It is often suggested that conducting IVIVC studies, i.e., developing a relationship between in vitro (dissolution) and in vivo (plasma drug level), are necessary for developing dissolution tests capable of reflecting or predicting plasma drug levels. Unfortunately, this is not a correct view, as explained below:
Author: Dr. Saeed Qureshi, Ph.D.
When an oral product, usually a tablet or capsule, is taken, it instantly goes into the stomach (gastric compartment). The gastric environment can be described as an acidic (mostly HCl-based) aqueous solution (pH 1 to 3) with a churning (moving and mixing) process. Assuming a disintegrating type product, the product will disintegrate into solid particles/aggregates. Once in this disintegrated form, the drug will behave exactly like granules in a dilute acidic solution with mild stirring in a beaker or flask. In the case of non-disintegrating type tablets, the drug will be released or leaked-out from the unit into the acidic solution.
If the drug is soluble, it will move into the intestine as a solution, otherwise as a slurry or suspension. The important thing to note here is that the drug will move into the intestinal component with some delay. Here the acid solution or suspension will be mixed with a strong buffer turning the acidic liquid to basic, more accurately less acidic in the pH range of 5 to 7. Considering the variability in contents and the rates of the entrance of the two solutions i.e. slurry from the stomach and the buffer from the pancreas, it is almost impossible to determine or establish the pH of the soup accurately. However, it is a well-established fact that pH in this area of intestine ranges between 5 and 7. Therefore, for all practical and standardization purposes, one can use pH of 6, an average of 5 and 7.
Suppose someone is given an assignment to obtain/extract propranolol (PL) from a mixture of microcrystalline cellulose (MC) and a propranolol·HCl (PL·HCl). In a sense, it will be a fractional extraction procedure where one would exploit differences in these two compounds’ chemical or physical nature. The first difference one observes will be their aqueous solubilities; MC is not soluble in water, but PL·HCl is. So, one can separate PL·HCl from MC by simply adding some water and filtering it. The PL·HCl will remain in solution form, but MC will be separated out as a precipitate. However, PL will still be in its hydrochloride form. To extract the PL, one may require a liquid-liquid extraction step. In this regard, one first needs to adjust the pH of the aqueous solution so that the HCl part can be neutralized and PL·HCl should be available as PL, which could then be extracted with an organic solvent (e.g., hexane or dichloromethane). Adding some alkaline solution to the PL solution will increase the pH of the solution to a much higher level, e.g., pH 12. Most of the PL will now be in undissociated form and can be extracted into the organic phase. One or two extraction repeats will transfer PL into the organic phase, which may be removed by evaporation, leaving behind pure PL in its native or basic form.
On the other hand, if one is unable to increase the pH of the solution to 12 to avoid potential complications, then a lower pH may be used. Then, the same extraction step can be used. However, one would require an increased number of extraction repeats to complete PL extraction from the aqueous solution. The end result will be the same i.e. complete extraction of PL form in its native or basic form in the organic solvent.
It is important to note that one can also perform the above-described extraction in one step (i.e. without separating MC by filtration first). In this case, adding some milder buffer having a pH around 7 to the mixture, to avoid any complications of higher pH with MC.
Now let us assume that this extraction process of PL is to occur in the intestinal tube rather than in a glass test tube, as explained above. The content of the intestinal tube is at a pH between 5 and 7 and contains multiple endogenous and exogenous compounds, including PL and MC. The organic phase for the test tube experiment is replaced with the lipid layers of the intestinal tube walls. When the undissociated PL comes into contact with the intestinal tube’s lipid part, it will get absorbed or extracted. This process will occur almost an infinite number of times (considering the vast surface area of the intestines). Once PL, or any other drug, gets absorbed, it will be transferred to the bloodstream.
This process of liquid-liquid drug extraction is commonly referred to as absorption of drugs in humans by passive diffusion. However, the process in both cases (in vitro vs in vivo) is almost the same, if not exactly the same. It is worth remembering that most drugs absorbed through the GI tract occur through a passive diffusion process.
This means that when someone takes a solid oral product (tablet and capsule), the drug must first come out of the product and then dissolve as a non-polar (undissociated) drug in the intestinal fluid. The drug does not have to be completely dissolved but to the extent that the continuous process of extraction/absorption occurs efficiently and sufficiently within the intestine.
