Fast solute release from photosensitive liposomes: an alternative to 'caged' reagents for use in biological systems

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

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

Photolipid Bilayer Permeability is Controlled by Transient Pore Formation

Controlling the release or uptake of (bio-) molecules and drugs from liposomes is critically important for a range of applications in bioengineering, synthetic biology, and drug delivery. In this paper, we report how the reversible photoswitching of synthetic lipid bilayer membranes made from azobenzene-containing phosphatidylcholine (azo-PC) molecules (photolipids) leads to increased membrane permeability. We show that cell-sized, giant unilamellar vesicles (GUVs) prepared from photolipids display leakage of fluorescent dyes after irradiation with UV-A and visible light. Langmuir-Blodgett and patch-clamp measurements show that the permeability is the result of transient pore formation. By comparing the trans-to-cis and cis-to-trans isomerization process, we find that this pore formation is the result of area fluctuations and a change of the area cross-section between both photolipid isomers.

Formation of nanopores in DiynePC-DPPC complex lipid bilayers triggered by on-demand photo-polymerization

Vesicles have unique characteristics that enable the release of drugs as well as encapsulation while maintaining biocompatibility. A photo-polymerizable liposome composed of 1,2-bis(10,12-tricosadiynoyl)-sn-glycero-3-phosphocholine (23:2 DiynePC) has been investigated as vehicles for triggered delivery of drugs to cells. In this study, we confirmed for the first time that supported lipid bilayers (SLBs) prepared with a 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC)/DiynePC mixture generated pores ca. 100-300 nm in size on the membrane after UV polymerization. This direct observation was done by analyzing the SLBs formed with the DPPC/DiynePC mixture by employing atomic force microscopy (AFM) in a liquid environment. However, photo-polymerization did not occur in the 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC)/DiynePC mixed bilayer and pores were not formed. A theoretical study was performed to explore the phase behavior of the lipid mixtures. A coarse-grained model of DiynePC was developed that is comparable with the Martini force field; the parameters were validated against atomistic simulations. Transition from fluidic to gel phase was observed only when DiynePC was mixed with DPPC, whereas the DOPC mixture remained fluidic over the entire domain. This implies a correlation between the formation of DiynePC-rich gel phase domains and the generation of pores after polymerization. The size of the pores were found to be controlled by the amount of polymerizable lipid which results in higher release rate of encapsulated calcein from the vesicles with larger pores.

Photo activation of HPPH encapsulated in "Pocket" liposomes triggers multiple drug release and tumor cell killing in mouse breast cancer xenografts

We recently reported laser-triggered release of photosensitive compounds from liposomes containing dipalmitoylphosphatidylcholine (DPPC) and 1,2 bis(tricosa-10,12-diynoyl)-sn-glycero-3-phosphocholine (DC(8,9)PC). We hypothesized that the permeation of photoactivated compounds occurs through domains of enhanced fluidity in the liposome membrane and have thus called them "Pocket" liposomes. In this study we have encapsulated the red light activatable anticancer photodynamic therapy drug 2-(1-Hexyloxyethyl)-2-devinyl pyropheophorbide-a (HPPH) (Ex/Em410/670 nm) together with calcein (Ex/Em490/517 nm) as a marker for drug release in Pocket liposomes. A mole ratio of 7.6:1 lipid:HPPH was found to be optimal, with >80% of HPPH being included in the liposomes. Exposure of liposomes with a cw-diode 660 nm laser (90 mW, 0-5 minutes) resulted in calcein release only when HPPH was included in the liposomes. Further analysis of the quenching ratios of liposome-entrapped calcein in the laser treated samples indicated that the laser-triggered release occurred via the graded mechanism. In vitro studies with MDA-MB-231-LM2 breast cancer cell line showed significant cell killing upon treatment of cell-liposome suspensions with the laser. To assess in vivo efficacy, we implanted MDA-MB-231-LM2 cells containing the luciferase gene along the mammary fat pads on the ribcage of mice. For biodistribution experiments, trace amounts of a near infrared lipid probe DiR (Ex/Em745/840 nm) were included in the liposomes. Liposomes were injected intravenously and laser treatments (90 mW, 0.9 cm diameter, for an exposure duration ranging from 5-8 minutes) were done 4 hours postinjection (only one tumor per mouse was treated, keeping the second flank tumor as control). Calcein release occurred as indicated by an increase in calcein fluorescence from laser treated tumors only. The animals were observed for up to 15 days postinjection and tumor volume and luciferase expression was measured. A significant decrease in luciferase expression and reduction in tumor volume was observed only in laser treated animal groups injected with liposomes containing HPPH. Histopathological examination of tumor tissues indicated tumor necrosis resulting from laser treatment of the HPPH-encapsulated liposomes that were taken up into the tumor area.

