BIOANALYTICX

An IVIVC (in vitro-in vivo co-relationship) is a terminology commonly used in the area of drug dissolution testing, desiring the relationship of in vitro (drug dissolution) results with in vivo characteristics such as drug levels in humans.

As the in vitro results are generally expressed as cumulated percent drug release, thus these profiles (results) are difficult to compare with in vivo results reported in concentration units. In addition, in vitro results only reflect a product’s release (dissolution) characteristics, while in vivo results reflect the combined effect of drug dissolution and absorption/elimination characteristics. To compare, either the in vitro dissolution results are manipulated (mathematically) using drug absorption/elimination characteristics to predict blood levels and compare with actual/observed drugs levels or by extracting in vivo dissolution results from actual drug levels in humans and comparing with those of others the observed in vitro results. The first approach is known as a convolution technique and the second as de-convolution. One of these techniques of data manipulation would be required to make the results comparable.

Once the desired results are derived, i.e., in vivo dissolution from drug levels in blood (de-convolution) or in vivo drug levels from in vitro dissolution tests (convolution), these are compared with corresponding/actual in vitro or in vivo results, respectively. Achieving level A IVIVC means that these comparisons of results be made point by point, i.e., in vitro and in vivo results are compared individually for each sampling time. More accurately plotting in vitro and in vivo results as a line and obtaining a coefficient of relationship (r) value approaching 1. This is where the difficulty is i.e., assumption of exact or matching time course in both in vitro and in vivo drug release. However, this is a well-known fact that predictability of accurate time course of drug in humans is very difficult, if not impossible.  For example, even when one would like to compare drug levels in humans alone, such as in bioequivalence studies, such point-by-point comparisons of drug levels are neither used nor required by regulatory agencies. The reason being variability in results (within or between human subjects) is expected to be very high. Therefore, how could it be possible to achieve point-by-point comparisons of in vitro and in vivo results? 

To address this difficulty in comparing the in vivo results for bioequivalence studies, one is required to use derived parameters from the drug levels, which are; highest observed drug (concentration) level and the area under the drug concentration-time profiles.  These parameters sort of normalize or reduce the observed variability of drug levels or profiles, thus offering a more reasonable approach for evaluating or comparing in vivo results.

 Thus, in short, one should keep in mind the limitations, in fact impractically, of point by point (Level A) comparison approach for IVIVC purposes. For a more detailed discussion on this aspect, in particular regarding developing IVIVC, one of my publications may be of interest (The Open Drug Delivery Journal, 2010, 4, 38-47, Link).

A common query concerning a dissolution test is how one should conduct the test for a drug. Further, in response to such queries, different suggestions are made for choosing the apparatus, rpm, medium etc. However, unfortunately, such queries and responses lack scientific merit and logical thinking. The reason is that a dissolution test is conducted for a product and not for a drug (active ingredient). That is why pharmacopeial monographs, particularly USP, do not have dissolution tests under drugs (active ingredients) but products.

Therefore, one can only suggest a testing procedure for products and not for drugs. Further, a testing procedure is not, or should not, be linked to a product because testing is done to evaluate a product. Therefore, the procedure must be product-independent. The question then becomes how one should set up the experimental conditions. The answer is that the dissolution testing conditions should reflect the GI tract environment, particularly intestinal. As the GI tract environment does not change from product to product, the testing procedure should be fixed.

 A typical testing environment may be water maintained at 37 °C, with some solubiliser to dissolve low aqueous solubility drugs and a simple stirring mechanism that should provide efficient/thorough product-medium interaction.  It is hoped that this suggestion will help in answering the common and frequent queries.

In a recent publication, USP describes prednisone based performance verification test (PVT) as,

“Lot P1 demonstrates sensitivity to test performance parameters (vessels and degassing)”.

It appears that the use of PVT has been reduced from establishing the appropriateness of vessels and degassing of the medium rather than a performance evaluation test for the apparatuses or procedure as it is supposed to. Even claims for sensitivities of the two suggested parameters may be of questionable merit. Because;

1. In a recent study from the FDA laboratory, it has been demonstrated that dissolution test does not appear to show sensitivity to vessel dimensions, stating “Geometric characteristics varied within and among the sets of vessels, but the overall averages and standard deviations of dissolution results (six vessels) showed no statistical significant differences among the vessel sets”.

2.  To be sensitive to a parameter (vessel geometry), there must be a link of dissolution results (response) to vessel geometry (action). For example, a mercury thermometer is based on heat-based expansion. Thus, the higher the temperature higher the expansion. How does vessel geometry (or variation in its contour) relate/link to dissolution results?

