Polymorphic forms of Compound (1) or a pharmaceutically acceptable salt thereof, wherein Compound (1) is represented by the following structural formula:are Form A of HCl salt of Compound (1)·1/2H2O, Form F of HCl salt of Compound (1)·3H2O, Form D of HCl salt of Compound (1), Form A of Compound (1), and Form A of tosylate salt of Compound (1). Such polymorphic forms are employed for treating influenza, inhibiting the replication of influenza viruses, or reducing the amount of influenza viruses in a biological sample or in a subject.