In conclusion, drug absorption occurs as a liquid-liquid extraction process for which dissolution of the drug within the GI tract is one of the most critical steps. However, it is generally not necessary or required that the drug has to be completely dissolved at a given time for successful and efficient drug absorption. Efficient and successful drug absorption can occur by continuously replenishing the extracted/absorbed portion of the drug, as often low solubility drugs (mostly non-polar) show quite a high and efficient drug absorption.
In a recent article, titled “Stage Appropriate Dissolution Methods in Formulation Development,” published in the above-mentioned journal, the author presented a view as to how dissolution method requirements change as a project advances in time (link). Unfortunately, not only is this view logically flawed but scientifically invalid as well.
A dissolution method is used to estimate drug release characteristics of a product, mostly tablets, and capsules. Therefore, by definition, a method just like any other scale or measuring method (thermometer, weighing scale, density, etc.) must remain constant. A product, or stage, dependent scale/method will be considered scientifically invalid for this reason.
Further, during the product development stage, a dissolution method is used for evaluating the impact of different variables (formulation and/or manufacturing) so that a product with appropriate drug release characteristics is developed. Therefore, again, a constant method is required during the product development exercise. If the suggestion is to keep changing the methods (scales) at every stage, then one wonders how one would establish dissolution characteristics/rate of a product or any product. For a more detailed explanation and discussion on the topic, please follow the links:
(1) Limitations of Some Commonly Described Practices in Drug Dissolution Testing and Suggestions to Address These. (link).
(2) Blog (link)
In my view, the author had provided information that is not scientifically valid and would not help develop useful dissolution methods.
Note: This post has been shared with the author of the article, who provided the following response which is greatly appreciated. Also, I took the opportunity to introduce Dr. Hawley to the newly suggested crescent shape spindle which may help develop a “universal” dissolution tester. Saeed
I appreciate the thoughtful comments above, and especially the opportunity to respond to them. However, I do disagree that the article we published was logically flawed and scientifically invalid. The crux of the argument above is that the dissolution method should be an invariant test which is representative of the in-vivo system. This method should remain in place throughout the product’s development from pre-clinical studies to commercial. As such, it acts as an absolute arbiter of the performance of the formulations developed. I have been extremely fortunate to work with many excellent scientists in industry in my career. Unfortunately, I have not yet met any who have been able to easily identify a dissolution method to unfailingly predict the drug release characteristics of any drug product early in development. If we could, I would agree with the critique above, this article would not need to have been written, and I would use this defined dissolution method without alteration through development.
In the course of developing a drug product however, we go through several phases. In the early stage, when there is a large likelihood that the project will fail, we preserve our resource and may use something simple such as intrinsic dissolution or disintegration to assess the performance of our drug product. Admittedly crude, yes; not really representative of the in-vivo situation, yes also. However, these tests can identify critical flaws in the formulation approach which would impede the success of the clinical study, which is the real goal for the project. Later on, we may have a totally different formulation approach as, for example, we move from a powder in capsule dosage form to a tablet. If we had infinite resource to invest early on, we could develop the understanding to come up with a decent, representative method at this time. However, I do not think that is wise deployment of either formulation or analytical resources. Instead, much as many other analytical assays, as we progress the project, we learn and we revise our methods to enable us to measure the properties we are interested in. As the project team comes closer to defining a formulation, we do lock in our dissolution method as soon as it is practical. This paper was written to show what kinds of considerations should be taken into account when going through this process.
The purpose of this paper was really to discuss how to use different dissolution tests methods (rotating disk dissolution, USP Dissolution, multi-compartment dissolution) at different periods of the product development cycle to understand the mechanism by which the formulation works in order to design a better formulation (and we would argue – a better, final dissolution method). The clinical model is the true unwavering test that we all need to perform against and that is the standard by which the performance of our formulations are really measured, not by the in-vitro assay. When we reach the state where we have the ability to develop a representative dissolution method using little time, little resource and little API early in development, I will happily agree with the statements above and follow the protocol prescribed above. Until then, however, I respectfully dissent.
Michael Hawley, Ph.D.
