P4-01-20: Specific Anti-Proliferative Profile of Endoxifen for MCF-7 Cell Compared to 4-OH Tamoxifen

Background: Endoxifen and 4-hydroxy tamoxifen are both active metabolites of tamoxifen and exert anti-tumor activity for ER+ breast cancer. However, only small numbers of paper reported biological profile of endoxifen. Methods: Cells were incubated with various concentration of endoxifen ( (Z)-4-Hydroxy-N-desmethyl Tamoxifen ( Results: IC50s of endoxifen for MCF-7 were 100 nM in estradiol (E2) deprivation condition and 500 nM in the presence of 1nM E2. These are significantly higher than those of 4-OH tamoxifen, which are 10 nM without E2 and 50 nM with E2, respectively. From the previous reports, plasma endoxifen concentrations in breast cancer patients were ranged from 20 to 200 nM according to CYP2D6 phenotype. Anti-proliferative index of 20 nM endoxifen for MCF-7 was 0.7 (indicating 30% reduction in proliferative activity), and 200 nM endoxifen was 0.5 in the absence of E2. These were 0.95 (20 nM) and 0.75 (200nM), respectively, when incubated with 1 nM E2. 10 nM 4-OH tamoxifen, which is mean concentration in patient plasma, showed about 0.6 anti-proliferative index with no regards to E2 condition. Without E2, endoxifen reduced ERα expression as previously reported, however, endoxifen increased ERα amount in the presence of E2 accompanied with decrease of HER-2 expression. This alternation of ERα and HER2 expression were identical to those observed using 4-OH tamoxifen. There was a weak tendency that ERβ expressing MCF-7 clones showed favorable response to endoxifen compared to parent MCF-7, but this was not significant. Conclusion: Endoxifen is active agent for MCF-7 cell, however, it requires 10-fold higher concentration compared to 4-OH tamoxifen to achieve IC 50. Difference of anti-proliferative effect between 20 nM and 200 nM of endoxifen are relatively small, and it might result in minor difference for clinical response, as expected from recent clinical studies of CYP2D6. This is the first time to show that endoxifen has opposite effect to ERα protein level depending on the presence or absence of E2. ERβ expression did not enhance activity of endoxifen in contrast to previous report. Citation Information: Cancer Res 2011;71(24 Suppl):Abstract nr P4-01-20.