Quantification of low amounts of antibody side-products

A method for determining avid-binding side-products of a bispecific antibody in a sample comprising correctly assembled bispecific antibody and one or more avid-binding side-product of the bispecific antibody using surface plasmon resonance,

• wherein the correctly assembled bispecific antibody comprises one or more binding site for a first antigen and one or more binding sites for a second antigen,
• wherein the avid-binding side-product of the bispecific antibody comprises two or more binding sites to the first antigen and more binding sites to the first antigen than the correctly assembled bispecific antibody,
• the method comprising the following steps:
• i) applying a loading solution comprising the correctly assembled bispecific antibody to an SPR surface on which the first antigen has been immobilized with at least 1000 RU to generate an SPR signal, applying thereafter a dissolution solution not comprising a compound binding to the first antigen to the SPR chip and recording the decay of the SPR signal,
• ii) applying a sample solution suspected to comprise an avid-binding side-product of the bispecific antibody to an SPR surface on which the first antigen has been immobilized with at least 1000 RU to generate an SPR signal, applying thereafter a dissolution solution not comprising a compound binding to the first antigen to the SPR chip and recording the decay of the SPR signal,
• iii) determining the avid-binding side-product of the bispecific antibody if the decay of the SPR signal determined in step ii) is slower than the decay of the SPR signal determined in step i).