TREATING AGENT FOR DECREASING BLOOD PLATELET

PURPOSE:To provide a highly safe thrombocytosis-treating agent containing a 3(2H)-pyridazinone derivative as an active ingredient and having an excellent activity against platelet excess. CONSTITUTION:The agent contains a compound of the formula (R , R are H, methyl, ethyl, normal propyl, isopropyl, tertiary butyl, etc.; X is Cl, Br) in an amount of 0.1-9.95wt.%, preferably 0.5-95wt.%, as an active ingredient. The agent can be prepared in the form of tablets, capsules, granules, injections, liquid agents, emulsions, suspensions, syrups, aerosol agent, ointments, etc., by conventional methods. The agent is administered in a dose of 1-1500mg for an adult per day. The compound of the formula includes 4-bromo-5-(3- ethoxy-4-methoxybenzylamino)-3(2H)-pyridazinone. The agent is useful for preventing and treating diseases due to abnormal increase in the number of blood platelets, diseases caused or developed by activated blood platelets, such as thrombocytosis, chronic myelocytic leukemia or various thrombosis.