Vivian A. Gray
V. A. Gray Consulting, Inc., Hockessin, Delaware, U.S.A.
INTRODUCTION
Dissolution Testing is a critical part of the characterization of the drug product. The test involves an elaborate sample preparation step, where the product dissolves under con-trolled conditions using prescribed equipment. This chapter will describe the equipment, sources of error when performing the test, how to validate the method and qualify the equipment, and lastly how to develop methods from simple dosage forms to the more novel dosage forms of today.
HISTORY OF DISSOLUTION TESTING
In the late 1800s, pill absorption was related to dissolution, and the earliest experiments with in vitro–in vivo correlations occurred in the 1930s. In the 1950s, disintegration testing became official in USP XV. The Kefauver–Harris drug amendments were passed in 1962 to ensure drug effectiveness as well as safety. A USP-NF Panel was created to examine physiologic availability and evaluate mechanisms to help assure drug effec-tiveness. The Panel recommended the need for dissolution testing and the rotating basket apparatus was chosen based on salicylic acid tablet performance. During the 1970s, there were 12 official monographs in USP using baskets. In the early 1980s, the USP proposed a single-point method, 75% in 45 minutes with water as medium. This specification was, in retrospect, mainly for the BCS Class I (highly soluble/highly permeable) compounds (1). In the 1990s, testing using profiles came into the mix with FDA requiring profiles in all the dissolution and drug release guidances. The FDA also pushed for specifications that were tighter than the 75% in 45 minutes, and instead required 80% in 30 minutes.
This was to assure there was manufacturing control. Today dissolution issues center around the poorly soluble drugs (BCS Class II—poorly soluble/highly permeable), since this type of product has become the norm. The call is for more clinically relevant specifications, and in particular, in vitro and in vivo correlations when appropriate. There are many novel dosages forms now seeking regulatory approval, these products require unique methods and apparatus. The concept of quality by design (QbD) is presently affecting the way analysts view the dissolution test. Does it add value?
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There are three stages in the dissolution process. The first is the disintegration of a gross tablet to particles of various sizes. This can be measured by the Disintegration Test in USP General Chapter< 701> (2). This stage also includes the rupturing of the capsule shell. Then there is the deaggregation step, where there is a breakdown of the dosage form into discrete particles that increases the surface area, providing solid-liquid interface and beginning dissolution. The dissolution process continues, and the rate is measured by the dissolution test.
The dissolution rate is represented mathematically by the Modified Noyes and Whitney Equation (3).
Rate¼ kDS=vh ðCs CtÞ
whereD is the diffusion rate constant, S is surface area, v is volume of the dissolution media,h is thickness of the saturated layer, Csis concentration of the API at saturation, k is the dissolution rate constant, and Ctis the concentration of the bulk solution. Special attention should be paid to the thickness of the saturated layer as this is where the influence of paddle or basket speed on the dosage unit boundary layer is evidenced. If sink conditions are met, the concentration of the bulk solution should be the concen-tration of the drug at saturation, diluted by at least a factor of three. It is clear from the equation that the drug substance surface area and hence particle size are very important factors in the dissolution rate. The typical dissolution test measures the rate at which a drug substance dissolves from the dosage unit. The term “in vitro release” is more appropriate in the case of an extended-release (ER) product, since drug is released from a matrix then dissolved in the media. The dissolution rate may be defined as the amount of active ingredient in a solid dosage form dissolved in unit time under standardized con-ditions or liquid-solid interface, temperature, and media composition. The dissolution results are typically expressed as a cumulative percent dissolved,Q, of the label claim, over time intervals, until at least 80% dissolution is obtained.
When approaching the dissolution of drug product, there are three aspects to consider: the solubility of API, which is typically an equilibrium process; the dynamic process of the dissolution rate; and lastly, but of major influence, the effect of excipients, and the manufacturing process. The later may enhance or impede the dissolution.
REGULATORY AND COMPENDIAL ROLE IN DISSOLUTION TESTING The Food and Drug Administration
A discussion of dissolution testing begins with the primary regulatory agency in the United States, the Food and Drug Administration (FDA). The role of the FDA regarding dissolution extends beyond the obvious role of approving drug products, thus approving dissolution and drug release tests. The FDA by law is the enforcer of the USP standards put forth in the Compendia. FDA has published many guidances related to dissolution.
