The purpose of this study is to test the efficacy of the combination of the study drugs tazemetostat with mosunetuzumab and see what effects it has on untreated follicular lymphoma. An additional purpose of this study is to understand what side effects people may have when they receive both drugs together.

Tazemetostat is a drug that inhibits EZH2, an enzyme known to drive the development of B-cell lymphomas, and inhibiting it appears to have many effects that slow down lymphoma growth and enhance the immune system’s ability to fight it. Tazemetostat is approved by the U.S. Food and Drug Administration (FDA) in previously treated follicular lymphoma, but is not yet approved for untreated follicular lymphoma and currently undergoing study in other lymphomas. 

Mosunetuzumab is a bispecific antibody therapy that is a therapeutic strategy that uses the immune system to fight lymphoma, called immunotherapy. Bispecific antibodies have two ends: one attaches to T cells in the immune system and the other attaches to lymphoma cells, helping guide our immune system to attack the cancer. Mosunetuzumab is approved by the FDA to be given intravenously (directly into a vein) but is not yet approved by the FDA as an injection under the skin, which is how it is given in this study.

The combination of these two drugs is not yet approved by the FDA.

Participants will take tazemetostat by mouth in addition to mosunetuzumab injections. Mosunetuzumab injections will occur in clinic, and participants will continue to take the pill tazemetostat at home. All study participants will receive the same doses of the study drugs.

Total study participation will last approximately 2 years.