Degradation of Phospholipid Vesicles By Phospholipases

Liposomes are drug delivery carries whose structure often need to be specifically tailored for each compound and target condition to maximize efficacy. In other words, drug delivery vesicles should remain stable to provide long-term blood circulation and release drug payloads immediately at the targeting sites. As one of the most important mechanisms, phospholipase catalyzed lipid degradation might be utilized to quickly hydrolyze liposomes. Activities of the phospholipases highly depend on physicochemical properties of the liposomes, which could have a wide spectrum of activity due to the plethora of possible phospholipid combinations. Therefore, empirical trials are not efficient and judicious understanding of the non-linear degradation kinetics is necessary. We have combined a few state-of-the-art techniques to investigate enzyme adsorption and catalyzed lipid degradation, which will provide fundamental details of liposome structure design and optimization.

Degradation kinetics of liposomes were measured in situ using dynamic light scattering with temporal resolution of one second. Morphologies of the giant unilamellar vesicles (GUVs) during degradation were observed via fluorescence microscopy. Molecular packing of lipid monolayers at the aqueous-air interface before and after enzyme adsorption and degradation were quantified using X-ray reflectivity and grazing incidence diffraction (GID).

 Threshold concentrations of phospholipases were identified: above the threshold concentration, liposomes are quickly degraded in a few seconds to minutes; below the threshold concentration, liposomes remain stable for a long period. Therefore, it is possible to optimize vesicle design to release therapeutic payloads at phospholipase (such as sPLA₂) concentrations around 1 mg/ml, corresponding to the enzyme concentration near cancer, inflammation, or bleeding sites.