Novel Pyrrolopyridone Bromodomain and Extra-Terminal Motif (BET) Inhibitors Effective in Endocrine-Resistant ER+ Breast Cancer with Acquired Resistance to Fulvestrant and Palbociclib

Acquired resistance to fulvestrant and palbociclib is a new challenge to treatment of estrogen receptor positive (ER+) breast cancer. ER is expressed in most resistance settings; thus, bromodomain and extra-terminal protein inhibitors (BETi) that target BET-amplified ER-mediated transcription have therapeutic potential. Novel pyrrolopyridone BETi leveraged novel interactions with L92/L94 confirmed by a cocrystal structure of 27 with BRD4. Optimization of BETi using growth inhibition in fulvestrant-resistant (MCF-7:CFR) cells was confirmed in endocrine-resistant, palbociclib-resistant, and ESR1 mutant cell lines. 27 was more potent in MCF-7:CFR cells than six BET inhibitors in clinical trials. Transcriptomic analysis differentiated 27 from the benchmark BETi, JQ-1, showing downregulation of oncogenes and upregulation of tumor suppressors and apoptosis. The therapeutic approach was validated by oral administration of 27 in orthotopic xenografts of endocrine-resistant breast cancer in monotherapy and in combination with fulvestrant. Importantly, at an equivalent dose in rats, thrombocytopenia was mitigated.

G1T48, an oral selective estrogen receptor degrader, and the CDK4/6 inhibitor lerociclib inhibit tumor growth in animal models of endocrine-resistant breast cancer

Purpose

The combination of targeting the CDK4/6 and estrogen receptor (ER) signaling pathways with palbociclib and fulvestrant is a proven therapeutic strategy for the treatment of ER-positive breast cancer. However, the poor physicochemical properties of fulvestrant require monthly intramuscular injections to patients, which limit the pharmacokinetic and pharmacodynamic activity of the compound. Therefore, an orally available compound that more rapidly reaches steady state may lead to a better clinical response in patients. Here, we report the identification of G1T48, a novel orally bioavailable, non-steroidal small molecule antagonist of ER.

Methods

The pharmacological effects and the antineoplastic mechanism of action of G1T48 on tumors was evaluated using human breast cancer cells (in vitro) and xenograft efficacy models (in vivo).

Results

G1T48 is a potent and efficacious inhibitor of estrogen-mediated transcription and proliferation in ER-positive breast cancer cells, similar to the pure antiestrogen fulvestrant. In addition, G1T48 can effectively suppress ER activity in multiple models of endocrine therapy resistance including those harboring ER mutations and growth factor activation. In vivo, G1T48 has robust antitumor activity in a model of estrogen-dependent breast cancer (MCF7) and significantly inhibited the growth of tamoxifen-resistant (TamR), long-term estrogen-deprived (LTED) and patient-derived xenograft tumors with an increased response being observed with the combination of G1T48 and the CDK4/6 inhibitor lerociclib.

Conclusions

These data show that G1T48 has the potential to be an efficacious oral antineoplastic agent in ER-positive breast cancer.

Electronic supplementary material

The online version of this article (10.1007/s10549-020-05575-9) contains supplementary material, which is available to authorized users.

Design and Synthesis of Basic Selective Estrogen Receptor Degraders for Endocrine Therapy Resistant Breast Cancer

The clinical steroidal selective estrogen receptor (ER) degrader (SERD), fulvestrant, is effective in metastatic breast cancer, but limited by poor pharmacokinetics, prompting the development of orally bioavailable, nonsteroidal SERDs, currently in clinical trials. These trials address local breast cancer as well as peripheral metastases, but patients with brain metastases are generally excluded because of the lack of blood-brain barrier penetration. A novel family of benzothiophene SERDs with a basic amino side arm (B-SERDs) was synthesized. Proteasomal degradation of ERα was induced by B-SERDs that achieved the objectives of oral and brain bioavailability, while maintaining high affinity binding to ERα and both potency and efficacy comparable to fulvestrant in cell lines resistant to endocrine therapy or bearing ESR1 mutations. A novel 3-oxyazetidine side chain was designed, leading to 37d, a B-SERD that caused endocrine-resistant ER+ tumors to regress in a mouse orthotopic xenograft model.

