Pharmacogenomics, sometimes called as pharmacogenetics, is the study of how genes affect a person’s response to drugs. It is a combination of two fields - pharmacology (the science of drugs) and genomics (the study of genes and their functions).
Just like how genes determine our eye color, height, etc. they also partly influence how our body responds to drugs. Some chemical changes in these genes can elicit unwanted side effects upon drug consumption.
The long-term goal of pharmacogenomic research is to design drugs best suited for each person, in order to avoid these undesirable side effects.
Genes influence multiple steps involved in your response to drugs. They include:
Drug Receptors: Some drugs require a type of protein called the receptors, to which they bind and get activated. Your genes can influence the number and effectiveness of these receptors.
Example: T-DM1 is a drug used to treat breast cancer. This drug works by attaching to a receptor called the HER-2 receptor. However, not all breast cancer cells express this receptor. So, this drug may not be effective for all individuals with breast cancer.
Drug Uptake: Certain drugs are activated only after they are taken into the cells and tissues. If your genetic makeup leads to reduced uptake of the drug, it may accumulate in other parts of the body.
Example: Statins are a class of drugs commonly used to treat high cholesterol levels. For the drug to work, it must be transported to and taken up by the liver efficiently. SLCO1B1 gene influences this process. A change in this gene results in a reduced transport of statins to the liver. This can result in statin buildup in muscles resulting in pain and weakness.
Drug breakdown/metabolism: If your genetic makeup results in a faster breakdown of drugs, it gets clear from the body faster. This may warrant an increased dosage of the drug or a different drug. On the other hand, if your drug metabolism is slow, it stays in your body for a longer period. In this case, a lower dosage may do the work.
Example: Amitriptyline is an antidepressant drug. Two genes, namely, CYP2D6 and CYP2C19, are involved in its metabolism. If you carry a change that slows down or boosts the metabolism, you may need to alter the drug dosage accordingly.
Patients can respond differently to the same medicine.
Commonly used drugs to treat some medical conditions need not be effective for everyone. Some examples are:
- Antidepressants drugs (SSRIs) are ineffective in as many as 38% of patients who are prescribed these drugs
- Asthma drugs are ineffective in as many as 40% of patients who are prescribed these drugs
- Diabetes drugs are ineffective in as many as 43% of patients who are prescribed these drugs
- Arthritis drugs are ineffective in as many as 50% of patients who are prescribed these drugs
- Alzheimer’s drugs are ineffective in as many as 70% of patients who are prescribed these drugs
- Cancer drugs are ineffective in as many as 75% of patients who are prescribed these drugs
- Cardiac Arrhythmias drugs are ineffective in as many as 40% of patients who are prescribed these drugs
Source: Brian B Spear, Margo Heath-Chiozzi, Jeffrey Huff, Clinical application of pharmacogenetics, Trends in Molecular Medicine, Volume 7, Issue 5, 2001, Pages 201-204, ISSN 1471-4914, https://doi.org/10.1016/S1471-4914(01)01986-4.
The purpose of pharmacogenomic testing is to find out if a medication is right for you. A pharmacogenomic test will help in knowing:
Efficacy - Whether a medication may be an effective treatment for you.
Dosage - What is the best dose for you for specific medications.
Toxicity - Whether you could have serious side effects from a medication.
CYP enzymes or the Cytochrome P450 enzymes are the major drug-metabolizing enzymes in the body. The P450 enzymes contain a protein called heme (iron-containing compound) and are commonly present in hepatocytes (cells of the liver). This is why drugs are mostly broken down or metabolized in the liver.
From a clinical perspective, the most commonly tested CYPs are:
Changes in CYP enzymes can influence the metabolism and clearance of drugs.
The CYP450 Test categorizes individuals into one of the four known metabolic profiles, called “predicted phenotypes.”
What are the limitations of a CYP test?
- Pharmacogenomic research is still in its infancy. Therefore, tests are available only for certain drugs.
- Any change in medication will require a new CYP test - this is because different enzymes are responsible for metabolizing different drugs
- The test reveals how genes affect the drugs and not what the drug does to the body (for example, we cannot determine how the drugs change certain receptors in the brain to alleviate the symptoms)
- Some drugs are metabolized and cleared by more than one CYP enzyme. For example, antidepressant drugs like the SSRIs (Selective Serotonin Reuptake Inhibitor) are metabolized by serotonin receptor molecules as well. This can limit the predictive value of the test.
Who should take the PGx test ?
If you answer yes to any of the below questions, you are an ideal candidate for a PGx test.
1. Are you currently taking four or more medications monthly?
2. Have you or anyone in your family ever been hospitalized for taking medication?
3. Have you or anyone in your family ever felt ill after taking a new medication?
4. Has your doctor changed your dose of medication due to a lack of response or a reaction to the medication?
5. Do you take your prescribed medication, and you still aren’t feeling better?
6. Are you taking or is your doctor considering prescribing to you pain medicine, tamoxifen, or Plavix?
7. Do you take herbal supplements regularly in addition to your medication?
Our traits are basically divided into genetic and environmental. Genetic factors are the ones a person is born with, and a large chunk of these factors are inherited from the previous generations. Environmental factors include chemical, physical, nutritional, infectious and behavioural factors. Many prevailing diseases such as diabetes and cancer are caused by the complex interplay of genetic and environmental factors. Both the factors will play a part in influencing the diseases. Some may be more influenced by the genetic factors while others will be largely affected by the environmental factors. But, most of the diseases are always associated with the genetic makeup and many inherited diseases can be influenced by environmental conditions.
If you have a genetic predisposition to antisocial behaviour, you may not demonstrate the trait until you experience abuse or neglect in your childhood. If you have had a stress-free and normal childhood, you will never express this specific genetic trait. The expression of a specific trait towards which you are genetically predisposed can be prevented by protective environmental factors. If you have a predisposition to alcohol abuse and live in an alcohol-prohibited environment, it may not express itself. Thus, protective genetic factors have a comparatively less significant effect if environmental exposure is strong.
Response to environmental exposure depends on the genotype, which is a term that defines your genetic make-up for a specific trait/disease. If you have had stress in your early life, it may cause depression in later years. This is only when certain genotypes are present. A person’s genotype can also determine their response to specific medications and their side-effects through various biochemical mechanisms. There is an entire branch of science that studies this called Pharmacogenomics.
If we can identify our genes and characterize their interactions with the environment, we can have intervention strategies to target them. Therefore, when studying the genetic make-up of individuals to determine their natural predisposition towards certain traits and diseases, it is very important to take into consideration the environmental factors like diet, lifestyle, work environment etc, to be able to characterize their inclination towards these traits and their risk for developing specific diseases.