Powder mixing in direct compression formulation by ordered and random processes.

The mixing processes taking place in a two component system involving a cohesive drug, tetracycline and a direct compression vehicle, spray dried lactose (SDL), have been studied by chemical analysis of powder samples and fluorescence microscopy. At a drug concentration level of 0.25% w/w, both random and ordered mixing is taking place. Mixture quality is better (Cv = 1%) using a fine grade of crystalline lactose (CL) than with SDL (Cv = 4%). Using fractionated material, it was found that SDL between 106 and 300 micron gave Cv values up to 12%, whereas with SDL below 106 micron values of 2% were obtained. The poor quality of the SDL mixtures is attributed to ordered unit segregation.