Timing genomic antigen loss in multiple myeloma treated with T-cell redirecting immunotherapies.

Genomic antigen loss is a recurring mechanism of resistance to chimeric antigen receptor T-cell (CAR-T) and T-cell engagers (TCE) in relapsed/refractory multiple myeloma (RRMM). Yet, it remains unclear whether these events are acquired under treatment or merely selected from pre-existing, undetectable clones. By leveraging chemotherapy mutational signatures as temporal barcodes within whole genome sequencing data, we could time genomic antigen escape in 4 out of 11 RRMM patients. In all cases, the biallelic loss was driven by genomic events acquired after exposure to BCMA- and GPRC5D-targeted CAR-T/TCE, and not present at baseline. Longitudinal digital PCR analysis corroborated that resistance mutations were undetectable at therapy initiation but emerged preceding relapse. Among 752 newly diagnosed patients only 2.7% and 9% had monoallelic inactivation of TNFRSF17 and GPRC5D, respectively, with no biallelic loss. Our findings suggest limited utility of mutational screening prior to CAR-T/TCE, while underscoring the importance of dynamic surveillance during therapy.