Acid-labile mPEG-vinyl ether-1,2-dioleylglycerol lipids with tunable pH sensitivity: synthesis and structural effects on hydrolysis rates, DOPE liposome release performance, and pharmacokinetics

Brønsted acid-catalyzed synthesis of spirocyclobutanes via heteroannulation of vinyloxyphenylbicyclobutanes with water

We report a perchloric acid-catalyzed heteroannulation for the synthesis of spirocyclobutanes using vinyloxyphenylbicyclobutanes with water. This metal-free reaction yields high product outputs and is consistent with the formation of a cyclobutene intermediate originating from an isomerization of a bicyclobutane.

Advanced Strategies for Overcoming Endosomal/Lysosomal Barrier in Nanodrug Delivery

Nanocarriers have therapeutic potential to facilitate drug delivery, including biological agents, small-molecule drugs, and nucleic acids. However, their efficiency is limited by several factors; among which, endosomal/lysosomal degradation after endocytosis is the most important. This review summarizes advanced strategies for overcoming endosomal/lysosomal barriers to efficient nanodrug delivery based on the perspective of cellular uptake and intracellular transport mechanisms. These strategies include promoting endosomal/lysosomal escape, using non-endocytic methods of delivery to directly cross the cell membrane to evade endosomes/lysosomes and making a detour pathway to evade endosomes/lysosomes. On the basis of the findings of this review, we proposed several promising strategies for overcoming endosomal/lysosomal barriers through the smarter and more efficient design of nanodrug delivery systems for future clinical applications.

Key Design Features of Lipid Nanoparticles and Electrostatic Charge-Based Lipid Nanoparticle Targeting

Lipid nanoparticles (LNP) have gained much attention after the approval of mRNA COVID-19 vaccines. The considerable number of currently ongoing clinical studies are testament to this fact. These efforts towards the development of LNPs warrant an insight into the fundamental developmental aspects of such systems. In this review, we discuss the key design aspects that confer efficacy to a LNP delivery system, i.e., potency, biodegradability, and immunogenicity. We also cover the underlying considerations regarding the route of administration and targeting of LNPs to hepatic and non-hepatic targets. Furthermore, since LNP efficacy is also a function of drug/nucleic acid release within endosomes, we take a holistic view of charged-based targeting approaches of LNPs not only in the context of endosomal escape but also in relation to other comparable target cell internalization strategies. Electrostatic charge-based interactions have been used in the past as a potential strategy to enhance the drug release from pH-sensitive liposomes. In this review, we cover such strategies around endosomal escape and cell internalization in low pH tumor micro-environments.

Proton-mediated burst of dual-drug loaded liposomes for biofilm dispersal and bacterial killing

Exposure of infectious biofilms to dispersants induces high bacterial concentrations in blood that may cause sepsis. Preventing sepsis requires simultaneous biofilm dispersal and bacterial killing. Here, self-targeting DCPA(2-(4-((1,5-bis(octadecenoyl)1,5-dioxopentan-2-yl)carbamoyl)pyridin-1-ium-1-yl)acetate) liposomes with complexed water were self-assembled with ciprofloxacin loaded in-membrane and PEGylated as a lipid-membrane component, together with bromelain loaded in-core. Inside biofilms, DCPA-H₂O and PEGylated ciprofloxacin became protonated, disturbing the balance in the lipid-membrane to cause liposome-burst and simultaneous release of bromelain and ciprofloxacin. Simultaneous release of bromelain and ciprofloxacin enhanced bacterial killing in Staphylococcus aureus biofilms as compared with free bromelain and/or ciprofloxacin. After tail-vein injection in mice, liposomes accumulated inside intra-abdominal staphylococcal biofilms. Subsequent liposome-burst and simultaneous release of bromelain and ciprofloxacin yielded degradation of the biofilm matrix by bromelain and higher bacterial killing without inducing septic symptoms as obtained by injection of free bromelain and ciprofloxacin. This shows the advantage of simultaneous release from liposomes of bromelain and ciprofloxacin inside a biofilm.

