PD-L1 targeting and subclonal immune escape mediated by PD-L1 mutations in metastatic colorectal cancer. AVETUX

In patients with microsatellite stable (MSS) metastatic colorectal cancer (mCRC), immune checkpoint blockade (ICB) is ineffective and combinatorial approaches enhancing immunogenicity need exploration. We treated patients with predominantly microsatellite stable, RAS/BRAF wild-type mCRC on a phase II trial combining chemotherapy with the epidermal growth factor receptor antibody cetuximab and the programmed cell death ligand 1 (PD-L1) antibody avelumab. Circulating tumor DNA showed rapid clearance in the majority of patients mirroring a high rate of early tumor shrinkage. In three of 13 patients expressing the high-affinity Fcγ receptor 3a (FcγR3a), tumor subclones with PD-L1 mutations were selected that led to loss of tumor PD-L1 by nonsense-mediated RNA decay in PD-L1 K162fs and protein degradation in PD-L1 L88S. As a consequence, avelumab binding and antibody-dependent cytotoxicity were impaired, while T cell killing of these variant clones was increased. Interestingly, PD-L1 mutant subclones showed slow selection dynamics reversing upon avelumab withdrawal and patients with such subclones had above-average treatment benefit. This suggested that the PD-L1 mutations mediated resistance to direct anti-tumor effects of avelumab, while at the same time loss of PD-L1 reduced biological fitness by enhanced T cell killing limiting subclonal expansion. Together, the addition of avelumab to standard treatment appeared feasible and safe. PD-L1 mutations mediate subclonal immune escape to avelumab in some patients with mCRC expressing high-affinity FcγR3a which appears as a subset experiencing most selective pressure by this antibody. Future randomized trials evaluating the addition of avelumab to standard treatment in MSS mCRC should selectively target this patient subpopulation.