 3. Even if the contour of a vessel has any effect, then shouldn’t its control be established using appropriate physical measurements? It is like suggesting monitoring or controlling the room temperature based on measuring the humidity in the room. This appears to be quite impractical and an irrelevant approach.

 4.  Concerning sensitivity to de-aeration, objective and practice also appears irrelevant for a number of reasons. For example (1) conducting a dissolution test using a de-aerated medium should make the test physiologically irrelevant as the physiological environment does not dictate the de-aerated medium. (2) For testing of products that are not sensitive to de-aeration, performing PVT with a de-aerated medium would be irrelevant. (3) Often, dissolution tests are conducted for longer than half an hour. Usually, during the testing, dissolution media become equilibrated with the dissolved gasses (air) and do not remain de-aerated. So, how would the analyst to maintain a consistent de-aeration level which apparently would be impossible?

 In short, it appears PVT in its current form does not appear to provide any useful purpose but a financial burden on the pharmaceutical industry. Therefore, its use may easily be discontinued.

In a recent publication, USP describes the dissolution procedure as,

“The procedure can function both as a quality control tool and, under specified circumstances, as a predictor of the dosage form’s performance in vivo.”

One may interpret this as an indication of very weak support for the continuation of the procedure, at least in its current format.

Stating that the procedure could predict the product’s in vivo performance under certain specified circumstances is a clear deviation from the commonly accepted understanding. Furthermore, the only reason a dissolution test was introduced was to replace a disintegration test which was considered a poor predictor of in vivo performance of the products. Therefore, now as per the publication, the current dissolution procedure appears to have the same limitation as that of the disintegration test, which obviously has limited use.

Further, if the procedure is to work in some cases, one would require some form of guidance in determining how these specified circumstances are to be determined or established.

On the other hand, describing the procedure as a quality control tool may also become redundant without in vivo relevance. In general, it is accepted that if a dissolution test, as a quality control tool, shows unexpected drug release, it would reflect potential unexpected in vivo drug release of the drug, leading to concerns about the quality of the product. However, if the in vitro and in vivo link is severed or weak, what would be the rationale for using a dissolution procedure as a quality control tool?

The publication appears to have added serious confusion regarding the usefulness of the PVT procedure and the current practices of dissolution testing.

In a recent publication from USP, it is concluded that,

“Apparatus 1 results are stable over time. Those in Apparatus 2 show a decrease over time in the geometric mean but show no trend in variability”.

If all things being equal for testing except differences in apparatuses used, then is it not obvious that instability in results would reflect the instability of Apparatus 2? The conclusion from the USP supports the observation/results reported in the literature regarding the poor performance of Apparatus 2. This poor performance in testing reflects the poor hydrodynamic environment within dissolution vessels, as described extensively in the literature. Thus, the use of Apparatus 2 would require caution.

USP calendar shows an entry of a planned webinar to address the problems in the dissolution testing area. Accordingly, USP is requesting the submission of questions so that issues and concerns may be addressed in this regard. The following are some suggested questions/concerns which USP may consider addressing during the webinar or later.

1. USP usually recommends the use of Paddle and Basket apparatuses for drug dissolution testing as a first choice. Are there any documented reasons (evidence) for these choices showing their superiority compared to other apparatuses? How have these apparatuses been validated as appropriate dissolution apparatuses to evaluate pharmaceutical products for human use?
2. As a general practice, USP suggests experimental conditions for individual products (monographs) to establish their drug dissolution characteristics. This aspect is also described in more detail in the chapter <1092>. The practice requires that an analyst know the product and its expected dissolution behaviors and adjust the experimental conditions accordingly to achieve the expected characteristics. However, the objective of a dissolution test is to know or establish the drug dissolution characteristics of a test product. How should this conflict be addressed?
3. Concerning the variability in dissolution results, how would an analyst determine (differentiate) whether the variability is related to the PVT (or a test product) or the apparatuses? Is the information on the contribution to variability due to apparatuses available?
4. USP supplies prednisone performance verification tablets (PVT), which results in dissolution depending on the apparatus and experimental conditions employed. What would be the “true” (“actual”) dissolution characteristics of PVT as a product which may be used as “reference”?
5. Apparently, the prednisone performance verification tablets may be characterized as a fast-release product but show a slower release type due to “cone” formation. The “cone” formation is the result of poor hydrodynamics and lack of product-medium interaction within dissolution vessels. Would it not be obvious that the products which will have similar characteristics like PVT would also be inaccurately characterized as slow-release type when in fact, they would be fast release type?
6. Why does the USP recommend that the dissolution medium be de-aerated?  What is the rationale for this suggestion, when apparently this suggestion would make the dissolution test irrelevant, as the physiological aspect does not require a de-aerated environment?
7. Pharmacopeial tests are often presented as tests for establishing batch-to-batch consistency in products’ quality, usually not as a bio-relevant test because the test often fails the IVIVC. If bio-relevancy or IVIVC is not the objective, then any other test, such as the disintegration test may be used for consistency checks. The dissolution test was introduced to replace disintegration test because of the lack of bio-relevancy of the latter. Most of the pharmacopeial dissolution tests are not bio-relevant, so how should one justify a dissolution test over a disintegration test?

