In vivo monitoring of angiogenesis inhibitory treatment effects by dynamic contrast-enhanced computed tomography in a xenograft tumor model

Abstract

Rationale and objectives: To evaluate the potential of dynamic CT enhanced by iohexol or a novel macromolecular contrast agent, PEG12000-Gen4-triiodo, to monitor microvascular changes in tumors treated with the angiogenesis inhibitor bevacizumab.

Materials and methods: Ten female nude rats with MDA-MB 435 xenograft tumors were treated with 1 mg intraperitoneal bevacizumab when tumors reached 1 cm diameter and, for 4 rats, treated again 7 days later. Just before and 24 hours after the first injection of anti-VEGF antibody, the tumors were imaged by dynamic CT scans enhanced with PEG12000-Gen4-triiodo (n = 3 rats) or iohexol (n = 3 rats). The other 4 rats underwent dynamic CT scans enhanced with PEG12000-Gen4-triiodo just before and 24 hours after the second injection of anti-VEGF antibody. Microvascular leakiness (K(PS)) was calculated for the tumors using a 2-compartment tissue model.

Results: PEG12000-Gen4-triiodo-enhanced CT scans showed progressive reductions in K(PS) from day 1 to 2 to 9 (from 2.55 to 1.27 to 0.69 microL min(-1) cm(-3), respectively, P < 0.005 for each comparison of day 1-2 and day 2-9). No significant difference was seen in the K(PS) estimates derived from iohexol-enhanced CT scans obtained before or after treatment (276 vs. 223.8 microL min(-1) cm(-3), respectively, P = 0.54). The microvascular leak (K(PS)) was significantly larger for iohexol than for PEG12000-Gen4-triiodo-enhanced CT, P < 0.05.

Conclusion: Dynamic macromolecular contrast-enhanced CT can be used to monitor serial decreases in tumor microvessel leakiness induced by repeated doses of an angiogenesis inhibitor drug.

Figure 1

Chemical structure of PEG₁₂₀₀₀-Gen4-triiodo conjugate

Figure 2

Dynamic CT derived from MDA-MB 435 tumor-bearing nude rats. (a) For CT scans enhanced by PEG₁₂₀₀₀-Gen4-triiodo, gradual enhancement of the tumor (white arrow) is seen over a period of 35 minutes as the contrast in the inferior vena cava (black arrows) is seen to diminish slowly over time. (b) However, for CT scans enhanced by iohexol, the tumor (white arrows) and inferior vena cava (black arrows) both enhance quickly and show rapid de-enhancement.

Figure 3

Representative semilog plots for blood and tumor enhancement (Hounsfield units, HU) in the inferior vena cava (○) and tumor (△) for (a) the macromolecular PEG₁₂₀₀₀-Gen4-triiodo contrast agent and (b) the low-molecular-weight contrast agent Iohexol.

Figure 4

Graph of K^PS as determined by dynamic contrast-enhanced CT scans for the macromolecular PEG₁₂₀₀₀-Gen4-triiodo contrast agent and the low-molecular-weight contrast agent Iohexol. Gray circles on the x-axis indicate the time of intraperitoneal injection of bevacizumab, an antiangiogenesis drug, which was given immediately following the day 1 and day 8 CT scans. A significant reduction in K^PS was seen between the pre- and post-treatment CT scans for the macromolecular contrast agent for both the first dose (Group 1, mean, 2.56 versus 1.28 μl min⁻¹ cm⁻³ on day 1 versus 2, respectively, p < 0.005) and second dose (Group 2, mean, 1.40 versus 0. 69 μl min⁻¹ cm⁻³ on day 8 versus 9, respectively, p < 0.005). Similarly, the K^PS remained significantly lower on day 8 than prior to the first dose of angiogenesis inhibitor drug (p<0.005), and was not significantly different between day 2 and day 8 (p = 0.50). No significant difference was seen for the K^PS between the pre- and post-treatment CT scans for iohexol (Group 3, mean, 276.0 versus 223.8 μl min⁻¹ cm⁻³ on day 1 versus 2, respectively, p=0.54)