Biomarkers validated by genetic and molecular biology methods can be classified into three types.
A biomarker can be any kind of molecule indicating the existence, past or present, of living organisms. In the fields of geology and astrobiology, biomarkers are also known as biosignatures. The term is also used to describe biological involvement in the generation of petroleum (see Biomarker (petroleum)).
In medicine, a biomarker can be a substance that is introduced into an organism as a means to examine organ function or other aspects of health. For example, rubidium chloride is used as a radioactive isotope to evaluate perfusion of heart muscle.
It can also be a substance whose detection indicates a particular disease state, for example, the presence of an antibody may indicate an infection (see biomarker (medicine)). More specifically, a biomarker indicates a change in expression or state of a protein that correlates with the risk or progression of a disease, or with the susceptibility of the disease to a given treatment. Once a proposed biomarker has been validated, it can be used to diagnose disease risk, presence of disease in an individual, or to tailor treatments for the disease in an individual (choices of drug treatment or administration regimes). In evaluating potential drug therapies, a biomarker may be used as a surrogate for a natural endpoint such as survival or irreversible morbidity. If a treatment alters the biomarker, which has a direct connection to improved health, the biomarker serves as a surrogate endpoint for evaluating clinical benefit.
In cell biology, a biomarker is a molecule that allows for the detection and isolation of a particular cell type (for example, the protein Oct-4 is used as a biomarker to identify embryonic stem cells).
A biomarker can also be used to indicate exposure to various environmental substances in epidemiology and toxicology. In these cases, the biomarker may be the external substance itself (e.g. asbestos particles or NNK from tobacco), or a variant of the external substance processed by the body (a metabolite). (See also: Bioindicator.)
Many new biomarkers are being developed that involve imaging technology. Imaging biomarkers have many advantages. They are usually noninvasive, and they produce intuitive, multidimensional results. Yielding both qualitative and quantitative data, they are usually relatively comfortable for patients. When combined with other sources of information, they can be very useful to clinicians seeking to make a diagnosis.
Cardiac imaging is an active area of biomarker research. Coronary angiography, an invasive procedure requiring catheterization, has long been the gold standard for diagnosing arterial stenosis, but scientists and doctors hope to develop noninvasive techniques. Many believe that cardiac computed tomography (CT) has great potential in this area, but researchers are still attempting to overcome problems related to “calcium blooming,” a phenomenon in which calcium deposits interfere with image resolution. Other intravascular imaging techniques involving magnetic resonance imaging (MRI), optical coherence tomography (OCT), and near-infrared spectroscopy are also being investigated.
Another new imaging biomarker involves radiolabeled glucose. Positron emission tomography (PET) can be used to measure where in the body cells take up glucose. By tracking glucose, doctors can find sites of inflammation because macrophages there take up glucose at high levels. Tumors also take up a lot of glucose, so the imaging strategy can be used to monitor them as well. Tracking radiolabeled glucose is a promising technique because it directly measures a step known to be crucial to inflammation and tumor growth.
Not all biomarkers should be used as surrogate endpoints to assess clinical outcomes. Biomarkers can be difficult to validate and require different levels of validation depending on their intended use. If a biomarker is to be used to measure the success of a therapeutic intervention, the biomarker should reflect a direct effect of that intervention.
An example from the 1980s demonstrates the pitfalls of depending too heavily on biomarkers. In the mid-1980s two new drugs, flecainide and encainide, were introduced to reduce ventricular arrhythmias in patients with histories of heart disease. The drugs did indeed reduce arrhythmias. A large trial, the CAST trial, was undertaken to test the efficacy of the drugs, but the trial was stopped after a year because patients taking the drugs were found to be more than twice as likely to die as patients taking placebos. Flecainide and encainide were recalled in 1991. Their example demonstrates that improving a biomarker does not necessarily translate into increased survival.
Biomarkers are gaining prominence in social science research, in particular in the field of consumer behaviour.