Often it is suggested that one should use a de-gassed or de-aerated dissolution medium. Unfortunately, conducting a dissolution test using such a condition makes the test and results obtained invalid, why?:

1. Dissolution tests are conducted to evaluate drug dissolution characteristics in vivo, i.e., in the human GI tract. The GI tract contents are not de-aerated or de-gassed, therefore, in vitro tests conducted in a de-gassed medium will not be reflective of the in vivo environment and thus the results will be invalid.
2. Commonly suggested procedure for preparing a de-aerated medium is by vacuum filtering of heated (41-45 ºC) dissolution medium. Dissolution tests are conducted at 37 ºC, thus dissolution medium temperature will change from ~45 to 37 ºC during testing. Therefore, not only does de-aeration create a non-physiological condition, but also introduces instability in the medium characteristics during testing.

For more appropriate drug dissolution testing, the medium should be equilibrated at 37 ºC, which would provide a more appropriate physiological condition and stability in the testing environment.

It is generally accepted that the pH of the aqueous phase within GI tract ranges from pH 1 to 7 (or 8). This range may further be divided into two sub-groups; one is of pH 1 (perhaps to 3) and a second of pH 5-7.  The segment having pH 1 represents the stomach and the other, in the range of 5-7, is the (small) intestinal part.

It is commonly accepted that most of the drug (or food) ingested, gets absorbed from the intestinal part. As absorption depends on dissolution, the majority of the drug should be available in the solution form in this segment of the GI tract. Obviously, for dissolution purposes, it is this environment of pH which appears to be relevant and critical. Therefore, dissolution tests are to be conducted in media having pH in the range of 5-7.

Conducting dissolution tests, thus, in acidic (HCl) media (pH ~1), does not appear to be an appropriate choice.

Drug dissolution tests are to be conducted using physiologically relevant experimental conditions. The physiologically relevant conditions do not imply that one needs to duplicate or simulate the anatomy or physiology of the GI tract in vitro, but that the environment or the process which facilitates the product/drug dissolution in vivo.

For example for dissolution testing, one is required to maintain a temperature of 37 °C, however, it is not necessary or required that this temperature be attained or maintained in vitro by biochemical or enzymatic means as it occurs in vivo. The in vitro temperature can be attained and maintained by any means such as using a water bath, heating coils or plates, etc. The critical aspect is that the temperature has to be 37 °C.

Similarly, for in vitro dissolution purposes, one would require a container or vessel (reflecting intestinal space), water or aqueous buffer (reflecting liquid in the intestine), and a stirrer (reflecting churning within GI tract). These parameters should be chosen in such a way that they may or may not look or feel similar to the physiology/anatomy, but their effects have to be similar to the in vivo effects. The more one deviates from producing the in vivo effects in vitro, the less reliable and useful the drug dissolution test will become and vice versa. It is to be noted that, as described in the literature extensively, the currently used paddle and basket apparatus lacks in providing mixing or stirring (equivalent of churning in vivo), therefore, dissolution results obtained using these apparatuses may be of limited use or value.

Furthermore, as the physiological environment or process remains the same or constant from product to product i.e. product independent, the in vitro experimental conditions, therefore, should remain product independent as well. If dissolution testing requires changing of experimental conditions from product to product (e.g. IR vs ER), then that testing procedure needs to be reconsidered, as the in vitro (dissolution) procedure may not be valid.

The only limitation for the volume of dissolution medium is that the expected amount of test drug must be freely soluble in the volume at 37 ºC. It is possible that the drug may not be soluble at room temperature, but at 37 ºC it may be freely soluble. In such situations, for dissolution testing purposes, the drug will be considered as freely soluble.

This freely soluble drug requirement/concept is often referred to as a “sink” condition. The sink condition may be any condition where drug is freely soluble. Often, in the literature, it is defined as 2, 3, 5 or more times the volume of the dissolution medium required to dissolve the expected amount of the drug. These choices may be personal preferences, however, in practice the drug should be freely soluble in the medium. Therefore, before conducting a dissolution test and choosing a volume, one must determine the solubility of the drug in the medium at 37 ºC. Once this volume is known, to be on the safe side, one may use 10% extra volume than needed.

