The influence of water content and drug solubility on the formulation of pellets by extrusion and spheronisation.

The influence of drug solubility in the range 14.3-1000 gl-1 on the formation of pellets by extrusion and spheronisation has been investigated by evaluating the performance of a series of model drugs mixed with an equal part by weight of microcrystalline cellulose. The optimum formulation in terms of pellet roundness and the maximum quantity within a limited size range was established by preparing samples with a range of water levels. The range of water levels over which pellets could be formed was found to be dependent on the model drug and its particle size. In general the force necessary to extrude the wet mass through the ram extruder was found to decrease as the quantity of water added increased. The optimum water level required to form the best quality pellets was found to decrease as a linear function of the natural logarithm of the water solubility of the drug. If allowance is made for the loss of solid by dissolution of the drug, there is an increase in the apparent water content necessary to form good spheres above a critical solubility between 350 and 400 gl-1.