An in vitro drug dissolution test is conducted to assess in vivo dissolution characteristics of products usually tablets and capsules. The reason or logic behind conducting this test is that if a drug is to exert its therapeutic effect after administration, it should be absorbed from the GI tract (mostly small intestine). Further, for a drug to be absorbed it should be available in dissolved (solution) form in the GI tract. Thus, measurement of the dissolution of a drug in the GI tract becomes one of the most critical steps for establishing and/or monitoring the efficacy and quality of the drug or its product.
Usually it is not possible that the in vivo dissolution of a drug be monitored directly. Therefore, it is monitored indirectly by measuring bioavailability of the drug, which is estimated from the drug concentrations in the blood or plasma, commonly known as plasma drug concentration-time profiles. On the other hand, as the in vitro dissolution test is used to evaluate in vivo dissolution which is monitored by bioavailability, the in vitro test dissolution test and bioavailability assessment thus becomes interlinked with each other.
It is very important to note that the only purpose or use of drug dissolution testing is to assess bioavailability of a drug from its product, which then reflects the quality of the product. It is also equally important to note that the link between dissolution testing and the quality of the product is through bioavailability assessment only. Otherwise, dissolution testing has no link to the quality of the product or its assessment. Often, it is promoted that dissolution testing is used and/or valuable for monitoring manufacturing efficiency/quality or its consistency. However, unfortunately, this is simply a false description or promotion of dissolution testing. A dissolution test has no ability to monitor anything but dissolution characteristics of a product, which then is used for bioavailability assessment.
It is now also well accepted that the current practices of dissolution testing have not been successful in predicting bioavailability assessment. Recently USP clearly described this situation by including the following statement in one of its general chapters, i.e. “Compliance with any of the [dissolution] tests does not assure bioequivalence or bioavailability” (link).
Obviously, it is clear that dissolution testing cannot be used for bioavailability assessment. What then is the purpose of such (pharmacopeial) dissolution tests? The answer is not much.
On the other hand, if one would address well known deficiencies of the currently suggested dissolution testers/apparatuses, in particular paddle/basket, then not only can dissolution testing be made bio-relevant, but also be vastly simplified and improved.
A modified apparatus/procedure has been suggested using the crescent-shape spindle, with a single set of experimental conditions, to address the above mentioned deficiencies. For further details in this regard, please follow the link.
