A pharmaceutical composition comprises, in powder form, (a) at least one water-soluble therapeutic agent selected from selective COX-2 inhibitory drugs and prodrugs and salts thereof, for example parecoxib sodium, in a therapeutically effective total amount constituting about 30% to about 90% by weight, (b) a parenterally acceptable buffering agent in an amount of about 5% to about 60% by weight, and optionally (c) other parenterally acceptable excipient ingredients in a total amount not greater than about 10% by weight, of the composition. The composition is reconstitutable in a parenterally acceptable solvent liquid to form an injectable solution. A lyophilization process is provided for preparation of such a composition.