Triggered Liposomal Release through a Synthetic Phosphatidylcholine Analogue Bearing a Photocleavable Moiety Embedded within thesn-2 Acyl Chain

Engineering aspects of lipid-based delivery systems: In vivo gene delivery, safety criteria, and translation strategies

Defects in the genome cause genetic diseases and can be treated with gene therapy. Due to the limitations encountered in gene delivery, lipid-based supramolecular colloidal materials have emerged as promising gene carrier systems. In their non-functionalized form, lipid nanoparticles often demonstrate lower transgene expression efficiency, leading to suboptimal therapeutic outcomes, specifically through reduced percentages of cells expressing the transgene. Due to chemically active substituents, the engineering of delivery systems for genetic drugs with specific chemical ligands steps forward as an innovative strategy to tackle the drawbacks and enhance their therapeutic efficacy. Despite intense investigations into functionalization strategies, the clinical outcome of such therapies still needs to be improved. Here, we highlight and comprehensively review engineering aspects for functionalizing lipid-based delivery systems and their therapeutic efficacy for developing novel genetic cargoes to provide a full snapshot of the translation from the bench to the clinics. We outline existing challenges in the delivery and internalization processes and narrate recent advances in the functionalization of lipid-based delivery systems for nucleic acids to enhance their therapeutic efficacy and safety. Moreover, we address clinical trials using these vectors to expand their clinical use and principal safety concerns.

Lipid cubic phase with an organic-inorganic hybrid structure formed by organoalkoxysilane lipid

A lipid cubic phase encompassing a cross-linked siloxane structure was formed by the self-assembly of a synthetic organoalkoxysilane lipid in water. The spontaneous sol-gel reaction of the alkoxysilane moiety on the lipid head group produced an organic-inorganic hybrid material with a double gyroid Ia3d cubic structure.

Design of light-responsive amphiphilic self-assemblies: A novel application of the photosensitive diazirine moiety

Diazirine is one of the smallest photo-sensitive moieties discovered to date. When incorporated in the structure of phospholipids, its minimal size has a low impact on the morphology of the resultant liposomes. A DMPC-diazirine analogue was designed and subsequently used to generate liposomes with a lower permeability and a lower phase-transition temperature compared to control DMPC liposomes. Contrary to control liposomes, in the absence of light, the photosensitive nanoparticles retained the cargo (calcein) for at least 10 days. However, upon irradiation, diazirine's conversion triggered the fluorophore release within minutes. The kinetics of the release could be tuned by the power and duration of the irradiation process. The same approach can be used on other nanomaterials, with the final goal of discovering a release profile appropriate not only for therapeutic applications, but also for agrochemicals delivery or cosmoceutics.

Photoactivatable Liposomes for Blue to Deep Red Light-Activated Surface Drug Release: Application to Controlled Delivery of the Antitumoral Drug Melphalan

Liposome-based nanoparticles able to release, via a photolytic reaction, a payload anchored at the surface of the phospholipid bilayer were prepared. The liposome formulation strategy uses an original drug-conjugated blue light-sensitive photoactivatable coumarinyl linker. This is based on an efficient blue light-sensitive photolabile protecting group modified by a lipid anchor, which enables its incorporation into liposomes, leading to blue to green light-sensitive nanoparticles. In addition, the formulated liposomes were doped with triplet-triplet annihilation upconverting organic chromophores (red to blue light) in order to prepare red light sensitive liposomes able to release a payload, by upconversion-assisted photolysis. Those light-activatable liposomes were used to demonstrate that direct blue or green light photolysis or red light TTA-UC-assisted drug photolysis can effectively photorelease a drug payload (Melphalan) and kill tumor cells in vitro after photoactivation.

