It appears that there is confusion in the description of these terminologies. In reality, these are one and the same thing. Let me explain:

A bio-relevant dissolution test is a test, which should be able to differentiate (discriminate) between the in vivo behaviour of two or more products. The in vivo behaviour means bioequivalency or bio-in-equivalency of the tested products. Therefore, by its very nature a bio-relevant test becomes a discriminatory test as well.

So, why are these two terminologies often referred to as different or separate? It is due to the fact that in the dissolution testing area, a discriminatory test is also described as, wrongly, an in vitro dissolution test, which may show formulation/manufacturing differences without their in vivo relevance or consequence. Such tests are often described as QC or consistency-check tests (e.g., in pharmacopeias). It is important to note that these tests do not relate to the in vivo characteristics of products, however, they are still expected to reflect quality of the products for humans use. How? It is not clear and is the most confusing part of current dissolution practices! In my opinion, these tests (QC or in vitro discriminatory), and their requirements, are not very useful and are conducted as a “tradition”. Continue reading →

It is often noted that drug dissolution results fail to reflect the in vivo behaviour of a product, i.e., lack of relationship to bioequivalence of two test products. Therefore, it is usually inquired as to how one should explain the lack of relevance and/or how to address this issue.

There could be a number of potential causes for such a problem (lack of bio-relevance). In addition, the problem could only be related to the specific product. In such cases, it is difficult for others to provide suggestions without knowing the details about the formulation of the product, which are often not available because of the proprietary nature of the information. Therefore, it is almost impossible that one can obtain a direct and/or a correct suggestion to address the problem. The formulator should be aware that he/she might obtain completely un-related suggestions which may end up wasting a lot of time. Keeping these thoughts in mind, a few very general suggestions are provided here which may be helpful in such situations.

There are usually two potential outcomes in such situations (lack of bio-relevance): (1) in vitro dissolution tests show different (the so called “discriminatory”) results, but the in vivo results are similar; (2) in vitro dissolution test show similar results, but the in vivo results are not similar i.e. bio-in-equivalent.

In the first scenario, the most likely cause is that the test may have been conducted using much softer conditions (in particular, related to stirring/mixing) under the “requirements” of obtaining “discriminatory” results. For example, if the formulations of the two products are such that one hides the drug (API) better than the other at the bottom of the vessel then one will observe different in vitro dissolution results, but most likely similar in vivo results. Drugs with low aqueous solubility and/or of low content would show such a problem. This is the most commonly observed issue with the use of paddle/basket apparatuses as these provide poor, and/or lack of, stirring and mixing required. Commonly such tests are referred to as “overly discriminatory”, however, in reality these tests are incorrect tests, mostly because of the incorrect choice of a dissolution apparatus.

In the second scenario, the most likely cause would be of chemical nature, such as an interaction between a drug and an excipient. It is possible that during dissolution testing a complex has been formed, or dissociated, with much higher solubility, which may not be the case in vivo. The likely suspect in this case may be the pH of the medium. One should make sure that the pH of the medium has not been inadvertently increased as this will certainly cause this problem (use of larger amounts of SLS may be a prime suspect here). Some focus on the chemistry aspect of the product (drug excipient interaction) would be very helpful.

The next consideration should be given to the use of the paddle/basket apparatuses themselves. If possible, avoid using these apparatuses as these can be the main cause of the problem. Even if you are going to try the suggestions provided above, the use of paddle/basket may hinder in establishing cause of the problem. Consider using an apparatus which will provide efficient and reproducible mixing and stirring.

As described, in one of my earlier posts, one can easily perform an analytical method validation by spiking the dissolution medium using solution of a drug (API, link). The suggested approach is scientifically correct and valid, however, I do see where dissolution scientists in general will face difficulties. Let me explain:

Suppose an analyst follows the suggestion made, and obtains a %RSD of “X” for the method (say less than 5) and the analyst is happy with it. Now the analyst proceeds to the next step and tests the tablets and gets a %RSD of “Y”.  Under normal circumstances, this will reflect the %RSD of the product which will include %RSD of the method as well. Usually there are no concerns as most often this value of “Y” comes out acceptable, between 5 and 10.

However, dissolution testing, in particular using paddle/basket, faces a unique problem, that it introduces one additional variability component which is very well documented in literature. This is because of the positioning effect of the tablet/capsule i.e., where it settles at the bottom of the vessel (link1, link2, link3). Unfortunately, people do not realise how such a minor variation can cause a big problem, but it does. As one cannot control this positioning effect, therefore, one cannot control variability due to this effect as well. It is totally random. The contribution from this random effect is reported to produce very high RSD, up to 37% (link). So, when it is asked what should be the expected variability for drug dissolution testing of a product, a safe bet/estimate is 37%. A product may have excellent repeatability/reproducibility of its drug dissolution characteristics (with extremely low %RSD), however, dissolution results may or may not reflect this low variability.

It is just like any other biased, but random phenomenon, where one may or may not succeed, however, one always sees advertisements of some examples of big winnings/successes. In dissolution terminology, one may observe some low %RSD values, at random, but overall variability using paddle/basket apparatuses will always be high. There are number of publications available describing this high variability aspect, which may be useful. In addition, some posts may also be useful in this regard e.g. see link.

If you are re-visiting this post, please make sure that you read the correction at the end of this post.

It appears that there is confusion that to develop IVIVC one is required first to de-convolute a plasma drug concentration-time (C-t) profile to obtain a so called “weighting function or factor” and then this function should be used to predict C-t profiles. The confusion appears to come from the way the concept and practice of IVIVC have been presented in literature.

