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

Lipid mimetics: A versatile toolbox for lipid biology and beyond

Being the principal component of biological membranes lipids are essential building blocks of life. Given their huge biological importance, the investigation of lipids, their properties, interactions and metabolic pathways is of prime importance for the fundamental understanding of living cells and organisms as well as the emergence of diseases. Different strategies have been applied to investigate lipid-mediated biological processes, one of them being the use of lipid mimetics. They structurally resemble their natural counterparts but are equipped with functionality that can be used to probe or manipulate lipid-mediated biological processes and biomembranes. Lipid mimetics therefore constitute an indispensable toolbox for lipid biology and membrane research but also beyond for potential applications in medicine or synthetic biology. Herein, we highlight recent advances in the development and application of lipid-mimicking compounds.

Hybrid bilayer membranes as platforms for biomimicry and catalysis

Hybrid bilayer membrane (HBM) platforms represent an emerging nanoscale bio-inspired interface that has broad implications in energy catalysis and smart molecular devices. An HBM contains multiple modular components that include an underlying inorganic surface with a biological layer appended on top. The inorganic interface serves as a support with robust mechanical properties that can also be decorated with functional moieties, sensing units and catalytic active sites. The biological layer contains lipids and membrane-bound entities that facilitate or alter the activity and selectivity of the embedded functional motifs. With their structural complexity and functional flexibility, HBMs have been demonstrated to enhance catalytic turnover frequency and regulate product selectivity of the O₂ and CO₂ reduction reactions, which have applications in fuel cells and electrolysers. HBMs can also steer the mechanistic pathways of proton-coupled electron transfer (PCET) reactions of quinones and metal complexes by tuning electron and proton delivery rates. Beyond energy catalysis, HBMs have been equipped with enzyme mimics and membrane-bound redox agents to recapitulate natural energy transport chains. With channels and carriers incorporated, HBM sensors can quantify transmembrane events. This Review serves to summarize the major accomplishments achieved using HBMs in the past decade.

Recent Advances in Bioorthogonal Click Chemistry for Biomedical Applications

Bioorthogonal click chemistry, first introduced in the early 2000s, has become one of the most widely used approaches for designing advanced biomaterials for applications in tissue engineering and regenerative medicine, due to the selectivity and biocompatibility of the associated reactants and reaction conditions. In this review, we present recent advances in utilizing bioorthogonal click chemistry for the development of three-dimensional, biocompatible scaffolds and cell-encapsulated biomaterials. Additionally, we highlight recent examples using these approaches for biomedical applications including drug delivery, imaging, and cell therapy and discuss their potential as next generation biomaterials.

Ultrasound-Triggered Delivery of Iproplatin from Microbubble-Conjugated Liposomes

The PtIV prodrug iproplatin has been actively loaded into liposomes using a calcium acetate gradient, achieving a 3-fold enhancement in drug concentration compared to passive loading strategies. A strain-promoted cycloaddition reaction (azide- dibenzocyclooctyne) was used to attach iproplatin-loaded liposomes L(Pt) to gas-filled microbubbles (M), forming an ultrasound-responsive drug delivery vehicle [M-L(Pt)]. Ultrasound-triggered release of iproplatin from the microbubble-liposome construct was evaluated in cellulo. Breast cancer (MCF-7) cells treated with both free iproplatin and iproplatin-loaded liposome-microbubbles [M-L(Pt)] demonstrated an increase in platinum concentration when exposed to ultrasound. No appreciable platinum uptake was observed in MCF-7 cells following treatment with L(Pt) only or L(Pt)+ultrasound, suggesting that microbubble-mediated ultrasonic release of platinum-based drugs from liposomal carriers enables greater control over drug delivery.

Photoresponsive transformation from spherical to nanotubular assemblies: anticancer drug delivery using macrocyclic cationic gemini amphiphiles

We developed NIR-light-responsive macrocyclic cationic gemini amphiphiles, one of which displayed various favorable properties of lipids. The NIR-light-mediated cleavage of the strained dioxacycloundecine ring led to the conversion of the spherical to a nanotubular self-assembly in the aqueous medium. This photo-mediated transformation from the spherical to nanotubular self-assembly resulted in the release of encapsulated hydrophobic anticancer drug molecule doxorubicin (Dox) in a controlled manner. The potent cationic gemini amphiphile also displayed lower cytotoxicity and efficient NIR-light-mediated Dox release efficacy to cancerous cells.

