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 Sezary syndrome (SS) remains unsatisfactory. Complete responses (CR) are typically Methods: This single-arm, multicenter study by the Cancer Immunotherapy Trials Network (CITN) enrolled 24 patients with MF/SS stages IB-IV, with at least one prior systemic therapy. Pembrolizumab was administered at 2 mg/kg every 3 weeks for up to two years. The primary endpoint was overall response rate (ORR) using global response criteria according to the ISCL/EORTC consensus guidelines. Skin responses were measured by mSWAT. Secondary endpoints were safety/tolerability, time to response (TTR), duration of response (DOR) and progression-free survival (PFS). Correlative studies included immunohistochemistry (IHC), mass cytometry, whole exome sequencing, gene expression profiling, and serum cytokine analysis. Results: Patients had advanced stage disease (23/24 with stage IIB-IV MF/SS), and were heavily pretreated (median of 4 prior systemic therapies). The ORR was 38% with 2 CR and 7 PR. Of the 9 responding patients, 6 had ≥90% improvement in skin disease by mSWAT. The median TTR was 11 weeks. Responses were durable, with 8 of 9 responses ongoing at last follow up (median DOR 64 weeks, range 32-153 weeks). One responding patient progressed 2 months after discontinuing treatment due to an adverse event (AE). The median PFS was not reached. Overall, the toxicity profile was similar to prior studies of pembrolizumab. Four patients discontinued treatment due to treatment related serious AEs of duodenitis, pneumonitis, hepatitis, and corneal ulcer. Skin flare reactions were observed early in the treatment course in 40% of patients with SS, but none with MF. The skin flare reactions did not result in any treatment discontinuation, and did not correlate with subsequent response to treatment. There was no significant association between response and clinical characteristics including stage, disease type (MF vs. SS), or number of prior therapies. IHC assessment of PD-1, PD-L1, and PD-L2 did not predict response. Treatment resulted in an increase of PD-L1 expression by both IHC and nanoString analysis. A nanoString18 gene signature of tumor inflammation that is predictive of response to pembrolizumab in other tumor types was not predictive in this cohort. High dimensional mass cytometry enabled precise identification and phenotyping of malignant T cells. There was a positive correlation between PD-1 expression on the malignant T cells and development of the skin flare reaction. Whole exome sequencing revealed genomic disruptions of PD-1 signaling including copy loss of PD-1. No associations were found between outcomes and genomic events involving the PD1/PD-L1 axis, total mutation number, or neoantigen numbers. Conclusions: The 38% ORR rate in this heavily pretreated population was 38% with a highly encouraging DOR of 64 weeks. No predictive biomarkers have emerged thus far, but additional studies are ongoing. Additional combination studies with pembrolizumab are warranted to improve response rates. Disclosures Khodadoust: Innate Pharma: Research Funding. Porcu: Innate Pharma: Consultancy. Foss: Miragen: Consultancy, Speakers Bureau; Seattle genetics: Consultancy; Spectrum: Consultancy; Mallinkrodt: Consultancy. Moskowitz: Bristol Myers-Squibb: Consultancy, Research Funding; Seattle Genetics: Consultancy, Honoraria, Research Funding; Incyte: Research Funding; Merck: Research Funding; Takeda: Honoraria; ADC Therapeutics: Research Funding. Shustov: Seattle Genetics: Research Funding. Sokol: Seattle Genetics: Consultancy; Mallinckrodt Pharmaceuticals: Consultancy; Spectrum Pharmaceuticals: Consultancy. Yearley: Merck: Employment. Horwitz: Mundipharma: Consultancy; Celgene: Consultancy, Research Funding; Seattle Genetics: Consultancy, Research Funding; ADC Therapeutics: Consultancy, Research Funding; Kyowa-Hakka-Kirin: Consultancy, Research Funding; Forty Seven: Consultancy, Research Funding; Millennium/Takeda: Consultancy, Research Funding; Infinity/Verastem: Consultancy, Research Funding; Aileron Therapeutics: Consultancy, Research Funding; Portola: Consultancy; Trillium: Consultancy; Innate Pharma: Consultancy; Spectrum: Research Funding; Corvus: Consultancy. Kim: miRagen: Research Funding; Horizon Pharma: Consultancy, Research Funding; Neumedicine: Consultancy, Research Funding; Takeda: Membership on an entity9s Board of Directors or advisory committees, Research Funding; Galderma: Research Funding; Soligenix: Research Funding; Eisai: Membership on an entity9s Board of Directors or advisory committees, Research Funding; Tetralogic: Research Funding; Seattle Genetics: Membership on an entity9s Board of Directors or advisory committees; Portola: Research Funding; Merck: Research Funding; Medivir: Membership on an entity9s Board of Directors or advisory committees; Kyowa-Kirin-Pharma: Membership on an entity9s Board of Directors or advisory committees, Research Funding; Innate Pharma: Consultancy, Membership on an entity9s Board of Directors or advisory committees, Research Funding; Forty Seven Inc: Research Funding.