Enzymatically activated microencapsulated liposomes can provide pulsatile drug release

Self-destructing "mothership" capsules for timed release of encapsulated contents

We describe a new class of hierarchical containers that are formed via single-step assembly and, at a later time, self-destruct because of their packaged contents. These containers are spherical capsules formed by electrostatic complexation of the anionic biopolymer, gellan gum, with the cationic biopolymer, chitosan. The capsules are termed "motherships" and are engineered to carry a cargo of much smaller containers (e.g., nanoscale liposomes ("babyships")), within their lumen. Additionally, we package an enzyme, chitosanase, in the capsule that is capable of degrading polymeric chitosan into short oligomers. Thereby, we create motherships that self-destruct, liberating their cargo of babyships into the external solution. The time scale for self-destruction can be engineered based on the internal concentration of enzyme. The motherships are stable when stored in a freeze-dried form and can be readily dispersed into water or buffer solutions at a later time, whereupon their "internal clock" for self-destruction is initiated. The above concept could be useful for the triggered release of a variety of payloads including drugs, biological therapeutics, cosmetics, and flavor ingredients.

Enzyme-directed assembly and manipulation of organic nanomaterials

Enzymes are the prime protagonists in the chemistry of living organisms. As such, chemists and biologists have long been fascinated by the array of highly selective transformations possible under biological conditions that are facilitated by enzyme-catalyzed reactions. Moreover, enzymes are involved in replicating, repairing and transmitting information in a highly selective and organized fashion through detection and signal amplification cascades. Indeed, because of their selectivity and potential for use outside of biological systems, enzymes have found immense utility in various biochemical assays and are increasingly finding applications in the preparation of small molecules. By contrast, the use of enzymatic reactions to prepare and build supramolecular and nanoscale materials is relatively rare. In this article, we seek to highlight efforts over the past 10 years at taking advantage of enzymatic reactions to assemble and manipulate complex soft, organic materials on the nanoscale. It is tantalizing to think of these processes as mimics of natural systems where enzymes are used in the assembly and transformation of the most complex nanomaterials known, for example, virus capsid assemblies and the myriad array of nanoscale biomolecular machinery.

In situ preparation and protein delivery of silicate-alginate composite microspheres with core-shell structure

The efficient loading and sustained release of proteins from bioactive microspheres remain a significant challenge. In this study, we have developed bioactive microspheres which can be loaded with protein and then have a controlled rate of protein release into a surrounding medium. This was achieved by preparing a bioactive microsphere system with core-shell structure, combining a calcium silicate (CS) shell with an alginate (A) core by a one-step in situ method. The result was to improve the microspheres' protein adsorption and release, which yielded a highly bioactive material with potential uses in bone repair applications. The composition and the core-shell structure, as well as the formation mechanism of the obtained CS-A microspheres, were investigated by X-ray diffraction, optical microscopy, scanning electron microscopy, energy dispersive spectrometer dot and line-scanning analysis. The protein loading efficiency reached 75 per cent in CS-A microspheres with a core-shell structure by the in situ method. This is significantly higher than that of pure A or CS-A microspheres prepared by non-in situ method, which lack a core-shell structure. CS-A microspheres with a core-shell structure showed a significant decrease in the burst release of proteins, maintaining sustained release profile in phosphate-buffered saline (PBS) at both pH 7.4 and 4.3, compared with the controls. The protein release from CS-A microspheres is predominantly controlled by a Fickian diffusion mechanism. The CS-A microspheres with a core-shell structure were shown to have improved apatite-mineralization in simulated body fluids compared with the controls, most probably owing to the existence of bioactive CS shell on the surface of the microspheres. Our results indicate that the core-shell structure of CS-A microspheres play an important role in enhancing protein delivery and mineralization, which makes these composite materials promising candidates for application in bone tissue regeneration.

A review on composite liposomal technologies for specialized drug delivery

The combination of liposomes with polymeric scaffolds could revolutionize the current state of drug delivery technology. Although liposomes have been extensively studied as a promising drug delivery model for bioactive compounds, there still remain major drawbacks for widespread pharmaceutical application. Two approaches for overcoming the factors related to the suboptimal efficacy of liposomes in drug delivery have been suggested. The first entails modifying the liposome surface with functional moieties, while the second involves integration of pre-encapsulated drug-loaded liposomes within depot polymeric scaffolds. This attempts to provide ingenious solutions to the limitations of conventional liposomes such as short plasma half-lives, toxicity, stability, and poor control of drug release over prolonged periods. This review delineates the key advances in composite technologies that merge the concepts of depot polymeric scaffolds with liposome technology to overcome the limitations of conventional liposomes for pharmaceutical applications.

