It was hoped some day that Genomic Analyses of cancer tumors would be able to identify in advance which patients will benefit from use of cancer drugs (clinical responders). The Agency for Healthcare Research and Quality (AHRQ) suggests that day is still a ways off.
A study looked at whether the presence of specific mutations in people who had breast and colon cancer and chronic myeloid leukemia determined if patients would respond to expensive new drugs commonly used to fight the diseases. Only in colon cancer did the mutation matter and in that case, while it ruled out the effectiveness of drug therapy, the relevant mutation only appears in a small percentage of cases.
In the finding, the AHRQ report found there was "no consistent associations" between breast cancer patients with the relevant CYP2D6 polymorphism and the outcome of tamoxifen therapy, whether as primary treatment or in as post-operative adjuvant therapy.
Estimates vary, but anywhere from 10 to 40 percent of women have the gene variant of CYP2D6 that is believed to slow the metabolism of tamoxifen and make it less effective.
http://cancerfocus.org/forum/showthread.php?t=3061
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