Abstract P5-09-08: A comparison of the non-genomic effects of endoxifen and tamoxifen in aromatase inhibitor resistant breast cancer: Differential effects on the estrogen receptor co-regulator SRC3 (AIB1) and identification of PKC and PI3K as endoxifen substrates
Authors: Goetz MP , Kuffel M , Reinicke KE , Huang Z , Bode AM , Cheng J , Hoskin T , Suman VJ , Artega C , Reid J , Hawse J , Ames M 
Published: August 2014

Background: Endoxifen (Endx) is being developed as a drug for patients with estrogen receptor (ER)+ breast cancer (BC). We previously demonstrated that Endx was superior to TAM in both a letrozole (L) sensitive and resistant MCF7/AC1 model. Tamoxifen (TAM) and its most commonly studied metabolite, 4-hydroxy TAM (4HT), are known to activate critical ER co-regulators such as SRC3, which are oncogenic, and contribute to acquired and de novo endocrine resistance. While both Endx and 4HT are more potent inhibitors of genomic ERα functions compared to TAM, the effects of Endx on non-genomic signaling have not been characterized and its efficacy as an anti-cancer agent compared to RAD001 in AI resistant BC is unknown.

Methods: We evaluated the 1 hour in vitro effects of TAM, 4HT, Endx, and L, in terms of activation of multiple signaling cascades using an MCF7/AC1 L-resistant cells and long term estrogen deprived (LTED) cells (MDA 361 and ZR75). Protein Docking studies (shape screening, pharmacophore modeling and ligand-protein docking) for oncogenic kinases, and their associated IC50's were determined for TAM and Endx. The in vivo activity of Endx (50 mg/kg, oral) was determined in MCF/AC1 L-resistant xenograft tumors and compared with L control (10 ug/day, sc), Exemestane (E) (250 ug/d, sc) and E+RAD001 (2.5 mg/kg/d, oral).

Results: In MCF7/AC1 L-resistant cells, 1 hr treatment with either TAM (100 nM or 5 uM) or 4HT (100 nM or 5 uM) phosphorylated SRC3 (AIB1) and resulted in downstream activation (phosphorylation) of IGF1R, PDK1, AKT, and p70S6K. In contrast, Endx (100 nM and 5 uM) did not phosphorylate SRC3 nor any downstream kinases but resulted in a dose dependent decrease in AKT phosphorylation. Similar findings were observed in LTED cells MDA 361 and ZR75. Docking studies identified a significant decrease in the binding energy (&gt;2 kcal/mol) of Endx compared to TAM for PKC and PI3K and subsequent kinase profiling demonstrated that Endx more potently inhibits PKCβ1 (IC50 357nM) and PI3KΔ (2.5uM) compared to TAM (IC50 4.8μM and 6.2μM respectively). Therefore, we evaluated the in vivo anti-tumor activity of Endx with E and E+RAD001 in MCF7 L-resistant xenograft tumors. After 9 weeks, the% change in tumor volume relative to baseline was significantly lower for both Endx (median -34%, p = 0.0002) and E+RAD001 (median +1.5%, p = 0.0002) as compared to L controls (median +301%). In contrast, E alone was non-significantly lower (median +172%, (p = 0.09). No significant differences were observed comparing Endx and E+RAD001 (p = 0.56).

Conclusions: Endx inhibits PKCβ1 and PI3KΔ at concentrations that are pharmacologically relevant, and unlike TAM and 4HT, does not activate SRC3 nor induce oncogenic signaling pathways in AI resistant cells. Importantly, Endx elicits substantial anti-tumor effects in AI resistant models that are similar to the current standard of care of E+RAD001. These data suggest that Endx may be superior to TAM for the treatment of ER+ BC and may result in clinical benefit for patients with AI resistant disease.

Citation Information: Cancer Res 2013;73(24 Suppl): Abstract nr P5-09-08.