Abstract 3738: Three-dimensional treatment-resistant breast cancer spheroids as a predictive model of in vivo response to endocrine therapy

1 out of every 8 U.S. women will develop invasive breast cancer during her lifetime, making it the second most common form of cancer affecting women. Breast cancer is also a leading cause of cancer related deaths for women in the U.S., second only to lung cancer. Activation of estrogen receptor alpha (ERα) is the primary proliferative mechanism of breast cancer cells, making it a logical target for therapy. ER ligands with antiestrogenic activity, such as the selective estrogen receptor modulator (SERM), tamoxifen, and selective estrogen receptor degrader (SERD), fulvestrant, have proven clinically successful as treatments for breast cancer; however, resistance in up to 50% of patients provides a therapeutic challenge. Once resistant, breast cancer cells become endocrine-independent, because of this, there is an urgent need for both novel therapy and improved models of resistant breast cancer. Our lab has created a panel of various endocrine-independent cell lines to mimic SERM and SERD resistance. Along with traditional 2D cell culturing, 3D spheroids have also been utilized to gain a better understanding of resistance. Importantly, the response to therapeutic agents, of cell lines in 2D versus 3D cultures is not identical. We observe that 3D cultures better replicate observations in mouse xenograft models, demonstrating that elements of the spheroid microenvironment, such as cell-cell interactions and the presence of extracellular matrix (EM), mimic aspects of the tumor microenvironment in vivo. Cells cultured as spheroids are therefore a suitable in vitro model for drug discovery, predictive of response in preclinical animal models, in contrast to 2D monolayer cell cultures.

Citation Format: Carlo I. Rosales, Jiong Zhao, Lauren M. Gutgesell, Rui Xiong, Debra A. Tonetti, Gregory R. Thatcher. Three-dimensional treatment-resistant breast cancer spheroids as a predictive model of in vivo response to endocrine therapy [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 3738.

Abstract 3744: Multiple endocrine-resistant breast cancer cell lines retain ER and sensitivity to endocrine therapy

Endocrine therapy is the standard of care for breast cancer expressing estrogen receptor (ER),

which occurs in 70% of patients. Unfortunately, acquired or de novo resistance to endocrine

therapy is observed in up to 50% of patients, leaving a significant portion of patients with

insufficient treatment options. Endocrine-resistance, usually defined as resistance to tamoxifen

and aromatase inhibitor (AI) therapy, can also include resistance to selective estrogen receptor

degraders (SERDs), since these also target ER. Since multiple mechanisms contribute to

resistance, development of multiple resistant cell lines is needed for drug discovery and to identify

characteristics that may suggest susceptibility to alternative and combination therapies. We have

developed 5 stable, endocrine-resistant cell lines from a parent MCF-7 cell line, which all retain

ER: one of these cell lines, MCF-7:CFR is resistant to the SERD fulvestrant. Clinical metastatic

breast cancers that have gained endocrine resistance are overwhelmingly ER+. In addition to ER,

progesterone receptor (PR) and glucocorticoid receptor (GR) status of these lines was assessed,

and correlated with the response of these cells in culture to four classes of endocrine therapeutics:

SERDs, selective ER modulators (SERMs), and selective human ER partial agonists (ShERPAs).

Growth of all 5 cell lines was endocrine independent, indicating resistance to AI therapy. Two of

the ER+, PR- cell lines were most resistant to the spectrum of endocrine therapies, but these cell

lines both showed sensitivity to ShERPAs, especially in combination with non-endocrine targeted

therapies, such as the PI3K inhibitor, alpelisib. Paradoxically, all endocrine-resistant cell lines

responded to at least one of the endocrine therapies tested, demonstrating that if ER is not lost

in the metastatic state, it remains a vulnerability suitable for therapeutic targeting.

Citation Format: Lauren M. Gutgesell, Rui Xiong, Jiong Zhao, Huiping Zhao, Debra A. Tonetti, Gregory R. Thatcher. Multiple endocrine-resistant breast cancer cell lines retain ER and sensitivity to endocrine therapy [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 3744.