It is important to note that, by definition, a drug dissolution test has to be a bio relevant test. A non-bio relevant dissolution test is just like a non-bio-relevant thermometer or non-bio relevant pair of eyeglasses i.e. such things have no practical use or purpose. However, unfortunately, in the pharmaceutical area, in particular for oral (tablet/capsule) products, not only does such non-biorelevant testing exist (e.g. pharmacopeial) but it is the norm, strongly promoted and defended, which causes enormous confusion and financial losses.
The reason for this confusion is that non-biorelevant methods are presented as biorelevant and in fancy wrappings, or with catchy phrases, e.g. the one mentioned in the title (“biorelevant performance testing”) or by confusing with other names such as BCS, IVIVC, bio-waivers, f2, QbD etc. In reality, the issue is not how dissolution testing is presented and described, but rather how the tests are conducted and evaluated.
For example: (1) the apparatuses currently used, even those recommended by regulatory authorities, have never been qualified and/or validated for dissolution testing purposes. In fact, it has been shown many times that the apparatuses provide irrelevant and unreliable results; (2) recommended experimental conditions are mostly selected arbitrarily lacking physiological or scientific rationale; (3) tests are conducted using product-specific (i.e., not product independent) procedures or experimental conditions thus results obtained are biased and cannot relate to the actual quality of a product; (4) there are no existing criteria or standards available which could be used to relate dissolution results for product quality. That is, no procedure is available to set physiologically relevant tolerances with scientific or statistical relevancy or credibility. For further details, see here.
In conclusion, if dissolution results have been obtained using traditional approaches/methods, then their interpretation and usefulness will be of questionable merit at best.
It is generally accepted that for a drug to be absorbed from the human gastrointestinal (GI) tract, it should be in a solution form established based on the drug’s solubility/dissolution characteristics. This in vivo dissolution is determined using in vitro drug dissolution tests.
It is also generally accepted that the higher the solubility of the drug, the higher the dissolution and absorption, and their corresponding rates, will be. In addition, it is also a well-established fact that absorption preferentially occurs from the non-polar or undissociated form of a drug. On the other hand, the undissociated, or non-polar moiety, of a drug often shows lesser aqueous solubility compared to its polar version.
For example, propranolol is a basic drug with a pKa value of 9.42 and its aqueous solubility is 61.7 mg/L or 1 part in ~16,000 (link). Therefore, propranolol should be considered to be a low solubility drug. However, its products are usually manufactured using the drug in its hydrochloride salt form,i.e., propranolol·HCl, which is freely or highly soluble in water. It would exist in its ionic/protonated form in water, which would be less absorbable than the native propranolol. On the other hand, propranolol is known to be highly absorbable/permeable (bioavailability higher than 90%), which suggests that in reality, the body sees propranolol as non-polar/undissociated moiety. Therefore, for in vivo dissolution/absorption purposes, the solubility of native propranolol should be considered, not of its salt form. This means that in reality, propranolol (and other similar drugs) is a BCS class II drug and not the class I drug, as commonly considered.
In conclusion, for drug dissolution and absorption evaluation purposes, one should consider solubility characteristics of a drug in its native form and not that of its salt form. For further discussion on the topic, the following links would be useful (1, 2, 3, and 4).
Drug dissolution tests are routinely conducted to evaluate drug release characteristics of pharmaceutical products such as tablets and capsules. These tests should be conducted to reflect in vivo drug release, which in turn is reflected by the observed plasma drug conc.-time (C-t) profiles in humans.
In this regard, a simple convolution-based method using spreadsheet software has been suggested to convert dissolution results into C-t profiles (link1, link2). This article provides another example describing the estimation of plasma drug levels from OROS-based nifedipine products using the suggested convolution approach.
There are about 500+ dissolution methods listed in the FDA database and about 600+ methods (monographs) in the USP. In addition to these, there are many more, perhaps in the hundreds, dissolution methods described in the literature. Moreover, as part of new product development exercises, it is a common and expected practice to develop additional new or revised methods.
It may be interesting to note that the objective of drug dissolution testing has never been to develop methods but to determine/estimate drug dissolution/release characteristics of products. By developing drug and/or product-specific dissolution tests, one, in fact, would never know or determine the actual dissolution characteristics of any product. The current practices of method development simply defeat the purpose of products evaluation.
For products evaluation, one requires a test/method which is independently developed and established. Therefore, current practices of method developments are scientifically invalid and useless and a waste of time and resources.
