Proposal of an endpoint for a phase III clinical study of essential thrombocythemia: Balancing between short term effects and long term benefits

7055 Background: Essential thrombocythemia (ET) is a chronic myeloproliferative neoplasm (MPN), covering broad spectrum of clinical scenarios, from asymptomatic patients with only isolated high platelets to highly morbid patients in late stage disease. Due to heterogenous patient population, designing clinical studies in ET is difficult, and after anagrelide there was no new drug approval to treat ET during the past 15 years, thus, high unmet medical to treat patients with ET remains. Interferons alpha (IFNa) are known to have beneficial effects in MPN (Kiladjian et al, 2016). P1101 is a next generation monopegylated IFNa, developed specifically to treat MPNs, including ET. Methods: External published clinical data in ET were analyzed to design optimal clinical study. Proposed endpoints are meant to cover all relevant clinical aspects of ET, and suffice for a regulatory relevant pivotal clinical study. Results: Composite primary endpoint scale is based on modified ELN criteria. Short term study endpoints should have clinical meaningfulness at time of measurement but also predict the later outcomes. The scale consists of: normalization of platelets (