LeonaBio's announcement of conference presentations on lasofoxifene represents a scientific visibility event, not an efficacy signal — the inputs contain zero efficacy, safety, or mechanistic data on the drug itself. The asset enters a biomarker-defined space where regulatory approval is achievable but HTA-recognized value has eluded every predecessor: elacestrant (Orserdu, Stemline Therapeutics), the only approved oral SERD in ER+/HER2-/ESR1-mutated metastatic breast cancer post-CDK4/6 inhibitor, achieved marketing authorization in September 2023 on the basis of a median PFS of 3.78 months versus 1.87 months (HR 0.55, 95% CI 0.39–0.77, p=0.0005) in ESR1-mutated patients in the EMERALD Phase 3 RCT, yet received ASMR V (no improvement) and SMR LOW from French HTA, primarily because the absolute PFS gain of 1.91 months was questioned for clinical meaningfulness and no OS benefit was demonstrated. [1] Capivasertib plus fulvestrant (CAPItello-291 Phase 3 RCT), a mechanistically distinct AKT inhibitor targeting a different pathway, also received ASMR V despite a 4.2-month PFS benefit in its altered-biomarker subgroup, owing to absent OS data and grade ≥3 adverse events of 39.7% versus 14.9%. [2] ELAINE-3's combination strategy with abemaciclib introduces three compounding uncertainties absent from both precedents: the comparator arm is undisclosed, preventing any assessment of trial design adequacy; abemaciclib is being re-challenged in patients who already progressed on a CDK4/6 inhibitor, for which the inputs explicitly note 'little data'; and without lasofoxifene monotherapy data, benefit attribution between the investigational ER-targeting agent and the CDK4/6 re-challenge component will be unanswerable at readout. [3] No precedent in the inputs combines ESR1-mutation targeting with a novel ER modulator in combination with a CDK4/6 inhibitor post-CDK4/6 inhibitor progression — the PPDD analysis explicitly confirms no closely comparable precedent exists at this mechanistic intersection. Elacestrant provides the nearest population-level fit but as monotherapy; MONARCH-2 (abemaciclib plus fulvestrant, Phase 3 RCT, HR 0.553 PFS, HR 0.757 OS) validates the abemaciclib backbone but enrolled a CDK4/6-inhibitor-naive broader HR+ population, rendering it inapplicable to the re-challenge question. [4][5] The sharpest risk: enrollment completes Q4 2026 with topline data only in H2 2027, leaving 12–18 months of zero efficacy visibility in a space where elacestrant's first-mover advantage is compounding and the target population is estimated at only 1,083–4,720 patients per year. [6]
ELAINE-3 is an ongoing Phase 3 trial with no interim, Phase 2, or safety data reported. The announcement is a scientific conference presence notice, not an efficacy readout; topline data are not expected until H2 2027.
| Indication | ER-positive (ER+), HER2-negative, ESR1-mutated Metastatic Breast Cancer |
| Drug | lasofoxifene |
| Company | LeonaBio, Inc. |
| Trial Phase | Phase 3 |
| Trial Acronym | ELAINE-3 |
| Category | Clinical Trial Event |
| Sub Category | Patient Enrollment Milestone |
| Therapeutic Area | Oncology |
| Conference Name | DAVA Oncology’s 4th Summit on Breast Cancer |
| Conference Dates | August 18-22, 2026 |
| Conference Location | Kona, Hawaii |
| Combination Partner | abemaciclib |
| Enrollment Completion Target | 4Q 2026 |
| Topline Data Expectation | 2H 2027 |
| Presenter 1 | Jessica Tao, M.D. |
| Presenter 2 | David Portman, M.D. |
LeonaBio to Showcase Lasofoxifene Data at Breast Cancer Summit
LeonaBio, Inc. announced it will feature its lead product candidate, lasofoxifene, in multiple presentations at DAVA Oncology’s 4th Summit on Breast Cancer from August 18-22, 2026. The presentations will focus on the clinical and scientific evidence supporting lasofoxifene as a potential treatment for ER-positive, HER2-negative, ESR1-mutated metastatic breast cancer. This includes insights from the ongoing Phase 3 ELAINE-3 clinical trial, where lasofoxifene is being evaluated in combination with abemaciclib. LeonaBio is on track to complete enrollment for ELAINE-3 in Q4 2026, with topline data anticipated in H2 2027, aiming to address significant unmet needs in this patient population.
