Prior Endocrine Therapy Research Articles

Abemaciclib plus non-steroidal aromatase inhibitor or fulvestrant in women with HR+/HER2- advanced breast cancer: Final results of the randomized phase III MONARCH plus trial.

In the interim analysis of MONARCH plus, adding abemaciclib to endocrine therapy (ET) improved progression-free survival (PFS) and objective response rate (ORR) in predominantly Chinese postmenopausal women with HR+/HER2- advanced breast cancer (ABC). This study presents the final pre-planned PFS analysis. In the phase III MONARCH plus study, postmenopausal women in China, India, Brazil, and South Africa with HR+/HER2- ABC without prior systemic therapy in an advanced setting (cohort A) or progression on prior ET (cohort B) were randomized (2:1) to abemaciclib (150mg twice daily [BID]) or placebo plus: anastrozole (1.0mg/day) or letrozole (2.5mg/day) (cohort A) or fulvestrant (500mg) (cohort B). The primary endpoint was PFS of cohort A. Secondary endpoints included cohort B PFS (key secondary endpoint), ORR, overall survival (OS), safety, and health-related quality of life (HRQoL). In cohort A (abemaciclib: n =207; placebo: n =99), abemaciclib plus a non-steroidal aromatase inhibitor improved median PFS vs . placebo (28.27months vs . 14.73months, hazard ratio [HR]: 0.476; 95% confidence interval [95% CI]: 0.348-0.649). In cohort B (abemaciclib: n =104; placebo: n =53), abemaciclib plus fulvestrant improved median PFS vs . placebo (11.41months vs . 5.59months, HR: 0.480; 95% CI: 0.322-0.715). Abemaciclib numerically improved ORR. Although immature, a trend toward OS benefit with abemaciclib was observed (cohort A: HR: 0.893, 95% CI: 0.553-1.443; cohort B: HR: 0.512, 95% CI: 0.281-0.931). The most frequent grade ≥3 adverse events in the abemaciclib arms were neutropenia, leukopenia, anemia (both cohorts), and lymphocytopenia (cohort B). Abemaciclib did not cause clinically meaningful changes in patient-reported global health, functioning, or most symptoms vs . placebo. Abemaciclib plus ET led to improvements in PFS and ORR, a manageable safety profile, and sustained HRQoL, providing clinical benefit without a high toxicity burden or reduced quality of life. ClinicalTrials.gov (NCT02763566).

Absolute lymphocyte count predicts efficacy of palbociclib in patients with metastatic luminal breast cancer

BackgroundAbsolute lymphocyte count (ALC) is a predictive and prognostic factor for various tumor types, including breast cancer. Palbociclib is a CDK4/6 inhibitor widely used for the treatment of metastatic estrogen receptor (ER)-positive, HER2-negative breast cancer. However, predictive biomarkers of the efficacy of palbociclib remain unelucidated. We conducted a retrospective study to examine the predictive value of the baseline ALC in patients treated with palbociclib.MethodsThe medical records of patients with ER-positive, HER2-negative breast cancer treated with palbociclib plus hormonal therapy between December 2017 and December 2021 were analyzed retrospectively. The cutoff value of ALC was set at 1800 cells/μL at the initiation of palbociclib treatment. The clinical benefit rate (CBR) was defined as the rate of complete or partial response or stable disease for at least 6 months. Progression-free survival (PFS) rates were estimated using the Kaplan–Meier method and compared using the log-rank test. Univariate and multivariate analyses were performed using Cox proportional hazards regression.ResultsAll of the 202 patients were women, with a median age of 59 years and a performance status (PS) of ≤ 2. The median numbers of lines of chemotherapy and endocrine therapy before palbociclib treatment were 0 (range, 0–9) and 1 (range, 0–7), respectively. Fifty-one patients had liver metastases. Forty-six patients tested negative for progesterone receptor (PgR) expression. The median follow-up time was 9.1 months. The CBR was significantly higher in the ALC-high group than in the ALC-low group (79% vs. 60%; P = 0.018). The median PFS was significantly longer in the ALC-high group than in the ALC-low group (26.8 months vs. 8.4 moths, respectively; P = 0.000013). ALC, age, PS, PgR status, prior chemotherapy, prior endocrine therapy, and liver metastasis were entered into the multivariate analysis. ALC was identified as an independent factor for PFS (P = 0.00085), along with liver metastasis (P = 0.0020), PS (P = 0.026), and prior endocrine therapy (P = 0.019).ConclusionALC can serve as a predictor of palbociclib efficacy in patients with metastatic ER-positive, HER2-negative breast cancer.

Trastuzumab deruxtecan (T-DXd) vs physician’s choice of chemotherapy (TPC) in patients (pts) with hormone receptor-positive (HR+), human epidermal growth factor receptor 2 (HER2)-low or HER2-ultralow metastatic breast cancer (mBC) with prior endocrine therapy (ET): Primary results from DESTINY-Breast06 (DB-06).

LBA1000 Background: T-DXd is approved for HER2-low (IHC 1+ or 2+/ISH-negative) mBC after ≥1 line of chemotherapy (CT). DB-06 (NCT04494425) evaluated T-DXd in pts with HER2-low or -ultralow (IHC 0 with membrane staining), HR+ mBC after disease progression (PD) on endocrine-based therapy and no prior CT for mBC. Methods: Pts with HER2-low or -ultralow, HR+ mBC were randomized 1:1 to T-DXd 5.4 mg/kg or TPC. Pts had no prior CT for mBC, with ≥2 lines of ET for mBC, or 1 line of ET for mBC if PD occurred ≤24 months (mo) of adjuvant ET or ≤6 mo of ET+CDK4/6i for mBC. Primary endpoint was progression-free survival (PFS) by blinded independent central review (BICR) in HER2-low. Key secondary endpoints were PFS in intent-to-treat (ITT = HER2-low and -ultralow) and overall survival (OS). Other endpoints included objective response rate (ORR) and safety. Results: As of Mar 18, 2024, 866 pts (HER2-low, n=713; HER2-ultralow, n=153) were randomized; 90.4% had prior CDK4/6i. TPC group pts were selected for capecitabine (59.8%), nab-paclitaxel (24.4%) or paclitaxel (15.8%). T-DXd significantly improved PFS vs TPC in HER2-low (HR, 0.62 [95% CI 0.51, 0.74], P<0.0001; median, 13.2 vs 8.1 mo). ITT and HER2-ultralow results were consistent with HER2-low (Table). Median treatment duration was 11.0 mo (T-DXd) vs 5.6 mo (TPC). OS was immature at first interim analysis (HER2-low HR, 0.83 [95% CI 0.66, 1.05], P=0.1181; median follow up, 18.6 mo). Grade (Gr) ≥3 drug-related adverse events occurred in 40.6% (T-DXd) vs 31.4% (TPC). Adjudicated interstitial lung disease / pneumonitis occurred in 49 (11.3%; 0.7% Gr 3/4, 0.7% Gr 5) vs 1 (0.2% Gr 2) pts receiving T-DXd vs TPC. Conclusions: T-DXd showed a statistically significant and clinically meaningful PFS benefit vs TPC (CT) in HER2-low mBC. HER2-ultralow results were consistent with HER2-low. Safety was in line with known profiles. DB-06 establishes T-DXd as a standard of care following ≥1 endocrine-based therapy for pts with HER2-low and -ultralow, HR+ mBC. Clinical trial information: NCT04494425 .[Table: see text]

SACI-IO HR+: A randomized phase II trial of sacituzumab govitecan with or without pembrolizumab in patients with metastatic hormone receptor-positive/HER2-negative breast cancer.

