Approximately 60-70% of invasive breast cancers express estrogen receptor (ER) and/or progesterone receptor and are termed hormone receptor positive (ER+). Endocrine therapy remains the therapeutic backbone for the treatment of ER+ cancers and although anti-estrogen therapies are initially frequently effective, 50% of ER+ patients develop resistance to hormonal manipulation within their lifetime, ultimately leading to therapeutic failure. Two emerging mechanisms of endocrine resistance include activation of growth signaling pathways such as the phosphatidylinositol 3-kinase (PI3K) pathway and more recently, the decoupling of cell cycle control from ER-signaling, via deregulation of the cyclinD-cyclin dependent kinase (CDK-4/6:INK4:Rb) pathway. In this study we hypothesized that combining an anti-estrogen (letrozole or fulvestrant) with a CDK-4/6 inhibitor (LEE011) and PI3K inhibitors (buparlisib [BKM120; pan-PI3K inhibitor] or BYL719 [α-specific PI3K inhibitor] would elicit an improved tumor response over agents inhibiting either pathway alone.

Four ER+ BC mouse models including, an ER+ patient primary human letrozole sensitive model (HBX34, PTEN/PIK3CA wild-type) and three ER+ cell lines; ZR75-1 (PTEN null), MCF7 (PIK3CA mutant) and KPL1 (PTEN/PIK3CA wild-type) were used for this study. Treatment was carried out daily at doses relevant to clinical exposures for a period of 4 weeks with fulvestrant combinations and 8 weeks with letrozole combinations followed by observation for tumor progression. Treatment doses were as follows: LEE011 (75 mg/kg), BKM120 (30mg/kg), BYL719 (35mg/kg), Letrozole (2.5mg), Fulvestrant (5mg/wk) either as single agents or in combinations (Hormone therapy (HT)+LEE011, HT+PI3Ki, HT+LEE011+PI3Ki).

Significant tumor growth inhibition (TGI) was observed for single agent treatment with the CDK-4/6 inhibitor in each of the four ER+ xenograft models. The addition of letrozole (HBX34) or fulvestrant (MCF7, ZR751 and KPL1) with LEE011 increased the TGI observed with single agent. The triplet combination with each PI3K inhibitor increased the (TGI) even further, inducing tumor regressions in each of the four models. Response was independent of PI3K/PTEN mutation status. Complete tumor regressions were observed for a subset of mice within each of the triple combination arms. Tumor regressions were maintained for up to four weeks post-interruption of treatment. No significant toxicities were observed with any of the triplet combinations.

These preclinical data highlight the potential efficacy and safety of targeting the ER, CDK-4/6 and PI3K signaling pathways in ER+ breast cancer. LEE011 in combination with an anti-estrogen is sufficient to inhibit tumor growth in vivo, while the addition of a PI3K inhibitor results in robust tumor regressions. The combination of LEE011 with an anti-estrogen and a PI3K inhibitor is a rational therapeutic approach that should be investigated in the clinic.

Citation Format: Neil A. O'Brien, Emmanuelle Di Tomaso, Raul Ayala, Luo Tong, Shawnt Issakhanian, Ronald Linnartz, Richard S. Finn, Samit Hirawat, Dennis J. Slamon. In vivo efficacy of combined targeting of CDK4/6, ER and PI3K signaling in ER+ breast cancer. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 4756. doi:10.1158/1538-7445.AM2014-4756