo-1. Following this successful demonstration, we next showed that the selectivity of the lipid switch could be altered among different metal cations by producing a liposomal platform for which release is induced through zinc binding. Our next goal was to develop metabolite-responsive liposomes in which switching is driven by molecular recognition events involving phosphorylated small molecules. In this work, screening of lipid switches designed to interact with phosphorylated metabolites led to the identification of liposomal formulations that selectivity release contents in the presence of adenosine triphosphate (ATP). Finally, we were able to modulate the metabolite selectivity by rationally designing a modified lipid switch structure that is activated through complexation of inositol-(1,4,5)-trisphosphate (IP₃). These projects show the progression of our approaches for liposome release triggered by molecular recognition principles, building from ion-responsive lipid switches to structures that are activated by small molecules. These "smart" liposomal platforms provide an important addition to the toolbox for controlled cargo release since they respond to ions or small molecules that are commonly overproduced by diseased cells.

Reactive Oxygen Species (ROS) Activated Liposomal Cell Delivery using a Boronate-Caged Guanidine Lipid

We report boronate-caged guanidine-lipid 1 that activates liposomes for cellular delivery only upon uncaging of this compound by reactive oxygen species (ROS) to produce cationic lipid products. These liposomes are designed to mimic the exceptional cell delivery properties of cell-penetrating peptides (CPPs), while the inclusion of the boronate cage is designed to enhance selectivity such that cell entry will only be activated in the presence of ROS. Boronate uncaging by hydrogen peroxide was verified by mass spectrometry and zeta potential (ZP) measurements. A microplate-based fluorescence assay was developed to study the ROS-mediated vesicle interactions between 1-liposomes and anionic membranes, which were further elucidated via dynamic light scattering (DLS) analysis. Cellular delivery studies utilizing fluorescence microscopy demonstrated significant enhancements in cellular delivery only when 1-liposomes were incubated with hydrogen peroxide. Our results showcase that lipid 1 exhibits strong potential as an ROS-responsive liposomal platform for targeted drug delivery applications.

Stimuli-responsive liposomal nanoformulations in cancer therapy: Pre-clinical & clinical approaches

The site-specific delivery of antitumor agents is of importance for providing effective cancer suppression. Poor bioavailability of anticancer compounds and the presence of biological barriers prevent their accumulation in tumor sites. These obstacles can be overcome using liposomal nanostructures. The challenges in cancer chemotherapy and stimuli-responsive nanocarriers are first described in the current review. Then, stimuli-responsive liposomes including pH-, redox-, enzyme-, light-, thermo- and magneto-sensitive nanoparticles are discussed and their potential for delivery of anticancer drugs is emphasized. The pH- or redox-sensitive liposomes are based on internal stimulus and release drug in response to a mildly acidic pH and GSH, respectively. The pH-sensitive liposomes can mediate endosomal escape via proton sponge. The multifunctional liposomes responsive to both redox and pH have more capacity in drug release at tumor site compared to pH- or redox-sensitive alone. The magnetic field and NIR irradiation can be exploited for external stimulation of liposomes. The light-responsive liposomes release drugs when they are exposed to irradiation; thermosensitive-liposomes release drugs at a temperature of >40 °C when there is hyperthermia; magneto-responsive liposomes release drugs in presence of magnetic field. These smart nanoliposomes also mediate co-delivery of drugs and genes in synergistic cancer therapy. Due to lack of long-term toxicity of liposomes, they can be utilized in near future for treatment of cancer patients.

ATP-Responsive Liposomes via Screening of Lipid Switches Designed to Undergo Conformational Changes upon Binding Phosphorylated Metabolites

Liposomal delivery vehicles can dramatically enhance drug transport. However, their clinical application requires enhanced control over content release at diseased sites. For this reason, triggered release strategies have been explored, although a limited toolbox of stimuli has thus far been developed. Here, we report a novel strategy for stimuli-responsive liposomes that release encapsulated contents in the presence of phosphorylated small molecules. Our formulation efforts culminated in selective cargo release driven by ATP, a universal energy source that is upregulated in diseases such as cancer. Specifically, we developed lipid switches 1a-b bearing two ZnDPA units designed to undergo substantial conformational changes upon ATP binding, thereby disrupting membrane packing and triggering the release of encapsulated contents. Dye leakage assays using the hydrophobic dye Nile red validated that ATP-driven release was selective over 11 similar phosphorylated metabolites, and release of the hydrophilic dye calcein was also achieved. Multiple alternative lipid switch structures were synthesized and studied (1c-d and 2), which provided insights into the structural features that render 1a-b selective toward ATP-driven release. Importantly, analysis of cellular delivery using fluorescence microscopy in conjunction with pharmacological ATP manipulation showed that liposome delivery was specific, as it increased upon intracellular ATP accumulation, and was inhibited by ATP downregulation. Our new approach shows strong prospects for enhancing the selectivity of release and payload delivery to diseased cells driven by metabolites such as ATP, providing an exciting new paradigm for controlled release.