- LeonaBio will showcase lasofoxifene at DAVA Oncology’s 4th Summit on Breast Cancer, with Dr. Jessica Tao presenting on the Phase 3 ELAINE-3 trial in combination with abemaciclib for ER+, HER2-, ESR1-mutated metastatic breast cancer. Additionally, Dr. David Portman will lead an industry lunch presentation discussing the development and potential of lasofoxifene.
- The company is making significant progress with its pivotal Phase 3 ELAINE-3 clinical trial, targeting the completion of patient enrollment by the fourth quarter of 2026. Topline data from this crucial trial is expected in the second half of 2027, which could potentially establish lasofoxifene in combination with CDK4/6 inhibition as a new standard of care.
- LeonaBio emphasizes the potential of lasofoxifene to redefine the treatment paradigm for patients with metastatic breast cancer, particularly those who develop resistance to existing therapies. The drug's differentiated mechanism of action and promising clinical profile are believed to offer a new therapeutic option for patients facing limited treatment choices.
Addressing Unmet Needs in ESR1-Mutated Metastatic Breast Cancer
ESR1 mutations represent a clinically significant mechanism of acquired resistance that emerges predominantly under the selective pressure of aromatase inhibitor therapy, rendering standard endocrine strategies substantially less effective. The management of ER+/HER2− metastatic breast cancer harboring these mutations is complicated by overlapping biological and clinical factors that collectively limit the durability and efficacy of available treatment options.
Ligand-independent ER activation driving endocrine resistance: ESR1 mutations — most notably D538G and Y537S — induce constitutive, ligand-independent transcriptional activity of the estrogen receptor by causing conformational changes in the ligand-binding domain that mimic the active, ligand-bound state. These mutations disrupt residue contacts critical for stabilizing the apo helix-12 conformation, enabling receptor activation without ligand engagement and altering the binding dynamics of agents such as tamoxifen.
Markedly poor outcomes following CDK4/6 inhibitor progression: Once secondary resistance develops after CDK4/6 inhibitor-based therapy, standard monotherapy with aromatase inhibitors or fulvestrant yields limited benefit. Data from the EMERALD trial control arm — in which all patients had prior CDK4/6 inhibitor exposure — demonstrated a median progression-free survival of only 1.9 months on endocrine therapy alone.
Intrinsic limitations of intramuscular fulvestrant and co-occurring pathway mutations: Recognized pharmacological and delivery limitations of intramuscular fulvestrant have driven development of novel oral selective estrogen receptor degraders (SERDs). Compounding this, co-occurring ESR1 and MAP kinase pathway mutations are associated with significantly inferior overall survival (p = 0.0092), and multiple co-occurring PIK3CA mutations correlate with shortened progression-free survival on fulvestrant (p = 0.0036).
Tumor heterogeneity and subclonal diversification: Advanced breast cancer exhibits marked geographic and temporal heterogeneity, with diverse subclonal resistance mutations that vary in clonal dominance both across genes and within ESR1 and PIK3CA hotspot mutations. This limits the reliability of single-timepoint tumor tissue biopsy sequencing for capturing the full mutational landscape.
Lack of established treatment sequencing frameworks: The optimal sequencing of therapeutic agents following progression on CDK4/6 inhibitor-based regimens remains undefined, introducing significant uncertainty into clinical decision-making for this patient population.
ELAINE-3: Designing a Pivotal Trial for ER+, HER2-, ESR1-Mutated MBC
The clinical landscape for ER+/HER2−, ESR1-mutated metastatic breast cancer (MBC) has been shaped by a series of phase I–II trials evaluating next-generation endocrine agents—both as monotherapies and in combination with CDK4/6 inhibitors. The trials summarized below span selective estrogen receptor degraders (SERDs), oral SERMs, and rechallenge strategies, each contributing distinct design innovations and endpoint data to inform pivotal trial development.
