Hormone receptor-positive (HR+), HER2-negative (HER2−) represents about 70% of all metastatic breast cancer cases, and patients with this type of breast cancer often exhibit resistance to endocrine therapy. 

Researchers have been hard at work developing strategies to overcome such resistance by means of precision interventions. Let’s take a closer look.

 

Resistance pathways

 

Often noted following treatment with aromatase inhibitors, according to Harvard researchers publishing in Clinical Cancer Research, a major mechanism of resistance is mutation of the ERα gene or ESR1.

Tolaney SM, Toi M, Neven P, et al. Clinical Significance of PIK3CA and ESR1 Mutations in Circulating Tumor DNA: Analysis from the MONARCH 2 Study of Abemaciclib plus Fulvestrant. Clin Cancer Res. 2022 Apr 14;28(8):1500-1506. doi: 10.1158/1078-0432.CCR-21-3276.

The majority of somatic ESR1 mutations occur at the level of the ESR1 ligand-binding domain within D538 or Y537. Whether fulvestrant is effective against such mutations remains to be elucidated. 

 

PI3K pathway dysregulation in metastatic breast cancer progression and endocrine resistance also plays a major role in endocrine resistance present in breast cancer progression.

“Most mutations occur at E542K or E545K of exon 9 (helical domain of p110α), and H1047R/L of exon 20 (activation loop in the kinase domain), representing ‘hotspot’ mutations,” write the Clinical Cancer Research authors. “Prior clinical studies have demonstrated HR+ breast cancers with mutations in either exon 9 or exon 20 exhibited decreased response to ET. To address this clinically, several targeted therapies have been developed to inhibit the PI3K pathway. Efficacy of the PI3Kα inhibitor alpelisib in an endocrine-resistant population with HR+, HER2− ABC [advanced breast cancer] was demonstrated in the phase III SOLAR-1 study, demonstrating improved median PFS in combination with fulvestrant in patients with PIK3CA-mutant tumors, although no statistical benefit in OS was observed.”

 

Targeting PI3K/AKT/PTEN

 

PIK3CA mutations occur in about 40% of patients with HR+, HER2− breast cancer. According to the results of a phase 3 study (n=572) published in the NEJM, investigators compared alpelisib plus fulvestrant with placebo plus fulvestrant in patients with HR+, HER2− advanced breast cancer who had received prior endocrine therapy.

Andre F, Ciruelos E, Rubovszky G, et al. Alpelisib for PIK3CA-Mutated, Hormone Receptor–Positive Advanced Breast Cancer. N Engl J Med. 2019; 380:1929-1940. DOI: 10.1056/NEJMoa1813904.

The PFS was 11.0 months for the alpelisib-fulvestrant group vs 5.7 months in the placebo-fulvestrant group.

 

“These results show improvements in patients’ outcomes with the addition of an α-specific PI3K inhibitor to standard treatment for PIK3CA-mutated, HR-positive, HER2-negative advanced breast cancer, findings that validate PIK3CA as an important treatment target in this population,” the investigators wrote. “Patients with PIK3CA-mutated, HR-positive, HER2-negative advanced breast cancer that had progressed during or after the receipt of endocrine therapy had significantly longer progression-free survival when they received alpelisib-fulvestrant than when they received placebo-fulvestrant,” they continued, noting an estimated 35% lower risk of progression or death.

“Progression-free survival was similar in the placebo groups in the two cohorts defined according to PIK3CA mutation status,” they concluded.

As detailed in another study published by NEJM, CAPItello-291 (n=708) is a phase 3, randomized, double-blind study that assessed the AKT inhibitor capivasertib in combination with fulvestrant vs placebo and fulvestrant in patients who had hormone receptor-positive metastatic breast cancer pretreated with an aromatase inhibitor followed by disease progression.

Turner NC, Oliveira M, Howell SJ, et al. Capivasertib in Hormone Receptor–Positive Advanced Breast Cancer. N Engl J Med 2023; 388:2058-2070. DOI: 10.1056/NEJMoa2214131.

The median PFS was 7.2 months in the capivasertib–fulvestrant group vs 3.6 months in the placebo-fulvestrant group. 

 

“Capivasertib–fulvestrant therapy resulted in significantly longer progression-free survival than treatment with fulvestrant alone among patients with hormone receptor-positive advanced breast cancer whose disease had progressed during or after previous aromatase inhibitor therapy with or without a CDK4/6 inhibitor,” the authors concluded.

Capivasertib (Truqap) is a potent inhibitor of all 3 AKT isoforms (AKT1, AKT2, AKT3), which is an effector of the PI3K/AKT/PTEN pathway that is downstream of PI3K and PTEN.

