Background: Peripheral T-cell lymphoma (PTCL) is a heterogeneous group of non-Hodgkin lymphomas associated with a poor prognosis for most subtypes. Anthracycline-based therapies such as cyclophosphamide, doxorubicin, vincristine, and prednisone (CHOP) are commonly used for frontline treatment of PTCL; however, durable remissions are uncommon with these regimens. Patients with relapsed or refractory (R/R) PTCL and platelet counts Belinostat (Beleodaq) is a potent, pan-histone deacetylase inhibitor that was recently approved in the United States for the treatment of patients with R/R PTCL. Approval was based on results from the pivotal Phase 2 BELIEF study of belinostat in R/R PTCL (N = 129 enrolled, N = 120 evaluable), which demonstrated durable clinical benefit (objective response rate [ORR] 25.8%) and tolerability. This analysis presents BELIEF study data for the subgroup of patients who had thrombocytopenia ( Methods: Patients with R/R PTCL received belinostat as a 1000 mg/m2 intravenous infusion on Days 1-5 of 21-day cycles. The primary endpoint of the study was ORR (complete response [CR] + partial response [PR]) determined by an Independent Review Committee. For this subset analysis, efficacy and safety data for the 20 evaluable patients with low baseline platelet counts were examined and compared to the overall study population. Results : For both the low baseline platelet count ( 5% of the evaluable patients with low baseline platelet counts were largely hematologic, including anemia (5.4% overall and 10.0% low platelet group), thrombocytopenia (4.7% and 15.0%), leukopenia (2.3% and 10.0%), and neutropenia (4.7% and 10.0%). Platelet counts tended to increase over time on study for the low baseline platelet subgroup (Figure). Conclusions: Complete and partial responses were seen with belinostat overall and in the subgroup of patients with low baseline platelet counts. Patients with baseline platelet counts Abbreviations: CI = confidence interval; CPRG = Central Pathology Review Group; DoR = duration of response; IRC = Independent Review Committee; ORR = objective response rate; OS = overall survival; PFS = progression-free survival; TTR = time to response Disclosures Horwitz:Celgene: Consultancy, Research Funding; Millenium: Consultancy, Research Funding; Infinity: Research Funding; Kiowa-Kirin: Research Funding; Seattle Genetics: Consultancy, Research Funding; Spectrum: Consultancy, Research Funding; Amgen: Consultancy; Bristol-Myers Squibb: Consultancy; Jannsen: Consultancy. Bhat:Spectrum Pharmaceuticals: Employment. Choi:Spectrum Pharmaceuticals: Employment. Allen:Spectrum Pharmaceuticals: Employment.