Promoting Safe and Effective Gene and Cell Therapies: The Importance of Lentivirus Testing

A mock sample was used for the generic cell validation to mimic a sample that could include common impurities, degradants, matrix, etc. Matrix inhibition was determined through the assessment of the specificity of the methodology, PBRT (PERT), and p24 ELISA endpoint testing. Positivity from the positive controls (PC) and the mock sample spike (SS) in these endpoint tests would indicate that the assay is capable of detecting replication competent lentivirus. If SS were negative in the endpoint tests, it could be an indicator of matrix inhibition. For both PERT and p24 ELISA in the generic validation, each PC and SS condition was positive, indicating that no matrix inhibition was observed with the tested mock sample.

The generic sample matrix used during the replication competent lentivirus (RCL) assay cell validation was a mock sample of HEK293 T cells transfected by a third-generation lentiviral vector carrying an enhanced green fluorescent protein (EGFP) transgene.

If assay controls worked properly (negative control [NC] for negative endpoints and positive control [PC] positive for endpoints) and replication competent lentivirus (RCL) amplification is not observed in the sample spike (SS) such that both PBRT (PERT) and p24 ELISA endpoints are not detected, then the sample matrix tested may have caused interference or matrix inhibition.

The mock sample matrix from the generic validation did not exhibit any adverse effects on the C8166 cells. Based on the composition of a hypothetical LVV sample matrix, impurities, degradants, and other components could cause cytotoxicity in the C8166 cells or lead to interference and inhibitory effects, resulting in false positives in endpoint testing.

The generic validation performed for both cell and supernatant samples acts as a platform assay for replication competent lentivirus (RCL) testing. A client may choose to further perform a product-specific qualification for their sample matrix using this method. Typically, samples are tested within the routine, platform assay.

The rapid replication competent lentivirus (RCL) PCR assay is available; however, it is important to note that this assay is not typically suitable for RCL detection on its own. The turnaround time (TAT) for the cell-based RCL assay is 84 days, which includes the time necessary for cell preparation in virology and the endpoint tests conducted in the analytical and molecular departments. To streamline this process, providing advanced notice of sample submissions can significantly reduce TAT. Typically, cells require three weeks for preparation. However, with prior notice, these cells can be prepared ahead of sample receipt, allowing the samples to proceed directly to testing and potentially reduce the overall processing time by a couple of weeks.

In consideration of the limited supply, it is essential to keep assay volumes low. The exact volume required for your drug product testing panel may vary depending on the specific protocols being evaluated. When referencing replication competent lentivirus (RCL) testing, it is advisable to adhere to FDA guidance Section III.B, Testing of Retroviral Vector-Based Human Gene Therapy Products for Replication Competent Retrovirus During Product Manufacture and Patient Follow-up, which stipulates that for supernatants, at least 5% of the amount should be used, with the specific volume determined by the equation outlined in the Appendix to demonstrate less than 1 RCL per dose. For cell samples, the recommendation is to use either 1% of the total volume or 10⁸ cells, whichever amount is less. Specifically for GP-V1245 (supernatant), we have typically received volumes ranging from 15 mL to 300 mL. For GP-V1245.1 (cells), the standard volume submitted for testing is approximately 10⁸ cells per milliliter.

We are pleased to assist you in getting started with your testing needs. To facilitate this process, we recommend utilizing the test panel generator available on our website. By answering a series of questions, you can receive a customized panel recommendation tailored to your specific requirements. Following this initial step, you will have the opportunity to directly contact the Sales team for further assistance.

The regulatory landscape for testing retroviral vector-based human gene therapy products varies between the EU and the US. In the EU, the key regulations are outlined in General Monograph 3186 and General Chapter EP 5.34, which mandate cell-based replication competent lentivirus (RCL) testing on vector materials, similar to FDA requirements. However, these regulations do not explicitly require testing of both harvest supernatants and end-of-production cells (EOPC). Instead, the EU guidelines allow for flexibility, permitting testing of in-process materials, purified bulk, or drug substances provided that no dilution or pooling of several harvests has occurred.

Conversely, the FDA's guidance specifically indicates that testing should encompass both the supernatant and cells from the crude bulk harvest and the corresponding EOPC. To ensure thorough safety measures, we recommend implementing testing on both supernatants and cells; however, this approach may be subject to discussion. For more comprehensive information, visit our Viral Vector webpage.

Proper sourcing, preparation, and titration of viruses are crucial for assay sensitivity. Replication-competent lentivirus (RCL) is used as a positive control (PC) for safety, utilizing attenuated HIV. Most sample vectors contain the VSV-G envelope protein, which increases tropism but poses significant safety risks due to replication competency. Although using HIV with the VSV-G envelope as a PC may seem advantageous, it presents hazards to personnel and the environment. Evidence is lacking to confirm that RCL shares similar growth and traits with HIV carrying the VSV-G envelope. Therefore, it's advisable to use wild-type HIV, attenuated HIV, or recombinant conditional replication lentiviral vectors that meet sensitivity requirements for RCL verification. Positive viruses are essential for validating detection methods, requiring detailed information on their source, preparation, functional elements, and biological titers, as well as stability during storage.