Microflow of fluorescently labelled red blood cells in tumours expressing single isoforms of VEGF and their response to VEGF-R tyrosine kinase inhibition

In this work we studied the functional differences between the microcirculation of murine tumours that express only single isoforms of vascular endothelial growth factor-A (VEGF), namely VEGF120 and VEGF188, and the effect of VEGF receptor tyrosine kinase (VEGF-R TK) inhibition on their functional response to the vascular disrupting agent, combretastatin A-4 phosphate (CA-4-P), using measurement of red blood cell (RBC) velocity by a ‘keyhole’ tracking algorithm. RBC velocities in VEGF188 tumours were unaffected by chronic treatment with a VEGF-R tyrosine kinase inhibitor, SU5416, whereas RBC velocities in VEGF120 tumours were significantly increased compared to control VEGF120 tumours. This effect was accompanied by a reduced tumour vascularisation. Pre-treatment of VEGF120 tumours with SU5416 made them much more resistant to CA-4-P treatment, with a RBC velocity response that was very similar to that of the more mature vasculature of the VEGF188 tumours. This study shows that vascular normalisation following anti-angiogenic treatment with a VEGF-R tyrosine kinase inhibitor reduced the response of a previously sensitive tumour line to CA-4-P.