Drug Discovery
attrition rate of drugs in later stages of develop- ment. Differentiation and expansion of human stem cells into hepatocytes for use in investigative toxicity studies could overcome the shortcomings of primary hepatocytes and immortalised cell lines. Use of stem cell-derived hepatocytes (and other cell types commonly used for toxicity stud- ies) offers a number of important advantages to investigative toxicity studies including:
lAvailability of a consistent source of cells that more closely match in vivo phenotype and physiology. l Elimination of reliance on donor sources which can have sporadic availability. l A more standardised, reproducible process for toxicity testing. l Reduction in the use of animal models and ani- mal tissue. l Improvement in the predictive capabilities of early toxicity studies leading to reduction in late stage attrition of drugs.
More efficient and predictive toxicity studies enabled by iPS-derived cells can be expected to reduce development costs associated with the late stage failure of drug candidates. Identifying drug candidates with toxicity concerns earlier in the dis- covery process can improve the safety and, ulti- mately, the success of clinical trials.
Engineering large-scale stem cell production
As more stem cell-based therapeutics progress towards clinical testing8, process-scale stem cell manufacturing systems must be engineered. Achieving ‘industrialised’ production of stem cells while meeting rigorous quality and regulatory standards will depend on further progress in the areas of cell culture and scale-up, characterisation, enrichment, purification and process control to deliver a consistent and reproducible supply of cells in a safe and cost-effective manner (Figure 3). Although industrial-scale cell culture methods exist for the manufacture of protein therapeutics, these processes are always directly transferrable to the culture of stem cells where the cell itself is the ‘product’.
Culture conditions
Industrial-scale stem cell cultures must adhere to Current Good Manufacturing Practice (cGMP) standards, incorporating well-defined, well-charac- terised media and supplements to support cell expansion and differentiation as desired. The FDA does not mandate the complete absence of animal
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products in stem cell cultures. However, there is interest in moving away from use of undefined serum, which is incompatible with the tightly con- trolled culture conditions necessary for producing human therapeutics.
Incorporation of cGMP-compliant supplements contributes to high quality, consistent, repro- ducible culture conditions. When manufactured under cGMP conditions, supplements enabling robust proliferation of stem cells in culture without the need for feeder layers are fully characterised and validated for activity, potency and purity. In addition, cGMP supplements are generally pro- duced in larger lot sizes, which contribute to the reproducibility of culture conditions. Although some cell culture supplements are cur- rently available in cGMP form, many are not, including the widely used epidermal growth factor and fibroblast growth factor. In those labs where stem cells are being cultured for evaluation in the clinical setting, researchers often develop their own cGMP supplements. This process can be time-con- suming and labour-intensive. It also doesn’t allow for standardisation of culture conditions across different labs and organisations.
Process monitoring and characterisation Once in vitro conditions are optimised, culture parameters and cell phenotypes must be continual- ly monitored to ensure conditions do not drift out- side established limits. Reproducibility and consis- tency of the stem cell ‘product’ is highly dependent on the ability to monitor culture conditions and cell phenotypes in real time (or as close to real time as possible). Media and external factors can be readily monitored and adjusted following standard practices that have been applied to the soluble components of protein production, but assessing phenotypic characteristics is more challenging. Control strategies for biopharmaceutical pro- duction using transformed cell lines or microbes are generally based on stabilisation of cultures at a steady state condition to optimise recombinant protein yield. However, stem cell therapy produc- tion is a dynamic process. Cell populations evolve with time. Therefore, dynamic, rather than static, controls will likely be more effective. Efforts are currently under way to create more timely feed- back loops that can assess the effects a process is having on the phenotypic characteristics of a cul- ture while there is still time to modify process.
Integrated cell processing systems
Consistent and reproducible stem cell manufactur- ing requires scalable, optimised solutions that
Drug Discovery World Spring 2011
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