PTCL with T-follicular helper phenotype (PTCL-TFH) has recurrent mutations affecting epigenetic regulators, which may contribute to aberrant DNA methylation and chemoresistance. This phase 2 study evaluated oral azacitidine (CC-486), a DNA methyltransferase inhibitor, plus CHOP as initial treatment for PTCL (ClinicalTrials.gov - NCT03542266). CC-486 at 300 mg daily was administered for 7 days prior to C1 of CHOP, and for 14 days before CHOP C2-6. The primary endpoint was end-of-treatment CR. Secondary endpoints included ORR, safety and survival. Correlative studies assessed mutations, gene expression and methylation in tumor samples. Grade 3-4 hematologic toxicities were mostly neutropenia (71%), with febrile neutropenia uncommon (14%). Non-hematologic toxicities included fatigue (14%) and GI symptoms (5%). In 20 evaluable patients, CR was 75%, including 88.2% for PTCL-TFH (n=17). At a median follow-up of 21 months, 2-yr PFS was 65.8% for all and 69.2% for PTCL-TFH, while 2-yr OS was 68.4% for all and 76.1% for PTCL-TFH. The frequencies of the TET2, RHOA, DNMT3A, and IDH2 mutations were 76.5%, 41.1%, 23.5% and 23.5%, respectively, with TET2 mutations significantly associated with CR (p=0.007), favorable PFS (p=0.004) and OS (p=0.015), and DNMT3A mutations associated with adverse PFS (p=0.016). CC-486 priming contributed to the reprograming of the tumor microenvironment by upregulation of genes related to apoptosis (p<0.01) and inflammation (p<0.01). DNA methylation did not show significant shift. This safe and active initial therapy regimen is being further evaluated in the ALLIANCE randomized study A051902 in CD30-negative PTCL.