Bioactive inorganic-materials/alginate composite microspheres with controllable drug-delivery ability

Alginate microspheres are considered a promising material as a drug carrier in bone repair because of excellent biocompatibility, but their main disadvantage is low drug entrapment efficiency and noncontrollable release. The aim of this study was to investigate the effect of incorporating mesoporous bioglass (MBG), nonmesoporous bioglass (BG), or hydroxyapatite (HAp) into alginate microspheres on their drug-loading and release properties. X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), and atomic emission spectroscopy (AES) were used to analyze the composition, structure, and dissolution of bioactive inorganic materials and their microspheres. Dexamethasone (DEX)-loading and release ability of four microspheres were tested in phosphate buffered saline with varying pH. Results showed that the drug-loading capacity was enhanced with the incorporation of bioactive inorganic materials into alginate microspheres. The MBG/alginate microspheres had the highest drug loading ability. DEX release from alginate microspheres correlated to the dissolution of MBG, BG, and HAp in PBS, and that the pH was an efficient factor in controlling the DEX release; a high pH resulted in greater DEX release, whereas a low pH delayed DEX release. In addition, MBG/alginate, BG/alginate, and HAp/alginate microspheres had varying apatite-formation and dissolution abilities, which indicate that the composites would behave differently with respect to bioactivity. The study suggests that microspheres made of a composite of bioactive inorganic materials and alginate have a bioactivity and degradation profile which greatly improves their drug delivery capacity, thus enhancing their potential applications as bioactive filler materials for bone tissue regeneration.

Optimizing the delivery of cancer drugs that block angiogenesis

Drugs that block angiogenesis are important components of first-line therapies for a number of human cancers. However, some of these agents have undesirable effects on the patient. Optimal delivery systems must be developed to maximize clinical benefits and minimize adverse effects in cancer patients. In this Perspective, we discuss these drug-related issues and propose ways to optimize antiangiogenic therapy by the development of new drug delivery systems.

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.

Microencapsulated liposomes in controlled drug delivery: strategies to modulate drug release and eliminate the burst effect

The release of fluorescein isothiocyanate labeled bovine serum albumin (FITC-BSA) from alginate-microencapsulated liposomes was studied to evaluate the properties of this system for controlled drug delivery. Liposomes composed of phosphatidylcholine (PC) and cholesterol (Chol) (molar ratio 7:3) and of PC, phosphatidylglycerol (PG), and cholesterol (6:1:3) were encapsulated in alginate (Alg) crosslinked with Ca(2+) (Ca-Alg), Al(3+) (Al-Alg), and Ba(2+) (Ba-Alg). Capsules were coated with poly(l-ornithine) followed by a final alginate coat. A rapid initial burst of protein release was observed from liposomes encapsulated in Ca-Alg and Al-Alg. No burst was observed when liposomes were encapsulated in Ba-Alg, indicating that the crosslinking ions could significantly affect the release of entrapped protein. Also, the release from encapsulated liposomes varied significantly with liposome composition, especially with Ca-Alg as observed with encapsulation of PC, dioleoylphosphatidylcholine (DOPC), and DOPC/Chol liposomes. Cholesterol increased the leakiness of the liposomes after encapsulation. In all cases, the release from microencapsulated liposomes was much faster than that from free liposomes suggesting an interaction between the liposomes and the alginate. Differential scanning calorimetry supports the hypothesis that alginate was inserted into the lipid bilayer resulting in a rapid release of protein from microencapsulated liposomes. Moreover, it was observed that the degree of interaction between liposomes and alginate varied with liposome composition.

Biodegradable injectable in situ forming drug delivery systems

The ability to inject a drug incorporated into a polymer to a localized site and have the polymer form a semi-solid drug depot has a number of advantages. Among these advantages is ease of application and localized, prolonged drug delivery. For these reasons a large number of in situ setting polymeric delivery systems have been developed and investigated for use in delivering a wide variety of drugs. In this article we introduce the various strategies that have been used to prepare in situ setting systems, and outline their advantages and disadvantages as localized drug delivery systems.

