Jesper Davidsen

Secreted phospholipase A2 as a new enzymatic trigger mechanism for localised liposomal drug release and absorption in diseased tissue

Polymer-coated liposomes can act as versatile drug-delivery systems due to long vascular circulation time and passive targeting by leaky blood vessels in diseased tissue. We present an experimental model system illustrating a new principle for improved and programmable drug-delivery, which takes advantage of an elevated activity of secretory phospholipase A(2) (PLA(2)) at the diseased target tissue. The secretory PLA(2) hydrolyses a lipid-based proenhancer in the carrier liposome, producing lyso-phospholipids and free fatty acids, which are shown in a synergistic way to lead to enhanced liposome destabilization and drug release at the same time as the permeability of the target membrane is enhanced. Moreover, the proposed system can be made thermosensitive and offers a rational way for developing smart liposome-based drug delivery systems. This can be achieved by incorporating specific lipid-based proenhancers or prodestabilisers into the liposome carrier, which automatically becomes activated by PLA(2) only at the diseased target sites, such as inflamed or cancerous tissue.

Biophysical mechanisms of phospholipase A2 activation and their use in liposome-based drug delivery

Secretory phospholipase A2 (PLA2) is a ubiquitous water‐soluble enzyme found in venom, pancreatic, and cancerous fluid. It is also known to play a role in membrane remodeling processes as well as in cellular signaling cascades. PLA2 is interfacially active and functions mainly on organized types of substrate, e.g. micelles and lipid bilayers. Hence the activity of the enzyme is modulated by the lateral organization and the physical properties of the substrate, in particular the structure in the nanometer range. The evidence for nano‐scale structure and lipid domains in bilayers is briefly reviewed. Results obtained from a variety of experimental and theoretical studies of PLA2 activity on lipid‐bilayer substrates are then presented which provide insight into the biophysical mechanisms of PLA2 activation on lipid bilayers and liposomes of different composition. The insight into these mechanisms has been used to propose a novel principle for liposomal drug targeting, release, and absorption triggered by secretory PLA2.

Synergistic permeability enhancing effect of lysophospholipids and fatty acids on lipid membranes

The permeability-enhancing effects of the two surfactants, 1-palmitoyl-2-lyso-sn-gycero-3-phosphocholine (lysoPPC) and palmitic acid (PA), on lipid membranes that at physiological temperatures are in the gel, fluid, and liquid-ordered phases were determined using the concentration-dependent self-quenching properties of the hydrophilic marker, calcein. Adding lysoPPC to lipid membranes in the gel-phase induced a time-dependent calcein release curve that can be described by the sum of two exponentials, whereas PA induces a considerably more complex release curve. However, when lysoPPC and PA were added simultaneously in equimolar concentrations, a dramatic synergistic permeability-enhancing effect was observed. In contrast, when both lysoPPC and PA are added to liposomal membranes that are in the fluid or liquid-ordered phases, no effect on the transmembrane permeation of calcein was observed.

Drug delivery by phospholipase A2 degradable liposomes

The effect of poly(ethylene glycol)-phospholipid (PE-PEG) lipopolymers on phospholipase A(2) (PLA(2)) hydrolysis of liposomes composed of stearoyl-oleoylphosphatidylcholine (SOPC) was investigated. The PLA(2) lag-time, which is inversely related to the enzymatic activity, was determined by fluorescence, and the zeta-potentials of the liposomes were measured as a function of PE-PEG lipopolymer concentration. A significant decrease in the lag-time, and hence an increase in enzymatic activity, was observed with increasing amounts of the negatively charged PE-PEG lipopolymers incorporated into the SOPC liposomes. The enhancement of the PLA(2) enzymatic activity might involve a stronger PLA(2) binding affinity towards the negatively charged and polymer covered PEG liposomes.

Lipid-based drug delivery systems

The principle objective of formulation of lipid-based drugs is to enhance their bioavailability. The use of lipids in drug delivery is no more a new trend now but is still the promising concept. Lipid-based drug delivery systems (LBDDS) are one of the emerging technologies designed to address challenges like the solubility and bioavailability of poorly water-soluble drugs. Lipid-based formulations can be tailored to meet a wide range of product requirements dictated by disease indication, route of administration, cost consideration, product stability, toxicity, and efficacy. These formulations are also a commercially viable strategy to formulate pharmaceuticals, for topical, oral, pulmonary, or parenteral delivery. In addition, lipid-based formulations have been shown to reduce the toxicity of various drugs by changing the biodistribution of the drug away from sensitive organs. However, the number of applications for lipid-based formulations has expanded as the nature and type of active drugs under investigation have become more varied. This paper mainly focuses on novel lipid-based formulations, namely, emulsions, vesicular systems, and lipid particulate systems and their subcategories as well as on their prominent applications in pharmaceutical drug delivery.

Enzymatic degradation of polymer covered SOPC-liposomes in relation to drug delivery

Polyethylenoxide (PEG) covered liposomes are used as lipid-based drug-delivery systems. In comparison to conventional liposomes the polymer-covered liposomes display a long circulation half-life in the blood stream. We investigate the influence of polyethyleneoxide-distearoylphosphatidylethanolamine (DSPE-PEG750) lipopolymer concentration on phospholipase A2 (PLA2) catalyzed hydrolysis of liposomes composed of stearoyloleoylphosphatidylcholine (SOPC). The characteristic PLA2 lag-time was determined by fluorescence and the degree of lipid hydrolysis was followed by HPLC analysis. Particle size and zeta-potential were measured as a function of DSPE-PEG750 lipopolymer concentration. A significant decrease in the lag-time, and hence an increase in enzyme activity, was observed with increasing concentrations of the anionic DSPE-PEG750 lipopolymer lipids. The observed decrease in lag-time might be related to changes in the surface potential and the PLA2 lipid membrane affinity.