Abstract 3603: Optimizing combination therapy against antiestrogen-resistance in estrogen receptor positive breast cancer

Approximately 75% of breast cancer incidences are Estrogen Receptor positive (ER+). Treatment of ER+ breast cancer with antiestrogen endocrine therapy targets the proliferative mechanisms of ER using selective ER modulators (SERMs), such as tamoxifen, and selective ER downregulators (SERDs), such as fulvestrant. The resistance to endocrine therapy, either innate or acquired, is clinically diagnosed in up to 50% of patients within 5 years of treatment. Antiestrogen therapy of ER+ mammary cancer cells in vitro and in xenografts does not cause complete cell death or regression. Through the use of combination therapy to target the remaining sub-population of cells, complete regression can be predicted. Our lab has developed endocrine tamoxifen-resistant models through prolonged exposure to tamoxifen, estrogen deprivation, and overexpression of PKCα, a pro-survival pathway component, in addition to a fulvestrant-resistant model through prolonged exposure to fulvestrant. Investigation of the resistance mechanisms will provide insight into potential, combinatorial targets for overcoming endocrine resistance in ER+ breast cancer. Use of spheroidal 3D cell culture provides a physiologically relevant model with sufficient throughput for drug discovery. Establishing spheroids and subsequent treatment predicts the efficacy of combination treatment on tumor regression. Targeting pathways associated with increased prevalence of resistance, such as CDK 4/6, can be employed for clinical therapy. Using both novel in-house and clinical SERMs and SERDs, we have used these multiple cell lines to discover combinatorial targets using both mechanistic and unbiased screening approaches. The use of combination endocrine therapy in tamoxifen-resistant models has shown antiproliferative synergy, and is able to enhance tumor regression in xenografts. By optimizing combinatorial endocrine treatment against both fulvestrant- and tamoxifen-resistance, novel therapeutic approaches are being developed for ER+ breast cancer.

Citation Format: Lauren M. Gutgesell, Rui Xiong, Jiong Zhao, Debra A. Tonetti, Gregory R. Thatcher. Optimizing combination therapy against antiestrogen-resistance in estrogen receptor positive breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 3603. doi:10.1158/1538-7445.AM2017-3603

Abstract P3-04-23: Combination therapy of targeted anticancer pathways and estrogen receptor ligands and their responses in de novo andtamoxifen resistant cell models

Approximately 75% of breast cancers are classified as Estrogen Receptor positive (ER+). Tamoxifen, a selective estrogen receptor modulator (SERM), is the standard of care for many of these ER+ breast cancer patients. Unfortunately, tamoxifen resistance occurs in almost 50% of patients within 5 years of treatment, and endocrine-independence accompanying resistance also negates the effects of aromatase inhibitors. Combination therapy is increasingly used in non-cytotoxic therapeutic approaches in many types of cancer. The potential exists that endocrine resistance can be lessened, eliminated, or overcome through targeted therapy in combination with endocrine therapy. Inhibition of kinase signaling (e.g. via CDK4/6 or PI3K) and other pathways (e.g. HSP90) are expected to be effective in combination with endocrine therapy. We have discovered a variety of ER ligands with potential as endocrine therapeutic agents, based upon a single chemical scaffold with a diverse set of pharmacological responses: including SERMs, selective ER downregulators (SERDs), selective estrogen mimics (SEMs), and selective human ER partial agonists (ShERPAs). To predict which agents in combination with these endocrine-targeted ligands would be of potential therapeutic benefit it was necessary to develop 3D spheroidal cultures of ER+ breast cancer cell lines: including endocrine-dependent lines; and cell lines made endocrine-independent either by extended exposure to tamoxifen or extended deprivation of estradiol. In contrast to 2D cultures, drug response in 3D spheroidal cell cultures was predictive of response to treatment in mouse xenograft studies. Growth of endocrine-dependent cell lines was, as expected, inhibited by SERDs; and endocrine-independent, tamoxifen-resistant cell lines were also sensitive to SERD treatment, although one cell line was largely resistant. Growth of all three tamoxifen-resistant cell lines was inhibited by SEMs/ShERPAs. Importantly, regardless of the type of endocrine therapeutic agent studied, concentrations leading to saturation of the target (ER) did not cause cell death. Equally, all endocrine therapies studied benefited from combination treatment with other agents, leading to enhanced cell death.

Citation Format: Gutgesell LM, Xiong R, Thatcher GRJ, Tonetti DA. Combination therapy of targeted anticancer pathways and estrogen receptor ligands and their responses in de novo andtamoxifen resistant cell models [abstract]. In: Proceedings of the 2016 San Antonio Breast Cancer Symposium; 2016 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2017;77(4 Suppl):Abstract nr P3-04-23.

