CLASSIFICATION OF TUMOR-MARKERS

Since the discovery of the first tumor markers more than a century ago (Bence-Jones proteins), a vast array of molecules have been described as being associated with cancer. These are generally naturally occurr ing biomolecules with the exception of neo-antigens expressed in certa in tumors induced by viruses. Tumor markers can be broadly classified into tumor specific antigens and tumor-associated markers. Most tumor markers were often heralded as highly tumor specific but subsequent st udies demonstrated their presence in normal tissues of the adult or in various stages of ontogeny. As a result, very few tumor-specific anti gens can be recognized. The idiotypes of immunoglobulins of B cell tum ors and certain neo-antigens of virus induced tumors are two examples that are strictly tumor specific. The vast majority of tumor markers a re in reality tumor-associated antigens and can be classified into two types based on their size. The low-molecular weight tumor markers (si milar to < 1000 Daltons) include some nucleosides, lipid associated si alic acid, polyamines, pseudouridine, pigment derivatives, and other m etabolites. The macromolecular tumor antigens are the most important s ub-type useful in the clinical management of cancer patients. The larg e cancer antigens are either enzymes, growth factors, hormones, recept ors, biological response modifiers, oncogenes and their products, of g lycoconjugates which include glycoproteins and glycolipids. Collective ly all the commercial tumor marker assays available to the oncologist for cancer patient management amount to an annual sales of > $1 billio n world wide. The demonstrated clinical usefulness and commercial succ ess of tumor markers have continued to fuel exciting research into the discovery and novel uses of new analyses.