Abstract 1362: Mechanistic evaluation of combination anti-angiogenic effects of anti-neuropilin-1 and anti-VEGF by in vivo imaging

Proceedings: AACR 101st Annual Meeting 2010‐‐ Apr 17‐21, 2010; Washington, DC Introduction. Anti-neuropilin 1 (anti-NRP1) has been shown preclinically to have an additive effect when used in combination with anti-VEGF. This has been hypothesized to be due in part to the inhibition of vascular maturation that results from anti-NRP1 treatment, which leaves vasculature in a highly anti-VEGF sensitive state. In this study, we examined the effects of sequencing these agents to evaluate this hypothesis. We investigated the effect of pre-treating tumors with anti-NRP1 before treating with anti-VEGF, and vice versa. To monitor tumor vasculature during ongoing treatment, in vivo magnetic resonance imaging was used to reveal information about therapeutic effects on tumor vasculature beyond simple tumor volume measurements. The vessel imaging MRI technique determines the fractional blood volume, mean vessel size, and Q (a dimensionless parameter related to vessel density) on a voxel-by-voxel basis. Methods. MRI scans were performed on a 4.7 T Varian MRI system (Varian Inc, Palo Alto, CA), for 25 mice with Calu6 human lung cancer tumors grown subcutaneously on the leg. Mice were randomized into three groups based on tumor volume caliper measurements: Group 1 (N=8) was treated with appropriate control antibodies twice a week for 3 weeks, Group 2 (N=8) was treated first with anti-VEGF twice a week for 1.5 weeks and then with anti-NRP1 twice a week for 1.5 weeks, and Group 3 (N=9) was treated first with anti-NRP1 twice a week for 1.5 weeks and then with anti-VEGF twice a week for 1.5 weeks. All mice were imaged after the first 10 days of treatment and again after the second 10 days of treatment. Vessel size index maps, blood volume maps, and Q maps were calculated voxel-by-voxel, and mean values of each parameter were calculated across the viable tumor tissue as determined using tissue segmentation analysis on the images. Results & Discussion. After 3 weeks, both treated groups had significantly reduced viable tumor volume relative to the control group. There was no difference in mean vessel size between the three groups. However, Group 3 had significantly reduced blood volume and Q (vessel density) relative to both Groups 1 & 2. This may indicate that initial treatment with anti-NRP1 makes tumor vasculature more susceptible to later treatment with anti-VEGF, while tumors that had been treated first with anti-VEGF did not benefit from later treatment with anti-NRP1. A repeat study with an additional group that was treated with both antibodies for the entire study duration confirmed these results and showed that the group pre-treated with anti-NRP1 showed equivalent reductions in blood volume and Q to the group treated with both antibodies for the full three weeks. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 1362.