Durable Responses with Pembrolizumab in Relapsed/Refractory Mycosis Fungoides and Sézary Syndrome: Final Results from a Phase 2 Multicenter Study

Background: Current treatment of advanced stage Mycosis fungoides (MF) and Sézary syndrome (SS) remains unsatisfactory. Complete responses (CR) are typically <10% and partial responses (PR) tend to be short-lived. Antibodies targeting PD1 expressed by healthy tumor-infiltrating T cells can induce long-lasting and deep remissions in cancer patients by activating the host immune response. However, the malignant T-cells of MF/SS also express PD-1. Additionally, genomic alterations involving PD-1, PD-L1, and PD-L2 have been reported in MF/SS, suggesting an important role for the PD-1/PD-L1 axis. We hypothesized that PD-1 checkpoint blockade would be an effective treatment in targeting a T cell malignancy that itself expresses PD-1. Here, we report the efficacy of pembrolizumab in relapsed/refractory MF/SS and correlative biomarker studies.