RESTORE & TARGET: a CONCEPTUALLY NOVEL TREATMENT APPROACH TO CLASSICAL HODGKIN'S LYMPHOMA

Introduction: Irrespective of its genomic B-cell origin, classical Hodgkin's lymphoma (cHL) is characterized by the virtual lack of gene products whose expression constitutes the B-cell phenotype. Epigenetic repression of B-cell-specific genes was previously postulated to contribute to the lost B-cell phenotype in cHL. Restoration of the B-cell phenotype may not only revert a hallmark of cHL but provide a new Achilles' heel by sensitizing cHL to clinically established antibody therapies targeting B-cell surface receptors as well as small compounds interfering with B-cell receptor (BCR) signaling. Methods: We engineered cHL cell lines to carry a CD19 reporter, and conducted a high-throughput pharmacological screening with more than 28,000 compounds to identify drugs that promote re-expression of the B-cell phenotype. Results: We found three chemicals to robustly enhance CD19 transcription. Since two of them reportedly interfere with epigenetic regulators, we performed chromatin immunoprecipitation assays, showing that these compounds lowered transcriptionally repressive lysine 9-trimethylated histone H3 (H3K9me3) levels at the CD19 promoter. Inhibition of the H3K9-methyltransferase EHMT2, a possible target structure of these two compounds, by BIX-01294 or shRNA-mediated knockdown resulted in increased CD19 transcript levels, suggesting that EHMT2 might be involved in repression of the B-cell phenotype in cHL. Furthermore, the anti-leukemic and differentiation-promoting agents arsenic trioxide (ATO) and all-trans retinoic acid (ATRA), both not part of the screened library, were found to reconstitute the silenced B-cell transcriptional program and impair viability of cHL cell lines. In combination with a screening-identified chemical, ATO evoked re-expression of the CD20 surface receptor, which could be therapeutically exploited by enabling CD20 antibody-mediated direct apoptosis and antibody-dependent cellular cytotoxicity of Hodgkin cells. Even more strikingly, restoration of the B-cell phenotype profoundly sensitized an expanded panel of eight cHL cells towards the B-cell Non-Hodgkin's lymphoma-tailored small compound inhibitors of BCR signaling, ibrutinib and idelalisib, where dramatic death rates were observed after priming with two restore agents. Further investigations, including pre-clinical mouse trials, are currently in progress and will be reported at the meeting. Conclusions: In essence, we present here a novel “Restore & Target” strategy that builds on the re-expression of a lost tumor cell phenotype in the first place followed by it specific exploration as druggable vulnerability in a subsequent step. Such a strategy would expand the arsenal of treatment options for cHL by a “chemo-free” combination, and might not only be of interest for relapsed or refractory patients. Keywords: B-cell receptor (BCR); CD20; Hodgkin lymphoma (HL).