Human mesenchymal stem or stromal cells (hMSCs) are an integral part of cell-based therapeutics, with over 400 clinical trials recently completed or in progress using hMSCs. As more research teams transition their stem cell-based regenerative technologies to the clinic, the use of serum in the cell production process has been and will continue to be, a necessary evil that must be managed. Luckily, pharmaceutical regulatory agencies, driven by the biologics industry over the last 30 years, have established guidances and guidelines that have helped to demystify and clarify some critical aspects of dealing with animal components. As it is important to have an understanding of how to manage serum during the clinical translation of hMSCs, we have focused this blog post on this specific topic.
(Note: when we refer to “clinical-grade” products below, this is not an official regulatory classification, it is meant to generally refer to materials that are destined for use in clinical testing of cell therapies.)
- FBS: Clinical-grade FBS must be derived from cattle herds grown in countries that are USDA approved for import, with well-monitored animal health status . The FBS should be processed under current good manufacturing practice (cGMP) standards that set minimum requirements for the facilities, materials and protocols used . Every batch or lot of FBS must be traceable back to its country, slaughterhouse and herd of origin. Finally, all lots must be tested for adventitious agents (viral contamination), sterility (bacterial and fungal elements), endotoxin levels, mycoplasma content and other constituents [2,5]. While regulatory agencies address safety, it is up to the cell manufacturer to establish metrics around performance, as FBS has traditionally been both a major cost driver and a source of process variability.
- Clinical-grade hMSCs: Clinical-grade hMSCs must be manufactured under cGMP standards, and this topic is covered extensively in the literature. As it pertains to serum use, each lot of serum used during the cell production process must be documented , and the final cell product must meet specific standards of identity, potency, purity, and safety. Purity standards include freedom from unwanted contaminants (such as other cell types, endotoxins, residual proteins, and animal serum) . The FDA Code of Regulations for Biologics provides a guideline for vaccines that animal serum levels must be under 1 ppm in the final product formulation when the serum is used in any part of the process [US FDA. 21 CFR 610.15 ]. While there is no direct guidance for cellular therapies, the 1ppm residual level has been used as a target in some cell therapy manufacturing processes  and is a good place to start when developing process specifications.
- HPL: HPL has been shown to induce a higher proliferation rate of hMSCs compared to FBS, due to a rich concentration of a variety of growth factors [16,17]. However, similar to FBS, its chemical composition is poorly defined and pooling of donors is used as a strategy to address lot-to-lot variability. Additionally, allogeneic HPL holds the risk of carrying human pathogens requiring extensive (and expensive) safety testing prior to use. Autologous HPL has expected performance variability and supplies limitations due to donor issues . To date, there has not been a thorough supply chain analysis performed to estimate the amount of HPL that would be available for cell therapy products brought to the market using HPL as a raw material.
- SFM: SFM is favorable due to its defined and consistent chemical composition and reduced risk for disease transmission. However, SFM formulations are often not able to elicit consistent biological function of hMSCs or support consistent cell proliferation in different cell culture environments, including during scale-up manufacturing [6,18]. This suggests that SFM formulations need to be optimized for every cell source and culture condition involved in a specific protocol, a process requiring an extensive amount of time and money . While SFM will be the long term solution for clinical-grade hMSC production, there are still several technology and business challenges to address prior to its wide-spread implementation.
- Hybrid strategies: One middle-of-the-road strategy is to combine serum and serum-free culture steps in the cell manufacturing process. Using serum during hMSC isolation and Master Cell Bank production, but subsequently transitioning to SFM for final therapeutic production is a documented and commonly-used technique that mitigates the aforementioned challenges to serum use . This strategy could significantly reduce the need for FBS (up to 99%) in clinical and commercial cell therapy manufacturing processes . Alternatively, RoosterBio has taken the approach of minimizing serum use by engineering a rich culture medium, similar to a chemically-defined SFM medium, supplemented with very low levels of high-quality serum to stabilize the formulation. Coupled with a streamlined manufacturing process, this approach reduces serum requirements by well over 90%, greatly extending the lifetime of qualified serum lots and bringing consistency to RoosterBio’s cell culture media products. Thus, RoosterBio hMSCs display consistent growth rates and functional characteristics across cell and media lots and our cell-media systems are amenable to scale-up manufacturing processes.
Michelle S DiNicolas, PhD
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