Photosensitive liposomes as 'cages' for laser-triggered solute delivery: the effect of bilayer cholesterol on kinetics of solute release

Advances in Liposome-Encapsulated Phthalocyanines for Photodynamic Therapy

This updated review aims to describe the current status in the development of liposome-based systems for the targeted delivery of phthalocyanines for photodynamic therapy (PDT). Although a number of other drug delivery systems (DDS) can be found in the literature and have been studied for phthalocyanines or similar photosensitizers (PSs), liposomes are by far the closest to clinical practice. PDT itself finds application not only in the selective destruction of tumour tissues or the treatment of microbial infections, but above all in aesthetic medicine. From the point of view of administration, some PSs can advantageously be delivered through the skin, but for phthalocyanines, systemic administration is more suitable. However, systemic administration places higher demands on advanced DDS, active tissue targeting and reduction of side effects. This review focuses on the already described liposomal DDS for phthalocyanines, but also describes examples of DDS used for structurally related PSs, which can be assumed to be applicable to phthalocyanines as well.

Photosensitive drug delivery systems for cancer therapy: Mechanisms and applications

Over the past three decades, various photosensitive nanoparticles have been developed as potential therapies in human health, ranging from photodynamic therapy technologies that have already reached clinical use, to drug delivery systems that are still in the preclinical stages. Many of these systems are designed to achieve a high spatial and temporal on-demand drug release via phototriggerable mechanisms. This review examines the current clinical and experimental applications in cancer treatment of photosensitive drug release systems, including nanocarriers such as liposomes, micelles, polymeric nanoparticles, and hydrogels. We will focus on the three main physicochemical mechanisms of imparting photosensitivity to a delivery system: i) photochemical reactions (oxidation, cleavage, and polymerization), ii) photoisomerization, iii) and photothermal reactions. Photosensitive nanoparticles have a multitude of different applications including controlled drug release, resulting from physical/conformational changes in the delivery systems in response to light of specific wavelengths. Most of the recent research in these delivery systems has primarily focused on improving the efficacy and safety of cancer treatments such as photodynamic and photothermal therapy. Combinations of multiple treatment modalities using photosensitive nanoparticulate delivery systems have also garnered great interest in combating multi-drug resistant cancers due to their synergistic effects. Finally, the challenges and future potential of photosensitive drug delivery systems in biomedical applications is outlined.

Triggered interactions between nanoparticles and lipid membranes: design principles for gel formation or disruption-and-release

Lipid bilayer vesicles offer exciting possibilities for stimulated response, taking advantage of the membrane's flexibility and impermeability. We show how synergistic interactions between vesicles and polymer-based nanoparticles can be triggered at the nanoscale using UV light. This interaction leads either to adhesion and a membrane-based gel, or to nanoscale wrapping of the particles by the membrane and then vesicle destruction. To map the response, we varied the particle-membrane interactions via their surface charge densities. We found a crossover from adhesion to destruction at a well-defined region in parameter space. We modeled these results by accounting for the electrostatic attraction and the energy of membrane bending. We then synthesized amphiphilic polymers containing a UV-responsive nitrobenzyl moiety that switches its charge, and showed how a trigger predictably led to either a vesicle gel or disruption and release. The results pave the way to a new triggering mechanism and new response modes in soft materials.

Photocontrollable Intermittent Release of Doxorubicin Hydrochloride from Liposomes Embedded by Azobenzene-Contained Glycolipid

Azobenzene-contained glycolipids GlyAzoCns, newly structured azobenzene derivatives, which have an azobenzene moiety between the galactosyl and carbon chains of various sizes, have been synthesized. The GlyAzoCns undergo reversible photoinduced isomerization in both ethanol solution (free state) and liposomal bilayer (restricted state) upon irradiation with UV and vis light alternately. The drug release of Liposome@Gly induced by isomerization was found to be an instantaneous behavior. The photoinduced control of DOX release from liposome was investigated in various modes. The Liposome@Glys have been found to keep the entrapped DOX stably in the dark with less than 10% leakage in 10 h but release nearly 100% of cargos instantaneously with UV irradiation. The molecular structure of GlyAzoCns and the property of the liposomal bilayer were considered as important factors influencing drug release. Among the synthesized GlyAzoCns, GlyAzoC7 was shown to be the most efficient photosensitive actuator for controlling drug release. A lower proportion of cholesterol in Liposome@Glys was conducive to promote the release amount. Results indicated that the synthesized GlyAzoCns could act as a role of smart actuators in the liposome bilayer and control the drug to release temporarily and quantitatively.

