Background: Only old evidence exist to back up the use of medroxyprogesterone acetate (MPA) in endocrine therapy. Therefore, this study aimed to explore the factors that influence the time to treatment failure (TTF) of MPA in real world settings as late-line treatment following aromatase inhibitors and fulvestrant. Methods: This was a cohort study that used the database of the Safari study, on estrogen receptor-positive (ER+) post-menopausal advanced breast cancer previously treated with fulvestrant (UMIN000015168). We created Kaplan-Meier curves for TTF treated with MPA. Further, univariate and multivariate analyses were performed using a Cox hazard model of the clinicopathological factors involved in the TTF of MPA. Results: Fist, we made Kaplan-Meier curves by treatment line for MPA in TTF analysis population 1 (n = 244), excluding HER2+ and HER2 with unknown status. The median TTF for MPA was 8.2 months (95% CI 5.1–14.9) for first- and second-line treatments, 3.0 months (95% CI 2.5–3.9) for third-line treatment, and 4.1 months (95% CI 3.5–5.0) for fourth or later treatment lines. The first- and second-line treatments had significantly longer TTF than the third-line treatment (P < 0.001) and fourth-line or later treatments (P < 0.001). No difference in TTF was observed between the third and fourth or later treatment lines. Similar results were obtained in the analysis population 2 (n = 203) for TTF, excluding cases in which MPA was considered to have been used in palliative care. The median TTF for MPA was 7.9 months (95% CI, 5.1-16.0) for first- and second-line treatments, 3.0 months (95% CI 2.8–4.6) for third-line treatment, and 4.3 months (95% CI 3.7–5.6) for fourth or later treatment lines. The first- and second-line treatments had significantly longer TTF than the third-line treatment (P < 0.001) and the fourth-line or later treatments (P < 0.001). No difference in TTF was observed between the third and fourth or later treatment lines. Second, Table 1 shows the clinicopathological factors involved in the TTF of MPA. In univariate analysis, long DFI (≥ 6 years), small nuclear or histological grade, and the presence of visceral metastases correlated with significantly long TTF (P < 0.05). Whereas PgR, adjuvant chemotherapy, and adjuvant endocrine therapy did not affect the TTF of patients treated with MPA. However, in the multivariate regression analysis, only longer DFI (≥ 6 years) was correlated with a significantly longer TTF. Third, we compared the clinicopathologic factors in the groups that received MPA as the fourth or later treatment lines and achieved a TTF of more than 1 year with those that did not. There were no characteristic clinicopathological factors distinct between the two groups. Conclusion: In actual clinical practice, patients treated with MPA alone as the fourth or subsequent treatment lines showed a TTF of 4 months, suggesting that there is merit in using MPA even in late treatment lines, especially in patients with long DFI and those who are difficult to treat with other antineoplastic agents. Table 1. Univariate and Multivariate analyses to investigate association between clinicopathological factors and TTF of MPA (n=170) Citation Format: Kaho Utsunomiya, Hidetoshi Kawaguchi, Yutaka Yamamoto, Shigehira Saji, Norikazu Masuda, Takahiro Nakayama, Kenjiro Aogi, Keisei Anan, Shoichiro Ohtani, Nobuaki Sato, Toshimi Takano, Eriko Tokunaga, Seigo Nakamura, Yoshie Hasegawa, Masaya Hattori, Tomomi Fujisawa, Satoshi Morita, Miki Yamaguchi, Toshinari Yamashita, Daisuke Yotsumoto, Masakazu Toi, Shinji Ohno. Retrospective study using database for the effectiveness of medroxyprogesterone acetate in patients with ER-positive/HER2-negative postmenopausal advanced breast cancer: An additional analysis of the JBCRG-C06 Safari study [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr P2-03-15.
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