Abstract PO1-05-05: Overall survival (OS) and subsequent therapy patterns in Japanese patients with HR+/HER2− advanced breast cancer (ABC) treated with palbociclib (PAL) plus letrozole (LET) in first-line setting (1L)

Abstract Background: PAL is a first-in-class cyclin-dependent kinase 4/6 inhibitor (CDK4/6i) for the treatment of HR+/HER2− ABC. In PALOMA-2 study, PAL plus LET demonstrated a numerically, not statistically significant, prolonged OS versus placebo plus LET (53.9 vs 51.2 months; HR, 0.956; 95% CI, 0.777–1.177) in patients with HR+/HER2− ABC in 1L. Subgroup analyses showed a similar trend in Asia/Pacific region (73.4 vs 55.1 months; HR, 0.744), however, data specific to Japanese subgroup have not been reported. An open-label, single-arm, multicenter Japanese phase 2 study (J-Ph2) investigated the efficacy and safety of PAL plus LET in postmenopausal Japanese women with HR+/HER2− ABC in 1L and showed a median progression-free survival (PFS) of 35.7 months (95% CI, 21.7–46.7) in the final analysis of PFS. However, OS data were immature at the data cutoff. Here, we report the findings from a follow-up study of J-Ph2 (NCT04735367) evaluating OS and subsequent therapy in these Japanese patients. Methods: Patients (N=42) who participated in J-Ph2 were enrolled in this follow-up study. The primary endpoint was OS, defined as the time from the first dose of PAL plus LET in J-Ph2 to date of death due to any cause. Secondary endpoints included chemotherapy-free survival (CFS) and type and duration of subsequent therapy. Median OS, CFS, duration of subsequent therapy and associated 95% CIs were estimated using the Kaplan–Meier method. Outcomes were also assessed for baseline demographic and disease characteristic subgroups including visceral or nonvisceral metastatic disease, disease-free interval (DFI; ≤12 months, 12 months, de novo metastatic), and duration of 1L PAL use (≤24 months or 24 months). Results: Patients had a median age of 62.5 years; 48% had visceral metastases; 33% had de novo disease; 48% had a TFI 12 months; 93% had an ECOG performance status (PS) of 0. At a median follow up of 89.7 months, the median OS was 85.4 months (95% CI, 64.3–not estimable NE). Median OS was longer in patients with nonvisceral versus visceral metastases (not reached NR vs 67.3 months), with TFI 12 months versus ≤12 months (85.4 vs 45.4 months), or with duration of 1L PAL use 24 months versus ≤24 months (NR vs 47.5 months). Median CFS was 69.1 months (95% CI, 24.2–85.4), and that was longer in patients with nonvisceral versus visceral metastases (77.5 vs 37.5 months). At the data cutoff, 3 patients (7.1%) were still receiving PAL plus LET (90.9–93.2 months). Subsequent therapy after disease progression was administered to 34 of 42 patients (81%). Of these patients, 28 (82%) received endocrine-based therapy, while 3 (9%) patients each received chemotherapy and other therapy, respectively. The median duration of the first subsequent therapy was 8.3 months (95% CI, 3.9–12.2), and that was similar among patients with nonvisceral versus visceral metastases (7.8 vs 8.3 months). Conclusion: This interim analysis showed a median OS of 7 years with 1L PAL plus LET. Patient demographics (geographic, racial factors), disease characteristics (ECOG PS, visceral metastases), and subsequent therapy decisions may have contributed to the extended median OS observed in this study. Further, this report provides insight into real-world subsequent treatment patterns following PAL plus ET. Table Citation Format: Masato Takahashi, Tomofumi Osako, Hiroyuki Yasojima, Kenichi Inoue, Masahiro Kawashima, Hideki Maeda, Mitsuya Ito, Yasuaki Sagara, Kan Yonemori, Masaya Hattori, Naohito Yamamoto, Eriko Tokunaga, Shozo Ohsumi, Yutaka Yamamoto, Akemi Ichikawa, Yasuaki Muramatsu, Norikazu Masuda. Overall survival (OS) and subsequent therapy patterns in Japanese patients with HR+/HER2− advanced breast cancer (ABC) treated with palbociclib (PAL) plus letrozole (LET) in first-line setting (1L) abstract. In: Proceedings of the 2023 San Antonio Breast Cancer Symposium; 2023 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2024;84(9 Suppl):Abstract nr PO1-05-05.