A biomarker is a characteristic that can be objectively measured and evaluated as an indicator of a physiological as well as a pathological process or pharmacological response to a therapeutic intervention. Examples of classical biomarkers are measurable alterations in blood pressure and blood glucose in diabetes mellitus. In the era of molecular medicine, biomarkers usually mean molecular biomarkers and can be divided into 3 broad categories (22):
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(1) Those that track disease progression over time and correlate with known clinical measures
(2) Those that detect the effect of a drug
(3) Those that serve as surrogate endpoints in clinical trials
A biomarker can be as simple as a laboratory test or as complex as a pattern of genes or proteins. From a practical point of view, the biomarker would specifically and sensitively reflect a disease state and could be used for diagnosis as well as for disease monitoring during and following therapy.
Despite all of the advances in neurology, particularly in the last decade of the 20th century (“Decade of the Brain”), there are serious deficiencies in our understanding of the pathomechanism of several neurologic disorders as well as our ability to diagnose and treat these disorders. Biotechnologies are being increasingly applied in neurology to address some of these deficiencies (23). Novel biomarker identification for neurologic disorders will address the current shortcomings in their diagnosis and therapeutics. With the introduction of digital technologies in neurology, digital biomarkers are also being used in clinical trials as well as in the practice of neurology.
Biomarkers are also used as drug discovery tools--not only to detect biological responses to experimental drugs but also to aid in the discovery of new targets for therapeutic intervention. Biomarkers are a common reference point for diagnosis, as well as therapy, and play an important role in the development of personalized medicine (24).
Desirable characteristics of a biomarker vary according to the disease. For an ideal biomarker of neurologic disorders:
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• The biomarker should be noninvasively (or minimally invasively) detectable in living subjects
• Results should be reproducible
• The biomarker should be positively correlated to the cause or progression of the disease.
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• There are thousands of biomarkers of various diseases including neurologic disorders, but not all of them have been validated.
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• Biomarkers in blood can provide early indicators of disease and help in understanding the pathomechanism of disease as well as determine prognosis.
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• Besides contributing to diagnosis, biomarkers contribute to integration of diagnosis with therapy and are useful for monitoring the course of disease as well as response to treatment.
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• Some biomarkers are potential targets for discovering new drugs for neurologic disorders and are useful for clinical trials during drug development.
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• Biomarkers are facilitating the development of personalized neurology.
Historical note and terminology
The first laboratory test for a protein cancer biomarker, the Bence Jones protein in urine was used in mid-19th century. The term “biomarker,” an abbreviation of biochemical biomarkers, started to appear in the literature during the early 1960s in connection with metabolites and biochemical abnormalities associated with several diseases. During the last decade of the 20th century, discovery of biomarkers was accelerated by mass spectrometry used for analysis of biological samples for biomarkers, applications of proteomics for molecular diagnostics, and emergence of metabolomics for the study of biomarkers. Completion of sequencing of the human genome in 2000 opened the way for discovery of gene biomarkers.
Since 2005, discovery and application of biomarkers has become a major activity in biotechnology and biopharmaceutical industries. The term “molecular biomarkers” is now applied to any specific molecular alteration of a cell on DNA, RNA, and metabolite or protein level.
Of the thousands of biomarkers that have been discovered, most remain to be validated. A biomarker is valid if:
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(1) It can be measured in a test system with well-established performance characteristics
(2) Evidence for its clinical significance has been established.