mTOR Inhibition Overcomes RSK3-mediated Resistance to BET Inhibitors in Small Cell Lung Cancer

Purpose: SCLC is a recalcitrant malignancy with limited treatment options. BET inhibitors have shown promising preclinical activity in SCLC, but their broad sensitivity spectrum limits their clinical prospects in this malignancy. Drug combination could be a solution. Experimental design: We performed high-throughput drug combination screens in SCLC cell lines to identify potential therapeutics synergizing with BET inhibitors. Validation was performed in SCLC cell lines and patient-derived xenograft models. Genome-wide RNA sequencing of xenograft tumors was performed to determine the mechanism underlying the synergy of the drug combination. Results: Inhibitors of the PI-3K-AKT-mTOR pathway were the top candidates from the screens. Among the therapeutics targeting this pathway, mTOR inhibitors showed the highest degree of synergy with BET inhibitors in vitro. Furthermore, the combination of these two classes of drugs showed superior antitumor efficacy and tolerability in vivo. Using both in vitro and in vivo SCLC models, we demonstrate that BET inhibitors activate the intrinsic apoptotic cascade, and mTOR inhibitors further enhance these apoptotic effects. Mechanistically, BET inhibitors activate the TSC2-mTOR-p70S6K1 signaling cascade by upregulating RSK3, an upstream kinase of TSC2. Activation of p70S6K1 leads to BAD phosphorylation and cell survival. mTOR inhibition blocks this survival signaling cascade and potentiates the antitumor effects of BET inhibitors. Conclusions: Our results demonstrate that RSK3 upregulation is a novel resistance mechanism of BET inhibitors in SCLC, and mTOR inhibition can overcome this resistance and enhance apoptosis. These findings provide a rationale to evaluate the combination of mTOR and BET inhibitors in patients with SCLC.