Improved Outcomes with Early Switch to Fulvestrant Plus Palbociclib in ESR1 Mutated Advanced Breast Cancer

SUMMARY: Breast cancer is the most common cancer among women in the US and about 1 in 8 women (12%) will develop invasive breast cancer during their lifetime. Approximately 290,560 new cases of breast cancer will be diagnosed in 2022 and about 43,780 individuals will die of the disease, largely due to metastatic recurrence. Approximately 70% of breast tumors express Estrogen Receptors and/or Progesterone Receptors. The most common subtype of metastatic breast cancer is Hormone Receptor-positive (HR-positive), HER2-negative breast cancer (65% of all metastatic breast tumors), and these patients are often treated with anti-estrogen therapy as first line treatment. However, resistance to hormonal therapy occurs in most of the patients, with a median Overall Survival (OS) of 36 months. With the development of Cyclin Dependent Kinases (CDK) 4/6 inhibitors, endocrine therapy plus a CDK4/6 inhibitor is the mainstay for the management of ER+/HER2- metastatic breast cancer as first-line therapy. Even with this therapeutic combination, most patients will eventually experience disease progression, including development of ESR1 (Estrogen Receptor gene alpha) mutations.

ESR1 is the most common acquired mutation noted in breast tumors as they progress from primary to metastatic setting. These mutations promote ligand independent Estrogen Receptor activation and have been shown to promote resistance to estrogen deprivation therapy. It appears that ESR1 mutations are harbored in metastatic ER+ breast cancers with prior Aromatase Inhibitor (AI) therapy, but not in primary breast cancers, suggesting that ESR1 mutations may be selected by prior therapy with an AI, in advanced breast cancer. In a recently published study (JAMA Oncol.2016;2:1310-1315), ESR1 mutations Y537S and D538G mutations detected in baseline plasma samples from ER+/HER- advanced breast cancer patients, was associated with shorter Overall Survival. In this study it was noted that there was a three-fold increase in the prevalence of these mutations in patients who had failed first line hormonal therapy for metastatic disease, compared with those who were initiating first line therapy for advanced breast cancer (33% versus 11%).

Fulvestrant (FASLODEX®) is an estrogen antagonist and like Tamoxifen binds to estrogen receptors (ERs) competitively, but unlike Tamoxifen causes rapid degradation and loss of ER protein (ER down regulator) and is devoid of ER agonist activity. Palbociclib (IBRANCE®) is a reversible, oral, selective, small molecule inhibitor of Cyclin Dependent Kinases, CDK4 and CDK6, and prevents RB1 phosphorylation. Palbociclib is the first CDK inhibitor approved by the FDA. It exhibits synergy when combined with endocrine therapies. The FDA in February 2016, approved Palbociclib in combination with Fulvestrant, for the treatment of women with HR-positive, HER2-negative advanced or metastatic breast cancer, with disease progression following endocrine therapy.

Patients with ESR1 mutations on Fulvestrant had improved Progression Free Survival (PFS) compared with Exemestane (AROMASIN®) in the SoFEA trial. The combination of Palbociclib and Fulvestrant improved PFS compared with Fulvestrant plus placebo in both ESR1 mutant and ESR1 wild-type patients, in the PALOMA3 trial.

The PADA-1 study aimed to show the efficacy of an early change in therapy based on a rising ESR1 mutation in the peripheral blood, while assessing the global safety of the combination Fulvestrant and Palbociclib. PADA-1 is a prospective, randomized, open-label, multicentre, Phase III trial in which 1017 patients with ER-positive, HER2-negative advanced breast cancer were included. These patients were monitored for a rising ESR1 mutation in the peripheral blood, while on first-line treatment with an Aromatase Inhibitor (Letrazole 2.5 mg, Anastrozole 1 mg or Exemestane 25 mg orally once daily, taken continuously) and Palbociclib 125 mg orally once daily on days 1-21 of a 28-day treatment cycle, at enrollment and every 2 months thereafter. Blood samples were monitored for several ESR1 mutations which included E380, P535, L536, Y537, and D538. The median time from trial enrollment to detection of the ESR1 mutation was 14.2 months. Patients with newly present or rising ESR1 mutation in the peripheral blood circulating tumor DNA and no synchronous disease progression (N=172) were randomly assigned (1:1) to continue with the same therapy (N=84) or to switch to Fulvestrant 500 mg IM on day 1 of each 28-day cycle and on day 15 of cycle 1, along with Palbociclib as previously dosed (N=88). Baseline characteristics were similar in both treatment groups. The median patient age was 61 years, and one-third of patients had prior treatment with an Aromatase Inhibitor. Patients were stratified according to visceral involvement (present or absent) and the time from inclusion to detection of ESR1 mutation in the peripheral blood (<12 months or 12 months or more). The co-Primary endpoints were Progression Free Survival and Grade 3 or more hematologic adverse events in all patients.

At a median follow up of 26.0 months from randomization, switching patients from an Aromatase Inhibitor to Fulvestrant, upon detection of ESR1 mutation in the peripheral blood was associated with a 39% reduction in the risk of disease progression or death. The median Progression Free Survival was 11.9 months in the Fulvestrant and Palbociclib group versus 5.7 months in the Aromatase Inhibitor and Palbociclib group (HR=0.61; P=0·004). The co-Primary endpoint of Grade 3 or more hematologic adverse events found no safety signals associated with switching from an Aromatase inhibitor to Fulvestrant. The most frequent Grade 3 or more hematological adverse events were neutropenia. lymphopenia, and thrombocytopenia. Dose reductions were similar in both randomized treatment groups.

The authors concluded that PADA-1 is the first prospective randomized trial to demonstrate that early therapeutic targeting of ESR1 mutation in the peripheral blood results in significant clinical benefit. The researchers added that the original design explored in PADA-1 might help with addressing acquired resistance to new drugs in future trials.

Switch to fulvestrant and palbociclib versus no switch in advanced breast cancer with rising ESR1 mutation during aromatase inhibitor and palbociclib therapy (PADA-1): a randomised, open-label, multicentre, phase 3 trial. Bidard FC, Hardy-Bessard AC, Dalenc F, et al. The Lancet Oncology. Published: September 29, 2022.DOI: