RET Receptor Tyrosine Kinase Promotes Breast Cancer Metastasis to the Brain and RET Inhibitors Pralsetinib and Selpercatinib Suppress Breast Cancer Brain Metastases

Patients with breast cancer brain metastases (BCBM) exhibit dismal prognosis, largely due to the insufficient biological understanding of BCBM and the scarcity of therapeutics that can penetrate the blood-brain barrier. This study was focused on Rearranged during transfection (RET) receptor tyrosine kinase that has been implicated in tumorigenesis and metastatic progression of several solid tumor types including, non-small cell lung cancer (NSCLC), thyroid carcinomas, and luminal breast cancer subtypes. FDA-approved selective RET inhibitors, pralsetinib and selpercatinib, have demonstrated potent intracranial activity in brain metastases from NSCLC and thyroid cancer; however, their efficacy in BCBM has not been investigated. Here, we report that RET activation is elevated in brain metastases of breast cancer patients compared to matched primary tumors (N=30), and in three brain-tropic breast cancer cell lines compared to the parental lines. High RET pathway activation is associated with shorter overall metastasis-free survival and brain metastasis-free survival in patients with HER2-enriched and triple-negative breast cancer (TNBC). Using isogenic TNBC cells lines RET overexpression, we demonstrated that RET strongly promotes their preferential metastasis to the brain in mice with intracardiac injections to tumors cells. Using intracranial tumor implantation of the isogenic lines, we further found that RET significantly enhances the formation and progression of brain tumors in vivo. Moreover, we report that selective RET inhibition using pralsetinib and selpercatinib significantly reduces cell viability, enhances apoptosis, and attenuates migration of brain-tropic breast cancer cells in vitro. Using two mouse studies that model multi-organ metastases and breast tumor formation in the brain, we observed that RET inhibition significantly prevented the circulating tumor cells from forming brain metastases and suppressed the growth of intracranially implanted tumor cells, but did not significantly inhibit the progression of well-established brain metastases. Together, our findings demonstrated that RET is highly activated in BCBM and functioning as a novel mediator of BCBM, and that RET plays a new role as a viable therapeutic target for BCBM.