Ultraviolet light-mediated drug delivery: Principles, applications, and challenges

UV light has been extensively employed in drug delivery because of its versatility, ease of manipulation, and ability to induce chemical changes on the therapeutic carrier. Here we review the mechanisms by which UV light affects drug delivery systems. We will present the challenges facing UV-induced drug delivery and some of the proposed solutions.

Nonphospholipid fluid liposomes with switchable photocontrolled release

We created novel nonphospholipid photosensitive liposomes from a mixture of a monoacylated azobenzene amphiphile (AzoC10N(+)) and cholesterol sulfate (Schol). This system belongs to the family of sterol-enriched nonphospholipid liposomes that were shown to form stable large unilamellar vesicles (LUVs) with enhanced impermeability. Fluid bilayers were successfully prepared from AzoC10N(+)/Schol (25/75 molar ratio) mixtures, and LUVs could be derived at room temperature using standard extrusion methods. The isomerization process of the bilayer-inserted AzoC10N(+) was characterized. Leakage from these liposomes could be induced by the photoconversion of AzoC10N(+) from its trans form to its cis form. This photocontrolled release from fluid liposomes contrasts with the case of phospholipid-based azo-containing liposomes, which are generally required to be in the gel phase to be photosensitive. It is proposed that the very high degree of conformational order of the monoalkylated amphiphile and the tight packing of the hydrophobic core of the AzoC10N(+)/Schol liposomes make them responsive to the presence of the bulky cis azo isomer. Interestingly, the liposome impermeability could be fully restored by the photoisomerization of the cis form back to the trans form, providing a sharp on-and-off control of payload release. In addition, these nonphospholipid liposomes display a very limited passive release. Therefore, it is shown that AzoC10N(+)/Schol LUVs can be used as nanocontainers, whose content can be released by light in a controlled and switchable manner.

Nanopreparations for organelle-specific delivery in cancer

To efficiently deliver therapeutics into cancer cells, a number of strategies have been recently investigated. The toxicity associated with the administration of chemotherapeutic drugs due to their random interactions throughout the body necessitates the development of drug-encapsulating nanopreparations that significantly mask, or reduce, the toxic side effects of the drugs. In addition to reduced side effects associated with drug encapsulation, nanocarriers preferentially accumulate in tumors as a result of its abnormally leaky vasculature via the Enhanced Permeability and Retention (EPR) effect. However, simple passive nanocarrier delivery to the tumor site is unlikely to be enough to elicit a maximum therapeutic response as the drug-loaded carriers must reach the intracellular target sites. Therefore, efficient translocation of the nanocarrier through the cell membrane is necessary for cytosolic delivery of the cargo. However, crossing the cell membrane barrier and reaching cytosol might still not be enough for achieving maximum therapeutic benefit, which necessitates the delivery of drugs directly to intracellular targets, such as bringing pro-apoptotic drugs to mitochondria, nucleic acid therapeutics to nuclei, and lysosomal enzymes to defective lysosomes. In this review, we discuss the strategies developed for tumor targeting, cytosolic delivery via cell membrane translocation, and finally organelle-specific targeting, which may be applied for developing highly efficacious, truly multifunctional, cancer-targeted nanopreparations.

Light-activated content release from liposomes

Successful integration of diagnostic and therapeutic actions at the level of individual cells requires new materials that combine biological compatibility with functional versatility. This review focuses on the development of liposome-based functional materials, where payload release is activated by light. Methods of sensitizing liposomes to light have progressed from the use of organic molecular moieties to the use of metallic plasmon resonant structures. This development has facilitated application of near infrared light for activation, which is preferred for its deep penetration and low phototoxicity in biological tissues. Presented mechanisms of light-activated liposomal content release enable precise in vitro manipulation of minute amounts of reagents, but their use in clinical diagnostic and therapeutic applications will require demonstration of safety and efficacy.

