Metastatic breast cancer (mBC) is a leading cause of cancer death in women. Most breast cancer patients are administered estrogen-receptor-targeted endocrine therapies to treat or prevent progressive metastatic disease. Development of endocrine resistance through acquisition of mutations in the estrogen receptor gene, ESR1, that constitutively activate the estrogen receptor leads to relapse. Complete antagonism of both WT and mutant ESR1 (mutESR1) with an oral therapeutic that persistently antagonizes ER-driven oncogenic transcriptional activities is a requirement for efficacy. Here, we describe our discovery of the investigational drug OP-1250 (palazestrant). OP-1250 is a potent complete estrogen receptor antagonist (CERAN) and selective estrogen receptor degrader (SERD) that is active in both WT and mutESR1 breast cancer tumors. OP-1250's effective induction of tumor regression either as a single agent or in combination with a CDK4/6 inhibitor has led to the rapid advancement of this compound into a Phase 3 clinical trial (OPERA-01).

Addition of OP-1250 to CDK4/6 inhibitors enhances tumor suppression and prolongs animal survival in ER+ breast cancer xenograft models. A, B, Percent change in tumor volume of individual animals in listed treatment groups at day 29 of the MCF7 xenograft study, normalized to day 1 tumor volume. C, D, Percent change in tumor volume of individual animals in listed treatment groups at day 29 of the ST941 xenograft study, normalized to day 1 tumor volume. E, F, Kaplan–Meier graphs of animal survival over time in listed treatment groups of mice implanted with the ESR1Y537S ST941 PDX model. Dosing ended on day 29 of the study, and animals were sacrificed when tumor volume reached 2000 mm3. G, H, Mean and SEM tumor volume over the full study interval with monotherapy and combination treatments in the ESR1Y537S ST941 PDX breast cancer model, with the dotted line representing tumor stasis. Dosing ended on day 29, as indicated by the dashed line.

OP-1250 acts as a selective ER degrader (SERD) in wild-type and ESR1 mutant cell lines. A, B, Representative blot images from the simple western assay shown in Figure 3 (see main article) after 24-h treatment with 300 nM compound or 1 nM E2 with indicated compound treatments and cell lines. C, D, Representative blot images from simple western assay of MCF7 cells treated for 2 h, 4 h, or 8 h with the listed concentrations of compounds in the presence or absence of MG-132 and simple western analysis of ERα protein levels after treatment with listed compounds. Samples were normalized to untreated control, with mean and SD across at least three independent experiments shown. E, F, ERα degradation in CAMA-1 cells with the indicated homozygous mutant as assessed by simple western assay after 24-h treatment with 300 nM compound or 1 nM E2. Samples were normalized to untreated control, represented by a dotted line, with mean and SD across at least three independent experiments shown. G, H, Representative blot images from simple western data shown in panels E and F with indicated compound treatments and cell lines.

Effects of E2 and antiestrogens on transcriptional activity and cellular proliferation in E2-depleted media. A, Volcano plot of genes differentially expressed by 15-min E2 treatment relative to vehicle in PRO-seq experiment. Each dot represents a gene in the annotation set. Adjusted p-value is plotted on the y-axis and fold-change relative to time-matched vehicle on the x-axis. The number in the top right-hand corner represents the number of genes changed, and red dots indicate genes with observed change in expression. B, PRO-seq read distribution at estrogen target genes CCND1 and MYC after 15 min of E2 treatment (red) compared to vehicle control (blue). The x-axis refers to the position along the genome, and the y-axis refers to the number of reads aligning to that position normalized for the depth of each sample. Replicates were averaged. Bars that are less than zero are reverse-strand reads; bars above zero are forward-strand reads. C, PRO-seq read distribution at estrogen target gene GREB1 after 6 h of OP-1250 treatment following E2 pretreatment (red) compared to 24-h E2 treatment (blue). The x-axis refers to the position along the genome, and the y-axis refers to the number of reads aligning to that position normalized for the depth of each sample. Replicates were averaged. Bars that are less than zero are reverse-strand reads; bars above zero are forward-strand reads. D, Heatmap of annotated G2/M checkpoint genes in CAMA-1 and MCF7 cells after 24 h of compound treatment in PRO-seq experiment. Red indicates high expression and blue low expression. Samples and genes with similar expression patterns are grouped together using hierarchical clustering, with sample relationships depicted by dendrograms at the top and left of graph. E, Chart showing number of differentially expressed genes compared to vehicle for each treatment group in CAMA-1 mRNA-seq experiment. Cells were treated with 100 pM E2 or 316 nM antiestrogen in E2-depleted media for 24 h.

