Initial safety analysis of a randomized phase II trial of nelipepimut-S + GM-CSF and trastuzumab compared to trastuzumab alone to prevent recurrence in breast cancer patients with HER2 low-expressing tumors.

The development of HER2-targeted therapy has decreased recurrence rates and improved survival, transforming the natural history of HER2-positive breast cancer. However only a minority of breast cancer patients benefit as these agents are not used in patients with tumors expressing low levels of HER2. Preclinical data suggests a synergistic action of HER2-targeted vaccination with trastuzumab. We report the initial safety interim analysis of a phase II trial that enrolled patients with HER2 low-expressing (IHC 1+/2+) breast cancer who were clinically disease-free after standard therapy. Patients were randomized to receive the HER2-peptide vaccine nelipepimut-S + GM-CSF with trastuzumab (vaccine arm) or trastuzumab + GM-CSF (control arm) and were followed for recurrence. A planned analysis that occurred after enrollment of 150 patients showed no significant differences in toxicity between the two arms, including cardiac toxicity. The clinical efficacy of this combination will be reported 6 months after the final patient was enrolled.