The term pharmacogenetics refers to the way that genetic variation between individuals affects the way they respond to drugs. Such information is already used in clinical practice in a variety of areas, and is relevant to anyone interested in the prescription of or use of drugs.
No drug is effective for everyone, and in a few patients, usually safe drugs can produce serious adverse effects (Factsheet 1 - When usual doses lead to adverse effects: Dose level relationships). Pharmacogenetic research is starting to provide the basis for predicting individual response to an increasing range of drugs:
Endocrine therapy, with drugs such as Tamoxifen, is often effective in breast cancer patients. Pharmacogenetic testing helps to identify the more likely responders and helps ensure that treatment of likely non-responders with alternative agents is not unnecessarily delayed.
Codeine acts as an effective painkiller after conversion to morphine, which is in turn detoxified and excreted. If too much morphine is produced or if its excretion is impaired, normal doses of codeine may become toxic. Individual genetic differences, as well as concomitantly prescribed drugs, may affect these metabolic pathways with clinical implications (Factsheet 2 - When usual doses lead to adverse effects: Polymorphisms of metabolic enzymes).
Cancer cells show high genetic instability. In fact, most cancer patients die following genetic changes in tumours that lead to drug resistance. Pharmacogenetics contributes to the development of better treatment strategies. In some cancers, such as chronic myelogenous leukaemia, positive treatment effects can be dramatic (Factsheet 3 - The cancer treatment revolution: Controlling the renegade cell and Factsheet 7-Gene expression signatures).
A few HIV-AIDS patients given Abacavir, an antiretroviral drug, develop a potentially lethal hypersensitivity reaction. Pharmacogenetic testing now allows us to identify many of them.
Tuberculosis is still one of the major killers. A recent threat is the emergence of extensively drug resistant (XDR) strains. Rapid aggressive therapy is required to cure the patient and also to prevent the spread of the disease. Testing allows rapid identification of XDR microorganisms (Factsheet 4 - Genotyping the invader and developing new vaccines).
Sometimes it is not easy to identify infective microorganisms. For example with Chlamydia infection, carriers may not know they are infected. Molecular testing allows rapid and reliable identification of the microorganism and use of effective antibiotic therapy.
Many new drugs are only modestly effective but enormously expensive; so much so that the National Institute of Clinical Excellence judge them to be not cost-effective for the National Health Service. Yet sometimes a few patients derive considerable benefit from such drugs. As pharmacogenetic testing becomes more accurate, targeting the likely responders would make prescribing more affordable.
Last updated: 5 July 2011
