Roadblocks for integration of novel biomarker concepts into clinical routine: the peptoid approach

Abstract

In the field of Alzheimer's disease, the development of novel biomarker assays is critically needed to improve the early diagnosis of the disease, to estimate the risk of developing the disease, to predict the rate of cognitive decline, and to monitor the response or effectiveness of a therapy. The molecular mechanisms of the disease are becoming more evident. This basic knowledge has yet to be translated into novel biomarker tools with a clinical value for general use by the community. There is therefore high interest in evaluating new technological approaches beside the classical immunoassay approach. The present paper discusses the hypothesis that there might be an adaptive immune response, unique to Alzheimer's disease, which can be visualized by the presence in body fluids of antibodies against specific analytes. Current technologies to identify such antibodies are reviewed. In addition, the major challenges to transfer discovery results of the novel antibody-based biomarker assays to a clinically relevant test will be discussed.

Figure 1

Schematic presentation of different approaches to develop novel biomarker assays.

Figure 2

Schematic presentation of the process employed to identify peptoids that bind to antibodies present at higher levels in Alzheimer’s disease patients. Serum samples were hybridized to an array of thousands of eight-mer peptoids spotted covalently onto chemically modified glass microscope slides. After washing and addition of a fluorescent-labeled secondary antibody, the signals were read at each spot. Peptoid spots that consistently captured at least three times more IgG antibodies from the serum of the Alzheimer’s disease patients than that from controls were sequenced by mass spectrometry to identify the structure.

Figure 3

Schematic representation of classical immunoassays that measure levels of nonantibody serum proteins and an assay that measures levels of antibody analytes. Schematic representation of (A) classical immunoassays that use antibody reagents to measure the levels of nonantibody serum proteins and (B) an assay that measures levels of antibody analytes. In (A), off-target binding of a nondisease-related protein will usually not generate a signal since the sandwich antibody is unlikely to bind to this protein. However, any nonspecific binding of an IgG ligand to nondisease-related antibodies will generate a signal, since the binding of any IgG is registered by the labeled secondary antibody. This places a high premium on selective ligands.