| Trial | Phase | Design | Key Patient Population | Primary Endpoint(s) | Key Efficacy Results |
|---|---|---|---|---|---|
| ELAINE 2 (NCT04432454) | II | Open-label; lasofoxifene 5 mg/day + abemaciclib 150 mg BID until progression or toxicity | ESR1-mutated, ER+/HER2− MBC; n=29; prior CDK4/6 inhibitor in 28/29 patients | Safety/tolerability | Median PFS: 56.0 weeks (95% CI 31.9–NE); CBR at 24 weeks: 65.5%; ORR (measurable disease): 55.6% |
| BioPER | II | Multicenter, open-label; palbociclib rechallenge (75–125 mg, 3 weeks on/1 week off) + endocrine therapy of physician's choice | HR+/HER2− advanced BC with prior clinical benefit on palbociclib + ET; n=33 | CBR and rate of baseline Rb protein loss | CBR: 34.4% (95% CI 18.6–53.2); Rb loss: 13.0%; Median PFS: 2.6 months |
| SERENA-2 (NCT04214288) | II | Open-label, randomized (1:1:1:1); camizestrant 75 mg, 150 mg, or 300 mg QD vs. fulvestrant 500 mg IM; stratified by prior CDK4/6i and liver/lung metastases | Post-menopausal women, ER+/HER2− MBC, ≥1 prior ET line, ≤1 prior ET in advanced setting; n=240 | Investigator-assessed PFS per RECIST v1.1 (ITT) | Median PFS: 7.2 months (camizestrant 75 mg) vs. 3.7 months (fulvestrant); HR 0.59 (90% CI 0.42–0.82; p=0.017) |
| SERENA-1 (NCT03616587) | I | Multi-part, open-label; camizestrant monotherapy (25–450 mg QD escalation; 75, 150, 300 mg expansion) | ER+/HER2− advanced/metastatic BC, refractory or intolerant to prior therapy; n=108 | Safety, tolerability, PK | 86.1% experienced TRAEs (82.4% grade 1–2); efficacy observed across all doses including post-CDK4/6i and post-fulvestrant, with and without ESR1 mutations |
| GO39932 (NCT03332797) | Ia/b | Open-label; giredestrant single-agent (10–250 mg) or giredestrant 100 mg ± palbociclib 125 mg ± LHRH agonist | ER+/HER2− locally advanced/MBC with prior ET; n=175 | Safety | No DLT observed; MTD not reached; CBR 48.6% (monotherapy) and 81.3% (+ palbociclib ± LHRH agonist), including ESR1-mutated tumors |
ELAINE 3: A Pivotal Test for ESR1-Mutated Breast Cancer
The landscape of ER-positive, HER2-negative metastatic breast cancer is continually evolving, yet a significant challenge persists: acquired resistance to endocrine therapies, often driven by mutations in the ESR1 gene. These ESR1 mutations render tumors less responsive to standard treatments, leaving patients with limited options after progression on therapies including CDK4/6 inhibitors. This is where lasofoxifene, a novel selective estrogen receptor modulator (SERM), emerges as a promising candidate.
LeonaBio's upcoming presentations underscore the growing body of evidence supporting lasofoxifene's potential. Preclinical studies have shown its ability to inhibit tumor growth and metastasis in models with common ESR1 mutations, even outperforming fulvestrant in some settings. More critically, the Phase 2 ELAINE 2 trial demonstrated compelling clinical activity for lasofoxifene in combination with abemaciclib in heavily pre-treated patients with ESR1-mutated mBC, achieving a median progression-free survival of approximately 13 months and a clinical benefit rate of 65.5%. These results are particularly encouraging given the patient population's prior exposure to CDK4/6 inhibitors.
However, the path forward is not without its considerations:
While promising in combination, the ELAINE 1 trial, which compared lasofoxifene monotherapy to fulvestrant monotherapy, did not show a statistically significant progression-free survival benefit, suggesting that combination strategies may be key to maximizing its impact.
Emerging preclinical data indicate that complex, dual ESR1 mutations could potentially reduce lasofoxifene's efficacy, highlighting a need for ongoing vigilance regarding resistance mechanisms.
The ongoing Phase 3 ELAINE 3 trial, comparing lasofoxifene plus abemaciclib against fulvestrant plus abemaciclib, sets a high bar. Its success will hinge on demonstrating a statistically significant improvement in progression-free survival over an established combination therapy.
With enrollment for ELAINE 3 on track for completion in Q4 2026 and topline data anticipated in H2 2027, the coming years will be pivotal. If ELAINE 3 confirms the promising signals from ELAINE 2, lasofoxifene could redefine the treatment paradigm for patients with ESR1-mutated, endocrine-resistant metastatic breast cancer, offering a much-needed oral, targeted therapy in a challenging disease setting.
