First USP Workshop of its Kind Focuses on Particulates in Biologics

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Along with particulate control and determination, speakers at the June conference in Rockville, Maryland, examined the role of protein aggregation and immunogenicity

 Particulates have become a major quality and safety issues, especially in injectable biologics. A workshop titled Control and Determination of Visible and Sub-visible Particulate Matter in Biologicswas held at The United States Pharmacopeial Convention (USP) headquarters in Rockville, Maryland, on June 26-27, 2017. The first USP workshop to focus specifically on particulates in biologic drugs, the program addressed many issues and challenges unique to biologics-including protein aggregation and immunogenicity-as they relate to particulate control and determination.

The workshop's goal was to provide a forum for stakeholders to come together, share information, and discuss key issues. This article summarizes highlights from the event. 

Particulate sources and types

Particulate matter in biologic drugs comes in many sizes, shapes, and materials. Generally, particles may be categorized as intrinsic, extrinsic, or inherent. Intrinsic particles are generated within the manufacturing process and may include silicone oil, rubber, glass, or stainless steel. Extrinsic particles come from outside the process and could include metal, human hair and skin, or dust.

Inherent particles, such as protein aggregates, are naturally present in protein therapeutics and may be acceptable with the appropriate control strategy. However, just because the particles are inherent does not mean they should be ignored or assumed to be safe. The safety aspects of inherent particles should be assessed throughout a product’s life cycle. Overall, it is important to distinguish between inherent particles and intrinsic or extrinsic particles

Another way to conceptualize particulate matter is to divide it into two categories: visible and subvisible. Detection of visible particulates (greater than 100–150 microns in diameter) depends on multiple factors such as the lighting, magnification, and container characteristics, as well as the technician’s training and visual acuity.

Subvisible particles are usually defined as particles that are 1–100 microns in size. In the area of subvisible particulates, there is increased focus on distinguishing between silicone oil and non-silicone oil particles.

Flow imaging can be used to visualize and count subvisible particles, which involves determining the aspect ratio to classify them. However, aspect ratio is not always reliable and it frequently misclassifies particles. Thus, a method is needed that will recognize patterns in the images.

Some companies are investing in the use of artificial intelligence to analyze thousands of particle images and categorize them. These algorithms, and equipment using them, scan images from a training set and “learn” to distinguish between the various particle types.

During the visual inspection process, when a particle is observed in a product, that unit is pulled and either discarded or set aside for further analysis. Several presenters at the workshop emphasized that it is important to have standard operating procedures (SOPs) in place for defects that have been identified during inspection, along with alert and action levels. Taking time to analyze rejected units can reveal clues that will allow manufacturers to prevent similar problems from happening in the future. More importantly, the same problem may be occurring in other lots or even other products, and the analysis may help improve these other materials too.

Particulates in drug products can form at any stage in the manufacturing process or during storage. Various environmental stress conditions (heat, shock, or vibration) can lead to particle formation. Thus, when developing or refining a formulation, stress conditions should be applied to learn about the formulation’s propensity to form particulates.

Presenters suggested that, in order to optimize the process and minimize particulate formation, it is best to follow a continuous improvement approach. Continuous improvement can be applied to formulation development, component compatibility, manufacturing procedures, and other processes.  All personnel involved should align on best practices and should also share accountability for the end product.


Protein aggregation and safety concerns

Protein aggregation in biologics can compromise the safety and efficacy of the product, noted an FDA presenter. Many triggers for aggregation have been identified, including oxidation, mechanical stress, impurities, pH shifts, and temperature changes. 

With blood products, another concern is platelet-derived extracellular vesicles (PEVs), which have a 50- to 100-fold higher pro-coagulant activity than resting platelets. Release of PEVs can be associated with the donor’s lifestyle, the collection procedure, and storage conditions. PEVs have a broad size distribution and are associated with adverse events including transfusion-related acute lung injury (TRALI). 

The gold standard methods for detecting protein aggregates and PEVs in biologics are flow cytometry and light obscuration; however, there is no regulatory guidance available on PEVs. It is always important to use orthogonal methods because these processes are very complex, and each method has limitations, according to the presenter.

