Temperature-dependent interaction of thermo-sensitive polymer-modified liposomes with CV1 cells

Temperature-sensitive polymers to promote heat-triggered drug release from liposomes: Towards bypassing EPR

Heat-triggered drug release from temperature-sensitive nanocarriers upon the application of mild hyperthermia is a promising approach to achieve site-specific delivery of drugs. The combination of mild hyperthermia (41-42 °C) and temperature-sensitive liposomes (TSL) that undergo lipid phase-transition and drug release has been studied extensively and has shown promising therapeutic outcome in a variety of animal tumor models as well as initial indications of success in humans. Sensitization of liposomes to mild hyperthermia by means of exploiting the thermal behavior of temperature-sensitive polymers (TSP) provides novel opportunities. Recently, TSP-modified liposomes (TSPL) have shown potential for enhancing tumor-directed drug delivery, either by triggered drug release or by triggered cell interactions in response to heat. In this review, we describe different classes of TSPL, and analyze and discuss the mechanisms and kinetics of content release from TSPL in response to local heating. In addition, the impact of lipid composition, polymer and copolymer characteristics, serum components and PEGylation on the mechanism of content release and TSPL performance is addressed. This is done from the perspective of rationally designing TSPL, with the overall goal of conceiving efficient strategies to increase the efficacy of TSPL plus hyperthermia to improve the outcome of targeted anticancer therapy.

Liposomes: structure, composition, types, and clinical applications

Liposomes are now considered the most commonly used nanocarriers for various potentially active hydrophobic and hydrophilic molecules due to their high biocompatibility, biodegradability, and low immunogenicity. Liposomes also proved to enhance drug solubility and controlled distribution, as well as their capacity for surface modifications for targeted, prolonged, and sustained release. Based on the composition, liposomes can be considered to have evolved from conventional, long-circulating, targeted, and immune-liposomes to stimuli-responsive and actively targeted liposomes. Many liposomal-based drug delivery systems are currently clinically approved to treat several diseases, such as cancer, fungal and viral infections; more liposomes have reached advanced phases in clinical trials. This review describes liposomes structure, composition, preparation methods, and clinical applications.

Hyperthermia and Temperature-Sensitive Nanomaterials for Spatiotemporal Drug Delivery to Solid Tumors

Nanotechnology has great capability in formulation, reduction of side effects, and enhancing pharmacokinetics of chemotherapeutics by designing stable or long circulating nano-carriers. However, effective drug delivery at the cellular level by means of such carriers is still unsatisfactory. One promising approach is using spatiotemporal drug release by means of nanoparticles with the capacity for content release triggered by internal or external stimuli. Among different stimuli, interests for application of external heat, hyperthermia, is growing. Advanced technology, ease of application and most importantly high level of control over applied heat, and as a result triggered release, and the adjuvant effect of hyperthermia in enhancing therapeutic response of chemotherapeutics, i.e., thermochemotherapy, make hyperthermia a great stimulus for triggered drug release. Therefore, a variety of temperature sensitive nano-carriers, lipid or/and polymeric based, have been fabricated and studied. Importantly, in order to achieve an efficient therapeutic outcome, and taking the advantages of thermochemotherapy into consideration, release characteristics from nano-carriers should fit with applicable clinical thermal setting. Here we introduce and discuss the application of the three most studied temperature sensitive nanoparticles with emphasis on release behavior and its importance regarding applicability and therapeutic potentials.

Poly( N-isopropylacrylamide)-Based Polymer-Inducing Isothermal Hydrophilic-to-Hydrophobic Phase Transition via Detachment of Hydrophilic Acid-Labile Moiety

