Vls-101 Is a Novel Therapeutic Antibody-Drug Conjugate (ADC) Targeting Receptor Tyrosine Kinase-like Orphan Receptor 1 (ROR1) in Richter's Syndrome (RS)

ROR1 is a transmembrane receptor with tightly controlled expression during development. It is present on multiple tumor types but not on normal adult tissues. Hematological malignancies are often ROR1-positive, including chronic lymphocytic leukemia (CLL), mantle cell lymphoma (MCL), and diffuse large B cell lymphoma (DLBCL). Given its unique pattern of expression, ROR1 represents a tumor-specific therapeutic target. The anti-ROR1 antibody, UC-961, is ahumanized IgG1 monoclonal antibody (mAb) that binds with high affinity to a specific extracellular epitope of human ROR1 receptor and can block Wnt5a-induced ROR1 signaling. Nonclinical studies document that UC-961 does not react with normal adult human tissues and selectively binds to tumor cells. Because of the antibody high specificity, rapid internalization, and trafficking to lysosomes, UC-961 appears ideally suited to serve as the targeting moiety for an anti-ROR1 ADC. Accordingly, we have developed VLS-101, a UC-961-linker-monomethyl auristatin E (MMAE) ADC that preserves the high-affinity binding and specificity of UC-961 and allows for ROR1-targeted intracellular release of MMAE. RS is an aggressive lymphoma, typically of DLBCL type, arising as transformation of CLL. Despite, progressive improvements in the therapy of CLL, very few effective treatment options exist for patients with RS. Using our recently established RS patient-derived xenografts (RS-PDXs), we explored the expression and signaling properties of ROR1 in RS and investigated the ex-vivo and in vivo effects of VLS-101. When assessed by flow cytometry (FACS), immunohistochemistry (IHC), and reverse-transcriptase-polymerase chain reaction (RT-PCR), 3 of 4 RS-PDXs showed ROR1 positivity (2 highly positive: 99% and 80% of cells; 1 medium/low positive: 25% of cells by FACS). The extent of ROR1 expression correlated among the 3 assays methods and was consistent with ROR1 expression data reported for non-RS DLBCL samples. When engaged by its ligand Wnt-5a, ROR1 activated downstream targets, Rac1 and RhoA, and induced phosphorylation of the p65 subunit of NF-kB and Jnk in RS cells. When cells purified from RS-PDX tumor masses were exposed to VLS-101 ex-vivo, the drug induced time- and dose-dependent apoptosis, as shown by increases in annexin V/propidium iodide and by Caspase-3 and PARP cleavage. VLS-101 efficacy was then assessed in vivo in both subcutaneous and systemic RS-PDX models. When palpable masses had formed in subcutaneous models, mi