Introduction: Mutations in TNFRSF17 extracellular domain were recently reported to mediate escape to anti-BCMA bispecific T cell engagers (TCE). The prevalence of these mutations and how they impact other anti-BCMA therapies remain undefined. Furthermore, it is unclear whether these mutations affect BCMA canonical ligand binding and signaling. We here investigated the impact of hotspot mutations in BCMA extracellular domain on the efficacy of anti-BCMA therapies and the emergence of oncogenic dependencies.
Methods: In order to define the frequency of TNFRSF17 alterations post anti-BCMA therapies and interrogate their functional significance, we collected CD138+ cells from patients treated with anti-BCMA CAR T/ TCE, and subjected the cells to WGS or single cell CNV analysis. Identified BCMA mutations were cloned and transduced into K562 myelogenous cell line for in vitro functional evaluation.
Results: Recipients of anti-BCMA therapy (CAR T n=5, TCE n=16, both n=3) were included in this analysis. In patients progressing on anti-BCMA CAR T, only one (20%) had biallelic deletion of TNFRSF17 with no BCMA extracellular domain mutations. In contrast, among patients progressing on anti-BCMA TCE (n=14), mutations in TNFRSF17 were observed in 42.8% including TNFRSF17 biallelic loss (n=1) and extracellular domain mutations (n=5). Monoallelic TNFRSF17 deletion were coupled with p.Arg27Pro in one case, while p.Pro34del, or p.Ser30del were observed in 3 and 2 patients, with one patient harboring both mutations, consistent with convergent evolution. None of these mutations were identified in pre-therapy samples using WGS (100x) and digital PCR (0.1% sensitivity). Lentivirally transduced K562 cells were used to characterize the impact of BCMA mutations on the efficacy of various TCEs and cross resistance to CAR T. Of interest, while these mutations differentially impacted the sensitivities of TCEs based on their geometry and valency, cross resistance was not carried out to CAR T cells. As such, while p.Arg27Po, p.Ser30del, and p.Pro34del abrogated teclistamab mediated T cell killing, T cells transduced with teclistamab based scFv CAR retained cytolytic activity. Therefore, TCE resistance derived from BCMA extracellular domain mutations does not preclude retreatment with another anti-BCMA TCE or CAR T. Functionally, while BCMA extracellular domain mutations abrogated the binding of most anti-BCMA TCEs, they did not impair the binding of Fc-tagged APRIL, p65 nuclear translocation, and ERK phosphorylation.
Conclusions: Collectively, our studies reveal that convergent evolution of MM clones harboring BCMA extracellular domain mutations selectively evade recognition by anti-BCMA TCEs while retaining the pro-survival signaling of the APRIL-BCMA-NFkB axis. Furthermore, these hotspot mutations do not equally confer resistance to all anti-BCMA TCEs or CAR Ts. Dynamic surveillance and functional evaluation of emerging mutant clones will be required to inform and guide therapy selection and sequence.
