Holly M. Alley

Systematic identification of signaling pathways with potential to confer anticancer drug resistance.

Pathway-centric screening reveals new mechanisms of drug resistance and combination therapeutic strategies. Finding New Targets for Drug-Resistant Cancers The development of drug resistance is a common problem in cancer patients. Knowing how drug resistance emerged in a tumor can inform clinical strategy. Martz et al. devised a drug screen to identify pathways of resistance when cancer cells were treated with drugs that are used in the clinic. Along with pathways known to mediate drug resistance, such as the MAPK and PI3K pathways, activation of the Notch1 pathway caused drug resistance in various types of cancer cells in culture. Inhibiting Notch1 signaling restored drug efficacy in cells in culture and in xenografts in mice. Intriguingly, Notch signaling mediated drug resistance to an estrogen receptor–targeted therapy used in breast cancer and to a kinase-targeted therapy used in melanoma, suggesting that this single pathway may be important in multiple types of drug-resistant cancers. Indeed, tumors of some patients with relapsed breast cancer or melanoma had increased markers of Notch1 signaling. In the Research Article by Winter et al. also in this issue, this screening method identified a pathway of drug resistance in bone marrow cancer. More generally, by screening entire signaling pathways instead of individual genes, the work of Martz et al. shows how we can quickly map pathways to the diverse properties of cancer cells. Cancer cells can activate diverse signaling pathways to evade the cytotoxic action of drugs. We created and screened a library of barcoded pathway-activating mutant complementary DNAs to identify those that enhanced the survival of cancer cells in the presence of 13 clinically relevant, targeted therapies. We found that activation of the RAS-MAPK (mitogen-activated protein kinase), Notch1, PI3K (phosphoinositide 3-kinase)–mTOR (mechanistic target of rapamycin), and ER (estrogen receptor) signaling pathways often conferred resistance to this selection of drugs. Activation of the Notch1 pathway promoted acquired resistance to tamoxifen (an ER-targeted therapy) in serially passaged breast cancer xenografts in mice, and treating mice with a γ-secretase inhibitor to inhibit Notch signaling restored tamoxifen sensitivity. Markers of Notch1 activity in tumor tissue correlated with resistance to tamoxifen in breast cancer patients. Similarly, activation of Notch1 signaling promoted acquired resistance to MAPK inhibitors in BRAFV600E melanoma cells in culture, and the abundance of Notch1 pathway markers was increased in tumors from a subset of melanoma patients. Thus, Notch1 signaling may be a therapeutic target in some drug-resistant breast cancers and melanomas. Additionally, multiple resistance pathways were activated in melanoma cell lines with intrinsic resistance to MAPK inhibitors, and simultaneous inhibition of these pathways synergistically induced drug sensitivity. These data illustrate the potential for systematic identification of the signaling pathways controlling drug resistance that could inform clinical strategies and drug development for multiple types of cancer. This approach may also be used to advance clinical options in other disease contexts.

Efficacy of SERD/SERM Hybrid-CDK4/6 inhibitor combinations in models of endocrine therapy resistant breast cancer

Purpose: Endocrine therapy, using tamoxifen or an aromatase inhibitor, remains first-line therapy for the management of estrogen receptor (ESR1)–positive breast cancer. However, ESR1 mutations or other ligand-independent ESR1 activation mechanisms limit the duration of response. The clinical efficacy of fulvestrant, a selective estrogen receptor downregulator (SERD) that competitively inhibits agonist binding to ESR1 and triggers receptor downregulation, has confirmed that ESR1 frequently remains engaged in endocrine therapy–resistant cancers. We evaluated the activity of a new class of selective estrogen receptor modulators (SERM)/SERD hybrids (SSH) that downregulate ESR1 in relevant models of endocrine-resistant breast cancer. Building on the observation that concurrent inhibition of ESR1 and the cyclin-dependent kinases 4 and 6 (CDK4/6) significantly increased progression-free survival in advanced patients, we explored the activity of different SERD– or SSH–CDK4/6 inhibitor combinations in models of endocrine therapy–resistant ESR1+ breast cancer. Experimental Design: SERDs, SSHs, and the CDK4/6 inhibitor palbociclib were evaluated as single agents or in combination in established cellular and animal models of endocrine therapy–resistant ESR1+ breast cancer. Results: The combination of palbociclib with a SERD or an SSH was shown to effectively inhibit the growth of MCF7 cell or ESR1-mutant patient-derived tumor xenografts. In tamoxifen-resistant MCF7 xenografts, the palbociclib/SERD or SSH combination resulted in an increased duration of response as compared with either drug alone. Conclusions: A SERD– or SSH–palbociclib combination has therapeutic potential in breast tumors resistant to endocrine therapies or those expressing ESR1 mutations. Clin Cancer Res; 21(22); 5121–30. ©2015 AACR. See related commentary by DeMichele and Chodosh, p. 4999

