In vitro experiments towards a mathematical model of solid tumour treatment by thermosensitive liposomes

Thermosensitive Liposomes (TSLs) are at a crossroads as a delivery platform. Recent attempts at clinical translation have had mixed outcomes. The HEAT and OPTIMA trials, using TSL doxorubicin and radiofrequency ablation (RFA) as a heating modality did not demonstrate improved outcomes compared to RFA alone. In contrast, the TARDOX trial demonstrated for the first time that the delivery of TSL doxorubicin with localised ultrasound-mediated mild hyperthermia of a deep-seated tumour was safe and feasible, as well as finding elevated levels of intratumoural doxorubicin after heating. Most notably, despite the fact many of the patients had diagnoses which would not ordinarily be expected to respond to doxorubicin therapy, a majority of patients had a localised radiological response following only a single cycle of treatment.
Several key parameters in the TARDOX trial (such as infusion duration, TSL dose, and heating duration) were chosen based on limited prior experimentation and practical considerations associated with an early stage trial. In this thesis, in vitro experiments and computational modelling are used to explore the effect of changing these variables on the likely treatment outcome, with a view to optimising the clinical translation of TSLs.
Three scales associated with the delivery of drug to tumours are identified: the clearance scale, which describes the concentration of drug in the systemic plasma; the delivery scale, which describes the extravasation of drug from the tumour vasculature and its transport through the extracellular space; and the pharmacodynamic scale, describing the cellular uptake and cytotoxic effect of the drug.
In the first chapter, a compartment model is used to demonstrate that literature data (from the delivery and pharmacodynamic scales) can be used to accurately resolve treatment of tumours with TSLs in man, comparing the model outputs to the clinical measurements obtained in the TARDOX trial. An initial optimisation is performed considering only the clearance and pharmacodynamic scales, and a sensitivity analysis shows that the predicted optimal treatment parameters are robust to fluctuations in parameters, however the predicted intratumoural concentrations do fluctuate more substantially with changing cellular uptake parameters.
The second research chapter therefore focuses on the pharmacodynamic scale, using experiments to quantify the beneficial effects of mild hyperthermia on cellular uptake and cytotoxicity of doxorubicin in CT26 cells, and provide improved parameters for the computational treatment model. It is found that the addition of mild hyperthermia results in significant increases in both uptake and cytotoxicity, with the cells exhibiting increased sensitivity to intracellular doxorubicin (compared to cells treated at normothermic conditions).
Finally, in the third research chapter, the aforementioned experimental data is used in a model which includes spatial heterogeneity (i.e., the delivery scale). It is demonstrated that spatial heterogeneity is not a substantial issue for doxorubicin TSLs, however in treatment with drugs that exhibit more rapid cellular uptake, this may be more limiting.
Overall, the work in this thesis demonstrates that TSLs should be able to bring a clinical benefit to patients, as increased delivery of drug is predicted with TSLs (compared to non-liposomal forms).

Keywords: thermosensitive liposomes
doxorubicin
solid tumours
Advisor: Coussios, CC
Payne, S
Carlisle, R