Excipients for High-Concentration Biologics

Published on: 
BioPharm International, BioPharm International, March 2022 Issue, Volume 35, Issue 3
Pages: 17–20, 37

Careful selection is essential for achieving viscosity reduction without protein destabilization.

High-concentration biologic formulations are accounting for an increasing percentage of the drug development pipeline as drug manufacturers seek to offer advanced therapeutics that can be administered by the patient or caregiver in the home setting. High concentrations, however, generally mean higher viscosities due to protein-protein interactions. Greater understanding of these interactions and the role that excipients can play in minimizing them without negatively impacting protein stability and other aspects of biologic formulations is driving a shift towards the use of carefully selected combinations of viscosity-reducing excipients.

Patient centricity driving interest in high-concentration biologics

The trend toward greater patient centricity underway prior to the COVID-19 pandemic has only been accelerated by this health crisis. There is greater interest than ever before in at-home medical care, from telehealth visits to administration of medications at home that were previously delivered in a hospital or clinic setting.

When it comes to parenteral formulations, patient preference for subcutaneous administration instead of receiving therapeutic treatments intravenously, especially for medications that require frequent dosing, is creating demand for higher-concentration biologic formulations, according to Danika Rodrigues, former drug product development scientist at Janssen R&D who is now pursuing her PhD.

That is because subcutaneous injections are limited to small volumes—traditionally 1 to at most 3 mL (1). When on-body delivery devices are employed, slightly larger volumes can be delivered over time, but even with this method, there are limits to what patients will bear. To convert an intravenous (IV) drug to one that can be delivered subcutaneously, therefore, requires an increase in concentration. This trend, Rodrigues says, is expected to continue. “Approximately one-third of all FDA-approved monoclonal antibody (mAb) products comprise high-concentration formulations (>100mg/mL), with about three-quarters of those approved since 2015,” she notes (2).

High concentrations create challenges for manufacturers and patients

The ability to deliver highly concentrated protein formulations remains a major challenge for biopharmaceutical companies, however, according to Robert Mahoney, chief scientific officer for Comera Life Sciences (formerly ReForm Biologics). Comera is focused on improving patient compliance, convenience, flexibility, and ultimately reducing patient pain and suffering through the development of a pipeline of products converted from hospital-based intravenous dosing to subcutaneous injection.

As the concentration of protein solutions increases, the viscosity also increases. Higher viscosity can make the production of biologics difficult, lead to potential quality and safety issues, and cause problems for patients.

During downstream processing, viscous protein solutions can bring challenges such as the potential for inadequate mixing and filter clogging during buffer exchange and protein concentration steps of the ultrafiltration/diafiltration (UF/DF) process, observes Laura Tanenbaum, drug product development scientist in the Biotherapeutics Development & Supply business of Janssen Research & Development. Pumping viscous solutions can also cause high shear stresses during drug product filling that can denature the protein and/or lower product recovery.

In fact, Rodrigues adds that high-concentration protein formulations typically have more stability challenges due to increased protein-protein interactions and potential for aggregation and/or opalescence. “Aggregates in the delivered product can potentially lead to decreased protein activity or unwanted immune responses in patients,” she explains. They can also potentially lead to increased formation of subvisible or visible particulates.

Some excipients may also be more prone to degradation in high-concentration protein products, Rodrigues comments. She points to polysorbates in particular, and notes that it is important that appropriate levels of such excipients be used to ensure stability.

Certain contaminants, such as host-cell proteins, from the bulk bioprocess fluid can be concentrated along with the protein during UF/DF processing, and high-concentration formulations may also carry more yellow/brown color, Rodrigues adds. “These challenges must be overcome to maintain product safety and quality,” she states. Furthermore, new analytical methods may need to be developed to assess the product quality of viscous, high-concentration formulations, according to Rodrigues.

From the patient perspective, high viscosities can limit the use of devices such as autoinjectors that may otherwise enable easier/faster administration and the potential for self-administration, according to Tanenbaum. Higher viscosities may also require injection forces too high for regular syringes to withstand and/or for some patients to achieve (3).

Protein-protein interactions are the main issue

The protein-protein interactions that can cause quality issues for high-concentration biologic solutions are the main cause of their increased viscosity. These interactions occur because the protein molecules are in greater proximity to one another. “In these crowded environments, short-range attractive interactions, including van der Waals interactions, hydrogen bonds, and dipole-dipole interactions, dominate over long-range repulsive interactions and lead to increased viscosity,” Rodrigues explains.

