Highly Sensitive Detection Method Reveals Significant Anti-Tumor Activity of Valemetostat in Patients with Relapsed/Refractory Adult T Cell Leukemia

Background: Enhancer of zeste homolog 2 (EZH2) and its close homolog, EZH1, catalyze the attachment of 3 methyl groups to histone H3 at lysine 27 (H3K27me3). H3K27me3 is an epigenetic mark involved in downregulating gene expression associated with tumor suppression and cell differentiation. Adult T-cell leukemia/lymphoma (ATL) is an aggressive non-Hodgkin lymphoma (NHL) subtype that arises from CD4 positive T cells associated with human T-lymphotropic virus type 1 (HTLV-1) infection. During asymptomatic period, genetic and epigenetic abnormalities in the HTLV-1 infected cells accumulates and finally leads to ATL development. Current therapy for aggressive type ATL is still unsatisfactory and prognosis of relapsed and refractory (r/r) ATL after lenalidomide or mogamulizumab treatment is poor. Dysregulation of EZH2/PRC2-mediated epigenome status has been reported to be highly involved in tumorigenesis in ATL as well as other tumors such as B cell lymphoma. Valemetostat is a potent dual inhibitor of EZH1 and 2 which are key catalytic subunits of PRC2. The safety and efficacy of valemetostat in patients with r/r ATL were evaluated in a multicenter, single-arm, open-label phase 2 study (NCT04102150) and the results were presented at ASH 2021. Here we show clinical biomarker data using peripheral blood obtained over time, accompanying clinical outcomes. Also we discuss relevance of the measured data in blood to clinical outcomes among assay formats.

Method: In the Ph2 study, valemetostat 200mg was orally administrated once daily in continuous 28-day cycles until disease progression or intolerance. Nine of 25 patients in the Ph2 study had abnormal lymphocytes in their peripheral blood. Peripheral blood from these 9 patients (7 acute and 2 unfavorable chronic) was drawn at pre-dosing and every 4-weeks for the analysis of this study. Abnormal lymphocytes (Ably) and soluble IL-2R (sIL-2R) are indicator of ATL prognosis. For HTLV-1 proviral load (PVL) detection, genomic DNA was extracted from PBMC from each patient. Droplet Digital PCR (ddPCR) was performed using probes specific for HTLV-1 provirus and RNase 5’ pyrophospgohydrolase (RPPH) gene. % PVL was determined from the copy number ratio of HTLV-1 to RPPH1 genes. For HTLV-1 Analysis System (HAS)-flow cytometry, frozen cell suspensions were thawed and stained with anti- CD3/4/7/14/CADM1 Abs. The data from live T cells was analyzed by CD7 versus CADM1 plot. CADM1⁺CD7^- population ratio was denoted as %N. We analyzed changes of each measured parameter in every patient in comparison with clinical outcome.

Result: In the 9 patients who met criteria in blood collection for blood biomarker evaluation at screening, 1 CR, 6 PRs and 1 SD were observed as the best response by blood assessment (not evaluable in one patient, who failed to meet criteria for blood assessment at pre-treatment evaluation). Significant decrease in Ably number and blood sIL-2R concentration was observed after 4-week valemetostat treatment in all 9 patients and this decrease was maintained during the treatment until PD. Valemetostat also decreased the %PVL and %N in HAS-flow. The reduction trends were weaker in comparison with Ably and sIL-2R. As for %PVL, most of the patients experienced modest reduction and sustained around 50%. Especially HAS-flow results exhibited two patient groups with either lower (<25%) or higher %N at pre-dosing. Two patients in the lower %N group showed CR or PR, and in overall response the remaining 7 patients with higher %N showed SD or PR.

Conclusion: Valemetostat significantly reduced markers of ATL progression. Ably and sIL-2R to very low levels in all patients. In contrast, PVL assay did not exhibit complete %PVL reduction. HAS-flow showed variable %N at pre-dosing and the patients with higher %N exhibited SD or PR throughout the treatment in overall response and blood assessment. These data suggest that PVL assay and HAS-flow can detect ATL cells with a high resolution, which cannot be detected by Ably and sIL-2R, when valemetostat has significant therapeutic efficacy in ATL patients. Further analyses, such as gene mutation, transcription, chromatin state and histone methylation, using the collected blood samples will be performed next.

Takayama:Daiichi Sankyo Co., Ltd: Current Employment. Hizukuri:Daiichi Sankyo Co., Ltd: Current Employment. Fujioka:Daiichi Sankyo Co., Ltd: Current Employment. Hashimoto:Daiichi Sankyo Co., Ltd: Current Employment. Ito:Daiichi Sankyo Co., Ltd: Current Employment. Yamada:Daiichi Sankyo Co., Ltd: Current Employment. Yamagishi:Daiichi Sankyo Co., Ltd: Honoraria, Research Funding; Bristol Myers Squibb: Research Funding; Astellas Pharma Inc.: Honoraria. Nakashima:Takeda Pharmaceutical Co., Ltd,: Honoraria, Research Funding. Suzuki:Daiichi Sankyo Co., Ltd: Research Funding. Nannya:Takeda Pharmaceutical Company: Speakers Bureau; Pfizer: Speakers Bureau; Chugai Pharmaceutical: Speakers Bureau; Sumitomo Pharma: Speakers Bureau; Astrazeneca: Speakers Bureau; Novartis: Consultancy, Membership on an entity's Board of Directors or advisory committees; Fuji Pharma: Honoraria; Kyowa-Kirin: Speakers Bureau; Asahi Kasei Pharma: Speakers Bureau; Nippon Shinyaku: Speakers Bureau; Janssen Pharmaceutical: Speakers Bureau; Filgen: Speakers Bureau; Otsuka Pharmaceutical: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Bristol Myers Squibb: Speakers Bureau; Daiichi Sankyo RD Novare: Research Funding; Daiichi Sankyo Co., Ltd: Research Funding. Uchimaru:Daiichi Sankyo Co., Ltd: Research Funding; Bristol Myers Squibb: Honoraria, Research Funding; Takeda Pharmaceutical Co., Ltd,: Honoraria, Research Funding; Kyowa Kirin Co., Ltd: Honoraria; Meiji Seika Pharma Co., Ltd.: Honoraria. Tsutsumi:Daiichi Sankyo Co., Ltd: Current Employment.