Production scale-up and validation of packaging cell clearance of clinical-grade retroviral vector stocks produced in Cell Factories

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Abstract
The clinical implementation of gene therapy requires large-scale production of viral vector stocks (VS) derived from packaging cell lines. Upon scaling-up, maintenance of high viral titers and filtration of the VS become significantly challenging. Thus, production schemes amenable to straightforward validation must be developed. To this end, we have established a semi-closed process to manufacture batches of 7 l or more of clinical-grade oncoretroviral VS using 10-tray Cell Factories. Using a peristaltic pump, the VS are collected on 3 consecutive days, filtered, pooled and stored frozen. To ensure the absence of viable vector-producing cells (VPCs) from each VS unit-dose, we undertook an orthogonal log-removal validation study to demonstrate the ability of both the filtration system to remove viable cells and the VS freezing process to inactivate them. We demonstrate a total VPC-reduction of 11.6 log, thus insuring the absence of contaminating VPCs in transduced clinical samples. We also show that this production process generates stable VS that can be stored at -80°C for more than 3 years. Importantly, this relatively simple and affordable process can be customized to generating large volume of VS for small animal or non-human primate studies. This methodology is not limited to the generation of cell-free clinical oncoretroviral VS, and can be applied to other types of vectors produced in packaging cell lines, such as lentiviral vectors.