Phase I:~Up to 18 patients will be recruited, using a conventional dose-escalation algorithm (3+3 patients per dose level) to identify the maximum tolerated dose (MTD) which will be deemed the RP2D. Patients will receive 8 cycles of RCHOP every 21 days and tazemetostat every day, starting on day 2 of cycle 1.~4 cohorts are defined, according to dose levels of tazemetostat: 400mg Twice a day (BID) (cohort 1, starting level), 600mg BID (cohort 2), 800mg BID (cohort 3), 200mg BID (cohort -1), depending on the observed toxicities.~Phase II:~Up to 184 patients (122 DLBCL and 62 FL) will be recruited and treated with tazemetostat at the MTD and RCHOP.~Patients will receive 6 cycles of RCHOP every 21 days and tazemetostat at the MTD every day, starting on day 2 of cyle 1, + 2 cycles of Rituximab+tazemetostat. For FL, a maintenance of tazemetostat (6 months) + rituximab (24 months) is expected
Phase I of the study is designed to determine the recommended phase II dose (RP2D) for tazemetostat in patients treated with 8 cycles of R-CHOP 21.~Phase II of the study is designed to determine the safety and the efficacy of tazemetostat in DLBCL and FL patients :~DLBCL : tazemetostat with 6 cycles of R-CHOP 21 + 2 cycles of Rituximab FL : tazemetostat with 6 cycles of R-CHOP 21 + 2 cycles of Rituximab then maintenance with 6 months of tazemetostat and 24 months of Rituximab