Phospholipid-Membrane-Based Nanovesicles Acting as Vaccines for Tumor Immunotherapy: Classification, Mechanisms and Applications

Membrane vesicles, a group of nano- or microsized vesicles, can be internalized or interact with the recipient cells, depending on their parental cells, size, structure and content. Membrane vesicles fuse with the target cell membrane, or they bind to the receptors on the cell surface, to transfer special effects. Based on versatile features, they can modulate the functions of immune cells and therefore influence immune responses. In the field of tumor therapeutic applications, phospholipid-membrane-based nanovesicles attract increased interest. Academic institutions and industrial companies are putting in effort to design, modify and apply membrane vesicles as potential tumor vaccines contributing to tumor immunotherapy. This review focuses on the currently most-used types of membrane vesicles (including liposomes, bacterial membrane vesicles, tumor- and dendritic-cell-derived extracellular vesicles) acting as tumor vaccines, and describes the classification, mechanism and application of these nanovesicles.

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.

Challenges of Current Anticancer Treatment Approaches with Focus on Liposomal Drug Delivery Systems

According to a 2020 World Health Organization report (Globocan 2020), cancer was a leading cause of death worldwide, accounting for nearly 10 million deaths in 2020. The aim of anticancer therapy is to specifically inhibit the growth of cancer cells while sparing normal dividing cells. Conventional chemotherapy, radiotherapy and surgical treatments have often been plagued by the frequency and severity of side effects as well as severe patient discomfort. Cancer targeting by drug delivery systems, owing to their selective targeting, efficacy, biocompatibility and high drug payload, provides an attractive alternative treatment; however, there are technical, therapeutic, manufacturing and clinical barriers that limit their use. This article provides a brief review of the challenges of conventional anticancer therapies and anticancer drug targeting with a special focus on liposomal drug delivery systems.

Triggered Drug Release From Liposomes: Exploiting the Outer and Inner Tumor Environment

Since more than 40 years liposomes have being extensively studied for their potential as carriers of anticancer drugs. The basic principle behind their use for cancer treatment consists on the idea that they can take advantage of the leaky vasculature and poor lymphatic drainage present at the tumor tissue, passively accumulating in this region. Aiming to further improve their efficacy, different strategies have been employed such as PEGlation, which enables longer circulation times, or the attachment of ligands to liposomal surface for active targeting of cancer cells. A great challenge for drug delivery to cancer treatment now, is the possibility to trigger release from nanosystems at the tumor site, providing efficacious levels of drug in the tumor. Different strategies have been proposed to exploit the outer and inner tumor environment for triggering drug release from liposomes and are the focus of this review.

Nanoplatforms for Targeted Stimuli-Responsive Drug Delivery: A Review of Platform Materials and Stimuli-Responsive Release and Targeting Mechanisms

To achieve the promise of stimuli-responsive drug delivery systems for the treatment of cancer, they should (1) avoid premature clearance; (2) accumulate in tumors and undergo endocytosis by cancer cells; and (3) exhibit appropriate stimuli-responsive release of the payload. It is challenging to address all of these requirements simultaneously. However, the numerous proof-of-concept studies addressing one or more of these requirements reported every year have dramatically expanded the toolbox available for the design of drug delivery systems. This review highlights recent advances in the targeting and stimuli-responsiveness of drug delivery systems. It begins with a discussion of nanocarrier types and an overview of the factors influencing nanocarrier biodistribution. On-demand release strategies and their application to each type of nanocarrier are reviewed, including both endogenous and exogenous stimuli. Recent developments in stimuli-responsive targeting strategies are also discussed. The remaining challenges and prospective solutions in the field are discussed throughout the review, which is intended to assist researchers in overcoming interdisciplinary knowledge barriers and increase the speed of development. This review presents a nanocarrier-based drug delivery systems toolbox that enables the application of techniques across platforms and inspires researchers with interdisciplinary information to boost the development of multifunctional therapeutic nanoplatforms for cancer therapy.

