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Pharmacogenetics – Genes and Drug Response Explained

Pharmacogenetics studies how genetic differences affect a person´s response to medications. It helps tailor therapies to individuals and prevent adverse drug reactions.

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Things worth knowing about "Pharmacogenetics"

Pharmacogenetics studies how genetic differences affect a person´s response to medications. It helps tailor therapies to individuals and prevent adverse drug reactions.

What Is Pharmacogenetics?

Pharmacogenetics is a branch of medicine and pharmacology that examines how a person´s genetic makeup influences their response to drugs. Because every individual carries a unique set of genes, the way the body absorbs, processes, and responds to a medication can vary significantly from one person to another. These differences can mean that a drug works very effectively for one patient, has little effect for another, or causes serious side effects in a third.

Together with pharmacogenomics – which takes the entire genome into account – pharmacogenetics forms the scientific foundation of personalized medicine, also known as precision medicine.

Mechanisms and Biological Basis

Drugs are processed in the body by enzymes, transporter proteins, and receptors. The genes that encode these proteins can vary slightly between individuals. These genetic variations are called polymorphisms, and when they are common in the population, they are referred to as Single Nucleotide Polymorphisms (SNPs).

Metabolizing enzymes: Enzymes such as the cytochrome P450 family (e.g., CYP2D6, CYP2C19) are responsible for breaking down many drugs in the body. Genetic variants can cause these enzymes to work too slowly, too quickly, or not at all.
Transporter proteins: These regulate how much of a drug enters or exits cells, affecting the concentration of the drug at its site of action.
Receptors: Genetic differences in receptors can influence how effectively a drug binds to its target and produces its effect.

Metabolizer Phenotypes

Based on their genetic profiles, patients can be classified into different metabolizer types:

Poor Metabolizer: The enzyme is barely active. Drugs are broken down slowly, leading to drug accumulation and increased risk of side effects.
Intermediate Metabolizer: Enzyme activity is reduced but still present, resulting in mildly altered drug levels.
Extensive Metabolizer (Normal Metabolizer): The most common category, with normal enzyme activity and expected drug response.
Ultrarapid Metabolizer: The enzyme is overactive. Drugs are broken down very quickly, potentially leading to insufficient therapeutic effect.

Clinical Relevance and Applications

Pharmacogenetics has practical implications across many medical specialties:

Oncology: Pharmacogenetic testing is used to determine optimal chemotherapy dosing and prevent severe toxicity. A well-known example is testing for DPYD variants before treatment with 5-fluorouracil.
Psychiatry and Neurology: Many antidepressants and antipsychotics are metabolized by CYP2D6 and CYP2C19. Testing helps guide drug and dose selection.
Cardiology: The antiplatelet drug clopidogrel requires CYP2C19 for activation. In poor metabolizers, the drug may fail to provide adequate protection against blood clots.
Infectious Diseases: In HIV therapy, patients are screened for the HLA-B*57:01 allele before initiating abacavir to prevent potentially life-threatening hypersensitivity reactions.

Pharmacogenetic Testing

Pharmacogenetic tests are typically performed on a blood or saliva sample. The patient's DNA is analyzed for specific genetic variants that are known to affect drug metabolism or response. The results help clinicians select the most appropriate drug and dose for that individual.

Pharmacogenetic testing is increasingly integrated into routine clinical practice in many countries, and certain tests are now included in official treatment guidelines and reimbursed by health insurance providers.

Benefits and Limitations

Pharmacogenetics offers significant advantages: it can improve treatment safety, reduce adverse drug reactions, and enhance therapeutic outcomes. However, there are also limitations. Not all drug responses are genetically determined – factors such as age, body weight, concurrent medications, and underlying diseases also play important roles. Additionally, pharmacogenetic databases are not yet equally representative of all population groups, which may limit applicability in diverse patient populations.

References

Relling, M. V. & Evans, W. E. (2015). Pharmacogenomics in the clinic. Nature, 526(7573), 343–350. https://doi.org/10.1038/nature15817
PharmGKB – The Pharmacogenomics Knowledgebase. https://www.pharmgkb.org (accessed 2024)
World Health Organization (WHO). Genomics and World Health: Report of the Advisory Committee on Health Research. WHO Press, Geneva.

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