The polymerization of N-isopropylacrylamide (NIPAAm) with ionizable monomers results in pH-responsive lower critical solution temperature (LCST) polymer which works in an ionization-dependent manner. However, gradual ionization of the comonomer occurs at a broad pH range due to the electrostatic field generated by the polymers, limiting the extent of LCST shift in response to pH change. Furthermore, excess introduction of comonomer may dull phase transition behavior. Here, we report the development of an ionization-independent LCST polymer that exerts a sharp isothermal hydrophilic-to-hydrophobic phase transition in response to slight pH change. Our polymer has a poly(NIPAAm/2-aminoisoprpylacrylamide (AIPAAm)) (P(NIPAAm/AIPAAm)) backbone that retains the continuous structural similarity of N-alkyl groups for preserving phase transition sensitivity, and primary amine for forming hydrophilic acid-labile 2-propionic-3-methylmaleic (PMM) amide linkage. The PMM moiety improves the polymer's hydrophilicity and drastically increases the LCST. Detachment of the PMM moiety in response to mild acidic condition (pH < 6.8) lowers the LCST to that of original P(NIPAAm/AIPAAm), permitting isothermal pH-responsive phase transition. Utilizing this mechanism, P(NIPAAm/AIPAAm) modified with PMM amide linkage exhibits a sharp hydrophilic-to-hydrophobic transition at a physiological temperature (37 °C) and, strikingly, facilitates interaction with cultured cells. Most importantly, our polymer showed significantly higher accumulation within a solid tumor after systemic injection compared to conventional PNIPAAm, which may be due to its phase transition responding to slightly acidic tumor microenvironment. Thus, this study provides a novel polymer that offers delicate control of LCST and pH-responsiveness suitable for use in even fuzzy biological environments.

Effect of Polymer Phase Transition Behavior on Temperature-Responsive Polymer-Modified Liposomes for siRNA Transfection

Small interfering RNAs (siRNAs) have been attracting significant attention owing to their gene silencing properties, which can be utilized to treat intractable diseases. In this study, two temperature-responsive liposomal siRNA carriers were prepared by modifying liposomes with different polymers-poly(N-isopropylacrylamide-co-N,N-dimethylaminopropyl acrylamide) (P(NIPAAm-co-DMAPAAm)) and poly(N-isopropylacrylamide-co-N,N-dimethylacrylamide) P(NIPAAm-co-DMAAm). The phase transition of P(NIPAAm-co-DMAPAAm) was sharper than that of P(NIPAAm-co-DMAAm), which is attributed to the lower co-monomer content. The temperature dependent fixed aqueous layer thickness (FALT) of the prepared liposomes indicated that modifying liposomes with P(NIPAAm-co-DMAPAAm) led to a significant change in the thickness of the fixed aqueous monolayer between 37 °C and 42 °C; while P(NIPAAm-co-DMAAm) modification led to FALT changes over a broader temperature range. The temperature-responsive liposomes exhibited cellular uptake at 42 °C, but were not taken up by cells at 37 °C. This is likely because the thermoresponsive hydrophilic/hydrophobic changes at the liposome surface induced temperature-responsive cellular uptake. Additionally, siRNA transfection of cells for the prevention of luciferase and vascular endothelial growth factor (VEGF) expression was modulated by external temperature changes. P(NIPAAm-co-DMAPAAm) modified liposomes in particular exhibited effective siRNA transfection properties with low cytotoxicity compared with P(NIPAAm-co-DMAAm) modified analogues. These results indicated that the prepared temperature-responsive liposomes could be used as effective siRNA carriers whose transfection properties can be modulated by temperature.

Enhanced cellular uptake and gene silencing activity of siRNA using temperature-responsive polymer-modified liposome

Short interfering RNA (siRNA) delivery systems using nanoparticle carriers have been limited by inefficient intracellular delivery. One drawback is the poor cellular uptake of siRNA/particle complexes through the plasma membrane and release of the nucleic acids into the cytosol. In this study, to develop the temperature-responsive liposome as a novel carrier for siRNA delivery, we prepared lipoplexes and assessed cellular uptake of siRNA and gene silencing activity of target genes, compared with those of a commercial transfection reagent, Lipofectamine RNAiMAX, and non-modified or PEGylated liposomes. The temperature-responsive polymer, N-isopropylacrylamide-co-N,N'-dimethylaminopropylacrylamide [P(NIPAAm-co-DMAPAAm)]-modified liposome induced faster intracellular delivery because P(NIPAAm-co-DMAPAAm) exhibits a lower critical solution temperature (LCST) changing its nature from hydrophilic to hydrophobic above the LCST. The temperature-responsive liposomes showed significantly higher gene silencing activity than other carriers with less cytotoxicity. Furthermore, we showed that the temperature-responsive lipoplexes were internalized mainly via microtubule-dependent transport and also by the clathrin-mediated endocytosis pathway. This is the first report that temperature-responsive polymer-modified liposomes thermally enhanced silencing activity of siRNA. The dehydrated polymer on the liposomes, and its aggregation caused around the LCST, can probably be attributed to effective cellular uptake of the lipoplexes for gene silencing activity by interaction with the cell membrane.