Lysine Auxotrophy Combined with Deletion of the SecA2 Gene Results in a Safe and Highly Immunogenic Candidate Live Attenuated Vaccine for Tuberculosis

Tuberculosis (TB) caused by Mycobacterium tuberculosis remains a major global health problem, despite the widespread use of the M. bovis Bacille Calmette-Guerin (BCG) vaccine and the availability of drug therapies. In recent years, the high incidence of coinfection of M. tuberculosis and HIV, as well as escalating problems associated with drug resistance, has raised ominous concerns with regard to TB control. Vaccination with BCG has not proven highly effective in controlling TB, and also has been associated with increasing concerns about the potential for the vaccine to cause disseminated mycobacterial infection in HIV infected hosts. Thus, the development of an efficacious and safe TB vaccine is generally viewed as a critical to achieving control of the ongoing global TB pandemic. In the current study, we have analyzed the vaccine efficacy of an attenuated M. tuberculosis strain that combines a mutation that enhances T cell priming (ΔsecA2) with a strongly attenuating lysine auxotrophy mutation (ΔlysA). The ΔsecA2 mutant was previously shown to be defective in the inhibition of apoptosis and markedly increased priming of antigen-specific CD8+ T cells in vivo. Similarly, the ΔsecA2ΔlysA strain retained enhanced apoptosis and augmented CD8+ T cell stimulatory effects, but with a noticeably improved safety profile in immunosuppressed mice. Thus, the M. tuberculosis ΔsecA2ΔlysA mutant represents a live attenuated TB vaccine strain with the potential to deliver increased protection and safety compared to standard BCG vaccination.

Evaluation of the pharmacological activities of RAD1901, a selective estrogen receptor degrader

Endocrine therapy, using tamoxifen or an aromatase inhibitor, remains a first-line treatment for estrogen receptor 1 (ESR1) positive breast cancer. However, tumor resistance limits the duration of response. The clinical efficacy of fulvestrant, a selective ER degrader (SERD) that triggers receptor degradation, has confirmed that ESR1 often remains engaged in endocrine therapy resistant cancers. Recently developed, selective ER modulators (SERMs)/SERD hybrids (SSHs) that facilitate ESR1 degradation in breast cancer cells and reproductive tissues have been advanced as an alternative treatment for advanced breast cancer, particularly in the metastatic setting. RAD1901 is one SSH currently being evaluated clinically that is unique among ESR1 modulators in that it readily enters the brain, a common site of breast cancer metastasis. In this study, RAD1901 inhibited estrogen activation of ESR1 in vitro and in vivo, inhibited estrogen-dependent breast cancer cell proliferation and xenograft tumor growth, and mediated dose-dependent downregulation of ESR1 protein. However, doses of RAD1901 insufficient to induce ESR1 degradation were shown to result in the activation of ESR1 target genes and in the stimulation of xenograft tumor growth. RAD1901 is an SSH that exhibits complex pharmacology in breast cancer models, having dose-dependent agonist/antagonist activity displayed in a tissue-selective manner. It remains unclear how this unique pharmacology will impact the utility of RAD1901 for breast cancer treatment. However, being the only SERD currently known to access the brain, RAD1901 merits evaluation as a targeted therapy for the treatment of breast cancer brain metastases.

PIK3CA mutations enable targeting of a breast tumor dependency through mTOR-mediated MCL-1 translation