Polymer-Modified Liposomes for Drug Delivery: From Fundamentals to Applications

Liposomes are highly advantageous platforms for drug delivery. To improve the colloidal stability and avoid rapid uptake by the mononuclear phagocytic system of conventional liposomes while controlling the release of encapsulated agents, modification of liposomes with well-designed polymers to modulate the physiological, particularly the interfacial properties of the drug carriers, has been intensively investigated. Briefly, polymers are incorporated into liposomes mainly using “grafting” or “coating”, defined according to the configuration of polymers at the surface. Polymer-modified liposomes preserve the advantages of liposomes as drug-delivery carriers and possess specific functionality from the polymers, such as long circulation, precise targeting, and stimulus-responsiveness, thereby resulting in improved pharmacokinetics, biodistribution, toxicity, and therapeutic efficacy. In this review, we summarize the progress in polymer-modified liposomes for drug delivery, focusing on the change in physiological properties of liposomes and factors influencing the overall therapeutic efficacy.

Liposomal PHD2 Inhibitors and the Enhanced Efficacy in Stabilizing HIF-1α

Prolyl hydroxylase domain-containing protein 2 (PHD2) inhibition, which stabilizes hypoxia-inducible factor (HIF)-1α and thus triggers adaptation responses to hypoxia in cells, has become an important therapeutic target. Despite the proven high potency, small-molecule PHD2 inhibitors such as IOX2 may require a nanoformulation for favorable biodistribution to reduce off-target toxicity. A liposome formulation for improving the pharmacokinetics of an encapsulated drug while allowing a targeted delivery is a viable option. This study aimed to develop an efficient loading method that can encapsulate IOX2 and other PHD2 inhibitors with similar pharmacophore features in nanosized liposomes. Driven by a transmembrane calcium acetate gradient, a nearly 100% remote loading efficiency of IOX2 into liposomes was achieved with an optimized extraliposomal solution. The electron microscopy imaging revealed that IOX2 formed nanoprecipitates inside the liposome’s interior compartments after loading. For drug efficacy, liposomal IOX2 outperformed the free drug in inducing the HIF-1α levels in cell experiments, especially when using a targeting ligand. This method also enabled two clinically used inhibitors—vadadustat and roxadustat—to be loaded into liposomes with a high encapsulation efficiency, indicating its generality to load other heterocyclic glycinamide PHD2 inhibitors. We believe that the liposome formulation of PHD2 inhibitors, particularly in conjunction with active targeting, would have therapeutic potential for treating more specifically localized disease lesions.

Tumor-Microenvironment-Responsive Nanomedicine for Enhanced Cancer Immunotherapy

The past decades have witnessed great progress in cancer immunotherapy, which has profoundly revolutionized oncology, whereas low patient response rates and potential immune-related adverse events remain major clinical challenges. With the advantages of controlled delivery and modular flexibility, cancer nanomedicine has offered opportunities to strengthen antitumor immune responses and to sensitize tumor to immunotherapy. Furthermore, tumor-microenvironment (TME)-responsive nanomedicine has been demonstrated to achieve specific and localized amplification of the immune response in tumor tissue in a safe and effective manner, increasing patient response rates to immunotherapy and reducing the immune-related side effects simultaneously. Here, the recent progress of TME-responsive nanomedicine for cancer immunotherapy is summarized, which responds to the signals in the TME, such as weak acidity, reductive environment, high-level reactive oxygen species, hypoxia, overexpressed enzymes, and high-level adenosine triphosphate. Moreover, the potential to combine nanomedicine-based therapy and immunotherapeutic strategies to overcome each step of the cancer-immunity cycle and to enhance antitumor effects is discussed. Finally, existing challenges and further perspectives in this rising field with the hope for improved development of clinical applications are discussed.

Cationic lipids for gene delivery: many players, one goal

Lipid-based carriers represent the most widely used alternative to viral vectors for gene expression and gene silencing purposes. This class of non-viral vectors is particularly attractive for their ease of synthesis and chemical modifications to endow them with desirable properties. Despite combinatorial approaches have led to the generation of a large number of cationic lipids displaying different supramolecular structures and improved behavior, additional effort is needed towards the development of more and more effective cationic lipids for transfection purposes. With this review, we seek to highlight the great progress made in the design of each and every constituent domain of cationic lipids, that is, the chemical structure of the headgroup, linker and hydrophobic moieties, and on the specific effect on the assembly with nucleic acids. Since the complexity of such systems is known to affect their performances, the role of formulation, stability and phase behavior on the transfection efficiency of such assemblies will be thoroughly discussed. Our objective is to provide a conceptual framework for the development of ever more performing lipid gene delivery vectors.

