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Pharmacogenomics

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Also listed as: Pharmacogenetics
Related terms
Background
Methods
Research
Implications
Limitations
Safety
Future research
Author information
Bibliography

Related Terms
  • Amplichip®, biotechnology, deoxyribonucleic acid, DNA, drug development, drugs and genetics, gene, genetic disease, genetic disorders, genetic research, genetic testing, geneticists, genetics, genome, genomics, heredity, human genome, inherited diseases, inherited disorders, personalized medicine, pharmacodynamics, pharmacogenetics, pharmacokinetics, pharmacology, phase I metabolism, phase II metabolism, polymerase chain reaction, PCR, targeted therapies.

Background
  • Pharmacogenomics is the study of how an individual's genes affect his or her response to drugs. It is a combination of the study of pharmacology (drugs) and genetics. Different patients can have a variety of responses to a single drug. When given a specific drug, for example, some patients may experience improvement in disease symptoms, while others may have no effects or may even experience mild to severe side effects. Causes of these different responses may be due to a number of factors, including age, diet, race, and the presence of disease. Genetic differences may also play a role in how patients are affected by drugs.
  • Genes are present in all human body cells. They are composed of deoxyribonucleic acid (DNA). The genes provide instructions for making proteins, which are responsible for the structure and function of cells, tissues, and organs. While any two individuals share about 99.9% of the same genes, differences do exist (except in identical twins). These differences are called gene variants, or polymorphisms. They are caused by small changes in the DNA code of the genes.
  • Changes in certain genes cause changes in proteins, which may be more or less active or even nonfunctional, compared to proteins of a different variant. Gene variants that affect proteins that interact with drugs may cause individuals to respond differently to certain drugs. For example, the drug may have decreased activity in the body or increased activity in the body, potentially placing an individual at risk for side effects.
  • Initially, individual gene variants were identified by the way a person responded to a drug. This is termed "pharmacogenetics." Researchers are now examining the entire genetic composition of an individual, which includes thousands of different genes, to evaluate how many different gene variants can affect a patient's response to a drug. This field of study is called "pharmacogenomics" (genomic refers to the entire genetic material of a person). Sometimes, these two words are used interchangeably.
  • Pharmacogenomics is based on the ability of researchers to identify gene variants. If scientists know that a gene variant causes a patient to respond in a certain way to a drug, doctors may decide not to prescribe that drug to the patient. As pharmacogenomics becomes more advanced, it may have significant effects on patient care. For instance, it may allow doctors to choose medications and doses of medications that are most appropriate for a patient based on his or her genetic makeup.

Methods
  • Gene variants can be detected by several different methods, including polymerase chain reaction (PCR), staining of tumor cells, and DNA microarray.
  • Polymerase chain reaction: Polymerase chain reaction (PCR) is the most commonly used method for pharmacogenomic testing. A small sample of cells is taken from a tumor that has been surgically removed, from a blood sample, or from a swab of the inside of the mouth. The genetic material is copied hundreds of times to create a larger sample. Molecules called "probes," which are DNA segments specifically designed to attach to a specific allele, are then used to tag the gene and identify it. Probes can be used to identify gene variants and to determine the amount of variant gene that is present. This method can be used to detect variations in genes that process and inactivate drugs in the liver.
  • Tissue staining: Tissue staining is an indirect method of detecting gene variants that is usually used on tissue that has been surgically removed, such as a tumor. Stains are created to specifically attach to one gene variant or the protein it produces. The stain is applied to a sample of tumor, and is examined under the microscope. If the gene variant is present, then the protein will also be present and the stain will be positive. This method is used to detect the presence of gene variants in some types of cancers.
  • DNA microarray: DNA microarray is a method of studying multiple genes at a single time. Microarrays are used to detect changes in dozens, hundreds, or thousands of genes and can therefore provide more information about a person's specific genetic makeup. Because microarrays are a relatively new technology, they are primarily used in research.

