Aromatase inhibitor (AI) + CDK4/6 inhibitor is settled down as the standard first line therapy for HR+/HER2- metastatic breast cancer and all three CDk4/6 inhibitors, palbociclib, ribociclib, and abemaciclib are currently available for same indications. However, there is no effective treatment strategy for patients who have progressed on AI+CDK4/6 inhibitor. In particular, the clinical efficacies of subsequent hormone therapy are lowered when ESR1 mutations, one of mechanisms of AI resistance occur. In the PADA-1 trial, when ESR1 mutations in ctDNA were detected in patients treated with AI+CDK4/6 inhibitor, AI was switched to fulvestrant even if disease progression was not confirmed clinically. As a result, the median PFS was prolonged by about 8 months in this switching group compared to the group in which AI was continued. The results of this study suggested that delaying the occurrence of ESR1 mutations and early response to them are necessary to increase the effectiveness of hormone therapy.
In SWOG S0226 study, fulvestrant + AI combination showed significant benefits in PFS and OS compared to AI monotherapy as the first line therapy. Based on these results, the NCCN guideline suggests fulvestrant + AI combination as one of the first line hormone therapy options. However, the clinical effect of AI + fulvestrant + CDK4/6 inhibitor has not been investigated yet. Therefore, the investigators are planning to compare the clinical efficacy of AI+ fulvestrant + CDK4/6 inhibitor and AI+CDK4/6 inhibitor, and to investigate if a triple combination regimen can delay the emergence of ESR1 mutations and modulate occurred ESR1 mutations.
Female ≥ 19 years of age
Histologically confirmed unresectable, locally advanced or metastatic invasive breast cancer with hormone receptor positive/HER2 negative
No previous history of systemic endocrine or chemotherapy for metastatic, advanced breast cancer.
If the patient has received AI as adjuvant endocrine therapy, the treatment free interval (TFI) should be more than 12 months after the end of adjuvant endocrine therapy. If the patient has received tamoxifen for adjuvant endocrine therapy, TFI less than 12 months will be allowed.
Patients with a history of previous treatment with a CDK4/6 inhibitor or other systemic treatment for advanced/metastatic breast cancer
Patients who have received prior treatment with fulvestrant and any investigational ER-directed therapy including SERDs (selective estrogen receptor degrader)
Patients who have disease recurrence on aromatase inhibitor treatment as adjuvant endocrine therapy
Patients who have symptomatic or untreated central nervous system metastasis