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Rationale & Hypothesis: Combretastatin A-4-phosphate (CA-4-P), a tubulin-binding, vascular-disrupting agent (VDA), induces rapid and selective tumor vascular shutdown and secondary tumor cell death. However, acute revascularization contributes to treatment resistance. A group of myeloid cells in tumors, tumor associated macrophages (TAMs) are known to drive angiogenesis and tumor progression. A potently pro-angiogenic subpopulation of TAMs has also been identified – Tie2 expressing monocytes (TEMs). We hypothesize that TAMs and/or TEMs infiltrate CA-4-P-treated tumors and promote tumor recovery. Methodology, Results & Conclusions: Late-stage Polyoma Middle T (PyMT) spontaneous murine breast adenocarcinomas were excised from CA-4-P- and saline-treated mice. Significant tumor necrosis was evident 24h following a single injection of 50mg/kg CA-4-P (19.37 ± 4.60%) compared with saline controls (1.76 ± 0.34%). This was accompanied by tumor infiltration of F4/80+ TAMs (5.61 ± 0.46% of tumor volume w.r.t. 0.71± 0.08% in controls). Significant increases in the number of MMP-9 expressing macrophages, primarily TEMs, were evident following CA-4-P treatment (5.72 ± 0.34% compared with 2.66 ± 0.25% in controls). TEMs were constitutive expressers of hypoxia-regulated CXCR4, which interacts via CXCL12 as a monocyte/macrophage chemoattractant. AMD3100, a chemical antagonist of CXCR4, was employed in combination with CA-4-P to prevent TEM recruitment into drug-treated tumors. TEM numbers were significantly depleted, which corresponded to heightened vascular damage (27% tumor necrosis w.r.t. 8% in CA-4-P alone). These results were replicated in a subcutaneous N202 murine mammary carcinoma model following 3 daily doses of 50mg/kg CA-4-P. Tumor growth was also significantly reduced at 72h in the combined treatment group (mean tumor volume ∼200mm3 w.r.t. ∼300mm3 in CA-4-P alone). In addition, specific TEM depletion was achieved in the N202 carcinoma model via genetic manipulation and bone marrow transplantation. Tumor necrosis was increased significantly when CA-4-P was administered in combination with TEM depletion (22.45%), compared to saline controls (0.37%) and CA-4-P therapy alone (12.89%). This was attributed to a significant reduction in MMP-9 positive TEMs (0.08 ± 0.18% compared with 1.37 ± 0.21% in saline controls and 3.67± 0.11% in CA-4-P treated tumors). Collectively, these data suggest that TEMs protect against CA-4-P or promote recovery of tumors following CA-4-P treatment. Therefore, targeting TEMs, in combination with vascular-disrupting agents such as CA-4-P may provide a novel therapeutic strategy for solid tumors. A.F. Welford was funded by a Cancer Research UK PhD studentship. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 2849. doi:10.1158/1538-7445.AM2011-2849