Evaluation of Recommendations for Replication-Competent Retrovirus Testing Associated with Use of Retroviral Vectors

With input from the gene therapy community, CBER is actively examining the recommendations for RCR testing during retroviral vector production, production of ex vivo-transduced cells, and in patients who receive such material. Our initial recommendations were made at a time when our experience with RCR detection assays and clinical use of retroviral vectors was limited. As the gene therapy field has matured, there is an increasing amount of data available on RCR detection assays and from monitoring of patients in clinical trials. The cumulative data give assurance that RCR detection assays in use are of sufficient sensitivity to provide a margin of safety to patients: no patients to date have evidence of RCR infection. However, CBER encourages members of the gene therapy community to continue to submit data to the FDA or to publish data that will enhance the cumulative data base on RCR testing assays, experience with different VPC, and patient monitoring.

Based on the analysis of data accumulated to date, and ongoing discussions with members of the gene therapy community, CBER is proposing to discuss changes to the current RCR testing recommendations, as summarized below.

RCR testing during production of retroviral vector and ex vivo-transduced cells

Development of characterized standards for RCR testing of supernatant and cells should allow comparison of assay sensitivity. One proposal under consideration is to apply statistical methods to determine how much material needs to be tested independent of the size of the production lot. Data and discussion are still needed to define a limit concentration and a value for probability of detection for RCR testing, while maintaining an appropriate margin of safety. These modifications of RCR testing strategies could lead to improvements in assay sensitivity. Additional discussion and data are also needed to evaluate the current recommendations of the testing for ex vivo-transduced cells: should both cells and supernatant be tested in all cases?

RCR testing during patient follow-up

The time points required for RCR testing during patient follow-up need examination. One proposal under consideration is to sample and assay at three time points during the first year of treatment (e.g., 4–6 weeks, 3 months, and 1 year post-treatment). Further discussion is needed to define appropriate additional follow-up.

Choice of assays to detect surrogate markers for RCR infection (i.e., serologic or PCR-based assays) should consider mode of vector administration and the patient population. Positive results with such assays should be pursued by direct culture assay to obtain and characterize the infectious viral isolate.

These proposals will be the focal point for the discussion at the Retroviral Vector Breakout Session at the 1997 FDA/NIH Gene Therapy Conference. After the 1997 FDA/NIH Gene Therapy Conference, CBER plans to propose revised recommendations for RCR testing for public comment.