Author: Dr. Saeed Qureshi, Ph.D.

Bio-waiver is a term used for establishing and accepting quality of a pharmaceutical product (tablet and capsule) based on in vitro drug dissolution testing without corresponding bioavailability/bioequivalence data.

The scientific rationale behind this practice is that, in general, the bioavailability of a drug product depends on its release and solution formation (dissolution) characteristics. The in vivo (physiological) dissolution is monitored or simulated in vitro by a dissolution test. Therefore, an assumption made here is that the in vitro dissolution test will appropriately and accurately reflect in vivo dissolution in humans and thus the bioavailability/bioequivalence characteristics of the products. This linkage is commonly referred to as an in vitro-in vivo (co)relationship or IVIVC. Therefore, a pre-requisite for bio-waiver practices is a well-established IVIVC.

It is, however, generally well recognized and documented in the literature that current practices of dissolution testing do not relate well to in vivo outcomes or often lack IVIVC. Therefore, presently, a bio-waiver should be considered as a scientifically weak case.

There are, however, guidances available for considering bio-waivers in certain, very specific cases.  For example, bio-waivers are considered for products having drugs with high aqueous solubilities and absorbabilities from the GI tract in humans. In addition, the products should show fast dissolution. The underlying thought is that if drugs are highly soluble and absorbable, and the products show fast and complete in vitro dissolution (usually in less than 30 minutes). The products are not expected to pose any potential problems in vivo, and thus they may be considered for bio-waivers. Therefore, it may be assumed that current practices of bio-waivers are based on faith rather than their scientific merits.

In short, for an appropriate and scientifically valid bio-waiver inference, one has to establish IVIVC, with evidence that the techniques and methodologies (dissolution apparatus, medium, and associated experimental conditions) employed are indeed capable of providing physiologically relevant results.

Often in literature and discussion, terminology of a discriminatory test is used to describe that a dissolution test can differentiate or discriminate between products based on formulation and/or manufacturing differences. However, the 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 is conducted using in vivo relevant experimental conditions, e.g., dissolution medium be aqueous having pH in the range of 1 to 7. Interestingly, the literature also well documented that dissolution results with formulation/manufacturing differences seldom reflect corresponding in vivo behavior.

It is, therefore, safe to consider that the use of the terminology of “discriminatory test” as commonly used does not appear to be correct. To be accurate, the discriminatory test terminology should clearly identify a test as an “in vitro discriminatory test” or “in vivo discriminatory test,” also known as a 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, the disintegration test may be considered a discriminating test if formulation/manufacturing differences are 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 that would relate differences in formulation/manufacturing of products to corresponding differences in vivo such as bioavailability/bioequivalence characteristics. An essential requirement for an in vivo discriminatory test is that the test be conducted using physiologically relevant experimental conditions. For example, an apparatus must provide a gentle but efficient stirring and mixing environment. The medium must be aqueous with pH in the range of 5-7 and maintained at 37C. 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.

Therefore, it is prudent to 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 the 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 the difference at each sampling time is 10% or less, then f2 value will be between 50 and 100. Therefore, a quick way to establish the similarity 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. Suggestions appear to have been made to lower the 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 conflict with the commonly accepted pharmacopeial (e.g., USP) standards, where a lot to lot testing (without formulation/manufacturing differences) the acceptable deviation is significantly higher than 10%. However, f2, which is generally suggested for evaluating products with formulation/manufacturing differences (for product developments and/or alterations), the allowance should be greater; thus, a wider range with a lower cut-off value than 50 may be more appropriate.

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

The quality control (QC) aspect of dissolution testing is linked to the release characteristics of the drug from its product, commonly tablet or capsule. This release characteristic, measured in vitro, is supposed to reflect/simulate drug release in vivo. Therefore, the QC test reflects drug release in vivo in humans, thus establishing the quality of the product. Such tests are conducted using experimental conditions that simulate human physiological conditions of GI tract as closely as possible. However, recent studies (see publication section) reflect that experimental conditions used (e.g., apparatuses) do not simulate an appropriate GI tract environment. They lack the needed mixing and stirring in the dissolution vessels. Therefore, current practices of dissolution testing may not reflect the quality of the products, and the test may not be considered a QC test.

