Cynthia A. Challener is a contributing editor to Pharmaceutical Technology.

Cynthia A. Challener

Medication adherence is a crucial issue today, and drug manufacturers are seeking to develop drug products that meet the needs of patients with respect to ease of use and palatability. For oral formulations, taste, appearance, and smell have a direct impact on medication adherence. Choosing the right excipients to provide optimum performance is therefore an important part of the drug development process. Various factors must be considered, from the API properties to the dosage form and the characteristics of the patient population.

Multiple approaches for different dosage forms

Oral dosage forms available today include traditional and fast-disintegrating tablets; capsules; chewables and gummies; lozenges and other suckable products; effervescent, buccal, sublingual, and thin-film formulations; suspensions, syrups, sprays, gels, sachets, stickpacks, and chewing gum; among others. The large number of different formulation types creates the need for many different approaches to improving taste and appearance. For solid formulations designed to be swallowed, particularly tablets, taste-masking and appearance are generally achieved using film coatings, according to Alen Guy, technical director IMCD Pharmaceuticals.

Such coatings prevent the API from being dissolved in saliva and interacting with the taste buds in the mouth before the drug product is swallowed, adds Nasrin Mahmoudi, AD&I scientist, Pharma Solutions with IFF. She also notes that coatings can help with appearance by providing a shiny, matte, or colorful look and hiding undesirable surface properties.

Capsule formulations are a little different from tablets. “Capsules generally don’t require taste masking unless they are designed to be opened and sprinkled onto a food or poured into a drink. Appearance is important for this oral dosage form, however, and color and design are often used to distinguish different products,” he observes.

For oral dosage products in which a bitter API does come in contact with saliva and taste buds in the mouth, different sweeteners are often used depending on the dosage form, Guy says. For instance, Elizabeth Tocce, AD&I scientist, Pharma Solutions at IFF, points to the use of sweeteners, often in combination with texture modifiers, in orally dissolving tablets (ODT), chewable tablets, oral thin films, and sachet formulations that stay longer in the mouth.

“Taste modification using flavors and other additives is more complicated, though,” Guy comments. Even so, it is often needed. “In many cases, and for very bitter high-dose drugs in particular, just changing the organoleptic properties is not sufficient, and other methods to create a physical or chemical barrier around the drug particles, such as microencapsulation and complexation, are used,” Tocce explains.

The method employed largely depends on the API characteristics and the intended dosage form, agrees Krizia M. Karry, head of global technical marketing for BASF Pharma Solutions. “Poorly water-soluble APIs with adequate dissolution rates (i.e., BCS [Biopharmaceutics Classification System] II APIs) tend to be micronized and compressed into tablets. At low dosages, adding flavors or sweeteners can suffice to ensure palatability as most tablets are not meant to disintegrate in the oral cavity, but flavors will not suppress bitter tastes,” she says. At high drug loadings and for orodispersible products, Karry adds that film coating with methacrylate copolymers that are insoluble at saliva pH is the most effective technology.

If the unpleasant taste is related to grittiness or chalkiness in chewable tables, ODTs, or suspensions, a common method is to use finer grades of the APIs and excipients, according to Tocce.

Liquid prescription drugs are particularly popular in the EMEA (Europe, the Middle East and Africa) market, where there is a preference for the dosing flexibility and a perception that they offer rapid relief, according to Karry. In North America, meanwhile, liquids are mostly offered for over-the-counter cold and flu medications. In most of these liquid formulations, she observes that highly concentrated sweeteners and flavors are typically employed to mask the bitter API taste. In formulations where multiple APIs are combined including some that are bitter or where product shelf-life is of concern, however, Karry observes that insoluble polymers are used to envelop API particles and allow their suspension in the continuous liquid phase.

For liquids, other options if sweeteners aren’t sufficient, in addition to microencapsulation and complexation, include using an insoluble form of the API and adjusting the pH of the solution to lower the API solubility, according to Mahmoudi. She adds that viscosity enhancers and texture modifiers are also used. Alternatively, taste and appearance issues with liquid formulations can be overcome by containing them in soft or hard capsules.

Many different types of excipients are used to improve taste and appearance. Because the most common taste challenge is the bitterness of APIs, sweeteners, both natural and artificial, are the most commonly used taste-modifying excipients. Beyond compounds such as sugar, mannitol and sucralose, actual flavoring agents (orange, mint, etc.), flavor-maskers, particularly bitter-maskers, texture modifiers (co-processed microcrystalline cellulose and guar gum), and polyols are used. Complexing agents such as cyclodextrins and ion-exchange resins are often used for improving the taste of liquid formulations and orally dissolving dosage forms, Mahmoudi states.

“Depending on the formulation, different approaches to taste and flavor modification are needed,” Guy says. In fast-dissolve forms, short intense flavor bursts are needed, while in longer lasting products such as chewables, longer-lasting flavor effects are needed.

Hydrophobic (ethyl cellulose) or hydrophilic (hydroxypropyl methylcellulose, polyvinylpyrrolidone) polymers, lipids, and sweeteners can be used as coating materials, alone or in combinations, to modify both taste and appearance, Mahmoudi adds.

In addition to preventing an unpleasant taste, Guy notes that some coatings may also include pore-formers to help promote dissolution of the drug substance to meet standard immediate-release requirements for the API. “In such formulations, a careful balance must be achieved between taste-masking and efficacy. It is therefore a rich area of development given the complexity of drug particles and granules, varying solubilities, the need to often achieve a specific location of delivery (enteric needs for instance), and excipient compatibility concerns,” he comments.

For appearance, color is most widely used to differentiate products and mask undesirable product properties. For solid dosage forms, color is applied within coatings. For liquids, water-soluble dyes are generally employed.

Ultimately, Guy contends that the choice of excipients for taste and appearance improvement comes down to the preferred dosage form and the external factors that can impact the effectiveness of the choice, which include excipient compatibility, resistance to mechanical influence (granulation and tableting), solubility, stability, etc.

Impact of need for patient-centric formulations

Because taste and appearance are important factors influencing the patient experience, it is essential for formulators to understand the preferences of different patient populations for drug formats and, within those preferred formats, taste and appearance qualities that will encourage good medication compliance. “Different patient groups based on their ages, disease states, cultures, and demographic locations may require certain taste, flavor, texture/mouthfeel, and drug product appearance properties, such as tablet size, color, and shape for solid products and the viscosity and grittiness of liquid formulations,” Tocce asserts.

Flavor companies, adds Guy, are today much more attuned to patient and consumer preferences by gender, age, and other factors. “This information can and does help formulators and brand/product managers to make smarter decisions for taste and appearance as it relates to the intended patient or therapeutic group,” he says.

In particular, Guy notes that the need to create fit-for-purpose formulations for aging populations and pediatric patients has put pressure on appearance as it relates to the size of oral solid dosage forms. For instance, important research conducted by academics and members of the European Pediatric Formulation Initiative has revealed that young children are more accepting of small or mini-tablets.

“Such a patient-centric approach definitely benefits patients, but it does present manufacturing challenges,” Guy states. “For example, it can be difficult when producing tablets that are 2 mm in diameter or smaller to achieve good flow through production equipment for APIs and excipients, which in turn impacts the ability to achieve good content uniformity and acceptable taste,” he says.

