All ADCs, payload and stable isotope labeled payload, anti-idiotype and anti-payload antibodies used were provided by AstraZeneca (Gaithersburg, MD). Anti hu-Fc capture antibody was purchased from Bethyl. Peptide internal standards were custom synthesized by Vivitide. The pooled plasma was purchased from BioIVT. The SMART IA magnetic beads, tris(hydroxymethyl)aminomethane (Tris) buffer, Phosphate-buffered saline (PBS), Zeba 7K MWCO spin column, formic acid (FA), trifluoroacetic acid (TFA), and sulfo-NHS biotin were all purchased from Thermo Scientific. Bovine serum albumin and papain were purchased from Sigma-Aldrich. The chromatographic columns (BEH C18 and BioResolve) were purchased from Waters. All other reagents were purchased from VWR.