BOS-371, a monoclonal antibody against IL1RAP: Characterization in preclinical models of AML.

Abstract

7030 Background: IL1 receptor accessory protein (IL1RAP) is a cell surface protein overexpressed on leukemia stem cells (LSC) and progenitors in AML, CML and high-risk MDS. It is an indispensable co-receptor for the IL1 superfamily, but its role reaches beyond anti-apoptotic pathways. IL1RAP directly interacts with and potentiates signaling of FLT3 and c-KIT which have significant roles in AML pathogenesis. High IL1RAP is correlated with poor prognosis and IL1 blockade attenuates expansion of LSCs. Thus, targeting IL1RAP has emerged as a promising immunotherapeutic approach for myeloid malignancies. BOS-371 is a humanized monoclonal antibody targeting IL1RAP that has been Fc-engineered to enhance its capacity to induce antibody directed cell cytotoxicity (ADCC) and deplete IL1RAP-expressing AML cells. Methods: The preclinical activity of BOS-371 was characterized using multiple assays. In vitro analysis included: binding affinity in IL1RAP-expressing cell lines; inhibition of IL1-dependent signaling as measured by IL1a -stimulated secretion of IL6 from MRC5 lung cancer cells; measurement of ADCC in co-cultures of human NK cells with either IL1RAP-expressing cell lines or primary AML cells; and cell growth by monitoring metabolic activity of primary AML cells. In vivo efficacy of BOS-371 was determined in AML models: MV4-11 and ML2 cells (CDX) implanted in NOG and NOD-SCID mice, respectively, and patient-derived (PDX) leukemia cells implanted in NOG mice. Results: BOS-371 binds with high affinity to IL1RAP-expressing cells (average EC50 337pM) leading to potent inhibition of IL1 signaling (IC50 600pM). Co-culture assays in the presence of BOS-371 showed potent induction of ADCC with an EC50 of 60-70pM (cell lines) or 4-11pM (primary cells) and 80-100% cell lysis (both cell lines and primary cells). The ability of BOS-371 to impact AML cell growth exclusively through inhibition of IL1 signaling was measured in a panel of 15 primary AML samples. Incubation with BOS-371 led to IC50s ranging from 20-700nM in responsive samples. Evaluation of BOS-371 activity in vivo showed that administration of BOS-371 resulted in significant anti-leukemic activity. In CDX studies, BOS-371 treatment led to > 50% decrease in systemic disease and engraftment in bones and organs. The results observed in CDX AML models were reproduced in PDX mouse models of AML, where decreases in both peripheral ( > 70%) and bone marrow ( > 50%) disease burden in response to BOS-371 were observed. Conclusions: These findings demonstrate that BOS-371 binds with high affinity to IL1RAP, potently inhibits IL1 signaling and induces ADCC, leading to anti-leukemic activity in vitro and in vivo. The preclinical results support the clinical investigation of BOS-371 for the treatment of myeloid malignancies. A Phase 1 trial is currently in development.