Trial Design: Randomised phase II trial that aims to evaluate the efficacy and safety of paclitaxel plus pembrolizumab relative to paclitaxel alone, in patients with locally advanced or metastatic oestrogen receptor positive, HER2-negative, Luminal B breast cancer who have received no prior chemotherapy for advanced or metastatic disease. Patients will be randomised 2:1 to either pembrolizumab plus paclitaxel vs paclitaxel alone. Randomisation will be stratified by centrally assessed PD-L1 expression and the presence of visceral metastases. Paclitaxel treatment will be continued for at least 6 cycles unless there is evidence of disease progression Pembrolizumab treatment will not exceed 2 years. On completion of study treatment patients will enter a survival follow-up period during which data on cancer therapy, disease status and survival status will be collected.