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.

Phototriggerable liposomes: current research and future perspectives

The field of cancer nanomedicine is considered a promising area for improved delivery of bioactive molecules including drugs, pharmaceutical agents and nucleic acids. Among these, drug delivery technology has made discernible progress in recent years and the areas that warrant further focus and consideration towards technological developments have also been recognized. Development of viable methods for on-demand spatial and temporal release of entrapped drugs from the nanocarriers is an arena that is likely to enhance the clinical suitability of drug-loaded nanocarriers. One such approach, which utilizes light as the external stimulus to disrupt and/or destabilize drug-loaded nanoparticles, will be the discussion platform of this article. Although several phototriggerable nanocarriers are currently under development, I will limit this review to the phototriggerable liposomes that have demonstrated promise in the cell culture systems at least (but not the last). The topics covered in this review include (i) a brief summary of various phototriggerable nanocarriers; (ii) an overview of the application of liposomes to deliver payload of photosensitizers and associated technologies; (iii) the design considerations of photoactivable lipid molecules and the chemical considerations and mechanisms of phototriggering of liposomal lipids; (iv) limitations and future directions for in vivo, clinically viable triggered drug delivery approaches and potential novel photoactivation strategies will be discussed.

Self-assembled liposomal nanoparticles in photodynamic therapy

Photodynamic therapy (PDT) employs the combination of non-toxic photosensitizers (PS) together with harmless visible light of the appropriate wavelength to produce reactive oxygen species that kill unwanted cells. Because many PS are hydrophobic molecules prone to aggregation, numerous drug delivery vehicles have been tested to solubilize these molecules, render them biocompatible and enhance the ease of administration after intravenous injection. The recent rise in nanotechnology has markedly expanded the range of these nanoparticulate delivery vehicles beyond the well-established liposomes and micelles. Self-assembled nanoparticles are formed by judicious choice of monomer building blocks that spontaneously form a well-oriented 3-dimensional structure that incorporates the PS when subjected to the appropriate conditions. This self-assembly process is governed by a subtle interplay of forces on the molecular level. This review will cover the state of the art in the preparation and use of self-assembled liposomal nanoparticles within the context of PDT.

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.

A novel class of photo-triggerable liposomes containing DPPC:DC(8,9)PC as vehicles for delivery of doxorubcin to cells

Success of nanoparticle-mediated drug delivery is subject to development of optimal drug release strategies within defined space and time (triggered release). Recently, we reported a novel class of photo-triggerable liposomes prepared from dipalmitoyl phosphatidylcholine (DPPC) and photopolymerizable diacetylene phospholipid (DC(8),(9)PC), that efficiently released entrapped calcein (a water soluble fluorescent dye) upon UV (254nm) treatment. To develop these formulations for in vivo applications, we have examined phototriggering of these liposomes by visible light, and the effect of released anticancer drugs on cellular toxicity. Sonicated liposomes containing various ratios of DPPC:DC(8),(9)PC and 4mol% DSPE-PEG2000 were loaded with calcein (Ex/Em, 485/517nm) or a chemotherapy drug, Doxorubicin (DOX, Ex/Em 490/590nm). Our initial experiments showed that 514nm laser treatment of liposomes containing 10 or 20mol% DC(8,9)PC for 1-3min resulted in significant release of calcein. Based on these results, we performed studies with DOX-loaded liposomes. First, biophysical properties (including liposome size and stability) and DOX encapsulation efficiency of the liposomes were determined. Subsequently, the effect of 514nm laser on DOX release, and cellular toxicity by released DOX were examined. Since liposomes using the 86:10:04 mole ratio of DPPC:DC(8),(9)PC:DSPE-PEG2000, showed highest encapsulation of DOX, these formulations were investigated further. We report that (i) liposomes retained about 70% of entrapped DOX at 37°C in the presence of 0-50% serum. (ii) 514nm laser treatment resulted in DOX release from liposomes in a wavelength-specific manner. (iii) Laser treatment of co-cultures containing DOX-loaded liposomes and cells (Raji and MCF-7) resulted in at least 2-3 fold improved cell killing as compared to untreated samples. Taken together, the photo-triggerable liposomes described here may provide a platform for future drug delivery applications. To our knowledge, this is the first report demonstrating improved cell killing following light-triggered release of an encapsulated anticancer agent from photosensitive liposomes.

