Araki K, et al. - In patients with human epidermal growth factor receptor 2 (HER2)-positive advanced breast cancer (ABC) treated with pertuzumab and trastuzumab (PT) in combination with eribulin (ERI) or nab-paclitaxel (Nab-PTX), researchers investigated the value of peripheral blood-based parameters (PBBPs) as prognostic indicators. For this retrospective-prospective study, data from 51 patients included in two single-arm, phase II trials were utilized. They found that prediction of progression-free survival was achieved by assessment of baseline absolute lymphocyte count in HER2-positive ABC treated with PT regardless of combined chemotherapy regimen. Tumor microenvironment as well as systemic peripheral circulating lymphocytes were suggested as the possible mediators of anti-tumor effects. For predicting the efficacy of PT treatment, a possible utility of baseline systemic parameters such as peripheral lymphocyte count was suggested, in addition to disease extent.