Frequently Asked Questions
References
- [1] Nagaraj G, Ma CX. Clinical Challenges in the Management of Hormone Receptor-Positive, Human Epidermal Growth Factor Receptor 2-Negative Metastatic Breast Cancer: A Literature Review. Advances in therapy. 2021 Jan. 33190190
- [2] Guttery DS, Page K et al.. Noninvasive detection of activating estrogen receptor 1 (ESR1) mutations in estrogen receptor-positive metastatic breast cancer. Clinical chemistry. 2015 Jul. 25979954
- [3] Kingston B, Cutts RJ et al.. Genomic profile of advanced breast cancer in circulating tumour DNA. Nature communications. 2021 Apr 23. 33893289
- [4] Chen SH, Tse KP et al.. Comprehensive genomic profiling and therapeutic implications for Taiwanese patients with treatment-naïve breast cancer. Cancer medicine. 2024 Jun. 38895905
- [5] Qureshi Z, Jamil A et al.. Elacestrant in the treatment landscape of ER-positive, HER2-negative, ESR1-mutated advanced breast cancer: a contemporary narrative review. Annals of medicine and surgery (2012). 2024 Aug. 39118705
- [6] Turner NC, Slamon DJ et al.. Overall Survival with Palbociclib and Fulvestrant in Advanced Breast Cancer. The New England journal of medicine. 2018 Nov 15. 30345905
- [7] Venetis K, Pepe F et al.. ESR1 mutations in HR+/HER2-metastatic breast cancer: Enhancing the accuracy of ctDNA testing. Cancer treatment reviews. 2023 Dec. 37864956
- [8] Li W, Feng C et al.. A comparative analysis of mutational profiles between triple-negative breast cancer and non-triple-negative breast cancer. Discover oncology. 2026 Jan 12. 41526581
- [9] Oliveira M, Pominchuk D et al.. Camizestrant, a next-generation oral SERD, versus fulvestrant in post-menopausal women with oestrogen receptor-positive, HER2-negative advanced breast cancer (SERENA-2): a multi-dose, open-label, randomised, phase 2 trial. The Lancet. Oncology. 2024 Nov. 39481395
- [10] Maloberti T, Poppi L et al.. ESR1 analysis of liquid biopsy in breast cancer, one-year routine experience of an Italian clinical referral center. The journal of liquid biopsy. 2025 Dec. 41142848
- [11] Neill NE, Mauro LA et al.. Novel Estrogen Receptor - Targeted Therapies in Hormone-Receptor Positive Breast Cancer. Current treatment options in oncology. 2025 Apr. 40163189
- [12] Mayne CG, Toy W et al.. Defining the Energetic Basis for a Conformational Switch Mediating Ligand-Independent Activation of Mutant Estrogen Receptors in Breast Cancer. Molecular cancer research : MCR. 2021 Sep. 34021071
- [13] Hamilton E, Oliveira M et al.. A phase I dose escalation and expansion trial of the next-generation oral SERD camizestrant in women with ER-positive, HER2-negative advanced breast cancer: SERENA-1 monotherapy results. Annals of oncology : official journal of the European Society for Medical Oncology. 2024 Aug. 38729567
- [14] Raphael A, Salmon-Divon M et al.. Alpelisib Efficacy in Hormone Receptor-Positive HER2-Negative PIK3CA-Mutant Advanced Breast Cancer Post-Everolimus Treatment. Genes. 2022 Sep 29. 36292649
- [15] McDougal DP, Pederick JL et al.. A ternary switch model governing ERα ligand binding domain conformation. Nature communications. 2025 Nov 24. 41285747
- [16] Liu Y, Su J et al.. Long-term efficacy of CDK4/6 inhibitors in early HR+, HER2- high-risk breast cancer: An updated systematic review and meta-analysis. Frontiers in pharmacology. 2025. 40717978
- [17] Jhaveri KL, Bellet M et al.. Phase Ia/b Study of Giredestrant ± Palbociclib and ± Luteinizing Hormone-Releasing Hormone Agonists in Estrogen Receptor-Positive, HER2-Negative, Locally Advanced/Metastatic Breast Cancer. Clinical cancer research : an official journal of the American Association for Cancer Research. 2024 Feb 16. 37921755
- [18] Kirmani N, De León-Fernández N et al.. Comparative efficacy and safety of novel oral selective estrogen receptor degraders in ER+/HER2- advanced breast cancer: An updated systematic review and meta-analysis of randomized controlled trials. Cancer treatment reviews. 2026 Feb. 41539087
- [19] Damodaran S, O'Sullivan CC et al.. Open-label, phase II, multicenter study of lasofoxifene plus abemaciclib for treating women with metastatic ER+/HER2- breast cancer and an ESR1 mutation after disease progression on prior therapies: ELAINE 2. Annals of oncology : official journal of the European Society for Medical Oncology. 2023 Dec. 38072513
- [20] Chainitikun S, Long JP et al.. The efficacy of first-line chemotherapy in endocrine-resistant hormone receptor-positive (HR+), human epidermal growth factor receptor 2-negative (HER2-) metastatic breast cancer. Breast cancer research and treatment. 2020 Oct. 32720114
Contact Us
Address
One Research Ct, Suite 450
Rockville, MD 20850
For General Inquiry
info@pienomial.com