They have led the scientific debate and issues by cosponsoring workshops with the American Association of Pharmaceutical Scientists (AAPS), USP, and other organ-izations. The formation of task force groups to address current issues has been a very powerful tool in drafting science-based regulations. For example, the task force on gelatin-coated product cross-linking (4) was able to propose addition of enzyme to dis-solution medium.
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The FDA labs perform off-the-shelf testing and validation of NDA methods. The compliance officers perform inspections; a major concern for the pharmaceutical industry is the FDA issuance of recalls, many of which are based on dissolution results. Also along these lines, the FDA issues 483 warning letters, some of which are concerned with dissolution issues.
The FDA Guidances
The main FDA guidances related to dissolution and drug release are listed below:
1. Dissolution Testing of Immediate Release Solid Oral Dosage Forms.
2. Extended release oral dosage forms: Development, evaluation, and application of in vitro/in vivo correlations.
3. SUPAC-IR: Immediate-release solid oral dosage forms: scale-up and post-approval changes: chemistry, manufacturing, and controls, in vitro dissolution testing, and in vivo bioequivalence documentation.
4. SUPAC-MR: Modified-release solid oral dosage forms: scale-up and post-approval changes: chemistry, manufacturing, and controls; in vitro dissolution testing and in vivo bioequivalence documentation.
5. SUPAC-SS: Nonsterile semisolid dosage forms: scale-up and post-approval changes:
chemistry, manufacturing, and controls, in vitro release testing and in vivo bioequi-valence documentation.
6. Waiver of in vivo bioavailability and bioequivalence studies for immediate-release solid oral dosage forms based on biopharmaceutics classification system.
United States Pharmacopeia
The influence of USP on dissolution testing has been critical; many initiatives for dis-solution testing, including equipment prototypes and the acceptance criteria, came from USP as the various committees and staff worked with the pharmaceutical industry as well as equipment manufacturers to promote accurate and reproducible dissolution tests. USP has several General Chapters devoted to the area of dissolution and drug release, but first a discussion of disintegration is needed.
General Chapter Disintegration< 701>
Disintegration testing has been in existence since 1950 (USP XV). The test was intro-duced when it was realized that tablets that were made very hard (so they would not chip) also would not disintegrate in the gastrointestinal tract. In 1997, an important discovery by Hoag (5) showed that many vitamin products containing folic acid were not meeting the standard of dissolving within an hour. The disintegration test was mandatory for oral dosage forms for 40 years, but its elimination and replacement with dissolution testing became a standard-setting issue in 1981 (6). This was because the disintegration test was not believed to correlate with in vivo performance (7). The apparatus is seen in Figure 1.
From 1990 to 1995, the disintegration tests in the USP were replaced with dissolution tests and the disks were removed.
Now it appears that the disintegration test is re-emerging as the test of choice for fast-dissolving products that have a disintegration test that can relate results to dissolution rates. This is shown in the ICH document Q6A, Decision Tree # 7 (8). As the debate of added value for the dissolution test continues, it may be that more disintegration tests will be the regulatory test for products where disintegration is the only critical release mechanism.
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The disintegration test is the method now being cited in the Nutritional Supplements section of the USP, with General Chapter < 2040> as the recommended procedure.
General Chapter< 711> Dissolution
This General Chapter describes the dissolution procedure to be used when testing a monograph product (9). Other than the official test procedure and diagrams of equipment, this chapter contains special notes and instructions on various topics. One of the more recent changes is the allowance of enzyme addition to the second dissolution test when a capsule or gelatin-coated product fails the dissolution test. This addition is an outcome of the FDA gelatin task force mentioned in the section on FDA. The chapter also includes special statements on deaeration/bubbles, calibration, apparatus dimensions, filters, sinkers, and automation. By the early 1990s, the exemptions for chewable tablets and soft gelatin capsules were removed.
In April 2006, the Chapter was officially harmonized with Japanese Pharmacopoeia (JP) and European Pharmacopoeia (EP). There are now elements of the General Chapter < 724> Drug Release within < 711>. Those elements are the ER Apparatuses 3 and 4. Apparatuses 5–7 remain in< 724>, with that chapter now applied to transdermal dosage form testing.
General Informational Chapters
The content of USP General Chapters above < 1000> is considered “informational,”
somewhat like a guidance. However, if these chapters are referenced in CMC filings, they FIGURE 1 USP disintegration apparatus.