OP-1250 specifically inhibits the ER without cross-reactivity with other hormone receptors. A, LanthaScreen co-activator recruitment assay using wild-type ERα LBD and fluorescein tagged PGC1a comparing OP-1250 and other approved and in-trial comparator molecules. Data are normalized to DMSO vehicle and 10 µM E2-treated wells (100% recruitment) and presented as mean and SD of at least two separate experiments. B, Reporter gene activity assay in CAMA-1 and MCF7 cells transiently transfected with a luciferase reporter gene construct containing an estrogen response element and minimal tk promoter. Cells were cultured in estrogen-depleted media and reporter gene activity was normalized to E2 vehicle. The dotted line represents activity in the absence of estrogen, the hashed line represents activity with E2 supplementation. Data shown represent mean and SD from a representative of at least three independent experiments. C, Reporter gene assay of ER-negative SK-BR-3 cells transiently transfected with wild-type or mutant ESR1 vectors along with a luciferase reporter gene cloned to 3× estrogen response element and TATA minimal promoter. Transcriptional activity was measured as luciferase activity. Cells were cultured in estrogen-depleted media and reporter gene activity was normalized to wild-type E2 vehicle and empty vector + E2. The dotted line represents activity with empty vector control, the hashed line represents activity following supplementation with 31.6 nM E2. Data shown represent mean and SD from a representative of at least three independent experiments. D, Reporter gene activity assay of HeLa cells transiently transfected with glucocorticoid, androgen, or progesterone receptor and a luciferase reporter cloned to 3× PRE and TATA minimal promoter, measuring the impact of OP-1250 on the glucocorticoid receptor in the presence and absence of 31.6 nM dexamethasone, progesterone receptor in the presence and absence of 500 nM MPA (medroxyprogesterone acetate), and androgen receptor in the presence and absence of 50 nM R1881. Data are normalized to the indicated amount of agonist (either MPA, dexamethasone, or R1881). Figures show data as the mean and SD from a representative of at least three independent experiments. E, Reporter gene assay of ER-negative SK-BR-3 cells transiently transfected with ESR1 or ESR2 along with a luciferase reporter gene cloned to 3× estrogen response element and TATA minimal promoter. Transcriptional activity was measured as luciferase activity. Cells were cultured in estrogen-depleted media and reporter gene activity was normalized to E2 vehicle. Data shown represent mean and SD from a representative of at least three independent experiments. F, Uterine wet weight assay following treatment with a dose response of OP-1250 or fulvestrant in the presence of E2 supplementation. Mean and SEM from six mice per treatment group are plotted with asterisks indicating significance relative to vehicle by one-way analysis of variance. * represents adjusted p-value <0.05, *** represents adjusted p-value <0.001, **** represents adjusted p-value <0.0001. G, Simple western analysis of ERα protein levels in Ishikawa cells after 72-h treatment with listed compounds. Samples are normalized to untreated control. Data shown represent mean and SD from a representative of at least three independent experiments. H, Representative blot image from simple western assay shown in panel G after 72-h treatment with 300 nM compound or 1 nM E2 with indicated compound treatments and cell line. AR=androgen receptor; GR=glucocorticoid receptor; PR= progesterone receptor; WT=wild-type.

OP-1250 demonstrates potent activity in an ER+ breast cancer intracranial xenograft and outperforms ribociclib. A, Schematic of intracranial xenograft experiment with data shown in B–D. B, Mean and SEM intracranial tumor volume over time with listed treatments as assessed by MRI in an ESR1Y537S ST941 PDX intracranial xenograft model. C, Percent change in intracranial tumor volume of individual animals in listed treatment groups at day 101 or at the last recorded timepoint in the ST941 intracranial xenograft study, normalized to day 1 tumor volume. D, Kaplan–Meier graphs of animal survival over time in listed treatment groups of mice implanted intracranially with the ST941 PDX xenograft. Dosing ended on day 100 of the study in the OP-1250 10 mg/kg group, as indicated by the shaded area. E, Representative intracranial MRIs from ST941-implanted mice with data shown in Figure 6 (see main article) at indicated timepoints and treatment groups. PO=orally.

