Adeline Siew is editor for Pharmaceutical Technology Europe. She is also science editor for Pharmaceutical Technology.

Adeline Siew, PhD

 drug release information that is routinely used for quality-control (QC) and quality-assurance (QA) purposes in the pharmaceutical industry. The quality-by-design (QbD) approach places strong emphasis on the role of dissolution testing in optimization of a formulation’s drug release rate and evaluation of critical process parameters (CPPs) in the manufacturing process that may affect performance of the dosage form, notes Bryan Crist, scientific affairs manager, Agilent Technologies. “Data from dissolution studies form an essential component in regulatory submissions and all stages of the approval process,” he says. “Post approval, the dissolution test provides continual assurance of batch-to-batch consistency in product performance as well as the ability to reject sub-performing batches. Additionally, scale-up and post-approval changes (SUPAC) guidance relies heavily on dissolution testing to assure that these routine post-approval activities do not impact product performance.”

	According to Marcel Arndt, head of Dissolution Laboratory, Evonik Health Care, a QC dissolution test should be discriminative for critical quality attributes (CQA) of a pharmaceutical product. The dissolution rate can significantly affect bioavailability, he highlights; therefore, the dissolution test and acceptance criteria should distinguish batches with unacceptable bioavailability. “Risk assessments and definition of quality target product profiles are used throughout development and lifecycle management to identify potentially high-risk formulation and process variables. Design of experiment (DOE) procedures, which include dissolution testing, can be used to achieve an improved product and process understanding to develop an appropriate control strategy. A design space can then be defined based on dissolution test results,” he explains.

	During product lifecycle management, the dosage form or formulation constitution might change, Andreas Gryczke, global technical marketing manager, Solubilization at BASF, points out. In such cases, it is important that the dissolution test is reviewed critically.

 spoke with Bryan Crist, Andreas Gryczke, and Marcel Arndt about best practices for dissolution testing of poorly soluble, immediate-release, and controlled-release formulations and about the different analytical approaches used.

 What are the challenges in dissolution testing?

Dissolution testing is still a model with constraints. The main challenge lies in establishing a proper 

 correlation due to differences between human beings--for example, pH, drug residence time in the gastrointestinal (GI) tract, and the viscosity caused by food consistency can vary from individual to individual, influencing the drug release and ultimately drug absorption.

 For QC purposes, the challenge is to design a dissolution test method that can distinguish between different product qualities as a surrogate for predicting acceptable or nonacceptable bioavailability. For development purposes, the challenge is to achieve a dissolution test method that is able to sufficiently simulate the GI physiology in order to predict how the formulation will behave in vivo (e.g., for de-risking clinical studies).

	There are also several technical challenges to overcome. Just to mention a few: Coning is an issue in dissolution tests, where undissolved material forms a mound in the stagnant zone below the paddle, which inhibits dissolution and can be overcome by either adapting the stirring speed or by using peak vessels. Capsules may require the use of a sinker (a stainless-steel wire helix) to prevent the dosage form from floating to the surface of the dissolution media, which would otherwise result in variable dissolution. Filter clogging can restrain online ultraviolet (UV) measurements or block systems using the United States Pharmacopeia (USP) IV test setup. Undissolved particles forming a turbid suspension limit the use of fiber optic UV dissolution systems. Additionally, conducting dissolution tests for lipid formulations, which are often filled in soft gelatin capsules, can result in the floating of the low-density lipid matrix and prevent adequate mixing in the USP I and II dissolution test setups.

 Manual dissolution testing can introduce technique-dependent variability into the test and for this reason, automation is often sought as a way to standardize the dissolution with less analyst-to-analyst variability. The five major areas of the dissolution test are typically: setup, executing the test, sampling, sample analysis, and cleaning. Testing, whether manual or automated, still relies heavily upon the analyst’s level of training and attention to detail contained in dissolution standard operating procedures (SOP) and analytical procedures.

	Setup challenges begin with media preparation, which must be exactly as dictated in the dissolution method, including deaeration if required. Observational checks must ensure that the apparatus is clean, in proper condition, properly adjusted, and that sources of environmental vibration are absent from the area. Manual or automated sampling equipment must be clean and ready for use with the proper disposable filters.

	Challenges for executing the test begin with the media being accurately measured and introduced into the vessels to control the volume within 1% of its total volume through the test period with proper evaporation covers. The timing begins with samples introduced into non-rotating media; the samples must settle to the bottom prior to rotation. Dosage units should be observed to confirm they are present and not floating.

	The main sampling challenge involves introducing dosages simultaneously because samples must be withdrawn within 2% of the time they were dropped. For a 30-minute sample, this means you have ± 36 seconds to pull all six samples at the correct midway position and filter all of them into their respective vials or tubes; this is one of the primary reasons the dissolution test may be automated.

	Analytical procedures (whether using UV or high-performance liquid chromatography [HPLC]) have their own set of challenges, and the testing concludes by performing a validated cleaning procedure to ensure that all traces of the product and media are removed from the dissolution equipment.

 What are the different approaches and techniques used in dissolution testing? What are the pros and cons of the different approaches and techniques?

 The basket (Type I) and paddle (Type II) USP apparatus are by far the most widely used dissolution test equipment. Both are extensively recommended in the individual USP monographs as QC methods. The basket and paddle apparatus can cover a wide range of oral dosage forms, with the basket apparatus having particular advantages in the testing of multiparticulate dosage forms. A limitation of both the USP I and USP II apparatus is the inability to use volumes other than in the 500-1000 mL range, which thereby restricts the use of biorelevant volumes under certain GI conditions (e.g., fasted state gastric volume) in contrast to the USP IV flow-through cell apparatus.

	When detailed GI transit with multiple test media is required, for example, for pH-sensitive modified-release dosage forms, the USP III reciprocating cylinder is far more appropriate than either the basket or paddle apparatus. Another advantage of the USP III apparatus is that the more disintegration tester like hydrodynamics, keep lipid formulations dispersed in the dissolution media, which may float without any mixing in a USP I or II setup. However, because the operating volume for each vessel of the reciprocating cylinder is comparably low, it may be difficult to generate sink conditions, which may be a crucial factor especially for poorly soluble drugs.

	The USP IV apparatus is described mainly for oral modified-release dosage forms testing, but can also be used for suppositories, parenteral depot dosage forms, suspension-type extended-release dosage forms, or microparticles. For this apparatus, however, certain variables including the size of the glass beads and filter material used to maintain the formulation in the sample cell may influence the hydrodynamic behavior, thereby, making it difficult to interpret the dissolution profile. A detailed review of the different dissolution equipment is given by Kostewicz et al. (1).