LBA1004 Background: Sacituzumab govitecan (SG) is a TROP2 antibody drug conjugate (ADC) with a topoisomerase I inhibitor payload (SN-38) approved for previously treated triple negative and hormone receptor-positive/HER2-negative (HR+/HER2-) metastatic breast cancer (MBC). Double-strand DNA breaks induced by SN-38 activate cGAS-STING, stimulating type I IFN production and T cell recruitment. SN-38 can upregulate MHC class-I and PD-L1 expression, enhance cytotoxic T cell effector functions and deplete regulatory T cells. To evaluate if SG synergizes with pembrolizumab (PD-1 inhibitor) we conducted a randomized, open-label phase 2 study comparing SG with or without pembrolizumab in HR+/HER2- MBC (NCT04448886). Methods: Eligible patients (pts) had unresectable locally advanced or metastatic HR+ (ER≥1% and/or PR≥1%), HER2- breast cancer treated with ≥1 prior endocrine therapy and 0-1 chemotherapy (CT) for MBC. Pts with brain metastases were eligible if locoregional therapy was completed and steroids discontinued ≥7 days before study therapy. Pts who received prior topoisomerase I inhibitor ADC, irinotecan or PD-1/L1 inhibitors were excluded. Pts were randomized 1:1 to Arm A [SG 10 mg/kg IV (D1, D8) + pembrolizumab 200 mg IV (D1), 21-day cycle] or Arm B (SG alone). The primary endpoint was progression-free survival (PFS); secondary endpoints included PFS in PD-L1+ pts (22C3 CPS ≥1), overall survival (OS), objective response rate (ORR) and toxicity (NCI CTCAE v5.0). Baseline and on-treatment biopsies were performed for correlative analyses. For this preliminary analysis, data were locked 1/12/24. Results: Between 03/2021-01/2024, 110 pts enrolled; 104 pts (52 Arm A; 52 Arm B) started study therapy and were included in the analysis. Median age was 57 yrs (range: 27-81); 102 pts (98.1%) were female. 80 pts (76.9%) received prior CDK4/6 inhibitor for MBC; 58 (55.8%) had no prior CT, 46 (44.2%) had 1 prior line of CT for MBC. At a median follow-up of 9.2 months (mo), median PFS was 8.4 mo in Arm A vs 6.2 mo in Arm B (HR 0.76, 95% CI 0.47-1.23, log-rank p=0.26); ORR 21.2% and 17.3%, respectively. OS data are immature with only 26 events to date; OS was 16.9 mo vs 17.1 mo (HR 0.65, 95% CI 0.30-1.41, log-rank p=0.28), respectively. The most frequent treatment-related toxicities (≥G2) in Arm A were neutropenia (67.3%), fatigue (36.5%), alopecia (36.5%), anemia (32.7%), leukopenia (26.9%), diarrhea (21.2%) and nausea (21.2%); in Arm B, neutropenia (59.6%), alopecia (38.5%), diarrhea (34.6%), nausea (32.7%), fatigue (32.7%) and anemia (21.2%). Conclusions: Addition of pembrolizumab to SG showed a non-significant trend toward improved PFS in unselected HR+/HER2- MBC at this preliminary time point. Final PFS and updated OS with further follow-up will be presented at the meeting. Exploratory outcome analyses by TROP2 and PD-L1 expression will be reported. Clinical trial information: NCT04448886 .

Real-world (RW) elacestrant use patterns and therapeutic outcomes in patients (pts) with hormone receptor-positive (HR+)/HER2-negative advanced breast cancer (aBC).

1071 Background: Elacestrant was the first novel endocrine therapy (ET) to receive FDA approval in over two decades. In the EMERALD study, pts with ESR1-mutant ( ESR1m) HR+/HER2-negative aBC had a median progression free survival (PFS) of 3.8 months (mos). Here, we use a large clinical-genomic database to assess the RW use of elacestrant and examine the impact of line of therapy (LOT) and number of ERS1m on pt outcomes. Methods: Pts with HR+/HER2- aBC were identified via the GuardantINFORM database; all pts with ESR1m detected on circulating tumor DNA (ctDNA) testing done in routine clinical care and treated with elacestrant after the FDA approval date were included. Real-world time to treatment discontinuation (TTD) and time to next treatment (TTNT) were used as proxies for PFS (measured in mos). For TTD/TTNT, Kaplan-Meier curves and median time-to-event with 95% confidence intervals (CI) were used. For outcomes specific to single versus >1 ESR1m, only pts with an ESR1m detected within the 6 months prior to elacestrant start were included. Results: 418 aBC pts were treated with elacestrant, with a median age of 62 years. Elacestrant was used across metastatic LOT (2nd: 12%, 3rd: 18%, 4tH: 17%, 5th+: 53%). Considering all prior therapies, pts commonly received prior aromatase inhibitors (AI) (89%), CDK4/6i (73%), fulvestrant (63%), and chemotherapy (49%). Of 312 pts with testing within 6 months of elacestrant start, 56% harbored a single ESR1m, with a maximum of 11 ESR1m detected in a single patient. TTD was 5.4 mos (95% CI: 4.2 - not reached [NR]) and TTNT was 6.2 mos (95% CI: 5.7 - NR); most pts were censored at the time of analysis (65% for TTD, 71% for TTNT). There were no significant differences in TTD or TTNT based on number of prior LOT or number of ESR1m mutations (Table 1). Data was not yet mature to assess differences in outcomes based on prior ET use, duration of prior CDK4/6i exposure, specific ESR1m, or co-occurring alterations. Conclusions: Using a clinical-genomic database, we present early RW insights of aBC pts treated with elacestrant. Initial outcomes, as assessed via TTD and TTNT, exceed the PFS reported in the EMERALD trial, perhaps due to multiple factors impacting patient selection and restaging protocols, which can differ in prospective trials versus routine clinical care. Factors such as prior LOT and number of ESR1m did not impact patient outcomes; additional factors such as prior ET use, duration of CDK4/6i treatment, specific ESR1m, and co-occurring mutations will be assessed as data matures. [Table: see text]

PionERA Breast Cancer (BC): Phase III study of first-line (1L) giredestrant vs. fulvestrant, both combined with a cyclin-dependent kinase 4/6 inhibitor (CDK4/6i), in patients (pts) with estrogen receptor-positive (ER+), HER2– locally advanced/metastatic BC (LA/mBC) with resistance to prior adjuvant (adj) endocrine therapy (ET).