From a personalized medicine standpoint, it is also important to understand specific age-related elements of physical condition that might impact drug performance and the appropriate excipient choices. “For pediatric medicines, factors to be considered include the fat and water content along with relevant genetic information. It is a tricky area from a regulatory standpoint, but it does help guide the selection of excipients by taking into consideration their potential impacts on young developing bodies,” observes Guy.

Age is indeed one of the most important factors guiding excipient selection. If a medication is intended to treat a wide patient age range, Guy recommends developing modified versions formulated for subsets of the population based on reasonable age groups, such as pediatric through adult and then elderly or long-term care patients.

The first consideration, though, Guy emphasizes, is therapeutic benefit and efficacy, followed by safety, ease of use for the patient, and manufacturability. “We need to understand the target drug product profile when selecting a taste-modifying agent/approach to ensure drug performance is not negatively affected,” Mahmoudi agrees. The characteristics of the API and the desired properties of the final product directly impact excipient selection, she states.

The extent and type of unpleasant taste associated with the API (bitter, sour, irritating) and its aqueous solubility determine which excipients will be appropriate. “It can be more challenging to cover the poor taste of a highly water-soluble API than that of a less-water-soluble active,” Mahmoudi says.

In addition, the particle size and shape of the API can influence the approach for taste-masking and appearance improvement. Specifically, Mahmoudi notes that spherical particles are easier to coat than irregularly shaped particles. The dose must also be considered; it is usually easier to taste-mask a low-dose formulation.

In film-coating approaches, adds Karry, the formulator should ensure complete drug solubilization and absorption to achieve optimum bioavailability. “Doing so is especially important for reverse-enteric polymers, as they should quickly dissolve in the acidic stomach media to allow drug diffusion out of the coated core and absorption in the small intestine. Fast polymer solubilization is critical because the tablets and pellets have a short residence time in the stomach, and these polymers are not soluble in non-acidic media. In other words, if the film does not dissolve in the stomach, it will not dissolve anywhere else in the body and as such, the drug will not diffuse out of the protected core,” she explains.

Appearance factors such as shape and color can play a role in determining the correct identification and/or correct dosing material (syringe or spoon, etc.). In some cases, a certain format can address appearance concerns. For instance, Guy points to single-unit doses such as stickpacks as a potential means for enhancing identification through improved naming and ‘branding’ of the unit itself. He does stress, however, that care must be taken to ensure that these adult/senior-friendly products are child-resistant.

Experience and knowledge of patients best guides

For some excipients, such as those designed to improve API solubility, formulators have developed predictive algorithms that facilitate more rapid formulation development. Taste and appearance are more challenging, however, because often these qualities vary from one individual to another.

“Structuring data from humans and how they respond takes a great many data points and statistical analysis and remains subject to less-well-understood error knowledge. While augmented intelligence approaches can certainly help formulators make better choices sooner rather than later, there are too many unknowns, and the need for structured data is too overwhelming for good models to work at this point,” says Guy.

The fact that there is no one method that can address taste and appearance properties in every oral medication makes predictive approaches difficult, Mahmoudi agrees. She does note, however, that evaluating literature reports and patient and caregiver surveys has revealed some general “dos and don’ts” that should be followed to achieve desirable taste and drug appearance.

For example, Mahmoudi says that the use of a coating on tablets is preferred among older patients because it can help to optimize both medication identification and taste. “Elderly patients generally take multiple medications, and having tablets that are visually appealing and easy to identify and handle, such as with a score line for easy breakage and optimized dimensions and shapes that address dexterity issues, is important,” she explains.“There are more preferences towards brightly colored tablets and identification based on shape and color for different indications to avoid medication errors,” she adds.

Karry disagrees with Guy and Tocce with regard to the ability to predict bitterness and believes the process for identifying if an API is bitter has rapidly progressed in the past few years. She points to BitterDB, a free database available to formulators with more than 1000 naturally bitter and synthetic compounds and the use of machine learning algorithms trained with known bitter API molecules that can predict the bitterness of new compounds. “Based on these predictions and additional physiochemical API properties, pharmaceutical formulators can proactively plan for their choice of taste-masking technology,” Karry contends.

The use of electronic tongues, however, has not been successful to date, says Karry, and the process to test and optimize taste-masked formulations continues to be reliant on trained human panels. E-tongues are expensive instruments based on capacitance sensors that are used to rank the bitterness of formulations, but have been shown to exhibit no correlation to the results obtained by human taste panels (1).

The use of human test panels presents its own set of challenges, including a new one that has recently arisen. A study published in 2017 by Dagan-Wiener et al. showed that 66% of drugs in clinical and experimental stages at the time were classified as bitter compounds (2). The situation remains the same today, according to Karry. Many of the drug candidates under development today, however, are different because they have much smaller differences between their therapeutic and toxic doses (NTI-drugs), she says. “For these drugs, recruiting healthy volunteers for taste panels and ensuring no side effects after evaluation of test formulations has become more complicated,” Karry remarks.

Currently, conventional film coating and the addition of sugars, flavors, and sweeteners remain the common approaches to taste masking, but numerous other taste-masking techniques have been developed to address this challenging task, according to Mahmoudi. “Microencapsulation, high shear mixing, freeze-drying, fluid-bed coating, and spray drying have been successfully used to modify taste and making various dosage forms,” she says.

In fact, Karry notes that there is a move away from the use of flavors and sweeteners to mask bitterness because studies have shown that sucrose-sweetened medicines are associated with increased dental issues in children. Addition of acids to generate sour liquid formulations rather than bitter medicines, meanwhile, affects tooth enamel and causes dental erosion.

“For these reasons,” Karry says, “film-coating approaches are increasingly used for taste masking of bitter compounds. Commonly used polymers include methacrylate copolymers such as Kollicoat Smartseal or Eudragit E-PO since they are insoluble in saliva (pH 6.2 –7.4), but readily soluble in the stomach.”

Hot-melt extrusion and 3D printing for personalized medicines are emerging pharmaceutical technologies that Mahmoudi believes can be efficiently used to mask the taste of very bitter APIs. In particular, Mahmoudi notes that 3D printing is a powerful technique with the potential to enable the design of visually appealing tablets with modified taste.

With respect to advances in excipient technology, Mahmoudi points to the introduction of several taste-masking polymers with pH-dependent and independent solubility, new excipients such as co-processed microcrystalline cellulose (MCC) and guar gum, colloidal MCC, and many different flavors and taste-masking agents developed by excipient manufacturers as being tremendously beneficial in modifying taste and drug appearance.

For Guy, the use of taste-masking agents as ingredients, rather than simply applying a coating as a physical barrier, is transforming flavor and flavor perception. “Such an approach has allowed us to tackle ‘off-notes’ in products, particularly in nutraceutical formulations. Specifically, combining bitter-maskers with an appropriate flavor combination helps to reduce or even eliminate off-notes to a background level that is tolerable,” he says.

Orally disintegrating and dispersible solid-dose formulation have increased in popularity in recent years, creating challenges for taste masking and appearance. “It can be difficult to navigate the possible excipient choices and strategies that will ensure the desired product performance, for which there are numerous requirements and constraints,” Guy observes. “Facing challenges is not unfamiliar territory, though, and formulators do have an extensive and growing toolbox of excipients that can be applied for these and other novel oral dose forms,” he concludes.

