Abstract 1586: Improving the radio-sensitizing effect of sunitinib by drug-specific scheduling.
Authors: Kleibeuker Esther A. , Castricum Kitty C. , van den Berg Jaap , Honeywell Richard , Griffioen Arjan W. , Slotman Ben J. , Verheul Henk M. , Thijssen Victor L. 
Published: August 2014

Introduction: Over 50% of all cancer patients receive radiotherapy during their treatment. Recent studies demonstrate that combining radiotherapy with angiostatic drugs may have a beneficial effect on tumor growth inhibition in vivo. However, efficacy depends on the type of drug, the dose and the scheduling. We adapted the chick chorioallantoic membrane (CAM) assay to allow rapid analysis of the effects of radiotherapy in combination with angiostatic drugs on in vivo angiogenesis. Using this model, we studied the effects of different treatment schedules with sunitinib on tumors grafted onto the CAM.

Methods: The CAMs of fertilized chicken eggs were treated daily with sunitinib (50 μL of 10 μM) starting at embryonic day of development (EDD) 6. Irradiation was given using a cobalt-60 source. On EDD10 the CAM vasculature was imaged and analyzed using specific software (HetCAM, DCI labs). For the in vivo tumor model, 5x106 HT29 or D384 cells were grafted on the CAM on EDD6. From EDD10 onwards, combination therapy was applied to the tumors using different treatment schedules. Tumor growth was measured daily until EDD17 after which tumor were dissected for further analysis.

Results: Single dose (SD) radiotherapy (4 Gy) on EDD6 induces an almost 50% reduction in the vessel length and the number of branchpoints. Within 3 days these vascular parameters normalize. Sunitinib treatment for 2 and 4 days results in significant reduction of the vascular parameters. Applying 4 Gy after sunitinib treatment has an additive effect on the vasculature. In the tumor grafts, SD radiotherapy (4 Gy) decreased tumor volume and weight significantly four days after irradiation which was mainly caused by impaired tumor cell growth rather than by vascular defects. Combining radiotherapy (SD of 4Gy) with sunitinib treatment (4x 50 uL of 0.8uM) only had a beneficial effect when irradiation was given within the first 3 days of sunitinib treatment. Furthermore, while SD irradiation with 2 Gy alone did not affect tumor growth, a beneficial effect was observed when 2 Gy irradiation was given at the start of sunitinib treatment.

Conclusion: The effect of combining radiotherapy with sunitinib on in vivo angiogenesis and tumor growth depends on the treatment schedule. Optimal scheduling allows a reduction in radiation dose. Preclinical analysis of optimal dose/scheduling is essential for the design of clinical trials and for translation of combination therapy to the clinic.

Citation Format: Esther A. Kleibeuker, Kitty C. Castricum, Jaap van den Berg, Richard Honeywell, Arjan W. Griffioen, Ben J. Slotman, Henk M. Verheul, Victor L. Thijssen. Improving the radio-sensitizing effect of sunitinib by drug-specific scheduling. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 1586. doi:10.1158/1538-7445.AM2013-1586