RNASeq dataset of gene pathway changes following treatment with OP-1250, palbociclib and ribociclib in an MCF7 xenograft model.

Supplementary text including supplementary materials and methods and references.

PROSeq analysis of gene expression changes following treatment of MCF7 and CAMA-1 cells with ER-alpha ligands.

Abstract The estrogen receptor (ER) is a well-established target for the treatment of breast cancer, with the majority of patients presenting as ER-positive (ER+). Endocrine therapy is a mainstay of breast cancer treatment but the development of resistance mutations in response to aromatase inhibitors, poor pharmacokinetic properties of fulvestrant, agonist activity of tamoxifen, and limited benefit for elacestrant leave unmet needs for patients with or without resistance mutations in ESR1, the gene that encodes the ER protein. Here we describe palazestrant (OP-1250), a novel, orally bioavailable complete ER antagonist and selective ER degrader. OP-1250, like fulvestrant, has no agonist activity on the ER and completely blocks estrogen-induced transcriptional activity. In addition, OP-1250 demonstrates favorable biochemical binding affinity, ER degradation, and antiproliferative activity in ER+ breast cancer models that is comparable or superior to other agents of interest. OP-1250 has superior pharmacokinetic properties relative to fulvestrant, including oral bioavailability and brain penetrance, as well as superior performance in wild-type and ESR1-mutant breast cancer xenograft studies. OP-1250 combines well with cyclin-dependent kinase 4 and 6 inhibitors in xenograft studies of ER+ breast cancer models and effectively shrinks intracranially implanted tumors, resulting in prolonged animal survival. With demonstrated preclinical efficacy exceeding fulvestrant in wild-type models, elacestrant in ESR1-mutant models, and tamoxifen in intracranial xenografts, OP-1250 has the potential to benefit patients with ER+ breast cancer.

Abstract OP-1250, a complete estrogen receptor antagonist (CERAN) and selective estrogen receptor degrader (SERD), is currently in Phase I/II clinical trials for the treatment of estrogen receptor positive (ER+)/human epidermal growth factor receptor 2 negative (HER2-) breast cancer. OP-1250 has previously demonstrated efficacy in both estrogen receptor 1 (ESR1) wild-type and Y537S mutant breast cancer xenograft models. Inhibitors of cyclin-dependent kinases 4 and 6 (CDK4/6) are a standard first-line treatment for ER+ advanced or metastatic breast cancer in combination with endocrine therapy. To characterize OP-1250 efficacy in this setting, we explored the combination of OP-1250 with CDK4/6 inhibitors palbociclib and ribociclib in ER+/HER2- breast cancer preclinical models, and profiled the transcriptional changes associated with single agent and combination therapy. OP-1250 and CDK4/6 inhibitors were tested alone and in combination in ESR1 wild-type MCF7 and ESR1 Y537S mutant ST941 xenograft models. Higher doses of OP-1250 and either CDK4/6 inhibitor, when given as a monotherapy, suppress tumor growth and increase animal survival, while lower doses of the compounds display a modest or minimal effect on tumor growth or survival. In combination, lower doses of OP-1250 and either CDK4/6 inhibitor result in significantly greater tumor growth suppression (both xenograft models), tumor regression (MCF7) and animal survival (ST491) than each compound individually. RNA sequencing revealed substantially more gene expression changes with combination treatment than monotherapy. In particular, pathways associated with cell cycle progression displayed greater changes with combination treatment, similar to what was observed with a much higher dose of monotherapy. This study supports the clinical evaluation of OP-1250 in combination with CDK4/6 inhibitors (palbociclib or ribociclib) and provides additional context to mechanisms of combination efficacy. OP-1250 is currently being evaluated in a Phase Ib clinical trial in combination with palbociclib and a study including evaluation of the combination of OP-1250 with ribociclib is planned to initiate in Q3 2022. Citation Format: Alison D. Parisian, Gopinath S. Palanisamy, Fabian Ortega, Judevin L. Sapugay, William J. Bodell, David Kulp, Peter Kushner, Cyrus Harmon. Combination of complete estrogen receptor antagonist, OP-1250, and CDK4/6 inhibitors enhances tumor suppression and inhibition of cell cycle-related gene expression [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr P2-24-07.

