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

Adeline Siew, PhD

Lipid-based formulations (LBFs) may improve oral bioavailability by exploiting the body’s lipid digestion and absorption pathways, offering a proven means of addressing the physicochemical and biological challenges of poorly soluble APIs. LBFs can be complex systems, so their development requires a multifaceted approach, and experience in how to approach their development provides significant benefits. With the availability of robust delivery systems, such as the softgel dosage form, LBFs can offer formulators potential benefits, provided that the most appropriate excipients are selected.

spoke with Karunakar Sukuru, vice-president of Product Development, Pharmaceutical Softgel, and Vincent Plassat, lead scientist, Softgel Product Development, both from Catalent, about the importance of excipient selection and stability testing in the development of LBFs. 

Can you discuss the challenges in the development of LBFs and the key considerations when working with these systems? 

LBFs provide a versatile platform to formulate APIs with a wide range of physicochemical properties. The excipients that can be used within these formulations have a wide range of properties themselves, accommodating lipophilic compounds to be solubilized in oil as well as hydrophilic compounds that can be solubilized in high hydrophilic–lipophilic balanced (HLB) surfactants or hydrophilic solvents. The development of successful formulations requires specialized formulation expertise to perform preformulation screening and assessments due to the great versatility and dynamic nature of LBFs 

	The first hurdle is choosing appropriate formulation excipients that not only have adequate solvent capacity to solubilize the entire dose, but which also ensure that the formulation maintains its solvent properties in the intestine after dilution and digestion. The balance between these two requirements is currently poorly understood, and there is a considerable risk of precipitation of drug during the various intermediate stages of drug transfer, for example, from the solution state to the micellar state. The extent of this precipitation is dependent upon the formulation-it is, therefore, crucial to conduct various 

studies to challenge the formulation and help predict the likelihood of precipitation and/or guide the appropriate excipient selection. 

	Some of the important parameters to consider in LBFs include: screening for solubility in excipients, biorelevant media and lipid-digestion products, excipient compatibility, and finally, the risk of precipitation upon dispersion and digestion.

	Another key consideration in oral formulation design is the safety and regulatory status of proposed lipid excipients. Not all lipid excipients are generally recognized as safe (GRAS), hence, specific attention to their maximum daily intake should be considered as soon as possible in the development of a LBF. This is a critical factor for new molecular entities because high doses of excipients could be required during dose escalation studies. 

How do you ensure the formulation is stable? And how do you test for stability?

The physicochemical stability of LBFs is just as crucial as with any other formulation. Chemical stability is handled the same as it would be with other dosage forms, with appropriate excipient selection based on a compatibility study with a mixture of API and a single or blend of excipient(s), along with analysis of the impurities formed (if any) during storage at various temperature and humidity conditions. Once the formulation is established, a formal stability study is performed on the dosage form in the proposed packaging at International Council for Harmonization (ICH) conditions to establish the shelf life. For LBFs, the differences come in testing for physical stability. For example, when LBFs are made with excipients that could be semisolid or that have different hydrophilicity or lipophilicity characteristics, these excipients can settle over time, especially during storage at 40 °C. It is, therefore, necessary to conduct stress studies to challenge the robustness of the formulation. Cycling studies are commonly used with cycles of high and low temperature to stress the formulation. Additional tests to challenge the LBF’s robustness to ensure the API does not precipitate out in 

Can you outline the different excipients used in LBFs and the role they play?

There are a wide range of excipients that can be used in LBFs. They can be classified under five different categories:

		Triglycerides are vegetable oils composed of triglyceride esters of fatty acids. They are a component of many foods and do not present safety issues. Triglycerides are foundational excipients for LBFs. Their solvent power is usually limited, but after digestion, the fatty acids released form mixed micelles with bile salts that can dissolve a portion of the API and thus become carriers for the now suspended API. Examples include corn oil and sesame oil.

		Mixed glycerides and polar oils are partially hydrolyzed triglycerides that are generally much better solvents than triglycerides. These excipients help to form self-emulsifying systems but can still be sensitive to digestion. Other esters such as propylene glycol or sorbitan esters of fatty acid are currently available and may be valuable additives in cases of chemical incompatibility. An example is glycerol monocaprylocaprate.