A synthetic polymer matrix for the delayed or pulsatile release of water-soluble peptides

The design of a polymeric peptide release system with a controlled delay time and a burst-free pre-release phase is described. In general, the system consists of a blend of a tyrosine-derived polyarylate and a fast-degrading copolymer of lactic and glycolic acid (PLGA). Due to the peptide-like structure of the polyarylate backbone, peptide-polymer interactions prevented the release of peptide from neat polyarylate films. The addition of PLGA acts as a 'delayed' excipient: as PLGA degrades, it generates acidic degradation products that cause a drop in the internal pH of the polyarylate matrix. This drop in pH weakens the peptide-polymer interactions and causes the release of peptide to commence. The initial molecular weight of PLGA can be used to control the length of time before degradation occurs. Consequently, this parameter can also be used to control the duration of the delay period prior to peptide release. As a specific model system, blends of poly(DTH adipate) with three different copolymers of lactic and glycolic acid were prepared and used for the delayed release of Integrilin, a synthetic water-soluble heptapeptide (clinically used in antithrombic injections) that acts as a highly potent glycoprotein IIb/IIIa antagonist. Blends composed of a 1:1 weight ratio of poly(DTH adipate) and PLGA and containing Integrilin (15%, w/w) were prepared. In vitro release studies were conducted in phosphate buffered solution at 37 degrees C and the release of Integrilin was followed by HPLC. As the initial molecular weight of PLGA varied from 12000 to 62000, the duration of the delay period prior to release increased from 5 to 28 days.

Pulsatile drug release control using hydrogels

Current research in the field of drug delivery devices, by which pulsed and/or pulsatile release is achieved, has been intensified. In this article several types of drug delivery systems using hydrogels are discussed that showed pulsed and/or pulsatile drug delivery characteristics. As is frequently found in the living body, many vital functions are regulated by pulsed or transient release of bioactive substances at a specific site and time. Thus it is important to develop new drug delivery devices to achieve pulsed delivery of a certain amount of drugs in order to mimic the function of the living systems, while minimizing undesired side effects. Special attention has been given to the thermally responsive poly(N-isopropylacrylamide) and its derivative hydrogels. Thermal stimuli-regulated pulsed drug release is established through the design of drug delivery devices, hydrogels, and micelles. Development of modified alginate gel beads with pulsed drug delivery characteristic is also described in this article.

Triggered release of calcium from lipid vesicles: a bioinspired strategy for rapid gelation of polysaccharide and protein hydrogels

The bioinspired strategy of triggered release of Ca2+ from liposomal compartments was used to induce rapid gelation of polysaccharide and protein-based hydrogels. Thermally triggerable liposomes were designed by entrapping CaCl2 within liposomes constructed of 90% dipalmitoylphosphatidylcholine and 10% dimyristoylphosphatidylcholine. These liposomes released greater than 90% of entrapped Ca2+ when heated to 37 degrees C. A precursor fluid containing liposomes suspended in aqueous sodium alginate remained fluid for several days at room temperature but gelled rapidly when heated to 37 degrees C, as a result of Ca2+ release and formation of crosslinked Ca-alginate. Alternatively, thermally triggered Ca2+ release from liposomes was used to activate enzyme-catalyzed crosslinking of proteins to form hydrogels. A mixture of Ca-loaded liposomes, fibrinogen, and a Ca2+-dependent transglutaminase enzyme (either human recombinant FXIII or guinea pig liver transglutaminase) remained fluid indefinitely when stored at room temperature, but gelled rapidly when heated to 37 degrees C. SDS-PAGE of the reaction mixture revealed that gelation was due to enzymatic crosslinking of the alpha and gamma chains of fibrinogen, and oscillating rheometry revealed gel formation within 10 min of heating to 37 degrees C. This new approach may be useful for developing rapidly gelling injectable biomaterials that can be stored at room temperature and injected in a minimally invasive manner into a body tissue or cavity, upon which rapid solidification would occur. This versatile bioinspired strategy could be utilized for the delivery of biomaterials for tissue repair and reconstruction, and local site-directed drug delivery.

Pulsatile release from subcutaneous implants

Pulsatile delivery of antigens and hormones from subcutaneous implants could have uses in the animal production and veterinary medicine. Development of single-shot vaccines which release both initial and booster antigen from a single administration and hormonal preparations that release in a similar manner to the natural secretion patterns are two areas with potential. Formulation approaches employed to produce subcutaneous implants with pulsatile release profiles are reviewed.

