Polyphor AG announced that its global Phase III FORTRESS study, evaluating balixafortide (POL6326) in combination with eribulin for the treatment of patients with HER2 negative, locally recurrent or metastatic breast cancer, did also not meet its primary endpoint. As communicated, the analysis was conducted following the company's decision to initiate the closure of the FORTRESS study. The FORTRESS study investigates the efficacy and safety of the combination of balixafortide and eribulin versus eribulin monotherapy in patients with HER2 negative metastatic breast cancer with progressive disease after one to four lines of chemotherapy. Progression free survival (PFS) is the key primary endpoint of the study. 432 patients were included in the FORTRESS study, thereof 348 having had at least two lines of prior chemotherapy, a prespecified subgroup of the study. The study did not meet its primary endpoint of PFS in the overall population (n=432 patients): HR=1.10 (0.85, 1.41); p=0.445; median PFS: 3.5 m (B+E) vs. 4.0 m (E) and also not in the patients with at least 2 prior lines of chemotherapy according to the assessment of the blinded independent review committee (n=348 patients): HR=1.07 (0.81, 1.41); p=0.6158; median PFS: 3.5 m (B+E) vs. 4.0 m (E). Investigator assessment of PFS was consistent with the independent review committee analysis (HR=0.99 for the overall population and HR=1.05 for the > second line population). At the time of the PFS analysis, a pre-specified interim analysis of overall survival (OS) was conducted as well. There were no statistically significant differences for survival in the overall population (230 events): HR=1.08 (0.71,1.63); p=0.5897; median OS: 10.9 m (B+E) vs. 11.7 m (E) or in the > second line population (181 events): HR=1.08 (0.66, 1.77); p=0.6126; median OS: 11.0 m (B+E) vs. 11.2 m (E). The analysis of the safety and tolerance of balixafortide in combination with eribulin in the FORTRESS study was consistent with the previously reported safety profile with a numerically higher number of patients having had serious adverse events (28.4% B+E vs. 26.5% E) or adverse events of a grade 3 or higher severity (68.3% B+E vs. 63.7% E). Based on these results, the FORTRESS study will be closed and the pre-specified secondary efficacy analysis of OS after reaching 284 events in the overall population will not be performed.