Inhibitors of BCL-XL, combined with inhibition of the mTOR/4E-BP axis, drive regressions of PIK3CA mutant breast tumors. Sneak attack on breast cancer’s defense The usual goal of cancer treatment is to kill malignant cells, not just slow down their growth. A class of drugs called BH3 mimetics serves this purpose by inhibiting antiapoptotic proteins and thus helping drive the cells toward apoptosis (programmed cell death). MCL-1 is an antiapoptotic protein that is not targeted by currently bioavailable BH3 mimetics, and it is often responsible for resistance to these drugs. Anderson et al. have discovered that breast cancers with the commonly observed PIK3CA mutations can be treated with mTOR inhibitors to suppress MCL-1, leaving the cells vulnerable to BH3 mimetics and subsequent induction of apoptosis, both directly and in combination with chemotherapy. Therapies that efficiently induce apoptosis are likely to be required for durable clinical responses in patients with solid tumors. Using a pharmacological screening approach, we discovered that combined inhibition of B cell lymphoma–extra large (BCL-XL) and the mammalian target of rapamycin (mTOR)/4E-BP axis results in selective and synergistic induction of apoptosis in cellular and animal models of PIK3CA mutant breast cancers, including triple-negative tumors. Mechanistically, inhibition of mTOR/4E-BP suppresses myeloid cell leukemia–1 (MCL-1) protein translation only in PIK3CA mutant tumors, creating a synthetic dependence on BCL-XL. This dual dependence on BCL-XL and MCL-1, but not on BCL-2, appears to be a fundamental property of diverse breast cancer cell lines, xenografts, and patient-derived tumors that is independent of the molecular subtype or PIK3CA mutational status. Furthermore, this dependence distinguishes breast cancers from normal breast epithelial cells, which are neither primed for apoptosis nor dependent on BCL-XL/MCL-1, suggesting a potential therapeutic window. By tilting the balance of pro- to antiapoptotic signals in the mitochondria, dual inhibition of MCL-1 and BCL-XL also sensitizes breast cancer cells to standard-of-care cytotoxic and targeted chemotherapies. Together, these results suggest that patients with PIK3CA mutant breast cancers may benefit from combined treatment with inhibitors of BCL-XL and the mTOR/4E-BP axis, whereas alternative methods of inhibiting MCL-1 and BCL-XL may be effective in tumors lacking PIK3CA mutations.

CDK4/6 Therapeutic Intervention and Viable Alternative to Taxanes in CRPC

Resistance to second-generation androgen receptor (AR) antagonists and CYP17 inhibitors in patients with castration-resistant prostate cancer (CRPC) develops rapidly through reactivation of the androgen signaling axis and has been attributed to AR overexpression, production of constitutively active AR splice variants, or the selection for AR mutants with altered ligand-binding specificity. It has been established that androgens induce cell-cycle progression, in part, through upregulation of cyclin D1 (CCND1) expression and subsequent activation of cyclin-dependent kinases 4 and 6 (CDK4/6). Thus, the efficacy of the newly described CDK4/6 inhibitors (G1T28 and G1T38), docetaxel and enzalutamide, was evaluated as single agents in clinically relevant in vitro and in vivo models of hormone-sensitive and treatment-resistant prostate cancer. CDK4/6 inhibition (CDK4/6i) was as effective as docetaxel in animal models of treatment-resistant CRPC but exhibited significantly less toxicity. The in vivo effects were durable and importantly were observed in prostate cancer cells expressing wild-type AR, AR mutants, and those that have lost AR expression. CDK4/6i was also effective in prostate tumor models expressing the AR-V7 variant or the AR F876L mutation, both of which are associated with treatment resistance. Furthermore, CDK4/6i was effective in prostate cancer models where AR expression was lost. It is concluded that CDK4/6 inhibitors are a viable alternative to taxanes as therapeutic interventions in endocrine therapy–refractory CRPC. Implications: The preclinical efficacy of CDK4/6 monotherapy observed here suggests the need for near-term clinical studies of these agents in advanced prostate cancer. Mol Cancer Res; 15(6); 660–9. ©2017 AACR.

Androgen receptor antagonism drives cytochrome P450 17A1 inhibitor efficacy in prostate cancer

The clinical utility of inhibiting cytochrome P450 17A1 (CYP17), a cytochrome p450 enzyme that is required for the production of androgens, has been exemplified by the approval of abiraterone for the treatment of castration-resistant prostate cancer (CRPC). Recently, however, it has been reported that CYP17 inhibitors can interact directly with the androgen receptor (AR). A phase I study recently reported that seviteronel, a CYP17 lyase–selective inhibitor, ædemonstrated a sustained reduction in prostate-specific antigen in a patient with CRPC, and another study showed seviteronel’s direct effects on AR function. This suggested that seviteronel may have therapeutically relevant activities in addition to its ability to inhibit androgen production. Here, we have demonstrated that CYP17 inhibitors, with the exception of orteronel, can function as competitive AR antagonists. Conformational profiling revealed that the CYP17 inhibitor–bound AR adopted a conformation that resembled the unliganded AR (apo-AR), precluding nuclear localization and DNA binding. Further, we observed that seviteronel and abiraterone inhibited the growth of tumor xenografts expressing the clinically relevant mutation AR-F876L and that this activity could be attributed entirely to competitive AR antagonism. The results of this study suggest that the ability of CYP17 inhibitors to directly antagonize the AR may contribute to their clinical efficacy in CRPC.

