A stable solid pharmaceutical composition consisting essentially of an effective amount of a salt of formula (II) ##STR00001## together with an alkaline-reacting component maintaining the pH preferably above 8, or a salt with a divalent metal cation; and at least one pharmaceutical excipient; said salt of formula (II) being essentially stable during storage at room temperature for a period of at least 3 years. A process for stabilizing the salt of formula (II). A crystalline salt of formula (II) and a process for preparing said salt.