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Such tests can already show how rapidly or slowly a person metabolizes drugs, for instance, helping doctors prescribe doses that are large enough to be effective, yet not so large as to cause harm in a particular patient with heart disease, depression, or schizophrenia.
Other gene-based tests can indicate whether a patient with an early tumor really needs to undergo chemotherapy—with its many side effects—after surgery. One test may soon detect the first signs of a particular cancer in a patient's blood before any symptoms appear and in time for a definitive cure by surgery.
Some of these tests will save money in the long run. Others will lead to expensive new drugs (some have already) that are targeted to specific subtypes of patients—drugs that not every patient can afford. “This is an enormous and challenging issue,” says Kenneth Offit, chief of the Clinical Genetics Service at Memorial Sloan-Kettering Cancer Center in New York City. He recently took part in an Institute of Medicine conference where, he notes, several speakers emphasized the need for “more evidence-based reviews and more cost-benefit analyses to inform the discussion of access to molecular medicine.”
Offit is one of many optimistic researchers who are convinced that the overall effect of gene-based tests will be to make treatment more efficient and effective, reducing the instances in which people take drugs that don't work.
“We now know that certain drugs work only in a particular subset of patients,” says Charles L. Sawyers, an HHMI investigator at the University of California, Los Angeles (UCLA), “but we don't yet know how to identify subsets in an easy way.” This is why Sawyers and other researchers are busy ferreting out differences in patients' genes, then using these distinctions to stratify people into smaller and smaller subgroups, each of which responds to drugs in its own particular way. This approach has already changed how certain diseases are treated, at least in leading hospitals and research centers. At the same time, it is inducing drug companies to take account of subtypes as they develop and test new treatments.
The fruits of personalized medicine are most evident in cancer. Sawyers's own work has helped revolutionize the treatment of chronic myeloid leukemia (CML) by aiming the precisely targeted and highly effective Gleevec (imatinib), and now the recently approved drug Sprycel (dasatinib), at specific genetic mutations in the cancer (see Personalized Medicine Made Real). Research by others has revealed the genetic mutations that make the drug Herceptin (trastuzumab) effective against certain types of breast cancer, and Iressa (gefitinib) effective against a small subset of lung cancers. New tests for these mutations can identify who is most likely to benefit from these costly drugs—and who should try something else, saving valuable time and expense as well as reducing unnecessary toxicities.
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