Functional liposomes in the cancer-targeted drug delivery

Cancer is considered as one of the most severe health problems and is currently the third most common cause of death in the world after heart and infectious diseases. Novel therapies are constantly being discovered, developed and trialed. Many of the current anticancer agents exhibit non-ideal pharmaceutical and pharmacological properties and are distributed non-specifically throughout the body. This results in death of the both normal healthy and malignant cells and substantially leads to accruing a variety of serious toxic side effects. Therefore, the efficient systemic therapy of cancer is almost impossible due to harmful side effects of anticancer agents to the healthy organs and tissues. Furthermore, several problems such as low bioavailability of the drugs, low drug concentrations at the site of action, lack of drug specificity and drug-resistance also cause many restrictions on clinical applications of these drugs in the tumor therapy. Different types of the liposomal formulations have been used in medicine due to their distinctive advantages associated with their structural flexibility in the encapsulation of various agents with different physicochemical properties. They can also mediate delivery of the cargo to the appropriate cell type and subcellular compartment, reducing the effective dosage and possible side effects which are related to high systemic concentrations. Therefore, these novel systems were found very promising and encouraging dosage forms for the treatment of different types of cancer by increasing efficiency and reducing the systemic toxicity due to the specific drug delivery and targeting.

Adaptive micro and nanoparticles: temporal control over carrier properties to facilitate drug delivery

Recent studies have led to significant advances in understanding the impact of key drug carrier properties such as size, surface chemistry and shape on their performance. Converting this knowledge into improved therapeutic outcomes, however, has proved challenging. This owes to the fact that successful drug delivery carriers have to navigate through multiple physiological hurdles including reticuloendothelial system (RES) clearance, target accumulation, intracellular uptake and endosomal escape. Each of these processes may require unique, and often conflicting, design parameters, thus making it difficult to choose a design that addresses all these hurdles. This challenge can be addressed by designing carriers whose properties can be changed in time so as to successfully navigate them through various biological hurdles. Several carriers have been reported that implement this strategy. This review will discuss the current status and future prospects of this emerging field of "adaptive micro and nanoparticles".

Protein adsorption on poly(N-isopropylacrylamide) brushes: dependence on grafting density and chain collapse

The protein resistance of poly(N-isopropylacrylamide) brushes grafted from silicon wafers was investigated as a function of the chain molecular weight, grafting density, and temperature. Above the lower critical solution temperature (LCST) of 32 °C, the collapse of the water-swollen chains, determined by ellipsometry, depends on the grafting density and molecular weight. Ellipsometry, radio assay, and fluorescence imaging demonstrated that, below the lower critical solution temperature, the brushes repel protein as effectively as oligoethylene oxide-terminated monolayers. Above 32 °C, very low levels of protein adsorb on densely grafted brushes, and the amounts of adsorbed protein increase with decreasing brush-grafting-densities. Brushes that do not exhibit a collapse transition also bind protein, even though the chains remain extended above the LCST. These findings suggest possible mechanisms underlying protein interactions with end-grafted poly(N-isopropyl acrylamide) brushes.

