Evolution Of The Myeloid-Derived Suppressor Cells In Advanced Breast Cancer And Comparative Analysis With A Healthy Population Cohort.

2543 Background: High levels of myeloid-derived suppressor cells (MDSCs) seem a negative prognostic factor in advanced breast cancer (ABC) patients (pts). Preclinical studies suggest an immunomodulatory effect of some classical anti-tumor agents through alteration of MDSCs homeostasis. We analyzed the association of MDSCs and clinical evolution of ABC pts, taking into account the systemic treatment (tx) modulation of MDSCs levels in pts from two studies (“A”: GEICAM/2015-04 PANGEA-BREAST, NCT03025880 “Efficacy and Safety of Pembrolizumab and Gemcitabine in HER2-negative ABC”, and “B”: PI-0502-2014 “Peripheral blood analyses of immune response induced by 1 st line tx of ABC according to clinical guidelines”). Methods: MDSCs (CD33+ CD11b+) levels were determined by flow-cytometry in peripheral blood samples at three time points (basal, at cycles 3 and 6) from: 39 HER2-negative heavily pretreated pts from study “A”, 43 non-pretreated pts (all subtypes) from study “B” and 20 women from a healthy cohort (HC), with no cancer diagnosis. MDSCs levels from the different cohorts were compared and correlated with pts with Clinical Benefit (CB: partial/complete response + disease stabilization) vs pts with Progressive Disease (PD). Results: Tx response was assessed in 33 pts (85%) from study “A” and 39 pts (91%) from study “B”. CB was observed in 11 pts (28%) from study “A” and in 34 (79%) from study “B” while PD was observed in 22 pts (56%) from study “A” and in 5 (12%) from study “B”. Basal MDSCs levels were significantly higher in ABC pts (studies “A”+”B”) than in HC (15.95 vs 0.81 cells/µl, p = 0.009). At cycle 6, MDSCs were considerably lower in pts with CB vs DP (2.90 vs 13.75 cells/µl, p < 0.001). This decrease was more pronounced in study “B” than in study “A” pts (p < 0.001 vs p = 0.074, respectively), probably due to differences in number of events, tumor subtypes and tx between both studies. Conclusions: Our results suggest that ABC pts show alterations in MDSCs and that their decrease along tx may have a positive predictive value, highlighting the importance that immune-competent status may play in the evolution of ABC. MDSCs may represent a target for therapeutic purposes in ABC.