Photochemical mechanisms of light-triggered release from nanocarriers

Over the last three decades, a handful of photochemical mechanisms have been applied to a large number of nanoscale assemblies that encapsulate a payload to afford spatio-temporal and remote control over activity of the encapsulated payload. Many of these systems are designed with an eye towards biomedical applications, as spatio-temporal and remote control of bioactivity would advance research and clinical practice. This review covers five underlying photochemical mechanisms that govern the activity of the majority of photoresponsive nanocarriers: 1. photo driven isomerization and oxidation, 2. surface plasmon absorption and photothermal effects, 3. photo driven hydrophobicity changes, 4. photo driven polymer backbone fragmentation and 5. photo driven de-crosslinking. The ways in which these mechanisms have been incorporated into nanocarriers and how they affect release are detailed, as well as the advantages and disadvantages of each system.

Photoresponsive liposomal nanohybrid cerasomes

An innovative photoresponsive cerasome is fabricated by sol-gel process in combination of self-assembly technique from a molecularly designed organoalkoxysilylated lipid containing an azobenzene unit, which is able to operate as a "valve" with an "on-off" function under specific stimuli to control the release of loaded guest molecules from the liposomal membrane.

Light-sensitive lipid-based nanoparticles for drug delivery: design principles and future considerations for biological applications

Radiation-based therapies aided by nanoparticles have been developed for decades, and can be primarily categorized into two main platforms. First, delivery of payload of photo-reactive drugs (photosensitizers) using the conventional nanoparticles, and second, design and development of photo-triggerable nanoparticles (primarily liposomes) to attain light-assisted on-demand drug delivery. The main focus of this review is to provide an update of the history, current status and future applications of photo-triggerable lipid-based nanoparticles (light-sensitive liposomes). We will begin with a brief overview on the applications of liposomes for delivery of photosensitizers, including the choice of photosensitizers for photodynamic therapy, as well as the currently available light sources (lasers) used for these applications. The main segment of this review will encompass the details of strategies used to develop photo-triggerable liposomes for their drug delivery function. The principles underlying the assembly of photoreactive lipids into nanoparticles (liposomes) and photo-triggering mechanisms will be presented. We will also discuss factors that limit the applications of these liposomes for in vivo triggered drug delivery and emerging concepts that may lead to the biologically viable photo-activation strategies. We will conclude with our view point on the future perspectives of light-sensitive liposomes in the clinic.

Photo-targeted nanoparticles

We report a novel and simple proof-of-concept of a nanoparticulate system that targets any tissue selectively upon illumination. Nanoparticles were covalently functionalized with the amino acid sequence YIGSR, which adheres to the beta1 integrins present on most cell surfaces. This peptide was masked with a caging group, rendering it biologically inert. Illumination with UV light released the caging group from the YIGSR, allowing binding to cells.

Laser-induced disruption of systemically administered liposomes for targeted drug delivery

Liposomal formulations of drugs have been shown to enhance drug efficacy by prolonging circulation time, increasing local concentration and reducing off-target effects. Controlled release from these formulations would increase their utility, and hyperthermia has been explored as a stimulus for targeted delivery of encapsulated drugs. Use of lasers as a thermal source could provide improved control over the release of the drug from the liposomes with minimal collateral tissue damage. Appropriate methods for assessing local release after systemic delivery would aid in testing and development of better formulations. We use in vivo bioluminescence imaging to investigate the spatiotemporal distribution of luciferin, used as a model small molecule, and demonstrate laser-induced release from liposomes in animal models after systemic delivery. These liposomes were tested for luciferin release between 37 and 45 degrees C in PBS and serum using bioluminescence measurements. In vivo studies were performed on transgenic reporter mice that express luciferase constitutively throughout the body, thus providing a noninvasive readout for controlled release following systemic delivery. An Nd:YLF laser was used (527 nm) to heat tissues and induce rupture of the intravenously delivered liposomes in target tissues. These data demonstrate laser-mediated control of small molecule delivery using thermally sensitive liposomal formulations.