Inhibiting androgen receptor nuclear entry in castration-resistant prostate cancer

Clinical resistance to the second-generation antiandrogen enzalutamide in castration resistant prostate cancer (CRPC), despite persistent androgen receptor (AR) activity in tumors, highlights the unmet medical need for next generation antagonists. We have identified and characterized tetra-aryl cyclobutanes (CBs) as a new class of competitive AR antagonists that exhibit a unique mechanism of action. These CBs are structurally distinct from current antiandrogens (hydroxyflutamide, bicalutamide, and enzalutamide), and inhibit AR-mediated gene expression, cell proliferation, and tumor growth in several models of CRPC. Conformational profiling revealed that CBs stabilize an AR conformation resembling an unliganded receptor. Using a variety of techniques, it was determined that the AR:CB complex was not recruited to AR-regulated promoters and, like apo AR, remains sequestered in the cytoplasm bound to heat shock proteins. Thus, we have identified third generation AR antagonists whose unique mechanism of action suggests that they may have therapeutic potential in CRPC.

Validation of histone deacetylase 3 as a therapeutic target in castration-resistant prostate cancer

Whereas the androgen receptor (AR) signaling axis remains a therapeutic target in castration‐resistant prostate cancer (CRPC), the emergence of AR mutations and splice variants as mechanisms underlying resistance to contemporary inhibitors of this pathway highlights the need for new therapeutic approaches to target this disease. Of significance in this regard is the considerable preclinical data, indicating that histone deacetylase (HDAC) inhibitors may have utility in the treatment of CRPC. However, the results of clinical studies using HDAC inhibitors (directed against HDAC1, 2, 3, and 8) in CRPC are equivocal, a result that some have attributed to their ability to induce an epithelial to mesenchymal transition (EMT) and neuroendocrine differentiation. We posited that it might be possible to uncouple the beneficial effects of HDAC inhibitors on AR signaling from their undesired activities by targeting specific HDACs as opposed to using the pan‐inhibitor strategy that has been employed to date.

Abstract P3-14-04: Effects of the dual selective CYP17 lyase inhibitor and androgen receptor (AR) antagonist, VT-464, on AR+ and ER+ tumor models in vitro and in vivo

VT-464 is a lyase-selective inhibitor of the dual-activity CYP17A1 enzyme that is required for the synthesis of androgens and estrogens in the gonads, adrenals, and tumors. In addition to its role as a CYP17A1 lyase inhibitor, we previously showed that VT-464 also functions as a direct and effective AR antagonist in prostate cancer models. In our current study, we evaluated the therapeutic potential of VT-464 in breast cancer by analyzing its effectiveness in several AR-positive breast cancer cell lines, including those that are ER-positive and ER-negative. Through in vitro assays and xenograft analysis, we compared the activity of VT-464 to enzalutamide, a second generation AR antagonist that is approved for the treatment of castration resistant prostate cancer and is in multiple Phase 2 breast cancer studies. Our results showed that VT-464 was highly effective in preventing proliferation of both ER-positive and ER-negative breast cancer cell lines in vitro. Importantly, significant inhibition was also observed in soft agar assays that assesses anchorage-independent growth. In mechanistic studies, VT-464 and enzalutamide both inhibited induction of AR target genes and recruitment of AR to target promoters, verifying direct AR antagonistic activity observed previously in prostate cancer cells. Furthermore, we evaluated the ability of VT-464 and enzalutamide to inhibit tumor formation in a tamoxifen-resistant model of breast cancer. Oral administration of either enzalutamide or VT-464 significantly decreased tumor growth in mice, with VT-464 achieving greater growth inhibition than enzalutamide. Together, these data provide further rationale for the future study of the AR as a viable therapeutic target in breast cancer, and importantly, suggest that AR inhibition can impact tamoxifen-resistant tumor growth. The dual effects of CYP17A1 lyase inhibition and AR antagonism that are achieved with VT-464 further supports its development as an effective oral therapy option for AR-positive breast cancer. A phase 1 / 2 clinical study of oral VT-464 in women with AR+ triple-negative breast cancer or ER+ cancer resistant to aromatase inhibitors will commence in 2015. Citation Format: Ellison SJ, Norris JD, Wardell S, Eisner JR, Hoekstra WJ, Stagg DB, Alley HM, Moore WR, McDonnell DP. Effects of the dual selective CYP17 lyase inhibitor and androgen receptor (AR) antagonist, VT-464, on AR+ and ER+ tumor models in vitro and in vivo. [abstract]. In: Proceedings of the Thirty-Eighth Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2015 Dec 8-12; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2016;76(4 Suppl):Abstract nr P3-14-04.