Clinical impact of particulates

Particles can have adverse effects on the drug product and/or the patient who ultimately receives it. The focus should be on the patient, presenters agreed. For example, if a relatively large number of subvisible particles is detected, one should characterize them and perform a clinical risk assessment. This assessment can reveal information about the impact of particles on the drug product’s clinical performance.

Important clinical factors to consider include the route of administration, which is often intravenous, and the intended patient population. The most vulnerable patient populations include neonates, young children, seriously ill patients, and the elderly. Protein aggregates and other particulate matter can be immunogenic and can also alter the drug’s efficacy.

In the worst-case scenario, the patient may develop an autoimmune response. This reaction is sometimes missed because the patient’s illness can mask the adverse effects of the therapeutic product.  It is unclear whether all aggregate types are potentially immunogenic, but there is growing evidence that the size of the aggregates has an effect. For example, vaccine components  are purposely aggregated to increase the immune response to a vaccine.


FDA speakers recommended that orthogonal techniques based on different separation and detection principles should be used to both enumerate and characterize particles. Presenters from the National Institute of Standards & Technology (NIST) described a standard they developed that is optically similar to protein particles. It mimics the fibrous nature of proteins and is made of abraded ethylene tetrafluoride (ETFE). Unlike particle sets for instrumentation that are spherical and do not resemble proteinaceous material, the ETFE standard is made of a more stable material than protein and also can be easily counted. The NIST study showed that abraded ETFE looked more like protein than did polystyrene beads.

Another concern raised was that during the final steps before a biologic drug is administered to a patient, particles can form if the drug is mishandled. Thus, it is critical to provide clear, explicit instructions on the administration of drug products. The instructions should describe how these drugs must be handled to protect their potency and efficacy.

One of the most important and challenging questions raised at the workshop was what is acceptable-how many particles are allowed?  There is no simple answer, said an FDA presenter, adding that FDA looks at the whole profile and cannot specify a limit that will be appropriate for every product. FDA’s evaluation is product specific and is done on a case-by-case basis.

However, FDA does tell sponsors to collect and provide as much information as possible about their product.  In general, there is no clear evidence regarding a threshold of criticality or cutoff point. How does one justify that a specific level of exposure to particulates is acceptable? In each case, it depends on the product and the patient.


Risk–benefit ratio of therapeutics

Assessing drug products for particulates can be challenging for a variety of reasons. Often, reference to the “safety” of a product is connected to particulates, but every product has a risk–benefit profile. To further complicate matters, different patients react differently to the same product.

One might assume that visible particulates pose a greater risk than subvisible ones, particularly with injectable medications, simply because the visible ones are larger. However, visible particles typically do not fit through the lumen of a needle and thus they do not get injected. For example, 100 microns is about the size of a 36-gauge needle. In contrast, the subvisible particles are very likely to be injected and thus could pose a greater risk.

In some clinical settings, in-line filters are used to catch any remaining particulates during infusion of the drug to the patient. This approach may sound promising as a final measure for protecting the patient, but in-line filters can be problematic because they can produce particles as well. In addition, hospital personnel might use incompatible filters, unless the appropriate filter is supplied with the medication. Hospitals generally take the position that the drug manufacturer needs to qualify and supply the filter if a filter should be used.

Some presenters said that in-line filters should only be used if there is a sound, clinically based reason to do so. In some cases, the use of a filter is the only option for a medically necessary drug, where no alternatives are available. However, the in-line filter for a product needs to be qualified as capable of reducing visible and subvisible particles.

A related issue is the potential for particulates to enter the drug product from the administration set, which is not a filter but rather the standard equipment used to infuse intravenous medications. Administration sets may be loaded with particles, negating the impact of any and all controls and tests designed to prevent particulate contamination.

If a patient does receive particulates, this has potential clinical implications. In many cases, there is no clinically discernable effect, but this does not mean that no effect has occurred. Particulates can be vaso-occlusive, blocking pulmonary capillaries, or they can act as irritants, leading to soft-tissue granulomas. The composition of the product in terms of organic and inorganic leachates can also be affected.