The human epidermal growth factor receptor 2 (HER2, also known as ERBB2) oncogene is overexpressed in approximately 25% of breast cancer tumors and is associated with a high rate of brain metastasis. Patients with brain metastases have a poor prognosis and limited treatment options available. Since about half of HER2+ tumors are also hormone receptor positive, and cross talk has been shown to occur between these two signaling pathways, it is plausible that these tumors could benefit from the addition of hormonal therapy on top of HER2 inhibition. OP-1250 is a complete estrogen receptor antagonist (CERAN) and selective estrogen receptor degrader (SERD), and has been shown to effectively inhibit estrogen receptor positive (ER+) breast cancer growth in both cell-based assays and xenograft studies. OP-1250 has favorable pharmacokinetic properties and demonstrated brain penetrance in non-clinical models. While extensively characterized in ER+/HER2- breast cancer models, OP-1250 had not previously been tested in a HER2+ context. Here we report OP-1250 inhibited estrogen receptor-driven proliferation and effectively degraded the estrogen receptor in multiple ER+/HER2+ cell lines. OP-1250 combined effectively with HER2 inhibitors tucatinib and trastuzumab, demonstrating that inhibition of both HER2 and estrogen receptor signaling was more effective in inhibiting breast cancer cell proliferation than inhibiting either pathway alone. The addition of OP-1250 to a HER2 inhibitor also resulted in greater tumor shrinkage in multiple ER+/HER2+ xenograft models. Our data suggests a potential clinical benefit in the addition of OP-1250 on top of HER2 inhibition for the treatment of ER+/HER2+ breast cancer and provides strong rationale for a clinical study evaluating the combination of OP-1250 and HER2 targeted agents, which is planned for 2022. Citation Format: Alison D Parisian, Leslie Hodges-Gallagher, Richard Sun, Susanna Barratt, Gopinath S Palanisamy, Julia Lawrence, Pamela Klein, David C Myles, Cyrus L Harmon, Peter J Kushner. The complete estrogen receptor antagonist OP-1250 can combine with HER2 inhibition to inhibit estrogen receptor-driven cellular proliferation and shrink xenograft tumors in ER+/HER2+ breast cancer models [abstract]. In: Proceedings of the 2021 San Antonio Breast Cancer Symposium; 2021 Dec 7-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2022;82(4 Suppl):Abstract nr P5-08-07.

OP-1250 is an orally-bioavailable complete estrogen receptor antagonist (CERAN) previously shown to shrink wild-type and Y537S mutant estrogen receptor-positive (ER+) tumors in multiple preclinical xenograft models. To better understand the effects of OP-1250 treatment on ER+ breast cancer cells, we used precision run-on sequencing (PRO-seq) to profile the direct transcriptional effects of OP-1250 in two commonly used breast cancer cell lines, MCF7 and CAMA-1. PRO-seq detects nascent RNA, and thus can detect changes at shorter timescales than traditional RNA-seq methods. We conducted PRO-seq 15 minutes, 1 hour, 6 hours and 24 hours after administration of OP-1250, estradiol, fulvestrant and tamoxifen to compare the effects of these compounds across the two cell lines. OP-1250 was administered in the absence of estradiol and after estradiol pretreatment to generate a detailed profile of the time course of its activity. We identified genes that display early transcriptional changes in response to the compounds tested, many of which have been previously identified as estrogen response genes. At later time points we observed transcriptional changes consistent with modulation of pathways involved in cell cycle progression. We show that OP-1250 treatment reverses the estradiol-induced transcriptional changes associated with estrogen receptor activation and chronicle the time course of these changes. Finally, we show greater similarity in the transcriptional changes induced by CERANs OP-1250 and fulvestrant compared to tamoxifen, as well as cell line-specific differences in estrogen response and response to anti-estrogen treatment. These data provide additional insights into the mechanisms of OP-1250 action on ER+ cell lines and patient tumors. OP-1250 is currently in a Phase I/II clinical trial for the treatment of ER+ breast cancer. Citation Format: Alison D. Parisian, Caitlin Miller, Fabian Ortega, Leslie Hodges-Gallagher, Susanna Barratt, Joey Azofeifa, Peter J. Kushner, David Kulp, Cyrus L. Harmon. Precision run-on sequencing (PRO-seq) analysis of a treatment time course in ER+ breast cancer cell lines reveals the transcriptional changes underlying response to complete estrogen receptor antagonist (CERAN) OP-1250 [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 5375.

