Document Type

Theses, Ph.D

Rights

This item is available under a Creative Commons License for non-commercial use only

Publication Details

A thesis submitted to the Dublin Institute of Technology for the Award of Doctor of Philosophy (PhD), 2018.

Abstract

Selenium (Se) is an essential micronutrient in both human and animal nutrition that exists in a wide array of different forms, both organic and inorganic. Selenoamino acids (SeAAs), such as selenocystine (SeCys2), selenomethionine (SeMet), and methylselenocysteine (MSC) are organic species with reported health benefits of cancer prevention, increased fertility and improved immunological status. However, supplementation of SeAAs can be challenging, due to their reported narrow therapeutic range of indices, low bioavailability and increased susceptibility to oxidation. To address this, SeAAs were encapsulated into nanoparticles (NPs) to lower their toxicity profiles in comparison to their native form, in addition to offering them protection from the harsh conditions present in the gastrointestinal tract. The ionotropic gelation method was employed to produce NPs, using the cationic polyelectrolyte chitosan (Cs) crosslinked with the polyanion tripolyphosphate (TPP). The NP physicochemical properties were optimised using a Design of Experiments (DoE) and mathematical modelling approach. First, a central composite design (CCD) was used to identify a feasible region in which unloaded NPs with target properties for oral delivery (size~300 nm, PDI < 0.5 and ZP > 30 mV) could be achieved, indicating that optimum conditions to be 6:1 mass ratios of Cs:TPP. Following that, SeMet was used as a prototype encapsulant and a second DoE approach was employed, namely a Box Behnken design (BBD). In that study, Cs:TPP ratio, Cs solvent pH, and drug load concentration were independently varied and the dependent variables assessed were encapsulation efficiency (EE%), particle size, PDI and ZP. The BBD highlighted the optimum conditions for NP production, although EE% remained relatively low (≈40 %). By varying the pH of the ionotropic solution components and coating the NPs with zein (a prolamine rich protein), EE% was doubled (≈80 %), diameters increased from 187±58nm to 377±47nm and PDI and ZP values were maintained. The formulation performed well indicating good suitability for oral delivery, in terms of stability, cytotoxicity (intestinal and liver cell lines) and release profiles, as determined by in vitro techniques. The models generated from the BBD were then applied to the SeAAs, SeCys2 and MSC and further refinement of the zein coating was investigated. NPs with similar physicochemical properties to those of SeMet loaded Cs were produced, indicating the strength of the DoE models and their usefulness in formulation design. These NPs also performed well in in vitro assessments for stability and release profiles. However, SeCys2 showed cytotoxicity to liver cell lines which was reduced after encapsulation within the NP matrix, inferring its protective effects and thus use as a delivery system.

Chitin was extracted from the cultivated mushroom Agaricus bisporus in order to derive fungal Cs (fCs) with equivalent physicochemical properties to the commercial Cs (CL113) that had been employed for NP production. fCs with comparable properties to CL113 were produced, although fCs possessed a higher molecular weight (MW) (180±19 vs 110±4 kDa). Trimethyl chitosan was then synthesised from the fCs and assessed for permeation enhancing capabilities on intestinal cell monolayer models, which implicated its potential application as a permeation enhancer. fCs was used for the production of synthetic SeCys2 loaded NPs using the knowledge gained from the previously developed DoE models and in vitro methodologies. Lastly, a preliminary study to assess the feasibility of Irish mushrooms providing Se species for supplementation purposes was conducted, implicating them as a viable source for SeCys2.

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