PROJECT SUMMARY Cisplatin is one of the most effective and widely used anti-cancer drugs. For small cell lung cancer (SCLC), the current platinum-based standard treatment has not changed for more than three decades. Robust initial clinical response is usually observed in SCLC patients but the majority of patients succumb to chemoresistant recurrence. Despite tremendous efforts have been made to understand how SCLC cells develop cisplatin resistance, the precise mechanism remains elusive. As a part of our larger effort to decipher the mechanism of chemo-resistance, we employed a kinome wide shRNA screening and identified microtubule-associated serine/threonine kinase 1 (MAST1) as a ?synthetic lethal? partner of cisplatin in SCLC. Using both SCLC cell lines and patient-derived tumors (PDX), we have demonstrated that MAST1 knockdown sensitizes ASCL1-high SCLC cells to cisplatin treatment in vitro and in vivo. Mining of CCLE database further shows that MAST1 expression is elevated in ASCL1-high subtype SCLC and positively correlates with cisplatin resistance in ASCL1-high SCLC cell lines. Through transcription factor profiling and unbiased datamining, we discovered that cisplatin physically binds and stabilize NFIB protein, a SCLC driver associated with disease progression, to promote MAST1 expression. Indeed, knockdown of NFIB blocked MAST1 induction by cisplatin, suggesting that MAST1 may be a downstream target of NFIB in SCLC. In addition, to identify the potential downstream effectors of MAST1 that contributes to cisplatin resistance in SCLC, we performed mass spectrometry-based proteomic studies and identified PLK1 as a potential binding partner of MAST1. In vitro kinase assay demonstrated that MAST1 directly phosphorylates PLK1 activation site T210. These findings identified MAST1 as a promising target to overcome cisplatin resistance in SCLC. However, no small molecule drug targeting MAST1 is currently available. Thus, we employed a ?drug repurposing? strategy and identified clinical trial-staged kinase inhibitor lestaurtinib as a novel MAST1 inhibitor. Lestaurtinib significantly restored cisplatin sensitivity in SCLC cell lines and PDX models with minimal toxicity. Our central hypothesis is that cisplatin stabilizes NFIB protein to promote MAST1-PLK1 signaling, leading to cisplatin resistance in SCLC. Thus, targeting MAST1-PLK1 signaling represents a promising anti-SCLC target in combination with cisplatin, particularly for ASCL1-high subtype SCLC. We will test our hypothesis through the following aims: (1) To determine whether and how cisplatin stabilizes NFIB to promote MAST1 expression and consequently lead to cisplatin resistance in SCLC. (2) To determine whether MAST1 confers cisplatin resistance to SCLC by activating PLK1 and promoting cell cycle progression in the presence of cisplatin. (3) To validate NFIB-MAST1-PLK1 axis as a therapeutic target for cisplatin-resistant SCLC.