Background: ER+, HER2- metastatic breast cancer (MBC) patients will receive sequential, endocrine based therapy, either as monotherapy or in combination with a targeted agent until endocrine resistance develops. OP-1250 is a small molecule Complete Estrogen Receptor Antagonist (CERAN) that completely inactivates ER by blocking the 2 activation functions of ER transcription (AF-1 & AF-2). While complete antagonism is believed to be the most critical mechanism of action, OP-1250 is also a strong degrader of the ER. OP-1250 demonstrates anti-cancer activity in preclinical models, including activity against brain metastases and in tumors with activating mutations in ESR1. OP-1250 is orally bioavailable with favorable pharmacokinetics (PK) yielding high and steady drug levels in multiple species. The characteristics of OP-1250 including PK, ability to cross the blood brain barrier, and complete antagonism of the ER suggest that OP-1250 may have superior efficacy over standard of care and experimental agents, both as a monotherapy and in combination with other targeted therapies. Methods: OP-1250-001 is a phase I/II first-in-human study to determine the Dose Limiting Toxicity (DLT), Maximum Tolerated Dose (MTD) and/or Recommended Phase II Dose (RP2D) and to characterize the safety, PK profile and preliminary efficacy in subjects with ER+, HER2- MBC. Eligible patients received prior endocrine therapy. OP-1250 was given orally, once a day continuously in 28-day cycles. Using a rolling 6 design, cohorts of 3 - 6 subjects were sequentially enrolled and monitored for DLTs. The study allows for expansion of 1 or 2 doses for further evaluation of safety and PK to best inform selection of the RP2D. The phase II portion will evaluate the preliminary activity of OP-1250 in 3 cohorts: 1) measurable disease; and 2 exploratory cohorts: 2) evaluable & non-measurable; and 3) central nervous system metastasis. Plasma is being collected for ctDNA sequencing to evaluate ESR1 and other relevant mutations. Results: Between August 5, 2020 and June 4, 2021, 28 subjects with a median age of 63 (range 37-82) have been enrolled. Of the 27 subjects for whom detailed data on prior therapy was reported, all subjects received > 1 prior endocrine therapy for MBC (74% received > 2 and 37% received > 3; range 1-6). All of the 27 subjects received a CDK4/6 inhibitor and 23 of 27 (85%) had received fulvestrant. 20 subjects of 28 (71%) subjects had visceral disease affecting the liver, lung, peritoneum, and/or pleura. OP-1250 was escalated through 5 dose cohorts (range 30 - 300 mg qd). 26 patients were evaluable for a DLT. As of the data cut-off date, no DLTs occurred. Most of the TEAE were grade 1 or 2. OP-1250 is orally bioavailable and extensively distributed with an effective half-life of approximately 2-3 days. Cmax and AUC show dose proportional increase. By cycle 2, exposure levels are at steady state with limited fluctuations and at all doses significantly higher than those seen with fulvestrant. Importantly, OP-1250 has now demonstrated clinical activity at doses that were well tolerated and within the predicted exposure windows where maximum efficacy was observed in preclinical models. Detailed response and safety data will be presented. Conclusion: OP-1250 is continuing evaluation in a phase I/II study. The population enrolled reflects the evolving standard of care in that all pts received prior CDK4/6 inhibitors and the majority also received fulvestrant. OP-1250 is well tolerated with a favorable PK across dose levels ensuring target coverage throughout the dosing interval. No MTD has been identified and selection of the RP2D will be based on consideration of long-term tolerability, efficacy, and the ability to combine with targeted agents. (NCT04505826) Citation Format: Manish Patel, Carlos Alemany, Zahi Mitri, Della Makower, Virginia Borges, Joseph Sparano, Trinh Le, Pamela Klein, Julia Lawrence, Peter Kushner, Demiana Faltaos, Cyrus Harmon, David Myles, JoAnne Zujewski, Erika Hamilton. Preliminary data from a phase I/II, multicenter, dose escalation study of OP-1250, an oral CERAN/SERD, in subjects with advanced and/or metastatic estrogen receptor (ER)-positive, HER2-negative breast cancer [abstract]. In: Proceedings of the 2021 San Antonio Breast Cancer Symposium; 2021 Dec 7-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2022;82(4 Suppl):Abstract nr P1-17-12.