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take on official status and must be followed. General Informational Chapter < 1088>
In Vitro and In Vivo Evaluation of Dosage Forms was the precursor to the FDA guidance, Extended Release Oral Dosage Forms: Development, Evaluation, and Application of In Vitro/In Vivo Correlations. Within this chapter, there is immediate/extended release in vitro evaluation or method development instructions. The chapter’s main focus is the in vivo evaluation of modified dosage forms and how to perform in vivo-in vitro correlations.
The General informational Chapter < 1090> In Vivo Bioequivalence Guidances mainly tells how to conduct bioequivalence tests and contains bioavailability protocols for certain products. This chapter merely repeats what is available from FDA and may be revised to serve some other purpose, probably that of interchangeability.
A very important chapter for all testing procedures is the General Informational Chapter< 1225> Validation of Compendial Methods. This chapter is not very informative for dissolution testing methods, and only targets a typical analytical finish to the test, that being chromatographic analysis, mainly by HPLC.
The New General Informational Chapter< 1092> the Dissolution Procedure:
Development and Validation
This chapter was official in August 2006 (10). This chapter is of utmost importance for dissolution testing and will be explored in greater depth in later sections. The chapter originated with an article written for the Pharmacopeial Forum (11) introducing the concept of a general dissolution chapter that gave guidance on method development and validation of those methods. It was based on industry practices on these topics. The original authors were Vivian Gray, Lew Leeson, Cindy Brown, and Jennifer Dressman; as it progressed to a proposal for USP, the feedback from the USP Expert Biopharmaceutics Committee and comments from PhRMA and other entities were incorporated. The chapter also encourages new technology and automation by instructing on how to vali-date these analytical methods.
USP Expert Committees and Panels
The standards related to dissolution and drug release issues are addressed by the USP Biopharmaceutics Expert Committee, which is elected every five years according to the revision cycle. The committee members for 2005–2010 are Thomas Foster (Chair), Clarence Ueda, Vivian Gray, Lew Leeson, Eli Shefter, Diane Burgess, Nhan Tran, Leon Shargel, Bryan Crist, Alan Parr, Johannes Kraemer, William Simon, James Polli, and Mario Gonzalez. There are also various Advisory Panels that are selected to address pertinent issues. In 2007, several Advisory panels are working on topics of performance verification testing (previously referred to as calibration) and performance testing for all forms of dosage form delivery.
Other Dissolution Regulatory Documents
The International Federation of Pharmaceutical Scientists issued Guidelines for Dissolution Testing of Solid Oral Products in 1996 (12), and there are regulatory docu-ments from both Europe (13) and Japan (14) that address dissolution topics. There are also Dissolution General Chapters in the WHO International Pharmacopoeia, EP, and JP.
The International Conference on Harmonization (ICH) mandated that the USP, EP, and the JP harmonize the general chapters on dissolution, disintegration, and drug release.
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release acceptance criteria that are appropriate and mentions disintegration testing in lieu of dissolution testing. The second decision tree points to specific test conditions and acceptance criteria that are appropriate for immediate release; the topic of a dissolution test with or without discriminatory power is specifically addressed. The third decision tree deals with appropriate specifications for extended release. The subject of in vitro-in vivo correlations and relationships is covered.
COMPENDIAL EQUIPMENT REVIEW AND SOURCES OF ERROR
The most important aspect of the dissolution equipment is that it provides undisturbed homogenous mixing leading to complete or near complete dissolution and also is designed so that the visual observations are easily obtained. Each aspect of equipment can be a source of error. The major components of the equipment are shown in Figure 2.
There is the dissolution tester “head” containing the drive belt, spindle assemblies, and electronics for the mechanical aspects of the equipment. Then there is a water bath that includes a circulator and inlet screen where the vessels are placed, and a top plate containing insert holes for the vessels. Sometimes the vessels are “jacketed” and heated through heating elements instead of water (15). The stirring mechanisms are shafts inserted in the spindle assemblies. These shafts are one entity with either a paddle stirring device (Fig. 3) or a basket attached (Fig. 4). The vessels are inserted into the water bath and filled with dissolution medium. The paddle apparatus is referred to as USP Apparatus 2 and the basket apparatus as USP Apparatus 1. Most commonly they are simply referred to as the “basket” and “paddle.”
