Trastuzumab-Reactive Antibodies (Tr-Abs) In Serum And Trastuzumab (Tzb) Benefit Prediction In Patients With Her2-Overexpressing Breast Cancer

77 Background: Trastuzumab (Tzb) is a humanized monoclonal antibody (MAb) approved for treatment of HER2-overexpressing breast cancer. Unfortunately not all patients benefit from it and lack of response cannot be predicted. We detected TR-abs in serum of mice and patients treated with Tzb. We hypothesized that TR-abs would associate with response to therapy.Direct/competition ELISA, dot blot, and mass spectrometry were used to detect and characterize TR-abs in sera from Tzb treated FVB(MMTV/HER2)transgenic mice, from hybridoma MAbs stemming from transgenic mice responding favorably to Tzb (992-18 mMAb), and in sera of 22 patients with metastatic breast cancer enrolled in a phase I clinical trial. WST-1 viability assay was used to assess biological activity of 992-18 on SKBR3 or BT474 human breast cancer cell lines.From 12 mice bearing HER2-overexpressing tumors and treated with Tzb, 5 responded to therapy and 7 exhibited progressive disease (PD). All 5 responders had elevated TR-abs, whereas TR-abs were low/undetectable with PD (p=0.002; Mann-Whitney two-tailed). This was confirmed in a second cohort of 16 mice, in which TR-abs were undetectable prior to treatment, but gradually detected with Tzb therapy and tumor regression. TR-MAb 992-18 directly targeted also SKBR3 and BT474 in cell-based ELISAs and immunofluorescence assays. Treatment with 992-18 reduced BT474 and SKBR3 cell viability in comparison to isotype-matched control Ab (p<0.0001). In sera from patients with metastatic breast cancer, higher concentrations of TR-abs were significantly associated with lower risk of disease progression (p=0.023, Cox regression, univariate analysis).Low serum TR-abs are associated with poor response to Tzb in mice and with shorter progression free survival in women with HER2 overexpressing stage IV breast cancer. In addition, TR-abs (e.g. 992-18) produced in response to therapy may be pharmacologically active. Results support prospective evaluation of patients undergoing treatment with therapeutic antibodies to determine if this non-invasive immunoassay detecting anti-therapeutic antibodies would predict benefit to therapy.