It is to be noted that if the volume required is 250 mL, then there is no harm in using larger than the required volume, for example 900 mL. The excess volume should not impact dissolution, however, lower than required should, and may make the dissolution test and results in valid. In the event, the excess volume provides differences in dissolution results, then this would be an indication of lack proper or efficient stirring in the vessel. In such situations, use of such an apparatus may not be appropriate and the results would not reflect true dissolution characteristics of the test product but an interaction of the product and the dissolution apparatus.

Having said that, as 900 mL is the commonly used or suggested volume and there is no harm in following this suggestion or tradition. In fact one should try to use this standardized approach of using 900 mL. In such cases, the analyst must make sure that expected amount of drug is freely soluble in 900 mL. If not then the use of a solublizing agent such as SLS may be considered, to establish that the drug is, indeed, freely soluble in the 900 mL medium.

The PVT (Performance Verification Test) which is for all practical purposes is a name change of an old test known as dissolution apparatus calibration. This test has been propagated to provide some sort of assurance about the performance of the drug dissolution apparatuses commonly known as Paddle and Basket. Almost from the beginning, the test has been faced with criticism about its relevance and usefulness. In fact, it is generally considered as a serious economical burden on pharmaceutical manufacturers. To minimize this burden, and a lack of scientific rationale, FDA recently provides an alternate to avoid the PVT.

On the other hand, USP which propagates the use of PVT, and the provider of the tablets used for PVT, has been making tremendous efforts in convincing that the PVT is indeed a useful practice. However, selling this idea appears to be becoming difficult, because generally analysts/scientists consider this as “an effort to maintain the status quo”. This status quo impression appears to have merit, because there has been no experimental evidence provided to address the concerns of the users or to establish the usefulness and relevance of the PVT.

At this stage question remains that if PVT fails (which it often does without any apparent reasons), what experimental evidence are available to reflect that indeed the apparatus was not functioning as expected? Otherwise, why propagate the use of PVT?

On the other hand, lack of evidence with regard usefulness of PVT, gives strength to the reported observations that problems with PVT are in fact reflections of problems of Paddle and Basket apparatuses (poor hydrodynamics, variable and unpredictable results) themselves.

Often in literature and discussion, terminology of a discriminatory test is used to describe that a dissolution test is capable of differentiating or discriminating between products based on formulation and/or manufacturing differences. However, implied understanding of this terminology is that these differences may reflect products’ in vivo differences, thus their quality in humans. The underlying implied assumption of in vivo relevance is emphasized by suggestions that dissolution testing be conducted using in vivo relevant experimental conditions, e.g., dissolution medium be aqueous having pH in the range of 1 to 7. Interestingly, it is also very well documented in the literature that dissolution results with formulation/manufacturing differences seldom reflect corresponding in vivo behavior.

It is, therefore, safe to consider that the use of terminology of “discriminatory test” as commonly used does not appear to be correct. To be correct, the discriminatory test terminology should clearly identify a test as “in vitro discriminatory test” or “in vivo discriminatory test” also known as bio-relevant test.

An in vitro discriminatory test would be the test to reflect differences in physical characteristics of the test products (formulation/manufacturing) with no direct or definite consequences in vivo. Such tests may be conducted using any of the experimental conditions necessary concerning apparatuses (paddle/basket, Erlenmeyer flask with magnetic stir etc.) and media (organic or aqueous solvents having any pH), etc. In this respect, disintegration test may be considered as a discriminating test, if formulation/manufacturing differences be linked to disintegrating time. It may be important to note that although often in vitro discriminatory dissolution tests are developed and used but their usefulness is limited and may be an unnecessary burden on the pharmaceutical industry and the regulatory agencies.

An in vivo discriminatory test or a bio-relevant test, on the other hand, would be a test which would relate differences in formulation/manufacturing of products to corresponding differences in vivo such as bioavailability/bioequivalence characteristics. For an in vivo discriminatory test, an essential requirement is that the test must be conducted using physiologically relevant experimental conditions. For example, an apparatus must provide gentle but efficient stirring and mixing environment, medium must be aqueous with pH in the range of 5-7 and maintained at 37C, and the medium must not be de-aerated but equilibrated with dissolved gasses. In addition, as the testing environment is linked to the GI tract physiology, which does not change from product to product (e.g. IR to ER), the experimental conditions should not be changed from product to product as well.