When evaluating the intended patient population for a new medication, the factors to consider may include: patient age, comorbid conditions, presence of ischemic tissue, altered immune status, and volume of drug product to be given over a certain time period. If any adverse events occur, finding the root cause and linking clinical effects with particle counts can be very challenging.


Cell therapy products

Cell therapy products differ from other types of drug products in several important ways.They may be grouped into the following categories:

  • Minimally manipulated-not requiring a biologics license application (BLA)

  • More than minimally manipulated-requiring a BLA

  • Autologous-coming from and going back to the same patient; because the amount available is limited, destructive testing cannot be done

  • Allogenic-coming from a single source and, typically, going to many patients.

During manufacturing, cell therapy products cannot be purified or filtered using the standard approaches to remove particulate matter from drug products. In addition, these products are often opaque rather than clear, limiting the effectiveness of subvisible and visual particle detection methods. 

Differentiating between contaminants and the product itself, which typically contains particulates too, is often complicated, noted one of the FDA attendees. There are many challenges involved in monitoring particulate matter, therefore emphasis should be placed on limiting introduction of particulate matter that may be in raw materials or introduced by the manufacturing process.

Particulates that affect cell therapy and gene therapy products do not come from one single source. Instead, particulates are generated by many sources, often with no consistency. Particles can come from the original biological components, such as solid tissue or bone marrow. Other sources are tubing, bags, vials, and stoppers that come in contact with the materials and could also contain and release particulates. Other sources for particulates include the environment, personnel, instruments, and excipients.

Although FDA recommends using orthogonal methods to evaluate and analyze biologic drug products, this is not always possible. In some cases, there is no orthogonal method available. In the area of cell therapy, there are no established industry-specific standards for particles.

In one presentation about cell therapy and gene therapy, the speaker emphasized the growing challenge of controlling particulates in cell/gene therapy products, and the need for new detection methods. More scientists and their companies are starting to admit that they are encountering problems and challenges with particulate control and determination. However, there is still a lack of data on this issue, and manufacturers are searching for solutions to various problems.

Control of particulates is as important as developing methods to reduce them. FDA presenters advised cell therapy manufacturers to first determine the number, size, and type of particles, and then try to control and/or reduce particles. The next step is identifying the likely contributing sources, then developing mitigation strategies such as:

  • switching to better-quality source material

  • evaluating and improving cleaning practices

  • finding ways to reduce the number of handling steps

Inspecting products for visible particles

As one member of USP’s visual inspection expert panel mentioned, before 2010, there were few standards for performing particulates inspection. Now, it appears, people are taking some consistent and similar approaches.

Today, performing a robust 100% inspection of the finished drug product is a must, according to members of the expert panel. The goal is to use a statistical approach to inspection to achieve a product that is essentially free of visible particulates. The concepts involved were defined in USP general chapter <790> “Visible Particulates in Injections.” 

However, striving for zero defects seems unrealistic, according to workshop attendees, because visual inspection is such a probabilistic process.

Another dilemma is knowing when to stop looking for particulates. USP inspection conditions have been defined and are now harmonized with the European Pharmacopoeia; they apply to extrinsic and intrinsic particles. In fact, as a number of workshop participants noted, the three major pharmacopeias are now aligned in many ways (i.e., USP chapter <790> versus EP 2.9.20 and JP 6.06), which is a big step forward.

Another aspect of inspection is the “difficult to inspect parenterals (DIPs).” A product may be a DIP due to the container type, even if the solution itself is clear. With DIPs, there is a diminished capacity for 100% visual inspection; the actual rate is more like 70%–80%, according to a presenter on this topic.  Stakeholders need to develop and use testing and controls that can detect particles and statistically significant changes throughout the lifecycle of a product.


Themes and concepts discussed in the workshop

  • Particle control involves many factors and should be characterized and monitored throughout the product lifecycle.

  • Even though FDA does not recommend specific orthogonal methods to detect particles, the consensus is that they are important, as is quality by design (QbD), which requires evaluating each step in a procedure.

  • More rigorous particle characterization is necessary. In particular, silicone oil poses many concerns because it can interact with products and be immunogenic.

Based on the issues raised during this workshop, USP will consider suitable revisions to the existing chapters or the creation of new standards for certain classes of biologics.