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. SCLC-A (ASCL1 positive) and SCLC-P (POU2F3 positive), which make up almost 80% of SCLC, frequently express high levels of BCL-2. 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 (WT) p53 tumor suppressor functions, but also acquire novel cancer-promoting activities (oncogenic, gain-of-function). A recent study with oncogenic mutant (Onc)-p53 knock-in mouse models of SCLC suggests gain-of-function activity can attenuate chemotherapeutic efficacy. Based on these observations, we hypothesize that Onc-p53 confers venetoclax resistance and that simultaneous inhibition of BCL-2 and Onc-p53 induces synergistic anticancer activity in a subset of SCLC-A and SCLC-P. We show here that (1) down-regulation of Onc-p53 increases the expression of a BH3-only pro-apoptotic BIM and sensitizes to venetoclax in SCLC-P cells; (2) targeting Onc-p53 by the HSP90 inhibitor, ganetespib, increases BIM expression and sensitizes to venetoclax in SCLC-P and SCLC-A cells. Although there are currently many combination studies for venetoclax proposed, the concept of simultaneous targeting of BCL-2 and Onc-p53 by the combination of venetoclax and HSP90 inhibitors would be a promising approach for SCLC treatment.

Breast cancer is the most commonly occurring malignancy in women and the second most common cause of cancer-related deaths. ER⁺ breast cancer constitutes approximately 70% of all breast cancer cases. The standard of care for ER⁺ breast cancer involves estrogen antagonists such as tamoxifen or fulvestrant in combination with CDK4/6 inhibitors such as palbociclib. However, these treatments are often not curative, with disease recurrence and metastasis being responsible for patient mortality. Overexpression of the epigenetic regulator, BRD4, has been shown to be a negative prognostic indicator in breast cancer, and BET family inhibitors such as ARV-825 and ABBV-744 have garnered interest for their potential to improve and prolong the response to current therapeutic strategies. The current work examined the potential of utilizing ARV-825 and ABBV-744 to increase the effectiveness of tamoxifen or fulvestrant plus palbociclib. ARV-825 was effective in both p53 wild-type (WT) breast tumor cells and in cells lacking functional p53 either alone or in combination with tamoxifen, while the effectiveness of ABBV-744 was limited to fulvestrant plus palbociclib in p53 WT cells. These differential effects may be related to the capacity to suppress c-Myc, a downstream target of BRD4.

Non–small cell lung cancer (NSCLC) cells with oncogenic mutant p53 (Onc-p53) alleles exhibit significantly higher levels of proteasome activity, indicating that Onc-p53 induces proteotoxic stress which may be leveraged as a therapeutic vulnerability. Proteasome inhibitors (PI) are most active in cells under proteotoxic stress, so we investigated whether PIs exhibit preferential cytotoxicity in Onc-p53 NSCLC cells. Indeed, bortezomib (BTZ) and other PIs exhibited IC50 values 6- to 15-fold lower in Onc-p53 cells versus wild-type (WT) p53 cells. BTZ cytotoxic effects in Onc-p53 cells were abrogated by antioxidants such as N-acetyl-L-cysteine, indicating that oxidative stress is the critical driver of BTZ-dependent cytotoxic effects in Onc-p53 cells. Importantly, we observed oxidative stress–dependent transcriptional induction of the proapoptotic BH3-only protein NOXA, leading to cleavage of caspase-3, consistent with an apoptotic mechanism of cell death in Onc-p53 but not in WT p53 cells treated with BTZ. BTZ-generated oxidative stress was linked to nuclear translocation of NRF2 and transcriptional activation of ATF3, which in turn was required for NOXA induction. Validating the translational potential of BTZ in Onc-p53 NSCLC cells, BTZ and the BH3-mimetic navitoclax were synergistically cytotoxic in Onc-p53 but not in WTp53 cells in vitro, and BTZ effectively limited the growth of Onc-p53 NSCLC xenografts when combined with navitoclax and carboplatin (a standard-of-care chemotherapeutic in NSCLC) in vivo. Our data therefore support further investigation of the therapeutic utility of PIs combined with BH3-mimetics and chemotherapy in Onc-p53 human NSCLC as a novel therapeutic strategy. Significance: NSCLC is the leading cause of cancer death due, in part, to a lack of active therapies in advanced disease. We demonstrate that combination therapy with a proteasome inhibitor, BH3-mimetic, and chemotherapy is an active precision therapy in NSCLC cells and tumors expressing Onc-p53 alleles.

Triple-negative breast cancer (TNBC) is associated with a generally poor prognosis due to its highly aggressive and metastatic nature, lack of targetable receptors, as well as the frequent development of resistance to chemotherapy. We previously reported that AU1, a small molecule developed as an inhibitor of BPTF (bromodomain PHD finger-containing transcription factor), was capable of sensitizing preclinical models of TNBC to chemotherapy in part via the promotion of autophagy. In studies reported here, we identify an additional property of this compound, specifically that sensitization is associated with the inhibition of the P-glycoprotein (P-gp) efflux pump. In silico molecular docking studies indicate that AU1 binds to active regions of the efflux pump in a manner consistent with the inhibition of the pump function. This work identifies a novel chemical structure that can influence multidrug efflux, an established mechanism of drug resistance in TNBC, that has not yet been successfully addressed by clinical efforts.

P53 has long been studied as a major regulator in cellular pathways, resulting in a plethora of information on the structure and function of this protein as a frequently mutated tumor suppressor. Recent studies have demonstrated how the p53 transcription activation domain (TAD) interacts with the BH3-binding pocket of BCL-2 to regulate cell survival/death. While the in vitro studies on p53 and BCL-2 have frequently used truncated and stabilized proteins of p53 to ensure crystallization, these mutated proteins are not naturally observed in cells. Thus, it becomes important to find a way in silico to simulate how a full-length monomer with the unaltered sequence of wild-type (WT) or missense mutant (MT) p53 interacts with BCL-2. Our objective is to provide a predictive insight into how p53 monomers might interact with BCL-2 through the combination of previously published algorithms. Using pre-established computational techniques in silico, the interactions between p53 variants and BCL-2 were compared to existing crystals to ensure the validity of the current method, and the affinities were predicted to explore the strength of these interactions. Here, we found that this protocol was able to replicate some of the amino acid interactions identified in the previous literature, as well as identify affinities between each WT/MT p53 and BCL-2. Most major MT p53 variants are predicted to directly interact with BCL-2, but have a decrease in affinity compared to WT p53, suggesting a potential increase in BCL-2 survival activity. Together, the method described here can potentially be useful as a predictive workflow to inform future studies in vitro and in vivo.