TPS1130 Background: Pts with ER+, HER2– LA/mBC who relapse on/after adj ET and who have endocrine resistance represent a population with a high unmet need; for pts who relapse on or <12 months of adj aromatase inhibitors, fulvestrant + a CDK4/6i is 1L standard of care. However, fulvestrant has limited efficacy in pts with ESR1-mutated ( ESR1m) tumors and requires monthly intramuscular (IM) injections that negatively impact quality of life (QoL). New treatment (tx) options, such as combinations of an oral selective ER degrader (SERD) + a CDK4/6i (beyond palbociclib), are needed to improve survival, QoL, tolerability, and adherence. Giredestrant is a highly potent, nonsteroidal, oral selective ER antagonist and degrader that achieves robust ER occupancy and is active regardless of ESR1m status. Giredestrant was well tolerated with promising activity, as monotherapy and in combination with any approved CDK4/6i, in Phase I/II studies in ER+, HER2– BC. pionERA BC is the first study to investigate an oral SERD (giredestrant) + investigator’s (inv’s) choice of CDK4/6i in pts with 1L ET-resistant ER+, HER2– LA/mBC. Methods: pionERA BC (NCT06065748) is a Phase III, global, randomized, open-label, multicenter study in pts with ER+, HER2– LA/mBC with resistance to prior adj ET. Pts will be randomized 1:1 to oral daily 30 mg giredestrant or IM 500 mg fulvestrant (Days 1 and 15 of Cycle 1, then once monthly), both in combination with inv’s choice of CDK4/6i (palbociclib, abemaciclib, or ribociclib), stratified by: prior adj CDK4/6i (yes vs. no); choice of CDK4/6i; ESR1m status by central testing ( ESR1m vs. no ESR1 mutation detected [ ESR1nmd]); disease site (visceral vs. non-visceral). Pts will receive tx until disease progression, limiting toxicity, death, or consent withdrawal. Eligibility: Female/male pts with ER+, HER2– LA/mBC; resistance to prior adj ET (± CDK4/6i); no prior systemic tx for LA/mBC; documented ESR1m status by circulating tumor DNA. Enrollment of pts with ESR1nmd tumors will be capped at 60% of the total study population. Co-primary endpoints (EPs): Inv-assessed progression-free survival (PFS) in the ESR1m subgroup and in the full analysis set. Secondary EPs: PFS ( ESR1nmd subgroup); overall survival (OS); inv-assessed confirmed overall response rate; duration of response; clinical benefit rate; time to chemotherapy; safety; pt-reported outcomes. The co-primary EPs will be compared between tx arms using a stratified log-rank test at an overall 0.05 significance level (two-sided). An interim OS analysis is planned at the time of the primary PFS analysis; an independent data monitoring committee will be in place for safety. Target enrollment: 1050 pts; recruitment ongoing. Clinical trial information: NCT06065748 .

Entinostat, a class I selective histone deacetylase inhibitor, plus exemestane for Chinese patients with hormone receptor-positive advanced breast cancer: An overall survival update and long-term safety from the randomised, double-blind, placebo-controlled, phase 3 trial.

1050 Background: Entinostat, a synthetic benzamide derivative HDAC inhibitor, selectively targets Class I HDAC enzymes. In a randomized, double-blind, phase 3 trial, entinostat with exemestane demonstrated a statistically significant progression-free survival (PFS) benefit compared to placebo, with a median PFS of 6.32 months versus 3.72 months (HR 0.76, 95% CI 0.58-0.98, p=0.046; Binghe Xu, et al. Acta Pharm Sin B. 2023). This analysis focus on the results of overall survival (OS) and long-term safety. Methods: Patients eligible for this study had HR+/HER2– advanced breast cancer (ABC) with disease relapse or progression following at least one prior endocrine therapy (ET). They were randomly assigned in a 2:1 ratio to receive either entinostat (5mg, weekly) or placebo in combination with exemestane until disease progression or unacceptable toxicity. The primary endpoint of the study was PFS. Secondary endpoints included OS, objective response rate, clinical benefit rate and safety. Median OS was estimated using the Kaplan-Meier method and the HR and its 95% CI were calculated using the multivariate Cox proportional hazard regression model. Results: A total of 354 patients were randomized to receive either entinostat (n=235) or placebo (n=119). As of the data cutoff date (July 31, 2023), 196 (55.4%) OS events had occurred, with a median follow-up of 31.83 months. There were 127 (54.0%) deaths in the entinostat arm compared to 69 (58.0%) deaths in the placebo arm. The entinostat demonstrated improved OS compared to placebo, with a median OS of 38.39 months versus 29.18 months (HR 0.837, 95% CI 0.624-1.124, p=0.237) in the full analysis set (FAS). The results of the per-protocol analysis set (PPS) were consistent with those of the FAS, showing a median OS of 39.01 months versus 33.98 months (HR 0.796, 95% CI 0.588-1.079, p=0.142). Notably, 179 (76.2%) patients in the entinostat arm and 100 (84%) patients in the placebo arm received new anti-cancer regimens, including CDK4/6 inhibitors (14.5% vs 24.4%), chemotherapy (54.9% vs 63.9%), endocrine therapy (40.4% vs 42.9%), and herbal medicines (9.8% vs 3.4%). After adjusting for the imbalance in the use of CDK4/6 inhibitors and chemotherapy between the treatment arms, the HR for overall survival was 0.78 (95% CI 0.58-1.05, p=0.097). The updated safety results were generally consistent with the previous analysis. Conclusions: The combination of entinostat with exemestane has provided clinically meaningful OS benefit in addition to the previously observed PFS benefit over exemestane alone in patients with pre-treated, ET-resistant ABC. These findings further support entinostat as an effective and safe treatment option for the specific patient population. Clinical trial information: NCT03538171 .

GOG3069: Phase 2 study of alpelisib and fulvestrant for PIK3CA-mutated estrogen receptor (ER)-positive endometrioid endometrial cancers.

TPS5639 Background: Treatment options for persistent, metastatic, or recurrent endometrial cancer continues to evolve as we learn more about the molecular landscape of this disease. Recent studies have shown a benefit to endocrine-based combinations such as aromatase inhibitors with mTOR or CDK4/6 inhibitors especially in patients who had not received prior chemotherapy ( 1– 3). The presence of oncogenic PIK3CA mutations have been shown to impact response to aromatase inhibitors in breast cancer. The combination of alpelisib and fulvestrant was approved for the treatment of metastatic hormone receptor-positive, HER2-negative breast cancer harboring oncogenic mutations in the PIK3CA gene that progressed on prior endocrine therapy( 4). PIK3CA mutations are commonly identified in endometrial cancers and may similarly confer endocrine resistance. GOG3069 is a phase II trial designed to evaluate the efficacy and safety of alpelisib and fulvestrant in a biomarker-selected endometrial cancer patient population. Methods: This is a phase II non-randomized, open label, multicenter, two-stage study. Eligible patients must have advanced (FIGO 2014 Stage III or IV), persistent, or recurrent endometrial carcinoma with measurable disease by RECISTv1.1 criteria. Patients cannot be curable by surgery or radiotherapy. Disease must be of endometrioid histology with positive expression of estrogen receptor (defined as ≥1% of tumor cells demonstrating positive nuclear staining by immunohistochemistry) and oncogenic PIK3CA mutation. Patients may have received no more than 3 prior lines of therapy including 1 prior line of systemic chemotherapy. Enrolled patients will receive treatment with alpelisib 300mg orally daily and fulvestrant 500mg IM on Day 1 and Day 15 of Cycle 1, then 500mg IM on Day 1 of each 28-day cycle. The primary endpoint is overall response rate as measured by RECISTv1.1-defined tumor response. Secondary endpoints include safety, tolerability, progression-free survival, duration of response, and 24-month overall survival rate. GOG3069 activated in November 2023 and expects to enroll across 15 US sites. The statistical plan uses a conditional stratified design factoring differing probabilities of response based on whether the patient had prior chemotherapy exposure. ClinicalTrials.gov Identifier: NCT05154487

HOPE: Harnessing olaparib, palbociclib, and endocrine therapy for BRCA1/2-associated HR+, HER2- metastatic breast cancer.

