How to Evaluate Cell Therapy Product Purity

Cell Therapy Product

Gene and stem cell therapies are becoming increasingly sought after to treat a wide range of diseases, ranging from cancer to neurological afflictions like Parkinson’s disease. The challenge though is to implement scalable and repeatable processes that don’t need the same costly operator intervention required by traditional methods. That’s why the industry’s key players are developing various manufacturing alternatives. Nevertheless, each process needs to generate a top quality product for cell therapy.

Potential Cell Therapy Purity Issues 

Developing cultures for cell therapy can be manually intensive because the cells need to be seeded, refreshed, passaged, and harvested. These days, there are other, more automated, ways to do this, including using bioreactors. Nevertheless, such processes can impact the characteristics and functionality of the cell types, as well as introduce potential impurities. 

As cellular therapies rely on the accuracy of the cells, it’s important the following potential issues are investigated during clinical trials and post-manufacturing for all types of cells: 

  • Residual impurities 
  • Unintended characteristics 

Residual Impurities 

Mesenchymal stem cells, isolated from bone marrow, are some of the most commonly used for cell therapy because of their pluripotent stem cell abilities. By being able to generate many different cell functions, they have a large potential role in cell therapy. Part of the purity cell therapy tests includes determining their cell content. This is particularly important for pluripotent cells such as mesenchymal stem cells. Fundamentally, the manufacturing process for these cell types often creates undifferentiated or partially differentiated cells. 

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First, purity tests check for impurities such as bovine serum albumin, cytokines, and viral vectors. Then, tests such as the Limulus Amebocyte Lysate (LAL) test determine the presence of certain bacteria known as endotoxins. 

Unintended Characteristics 

Multiple analyses can verify the content of any cell therapy sample. Even then, research is underway to keep improving the accuracy of the various options. For example, light obscuration and manual microscopy can give some insight into particle count and size. 

In terms of morphology though, micro-flow imaging is more successful at confirming cell therapy samples. Furthermore, immunostaining with cell surface markers can further highlight the presence of cells with unwanted characteristics. 

Other Cell Therapy Quality Issues and How to Verify Them 

All approved biological products for cell therapy must have their purity confirmed. Moreover, they will also need to have the following verified for any clinical use: 

  • Identity 
  • Potency 
  • Safety 


Morphology of the stem cells being used for cell therapy is clearly important for its success. That’s why microscopy and flow cytometry check morphology and relevant stem cell markers. These tests essentially overlap with the purity test because they are checking the actual characteristics of the cells.

Another option is gene expression profiling. These tests are still being researched but present exciting opportunities as stem cell therapy and gene therapy approaches are increasingly being combined. Another point to note though is that cell identity can change during the manufacturing process, especially for pluripotent cells. Therefore, a protein expression profile, along with genetic analysis, could give greater confidence of quality. 


As expected, it’s critical to ensure that the product actually has the right biological function for its intended cell therapy. Due to the variety of diseases in cell therapy, some stem cells are used because of their impact on the immune system whilst others are more specific for regenerative medicine to heal joints or for encouraging neuron development. 

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It’s important to note that the assay tests for potency aren’t about verifying the efficiency of the stem cell and therefore the expected success of the cell therapy. On the contrary, the tests are simply to verify what the cell actually does. That’s why the assays will be designed specifically for whatever cell types are produced.

However, you can usually expect to see an enzyme-linked immunosorbent assay (ELISA) to test for substances such as cytokines. These proteins are an important part of activating the growth of immune system cells and blood cells. Another test often seen is the High-Pressure Liquid Chromatography (HPLC) that essentially confirms the stem cells are what they are. 

Biological Safety 

One aspect of cell therapy safety requires testing for impurities. Other potential issues could be genetic recombination between packaging cell lines or plasmids. Moreover, the product will need to prove that it is free from viruses and other unwanted structural or genetic variants. In general, sterility, mycoplasma, and adventitious viral agent tests should be conducted. 

Final Thoughts for Cell Therapy Product Purity 

Cell therapy is currently going through FDA approval for various treatments of diseases which requires proving that the product is of the highest quality. That’s why the traits purity, potency, identity, and biological safety are the key themes. Although, further consideration for manufacturing processes is how repeatable and accurate the process is for meeting those criteria. That will be the difference between those manufacturers who make it in the long run because they will stand out in terms of effectiveness and quality assurance. 



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