A method of manufacturing a flexible container housing a drug substance is disclosed that includes forming a first compartment from a flexible sheet-like material, filling a liquid into the first compartment, sealing the first compartment, forming a second compartment from the flexible sheet-like material, filling a dry drug formulation into the second compartment, and sealing the second compartment. The method further involves lyophilizing the drug formulation inside a tubular cartridge, filling the second compartment by introducing the tubular cartridge through an opening of the second compartment such that an open end of the tubular cartridge is positioned distant from the opening of the second compartment, providing the drug formulation from the open end of the tubular cartridge into the second compartment, and withdrawing the tubular cartridge from the second compartment. The first and second compartments are separated by a frangible seal which opens when the first compartment is compressed.