BNP and proBNP are reliable markers of heart failure (HF) widely used in clinical practice. Several types of sandwich immunoassays (conventional assays) utilizing two mono- or polyclonal antibodies, specific to different epitopes of BNP or BNP-fragment of proBNP molecule are described in literature.
BNP molecule is known as an extremely unstable molecule rapidly losing its immunological activity in water solutions. This loss of activity is usually associated with proteolytic degradation of the peptide. Sandwich immunoassays commonly used for qualitative or quantitative antigen immunodetection utilize two or more antibodies specific to two or more different epitopes. The longer is the distance between the epitopes, the higher is the probability that sites of proteolysis would be located between the epitopes of the antibodies, thus increasing the sensitivity of the assay to proteolytic degradation of the antigen. And vice versa, the closer are the epitopes to each other, the smaller is the probability of the proteolytic cleavage of the molecule between the epitopes.
Immunoassay methods for very small molecules have been described, including the application of so called anti-metatype antibodies. Such methods are disclosed, e.g. for detecting digoxin (Self et al., 1994, Clin. Chem. 40:2035-2041), and angiotensin II (Towbin et al., 1995, J. Immunol. Meth. 181:167-176).
However, it is not an easy task to apply this type of method to different analytes, since very specific monoclonal antibodies are required in such a method.