		Water-insoluble surfactants include non-ionic polyethoxylated or polyglycerylated esters of fatty acid that are not hydrophilic enough to be soluble in water but form a good oil/water interface. They are considered dispersible in water and are therefore commonly used to create selfemulsifying systems. Examples include linoleoyl polyoxylglycerides.

		Water-soluble surfactants are the most commonly used excipients for formulation of self-emulsifying drug delivery systems (SEDDS) or selfmicro-emulsifying drug delivery systems (SMEDDS). Above their critical micelle concentration, these excipients spontaneously form micellar solutions that help to solubilize the API. Examples includes polysorbate 20 and 80.

		Co-solvents are water-soluble solvents such as ethanol, propylene glycol, and polyethylene glycol. They have multiple roles in LBFs. They increase the solvent capacity of the formulation for drugs and aid the dispersion of systems containing a high proportion of water-soluble surfactants. However, because they lose solvent power during dilution in gastrointestinal fluids, their use is limited. Because lipids are prone to lipid peroxidation, which generates free radicals that can adversely affect API stability, liposoluble antioxidants such as tocopherols and butylated hydroxytoluene/hydroxyanisole are sometimes also needed as additives in LBFs.					 	

Can you tell us about the variability of lipid excipients and how it can affect the formulation? What must formulators do to address this issue?

Due to their natural origin, some excipients can have a variable composition. Subsequent chemical modifications on excipients that are inherently variable, such as hydrolysis and esterification, can lead to even greater variability and challenges. The formulator must have a good understanding of the exact excipient specifications to select the one most suitable for the formulation. The formulator must also understand and accept that there will be small variations between batches of the same product. The formulation must, therefore, be robust enough not to be sensitive to these small variations in the composition of the excipients. If the LBF cannot withstand small variations, a strategy to mitigate the impact from such variations should be put in place.


	Vol. 42, No. 10
	October 2018
	Pages: 26–29

	When referring to this article, please cite it as A. Siew, “Developing Lipid-Based Formulations,” 

	Adeline Siew was previously editor for 

Articles

Lipid-based formulations offer a means of addressing the physicochemical and biological challenges of poorly soluble APIs.

Developing Lipid-Based Formulations

	It is well recognized that bitter APIs can have a negative effect on patient compliance. As a result, there is an increasing demand for taste-masking technologies that address this issue. Senomyx uses an alternative approach, known as taste-blocking, to address the challenges of bitter APIs. “The company was founded in 1998 based on the work of Charles Zuker from the University of California, San Diego, who is credited with the discovery of taste receptors in rodents,” Kenneth J. Simone, vice-president of Pharmaceutical Business Development, Senomyx, told 

. “Since then, we have replicated this work in human taste receptors and determined the mechanism of action for sweet and bitter taste.” 

	Simone explains that in the human genome, there are 25 genes localized on three different chromosomes, which code for bitter taste receptors that are expressed in taste buds on the tongue. “Senomyx has developed proprietary assays for 22 of these receptors,” he says. “Using these assays with classical biotechnology approaches such as high throughput screening, chemical optimization, and taste tests, we have developed novel bitter taste blockers for food, beverages, over-the-counter (OTC), and pharmaceutical applications.” 

spoke to Simone about Senomyx’s taste-blocking technology.

How does taste-blocking work, what is the mechanism of action or the principle behind the technology? 

Our bitter taste blockers act as genuine receptor antagonists. They bind to the bitter receptor eliciting the bitterness of the active ingredient and keep the receptor inactive, resulting in a reduced level of bitter sensation being relayed from the receptor to the nerves, even in the presence of the bitter active ingredient. We also have the ability, through our proprietary assays, to screen compounds, such as APIs, to determine which bitter receptors the compound activates. This information allows us to focus on the dominant receptors and achieve maximum blocking effect.

How is it different than other taste-masking approaches? 

The materials Senomyx has developed are blockers, not maskers. Our bitter taste blockers have no inherent taste on their own at usage levels and specifically block bitterness at the receptor level. Masking is typically approached by adding sweeteners or other flavoring agents, at high levels, to impart other taste characteristics in a formulation. The masking approach does not act specifically to attenuate bitterness, and as such, only limited success is achieved when trying to mitigate the bitterness of APIs. Many APIs are several hundred times more bitter than sources from food and beverages, like coffee.