Laser photolysis of dye-sensitized nanocapsules occurs via a photothermal pathway

Light-addressable nanocapsules offer a powerful method for delivering spatiotemporally precise signals to cells. Thus far, the mechanism involved in the photolysis of nanocapsules has been opaque. This paper presents experimental evidence that rules out a photochemical pathway in favor of a photothermal mechanism in the far-red photolysis of dye-sensitized, lipid-vesicle based nanocapsules. Photolysis efficiency was unaffected by the presence of radical inhibitors, and mass spectrometry measurements confirmed that the photolytic process did not produce dye radicals. Measurements of dye quantum yield in the lipid membrane showed an inverse correlation between quantum yield of the dye and photolysis efficiency of the vesicle. The result is consistent with the notion that a decrease in quantum yield translates into more vibrational relaxation and thermal motion of the dye molecules in the membrane and thus more efficient photothermal disruption of the vesicle. Furthermore, we observed that the decrease in quantum yield and increase in photolysis efficiency was caused by the formation of raftlike domains that clustered the dye molecules into concentrated regions. On the basis of this information, we were able to design new nanocapsules using ternary mixtures of lipid and cholesterol that promoted the formation of raft domains and dye clustering. These nanocapsules showed improved photolysis efficiency over the best results we obtained previously.

Lipid-based nanoparticles as pharmaceutical drug carriers: from concepts to clinic

In recent years, various nanotechnology platforms in the area of medical biology, including both diagnostics and therapy, have gained remarkable attention. Moreover, research and development of engineered multifunctional nanoparticles as pharmaceutical drug carriers have spurred exponential growth in applications to medicine in the last decade. Design principles of these nanoparticles, including nanoemulsions, dendrimers, nano-gold, liposomes, drug-carrier conjugates, antibody-drug complexes, and magnetic nanoparticles, are primarily based on unique assemblies of synthetic, natural, or biological components, including but not limited to synthetic polymers, metal ions, oils, and lipids as their building blocks. However, the potential success of these particles in the clinic relies on consideration of important parameters such as nanoparticle fabrication strategies, their physical properties, drug loading efficiencies, drug release potential, and, most importantly, minimum toxicity of the carrier itself. Among these, lipid-based nanoparticles bear the advantage of being the least toxic for in vivo applications, and significant progress has been made in the area of DNA/RNA and drug delivery using lipid-based nanoassemblies. In this review, we will primarily focus on the recent advances and updates on lipid-based nanoparticles for their projected applications in drug delivery. We begin with a review of current activities in the field of liposomes (the so-called honorary nanoparticles), and challenging issues of targeting and triggering will be discussed in detail. We will further describe nanoparticles derived from a novel class of amphipathic lipids called bolaamphiphiles with unique lipid assembly features that have been recently examined as drug/DNA delivery vehicles. Finally, an overview of an emerging novel class of particles (based on lipid components other than phospholipids), solid lipid nanoparticles and nanostructured lipid carriers will be presented. We conclude with a few examples of clinically successful formulations of currently available lipid-based nanoparticles.

Accelerated photostability study of tretinoin and isotretinoin in liposome formulations

The photodegradation of retinoic acids, tretinoin and isotretinoin, in ethanol and liposomes was studied. The light irradiation was performed according to the conditions suggested by the ICH Guideline for photostability testing by using a Xenon lamp within a wavelength range of 300-800 nm. The photodegradation process was monitored by UV spectrophotometry. In ethanol solution, tretinoin and isotretinoin undergo complete isomerization just within a few seconds of light exposure to give 13-cis and 9-cis isomers, respectively. The 13-cis isomer from tretinoin undergoes in turn a slow isomerization to the same 9-cis isomer. Both retinoic acids incorporated in liposome complexes showed an increased stability in comparison to the ethanol solutions. In particular for tretinoin, a residual concentration of 60% was still present after a light irradiance of 3470 kJ/m(2), by means of a 250 W/m(2) light power for 240 min, versus a residual value of just 8% measured at the same time in ethanol solution. Moreover, the isomerization rate in liposomes resulted reduced for isotretinoin and practically irrelevant for tretinoin. The degradation rate was found to be dependent on the drug concentration. The better stability of the tretinoin in liposome complex was supposed to be related to its higher incorporation value due to the linear structure of the molecule.

