Objective response rate in a phase II multicenter trial of pertuzumab (P), a HER2 dimerization inhibiting monoclonal antibody, in combination with trastuzumab (T) in patients (pts) with HER2-positive metastatic breast cancer (MBC) which has progressed during treatment with T

Abstract

1004 Background: T and P bind to different epitopes on the extra cellular domain of HER2. Unlike T, P binds to the dimerization domain and blocks homo- and hetero-dimerization of HER2 with other HER kinase family members. Xenograft models support the hypothesis that the complementary mechanisms of action could result in augmented efficacy when T and P are combined. Methods: Two-stage design, criteria to proceed to the 2 nd stage were: ≥ 2 partial responses (PR) or 1 PR and 12 stable disease (SDs) or 13 SDs. Eligibility included: measurable, centrally-tested HER2 positive breast cancer; up to 3 lines of prior chemotherapy plus T (including adjuvant chemotherapy plus T); disease progression during T as most recent treatment for metastatic disease; baseline left ventricular ejection fraction (LVEF) ≥ 55% and no decrease of LVEF to below 50% during prior T treatment. Consenting Pts received T i.v. weekly or every 3 weeks at 2 mg/kg or 6 mg/kg respectively (with re-loading dose if required) plus 420mg fixed dose of P i.v. every 3 weeks following loading dose 840mg. Study treatment was initiated within 9 weeks of the last dose of T given as most recent therapy. An independent data safety monitoring board has overseen the 1 st stage safety data. Results: Recruitment into 1 st stage is complete. The main adverse events were diarrhea (71%), fatigue (46%), nausea/vomiting (38%) and rash (25%). Most AE’s were mild to moderate (there was 1 case of Grade 3 diarrhea) and none was treatment-limiting. There were no clinical cardiac events, and central review revealed no case of fall in LVEF of ≥10% and to ≤50%. Response status: 5 confirmed PR (21%); 12 SD (50%). Responses have been observed in lymph node and liver metastases. Recruitment into the 2 nd stage of the trial has commenced. Conclusions: The combination of the P and T is active and well tolerated in patients with pre-treated HER2 positive breast cancer which has progressed during treatment with T. No significant financial relationships to disclose.