Elastin-like polypeptides: Therapeutic applications for an emerging class of nanomedicines

Abstract

Elastin-like polypeptides (ELPs) constitute a genetically engineered class of 'protein polymers' derived from human tropoelastin. They exhibit a reversible phase separation whereby samples remain soluble below a transition temperature (T_t) but form amorphous coacervates above T_t. Their phase behavior has many possible applications in purification, sensing, activation, and nanoassembly. As humanized polypeptides, they are non-immunogenic, substrates for proteolytic biodegradation, and can be decorated with pharmacologically active peptides, proteins, and small molecules. Recombinant synthesis additionally allows precise control over ELP architecture and molecular weight, resulting in protein polymers with uniform physicochemical properties suited to the design of multifunctional biologics. As such, ELPs have been employed for various uses including as anti-cancer agents, ocular drug delivery vehicles, and protein trafficking modulators. This review aims to offer the reader a catalogue of ELPs, their various applications, and potential for commercialization across a broad spectrum of fields.

Keywords: Biomedical engineering; Drug delivery; Elastin-like polypeptides; Nanomedicine.

Figure 1. Depiction of reversible phase separation by Elastin-like polypeptides (ELPs)

A. ELPs are soluble below a transition temperature, T_t, but undergo coacervation at temperatures above T_t. B. The linear relationship between T_t and concentration can be studied by measuring optical density as a function of temperature. Three different ELPs of varying length and hydrophobicity phase separate above the indicated lines.

Figure 2. Uptake and degradation of ELP nanoparticle

Rhodamine-conjugated S48I48 (red) was incubated with transformed murine hepatocytes for 0, 2, 4, 24 h at 37°C and imaged using live-cell confocal fluorescence microscopy. Cells were counterstained with lysotracker green to show low pH compartments associated with lysosomal protease activity. Differential interference contrast (DIC) imaging illustrated cell morphology. ELP nanoparticles were visible within and on the cell surface after 2 h; intracellular staining significantly decreased after 24-h incubation. The arrow indicates internalized nanoparticles with the lysosomes. Scale bar: 10 μm. Reprinted from [92] with permission of John Wiley and Sons.

Figure 3

The various applications of ELPs as nanomedicines

Figure 4. Design of Elastin-like polypeptide (ELP) nanoparticles that carry an anti-proliferative drug at both their core and corona

(A) High-avidity interaction between a small molecule drug (rapamycin) and its cognate target protein (FKBP) decorated at surface of an ELP nanoparticle. The nanoparticles assemble nanoparticles above a critical micelle temperature (CMT). (B) Dynamic light scattering of FKBP-decorated FSI and plain SI nanoparticles shows that protein modification minimally affects CMT or hydrodynamic radius. (C) Tumor growth inhibition by FSI-rapamycin versus free rapamycin (0.75 mg/kg BW). Free rapamycin mice were sacrificed at Day 24 due to toxicity. Reprinted from [66] with permission of Elsevier.

Figure 5. Lacritin-ELP nanoparticles heal corneal wounds in mice

A 2 mm defect in the corneal epithelium of female non-obese diabetic (NOD) mice was monitored using fluorescein staining at 0, 12 and 24 h with or without treatment by LSI, SI, and a positive control EGF + BPE. (A) Representative images showing the time-lapse healing of the corneal wound. (B) LSI at both 12 and 24 h significantly (***p = 0.001, n = 4) decreased the percentage of initial wound area (PctArea) compared to SI, EGF + BPE, and no treatment groups. (C) After 24 h, corneas were fixed, sectioned across the defect, and stained by hematoxylin and eosin. The corneal epithelium of the LSI treatment group revealed normal pathology. Although reduced fluorescein staining was observed at late times in the SI group, the epithelium did not recover fully, as evidenced by its irregular surface (black arrows). Reprinted from [124] with permission of The Royal Society of Chemistry.

Figure 6

NIR tomography images of 175 nmol/kg (GLP-1)-ELPSol, 175 nmol/kg (GLP-1)-ELPDepot, and 700 nmol/kg (GLP-1)-ELPDepot at 0, 24, 72 and 120 hr after subcutaneous injection. Reprinted from [138] with permission of Elsevier.

Figure 7. Serum half-life of ^NtTNF-V_HH_ELP and neutralization of LPS/D-Gal toxicity by ^NtTNF-V_HH_ELP

(A) Three mice were intravenously injected with 100 μg plant-derived ^NtTNF-V_HH_ELP and for control with 100 μg ^EcTNF-V_HH. Serum samples were prepared over a time range post-injection. (B) LPS/D-gal-induced septic shock was blocked by ^NtTNF-V_HH_ELP and ^EcTNF-V_HH. Survival was monitored 24-h post-injection. Reprinted from [152] with permission of John Wiley and Sons.