The specific interactions, notes Tanenbaum, depend on both the properties of the individual protein and the formulation. “Some molecules may have more solvent-exposed hydrophobic patches that permit more of these short-range attractions, resulting in solutions with higher viscosities, while molecules with hydrophobic patches that are more hidden may not have this same viscosity concern,” she says. Furthermore, Tanenbaum observes that formulation pH can alter the overall charge of the protein, with pH values closer to the isoelectric point potentially causing increased viscosity.

Typically, formulations with protein concentrations below 75 mg/mL do not have viscosity issues (4). Some but not all proteins at concentrations of 100 to 200 mg/mL will exhibit measurable increases in viscosity. When concentrations surpass 200 mg/mL, most if not all protein solutions have higher viscosities simply due to the forced physical closeness of the protein molecules.

Excipients can help


The patient experience of a subcutaneous injection does not depend solely on the formulation viscosity. The size and length of the needle, the diameter of the syringe barrel, and the injection rate also determine the injection force. The insertion rate of the needle, the temperature and pH of the product, and the stress level of the patient are additional factors.

Formulators cannot control many of these factors, but they can minimize their importance by, when possible, reducing the viscosity of high-concentration formulations using appropriate excipients. The key to effective excipient use, Tanenbaum underscores, is understanding protein-excipient interactions.

Salts and amino acids have been used as viscosity reducers. One proposed mechanism of action, according to Tanenbaum, is the binding of charged excipients at protein hot spots such that the presence of that excipient disrupts protein-protein interactions that would otherwise lead to increased viscosity.

“Different protein molecules have varied numbers of ‘high-affinity’ binding sites for specific excipients, which can lead to different levels of viscosity modulation,” Rodrigues notes. For that reason, she says that while it is generally favorable to minimize the total number of excipients in a given formulation, there may be cases where combinations of excipients provide synergistic effects. “Researchers across the biopharma value chain continue to work and innovate in this space, including novel excipients and excipient combinations,” adds Tanenbaum.

There are some limitations with current excipients

There are limitations to what excipients can achieve with respect to viscosity reduction, in part because of how they function and the protein-specific nature of viscosity changes with concentration. “There is still a limit to the amount that viscosity can be reduced through addition of excipients, as viscosity increases exponentially as a function of protein concentration,” states Rodrigues.

One of the challenges with using excipients is their potential to impact protein stability. “The mechanism by which the excipients interact with proteins to lower their viscosity can also change the conformations of the proteins,” Rodrigues explains. “While increasing excipient concentration could further lower the formulation viscosity through interference of protein-protein interactions, it could potentially act as a protein destabilizer simultaneously; thus, it can be important to balance viscosity-reducing and protein-stabilizing capabilities during excipient selection,” she concludes.

Osmolality must also be monitored, according to Tanenbaum, because the addition of excipients to protein formulations may increase this property, which should ideally be kept near isotonic conditions. The effect of excipients is also dependent on the pH of the formulation, she comments.

During UF/DF processes, meanwhile, the addition of excipients to a protein formulation can cause the effective excipient concentration and formulation pH to be shifted from the target conditions due to the retention of charge (from the protein) on a particular side of the UF/DF membrane or steric limitations associated with a high number of protein molecules in a given volume, says Tanenbaum. “It is important in order to understand the molecule and the formulation space, to specifically evaluate each excipient and excipient concentration and the impacts they have,” she contends.

It is also important, observe Rodrigues, to be aware that the magnitude of viscosity reduction is a protein-specific phenomenon, as some proteins maintain relatively low viscosities at high concentrations without the need for added excipients, whereas others may have high viscosity despite the addition of excipients. “Although much effort is spent on reducing the viscosity of formulations to develop high-concentration products, lowering the viscosity with the addition of excipients may not necessarily mean that we can achieve higher protein concentrations in the concentrating process due to protein solubility limits,” she emphasizes.

Shift toward excipient combinations

One recent approach to overcoming these challenges has been to investigate the impact of different excipients individually and in combination with one another. Two companies active in this area have been Comera Life Sciences and MilliporeSigma.