Reactive Oxygen Species-Responsive Liposomes via Boronate-Caged Phosphatidylethanolamine

Liposomes have proven to be effective nanocarriers due to their ability to encapsulate and deliver a wide variety of therapeutic cargo. A key goal of liposome research is to enhance control over content release at diseased sites. Though a number of stimuli have been explored for triggering liposomal release, reactive oxygen species (ROS), which have received significantly less attention, provide excellent targets due to their key roles in biology and overabundance in diseased cells. Here, we report a ROS-responsive liposome platform through the inclusion of lipid 1 bearing a boronate ester headgroup and a quinone-methide (QM) generating self-immolative linker attached onto a dioleoylphosphatidylethanolamine (DOPE) lipid scaffold. Fluorescence-based dye release assays validated that this system enables release of both hydrophobic and hydrophilic contents upon hydrogen peroxide (H₂O₂) addition. Details of the release process were carefully studied, and data showed that oxidative removal of the boronate headgroup is sufficient to result in hydrophobic content release, while production of DOPE is needed for hydrophilic cargo leakage. These results showcase that lipid 1 can serve as a promising ROS-responsive liposomal delivery platform for controlled release.

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.

Stimuli-Responsive Materials for Tissue Engineering and Drug Delivery

Smart or stimuli-responsive materials are an emerging class of materials used for tissue engineering and drug delivery. A variety of stimuli (including temperature, pH, redox-state, light, and magnet fields) are being investigated for their potential to change a material's properties, interactions, structure, and/or dimensions. The specificity of stimuli response, and ability to respond to endogenous cues inherently present in living systems provide possibilities to develop novel tissue engineering and drug delivery strategies (for example materials composed of stimuli responsive polymers that self-assemble or undergo phase transitions or morphology transformations). Herein, smart materials as controlled drug release vehicles for tissue engineering are described, highlighting their potential for the delivery of precise quantities of drugs at specific locations and times promoting the controlled repair or remodeling of tissues.

Calcium-responsive liposomes: Toward ion-mediated targeted drug delivery

Liposomes are clinically approved supramolecular drug delivery platforms due to their ability to enhance the pharmacokinetic properties of encapsulated therapeutic agents. A key point for advancing liposomal drug delivery would be to control the timing and location of cargo release to maximize drug potency and minimize side effects. Toward this end, triggered release approaches have been developed that exploit either pathophysiological stimuli (passive release) including pH or external stimuli (active release) such as light. Here, we present a novel approach for triggering release of contents from liposomes driven by increased calcium at target sites, which plays an important role in biology related to certain diseases. In this chapter, we provide detailed experimental procedures for this project, including synthesis of calcium-responsive lipid switch 1, evaluation of dye release properties and selectivity via fluorescence-based release assays as well as studies of morphology changes during release process by dynamic light scattering (DLS) and scanning transmission electron microscopy (STEM).

Formation of lipid vesicles in situ utilizing the thiol-Michael reaction

Synthetic unilamellar liposomes, functionalized to enable novel characteristics and behavior, are of great utility to fields such as drug delivery and artificial cell membranes. However, the generation of these liposomes is frequently highly labor-intensive and time consuming whereas in situ liposome formation presents a potential solution to this problem. A novel method for in situ lipid formation is developed here through the covalent addition of a thiol-functionalized lysolipid to an acrylate-functionalized tail via the thiol-Michael addition reaction with potential for inclusion of additional functionality via the tail. Dilute, stoichiometric mixtures of a thiol lysolipid and an acrylate tail reacted in an aqueous media at ambient conditions for 48 hours reached nearly 90% conversion, forming the desired thioether-containing phospholipid product. These lipids assemble into a high density of liposomes with sizes ranging from 20 nm to several microns in diameter and include various structures ranging from spheres to tubular vesicles with structure and lamellarity dependent upon the catalyst concentration used. To demonstrate lipid functionalization, an acrylate tail possessing a terminal alkyne was coupled into the lipid structure. These functionalized liposomes enable photo-induced polymerization of the terminal alkyne upon irradiation.