Photoisomerisable cholesterol derivatives as photo-trigger of liposomes: Effect of lipid polarity, temperature, incorporation ratio, and cholesterol

Three cholesterol derivatives containing an azobenzene moiety with different polarities were designed and synthesized (AB lipids 1 to 3). The effects of structure, temperature and incorporation ratio on liposomes were studied, with the results showing that the polarity in 4-substituent and in some cases, 4'-substituent may be important for their incorporation feasibility and photoisomerizability in liposomes. Liposomes incorporated with AB lipid 3 could release multi-pulsatilely upon UV and visible light irradiation both in gel state and liquid crystal state of liposomes. An increase in the incorporation ratio of AB lipid 3 enhanced the amount of drug released greatly. Unlike other azobenzene photo-triggers reported, AB lipid 3 did not increase the spontaneous release of liposomes. Furthermore, cholesterol suppressed the spontaneous release of liposomes.

Reversible photoswitching in a cell-sized vesicle

A photosensitive amphiphilic molecule can switch the shape of an assembled vesicle as determined by microscopic observation. Photoisomerization induces a change in membrane fluctuation behavior or a morphological transition between ellipsoid and bud shapes, depending on the asymmetrical degree of the initial shape. The mechanism of this reversible photoswitching in the vesicle morphology is interpreted in terms of a change in the effective cross-sectional area of the photosensitive molecule.

UV-induced drug release from photoactive REV sensitized by suprofen

In order to achieve local administration of drugs, calcein (CAL) encapsulated reverse phase evaporation vesicles (REV) carrying photoactive destabilization agent suprofen (SPF) in the lipid bilayer were prepared. Effect of both UV-A and UV-B photoactivation of liposomal membrane incorporated SPF on the destabilization of the liposome bilayer and the release of encapsulated CAL was investigated. Standard REV of phosphatidylcholine (PC):cholesterol (CHOL) in 7:3 molar ratio, and photoactive REV of PC:CHOL:SPF, and DPPC:CHOL:SPF in 7:3:3 molar ratio were prepared. CAL encapsulation efficiency (EE (%)) and in situ release was studied. SPF incorporation in the PC REV membrane led to approximately 5% increase in the EE (34%) in comparison to standard REV (29%). EE decreased (21%) when DPPC was used to replace PC. Exposure to UV-B caused the highest CAL release. The lowest release was from the unexposed REV. DPPC led to a higher liposomal membrane stability (lower CAL release) than PC. A linear relationship was observed between UV-B exposure duration and REV permeability. This study revealed that membrane destabilization of SPF incorporated REV was best achieved upon photoactivation of the membrane-localized SPF by a 40 min exposure to UV-B.

H-aggregation of azobenzene-substituted amphiphiles in vesicular membranes

Photochemical switching has been studied of double-tailed phosphate amphiphiles containing azobenzene units in both tails in aqueous vesicular dispersions and in mixed vesicular systems with 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC). Since the ease of switching depends on the strength of the bilayer packing, particular emphasis has been placed on the occurrence of H-aggregation in the hydrophobic core of the vesicles. UV-vis spectrometry was employed to monitor H-aggregation and showed how this process depends on the ionic strength and on the mode of preparation of the vesicles. Two types of H-aggregates were observed in mixed DOPC vesicles with 5 mol % of azobenzene phosphate: one with lambda(max) at around 300 nm and one with lambda(max) at 305-320 nm. Those with lambda(max) at 300 nm could not be trans-cis photoisomerized, whereas those with lambda(max) at 305-320 nm are more loosely packed and can be photochemically switched. The permeability of the vesicular bilayers, as probed with leakage experiments using calcein as a fluorescent probe, was examined as another measure for the strength of bilayer packing. Leakage occurred only for DOPC vesicles containing more than 20 mol % of azobenzenephosphate, irradiated with UV light to induce trans-cis photoisomerization. We contend that detailed information on bilayer packing will be of crucial importance for fine-tuning the lateral pressure in vesicular membranes with the ultimate aim to steer the opening and closing of mechanosensitive protein channels of large conductance.

