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

• Published in: Blood 2018-11, Vol.132 (Supplement 1), p.1862-1862
• Main Authors: Mian, Yousaf A., Widhopf II, George F., Vo, Thanh-Trang, Jessen, Katti, Rassenti, Laura Z., Kipps, Thomas J.
• Format: Article
• Language: English
• ISSN: 0006-4971; 1528-0020
• DOI: 10.1182/blood-2018-99-119447

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). F