Multimodal antigenic escape to GPRC5D-targeted T cell engagers in multiple myeloma

Tumor-intrinsic adaptations with emerging resistant clones following T cell-targeted immunotherapies pose a major barrier to durable remissions in multiple myeloma. Through integrated genomic, transcriptomic and epigenomic interrogation of clonal plasma cells, we observed antigenic drift in 68.4% of relapsed cases following anti-GPRC5D T cell-engager (TCE) therapy (n = 21). These escape events were driven by three distinct mutational mechanisms involving (1) focal to large biallelic deletions at the GPRC5D gene locus; (2) monoallelic deletion coupled with GPRC5D single-nucleotide variants or insertions/deletions (indels) on the remaining allele; as well as (3) epigenetic GPRC5D promoter/enhancer silencing. Beyond biallelic deletions resulting in complete antigenic loss, we demonstrate that GPRC5D single-nucleotide variants and indels mutate anti-GPRC5D TCE-binding epitopes or more commonly affect G-protein-coupled receptor family conserved motifs critical for protein membrane trafficking resulting in endoplasmic reticulum GPRC5D trapping. Multiple subclones bearing distinct genomic alterations at GPRC5D locus co-emerged within individual cases, depicting their convergent evolutionary trajectories. Of note, anti-GPRC5D TCEs with varying epitope specificity, affinity and valency differentially targeted mutant subclones, underscoring their nonredundant functional roles in overcoming resistance. Genomic and transcriptomic analysis of samples from patients with multiple myeloma, followed by in vitro validation, indicate mechanisms of antigen escape in response to GPRC5D T cell-engager talquetamab, including biallelic deletions, small nucleotide variants, insertion-deletions and chromatin silencing.