Ask the Expert: Formulation Strategies for AAV-Based Gene Therapies

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Although formulation and drug-product activities are critical to developing gene therapies, much remains to be learned about their degradation mechanisms, and firm criteria still need to be established for buffer and excipient selection. Sarathi Vijay Boddapati (associate director of formulation and drug development at Catalent Cell and Gene Therapy) joined BPI on 25 March 2021 to present what his company continues to learn about formulating gene therapies by adapting methods used for other biologics.

Boddapati’s Presentation
Adenoassociated virus (AAV) remains the most common vector for gene therapies. As with biologics, formulation and drug-product activities for AAV products fall into three categories. The most critical work involves formulation development and product characterization, which help to investigate degradation propensities and identify mitigation strategies. Drug product (DP)-related activities determine how a product will be stored, handled, and administered. Freeze–thaw and stability tests are critical because AAV products are cryopreserved. Assessment of packaging, storage, and usage requirements is the final piece.

AAV and protein products require similar development activities, with some differences in their prioritization. Workflows for protein drugs frontload biophysical characterization, followed by preformulation studies, container–closure screening, and fill–finish studies. Supporting drug substance optimization activities occur concurrently with other formulation activities.

AAV formulation and drug-product workflows prioritize preformulation development, including buffer compatibility, freeze–thaw, and stability studies. Gene therapy products are stored at –80 °C. Thus, initial development and testing focus on freeze–thaw conditions and temperature excursions that are likely to occur outside normal storage conditions (e.g., –40 °C and –20 °C). Those are followed by compatibility testing for tubing and other devices for clinical administration. Biophysical characterization occurs as needed, with minimal testing performed.

Formulation Development: After relating the scope of AAV formulation and drug-product activities, Boddapati described methods used during characterization and formulation development. High-performance liquid chromatography (HPLC) and analytical ultracentrifugation (AUC) have become standard for quantifying full, empty, and partial capsids. Other characterization methods include differential scanning calorimetry (DSC) and dynamic light scattering (DLS). Forced degradation studies assess the effects of temperature, pH, process conditions, and oxidation on product quality.

Data gathered from characterization and forced degradation studies are used to enable buffer and excipient screening. Currently, buffers for AAV products are restricted to pH 7–8. Neutral pH is preferred because AAV products are delivered to tissues that are sensitive to osmolality changes. Buffer and excipient screening must account carefully for the osmolality of salts, sugars, detergents, and amino acids.

Learning from Proteins: Methods that have helped to optimize protein-drug formulation are being adapted for AAV products. Analysts are using differential scanning fluorimetry (DSF) in place of DSC. Scientists once needed different instruments to gather DSC, fluorescence, and aggregation data. Now, new tools such as the Uncle DSF system (Unchained Labs) combine several methods in one instrument to assess multiple parameters. Such systems also use lower sample volumes (e.g., 10 µL) than standard DSC units (e.g., 500 µL).

Device Compatibility: Gene therapies often require multicomponent infusion systems. Pumps deliver DP through a combination of syringes and cannulas. Narrow-gauge cannulas can create resistance during DP injection, so they are investigated during device-compatibility studies. Such studies are performed to confirm infusion times and dosing accuracies. Delivery-device components are evaluated for interactions with AAV vectors. Hold times studies also are considered to establish — and to inform therapy-administering clinicians about — appropriate timing options for dosing.

Currently, the scope of formulation and drug-product activities is limited for AAV products. That will need to enlarge as new products with larger numbers of doses enter the market. Such products must be formulated for different storage and handling conditions. Formulation and drug-product activities will become more critical when that transition occurs. Adapting learnings, including strategies and methods used for biologics formulation, will help meet future growth in development activities.

Questions and Answers
How do process development and formulation and drug-product activities overlap? Formulation and drug-product scientists receive materials from process development teams, so both groups collaborate during drug substance purification to ensure that material is of requisite quality — especially when a product uses an atypical AAV serotype.

What measures would help develop platforms for AAV formulation? Formulation and drug-product teams are working to enhance AAV product stability (e.g., by increasing pH ranges). Although platforms have been difficult to establish because availability of AAV is limited, advances in methods such as DSF now require lower AAV quantities than previous methods have.

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