Effect of Ca2+-alginate gel dissolution on release of dextran with different molecular weights

Release of macromolecular drug from calcium-alginate gel beads was investigated. Fluorescein isothiocyanate-labeled dextrans with molecular weight ranging from 9400 to 145000 were used as a model macromolecular drug. Dextran release was observed to be molecular weight-dependent, with the release pattern changing from pseudo first-order for dextran with 9400 molecular weight to sigmoidal for dextran with molecular weight of 145000. Release of a lower molecular weight dextran is mainly governed by the drug diffusion through the calcium-alginate gel matrix. With increasing dextran molecular weights, dextran release was strongly influenced by the dissolution of alginate matrix through the exchange of Ca2+ ions which act as a cross-linker. Rapid and complete release was achieved for dextran with 145000 molecular weight in a "burst" fashion showing a initial lag time. Minimal dextran release was observed in pseudo-gastric fluid at pH 1.2, while rapid dextran release within a narrow time range was achieved in simulated intestinal fluid at pH 6. 8. These results strongly suggest that calcium-alginate gel is a useful vehicle for pulsatile release of macromolecular drugs in oral drug delivery.

Cascade liposomal triggering: light-induced Ca2+ release from diplasmenylcholine liposomes triggers PLA2-catalyzed hydrolysis and contents leakage from DPPC liposomes

We have previously reported a direct triggering approach [Thompson, D. H., et al. (1996) Biochim. Biophys. Acta 1279, 25-34; Gerasimov, O. V., et al. (1997) Biochim. Biophys. Acta 1324, 200-214] based on the facile degradation of plasmenylcholine and diplasmenylcholine vinyl ether linkages by either photooxidation or low-pH environments. This report describes a novel, cascade-type triggering technique that utilizes liposome photooxidation and contents release to activate an enzyme capable of destabilizing conventional phosphatidylcholine liposomes. Our application of this concept employs a mixture of two different liposome populations, one composed of synthetic diplasmenylcholine (1, 2-dihexadec-1'-enyl-sn-glycero-3-phosphocholine, DPPlsCho) containing Ca2+ as a signaling agent for phospholipase A2 (PLA2) and the second composed of 1, 2-dihexadecanoyl-sn-glycero-3-phosphocholine (DPPC) with encapsulated calcein as the reporter molecule. Bacteriochlorophyll (BChl)-sensitized photorelease of Ca2+ from PLA2-resistant DPPlsCho liposomes activates extravesicular PLA2, thereby promoting catalyzed DPPC hydrolysis in a secondary triggering reaction, leading to calcein release. BChl/DPPlsCho/DHC/DPPE-PEG5000/Ca2+IN (0.5:85:10:5) liposomes can be phototriggered using 800 nm excitation, resulting in Ca2+ release (t50% release = 15 min) that cocatalyzes the release of calcein (t50% release = 40 min) from DPPC liposomes (1.5 mM total lipid in DPPlsCho liposomes, 0.18 mM DPPC, 210 micro M final Ca2+ concentration, 90 units of PLA2/ml, 50 mM calcein, and 36 micro M EDTA). No appreciable calcein release occurs in the absence of either PLA2 or BChl/DPPlsCho/DHC/DPPE-PEG5000/CaIN liposomes. The implications of this cascade triggering technique on drug delivery approaches are briefly discussed.

Acacia-gelatin microencapsulated liposomes: preparation, stability, and release of acetylsalicylic acid

Liposomes of dipalmitoylphosphatidylcholine (DPPC) containing acetylsalicylic acid (ASA) have been microencapsulated by acacia-gelatin using the complex coacervation technique as a potential oral drug delivery system. The encapsulation efficiency of ASA was unaltered by the microencapsulation process. The stability of the microencapsulated liposomes in sodium cholate solutions at pH 5.6 was much greater than the corresponding liposomes. The optimum composition and conditions for stability and ASA release were 3.0% acacia-gelatin and a 1- to 2-hr formaldehyde hardening time. Approximately 25% ASA was released in the first 6 hr from microencapsulated liposomes at 23 degrees C and the kinetics followed matrix-controlled release (Q varies; is directly proportional to t1/2). At 37 degrees C, this increased to 75% released in 30 min followed by a slow constant release, likely due to lowering of the phase transition temperature of DPPC by the acacia-gelatin to near 37 degrees C. At both temperatures, the release from control liposomes was even more rapid. Hardening times of 4 hr and an acacia-gelatin concentration of 5% resulted in a lower stability of liposomes and a faster release of ASA. It is concluded that under appropriate conditions the microencapsulation of liposomes by acacia-gelatin may increase their potential as an oral drug delivery system.

