Background: Downregulation of Ki67 by neoadjuvant endocrine therapy predicts activity of endocrine treatments in hormone receptors positive breast cancer. NA-PHER2 is an exploratory phase II study (NCT02530424) assessing Ki67 changes in patients with HER2+ and ER+ breast cancer undergoing dual HER2 block and palbociclib. Cohort A of the NA-PHER2 showed significant decrease of Ki67 at week 2 and at surgery and pathological complete response (pCR) in 27% of patients (Lancet Oncol 2018). Methods: After completing cohort A two additional cohorts were started. In Cohort B cases with HER2 3+ or amplified unilateral breast cancer received therapy with dual block and palbociclib without fulvestrant. In Cohort C tumors with Ki67 >20% and HER2 low (1+/2+, no amplification) received also fulvestrant. Trastuzumab and pertuzumab q3 wks were dosed for 6 cycles and palbociclib for 5 cycles (125 mg po q.d. 3q4 wks). Fulvestrant in Cohort C was given im 500 mg q4 wks for 5 cycles. Primary endpoint was Ki67 change from baseline to 2 weeks and at surgery. Results: 26 eligible patients in cohort B and 23 in cohort C with centrally confirmed HER2 and ER status were recruited. Ki67 was centrally assessed. Main results are reported in the table. Clinical trial information: NCT02530424 The most frequent G >=3 adverse events were neutropenia (36%) and gastrointestinal disorders (12%). Conclusions: Dual block of HER2 and palbociclib caused robust persistent decrease of Ki67 as in cohort A. In cohort B without endocrine therapy there also were pCR and high objective response rate. Effects on Ki67 and ORR were similar in HER2 low tumors. The chemo-free approach of NA-PHER2 leads to promising therapeutic effects and deserves investigation in ER+ HER2+ tumors to spare the toxicity of chemotherapy, and in HER2-low tumors, in which functional activation of HER2 may lead to resistance to endocrine therapy. Supported in part by unrestricted grants of Pfizer Italia S.r.l. and Roche S.p.a. Italia.