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.

Synthesis, conjugating capacity and biocompatibility evaluation of a novel amphiphilic polynorbornene

Polynorbornenes, prepared by the ‘living’ and ‘controlled’ ring-opening metathesis polymerization (ROMP) method, have emerged as a stimuli-sensitive new class of polymer carriers. Herein, we reported a novel amphiphilic diblock polynorbornene, PNCHO-b-PNTEG, containing active benzaldehyde units, which exhibited good conjugating capacity to amino-containing molecules (e.g., doxorubicin (DOX)) via the pH-sensitive Schiff base linkage. The copolymer and its conjugate with DOX, DOX-PNCHO-b-PNTEG, were adequately analyzed by various techniques including ¹H NMR, ¹³C NMR, gel permeation chromatography, etc. Especially, the formed conjugate of DOX-PNCHO-b-PNTEG could self-assemble into near-spherical micelles with the diameter of 81 ± 10 nm, and exhibit acid-triggered DOX release behavior, and the release rate could be adjusted by changing the environmental pH value. The excellent biological safety of PNCHO-b-PNTEG was further demonstrated by the results from both in vitro toxicity evaluation to murine fibroblast cells (L-929 cells) and in vivo evaluation of acute developmental toxicity and cell death in zebrafish embryos. Hence, the present polynorbornene-based PNCHO-b-PNTEG possesses great potential application as a biocompatible polymeric carrier and could be employed to fabricate various pH-sensitive conjugates.

SYL3C aptamer-anchored microemulsion co-loading β-elemene and PTX enhances the treatment of colorectal cancer

The aim of this study is to construct a SYL3C aptamer-anchored microemulsion based on β-elemene and PTX (SYL3C/EP-MEs) for enhancement on colorectal cancer therapy. Such microemulsion is consist of encapsulated drugs (β-elemene and PTX), tumor targeting ligand (3’-end thiolated SYL3C aptamer), thiol conjugated site (maleimide-modified PEGylated 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine, mal-DOPE-PEG), pH-sensitive component (DOPE) and other necessary excipients. SYL3C/EP-MEs showed a spherical particle with an average particle size around 30 nm and a high encapsulation efficiency (>80%) for both drugs. β-elemene and PTX could be released controllably from SYL3C/EP-MEs as pH values changed. SYL3C/EP-MEs displayed a selective affinity to HT-29 cells, leading to an obvious increase in cellular uptake, cell apoptosis and cytotoxicity. In the HT-29 tumor xenograft-bearing nude mice model studies, SYL3C/EP-MEs showed an overwhelming tumor growth inhibition, the longest survival time and the lowest systemic toxicity among all the treatments. The potential mechanism of enhanced anti-cancer ability was probably associated with the induction of M1 macrophage polarization, the downregulation of mutant p53 protein and the reduction of bcl-2 protein expression. Collectively, the microemulsion codelivery of β-elemene and PTX using functionalization with SYL3C aptamer provides a novel approach for combinational colorectal cancer-targeted treatment.

Microgels in biomaterials and nanomedicines

Microgels are colloidal particles with crosslinked polymer networks and dimensions ranging from tens of nanometers to micrometers. Specifically, smart microgels are fascinating capable of responding to biological signals in vivo or remote triggers and making the possible for applications in biomaterials and biomedicines. Therefore, how to fundamentally design microgels is an urgent problem to be solved. In this review, we put forward our important fundamental opinions on how to devise the intelligent microgels for cancer therapy, biosensing and biological lubrication. We focus on the design ideas instead of specific implementation process by employing reverse synthesis analysis to programme the microgels at the original stage. Moreover, special insights will be, for the first time, as far as we know, dedicated to the particles completely composed of DNA or proteins into microgel systems. These are discussed in detail in this review. We expect to give readers a broad overview of the design criteria and practical methodologies of microgels according to the application fields, as well as to propel the further developments of highly interesting concepts and materials.