Zinc Triggered Release of Encapsulated Cargo from Liposomes via a Synthetic Lipid Switch

Liposomes are effective nanocarriers due to their ability to encapsulate and deliver a wide variety of therapeutics. However, therapeutic potential would be improved by enhanced control over the release of drug cargo. Zinc ions provide exciting new targets for stimuli-responsive lipid design due to their overly abundant concentrations associated with diseased cells. Herein, we report zinc-triggered release of liposomal contents exploiting synthetic lipid switches designed to undergo conformational changes in the presence of this ion. Initially, Nile red leakage assays were conducted that validated successful dose-dependent triggering of release using zinc-responsive lipids (ZRLs). In addition, dynamic light scattering and confocal microscopy experiments showed that zinc treatment led to morphological changes in lipid nanoparticles only when ZRLs were present in formulations. Next, zinc-binding experiments conducted in a solution (NMR, MS) or membrane (zeta potential) context confirmed ZRL-Zn complexation. Finally, polar cargo release from liposomes was achieved. The results from these wide-ranging experiments using four different compounds indicated that zinc-responsive properties varied based on ZRL structure, providing insights into the structural requirements for activity. This work has established zinc-responsive liposomal platforms toward the development of clinical triggered release formulations.

Demolish and Rebuild: Controlling Lipid Self-Assembly toward Triggered Release and Artificial Cells

The ability to modulate the structures of lipid membranes, predicated on our nuanced understanding of the properties that drive and alter lipid self-assembly, has opened up many exciting biological applications. In this Perspective, we focus on two endeavors in which the same principles are invoked to achieve completely opposite results. On one hand, controlled liposome decomposition enables triggered release of encapsulated cargo through the development of synthetic lipid switches that perturb lipid packing in the presence of disease-associated stimuli. In particular, recent approaches have utilized artificial lipid switches designed to undergo major conformational changes in response to a range of target conditions. On the other end of the spectrum, the ability to drive the in situ formation of lipid bilayer membranes from soluble precursors is an important component in the establishment of artificial cells. This work has culminated in chemoenzymatic strategies that enable lipid manufacturing from simple components. Herein, we describe recent advancements in these two unique undertakings that are linked by their reliance on common principles of lipid self-assembly.

Caged Oxime Reactivators Designed for the Light Control of Acetylcholinesterase Reactivation†

Despite its promising role in the active control of biological functions by light, photocaging remains untested in acetylcholinesterase (AChE), a key enzyme in the cholinergic family. Here, we describe synthesis, photochemical properties and biochemical activities of two caged oxime compounds applied in the photocontrolled reactivation of the AChE inactivated by reactive organophosphate. Each of these consists of a photocleavable coumarin cage tethered to a known oxime reactivator for AChE that belongs in an either 2-(hydroxyimino)acetamide or pyridiniumaldoxime class. Of these, the first caged compound was able to successfully go through oxime uncaging upon irradiation at long-wavelength ultraviolet light (365 nm) or visible light (420 nm). It was further evaluated in AChE assays in vitro under variable light conditions to define its activity in the photocontrolled reactivation of paraoxon-inactivated AChE. This assay result showed its lack of activity in the dark but its induction of activity under light conditions only. In summary, this article reports a first class of light-activatable modulators for AChE and it offers assay methods and novel insights that help to achieve an effective design of caged compounds in the enzyme control.

Strategies for altering lipid self-assembly to trigger liposome cargo release

While liposomes have proven to be effective drug delivery nanocarriers, their therapeutic attributes could be improved through the development of clinically viable triggered release strategies in which encapsulated drug contents could be selectively released at the sites of diseased cells. As such, a significant amount of research has been reported involving the development of stimuli-responsive liposomes and a broad range of strategies have been explored for driving content release. These have included the introduction of trigger groups at either the lipid headgroup or within the acyl chains that alter lipid self-assembly properties of known lipids as well as the rational design of lipid analogs programed to undergo conformational changes induced by events such as binding interactions. This review article describes advances in the design of stimuli-responsive liposome strategies with an eye towards emerging trends in the field.