Protein assays for monoclonal antibodies

Several techniques were evaluated for the quantitation of the total protein content of an IgG2a monoclonal antibody, KS1/4, and its deacetylvinblastine (DAVLB) conjugate. The UV assay is rapid, but it requires an extensive calibration of the response factor, and impurities may cause a high bias. Amino acid analysis (AAA) is an absolute method that has few interferences, but it requires evaluation of hydrolysis recovery factors. Kjeldahl nitrogen is very sensitive to minor impurities, and it requires a conversion factor to calculate percent protein. The Kjeldahl assay also is less precise (observed RSD of 3-4%) than the UV and AAA assays (observed RSD of 2-3% for both). The bicinchoninic acid assay, a representative of colorimetric assays, inherently requires comparison to a calibrated standard of the same material and tends to be less precise than the other assays. Thus, the UV and AAA assays are the techniques of choice for measurement of the total protein content of KS1/4 and its DAVLB conjugate.