ROR1 targeting with the antibody drug-conjugate VLS-101 is effective in Richter syndrome patient-derived xenograft mouse models

Richter syndrome (RS) represents the transformation of chronic lymphocytic leukemia (CLL), typically to an aggressive lymphoma. Treatment options for RS are limited and the disease is often fatal. Receptor tyrosine kinase-like orphan receptor 1 (ROR1) is expressed on CLL cells and other cancers but not on normal adult tissues, making it an attractive, tumor-specific therapeutic target. VLS‑101 is being developed as an antibody-drug conjugate (ADC) for therapy of ROR1-expressing (ROR1+) cancers. VLS-101 comprises UC‑961 (a humanized immunoglobulin IgG1 monoclonal antibody that binds an extracellular epitope of human ROR1), a maleimidocaproyl-valine-citrulline-para-aminobenzoate (mc-vc-PAB) linker, and the anti‑microtubule cytotoxin monomethyl auristatin E (MMAE). VLS‑101 binding to ROR1 results in rapid cellular internalization and delivery of MMAE to induce tumor cell death. We studied 4 RS patient-derived xenografts (RS‑PDXs) with varying levels of ROR1 expression (11%, 32%, 85%, 99% of cells). VLS-101 showed no efficacy in the lowest-expressing RS-PDX but induced complete remissions in those with higher levels of ROR1 expression. Responses were maintained during the post-therapy period, particularly after higher VLS-101 doses. In systemic ROR1+ RS-PDXs, VLS-101 dramatically decreased tumor burden in all RS-colonized tissues and significantly prolonged survival. Animals showed no adverse effects or weight loss. Our results confirm ROR1 as a target in RS and demonstrate the therapeutic potential of using an ADC directed toward ROR1 for the treatment of hematological cancers. A Phase 1 clinical trial of VLS‑101 (NCT03833180) is ongoing in patients with RS and other hematological malignancies.