 Dissolution and drug release apparatus are quite flexible to accommodate numerous dosage forms, and the requirements for testing have continued to evolve well beyond traditional oral medicines requiring only the paddle or basket apparatus. Due to alternative routes of drug administration and delivery modifications, special apparatus have been developed over the years to address products ranging from topical forms (e.g., transdermal systems, ointments, creams, and gels) to implants (e.g., drug eluting stents, pacemaker leads, and mesh); as well as injectable dosage forms incorporating micro- and nanomedicines.

	Many of these injectables or implants rely on targeted drug delivery that contain very small quantities of API. To address these needs, typical apparatus have been miniaturized to allow dissolution in small-volume vessels in 50-200 mL or less, with reciprocating holder apparatus now capable of precise and accurate drug release testing in only 5 mL. In contrast, veterinary bolus dosage forms containing grams of active drug may require 2 L or even larger-volume systems to accommodate sink conditions required to solubilize the drug during the dissolution test.

	These alternatives have provided essential product performance testing platforms to the pharmaceutical industry usually through joint collaborations with drug product and dissolution apparatus development teams. Because many of these apparatus modifications are made for new and proprietary drugs, however, the knowledge of these alternative methods of analysis unfortunately remains non-available to the public even though the new apparatus platforms are commercially available.

 Nowadays, many different dissolution test models exist. Some models such as the USP standard methods are well established, while others are complex because they try to replicate in detail in-vivo GI conditions, including the aspect of drug absorption. For QC of controlled- and immediate-release dosage forms, methods such as the USP Dissolution Apparatus 1 and 2 are suitable; whereas methods such as the reciprocating cylinder and the flow-through cell are suitable to reflect the requirements of the major challenges in achieving a sufficient bioavailability, such as in poorly soluble API formulations using polymers as a solubilizer.

	At BASF, we collaborate closely with academia (e.g., University of Copenhagen) to implement more sophisticated dissolution test models for poorly soluble drugs. An example is the so-called acceptor-donor-model that includes the aspect of biphasic dissolution, which we use for both polymer-based amorphous solid dispersions and triglyceride-based self-emulsifying drug-delivery systems.

	New approaches such as Permeapad, a multiwell plate system from the University Odense, Denmark, help to study the aspect of drug absorption better. For soft-gelatin capsule formulations, we use the same dissolution test model as for amorphous solid dispersions, but with the addition of a lipolysis model. For dermatologic formulations, the ‘Franz Cell’ apparatus is a suitable method.

	Beside the apparatus, the media play an important role. In our lab, for example, we have moved away from using simple buffer and hydrochloric acid and now use biorelevant media containing sodium taurocholate, lecithin, and pepsin, which mimic a fasted state or fed state. Lastly, the aspect of enzymatic degradation can be reflected by using a pancreatin-based lipolysis model if the formulation requires this aspect. These complex test media provide the advantage of measuring dissolution in a more biorelevant environment. Using the in-vitro and in-vivo data together with the available performant statistical test methods allow us to build complex and precise computer models that can more accurately predict the drug release performance of a dosage form.

 What would you describe as acceptable analytical practices for dissolution testing of poorly soluble, immediate-release, and controlled-release formulations?

 Most solid oral-dosage forms fall into the categories of immediate release and some form of controlled release--extended, modified, or sustained. Immediate-release products are formulated to dissolve the API(s) within 30 minutes, at which time a single time point requiring at least 80%, typically, of the label claim to be in solution is demonstrated. Immediate-release drugs that are designed to release up to an hour will require a second time point to verify a release profile through that period.

	Controlled-release testing requirements may vary greatly due to the wide variety of dosage forms that fall into this category. Most require drug release profiles consist of at least three time points: initial, midpoint, and final. The initial point early in the dosing period ensures that there is no dose dumping (i.e., a sudden and uncontrolled release of the drug). The midpoint determination ensures that the profile is controlling the release as expected about halfway through the release period, by concentration with most extended-release products, or by time with most sustained-release products. The final time point, usually near the end of the dosing interval, serves to indicate that typically a minimum of 80% of the drug is in solution and is available for absorption.

	Drug solubility plays heavily into the suitability of the drug product formulation to fully release the active component(s) when expected, whether it is an immediate- or controlled-release product. Poor API solubility can be improved by providing salt or amorphous forms to increase its rate. Many poorly soluble drugs, however, require additional dissolution practices to solubilize the drug product, including the addition of surfactants or increased agitation. Higher volumes of dissolution media are occasionally required and used in special 2-L dissolution apparatus that are available for these purposes.

 In immediate-release formulations, such as pellets of bitter tasting drugs coated with a polymer for taste-masking and moisture protection (e.g., Kollicoat Smartseal), the polymer dissolution is pH dependent. Therefore, it is necessary to establish a realistic pH-over-time profile.

	The challenge of controlled-release formulations containing freely soluble APIs and based on sustained-release polymers is to guarantee that under all circumstances (e.g., low and high alcohol concentrations, fasted and fed state), the dissolution release profile is constant. In this case, the dissolution test needs to reflect not only on media constitution, such as ethanol concentration and bile salt concentration, but also on hydrodynamic influencing parameters, such as paddle speed.

	For poorly soluble APIs, the two core aspects of formulation are:

		To bring the API into the saturated solution (e.g., by using vinylpyrrolidone-vinyl acetate copolymers in amorphous solid dispersions)

		To maintain the API in a supersaturated state until it is absorbed (e.g., by using solubilizers).

	The challenge for poorly soluble materials is to generate a suitable calibration curve. Because this curve cannot be developed in water, typically an organic solvent-based HPLC method is used to determine the correct API concentration in different resulting phases--for example, aqueous phase, lipid phase if adequate, and pellet phase for crystallites. In summary, we can say that one needs to specify first what the critical parameters for the dissolution test are and then develop the corresponding method.

 For development projects with poorly soluble drugs, biorelevant media such as Fasted (FaSSIF) or Fed State Simulated Intestinal Fluid (FeSSIF) should be considered. These media contain solubilizing ingredients such as bile salts and phospholipids at physiological concentrations, and therefore, are more precise in simulating the in-vivo solubility and dissolution rate of poorly soluble compounds than pure buffer media. Also, dissolution volumes used in the in-vitro test can be adapted to better reflect the physiological situation. Dissolution testing for QC purposes may require a non-physiological pH or the addition of solubilizers, such as sodium lauryl sulfate, to enable different product qualities to be differentiated based on the dissolution behavior.

	For dissolution testing of standard immediate-release formulations using either the USP apparatus I or II, the test setup is in general less complex and of shorter duration compared to the test setup for controlled release dosage forms. According to the European Pharmacopoeia (Ph.Eur.), one suitable dissolution medium needs to be used for conventional release solid-dosage forms. In contrast, a two-stage test is required for delayed release dosage forms. An acid stage of 2 hours, in which the formulation should release less than 10%, is followed by a buffer stage at pH 6.8, in which rapid release is targeted. Controlled-release formulations, which target a release in the colon, could even require another buffer stage at pH 7.2, for example. Dissolution testing with increased change of media can be more convenient using USP III apparatus. However, there are also controlled-release dosage forms for which the release rates are robust to variations in GI physiology (e.g., many osmotic pump formulations aim for a prolonged release at zero-order, and therefore, do not require a change of dissolution medium).

