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PURPOSE Imlunestrant is a next-generation oral selective estrogen receptor (ER) degrader designed to deliver continuous ER target inhibition, including in ESR1- mutant breast cancer. This phase Ia/b trial determined the recommended phase II dose (RP2D), safety, pharmacokinetics, and efficacy of imlunestrant, as monotherapy and in combination with targeted therapy, in ER-positive (ER+) advanced breast cancer (ABC) and endometrial endometrioid cancer. The ER+/human epidermal growth factor receptor 2–negative (HER2–) ABC experience is reported here. METHODS An i3+3 dose-escalation design was used, followed by dose expansions of imlunestrant as monotherapy or in combination with abemaciclib with or without aromatase inhibitor (AI), everolimus, or alpelisib. Imlunestrant was administered orally once daily and with the combination partner per label. RESULTS Overall, 262 patients with ER+/HER2– ABC were treated (phase Ia, n = 74; phase Ib, n = 188). Among patients who received imlunestrant monotherapy (n = 114), no dose-limiting toxicities or discontinuations occurred. At the RP2D (400 mg once daily), patients (n = 51) reported grade 1-2 nausea (39.2%), fatigue (39.2%), and diarrhea (29.4%). Patients at RP2D had received previous cyclin-dependent kinase 4/6 inhibitor (CDK4/6i; 92.2%), fulvestrant (41.2%), and chemotherapy (29.4%) for ABC and achieved a median progression-free survival (mPFS) of 7.2 months (95% CI, 3.7 to 8.3). Among patients who received imlunestrant + abemaciclib (n = 42) and imlunestrant + abemaciclib + AI (n = 43), most (69.4%) were treatment-naïve for ABC; all were CDK4/6i-naïve. Patients treated with imlunestrant + everolimus (n = 42)/alpelisib (n = 21) had received previous CDK4/6i (100%), fulvestrant (34.9%), and chemotherapy (17.5%) for ABC. No new safety signals or interactions with partnered drugs were observed. The mPFS was 19.2 months (95% CI, 13.8 to not available) for imlunestrant + abemaciclib and was not reached for imlunestrant + abemaciclib + AI. The mPFS with imlunestrant + everolimus/alpelisib was 15.9 months (95% CI, 11.3 to 19.1)/9.2 months (95% CI, 3.7 to 11.1). Antitumor activity was evident regardless of ESR1 mutation status. CONCLUSION Imlunestrant, as monotherapy or in combination with targeted therapy, had a manageable safety profile with evidence of preliminary antitumor activity in ER+/HER2– ABC.

1027 Background: Imlunestrant is a next-generation, oral SERD designed to deliver continuous ER-target inhibition. In the first-in-human Phase 1a/b EMBER study (NCT04188548), imlunestrant demonstrated favourable safety, PK, and clinical benefit when administered as monotherapy or with targeted therapy in ER+, HER2- aBC. Here we present clinical data of imlunestrant with HER2 targeted therapy, in patients (pts) with ER+, HER2+ aBC enrolled in EMBER. Methods: Pts were randomized to imlunestrant (400mg po daily) + trastuzumab (H), with or without (±) abemaciclib (Part C) or received maintenance treatment with imlunestrant + H + pertuzumab (P) (Part E), at standard doses in 21-day cycles. Key eligibility (Part C): ≥ 2 prior HER2 directed regimens, no prior CDK4/6i or fulvestrant; (Part E): received 1st line induction taxane chemotherapy (any duration) + H + P, without disease progression and ≤1 prior therapy for aBC. Key endpoints included safety, PK, ORR, CBR and PFS. Results: Overall, 45 pts with ER+/HER2+ aBC received the following combinations: imlunestrant + H (n=18), imlunestrant + H + abemaciclib (n=21; 1 pt received 800 mg imlunestrant), and imlunestrant + H + P (n=6). Baseline characteristics, safety, and preliminary efficacy are presented below (Table). Most TEAEs were grade 1-2. Common TEAEs with imlunestrant + H were fatigue (33%) and nausea (28%); with imlunestrant + H + abemaciclib were diarrhea (100%), neutropenia (57%), fatigue (52%) and nausea (48%); and with imlunestrant + H + P were diarrhea (50%) and arthralgia (50%). Imlunestrant PK was consistent with those previously reported with no drug-drug interactions observed with the combinations. From Part C, sequencing of baseline plasma ctDNA samples (n=37) identified prevalent mutations in TP53 (49%), PIK3CA (30%), ESR1 (24%), and GATA3 (14%). Clinical activity of imlunestrant in this ER+/HER2+ population, while based on small numbers, is promising. Conclusions: Imlunestrant in combination with trastuzumab ± abemaciclib or pertuzumab was well tolerated, showed no drug-drug interactions, and demonstrated preliminary antitumor activity in pts with ER+/HER2+ aBC. Clinical trial information: NCT04188548 . [Table: see text]

Abstract Hepatocellular carcinoma is one of the most lethal cancers in the world. Most of the patients are diagnosed at advanced stage, and they receive sorafenib as their treatment modality. However, sorafenib can only extend patients’ survival to a median of 3-4 months. Hence, there is an urgent need to identify a new therapeutic target for treatment of this disease. Increasing evidence showed that tumor-initiating cells (T-ICs) are intrinsically resistant to conventional treatments. Identification of the signalling pathways that maintain the functions of T-ICs provides potential targets for treatment of HCC. For this purpose, we cultured serial passages of hepatospheres combined with chemotherapeutic regimens as a strategy to enrich liver T-IC population. Using cDNA microarray, we found upregulation of several key enzymes in lipogenesis in enriched T-IC population, among which Stearoyl CoA desaturase-1 (SCD1), the enzyme involved in the conversion of saturated into monounsaturated fatty acids, was the most significant. In HCC clinical samples, 62% (36/58) was found to be overexpressed (> 2-fold) when compared with non-tumor counterparts and patients with SCD1 overexpression had shorter disease free survival. Using overexpression and knockdown approaches, SCD1 was found to regulate the traits of T-ICs, including tumorigenicity, self-renewal, drug resistance and expression of liver T-IC markers. SCD1 was found critical in survival of T-ICs, as enriched T-IC population are more sensitive to the inhibition of SCD1 compared to adherent lineages. Interestingly, SCD1 was found to be upregulated in sorafenib-resistant HCC cells. Pharmacological inhibition of SCD1 by A939572 not only suppressed self-renewal ability but also consistently enhanced the sensitivity towards sorafenib. Using a patient-derived xenograft model, we found that a new inhibitor against SCD1 demonstrated significant growth suppressive effect. By comparing the genetic profiles between SCD1 knockdown cells and control cells by RNA sequencing analysis, we found that upregulation of ER stress signature genes may be the potential downstream targets of SCD1 to maintain the properties of T-ICs. In conclusion, we demonstrated the crucial role of SCD1 in maintenance of liver T-ICs, and targeting SCD1 in combination with sorafenib might be a novel therapeutic regimen against HCC. Citation Format: Kin Fai Ma, Jessica Lo, Eunice Yuen-Ting Lau, John A. Copland, Irene Oi-Lin Ng, Terence Kin-Wah Lee. Targeting liver-tumor initiating cells via hampering the lipogenesis pathways through stearoyl - CoA desaturase. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 1003.