Boronic acid liposomes for cellular delivery and content release driven by carbohydrate binding

Boronic acid liposomes enable triggered content release and cell delivery driven by carbohydrate binding. Dye release assays using hydrophilic and hydrophobic fluorophores validate dose-dependent release upon carbohydrate treatment. Microscopy results indicate dramatic enhancements in cell delivery, showcasing the prospects of boronic acid lipids for drug delivery.

Liposomes formed from photo-cleavable phospholipids: in situ formation and photo-induced enhancement in permeability

Photocleavable liposomes were formed in situ through the coupling of an o-nitrobenzyl-containing azide tail precursor and an alkyne-functionalized lysolipid by the copper-catalyzed azide-alkyne cycloaddition (CuAAC) reaction. Inclusion of the photolabile o-nitrobenzyl-structure enables control over the permeability and morphology of the liposomes. Photolysis of the o-nitrobenzyl group changes the molecular structure of the photolabile phospholipids, inducing phase transitions and permeability increases in the bilayer membrane, ultimately disrupting the liposome entity.

Calcium-Responsive Liposomes via a Synthetic Lipid Switch

Liposomal drug delivery would benefit from enhanced control over content release. Here, we report a novel avenue for triggering release driven by chemical composition using liposomes sensitized to calcium-a target chosen due to its key roles in biology and disease. To demonstrate this principle, we synthesized calcium-responsive lipid switch 1, designed to undergo conformational changes upon calcium binding. The conformational change perturbs membrane integrity, thereby promoting cargo release. This was shown through fluorescence-based release assays via dose-dependent response depending on the percentage of 1 in liposomes, with minimal background leakage in controls. DLS experiments indicated dramatic changes in particle size upon treatment of liposomes containing 1 with calcium. In a comparison of ten naturally occurring metal cations, calcium provided the greatest release. Finally, STEM images showed significant changes in liposome morphology upon treatment of liposomes containing 1 with calcium. These results showcase lipid switches driven by molecular recognition principles as an exciting avenue for controlling membrane properties.

Liposomes and polymersomes: a comparative review towards cell mimicking

Minimal cells: we compare and contrast liposomes and polymersomes for a bettera priorichoice and design of vesicles and try to understand the advantages and shortcomings associated with using one or the other in many different aspects (properties, synthesis, self-assembly, applications).

Vesicles from Amphiphilic Dumbbells and Janus Dendrimers: Bioinspired Self-Assembled Structures for Biomedical Applications

The current review focuses on vesicles obtained from the self-assembly of two types of dendritic macromolecules, namely amphiphilic Janus dendrimers (forming dendrimersomes) and amphiphilic dumbbells. In the first part, we will present some synthetic strategies and the various building blocks that can be used to obtain dendritic-based macromolecules, thereby showing their structural versatility. We put our focus on amphiphilic Janus dendrimers and amphiphilic dumbbells that form vesicles in water but we also encompass vesicles formed thereof in organic solvents. The second part of this review deals with the production methods of these vesicles at the nanoscale but also at the microscale. Furthermore, the influence of various parameters (intrinsic to the amphiphilic JD and extrinsic-from the environment) on the type of vesicle formed will be discussed. In the third part, we will review the numerous biomedical applications of these vesicles of nano- or micron-size.

Scalable Synthesis and Purification of Acetylated Phosphatidyl Choline Headgroup

The acetylated headgroup of the most abundant mammalian phospholipid, 1,2-diacetyl-3-sn-phosphatidyl choline (DAcPC), has several important applications in research. For instance, it can be dissolved in the same amount of water as in the fluid PC bilayer, to create a surrogate of a PC headgroup stratum, for studying solvation of small molecules and the influence of their structure on the process. In contrast to PC derivatives with longer acyl chains, DAcPC does not self-aggregate, rendering the aqueous solution homogeneous and suitable for simplified analyses of interactions of molecules with the headgroups. Several studies have been published where DAcPC was used in a crudely purified form. Here we describe a one-step preparation of DAcPC from commercially available bulk chemicals and purification of the product by crystallization and washing. The process gives a good yield and is easily scalable. The availability of enantiopure, crystalline DAcPC could open the door to more extensive biochemical, pharmacological, and nutritional studies of this interesting chemical.