 Can you outline how to develop and validate a discriminating in-vitro dissolution test?

 A dissolution test procedure intended to be used as a routine control test for drug products should be robust, reproducible, and discriminatory to ensure consistent product quality. The formulation prototype should, therefore, be tested during development under various conditions in-vitro (e.g., media, pH, apparatus, agitation) to identify a suitable method. Preliminary tests, such as API solubility and stability in the respective media, and filter compatibility studies can contribute to the test setup. Testing conditions, including sampling time points and frequency, should be chosen to provide the most suitable discrimination. If a surfactant is used in the dissolution media, the amount needed should be justified. After the identification of an appropriate QC dissolution test which is discriminative for CQAs of the drug product, the validation process can be initiated; it should include the following parameters.

	The validation of the dissolution method should involve specificity testing to ensure that there is no interference with other components (e.g., excipients or dissolution media ingredients). Also, accuracy as well as linearity over the calibration range need to be examined and should cover the lowest expected concentration to more than the highest concentration during release testing. Precision testing (by performing replicate measurements of standard solutions) and intermediate precision/ruggedness testing (e.g., by repeating a run by at least two different analysts and different equipment from the same laboratory) also need to be performed as well as stability testing of standards and sample solutions. In later development phases, robustness should be part of the validation as well, which involves deploying deliberate changes to the dissolution conditions to determine an analytical design space.

 The complexity of developing a discriminating dissolution test is highest with new innovative products and novel delivery systems that have little or no public information. It is less complex for generic-drug products, for which there may be an abundance of public information available.

	Development requires information including solubility determinations, intrinsic dissolution, sink condition (which will assist with volume considerations), and selection of the intended biorelevant media similar to fluid at in-vivo sites where dissolution occurs. This information will ultimately help with apparatus selection and the required agitation to provide a discriminating dissolution profile.

	Sample filtration ensures that the integrity of the sample obtained during the test is preserved by removing undissolved drug substance and clarifying the sample without binding drug substance for analytical measurement. Additional method development tasks include setting specifications of specific dissolution time points, defining analytical methods of analysis, identifying the need to use sinkers, and justification for alternative test methods or components other than established harmonized pharmacopeial dissolution and drug release procedures.

	When the crucial steps have been developed and a method is written, it must be validated to ensure it is accurate and precise and as robust as possible. Validation steps should challenge media preparation and deaeration, filters and cleaning procedures, analytical solution stability, and calculations accounting for analyte and volume loss. The analytical method must demonstrate linearity, range, specificity, accuracy, recovery, and precision. It is crucial that the suitably developed dissolution method will ultimately discriminate between a good batch and a bad one and ensure that batch-to-batch consistency (in terms of drug product performance) is maintained throughout the product’s lifecycle including any changes that may occur to the formulation post approval. The USP General Chapter <1092> The Dissolution Procedure has captured many of the steps required for development and validation of dissolution methods and is an excellent resource.

 Firstly, the purpose needs to be determined--whether it is a QC method, one that measures pharmacokinetic performance, or both at the same time. Secondly, it has to be decided if all the different critical test parameters will be assessed in one method or if separate tests for particular aspects needs to be developed.

	As an example, for lipid-based formulations of poorly soluble APIs, the specific aspects of enzymatic degradation, supersaturation (high kinetic solubility), and potential precipitation of the API from supersaturated state all need to be considered. One solution to investigate the dissolution and drug release is to use a multi-compartment model (acceptor-phase and donor phase). The sample is retrieved with the complex matrix of free fatty acids, bile salts, and pancreatin, for example. The API is then separated from the matrix by HPLC to determine the API concentration in the different phases of the release process. For daily use in a normal lab, this approach is quite a complex model although still manageable. 

	As discussed, there are several approaches and techniques used in dissolution testing, but it important that key considerations such as the API, formulation, and methodology are taken into account when designing a dissolution test. 

	When referring to this article, please cite it as A. Siew “Understanding Dissolution Testing,” 

Articles

Industry experts discuss best practices for dissolution testing of poorly soluble, immediate-release, and controlled-release formulations and the different analytical approaches used.

Understanding Dissolution Testing

Unlike small-molecule drugs, which are chemically synthesized, biologics are produced in living systems such as plant or animal cells, often using recombinant DNA technology. The complexity and size of these macromolecules not only present challenges in the manufacturing step, but also in formulation development. Delivering biologics is not as straightforward as their small-molecule counterparts, and despite their blockbuster success, most biopharmaceuticals are administered by injection--a route that is not considered ideal because it is associated with pain, and therefore, affects quality of life and patient compliance.

	There is a lot of interest in oral delivery of biologics because of the convenience it offers both for the patient and manufacturers; however, success has been limited and there is still no proven technology for delivering biologics orally. Alternative routes of administration are now being explored, one of which is the potential of delivering a biologic drug through the skin using microneedles to replace the traditional syringe delivery method; and the other, by developing biopharmaceutical formulations for inhaled or nasal delivery.

	“Microneedle delivery can be an alternative to standard syringes and needles or autoinjectors,” says Lisa Dick, PhD, MTS new technology and product development manager, 3M Drug Delivery Systems. Targeting the transdermal route offers the possibility of a faster onset of action compared to subcutaneous administration, she says. “More importantly, for needle-phobic patients, microneedles are less threatening than standard injection needles,” Dick notes. “If you consider microneedle systems where the drug is coated on the microneedle patch, these systems can be virtually painless, easy to apply, and can also be potentially stable at room temperature.”

	“For transdermal products, the drug molecule needs to be relatively small (i.e., several hundred atomic mass units) and permeable through the stratum corneum,” Mark Tomai, PhD, head of TLR and MTS Business Development, 3M Drug Delivery Systems, points out. He explains that because most biologic drugs are too large for passive intradermal delivery, microneedle-based delivery systems have been developed. 3M, for example, has developed hollow and solid microneedle systems, which use biocompatible polymeric microneedles to bypass the barrier properties of the stratum corneum.

 The hollow microneedle technology by 3M is a single-use delivery system designed for self-administration. Liquid formulations are contained in a reservoir and delivered into the skin via a 1-cm

 microneedle array. The microneedles can accommodate a range of molecules and drug volumes of up to 2 mL, as long as stability in the liquid form is maintained (1). “For liquid formulations delivered through hollow microneedles, skin tolerability and viscosity are keys considerations during formulation development,” says Tomai.

