A phase 3b, multicenter, open-label extension study of the long-term safety of anagrelide in Japanese adults with essential thrombocythemia

Cytoreductive therapy is used in high-risk essential thrombocythemia (ET) to reduce risk of thrombohemorrhagic complications. Anagrelide is an orally active, quinazolone-derived platelet-lowering agent approved for first-line treatment of high-risk ET in Japan. Long-term safety and efficacy data were collected from 53 Japanese high-risk ET patients (Study 308); 41 patients who completed Study 308 entered this phase 3b, open-label extension (Study 309; NCT01467661). Reductions in mean platelet counts occurred throughout the study, from 1021.6 × 10⁹/L (at Study 308 baseline) to 675.4 × 10⁹/L at final assessment. At month 48 (since Study 308 enrollment), mean platelet count was 444.5 × 10⁹/L in the 10 patients who completed 4 years of therapy. Overall, platelet counts decreased from 1088.3 × 10⁹/L at Study 308 baseline (n = 33) to 473.5 × 10⁹/L at final assessment (n = 31). Long-term platelet count reductions were maintained without marked changes in mean anagrelide dose. Anagrelide was generally well tolerated, with anemia (54.7%) and headache (49.1%) as the most frequent adverse events. These findings indicate that anagrelide effectively reduces platelet counts in high-risk Japanese ET patients, with titration resulting in a well-tolerated, effective and sustainable dose. In conclusion, these results support anagrelide administration to high-risk Japanese ET patients using individualized dosing strategies defined in instructions previously approved in Europe and the USA.