Can you provide some examples of pharmaceutical applications of your taste-blocking technology? 

Senomyx’s Bittermyx BB68 bitter blocker is already being used in API applications to attenuate bitterness. It is being evaluated for use in additional products. The work is focused on pediatric and geriatric solutions (i.e., liquid or dissolvable formats, and not encapsulated or coated tablets). 

studies have demonstrated that BB68 can attenuate bitterness in cough and cold remedies using acetaminophen, guaifenesin, and dextromethorphan. 


	Vol. 42, No. 9
	September 2018
	Page: 32

	When referring to this article, please cite it as A. Siew, “Taste-Blocking-An Alternative to Tastemasking,” 

Senomyx uses an approach, known as taste-blocking, to address the challenges of bitter APIs. Kenneth J. Simone, vice-president of Pharmaceutical Business Development, Senomyx, speaks with Pharmaceutical Technology about this technology. 

Taste-Blocking—An Alternative to Tastemasking

Coating is one of the approaches used to mask the taste of bitter drugs. Coating for tastemasking purposes is a well-established technology used throughout various dosage forms, observes Felix Hofmann, technical sales manager, Evonik. “It ensures a reliable outcome of the manufacturing process, and has obvious advantages, such as short processing times, easy application, and straightforward upscaling on standard equipment,” he says. “Safe tastemasking properties can be achieved by covering the entire surface of the dosage form by a polymer film so that direct contact of the bitter substance with the taste receptors on the tongue is prevented.”

	“From the patient’s view, coating not only masks bad taste and odors, but also makes the tablet easier to swallow and aids in recognition and differentiation between tablets, thus reducing the potential risk of medication errors,” Dr. Ali Rajabi-Siahboomi, vice-president and chief scientific officer, Colorcon, highlights. He notes that the coating also increases the mechanical strength of tablets and, therefore, reduces friability, breakages, or dusting associated with uncoated tablets. 

	“The selection of a tastemasking method is often dictated by the API’s physicochemical properties and taste profile,” explains Rajabi-Siahboomi. Hofmann adds that other characteristics such as drug load, shelf-life, the desired dosage form, and field of application also play a crucial role in selecting the appropriate tastemasking technology. 

	“There are various options available for consideration, such as the use of flavors and sweeteners, the application of barrier membrane coatings on drug particles, and the use of ion exchange resins-all of which can be applied individually or in combination to overcome the undesirable taste of an API,” says Rajabi-Siahboomi. “While these options exist, coating of tablets and multiparticulates, remains the simplest and preferred method for tastemasking of bitter APIs for many reasons, including ease of manufacturing, accurate dose administration, good product stability, convenience, good patient acceptability, and it delivers improved patience experience.” 

	Hofmann concurs that for single- and multiple-unit dosage forms, coating is generally considered the preferred tastemasking approach because of its specific benefits. “Overall, if the dosage form has a smooth surface that allows proper coating, we would certainly go for it,” he says. He also points out that film coatings for tastemasking purposes can even be applied for liquid dosage forms. “Usually, they are prepared as dry syrups that are going to be reconstituted before administration,” he explains. “Of course, in such set up, the shelf life of the ready-to-use dosage form is limited by potential diffusion of the active ingredient through the film as compared to solid drug products.”

	A crucial consideration for formulators, according to Rajabi-Siahboomi, is to determine the level of tastemasking required, which depends on the API properties and the dosage form design. “In some cases, the API is only slightly bitter and may be masked with flavors and sweeteners. In other cases, the API has a very bitter taste requiring additional tastemasking techniques,” he says. “In general, for tablets that are swallowed whole by the patient, an immediate-release film coating is sufficient to mask an unpleasant taste of API, but it may not mask the unpleasant odor of the drug. With bitter APIs, however, even a small amount of exposure is sufficient to give the patient a poor experience. In these cases, formulators should consider using a barrier membrane film coating as their first choice to taste-mask the drug particles or granules,” he explains. 

