Safety and efficacy of the HER2-derived GP2 peptide vaccine in combination with trastuzumab for breast cancer patients in the adjuvant setting.

3096 Background: GP2 is a 9 amino acid HLA-A2/A3 restricted HER2-derived peptide. GP2 + GM-CSF has been shown to be safe and effective in eliciting anti-HER2 immune response in breast cancer patients. Preclinical data has demonstrated that pretreatment of cells with trastuzumab (Tz) enhances susceptibility to lysis by GP2-specific cytotoxic T lymphocytes (CTLs). We conducted a phase Ib study to evaluate the combination of the GP2 vaccine and Tz. Methods: HLA-A2/A3 + patients with HER2 overexpressing breast cancer receiving Tz as standard therapy were enrolled. The study was designed as a 3+3 dose escalation trial with an expansion cohort evaluating 4 dose levels of the vaccine administered as 6 inoculations given every 3 weeks in combination with Tz (6mg/kg). Toxicity was graded 48-72 hr post vaccination using NCI Toxicity Criteria. Ejection fraction (EF) was monitored every 3 mo. Immunologic response was assessed in vivo by injection site local reaction (LR) and in vitro by quantifying the number of GP2-...