Zagouri, Flora

1001 Background: Imlunestrant (imlu) is a next-generation, brain-penetrant, oral selective estrogen receptor degrader. The EMBER-3 trial, in patients (pts) with ER+, HER2- ABC who had disease progression on or after aromatase inhibitor-based therapy, showed significant progression free survival (PFS) improvement with imlu vs standard therapy (SOC, fulvestrant or exemestane) in pts with ESR1 mutations ( ESR1 m), and with imlunestrant+abemaciclib (imlu+abema) vs imlu in all pts, regardless of ESR1 m. Exploratory PRO analyses are presented here. Methods: EORTC QLQ-C30 was administered at baseline (BL) and every 8 weeks until treatment discontinuation. Prespecified QLQ-C30 analysis used a longitudinal mixed model for repeated measures to calculate mean change from BL in pts with BL and ≥1 post-BL score. PRO-CTCAE (diarrhea frequency) was administered weekly, reporting 0 (never) to 4 (almost constantly). PRO-CTCAE (injection site reaction [ISR]) was administered to fulvestrant recipients weekly for 2 weeks post-injection, reporting yes/no (pain, swelling, redness). Descriptive analysis was used for PRO-CTCAE. Results: In pts with ESR1 m, imlu monotherapy was associated with many improved or maintained EORTC QLQ-C30 scores, whereas scores with SOC were declined or maintained. Specifically, pts with ESR1 m on imlu had improved global health status (GHS)/quality of life (QOL) and physical function (PF) scores, while scores with SOC declined (mean change differences between treatments: 9.9 [0.1, 19.7] and 6.2 [-0.8, 13.1], respectively). These PRO findings mirror the PFS findings in this group. In the overall population, GHS/QOL scores declined similarly with imlu vs SOC (mean change differences: 0.5 [-4.7, 5.7]), while PF scores were maintained with imlu vs a slight decline with SOC (mean change difference: 2.5 [-1.1, 6.1]). Most fulvestrant recipients (72%) reported ISR at any time while on treatment, with a mean of 31% during the first week of the first 6 cycles. Imlu+abema vs imlu showed broadly similar declines in all pts, with minimal mean change differences in GHS/QOL and PF scores (0.8 [-7.4, 5.9]; -2.2 [-6.6, 2.2], respectively). Pts reported similarly low rates of “frequent” or “almost constant” diarrhea with imlu (3%) and SOC (2%) and higher rates with imlu+abema (22%). Conclusions: PROs from EMBER-3 demonstrated that patients with ESR1 m had better GHS/QOL and PF with imlu vs SOC, mirroring efficacy results. While the frequency of CTCAE defined ISRs was low, the high rate of PRO-CTCAE ISR demonstrates that this clinically relevant adverse event is underappreciated by physicians. Additionally, all pts had generally comparable GHS/QOL and PF with imlu+abema vs imlu. Overall, these results support the efficacy and safety of imlu compared to existing SOC. Clinical trial information: NCT04975308 .