Targeting SIK2 with GRN-300 Potentiates Paclitaxel Efficacy in Triple-Negative Breast Cancer

Breast cancer is the most frequently diagnosed cancer worldwide, with approximately 15% classified as Triple-Negative Breast Cancer (TNBC). TNBC is characterized by the absence of estrogen receptor (ER) and progesterone receptor (PR), and the lack of HER2 overexpression, limiting use of targeted therapies. Current TNBC treatment relies heavily on chemotherapy, most commonly taxanes including paclitaxel that stabilize microtubules, disrupt chromosome separation and induce apoptosis. TNBCs frequently develop chemoresistance after multiple treatment cycles, highlighting a critical unmet need for novel therapeutic strategies. This study addresses this challenge by targeting salt-inducible kinase 2 (SIK2), which is overexpressed in 85% of TNBCs compared to normal breast tissue.

In collaboration with Arrien Pharmaceuticals and Greenfire Biologics, we developed ARN-3261/GRN-300, a novel orally bioavailable SIK2 inhibitor and evaluated its ability to sensitize TNBC cells to paclitaxel in vitro and in vivo.

GRN-300 demonstrated strong synergy with paclitaxel in all eight TNBC cell lines tested, as indicated by favorable combination indices. In xenograft models, the combination therapy significantly enhanced tumor growth inhibition and prolonged survival compared to either agent alone. Mechanistic studies showed that GRN-300 disrupts the anaphase-promoting complex/cyclosome (APC/C) pathway by downregulating key mitotic regulators, including CDC27, CDK1, and PLK1, thereby potentiating G2/M cell cycle arrest and apoptosis.

Together, these findings establish GRN-300 as a promising therapeutic agent that enhances paclitaxel efficacy through complementary disruption of mitotic regulatory pathways, providing strong preclinical rationale for clinical development in TNBC.