Products that appear promising in early clinical trials of somatic-cell or gene therapy will usually enter a commercial development process with two important parallel components. The safety and efficacy of the products are tested in clinical trials of appropriate design. Concurrently, the manufacture and testing of the biologic product itself are refined to permit large-scale production and distribution of a pure material with reproducible qualities. Although the clinical aspect of this process is the focus of public attention, the product-development component is equally important.
The need for appropriate control of biologic products has been recognized since their first large-scale use in the late 19th century. Therapeutic antiserums were found to be effective in treating certain infectious diseases, but their potency and purity varied widely. In 1901, 13 children in St. Louis died of tetanus after they had been injected with diphtheria antitoxin. Their deaths were traced to tetanus contamination of the equine serum from which the antitoxin was prepared13. This and other less dramatic incidents led to the enactment of the Biologics Control Act of 1902 — also known as the Virus, Serum, and Toxin Act — which mandated the federal regulation of biologic products14. Since that time, the manufacturers of biologic products have been required to hold licenses both for the product and for all manufacturing facilities.
The control of biologic products has been progressively refined since the 1902 act was passed. Three principles are central: control of the biologic source or sources, control of the production process, and control of the bulk and final product. These principles have been successfully applied to quality control for products as diverse as human blood and vaccines against viruses, and they are also crucial to controlling the quality of products for somatic-cell and gene therapy.
Products for somatic-cell and gene therapy may be derived from a variety of biologic sources, including directly harvested autologous, allogeneic, or xenogeneic cells; cultured cell lines; genetically modified cell lines; and viral vectors. Product safety requires that such sources be well characterized, uniform, distinguishable from the sources of similar materials, and not contaminated by hazardous adventitious agents. At the time of the 1902 act, the control of biologic materials centered around microbiologic testing and animal husbandry. The importance of such controls was illustrated when foot-and-mouth disease occurred in animals used to produce smallpox vaccine13. Subsequently, the development of new forms of technology, beginning with the production of viral vaccines by tissue culture, generated additional scientific challenges. Viral seed-lot systems, which set the permissible number of passages from the well-characterized parent virus through vaccine production, were developed to control potential reversion to virulence by attenuated viral strains. In addition, the concept of the production-cell substrate, a defined cellular source material used to produce biologic agents, was developed. Strategies were devised to test for contaminants originating in cell substrates — for example, the simian virus 40 found in the monkey-kidney-cell cultures used to produce poliovirus vaccine. Adventitious viruses continue to be a problem in today’s cell substrates. Currently, cell-banking and testing algorithms are used to evaluate the cell substrates used in the production of biologic agents such as vaccines, monoclonal antibodies, and recombinant-DNA products, as well as certain forms of somatic-cell and gene therapy.
Cells directly removed from humans may be used in somatic-cell and gene therapy and pose additional problems in preventing source-related contamination by adventitious agents. Safety issues related to the use of fresh cells first emerged with the advent of blood transfusion. Banking blood for transfusion saved countless lives during World War II, and whole blood subsequently became the first cellular material approved as a biologic product by the FDA. However, the widespread use of a human-derived cellular product raised unique issues of quality control related to the transfusion-associated transmission of disease. Because blood could not be sterilized by filtration or other means, the development of strategies to control or prevent viral and bacterial contamination was essential. As a result, the evaluation of donor health through history taking, physical examination, and laboratory testing became central to protecting the safety of the blood supply. The recent emergence of HIV reinforces the importance of donor screening and testing procedures when human-derived biologic materials are used.
The concept of controlling the manufacturing process is the second cornerstone in ensuring the quality of biologic products. Rigorous control of the process is essential because of the difficulties inherent in assessing and controlling the consistency of biologic products. Source materials, such as cells, viruses, and blood, are often not uniform. In addition, seemingly minor changes in the conditions of cell cultures or in purification processes may significantly alter the biologic characteristics of the final product. Because of the complex nature of final products that consist of cells, microorganisms, or macromolecules, testing of final products alone cannot reliably detect, test, or control for variability. Manufacturers must therefore rely on controlled, reproducible manufacturing procedures and environments to produce a uniform product. The degree of reliance on a controlled process varies according to the nature of the product. For example, in the case of certain products containing living cells that may be prepared in single-donor, single-recipient batches, the small size of each batch and the need for timely administration of the cells impose special limitations on testing. As a consequence, control over the process and the facility has been particularly emphasized.
The third central principle of controlling biologic products involves control of the bulk and final product. Because the complete chemical characterization of biologic products is not ordinarily feasible for quality control, the testing of biologic potency receives particular emphasis. Controlling the potency of somatic-cell therapies will be particularly challenging and will probably require the development of new approaches.
As the preceding examples demonstrate, control of the production of biologic agents has had a key role in quality assurance from the earliest biologic therapeutic agents through today’s scientifically complex interventions. The technical standards developed for the commercial production of somatic-cell and gene therapy will be based on these existing manufacturing and control principles.
Advanced therapy medicinal products (ATMPs) are medicines for human use that are based on genes, tissues or cells. They offer groundbreaking new opportunities for the treatment of disease and injury.