On the other hand, considering this lack of QC aspect, commonly dissolution test is presented as a test for consistency check for batch to batch evaluations. Still, it appears to be implied as a QC test. This obviously creates significant confusion in properly describing and/or differentiating the test as a QC or consistency-check test. As stated above, in its current form dissolution test does not appear to be a QC test. Therefore, it should be considered a consistency check without its link to in vivo release and the quality of the product.

A consistency-check test may be performed using any of the experimental conditions that may or may not be physiologically relevant – for example, organic solvents vs aqueous-based, higher or lower temperatures vs 37C, any other type of stirring device (magnetic bar, shakers, propeller with high-speed motors, etc) vs commonly used paddle and basket apparatuses. Further, one may report the results for any sampling time which appear to be most stable and reproducible. This has never been the intent of the dissolution test to be conducted in this manner, particularly as a QC test.

Therefore, to conduct a dissolution test as a QC test, as was originally intended, the test must be conducted by creating or simulating a more appropriate physiological environment, i.e., improved stirring and mixing. This improved stirring and mixing aspect indeed appears to address the limitations of current practices and their artifacts. For further discussion on this topic, please see the recent literature under the publication section.

A common and frequent response to a 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 the 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, in good faith, most respondents suggest what it may be, not what it is or should be because no one knows what it is and what 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 is indeed validated apparatuses? How was this shown that paddle/basket apparatuses are suitable for their purpose (QC, discriminatory, IVIVC etc.)? If we have that procedure, we may follow the procedure to establish the validity of other secondary steps (changes, alteration, improvements, etc.). However, as we, to our knowledge, do not have that initial procedure that was used to establish the validity of paddle/basket, we cannot perform a secondary validation.

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

It is hoped that this will help and simplify your future dissolution work and validation steps.

There are four apparatuses generally recognized by pharmacopeias (e.g. USP) and other regulatory bodies that 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 that meets the product’s expected behavior. This approach has two flaws: (1) indeed, these apparatuses are not measuring the same property (dissolution). Otherwise, the 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).

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

Current practices, however, seek and provide experimental conditions such as choice of the 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 gastrointestinal (GI) tract fluid. 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 a critical step for developing and assessing the quality of products linked 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, the dissolution technique is based on the extraction process with rather restrictive solvents, temperature, and stirring/shaking. The extraction solvents used in dissolution testing are limited to water or aqueous-based solutions with 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 the extraction process within the GI tract.

It is important to note that dissolution extractor/tester, which is commonly based on a vessel and stir the 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 common methods such as chromatographic/spectroscopic. The results are commonly obtained in the units of amount/volume (e.g. mg or ug/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 the Soxhlet apparatus or dissolution technique are reported in percentage of the extracted amount based on the total amount of the matrix (Soxhlet) or the total expected amount of the drug present in the product.

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

The other day someone indicated that even products of drugs from BCS class II (low solubility and high permeability) had not shown successful IVIVC. These drugs, at least in theory, provide the best-case scenario for successful IVIVCs. The question was then asked what may be the reason for such a general lack of success.

For any successful IVIVC, one needs to conduct dissolution tests by mimicking the in vivo environment as closely as possible. This is usually done by conducting a dissolution test using water or aqueous buffers having pH in the range of 5 to 7 maintained at 37C. These conditions represent the GI tract (intestinal) environment.

 On the other hand, the tests are conducted mostly using paddle and basket apparatuses to simulate mixing and stirring environment. Unfortunately, the stirring and mixing environment of these apparatuses lack simulation of the in vivo environment. In fact, these apparatuses almost provide no stirring and mixing. Therefore, because of this mismatch, one should not expect successful IVIVC. For successful IVIVC, one requires an efficient (gentle but thorough) stirring environment. One such possibility to address this issue may be the use of a crescent-shaped spindle. For further discussion on the use of a crescent-shaped spindle, one may search this site or literature in general.

 In short, one should not expect success in developing IVIVC using paddle and basket apparatuses.