Photo-triggerable liposomal drug delivery systems: from simple porphyrin insertion in the lipid bilayer towards supramolecular assemblies of lipid–porphyrin conjugates

Light-responsive liposomes are considered nowadays as one of the most promising nanoparticulate systems for the delivery and release of an active pharmaceutical ingredient (API) in a spatio-temporal manner.

Stimuli-responsive liposomes for drug delivery

The ultimate goal of drug delivery is to increase the bioavailability and reduce the toxic side effects of the active pharmaceutical ingredient (API) by releasing them at a specific site of action. In the case of antitumor therapy, association of the therapeutic agent with a carrier system can minimize damage to healthy, nontarget tissues, while limit systemic release and promoting long circulation to enhance uptake at the cancerous site due to the enhanced permeation and retention effect (EPR). Stimuli-responsive systems have become a promising way to deliver and release payloads in a site-selective manner. Potential carrier systems have been derived from a wide variety of materials, including inorganic nanoparticles, lipids, and polymers that have been imbued with stimuli-sensitive properties to accomplish triggered release based on an environmental cue. The unique features in the tumor microenvironment can serve as an endogenous stimulus (pH, redox potential, or unique enzymatic activity) or the locus of an applied external stimulus (heat or light) to trigger the controlled release of API. In liposomal carrier systems triggered release is generally based on the principle of membrane destabilization from local defects within bilayer membranes to effect release of liposome-entrapped drugs. This review focuses on the literature appearing between November 2008-February 2016 that reports new developments in stimuli-sensitive liposomal drug delivery strategies using pH change, enzyme transformation, redox reactions, and photochemical mechanisms of activation. WIREs Nanomed Nanobiotechnol 2017, 9:e1450. doi: 10.1002/wnan.1450 For further resources related to this article, please visit the WIREs website.

Recent Advances in Endogenous and Exogenous Stimuli-Responsive Nanocarriers for Drug Delivery and Therapeutics

Significant progress has been achieved in the development of stimuli-responsive nanocarriers for drug delivery, diagnosis, and therapy. Various types of triggers are utilized in the development of nanocarrier delivery. Endogenous factors such as changes in pH, redox, gradient, and enzyme concentration which are linked to disease progression have been utilized for controlling biodistribution and releasing drugs from nanocarriers, as well as increasing subsequent pharmacological activity at the disease site. Nanocarriers which respond to artificially-induced exogenous factors (such as temperature, light, magnetic field, and ultrasound) have also been developed. This review aims to discuss recent advances in the design of stimuli-responsive nanocarriers which appear to have a promising future in medicine.

Cleavable Polyethylene Glycol: 3,4-Epoxy-1-butene as a Comonomer to Establish Degradability at Physiologically Relevant pH

Polyethylene glycol (PEG) has been used for decades to improve the pharmacokinetic properties of protein drugs, and several PEG-protein conjugates are approved by the FDA. However, the nondegradability of PEG restricts its use to a limiting molecular weight to permit renal excretion. In this work, we introduce a simple strategy to overcome the nondegradability of PEG by incorporating multiple pH-sensitive vinyl ether moieties into the polyether backbone. Copolymerization of 3,4-epoxy-1-butene (EPB) with ethylene oxide via anionic ring-opening polymerization (AROP) provides access to allyl moieties that can be isomerized to pH-cleavable propenyl units (isoEPB). Well-defined P(EPB-co-EG) copolymers (Đ = 1.05-1.11) with EPB contents of ∼4 mol% were synthesized in a molecular weight range of 3000 to 10000 g mol^-1. ¹H NMR kinetic studies served to investigate acidic hydrolysis in a pH range of 4.4 to 5.4 and even allowed to distinguish between the hydrolysis rates of (E)- and (Z)-isoEPB units, demonstrating faster hydrolysis of the (Z)-isomer. SEC analysis of degradation products revealed moderate dispersities Đ of 1.6 to 1.8 and consistent average molecular weights M_n of ∼1000 g mol^-1. The presence of a defined hydroxyl end group permits attachment to other functional molecules. The novel pH-degradable PEGs combine various desirable properties such as excellent long-term storage stability and cleavage in a physiologically relevant pH-range that render them promising candidates for biomedical application.

