Objective

Bruton's tyrosine kinase (BTK) is crucial for B cell maturation. Its activation has also been considered as a major oncogenic driver for various B cell-derived lymphoid cancers. A BTK inhibitor, ibrutinib, has been approved for mantle cell lymphoma, chronic lymphocytic leukemia, and Waldenström's macroglobulinemia, a form of non-Hodgkin lymphoma (NHL). Ibrutinib has also shown encouraging efficacy in defuse large B cell lymphoma (DLBCL), specifically in the activated B cell-like (ABC) subtype, but not in the germinal center B cell-like (GCB) subtype¹.

We have established a series of patient-derived xenograft (PDX) models, recapitulating the diverse genotypes of de novo DLBCL patients, including two MYD88^L265P/
CD79B^Y197N double mutants (LY2298, LY2264), one MYD88^L265P single mutant (LY0257), and a variety of wild type models (LY2214, LY3604, and LY6934, etc.). This cohort of PDX models serves as the perfect preclinical tool to investigate ibrutinib response in different genotypes. Our studies demonstrated the moderate to robust efficacy of ibrutinib in the MYD88/CD79B double mutants, suggesting that CD79B mutation is a predictive biomarker for chronic activation of B cell receptor (BCR) signaling, as well as of generally good response to BTK inhibitors. Interestingly, the MYD88 single mutant model LY0257 (where the disease is likely driven by constitutive activation of MYD88 signaling) also showed response to ibrutinib when dosed in drinking water. In the wild type PDX, four of the nine models tested so far are either partially responsive or sensitive to Ibrutinib, while the others are resistant to the treatment. It seems that ibrutinib response does not always require CD79B mutation, and BCR pathway addiction may occur by other means in DLBCL, even when MYD88 signaling is constitutively active.

Notably, all 5 wild type DLBCL PDX that are non-responsive to ibrutinib are EBV positive, verified by RNAseq. These EBV-transformed B lymphoma PDX models share similar histopathology with de novo DLBCL, but have distinct molecular pathology signatures and different pathogenesis. In efficacy studies, none of the transformed B lymphoma PDX showed response to ibrutinib. Furthermore, western blot analysis of BTK phosphorylation following a single dose of ibrutinib demonstrated that ibrutinib highly inhibited the phosphorylation of BTK in de novo lymphoma, while no BTK phosphorylation and low t-BTK were found in EBV+ lymphoma.

Taken together, our DLBCL and EBV-induced lymphoma PDX models provide a valuable preclinical platform for evaluating BTK inhibitors, as well as future drug discovery efforts on other targets in the BCR and MYD88 pathways, such as PI3K, SYK, and IRAK4.