10616 Background: Thepoly (ADP-ribose) polymerase inhibitor olaparib yields superior progression-free survival, response rates and patient-reported outcomes compared with chemotherapy in patients with BRCA1- or BRCA2- ( BRCA1/2) associated metastatic breast cancer (MBC). Fulvestrant and palbociclib improve outcomes in patients with hormone receptor-positive (HR+) MBC. HOPE (NCT03685331) is a phase I trial to evaluate combination olaparib (O), fulvestrant (F), and palbociclib (P) in patients with BRCA1/2-associated HR+, HER2 negative MBC. Methods: A 3+3 dose escalation was used to assess safety and preliminary efficacy of O, F and P. Eligible participants (pts) had HR+ MBC, a germline (g) or somatic (s) mutation in BRCA1/2, measurable/evaluable disease, any/no prior endocrine therapy, and 0-2 prior lines of chemotherapy for MBC. Prior PARPi and CDK4/6i were permitted but not in combination. Palbociclib was added after a 28-day safety run-in (Cycle 0) of O + F alone. Fulvestrant dose was 500mg IM on day 1 of each 28-day cycle. Dose level 0 (DL0, starting level) was F, O 300mg orally BID continuously, P 75mg orally daily on days 1-21; DL-1 was F, O 250mg orally BID, P 75mg orally daily on days 1-21. The primary endpoint was MTD (adverse events (AEs) based on CTCAE v5.0). Dose limiting toxicity (DLT) period included Cycles 0 and 1. A 30% DLT rate was acceptable; 6 pts had to be treated at a dose for it to be declared MTD. RECIST v1.1 was used to evaluate response. Results: Eight treated pts (s BRCA1:1 ;g BRCA2:6; s BRCA2:1 ) had median (M) age 47; M 0 (0-2) prior lines of endocrine therapy and 0 (0-1) prior chemotherapies for MBC. Three pts each had received O, F, and CDK4/6i as part of their standard of care treatment before study. At DL0, 2/2 pts had DLT with persistent grade(G) 3 neutropenia. Among 6 pts treated at DL-1, none experienced DLT, but 3 pts missed doses of study treatment due to cytopenias (G3 neutropenia, G3 leukopenia, G3 thrombocytopenia) during the DLT period. Following the DLT period, 1 pt reduced O to 200mg orally daily and 1 discontinued P based on provider discretion due to cytopenias (G3 neutropenia, G3 thrombocytopenia). Grade 3 AEs occurring in >1 pt related to study treatment were: neutrophil count decrease (G3/2, n=5/2); white blood cell decrease (G3/2/1, n=3/2/2); platelet count decrease (G3/2/1, n=2/0/1). Overall related AEs in >50% of pts were anemia (n=7), fatigue (n=6) and nausea (n=5). Best responses were CR/PR/SD/PD in 1/1/5/1 pts. Conclusions: MTDof O, F and P (DL-1) was reached and demonstrated antitumor activity. Although no AEs meeting protocol-specified DLT criteria were observed at MTD, hematologic toxicity at this dose complicated drug delivery and required close monitoring during and after the DLT period. Combinations utilizing PARP1 selective inhibitors, lower doses of therapy or sequential rather than combination therapy may improve feasibility. Clinical trial information: NCT03685331 .

Bireociclib plus fulvestrant for advanced HR+/HER2- breast cancer progressing after endocrine therapy: Interim analysis of a phase 3 trial (BRIGHT-2).

1058 Background: Bireociclib is a new selective inhibitor of CDK4/6 which showed promising clinical activity and manageable tolerability both as monotherapy and in combination with endocrine therapy (ET). This phase III study (BRIGHT-2, NCT05077449) was to evaluate the efficacy and safety of Bireociclib plus fulvestrant in HR+/HER2- advanced breast cancer (ABC) that progressed on or after previous ET. Methods: BRIGHT-2 study was a multicenter, randomized, double-blinded, phase 3 trial in patients with HR+/HER2- ABC who had progressed on or after prior ET. Eligible patients were randomized 2:1 to receive Bireociclib (360 mg po bid) or placebo with fulvestrant (500 mg im, cycle1 d1, d15, then d1) on each 28-day cycle. The primary endpoint was PFS based on investigator’s assessment per RECIST v1.1. The interim analysis was conducted when 70% PFS events occurred. Results: A total of 305 Chinese female patients were randomly allocated to receive Bireociclib plus fulvestrant (BF, n = 204) or placebo plus fulvestrant (F, n = 101). 208 (68.2%) patients had visceral metastases, 78 (25.6%) patients with primary resistance to ET and 73 (23.9%) patients had received chemotherapy in metastatic setting. As of March 28, 2023, with a median follow-up of 8.7 mo, BF significantly prolonged the PFS per investigator with 12.94 (95%CI, 11.07-NR) mo compared to 7.29 (95%CI, 5.45-11.04) mo in the F group (HR, 0.561; 95%CI, 0.393-0.799; P= 0.0012). The median PFS assessed by blinded independent central review (BICR) were no-reached (95%CI, NR-NR) in BF group and 7.46 (95%CI, 5.49-NR) mo in F group (HR, 0.461; 95%CI, 0.312-0.683; P<0.0001) which were consistent with the investigator’s assessment. The subgroup analyses revealed that in BF group, PFS was improved compared across all subgroups with HRs < 1. For patients with primary resistance to ET, with liver metastases and with bone-only metastases, the HRs were 0.254 (95%CI: 0.128-0.503), 0.381 (95%CI: 0.226-0.644) and 0.234 (95%CI: 0.070-0.787) respectively. In BF group, the most common TEAEs were diarrhea [92.6% (all grades); 5.4% (≥grade 3)], neutropenia [87.3%; 31.4%], leukocytopenia [83.3%; 18.6%] and anemia [66.7%; 10.8%]. Most of AEs were grade 1 or grade 2 and could be alleviated or cured by dose adjustment or symptomatic treatment. Treatment discontinuation due to TEAE was reported in 3 (1.5%) patients with BF. As of March 28, 2023, 120(58.8%) patients in BF were receiving treatment and the trail were ongoing. Conclusions: Bireociclib plus fulvestrant showed superior efficacy for pretreated HR+/HER2- ABC, especially in patients with primary resistance to ET, liver metastases or bone-only metastases. The toxicity of Bireociclib plus fulvestrant were well tolerated and manageable with no new safety signal. Clinical trial information: NCT05077449 .

Effect of timing of recurrence on outcomes in patients with metastatic breast cancer treated with CDK4/6 inhibitors.

e13067 Background: CDK4/6 inhibitors (CDKi) + endocrine therapy (ET) comprise the standard 1st line treatment for ER+ HER2- metastatic breast cancer (MBC). However, there is a gap in understanding how timing of MBC recurrence affects clinical outcomes on CDKi. We conducted a study to compare clinical outcomes of MBC patients on CDKi based on timing of MBC recurrence. Methods: This retrospective study enrolled ER+ HER2- MBC patients diagnosed from Jan 2015 to Jan 2023, who received CDKi + ET as 1st line treatment for MBC. Patients were categorized as de novo MBC (dMBC), if they had 1. Stage 4 disease at initial diagnosis or 2. MBC diagnosed < 6 months of the primary breast cancer (PBC) diagnosis and before starting any systemic treatment for PBC. Patients with distant recurrence occurring < 5 years, 5-10 years, and >10 years from PBC were categorized as early, intermediate, and late MBCs (eMBC, iMBC, laMBC). Endpoints were Progression-Free Survival (PFS) and Overall Survival (OS) from MBC diagnosis. Baseline variables were compared using t-tests (continuous) and Chi-square tests (categorical). Endpoint analyses were conducted using Kaplan-Meier method and compared using Log rank test. Multivariate analyses were performed using Cox models. Results: Out of 280 patients enrolled, 67 were dMBC, 75 eMBC, 56 iMBC, and 82 laMBC. The laMBC group had a higher mean age (years) compared to other groups (71 vs. 62 in dMBC+ eMBC + iMBC, p<0.001). 64.5% of laMBC had visceral disease compared to 50.5% of dMBC + eMBC + iMBC (p=0.031). 65.7% of dMBC patients were treated with palbociclib compared to 80.8% of eMBC + iMBC + laMBC (p=0.010). Higher % of eMBC were PR- vs other cohorts (41.3 vs 23.9 dMBC + iMBC + laMBC, p<0.001). Among the 3 non dMBC groups, prior ET sensitivity (no recurrence for 2 years on adjuvant ET) was lower in eMBC (45.3%) compared to iMBC + laMBC (81.9%, p<0.001). 61.3% of eMBC patients were on adjuvant ET at recurrence compared to 17.4% of iMBC + laMBC (p<0.001). Other variables were similar. Table shows unadjusted endpoint analyses. In multivariate analyses, the risk of PFS events were not significantly different among cohorts (age<=65, ECOG performance status (PS) >=2 and PR- were independently associated with shorter PFS); risk of death was higher in eMBC vs laMBC (HR 1.73, p=0.047). Conclusions: In this retrospective study of ER+ HER2- MBC patients treated with 1st line CDKi + ET, timing of MBC recurrence was not associated with PFS differences after adjusting for other variables. However, patients with MBC recurring < 5 years of PBC had the higher likelihood of death compared to those recurring >10 years of PBC. [Table: see text]

Baseline (BL) characteristics and efficacy endpoints for patients (pts) with node-negative (N0) HR+/HER2− early breast cancer (EBC): NATALEE trial.