Estrogen receptor positive (ER+) breast cancers represent about 70-75% of all breast cancer. In general, these cancers are effectively treated with adjuvant endocrine therapies, with or without the addition of CDK4/6 inhibitors. However, many patients develop resistance and eventually progress to metastatic disease (MBC). Acquired mutations in ESR1, which encodes estrogen receptor alpha (ERα), contribute to 20-40% of endocrine therapy resistant MBC. In patients, breast cancer metastasis occurs in the liver, lungs, brain and bone. OP-1250 is a novel compound that is a complete ERα antagonist (CERAN). Previous studies have shown that OP-1250 effectively blocks both the AF-1 and AF-2 activation functions of ERα. In the current study, we used a mutant ERα MBC endocrine therapy resistant xenograft model to examine the efficacy of OP-1250 in a metastatic context. MCF7 cells engineered to express one of the most common and aggressive ERα mutations, Y537S, were labeled with luciferase and injected via the nipple (mammary intraductal MIND model) into NSG mice. Mice were treated with 3 and 10 mg/kg of OP-1250 alone or in combination with palbociclib at 70mg/kg and tumor growth was monitored via a Xenogen IVIS imager. At study end point, mice were sacrificed and excised organs were imaged ex-vivo and further processed for immunohistology analysis. OP-1250 at both doses inhibited primary tumor growth as well as metastasis to the lung, liver, brain and bone, with greater effect at 10 mg/kg, and was significantly more effective than Faslodex. When combined with palbociclib at 70 mg/kg, an additional significant decrease in metastasis was observed, compared to OP-1250 or palbociclib alone. Our results also suggest that the efficacy seen in the combination study was driven primarily by OP-1250. In total, these results demonstrate the potential use of OP-1250, both alone and in combination with a CDK4/6 inhibitor, to inhibit tumor growth and metastasis in a model of aggressive mutant ER+ MBC. Citation Format: Muriel Laine, Marianne E. Greene, Tiffany Leng, Sophia Li, Gopinath S. Palanisamy, Cyrus L. Harmon, Leslie Hodges-Gallagher, Peter J. Kushner, Geoffrey L. Greene. OP-1250 prevents tumor spread in a model of metastatic mutant ERα+ breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2022; 2022 Apr 8-13. Philadelphia (PA): AACR; Cancer Res 2022;82(12_Suppl):Abstract nr 1618.

Antiestrogens are widely used to treat ER+, HER2- breast cancer however these may produce estrogen-like agonist effects in a cell and gene-specific manner. Furthermore, this partial agonism has been implicated in the development of tumor resistance. In contrast, a complete ER antagonist lacks agonist activity in all tissues. Given the high risk of recurrence in ER+ patients treated with endocrine therapies, we hypothesize that drugs that completely antagonize ER-mediated signaling will have a clinical advantage in treating resistant tumors, particularly in tumors for which the ER has acquired mutations conferring estrogen-independent growth. We previously identified OP-1250 in our laboratory as a complete antagonist and degrader of wild type and mutant ERalpha in breast and uterus cells. Here we explore the pharmacokinetic properties of orally administered OP-1250 and its ability to shrink breast tumors from xenograft models expressing both wild type and mutant estrogen receptors. Orally administered OP-1250 produced high and stable levels in multiple species, suggesting that once daily dosing may be sufficient to produce excellent drug exposure in patients. OP-1250 shrank ER+ tumors in multiple patient-derived tumor xenografts in mice at daily doses of 3 mg/kg, including in tumors expressing the Y537S allele of ERalpha that confers estrogen-independent growth and tumor resistance. Furthermore, OP-1250 accumulated within tumor specimens, indicating excellent target penetration was achieved with low daily doses. These preclinical studies indicate that OP-1250 has the potential to be a best-in-class antiestrogen due to its ability to complete antagonize ER, potent efficacy in shrinking tumors, and robust drug exposure. A Phase 1/2 dose escalation and expansion study in patients with ER+, HER2- metastatic or advanced BC previously treated with an endocrine therapy will be initiated this year. Citation Format: Leslie Hodges-Gallagher, Richard Sun, David C Myles, Pamela M Klein, Jo Anne Zujewski, Cyrus L Harmon, Peter J Kushner. The complete estrogen receptor antagonist OP-1250 shrinks tumors in xenograft models and has favorable preclinical pharmacokinetic attributes [abstract]. In: Proceedings of the 2020 San Antonio Breast Cancer Virtual Symposium; 2020 Dec 8-11; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2021;81(4 Suppl):Abstract nr PS18-16.