Doctors have long known that people sometimes differ in how a drug works in their bodies. What accounts for these differences? The field of pharmacogenetics (or pharmacogenomics) explores this question by focusing how genes affect the way people respond to medicines. The idea is to create very specific medications that will be effective in people with a certain gene while sparing use—including the potential side effects of use—in people without the gene. The ultimate goal is to create “designer drugs” matched to unique genetic profiles.
The genes of many
differ from noncancer genes in the same individual. These cancer genes are often very active, and this activity potentially provides scientists with targets for effective cancer treatment. For example, the cancer drug trastuzumab (Herceptin) is prescribed to treat metastatic breast cancer in which the tumors contain an overactive HER-2 gene. It is estimated that nearly 30% of all breast cancer patients have tumors with this type of abnormal gene activity. If doctors can determine excessive HER-2 activity in a tumor, trastuzumab can be used to block the effects of the HER-2 gene and improve cancer survival. While trastuzumab can be lifesaving in some patients, it also has serious potential side effects which must be balanced against its benefits.
drug imatinib (Gleevec) is used in people with chronic myeloid leukemia whose cancers have a gene that makes an abnormal leukemia-causing protein, BCR-ABL. This medicine may also be effective in treating other types of cancer that affect the blood cells, as well as gastrointestinal stromal tumors.
Identifying Natural Variations
Even though we all have the same overall number of genes, what contributes to our individuality, including our susceptibility to disease and reactions to medicines are the unique variations within our own set of genes. These are naturally occurring differences, known as single nucleotide polymorphisms (SNPs). Among the more than three billion pairs of DNA building blocks in the human genome, a very small fraction of these pairs vary from person to person. Yet these genetic variations are at the heart of pharmacogenetics research.
By digging deeper into our molecular blueprints, medicine will become more tailored to groups or individuals with certain genetic flags. Scientists hope that understanding these genetic variations will increasingly explain individual differences in the way that drugs are absorbed and metabolized, their side effects, and their overall effectiveness. Some of this understanding has begun to find its way into clinical applications.
For example, AmpliChip CYP450 is a test that measures variations in two genes that play a role in the metabolism of some commonly prescribed drugs. The test’s manufacturer claims AmpliChip can reduce the chances of unwanted drug reactions if doctors use it to guide their prescriptions of drugs known to be metabolized through one of the two measured genes. Test results may also allow dosages to be adjusted for those persons whose genes lead them to metabolize drugs unusually rapidly or unusually slowly.
Earlier Diagnosis, Tailored Treatment
Advances in molecular analysis offer the promise of improvements not only in individualized treatment, but in early disease diagnosis, as well. If researchers know the genes or gene products to look for, diseases may be found earlier, potentially even before symptoms are apparent or the disease process really gets going—an obvious advance in the case of monitoring for recurrent cancers, for example. The Oncotype DX Breast Cancer Assay is a panel test designed to detect the presence of 21 cancer-related genes. The National Cancer Institute is currently doing a large study involving over 7,000 women with early stage breast cancer. The researchers are studying whether certain genes that are linked to cancer recurrence can be used as a basis to select individualized treatment plans for the best outcomes.
Personalized medicine may be able to:
- Identify people
earlier intervention—Researchers hope that quicker treatment may translate into more effective treatment and better outcomes for patients.
better drugs more quickly—As scientists understand the genetic variations and molecular pathways involved in a disease, pharmaceutical companies hope to develop highly targeted drugs more quickly than is the norm today.
Before personalized medicine can be widely applied, however, much more research is needed in the field of pharmacogenetics.