It is important to note that the emphasis is on the “practice” and not the drug dissolution testing itself. A drug dissolution test is an important and extremely useful test and is required for the development and evaluation of pharmaceutical products, in particular tablets and capsules. However, how the test is conducted, referred here as the “practice,” is very different from dissolution testing itself. One should keep this difference in mind. 

There are numerous issues with the practice (or conducting a test). For example:

The test is to be done to determine drug dissolution characteristics of the product, but in practice it is done to establish experimental conditions to obtain pre-set (or preconceived) dissolution characteristics of the product. The analyst would never know the true drug dissolution characteristics of the product, thus its quality.

The most commonly used apparatuses, particularly paddle and basket, have never been validated to establish their suitability for drug release (or product) evaluation. These apparatuses usually do not even differentiate a slow-release product from a fast release product and vice versa. Thus, separate methods for these types of products are required. However, their use within the same product type is required to differentiate drug release based on manufacturing/formulation attributes.

Commonly, the practice is rationalized to monitor batch-to-batch consistency in product manufacturing. However, the consistency of the test (repeatability and reproducibility of the test itself) has never been established.

Often, tests are expected to be conducted using irrelevant experiment conditions, de-aeration in particular with a non-verifiable parameter or end point.

A product is expected to have more than one value (result) of the parameter (dissolution). Even USP performance verification tablets (USP) have two sets of dissolution results based on apparatuses used. The question is, what are the correct (true) dissolution characteristics of USP performance verification tablets?

The reason for these problems appears to be that for the past number of years, the focus has been to rationalize the use of paddle and basket apparatuses, which have been known to be flawed.

For appropriate testing, the focus must be on obtaining relevant and reproducible dissolution results of a product. The practice of adjusting experimental conditions to obtain certain desired or pre-conceived dissolution results should be discouraged.

Hello Vivian:

As suggested, I am posting your response to the post mentioned above, along with an Editorial note from myself. Regards. Saeed

Editorial Note:

To address this response, I would like to clarify that I was referring to a potentially diminished role in the future in my post. Also, I assume that your comments reflect your personal views and not those of the USP or the new Expert Advisory Committee.

Response:

Dear Saeed, I feel I have to respond your recent posting on your website about Changes at USP. Please be sure you have seen the article in the August issue of Dissolution Technologies that gives the make up of the new USP committee. This information in is the USP update with Tom Foster achieving the Beal award. In that article the new members of the new committee are listed. There are five members of the BPC committee that are now on this new committee. They are Mario Gonzalez, Johannes Kramer, Tom Foster, Alan Parr and I. These people were very much involved with the PVT and I can assure you that the work will continue in the new committee and at the USP labs. The USP has published many articles in Pharmacopeial Forum (PF) about the PVT and evaluations of variables using the Prednisone Standard Tablets. Also if you look in Dissolution Technologies (DT), www.dissolutiontech.com back issue there are a plethora of articles on the dissolution variables and the USP Prednisone Tablets. Also in DT we have reprinted many of the PF articles about the PVT and the new criteria. So the “absence of clear information” is not accurate. If you go to the USP website there are many places for information on the subject including the USP Tool Kit on mechanical calibration. As a point of interest, the PF will be available on line for free in 2011.

I was hoping that I could respond on your website but the comments are closed. Please consider posting my reply to you in this public forum.

