In an analysis of the KARISMA trial investigating tamoxifen response by CYP2D6 metabolizer status we found that discontinuation was higher in poor CYP2D6 metabolizers compared to less efficient metabolizers.
One in three ultrarapid CYP2D6 metabolizers discontinued tamoxifen therapy within 1 month, which is substantially higher than in poor metabolizers.
In addition we demonstrated that poor CYP2D6 metabolizers experienced no mammographic density reduction whereas ultrarapid CYP2D6 metabolizers experienced statistically significant mammographic density reduction, higher endoxifen level and more side-effects.
He, et al Ann Oncol. 2021 Oct;32(10):1286-1293. doi: 10.1016