In vivo measurement of vascular modulation in experimental tumors using a fluorescent contrast agent

We compared the effectiveness of three optical techniques based on fluorescence imaging and spectroscopy with indocyanine green (ICG) contrast agent to evaluate in vivo the disruption of the active vasculature induced by a vascular targeting agent. The blood perfusion of the MDA-MB-435 tumor model transplanted in nude mice was estimated from the signal of the contrast agent measured immediately after its systemic injection in mice. Optical measurements were performed using a fluorescence imaging setup and a fiber-based time correlated single photon counting (TCSPC) apparatus. This latter apparatus was used to measure the tumor fluorescence in transmittance geometry and the change in the basal optical absorption induced by the contrast agent, thus providing an alternative estimation of the blood content in the tumor. Mice were divided into four groups. Three groups were treated with different doses of the vascular disrupting agent ZD6126, the fourth group (control group) received the drug vehicle only. Optical measurements were carried out 3 h after pharmacologic treatment. After 24 h, mice were killed, tumors were excised and the extent of necrosis was evaluated with standard histologic analysis. On fluorescence imaging ICG emission from tumors of mice treated with ZD6126 significantly was lower compared with the emission from control mice. The histologic sections also showed a significantly higher amount of necrosis in tumors of treated mice. Both these findings, which correlate with each other, indicate an effective vascular shutdown induced by the drug. However, ICG fluorescence measured with the TCSPC apparatus in transmittance geometry and the estimate of the change in optical absorption did not allow a statistically significant differentiation between treated and control groups.