Localized delivery to CT-26 tumors in mice using thermosensitive liposomes

A heat-sensitive liposomal drug delivery system was tested using Colon-26 (CT-26) cultured cells and tumors in mice. Lucifer yellow iodoacetamide (LY) was used as a fluorescence marker. The heat-sensitive liposomes exploit the temperature-dependence of critical micellar concentrations of the poloxamer, F127. LY release from unilamellar liposomes at different temperatures was measured. Onset of LY release occurred near 33 degrees C, and reached plateau above 42 degrees C when 90% of the LY was released. Temperature-treated liposomes were mixed with CT-26 cells to measure the binding of the released LY to cell surface. Temperature-dependency of cell-bound LY corresponds to the release curve. CT-26 tumors were grown subcutaneously in both hind legs of Balb/c mice. Mice received heat-sensitive or plain liposomes via tail vein injections, or no liposomes. For each mouse, one tumor was kept at 31.5 degrees C, while the counterlateral tumor was heated to 42 degrees C during injection and for 30min after. LY released in tumors was determined from fluorescence intensity. Tumors receiving heat-sensitive liposomes plus heat treatment showed 2.5-fold greater fluorescence than all other tumors, which were at the background level. This study demonstrates the possible use of poloxamer-containing liposomes as a heat-sensitive drug delivery system in vivo.

Spatially and temporally resolved delivery of stimuli to single cells

This Communication describes the photolysis of individual optically trapped vesicles for delivering spatially and temporally resolved stimuli to single cells. A submicrometer-sized vesicle, which encapsulates the stimuli of interest, is precisely positioned with respect to a select cell by optical trapping. Application of a single focused UV laser pulse photolyzes the trapped vesicle and releases the stimuli onto a localized region of the cell. The spatial resolution of this technique is on the order of several hundreds of nanometers, while the temporal resolution lies in the sub-microsecond range.

Wavelength-programmed solute release from photosensitive liposomes

Liposomes of dipalmitoylphosphatidylcholine containing a photochromic lipid "Bis-Azo PC" release entrapped solutes on exposure to UV light. We have now demonstrated that on addition of cholesterol (up to 25 mol%) to the liposomal membrane the liposomes also release their contents in response to visible light in the region of 470 nm, to which liposomes lacking steroid are insensitive. In a mixed population of liposomes prepared with and without cholesterol, this enables wavelength-dependent release of entrapped solutes on sequential exposure to visible and UV light. Furthermore, the cholesterol-containing liposomes allow stepped partial release of entrapped solute following multiple periods of short visible illumination. It is suggested that the cholesterol-containing liposomes may be potentially useful for drug delivery and for "caging" of reagents.

Active uptake of drugs into photosensitive liposomes and rapid release on UV photolysis

Liposomes containing high concentrations of the anticancer drug doxorubicin, prepared by active-loading techniques, have been intensively investigated as potential agents for chemotherapy. The present study investigates the possibility of active uptake and photoinduced release of such solutes from liposomes incorporating a photoisomerizable lipid. The active loading of acridine orange and doxorubicin was investigated using liposomes containing entrapped ammonium sulfate. The liposomes were prepared with dipalmitoyl-L-alpha-phosphatidyl choline (DPPC) and a photochromic lipid, (1,2-(4'-n-butylphenyl)azo-4'-(gamma-phenylbutyroyl))-glycero-3- phosphocholine (Bis-Azo PC), which isomerizes on exposure to near-UV light with resulting changes in membrane permeability to solutes. The rate of loading of the vesicles below the phase transition temperature of DPPC was investigated as a function of Bis-Azo PC and cholesterol concentrations in the liposome. The rate of doxorubicin uptake was found to be greatly decreased in the presence of cholesterol, while below 30 degrees C the rate of acridine orange uptake was increased in the presence of cholesterol. On exposure to a single UV laser pulse, actively loaded acridine orange was rapidly released from liposomes containing Bis-Azo PC at a rate similar to that found for the indicator dye calcein. However while cholesterol had previously been shown to greatly enhance the rate of photo-induced calcein leakage, it had no significant effect on the rate of acridine orange release. After active loading into DPPC vesicles containing Bis-Azo PC, doxorubicin was also released after exposure to a single laser pulse, but at a rate slower than for acridine orange and calcein. The difference in behavior between these systems is ascribed to the interactions of acridine orange and doxorubicin with the liposome bilayer. Photoinduced release of pharmacologically active materials from sensitized liposomes might provide a useful adjunct or alternative to conventional photodynamic therapy.