Cu(I)-Catalyzed Click Chemistry in Glycoscience and Their Diverse Applications

Copper(I)-catalyzed 1,3-dipolar cycloaddition between organic azides and terminal alkynes, commonly known as CuAAC or click chemistry, has been identified as one of the most successful, versatile, reliable, and modular strategies for the rapid and regioselective construction of 1,4-disubstituted 1,2,3-triazoles as diversely functionalized molecules. Carbohydrates, an integral part of living cells, have several fascinating features, including their structural diversity, biocompatibility, bioavailability, hydrophilicity, and superior ADME properties with minimal toxicity, which support increased demand to explore them as versatile scaffolds for easy access to diverse glycohybrids and well-defined glycoconjugates for complete chemical, biochemical, and pharmacological investigations. This review highlights the successful development of CuAAC or click chemistry in emerging areas of glycoscience, including the synthesis of triazole appended carbohydrate-containing molecular architectures (mainly glycohybrids, glycoconjugates, glycopolymers, glycopeptides, glycoproteins, glycolipids, glycoclusters, and glycodendrimers through regioselective triazole forming modular and bio-orthogonal coupling protocols). It discusses the widespread applications of these glycoproducts as enzyme inhibitors in drug discovery and development, sensing, gelation, chelation, glycosylation, and catalysis. This review also covers the impact of click chemistry and provides future perspectives on its role in various emerging disciplines of science and technology.

UVA-Triggered Drug Release and Photo-Protection of Skin

Light has attracted special attention as a stimulus for triggered drug delivery systems (DDS) due to its intrinsic features of being spatially and temporally tunable. Ultraviolet A (UVA) radiation has recently been used as a source of external light stimuli to control the release of drugs using a "switch on- switch off" procedure. This review discusses the promising potential of UVA radiation as the light source of choice for photo-controlled drug release from a range of photo-responsive and photolabile nanostructures via photo-isomerization, photo-cleavage, photo-crosslinking, and photo-induced rearrangement. In addition to its clinical use, we will also provide here an overview of the recent UVA-responsive drug release approaches that are developed for phototherapy and skin photoprotection.

Application and design of esterase-responsive nanoparticles for cancer therapy

Nanoparticles have been developed for tumor treatment due to the enhanced permeability and retention effects. However, lack of specific cancer cells selectivity results in low delivery efficiency and undesired side effects. In that case, the stimuli-responsive nanoparticles system designed for the specific structure and physicochemical properties of tumors have attracted more and more attention of researchers. Esterase-responsive nanoparticle system is widely used due to the overexpressed esterase in tumor cells. For a rational designed esterase-responsive nanoparticle, ester bonds and nanoparticle structures are the key characters. In this review, we overviewed the design of esterase-responsive nanoparticles, including ester bonds design and nano-structure design, and analyzed the fitness of each design for different application. In the end, the outlook of esterase-responsive nanoparticle is looking forward.

Impacting Pancreatic Cancer Therapy in Heterotypic in Vitro Organoids and in Vivo Tumors with Specificity-Tuned, NIR-Activable Photoimmunonanoconjugates: Towards Conquering Desmoplasia?

Despite untiring efforts to develop therapies for pancreatic ductal adenocarcinoma (PDAC), survival statistics remain dismal, necessitating distinct approaches. Photodynamic priming (PDP), which improves drug delivery and combination regimens, as well as tumor photodestruction are key attributes of photodynamic therapy (PDT), making it a distinctive clinical option for PDAC. Localized, high-payload nanomedicine-assisted delivery of photosensitizers (PSs), with molecular specificity and controlled photoactivation, thus becomes critical in order to reduce collateral toxicity during more expansive photodynamic activation procedures with curative intent. As such, targeted photoactivable lipid-based nanomedicines are an ideal candidate but have failed to provide greater than two-fold cancer cell selectivity, if at all, due to their extensive multivariant physical, optical, and chemical complexity. Here, we report (1) a systematic multivariant tuning approach to engineer (Cet, anti-EGFR mAb) photoimmunonanoconjugates (PINs), and (2) stroma-rich heterotypic PDAC in vitro and in vivo models incorporating patient-derived pancreatic cancer-associated fibroblasts (PCAFs) that recapitulate the desmoplasia observed in the clinic. These offer a comprehensive, disease-specific framework for the development of Cet-PINs. Specificity-tuning of the PINs, in terms of PS lipid anchoring, electrostatic modulation, Cet orientation, and Cet surface densities, achieved ∼16-fold binding specificities and rapid penetration of the heterotypic organoids within 1 h, thereby providing a ∼16-fold enhancement in molecular targeted NIR photodestruction. As a demonstration of their inherent amenability for multifunctionality, encapsulation of high payloads of gemcitabine hydrochloride, 5-fluorouracil, and oxaliplatin within the Cet-PINs further improved their antitumor efficacy in the heterotypic organoids. In heterotypic desmoplastic tumors, the Cet-PINs efficiently penetrated up to 470 μm away from blood vessels, and photodynamic activation resulted in substantial tumor necrosis, which was not elicited in T47D tumors (low EGFR) or when using untargeted constructs in both tumor types. Photodynamic activation of the Cet-PINs in the heterotypic desmoplastic tumors resulted in collagen photomodulation, with a 1.5-fold reduction in collagen density, suggesting that PDP may also hold potential for conquering desmoplasia. The in vivo safety profile of photodynamic activation of the Cet-PINs was also substantially improved, as compared to the untargeted constructs. While treatment using the Cet-PINs did not cause any detriment to the mice's health or to healthy proximal tissue, photodynamic activation of untargeted constructs induced severe acute cachexia and weight loss in all treated mice, with substantial peripheral skin necrosis, muscle necrosis, and bowel perforation. This study is the first report demonstrating the true value of molecular targeting for NIR-activable PINs. These constructs integrate high payload delivery, efficient photodestruction, molecular precision, and collagen photomodulation in desmoplastic PDAC tumors in a single treatment using a single construct. Such combined PIN platforms and heterocellular models open up an array of further multiplexed combination therapies to synergistically control desmoplastic tumor progression and extend PDAC patient survival.

