Silk-fibroin Grafted with Chitosan is a Promising Polymer-based Nanoparticle
for Drug Delivery
Drug delivery systems are crucial to promote the efficacy of pharmaceutical compounds, as these carriers contain the active drug. The drug can be dissolved, dispersed, encapsulated, absorbed, or attached to the carrier. Therefore, the material used in the composition of the carrier is carefully engineered to elicit the desired drug profile and release rate of the drug. These drug-controlled release systems can be categorized into different classes: nanoparticles, microspheres, microcapsules, pills, emulsions, etc. Their classification can then be further sub-categorized based on composition of the drug delivery system.
Recently, the emerging field of nanoscience has propelled bioengineering discoveries, such as, the foundational contributions of Birrenbach and Speiser in the 1970s who developed the first nanoparticles as a drug delivery device. Birrenbach and Speiser noted that due to the “size effect” of these particles, between 1-1000 nm, unique characteristics such as protection from degradation, increased absorption, enhanced stability and bioavailability, longer retention, and better intracellular penetration could be observed when drugs were encapsulated inside nanoparticles. Depending on the composition and surface properties of the particle, different drug release profiles can be achieved.
On the drug market over the last decades are a vast variety of FDA-approved nanoparticles used in subcutaneous, intra-nasal, intravenous, delivery to treat cancers, ______________. The different flavors of nanoparticles can be engineered from lipids, ceramics, carbon, metal, polymers, and dendrimers. Lipid-based nanoparticles have been well-characterized due to their ease of preparation. Most commonly, crude liposomes can be prepared through thin-film hydration or size extrusion, and further sonicated to yield smaller unilamellar vesicles. The unique ability of lipogenic systems to self-assemble and entrap lipophilic compounds in the bilayer membrane and hydrophilic compounds in the aqueous center makes lipogenic nanoparticles a promising approach to encapsulate large charged molecules. Modifications to these liposomes such as____ were explored to increase stability__. Techniques such as ___________ were used to improve ___. An example of a lipid-based nanoparticle is ________ that has been modified to ————–. However, the high cytotoxicity, poor encapsulation efficiency and instability limit liposomes for the delivery of certain drugs.
Consequently, there is a growing interest in developing polymer-based nanoparticles as a better alternative to traditional plain liposomes. A few advantages of using specifically biodegradable polymers in nanoparticles include excellent biocompatibility, better encapsulation and controlled drug release properties. Protein-based carriers present as an attractive nano-delivery system due to their biodegradable and non-antigenic properties. Functional groups of the protein polymer can also be exploited in the design of a nanoparticle to trigger a desired biological signaling pathway in cells or allow for covalent conjugation of ligands/drug to the surface proteins of the nanocarrier. Natural polymers such as cellulose, chitosan, hyaluronic acid, alginate, dextran, collagen, gelatin, elastin, soy, keratin, corn zein and silk fibroin have been used in creating drug delivery matrixes, hydrogels, as well as nanoparticles. New emerging technologies for engineering polymer-based particles allows for the exploration of different nanoparticle compositions, including combining and grafting different polymers to enhance therapeutic efficiency. This review paper will focus on a combinatorial protein-polymer based nanoparticle design which uses silk-fibroin and chitosan as a carrier for drug delivery. The properties of the polymers, preparation techniques, and their applications as carriers for drug delivery are discussed.
II. Silk Fibroin + Chitosan
The silk-fibroin and chitosan polymers have been individually demonstrated as successful drug delivery systems. Silk fibroin (SF) is approved as a biomaterial by the US Food and Drug Administration (FDA) because the biomacromolecule elicits almost no immunological response. The natural protein polymer can be isolated from a variety of species including silkworms like the Bombyx mori or the spider Nephila clavipes. Due to the restrictions on commercial production of spider silks, silkworms are the current predominant supply chain for silks. The silk-fibroin collected from domesticated silkworm Bombyx mori generally is a heterodimeric protein with a heavy and light chain connected by a single di-sulfide bond between cys-172 and c-20 of the light and heavy chain, respectively. SF is comprised of large repetitive modular hydrophobic domains with inter-dispersed smaller hydrophilic regions. These hydrophobic regions on the heavy chain (~325kDa) of SF are comprised of Gly-X repeats, where X can be Alanine, Serine, Threonine and Valine amino acid residues. Functionally, these repeats provide stability and contribute to the lusty mechanical properties of silk fiber through formation of anti-parallel ?-sheets and crystalline (block) domains. In contrast, the light chain (~25kDa) with polar, bulky side chains (in particular tyrosine, valine and acidic amino acids) is hydrophilic in nature and contributes to the elasticity of silk through the formation of amphorous (line) domains. Together, the amphorous and block parts make up the secondary structure of silk.
USES as NANOPARTICLE
Chitosan is a cationic polysaccharide derived from chitin, a biodegradable and biocompatible polymer that can isolated from the exoskeleton of crustaceans such as prawns or crabs, or from the cell walls of some fungi. Chitosan (CS) is produced by removing the acetate moiety from chitin. It is approved by the FDA for tissue engineering and drug delivery and is composed of N-acetyl-D-glucosamine and D-glucosamine residues. The pKa of the glucosamine monomer in chitosan is ?6.5 . The cationic nature of chitosan in acidic environments, is a valuable asset that can be manipulated for pH-responsive controlled release functionality. For example, in the gastrointestinal (GI) tract (highly acidic in the stomach, to about pH 6 in the duodenum 50) or in the intracellular endosomes (pH 5–6 51), and has been successfully used in hydrogels.
The CS nanoparticles can resist gastric chemical and enzymatic degradation due to the
a. SFCS combination nanoparticle
t Chitin, however, delivers drugs too rapidly. It is more ideal to have a controlled release, which can be achieved through grafting of polymers; in this case, silk. The grafting will create a more stable nanoparticle, creating a controlled release, which is ideal. The optimized chitin-silk nanoparticle creates a delivery system that can be used not only for current drugs on the market, such as antifibrotics, but others as well. Biopolymer nanoparticles are shown to be a more stable drug delivery system when compared to classical lipid nanoparticles
Chemical grafting – increase stability with block species connected as a sidechain to the main chain of polymeric chitosan np.
b. Preparation methods of the polymers
Natural silk fibroin is a linear, water-insoluble polymer protein that must be dissolved prior to processing it into films, gels, scaffold matrixes, or nanoparticles. Dissolution of silk-fibroin can be performed by a
Chitosan is produced through the N-deacetylation of purified chitin. Raw unpurified chitin must be removed of proteins and minerals that are naturally associated with the polysaccharide. Different processes of acidification and alkalization to purify chitin can affect the molecular weight and pKa of the resultant chitosan.
c. Conjugation of bioactive groups on SFCS nanoparticle