Subcutaneous Drugs: Challenges and Considerations in Downstream Manufacturing

The last decade has seen an increasing trend towards the development of biologic products across many disease areas that can be delivered by subcutaneous (Sub-C) injection as an alternative to conventional intravenous (IV) formulations. Subcutaneous delivery provides the option of self-administration of the drug by the patient in a non-clinical setting, improving both the patient experience, and reducing healthcare costs associated with the longer, more complex IV infusion.

While this delivery method has obvious benefits, Sub-C formulations require considerably higher protein concentrations within a small volume (at or below 2 mL) compared to IV delivery, which can increase fluid viscosities that affect the stability, manufacturability, and delivery/administration of the therapeutic drug. The higher viscosities directly impact manufacturing unit operations particularly in downstream processing.

Here, we will provide a brief overview of a white paper by Pall Corporation on this topic, titled High Concentration Monoclonal Antibody Drugs – Manufacturing Challenges. The publication does an excellent job of highlighting key challenges and unique considerations in the development of high concentration Sub-C biologics, such as monoclonal antibodies (mAbs), with respect to formulation and process steps including ultrafiltration/diafiltration (UF/DF), sterile filtration, and fill/finish and storage/transportation.


During drug development, much of the focus is on the active pharmaceutical ingredient (API), and rightfully so. However, optimization of the formulation platform is a necessary step to assure bioavailability of Sub-C drug formulations. A typical formulation consists of the active ingredient (the antibody or recombinant protein) and inactive ingredients or excipients. A number of different excipients can be considered during optimization that can help overcome some of the challenges associated with high viscosity, high protein concentration solutions. Excipients are added to the formulation for preservation, stabilization and also to condition the active so that it is suitable for delivery.¹ Sugars could stabilize proteins by a variety of alternative mechanisms.² The amino acids arginine and histidine have been shown to significantly reduce the viscosity at high co-solute concentrations without adversely impacting the mAb stability. Surfactants such as, polysorbate 20, are used to reduce protein-protein interactions to inhibit aggregation. Additional formulation ingredients such as hyaluronidase are beneficial to improve drug adsorption and bioavailability, which can allow for greater subcutaneous injection volumes and supports better and quicker dispersal of the therapeutic drug in subcutaneous tissue.

Additionally, maintaining product homogeneity in a more viscous fluid can be a challenge. Biologic drug substances are often sensitive to shear stress, so the mixing and pumping during formulation must be powerful yet gentle to prevent uneven mixing and any damage to the drug substance.


Process operations designed for lower concentration manufacturing platforms can result in undesirable quality risks, or reduce yields when applied to high concentration products, which is why re-optimization of an existing platform technology or investigating alternative approaches can be required. During filtration operations, highly concentrated and viscous solutions can increase the back pressure and lower flux that can increase processing times. There is also a greater risk of shear-related damage and aggregation resulting from extended processing times and increasingly high protein concentrations that must be addressed.

UF/DF: Product concentration and buffer exchange into the desired formulation conditions is the first stage where the preparation of the high concentration drug substance begins. For traditional lower concentration products, flat-sheet tangential flow filtration (TFF) cassettes and a product recirculation are commonly used to slowly increase the retentate concentration towards the target, which is typically 3-5-fold. However, there is a requirement to achieve a 10-20-fold increase to greater than 100 g/L for high-concentration drug substances. Differential and transmembrane pressures, potential membrane fouling and protein aggregation are all pitfalls of this method.

Single-pass tangential flow filtration (SPTFF) may offer significant advantages over recirculating TFF for high concentration feeds. SPTFF allows direct flow-through in-line concentration with no recirculation of product with lower hold-up volumes that mitigates product loss and achieves a higher step yield at higher concentration than conventional TFF. There are fully controllable SPTFF systems capable of precisely controlling the concentration factor.

Sterile Filtration: Sterile filtration is essential to control process bioburden before final filling and impacts the quality and safety of the drug product. The filter selected needs to be fully validated with a full assessment of process risk to ensure it will be effective and that contaminants are excluded in accordance with regulatory guidelines while minimizing loss of highly valuable product. Additional risk factors associated with higher concentration and formulation components (i.e., viscosity and surfactants) and potential adsorption of components to the sterile filter must be well characterized. When retention performance of the filter options appears equal, then the impact of the filter on other parameters such as throughput and fluid loss (yield) will influence decision-making when designing the process.

Fill/Finish Operations

The final filling operation in the drug product manufacturing process can lead to problems with inaccurate dosing, or even interruption of the filling run, both risking significant losses of high value product without adequate consideration. The higher protein concentration and higher viscosity requires dispensing with larger bore needles that are prone to dripping or clogging caused by protein crystallization at the needle tip. Therefore, needle choices with materials that reduce drip formation, coupled with optimal bores and filling flow regimes guard against filling issues and safeguard valuable product at a critical point in the process.


The lower volume and higher drug value per mL amplifies any product losses during storage and transportation. It is common for the bulk drug substance to be frozen to avoid degradation that can impact shelf life and quality. The higher concentration does pose a greater risk of aggregation and the highest degree of control over freezing kinetics is essential to prevent variations that can impact product quality. Plate freezing systems with 2D biocontainer bags offer faster freezing and consistent temperature control independent of batch volume compared to 3D bottles. Identifying solutions that can protect frozen biocontainer bags during storage and transportation can also mitigate risk and product loss resulting from temperature excursions.

Overall, while many of the manufacturing process operations for high-concentration biologics resemble those for traditional drug substances, challenges imposed by the higher viscosity and protein concentration can lead to significant product loss. The increasing value of these drug substances makes any losses highly undesirable, which is why re-optimization of an existing platform technologies or adopting alternative approaches and system designs for the same operation are worthwhile, both to overcome quality risk and loss in yield.

Download the Pall Corporation white paper here: High Concentration Monoclonal Antibody Drugs – Manufacturing Challenges


  1. Holstein et. al, Strategies for high-concentration drug substance manufacturing to facilitate subcutaneous administration: A review, Biotechnology Bioengineering, Vol. 117 (11), 3591-3606, 2020
  2. Wang and Ohtake, Review: Science and art of protein formulation development, International Journal of Pharmaceutics 568 (2019) 118505
  3. Hung et. al, High concentration tangential flow ultrafiltration of stable monoclonal antibody solutions with low viscosities, Journal of Membrane Science, Vol. 508, 113-126, 2016

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