Characterization of lymphoma models for the surface ROR1 expression

Receptor tyrosine kinase-like orphan receptor 1 (ROR1) is a developmental antigen aberrantly expressed in several B-cell malignancies and represents an attractive therapeutic target. Here, we systematically characterized ROR1 expression in a large panel of B-cell lymphoma cell lines and assessed the activity of a ROR1-targeting antibody-drug conjugate (ADC). ROR1 RNA expression was first evaluated using a previously generated total RNA-Seq dataset from 47 B-cell lymphoma cell lines. ROR1 transcripts were detectable in most models, with 19 cell lines showing moderate to high expression. To determine whether ROR1 is present on the cell surface, we next analyzed 29 cell lines by flow cytometry using a PE-conjugated anti-ROR1 antibody. Only a subset of cell lines expressed appreciable levels of ROR1 on the cell membrane, with the highest expression observed in mantle cell lymphoma (MCL) models compared with other B-cell lymphoma subtypes, and detectable expression in a few diffuse large B-cell lymphoma (DLBCL) and one marginal zone lymphoma (MZL) line. ROR1 RNA and surface protein levels were highly correlated. As a proof of principle, four ROR1-positive DLBCL cell lines and one cell line with low surface protein were exposed to the ROR1-targeting ADC zilovertamab vedotin for 120 hours. All ROR1-positive models were sensitive, with IC50 values ranging from 28 to 58 nM, whereas the remaining one was largely resistant. In summary, we defined a set of well-characterized lymphoma models suitable for preclinical studies of ROR1-directed agents, confirming the functional relevance of ROR1 surface expression for the activity of zilovertamab vedotin.

The gene coding for the receptor tyrosine kinase-like orphan receptor 1 ( ROR1 ) was initially identified while looking for genes encoding proteins with tyrosine kinase-like domains ^1,2 . It is now understood that ROR1 possesses intrinsic tyrosine kinase activity and also functions as a receptor for Wnt5a, which activates β-catenin-independent non-canonical pathways, sustaining cell survival and oncogenesis ^2,3 . In adult tissues, including normal B lymphocytes, ROR1 is preferentially expressed by cancer cells rather than normal cells and has a role in sustaining the growth and survival of neoplastic cells ^2,4 . Several therapeutic strategies targeting ROR1 have been developed, including naked monoclonal antibodies, antibody-drug conjugates (ADCs), small molecule inhibitors, bispecific T cell engager (BiTE), and chimeric antigen receptor (CAR) T cells have been developed ² , with some currently in advanced clinical evaluation ² .

Here, we characterized a vast panel of lymphoma cell lines for their ROR1 RNA level and ROR1 cell surface protein expression, and showed, using zilovertamab vedotin as proof-of-principle, that these models can be exploited to study this promising class of anti-cancer agents.