Biologic drugs have revolutionized the treatment of many chronic diseases. Used to treat certain cancers, autoimmune diseases like multiple sclerosis, and several other complex disorders, many modern biologic drugs are essentially proteins in their makeup—either naturally derived, derived from recombinant technology or, in some cases, chemically synthesized.
Protein therapies have become the treatments of choice in certain diseases since they do not generate side effects encountered with metabolism into undesirable compounds.
Nevertheless, a common concern among patients, clinicians, and manufacturers regarding therapeutic proteins is the development of unwanted immunogenic responses associated with their use. Unintended immunogenicity in patients must be monitored during the clinical development of a drug as well as during post-market surveillance, especially since manufacturing process changes are made throughout the life cycle of a drug. Testing for immunogenicity is directly influenced by good assay design, assay reagents, how assays are executed, and how assay data are analyzed.
An unwanted immunogenic response can result in clinical events ranging from the inability of the drug to deliver any therapeutic effect to severe adverse events. The relatively large molecular makeup of a protein drug is key to triggering an unintended immune response in the body. At times, the body can confuse the presence of a large molecule drug with that of another foreign substance, thus raising a flag for the body to respond accordingly.
The tendency for proteins to aggregate also increases the likelihood that the body will recognize the protein drug as another kind of foreign body comparable in size (e.g., a virus).
Several key factors can influence whether or not the administration of a protein therapy will induce an immune response in the patient. These include the structure of the protein product itself, its variants, the immune status and genetic makeup of the patient receiving the treatment, and the dosing route and regimen used in a clinic.
For example, a cancer patient receiving a protein therapy during a course of chemotherapy can have a suppressed immune system resulting in reduced likelihood of an immunogenic response to the protein drug.
Even the most subtle changes in the manufacture of a biologic might influence the likelihood that a protein drug will elicit an immunogenic response. Some changes may be so difficult to detect that no biochemical test can be applied to link such a change to the potential for immunogenicity.
Generally, subcutaneous injection of a drug is perceived to be a more immunogenic route of exposure than intravenous administration. Thus, the overall context in which a particular drug is being administered must be taken into consideration when managing immunogenicity risk for a patient.
Unwanted immunogenicity associated with a protein therapy can become apparent with the formation of anti-drug antibodies (ADA). Various types of ADA responses can develop in either nonclinical or clinical studies. In some cases, ADA can cover the binding sites on a protein drug, preventing it from binding to its therapeutic target. This can cause the drug’s eventual clearance from the body as well as a loss of efficacy.
In other cases, an immune response to a therapeutic protein can involve a cross-reaction with endogenous (or native) proteins in the body and potentially shut down biochemical processes that may be essential to life.
In 1998, an example of this type of cross reaction involved a reformulation of the protein epoetin, a recombinant version of the naturally occurring hormone erythropoetin (EPO). A slight change in the manufacture of epoetin resulted in the production of anti-EPO antibodies in some patients. These antibodies caused a suppression of endogenous erythropoietin, thus leaving some patients with dangerously low red blood cell counts.
Immunoassay methods for ADA detection generally are complex and require a broad understanding of multiple technical challenges. The U.S. Pharmacopeial Convention (USP) has recently proposed a set of best practices for manufacturers regarding ADA screening activities in immunogenicity testing.
While several regulatory guidance documents and published studies related to immunogenicity testing currently exist, USP’s newly proposed informational standard, USP General Chapter <1106> Immunogenicity Assays—Design and Validation of Immunoassays to Detect Anti-Drug Antibodies, attempts to provide an inclusive set of detailed recommendations for manufacturers with regard to ADA screening.
Above-1000 general chapters such as <1106> are informational, and contain no mandatory requirements unless specifically referenced in a monograph, General Notices, or a general chapter numbered below 1000 in USP’s official compendia, United States Pharmacopeia-National Formulary. General Chapter <1106> is the first of several such USP informational chapters to be developed around the topic of immunogenicity.
General Chapter <1106> outlines proposed strategies for the design, development, validation, and analysis of binding immunoassays that measure unwanted immunogenicity responses following the administration of a biotherapy product. In any testing scheme, a screening assay often serves as the first assessment step. It is critical that screening assays be designed to have a certain false positive (rather than false negative) rate in order to maximize the sensitivity for detecting ADA.
By defining a specific false positive rate, analysts can help ensure that the false negative rate essentially approaches zero. As a follow-up, it is common to confirm positive samples with a second assay. This is intended to demonstrate that a positive signal seen in the ADA screening assay is, in fact, caused by the presence of drug-specific antibodies, instead of other contributing factors such as nonspecific binding proteins contained in serum.
The chapter goes on to provide approaches to the validation of immunoassays. Validation is the process of demonstrating, by the use of specific laboratory investigations, that the performance characteristics of an analytical method are suitable for its intended use. Validation activities include the establishment of assay cut-points or decision thresholds used to discriminate between ADA-positive and -negative samples.
Additional elements related to validation addressed in the chapter include the establishment of system suitability criteria to help confirm that an analytical procedure remains valid for use, the assessment of assay sensitivity, and the confirmation of method specificity.
USP’s process for developing public quality standards relies on input from industry and other interested parties. As a growing number of biologic drugs enter the market, the need for good quality standards that support their manufacture and production will continue to expand. Given the critical need for manufacturers to closely monitor and measure the unintended immunogenic activity of protein therapies, USP’s newly proposed informational standard provides the biologics community with an opportunity to help shape this developing area.