Emerging Applications of Polymersomes in Delivery: from Molecular Dynamics to Shrinkage of Tumors

Abstract

Polymersomes are self-assembled shells of amphiphilic block copolymers that are currently being developed by many groups for fundamental insights into the nature of self-assembled states as well as for a variety of potential applications. While recent reviews have highlighted distinctive properties - particularly stability - that are strongly influenced by both copolymer type and polymer molecular weight, here we first review some of the more recent developments in computational molecular dynamics (MD) schemes that lend insight into assembly. We then review polymersome loading, in vivo stealthiness, degradation-based disassembly for controlled release, and even tumor-shrinkage in vivo. Comparisons of polymersomes with viral capsids are shown to encompass and inspire many aspects of current designs.

Keywords: amphiphile; block copolymers; controlled release; liposomes; nanoparticles.

Figure 1

Polymersome assembly and disassembly by computer simulation and in experiment. (A) Atomistic representation of lipid (left) and amphiphilic diblock copolymer (right) and the potential-matching scheme used to derive an accurate coarse-grain model. (B) Membrane assembly of the coarse grain polymer within a periodic box of water. (C) Polymer nano-vesicle assembly simulated by dissipative particle dynamics using a simplified coarse grain polymer that allows larger scale simulations, including (D) disassembly and release of encapsulants. (E) Molecular weight series of diblock copolymer polymersomes imaged by cryo-TEM, which allows visualization of the hydrophobic core of PBD. The thickness of the hydrated and invisible PEO brush, d_brush, is estimated by measuring the closest distance between adjacent vesicles and dividing by two. (F) Fluorescently labeled 100 nm nano-vesicles after dilution can be visualized as small dots by fluorescence microscopy. Dynamic Light Scattering provides an accurate measurement of vesicle size and, after periodic measurements, shows that PEO-PBD polymersomes are stable whereas vesicles composed with the degradable copolymer PEO-PLA diassemble over a period of days at room temperature.

Figure 2

Stable insertion of a mimetic protein pore into either thin or thick block copolymer membranes, using coarse grain simulation methods. In the thicker membrane, polymer chains are sufficiently flexible to deform and accomodate the pore.

Figure 3

Polymersome interactions in the circulation. (A) Electrophoretic analysis of plasma proteins adsorbed to polymersomes incubated in blood plasma shows that select proteins interact with polymersomes and are enriched after 18 h, despite a hydrated PEO (or PEG) brush. Albumin is not detectable, but a protein around 40 kD is detected after separation of the polymersomes by centrifugation followed by vesicle solubilization with detergent. (B) Plasma-incubated polymersomes, P, will adhere to blood neutrophils, N, that will eventually engulf or ‘phagocytose’ such vesicles, leading to complete internalization. Adjacent red cells, R, do not adhere or interact with N or P. (C) Polymersomes have a 100% PEG brush, which can far exceed the PEG covering of PEGylated-liposomes, but the molecular weight of the PEG can be similar. Injections of 100-nm polymersomes into rats thus allow the circulation half-life to be determined as a function of %PEGylation. This kinetic measure accumulated from data in the literature fits to a simple binding isotherm, suggestive of inhibition with an inhibition constant K₂ as calculated. (D) For polymersomes with 100% PEGylations, the circulation time in rats increases weakly with the number-average molecular weight of PEG. The scaling exponent is close to that estimated for the PEG brush thickness.

Figure 4

Dual drug-loaded degradable polymersomes shrink tumors. (A) Degradable polymersomes composed with PEG-PLA possess a membrane core that can be loaded with hydrophobic paclitaxel (TAX) and, within the interior, soluble doxorubicin (DOX) as a second anti-cancer drug. The latter is shown in cryo-TEM to precipitate or crystallize. The simulation snapshot at left illustrates the thickness of the hydrophobic core, which is about 3 times thicker than the core of a lipid vesicle and so polymersome membranes can carry more TAX. (B) Injection of these dual drug nano-carriers into mice bearing tumors of human-derived breast cancer cells shows rapid shrinkage of the tumor. The inset plot shows that empty polymersomes have no effect on tumor size when compared to saline injections; in addition, injection of free drugs only slows tumor growth initially. (C) Imaging of tumor sections after 1 day allows DOX, which is fluorescent, to be visualized within the tumors. Delivery of free drug invariably shows diffuse staining of the tumors, whereas delivery of P'some-drugs leads to punctate images that suggest localization of the carriers to the tumors. Consistent with localization and tumor shrinkage, the drug-laden polymersomes induce massive cell death based on positive staining for apoptosis (TUNEL method).