Best regards, Vivian

9 Yorkridge Trail
Hockessin, DE 19707 USA
Tel. 302-235-0621
Fax 443-946-1264

vagray@rcn.com
www.vagrayconsulting.net
www.dissolutiontech.com (now searchable, check it out)

Current practices of drug dissolution testing require that the experimental conditions, such as medium and its volume and apparatus and its associated stirrer rotation speed, be established for each test product to achieve certain ‘expected’ dissolution characteristics or results. In reality, however, the purpose of dissolution testing should be to determine potentially unknown dissolution results reflective of a test product based on its formulation and/or manufacturing attributes. For appropriate testing, particularly for comparative purposes, the experimental conditions must be the same or consistent from product to product, i.e., product independent. This article describes a newly developed spindle, known as crescent-shaped, which can easily be installed in the vessel-based dissolution apparatuses (basket and paddle) to provide a product-independent dissolution testing approach for improved drug dissolution assessments. The new spindle provides an improved stirring and mixing environment, leading to better characterization of pharmaceutical products. The use of the crescent-shaped spindles offers additional significant advantages over the current practices, such as (1) allows analyses using a single method, compared to hundreds as currently required, for both immediate and extended-released products having the same or different active ingredients; (2) provides improved dissolution characteristics of products by avoiding false slow-release properties for fast release type products; (3) simplifies testing by avoiding the necessity of developing separate QC and bio-relevant dissolution methods; (4) provides a rugged testing environment free from common sensitivities, in particular to de-aeration and vibration effects. (Link to the article).

It may be said that analysts performing drug dissolution tests are in a pretty difficult situation. They are expected to conduct appropriate dissolution tests to determine the quality of a product based on its in vitro drug release characteristics. However, procedures described in the literature (e.g., USP <1092>) or commonly taught in courses provide suggestions for choosing/selecting experimental conditions to achieve (match) a desired or pre-set dissolution outcome. These desired dissolution characteristics are commonly obtained by selecting apparatuses (mostly between basket and paddle) and/or adjusting rpm, pH, or molarity of medium/buffer and/or solubilizer (nature or amount). Therefore, it is important to note that the analyst, following the currently suggested procedures, will never know the true or actual nature of drug dissolution characteristics of the product, thus its quality.

A similar confusing situation exists when one is expected to establish bio-relevant characteristics of a product. Here, the analyst also does not determine the potential in vivo dissolution characteristics of a product but requires that in vitro dissolution results match those obtained from a bio-study by adjusting the experimental conditions. An even more confusing aspect of this practice is that the analyst is expected to describe this matching exercise as developing a bio predictable dissolution method. Sometimes such confusion and misunderstanding are quite obvious, for example, as described in a recent publication.

“Adjusting dissolution testing conditions to match the behavior of the formulations in vitro with that in vivo by taking into account the properties of the drug and the formulation is a straightforward and useful approach in identifying a predictive method in the development of the IVIVC. These investigations will definitely help by derisking of new formulations as well as by rating changes in existing formulations with regard to their impact on bioavailability before entry into human”.

Such described methods are neither bio-relevant nor predicable, instead, it is simply obtaining expected results by adjusted experimental conditions. A simple analogy to such an approach would be that one could start dissolution testing with a high-speed stirrer (blender) and keep adjusting the stirring speed lower and lower until desired dissolution results are obtained. Obviously, this will not be considered a predictable of the bio-relevant test. The analysts must work with a standard and common procedure independent of the test product for appropriate testing. If a product does not provide relevant or expected dissolution characteristics, then the product attributes (formulation/manufacturing) are to be changed, not the dissolution method to achieve or match certain desired results. This is not a requirement for just dissolution testing but a standard scientific principle and procedure of analytical chemistry. This is where current practices of dissolution testing have deviated from standard scientific principles.

The reason for this deviation is because of the recommended dissolution apparatuses, in particular paddle and basket, which are flawed. There are numerous examples/studies provided in the literature describing their flaws. These apparatuses are not capable of providing relevant and reproducible dissolution results. However, the analysts and manufacturers are required to use these flawed apparatuses and experimental procedures to meet regulatory requirements. Thus, the dissolution results reported are often fulfilled the regulatory requirements. These results often do not reflect the characteristics or quality of the products.

As it stands now, therefore, analysts face a tough situation where, in principle, s/he is expected to establish the quality of drug products, but with the use of apparatuses that are known to be flawed. A more detailed discussion on this topic is provided in the literature and in various posts on this blog (please see under sub-menu “useful lists“).

 A  modified spindle has been proposed to address the flaws of the basket and paddle apparatuses. The use of the crescent-shaped spindle with a single set of product-independent experimental conditions simplifies the dissolution practice significantly. In addition, it provides more appropriate and relevant dissolution characteristics of the products. The details about crescent-shaped based approach may be found in the literature (e.g. Link), on this website/blog and the upcoming course.