Pharmacogenetics: DNA Testing for Drug Response and Overdose Prevention

An exciting new field is on the horizon in science–one which will completely revolutionize healthcare as we know it. One day soon, doctors will routinely choose, prescribe, and dose many drugs based on an individual patient’s genome, able to predict ahead of time using the patient’s tested genetic code the efficiency of the body’s ability to process a drug, and even how much of the drug is required by that patient to reach the therapeutic window where the drug works.

What is pharmacogenetics?

Pharmacogenomic testing is a new field whereby a patient’s response to a drug is determined by their genetic makeup. Whilst up until now, it was very much assumed that all individuals would have the same response to a drug and that a given dose was the ideal dose for adults or children, pharmacogenetic DNA testing has shed a new and different light on the way medicines are prescribed. A pharmacogenetic DNA test will not only help determine a patient’s response to a drug but also make it possible for the doctor to determine what the ideal dose should be – an dosage is an important part of effective medical care. Depending on your genes, some drugs might work better for you than others and whilst a dose of the same drug might work well for someone, it could have catastrophic effects on somebody else, perhaps even causing an overdose. Pharmacogenetics can help determine how a drug will be metabolized, whether its metabolism will be effective and complete or whether the person will only partially metabolize the drug, perhaps increasing the chance of an overdose. The trial and error approach that is so common in medical treatment and causes so much frustration to doctors and patients may soon be a thing of the past. Let us take the treatment of depression – typically the patient suffering from antidepressants will try a whole range of different medications including SSRIs, NSRI and tricyclics.  To feel any effects, positive or negative, the patient will need to wait at least 3 weeks. If that medication does not work, the psychiatrist might alter the dose, choose to combine another drug or change medication altogether. Again, the patient has to wait a couple of weeks to notice any significant changes and in some cases, no medication will work or have any positive effect on the patient. With pharmacogenetic testing, a psychiatrist can work out the optimal dose and whether a particular medication is likely to work, thus helping patients and even eschewing risky overdoses such as what is known as Serotonin toxicity, a result of the administration of serotonergic drugs which causes an excess build up of serotonin in the nervous system.

The field of pharmacogenetics is quickly developing and will be part of our personalized medicine in not too long. It will compliment genetic predisposition health testing, a type of test which evaluates a person’s risk of disease. Genetic health testing is already offered by leading DNA testing companies like easyDNA UK and The Genetic Testing Laboratories.

Intubation and pharmacogenetics

An example happens during intubation, which is a medical procedure often performed when a patient needs airway intervention, such as during surgery or after an emergency when the patient’s breathing is compromised. The intubation involves snaking a mechanism usually through the mouth and down the throat of the person. This can obviously be painful and requires not only relaxed muscles but pain relief as well as usually putting the patient under. A special kind of doctor called an anesthesiologist gives such patients a medication called succinylcholine in order to relax the airway of the person and cause slight paralysis so that the tube can be placed, and the common scenario is that after the surgery, the tube can be removed, the body has processed the medication, and the patient can breathe on their own. Succinylcholine is a paralyzing medication usually given with several other drugs that will treat pain and force the patient to lose consciousness as succinylcholine doesn’t—that’s right, a patient can be paralyzed and yet feel pain and be entirely awake yet incapable of breathing.  This is why the other medications are given in concert with succinylcholine–the strong muscle relaxer.

A problem happens for a small percentage of people. These people have inherited one or more variants or alleles in a gene called BCHE, a gene which controls how the liver processes a drug class that includes succinylcholine. These people and many of their close family members are said to have pseudocholinesterase deficiency; their bodies cannot efficiently break the medication down and get rid of it and therefore cannot get rid of the paralyzing effect in the normal ten minutes. Instead, these people stay paralyzed and unable to breathe on their own for hours. If a clinician orders succinylcholine for someone and they don’t know that person’s genome, signs of extended paralysis are an immediate signal they need to get the patient on artificial breathing as well as rapidly order tests and of course try to soothe the patient who is not only paralyzed by very, very awake and very much able to feel pain yet who is unable to breathe and who without medical intervention can even die. But if the doctor knew this ahead of time, she could simply prescribe an alternate medication such as rocuronium, which is processed through a different route in the body.

Pharmacogenetics tend to focus on one gene to one drug mechanism such as in this example of pseudocholinesterase deficiency, whereas pharmacogenomics such as might be used to understand a specific patient’s metabolizing or breaking down of blood thinner drugs like warfarin or coumadin, rely on plural, complex, and overlapping gene polymophisms (or highly variable regions) and how those affect the interaction of the body with a medication. Physicians are already using genetic testing in tangent with prescribing medication on an individual basis, and very soon routine pharmacogenomic testing will be an exciting new way to raise best practice in healthcare.

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