Abstract only 8506 Background: Mogamulizumab (Moga), a defucosylated humanized anti-CCR4 antibody, was approved for the treatment of relapsed/refractory ATL in Japan in 2012. This multicenter, randomized, phase II trial was conducted to examine the efficacy of the combination of Moga with standard chemotherapy for untreated aggressive ATL. Methods: Previously untreated patients (pts) with CCR4-positive ATL were randomly assigned to receive mLSG15 plus Moga (arm A) or mLSG15 alone (arm B). The primary endpoint was CR rate (%CR), and secondary endpoints included ORR, PFS, OS and safety. Pts received 4 courses of mLSG15 regimen, with or without a total of 8 doses of Moga (1.0 mg/kg) once every 2 weeks. The planned sample size, 22 pts per arm, provided a probability of 80% that %CR in arm A would have larger %CR when true %CR for arm A is 15% better than that for arm B. Results: Of 54 pts randomized, 53 were treated (arm A: 29; arm B: 24). Male/female ratio was 53/47%, median age was 63 (37-81), and subtype was acute/lymphoma/unfavorable chronic, 70/25/6%. %CR and ORR in arms A and B was 52% (95%CI CI; 33, 71) vs. 33% (CI; 16, 55) and 86% (CI; 63, 96) vs. 75% (CI; 53, 90), respectively. The results in arm B were similar to the previously reported %CR of 40% and ORR of 72% with mLSG15 (Tsukasaki et al, JCO 2007). ORR according to the disease subtype, in arms A and B, was 55% vs. 29% for acute, 50% vs. 43% for lymphoma and 33% vs. 0% for unfavorable chronic. Median PFS was 259 days (CI; 197, -) for arm A and 192 days (CI; 147, -) for arm B. Median OS was not reached in both arms. The most common treatment-related AEs in each arm were neutropenia (100%, 96%), thrombocytopenia (100%, 96%), leukopenia (100%, 92%), lymphocytopenia (97%, 96%), anemia (97%, 92%) and febrile neutropenia (90%, 88%). In arm A, skin disorders were more frequent but manageable, and no serious skin disorder like Stevens-Johnson syndrome was observed. There was one treatment-related death, which was not related to Moga. Conclusions: The combination of Moga with mLSG15 was well tolerated and the study met its primary endpoint. These results suggest that Moga with mLSG15 is a rational treatment option for newly diagnosed aggressive ATL. Clinical trial information: NCT01173887.