Modeling retrovirus production for gene therapy. 2. Integrated optimization of bioreaction and downstream processing

In this work a model envisaging the integrated optimization of bioreaction and downstream processing is presented. This model extends the work present ed in part 1 of this pair of papers by adding ultrafiltration to process op timization. The new operational parameters include ultrafiltration time, pr essure, and stirring rate. For global optimization, the model uses as const raints the final product titer and quality to be achieved after downstream processing. This extended model was validated with the same system used in part 1, i.e., PA317 cells producing a recombinant retrovirus containing the LacZ gene as a marker in stirred tanks using porous supports. Optimization of the extended model led to the conclusion that bioreaction should have t wo steps, batch and perfusion, similar to what was found in part 1. Ultrafi ltration in a stirred cell should be performed at low pressures and stirrin g rates to reduce the losses of infective retroviruses. Sensitivity analysi s performed on the results of the integrated optimization showed that under optimal conditions the productivity is less sensitive to the parameters re lated to ultrafiltration than to those associated with bioreaction. These r esults were interpreted as reflecting the high yield of ultrafiltration (90 %). The relevance of the model extension to perform integrated optimization was also demonstrated since a restriction in the specific ultrafiltration area in downstream processing conditioned perfusion duration and perfusion rate in bioreaction. This clearly indicates that overall process optimizati on cannot be achieved without integrated optimization.