The researchers found higher breast cancer event rates in patients with reduced or absent CYP2D6 function vs. extensive metabolism patients. "At 9 years of follow-up, the recurrence rates were 14.9 percent for extensive metabolizers, 20.9 percent for heterozygous extensive/intermediate metabolizers, and 29.0 percent for poor metabolizers, and all-cause mortality rates were 16.7 percent, 18.0 percent, and 22.8 percent, respectively," the authors write. Compared with extensive metabolizers, heterozygous extensive/intermediate metabolizers had a 40 percent increased risk of recurrence; poor metabolizers had nearly twice the risk.

"Compared with extensive metabolizers, those with decreased CYP2D6 activity (heterozygous extensive/intermediate and poor metabolism) had worse event-free survival and disease-free survival, but there was no significant difference in overall survival."

"Genotyping has the potential for identification of women who have the CYP2D6 poor metabolism phenotype and for whom the use of tamoxifen is associated with poor outcomes, thus indicating consideration of alternative forms of adjuvant endocrine therapy," the authors conclude.

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