	“Non-viscous formulations flow readily through narrow flow paths in microneedle-based devices, whereas viscous formulations are more prone to plugging in the narrow channels of traditional single-channel devices or require a long delivery time,” Dick observes. According to her, it is possible to deliver more viscous solutions of up to approximately 25 centipoise (cp) with the 3M hollow microstructured transdermal system (hMTS), and there are options for delivery of up to 80 cp solutions in the future.

	“The drug product formulation must also be chemically compatible with the microneedle arrays and device, including storage over time,” says Tomai. “Also, drug molecules in a liquid formulation that are delivered using a hollow microneedle injector must be physically able to withstand shear forces when flowing through the device. Such physical compatibility considerations are especially important for biological drugs, which can denature or become inactive.”

 3M’s solid microneedle technology is based on tiny needles mounted on a wearable patch, which has been designed to deliver proteins, vaccines, or other solid formulations up to 300 mcg (2). “With solid coated microneedles, there are several formulation considerations that need to be taken into account, for example, the balance between chemical stability, the ability to coat onto solid microneedles, and the ability of the formulation to release into the skin,” Tomai highlights. “Chemical stability considerations are similar to those of other dosage forms. The ability to coat a formulation onto solid microneedles is drug dependent and relies on solubility, target dose, and viscosity. For the ability of a formulation to release into the skin, it needs to be released from the microneedle surface through its inherent solubility in the dermal interstitial fluid.” Tomai adds that for every drug molecule, there may be more than one acceptable formulation composition, therefore it is a matter of testing several formulations for manufacturability and the eventual outcome for effective treatment.

Analytical testing of transdermal drug-delivery systems.

 Analytical testing for transdermal products is phase-appropriate, notes Dick. Fewer tests are carried out during early feasibility investigations and the limits are wider. “However, as more methods are added, the specifications on the results are narrowed,” she continues. 
	“The analytical testing of drug product delivered with microneedles is similar in scope to other dosage forms,” Tomai adds. “Regulatory guidance such as the International Council for Harmonization (ICH) Q1, Q3, Q6, and Q8 are consulted in addition to the United States Pharmacopeia (USP). Tests often include the following: drug content, content uniformity, impurities, microbials, and sterility.”

	For microneedle products, some device-specific requirements and testing may also be required, according to Dick. “Materials of construction are often USP Class VI and evaluated according to ISO 10993 (3),” she says. “Human factors design and testing are completed at both formative and summative stages. Device performance testing, packaging systems, and sterility controls are also considered as needed for drug products.”

	Another alternative for delivering biologics is the inhaled or nasal route, which has the potential to provide good bioavailability and direct delivery of the drug to the respiratory system (4). “There are two principle drivers towards inhaled or nasal delivery for biologics: firstly, the potential for topical delivery, and secondly, as an alternative systemic delivery route,” explains Mark Parry, technical director, Intertek Melbourn. “Topical delivery allows targeting of the product to the location of action with a potentially lower dose than would be needed for systemic intravenous dosing (e.g., treatment of a lung infection or delivery of a gene therapy to the lungs of a cystic fibrosis sufferer). As an alternative systemic dosing route, inhaled and nasal delivery presents a more patient-friendly alternative to parenteral delivery, as well as potential advantages in pharmacokinetics profiles compared to other options such as subcutaneous injection.”

 Devices used for delivering biologics by the inhaled or nasal route range from nasal sprays or nasal dry powders, to dry powder inhalers (DPI), pressurized metered-dose inhalers (pMDI), nebulizers, and soft mist devices. “Device selection needs to consider a number of factors that relate to both the properties of the compound being developed as well as the intended usage model of the product,” highlights Ashleigh Wake, director of Biological Services, Intertek Manchester. She adds that there are also related commercial considerations to be factored in.

	“Some compounds may need to be present as a solid material because they lack long-term stability in solution, but engineering a DPI formulation requires development of a suitable spray-drying process (or other suitable particle engineering technique), which adds complexity and cost to the product and its development,” Parry explains. “As a result, nebulized and nasal solution and reconstituted solution products tend to dominate the development; however, there are advantages in terms of a simpler, shorter inhaled dosing processes as well as improved shelf lives that can make DPI products more commercially desirable.”

	“In terms of device compatibility, one of the early parts of development is to look at the compatibility of the product with the materials and methods of operation of the chosen devices,” says Wake. “For example, the aerosolization of solutions for nebulization can impart significant shear forces as well as increased exposure to oxygen in the air, which must be assessed in order to select an appropriate delivery device.”

 According to Parry and Wake, the range of excipients proven for use in inhaled and nasal products is more limited than those used for oral formulations. They note that some existing parental formulations contain excipients not used in inhaled products. “This means that the choice of excipients must consider the impact on the required toxicological assessments on any new inhaled excipients,” says Parry.
	“Typical developments for aqueous solutions will need to consider tight controls of pH and ionic strength,” Wake explains. “But the use of surfactants is very common in order to control interactions with container and device surfaces.”

	Analytical testing of inhaled and nasal products. Besides the typical quality control criteria of assay, purity, and impurities, inhaled and nasal products requires specific testing to look at the product performance, Parry points out. “These tests include measuring the delivered dose (i.e., the actual amount delivered by the device to the patient) and the particle or droplet size, which affects where the product will deposit,” he says. “These parameters are critical for inhaled and nasal product performance as they ensure that the right amount of drug is delivered to the right part of the nose or lungs, and are key stability indicating tests for development and routine testing of these product types.”

	Biologics, in general, can suffer from changes in activity and structure when exposed to extremes of condition, as typical of a nebulized or nasal system, observes Wake. “When considering inhaled or nasally delivered biologic materials, it is also necessary to include analytics to ascertain if the delivery mechanism has adversely effected such parameters,” she says.

	“Consequently, it is necessary in any quality control program supporting inhaled/nasally delivered biologics to combine chromatographic methods (e.g., reversed-phase size exclusion chromatography [RP-SEC] and/or SEC) used to provide precise and accurate measure of delivered dose with analysis to confirm maintenance of biological activity (typically a cell-based potency assay) and absence of structural change, such as aggregation (SEC/dynamic light scattering [DLS]/analytical ultracentrifugation [AUC]), which can have a detrimental impact on both product efficacy and safety,” Wake explains. “The complexity of biologics analytics in any application often requires the application of multiple techniques to overcome the limitation of any one individual approach.”

Drug Delivery Systems-Hollow Microneedle Technology

Drug Delivery Systems-Solid Microneedle Technology

, accessed April 5, 2017.
	3. ISO 10993-1, Biological evaluation of medical devices--Part 1: Evaluation and testing within a risk management process (2009).
	4. M. Parry et al., 

Formulation of Biologics for Inhaled and Nasal Delivery

	When referring to this article, please cite it as A. Siew, “Alternative Routes for Delivering Macromolecules," 

Transdermal and inhaled/nasal delivery provide alternative routes of administration for macromolecules.