Therefore, it is prudent that one should clearly indicate the nature of the test described whether it is an in vitro or in vivo discriminatory type so that proper evaluation and use of the test be considered.

An f2 parameter is commonly used to establish similarity of two dissolution profiles. The formula and procedure to obtain f2 value is described in one of the publications.

In short, two profiles are considered identical when f2=100. An average difference of 10% at all measured time points results in a f2 value of 50. A public standard of f2 value between 50-100 to indicate similarity between two dissolution profiles. Another way of saying this is that on average if difference at each sampling time is 10% or less then f2 value will be between 50 and 100. Therefore, a quick way to establish similarly of two profiles is to see if differences in dissolution results at each sampling time are less than 10%.

In general, there is not much literature or debate available reflecting its relevance or link to the assessment of the compared products for their qualities or release characteristics in vitro (dissolution) or in vivo (bioequivalence). At best, the parameter (f2) appears to be a rule-of-thumb type gauge, and perhaps appears to add some extra burden for evaluating and reporting dissolution results without real gain in data analysis.

There has been some discussion on the tightness of the standard, which may result in rejection of products of similar drug release characteristics and suggestion appears to have been made to lower limit from 50 to say 30.

It also appears that this limit of 50-100 range for f2 (or a difference of less than 10% in dissolution results) may be in conflict with the commonly accepted pharmacopeial (e.g. USP) standards, where lot to lot testing (without formulation/manufacturing differences) the acceptable deviation is significantly higher than 10%. However, f2 which in general is suggested for evaluating products with formulation/manufacturing differences (for product developments and/or alterations), the allowance should be greater, thus wider range with a lower cut-off value than 50 may be more appropriate.

Settling of particles at the bottom of the vessel in case of paddle, or left over in case of basket, is reflection of poor product/medium interaction resulting in inefficient or slower dissolution. These are artefacts of the paddle and basket apparatuses. Therefore, one should expect slower dissolution rate, reduced extent (e.g. 80%), and highly variable results using these apparatuses. Thus, this observation appears expected and normal. Hope this is helpful.

A common and frequent response to number of different queries, regarding choices in apparatuses, media or other experimental conditions, is that the changes and choices must be validated. The responses are as varied as number of respondents and their views. This leaves people usually even more confused than before asking the question. The reason for this confusion is that one cannot validate an apparatus or method using current practices of dissolution testing. Therefore, mostly respondents, in good faith, suggest what it may be, not what it is or should be, because no one knows what it is and what exactly is expected.

The question may only be answered, if one has a procedure, or lead to a procedure, as to how an apparatus was validated to start with. For example, how was it established that paddle/basket are indeed validated apparatuses? That is, how was this established that paddle/basket apparatuses are good for their purpose (QC, discriminatory, IVIVC etc)? If we have that procedure, then we may follow the procedure to establish validity of other secondary steps (changes, alteration, improvements etc). As we, to our knowledge, do not have that initial procedure which was used to establish the validity of paddle/basket, we cannot perform a secondary validation.

Thus, it should be kept in mind that current practices of validation in this respect are more like rituals/traditions than based on facts from experimental science.

Hope this will help and simplify your future dissolution work and validation steps.

There are four apparatuses which are generally recognised by pharmacopeias (e.g. USP) and other regulatory bodies which may be used for drug dissolution testing for product evaluation. However, choosing one of these apparatuses, or any other, is difficult as there are no appropriate scientific or rationale criteria available for such selections. 

All four apparatuses usually provide different results for the same product and choice is left to the analyst to select one which meets the expected behaviour of the product. This approach has two flaws: (1) certainly, these apparatuses are not measuring the same property (dissolution), otherwise, results would have been the same. A product cannot have four or more values for the same parameter; (2) a product, or its property, cannot be evaluated using itself as a reference to select a technique (apparatus).

