Abstract
Over the past two decades, high sensitivity to HER2-amplified primary breast cancers has been achieved with HER2-targeted therapies. CDK4/6 inhibitors have long been identified as a potential treatment option for advanced breast cancer patients. However, acquired HER2 heterogeneity leading to resistance during the treatment has been identified as a bottleneck. This review focuses on the recent resistance mechanisms identified and potential therapeutic targets for conventional and combination endocrine therapies with CDK4/6 inhibitors by various breast cancer clinical trials and research groups in HER amplified and/or mutated breast cancer tumour. Activating HER2 alterations, JNK pathway, hyperactivated TORC1, co-mutations in HER2 and HER3, phenotypic changes of HER2, and few other advanced findings are identified as potential therapeutic targets in treating current HER2 endocrine therapy-resistant tumour. Along with the HER2-focused resistance mechanisms, we also describe how the microbiome may play a role in breast cancer therapy and its potential for new therapeutic strategies to overcome drug resistance in breast cancers.