Abstract Background: Glioblastoma is the most common malignant neoplasm of the central nervous system. FPR1 is a seven-transmembrane G protein coupled receptor, present on a highly malignant human astrocytoma cell line U87, where it promotes cell motility, growth, and angiogenesis. Chemotaxis Inhibitory Protein of S. aureus (CHIPS) is a potent FPR1 inhibitor. We therefore explored the in vivo activity of CHIPS in U87 tumor bearing mice. To determine the relevance of targeting FPR1 in gliomas, we analyzed FPR1 expression in human glioma specimens. Methods: Male, nonobese diabetic (NOD)/severe combined immune deficient (SCID) mice (total 16) were pre-irradiated with 2 Gy and subcutaneously implanted in the dorsal flank with 5x106 U87 cells in 100 μL PBS. Tumor size was measured 3 times a week and volumes were calculated with the formula Lw2/2. The survival of mice was evaluated with a Log-Rank (Mantel Cox) test. When reaching the humane endpoint (tumor size > 2 cm3) tumors were excised, formalin-fixed and paraffin-embedded. Tumor sections were subjected to H&E staining and immunohistochemical analysis of expression of anti-apoptotic markers BCLxL, survivin and MCL-1, apoptotic marker cleaved caspase-3 and proliferation marker Ki67. All xenograft sections were first stained with GLUT-1 to distinguish hypoxic from normoxic areas. In addition, the expression of FPR1 on different grades of human astrocytoma tissues was evaluated by immunohistochemistry. Results: Tumor volumes of CHIPS-treated and placebo-treated mice did not differ. However, CHIPS-treated animals did survive longer as compared to control animals (41 versus 47 days, P = 0.0019). No differences in BCLxL, survivin, MCL-1 and Ki67 expression were detected between treated and untreated tumors in both hypoxic or normoxic areas. However, in normoxic areas we found an increased expression of cleaved caspase-3 in the treated group (1.4 ± 1.1 % positive cells) compared to untreated group (0.7 ± 0.4 % positive cells, P = 0.026). Furthermore, in specimens of grade I and II astrocytomas, FPR1 expression was detected in over 70% of tumor cells, with a homogeneous cytoplasmic staining pattern and moderate to high staining intensity. In grade III astrocytomas, 70% of tumor cells were FPR1 positive, but the expression pattern was less homogeneous although with high intensity. In grade IV astrocytomas, 40% of tumor cells were FPR1 positive. Conclusions: Targeting FPR1 with CHIPS prolongs survival of U87 tumor-bearing mice. FPR1 is a relevant target in glioma patients expressed in all grades of human astrocytomas. Supported by grant RUG 4622 of the Dutch Cancer Society Citation Format: Jennifer Carla Boer, Urszula Domanska, Carla de Haas, Jos van Strijp, Evelien Jongeneel, Elisabeth de Vries, Wilfred Den Dunnen, Annemiek Walenkamp. Inhibitory effects of chemotaxis inhibitory protein of S. aureus (CHIPS) in U87 glioma mouse xenografts. [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 5537. doi:10.1158/1538-7445.AM2013-5537