As described in some earlier posts (link1, link2, link3, link4), and a publication (link), to develop or evaluate products one does not require IVIVC. The IVIVC is a step to relate in vitro dissolution to in vivo dissolution/absorption this is why one requires a de-convolution step to obtain in vivo dissolution from a C-t profile. However, it is very important to note that during the product development and evaluation stage one does not have C-t profiles, and the formulator is required to predict/estimate C-t profiles using experimentally observed in vitro dissolution results of test products. Therefore, at this stage the formulator cannot use the de-convolution step.

On the other hand, as stated above, at the product development stage, one needs to predict C-t profiles, for this purpose the only option is to use the convolution method. Mathematically to use the convolution method one would require a “weighting function or factor”, which in reality is the drug elimination rate equation, following drug administration using IV bolus. This weighting function or elimination rate equation can be obtained from literature. For most drugs, the elimination rate equation can easily be derived using the elimination half lives. Thus, there is no reason to conduct a bio-study to obtain this weighting function or elimination rate equation, as literature often suggests.

To conclude, for predicting C-t profiles one only requires a one step convolution method. The convolution method requires the use of a weighting function, which in reality is the elimination rate equation of the drugs, which can be obtained from literature. Combining the dissolution results with the elimination rate equation (weighting function) along with the volume of distribution and bioavailability values of the drug, also obtained from literature, and using the suggested spreadsheet software provide the required C-t profiles.

Note: Earlier it was stated that one obtains the “input function” from the deconvolution step. However, it was meant to be the “weighting function or factor” which one obtains from the deconvolution step. Therefore, accordingly, the wordings have been changed in the post. My apologies for this oversight (March 18, 2012).

It is commonly suggested that a dissolution medium should be de-aerated or de-gassed which presumably helps in reducing the variability in dissolution results. It is to be noted that it is not the presence of air, or gas, in the medium which causes the problem. It is the formation of the bubbles from these gases which may cause the problem. The question is why and how these bubbles are formed. If the source of bubble formation is established and then removed, only then this problem can be addressed.

The source of the bubble formation may be explained as follows: Drug dissolution tests are conducted using media maintained at 37 °C, however, media used are generally stored at room temperature which is lower than 37 °C commonly around 20 °C. Therefore, when a medium is transferred to dissolution vessels/baths and heated up to 37 °C, there is a change in solubility of the dissolved gasses, which are from the air thus de-aeration terminology, from higher to lower solubility. The decrease in solubility of the gasses at higher temperatures causes the dissolved gasses to come out of the medium in the form of tiny bubbles which tend to stick at random to the vessel and spindle surface, and may be to the product itself. However, once the medium is equilibrated at 37 °C the formation of the bubbles stops. Therefore, the answer to the question of why and how the bubbles are formed, is because of a transitory stage during the heating process of the dissolution medium. A simple solution to avoid this problem is to remove the temperature gradient effect, i.e., avoid transferring low temperature medium directly into the dissolution vessels. Therefore, the analysts should heat the medium to 37 °C out side the dissolution vessel or give sufficient time for the medium to equilibrate in a dissolution vessel at 37 °C with moderate stirring.

However, unfortunately, a practice of de-aeration or de-gassing has been introduced to address this problem of bubble formation. It is a practice which does not appear to be well thought out. The practice not only has its practical limitations, but also makes the drug dissolution testing irrelevant and unpredictable. For example:

1. Physiological environment does not require a de-aerated medium. Obviously, if the results obtained using a de-aerated medium they will not relate well with the physiological characteristics of the product.
2. The commonly suggested procedure of de-aerating, which is based on heating/vacuum steps, is without a measurable endpoint. Therefore, the de-aerating step will always be variable and unpredictable, thus it will introduce variability in testing.
3. In addition, no matter how reproducible one tries to be with the de-aeration step, after de-aeration the medium will quickly start equilibrating itself with the atmospheric gasses. Therefore, until this equilibrium is reached, the system will remain unstable and unreliable.
4. Often media containing detergents such as SLS are difficult, if not impossible, to de-aerate due to excessive foam formation. Therefore, one may not be able to work with de-aerated medium using SLS.

On the other hand, the medium equilibrated at 37 °C within a dissolution apparatus, or external to it such as by keeping in a water bath, provides a simple, stable, reproducible and physiologically relevant alternative.

One of the requirements to conduct an appropriate drug dissolution test is to use a sufficient volume of dissolution medium, which should be able to dissolve the expected amount of drug released from a product. This ability of the medium to dissolve the expected amount of drug is known as a “sink condition”. It is important to note that a dissolution test should not be conducted in a volume of the medium which would not dissolve the expected amount of the drug completely and freely. This is because of the obvious reasons that even if a product contains and releases 100% of the drug as expected, one cannot measure (quantify) it because for quantitation/sampling  the drug has to be in the solution. Therefore, it should be noted that this requirement of “freely soluble” or “sink condition” is the requirement for the quantitation (analytical chemistry) and has nothing to do with the physiological aspect. Considering that often times limited volume is available for in vitro dissolution testing such as with the vessel based apparatuses, one is required to add some solubilising agent (e.g., SLS) to create the sink condition for quantitation of the drug.

It is to be noted that a physiological environment deals with the availability of limited volume in a completely different manner. Here, the drug is continuously absorbed into blood, metabolised and eliminated, which provides a very efficient mechanism for providing a high solubility equivalent. Continue reading →