A reasonable starting point for any discussion of biomarkers applications is with a general definition of the term 'biomarkers.' In recent years a formal definition has been crafted by a committee termed the Biomarkers and Surrogate Endpoint Working Group; the definition of 'biological marker (biomarker)' they have put forth reads as follows: a characteristic that is objectively measured and evaluated as an indicator of normal biological processes, pathogenic processes, or pharmacologic responses to a therapeutic intervention.1 This definition is broad by necessity, as the field has impacts across biological systems and disease areas and can incorporate any of the technologies that are applicable in basic biological or clinical research. Although there has been an increase in attention to and formalization of the biomarkers field, in practice biomarkers have been an integral component of biomedical research for decades. The primary focus has historically been on biomarkers of disease; indeed, it has been proposed that the initial application of biomarkers may date to the practice of culturing infections to improve cure rates when antibiotics were first introduced during World War II.2 In the broadest sense, all of the symptoms that are typically associated with the presence of a disease state may be considered as disease biomarkers. With advancements in biochemical and molecular techniques, more specific, quantifiable, and mechanistically relevant types of markers have been characterized (herein referred to as molecular biomarkers). A few examples of clinically useful molecular biomarkers are: glucose and hemoglobin A1c levels in diabetes, circulating viral load in viral infections, and cholesterol, low-density lipoproteins (LDL), and high-density lipoproteins (HDL) levels in cardiovascular disease. Each of these endpoints is diagnostic or prognostic of a disease state, and also can be measured to indicate an effect of a therapeutic intervention, thus fitting the criteria for the biomarkers definition as listed above. Clinically relevant and disease-associated biomarkers such as these are very useful in the clinical testing of drug candidates, as their monitoring can simultaneously confer information about both the mechanistic or biological effect(s) of an intervention (pharmacodynamic effect) as well changes in the status of disease.
The potential for biomarkers to enhance biological understanding of disease processes and drug targets, coupled with their potential to improve the efficiency of decision making during the drug discovery and development process, has led to heightened interest in biomarker applications in the current environment. The impact of this interest may be noted by the observation that, in early 2005, a search of the World Wide Web using the Google search engine yields approximately 1 450 000 matches for the term 'biomarkers.' Similarly, a search for the text word 'biomarker' or 'biomarkers' in the Pubmed biomedical literature database at this time yielded a total of more than 12000 citations, dating back to the late 1970s. Of the 12 000, more than 8300 were dated in the year 2000 or later. The increased interest can be linked to breakthroughs in molecular technologies in areas such as genomics and proteomics as well as bioinformatics, and also to the increasing sophistication of drug discovery processes and continued emphasis on the rational targeting of specific molecular pathways (e.g., in the area of cancer treatment in particular, a rapidly emerging theme is the development of so-called 'molecularly targeted' therapeutics). Applications of biomarkers are now becoming standard in many phases of drug discovery and target characterization, and are leading to a more comprehensive understanding of drug effects; this ultimately can enable more effective and efficient drug development.
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