Synthesis, loading, and application of individual nanocapsules for probing single-cell signaling

This paper describes the synthesis and loading of silica and polystyrene-acrylic based nanocapsules with small molecules. The nanocapsules are used for delivering defined packages of stimuli to single cells with both high spatial and temporal resolutions. To introduce molecules into the capsules, we characterized two approaches. The first approach is based on a base-swell process in which the shell of the capsule is swelled so small molecules can diffuse into the interior of the capsule and be trapped inside once the capsules are de-swelled. The second approach is based on a dry-swell-dry process in which the solution containing the molecules of interest and the nanocapsules is physically dried to promote more molecules to enter into the interior of the capsule. We characterized both methods by monitoring the content of and the release from individual capsules with confocal microscopy and wide-field imaging. To illustrate the biological applications of such nanocapsules, we used optical trapping to position a single carbachol-loaded capsule adjacent to a single CHO cell that has been transfected with muscarinic acetylcholine (M1) receptors, released the carbachol from the capsule with a single 3-ns N2 laser pulse, and then monitored the subsequent intracellular signaling triggered by the binding of carbachol to the M1 receptors.

Design and monitoring of photostability systems for amlodipine dosage forms

Photostability of amlodipine (AML) has been monitored in several pharmaceutical inclusion systems characterized by plurimolecular aggregation of the drug and excipients with high molecular weight. Several formulations including cyclodextrins, liposomes and microspheres have been prepared and characterized. The photodegradation process has been monitored according to the conditions suggested by the ICH Guideline for photostability testing, by using a light cabinet equipped with a Xenon lamp and monitored by spectrophotometry. The formulations herein tested have been found to be able to considerably increase drug stability, when compared with usual pharmaceutical forms. The residual concentration detected in the inclusion complexes with cyclodextrins and liposomes was 90 and 77%, respectively, while a very good value of 97% was found for microspheres, after a radiant exposure of 11,340 kJm(-2).

Formulation and stability testing of photolabile drugs

Exposure of a drug to irradiation can influence the stability of the formulation, leading to changes in the physicochemical properties of the product. The influence of excipients of frequently used stabilizers is often difficult to predict and, therefore, stability testing of the final preparation is important. The selection of a protective packaging must be based on knowledge about the wavelength causing the instability. Details on drug photoreactivity will also be helpful in order to minimize side-effects and/or optimize drug targeting by developing photoresponsive drug delivery systems. This review focuses on practical problems related to formulation and stability testing of photolabile drugs.

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.

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

Liposomes containing acyl chains incorporating azobenzene chromophores have been investigated as potential 'caging' agents for fast solute release. On photolysis, trapped marker dye can be released from gel-phase liposomes within milliseconds. Solute release is markedly sensitive to the presence of cholesterol in the bilayer. Phospholipids bearing one saturated acyl chain and an azobenzene-substituted chain are ineffective as sensitisers unless cholesterol is present, while doubly substituted phospholipids sensitise release in its absence. Cholesterol markedly affects the temperature profile of solute release depending on the host phospholipid chain length. Solute release is not seen for lipid hosts with unsaturated acyl chains.

Fast laser-induced solute release from liposomes sensitized with photochromic lipid: effects of temperature, lipid host, and sensitizer concentration

Liposomes of gel-phase phospholipid have been prepared containing a photochromic lipid sensitizer. A fast UV laser pulse isomerizes the sensitizer destabilizing the lipid bilayer structure and causing release of trapped solute. The kinetics of solute release have been investigated as a function of host lipid chain length, sensitizer concentration, and temperature, and the limits of liposome stability have been established. At low concentrations of sensitizer, pulsed laser irradiation induces some solute release when continuous UV illumination is ineffective. Although rates of solute release usually increase with temperature, at low sensitizer concentration in a rigid host, leakage at first increases but then decreases rapidly above a threshold temperature. The results presented are relevant to the design of photostimulated drug delivery systems and to potential applications of photosensitive liposomes as caging agents for biological effectors.