Genetic Testing Valuable Tool In Medication Decisions
In his January State of the Union address, President Barack Obama announced a new White House initiative called precision medicine.
This proposal could have huge implications in the future as to how medicines are selected and dosed to treat individual patients with a wide variety of medical conditions.
We currently select and dose most medications based upon how large groups of patients respond to drugs in clinical trials. However, the manner in which a specific patient will respond to a particular drug is largely determined by that person’s individual genetic makeup. In order to find the most-effective and least-toxic medications to treat a particular problem, we often have to try several different types of medications in order to find a regimen that will work effectively without producing undesirable side effects.
We have known for years that some people respond to certain drugs better than other people. In addition, some patients will experience serious side effects from certain medications while other patients will not.
For example, in order to be an effective analgesic, codeine must be metabolized in our bodies to morphine. However, due to genetic variations, a significant percentage of the population is not able to make the drug-metabolizing enzymes necessary for this transformation to occur. As a result, these people will not obtain adequate pain relief from codeine and may have uncontrolled pain until another effective pain medication is used.
Genetic testing is making it possible to know in advance who will have a favorable therapeutic response to some medications before they are even given to a patient. This same technology may also help to identify in advance those patients who could experience serious side effects or toxicity from certain medications.
For example, there is a genetic test that helps to identify patients who may be at increased risk of severe toxicity from the cancer drug irinotecan. Irinotecan, also known as Camptosar, is indicated for the treatment of colorectal cancer. A significant percentage of patients who receive irinotecan can experience severe diarrhea and very low white blood cell counts. The diarrhea can be so severe that it can be fatal in some patients. The low white blood cell counts can put the patient at risk for serious infections.
This toxicity has been associated with the buildup of a toxic by-product of irinotecan. Some individuals genetically lack the ability to effectively metabolize this toxic metabolite. This genetic test helps to identify these individuals in advance, which will then allow for dosage modifications or considerations for other types of therapy.
A study was just published which identified a genetic variation which puts children being treated for a certain form of leukemia with the cancer drug vincristine at higher risk for developing a severe form of nerve damage. Knowing in advance that a patient is at increased risk for this debilitating side effect may allow other options to be explored.
So don’t be surprised if in the future your doctor orders genetic testing before selecting a drug regimen to treat you with. This ability to classify patients according to their underlying genetic characteristics will be an important step toward the goal of truly individualizing drug therapy.
These developments will allow greater precision in the selection of the most effective and least toxic drugs and an enhanced ability to dose them appropriately in individual patients.