Dissolution is the process by which a solid solute enters a solution. In the pharmaceutical industry, it may be defined as the amount of drug substance that goes into solution per unit time under standardized conditions of liquid/solid interface, temperature and solvent composition. Dissolution is considered one of the most important quality control tests performed on pharmaceutical dosage forms and is now developing into a tool for predicting bioavailability, and in some cases, replacing clinical studies to determine bioequivalence. Dissolution behaviour of drugs has a significant effect on their pharmacological activity. In fact, a direct relationship between in vitro dissolution rate of many drugs and their bioavailability has been demonstrated and is generally referred to as i n vitro-in vivo correlation, IVIVC 38 .
Solid dosage forms may or may not disintegrate when they interact with gastrointestinal fluid following oral administration depending on their design (Figure 1). For disintegrating solid oral dosage forms, disintegration usually plays a vital role in the dissolution process since it determines to a large extent the area of contact between the solid and liquid. However it is well known that considerable dissolution of the drug can take place before complete disintegration of the dosage form, a phenomenon which depends largely on the mechanism of disintegration and certain physicochemical properties of the drug, such as its solubility. This could be important when considering the motility of the drug or dosage form, and the release of the drug at specific sites, in the gastrointestinal tract. Thus, correlations have been established between disintegration times and dissolution rates for various pharmaceutical tablets 7, 39-42 . It should be noted, however, that there is not always an automatic correlation between disintegration and dissolution, especially for drugs with very low dissolution rates.
For many drugs, particularly those that are poorly soluble in the gastric fluid, the rate-limiting step in the absorption process is the dissolution rate and a dissolution rate determination can therefore be a useful guide to comparative bioavailability 43 . Since drug absorption and physiological availability depend on the availability of the drug substance in the dissolved state, suitable dissolution characteristics are important property for a satisfactory tablet. The dissolution test measures the amount of time required for certain percentage of the drug substance in a tablet to go into solution under a specified set of conditions. It describes a step towards physiological availability of the drug substance, but it is not designed to measure the safety or efficacy of the tablet being tested 44 . It provides in vitro control procedure to eliminate variation among production batches. The dissolution medium must be aqueous and the pH of the medium should be controlled and should simulate in vivo conditions. The dissolution medium should be 0.1M HCl and pH 6.8 buffer to simulate the biological extremes. The possible role of bile salts in absorption of highly insoluble drugs suggests the inclusion of physiological concentrations of sodium taurocholate in the mildly acid or alkaline media. Studies have shown that low agitation must be used (i.e. in the order of 50rpm) and that the tablet must not be subjected to abrasion in keeping with the mild agitation in the gastrointestinal tract 45 .
Figure 1: Schematic diagram of the dissolution process
Dissolution kinetics is important in determining the bioavailability of a drug 39 . Levy 45 and some other workers 46 reported that the dissolution rate controls the rate of build up of certain drugs in the blood stream. It was thus recognized that in-vitro dissolution kinetics provides useful information on the availability of drugs and their subsequent therapeutic effects in-vivo 45 . This led to the inclusion of dissolution tests in the United States NF XIII (1970) and USP XVIII (1970) monographs for one capsule and twelve tablet preparations. In 1975, dissolution tests were included in the British Pharmacopoeia (amendment to BP 1973) for digoxin tablets. The various pharmacopoeias contain specifications on the dissolution requirements of various drugs. A variety of designs of apparatus for dissolution testing have been proposed and tested, varying from simple beaker with stirrer to complex systems with lipid phases and lipid barrier where an attempt is made to mimic the biological milieu. The choice of the apparatus to be used depends largely on the physicochemical properties of the dosage form 47 .