Therapeutic antibodies are the fastest growing drug class, with continuous increases in the number of innovator molecules approaching the market. As the number of biotherapeutics undergoing development increases there is a consequent need to be able to effectively monitor the level and distribution of these drugs in pre-clinical and clinical samples. To do this we must have a way to specifically detect and quantify biotherapeutics within serum samples.
But this isn’t so simple; within serum there is approximately 12mg/ml of our natural IgG antibodies, and most therapeutic antibodies are present only at ng/ml levels. The majority of biotherapeutics consist of human or humanised antibodies which are highly homologous to the natural antibodies present in the serum. To be able to specifically identify the therapeutic antibody, amongst this million-fold excess of IgG, highly sensitive affinity reagents are essential.
As many of the major blockbuster antibody therapeutics are beginning to come off patent an increase has been observed in the number of biosimilar antibodies under development. Moreover, in addition to the expansion in innovator and biosimilar therapeutic antibodies, advances in the field of antibody engineering have led to the development of antibody-derived molecules, such as bispecific antibodies and antibody drug conjugates, which are now approaching the clinic for use as antibody drugs. All of these molecules require the ability to specifically, sensitively and reproducibly track their concentration and bioavailability in pharmacokinetic assays, during the drug development and clinical trials process.
Anti-idiotypic antibodies and anti-idiotypic antibody mimetics are the specific tools that allow us to monitor therapeutic antibody levels and distribution within PK assays. An idiotope is a specific set of unique antigenic determinants, typically found in the variable portion an antibody, that define that particular antibody. This is in contrast to an antibody allotype, in which the variation is found within the constant region. An anti-idiotypic antibody can therefore specifically bind to the idiotope of the target antibody, such as a therapeutic antibody, to act as capture and detection reagents in PK assays.
Anti-idiotypic antibodies can bind to a therapeutic antibody for specific recognition in three different modes.
(1) Binding to the binding site of the therapeutic antibody to inhibit the target binding of the therapeutic antibody.
(2) Binding to a region close to the binding site of the therapeutic antibody without inhibiting target binding.
(3) Binding to a complex of therapeutic antibody and its target antigen.
Bioanalytical scientists are able to gain different information, such as bound drug or total drug levels, regarding the activity of a therapeutic antibody from each of the different methods of anti-idiotypic antibody binding. If the anti-idiotypic antibody binds in a neutralising fashion to the therapeutic antibody paratope, this allows us to measure only the levels of free antibody in a sample. Binding to an antibody-antigen complex allows the measurement of only the bound therapeutic antibody within a sample. Finally, binding to a region close to the paratope, yet not the paratope itself, allows the measurement of total antibody levels, both free and bound, within a sample.
Many biotherapeutic targets are naturally present in serum, (e.g. TNF-? for adalimumab in the treatment of arthritis). Consequently, it is important that the anti-idiotypic antibodies or antibody mimetics used for analysis contain examples of non-neutralising antibodies, to ensures the accurate detection of the target-bound biotherapeutics in addition to free biotherapeutic molecules within any clinical samples.
It is clear that for use in clinical settings any anti-idiotypic antibodies or antibody mimetics must fulfil high quality standards. High specificity, sensitivity and reproducibility in assay performance are key for any anti-idiotypic reagent, in addition to consistent batch-to-batch control, because any specific anti-idiotypic reagent may be used for several years over the lifetime of the drug evaluation process.
In this respect, the development of recombinant anti-idiotypic reagents, such as Affimer binders, is ideal, as it allows guided selection to achieve binders with sufficient specificity and sensitivity to be able to identify a biotherapeutic target against a complex background of serum. Being able to drive the specificity of the anti-idiotypic binder to the idiotope of the biotherapeutic, in this way, is difficult with the use of traditional animal methods for the generation of these reagents. The in vitro selection process for specific Affimer binders includes the ability to deselect against homologous targets and serum constituents, which focuses selection upon the idiotypic region of the biotherapeutic and prevents matrix issues arising during use.
The reproducibility benefits of recombinant reagents over traditional monoclonal and polyclonal antibodies have been extensively stated. Preventing the loss, through batch-to-batch variation, of a specific anti-idiotypic binder is crucial to the continued reproducible monitoring of any candidate biotherapeutic during the drug development process.
Finally, an additional advantage that Affimer technology can provide is in their rapid development of anti-idiotypic reagents. Affimer binders can be developed within 12 weeks, preventing project delay and improving speed to market, which is important, particularly in the race to market for biosimilar biotherapeutics, where being first to market offers significant benefits.
We have produced a number of application notes around our anti-idiotypic data.