Alternative Routes for Delivering Macromolecules

The process, equipment, and technology for developing and manufacturing oral solid-dosage forms are well defined. Nonetheless, designing a drug product formulation that achieves the desired properties of the target profile both in magnitude and robustness is a multi-dimensional, and generally, constrained optimization problem, observes Aaron Goodwin, principal investigator, Research and Development, Capsugel. The ultimate goal in formulation development is to deliver the drug to the right place, at the right time, in the right concentration so that a beneficial therapeutic effect is achieved, John McDermott, executive director, Drug Product Optimization, Quotient, explains.

	“Historically, the focus of formulation optimization has been placed upon the manufacturing process because molecules and drug-delivery requirements were arguably more straightforward,” says McDermott. He points out that most new chemical entities (NCEs) entering the development pipeline, however, often have complex physicochemical properties that require enabled formulation technologies to overcome solubility and/or permeability challenges, or a modified-release technology to achieve a suitable treatment regime or protect patients from adverse events associated with peak plasma concentrations. “In all of these cases, optimization of the formulation composition is essential to ensure successful and consistent drug delivery without compromising other criteria, such as dosage form size, or wasting valuable drug,” he says.

	It is, therefore, imperative that key formulation and process parameters be identified and selected based on a quantitative and phase-appropriate risk assessment with regard to the clinical dosage form having suitable properties, Goodwin stresses. “This approach is crucial to ensuring a high probability of achieving the target pharmacokinetic (PK) profile and thus the desired efficacy and patient safety, as well as manufacturability and cost,” he says.

	For every drug product, the parameters that have to be optimized are different, notes Ravinder Kodipyaka, senior director and head, Formulation R&D, Custom Pharmaceutical Services, Dr. Reddy’s Laboratories. “For example, for a simple immediate-release tablet that is wet granulated, key optimization parameters for formulation would be granulation end-point, blend particle size distribution, granulating media volume, blend moisture content, and granule size distribution. Key process parameters for optimization would be the mixing time, granulation time, and drying parameters, among others,” he says. “On the other hand, for a modified-release pellet coated with a polymer, the key optimization parameters for the formulation would be the film forming temperature, uniformity, and integrity of the film and its impact on drug dissolution and drug release. Key process parameters for optimization would be spray atomization, product temperature, and humidity control.”

	Although each drug product’s key parameters are unique to the individual drug molecule and delivery challenge, there are several key metrics that are ubiquitous throughout oral solid-dosage form development, Goodwin points out for example, the pharmacokinetics (typically determined by the drug absorption, distribution, metabolism, and elimination [ADME] properties and drug release from the dosage form), drug substance and product stability, patient compliance, as well as manufacturability, processing throughput, and cost. “By establishing the relationships between formulation and process factors and the identified key parameters within these categories, formulators are able to not only study and address potential risks to key parameters, but also understand tradeoffs between them for a given formulation change,” he says.

	According to Goodwin, a typical optimization process for an oral solid-dosage formulation involves establishing multivariate relationships between formulation and process inputs (or factors) and output product properties (or responses) in a quantifiable manner. “Basically, you need to identify critical parameters, establish workable ranges, conduct experiments as per study design, and understand factor-response effects,” adds Kodipyaka.

	“Generally, the optimization process is initiated during the proof-of-concept stage and continues up to commercialization,” says Goodwin. An empirical approach to optimization is common, observes Kodipyaka, but there is a rapid shift towards the use of mathematical models and software that can provide insights into product variables (dependent or independent), including the identification of constraints/limits and the design of experimental space.

	Industry experts agree that design of experiments (DoE) is an essential element in drug product development. “DoE is an effective method of evaluating product parameters and identifying critical parameters that need consideration during product optimization,” says Kodipyaka. “Unlike the conventional method of evaluating one variable/parameter at a time, which can sometimes provide incomplete information when other parameters exist, DoE allows for simultaneous evaluation of various parameters, as well as their interactions and influence on target product parameters (TPP),” he adds. “By using DoE, there is an increased probability of product success because of large datasets that are generated systematically and evaluated statistically.”

	According to Kodipyaka, inputs from the early stages of development, such as the selection of excipients, manufacturing technology, and packaging, among others, inevitably have a significant impact on the optimization activities that have to be performed as the product progresses through further stages of development. “You need to have a good understanding of R&D and product scale operations, the impact of upscaling and downscaling, and factors that influence product robustness,” he stresses. Kodipyaka believes that a combination of an empirical approach (experience-based) and software/artificial intelligence is the best way to optimize drug products. 
	Among the benefits to drug product formulation optimization are improved patient safety and compliance, reduced unit cost, and reduced probability of a failed batch, notes Goodwin. However, there are downsides, he says. “Optimization studies can be costly and increase time to market. Moreover, optimization can be somewhat recursive if done in a univariate manner without regard for tradeoffs between key parameters.”

	There are many strategies to reduce the time and cost associated with optimization, observes Goodwin. “Capsugel, for example, uses two strategies when optimizing oral solid-dosage forms containing amorphous spray dried dispersions (SDDs),” he says. “The first approach is the integration of drug product and drug intermediate, while the second is the application of modeling and simulation.”

	The first strategy, according to Goodwin, is difficult to apply if the drug is in the crystalline form, but works well when the amorphous form is used. “This is because many of the crystalline drug attributes that are set by its crystal structure and habit cannot be ‘tuned’ or modulated significantly by the crystallization process. For this reason, the drug substance and drug product are traditionally developed and optimized independently,” he explains.

	On the other hand, producing an amorphous drug form by spray drying can substantially change the drug attributes. The development and optimization of the drug substance and drug product can, therefore, be efficiently integrated, Goodwin points out. In doing so, it is often possible to significantly reduce the risk for multiple drug product and drug intermediate key parameters more so than conducting two independent optimization studies, he says, and this can save experimental and testing time and the cost of an additional optimization study.

	The second strategy uses modeling and simulation, and is employed twofold, according to Goodwin. “First, the approach is leveraged to think fundamentally about process and formulation, and generate hypotheses and constraints about a given optimization problem statement. This type of thinking allows hypothetical correlations between inputs and outputs to guide efficient experimental design. Outputs allow these models to be refined as a fundamental knowledge base is built,” he says. Goodwin explains that this strategy saves time, cost, and drug substance because it not only reduces the total amount of experiments, but also maximizes the information from each. Second, the science of scale is taken into account. “The use of scale up and down predictive models enable a higher degree of confidence when scaling processes,” Goodwin says. “It also allows the formulator to be proactive about how a formulation will scale, even at the prototype stages.”

