Abstract
Cancer is one of the leading causes of mortality and morbidity in the Western world. The anguish caused by cancer is exacerbated by current chemotherapeutic treatments as severe adverse effects can be caused by off-target cytotoxicity against some healthy tissues. This limits the patients these drugs can be used in and importantly the doses that can be used, impacting on therapeutic efficacy. Therefore, tumour specific delivery of cytotoxic agents by environmentally sensitive nanoparticles to minimise off-target effects may be an effective strategy.
Another approach to treat cancer would be to utilise a patient’s immune system to more naturally treat cancer. However such immunotherapies have struggled to produce effective results as a comprehensive review of cancer vaccines demonstrated an overall response rate (as defined as the shrinkage of the tumour) of 3.6% (1). One of the challenges with cancer vaccines is the inefficient and ill-defined process of ‘cross-presentation’ which occurs rarely in antigen presenting cells. As cross-presentation is involved in the production of anti-cancer cytotoxic T cell responses, the delivery of antigen directly to the cytoplasm to bypass cross-presentation has been suggested as a strategy to improve vaccine efficacy.
Two strategies and nanoparticle formulations were investigated in this thesis where the release of therapeutic agents in acidic conditions was desired. In the first strategy, a formulation of pH responsive polymeric nanoparticles using PDMS-b-PDMAEMA was examined. This polymer was previously reported by Car et al. (2) due to the pH dependent ionisation of PDMAEMA. This ionisation is hypothesised to destabilise the nanoparticle in acidic conditions to release loaded chemotherapy agents, such as doxorubicin (DOX), in the acidic tumour microenvironment for tumour specific DOX release.
Secondly, liposomes modified with polyethyleneimine (PEI) and cyclodextrin based ion channels were developed. PEI confers pH responsive release from liposomes due to the ‘proton sponge effect’. The effect occurs when liposomes are internalised within the acidic endosomes of cells, and the buffering effect of PEI leads to an increase in counterion concentration within the liposomes and endosomes. This increase in ion concentration results in osmotic lysis of endosomes and liposomes resulting in release of actives into the cytoplasm. Cyclodextrin ion channels, previously reported by Chui et al. (3), were included in the formulation to facilitate the movement of ions across the liposomal membrane to allow the effect to occur. Delivery of DOX and an immunotherapeutic vaccine combined with an immunostimulant, 5,6-dimethylxanthenone-4-acetic acid (DMXAA) was examined.
Both types of nanoparticles were produced by the thin film hydration method. Uptake and release in vitro was determined using a light scattering based assay. Cell culture with murine melanoma cells and an in vivo murine melanoma model were utilised to assess biological activity.
PDMS-b-PDMAEMA nanoparticles demonstrated enhanced DOX release in acidic endosomal conditions. However, it was found to be incompatible with the model protein OVA preventing its use in vaccines. Whereas, modified liposomes demonstrated acid specific lysis which was then associated with improved cytoplasmic delivery in dendritic cells. In vitro dendritic cell stimulation, T cell proliferation assays, and an in vivo murine melanoma experiment established the efficacy of the modified liposomes as a viable vaccine carrier.
1. Klebanoff CA, Acquavella N, Yu Z, Restifo NP. Therapeutic cancer vaccines: are we there yet? Immunological reviews. 2011;239(1):27-44.
2. Car A, Baumann P, Duskey JT, Chami M, Bruns N, Meier W. pH-Responsive PDMS-b-PDMAEMA Micelles for Intracellular Anticancer Drug Delivery. Biomacromolecules. 2014;15(9):3235-45.
3. Chui JKW, Fyles TM. Cyclodextrin ion channels. Organic & Biomolecular Chemistry. 2014;12(22):3622-34.