The Development of Hydrogels and Nanoparticles for Drug and Cell Delivery to the Distal Airways
Chronic Obstructive Pulmonary Disease (COPD) is a major public health issue, affecting 64 million people globally. COPD is defined as a lung disease characterised by chronic obstruction of lung airflow that interferes with normal breathing and is not fully reversible. This obstruction is due to a combination of airway and parenchymal damage, associated with an enhanced chronic inflammatory response to noxious particles present in tobacco smoke. Despite the significant evolution of medical treatment options for COPD in the last two decades, it is still an incurable disease. Current interventions include pharmacological agents, which primarily aim for symptomatic control, and lung transplantation for those in end-stage disease. However, neither of these options treat or address the underlying disease state or progressive damage being caused, meaning there is an unmet clinical need for the investigation of novel strategies that can achieve this at a much earlier stage in disease development.
The overall objective of this research was to develop novel delivery systems suitable for minimally invasive drug and cell delivery that could have potential applications in the treatment of COPD. Our central hypothesis was that hydrogels and solid lipid nanoparticles can be used as drug and cell delivery vectors, which are then capable of exerting an anti-inflammatory effect on the local environment present in COPD. It was hypothesised that Human Mesenchymal Stem Cells (hMSCs) could be utilised to exert an anti-inflammatory effect via paracrine actions, and all trans-Retinoic Acid (atRA) as an anti-inflammatory signalling molecule. Strategies such as these may provide alternatives to current therapies which have suboptimal effects and also enable loco-regional delivery of therapeutics, which could significantly enhance clinical outcomes.
This thesis initially investigated a methylcellulose, collagen and beta-glycerophosphate hydrogel for its thermoresponsive properties and ability to maintain viability of encapsulated stem cells. This hydrogel (“Respiragel”) underwent sol-gel transition at 37oC and was physically robust in nature. Respiragel could be delivered through a range of common clinical devices, facilitated encapsulation of hMSCs and also maintained their survival and proliferation. Respiragel could be sterilised using gamma irradiation, which did not adversely affect either its thermogelation properties or cell encapsulation ability, establishing it as a highly suitable delivery vector for this cell type.
In order to assess further hydrogel biomaterials for their ability to support cell encapsulation, novel self-assembling co-polypeptides, Star-PLL-PLT and Linear-PLL-PLT, were developed. Both polymers were capable of forming hydrogels spontaneously on addition of aqueous media. The Star-PLL-PLT showed superior rheological properties and physical robustness than the Linear-PLL-PLT. Both hydrogels could be pushed through a range of clinical devices, indicating promise for minimally invasive delivery. However, both hydrogels were cytotoxic on encapsulation of hMSCs, demonstrating their lack of suitability as a cell delivery vector in their current form.
Solid lipid nanoparticles (SLNs) were formulated which were capable of a high degree of encapsulation of all trans-Retinoic Acid (atRA). SLNs were biocompatible and enabled sustained release of atRA. Suspension of atRA SLNs within the previously developed Respiragel was possible with no resulting negative effects on thermoresponse or atRA release. Thus, atRA SLNs suspended in Respiragel forms a promising combinatorial drug delivery system which has the potential to enable loco-regional delivery of atRA to the lung using minimally invasive delivery device technology.
Evaluation of atRA SLNs and atRA SLN/Respiragel formulations in an in vitro model of inflammation in COPD was performed, with both formulations demonstrating an anti-inflammatory effect through a reduction in IL-6 and IL-8 concentrations. This data has shown that atRA could modulate the inflammatory environment in COPD, which may result in a potential reduction in airway destruction. The hMSC/Respiragel formulation however, resulted in an increase in IL-6 and IL-8 – possibly indicating a pro-inflammatory reaction of MSCs when placed in an already inflammatory environment.
Collectively, the research presented in this thesis has resulted in the investigation of multiple formulations as potential cell or drug carriers which can be delivered in a minimally invasive manner. This thesis has highlighted potential shortcomings of the use of MSCs in the treatment of chronic inflammatory conditions, which need to be verified in further detailed studies. However, both atRA SLNs and a thermoresponsive hydrogel (Respiragel), when used in combination, provide unique potential for the loco-regional delivery of anti-inflammatory therapeutic agents in COPD.