A review of stimuli-responsive nanocarriers for drug and gene delivery

Nanotechnology has shown tremendous promise in target-specific delivery of drugs and genes in the body. Although passive and active targeted-drug delivery has addressed a number of important issues, additional properties that can be included in nanocarrier systems to enhance the bioavailability of drugs at the disease site, and especially upon cellular internalization, are very important. A nanocarrier system incorporated with stimuli-responsive property (e.g., pH, temperature, or redox potential), for instance, would be amenable to address some of the systemic and intracellular delivery barriers. In this review, we discuss the role of stimuli-responsive nanocarrier systems for drug and gene delivery. The advancement in material science has led to design of a variety of materials, which are used for development of nanocarrier systems that can respond to biological stimuli. Temperature, pH, and hypoxia are examples of "triggers" at the diseased site that could be exploited with stimuli-responsive nanocarriers. With greater understanding of the difference between normal and pathological tissues and cells and parallel developments in material design, there is a highly promising role of stimuli-responsive nanocarriers for drug and gene delivery in the future.

Multifunctional polyvinylpyrrolidinone-polyacrylic acid copolymer hydrogels for biomedical applications

Copolymers of N-vinylpyrrolidinone and acrylic acid, crosslinked with ethylene glycol dimethacrylate and polyethylene glycol 600 dimethacrylate were prepared by UV-polymerisation. These polymers were analysed for their extractable content by Soxhlet extraction of the samples at 100 degrees C for 72 h. Aspirin and paracetamol were incorporated into the polymer structure at 25 wt.% during the curing process and their presence confirmed by Fourier transform infrared spectroscopy. It was found that the release rate of the drug from the polymer matrix was dependent on intermolecular bonding between the polymer and active agent with aspirin being released slower than paracetamol in all cases. Results showed that paracetamol was completely released after 24h whereas complete release of aspirin took up to 70 h. Finally preliminary in vitro biocompatibility testing was performed for crosslinked polyvinylpyrrolidinone, by determining human hepatoma HepG2 cell viability in the MTT assay and DNA damage in the comet assay following direct contact with various concentrations of polyvinylpyrrolidinone-containing media. Cytotoxicity data suggests a dose-dependent effect for both crosslinkers, with concentrations in the range 0.025-2.5 mg ml(-1) showing a marginal decrease in viability to, at most, 70% that of untreated cells. Again DNA migration in the comet assay following short-term exposure to EGDMA crosslinked hydrogels correlates with MTT data.

Synthesis and Biological Activity of Anticancer Ether Lipids That Are Specifically Released by Phospholipase A2 in Tumor Tissue

The clinical use of anticancer lipids is severely limited by their ability to cause lysis of red blood cells prohibiting intravenous injection. Novel delivery systems are therefore required in order to develop anticancer ether lipids (AELs) into clinically useful anticancer drugs. In a recent article (J. Med. Chem. 2004, 47, 1694) we showed that it is possible to construct liposome systems composed of masked AELs that are activated by secretory phospholipase A2 in cancerous tissue. We present here the synthesis of six AELs and evaluate the biological activity of these bioactive lipids. The synthesized AEL 1-6 were tested against three different cancer cell lines. It was found that the stereochemistry of the glycerol headgroup in AEL-2 and 3 has a dramatic effect on the cytotoxicity of the lipids. AEL 1-4 were furthermore evaluated for their ability to prevent phosphorylation of the apoptosis regulating kinase Akt, and a correlation was found between their cytotoxic activity and their ability to inhibit Akt phosphorylation.

Advanced strategies in liposomal cancer therapy: problems and prospects of active and tumor specific drug release