	Quotient has also developed a platform to enable formulations to be designed, manufactured, and clinically evaluated rapidly. Traditionally, formulation development studies involve expensive and time-consuming screening of multiple prototypes in preclinical species to identify a limited number of pseudo-optimized systems for human clinical PK studies, notes McDermott. The success of this approach, however, is contingent on the preclinical model being predictive of the human response, and one of the selected formulations achieving acceptable performance in man. “There is a lack of predictability between animal bioavailability and human bioavailability owing to substantial inter-species physiological differences affecting dissolution, absorption, and metabolism of the drug molecule,” says McDermott. “A frequent outcome from the traditional approach is the need to conduct a further cycle of in-vitro and in-vivo studies, resulting in additional delays and cost to the development program.”

	According to McDermott, Quotient’s approach has been built on two premisesfirstly, the desire to make decisions based on human PK data rather than non-representative preclinical models; and secondly, the need to expedite the critical path for chemistry, manufacturing, and control (CMC) activities for prototype screening in humans by removing non-value added tasks and activities. “By using this approach, a drug product can be manufactured within seven days of dosing, removing stability package generation from the critical path to obtain clinical data on product performance,” he says. “Integrated GMP manufacturing with clinical testing allows clinical data from one study period to be used to select the product to be manufactured for evaluation in the next period.” Quotient’s rapid formulation development and clinical testing (RapidFACT) approach enables companies to select and optimize formulation technology in human subjects quickly, and according to McDermott, increases the accuracy of the optimization process.

	“The product development approach taken by Dr. Reddy’s consists of three phasesproduct design; design testing and optimization; and production,” Kodipyaka highlights. Formulation optimization is done during the product design phase as the company believes this approach helps to bridge the gap between R&D and commercial production of the final drug product. Kodipyaka explains that all activitiesfrom characterizing the input materials to defining the target features of the final productwill be divided into a unit stage and assessed both in-part and holistically, to identify important parameters. “A tentative control strategy is designed for each parameter, and then it is assessed whether that parameter is major, minor, or critical. Accordingly, an optimization plan is devised for the product composition and the manufacturing process,” says Kodipyaka. “By using this approach, we are able to see a virtual product on paper. Unlike trial-and-error methods, a systematic product design approach that uses simulations, software, and algorithms can reduce conventional experimentation methodology, hence bringing substantial time and cost benefits when optimizing a formulation.”

	When referring to this article, please cite it as A. Siew, " Designing Optimized Formulations," 

Design of experiment plays a crucial role in the optimization process of formulation development.

Designing Optimized Formulations

	David Fulper, PhD, director of technology support at Catalent, shares insights on the challenges and opportunities for cannabis-based therapeutics.

 Why is there an interest in cannabis-based therapeutics?

 The discovery of endogenous cannabinoid receptors has led to studies to investigate the potential effects of cannabinoids in the treatment of diseases such as cancer, diabetes, and neuromuscular disorders. There are potential benefits in pain relief, for use as a muscle relaxant, and for nausea and loss of appetite. With further research and studies, it is possible cannabis could help in the treatment of cancer, HIV, arthritis, multiple sclerosis, and other medical conditions. Movements by individual US states to legalize the medical use of cannabis also have created a medicinal market and significant interest.

 Could you tell us more about the physicochemical properties of cannabis and the challenges in formulation development?

 Cannabinoids are typically highly lipophilic compounds with poor solubility. Tetrahydrocannabinol (THC) exists as a viscous, oily resin that is oxygen-sensitive and requires special handling and formulation considerations to provide adequate stability. Other cannabinoids can exist in a crystalline state, but are often administered with THC or cannabis entourage materials that, again, result in an oily liquid. As a plant extract, there is also the challenge of standardization of cannabis content to assure consistent dosing. Lastly, cannabis’ status as a Drug Enforcement Agency (DEA) Schedule I drug greatly complicates working with these compounds, making it much more difficult to perform formulation studies compared to other compounds.

 In the European Union, there are cannabinoids currently available as pharmaceutical formulations. Are there any limitations with these marketed formulations? Do you see a need for an improved dosage form?

 Oral THC formulations generally suffer from poor or variable bioavailability due to first-pass metabolism. Disease state can also play a role in the type of delivery profile desired. Some of these issues can be addressed by route of delivery. GW Pharmaceuticals’ Sativex oromucosal spray attempts to avoid first-pass metabolism through absorption via the lining of the mouth. Teva has signed a marketing agreement with Syqe Medical to distribute Syqe’s high-tech vaporizing inhaler (Syqe Inhaler), which, similar to vaping, avoids first-pass metabolism and has a fast onset of action. There is also interest in topical and suppository formulations. However, there will always be a desire and need for oral cannabis, and lipid-based formulations are considered as a top choice due to the lipophilic nature of cannabinoids. Additional work is still needed to further improve the oral performance.

 What is a suitable drug-delivery approach for cannabis-based therapeutics? What would you consider an effective formulation to deliver cannabis molecules?

 Formulation development can be complicated, as cannabis-based therapeutics could include a single entity API, a combination of APIs, an API in combination with entourage materials, or as the plant extract. Disease states also could include situations where it would be desirable to allow the patient the ability to titrate their dose. Cannabis has a history of being delivered by smoking, and while this route of administration can effectively deliver cannabis, it also brings with it a number of undesired consequences, including concerns that this delivery route may be carcinogenic and that it may damage the lung’s epithelia over time; a regulator will, therefore, want to ensure that these concerns have been adequately addressed in any cannabis formulation. As a result, there is a strong desire to develop oral dosage forms to avoid the negative effects of smoking or vaporization. For any given API scenario, there will be various delivery approaches that will make more sense than others based on the disease being treated, patient needs, absorption, distribution, metabolism, and excretion (ADME) characteristics, and regulatory acceptance.

 There is evidence that fats and lipid excipients can increase systemic exposure to orally administered cannabis--can you discuss exploiting this pathway in formulation development?

 Natural cannabinoids tend to fall into the Developability Classification System (DCS) IIb category where lipid formulations are known to improve absorption. Good absorption, however, does not always translate into sufficient systemic exposure. Gershkovich et al. showed that a formulation that promoted lymphatic absorption significantly increased the bioavailability of the synthetic cannabinoid PRS-211,220 from 13% to 35%, compared with dexanabinol using long-chain triglyceride formulations (1). THC is highly metabolized by the liver and could benefit from formulations that promote lymphatic absorption and, by doing so, avoid first pass metabolism. More work is needed to be able to screen compounds for lymphatic absorption and to optimize formulation systems for lymphatic absorption.

	When referring to this article, please cite it as A. Siew, "Developing cannabis-based therapeutics” sidebar to “Designing Optimized Formulations," 

David Fulper, PhD, director of technology support at Catalent, shares insights on the challenges and opportunities for cannabis-based therapeutics.

Developing cannabis-based therapeutics

Iñaki Bueno, formulation and manufacturing manager at Idifarma, and Rob Harris, chief technical officer at Juniper Pharma Services, share insights on the challenges of capsule filling.

 What type of formulations are filled in a capsule? What formulations are not suitable for capsule filling?

