Advanced drug delivery systems for enhanced antibacterial and anticancer therapy

Abstract

Antibiotics—once the main line of defence against bacterial infections—have started losing their effectiveness in the treatment of bacterial infections owing to antimicrobial resistance (AMR) caused by long-term abuse and overprescription. With an exponential rise in AMR of bacteria, there is currently a crucial need to develop alternative antimicrobial agents capable of effectively containing their toxic effects while achieving a therapeutic function. While most bacteria poses a threat to public health and need to be treated, there are certain ‘friendly’ bacteria —mostly lactic acid bacteria (LAB)—with promising therapeutic functions. This project focuses on developing advanced drug delivery systems to kill harmful bacteria while making use of healthy bacteria to treat cancer.

Chapter 1 delves into the structure of Gram-positive and Gram-negative pathogenic bacteria and outlines in detail the distinct features that may contribute to bacterial infections. Next, the targeting mechanisms of action of currently available antibiotics are discussed and their limitations are presented. As alternative antibacterial treatment options, nanomaterials and hydrogels are introduced and the current state-of-art research is described and supplemented with examples. Then, the potential use of healthy bacteria in therapy is summarised with a strong focus on anticancer applications. After highlighting some of the drawbacks associated with current drug delivery systems, the rationale, aim, and objectives of this PhD thesis are established.

Chapter 2 provides an insight into the imaging and characterisation techniques used in this thesis. A brief description of their operating principles is provided and their benefits and drawbacks are highlighted. For some of the techniques, the optimised operating modes for characterisation of the drug delivery systems synthesised are described.

Chapter 3 is a collaboration work which focuses on photodynamic-based therapy to treat methicillin-resistant Staphylococcus Aureus (MRSA). Chlorin e6 (Ce6), a common photosensitiser, was covalently conjugated to a zeolitic imidazolate framework-8 (ZIF-8) nanoparticle (NP) termed as MOF-Ce6 and incubated with bacteria. After proving their antibacterial efficacy on the bacteria upon irradiation with a 650 nm LED light using optical density and CFU measurements, confocal imaging was used to provide a visual representation of the results. An insight into the mechanism of its antibacterial activity was provided.

Chapter 4 further explores photodynamic therapy of two Ce6-conjugated NPs, namely, MOF-Ce6 and silica-Ce6 (MCM-Ce6) NPs to understand the importance of cellular uptake of NPs in their photodynamic therapy-based antibacterial applications. The effects of two times light irradiation on the antibacterial performance of these NPs were studied and compared with that of free Ce6 molecules. Results from this experiment revealed that larger NPs are unable to enter MRSA bacteria and only act at their surface, thus showing bacterial growth after 12 h. Free Ce6 molecules, on the contrary, were still effective after 12 h and able to completely eradicate most bacteria after the second LED light irradiation owing to their ability to penetrate and remain within the bacteria.

Chapter 5 proposes a new generation of hand washing hydrogel with chitosan NPs acting as the antibacterial agent. Chitosan NPs with sizes ranging from 50 – 200 nm were synthesised using an ionotropic gelation method. The hydrogel formulation was prepared and the steps involved in its optimisation were described. Results from this work demonstrated good antibacterial activity against MRSA and streptomycin-resistant Escherichia Coli (E. Coli dB 3.1).

Chapter 6 investigates two LABs, namely, Lactococcus Lactis (L. Lactis) and Lactobacillus Casei (L. Casei) as potential carriers of drug-loaded NPs for the potential anticancer therapy of breast cancer cells. Mesoporous silica NPs (MSNs) were separately loaded with the chemotherapeutic drug, Doxurubicin HCl (DOX) and the photosensitising dye, methylene blue (MB). The drug5 loaded NPs were then coated with metal phenolic networks (MPNs). Nano biohybrids were formed by the incubation of the nanocomposites synthesised with each type of probiotic bacteria. These were then characterised by TEM and the coating of the NPs on the surface of the bacteria was optimised after testing under different incubation conditions. The potential benefits of using the probiotic bacteria-based nanohybrids for cancer cell targeting as well as coating of the NPs with MPNs for the long-term and sustained release of the drug was highlighted.

The closing chapter, Chapter 7 provides a summary of the key results obtained in the experiments carried out in this project. It draws a conclusion on their efficacies and their novelty as well provides a critical insight into their potential translation into commercial use.