Mechanisms of Light-induced Liposome Permeabilization

Liposomes have been widely studied for drug delivery applications. The inclusion of photoactive molecules into liposomes opens the possibility of light‐controlled cargo release to enhance drug biodistribution or bioavailability at target sites. Membrane permeabilization induced by light can be an effective strategy for enhancing cargo delivery with spatial and temporal control, which holds potential for chemophototherapy approaches. Several diverse mechanisms have been reported including light‐induced oxidation, photocrosslinking, photoisomerization, photocleavage, and photothermal release. Here, we review selected recent reports of light‐triggered cargo release from liposomes.

Photoinduced Vesicle Formation via the Copper-Catalyzed Azide-Alkyne Cycloaddition Reaction

Synthetic vesicles have a wide range of applications from drug and cosmetic delivery to artificial cell and membrane studies, making simple and controlled formation of vesicles a large focus of the field today. Here, we report the use of the photoinitiated copper-catalyzed azide-alkyne cycloaddition (CuAAC) reaction using visible light to introduce spatiotemporal control into the formation of vesicles. Upon the establishment of the spatiotemporal control over vesicle formation, it became possible to adjust initiation conditions to modulate vesicle sizes resulting in the formation of controllably small or large vesicles based on light intensity or giant vesicles when the formation was initiated in flow-free conditions. Additionally, this photoinitiated method enables vesicle formation at a density 400-fold higher than initiation using sodium ascorbate as the catalyst. Together, these advances enable the formation of high-density, controlled size vesicles using low-energy wavelengths while producing enhanced control over the formation characteristics of the vesicle.

Near-infrared light-sensitive liposomes for controlled release

A 6-bromo-7-hydroxy-4-hydroxymethylcoumarin containing amphiphilic lipid was synthesized and applied as a near-infrared light triggered controlled release system.

Photolysis of a bola-type supra-amphiphile promoted by water-soluble pillar[5]arene-induced assembly

A novel monolayer supramolecular vesicle assembled from a pillararene-based bola-type supra-amphiphile was successfully constructed, which showed excellent photodegradable properties and might have potential applications in phototherapy.

A clickable and photocleavable lipid analogue for cell membrane delivery and release

For drug delivery purposes, the ability to conveniently attach a targeting moiety that will deliver drugs to cells and then enable controlled release of the active molecule after localization is desirable. Toward this end, we designed and synthesized clickable and photocleavable lipid analogue 1 to maximize the efficiency of bioconjugation and triggered release. This compound contains a dibenzocyclooctyne group for bioorthogonal derivatization linked via a photocleavable 2-nitrobenzyl moiety at the headgroup of a synthetic lipid backbone for targeting to cell membranes. To assess delivery and release using this system, we report fluorescence-based assays for liposomal modification and photocleavage in solution as well as through surface immobilization to demonstrate successful liposome functionalization and photoinduced release. In addition, fluorophore delivery to and release from live cells was confirmed and characterized using fluorescence microscopy and flow cytometry analysis in which 1 was delivered to cells, derivatized, and photocleaved. Finally, drug delivery studies were performed using an azide-tagged analogue of camptothecin, a potent anticancer drug that is challenging to deliver due to poor solubility. In this case, the ester attachment of the azide tag acted as a caging group for release by intracellular esterases rather than through photocleavage. This resulted in a dose-dependent response in the presence of liposomes containing delivery agent 1, confirming the ability of this compound to stimulate delivery to the cytoplasm of cells.

Recent progress of liposomes in nanomedicine

Liposome formulations are highlighted focusing on their chemical modification, interaction with cells, and use in substrate-mediated drug delivery and cell mimicry.