 In principle, any type of formulation may be dosed into hard capsules, from blends or granules, to coated pellets, to other oral dosage forms such as tablets, micro-tablets, smaller capsules, including various combinations of any of the mentioned forms. It is also possible to fill liquids, provided that the material of the capsule (generally gelatin, although there are other alternatives) is not soluble in the solvent used in the formulation.

	Due to the need for a plasticizer in the capsule shell formulation, conventional hard gelatin capsules have a high water content (13-16%). For this reason, hygroscopic products can absorb moisture from the capsule, causing the capsules to become brittle, which can break under mechanical stress. On the other hand, such transfer of moisture to the contents of the capsule could generate problems of physical stability (crystalline form) or chemical stability of the API.

	To solve these problems, capsules with a low moisture content can be developed, either by using plasticizers other than water or  other polymers, such as hydroxypropyl methylcellulose (HPMC), which is the most widespread alternative.

	Finally, mixtures with materials containing reactive aldehydes are not suitable for capsule filling, as they favor the “cross-linking” effect experienced by the gelatin, reducing the capsule’s solubility.

 It is possible to fill powders, granules, non-aqueous liquids, non-aqueous gels, and thermo-setting formulations into capsules. For solids, powders with poor flow properties can be problematic because of poor fill weight control. Liquids with very low viscosity can leak from two-piece shells soon after filling. Hygroscopic liquids can cause embrittlement in capsule shells.

 How do you decide on capsule size and fill weight per capsule?

 The choice of capsule size and fill weight is dictated by the unit dose requirements and the formulation to be used.

 Once the formula of the contents of the capsule is developed and the weight to be dosed is known, it is easy to define the most appropriate capsule size. The body of each size of capsule has a defined volume; and by knowing the density of the mixture to fill, we can establish the volume that will occupy the weight that must be filled in each capsule. In the case of mixtures (powder or granulate), knowing the tapped density makes it possible to determine the most suitable capsule size. For pellets or micro-tablets, where there is no possibility of compaction, it is necessary to use bulk density instead. All capsule suppliers provide capsule size tables that facilitate the choice of capsule size.

 How is capsule filling for clinical studies different from that for commercial production?

 Capsule filling provides great potential for clinical trials, whether it’s filling just the API for first preclinical or clinical trials to filling other dosage forms in capsules (tablets or smaller-sized capsules, for instance) for blind or double-blind clinical trials. Capsules intended for clinical trials often have different geometries or dimensions from the capsules used in production, and therefore, might require specific machine formats. Typically, the batch sizes for clinical trials are smaller than for commercial production, so in many cases, it is possible to fill capsules manually.

 For small numbers of capsules for clinical studies, it may be feasible to hand-fill capsules, either with neat drug or with a simple powder blend, or use a precision powder dispenser, such as the Xcelodose (Capsugel). Another approach for relatively small numbers of capsules is to ‘flush-fill,’ which involves holding an array of opened capsule bodies in a perspex frame and adding/spreading out the drug or formulation across the capsule shells to fill them all to the brim, followed by adding the caps and closing the capsules. Difficult to handle powders (e.g., micronized drug, poor flow powders) can be filled into capsules either by hand-filling or by flush-filling (although due consideration should be given to the quantities of capsules required).

	For larger clinical trials, automation becomes important. For a batch size greater than a few thousand capsules, an automated filling approach is required, which demands the use of a suitable, free-flowing powder blend or granular formulation.

	For commercial production of capsule products, speed and ease of manufacture is important and, therefore, automated capsule filling machines are used. There are two types of automated capsule filling machine-the dosator-type and the dosing disc type.

 What are the challenges of capsule filling for liquid and powder formulations?

 For liquid formulations, very low viscosity liquids should be avoided due to the potential for leakage. For powder formulations, good flow properties are important to ensure satisfactory fill weight control.

 The main challenge when filling liquids in hard gelatin capsules is to find the right solvent (i.e., one that  does not interact with the capsule material). From a technological standpoint, the capsule filling machine requires a specific station to fill liquids, and the capsules must undergo an additional subsequent band-sealing process to prevent leakage of the contents of the capsule.

	As far as powder filling is concerned, the main challenge is to achieve a formulation with good flow properties, which guarantees the correct filling of the dosing systems, and therefore, a good uniformity of mass of the contents of the capsule.

 Can you discuss the differences between manual and automated capsule filling, and the pros and cons of each method?

 Unlike an automatic filling machine, where there is a transfer of the product to the capsule, the manual filling of powder is done volumetrically. This implies that the whole body of the capsule must be filled. Changes in API or final blend density may make it impossible to fill the exact amount of blend. In cases where the density is lower, it is possible to use manual compaction systems, but they are ineffective and introduce a high variability to the contents of the capsule.

	Obviously, the main advantage of manual encapsulation is that the investments in equipment and formats are much lower and that the times dedicated to associated activities, such as adjustment and cleaning, are drastically reduced. Another advantage of manual filling is that there is no dead volume, which makes it possible to encapsulate all the powder, something that is really useful when manufacturing very small batches for clinical trials or conducting formulation tests.

	Speed, reliability, and the ability to adjust the dose are the main advantages of automatic machines. Its main disadvantages include the need to invest in expensive equipment, the longer time needed to perform adjustments to the batch, format changes and cleaning, and the need to manufacture bigger batch sizes compared with manual filling.

 What are the common things that can go wrong in capsule filling that manufacturers should be aware of?

 When powder filling, the main challenge is poor capsule fill weight control. When filling liquid or semi-solid formulations, splashing of the liquid during the capsule filling can occur, making it necessary to reduce machine output speed or increase time for the pump stroke. In addition, ‘tailing’ during filling stroke for gels and semi-solids can lead to variable fill weights and contamination of capsules, making it necessary to adjust the speed of the pump stroke and/or increase the temperature of the formulation during filling.

 Capsule filling is a complex process, so there may be numerous challenges. It is important that the product to be encapsulated is well developed, with an acceptable flow and a reproducible batch-to-batch density to avoid problems of mass uniformity.

	In addition, an automatic encapsulating machine is a complex equipment. Each capsule format is composed of numerous pieces, which must be perfectly aligned and adjusted to avoid the opening or breaking of capsules. When capsules are broken, they can release their contents, staining the entire batch produced.

	Finally, hard gelatin capsules eventually undergo a cross-linking reaction that renders the gelatin less soluble and may affect the release of the capsule contents.


	Vol. 41, No. 3
	March 2017
	Pages: 30–34

	When referring to this article, please cite it as A. Siew, "Key Considerations in Capsule Filling," 

Capsule filling is a complex process, and the product to be encapsulated must be well developed to ensure mass uniformity.

Key Considerations in Capsule Filling

Dissolution testing is an important tool for characterizing the performance of oral solid dosage forms. Its significance is based on the fact that for a drug to be effective, it must first be released from the product and dissolve in the gastrointestinal fluids before absorption into the bloodstream can happen. In other words, the rate and extent of drug absorption are determined by its dissolution from the dosage form.