When formulating any type of drug and using excipients of any kind, including
for the modification of taste and appearance, the safety, regulatory acceptance, and daily allowable intake of each excipient must clearly be considered in addition to process and cost, according to Mahmoudi. “Beyond that, patient-centric drug design is paramount. There are great taste-masking excipients/texture modifiers and techniques that should be considered from the early formulation development steps when pursuing patient-centric solutions,” she says.

In addition to ensuring safety and efficacy, Guy reiterates that the most important thing formulators must do for any drug product is to ensure that the patients and care-givers that will be using/administering the medicine will not be afraid or put-off by the nature of the drug. “It is essential to have and apply good people knowledge and consider different formulation options for different patient populations as dictated by their differing needs. Patients are people, and that must always be top of mind,” he insists.

1. Senopsys, “Case Study: Correlation of Electronic Tongue (E-Tongue) and Human Taste Panel Data,” Senopsys.com, Accessed Nov. 24, 2021.

2. A. Dagan-Wiener, et al. Sci Rep 7, 12074 (2017).

Cynthia A. Challener, PhD, in contributing editor to 

 
Vol. 46, No. 1 
January 2022 
Pages: 26–27, 45

When referring to this article, please cite it as C. Challener, “Taste and Appearance: Selecting the Right Excipients,” 

Articles

Dosage form and patient needs drive excipient choice.

Taste and Appearance: Selecting the Right Excipients 

A large percentage of small-molecule APIs in development today suffer from poor solubility and bioavailability. Stabilizing these APIs in their amorphous forms can help overcome these issues because a supersaturated state is achieved upon dissolution and delivery. Such a state can result in enhanced absorption across gastrointestinal membranes and improved bioavailability, according to Lindsay Johnson, global technical marketing manager for BASF’s Pharma Solutions business.

“The high thermodynamic energy of an amorphous, supersaturated state is inherently unstable, however, and such systems are driven towards crystallization, liquid-liquid phase separation, or non-descript precipitation,” Johnson says. Tendencies to recrystallize, absorb moisture, and participate in side
reactions that generate impurities must also be overcome.

Solid dispersions lead the stabilization way

Depending on the chemical nature of API candidates, there are a variety of mechanisms that can be used to overcome solubility challenges. Amorphization of the API is one mechanism that can be pursued, observes Johnson, and commonly amorphous APIs are stabilized in amorphous solid dispersions (ASDs) formed via solvent evaporation processes such as spray drying (SD) or fluid-bed coating or melt processes such as hot-melt extrusion (HME).

These methods for stabilizing amorphous formulations are polymer-based, notes Daniel Joseph Price, strategic marketing manager, excipients for solid dosage at MilliporeSigma. “Although different processing methods are used and formulations vary, the end result is the same: a polymeric matrix, with hopefully a homogenous distribution of API throughout the polymer,” he adds. The distribution of the API in the polymer essentially immobilizes the API in the amorphous form, preventing re-crystallization.

Currently, there are 17 commercially available spray-dried dispersion amorphous drug products and 13 approved HME products (1,2). “Both of these technologies are established in the industry, easily scaled-up, and continuous,” comments Molly Adam, an R&D engineer at Lonza.

Both methods have advantages and disadvantages, too. Spray drying does not expose the API to high temperatures, but requires the use of large quantities of organic solvents and nitrogen gas and can have low throughputs, all of which make the process costly, Adam says. While extrusion has a lower footprint and a higher throughput, the process involves high temperatures and shear stresses that can degrade some APIs.

Generally speaking, the selection of amorphous formulation technologies should be guided by the glass-forming ability of the API, according to Price. “The glass-forming ability essentially describes how likely it is for a molecule to re-crystallize,” he explains. “Poor glass formers are molecules that have very poor stability in the amorphous form, and so have an inherent re-crystallization risk. It is therefore recommended to only use polymeric amorphous solid dispersions when working with good or moderate glass formers,” he continues.

The choice of spray drying or HME depends on several factors, including processing space, scale-up needs, performance drivers, and the properties of the API, says Gereint Sis, an R&D scientist with Lonza. Important physicochemical properties that drive API stabilization are the melt and glass-transition temperatures (Tm and Tg, respectively). APIs with significantly high melting points are not typically amenable to HME due to degradation, while APIs with poor solvent solubility are challenging for spray drying.

Ultimately, the amorphous system (API-polymer interactions, weight percent of API, temperature, humidity, and the region of the phase diagram the formulation occupies) will drive the extent of stabilization (3).

“The optimum selection strategy involves cooperatively conducting high-throughput, small-scale manufacturing, iterative in silico modeling, bench-top testing, polymer screening, and discriminating in vitro testing,” Sis states.The resulting optimum stabilization method should ensure performance, manufacturability, physical and chemical stability, and enable patient compliance, he adds.

One challenge, Sis observes, is the difficulty of striking a balance between these metrics while alleviating complexity and lowering costs. “Strategic and agile technology selection methodologies can help manage these issues,” he says. In addition, adding robust data sets coupled with machine learning to current selection strategies will likely ensure shorter timelines and alleviate cost burdens to drug-product-development lifecycles (4).

Common excipients include polyvinylpyrrolidones (PVPs), their copolymers, and their cross-linked derivatives, polyvinyl acetates (PVAs), and some functionalized cellulosic materials such as hydroxypropyl methylcellulose (HPMC) and hydroxypropyl methylcellulose acetate succinate (HPMCAS).

Typically, a binary API-polymer system that affords favorable API-polymer interactions to provide the stabilization is used in an ASD (3). These excipients are usually required to have specific material properties, including a relatively high Tg (usually 70 °C or higher), according to Johnson. Polymers with high Tg values are great at stabilizing amorphous APIs because they increase the Tg of the dispersion and decrease mobility, Adam remarks.

In addition to the Tg, other excipient properties to consider include the degradation temperature, solubility in organic solvents, water uptake, and typical sustainment performance during dissolution, according to Adam. She says that polymers with a wide processing temperature range are great for HME because they have a lower chance of degradation.

Poloxamers or polyethylene glycol (PEG) are also common in HME to improve processability by plasticizing the system, which decreases its melt temperature and melt viscosity. For SD, polymers with good organic solubility and low solution viscosity when dissolved are crucial to enable sufficient solubility for both the API and polymer.

Depending on the stabilizing mechanism, Johnson also notes that excipients for ASDs may contain functional groups that can participate in specific polymer-drug intermolecular interactions or have a rough solubility parameter that matches the hydrophilic/hydrophobic balance of the API in order to offer non-specific interactions that can be stabilizing.

Furthermore, polymers can aid in stabilizing the drug in gastrointestinal fluids in vivo and in the solid state during storage, according to Adam. Neutral polymers like PVP grades and HPMC can maximize dissolution rates throughout the whole range of pH of the GI tract. Enteric polymers such as HPMCAS and Evonik’s Eudragit L functional delayed release polymer can enable acid-sensitive APIs to be released in the intestine rather than the stomach if they degrade at low pH or have low solubility in the stomach.

