Formulation and evaluation of oxygen microbubbles stabilised in a hydrogel to potentiate radiotherapy

BACKGROUND: Tumour hypoxia poses a significant challenge in cancer treatment. There is mounting evidence that reoxygenating tumours increases their sensitivity to conventional cancer therapies. Oxygenated microbubbles (OMB) show promise for this application but suffer from poor stability and rapid clearance. Embedding OMB in a thermosensitive hydrogel (OMBHG) may prolong tumour oxygenation and improve therapeutic outcomes. OBJECTIVES: To formulate and evaluate OMB loaded in a temperature sensitive hydrogel on an in vitro model of tumour hypoxia. METHODS: OMB generated from a liposomal precursor were dispersed at various concentrations in a poloxamer hydrogel. OMB size, hydrogel rheology, injectability, oxygen loading/release, and impact on efficacy of radiotherapy against HCT116 colon cancer cells under hypoxia/normoxia were evaluated. RESULTS: DSPC:DSPE-PEG2000 (94:6 molar ratio) liposomes dispersed in a poloxamer 407: poloxamer 188 (21:6.5 % w/w) hydrogel generated OMB predominantly sized < 1 µm. OMBHG formulations were deemed injectable (force to inject < 38 N) at 20 °C and gelled before 37 °C and demonstrated both greater oxygen loading and prolonged oxygen release than OMB alone. Cancer cells were significantly less sensitive to radiotherapy under hypoxic conditions. Pre-treatment of the cells with OMB or OMBHG enhanced radiotherapy significantly, reducing clonogenic survival rates in HCT116 cells by 78 % in hypoxic conditions and by 68 % in normoxic conditions (p < 0.0001 in both cases). Notably, this treatment restored the radiotherapy sensitivity of hypoxic cells to the levels seen with normoxic cells. CONCLUSION: Reoxygenation with a newly developed OMB hydrogel formulation effectively sensitised HCT116 to radiotherapy in vitro. Ongoing studies are exploring the importance of reoxygenation rate and extent for optimal tumour sensitisation.