Therapeutics
reagents for use in PK assays, with the FDA, OECD and Clinical and Laboratory Standards Institute guidelines all emphasising the need to characterise reagents appropriately and providing guidance on key characteristics13-15. Consequently, a standard battery of bioanalytical assays has been developed to meet regulatory authority criteria for the approval of potential new therapeutics. While subtle differences exist between the given regulatory requirements for PK assay reagents from the various controlling regula- tory authorities, all available guidance documents require demonstration of selectivity, accuracy, pre- cision, sensitivity, stability, a lack of matrix effect and reproducibility for any specific reagent that may be incorporated into such assays16. Significant collaboration and communication across industry and the various health authorities has existed for more than 25 years, resulting in the generation of evidence-based regulations and recommendations governing the use of ligand- binding assays13-15. Until recently these recom- mendations have remained largely region specific, but the formation of the Global Bioanalysis Consortium in 2010 brought together scientists from various associations and different countries with the aim ‘to harmonise and merge existing or emerging bioanalytical guidance to create one, unified consensus document that can be presented to the regulatory bodies/health authorities in var- ious countries’17.
The fundamental nature of critical reagent con- trol to the quality and long-term performance of ligand-binding assays makes the consensus and regulation of these reagents throughout the drug lifecycle essential to safety. However, despite the global convergence on regulatory authority guide- lines for the approval of critical assay reagents, there has previously been an apparent lack of guidance on their lifecycle management and stabil- ity. To address this, a panel of industry experts formed the Large Molecule L4 Harmonisation Team of the Global Bioanalysis Consortium to lay out clear recommendations and best practices for the lifecycle management of critical reagents in bioanalysis assays, for both internally-developed and commercially-available reagents8. These rec- ommendations detail the management of lot changes for any critical reagent, stability manage- ment and the documentation of critical reagents within ligand-binding assays. The guidelines high- light the very real challenges presented by the lack of reproducibility in PK assay reagents and the need for new reagents that can overcome these pit- falls in lot-to-lot variability.
Drug Discovery World Winter 2017/18
Available anti-idiotypic reagents An array of anti-ID reagents has been developed to specifically identify and measure the concentration of potential biological therapeutics within PK assays (Figure 1). Each of these is associated with different advantages and disadvantages, with newer reagents having been developed to overcome many of the drawbacks of traditional reagents18.
Antigens
The capture of a potential therapeutic agent from a patient sample within a PK assay was tradition- ally achieved using the original protein or biologi- cal antigen to specifically bind the antibody thera- peutic. This method of capture can offer specificity for the target antibody so as to allow subsequent quantification as part of a PK assay, though matrix effects are often observed with human serum sam- ples, complicating the interpretation of any results. By design, capturing the antibody therapeutic via its antigen only allows for the capture of free antibody molecules, not antibody-target complex, preventing full interpretation of the therapeutic antibody’s metabolism. Another significant draw- back of the use of antigen as anti-ID reagents with- in PK assays can be the availability of sufficient quantities of the antigen at acceptable cost. Even with the use of recombinant antigens, supply issues may remain, due to the inherent variability of dif- ferent antigen proteins19.
Monoclonal antibodies
Monoclonal antibodies can offer the benefits of high target affinity and specificity for use as PK assay reagents. Furthermore, they can be devel- oped to recognise both bound and free forms of any therapeutic antibody, thus allowing the full repertoire of therapeutic biologic to be analysed within any patient sample, providing additional bioanalytical information.
A number of commercial laboratories, including Genscript, Creative Biolabs and R&D Systems, have shown success with this strategy, producing monoclonal antibodies for use as PK assay reagents. Each of these companies offers custom anti-ID antibody production from a range of species, in addition to a catalogue of standard anti- ID antibodies targeting the most common antibody therapeutics, for use in the growing field of biosim- ilar therapeutics development. Furthermore, R&D Systems offers additional characterisation services to ensure any selected antibodies function as desired within a specific assay system, and the abil- ity to convert any polyclonal or monoclonal hybridoma into recombinant antibodies.
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