What to consider when formulating bacteriophages

The menace of multidrug-resistant (MDR) bacteria has been growing every year, calling for new approaches to combat the lethal infections they cause. Bacteriophages, a group of viruses discovered over 100 years ago, have re-emerged as a subject of interest, drawing significant attention from the biotechnology industry. Despite being obscured by the practicality and broad spectrum of antimicrobials in the 1940s, these viruses have a unique characteristic that most antimicrobials lack: the ability to target specific bacteria, thus preserving the native microbiome.

This particular attribute  also makes bacteriophages interesting prospects for other applications, such as microbiome regulation, which has been linked to several autoimmune disorders that affect both the gut and the skin.^1-4 Additionally, engineered phages invite an even broader spectrum of applications, with potential for use as a delivery system for engineered proteins⁵ and to help modulate the tumour microenvironment for mucosal cancers, such as colorectal cancer.⁶ Moreover, personalised phage therapies would have still greater value for their ability to be tailored to each patient.⁷ They can also act in a synergistic manner with traditional antimicrobials by targeting bacterial resistance mechanisms from the inside.

Bacteriophages have already proven their utility in the drug discovery space, through what is known as phage display,⁸ and have recently been making a comeback in the therapeutic space. As of October 2021, there are approximately 20 clinical trials recruiting patients with a phage therapy as intervention, one of which is Phase III. This comeback is a result of the tremendous advances that have taken place in the biotechnology industry. The field, which in the last 40 years has been growing – driven by innovation and under strict regulation – has accumulated a breadth of knowledge on how to manufacture biological entities. One hundred years ago, when phages were discovered, knowledge on how to mass produce biological entities was limited. However, as the pool of tools at our disposal expanded, it became far more feasible to bring phage therapies to the clinic.

Nonetheless, developing a phage-based product is not an easy task and requires careful deliberation, as phages have unique characteristics and must overcome numerous challenges imposed by regulators.^9,10 On top of the challenges posed by drug substance manufacturing, the formulation stage is particularly critical. Despite being one of the last stages in the manufacturing process, it should be carefully considered early on.

Formulation selection

The first step in drug development is designing a target product profile (TPP), which is a document detailing its purpose and use. One particularly important decision to make about the drug relates to its dosage form.¹¹ Is it liquid, solid or semi-solid? Does it require a special coating? Is delayed delivery desired? Will it be part of a device? Additionally, there are several factors that are important to assess when designing the formula and the process for formulation, independent of the formulation type. Unique to the case of phages, some factors appear to play an important role. These fall into two categories – process-related and formulation-related factors – and are enumerated in Table 1.

Table 1. Process- and formulation-related factors that can impact phage infectivity.^12-15

Furthermore, choosing the right excipients requires a deep understanding of the physicochemical properties of bacteriophages. Each bacteriophage has unique characteristics that must be preserved in order to maintain their infectivity. Table 2 compiles several formulations that were tested by different groups for different phages.

Table 2. Formulations for phage solutions across multiple types. A few common elements include buffering agents and cryopreservants.

Something that can be challenging is designing a product that consists of a cocktail of multiple phages. For example, it is important to pay close attention to the charge of the phage/phages in question, as it could influence mixing/aggregation and could impact interactions with the excipients.²⁰ Aggregation can be detrimental, as it can damage the tail fibres, invalidating the phage infectivity.²¹

Solid formulations

This is one of the most commonly used formulations for traditional pharmaceuticals and there is extensive literature on the subject. Nonetheless, this type of formulation is uncommon for biopharmaceuticals and consequently it is important to consider the use of coating or encapsulation to avoid degradation in the gastrointestinal (GI) tract.¹⁹ Numerous authors have investigated encapsulating bacteriophages in polymers, such as alginate, as well as in liposomes.^19,22 Overall, the use of encapsulation techniques can improve delivery, as well as reduce losses in yield during drying processes.

Nonetheless, solid formulations are not limited to pills as they also include devices. For example, catheters and bandages can have the drug adsorbed in their surface, which offers another promising application for phage therapy.

Liquid formulations

When compared to the other two types of formulations, liquid formulations are the simplest.

A caveat to this simplicity is when the desired use targets the respiratory and GI tract. Unfortunately, many excipients commonly used in liquid formulations are not safe for use in the respiratory tract.¹⁵ and the harsh environment encountered in the stomach and gut calls for special protection.¹⁹ One possible solution could be encapsulation, as mentioned above.

Semi-solid formulations

This type of formulation substantially expands the potential application of phage therapies, from acne to pulmonary infections.²³ Nonetheless, this particular type of formulation requires careful assessment due to its diversity. The first consideration regards the type of semi-solid formulation; for example, creams, gels and ointments have different ratios between polar and non-polar excipients, which could impact solubility as phages are polar in nature.²⁴ Still on the choice of excipients, in the case of phage therapies it seems important to avoid ionic bases, as these can interact with the pages and impact their infectivity.²³

Additionally, the selected excipients must take account of the following:

• Target tissue
• Levels of permeation required
• Irritation
• Compatibility with container-closure system
• Active pharmaceutical ingredient (API) stability.

Finally, the mixing process can also present challenges, as semi-solid formulations are often more viscous than liquid formulations. Highly viscous fluids tend to offer high shear stress, which can impact phage infectivity.^25,26

Scale-up

After designing the desired formulation and optimising its parameters to meet optimal phage stability, one more challenge arises: the formulation process scale. This prospect addresses how to properly scale the process to meet product demand, while maintaining yield and speed. Most of the time for biologics, and the same applies to phages, challenges tend to appear within the drying process.²⁷ The commonly used freeze-drying process is complicated and involves high complexity heat and mass transfer, which makes the process hard to model. A few factors that are key in this process include: load size, chamber pressure, ramp rate and shelf temperature.²⁸ A systematised approach is needed, but so far there is no well-defined framework in place for drying processes.²⁷ A new alternative that has been gaining attention is the use of spray-drying. This technology offers several advantages over the traditional approach, the main ones being its scalability and mode of operation (continuous versus batch). Nonetheless, attention must be paid to the process parameters to avoid protein degradation and losses in yield. As noted by Leung et al.,¹⁶ spray drying can cause yield losses as the process can expose phages to high frequencies.

Conclusion

This short paper focused on formulation considerations for bacteriophages. Nonetheless, to bring a phage therapy product to market, it is paramount to have a deep understanding of the whole picture, as each step of the manufacturing process has its own challenges. In addition, analytical and quality control are also critical elements to successfully meet regulators’ requirements and expectations. Therefore, it is the opinion of the authors, that process development should begin early in the drug development process, given its impact on the final product.

References

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