Expression and treatment of ROR1+ cells with bispecific T cell engagers in pediatric acute lymphoblastic leukemia

Receptor tyrosine-like orphan receptor (ROR)1 is overexpressed in some hematological cancers but has low expression in normal tissues, making it a potential therapeutic target. We investigated this therapeutic potential in childhood B cell precursor (BCP) and T cell acute lymphoblastic leukemia (ALL) cases. The proportion of ROR1+ cells was significantly higher in T-ALL (median 13.8%, range 2.9-87%) than BCP-ALL (6%, 0.3-83%, P=0.02). Antigen density was also lower in BCP-ALL (median 1027, range 876-2588) compared to T-ALL (1089, 865-1527). In leukemia propagating cells (LPC), ROR1 levels were highest in CD34-/CD19+ and CD34-/CD7+ subpopulations. Notably, ROR1+ LPC, in both BCP- and T-ALL, survived induction therapy and their numbers increased post treatment. Subsequently, ROR1 bispecific T cell engagers (Teng) were tested on primary cases in vitro and in vivo. Addition of ROR1 Teng in vitro reduced ALL survival to 44% in BCP- and 58% in T-ALL, compared to T cells alone (94% and 84%, respectively P≤0.01). When NOD.Cg-PrkdcscidIl2rγtm1Wjl/SzJ mice engrafted with primary leukemia were treated with ROR1 Teng, disease burden was reduced by up to 520-fold (from 15.6% to 0.03%) in ROR1+ cells and 68-fold (58% to 0.9%) in CD19+ cells in BCP-ALL. In T-ALL cases, there was a 4-fold reduction (1.2% to 0.3%) in ROR1+ and 2.3-fold (83.7% to 36.7%) in CD7+ levels. This resistance of ROR1+ cells to current therapies makes it an important target. Moreover, as ROR1 Teng were at least comparable to CD19 Teng in vivo, they could be considered for the treatment of refractory BCP-ALL.