What question did this study set out to answer?

This trial examines whether micronised progesterone can enhance the suppression of tumor proliferation in early-stage, ER-positive breast cancer.

July 22, 2026npj Breast Cancer0 citationsOpen Access

The WinPro trial: window-of-opportunity study of endocrine therapy with micronised progesterone in ER-positive breast cancer

LHLucy Haggstrom KMKate Middleton APAndrew Parker

Key Points

This trial examines whether micronised progesterone can enhance the suppression of tumor proliferation in early-stage, ER-positive breast cancer.
Randomised, multi-centre, phase 2 trial with 244 post-menopausal women enrolled.
Participants received either letrozole, letrozole and micronised progesterone, or tamoxifen and micronised progesterone for 14 days preoperatively.
Primary endpoint focused on percent reduction in Ki67 as a measure of tumor proliferation.
No significant difference in Ki67 suppression between letrozole (88.2%) and letrozole + MP (89.2%) with p=0.39.
Ki67 suppression was significantly lower for tamoxifen + MP at 61.5%.
Hot flushes were less frequent with letrozole + MP (13.3%) compared to letrozole (22.4%) and tamoxifen + MP (20.5%).

Abstract

Abstract Progesterone can inhibit tumoural proliferation in oestrogen receptor positive (ER+) breast cancer. The WinPro trial (Australian New Zealand Trials Clinical Trials Registry number: ACTRN12618000928213, date of registration 01/06/2018) was a randomised, multi-centre, phase 2, window-of-opportunity trial evaluating micronised progesterone (MP) in post-menopausal women with early-stage, ER+, progesterone receptor (PR) positive, HER2- breast cancer. Patients were randomised to letrozole, letrozole and MP, or tamoxifen and MP for 14 days preoperatively. The primary endpoint was the percent proportional reduction in Ki67 (‘Ki67 suppression’) between the two letrozole groups in the per protocol population. From February 2018 to June 2024, 244 patients were enrolled. 189 patients completed per protocol: letrozole ( n = 66, 34.9%), letrozole + MP ( n = 64, 33.9%), and tamoxifen + MP ( n = 59, 31.2%). There was no difference in Ki67 suppression between letrozole (88.2%) versus letrozole + MP (89.2%) ( p = 0.39). Ki67 suppression appeared lower with tamoxifen + MP (61.5%). Hot flushes appeared less frequent with letrozole + MP (13.3%) versus letrozole (22.4%) or tamoxifen + MP (20.5%).

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