	For a commercial product, this test is routinely used for quality-control and quality-assurance purposes, to ensure consistency between production batches, or to justify scale-up and post-approval changes made to the manufacturing process (1). In early phase drug development, dissolution testing aids formulation design and drug delivery optimization (2); it is primarily carried out to assess product stability, monitor formulation changes over time, and establish 

 correlations have been demonstrated, dissolution can be used as a surrogate test to predict the 

 performance of drug products (4). From a regulatory perspective, dissolution testing plays a major role in the decision-making process, particularly in the development and approval of generic dosage forms, where unnecessary human studies can be avoided without compromising the quality of the generic drug products (5).

	A dissolution test measures the amount of drug that goes into solution over a period of time under standardized conditions. Factors that affect the dissolution of a drug product include the intrinsic properties of the API (e.g., solubility, wettability, particle size, surface area, morphology, polymorphs), the formulation composition and characteristics (e.g., excipients, hardness, manufacturing process), and the dissolution method used for its assessment (e.g., apparatus, medium, test conditions, sampling, and sample analysis) (6).

	In dissolution testing, the aim is to develop a discriminatory method that is sensitive to variables that affect the dissolution rate, and consequently, the 

 performance of the drug product. The method must be able to distinguish between drug products manufactured under target conditions and formulations with meaningful variations for the most relevant critical manufacturing variables, such as drug substance particle size, compression force, and tablet hardness, for example (7). The dissolution method should also be sufficiently rugged and reproducible for daily operations as well as transferable between laboratories (7).

 describe four different dissolution apparatuses that can be used to develop an appropriate dissolution method for oral solid-dosage forms based on the drug product characteristics (8, 9).

	Apparatus 1 (basket) and Apparatus 2 (paddle) are most commonly used methods in dissolution testing. Apparatus 1 consists of a vessel made of glass or other inert, transparent material and a cylindrical basket attached to the lower part of a rotating stirrer. The set up for Apparatus 2 is generally the same except that a paddle formed from a blade and a shaft is used as the stirring element. Sinkers may be used for dosage forms that would otherwise float. These apparatuses are simple, robust, well standardized, and flexible enough to allow dissolution testing for a wide variety of oral solid-dosage forms (1). Baskets and paddles are recommended for dissolution method development unless they have been shown to be unsatisfactory (1). One disadvantage with the basket and paddle methods is that the volume for dissolution is fixed. In addition, simulation of gastrointestinal transit condition is not easily possible.

	Apparatus 3 (reciprocating cylinder) was develop in recognition of the need for a system that sequentially alters the dissolution conditions to mimic the gastrointestinal tract, so that 

correlations can be established. Apparatus 3 consists of a set of cylindrical, flat-bottomed glass vessels, a set of glass reciprocating cylinders with inert fittings and screens at the top and bottom of the cylinders, and a motor and drive assembly to reciprocate the cylinders vertically inside the vessels; and if desired, the reciprocating cylinders can be moved horizontally to different rows of vessels (8, 9). Unlike the baskets and paddles, agitation in Apparatus 3 comes from dipping within the vessel, rather than through a stirred media approach. The advantages of the reciprocating cylinder include the ability for pH profiling, which enables simulation of pH changes in the gastrointestinal tract; and the system can be programmed to run dissolution in different media and at different speeds at various times (i.e., programmable dip speeds at each interval and programmable interval time). Such flexibility allows for better 

	Apparatus 4 (flow-through cell) consists of a reservoir containing the dissolution medium, a pump that forces the medium upwards through the vertically positioned flow-cell, and a water bath to maintain the temperature of the dissolution medium (8, 9). The flow-through cell apparatus can operate in two different modes-an open system with continuous flow of solvent passing through the cell from the reservoir, and a closed system where a fixed volume of media is recirculated. The open system is suitable for poorly soluble drugs, which require a high volume of media for dissolution. The closed system, on the other hand, is used when a low volume of medium is required.

	Dissolution testing should be carried out under physiological conditions (4). Media selection will be based on the purpose of the dissolution test, taking into account the solubility of the API. Ideally, the dissolution media should meet sink conditions, ensure that the drug is stable for at least 24 hours, preferably avoid the use of surfactants and alcohol where possible, and be biologically relevant for the site of dissolution 

 (i.e., pH 1.2–6.8 for immediate-release dosage forms and pH 1.2–7.5 for modified-release formulations) (3, 10).

	Sink conditions are described as the volume of the media being at least three times of that required to form a saturated solution of the drug substance. Sink conditions ensure that the amount of drug already dissolved in the media does not affect the dissolution rate as the run progresses. If sink conditions are not met, the dissolution rate will artificially slow down as the API nears the saturated solution state, making the dissolution test overly discriminatory and not reflective of 

 environment (10). However, although recommended, sink conditions are not always an essential requirement; failing to provide sink conditions may be justifiable in some cases. For example, when dissolution testing is used to indicate the biopharmaceutical properties of the dosage form, it is more important that the proposed biorelevant test closely simulate the gastrointestinal environment than necessarily produce sink conditions (3).

	The use of surfactants has to be justified, including the type and concentration of surfactant added into the dissolution media. For poorly soluble drugs, it may not be possible to achieve sufficient dissolution with aqueous solutions within physiological pH ranges; therefore, for these compounds, a surfactant may be required to enhance the drug solubility (3).   

	Once a dissolution test has been developed, it is important to have a good understanding of the method’s characteristics and ruggedness. It generally involves testing the method with different lots of the drug product and anticipating the variations that can be expected (1). 

 17 (1) 8-11 (2011). 
	2. X. Lu, “Filling in the Gaps for 

 Release and Dissolution Testing of Solid Oral Dosage Formulations,” AAPS Blog, Aug. 16, 2016.
	3. C. Brown et al., 

Guidance for Industry: Dissolution Testing of Immediate Release Solid Oral Dosage Forms

 (Rockville, MD, Aug. 1997)
	5. O. Anand et al., 

13 (3) 328-335 (2011). 
	6. J. Kochling, “Approaches to the Investigation of Dissolution Testing Changes and Failures,” AAPS Webinar, May 23, 2013. 
	7.

 General Chapter <1092>, “The Dissolution Procedure: Development and Validation,” (US Pharmacopeial Convention, Rockville, MD, 2013).
	8. 

 General Chapter <711>, “Dissolution,” (US Pharmacopeial Convention, Rockville, MD, 2011).
	9. 

, Chapter 2.9.3, “Dissolution Test for Solid Dosage Forms,” (EDQM, Strasbourg, France, 2008).
	10. K. Boda, “

Transferring Dissolution Methods--Building a Robust Dissiolution Method

Principles of dissolution testing, including method development and testing apparatus, are reviewed.