Slow release of molecules in self-assembling peptide nanofiber scaffold
Introduction
Self-assembling peptide nanofiber based hydrogels can be used in a broad range of biomedical and biotechnological applications ranging from 3D scaffolds for tissue engineering to drug delivery vehicles [1], [2], [3]. Short peptides (8–16 residues or 2.5–5 nm in length) are chemically synthesized and form β-sheet structures in water [4], [5], [6], [7]. Depending on the pH and the ionic strength of the medium these peptides self-assemble into nanofibers, which in turn form a hydrogel. These hydrogel systems are well characterized and have already been employed in a variety of 3D tissue cell cultures and tissue engineering research applications [8], [9], [10], [11], [12], [13], [14].
A significant increase in therapeutic efficacy can be realized through incorporating controlled release strategies into the design of drug delivery systems [15], [16], [17], [18], [19]. Developing a drug release system that is not only efficient, biocompatible, robust, but also useful for diverse applications requires a material that can deliver active compounds, at specific rates, throughout the entire therapy regimen. Thus, controlling the release rates of small molecules and peptides/proteins through various hydrogels is crucial. Self-assembling peptide hydrogels are an important class of hydrogels, which are potentially good candidates for providing a robust drug delivery system. When compared to chemically synthesized polymer materials, self-assembling peptide nanofiber hydrogels are generally more biocompatible [1], [2], able to respond to external stimuli under various physiological conditions and maintain a high water content (i.e., ca. 99.5% w/v): the latter may allow for the diffusion of a wide range of molecules. Furthermore, self-assembling peptide nanofiber hydrogels are amenable to molecular design and can be tailored for the specific needs of the application.
Herein, we investigated the effect of the model drug properties (charge and structure) on their release kinetics through self-assembling peptide hydrogels (Fig. 1). Phenol red, bromophenol blue, 8-hydroxypyrene-1,3,6-trisulfonic acid trisodium salt (3-PSA), 1,3,6,8-pyrenetetrasulfonic acid tetrasodium salt (4-PSA), and Coomassie Brilliant Blue G-250 (CBBG) were chosen as model drugs for several reasons: (1) they are well characterized dyes that have been used to investigate drug interactions with the liver (phenol red and bromophenol blue) [20], [21] and as anionic probes for sensor applications (3-PSA and 4-PSA) [22], [23]; (2) these molecules should allow for the systematic investigation of charge effects on drug diffusion through peptide hydrogels due to both their physical properties and the specific amounts of sulfonic acid and amine groups present (Fig. 2). The release kinetics of compounds dispersed throughout a hydrogel are predominantly controlled by diffusion [24]. The diffusion coefficients of these model compounds through our hydrogel will be calculated using standard methodologies [25], [26], [27]. Furthermore, by correlating the model drug properties to the resulting diffusion coefficients it is possible to discuss the release mechanism.
Section snippets
Materials
The self-assembling peptide RADA16 (Ac-RADARADARADARADA-CONH2, PuraMatrix™) 3% (w/v in PBS, pH = 3) was obtained from 3DM Inc. (Cambridge, MA, USA). Phenol red (MW = 354.4), bromophenol blue sodium salt (MW = 691.9), 8-hydroxypyrene-1,3,6-trisulfonic acid trisodium salt (3-PSA, pyranine, MW = 524.4), 1,3,6,8-pyrenetetrasulfonic acid tetrasodium salt (4-PSA, MW = 610.4) and Coomassie Brilliant Blue G-250 (CBBG, MW = 854.0) were purchased from Sigma-Aldrich and dissolved in Milli-Q water to prepare 0.8 and
Theory
Release experiments, utilizing thin hydrogel films containing molecularly dispersed ‘drug’, provide a route for calculating the hydrogel-specific apparent diffusion coefficients. Assuming an adequate diffusion sink, with a significantly larger volume and a significantly greater drug diffusion coefficient than that of the hydrogel, we can ignore the transport within the sink when calculating the overall release rate of the drug from the hydrogel (Fig. 3). Given these conditions, the 1D
Result and discussion
In order to interpret the results obtained for the diffusion of these dyes through the self-assembling peptide nanofiber hydrogel, it is imperative that the physicochemical characteristics of the peptide hydrogel and the structural, and the chemical, properties of the dye molecules are taken into consideration. RADA16 nanofibers have a hydrophilic surface composed of alternating arginine (positive charge) and aspartic acid (negative charge) residues [1], [34] that intertwine to form a hydrogel (
Conclusions
As controlled drug release enters into more and more applications in medicine there is a growing interest in developing a system that is amenable to molecular design and can be tailor-made for the specific drugs. We studied the possibility of using a hydrogel consisting of self-assembling peptides as a carrier for controlled drug release. The diffusing molecules were chosen so as to model the diffusion of drug compounds that have similar sizes and molecular structures, but varying charge
Acknowledgement
We thank Dr. Jiyong Park and Dr. Wonmuk Hwang for kindly sharing with us their unpublished results and stimulating discussions on the molecular modeling of the RADA16 peptide and to Aki Nagai for helpful discussions on the experimental details. Y.N. gratefully acknowledges the generous support from Menicon Ltd. Japan and S.K. was supported by the HighQ Foundation.
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