In some cases, individuals who are given a certain therapeutic drug to treat symptoms or to keep symptoms from occurring, have a very violent reaction to the drug or feel no affect whatsoever. In some cases, this can be traced to the genetic makeup of the individual and the study of this phenomenon is called “pharmacogenomics” or “pharmacogenetics.”
As an example, a woman who had surgery to remove a tumour was given codeine as a pain reliever. Although she was doing well after the surgery, as soon as she began treatment with codeine she developed a full-body rash, difficulty breathing, and an irregular heartbeat. When she was taken off the codeine, her reaction disappeared. Upon further study, it was found that she lacked the enzyme in her blood that metabolised (broke it down into different components) the codeine into morphine and other substances, so she was essentially being overdosed with codeine. The lack of the enzyme was directly related to a polymorphic variation in one of the cytochrome genes (CYP2D6) gene that produced it. Sometimes these polymorphisms can cause a very serious reaction in an individual that could lead to death.
In other cases, individuals “hyper metabolise” drugs. This occurs when there is too much of an enzyme present that breaks down the helpful drug too quickly, leading to a lack of response to the drug. This can happen when too many copies of the gene are present and too much enzyme is produced. In other cases, the special receptor that the drug binds to on cells or tissues is missing, again because of a variation in the gene that makes the receptor protein. When there is no receptor to bind the drug, the drug may not have any affect on the cells or tissues that it should.
Genetic testing to determine the polymorphisms that play a role in our response to a drug is now available for a number of conditions including an individual’s resistance or sensitivity to the effectiveness of drugs used in viral therapy for e.g. HIV or Hepatitis C.