Development of Cirmtuzumab Antibody-Drug Conjugates (ADCs) Targeting Receptor Tyrosine Kinase-like Orphan Receptor 1 (ROR1)

ROR1 is an onco-embryonic surface antigen expressed on chronic lymphocytic leukemia (CLL) and a variety of other cancers, but not on most normal adult tissues. We generated a humanized IgG1 monoclonal antibody (mAb) cirmtuzumab (formerly UC-961) that binds with high affinity to a specific extracellular epitope of human ROR1 and that can block Wnt5a-induced ROR1 signaling (Yu, J et al, J Clin Invest126:585, 2016; Yu, J et al, Leukemia31:1333, 2017). Preclinical studies found that cirmtuzumab did not react with normal post-partem cells and had a pharmacokinetic (PK) volume distribution in primates consistent with a lack of off-target binding to normal tissues. We evaluated cirmtuzumab in a phase I clinical trial involving patients with relapsed-refractory CLL (Choi MY, et al, Cell Stem Cell22:951, 2018); the drug was well-tolerated at doses ≤20 mg/kg (highest dose tested) without dose-limiting toxicity. PK studies showed cirmtuzumab had a half-life of 32.4 days with no evidence for development of neutralizing antibodies or off-target sequestration of infused antibody. Furthermore, cirmtuzumab effected partial down-modulation of leukemia-cell ROR1 in patients treated with doses ≥2 mg/kg. In vitro confocal microscopy studies showed that this down-modulation was caused by internalization of cirmtuzumab-ROR1 complexes into lysosomal compartments and concomitant steady-state re-expression of nascent surface ROR1. Because of its high specificity, in vivo stability, long serum half-life, and potential capacity to concentrate conjugated drugs into lysosomal compartments, cirmtuzumab appeared ideally suited to serve as the targeting moiety in anti-ROR1 ADCs. We therefore examined cirmtuzumab-based ADCs in collaboration with VelosBio Inc., evaluating multiple linker/payload chemistries, both as single agents and in combinations. We selected for further testing cirmtuzumab-ADC-7, a cirmtuzumab-linker-monomethyl auristatin E (MMAE) ADC that preserves the high-affinity binding specificity of cirmtuzumab and allows for ROR1-targeted intracellular release of MMAE. We found cirmtuzumab-ADC-7 was selectively cytotoxic for ROR1⁺ CLL and mantle-cell lymphoma (MCL) cell lines at nM concentrations in vitro. Moreover, cirmtuzumab-ADC-7 caused dramatic and sustained in vivo clearance of adoptively-transferred ROR1⁺ leukemia cells generated from ROR1xTCL1 transgenic mice (Widhopf G, et al, PNAS111:793, 2014), ROR1⁺ MCL-xenografts, or ROR1⁺ cancer patient-derived xenografts (PDX). Further, treatment caused dose-dependent and statistically significant decreases in total cancer burden with complete regressions of tumor in multiple animals; no effect on tumor-clearance was observed in mice treated with a control MMAE-ADC of irrelevant specificity. Recently we identified that miR-15a/16-1, which commonly are deleted/downregulated in CLL, target both BCL2 and ROR1, thereby accounting in part for the direct relationship we observed between the levels of BCL2 and levels of surface ROR1 expressed by CLL of different patients (Rassenti, LZ, et al,PNAS114:10731, 2017). Because high level expression of BCL2/ROR1 may mitigate the cytotoxic activity of the BCL2-antagonist venetoclax, but potentially enhance the cytotoxicity of cirmtuzumab-ADC-7, we treated ROR1⁺ leukemia/lymphoma cell lines with venetoclax and/or cirmtuzumab-ADC-7. Chou-Talalay combination indices were <0.5 in all ROR1⁺ cell lines tested, indicating strong antitumor synergy with these two agents. Collectively these data support the rationale for clinical development of a cirmtuzumab-based ADC for treatment of patients with ROR1⁺ malignancies.