	Various barrier membrane coating options are available. Rajabi-Siahboomi cites the example of the active substance, raltegravir, which has an intensely bitter taste and hence, tastemasking was seen as an essential part of the development program. “Several tastemasking options were evaluated by coating the raltegravir granules prior to processing into the final dosage form,” he says. “The chosen option was to use a coating comprised of an aqueous ethylcellulose dispersion in combination with a hypromellose-based film-coating system. Insoluble in water, ethylcellulose acts as the tastemasking agent by delaying the release of the drug in the mouth, while hypromellose is used as a pore-former in the coating, which allows immediate release of the drug once in the stomach.”

	Rajabi-Siahboomi highlights that the dosage strength will also dictate whether a specific tastemasking formulation strategy is suitable. “Low-dose APIs are easier to mask, while high-dose APIs may be challenging because more material (with a greater surface area) needs to be coated,” he says, noting that this is especially true for formulations with fast-dissolving bases, which may leave the patient with coated drug particles in the mouth for a longer time.

	As a production technique, coating typically has gentle processing conditions, with rather low temperature and energy intake, observes Hofmann. Nonetheless, he points out that careful adjustment of the process humidity and temperature must be considered for moisture- and/or temperature-sensitive drugs. “Solvent-based coatings can be applied for moisture-sensitive active if the parameter control strategy for aqueous processing fails,” he says. “Formulation adaptations might be required for free acidic or basic drug substances to circumvent ionic interactions depending on the coating polymer choice.” 

	For particle coating, Hofmann notes that particle size, morphology, flowability, and solubility of the active ingredient can have an impact on the overall processability. “Pre-treatment strategies such as granulation might be necessary for acceptable process performance and overall process time,” he explains. “In this respect, coating of tablets is generally less complex as the surface is well-defined and the drug substance is generally diluted with tableting excipients. Moreover, tastemasking of tablets can be considered less challenging compared with multiparticulate systems due to their shorter residence time in the mouth cavity.” 

	Different types of polymers are used in coating applications for tastemasking purposes. “Polymers can be clustered in natural and synthetic origin,” says Hofmann. “Among the natural-based polymers, cellulosics such as hydroxypropyl methylcellulose (HPMC) are often used for tastemasking applications. Typical characteristics of these polymers are the highly viscous spraying suspensions they produce, resulting in high spraying pressures, high product bed temperatures during spraying processes, and limited tastemasking capabilities due to their solubility in the saliva. Because of these limitations, poly (meth)acrylate-based coatings have become more popular recently.” Hofmann highlights that the (meth)acrylic copolymers can be applied as a low-viscous spraying suspension with low atomizing air pressures and low product temperatures. “The resulting films will stay intact at high pH values in the saliva resulting in superior tastemasking properties even if particles may stay in the mouth for a while,” he says. “The coating dissolves rapidly in the acidic conditions of the stomach and, therefore, has no impact on bioavailability of the drug.”

	According to Andrea Vescovi, head of Global Technical Marketing Pharma Solutions, BASF, using pH-dependent polymers that are insoluble in the neutral pH of the mouth, but soluble in a pH range between 1 to 5 in the stomach where the coating immediately releases the API, is an efficient way of tastemasking. “In this case, the pH at which the respective polymer dissolves is an important factor because it determines the release of the API,” he explains. “Additionally, when coating a tablet with polymers that dissolve at a lower pH range, the residence time in the stomach needs to be regarded to allow the film to dissolve and subsequently release the API.” Rajabi-Siahboomi, however, cautions that there are some concerns around the potential failure of the system to release the drug if the pH of the stomach is elevated due to a patient taking other medications (such as proton pump inhibitors), or the presence of food in the stomach, affecting its bioavailability. 

	Even so, Vescovi believes that this effect can be minimized by using innovative polymers with wider pH-dependent solubility range (pH 1–5). He adds that besides addressing tastemasking challenges, polymers also offer the opportunity to provide additional functionality such as different release profiles or moisture protection. 

	The quality-by-design (QbD) considerations for tablet coating is well established, notes Rajabi-Siahboomi. “The working design space identified for immediate release coating formulations also apply for tastemasking,” he says, but adds that film-coating formulations and processing of particles and granules for tastemasking are more complex. “Having said this, as the particle size of drug particles or granules increases, and their distribution becomes narrower, the level of complexity and difficulty to coat them decreases significantly.”

	As for coating formulations, it is important to consider choice and level of polymers, in addition to processing aids used for the coating process, observes Rajabi-Siahboomi. “These should provide suitable viscosity dispersion, correct ratios of components to provide functionality, and ease of processing without too much agglomeration of the particles.” He points out that the coating process should be designed to avoid granulation and agglomeration of drug particles and ensure individual particle coating with the barrier membrane system. “Critical parameters affecting optimum and consistent coating are: choice of bottom or top spray technology, spray gun set-up, and processing conditions such as spray rate, atomizing air, fluidizing air, and volume of air, among others,” he says. 

	According to Hofmann, the critical quality attributes (CQAs) and critical process parameters (CPPs) should be identified and incorporated into the development process at an early stage. “For functional coatings for tastemasking purposes, dissolution both in neutral and acidic pH is the most important CQA, as it reveals tastemasking efficiency through the lag time and bioavailability through the release,” he underlines. 

	Fully synthetic polymers consistently have low batch-to-batch variability and can be produced with tight specifications, Hofmann notes. “They are, therefore, less critical than naturally based polymers where batch-to-batch differences in molecular weight and substitution degree have to be considered as CQAs of the tastemasking polymer.” He adds that further to specific polymer properties, polymer amount, type, and individual quality have to be evaluated for optimum risk assessment. “Moreover, the amount and type of film-coating excipients have to be added to the CQAs of the coating formulation,” he says. “Among the CQAs of the substrate material, particle size distribution, specific surface area, and bulk density are key for multiparticulates; whereas for tablets, friability and weight variation are important.” 

	Hofmann adds that the setup of spraying process parameters will also have crucial influence on film quality and, therefore, the final product tastemasking performance. “Product bed temperature, spray rate, atomizing air pressure, inlet air temperature/humidity, and drying air capacity are among the CPPs. Their impact on film quality should, therefore, be evaluated carefully to create a robust and reliable design space for the individual coating process.


	Vol. 42, No. 9
	September 2018
	Pages: 26, 30, 32

	When referring to this article, please cite it as A. Siew, “Coating for Tastemasking,” 

The level of tastemasking required will depend on the API properties and the dosage form design.

Coating for Tastemasking

Antibody-drug conjugates (ADCs) are targeted therapies that combine a specific antibody or antibody fragment linked to a drug. ADCs have revolutionized the treatment of a number of diseases by enabling more specific drug targeting with fewer side effects. Despite the growing ADC pipeline, Charlie Johnson, CEO of ADC Bio, notes that many promising ADCs are being overlooked because of critical aggregation control problems. 

spoke to Johnson about the limitations of conventional manufacturing techniques used in the production of ADCs and why preventing aggregation at its source is the best solution.

What are the causes of aggregation in ADCs?

Aggregation of biomolecules can occur for a variety of reasons. Some antibodies are inherently prone to aggregation, especially when they are maintained in solution under conditions that promote aggregation, for example:

		Unfavourable buffer conditions where either the salt type and/or salt concentration is too low or too high can cause aggregation.

		Aggregation can occur if the system is held at a pH that coincides with the isoelectric point of the antibody (this is the point where the biomolecule carries no net charge and represents the point of least aqueous solubility). This pH may be required, for example, when a particular conjugation chemistry requires a specific pH.

		It is often the case during conjugation that solvents are required to solubilize otherwise poorly soluble payload-linkers, in particular highly hydrophobic ones. Some solvents are known to promote aggregation.

	However, the most significant issue that increases the propensity of antibodies to aggregate is the modification of their surfaces through chemical conjugation to hydrophobic payload linkers. Once the payloads are conjugated to the antibody, they present hydrophobic patches that attract hydrophobic areas on other antibodies, thus initiating the aggregation process. Once formed, these low molecular weight aggregates form nuclei sites where further aggregation can occur, eventually leading to high molecular weight aggregates and ultimately precipitation from solution.

What are the limitations of conventional manufacturing techniques used in ADC production and why do they fail in preventing aggregation?

Conventional conjugation of ADCs occurs in dilute aqueous buffered solutions. The buffer conditions and pH are adjusted to suit the chemical conjugation conditions for attachment of the payload-linker to the antibody. Historically, ADCs have been prepared by conjugation of either naturally occurring lysine or cysteine residues on the antibody, although more recent developments see the use of ‘site-specific’ residues, some of which focus on engineered-in residues (e.g., thiomabs) or through modification of naturally occurring sites on the antibody (e.g., glycan modification). What is common to all conventional conjugations is that eventually the antibody or modified antibody will be exposed to a payload-linker to produce the ADC, and that is commonly the most unstable point in the process for the antibody, because conditions such as buffer composition, concentration, pH, and addition of co-solvents are selected to optimize the conjugation chemistry, not the stability of the antibody. 