For poor glass-forming APIs with inherent mobility, Price points out that mesoporous silica has emerged in recent years as an excellent excipient for stabilization. “The steric confinement of the API in nanosized pores after loading substantially reduces the mobility of the API in the formulation,” he explains.

Opportunities for instability still exist

Even though amorphous APIs are stabilized in ASDs, instability can remain an issue during storage and once the formulation has been administered to the patient. “The one downside to polymeric ASDs is the inherent mobility of both the polymer and the API in the formulations, which can lead to re-crystallization of poor glass-forming APIs,” Price states.

The mechanisms by which amorphous forms destabilize are phase separation and crystallization, according to Sis. “When an amorphous solid dispersion phase separates, distinct polymer-rich and API-rich amorphous phases form, which can make the amorphous API more prone to crystallization. Phase separation may or may not be a precursor to crystallization, with the rate-limiting step for crystallization being either diffusion or crystal nucleation,” he explains. Nucleation occurs when small seed crystals of the API form that then grow until a thermodynamically stable polymorph is obtained, according to Price.

Internal factors affecting the extent of stabilization of ASDs include the API loading, the polymer properties, and the drug-polymer interactions within the ASD, Sis outlines. For instance, hydroscopic excipients that absorb moisture will have negative consequences, as will formulations that contain plasticizers that increase polymer and API mobility, according to Johnson.

Environmental factors such as humidity and temperature can thus also affect the chemical and physical stability of the amorphous API within the formulation by increasing the kinetics of unwanted reaction pathways, such as degradation in the presence of water. Increased API mobility leads to a shift in the phase diagram, phase separation, and API recrystallization (3). “Smart formulation decisions and storage strategies are necessary to mitigate these risks,” Sis contends.

The behavior of the amorphous form upon dissolution in the body must also be considered, says Price. “When an amorphous API is dissolved, supersaturation generally occurs, and the API is in solution at concentrations higher than its thermodynamic solubility. Supersaturation is not a stable condition and is thermodynamically unfavorable. The API may precipitate out of solution and undergo nucleation and crystal growth, or API-rich liquid domains may separate out of solution,” he says.

Upon dissolution, observes Johnson, the stability of amorphous APIs is often segmented by their relative molecular and physiochemical properties, which will dictate which formulation approach is likely to be successful at stabilization. “In order to be amorphous and solubilized, any drug molecule needs to dissociate from its crystal lattice, and the dissolving media needs to solvate the drug molecule at the molecular level. Both of these processes require energy to either overcome drug-drug intermolecular forces or solvate the compound, respectively,” she explains.

Depending on which step is more solubility-limiting for a particular API, Johnson continues, the molecule can be referred to as a “brick dust” molecule or a “grease ball”. “Grease ball” and “brick-dust” APIs are associated with low and high melting points, respectively, and thus Tm and lipophilicity (log P) are often predictors of this categorization. In fact, Tm and log P can be used in combination with an API’s amorphous solubility to steer the selection of formulation type, according to Johnson.

“The supersaturation concentration achieved relative to that amorphous solubility is what dictates the destabilization process—whether direct crystallization that occurs below the amorphous solubility threshold or liquid-liquid phase separation that occurs above the amorphous solubility threshold,” Johnson concludes.

It is therefore important, stresses Price, to ensure not only the stability of the amorphous form in the solid state, but also stabilization of the supersaturated state upon dissolution.

Increasing interest in co-amorphous systems

Recent literature has expanded the interest in the field in co-amorphous systems, specifically ones that are not cocrystal forming. These co-amorphous systems include the API and a low-molecular-weight molecule that may be another pharmacologically relevant API or an excipient, Johnson observes.She adds that such formulations are commonly produced via mechanical milling, melt quenching, or solvent evaporation.

As an example, Price points to the spray drying of a mixture of an API with an amino acid, with the latter stabilizing the API in the amorphous form. “The two main benefits of this approach are processing and quality. For small-molecule coformers, it is easier to identify cosolvents for spray drying, and from a quality perspective, a small-molecule coformer is more well-defined and easier to control than a polymer,” he states.

Co-amorphous forms may also benefit from higher intermolecular interactions that prevent crystallization, higher conformational flexibility to prevent phase separation, and the anti-plasticizing effect of small molecules (5). They have also been shown to have increased solubility, dissolution rates, and physical stability compared to polymer ASDs with weak intermolecular interactions, Adam comments.

There are challenges to the formation of co-amorphous systems, however. Their typically higher molecular mobility, for instance, can lead to a greater tendency to crystallize during processing and storage, according to Johnson. She notes that significant performance variability can result, so co-amorphous formulations must be extensively studied in order to mitigate these impacts.

Recent advances in ASD technology are helping to overcome some of the limitations posed by the nature of poorly soluble APIs. High-shear mixing as an alternative to HME has the benefit of fast processing times combined with reduced exposure to high temperatures, according to Adam. “This technology has the potential to greatly impact the use of amorphous APIs by opening up the processing space where both SD and HME fall short, because it can be used for APIs with low organic solubility and high melt temperatures without degrading them,” she comments.

For APIs with poor solubility in common SD solvents, Adam points to temperature-shift spray drying as another important manufacturing advance. In this process, a slurry of the API is rapidly heated using an inline heat exchanger before atomization to increase solubility and throughput around 10-fold (6).

Another development noted by Adam involves increasing the drug loading for ASDs while improving stability and maintaining performance. High loaded dosage form (HLDF) architecture takes advantage of a high Tg polymethacrylate-based copolymer to significantly improve the stability of the ASD and HPMCAS to help sustain supersaturation upon dissolution. “This technology is applicable in formulations where the API tends to crystallize in gastrointestinal fluids and a polymethacrylate copolymer alone cannot effectively stabilize the amorphous API under these conditions,” she explains. “HLDF not only allows for higher ASD drug loadings, but also decreases tablet size and tablet burden,” she adds.

With traditional ASDs, the API is already in the amorphous state before it is processed into tablet form. Induced amorphization is the process of amorphizing crystalline API within a dosage form, thus converting from crystalline to amorphous phase during tableting.Deliberately utilizing this technique on a final dosage form can be described as in situ amorphization (ISA) and can be achieved using microwave radiation and other methods (7,8).

Price points to several examples in which APIs have been combined with small-molecule coformers such as amino acids, compressed into a tablet, and irradiated with microwaves to trigger ISA. One of the benefits of this approach is the ability to avoid using large weight fractions of polymer excipients, which dilute the concentration of API in the product, according to Sis. Physical stabilization of ASD formulations typically involves employing large weight fractions of polymers or other excipients in the final dosage form, which in some cases causes increased tablet burden, possibly lowering patient compliance (8). “Decreasing the need for long-term amorphous API stabilization results in final dosage forms containing only crystalline API and having dramatically reduced excipient fractions,” Sis explains.

In addition, eliminating the need to manufacture SD or HME formulations could avoid flowability issues associated with ASDs and drive down manufacturing costs (7). “Other important advantages to this approach relate to the stability and shelf-life of the product,” Price comments. “If the API is converted to the amorphous form directly at the point of care (pharmacy or hospital), the shelf-life relating to the stability of the amorphous form is less important,” he posits.

