Metronomic chemotherapy has shown promising antitumor activity in a number of malignancies. For example, we previously reported (Allegrini et al., Angiogenesis (2012) 15(2):275-86) a phase II clinical trial of metronomic UFT (a 5-fluorouracil prodrug; 100 mg/twice per day p.o.) and cyclophosphamide (CTX; 500 mg/mq2 i.v. bolus on day 1 and then 50 mg/day p.o.) plus celecoxib (200 mg/twice a day p.o.) in 38 patients with advanced refractory gastrointestinal tumors. The mechanisms of action of metronomic chemotherapy include upregulation of the angiogenesis inhibitor Thrombospondin-1, the suppression of bone marrow derived endothelial progenitor cells and, at least for drugs such as CTX, activation of the immune system. To further evaluate the latter, we carried out an immune system multiplex 14-cytokine profiling of plasma samples that were available (for day 0, day 28, and day 56) from 31 of the 38 patients in the above noted (Allegrini et al) clinical trial. Our results show that pre-treatment plasma level cut-offs of interferon-gamma (>12.84pg/ml), sCD40L (55.11pg/ml), and IL-17a ( Citation Format: Paloma Valenzuela, Karla Parra, Derrick Oaxaca, Luis Reza, Jose Lopez, Montserrat Garcia Arreguin, Diana Garcia, Georgialina Rodriguez, Alfredo Falcone, Giacomo Allegrini, Teresa Di Desidero, Guido Bocci, Robert Kirken, Giulio Francia. Pharmacodynamic biomarkers in metronomic chemotherapy: Multiplex cytokine measurements in gastrointestinal cancer patients [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 784. doi:10.1158/1538-7445.AM2017-784