Access Type

Open Access Dissertation

Date of Award

January 2013

Degree Type

Dissertation

Degree Name

Ph.D.

Department

Pharmaceutical Sciences

First Advisor

David Oupicky

Abstract

Our long term goal is to develop a versatile and robust injectable carrier based on Mesoporous Silica Nanoparticles (MSN) for drug/drug combination therapies. The objective of my dissertation was to optimize surface functionality, particle shape and methods of colloidal stabilization of mesoporous silica by PEG for delivery of drug/drug combinations. This was achieved by pursuing the following three aims:

1. Optimize surface functionality of MSN for high drug loading and controlled release

2. Investigate the effect of particle shape and PEGylation on drug delivery in hypoxic tumor cells

3. Develop MSN capable of delivering drug/drug combinations

To investigate the effect of surface functionalization of MSN on crystallization, loading, release and activity of mitoxantrone (MTX), we synthesized thiol (SH), mixed thiol/amine (SH/NH2) and amine (NH2) functionalized MSN by sol-gel process. We observed that NH2

modification of MSN has limited MTX loading. In contrast, modifications of MSN with SH resulted in significant enhancement of MTX loading. MTX loading was controlled by pH of the loading media and surface charge of MSN. The SH-MSN particles maintained a strong negative charge due to silanol and thiol surface groups while amine modified MSN resulted in an overall positive charge. Differences in the loading were due to decrease in the electrostatic interaction between MTX and MSN. Based on the observed pH-dependence of MTX loading, we hypothesized that surface functionalization will provide a simple method of pH controlled MTX

release from MSN. Indeed, we observed strong pH-dependence of MTX release in SH-MSN with rapid release in acidic pH and very slow release in neutral pH. In comparison, MTX was released

rapidly from NH2-MSN regardless of the pH. Similar to loading, the differences in the release behavior are due to differences in interactions of the drug with the silica matrix. Another important aspect of mesoporous silica is amorphization or crystalline-to-amorphous transformation of drugs in the porous nanoconfinement. We found strong effect of surface functionalization of MSN on crystallization of MTX. In SH-MSN particles, MTX was found to

be in the amorphous form while semi-crystalline MTX was present in the NH2-MSN. This is a very interesting feature as surface functionalization serves as a simple tool to allow control over MTX loading, its crystallization and release profile.

After optimizing surface functionality, we investigated the effect of surface PEGylation and particle shape on loading and release profile of MTX in hypoxic tumor cells. We synthesized thiol functionalized mesoporous silica nanorods (MSNR) and stabilized them with different amount of covalently attached PEG. The focus on rod-shaped particles was based on studies showing beneficial properties of nanorods for increased blood circulation and tumor

accumulation compared with spherical particles. We found that PEGylation decreased zeta potential of MSNR with improved colloidal stability but reduced overall MTX loading as a function of PEG content. PEGylation also increased the rate of MTX release compared to release from non PEGylated MSNR. We found that MTX had better anticancer activity in hypoxic than normoxic conditions, while no clear effect of particle shape was observed. Flow cytometry study confirmed that increased activity of MTX formulations in hypoxic conditions was due to increased cell uptake and retention of the drug in the cells.

To achieve our long-term goal of developing a robust and versatile drug delivery carrier, we investigated feasibility of co-loading of hydrophilic-hydrophobic and hydrophobic-hydrophobic drug combinations in MSN. We found that loading of hydrophobic molecules such as PTX and 17-AAG depends on the polarity of solvent, with less polar solvents improving drug loading. We have successfully co-loaded PTX and 17-AAG into the same particles. We observed that PEGylation decreases loading of hydrophobic molecules. We hypothesize that this decrease in the drug loading was due to blocking of access to the pores. This is a new observation and we are currently investigating alternative ways to stabilize co-loaded MSN without compromising drug loading.

Inorganic particles like MSN offer an interesting alternative to organic drug delivery systems like polymeric nanoparticles, micelles and liposomes due to high drug loading capacity and biocompatibility. We demonstrated the effect of surface functionalization and PEGylation of MSN on crystallization, drug loading and release, and colloidal stability. We showed effective delivery of MTX using PEGylated MSN in hypoxic conditions that has significant promise in the treatment of clinically important triple negative as well as estrogen positive breast cancers. We also showed MSN are capable of delivering drug-drug combinations for cancer treatment.

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