512 Background: NATALEE assessed ribociclib (RIB) + non-steroidal aromatase inhibitor (NSAI) vs NSAI alone in pts with HR+/HER2− EBC at increased risk of recurrence, including pts with N0 disease, and showed a statistically significant invasive disease-free survival (iDFS) benefit. We report BL characteristics, efficacy, and safety for the N0 subgroup. Methods: Pts with stage II/III HR+/HER2− EBC were randomized (R) 1:1 to RIB 400 mg/d (3 wk on/1 wk off for 36 mo) + NSAI (letrozole or anastrozole for ≥60 mo) or NSAI alone. Men and premenopausal women received goserelin. Pts could receive any standard (neo)adjuvant endocrine therapy (ET) ≤12 mo before randomization. In the intent-to-treat (ITT) population, 78% of pts completed 3y of RIB treatment (tx) or discontinued early with 21% still on tx at the time of this analysis (data cutoff: July 21, 2023; median follow up, 33 mo). The N0 subgroup included pts with T2N0 (stage IIA; with grade [G]3, or G2 disease and Ki-67 ≥20% or high genomic risk [HGR]), T3N0 (stage IIB), and T4N0 (stage IIIB); T1N0 was excluded. Results: Of 2549 pts R to RIB + NSAI and 2552 to NSAI alone, 285 (11%) and 328 (13%) had N0 disease, respectively. BL characteristics for the N0 subgroup were balanced across arms. Most pts were premenopausal (RIB + NSAI: 67% vs NSAI alone: 63%). Pts with N0 disease had anatomic stage IIA (RIB + NSAI: 74% vs NSAI alone: 73%), stage IIB (17% vs 13%), or stage IIIB (9% vs 12%) disease (Table). Most pts received prior chemotherapy (RIB + NSAI: 72% vs NSAI alone: 71%); prior ET was received by 59% vs 62% of pts with RIB + NSAI vs NSAI alone. For pts with T2N0 disease treated with RIB + NSAI vs NSAI alone, 49% vs 45% had G3 disease and 49% vs 53% had G2 disease (54% vs 56% with T2N0, G2 & Ki-67 > 20%; 13% vs 23% with T2N0, G2, Ki-67 ≤20%/missing & HGR). Consistent with the ITT population, RIB + NSAI improved iDFS (HR, 0.72; 95% CI, 0.41-1.27; 3y rate with RIB + NSAI vs NSAI alone, 93.2% vs 90.6%), distant disease–free survival (DDFS; HR, 0.70; 95% CI, 0.38-1.29; 3y rate, 94.3% vs 91.5%), and distant recurrence–free survival (DRFS; HR, 0.58; 95% CI, 0.29-1.17; 3y rate, 96.3% vs 92.5%) in pts with high-risk N0 disease. The safety profile of RIB in the N0 subgroup was consistent with the ITT population. The rate of discontinuation due to all grade adverse events was 24% vs 8% with RIB + NSAI vs NSAI alone. Conclusions: This analysis showed a consistent efficacy benefit and manageable safety profile of RIB + NSAI vs NSAI alone in pts with N0 disease in NATALEE. The 3y iDFS, DDFS, and DRFS rates in the control arm at this early follow-up underscore the risk of recurrence for the N0 subgroup. These findings support the use of RIB in pts with HR+/HER2− EBC at increased risk of recurrence, including pts with N0 disease. Clinical trial information: NCT03701334 . [Table: see text]

Abemaciclib plus a nonsteroidal aromatase inhibitor as initial therapy for HR+, HER2− advanced breast cancer: final overall survival results of MONARCH 3

In MONARCH 2, the addition of abemaciclib to fulvestrant significantly improved both progression-free survival (PFS) and overall survival (OS) in patients with HR+, HER2- advanced breast cancer (ABC) with disease progression on prior endocrine therapy (ET). In MONARCH 3, the addition of abemaciclib to a nonsteroidal aromatase inhibitor (NSAI) as initial therapy for HR+, HER2- ABC significantly improved PFS. Here, we present the prespecified final OS results for MONARCH 3. MONARCH 3 is a randomized, double-blind, phase 3 study of abemaciclib plus NSAI (anastrozole or letrozole) versus placebo plus NSAI in postmenopausal women with HR+, HER2- ABC without prior systemic therapy in the advanced setting. The primary objective was investigator-assessed PFS; OS was a gated secondary endpoint, and chemotherapy-free survival (CFS) was an exploratory endpoint. A total of 493 women were randomized 2:1 to receive abemaciclib plus NSAI (n = 328) or placebo plus NSAI (n = 165). After a median follow-up of 8.1 years, there were 198 OS events (60.4%) in the abemaciclib arm and 116 (70.3%) in the placebo arm (hazard ratio, 0.804; 95% confidence interval [CI], 0.637-1.015; P = 0.0664, non-significant). Median OS was 66.8 versus 53.7 months for abemaciclib versus placebo. In the subgroup with visceral disease (sVD), there were 113 OS events (65.3%) in the abemaciclib arm and 65 (72.2%) in the placebo arm (hazard ratio, 0.758; 95% CI, 0.558-1.030; P = 0.0757, non-significant). Median OS was 63.7 months versus 48.8 months for abemaciclib versus placebo. The previously demonstrated PFS benefit was sustained, and CFS numerically improved with the addition of abemaciclib. No new safety signals were observed. Abemaciclib combined with an NSAI resulted in clinically meaningful improvement in median OS (ITT: 13.1 months; sVD: 14.9 months) in patients with HR+ HER2- ABC; however, statistical significance was not reached.