As a regulatory test, dissolution must be accurate and practical. Justification would be provided for atypical conditions. The test should have low variability and a good profile. Test results should show changes in the formulation and, ideally, an in vivo-in vitro relationship should exist.
FIGURE 2 Example of modern dissolution test equipment.
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The essentials of the test are accuracy of results and robustness of the method.
Aberrant and unexpected results do occur, however, and the analyst should be well-trained to examine all aspects of the dissolution test and watch the equipment in operation.
When performing dissolution testing, there are many ways that the test may gen-erate erroneous results (16). The testing equipment and its environment, sample handling, formulation, in-situ reactions, automation, and analytical techniques may be the cause of errors and variability. The physical dissolution of the dosage form should be unencum-bered at all times. Certain aspects of the equipment calibration process, as well as a close visual observation of the test, may reveal these errors.
Knowledge of drug properties, especially solubility in surfactants or as a function of pH, is essential. One could anticipate precipitation of the drug as the solution pH changes or as the amount of drug increases. Be aware that complete dissolution of the drug in the standard solution may be more difficult than expected. It is customary to use a
FIGURE 3 USP Apparatus 1: Basket.
FIGURE 4 USP Apparatus 2: Paddle.
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prepared properly.
Highly variable results indicate that the method is not robust, and this can cause difficulty in identifying trends and the effects of formulation changes. Two major causal factors influence variability, mechanical and formulation. Mechanical causes can arise from the dissolution conditions chosen. Carefully observe the product as it dissolves. An apparatus or speed change may be necessary.
The formulation can have poor content uniformity, and reactions or degradation may be occurring in situ. The film coating may cause sticking to the vessel walls. Upon aging, capsule shells are known to form pellicles, and tablets may become harder or softer, affecting the dissolution and disintegration rate depending upon the excipients and drug interaction with moisture.
Equipment Variables
The major components of dissolution equipment are the tester, water bath, paddles, baskets and shafts, vessels, samplers, and analyzers.
Mechanical aspects, such as media temperature, paddle or basket speed, shaft centering and wobble, and vibration can all have a significant impact on the dissolution of the product. Mechanical and chemical calibration should be conducted periodically, usually every 6 months, to ensure that the equipment is working properly.
The USP General Chapter on Dissolution < 711> contains a requirement for the analyst to perform the Apparatus Suitability Test using USP Calibrator Tablets. USP Calibrator Tablets come with certificates identifying appropriate ranges. The Apparatus Suitability Test is designed to detect sources of error associated with improper operation and inadequate condition of the equipment (17–19). Two calibrators are used, USP Prednisone tablets, 10 mg, and USP salicylic acid tablets, 300 mg. Use of each of these types of Calibrator Tablets involves unique considerations.
The salicylic acid tablets should be brushed before use to remove fine particles.
This should be done in the hood to avoid breathing the irritating dust. Whole tablets are used, but the tablets can be chipped or nicked. Since this tablet dissolves through erosion and is pure compressed salicylic acid, minor chips or nicks have no significant effect on the dissolution rate. The buffer should be prepared according to USP Reagent (Buffers) section.
Deaeration
The Prednisone tablets use deaerated water as the medium. There are numerous methods for deaeration of medium (20–23). Automated methods are also available. The method described inUSP 29 uses heat, filtration, and vacuum. Helium sparging is also a typical method for deaeration. The level of dissolved oxygen and other gases is related to the presence of bubbles. Bubbles are common and will cause problems in non-deaerated medium. USP General Chapter on Dissolution< 711> states that bubbles can interfere with dissolution test results and should be avoided. Dissolved air can slow down dis-solution by creating a barrier; bubbles may adhere to either the tablet surface or to basket
The Prednisone tablets use deaerated water as the medium. There are numerous methods for deaeration of medium (20–23). Automated methods are also available. The method described inUSP 29 uses heat, filtration, and vacuum. Helium sparging is also a typical method for deaeration. The level of dissolved oxygen and other gases is related to the presence of bubbles. Bubbles are common and will cause problems in non-deaerated medium. USP General Chapter on Dissolution< 711> states that bubbles can interfere with dissolution test results and should be avoided. Dissolved air can slow down dis-solution by creating a barrier; bubbles may adhere to either the tablet surface or to basket