Abstract P4-06-07: Activity of nivolumab alone or in combination with targeted therapies in a humanized BLT-mouse model of human breast cancer

Background : Recent advances in the field of cancer immunotherapy have increased demand for reliable preclinical models to inform patient selection and rational drug combination strategies. The development of the bone marrow-liver-thymus (BLT) mouse may provide the opportunity to study the complex interactions of human tumor and host immune systems in vivo . Other models are limited by the rapid onset of graft versus host disease (GVHD) and a lack of orderly maturation and trafficking of human T and B cells. In BLT mice, implantation of human fetal liver and thymus fragments beneath the kidney capsule of NSG (NOD/SCID/IL-2R Υ -/-) mice followed by engraftment of matched ex vivo expanded CD34+ cells supports the production of an almost complete human immune system. In this study, we used this model to assess the efficacy of the anti-PD-1 therapeutic antibody, nivolumab, in combination with targeted therapeutics in specific breast cancer sub-types. Materials and Methods : For triple negative breast cancer (TNBC), the activity of nivolumab was assessed in combination with the PARP1/2 inhibitor, talazoparib, in humanized BLT mice. Xenografts were established by subcutaneous injection of 5.0x10 6 MDA-231 (TNBC) cells. Mice (n=5) were randomized into treatment groups as follows; 1) Vehicle control (PBS), 2) nivolumab (10mg/kg QW), 3) talazoparib (0.33mg/kg Q5/2D) and 4) nivolumab+talazoparib. After 21 days of treatment, tumor tissue, serum and PBMCs were collected for biomarker analysis. Results : Successful reconstitution of mature human T and B cells was confirmed in BLT mice 12-weeks post engraftment of donor tissue and CD34+ hematopoietic stem cells. MDA-231 cells injected subcutaneously into the flank of these mice formed palpable tumors (150-200mm 3 ) within 9 days of injection. For vehicle control treated mice, tumors grew (2.5-fold) throughout the 21-day study. Single agent nivolumab induced significant tumor growth inhibition (TGI) relative to vehicle control treated mice at Day 21. Single agent talazoparib also induced comparable levels of TGI as did the combination of nivolumab plus talazoparib. Nivolumab treated mice continued to gain weight throughout the study without overt signs of toxicity. Reversible weight loss was observed in the talazoparib and combination treated arms. Overt signs of GVHD were not observed in any of these animals. Preliminary tissue analysis identified high levels of cell surface PD-L1 protein in control treated MDA-231 xenografts. Further analysis of the treated tumors will provide valuable insight into the mechanism of action of this class of molecule. We are establishing xenograft models of hormone receptor (ER+) positive breast cancer to measure the activity of nivolumab+/-CDK4/6-inhibition in humanized BLT mice and these data will also be presented. Discussion : The data presented here highlight the potential of the PD-1 antibody nivolumab to have activity in TNBC. Furthermore, these findings illustrate the potential of the humanized BLT-mouse to model responses to immune check-point in the preclinical setting. Expanded use of this model may help to identify response biomarkers and inform design of combination therapies using immune oncology molecules and approved targeted therapies. Citation Format: O9Brien NA, Luo T, Ayala R, Salgar S, Conklin D, McDermott M, Kitchen S, Rezek V, Horak C, Dugan U, Slamon DJ. Activity of nivolumab alone or in combination with targeted therapies in a humanized BLT-mouse model of human breast cancer [abstract]. In: Proceedings of the 2018 San Antonio Breast Cancer Symposium; 2018 Dec 4-8; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2019;79(4 Suppl):Abstract nr P4-06-07.