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 (<400 G/L) and leukocytes (<9.5 G/L); normalization in size or non-progression of palpable spleen; lack of major cardiovascular event during the observation period; and improvement of Myeloproliferative Neoplasm Symptom Assessment Form total symptom score (MPN-SAF TSS). MPN-SAF TSS is a 10 items questionnaire, allowing concise, valid, and accurate assessment of MPN symptom burden over time (Emanuel et al, 2012). For TSS-10 score, to qualify for response, following rules were elaborated: 10 points or higher reduction for patients with baseline score of ≥20, for patients with baseline score 15-19 – 5 points reduction, baseline scores 10-14 – reduction below 10 points, and for TSS baseline score <10 – stay <10. Bone marrow analysis is not to be a mandatory test for assessment of overall benefit of therapy. Extensive genetic workup is planned to document any change in observed genetic and chromosomal abnormalities, including level of circulating mutant CALR. This would allow for objective evaluation of IFNa’s ability to modify the disease. Long term observation, going beyond the 12 month of initial observation, is planned. Conclusions: Authors conclude that proposed endpoint scale covers well all clinically relevant aspects of ET, in order to make clinically relevant conclusions on durable benefits and risks of P1101 therapy.