Howell SJ, Casbard A, Carucci M, et al. Fulvestrant plus capivasertib versus placebo after relapse or progression on an aromatase inhibitor in metastatic, oestrogen receptor-positive, HER2-negative breast cancer (FAKTION): overall survival, updated progression-free survival, and expanded biomarker analysis from a randomised, phase 2 trial. Lancet Oncol. 2022 Jul;23(7):851-864. DOI: 10.1016/S1470-2045(22)00284-4.

 

 

 

Paclitaxel/bevacizumab

 

ESR1 mutations are linked to increased mortality. Dutch researchers assessed the effects of ESR1 mutations detected by circulating tumor DNA (ctDNA) for impact on outcome to taxane-based chemotherapy using paclitaxel/bevacizumab in advanced breast cancer patients, publishing their findings in Breast Cancer Research and Treatment.

Bos MK, Lam SW, Motta G, et al. Plasma ESR1 mutations and outcome to first-line paclitaxel and bevacizumab in patients with advanced ER-positive/HER2-negative breast cancer. Breast Cancer Res Treat. 2023 Jul;200(2):271-279. DOI: 10.1007/s10549-023-06965-5.

 

PFS at 6 months was 86% in patients with an ESR1 mutation detected and 85% in wildtype ESR1 patients, while median PFS was 8.2 months for ESR1 mutant patients compared with 8.7 months for ESR1 wildtype patients (p = 0.47). The median OS was 20.7 months for ESR1 mutant patients compared with 28.1 months for ESR1 wildtype patients (p = 0.27). Patients with ≥ 2 ESR1 mutations exhibited a significantly worse OS vs patients who did not (p = 0.003), although PFS was unaffected.

“Presence of ESR1 mutations in baseline ctDNA might not be associated with inferior PFS and OS in advanced breast cancer patients treated with paclitaxel/bevacizumab,” the researchers concluded.

 

Elacestrant

 

In the EMERALD trial, examined in an article from the Journal of the National Comprehensive Cancer Network (JNCCN), investigators compared the use of elacestrant vs endocrine therapy of physician choice (fulvestrant or aromatase inhibitor) in patients (n=477) who were previously administered 1 or 2 lines of endocrine therapy, including 1 line containing a CDK4/6 inhibitor and up to 1 line of chemotherapy in the metastatic setting.

Gradishar WJ, Moran MS, Abraham J, et al. NCCN Guidelines® Insights: Breast Cancer, Version 4.2023: Featured Updates to the NCCN Guidelines. J Natl Compr Canc Netw. 21(6), 594-608. DOI: 10.6004/jnccn.2023.0031.

ESR1 mutational status was assessed via cell-free circulating tumor DNA (ctDNA).

 

The NCCN’s formal recommendation reads, “In the recently updated guidelines, the panel has included elacestrant as a new treatment option for postmenopausal females or adult males with ER-positive, HER2-negative, ESR1-mutated tumors after disease progression on 1 or 2 prior lines of endocrine therapy, including 1 line containing a CDK4/6 inhibitor. The panel recommends evaluating ESR1 mutational status using next-generation sequencing or by assessing the ctDNA in the blood using PCR. Because ESR1 mutations are acquired during treatment, primary archived breast cancer should not be used as a source of tumor tissue for ESR1 mutation testing.”

 

Abemaciclib plus fulvestrant

 

The Harvard researchers performed an exploratory analysis to determine the efficacy of abemaciclib plus fulvestrant in patients with or without PIK3CA or ESR1 mutations in MONARCH 2.

Abemaciclib plus fulvestrant lengthened PFS compared with placebo plus fulvestrant in both PIK3CA-wildtype (median 16.9 months vs 12.3 months) and PIK3CA-mutant subgroups (median 17.1 months vs 5.7 months). The same was seen in ESR1-wildtype (median 15.3 months vs 11.2 months) and ESR1-mutant subgroups (median 20.7 months vs 13.1 months). Regardless of PIK3CA or ESR1 mutation status, OS and other outcomes also improved following treatment with abemaciclib plus fulvestrant.

“In this exploratory analysis,” they wrote, “we show in HR+, HER2− ABC, abemaciclib plus fulvestrant improved both PFS and OS, regardless of PIK3CA or ESR1 mutation status. In addition, abemaciclib plus fulvestrant improved other endpoints, including TTC, CFS, and PFS2, regardless of PIK3CA or ESR1 mutation status." The authors noted, however, that their analysis was limited by sample size.

They continued: “While acknowledging the limitations of cross study comparisons, the similarity of outcomes for the single-agent fulvestrant control arms in PIK3CA-mutant cancers between MONARCH 2 and SOLAR-1 and the magnitude of improvement in PFS and OS demonstrated in MONARCH 2 also highlights a potential therapeutic strategy for treatment of HR+, PIK3CA-mutant metastatic breast cancer and support further evaluation in prospective, and suitably powered, clinical trials.”