There are significant hurdles that must be overcome before such a dosage form is realized, however, particularly with respect to regulatory and quality concerns, Price temporizes. The technology is currently in its infancy, however, Sis notes.

In at least one example, some seed crystals were still present following microwaving of a tablet formulation (8). For some drug products, that may not affect the dissolution rate, but for others it could have a negative impact.

Overall, Sis believes ISA technology appears promising and warrants further development and investigation, with efforts focused on complete crystalline to amorphous conversion. He observes that considerations should also be taken to develop and understand the landscape in which FDA approval could occur. “ISA process technology must first be proven safe, effective, and scalable,” he concludes.

Rapidly evolving characteristics for candidate drug substances are driving the need for continued advances in technologies for the stabilization of amorphous APIs. Already formulators are challenged by the poor solubility of Biopharmaceutics Classification System II and IV APIs in the development pipeline.

“The ideal scenario for each of these drug substances would be to achieve a kinetic (beyond physio-relevant timelines) or thermodynamic increase in solubilization that allows for effective bioavailability,” contends Johnson. To achieve that goal, she believes continued investment in novel excipients that balance the need for increasing solubility while not hindering absorption due to competing thermodynamic or partitioning mechanisms will be necessary.

Adam points to the increasing number of “brick dust” candidates in the pipeline that are poorly soluble in both water and typically used organic solvents. That makes SD infeasible due to poor throughput or high emissions of harsh solvents. Because they often have higher Tm values, HME is also often challenging or not possible. “Strategies that can overcome the manufacturing complications with brick dust compounds will drive future formulation opportunities for needed therapeutics,” she concludes.

Drug loading also remains an issue for amorphous API dosage forms because they sometimes require high amounts of polymers to help with stabilization. In addition to HLDF architecture, Adam remarks that further improvements in stabilization technologies that enable decreased polymer amounts are needed to address this issue.

For Price, growing numbers of proteolysis targeting chimeras (PROTACs) and beyond rule-of-five (bRo5) compounds with molecular weights greater than 500 Daltons making their way through the clinic will be an important driver of innovation in amorphous technologies. “These larger molecules have the potential to totally transform the treatment of disease, in much the same way that mRNA is currently doing. The big difference is that PROTACs and bRo5 molecules have a high likelihood of being orally bioavailable and thus can be delivered orally. Development efforts must therefore be directed at identifying amorphous technologies for the formulation and stabilization of these novel and exciting modalities to ensure acceptable absorption from the GI tract,” he says.

1. J.M. Baumann, M.S. Adam, J.D. Wood,Annual Review of Chemical and Biomolecular Engineering 2021.

2. S.V. Bhujbal, et al., Acta Pharmaceutica Sinica B 11, 2505-2536 (2021).

3. D. T. Friesen, et al.,Mol. Pharm., 5, 1003-1019 (2008).

4. Y. Kapoor, et al., Molecular Pharmaceutics, 18, 2455-2469 (2021).

5. R. B. Chavan, et al., International Journal of Pharmaceutics, 515, 403-415 (2016).

6. D. T. Friesen, et al., Spray-drying process. Patent US2017333861A1, 2017.

7. W. Qiang, et al., Pharmaceutics, 12, 655 (2020).

8. M. Doreth, et al., International Journal of Pharmaceutics, 519, 343-351 (2017).


Volume 45, Number 12
December 2021
Page: 20–24

When referring to this article, please cite it as C. Challener, “Stabilization of Amorphous APIs,” 

Polymeric amorphous solid dispersions are the most commonly used technology, but amorphous APIs remain inherently unstable.

Stabilization of Amorphous APIs

Downstream processing operations have a direct impact on biopharma product quality and cost. Therefore, optimization of downstream biologics purification steps is not surprisingly focused on increasing productivity and reducing cost, while maintaining the highest possible quality. Two specific areas receiving attention, according to Martin Vollmer, biopharma program manager in the Lifescience Analysis Group at Agilent Technologies, are continuous manufacturing with automation/integration of associated analytics and the adoption of single-use technologies, including a shift from traditional column chromatography to membrane-based techniques.

Capture chromatography, adds Darren Verlenden, head of bioprocessing, MilliporeSigma, has been an early target for improvement due to the high cost of goods associated with this operation. Adoption of advanced capture methods, such as multi-column capture, has been slower than expected, however, due to complexity and regulatory concerns, he observes. The focus has therefore shifted to flow-through polishing. “Analytics in this space are needed to move from monitoring critical process parameters to monitoring critical quality attributes, with in-line or at-line aggregate analysis seen as an important need,” Verlenden says.

Indeed, the biopharmaceutical industry continues to express interest in technologies that enable process control while improving process development times and quality. There is particular need for advances downstream due to the dramatically higher productivity achieved in upstream manufacturing today, according to Phil Vanek, chief technology officer at Gamma Biosciences.

“Increasing interest in automation and continuous manufacturing and the advent of manufacturing strategies for advanced therapies (e.g., cell and gene) are compelling more chained or continuous manufacturing methods to allow for more walk-away operations,” Vanek continues. “These approaches not only improve cost efficiency, but when properly implemented can improve product quality and reduce risk through closed operations and increased use of automation,” he asserts.

Full integration of analytical technologies helps to provide real-time answers, reduce costs, avoid costly failures, and makes processes much more efficient, notes Vollmer. “Online and inline process analytical technologies will provide continuous insight into the process and into the quality of the drug substance,” he states.

Overall, therefore, the incorporation of rapid, high-throughput analytics enables more informed decision making in process development, resulting in improved processes, Verlenden summarizes. However, for these high-throughput development and analytics to also reduce development timelines, he cautions that novel approaches to data collection and processing are needed to overcome the inefficient, largely manual, paradigm of today.

While advanced control is lagging for a variety of reasons, including the different control architectures within the diversity of equipment being deployed in manufacturing today, Jonathan Hartmann, president and CEO of Nirrin Technologies, believes that with new technologies and the market demand for integrated control in operations, the control aspects (for full automation) will quickly catch up.

In addition, Hartmann observes that for next-generation therapies in particular, process analytics promise to improve not only their therapeutic potential and potency, which is often predicated on the manufacturing process itself, but also the ability to assure their safety through process consistency and control, as well as their cost effectiveness through automation and simplified manufacturing.

With Chinese hamster ovary (CHO) cell productivity routinely exceeding titers of 10 g/L, Vanek notes that many biologics manufacturers are looking to address knock-on effects such as column fouling and protein aggregation. “The ability to continuously monitor downstream unit operations and operate under conditions that avoid product yield losses, especially after affinity and polishing steps when the value of the product is at its highest, are of extreme importance,” he comments.

Particularly for advanced therapies, Vanek says that real-time monitoring and control of critical quality attributes, such as full capsid adeno-associated virus productivity, can have a positive and significant impact on product potency, cost, and safety.

Most analytical technologies employed today, however, are still used in an offline manner, and results are not available in real time, which causes delays. Samples are removed from the stream and submitted for testing with results produced hours or days later.

“With those techniques, immediate reaction on fractionation or in optimizing yield and recovery is not possible; the approach today can be compared to performing a post-mortem analysis,” Vollmer explains. “Such delays preclude decision making to inform the next round of experiments and slows overall development,” adds Verlenden.

