Monitoring the effects of bevacizumab beyond progression in a murine colorectal cancer model: a functional imaging approach

Clinical studies have shown that bevacizumab beyond progression to first line therapy is beneficial for overall survival in advanced stage colorectal cancer. We studied the utility of several functional imaging modalities to assess the efficacy of bevacizumab beyond progression (BBP). All BALB/c mice with s.c. LS174T xenografts were treated with capecitabine, oxaliplatin and bevacizumab combination therapy. Tumor volume was assessed using caliper measurements. Increase of 1.5 times the initial volume on two subsequent measurements, was considered progression. In half of the mice bevacizumab treatment was continued (n = 13) after progressive disease was established, while the others received saline injections (n = 12). Within 3 days after progression, multi-modal imaging was performed using FDG-PET, diffusion weighted imaging, T2* and dynamic contrast enhanced MRI. Measurements were repeated 7 and 10 days after the first measurements. Afterwards, tumors were analyzed for expression of carbonic anhydrase IX, glucose transporter 1, 9 F1 to stain the vasculature and Ki67 to assess proliferation. In the BBP group tumor growth after progression was reduced compared to the control group (p < 0.01). FDG-PET showed a trend towards lower FDG uptake in the BBP group (p = 0.08). DWI, T2* and DCE-MRI parameters were not significantly different between both groups. The immunohistochemical analyses showed higher CAIX-positive fraction (p < 0.01) and lower Ki67 expression (p = 0.06) in the BBP group. The relative vascular area was significantly lower in the BBP group (p = 0.03). GLUT-1 expression and vascular density did not significantly differ between both groups. Bevacizumab after progression resulted in significant changes in the tumor proliferation and microenvironment compared to discontinuation of bevacizumab. FDG-PET may be sensitive to BBP-induced effects.

Reproducibility and biological basis of in vivo T(2)* magnetic resonance imaging of liver metastasis of colorectal cancer

In this study, the reproducibility of T2* MR imaging in colorectal liver metastases was assessed and T2* values were correlated with the expression of the hypoxia-related markers GLUT-1 and CA-IX as well as the relative vascular area, and the vessel density in resected tumors. The reproducibility of T2* was analyzed in 18 patients with in total 22 colorectal liver metastases using the Bland and Altman method for the 16th, 50th, and 84th percentile values. Immunohistochemical staining was performed on 17 resected tumors obtained from 16 patients. The median T2* of all liver metastases was 25.0 ± 5.6 ms vs. 23.0 ± 4.1 ms (median ± st.dev.) in normal liver. The coefficient of repeatability was 11.2 ms and the limits of agreement were -13.2 ms and 9.1 ms for median T2* values. On average, T2* showed fair reproducibility. No correlations between T2* values, hypoxia- and vascularity-related markers were observed.

SU-E-T-06: A Mathematical Explanation to Tumor's Response to Perfusion and Hypoxic Fraction after Radiation

PURPOSE

To develop a dynamic model that explains oxygen dynamics between the microvascular perfusion and the hypoxic cell population inside a tumor.

METHODS

Bussink et al (Radiat Res 153(4), p.398 (2000)) observed fast oxygen dynamics, faster than cell-death. Based on a simplified three-compartment-model: the microvasculature, well-oxygenated, and hypoxic tumor cell populations. We applied a first-order differential model for the tumor's transient response as a function of oxygen content within the blood vessels. The sink terms in our model for each compartment are fast changing parameters because radiation rapidly changes the oxygen consumption of the tumor cell in a time scale which is much faster than the population changes of the tumor. Transportation balance condition is also applied for each compartment.

RESULTS

Our simulation results can explain the experimental data in Bussink et al's (Radiat Res 153(4), p.398 (2000)) paper. We provide an explanation for the relative complex behavior of the microvascular perfusion after radiation that emphasizes the role of dynamic metabolic changes in addition to population changes.

CONCLUSIONS

A newly developed dynamic model leads our understanding to the interrelationship between microvascular oxygen content within the blood vessels and the hypoxia state of the tumor to a deeper level, which has the potential to provide the theoretical foundation for the patient' specific adaptive radiotherapy.

Tumor hypoxia as a mechanism of resistance to bevacizumab in a murine model

e13111

Background: Despite the promise of preclinical and early phase clinical studies, the efficacy of bevacizumab in solid tumors is more limited than expected. One of the presumed reasons is the induction of tumor hypoxia by the anti-angiogenic effects of bevacizumab, leading to therapy resistance. The aim of this study was to assess the effect of bevacizumab on tumor hypoxia in vivo in a colorectal cancer model, using functional imaging techniques. Methods: Nude mice with s.c. LS174T colon carcinoma xenografts (0.05 - 0.3 cm3) were treated with bevacizumab (5 mg/kg; 2/wk, i.p.) or saline as a control. To assess tumor hypoxia in vivo 18F-MISO-PET microPET or T2*-MRI images were acquired of separate groups of mice (n=5) before treatment and at day 2, 6 and 10 days after start of treatment. Tumors were harvested directly after imaging to microscopically assess the hypoxic fraction (pimonidazole staining) and vascular density (9F1 staining). Results: Linear regression analyses showed that FMISO uptake increased significantly more over time in the control group than in the bevacizumab group (beta -0.44, p=0.02), indicating that bevacizumab reduced the inherent increase in tumor hypoxia over time. T2* time increased significantly less in the bevacizumab group (beta -0.45, p=0.01), indicating a higher deoxyhemoglobine concentration, which might indicate a higher perfusion of the tumor and thus less hypoxia. The hypoxic fraction did not change over time and no difference was observed between the tumors in the treated and the control group. Vessel density significantly decreased over time in the bevacizumab group (beta -0.25, p=0.05), while the hypoxic fraction remained unchanged in the control group. Conclusions: The bevacizumab-induced changes in the tumor were most prominent at 10 days after treatment initiation, implying a build-up effect of repeated bevacizumab administration. The treatment induced changes could be detected with both T2*-MRI as well as with FMISO microPET. Bevacizumab did not induce tumor hypoxia, despite the observed decrease in vascular density. Therefore, induction of tumor hypoxia as a resistance mechanism to bevacizumab treatment seems unlikely.

Quantification of tumour hypoxia. Functional histology and autoradiography

Tumor cell hypoxia is considered one of the important causes for radiation resistance. The introduction of IMRT(intensity modulated radiotherapy) allows specific boosting of tumor subvolumes that may harbour these radioresistant tumour cells. PET imaging of these subvolumes can be incorporated into treatment planning. However, at this moment microenvironmental changes visualized and quantified by means of PET-imaging need to be validated by high-resolution microscopic techniques. This will allow interpretation of imaging techniques with intermediate resolution (such as PET/CT) in relation to complex cellular signaling in response to anti-cancer treatments.