Lenvatinib+Letrozole Versus Fulvestrant in Metastatic ER+/HER2- Breast Cancer, Post Progression on Al + CDK4/6 Inhibitor

Breast cancer is the commonest cancer among females in Singapore and worldwide. Approximately 60-70% of breast cancers are hormone receptor positive and thus potentially sensitive to endocrine therapy. However, both primary and acquired resistance to endocrine therapy exists, and better combinations are constantly being explored to delay endocrine resistance and improve treatment outcome. Several known mechanisms of endocrine resistance have been proposed, and include deregulation of various components of the ER pathway itself, alterations in cell cycle and cell survival signaling molecules, and the activation of escape pathways that can provide tumors with alternative proliferative and survival stimuli. Endocrine blockade in breast cancer can be achieved by reducing the levels of estrogens through ovarian ablation (medical, surgical, or through radiation) in pre-menopausal women or with the administration of aromatase inhibitors (AI) in post-menopausal women. Direct inhibition of estrogen receptors can be achieved by administering selective estrogen receptor modulators, such as tamoxifen, or a pure estrogen receptor antagonist such as fulvestrant. Sequential single agent endocrine therapy used to be the standard of care in advanced hormone receptor positive, HER2 negative breast cancer patients without visceral crisis and low tumor burden. However, the development of CDK4/6 inhibitors (palbociclib, ribociclib and abemaciclib) since 2014 has resulted in endocrine therapy + CDK4/6 inhibitor becoming a new standard of care in the first- or second-line setting for these patients, even in those with visceral disease (though not visceral crisis) and substantial tumor burden. The addition of CDK4/6 inhibitors has resulted in consistent improvement in progression-free survival 2-4, and a recent meta-analyses has also confirmed improvement in overall survival outcomes. There is limited literature on the median progression-free survival (PFS) of single agent fulvestrant in patients who have failed first-line AI + CDK4/6 inhibitors. However, in several contemporary phase II/III randomized trials of second-line endocrine therapy in patients who failed prior AI (majority or all had no prior CDK4/6 inhibitors), e.g., FAKTION, SANDIPIPER, BELLE2 trials, median PFS in the control arm of single agent fulvestrant is fairly consistent at 4.5 to 5.6 months. Median PFS of fulvestrant post-progression from first-line AI + CDK4/6 inhibitors is expected to be shorter than 4.5 to 5.6 months as these patients are likely to be more endocrine-resistant. In a subgroup analysis of patients who failed prior CDK4/6 inhibitors (n=20) and who received single agent fulvestrant as second-line endocrine therapy in the phase III randomized SOLAR-1 trial, median PFS was 1.8 months and 1-year PFS was \<10%. RET is an estrogen response gene, and preclinical studies have demonstrated cross talk between RET and ER. Significant interactions between RET and ERa pathways have been described, with increased response to estrogen stimulation observed in the presence of functional RET. RET is associated with resistance to tamoxifen and aromatase inhibitors, and increased RET expression has been demonstrated in hormone resistant cell lines and primary tumors. Combined anti-estrogen and anti-RET therapy in luminal breast cancer had a greater effect on cell growth than either therapy alone. The two classes of drugs have different mechanisms of action; a RET tyrosine kinase inhibitor (TKI) reduced growth through induction of apoptosis, while anti-ERa reduced cell proliferation, forming the biologic basis for dual treatment. Dual therapy with tamoxifen and vandetinib, a RET inhibitor, resulted in greater reduction in tumor growth rate in MCF7 xenografts in mice. RET has been reported to be over-expressed in up to 75% of ER+ breast cancers (n=20), compared to only 10% of ER-negative breast cancers (n=10) in a small study. Preclinical experiments from laboratory showed lenvatinib to have activity in ER positive breast cancer cell lines, regardless of levels of RET expression. Lenvatinib was at least additive with tamoxifen in all 6 ER positive breast cancer cell lines tested, with the combination resulting in =50% cell kill compared to single agent tamoxifen in BT474, CAMA1, and T47D cell lines supporting the potential role of lenvatinib in combination with endocrine therapy in the treatment of ER positive breast cancers. A phase Ib with dose expansion study was carried out at the National University Cancer Institute, Singapore with the use of lenvatinib together with letrozole in the treatment of patients with estrogen receptor positive, HER2 negative breast cancer. This study was initiated in patients for neoadjuvant therapy, but expanded to include patients with metastatic breast cancer. In total, 47 patients were enrolled, among whom the first 4 patients received neoadjuvant therapy for locally advanced disease, and the remaining 43 patients received treatment in the metastatic setting. Recommended phase 2 dose of lenvatinib was established at 14mg daily in combination with letrozole 2.5mg daily. Thirty-one patients were enrolled in the dose expansion cohort at recommended phase 2 dose (RP2D). At interim analyses, patients enrolled had a median of 4 prior lines of therapy (range 0-11), with 78.9% of patients having prior exposure to CDK4/6 inhibitor therapy. In the intention-to-treat population, an overall response rate (ORR) of 36.8% was observed, with 28.9% patients remaining progression-free (PPF) at 1 year. Among patients with partial response or stable disease, median duration of response was 16.1 months (range 5.4 to 21.7 months). Amongst these patients, 78.9% (n=30) had prior CDK4/6 inhibitor treatment, and 12 patients received CDK4/6 inhibitors as immediate prior line of therapy. In the subset of patients with prior CDK4/6 inhibitor exposure, similar encouraging results were observed, with a ORR of 26.7, 23.3% PPF at 1 year, and duration of response of 13.2 months (range 3.8 to 21.3 months). Additionally, on-target effect, downregulation of RET expression among tumors were found to have positive RET expression at baseline. Main toxicities observed included hypertension (all grade 59%, grade3 8%), hypothyroidism (all grade 46%, grade 3 0%), and fatigue (all grade 43%, grade 3 5%), all of which were well controlled with optimal medical therapy. No grade 4-5 toxicities were observed. The promising results from the phase Ib study forms the basis of this randomized phase II study.