Using the crescent shape spindle with a common set of experimental conditions is suggested to address the current difficulties. The suggested approach practically eliminates the need for method developments, particularly product dependent, and provides a scientifically sound and valid drug dissolution testing and product evaluation approach (e.g., see link, link2).
The following links may be useful for further information regarding the difficulties of the current practices:
- Drug Dissolution Testing – A serious concern! (link)
- Costly mistake formulators/analysts often make, i.e., developing a product dependent dissolution test (link)
- Selecting a Dissolution Apparatus – Some Practical Considerations (link).
- (Developing) a discriminatory vs. bio-relevant test (link)
- Method Validation: A Unique Problem Concerning the Drug Dissolution Testing (link)
- Where does 20% of the drug go? (link)
- Dissolution method development – a practice that causes confusion and hinders in product evaluation. (link)
- Drug Dissolution Testing Mosaic. (link)
- Product dependent dissolution testing – a scientifically invalid practice. (link)
- How to conduct a dissolution test? A simple question but confusing answers. (link)
- An incorrect reason for developing and conducting a dissolution test. (link)
- Method development practices: Are these achieving their intended purpose? (link)
- Dissolution method development – what it is not! (link)
- Generics and discriminatory dissolution tests (link)
- Validation (Method/Apparatus) Practices (link).
- Selecting an apparatus and its associated experimental conditions for testing (link).
- The science of drug dissolution testing: Testers or apparatuses, experimental conditions, and interpretation of results – A systematic approach for learning (link)
Developing an IVIVC and its applications are often described in the literature as follows (e.g., see link):
“In vitro – in vivo correlation (IVIVC) allows prediction of the in vivo performance of a drug based on the in vitro drug release profiles. To develop an IVIVC, the physicochemical and biopharmaceutical properties of the drug and the physiological environment in the body must be considered. Key factors include drug solubility, pKa, drug permeability, octanol-water partition coefficient, and pH of the environment.”
There is a number of deficiencies in the description mentioned above. For example:
- “in vivo performance of the drug,” IVIVC studies are commonly conducted for products (such as tablets and capsules) and not for drugs.
- An IVIVC does not allow the prediction of in vivo performances from in vitro results. Therefore, in vitro studies (testing) are conducted based on the assumption that the IVIVC already exists.
- Furthermore, considering the existence of IVIVC, in vitro (dissolution) results are used to reflect or predict expected plasma drug concentration-time profiles.
- The mathematical approach used to predict plasma concentration-time profiles is not the IVIVC but the convolution technique. This (convolution) is the only technique that can be used or applied for the prediction of plasma drug profiles of products.
- The parameters mentioned above such as, drug solubility, pKa, drug permeability, and octanol-water partition coefficient, are all drug characteristics and not those of the products for which dissolution tests are conducted. Therefore, these parameters often remain constant or are kept constant to evaluate the impact of formulation and/or manufacturing attributes on the release/dissolution characteristics of a product.
- Regarding the “pH of the environment”, this is linked to GI tract physiology and is independent of the drugs and products. Thus for drug dissolution testing, the environment must also remain constant and independent of products and/or drugs.
Therefore, the IVIVCs as currently conducted or promoted are not of any practical use and can easily be ignored or avoided.
Drug dissolution tests are conducted to determine the dissolution/release characteristics of a product. Therefore, one requires a pre-established set of experimental conditions (apparatus, rpm, medium volume or pH, etc.) independent of the product to determine the actual or true characteristics (i.e. dissolution).
However, current practices, in particular using paddle and basket apparatuses, require that the analyst MUST first know, or anticipate, dissolution characteristics of the test product and then ADJUST experimental conditions to achieve the desired or anticipated results. As dissolution method development practices, selections or adjustments of such experimental conditions are then described or promoted incorrectly. Almost every product came with its own set of experimental conditions and expected dissolution results (commonly referred to as Tolerances). At present, one cannot know or determine the actual or true dissolution characteristics of the products. It is, therefore, very important and critical to note that current practices of dissolution testing are practically a complete waste of time and resources.
The suggestion of dissolution testing using the crescent-shaped spindle, along with a single set of experimental conditions (which are product independent as well) addresses the current issues and provides a simple, practical, and scientifically valid approach for dissolution testing. For further detail, please see these links (1, 2, 3).