- The UGT1A1 gene variant predisposes people to a decrease in white blood cells if they are prescribed Camptosar, a Pfizer drug for colon cancer. A lower starting dose should be set tor the 10 percent of people who carry this gene variant.
- The TPMT gene is associated with slow metabolism of 6-mercaptopurine, or 6MP, a drug for childhood leukemia and inflammatory bowel diseases. In people who carry two copies of the TPMT gene variant, 6MP can also cause a deadly decrease in white blood cells.
- High levels of the Her2 protein in breast cancer tumors indicate treatment with Herceptin.
- Specific HIV mutations induce resistance to particular drugs.
- Genes for the cytochrome P450 enzyme family are involved in drug metabolism.
- The 2D6 enzyme metabolizes codeine into morphine. Fast metabolizers can overdose on relatively small doses of codeine while slow metabolizers would gain little relief from pain. A Roche DNA chip can detect variations in genes for 2D6 and 2C19.
- Ten percent of Caucasians have a variation of 2D6 that makes it difficult for them to eliminate the antidepressants Prozac and Paxil; alternative treatment uses Celexa or Lexapro because they are metabolized by 2C19.
- The 2C9 and vitamin K epoxide reductase enzymes are involved in determining a person’s sensitivity to warfarin, a drug for the prevention of blood clots.
As it stands, many doctors and health insurers believe genetic testing is too costly. Some even think that if genetic testing is required before prescribing a specific medication, doctors would opt for another drug instead. For people who can afford it, private DNA testing companies offer tests directly to consumers who are then responsible for the (mis)interpretation of results.
Until prospective clinical trials can be done that demonstrate both the economic and health benefits of DNA testing prior to prescribing drugs, public health will have to do without pharmacogenetics (or pharmacogenomics or theranostics).
San Francisco Chronicle, November 13, 2005
In a way, this personalized medicine is a practiced this time and it helps patients to get well quicker. Each and every doctor, the serious ones, I mean, try to cure the patient not the disease. It is the fact that they have to know the history of a patient, so that he could make him better.
[...] From what I’ve seen, doctors often don’t believe genetic tests are advanced enough to be incorporated into the care of most diseases. In addition, some believe that it’s too costly and troublesome. Hopefully, as the genomic revolution develops more diagnostic tests and personalized medicine, doctors will be more willing to learn more about genetics. [...]