Calcium alginate beads as a slow-release system for delivering angiogenic molecules in vivo and in vitro

A method previously used in this laboratory for entrapment of tumor cells in alginate beads has been extended to provide a slow release delivery system for growth factors with known in vivo angiogenic activity. Protein growth factors were entrapped in alginate beads in amounts sufficient to cause incorporation of 3H-thymidine by COMMA-D cells in vitro, and in vivo neovascularization when injected subcutaneously into Balb/c mice. Entrapment of 125I-labelled growth factors showed that the amount of molecule entrapped in alginate beads may vary with the charge of the molecule. In vitro cell proliferation studies showed that entrapment in alginate beads may provide a slow-release system or a stabilizing environment for the protein. In some cases biological activity of the growth factor in solution was increased by the presence of control alginate beads. When alginate-entrapped growth factors were injected into Balb/c mice, induction of new blood vessels could be monitored qualitatively by macroscopic photography and assessed quantitatively by measuring the pooling of radiolabelled red blood cells at the experimental site. Subcutaneous injection of purified angiogenic factors not entrapped in alginate beads did not cause neovascularization. Diffusion of 125I-labelled growth factors from alginate beads in the animal showed that release in vivo may depend on the charge of the protein molecule. These results indicate that injection of purified molecules entrapped in alginate beads provides an effective localized and slow-release delivery of biologically active molecules. This delivery system may extend the time of effectiveness of biologically active molecules in vivo compared to direct injection without alginate entrapment. The method of entrapment and injection has potential for identifying active factors in tumor-induced angiogenesis and testing new compounds as modulators of neovascularization.

The pharmacokinetics of, and humoral responses to, antigen delivered by microencapsulated liposomes

The feasibility of creating a s.c. depot for sustained protein delivery with the goal of enhancing antigen immunogenicity was investigated. The depot was designed as antigen-laden liposomes of hydrogenated egg phosphatidylcholine and cholesterol (1:1 molar ratio) encapsulated in alginate-poly(L-lysine) microcapsules and evaluated using iodinated bovine serum albumin (BSA) as a model antigen. The in vivo release behavior of the liposomes and microencapsulated liposomes (MELs) was evaluated from the BSA serum concentration profiles after s.c. injection into rats and the pharmacokinetic parameters of 125I-labeled BSA appearance after s.c. or i.v. injections of BSA in saline. Maximal BSA concentrations were detected 11 h after s.c. injection in all rats. The BSA serum concentrations decreased rapidly in rats injected with BSA in saline or Freund's adjuvant and less rapidly in rats injected with BSA in liposomes or MELs. Four to 5 weeks after injection, BSA-associated radioactivity was detected only in sera of rats injected with BSA in liposomes or MELs. Fifty days after injection, 50% of the originally injected BSA was recovered form the s.c. sites of rats injected with BSA in MELs; no radioactivity was recovered from the other three groups of rats. The antigen-reactive antibody levels induced in rats immunized with BSA in MELs were 2- to 3-fold higher than those obtained in rats immunized with BSA in liposomes, saline, or Freund's adjuvant. More significantly, high antibody levels were maintained for more than 150 days after a single injection of BSA in MELs, suggesting that MELs can serve as a long-term single-dose immunization vehicle.

Lipid-alginate interactions render changes in phospholipid bilayer permeability

Lipid vesicles, e.g. liposomes, generally release their contents in a continuous manner. However, when these vesicles are entrapped in Ca-alginate and coated with poly(L-lysine), they release their contents in an unusual fashion, in 'bursts'. Molecular-level studies indicated that lipid-alginate interactions are responsible for changes in the barrier properties of lipid vesicles. Differential scanning calorimetry revealed that exposure of liposomes to alginate resulted in a 4-fold reduction in the phase transition enthalpy, with no change in the melting temperature. Size-exclusion chromatography of liposomes-in-alginate gave an additional liposomal peak with a smaller elution volume. These studies suggested that alginate is inserted into the lipid bilayer of vesicles. Lipid-alginate interactions were highly dependent on phospholipid head group charge and the phase transition temperature of the phospholipid. Based on these interactions, a mechanism to explain the 'burst' from these entrapped liposomes is suggested.

New methods of drug delivery

Conventional forms of drug administration generally rely on pills, eye drops, ointments, and intravenous solutions. Recently, a number of novel drug delivery approaches have been developed. These approaches include drug modification by chemical means, drug entrapment in small vesicles that are injected into the bloodstream, and drug entrapment within pumps or polymeric materials that are placed in desired bodily compartments (for example, the eye or beneath the skin). These techniques have already led to delivery systems that improve human health, and continued research may revolutionize the way many drugs are delivered.