Mass spectrometry for the quantification of bioactive peptides in biological fluids

The study of pharmacologically active peptides is central to the understanding of disease and development of novel therapies. It would be advantageous to monitor the fate of bioactive peptides in biological fluids and tissues following their in vivo administration (exogenous administration) or the modulation of endogenous factors (e.g., peptide hormones) affected by the administration of a pharmacological agent. Measurement of administered compounds (small molecules) in plasma is a mature field. However, measurement of pharmacologically active peptides presents particular problems for quantitative mass spectrometry, including challenges from selectivity and sensitivity perspectives. Current approaches towards peptide quantification in biological fluids include immunoassays and mass spectrometric techniques. Immunoassays, although sensitive, lack the necessary selectivity for distinction between peptide and metabolites. Modified molecules induced by metabolic transformations (e.g., N- or C-terminal truncation of the peptide) might not be differentiated by the antibody used in the assay, leading to cross-reactivity. However, although it is generally accepted that mass spectrometry is an ideal technique for the quantification of trace levels of analytes in biological fluids, immunological techniques are still characterized by better limits of peptide detection. In this review article, novel mass spectrometric approaches and strategies on peptide quantification will be described. The current capabilities and prospects for advances in this critical area of research will be examined.