"Sheddable" PEG-lipid to balance the contradiction of PEGylation between long circulation and poor uptake

PEGylated lipids confer longer systemic circulation and tumor accumulation via the enhanced permeability and retention (EPR) effect. However, PEGylation inhibits cellular uptake and subsequent endosomal escape. In order to balance the contradiction between the advantages of long circulation and the disadvantages of poor uptake of PEGylated lipids, we prepared a "sheddable" PEG-lipid micelle system based on the conjugation of PEG and phosphatidyl ethanolamine (DSPE) with a pH sensitive benzoic imine bond. In a physiological environment, the PEG-protected micelles were not readily taken up by the reticuloendothelial system (RES) and could be successfully delivered to tumor tissue by the EPR effect. In a tumor acidic microenvironment, the PEG chains detached from the surfaces of the micelles while the degree of linker cleavage could not cause a significant particle size change, which facilitated the carrier binding to tumor cells and improved the cellular uptake. Subsequently, the "sheddable" PEG-lipid micelles easily internalized into cells and the increased acidity in the lysosomes further promoted drug release. Thus, this "sheddable" PEG-lipid nanocarrier could be a good candidate for effective intracellular drug delivery in cancer chemotherapy.

Mechanisms and biomaterials in pH-responsive tumour targeted drug delivery: A review

As the mainstay in the treatment of various cancers, chemotherapy plays a vital role, but still faces many challenges, such as poor tumour selectivity and multidrug resistance (MDR). Targeted drug delivery using nanotechnology has provided a new strategy for addressing the limitations of the conventional chemotherapy. In the last decade, the volume of research published in this area has increased tremendously, especially with functional nano drug delivery systems (nanocarriers). Coupling a specific stimuli-triggered drug release mechanism with these delivery systems is one of the most prevalent approaches for improving therapeutic outcomes. Among the various stimuli, pH triggered delivery is regarded as the most general strategy, targeting the acidic extracellular microenvironment and intracellular organelles of solid tumours. In this review, we discuss recent advances in the development of pH-sensitive nanocarriers for tumour-targeted drug delivery. The review focuses on the chemical design of pH-sensitive biomaterials, which are used to fabricate nanocarriers for extracellular and/or intracellular tumour site-specific drug release. The pH-responsive biomaterials bring forth conformational changes in these nanocarriers through various mechanisms such as protonation, charge reversal or cleavage of a chemical bond, facilitating tumour specific cell uptake or drug release. A greater understanding of these mechanisms will help to design more efficient drug delivery systems to address the challenges encountered in conventional chemotherapy.

Nonfouling NTA-PEG-Based TEM Grid Coatings for Selective Capture of Histidine-Tagged Protein Targets from Cell Lysates

We report the preparation and performance of TEM grids bearing stabilized nonfouling lipid monolayer coatings. These films contain NTA capture ligands of controllable areal density at the distal end of a flexible poly(ethylene glycol) 2000 (PEG2000) spacer to avoid preferred orientation of surface-bound histidine-tagged (His-tag) protein targets. Langmuir-Schaefer deposition at 30 mN/m of mixed monolayers containing two novel synthetic lipids-1,2-distearoyl-sn-glycero-3-phosphoethanolamine-N-[(5-amido-1-carboxypentyl)iminodiacetic acid]polyethylene glycolamide 2000) (NTA-PEG2000-DSPE) and 1,2-(tricosa-10',12'-diynoyl)-sn-glycero-3-phosphoethanolamine-N-(methoxypolyethylene glycolamide 350) (mPEG350-DTPE)-in 1:99 and 5:95 molar ratios prior to treatment with a 5 min, 254 nm light exposure was used for grid fabrication. These conditions were designed to limit nonspecific protein adsorption onto the stabilized lipid coating by favoring the formation of a mPEG350 brush layer below a flexible, mushroom conformation of NTA-PEG2000 at low surface density to enable specific immobilization and random orientation of the protein target on the EM grid. These grids were then used to capture His6-T7 bacteriophage and RplL from cell lysates, as well as purified His8-green fluorescent protein (GFP) and nanodisc solubilized maltose transporter, His6-MalFGK2. Our findings indicate that TEM grid supported, polymerized NTA lipid monolayers are capable of capturing His-tag protein targets in a manner that controls their areal densities, while efficiently blocking nonspecific adsorption and limiting film degradation, even upon prolonged detergent exposure.