Any reduction in this timeline should speed development, according to Verlenden. “As biopharma moves to connected and continuous processing, the ability to monitor critical quality attributes in real (or near-real) time will further enable optimization and streamline key decision points,” he states. Vollmer agrees that the goal must be to get results when there is still time to take action in response. He notes that connecting liquid chromatography (LC) or spectroscopy online or inline will provide this capability.

“Using advanced analytical approaches including machine learning,” adds Vanek, “will deliver data that can identify process pitfalls and opportunities for improvement addressing quality and yield, as well as set the stage for predictive analytics that could allow users to intervene or abort processes that are likely to produce out of specification results.” Additionally, real-time data collection and analysis can be coupled to feedback controllers to enable automation and hands-free process management in the future, contends Vanek.

While there are some inline sensor technologies available on the market today, Vanek laments that they can be costly, slow, and/or complex, and often measure surrogate events in the process that are then indirectly correlated to the desired metric. It is important, stresses Hartmann, to consider the physical time needed to not only capture data, but also analyze and integrate the results of that analysis into a real-time control instruction set, to allow for process automation.

Digital innovation and analytical advances now essential and enabling

The desire for faster and more flexible analytical solutions for downstream processing is driving innovation. New “Industry 4.0” digital capabilities will be essential before fully automated feedback loops can be leveraged, according to Vollmer.

One of the key challenges of existing downstream-processing analytics, Vanek observes, is the need to sample product and analyze the material off-line, as many of the on-line technologies measure only surrogate analytes or cannot process the data in real-time. “As new applications are developed, or existing methodologies are being adapted to on-line measurements, it becomes easier to realize improvements in performance. In doing so, integration of the data into manufacturing execution systems and process chaining for continuous manufacturing, together with increased automation, become enabled,” he explains.

Advances in connected and continuous manufacturing, in particular, are increasing expectations for rapid decision making, agrees Verlenden. “Success in this area cannot be fully realized without an advancement in analytics,” he observes. New robust analytical equipment fully integrated into the process and specifically designed for this purpose helps to facilitate adoption as well, Vollmer agrees.

Regulatory is also a key driver with expanded data package requirements, Verlenden says. Design-of-experiment approaches, for instance, require additional analytical data in downstream-processing applications.

“In essence,” Verlenden states, “improving analytics will allow for shorter development cycles to enable manufacturers to accelerate speed to market and reduce costs, while in the future, these analytics will enable the transition to process control based on critical quality attributes.”

As with the adoption of any new technology, the conservative nature of the biopharmaceutical industry has led to slower-than-desired action with respect to the implementation of more advanced analytics that can facilitate downstream process optimization. “Our industry tends to conservatively adopt new technologies, and this continues to slow down adoption,” says Verlenden.

There are other challenges as well. “Inertia within the industry to switch, primarily caused by regulatory constraints and the high cost of implementation, is a key hurdle,” Vollmer notes. In addition, as analytical technologies move from established off-line methods to at-line or inline methods, there is likely to be a period of decreased efficiency where, for instance, methods are run both off-line and at-line or inline, according to Verlenden.

“While this duplication will be a necessary step to realize the future potential of at-line and inline analytical testing, it will require investment and acceptance of this intermediate inefficiency,” Verlenden comments. Ultimately, however, Vollmer believes that the pay-off will be lower-cost production and better drug quality.

Vanek is clear that implementing new technologies into regulated manufacturing environments can introduce many challenges, from data compatibility across legacy systems to methods re-validation and even respecifying acceptance criteria for a product—all of which can be regulatory headaches or worse. “To make it worth the investment from a cost- and risk-perspective, adopters of new technology have to have confidence that the method will be reproducible, reliable, and scalable, with the added information providing significant advantages over existing methods,” he explains.

Advances, while not numerous, are being introduced and demonstrating their benefits. Some downstream process innovations are being spurred by upstream process improvements in conventional biologics manufacturing, such as productivity increases and continuous manufacturing, according to Vanek. The demand is also increasing, he notes, due to the broader array of biologics being manufactured including plasmids, RNA, and viral vectors; the physical nature of these materials makes purification with conventional downstream methods challenging. 

“This diversity of products is one key element driving innovation. Real-time analysis using rapid methods and their integration into existing workflows is another innovation driver. Analysis that addresses not only product quantity, but simultaneously product quality, can be a real-time and cost-saver in the steps towards releasing a therapeutic product,” Vanek adds.

Vollmer points to online LC, new Raman spectroscopy solutions, and new near-infrared (NIR) instruments and LC/capillary electrophoresis (CE)-based analyzers that target very specific single-attribute applications.

Implementation of Raman spectroscopy for bioreactor feedback control, Verlenden notes, is contributing to increased efficiency, productivity, and quality through inline analysis, chemometric analysis, and feedback control. One specific example is MilliporeSigma’s Procellics Raman Analyzer with Bio4C PAT Raman Software, which currently enables upstream scientists to deliver harvest material with more highly controlled quality attributes that should,in turn, reduce downstream purification challenges.

“Eventually this technology should be useful for streamlining future adoption of Raman and other spectroscopic techniques for monitoring critical quality attributes such as concentration, aggregates, and formulation composition,” Verlenden believes.

Other technologies are under development with the goal of moving analytics from offline to at-line and online and enabling the monitoring of not just critical process parameters but also critical quality attributes.

“Process control based on critical quality attributes will provide additional degrees of freedom that are not possible today,” observes Verlenden. “For example, if aggregate levels could be monitored in real time, an excursion early in the process that today would require rework or a deviation could be corrected downstream within identified parameters to meet process purity goals. This type of approach would represent a fundamental change in how we develop and manufacture biologics,” he adds.

Agilent is focused on the development of online-LC solutions because this technology offers significant versatility, according to Vollmer. “Online LC is a very promising technology since it has the power to analyze multiple process and product-related attributes. With LC, it is possible to measure a multitude of parameters and get high-quality data from the analysis. LC is also a technology widely used and well-known in the pharma industry, and it can be connected to a variety of different detectors that provides different angles of insight,” he explains.

The key to successful online LC, Vollmer stresses, is to develop analytical instrumentation that seamlessly plugs into the overall process software environment.

Nirrin Technologies, meanwhile, utilizes the strengths of NIR, including rapid reliable results with a very high dynamic range, by combining new sensor deployment designs with novel laser technology, contends Hartmann.

While many of these developments are still in the early stages, scientist involved in the development of analytical solutions for downstream processing continue to set their sights well beyond what might be possible in the near term.

For instance, Vollmer would like to see the introduction of online LC-mass spectrometry (MS) technologies for downstream processing applications. “Online LC/MS would provide an additional layer of insight into the process,” he says. First, however, simplification and user friendliness must be improved for MS so that operators working on downstream processing lines who may not be analytical experts can operate these platforms with ease.


Volume 45, Number 12
December 2021
Page: 30–32

When referring to this article, please cite it as C. Challener, “Analytics Advances for Optimizing Downstream Processes,” 

Simple, inexpensive real-time analytics are urgently needed for high value products.

Analytics Advances for Optimizing Downstream Processes

With the majority of APIs under development considered poorly soluble and/or poorly permeable, formulators have been forced to develop new solutions for overcoming these key issues. Lipid-based drug delivery is one of only a few methods effective for increasing both the solubility and permeability of APIs.