Research
  • Pharmacogenomic research is currently being performed for a wide variety of diseases and conditions. Initially, research focuses on drugs that have different effects in different patients. When doctors recognize this difference in the effect of a drug, researchers perform tests to determine if there is a genetic cause for the differences.
  • Drug metabolism: An area of active research in pharmacogenomics is focused on drug metabolism. Drug metabolism is the process by which the body breaks down a drug. This process most often inactivates the drug and prepares it to be removed from the body by the kidneys.
  • For example, certain drugs are metabolized in the liver by a protein called cytochrome P450 in a two-step process. The first step, called phase I metabolism, causes the drug to change shape so that it becomes inactive. The second step, called phase II, attaches the inactive drug to another molecule that allows it to be excreted by the kidneys in the urine or in the stool. Gene variants in the cytochrome P450 system may cause patients to metabolize drugs too slowly, thereby decreasing the drug's activity in the body, or too fast, thereby increasing the drug's activity and potentially causing side effects.
  • The drug warfarin (Coumadin®) is commonly prescribed for patients who are at risk of forming excessive clots in the blood vessels. Warfarin is metabolized by a cytochrome P450 enzyme called CYP2D6. Some patients have a mutation or mutations in the gene that provides instructions for making the CYP2D6 enzyme, such that the gene does not produce enough of the enzyme, or the enzyme that is produced is less active. This causes them to metabolize warfarin more slowly and may place them at higher risk of bleeding when they take warfarin. Patients with this genetic mutation may therefore require a lower-than-normal dose of warfarin.
  • Variants of other genes are also being studied to identify those that affect how drugs work. For example, researchers are currently studying the genes responsible for the metabolism of antibiotics and anti-inflammatory medications.
  • Targeted therapies: More recently, drug companies have started to develop drugs that act on a single gene or protein. These drugs are called targeted therapies because they are designed to act on a specific target within the cell. Pharmacogenomic testing is used in these cases to identify patients who will benefit from the drug. For instance, Herceptin® is a drug used to treat breast cancer. It acts on a specific gene variant that occurs when tumor cells produce too much of a gene called Her2/neu. Herceptin® only works in patients who are positive for this variant. Genetic tests are performed on patients before they receive the drug, and if they are negative, another drug is used instead.

Implications
  • The ability to identify patients with specific gene variants will allow doctors to prescribe drugs that are safer and more effective. Pharmacogenomic testing may be used to identify patients who may benefit from specific drugs. It may also allow doctors to identify patients who are at risk for side effects.
  • Doctors may be able to use pharmacogenomic information to prescribe appropriate doses of medications to individuals. Instead of adjusting the dose of certain medications over several days or weeks, doctors may be able to determine who should get a higher dose and who should get a lower dose based on a single test.
  • Pharmacogenomic tests may one day be used to detect genetic variants that place a patient at risk for a disease later in life. Physicians may be able to use this information to begin screening for the disease earlier, or begin preventive treatment.
  • Drug companies may be able to use pharmacogenomic information to create drugs that are more effective and have fewer side effects. For example, they may be able to identify a gene variant that is present in cancer cells and to create a drug that specifically targets that variant, thereby effectively treating the cancer without affecting normal cells.
  • Pharmacogenomic testing can be used in human studies to identify and enroll patients who are more likely to benefit. Before a clinical study begins, the mechanism of action of the drug within the body is usually known. In the future, it may be possible to detect genes that interact with the drug to determine which variants are most likely to benefit.
  • By studying drugs only in patients who have a gene variant that will respond to the drug, human studies may be smaller, faster, and cheaper, which may translate into lower prescription drugs costs.

Limitations
  • Cost: Pharmacogenomic testing can be expensive. In order for these tests to be used on a regular basis, it will be necessary to demonstrate the potential benefit to the patient outweighs the cost of testing. Alternatively, cheaper methods of performing genetic tests may be developed.
  • Difficulty in identifying gene variants: The human genome (the whole body of genetic information in a human cell) is very large, and the process of detecting a specific gene or group of genes that affect an individual's response to a drug may be complicated and expensive. Many of these gene variants result from changes in a single molecule within the genetic material. In order to develop useful pharmacogenomic tests, researchers must first know what genes are involved in the body's response to a drug. They can then study each gene to determine what variants are present. This process may take years and may cost billions of dollars.

Safety




Future research
  • Researchers are attempting to identify gene variants in a large number of different conditions. Hopefully, identifying gene variants will allow drug companies to develop drugs that target these variants. Some conditions that are being studied for this purpose include Alzheimer's disease, inflammatory bowel diseases, rheumatoid arthritis, and cancer.
  • For drugs that are currently available, research is being conducted to determine if gene variants are responsible for the differences in effects between patients. This is currently being studied in drugs used to treat cardiovascular disease, infection, and psychiatric illnesses. If genetic variants are responsible for the different responses, pharmacogenomic tests may be used before prescribing medication to determine who will actually benefit from the drug.

Author information
  • This information has been edited and peer-reviewed by contributors to the Natural Standard Research Collaboration (www.naturalstandard.com).

Bibliography
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Copyright © 2011 Natural Standard (www.naturalstandard.com)


The information in this monograph is intended for informational purposes only, and is meant to help users better understand health concerns. Information is based on review of scientific research data, historical practice patterns, and clinical experience. This information should not be interpreted as specific medical advice. Users should consult with a qualified healthcare provider for specific questions regarding therapies, diagnosis and/or health conditions, prior to making therapeutic decisions.

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