Internal standards in the quantitative determination of protein biopharmaceuticals using liquid chromatography coupled to mass spectrometry

Following the increase in development of protein biopharmaceuticals, there is a growing demand for the sensitive and reliable quantification of these proteins in complex biological matrices such as plasma and serum to support (pre)-clinical research. In this field, ligand binding assays (LBAs) are currently the standard analytical technique, but in recent years, there is a trend towards the use of liquid chromatography hyphenated with (tandem) mass spectrometry (LC–MS/MS). One of the reasons for this trend is the possibility to use internal standards to correct for analytical variability and thus improve the precision and accuracy of the results. In the LC–MS/MS bioanalysis of small molecules, internal standardization is quite straightforward: either a stable-isotope labeled (SIL) form of the analyte or a structural analogue is used. For the quantification of biopharmaceutical proteins, the situation is more complex. Since the protein of interest is digested to a mixture of peptides, one of which is subsequently used for quantification, there are more options for internal standardization. A SIL form or a structural analogue of either the intact protein or the signature peptide can be used. In addition, a modified form of the SIL-peptide internal standard, containing one or more cleavable groups is a possibility, and an internal standard can be generated during the analysis by using differential derivatization techniques. In this paper we provide an overview of the different options for internal standardization in the field of absolute targeted quantification of protein biopharmaceuticals using LC–MS/MS, based on literature from 2003 to 2011. The advantages and disadvantages of the different approaches are evaluated both with regard to the correction they provide for the variability of the different steps of the analysis and with regard to their generic availability. As most of the approaches used lead to acceptable results in terms of accuracy and precision, we conclude that there currently is no clear preferable method for internal standardization in the field of protein quantification by LC–MS/MS. It is essential, however, that any step in the analysis that is not covered by the internal standard chosen, should be carefully optimized and controlled.