Therefore, it appears that current practice of product dependent choices of dissolution apparatuses, and its associated experimental conditions, is not a valid practice. Thus, results obtained in such manners to reflect true dissolution characteristics of the product whether test is for QC or IVIVC purpose or any other, may not be valid as well.  

Before selecting an apparatus, one has to establish its relevancy by conducting a dissolution test with a reference product of known and accepted drug dissolution characteristics. This reference may be from a third party or in-house product, but not the test product, and must have known and acceptable drug dissolution characteristics. Then to establish the dissolution properties of the test product, the apparatus with its associated experimental conditions be used without any changes.

Drug dissolution testing is commonly performed using vessel based apparatuses with Paddle/Basket spindles. The objective of testing is to establish dissolution or release characteristics of a test product.

Current practices, however, seek and provide experimental conditions such as choice of spindle, rpm, dissolution medium (nature and strength) to define such characteristics based on analysts’ expectations. Another way of saying the same thing is that, an analyst sets the experimental conditions to obtain desired quality of results, or products, e.g. less variable, discriminatory or not, slow or fast, bio-relevant or not. The analyst would never know the actual release characteristics of a product, thus its quality.

The reason for such inadequacy with the use of Paddle/Basket is that they do not provide an efficient mixing and stirring environment within dissolution vessels, the most critical and necessary process for the dissolution itself. Therefore, their future use certainly warrants caution.

Dissolution tests are employed to establish drug release characteristics of solid oral products, such as tablets and capsules. The rationale for conducting these tests is that for a product to be therapeutically effective, the drug must be released from the product and should generally be dissolved in the fluid of the gastrointestinal (GI) tract. The drug in solution form facilitates the absorption of the drug from the GI tract into the systemic (blood) circulation to reach its desired target (site of action) to exert its effect.  Therefore, a dissolution test may be considered as a critical step for the development and assessment of the quality of products linking to their safety and efficacy attributes. Thus, drug dissolution studies are conducted at every stage of a product’s life, including obtaining approval for marketing in a country from local regulatory authorities.

In reality, dissolution testing may be considered as an extraction technique such as a Soxhlet extractor for extracting compounds from their matrixes or perhaps a simple shake-flask technique for solubility determination. It is not to say that dissolution apparatuses may be replaced or substituted by apparatuses for the two types of techniques mentioned, but highlighting the fact that they all work on the same principle but with different objectives. The extraction techniques mentioned concerns with extraction/dissolution to the maximum of the test compounds using rather harsh experimental conditions such as boiling liquids, vigorous shaking and/or stirring at very high speeds. On the other hand, dissolution technique is based on extraction process with rather restrictive choices of solvents, temperature and stirring/shaking. The extraction solvent used in dissolution testing is limited to water or aqueous based solutions having pH in the range pH 5 to 7 and maintained at 37 °C. The stirring and mixing must be thorough but gentle to avoid any harsh abrasive impact on the product. The chosen solvent and experimental conditions are representative of the fluid present in the gastrointestinal (GI) tract to simulate extraction process within the GI tract.

It is important to note that dissolution extractor/tester, which is commonly based on a vessel and stir combination, does not reflect the GI physiology but the environment and process of the extraction within the human GI tract. Following the extraction, as for the other techniques mentioned above, samples are withdrawn, filtered and quantified using any of the common methods such as chromatographic/spectroscopic. The results are commonly obtained in the units of amount/volume (e.g. mg or ?g/mL). However, these results are further converted to other units, for example for solubility expression; they are reported as amount/100 mL. However, results from Soxhlet apparatus or dissolution technique are reported in percentage of the extracted amount based on the total amount of the matrix (Soxhlet) or total expected amount of the drug present in the product.

The preceding discussion about the similarity of the dissolution testing to other extraction techniques is presented to highlight the fact that the drug dissolution testing is a relatively simple analytical technique. It should not require any more elaborate method development/validation steps or reporting of results than for other simple analytical techniques such as the two described above. Such an understanding of the underlying principle of dissolution testing will help in critically evaluating current complex practices of reporting and evaluation the dissolution results and further simplifying them.