Analytical and pharmacological consequences of the in vivo deamidation of trastuzumab and pertuzumab

A liquid chromatography-tandem mass spectrometry method is presented for the quantitative determination of the in vivo deamidation of the biopharmaceutical proteins trastuzumab and pertuzumab at an asparagine in their complementarity determining regions (CDRs). For each analyte, two surrogate peptides are quantified after tryptic digestion of the entire plasma protein content: one from a stable part of the molecule, representing the total concentration, and one containing the deamidation-sensitive asparagine, corresponding to the remaining non-deamidated concentration. Using a plasma volume of 10 µL and a 2-h digestion at pH 7, concentrations between 2 and 1000 µg/mL can be determined for the various protein forms with values for bias and CV below 15% and without unacceptable in vitro deamidation taking place. A considerable difference between the total and non-deamidated concentrations, and thus a substantial degree of deamidation, was observed in plasma for both trastuzumab and pertuzumab. After a 56-day forced deamidation test 40% of trastuzumab and 68% of pertuzumab was deamidated, while trastuzumab and pertuzumab showed up to 47% and 35% of deamidation, respectively, in samples collected from breast cancer patients during treatment with a combination of both drugs. A good correlation between the non-deamidated concentration results and those of a receptor binding assay indicate a loss of receptor binding for both trastuzumab and pertuzumab along with the deamidation in their CDRs. Deamidated trastuzumab also lost its capability to inhibit the growth of breast cancer cells in a cell-based viability assay, suggesting a relation between the degree of deamidation and pharmacological activity.