Designing and Conducting Stability Studies for Cell and Gene Therapy Products

Cell and gene therapy (CGT) products are among the most complex and promising innovations in modern medicine. Unlike traditional biologics, CGTs often contain living cells or genetic material that are highly sensitive to environmental changes, requiring unique approaches to stability testing. This tutorial outlines the challenges, regulatory considerations, and best practices for designing stability studies that support product development, clinical use, and commercialization of cell and gene therapy products.

Why Stability Testing Is Critical for CGT Products

Stability studies ensure that a CGT product maintains its identity, purity, potency, and safety from the point of manufacture to the point of administration. These studies help determine:

• Appropriate storage and shipping conditions
• Shelf life for both frozen and fresh preparations
• Potency retention and cell viability over time
• Stability during hold times, thawing, and reconstitution

Due to short shelf lives, limited batch sizes, and complex storage logistics, CGT products require specialized and often real-time stability strategies.

Types of CGT Products and Their Unique Stability Requirements

1. Cell-Based Therapies

Examples include CAR-T cells, dendritic cells, mesenchymal stem cells (MSCs), and hematopoietic stem cells. Stability testing must assess:

• Viability (typically >70%) over time
• Functional assays (e.g., cytotoxicity, cytokine secretion)
• Surface marker expression stability
• Genomic stability (e.g., karyotyping)

2. Gene Therapies

These include viral vectors (e.g., AAV, lentivirus) or non-viral delivery systems that deliver therapeutic genes. Stability testing must assess:

• Vector genome integrity (qPCR or ddPCR)
• Capsid or envelope protein stability
• Infectivity and transduction efficiency
• Potency assays using target gene expression

3. Combined ATMPs (Advanced Therapy Medicinal Products)

These products include both cells and gene delivery elements, requiring comprehensive testing for each component as well as the finished product.

Step-by-Step Guide to Stability Testing for CGT Products

Step 1: Define Product Format and Storage Strategy

Identify whether the product is:

• Cryopreserved: Stored at ≤ -130°C (e.g., vapor phase liquid nitrogen)
• Refrigerated: Stored at 2–8°C for limited durations
• Room Temperature or Controlled Ambient: Mostly used for intraoperative or short-lived autologous therapies

Determine stability goals for both long-term storage and in-use conditions (e.g., after thawing).

Step 2: Establish Stability-Indicating Parameters

Unlike traditional biologics, stability in CGTs must reflect functional and structural integrity. Key parameters include:

• Viability: Trypan blue exclusion, flow cytometry, ATP assays
• Potency: In vitro functional assays (e.g., IFN-γ secretion, cytotoxicity)
• Genomic integrity: qPCR for vector copy number or DNA degradation
• Sterility and endotoxins: To ensure compliance with USP and
• Appearance and pH: For visual consistency and formulation performance

Step 3: Perform Forced Degradation Studies (Where Applicable)

Although difficult for living therapies, stress testing may be applied to gene therapy vectors and excipients. Conditions may include:

• Freeze-thaw cycles
• Thermal excursions (e.g., 2–8°C for 24–48 hours)
• UV or light exposure for vector formulations

Document degradation kinetics to inform formulation improvements or shelf-life limits.

Step 4: Design Real-Time and Accelerated Stability Protocols

While ICH Q5C may apply in principle, stability programs for CGTs are typically customized. Recommended practices include:

• Real-time testing: At intended storage condition (e.g., -80°C or 2–8°C)
• Accelerated testing: Only if product and matrix are stable enough to permit predictive modeling
• In-use stability: Post-thaw or reconstitution time windows (e.g., 2–6 hours at RT)

Step 5: Establish Shelf Life and Hold Time Limits

Use viability, potency, and purity data to define product shelf life under various conditions. For example:

• CAR-T therapy: 12-month shelf life at -150°C, 4-hour in-use window post-thaw
• AAV vector: 24-month stability at -80°C, stable 24 hours at 2–8°C after thawing

Analytical Methods Commonly Used in CGT Stability Testing

• Flow cytometry – cell identity, purity, and viability
• qPCR/ddPCR – vector genome copy number
• ELISA or reporter assays – transgene expression
• TCID₅₀ or infectivity assays – functional titer for viral vectors
• Western blotting – capsid/envelope integrity

Ensure all methods are validated or qualified for their intended use, particularly for potency and identity assays.

Regulatory Guidance on Stability for CGTs

• FDA: Guidance for Chemistry, Manufacturing, and Control (CMC) of Human Gene Therapy INDs
• EMA: Guideline on quality, non-clinical and clinical aspects of gene therapy medicinal products
• WHO: Points to consider for manufacturing and control of cell therapy products

Agencies expect a science-based, risk-informed justification of storage conditions and shelf-life assignment. All protocols should be captured in your Pharma SOP system and referenced in regulatory modules.

Case Study: CAR-T Cell Therapy Stability Assessment

A CAR-T manufacturer designed a stability protocol using batches stored at -150°C. Monthly testing over 12 months included viability (flow cytometry), potency (CD19+ tumor cell killing), and sterility. In-use studies post-thaw showed cell viability dropped below 70% after 6 hours at room temperature. Final label included a 12-month frozen shelf life and a 4-hour administration window post-thaw.

Checklist: CGT Stability Testing Implementation

1. Define product format and storage requirements
2. Select viability and potency assays suitable for real-time use
3. Design real-time and in-use stability protocols
4. Include microbial and endotoxin testing in stability plan
5. Submit risk-based shelf-life justification to regulators

Common Pitfalls to Avoid

• Assuming ICH stability designs are sufficient without customization
• Neglecting in-use or post-thaw stability testing
• Failing to define and validate rapid potency assays
• Not linking stability trends to clinical dose and product delivery timing

Conclusion

Stability testing in cell and gene therapies is uniquely complex, requiring customized protocols, rapid turnaround analytics, and integration with logistics planning. By focusing on viability, potency, and degradation trends, manufacturers can ensure that patients receive safe, effective, and consistent treatments—despite short shelf lives and challenging delivery chains. For resources, regulatory templates, and testing strategies, visit Stability Studies.