Abstract PO1-13-02: Circulating tumor DNA mutation landscape in HR+/HER2− patients with mBC treated with cyclin-dependent kinase 4/6 inhibitors in the SCRUM-Japan MONSTAR-SCREEN study

Abstract Introduction: Combination of cyclin-dependent kinase 4 and 6 inhibitors (CDK 4/6i) + endocrine therapy (ET) is recommended as first- or second-line treatment (1L or 2L Tx) for hormone receptor-positive, human epidermal growth factor receptor-2-negative metastatic breast cancer (HR+/HER2− mBC). In this study, we assessed cancer-specific genetic alterations in circulating tumor DNA (ctDNA) and tissues in abemaciclib-treated patients with HR+/HER2− mBC in Japan. Methods: MONSTAR-SCREEN is a multicenter study and part of a nation-wide cancer genome screening project for patients with advanced solid tumors (SCRUM Japan). Regardless of the Tx line, the current analysis included abemaciclib-treated patients with HR+/HER2− mBC (N=97). Data were collected between Jan 2020 and Dec 2021. Blood samples collected at ≤21 days before abemaciclib initiation (baseline; n=77) and at disease progression or Tx discontinuation (paired post-Tx; n=33) were tested with the FoundationOne® Liquid Companion Diagnostic panel. Additionally, archival or fresh tissue biopsy samples (n=79) were tested with the FoundationOne® Companion Diagnostic panel. Patient characteristics, baseline genetic alterations, neoplastic burden (measured by shedding rate and maximum variant allele frequency [VAF]), and alterations at clinical progression were reported. Results: The total population (N=97) included patients with next generation sequencing ctDNA and/or tissue data. All were female with median age of 57 years (interquartile range 50, 67). Bone, either alone or with other sites, was the most common metastatic site (61%). 78% of patients received abemaciclib in the first or second line of Tx (LoT). Fulvestrant with or without gonadotropin-releasing hormone (55%), letrozole (24%), and anastrozole (11%) were the most common concomitant drugs. Among patients with baseline ctDNA data (n=77), 30% and 21% had received prior ET and prior chemotherapy, respectively. In baseline ctDNA samples, PIK3CA (37%) and TP53 (28%) alterations were detected most frequently across all LoT; while ESR1 (16%), GATA3 (11%), and FGF3/4/19 (9%) were more frequently detected in samples from later LoT. The frequency of ESR1 alterations (p< 0.01), shedding rate, and maximum VAF (both p< 0.05) were significantly higher in prior ET recipients than those who did not receive prior ET. Frequency of post-Tx ESR1 alterations numerically decreased from baseline in patients on fulvestrant (23% to 18%) and numerically increased in patients on aromatase inhibitors (27% to 45%). Other genes showed either a numeric increase or similar alteration frequency between the baseline and post-tx. Newly acquired post-Tx alterations were detected in FGF3/4/19 (18%); PIK3CA, TP53, and RB1 (all 15%) and ESR1 (12%). The table shows PIK3CA and ESR1 alterations detected at baseline and post-Tx. In the tissue samples (n=79), TP53 (32% vs 20%) and ESR1 (14% vs 7%) alterations were numerically more frequent in metastatic versus primary lesions. Other gene alteration frequencies were similar. Conclusion: PIK3CA alterations were the most frequently detected genetic alterations in both pre- and post- abemaciclib Tx patients with HR+/HER2− mBC. Patients with prior ET had more frequent ESR1 alterations and higher neoplastic burden. Evidence from this study provides insight into the ctDNA dynamics and potential resistance mechanisms in patients with HR+/HER2− mBC. PIK3CA and ESR1 alterations detected in circulating tumor DNA at baseline and post abemaciclib treatment Citation Format: Masaya Hattori, Victoria Serelli-Lee, Yoichi Naito, Takashi Yamanaka, Hiroyuki Yasojima, Rikiya Nakamura, Takao Fujisawa, Mitsuho Imai, Yoshiaki Nakamura, Hideaki Bando, Tsutomu Kawaguchi, Takayuki Yoshino, Hiroji Iwata. Circulating tumor DNA mutation landscape in HR+/HER2− patients with mBC treated with cyclin-dependent kinase 4/6 inhibitors in the SCRUM-Japan MONSTAR-SCREEN study [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 PO1-13-02.

Abstract PS15-03: Vepdegestrant, a PROteolysis TArgeting Chimera (PROTAC) estrogen receptor (ER) degrader, plus palbociclib in ER–positive/human epidermal growth factor receptor 2 (HER2)–negative advanced breast cancer: phase 1b cohort

Abstract Background: Vepdegestrant (ARV-471) is an oral PROTAC ER degrader. In a phase 1/2 study (NCT04072952), vepdegestrant monotherapy had a favorable safety profile and encouraging clinical activity, and showed robust ER degradation. The phase 1b cohort of this study is evaluating vepdegestrant in combination with the cyclin-dependent kinase (CDK)4/6 inhibitor palbociclib (palbo). In xenograft models, vepdegestrant plus palbo showed substantially greater tumor growth inhibition vs fulvestrant plus palbo, supporting investigation in patients with breast cancer. Methods: Eligible patients for the phase 1b combination cohort had ER+/HER2- locally advanced/metastatic breast cancer and had received ≥1 prior endocrine therapy and ≤2 chemotherapy regimens for advanced disease; prior CDK4/6 inhibitor treatment was permitted. Vepdegestrant was given orally once daily (QD) continuously at doses of 180 mg (n=2), 200 mg (n=21), 400 mg (n=3), or 500 mg (n=20); palbo 125 mg was given orally QD for 21 days followed by 7 days off treatment in 28-day cycles. The primary endpoints were dose-limiting toxicities (DLTs) in the first cycle and safety (adverse events [AEs] and laboratory abnormalities). Enrollment in these 4 dose levels is complete; we report data as of June 6, 2023. Results: Across 46 patients in the phase 1b cohort, 45 (97.8%) patients were female with a median age of 62.0 y (range: 29–78). Patients had received a median of 4 prior therapies (range: 1–11) in any setting, including 87.0% prior CDK4/6 inhibitors (78.3% prior palbo), 80.4% prior fulvestrant, and 76.1% prior chemotherapy (45.7% in the metastatic setting). There were no DLTs. Treatment-emergent AEs (TEAEs) leading to dose reductions or discontinuation of vepdegestrant occurred in 5 and 4 patients, respectively. TEAEs leading to dose reductions or discontinuation of palbo occurred in 34 and 8 patients, respectively. Grade 3/4 treatment-related AEs (TRAEs) to either vepdegestrant or palbo in ≥10% of patients were neutropenia (89.1%), decreased white blood cell count (15.2%), and decreased platelet count (10.9%); there were no grade 5 TRAEs and no patients had febrile neutropenia. The clinical benefit rate (rate of confirmed complete response, partial response, or stable disease ≥24 wks) in patients treated with vepdegestrant plus palbo was 63.0% (95% CI: 47.5–76.8). The objective response rate in evaluable patients with measurable disease at baseline (n=31) was 41.9% (95% CI: 24.5–60.9). Pharmacokinetics (PK) showed dose-dependent exposure for vepdegestrant, consistent with data for vepdegestrant administered as monotherapy; palbo exposure was similar across dose levels of vepdegestrant and modestly higher compared with historical palbo PK data. Circulating tumor DNA levels were evaluated in patients who received vepdegestrant 200 mg QD plus palbo and demonstrated substantial and sustained decreases in ESR1 mutant allele fraction across multiple treatment cycles. Conclusions: The combination of vepdegestrant plus palbo showed promising clinical activity in heavily pretreated patients with ER+/HER2- advanced breast cancer who had received extensive prior treatment. The safety profile of vepdegestrant plus palbo was generally consistent with the known safety profiles of the 2 agents except for an increased occurrence of grade 3/4 neutropenia, which was readily managed with laboratory monitoring and dose reductions of palbo. There is an ongoing study lead-in to the global VERITAC-3 study (NCT05909397) that is evaluating 2 doses of palbo (100 mg and 75 mg) in combination with vepdegestrant 200 mg QD to determine the recommended phase 3 combination to compare with letrozole plus palbo as first-line treatment for ER+/HER2- advanced breast cancer. Citation Format: Erika Hamilton, Rinath Jeselsohn, Sara Hurvitz, Dejan Juric, Hyo Han, Melinda Telli, George Zahrah, Rita Nanda, Yuanyuan Zhang, Weiwei Tan, Elizabeth Duperret, Eric Zhi, Cecile Mather, Anne Schott. Vepdegestrant, a PROteolysis TArgeting Chimera (PROTAC) estrogen receptor (ER) degrader, plus palbociclib in ER–positive/human epidermal growth factor receptor 2 (HER2)–negative advanced breast cancer: phase 1b cohort [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 PS15-03.

