Vulnerability of small cell lung cancer to apoptosis induced by the combination of BET bromodomain proteins and BCL2 inhibitors

10% to 15% of all lung cancers are small cell lung cancer (SCLC). SCLC usually grows and metastasizes before it is diagnosed and relapses rapidly upon treatment. Unfortunately, no new targeted agent has been approved in the past 30 years for patients with SCLC. The BET (bromodomain and extra-terminal) proteins bind acetylated histones and recruit protein complexes to promote transcription initiation and elongation. BET proteins have been shown to regulate expression of key genes in oncogenesis such as MYC, CCND2, and BCL2L1. Here, we demonstrate that ~50% of SCLC cell lines are exquisitely sensitive to growth inhibition by the BET inhibitor, ABBV-075. The majority of these SCLC cell lines underwent apoptosis in response to ABBV-075 treatment via induction of caspase 3/7 activity. ABBV-075 enhanced the expression of proapoptotic protein BIM and down-regulated antiapoptotic proteins BCL2 and BCLxl to a lesser extent. Furthermore, BET inhibition increased BCL2-BIM complex, thus priming the cells for apoptosis. Indeed strong synergy was observed both in vitro and in vivo when co-treating the cells with BET inhibitor and the BH3-mimetic, BCL2 inhibitor venetoclax (ABT-199).  ABBV-075 interaction with venetoclax positively correlated with BCL2 expression.  Taken together, our studies provide a rationale for treating SCLC with BET and BCL2 inhibitors in tumors with high BCL2 protein expression.

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