I. Brief Overview of Historical Uses of Antibodies Used for Therapeutic Purposes
While we tend to think about the use of antibodies for therapeutic purposes as a relatively recent development, in fact, animal antisera for protection against diphtheria dates back to the 1890s. The uses of animal antisera to provide passive immunity provided new treatments, however, due to the use of non-human antisera, the development of serum sickness from the response to the foreign gamma globulins became a significant issue. Human gamma globulin therapies were developed in the 1940s to provide protection to immunodeficient and immunocompromised individuals, a therapy that is still common today. These therapies were developed without a clear understanding of the immune response and almost no knowledge of antibody structure. The major advantages and advancement in antibody-based therapeutics followed the development of methods for monoclonal antibody isolation and production.
The vision of using monoclonal antibodies for treating disease in a targeted way became immediately apparent following the development of hybridoma technology to isolate antibodies with very specific functional characteristics. A mere 10 years after the development of the mouse monoclonal antibody technology, muromonab (marketed as Orthoclone by Janssen-Cilag) was approved for use as an immunosuppressant for the reduction of organ transplant graft rejections. This was a mouse monoclonal antibody that targeted human CD3 on T cells. While it demonstrated the ability of antibodies to be used as targeted therapies, several side-effects were identified including the generation of tachyphylaxis (reduced effectiveness) due to the generation of an anti-mouse immunoglobulin response following repeated doses. Essentially, patients were immunized with a foreign (mouse) protein and would generate an antibody response that would clear the drug from the system, and therefore lose its effectiveness.
II. Developing “Humanized” Monoclonal Antibodies for Therapeutic Uses
In the early 1980’s and 1990’s, mouse hybridoma technology for monoclonal antibody development was the primary source of monoclonal antibodies. However, as had been demonstrated by muromonab, using mouse antibodies for human therapies has significant issues with responses to the “foreign” antibody. The approach to “humanize” antibodies, or develop antibodies that looked more human to the patient’s immune system, was explored to develop more effective and tolerable antibody therapeutics. The initial approach used the mouse heavy chain and light chain variable regions recombinantly fused to human heavy and light chain immunoglobulin constant regions (Figure 1). The resultant antibody had the original specificity and affinity of the mouse monoclonal, but with the added advantage of the human constant regions to allow for decreased antigenicity and increased functionality. However, this still contains significant mouse immunoglobulin structure and, therefore, further modifications were explored to reduce the foreign nature of the recombinant antibody. The CDR region defines the specificity of the antibody and therefore mouse CDRs were “grafted” onto Human immunoglobulin backbones to minimize any residual mouse sequence and structure. More often than not, this requires additional structural modification to maintain the affinity and specificity of the original mouse monoclonal antibody. Numerous methods for accomplishing these goals have been developed and advances in computational modeling have allowed for significant increases in the efficiency of generating humanized monoclonal antibodies.
Human monoclonal antibodies, such as adalimumab (marketed as Humira by AbbVie), have been developed generally through the development of libraries and screened using recombinant techniques, such as phage display. These antibodies, structurally speaking, are fully human. Overall the humanized and fully human antibodies are much more readily “tolerated” by the patient’s immune system. They still can be antigenic due to glycosylation pattern differences, aggregation issues, or due to repeated exposures. Modification of structures and expression systems are being explored to try and minimize these issues.
III. Increased Development of Therapeutic Antibodies
With the development of advanced molecular and computational methods; the identification, isolation, and production of antibodies for therapeutic use has dramatically increased over the past two decades. A search of the FDA approved drug database revealed there are 63 approved monoclonal antibodies currently used, or available for use, as therapeutics. A vast majority of these are for the treatment of cancer or inflammatory conditions. Based on a recent report (Antibodies to Watch in 2016), more than 50 (phase III) clinical trials were underway in November of 2015, as compared with 39 in 2014. This trend of increasing numbers of approvals, as well as more clinical trials, appears to be continuing. It is likely that antibody-based therapeutics will dominate the biotherapeutic market for the foreseeable future.
A new market of biosimilars has just recently developed. This is the development of the equivalent of generic therapeutic antibodies. Biosimilars are antibodies with the same clinical characteristics as the original marketed biologic therapeutic, yet are developed and marketed by a different company.
As with any drug treatment, studies must be conducted to measure effectiveness, stability, toxicity, and half-life of the drug in patients. Therefore it is critical to be able to quantitatively measure the drug in patients. With antibodies, this poses a particular problem of measuring a specific antibody in a sea of antibodies in circulation. In order to do this, anti-idiotype antibodies are used to detect the specific antibody. Monoclonal antibodies targeted towards the CDR region, which defines the idiotype or specific antibody, allow for the detection and measurement of specific antibodies in a complex mixture of immunoglobulins. The anti-idiotype antibodies are generally a critical reagent for use in clinical trials, as well as therapeutic management.
BBI has extensive experience in generating anti-idiotype monoclonal antibodies for use in PK and ADA assays. In addition to hybridoma development, we can provide assistance in the characterization of antibodies for their use in immunoassays. We can help to develop matched pair, bridging, or blocking assays to assist in immunoassay development necessary for preclinical or phase III trials of therapeutic antibodies.