Solid Dispersion Research Paper

1.4. SOLID DISPERSIONS

The term solid dispersion (SD) refers to a group of solid products consisting of at least two different components, generally a hydrophilic matrix and a hydrophobic drug. The matrix can be either crystalline or amorphous, whereas the drug can be dispersed molecularly, in amorphous particles (clusters), or as crystalline particles. Based on their molecular arrangement, six different types of SD can be distinguished as shown in Table II (19).

Table II: Types of solid dispersions (19)

Solid dispersion type Matrix

* Drug

** Remarks No. phases

I Eutectics C C The first type of solid dispersion prepared. 2

II Amorphous precipitations in a crystalline matrix C A Rarely encountered. 2

III Solid solutions Continuous

Which will prevail depends mainly on the composition and preparation method of solid dispersions. The molecular dispersion is the last stage in the process of particle size reduction. In this way, the surface area of the particle available for dissolution is increased to the maximum. Consequently, the rate of dissolution and solubility of the drug increases (19). The specific surface area of a drug can be increased by using a porous carrier in the process of solid dispersion making, on which the substance can be absorbed (20). Better wetting is a large contribution to improvement of dissolution that appears both in the media with surface activity, as well as in those without it, because each drug particle is completely surrounded with a water soluble carrier, which in contact with water dissolves rapidly (21). In solid dispersion the drug is often in an amorphous state. Less power is required for the dissolution of the active ingredient in an amorphous state, because it is not consumed for degradation of the crystalline structure during the process of dissolution (19, 22, 23). Furthermore, the lack of aggregation and agglomeration of crystals of pure hydrophobic drugs plays an important role on increasing the dissolution rate. It is surrounded by a carrier in a solid dispersion, which prevents convergence and aggregation of particles. The dissolution rate

The reasons for that are poorly defined, thus expensive manufacturing methods, which involve use of high temperatures or large amounts of organic solvents, low reproducibility of production, problems in manufacturing adequate dosage forms, limitations on the increasing of production batches and the physical and chemical instability might be among the reasons. The difficulties in designing adequate dosage forms occur due to stickiness, poor flow properties and incompressibility of the SD, fragmentation of too soft particles and poor disintegration of tablets. The biggest weakness of the SD is the possibility of converting an amorphous form of active substances into a crystal, which is less water soluble