Phase IB & II Study of Ribociclib With Trastuzumab Plus Letrozole in Postmenopausal HR+, HER2+ Advanced Breast Cancer Patients

Survival benefit and quality of life are two key elements that should be kept in mind in the treatment of metastatic breast cancer. In this regards, endocrine therapy (ET) is strongly recommended in hormone receptor (HR) positive patients unless there is visceral crisis even though there is no concrete evidence that it is better than chemotherapy in terms of survival. HER2 positive breast cancer is a subtype of breast cancer that showed the greatest improvement in terms of survival during the last decade due to trastuzumab based therapy. Recently, taxane and HER2 directed doublet including trastuzumab and pertuzumab (THP) is considered as standard of therapy based upon randomized phase 3 clinical trial (CLEOTATRA). HER2 positive breast cancer can be divided into HER2 enriched subgroup (HR-HER2+) and luminal B subgroup (HR+HER2+) in biologic viewpoint because they are distinctly different subgroups in gene expression analysis. Accordingly, we are currently treating biologically different subtypes in a same way, which is CTx and anti-HER2 combination therapy (THP). Luminal HER2+ subgroup has actually been tested with endocrine therapy (ET) and anti-HER2 therapy showed better PFS than ET alone (TAnDEM trial and trial comparing lapatinib plus letrozole versus letrozole alone) \[2\],\[3\] confirming existence of cross talk between ER and HER2 pathways in clinical setting. However, the combination regimen between ET and anti-HER2 therapy is not widely used in current practice in ER+HER2+ MBC patients because PFS seemed to be relatively shorter compared with chemotherapy based combination with anti-HER2 therapy even though several guidelines recommend it to be used as an initial treatment unless there is visceral crisis as they recommended ET alone first in ER+HER2- MBC (NCCN 2018). Recently, various CDK4/6 inhibitors including palbociclib, abemaciclib, and ribociclib were approved by FDA based on the clinical trial results demonstrating prolonged PFS over ET alone whe