more uniformly control the residual tertiary butyl alcohol in the freeze dried product optimum stability was achieved when freeze drying from a 17 25 tba water mixture the method for preparing a stabilized freeze dried formulation of pge 1 controls key parameters which affect product stability including the following the level of lactose diluent present the apparent ph of the lyophilized cake the moisture content the use of the co solvent tertiary butyl alcohol during processing the freezing rate and methodology prior to lyophilization the freeze drying rate and the size of the vial used to manufacture the product lyophilization of a buffered lactose formulation of pge 1 from a tertiary butyl alcohol tba water mixture provides superior product stability than when freeze drying from only an aqueous system the level of tba which afforded the product maximum stability appeared to be when the tba amount ranged from 17 25 v v the 20 tba level was selected as the amount of co solvent for the pge 1 formulation since it fell within the optimum co solvent range the lower the level of tba used also reduced the flammability potential reduced the amount of tba waste which would be generated after lyophilization reduced the level of isopropyl alcohol a process impurity in the tba in the product and lowered the precipitation potential during manufacturing for the lactose from the co solvent system stability data clearly indicated that lyophilization of the buffered lactose formulation of pge 1 from a tba.