Formulations that are designed to spontaneously emulsify upon contact with aqueous media, including self-emulsifying drug delivery system (SEDDS) and self-microemulsifying drug delivery system (SMEDDS), are often preferred because they are relatively easy to formulate, can potentially decrease first-pass metabolism, minimize food effects (minimize the difference in API absorption in the fed and fasted states), and can protect APIs sensitive to degradation in aqueous environments. In addition, because the API is dissolved in a pre-concentrate and not subject to amorphous-to-crystal transitions, which can occur over time with other technologies, SEDDS formulations are relatively more stable.

The key to successful SEDDS formulation is the choice of the right combination of lipid excipients for the particular API and route of administration.

Lipid-based formulations are generally classified into four categories, according to Philippe Caisse, scientific director pharmaceuticals at Gattefossé. Type I are composed 100% of lipids. Type II are SEDDS without water-soluble components that consist of 40–80% oils and 20–60% surfactants with low hydrophilic-lipophilic balance (HLB) values, have a turbid oil-in-water dispersion aspect, and are easily digested.

Type III lipid formulations are SEDDS/SMEDDS composed of <20–80% oils, 20–50% high-HLB surfactants, and 0–50% hydro cosolvents that have water-soluble components, form clear of bluish dispersions, and are less easily digested. Type IV systems are composed of 0–20% low-HLB surfactants, 30–80% high-HLB surfactants, and 0–50% hydro cosolvents and able to form clear micellar solutions but may not be digested.

Typically, SEDDS are isotropic and kinetically stable (SMEDDS are thermodynamically stable) formulations of functional lipids containing one or APIs for systemic delivery, according to John K. Tillotson, pharmaceutical technical business director (Americas) for ABITEC. SEDDS compositions, adds Nitin Swarnakar, North America Application Laboratory manager within BASF Pharma Solutions, comprise precise combinations of oil, surfactant, and cosurfactants to yield low-viscosity, isotropic mixtures.

The nature and selection of each of these components will, Swarnakar says, significantly affect properties such as droplet size, speed of dispersion, digestibility of the droplets, and API absorption.For example, he notes that a less digestible mixture can be formulated by including a lipid with a long carbon chain or by increasing the concentration of a less digestible surfactant.“Depending on the goals of the formulation, an optimal composition can be targeted,” he states.

As of 2019, there were at least 15 commercially available small-molecule drugs formulated as SEDDS (1,2). The relatively simple need to make a small-molecule API soluble to improve drug delivery—rendering it orally available or capable of getting across the lining of the gut—involves differences in lipid structure, according to Jamie Grabowski, vice president, portfolio and sourcing at Curia (formerly AMRI).

The most common classes of lipids employed are solubilizers, emulsifiers, surfactants, and potentially co-surfactants, Tillotson comments.Medium-chain triglycerides serve as solubilizers; mono- and di-glycerides as solubilizers and emulsifiers; and pegylated esters, polysorbates, and ethoxylated oils as surfactants and co-surfactants.

Surfactants can be water-insoluble (e.g., propylene glycol esters), water-dispersible (e.g., linoleyl polyoxyl-6 glycerides), or water-soluble (e.g., polyoxyl-based esters), according to Caisse. Diethylene glycol monoethyl ether is the most common hydrophilic cosolvent.

“These functional lipids are preferred based on their efficacy with regard to solubilization and emulsification capabilities,” says Tillotson. For example, medium-chain triglycerides have a high solubilizing capacity for lipophilic drugs and are especially easy to emulsify using suitable surfactants, such as castor oil derivatives, according to Swarnakar.

SEDDS formulations start as isotropic mixtures that contain the API(s) dissolved in the functional oil, solubilizer, and surfactants. When the SEDDS formulation enters an aqueous environment, such as the GIT, the SEDDS forms API-containing droplets, with the API(s) contained in the hydrophobic interior and the emulsifiers, surfactants, and co-surfactants stabilizing the discontinuous oil phase inside the continuous aqueous phase.

For example, Tillotson notes that greater amounts of hydrophobic lipids tend to increase API solubility in the system for some APIs; in contrast, greater amounts of less hydrophobic lipids, such as emulsifiers and surfactants, tend to reduce globule size and generate micro-emulsions.“The challenge is determining the optimum concentrations of each functional lipid in the pre-concentrate with regard to maximizing API solubilization and emulsion performance,” he concludes.

Often SEDDS formulations may include three, four, or five excipients along with the API, according to Caisse. Formulating an optimal SEDDS may thus require numerous trials and formulation variations. He also notes that some all-in-one self-emulsifying excipient systems are available that can simplify the preparation of type II and Type III lipid-based formulations.

Multiple factors influence lipid selection

Generally, lipids with chain lengths of C8 to C18 are reported in the literature as being ideal for SEDDS formulations, according to Swarnakar. She adds that specific lipids are chosen based on the melting point and crystal lattice properties of the API in question.

Most poorly water-soluble drugs are lipophilic, and thus the solubility of the API in the lipid components of the system will be the first parameter considered, according to Caisse. For highly lipophilic drugs, oils or mixed mono, di-, and triglycerides are often used. For APIs with medium lipophilicity, low HLB (≤ 9) surfactants are often preferred, he says. For APIs with low hydrophilicity, high HLB (> 10) surfactants and hydrophilic solvents are often required.

Another very important consideration is the desired emulsion performance and characteristics of the SEDDS formulation, says Tillotson. “The ideal SEDDS formulation optimally balances the overall solubility of the API(s) while realizing the desired emulsion characteristics, such as globule size and dispersibility. The goal is to develop a system that provides for maximal API loading, while also generating a rapidly-dispersing micro emulsion,” he explains.

SEDDS formulations should also take into account the type of API being delivered, according to Tillotson. For example, Biopharmaceutics Classification System (BCS) Class II APIs are poorly soluble but readily permeable.Therefore, the focus in a BCS Class II carrying SEDDS is on lipids that provide the greatest solubility/carrier capacity for the API.In contrast, a BCS Class IV API is both poorly soluble and poorly permeable.In this case, the SEDDS needs to not only address API solubility in the functional lipids, but also, if possible, permeability issues.

For this reason, Tillotson says a BCS Class IV API-carrying SEDDS may include lipids that open tight junctions between enterocytes (functional lipids composed of C8 and C10 fatty acids) or lipids that inhibit the activity of P-glycoprotein (PGP) efflux pumps (certain mono- and di-glycerides and certain macrogolglycerides).

In addition to the nature of the API, lipids for SEDDS formulations are also selected depending on the delivery strategy, Swarnakar adds. With respect to the API, “like-dissolves-like” is the rule of thumb for choosing the lipid. “Generally, very hydrophobic drugs (log P > 5) can be solubilized in more lipophilic lipids with longer carbon chains,” he says.

Specifically, longer and fully saturated carbon chains are more stable and less digestible within the gastrointestinal tract (GT). “This less digestible nature can be beneficial to the absorption profile by providing a secondary, lymphatic route of absorption in addition to the standard portal vein absorption. This additional route of absorption can be used to enhance the bioavailability of specific APIs and increase API absorption times,” observes Swarnakar.

