Pre-clinical Evaluation of Therapeutic Microbubbles for the Treatment of Colorectal Cancer

Colorectal cancer (CRC) is the third most common cancer worldwide. Despite improvements in the overall 5-year survival rate, success has not been reflected in those over 75-years of age or those diagnosed with advanced stages of the disease. It has been proposed that a targeted, triggered drug-delivery system would optimise drug delivery to tumours, a fundamental yet difficult requirement for the effective treatment of cancer. Microbubbles (MBs), 2 μm in diameter and consisting of a lipid stabilised gas core, were engineered to carry a low dose payload of the chemotherapy prodrug irinotecan and subsequently its active metabolite SN38. These drug-loaded MBs were termed therapeutic MBs (thMBs) and were specifically targeted to tumour vasculature using the pro- angiogenic receptor expressed by endothelial cells, vascular endothelial growth factor receptor-2 (VEGFR2). Once injected (intravenously), a short, low-frequency pulse of ultrasound (US) was deployed externally to the tumour and used as a ‘trigger’ to destroy the thMBs, releasing the drug at the target site and increasing intracellular drug delivery to the tumour tissue. A liquid chromatography tandem mass spectrometry method was developed and validated to determine concentrations of irinotecan and/or SN38 and their metabolites from murine CRC xenograft tumours, tissues and blood. Pharmacokinetic studies were performed to compare biodistribution after multiple doses of thMBs to normal delivery methods. Irinotecan thMBs enhanced tumour drug accumulation and/or retention and increased efficacy compared to free drug. The more potent SN38 was then investigated in vitro, and thMB drug delivery was developed for the more efficient treatment of CRC in vivo. SN38 thMBs resulted in 93% tumour growth inhibition compared to the control and drug was detected in tumour tissues 72-hours post final treatment, with significant tumour pharmacodynamic response determined. Finally, an orthotopic murine model of human CRC liver metastases was established using US guided injection to offer a novel, minimally-invasive model of advanced disease for more translatable pre-clinical testing of anti-cancer agents.