	A degree of aggregation is, therefore, always expected, although the degree to which the antibodies aggregate is mainly determined by the inherent hydrophobicity of the payload itself. Efforts have been made to reduce hydrophobicity of payloads either through modification with solubilizing linkers or modification of the payload structure, but these efforts have not fully solved the problems of aggregation. Aggregation of ADCs is not simply an issue of yield inefficiencies and costs, although these issues can be severe and lead to the unviability of a product. 

	Aggregates and, in particular, soluble high molecular weight aggregate must be removed as they are immunogenic and can cause severe allergic reactions if administered. Historically, aggregates are removed from ADCs via application of chromatographic methods such as size exclusion or hydrophobic interaction separation techniques. These approaches adds further processing time and cost, and it reduces the yield of the system, which all adds to the cost of producing an ADC. 

What’s the solution to such aggregation problems? 

The best solution to the aggregation problems prevalent with conventional conjugation methodology is to prevent aggregation at its source. If antibodies are held physically separate from each other during the ‘unfavourable’ conditions of conjugation, they cannot aggregate in the first place. 

	An obvious way to prevent this aggregation is through immobilization of the antibodies on a solid-phase support, such as a resin, and carry out the conjugation of the payload-linker while the antibodies are bound to that support. ADC Bio’s Lock-Release technology uses this approach and can be performed in either batch or flow mode. It is easily scalable in the same way that other chromatography methods are. 

	The initial step is antibody capture onto the resin, which is analogous to the protein A capture step in downstream processing of antibody manufacture. Once immobilized, the antibodies are sequentially exposed to process reagents and the payload-linker to produce the ADC, with intermediary washing steps to remove excess reagents. At the end of the process, the immobilized ADC is released from the resin into a stabilized buffer matrix that prevents any subsequent aggregation. 

	Lock-Release processing provides a unique way of preventing aggregation of ADCs at source while ensuring that the ADCs produced are free of contaminants (e.g., free payload-linkers and solvents) that can interfere with 

read-outs and also lead to enhanced toxicity when administered to patients.

	ADC manufacturing using Lock-Release is significantly simplified compared to conventional manufacturing, because the process equipment train used is the same irrespective of the ADC being made. Conventional manufacturing, in contrast, will have different requirements in terms of process equipment and hence the manufacturing footprint required depending on the process developed.

Can you tell us more about the locking mechanism?

Lock-Release at its most basic level is a conceptual way of constructing ADCs on solid supports, designed to improve both the quality of ADCs, the yield, and as a result, the economics of production. ADC Bio has patented multiple methods of Lock-Release. The favoured technique to date is based on affinity capture, using mimetic resins. The specific resins used depend on the process in question and, to some extent, where on the antibody the conjugation of the payload-linker is required. As with most affinity capture methods, the release method relies on varying pH and salt concentration. The demonstrated capacity of our Lock-Release resins is in the range 5 to 50 mg per mL of resin, but we expect that this range can be enhanced further. 

	A significant advantage of using affinity-based capture techniques is that the resin can be re-used over multiple batches, and that the quality of product is extremely high and consistent over multiple batches.

	Lock-Release can be run in either stirred batch or column (flow-mode). For manufacturing purposes, flow mode is preferred because it offers predictable linear scale-up as with other chromatography-based techniques. As such, the required manufacturing footprint and equipment train remain similar for all Lock-Release-based processes, which offers significant productivity and capital cost advantages over conventional manufacturing techniques at commercial manufacturing scale.

	When referring to this article, please cite it as A. Siew, " Tackling Aggregation Challenges in ADC Production," 

Pharmaceutical Technology APIs, Excipients, & Manufacturing 

Immobilizing the antibodies on a solid-phase support, such as a resin, and carrying out the conjugation of the payload-linker while the antibodies are bound to that support will prevent aggregation at its source.

Author | Adeline Siew, PhD