Bioanalysis of therapeutic peptides: Differentiating between total and anti-drug antibody bound drug using liquid chromatography–tandem mass spectrometry quantitation

An acylated peptide with MW ∼4.5 kDa was measured in samples from pharmacokinetic, toxicology and clinical studies using liquid chromatography–tandem mass spectrometry (LC–MS/MS). Lower limits of quantitation of 2 ng/mL and 50 pg/mL were achieved for animal and human plasma, respectively. Repeated drug administration may lead to anti-drug antibodies (ADA) which can inactivate the drug by formation of drug-ADA complexes. Hence, the LC–MS/MS assay incorporated cleavage of potential drug-ADA complexes to quantify the total plasma concentration. To obtain information on active drug levels, an assay that measures the free concentration or alternatively the ADA-unbound concentration would be needed. Ultrafiltration experiments through 100 kD cutoff membranes to remove Ig-bound peptide were not successful due to nonspecific binding. Extraction of Ig-bound drug using Protein A or G (bacterial cell wall proteins with high affinity to the Fc region of IgG) was suitable to distinguish between ADA-bound drug and [free + protein bound (not ADA-bound)] drug and correlated with findings from ELISA ADA measurement.