The challenge and opportunity of cell sourcing in the development of drugs and biologics
In 2019, anyone who has gone beyond basic research in regenerative medicine, toxicology, or drug discovery has likely come to grips with two hard facts: 1) some aspects of development depend on the use of human cells or tissues in the assessment of safety and efficacy, or as components of final products; and 2) human donor heterogeneity is a real thing. No experienced biologist buys into claims that a tightly controlled cell isolation process or subsequent laboratory manipulations will render a cell type ‘the same regardless of donor of origin’. While controlling as many factors as possible during sample processing minimizes process-related variability, donor variability is out of the control of the isolationist. Compounding this complexity further, cells of the same general type (i.e., endothelial cells, fibroblasts, or mesenchymal stromal/stem cells (MSCs)) that are derived from distinct tissue locations within the same donor, are inherently different1-3.
As scientists, we are hard-wired to want tight specifications and a high degree of reproducibility. Many of these expectations have their roots in the era of high throughput drug discovery where well established cell lines were employed in cell-based assays that delivered hit after hit in short order with tiny standard deviations, perhaps trading off some biological relevance for statistical significance4,5. A generous portion of our history may have been spent asking all the right questions of the wrong model systems. The cost of convenience includes the downstream consequences of making strategic or programmatic decisions based on biologically skewed data – unpredicted toxicity or lackluster efficacy in the final product.
We are at a crossroads with respect to the effective use of human cells and tissues in research and development. One path forward is to pursue the use of cells, such as MSCs, that are more easily isolated and banked to large numbers so that we can use the same stocks over and over from a small number of donors. This is a great option for many applications, and this strategy is at play worldwide with >198 actively recruiting clinical trials listed on clinicaltrials.gov investigating clinical utility of MSCs for vast array of indications, from the treatment of osteoarthritis to rotator cuff repair. As viable as the general-use, expandable/bankable cell populations are for immune suppression or regeneration in the acute injury setting, they are unlikely to play a major role in other areas of unmet need – for example, in the direct replacement of highly specialized cell types (insulin-producing beta cells, dopaminergic neurons, hepatocytes) or for some uses in vitro, such as the prediction of hepatotoxicity or neurotoxicity of compounds to support an IND submission. Thinking broadly, when it comes to the need for human cells, there isn’t one blanket solution for everything. This takes us down the second path – invest the time and effort to embrace and understand donor and tissue heterogeneity, ultimately leveraging the amazing (and quite beneficial) diversity that exists within the human donor, tissue, and cell populations to better serve the needs of science and patients.
Great in concept, maybe tough in execution, right? Fortunately, the past decade has delivered notable advancement in ‘omics’ and the relative ease (at least from the perspective of us pre-kit-generation kids who poured sequencing gels between giant glass plates held together with metal binder clips, trying not to think about where the surplus polyacrylamide was going) with which we can capture the genome, gene expression patterns, epigenetic modifications, secretome, and myriad biological information about a donor or an isolated specimen. Big data? You bet. What a glorious problem to solve using big data analysis, in silico modeling, and emerging analytical tools that can change the way we see and use the data that we have generated. Doing drug discovery for early intervention in Non-Alcoholic Fatty Liver Disease (NAFLD)? How about donor-matched hepatic stellate cells and Kupffer cells that carry the epigenetic markings associated with NAFLD risk and were isolated from an individual with confirmed disease? Developing an allogeneic cell-based therapy for chronic wound healing? Select donor lots based on their characterization profile and ability to support tissue remodeling and vascularization instead of trial and error experimentation. Have a rare and precious cell type that could save someone’s life, if you only had enough of them for the regulator-required safety testing and patient treatment? Qualify the cell lot based on deep characterization data extracted from 105 cells, instead of running a full slate of in vivo and in vitro studies that consume over 109 cells and take months to years.
These are big goals and will require investment and robust interdisciplinary all-on-board strategies to achieve. It will mean sharing our data and ideas, and developing a unified vision for how this will ultimately serve the industry, the regulators, and the patients. Achievable? Yes, I believe so, with impact beyond what we imagine today. In fact, it is already happening in pioneering labs around the world selecting blastocysts for transfer6, identifying MSCs that are capable of chondrogenesis for cartilage repair7, and understanding the phenotypic changes associated with post-implantation engraftment of hepatocytes and endothelial cells8. Keep going!
References Cited:
1 Foote, A. G., Wang, Z., Kendziorski, C. & Thibeault, S. L. Tissue specific human fibroblast differential expression based on RNAsequencing analysis. BMC Genomics 20, 308, doi:10.1186/s12864-019-5682-5 (2019).
2 Marcu, R. et al. Human Organ-Specific Endothelial Cell Heterogeneity. iScience 4, 20-35, doi:10.1016/j.isci.2018.05.003 (2018).
3 Menard, C. et al. Integrated Transcriptomic, Phenotypic, and Functional Study Reveals Tissue-Specific Immune Properties of Mesenchymal Stromal Cells. Stem Cells, doi:10.1002/stem.3077 (2019).
4 Kaur, G. & Dufour, J. M. Cell lines: Valuable tools or useless artifacts. Spermatogenesis 2, 1-5, doi:10.4161/spmg.19885 (2012).
5 Olsavsky, K. M. et al. Gene expression profiling and differentiation assessment in primary human hepatocyte cultures, established hepatoma cell lines, and human liver tissues. Toxicol Appl Pharmacol 222, 42-56, doi:10.1016/j.taap.2007.03.032 (2007).
6 Storr, A., Bilir, E., Cooke, S., Garrett, D. & Venetis, C. A. Fine-tuning blastocyst selection based on morphology: a multicentre analysis of 2461 single blastocyst transfers. Reprod Biomed Online, doi:10.1016/j.rbmo.2019.06.008 (2019).
7 Lam, J. et al. Functional Profiling of Chondrogenically Induced Multipotent Stromal Cell Aggregates Reveals Transcriptomic and Emergent Morphological Phenotypes Predictive of Differentiation Capacity. Stem Cells Transl Med 7, 664-675, doi:10.1002/sctm.18-0065 (2018).
8 Aizarani, N. et al. A human liver cell atlas reveals heterogeneity and epithelial progenitors. Nature 572, 199-204, doi:10.1038/s41586-019-1373-2 (2019).