Targeting gain-of-function p53 and BCL-2 for small cell lung cancer treatment - Project Summary/Abstract Through a unique genomics and drug screening platform with ~800 solid tumor cell lines, we have found a subset of SCLC cell lines are hypersensitive to venetoclax, an FDA-approved inhibitor of BCL-2. These sensitive SCLCs have invariably high BCL-2 expression. Based on this data, a phase I/II clinical trial with venetoclax in SCLC patients has been designed at VCU (NCT04543916). It has been recently proposed that SCLC can be classified in four subtypes based on the expression of key transcription factors: SCLC-A (ASCL1 positive), SCLC-N (NeuroD1 positive), SCLC-P (POU2F3 positive), and SCLC-Y (YAP1 positive). Among them, SCLC-A and SCLC-P, which make up almost 80% of SCLC, frequently express high levels of BCL-2. We noticed that BCL-2 high and venetoclax-sensitive cell lines that we examined all belong to SCLC-A. However, we found that a subset of SCLC-A and SCLC-P showed high BCL-2 expression but were venetoclax-resistant. In addition, most of these SCLC cell lines have TP53 missense mutations, which make a single amino acid change. These mutants not only lose wild-type p53 tumor suppressor functions, but also acquire novel cancer-promoting activities [gain-of- function (GOF)]. It has been shown that GOF p53 expression is associated with chemo- and radio-resistance in a variety of cancers. Furthermore, a recent study with GOF p53 knock-in mouse models of SCLC suggests gain- of-function activity can attenuate chemotherapeutic efficacy. Based on these observations, we hypothesize that GOF p53 confers venetoclax-resistance and that simultaneous inhibition of BCL-2 and GOF p53 induces synergistic anticancer activity in a subset of SCLC-A and SCLC-P. Our preliminary data support this hypothesis: (1) Down-regulation of GOF p53 increases the expression of a BH3-only pro-apoptotic BIM and sensitizes to venetoclax in SCLC-P cells; (2) targeting GOF p53 by the HSP90 inhibitor, ganetespib, increases BIM expression and sensitizes to venetoclax in SCLC-P and SCLC-A cells. Although currently there are many combination studies for venetoclax proposed, the concept of simultaneous targeting of BCL-2 and GOF p53 is innovative and the combination of venetoclax and HSP90 inhibitors would be promising approach for SCLC treatment. In this proposal, we will determine the mechanism of action of this combination in vitro and the efficacy of this combination in a subset of SCLC-A and SCLC-P subtypes using cell line xenograft and patient-derived xenograft (PDX) models. Aim 1 will determine the mechanisms of cell death induced by venetoclax and ganetespib combination in SCLC cell lines in vitro. Aim 2 will define the efficacy of venetoclax and ganetespib combination treatment in mouse models of SCLC. Since venetoclax is FDA-approved and HSP90 inhibitors are being investigated in clinical trials including SCLC, the successful completion of the proposed project will lead to developing clinical trials at VCU Massey Cancer Center and Holmes Hunter VA Medical Center.
