Selective Clearance of Chronic Lymphocytic Leukemia Cells in Vivo Following Treatment with UC99961, an Anti-ROR1 Monoclonal Antibody

Abstract 3886 ROR1 is a receptor-tyrosine kinase like protein expressed on the surface of chronic lymphocytic leukemia (CLL) B cells, but not on normal mature B cells, suggesting that it may be a promising therapeutic target. We have generated a chimeric monoclonal antibody (mAb), UC99961, which binds to an intradomain epitope of human ROR1 (hROR1). UC99961 binds the same epitope as the murine anti-hROR1 mAb, UC D10–001, which has direct cytotoxic effects on hROR1 positive CLL cells. In this study we investigated the in-vivo anti-leukemic activity and tolerability of UC99961 on ROR1+ primary patient CLL cells and human cord-blood-derived B cells and T cells, respectively. For these studies, immunodeficient RAG2−/−γc−/− neonatal mice were reconstituted with a human immune system by intrahepatic xenotransplantation of 1×105 CD34+ human cord blood progenitor cells. Eight to ten weeks post transplantation, cord blood engraftment was verified by peripheral blood screening, at which point the mice received an intraperitoneal transplantation of 2×107 primary patient ROR1+ CLL cells. Twenty-four hours after CLL transplantation, five animals per group were each treated with a single intraperitoneal injection (10mg/kg) of UC99961, UC D10–001, or control IgG. Seven days following mAb treatment, the animals were sacrificed and marrow, spleen, thymus, and peritoneal lavage samples were collected and analyzed by flow cytometry for CLL cells, as well as normal cord-blood-derived B cells and T cells. To confirm mAb administration according to the study design, serial residual ROR1 plasma antibody levels were determined by ELISA. Results from three consecutive experiments using leukemia cells from two different patients showed that the vast majority of CLL B cells remained in the peritoneal cavity of the animals and did not migrate to other hematopoietic organs. Both anti-hROR1 mAbs UC99961 and UC D10–001 significantly reduced the average number of harvested CLL cells in the peritoneal lavage compared to control IgG (99% and 71% reduction respectively), while cord-blood-derived T cells (CD45+3+) in thymus remained unaffected by the mAb treatment. For the majority of cord-blood-derived B cells in marrow and spleen, no significant reduction could be observed after UC99961 or UC D10–001 mAb treatment. A small CD19+ROR1+CD34− cord-blood-derived B cell population was identified in marrow and spleen that was reduced after UC99961 and UC D10–001 mAb treatment. This study demonstrates that the anti-human ROR1 specific mAbs have in vivo anti-leukemic activity with minimal impact on human cord-blood-derived B cells and T cells. From these results, UC99961 appears to be an excellent candidate antibody for future clinical studies for patients with CLL. Disclosures: No relevant conflicts of interest to declare.