Novel Selective Estrogen Receptor Downregulators (SERDs) Developed against Treatment-Resistant Breast Cancer

Resistance to the selective estrogen receptor modulator tamoxifen and to aromatase inhibitors that lower circulating estradiol occurs in up to 50% of patients, generally leading to an endocrine-independent ER+ phenotype. Selective ER downregulators (SERDs) are able to ablate ER and thus, theoretically, to prevent survival of both endocrine-dependent and -independent ER+ tumors. The clinical SERD fulvestrant is hampered by intramuscular administration and undesirable pharmacokinetics. Novel SERDs were designed using the 6-OH-benzothiophene (BT) scaffold common to arzoxifene and raloxifene. Treatment-resistant (TR) ER+ cell lines (MCF-7:5C and MCF-7:TAM1) were used for optimization, followed by validation in the parent endocrine-dependent cell line (MCF-7:WS8), in 2D and 3D cultures, using ERα in-cell westerns, ERE-luciferase, and cell viability assays, with 2 (GDC-0810/ARN-810) used for comparison. Two BT SERDs with superior in vitro activity to 2 were studied for bioavailability and shown to cause regression of a TR, endocrine-independent ER+ xenograft superior to that with 2.

Abstract 3485: Estrogen receptor ligands and their responses in de novo and tamoxifen resistant cell models

Approximately 75% of breast cancer incidences are Estrogen Receptor positive (ER+). Tamoxifen, a selective estrogen receptor modulator (SERM), is the standard of care for many of these ER+ breast cancer patients. Unfortunately, tamoxifen resistance occurs in almost 50% of patients within 5 years of treatment, and endocrine-independence accompanying resistance negates the effects of aromatase inhibitors. Paradoxically, estradiol (E2) has shown clinical efficacy in patients with resistant breast cancer. Understanding the antiproliferative role of E2 and ER signaling in resistant ER+ cell lines is essential to gain a better understanding of paradoxical clinical efficacy and for the appropriate biomarker-assisted selection of endocrine therapy for various stages of ER+ breast cancer. We have discovered ER ligands, based upon a single chemical scaffold with a diverse set of pharmacological responses, which can be used to better understand the role of ER signaling in resistance and therapy: selective ER modulators (SERMs), selective ER downregulators (SERDs), selective estrogen mimics (SEMs), and selective human ER partial agonists (ShERPAs). These compounds were initially classified using an ERE luciferase reporter assay and affinity for ER confirmed by biochemical assays. The effects of these novel ER-directed chemical probes on cell viability were further examined in multiple 2D and 3D spheroid models of tamoxifen resistance. Finally, ERα localization upon administration of these ligands in tamoxifen-sensitive and tamoxifen-resistant cells was studied. While SERDs and SEMs showed growth inhibition in tamoxifen resistant cell lines, both molecules had different responses and mechanisms of growth inhibition. ERα was localized to extranuclear sites upon administration of E2, SEMs, and ShERPAs, an observation specific to the resistant phenotype and mechanistically associated with spheroid disintegration. SERDs inhibited the antiproliferative actions of E2, but were antiproliferative in resistant cell lines. Further dissection of the role of ER in resistance and survival is needed to define the appropriate ER-directed, endocrine therapy in ER+ breast cancer.

Citation Format: Lauren M. Gutgesell, Gregory R. J. Thatcher, Hitisha Patel, Rui Xiong. Estrogen receptor ligands and their responses in de novo and tamoxifen resistant cell models. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 3485.

Selective Human Estrogen Receptor Partial Agonists (ShERPAs) for Tamoxifen-Resistant Breast Cancer

Almost 70% of breast cancers are estrogen receptor α (ERα) positive. Tamoxifen, a selective estrogen receptor modulator (SERM), represents the standard of care for many patients; however, 30-50% develop resistance, underlining the need for alternative therapeutics. Paradoxically, agonists at ERα such as estradiol (E2) have demonstrated clinical efficacy in patients with heavily treated breast cancer, although side effects in gynecological tissues are unacceptable. A drug that selectively mimics the actions of E2 in breast cancer therapy but minimizes estrogenic effects in other tissues is a novel, therapeutic alternative. We hypothesized that a selective human estrogen receptor partial agonist (ShERPA) at ERα would provide such an agent. Novel benzothiophene derivatives with nanomolar potency in breast cancer cell cultures were designed. Several showed partial agonist activity, with potency of 0.8-76 nM, mimicking E2 in inhibiting growth of tamoxifen-resistant breast cancer cell lines. Three ShERPAs were tested and validated in xenograft models of endocrine-independent and tamoxifen-resistant breast cancer, and in contrast to E2, ShERPAs did not cause significant uterine growth.