ATMPs can be classified into three main types:
- gene therapy medicines: these contain genes that lead to a therapeutic, prophylactic or diagnostic effect. They work by inserting ‘recombinant’ genes into the body, usually to treat a variety of diseases, including genetic disorders, cancer or long-term diseases. A recombinant gene is a stretch of DNA that is created in the laboratory, bringing together DNA from different sources;
- somatic-cell therapy medicines: these contain cells or tissues that have been manipulated to change their biological characteristics or cells or tissues not intended to be used for the same essential functions in the body. They can be used to cure, diagnose or prevent diseases;
- tissue-engineered medicines: these contain cells or tissues that have been modified so they can be used to repair, regenerate or replace human tissue;
In addition, some ATMPs may contain one or more medical devices as an integral part of the medicine, which are referred to as combined ATMPs. An example of this is cells embedded in a biodegradable matrix or scaffold.
For detailed definitions of the different groups of advanced therapy medicinal products, refer to Regulation (EC) No 1394/2007 and Directive 2001/83/EC.
In April 2020, EMA’sCommittee for Advanced Therapies (CAT) advised patients and the general public to beware of unproven cell-based therapies.
This followed the appearance of advertisements for cell therapies as cures for serious conditions across the European Union in early 2020.
In its statement, the CAT warned against the use of unregulated cell-based therapies, which may be ineffective and increase the risk of serious adverse reactions.
Healthcare providers should explain the benefits and risks of the cell-based therapies that they are providing to patients, as well as confirming that regulatory authorities have approved their use.
Anyone with doubts can contact EMA or their national competent authority.
The CAT’s statement replaces a statement it issued in 2010 following reports of unregulated stem-cell therapies being offered to patients.
Gene therapy works by altering the genetic code to recover the functions of critical proteins. Proteins are the workhorses of the cell and the structural basis of the body’s tissues. The instructions for making proteins are carried in a person’s genetic code, and variants (or mutations) in this code can impact the production or function of proteins that may be critical to how the body works. Fixing or compensating for disease-causing genetic changes may recover the role of these important proteins and allow the body to function as expected.
Gene therapy can compensate for genetic alterations in a couple different ways.
- Gene transfer therapy introduces new genetic material into cells. If an altered gene causes a necessary protein to be faulty or missing, gene transfer therapy can introduce a normal copy of the gene to recover the function of the protein. Alternatively, the therapy can introduce a different gene that provides instructions for a protein that helps the cell function normally, despite the genetic alteration.
- Genome editing is a newer technique that may potentially be used for gene therapy. Instead of adding new genetic material, genome editing introduces gene-editing tools that can change the existing DNA in the cell. Genome editing technologies allow genetic material to be added, removed, or altered at precise locations in the genome. CRISPR-Cas9 is a well-known type of genome editing.
Genetic material or gene-editing tools that are inserted directly into a cell usually do not function. Instead, a carrier called a vector is genetically engineered to carry and deliver the material. Certain viruses are used as vectors because they can deliver the material by infecting the cell. The viruses are modified so they can’t cause disease when used in people. Some types of virus, such as retroviruses, integrate their genetic material (including the new gene) into a chromosome in the human cell. Other viruses, such as adenoviruses, introduce their DNA into the nucleus of the cell, but the DNA is not integrated into a chromosome. Viruses can also deliver the gene-editing tools to the nucleus of the cell.
The vector can be injected or given intravenously (by IV) directly into a specific tissue in the body, where it is taken up by individual cells. Alternately, a sample of the patient’s cells can be removed and exposed to the vector in a laboratory setting. The cells containing the vector are then returned to the patient. If the treatment is successful, the new gene delivered by the vector will make a functioning protein or the editing molecules will correct a DNA error and restore protein function.
Gene therapy with viral vectors has been successful, but it does carry some risk. Sometimes the virus triggers a dangerous immune response. In addition, vectors that integrate the genetic material into a chromosome can cause errors that lead to cancer. Researchers are developing newer technologies that can deliver genetic material or gene-editing tools without using viruses. One such technique uses special structures called nanoparticles as vectors to deliver the genetic material or gene-editing components into cells. Nanoparticles are incredibly small structures that have been developed for many uses. For gene therapy, these tiny particles are designed with specific characteristics to target them to particular cell types. Nanoparticles are less likely to cause immune reactions than viral vectors, and they are easier to design and modify for specific purposes.
Researchers continue to work to overcome the many technical challenges of gene therapy. For example, scientists are finding better ways to deliver genes or gene-editing tools and target them to particular cells. They are also working to more precisely control when the treatment is functional in the body.
Scientific journal articles for further reading
Bulcha JT, Wang Y, Ma H, Tai PWL, Gao G. Viral vector platforms within the gene therapy landscape. Signal Transduct Target Ther. 2021 Feb 8;6(1):53. doi: 10.1038/s41392-021-00487-6. PMID: 33558455. Free full-text article from PubMed Central: PMC7868676.
Duan L, Ouyang K, Xu X, Xu L, Wen C, Zhou X, Qin Z, Xu Z, Sun W, Liang Y. Nanoparticle Delivery of CRISPR/Cas9 for Genome Editing. Front Genet. 2021 May 12;12:673286. doi: 10.3389/fgene.2021.673286. PubMed: 34054927. Free full-text article from PubMed Central: PMC8149999.
Credit: U.S. National Library of Medicine