Abstract PO2-27-05: Composite biomarkers for the prediction of progression-free survival with CDK4/6 inhibitors in metastatic HR+/HER2- breast cancer

Abstract CDK4/6 inhibitors are widely used in combination with endocrine therapy in the treatment of HR+/HER2- metastatic breast cancer. Generally the CDK4/6 inhibitors lead to a doubling of progression-free survival (PFS) over single agent endocrine therapy. In spite of a known mechanism of action, biomarkers to define the duration of response to CDK4/6 inhibitors with either aromatase inhibitors (AI) or fulvestrant (FUL) remain lacking. A cohort of over 300 patients receiving standard of care CDK4/6 inhibitor combination therapy for metastatic disease were consented for participation (NCT04526587). The duration of PFS in both AI and FUL treated sub-groups were comparable to that observed in randomized clinical trials. In this cohort progesterone receptor-status (HR 0.70, p=0.0123), visceral disease (HR 1.55, p=0.0013), prior endocrine therapy (HR 2.34, p< 0.001), and type of endocrine therapy (HR 2.16, p< 0.001) were associated with duration of PFS. To develop biomarkers, gene expression analyses was performed on 313 tumor samples representing over 200 individual patients. Focusing on the pre-treatment metastatic biopsy, established signatures associated with prognosis in HR+/HER2- breast cancer were employed to determine association with response to combination therapy in the metastatic setting. Proliferation-associated signatures were associated with shorter PFS. For example, an established RB loss signature was associated with PFS in both AI (HR 1.9, p=0.009) and FUL (HR 2.8, p=0.007) treated patients. Clinically employed signatures (e.g. OncotypeDx and Mammaprint) harbored variable associations with outcomes. Random Survival Forest feature selection was applied to individual and combined prognostic signatures. This approach resulted in a potent classifier for progression-free survival (HR 10, p< 1E-20). To enable applicability to different manifestations of disease, significant clinical pathological variables were incorporated into the model so that there was equivalent performance in AI and FUL treated patients for prediction of PFS. This predictive algorithm was also strongly associated with overall survival (HR 4.6, p=2.7E-8). Ongoing studies are evaluating the algorithms prospectively in NCT04526587. Citation Format: Agnieszka Witkiewicz, Jianxin Wang, Erik Knudsen, Ellis Levine, Tracey O'Connor. Composite biomarkers for the prediction of progression-free survival with CDK4/6 inhibitors in metastatic HR+/HER2- breast cancer [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 PO2-27-05.

Abstract PO1-23-10: Multi-omics analysis identifies the mechanism in cross-resistance to endocrine therapy and CDK4/6 inhibitor in breast cancer

Abstract Background In advanced/metastatic breast cancer (MBC) or recurrent breast cancer on prior endocrine therapy (ET), selective estrogen receptor downregulator (SERD) of fulvestrant and CDK4/6i are the mainstream therapy regimen. In the clinical, fulvestrant combined with CDK4/6i is the second-line therapy for breast cancer (BC) that had progressed during previous endocrine therapy. The alternation of backbone from aromatase inhibitor or tamoxifen (endocrine sensitive) to fulvestrant (endocrine resistance) partially comes from the mechanism of ET resistance. Moreover, in the PALOMA-2 trial, approximately 30% of patients developed into recurrence after two years of CDK4/6i treatment, indicating that the ET backbone may affect CDK4/6i efficacy, bringing an emerging clinical issue of cross-resistance. However, whether the cross-resistance of ET would be more sensitive to CDK4/6i or impair the efficacy of CDK4/6i is still an ambiguous and heterogeneous issue. Also, in CDK4/6i resistance, whether the mechanism of ET resistance is associated with the dysregulation of the cell cycle or activation of other “bypass” signaling pathways is not fully understood. Methods We use a 3D and 2D culture of ET-sensitive cell S0.5, tamoxifen-resistant cell TAM, and fulvestrant-resistant cell 182R6 as our model. The 3D culture was performed by using the Tools 3d culture plus kit. Drug responses to fulvestrant and ribociclib were conducted by using CCK-8 assay. The potential target and pathway involved in the mechanisms of cross-resistance were analyzed by using multi-omics analysis of sequential window acquisition of all theoretical mass spectra (SWATH-MS), RNA-seq, micro-western array (MWA), traditional western blots, and open access database. Results In comparison with S0.5, the sensitivity to fulvestrant was indeed reduced in 182R6 cell; however, we found the sensitivity to ribociclib was also reduced in 182R6 cell, which was in contrast to the ribociclib-sensitive TAM cell. The different response to ribociclib between 182R6 and TAM, indicating that cross-resistance between fulvestrant and ribociclib existed in the model of 182R6 and TAM would be another control for 182R6 to find the target with or without cross-resistance. With multi-omics analysis of SWATH-MS, RNA-seq, MWA, western blots, and open access database, we found some unique or high expression of targets and pathways including EGFR, HER2, CDK6, CDK2, MAPK, and TNF-α pathways in cross-resistance R6 cell than in parental and TAM cell. We reconfirmed the above targets by 3D culture and found MAPK signaling pathway of p-ERK was not affected under ribociclib treatment, indicating the important role of the MAPK signaling pathway in ribociclib resistance. Conclusion To our knowledge, these results provide the first preliminary mechanism of cross-resistance between fulvestrant and ribociclib. We found fulvestrant resistant cell R6 exhibited the phenomenon of cross-resistance to ribociclib, which was in contrast to ribociclib-sensitive TAM cell. We also found some unique or high expression of target and pathway in cross-resistance R6 cells than in parental and TAM cells. In the future, with compound library screening and in vivo animal models, we expect to discover an FDA-approved drug or potential compound to overcome the cross-resistance between fulvestrant and ribociclib. Citation Format: Ming-Feng Hou, Chung-Liang Li, Chih-Po Chiang. Multi-omics analysis identifies the mechanism in cross-resistance to endocrine therapy and CDK4/6 inhibitor in breast cancer [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 PO1-23-10.

Abstract LBO1-04: Primary results from a phase 2a study of zanidatamab (zani) + palbociclib (palbo) + fulvestrant (fulv) in HER2+/HR+ metastatic breast cancer (mBC)

