Drug dissolution from mesoporous silica systems

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McCarthy, Carol A.
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University College Cork
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Mesoporous silica materials have been investigated as novel formulation aids for oral drug delivery due to their drug solubility enhancing characteristics. However, the mechanism of drug release from these systems is not well understood. Several studies have reported unexplained incomplete release from mesoporous silica carriers. It has been reported, in other research fields, that passive drug adsorption onto the silica surface is possible. However, the implications for this behaviour on drug release from silica systems has not been considered to date. Dissolution studies involving these formulations are generally conducted using Type II dissolution apparatus under sink conditions with traditional simple buffer media. In this thesis, the suitability of this dissolution approach for mesoporous silica systems is considered. The overall aim of this thesis was to investigate factors influencing drug adsorption and release from mesoporous silica systems to enhance understanding of their drug release profiles. This thesis began with a comprehensive overview of the literature which identified the gaps in knowledge in this area. Based on these findings, the hypothesis, aims and objectives were developed. The four research chapters were each dedicated to factors which could potentially affect drug release; formulation excipients, dissolution medium, drug/silica interactions and dissolution apparatus. The role of drug adsorption on the silica surface was explored across several of the chapters using adsorption isotherms, adsorption models and spectroscopic techniques. Several aspects of dissolution experimental design were investigated including sink and supersaturating conditions, traditional simple buffer media versus biorelevant media and Type II (paddle) apparatus versus Type IV (flow-through cell) and a Transfer model (incorporating an SGF to FaSSIF-V2 media transfer). Finally, the results of in vitro dissolution studies were compared to in vivo performance in a fasting pig model. The literature review demonstrated the gap in knowledge concerning the mechanism of drug release from mesoporous silica systems. This informed the central themes of the thesis which were explored in four research chapters. In Chapter 3, it was determined that formulation excipients which can reduce surface tension of the dissolution media (e.g. surfactants) can significantly increase drug release from mesoporous silica carriers. Passive drug adsorption and competitive adsorption involving drug and surfactant molecules on the silica surface was also observed. Chapter 4 built on work from the previous chapter and demonstrated that components of biorelevant media that reduced surface tension can also enhance drug release from silica systems. This chapter established that the influence of biorelevant media extends beyond its impact on drug supersaturation promotion and that its use should also be recommended under sink conditions. In Chapter 5, the focus was placed on investigating drug/silica interactions under supersaturating conditions. It was determined these interactions occur through a hydrogen bonding process and not via non-specific hydrophobic interactions. It was determined that the dynamic equilibrium which exists between adsorbed and free drug during passive adsorption and dissolution can be related to the drug’s activity in solution. Finally, in Chapter 6, it was observed that dissolution experimental design can influence in vitro drug release from mesoporous silica systems. It was established that the Type IV apparatus incorporating an SGF -> FaSSIF-V2 transfer is the best predictor of in vivo performance. The findings of this thesis have made a significant contribution to enhancing knowledge on drug release from mesoporous silica systems. It provides robust recommendations for the design of in vitro dissolution studies involving mesoporous silica formulations including choice of dissolution media, drug supersaturation level and dissolution apparatus. Interesting results concerning the influence of drug activity in solution on the equilibrium process observed during drug adsorption and dissolution from mesoporous silica materials were documented. These findings open up interesting new avenues for future research in the field of mesoporous silica carriers for oral drug delivery.
Dissolution , Adsorption , Solubility enhancement , Formulation development , Biorelevant media , Isotherm
McCarthy, C. A. 2018. Drug dissolution from mesoporous silica systems. PhD Thesis, University College Cork.