LBAs for Macromolecules

The true specificity of LBAs for macromolecules is rarely addressed in depth, primarily for the reason that breakdown products of the macromolecule (metabolites and catabolites) are rarely available as reference standards for inclusion in crossreactivity studies. Where possible, attempts are made to derive some data to support... 

Reference standards for macromolecule LBAs are inherently heterogeneous and complex, in contrast to the homogeneity of low molecular weight drugs. This subject is elaborated in Chapter 9, accompanied by a number of illustrative examples. Characterization of USP and non-USP standards is discussed in this chapter in terms of such parameters as purity, potency, and stability. Case studies illustrate the assay effects of variability in reference standard quality. 

To facilitate the understanding of the specific applications and interpretation of validation parameters for LBAs, it is useful to consider first some important differences between low molecular weight compounds and macromolecules, especially differences in the in vivo disposition of these two classes of molecules and between the chromatographic assays and LBAs used for their respective quantitation. 

Given the foregoing discussion of some of the unique characteristics of macromolecules that lead to clear differences in their pharmacokinetics compared to those typical of small-molecule drugs, there is a subset of the entire group of bioanalytical assay validation parameters that are of key importance in support of pharmacokinetics of candidate macromolecular therapeutics. Assuming demonstration of accuracy and precision of sufficient quality for the intended application of the assay (e.g., non-GLP discovery support or GLP toxicokinetic support, as discussed above), the most important characteristics of a given assay in support of pharmacokinetic studies are likely to be selectivity, specificity, and reproducibility for analysis of incurred samples. These are all related to the ability of the LBA to detect and quantitate solely, or as closely as possible to solely, the analyte of interest. 

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