Abstract 1541: Evaluation of ROR1 targeted antibody drug conjugate in ROR1 positive leukemia

Introduction: Receptor tyrosine kinase like orphan receptor 1 (ROR1) is expressed on the surface of multiple blood and solid tumors. There have been increased interest in ROR1 as a potential therapeutic target in ROR1 positive leukemia and lymphomas. In this study, we present a novel first-in-class anti-ROR1 monoclonal antibody drug conjugate (ADC) based on chimeric rabbit/human mAb “XBR1-402” or its humanized version “huXBR1-402”. Conjugated to a highly potent anthracycline derived toxin, PNU-159682 (PNU) via a non-cleavable peptide/amide linker, it shows both in vitro and in vivo efficacy at eliminating ROR1 positive chronic lymphocytic leukemia (CLL), mantle cell lymphoma (MCL), and pre B cell acute lymphocytic leukemia (pre B cell ALL) cells. Results: We saw a decrease in viability for all ADC treated ROR1 positive leukemic cell lines. We saw a significant decrease in viability of the ROR1 positive pre B cell ALL cell lines 697 and Kasumi-2 when treated with XBR1-402-PNU (EC50 = 77 and 1 ng/mL) when compared to treatment with isotype-matched control Trastuzumab-PNU (Tras-PNU) (EC50 = 2987 and 5330 ng/mL). In in vitro models of MCL, we saw decreased viability for huXBR1-402-PNU treated ROR1 positive Jeko and Mino cell lines but not ROR1 negative Mec-1 cell line when compared to Tras-PNU. While we were unable to observe significant direct cytotoxicity in primary CLL cells, we show that PNU ADCs mediates antibody dependent cellular cytotoxicity and phagocytosis in vitro. We also show that PNU ADCs mediates a G2/M cell cycle arrest in affected cell lines and hypothesize that this is necessary for direct cytotoxicity. As ex vivo primary CLL cells are non-proliferative, we postulate that PNU would target actively dividing cells in proliferation centers (e.g. secondary lymphoid organs). To test this hypothesis, we set up in vivo studies to test with XBR1-402-PNU and huXBR1-402-PNU in disseminated and aggressive ROR1+ ALL and CLL models, respectively. In vivo studies on a murine 697 ALL model suggested XBR1-402-PNU treatment increased overall survival when compared to treatment with Tras-PNU control (Median survival of 32 and 23 days post implantation, p=0.021, n=6). In vivo CLL studies with engrafted murine human-ROR1 expressing TCL1 leukemic cells showed that huXBR1-402-PNU treatment (3 times/week for 1 week) both suppressed leukemia burden and increased overall survival when compared with Tras-PNU control (Median survival of 62 and 41 days post implantation