What is pharmacogenetics?
Pharmacogenetics, also known as pharmacogenomics, refers to the field of research and medicine that examines how genes, and their expression, affects an individual’s response to medicine. An individual’s genetic make up is expressed not only as physical features, such as height, skin color, and others, but can also affect how people interact with medicines. Genes can affect how the drug is absorbed, distributed, or metabolized by the body.
The goal of pharmacogenetics is to gain a complete understanding the genetic basis of drug response, to better enable doctors to create drug regimens that are best suited for each person’s unique genetic code. Pharmacogenetics and pharmacogenomics are often used interchangeably, although pharmacogenetics focuses more on single drug-gene interactions, whereas pharmacogenomics focuses on a more genome-wide approach, assessing the effects of multiple genes on drug interaction.
Why do we need pharmacogenetics?
For the most part, standard drug regimens and doses are safely tolerated by most people. However, in some cases, drugs can interact with certain individuals in an adverse manner due to their genotype.
Pharmacogenetics seeks to maximize the efficiency of drug therapy by ensuring that physicians prescribe medications best suited to an individual’s genes. Drug regimens under pharmacogenetics can be considered as a personalized, rather than the traditional mass-produced, standardized regimen that is usually prescribed.
How do doctors gather information needed for pharmacogenetics?
There is already a wide field of information available on the biochemical pathways of drug interaction and drug metabolism within cells. In particular, scientists have been able to demonstrate how certain enzymes metabolize drugs, leading to either a good outcome, or toxic effects.
For example, research has allowed pharmacogeneticists to identify genetic variants of enzymes that can cause potentially harmful reactions to drugs, such as the irinotecan, an anti-cancer drug and codeine, a painkiller. By testing patients for these genetic variants, physicians can adjust the drug dosage and regimen to take into account how patients
would interact with drugs.
Large-scale clinical drug trials have also been useful in the field of pharmacogenetics. Volunteers in these drug trials are examined, and DNA samples are collected and analyzed to identify the genetic sequences that are correlated with either effectiveness or toxicity of the drug. The mapping of the human genome in 2003 has also given researchers a great boost in the field of pharmacogenetics.
How is pharmacogenetics useful to me?
Undergoing pharmacogenetic testing can be useful in identifying if you have genetic variations that can cause you to react adversely to a certain drug, or to require a limited dosage. For example, patients with identified glucose-6-phosphate-dehydrogenase (G6PD) deficiency should not be prescribed sulfonamides, or anti-malarial drugs such as chloroquine.
Patients that have been identified to have genetic variations that affect the expression of Cytochrome P450 enzymes, which is one of the most significant drug-metabolizing enzymes, will have different rates of metabolism for drugs. The most commonly tested cytochrome P450 enzymes include CYP2D6, CYP2C19, CYP2C9, CYP3A4 and CYP3A5, which can account for the metabolism of approximately 80-90% of prescription drugs.
Based on individual genotype testing, patients can be categorized into four (4) general classifications, namely: extensive metabolizers (normal metabolic activity), intermediate metabolizers, (reduced metabolic activity), poor metabolizers (little to no metabolic activity), and ultra-rapid metabolizers (substantially increased metabolic activity). The rate at which an individual metabolizes the drug can affect the appropriate dose that should be prescribed to the patient.
What drugs can benefit from pharmacogenetic testing?
Statins, which are cholesterol-lowering drugs, are the most widely prescribed drugs in the world. They have highly variable responses, and require several adjustments of dosage to find the appropriate level for each person. The use of pharmacogenetic testing can be useful in identifying the appropriate dose at the onset.
Anti-cancer therapy can also benefit greatly from pharmacogenetic testing. The analysis of specific genes in a cancer will demonstrate the genetic susceptabilities of the tumor, allowing the physician to prescribe the combination of chemotherapeutic drugs and radiotherapy that will have the greatest effect on the neoplasm.
HIV patients can also be tested to reveal the most appropriate drug regimen. There exist genotyping kits that identify genetic variants of the HIV virus that make it susceptible or resistant to particular anti-retroviral drugs. Knowledge of the virus’ response to drugs allows the physician to tailor the drug regimen to include the most effective anti-retrovirals.
Clinical depression is treated with numerous medications, such as serotonin-specific reuptake inhibitors (SSRIs), and it is often difficult and time-consuming to find the combination of drugs that works best. The use of genetic testing can identify at the onset how the patient metabolizes drugs, allowing the better prescription of drugs at most efficient dosages.
Is pharmacogenetic testing required?
Currently, pharmacogenetic testing is optional. Sampling of DNA for pharmacogenetic testing is usually done by swabbing the inside of the cheek to collect cells.
This may cost up to several hundred dollars, and is typically covered by insurance. The information from pharmacogenetic testing is kept confidential by the Health Insurance Portability and Accountability Act (HIPAA), which was passed by the US Congress in 1996.
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 Cappellini MD, Fiorelli G (January 2008). “Glucose-6-phosphate dehydrogenase deficiency”. Lancet 371 (9606): 64–74. doi:10.1016/S0140-6736(08)60073-2. PMID 18177777.