Clinical Evaluation of the European LeukemiaNet Criteria for Resistance/Intolerance to Hydroxyurea In Essential Thrombocythemia

Abstract 4086

Definition of resistance/intolerance to hydroxyurea (HU) in essential thrombocythemia (ET) has been proposed by the European LeukemiaNet (ELN) however its clinical utilility has not been validated yet. We have retrospectively evaluated such criteria in 166 ET patients treated with HU for a median of 4.5 years. Response to HU treatment was categorized using the ELN criteria. The ELN definitions of resistance/intolerance to HU required the fulfillment of at least one of the following criteria: platelet count greater than 600 × 10⁹/L after 3 months of at least 2 g/day of HU (2.5 g/day in patients with a body weight over 80 kg); platelet count greater than 400 × 10⁹/L and leukocytes less than 2.5 × 10⁹/L or hemoglobin (Hb) less than 100 g/L at any dose of HU; presence of leg ulcers or other unacceptable mucocutaneous manifestations at any dose of HU; HU-related fever. Survival and time-to-event curves were estimated using the Kaplan-Meier method.Variab les attaining a significant level at the univariate analysis were included in a Cox proportional hazards model. Overall, 134 patients achieved a complete clinicohematologic response (CR) and 25 a partial response. Thirty-three patients met at least one of the ELN criteria defining resistance (n=15) or intolerance (n=21) to HU. Fifteen cases developed anemia with thrombocytosis. Other definitions of resistance were less useful. When compared with the others, resistant patients were more likely to display hyperproliferative features at ET diagnosis, such as higher levels of leukocytes (p= 0.05), platelets (p=0.004) and serum LDH (p=0.02). Eleven patients developed leg ulcers leading to a permanent discontinuation of the drug in 8 cases. No distinctive clinical profile could be ascribed to patients exhibiting leg ulcers, with the exception of a high prevalence of cardiovascular risk factors. Other unacceptable mucocutaneous manifestations occurred in 9 patients. Hematologic and mucocutaneous complications were unrelated, with only two patients presenting both types of toxicities. With a median follow-up from ET diagnosis of 7 years (range: 0.5–23), 38 patients (23%) had died, resulting in a survival probability of 65% at 10 years from HU start. The risk of death from any cause was increased by 6.2-fold (95%CI: 2.3–16.7, P<0.001) in patients who met any of the ELN criteria of resistance to HU. Anemia was in all instances the first finding qualifying for resistance to HU, with the median subsequent survival of patients with anemia being only 2.4 years (range, 0.01–4.9). A remarkable incidence of myelofibrosis was observed in patients fulfilling the ELN criteria for resistance, since this complication was recorded in 7 of 15 such cases (p>0.001). In conclusion, the best discriminating ELN criterion of resistance to HU is based on the detection of anemia. Moreover, such criterion is particularly useful since it also identifies a subset of ET patients with a poor prognosis.