Background: In Asian countries, BC incidence rates are rising, with a higher proportion of pre/perimenopausal (pre/peri-M) patients (pts). Data on treatment options for pre/peri-M pts are limited. Adding P to endocrine therapy (ET), such as aromatase inhibitor or fulvestrant, has improved progression-free survival (PFS) in phase 3 studies. However, the efficacy and safety of P in combination with TAM are still unclear in pts with HR+/HER2− ABC regardless of M status. This combination was investigated in PATHWAY (NCT03423199): a double-blind randomized phase 3 trial conducted in Japan, Korea, Taiwan and Singapore. The study was conducted as a Clinical Research Collaboration with the National Cancer Center Hospital being the regulatory sponsor and Pfizer providing drug and financial support. Methods: Women with HR+/HER2− ABC were randomly assigned 1:1 to receive either P (125 mg once daily, days 1-21 of a 28-day cycle) or placebo in combination with TAM (20 mg once daily, continuously) as 1st or 2nd line treatment for ABC. Pre/peri-M women received concurrent ovarian function suppression with goserelin. Pts were stratified by 1st vs 2nd line ET and M status. The primary endpoint was PFS as assessed by investigators. Secondary endpoints include overall survival (OS), objective response, safety, and patient-reported outcomes. Results: A total of 184 pts were assigned to P + TAM (91 pts) and placebo + TAM (93 pts). At data cutoff date (Sep 15, 2022), 138 PFS events had occurred. Median follow-up was 40.9 months for censored pts. Median PFS was 24.4 months (95% CI: 13.1, 32.4) with P + TAM and 11.1 months (95% CI: 7.4, 14.6) with placebo + TAM (hazard ratio [HR]: 0.602 [95% CI: 0.428, 0.848], 1-sided p-value from stratified log rank test: 0.002). The HRs for PFS of the subgroups by stratification factors were as follows: for pts with 1st line ET (HR: 0.521 [95% CI: 0.332, 0.817]) or with 2nd line ET (HR: 0.707 [95% CI: 0.421, 1.189]), and for pre/peri-M pts (HR: 0.378 [95% CI: 0.192, 0.742]) or post-M pts (HR: 0.677 [95% CI: 0.456, 1.005]). While OS data were still immature, the primary analysis showed a 27% reduction in overall risk of death (median OS: not reached in both arms, HR: 0.73 [95% CI: 0.442, 1.207]). 93.4% of pts with P + TAM vs. 20.4% of pts with placebo + TAM had grade (G) ≥3 treatment-emergent adverse events (TEAEs). The most frequently observed G ≥3 TEAE was neutropenia (89.0% in P + TAM arm and 1.1% in placebo + TAM arm). There were no G5 TEAEs in either arm. Conclusions: The study achieved its primary endpoint, demonstrating a significant and clinically meaningful improvement in PFS for P + TAM compared with placebo + TAM for pts with HR+/HER2− ABC. Early OS data with P+TAM is encouraging. TEAEs were generally consistent with the known safety profile of P and ET. Clinical trial information: NCT03423199.