Chemoselective triazole-phosphonamidate conjugates suitable for photorelease

Herein, we describe a new method for the conjugation of azide-containing target compounds that can be readily released as amines by irradiation with near UV light. This concept is based on a two-step protocol employing the chemoselective CuAAC and Staudinger-phosphonite reactions to deliver photo-cleavable phosphonamidate conjugates in high yields starting from 2-nitrobenzyl substituted phosphonites.

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.

Photocontrolled Release of Doxorubicin Conjugated through a Thioacetal Photocage in Folate-Targeted Nanodelivery Systems

Despite their proven ability for precise and targeted release, nanoplatform systems for photocontrolled delivery often face formidable synthetic challenges, in part due to the paucity of advanced linker strategies. Here, we report on a novel linker strategy using a thioacetal ortho-nitrobenzaldehyde (TNB) cage, demonstrating its application for delivery of doxorubicin (Dox) in two nanoscale systems. This photocleavable linker, TNB(OH), which presents two identical arms, each terminated with a hydroxyl functionality, was prepared in a single step from 6-nitroveratraldehyde. TNB(OH) was used to cross-link Dox to a folate receptor (FAR)-targeting poly(amidoamine) dendrimer conjugate G5(FA)_n=5.4(Dox)_m=5.1, and also used to prepare an upconversion nanocrystal (UCN) conjugate, UCN-PPIX@(Dox)(G5FA), a larger core/shell nanostructure. In this core/shell nanostructure, the UCN core emits UV and visible light luminescence upon near-infrared (NIR) excitation, allowing for the photocleavage of the TNB linker as well as the photostimulation of protoporphyrin IX (PPIX) coupled as a cytotoxic photosensitizer. Drug-release experiments performed in aqueous solutions with long-wavelength ultraviolet A (UVA) light showed that Dox release occurred rapidly from its TNB linked form or from its dendrimer conjugated form with comparable decay kinetics. Cellular toxicity studies in FAR-overexpressing KB carcinoma cells demonstrated that each nanoconjugate lacked intrinsic cytotoxicity until exposed to UVA or NIR (980 nm) (for the UCN nanoconjugate), which resulted in induction of potent cytotoxicity. In summary, this new TNB strategy offers synthetic convenience in drug conjugation chemistry with the ability for the temporal control of drug activation at the delivery site.

Artificial Membrane Fusion Triggered by Strain-Promoted Alkyne-Azide Cycloaddition

Artificial systems for controlled membrane fusion applicable for drug delivery would ideally use triggers that are orthogonal to biology. To apply the strain-promoted alkyne-azide cycloaddition (SPAAC) to drive membrane fusion, oxo-dibenzocyclooctyne (ODIBO)-lipid 1 was designed, synthesized, and studied alongside azadibenzocyclooctyne (ADIBO)-lipids 2-4 to assess fusion with liposomes containing azido-lipid 5. Lipids 1-2 were first shown to be effective for liposome derivatization. Next, fusion was evaluated using liposomes containing 1 and varying ratios of PC and PE via a FRET dilution fusion assay, and a 1:1 PC-to-PE ratio yielded the greatest signal change attributed to fusion. Finally, lipids 1-4 were compared, and 1 yielded the greatest triggering of fusion, while 2-4 yielded varying efficacies depending on the structural features of each lipid. Fusion was further validated through STEM studies showing larger multilamellar assemblies after liposome mixing, and FRET assay results supporting the mixing of liposome aqueous contents. This work provides a platform for triggered fusion toward drug delivery applications and an understanding of the effects of lipid structure and membrane composition on fusion.

Functionalized lipids and surfactants for specific applications

Synthetic lipids and surfactants that do not exist in biological systems have been used for the last few decades in both basic and applied science. The most notable applications for synthetic lipids and surfactants are drug delivery, gene transfection, as reporting molecules, and as support for structural lipid biology. In this review, we describe the potential of the synergistic combination of computational and experimental methodologies to study the behavior of synthetic lipids and surfactants embedded in lipid membranes and liposomes. We focused on select cases in which molecular dynamics simulations were used to complement experimental studies aiming to understand the structure and properties of new compounds at the atomistic level. We also describe cases in which molecular dynamics simulations were used to design new synthetic lipids and surfactants, as well as emerging fields for the application of these compounds. This article is part of a Special Issue entitled: Biosimulations edited by Ilpo Vattulainen and Tomasz Róg.