Tumor specific drug delivery has become increasingly interesting in cancer therapy, as the use of chemotherapeutics is often limited due to severe side effects. Conventional drug delivery systems have shown low efficiency and a continuous search for more advanced drug delivery principles is therefore of great importance. In the first part of this review, we present current strategies in the drug delivery field, focusing on site-specific triggered drug release from liposomes in cancerous tissue. Currently marketed drug delivery systems lack the ability to actively release the carried drug and rely on passive diffusion or slow non-specific degradation of the liposomal carrier. To obtain elevated tumor-to-normal tissue drug ratios, it is important to develop drug delivery strategies where the liposomal carriers are actively degraded specifically in the tumor tissue. Many promising strategies have emerged ranging from externally triggered light- and thermosensitive liposomes to receptor targeted, pH- and enzymatically triggered liposomes relying on an endogenous trigger mechanism in the cancerous tissue. However, even though several of these strategies were introduced three decades ago, none of them have yet led to marketed drugs and are still far from achieving this goal. The most advanced and prospective technologies are probably the prodrug strategies where non-toxic drugs are carried and activated specifically in the malignant tissue by overexpressed enzymes. In the second part of this paper, we review our own work, exploiting secretory phospholipase A2 as a site-specific trigger and prodrug activator in cancer therapy. We present novel prodrug lipids together with biophysical investigations of liposome systems, constituted by these new lipids and demonstrate their degradability by secretory phospholipase A2. We furthermore give examples of the biological performance of the enzymatically degradable liposomes as advanced drug delivery systems.

Steric stabilization of liposomes by pH-responsive N-isopropylacrylamide copolymer

The aim of this study was to characterize a pH-sensitive liposome formulation bearing a terminally alkylated N-isopropylacrylamide (NIPAM) copolymer with regard to its pH responsiveness, surface properties, and pharmacokinetics. The interacting forces between two lipid bilayers bearing the anchored NIPAM copolymer were measured with a surface force apparatus. The pH-triggered content release was evaluated in buffer before and after incubation in human serum. The pharmacokinetics was determined in rats following the intravenous injection of 67Ga-loaded liposomes with or without the polymer coating. The force measurements between lipid bilayers showed that NIPAM copolymers provide a steric barrier that was dependent on pH. The pH-sensitive liposomes maintained their pH sensitivity after incubation in serum. In vivo, the polymer-coated liposomes exhibited a prolonged circulation time in rats, with an area under the blood concentration-time curve that is 1.6-fold higher than the control formulation. This study showed that liposomes can be rendered pH sensitive by anchoring a terminally alkylated NIPAM copolymer at their surface. At neutral pH, the polymer provides a steric barrier that increases the liposome circulation time in vivo.

Thermosensitive polymer-modified liposomes

Temperature-sensitive liposomes are considered to be a promising tool to achieve site-specific delivery of drugs. These liposomes have been prepared using lipids whose membranes undergo a gel-to-liquid crystalline phase transition a few degrees above physiological temperature. However, recently, temperature-sensitization of liposomes has been attempted using thermosensitive polymers. So far, functional liposomes whose contents release behavior, surface properties, and affinity to cell surface can be controlled in a temperature-dependent manner, have been developed according to this strategy. The design and function of these thermosensitive polymer-modified liposomes have been outlined in this review.

Design of fusogenic liposomes using a poly(ethylene glycol) derivative having amino groups

As a novel fusogenic liposome, we designed liposomes modified with poly(glycidol) having beta-alanine residues, which is a poly(ethylene glycol) derivative with positively charged groups. The polymer-modified liposomes of egg yolk phosphatidylcholine (EYPC) and dioleoylphosphatidylethanolamine (DOPE) were prepared by reverse phase evaporation. Fusion of the polymer-modified liposomes with anionic liposomes consisting of phosphatidic acid and DOPE was investigated. Fusion ability of the polymer-modified liposomes increased with increasing amount of the polymer fixed on the liposome. Also, inclusion of DOPE was necessary for the generation of the fusion ability of the polymer-modified liposomes. CV1 cells treated with the polymer-modified DOPE/EYPC liposomes containing calcein displayed diffuse fluorescence, suggesting that calcein was introduced into the cytoplasm. In contrast, only punctual fluorescence was observed in the cells treated with the polymer-modified EYPC liposomes containing calcein, indicating that calcein remained in the endosome and/or lysosome. In addition, COS1 cells were transfected efficiently by treatment with the polymer-modified EYPC/DOPE liposomes containing pSV2cat plasmid, whereas the transfection was not induced by treatment with the polymer-modified EYPC liposomes. Close correlation between fusion ability of the polymer-modified liposomes and their ability to deliver their contents to the cytoplasm implies that membrane fusion plays an important role in the liposome-mediated cytoplasmic delivery.