Comera Life Sciences has identified caffeine as an excipient to be used alone or in combination with secondary excipients to significantly reduce the viscosity of highly concentrated therapeutic protein solutions (5). It has also developed antibody formulations using this compound to significantly enhance viscosity reduction (6). Comera has established individual agreements with Astellas Pharma Inc., Bayer AG, and KBI Biopharma, Inc. to enhance formulations of biologics based on Comera’s patented technology (5).

In February 2020, Comera Life Sciences also entered into a global licensing and collaboration agreement with MilliporeSigma to commercialize Comera’s excipients used in biotherapeutic formulations (7). That agreement has since been completed.

MilliporeSigma separately identified suitable excipient combinations for viscosity reduction and launched its Viscosity Reduction Platform of patented excipients and excipient combinations designed to reduce viscosity while maintaining protein stability in October 2021 (7). The Platform is based on six excipients that are intended to be used in up to nine different combinations, always including one amino acid and one anionic excipient, the latter of which are the ones most likely to negatively impact protein stability (7). Using these combinations results in more efficient viscosity lowering and enables formulators to better maintain protein stability while achieving viscosity reduction.

Importantly, all of the excipients in MilliporeSigma’s Viscosity Reduction Platform have been used in parenteral formulations in the past as parenteral nutrition agents (considered by the company to be APIs), counter ions to APIs, or for pH adjustment (8). They have all been subjected to the Hen’s Egg Test–Chorioallantoic Membrane (HET-CAM) in-vitro skin irritancy test with no irritancy detected. A formal toxicological and safety evaluation has also been performed by a European registered toxicologist.

Still a need for novel excipients

At Janssen, formulation approaches can rely on historical knowledge, computational modeling, and high throughput experimental techniques to identify promising excipients for viscosity reduction, according to Tanenbaum. “The optimized excipient or combination is highly protein-specific and must be evaluated on a case-by-case basis to ensure stability in the final
formulation,” she says. Once promising excipients or combinations are identified, the required concentration of the excipient can be further refined.

However, Tanenbaum also recognizes that currently all FDA-approved high-concentration products use excipients that have been traditionally employed due to regulatory challenges with using a novel excipient.

“There is a lot of exciting research and progress ongoing in this area to enable patient-centric drug product development. We recognize that not all excipients alter viscosity in the same way across different protein molecules nor across protein concentrations of a single molecule, which drives the need for product-specific formulation. Furthermore, lowering the viscosity through the addition of some excipients does not necessarily permit the ability to achieve high or ultra-high concentrations of the protein formulation. There is consequently a need for novel excipient solutions and a means for enabling their use in drug formulations,” Tanenbaum believes.

For that reason, Janssen is involved with the recently launched FDA “Novel Excipient Review Pilot Program,” which Tanenbaum says could encourage excipient manufacturers and pharmaceutical companies to explore novel excipients in formulations for viscosity reduction and other formulation purposes.


1. I. Usach, et al., Advances in Therapy 36, 2986-2996 (2019).
2. S. Shouye Wang, Y. (Susie) Yan, K. Ho, Antibody Therapeutics, 4 (4) 262–272 (October 2021).
3. R. P. Watt, H. Khatri, and A. R.G, Dibble, Int. J. Pharm. Jan 10 554:376-386 (2019).
4. S. Braun, et al., “The Viscosity Reduction Platform: Viscosity-reducing excipients for protein formulation,” MilliporeSigma White Paper, 2021.
5. ReForm Biologics, “ReForm Biologics Announces Issuance of U.S. Patent on Additional Excipients for Therapeutic Protein Formulations,” Press Release, Nov. 20, 2019.
6. Yuhong Zeng, et al., J. Pharm. Sci. 110, 3594-3604 (2021).
7. BioPharm International Editors, “MilliporeSigma and ReForm Collaborate on Excipient Commercialization,” BioPharm International, Feb. 12, 2020.
8. BioPharm International Editors, “MilliporeSigma Launches Excipient Platform for Protein-Based Therapies,” BioPharm International, October 6, 2021.

About the author

Cynthia A. Challener, PhD, is a contributing editor to BioPharm International.

Article Details

BioPharm International
Vol. 35, No. 3
March 2022
Pages: 17–20, 37


When referring to this article, please cite it as C. Challener, “Excipients for High-Concentration Biologics,” BioPharm International, 35(3) 2022.