Physical and chemical changes of medicinals in mixtures with adsorbents in the solid state. III. Determination of vapor pressure of solid drugs by steam distillation

The vapor pressures of solid drugs were determined by the steam distillation method. Experiments using a series of benzoic acid derivatives indicated that this method offered consistent and satisfactory values for vapor pressure at about 100 degrees C. The vapor pressures of two nonsteroidal antiinflammatory drugs, flufenamic acid (FFA) and mefenamic acid (MFA), were found to be 3.8 x 10(-3) and 1.8 x 10(-4) mmHg, respectively. These values explain the difference in the rate of change to the amorphous state between mixtures of each with magnesium aluminum silicate (MAS) when they were stored under reduced pressure. It was concluded that the vapor pressures of solid drugs at about 100 degrees C, as determined by steam distillation, may be suitable indices for predicting whether or not these drugs have an inherent propensity to become amorphous readily in mixtures with an adsorbent.