Biodegradable hyperbranched polyether-lipids with in-chain pH-sensitive linkages

Hyperbranched polyether-based lipids with cleavable acetal units were obtained via copolymerization of the epoxide inimer 1-(glycidyloxy)ethyl ethylene glycol ether (GEGE) and glycidol, using anionic ring-opening polymerization.

Switchable Lipids: Conformational Change for Fast pH-Triggered Cytoplasmic Delivery

We report the use of switchable lipids to improve the endosomal escape and cytosolic delivery of cell-impermeable compounds. The system is based on a conformational reorganization of the lipid structure upon acidification, as demonstrated by NMR spectroscopic studies. When incorporated in a liposome formulation, the switchable lipids triggered bilayer destabilization through fusion even in the presence of poly(ethylene glycol). We observed 88 % release of sulforhodamine B in 15 min at pH 5, and the liposome formulations demonstrated high stability at pH 7.4 for several months. By using sulforhodamine B as a model of a highly polar drug, we demonstrated fast cytosolic delivery mediated by endosomal escape in HeLa cells, and no toxicity.

Micro- and nanogels with labile crosslinks – from synthesis to biomedical applications

We emphasize the synthetic strategies to produce micro-/nanogels and the importance of degradable linkers incorporated in the gel network.

Exit-strategies - smart ways to release phospholipid vesicle cargo

This highlight describes recent trends in fundamental phospholipid research towards possible future drug delivery technology. In particular it focuses on synthetic phospholipids and their vesicular constructs and describes selected "smart" ways to release cargo from liposomes. Various chemical and physical release triggers are discussed such as temperature changes, application of ultrasound, enzyme degradation, changes in pH, redox reactions, photochemical reactions, as well as the effects of shear stress on vesicles.

Polymeric topology and composition constrained polyether-polyester micelles for directional antitumor drug delivery

Amphiphilic linear and dumbbell-shaped poly(ethylene glycol)-poly(lactide-co-glycolide) (PEG-PLGA) copolymers were simply synthesized by the ring-opening polymerization of lactide and glycolide using PEG or tetrahydroxyl-functionalized PEG as the macroinitiator and stannous octoate as the catalyst. The copolymers spontaneously self-assembled into spherical micelles in phosphate-buffered saline at pH 7.4. The self-assembly behavior was dependent on both the polymeric topology and composition. Doxorubicin (DOX), an anthracycline antitumor drug, was loaded into micelles through nanoprecipitation. The in vitro release behavior could be adjusted by regulating the topology or composition of the copolymer, or the pH of the release medium. The effective intracellular DOX release from DOX-loaded micelles was confirmed by confocal laser scanning microscopy and flow cytometry in vitro. DOX-loaded micelles displayed great cellular proliferation inhibition efficacies after incubation for 24, 48 or 72 h. The hemolysis ratio of DOX was significantly reduced by the presence of copolymer. These properties indicated that the micelles derived from linear or dumbbell-shaped copolymers were promising candidates as smart antitumor drug carriers for malignancy therapy.

pH-responsive biodegradable assemblies containing tunable phenyl-substituted vinyl ethers for use as efficient gene delivery vehicles

