Temperature-induced unloading of liposomes bound to microgels

Highlights

• Anionic liposomes preserve their integrity after binding to cationic microgels.

• Thermo-induced collapse of microgel core destroys the outer liposome layer.

• Liposome disruption is accompanied by a fast release of encapsulated drug.

• Sharp rise in the released drug was observed in between 39 and 41 °C.

• Rigid latex particles bound to microgels form incompressible outer shell.

Abstract

Bilayer lipid vesicles (liposomes) composed of zwitter-ionic (electroneutral) egg yolk lecithin and anionic phosphatidylserine were loaded with a water-soluble antitumor antibiotic Dox and electrostatically adsorbed onto the surface of thermo-sensitive microgels synthesized from 3-(N,N-dimethylamino)propylmethacrylamide, N,N-methylenebisacrylamide and 2,2′-azobis(2-methylpropionamidine) dihydrochloride. The resulting saturated microgel-liposomes complex with ultimately adsorbed 40 liposomes retains encapsulated drug in water solution at 25 °C. An increase in temperature induces a collapse of the microgel core and squeezing of the liposome shell, followed by liposome disruption and a release of encapsulated Dox. An intensive Dox leakage begins when temperature reaches 39 °C; a maximum amount of Dox is released within 5 min after starting the temperature trigger. The results are of importance for constricting of thermo-sensitive drug carries and developing protocols for the most productive drug release.

Introduction

Spherical bilayer lipid vesicles (liposomes) are widely used as containers for encapsulation, delivery and release of drugs [1], [2], [3], [4], [5], [6], [7], [8], [9]. A typical size of liposomes, 50–100 nm in diameter, significantly restricts their capacity to water-soluble drugs and complicates the joint encapsulation of different drugs and their long-term storage [5], [6], [7]. With the purpose of increasing a useful volume of the liposomal container, liposomes have been attached to the surface of micro-sized polymer particles [10], [11], [12], [13], [14], [15], or incorporated into polymeric microgels [16], [17]. The former “open” approach seems to be more attractive: here liposomes with encapsulated individual drugs are mixed at a desirable ratio and a resulting mixture is landed onto the surface of suitable polymer particles, whereas the latter “closed” approach includes new synthetic, isolation and purification procedures every time when multi-liposomal containers are fabricated. A simple electrostatic binding of liposome mixture to the polymer particle surface is also a step towards the personalized medicine when a combined therapeutic agent is designed for a specific patient [18].

Stimuli-responsive microhydrogels are a different class of materials that are relevant for biomedical applications [19], [20], [21], [22], [23]. It has been demonstrated that thermoresponsive microgels based on N-isopropylamide (NIPAM) provide interesting properties with respect to drug delivery applications [24], [25], [26], [27]. Recently, an electrostatic adsorption of anionic liposomes onto the surface of cationic thermo-sensitive microgels (μGs) and a liposome-microgel complex formation have been described [15], [28], [29]. Prior to this, the microgels proved to be effective sorbents for various compounds, including multi-charged objects like linear polyelectrolytes [30]. An increase in temperature stimulated a release of doxorubicin (Dox), a water-soluble antitumor drug, from liposomes to a surrounding solution, while the kinetics of drug leakage was not monitored [28]. Later, the drug release was controlled but at a constant temperature which was randomly taken within an interval of thermo-induced μG collapse [29]. The results favored understanding the driving force and mechanism of the thermo-mediated drug release, although the details are still unclear. At the same time, drug delivery systems that respond to an increase in temperature are of great interest for the treatment of various diseases [31]. The multi-liposomal constructs offer new possibilities for manufacturing capacious and controlled dosage forms.

In this article, we quantify the electrostatic complexation of anionic liposomes with cationic μG particles in aqueous solution, the stability of resulting complexes in the presence of simple salts (in water-salt solutions), the thermo-induced collapse of the μG-liposome complexes, and the kinetics of the thermo-mediated drug release. The special attention is paid to the correlation between the temperature range responsible for the microgel collapse and drug release, on the one hand, and the mechanism of the temperature-activated leakage of a water-soluble drug from liposomes to surrounding solution, on the other. Together with anionic liposomes, we use model objects, anionic polymeric (latex) nanoparticles whose size is comparable with the size of liposomes. Incompressible and indestructible latex particles allow the visualization of their complexation with the microgel and the composition of the resulting complexes. The obtained results are important for understanding the prospects for the practical use of the multi-liposomal containers.