Other factors related to the API in addition to low in vivo permeability may also be of importance, observes Caisse, such as heat sensitivity and a high first-pass metabolism. “Hence the design of a self-emulsifying formulation as an efficient delivery system for a given API is also related to the targeted strategy for its bioavailability enhancement or physical limits of its manufacturing process,” he says.

The final dosage form should also be considered. Caisse notes that for soft-gel capsules and liquid- filled hard capsules, liquid/low-viscosity formulations are best, while for solid-filled hard capsules, semi-solid/solid excipients are preferred as the main components, although up to 20% liquid excipient is feasible.

Despite the general understanding of how different lipids impact solubility and permeability, formulators have always struggled to predict the best SEDDS formulation prior to costly in vivo and clinical work, according to Swarnakar. “Various reported in vitro methods, such as [United States Pharmacopeia] USP type 2 dissolutions, provide limited discrimination of SEDDS formulation behavior.The interference of turbidity and biphasic media make conventional in vitro screening methods inaccurate for SEDDS formulations,” he explains.

To address this issue, BASF, in partnership with Professor Anette Müllertz at the University of Copenhagen, has recently established a robust in vitro-in vivo correlation of 10 ready-to-use SEDDS compositions using the MacroFlux device from Pion for determining the absorption potential of formulations and finished drug products in vitro. The ready-to-use compositions are categorized based on their compositional HLB values and performance-indicating target product profile attributes, including microemulsion droplet size and enzymatic digestibility.

“Through careful testing and consideration of the chemistries in these formulations, the formulator is able to pre-screen a range of formulations and select according to their preferred API absorption behavior,” says Lindsay Johnson, global technical marketing manager–Pharma Solutions at BASF.She believes this tool will help formulators avoid costly pre-clinical studies and ensure continuity of product quality and performance during product development. “Overall, these tools will enable formulators to choose the best SEDDS formulation based on the API properties for preclinical and clinical trials and accelerate the product development timeline,” she asserts.

New developments with SEDDS are focused on extending the efficacy of the dosage form beyond simple improvements in solubility. Areas of research, according to Tillotson, include chylomicron signaling for tissue targeting, long-chain lipid inclusion promoting lymphatic transport and reduced first pass metabolism, and employing lipids as a delivery system for more specific targeting such as conjugated antibody targeting with APIs.

Liquid SEDDS formulations for oral administration are generally loaded into liquid-filled soft-gelatin or hard-gelatin capsules. “An ongoing challenge is how to administer SEDDS on higher throughput dosage forms, such as tablets,” says Tillotson.

There are many drivers for the development of solid or semi-solid SEDDs formulations in addition to the ability to easily incorporate them into tablets. They may also offer improved stability and enable sustained-release or abuse-deterrent formulations. Liquid SEDDs, according to Caisse, are susceptible to degradation during long-term storage and suffer from in vivo precipitation issues and handling complexity.

Research is ongoing in this application at multiple institutions.“The primary difficulty is generating tablets at industry tableting speeds with minimum or no sticking to the punches that also release the SEDDS formulation,” he observes. Other solid SEDDS technologies are also being developed such as powder and granular SEDDS.

SEDDS compositions formulated as solids can be achieved, according to Swarnakar, using a variety of methods, including adsorption onto solid carriers, freeze drying, spray drying, and melt granulation. Caisse adds that wet granulation and extrusion/spheronization are other solidification techniques used for converting liquid SEDDS into solid SEDDS.

Of these methods, Johnson notes that adsorption onto an inert solid carrier is most common. In this case, a liquid SEDDS solution is mixed onto various solidifying agents such as mannitol, lactose, or calcium carbonate.

The key to this strategy is to retain the solubilization and dissolution enhancing properties of the SEDDS formulations once they are absorbed on the solid carrier materials, Caisse observes.The resulting powders, he says, can be subsequently filled into capsules or formulated as solid dosage forms such as tablets, granules, or pellets in sachets.

Progress has been dramatic in the past few years particularly with respect to the development of lipids that facilitate the absorption of large molecules—notably biologics, according to Grabowski. “Driving the development of these lipids for SEDDS is the need for drug products with expanded methods of administration, notably oral. This is a big challenge for biopharmaceutical companies that want to offer patients the choice of an oral drug instead of an injectable,” he says.

Specifically, Grabowski notes that developers are moving away from relying on off-the-shelf lipids to get hydrophobic drug substances into solution or improve their stability. Instead, they are turning to complex cationic lipids that are actually helping with the functionality and efficacy of biologics by altering their bioavailability and pharmacokinetics, both in SEDDS and lipid nanoparticles (LNPs) such as those used in the formulation of mRNA vaccines against the SARS-CoV-2 virus, he says.

Cationic lipids improve the solubility, oral absorption, bioavailability, and pharmacokinetics of biologic drug substances, according to Grabowski. In LNPs, which have a much more complex structure than SEDDS, they are used along with cholesterol, a minor lipid, and one other typically proprietary compound.

Lipid nanoparticles such as cubosomes, adds Tillotson, contain both hydrophilic and hydrophobic regions and are readily absorbed by cells through typical lipidomic pathways.“The amphiphilic nature of these lipid carriers allows for the incorporation of proteins, RNA and both hydrophilic and hydrophobic APIs.For this reason, lipid nanoparticles composed of high-purity, functional lipids are ideal carriers for biologics and small-molecule actives,” he contends.

Across all research regarding lipid-based delivery, the main focus is on the purposeful design of lipids with specific structural and physiochemical properties. “Ultimately,” asserts Grabowski, “the industry will stop using off-the-shelf compounds such as cholesterol for LNPs and switch to carefully designed lipids with improved and diverse structures that enable fine tuning of the intended pharmacological impacts.”

For instance, Grabowski notes that assessing the structures of cationic lipids through structure-activity relationships will help improve the pharmacokinetics of drug delivery. “It will become less about simply being able to form micelles and more about making lipids that allow the drug substance to get across the gut lining and improve pharmacokinetics. That’s going to be the big issue,” he asserts.

Similarly, Tillotson sees emerging research on lipid-based drug delivery as being focused on the design and manufacture of high-purity lipids for specific applications, such as incorporation into LNPs for the systemic delivery of biological therapeutics. He also notes that ongoing lipidomics research seeks to identify novel lipids and lipid metabolites that can be potentially employed in biomarker discovery programs for specific disease states.

Greater expectations for GMP lipid manufacture

In many advanced lipid-based formulations, including SEDDS/SMEDDS, the lipids are not inactive ingredients, but functional excipients that impact the efficacy of the drug product. Both drug manufacturers and regulators are responding by treating these types of functional excipients more like APIs, according to Grabowski.

“Increasingly drug manufacturers want lipids used in their drug formulations to be made according to [current good manufacturing practice] CGMP requirements,” Grabowski says. “While lipids may not need to be produced in a CGMP environment for early-phase research, if regulators potentially consider them to be additional ‘APIs’ because they affect the bioavailability of the drug substance or efficacy of the drug product, the lipid supplier will be expected to have the ability to produce GMP lipids.” As an example, Grabowski notes that there is movement toward treating cationic lipids as APIs.

SEDDS and SMEDDS improve solubility and permeability while expanding efficacy and applicability.