Novel pH-responsive assemblies (PEG-lipid:DOPE liposomes) containing tunable and bifunctional phenyl-substituted vinyl ether (PIVE) cross-linkers were prepared. The assemblies consisted of 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine (DOPE), acid-cleavable poly(ethylene glycol) (PEG)-conjugated lipids, pDNA, and protamine sulfate (PS). The PIVE linkage was designed to hydrolyze under acidic conditions, and the hydrolysis studies of PEG-lipid compounds containing PIVE at pH 4.2, 5.4, and 7.4 indicated that the hydrolysis rates of PIVE linker were influenced by the substitution of electron withdrawing or electron donating groups on the phenyl ring. Acid-catalyzed hydrolysis of PIVE leads to destabilization of the acid labile PEG-PIVE-lipid:DOPE liposomes via dePEGylation, thereby triggering content release. Content release assays showed that dePEGylation was highly pH-dependent and correlated with the PIVE proton affinity of the phenyl group. These results indicated that the dePEGylative triggering based on a new pH-sensitive PIVE linkage can be controlled. In vitro transfection studies on the pH-responsive assemblies containing mPEG-(MeO-PIVE)-conjugated 1,3-dioctadecyl-rac-glycerol lipids (mPEG-(MeO-PIVE])-DOG) showed higher transfection efficiency compared to that of polyethylenimine (PEI), a positive control, on HEK 293 and COS-7 cells. In addition, lower cytotoxicity of PEG-PIVE-lipid:DOPE liposomes/PS/DNA was observed in comparison to PEI. These results suggest that PEG-PIVE-lipid:DOPE liposomes can be considered as nonviral vehicles for drug and gene delivery applications.

Self-assembled pH-sensitive cholesteryl pullulan nanogel as a protein delivery vehicle

A self-assembled nanogel, derived from an acid-labile cholesteryl-modified pullulan (acL-CHP), was prepared by grafting vinyl ether-cholesterol substituents onto a 100 kD pullulan main chain polymer backbone. Stable nanogels are formed by acL-CHP self-assemblies at neutral pH. The hydrodynamic radius of the nanogels, observed to be 26.5 ± 5.1 nm at pH 7.0, increased by ~135% upon acidification of the solution to pH 4.0. SEC analysis of the acL-CHP nanogel at pH 4.0 showed that the grafts were nearly 80% degraded after 24 h, whereas little or no degradation was observed over the same time period for a pH stable analog (acS-CHP) at pH 4.0 or the acL-CHP at pH 7.0. Complexation of BSA with the acL-CHP nanogel was observed at pH 7.0 with subsequent release of the protein upon acidification. These findings suggest that stimuli-responsive, self-assembled nanogels can release protein cargo in a manner that is controlled by the degradation rate of the cholesterol-pullulan grafting moiety.

Acid-triggered release via dePEGylation of fusogenic liposomes mediated by heterobifunctional phenyl-substituted vinyl ethers with tunable pH-sensitivity

A new family of heterobifunctional phenyl-substituted vinyl ether (PIVE) coupling agents with tunable acid-sensitivity has been developed. The PIVE compounds are designed to hydrolyze under acidic conditions with hydrolysis rates that can be varied by rational selection of the phenyl ring substituent. These reagents were incorporated within 2-methoxypoly(ethylene glycol) PEG-conjugated 1,3-dioctadecyl-rac-glycerol lipids to produce the acid-cleavable lipopolymers mPEG-[H-PIVE]-DOG, mPEG-[F-PIVE]-DOG, mPEG-[Me-PIVE]-DOG, and mPEG-[MeO-PIVE]-DOG. These lipopolymers were hydrolyzed under acidic conditions (pH 3.5 or 4.5) at rates that were dependent on the electron donating or withdrawing character of the α-phenyl vinyl ether substituent, while remaining stable at pH 7.4. Blending of these compounds with 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine (DOPE) in a 10:90 mPEG-PIVE-Lipid:DOPE ratio produced stable liposomes at neutral pH; however, acidification of the solution led to dePEGylation and release of the liposomal cargo in a manner that correlated with the PIVE proton affinity. Specifically, we observed 70% calcein release within 12 h from mPEG-[MeO-PIVE]-DOG-containing liposomes at pH 4.5, whereas only 22% calcein release was observed from mPEG-[F-PIVE]-DOG:DOPE liposomes over this same time scale and pH. These results indicate that dePEGylation following acidification is a triggering mechanism that can be rationally designed and controlled through the appropriate selection of PIVE moieties.