Abstract P6-11-13: Phase Ib study evaluating the safety and clinical activity of lumretuzumab combined with pertuzumab and paclitaxel in HER2-low metastatic breast cancer
Authors: Schneeweiss A , Park-Simon T-W , Albanell J , Lassen U , Cortes J , Dieras V , May M , Schindler C , Marmé F , Cejalvo JM , Martinez-Garcia M , Gonzalez I , Lopez-Martin J , Welt A , Joly F , Michielin F , Jacob W , Adessi C , Moisan A , Meneses-Lorente G , James I , Ceppi M , Hasmann M , Weisser M , Cervantes A 
Published: March 2017

Background: Inhibition of HER2 and HER3 heterodimerisation is a novel treatment concept in HER2-”low” expressing breast cancer (BC). Lumretuzumab, a glycoengineered monoclonal anti-HER3 antibody, in combination with pertuzumab has demonstrated synergistic anti-tumor activity in preclinical HER2–low expressing preclinical BC models.

Methods: This open-label, multicenter phase I study selectively enrolled metastatic BC patients (pts) expressing HER3 protein and low levels of HER2 (defined as IHC 1+ and 2+ and ISH-negative) in a formalin-fixed paraffin-embedded pretreatment tumor biopsy sample. Eligible pts were treated with a combination of paclitaxel (PA) qw plus lumretuzumab (L) and pertuzumab (P) q3w in three dose cohorts. The safety, antitumor activity and tumor biomarkers including protein expression (IHC, MS) and mutational data (NGS) in association with clinical activity were evaluated.

Results: Overall, 35 pts were included in this study. The median age was 60 (range: 33 to 77) years. The median number of prior treatments for metastatic disease ranged from 0 to 5 with 23 pts (65.7%) without prior chemotherapy for metastatic disease. Cohort 1 was treated with PA at 80 mg/m2, L at 1000 mg and P at 840 mg for Cycle 1 followed by 420 mg for the following cycles. This cohort was stopped after two pts both experienced grade 3 diarrhea within the first treatment cycle which was considered a dose-limiting toxicity (DLT). For Cohort 2 the dose of L was reduced to 500 mg based on PK modelling and simulation data. No DLTs were seen for the first 6 pts. A total of 20 pts were recruited with an objective response rate (ORR) and disease control rate (DCR) of 30% and 75%, respectively, and 56% and 78%, respectively, for 1st-line pts (n=9) in this cohort. Diarrhea (≥G3) and hypokalemia (≥G3) occurred in 50% and 55% of pts, respectively, and all pts experienced chronic diarrhea throughout the course of treatment. For Cohort 3 the dose of L was maintained at 500 mg, PA at 80 mg/m2, and P was administered at 420 mg at all cycles. In addition, a prophylactic loperamide regimen was introduced. Altogether, 13 pts - all 1st-line for metastatic disease - were treated. No DLTs were seen for the first 6 pts. Diarrhea (≥G3) and hypokalemia (≥G3) were reduced to 31% and 15%, respectively, but chronic diarrhea was still observed throughout the treatment in all pts. The ORR and DCR were 31% and 77%, respectively. Preliminary mechanistic safety experiments revealed HER2/HER3-dependent chloride channels in the intestine as likely cause of diarrhea. Biomarker data will be presented along with updated clinical and safety data.

Conclusions: The combination of L, P and PA was associated with high rates of persistent diarrhea. Dose modifications and prophylactic anti-diarrheal medication led to significantly reduced diarrhea intensity but did not change the incidence and persistence of diarrhea overall. Despite encouraging clinical activity especially in 1st line pts, the therapeutic window of this combination is too low to warrant further clinical development.

Citation Format: Schneeweiss A, Park-Simon T-W, Albanell J, Lassen U, Cortes J, Dieras V, May M, Schindler C, Marmé F, Cejalvo JM, Martinez-Garcia M, Gonzalez I, Lopez-Martin J, Welt A, Joly F, Michielin F, Jacob W, Adessi C, Moisan A, Meneses-Lorente G, James I, Ceppi M, Hasmann M, Weisser M, Cervantes A. Phase Ib study evaluating the safety and clinical activity of lumretuzumab combined with pertuzumab and paclitaxel in HER2-low metastatic breast cancer [abstract]. In: Proceedings of the 2016 San Antonio Breast Cancer Symposium; 2016 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2017;77(4 Suppl):Abstract nr P6-11-13.