Abstract 4998: Selective PLK4 inhibition demonstrates synthetic lethality in TRIM37 amplified neuroblastoma and breast cancer models while less selective inhibitors do not

Amplification and copy number gains of the 17q23 amplicon are common in breast cancer and neuroblastoma and have been associated with early relapse and poor prognosis (Ganesan et al 2012; Takita et al. 2017). A synthetic lethal interaction of PLK4 with 17q23 amplicon-driven overexpression of TRIM37 was discovered in these tumor types (Meitinger et al. 2020; Yeow et al. 2020). High levels of TRIM37 prevented acentrosomal spindle assembly and rendered cells mitotically vulnerable to inhibition of polo-like kinase 4 (PLK4), a serine/threonine protein kinase that controls centriole duplication. We have discovered that exquisitely selective small molecule inhibitors of PLK4, which are highly selective against the kinome including against the closely related aurora kinases and PLK1-3, display this synthetic lethal interaction with TRIM37, while less selective inhibitors do not. PLK4 protein levels are regulated through proteasomal degradation induced by PLK4 trans-autophosphorylation of the phosphodegron. PLK4 inhibition results in blocked trans-autophosphorylation leading to stabilization of PLK4, thus directly demonstrating target engagement in cells. Importantly, PLK4 protein stabilization correlated with cell viability for selective PLK4 inhibitors but not for less selective compounds, providing a quantifiable pharmacodynamic (PD) association with antitumor activity. Cell viability assessment in cancer cell lines revealed that highly selective PLK4 inhibitors showed greater potency in TRIM37 high cancer cell lines as compared to TRIM37 low cell lines. In contrast, less selective compounds, including from the clinical literature, did not display differential potency in TRIM37 high versus low cancer cell lines. Additionally, selective PLK4 inhibition induced significantly greater apoptosis in TRIM37 high versus low cancer cell lines as measured with a caspase 3/7 assay. We confirmed that only selective PLK4 inhibitors are synthetic lethal with TRIM37 amplification using an engineered cell line system of PLK4 G95L, in which the Leucine mutation blocks compound binding but allows the PLK4 enzyme to function. In cell viability assays, selective PLK4 inhibitors were potent in the parental G95 cells and lost activity in L95 cells, unlike less selective inhibitors whose potency did not depend on PLK4. Oral dosing of a selective PLK4 inhibitor resulted in tumor growth inhibition in TRIM37 high xenograft tumors with no body weight loss. In summary, we have discovered