Cell and gene therapies represent the most complex drugs ever created. Acknowledging this, we are unlikely to be able to fully characterize or homogenize these products in the manner achievable for small-molecule chemical drugs or peptides. The best we can do is harmonize and consistently replicate the manufacturing process itself. Therefore, ideally, every step in CGT manufacture should be carefully considered and optimized.
Adherent cell culture has numerous benefits, including substantially greater purity of the starting harvest material, which reduces the number of downstream steps required and helps minimize product degradation. This is especially true for secreted products, such as coagulation factors and lentiviral (LV) or retroviral (RV) vectors, where the adherent cells are naturally segregated from soluble proteins and vectors without the need for additional separation techniques — often at a cost of introducing cell contaminants into the harvest.
A significant amount of biopharmaceutical discovery and proof of concept (PoC) science occurs using adherent cell platforms. Continuing to utilize an adherent platform throughout development allows the manufacturer to use one certified master cell bank (MCB) throughout development. Still, many organizations opt to switch to a suspension cell culture as they scale up, believing it to be the superior — or only — choice for CGT clinical and commercial production.
To some extent, this perception may stem from the industry’s early experience with recombinant cytokines, monoclonal antibodies (mAbs), and coagulation factors. For many first-generation recombinant protein products, upstream manufacturing was performed using adherent cells in roller bottles. Quickly, the scale-up cost and complexity of roller bottle facilities became apparent, and several companies began transitioning to single-cell suspension culture in stirred tank bioreactors.
The need to produce larger lots and decrease cost of goods while maintaining quality led to the creation of progressively larger bioreactors capable of producing several thousand liters of material. As product and product candidate pipelines grew exponentially, many CDMOs manufacturing such products also explored the upper limits of single-use disposable technology in this familiar setting.
However, additional downstream process complexity is inherent in suspension cell harvesting, which requires additional steps to separate suspended cells from soluble product. Further, per-cell productivity can be lower using suspension cells versus adherent cells. And, it often is a difficult and expensive decision when and how to transition product development into a suspension platform, knowing there are likely to be challenges relating to one or more aspects of product manufacturing processes, yields, and quality attributes.
Continued use of adherent culture throughout development provides sponsors the ability to maintain upstream process fidelity through later phases of product development (i.e., no need to perform bridging studies to establish equivalency between materials produced in the earlier studies, using adherent cells, and those produced later using suspension). While up-scaled adherent bioreactor systems either did not exist or existed in a form that was not sufficiently beneficial (e.g., microcarriers) in the past, introduction of systems such as the Corning® Ascent® Fixed Bed Bioreactor System have changed this paradigm. The Ascent FBR system preserves the performance of adherent cells and is scalable on par with common stirred tank bioreactors in a single-use format.
Ultimately, every cell and gene therapy is a unique entity ill-suited to “plug-and-play” application of any manufacturing platform. Careful consideration must be made regarding the time available for process change development, anticipated challenges, and quantities of material expected to be required through clinical development and commercialization.
In some cases, a transition to suspension technologies may fit within these constraints or may even be required to meet product demands. However, many product development programs may be better served by applying new adherent platforms, preserving time and money that would otherwise be wasted in the attempt to transition to suspension cultures. Regardless, continued innovation can be expected to positively impact both platforms moving forward.