Targeting the tumour microenvironment : characterizing the anti-vascular effects of the hypoxic cytotoxin tirapazamine

Tirapazamine (TPZ) is a bioreductive prodrug with greater toxicity to hypoxic cells in vitro, and anti-vascular activity in tumours grown in vivo in mice. Considerable inter- and intra-tumour heterogeneity occurs in response to the anti-vascular effects of TPZ. The main hypothesis for the work described in this thesis is that features of the tumour microenvironment confer tumour sensitivity to TPZ-mediated vascular dysfunction. Tumours exhibiting less sensitivity to the anti-vascular effects of TPZ had evidence of greater pre-treatment vascular function, including greater blood flow or permeability measured using DCE-MRI derived biomarkers and tumour mapping data of high molecular weight fluorescent dyes injected intravenously. Modulation of nitric oxide (NO) levels decreased the density of perfused blood vessels and sensitized tumours to the anti-vascular effects of TPZ. Additional vascular phenotype features such as relatively poor vascular maturity were also found to correlate with greater tumour sensitivity to TPZ-mediated vascular dysfunction. Greater toxicity of TPZ to hypoxic cells in vitro led to the hypothesis that blood oxygenation may have an impact on tumour sensitivity to TPZ-mediated vascular dysfunction in vivo. Tumours from mice that had moderate bleeding-induced anemia or that were breathing lowered (7-10%) oxygen were sensitized to the anti-vascular effects of TPZ. In vitro assays showed that human microvascular tube structures are sensitive to damage by TPZ at clinically relevant concentrations and oxygen levels. TPZ may be reduced by cellular nitric oxide synthase (NOS), and NOS is competitively inhibited by TPZ to result in decreased amounts of NO. Enhanced anti-cancer effects have previously been observed for other vascular disrupting agents (VDAs) and hypoxic cytotoxins when combined with NOS inhibitors. These findings led to studies presented in this thesis, which show that TPZ-mediated vascular dysfunction is enhanced by co-administration with a NOS inhibitor, and this combined activity can lead to reduced cancer growth. Results from this thesis suggest that features of the tumour microenvironment, including tumour vascular phenotype, blood oxygenation and tumour hypoxia impact tumour sensitivity to TPZ-mediated vascular dysfunction. In addition, inhibiting NOS in combination with TPZ is a therapeutically advantageous strategy that merits further investigation.Medicine, Faculty ofGraduat