Background
Tazemetostat (EPZ-6438) is a selective inhibitor of the histone methyltransferase EZH2, currently in clinical development for non-Hodgkin lymphoma and genetically defined tumors.
Procedures
Tazemetostat was tested against the PPTP solid tumor xenografts using a dose of 400 mg/kg administered twice-daily by oral gavage for 28 days. H3K27me3:H3 ratios were determined in control and treated tumors.
Results
Tazemetostat induced significant differences in event free survival (EFS) distribution compared to control in 9 of 30 (30%) of the xenografts studied. Significant differences in EFS distribution were observed in 5 of 7 (71%) rhabdoid tumor xenograft lines compared to 4 of 23 (17%) non-rhabdoid xenograft lines [chi square (χ2) test p=0.006]. Tazemetostat induced tumor growth inhibition meeting criteria for intermediate and high EFS treated to control (T/C) activity in 2 of 25 (8%) and 1 of 25 (4%) xenografts, respectively. Intermediate and high activity for the EFS T/C metric was observed exclusively among rhabdoid tumor xenografts (3 of 5 rhabdoid tumor versus 0 of 22 non-rhabdoid tumors (χ2 test p<0.001). One rhabdoid tumor xenograft (G401) showed stable disease. For one rhabdoid tumor (G401), delayed tumor regression to tazemetostat was noted following 1 week of tumor growth. Tazemetostat induced significant reduction of H3K27me3 levels in the majority of tumors compared to controls.
Conclusions
Tazemetostat demonstrated significant antitumor activity in rhabdoid tumor models, but showed no consistent activity against any other histology. Tazemetostat reduced H3K27me3 levels irrespective of tumor response. Further preclinical testing to evaluate tazemetostat in combination with other anticancer agents is warranted.