Abstract Background: HER2+ mBC remains incurable, thus novel HER2-directed therapies including chemotherapy-free options are needed. Approximately 50% of HER2+ mBC is also HR+, making the estrogen pathway an additional therapeutic target. The CDK4/6 inhibitor palbo + endocrine therapy fulv is approved for HER2−/HR+ mBC. Targeting all 3 pathways may further improve outcomes in pts with HER2+/HR+ mBC. Zani is a bispecific HER2-targeted antibody that binds HER2 in a unique trans configuration, driving multiple mechanisms of action. A prior analysis of the current single-arm, phase 2a study (NCT04224272) demonstrated antitumor activity and a tolerable safety profile for zani + palbo + fulv in heavily pretreated pts with HER2+/HR+ mBC. Enrollment has been completed; here we report the primary endpoint of PFS at 6 mo (PFS6) and other endpoints. Methods: Eligible pts had HER2+ (by local HER2 testing) and HR+, unresectable, locally advanced or mBC; ECOG PS ≤1; prior treatment with at least trastuzumab, pertuzumab, and T-DM1; and no prior CDK4/6 inhibitor. Pts received zani (20 mg/kg Q2W) + palbo + fulv (standard doses)—recommended doses determined in Part 1 assessment. The Part 2 primary endpoint was PFS6. Other endpoints included median PFS (mPFS), confirmed objective response rate (RECIST v1.1), disease control rate, and duration of response. PAM50 analysis was exploratory. A centrally confirmed HER2+ (ccHER2) subset was analyzed post hoc. Results: As of August 3, 2023, 51 pts (median age [range] 54 yr [36-77]) received zani + palbo + fulv treatment with a median follow-up time of 16.1 mo. Of the 51 pts, 32 (63%) were ccHER2+. Nine pts (18%) remained on treatment; median (range) duration of zani treatment was 8.4 (1.0-29.5) mo. In the metastatic setting, pts received a median (range) of 4 (1-12) prior systemic regimens, 3 (1-10) prior different HER2-targeted therapies, and 1 (0-5) prior endocrine therapy; 12 (24%) pts had prior T-DXd and 11 (22%) had prior fulv. The primary endpoint of PFS6 was 67% (69% in ccHER2+ subset). The mPFS was 11.7 mo (14.9 mo in ccHER2+ subset). See Table 1 for other efficacy endpoints. PAM50 subtyping was available for 29 pts (57%; 1 basal like; 16 HER2 enriched; 12 luminal B). Compared with HER2 enriched, luminal B was associated with numerically, but not statistically, longer mPFS (11.7 vs 9.3 mo; P=0.74) and similar PFS6 (66.7% vs 62.5%). The most common (>20%) treatment (zani, palbo, and/or fulv)-related adverse events (TRAEs) were diarrhea (80%), neutrophil count decrease/neutropenia (59%), nausea (39%), stomatitis (37%), anemia (29%), vomiting (25%) and asthenia (24%). Grade ≥3 TRAEs in ≥2 pts were neutrophil count decrease/neutropenia (53%), diarrhea (14%), anemia (10%), thrombocytopenia (6%), hypokalemia (4%), and hypomagnesemia (4%). One serious TRAE (transaminases increased) was reported. AEs of special interest: 6 pts with cardiac events (all LVEF decrease; 5 pts with grade 1 or 2 events, 1 pt with a grade 3 event) and 2 pts with infusion-related reactions (both grade 1). Three pts discontinued palbo due to an AE; 1 pt discontinued zani and fulv due to an AE. No treatment-related deaths were reported. Conclusions: Zani + palbo + fulv showed a promising PFS6 and mPFS with durable responses. The safety profile was manageable. These results support further development of a novel chemotherapy-free treatment regimen for heavily pretreated pts with HER2+/HR+ mBC. Table 1. Efficacy of Triplet Regimen (Zani + Palbo + Fulv) for HER2+/HR+ mBC a Patients with measurable disease (n=46 all pts; n=29 ccHER2+ subset). b Defined as best response of CR, PR, non-CR/non-PD (for patients who have only non-target lesions) or SD per RECIST 1.1. c Patients with DOR (n=16 all pts; n=14 ccHER2+ subset). cBOR, confirmed best overall response; ccHER2+, centrally confirmed HER2+; CI, confidence interval; cORR, confirmed objective response rate; CR, complete response; DCR, disease control rate; DOR, duration of response; HER2+, human epidermal growth factor receptor 2 positive; HR+, hormone receptor positive; mBC, metastatic breast cancer; mo, month; PD, progressive disease; PFS, progression-free survival; PFS6, progression-free survival at 6 months; PR, partial response; RECIST 1.1, Response Evaluation Criteria in Solid Tumors version 1.1; SD, stable disease. Citation Format: Santiago Escrivá-de-Romani, Juan Miguel Cejalvo, Emilio Alba, Jennifer Friedmann, Álvaro Rodríguez Lescure, Marie-France Savard, Rossanna C. Pezo, Maria Gion, Manuel Ruíz - Borrego, Erika Hamilton, Timothy Pluard, Marc Webster, Muralidhar Beeram, Hannah Linden, Cristina Saura, Diana Shpektor, Bob Salim, Phoebe Harvey, Sara Hurvitz. Primary results from a phase 2a study of zanidatamab (zani) + palbociclib (palbo) + fulvestrant (fulv) in HER2+/HR+ metastatic breast cancer (mBC) [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 LBO1-04.

Abstract PO2-05-05: ELECTRA: An open-label, multicenter, phase 1b/2 study of elacestrant in combination with abemaciclib in patients with brain metastasis (mets) from estrogen receptor-positive (ER+), HER2-negative (HER2-) breast cancer (BC)

Abstract Background: Currently, there are no approved systemic treatment options specifically for patients with ER+/HER2- BC who have brain metastasis. The prevalence of brain metastases from ER+/HER2- BC is approximately 14% of women (Aversa, 2014). Elacestrant is a next-generation oral SERD that was evaluated in a pivotal phase 3 trial, EMERALD, as monotherapy vs. standard of care (SOC) endocrine therapy. The EMERALD trial reported significantly prolonged progression-free survival (PFS) with elacestrant vs SOC endocrine therapy in patients with ER+/HER2− ESR1 mutated (ESR1-mut) mBC following progression on prior endocrine therapy (Bidard, 2022). Elacestrant was well tolerated with a manageable safety profile. Most adverse events, including nausea, were low-grade and consistent with other endocrine therapies. A clinical study in healthy postmenopausal women demonstrated the ability of elacestrant to cross the blood-brain barrier (Conlan, 2020). Abemaciclib, a CDK4/6i, has been shown to achieve therapeutic concentrations in brain metastases tissue (Tolaney, 2020). Given the lack of current systemic treatments, elacestrant is now being evaluated with abemaciclib in patients with ER+/HER2- BC and brain metastasis (NCT05386108). Methods: ELECTRA (NCT05386108) is an open-label phase 1b/2, multicenter study evaluating elacestrant in combination with abemaciclib in patients with brain metastasis from ER+, HER2- BC. Eligibility are patients with ER+/HER2− locally advanced or mBC and measurable brain metastasis. For phase 1b, the presence of brain metastases is allowed but not required for eligibility. For phase 2, patients must have ≥ 1 active and measurable brain metastasis per RECIST v1.1. Patients must have received prior therapy in the metastatic setting, including ≥ 1 ET, ≤2 chemotherapy regimens, and 0-2 prior CDK4/6i (excluding abemaciclib). The primary objectives of Phase 1b are to determine the recommended Phase 2 dose (RP2D). Phase 1b secondary objectives include safety, PK, objective response rate (ORR), duration of response (DoR), clinical benefit rate (CBR), PFS, and overall survival (OS). Phase 2 primary objective is ORR per RECIST version 1.1. Phase 2 secondary objectives include intracranial response rate (RR), DoR, CBR, PFS, OS, PK, and quality of life (QoL). The phase 1b portion evaluated dose-limiting toxicities (DLTs) of the combination during the first cycle (28 days) of treatment in up to three 6-patient cohorts, starting at elacestrant 300 mg daily plus abemaciclib 100 mg twice daily. Here we report initial safety results from the phase 1b portion. Results: As of June 2023, 15 patients have been enrolled in the phase 1b portion of the trial. At dose level 1, elacestrant 300 mg QD + abemaciclib 100 mg BID, no patient experienced DLTs, and the combination was feasible. Dose level 2, elacestrant 400 mg QD + abemaciclib 100 mg BID has been completed, and no patients experienced DLTs. In dose level 1, 4/7 patients remain on treatment at cycle 5. Additional safety and PK data for the cohorts will be provided. Conclusion: In the phase 1b portion of ELECTRA, dose level 1 and dose level 2 have been completed. No patients experienced DLTs in either dose level 1 or dose level 2, and the combination was considered feasible in the respective dose levels. Dose level 3 (elacestrant 400 mg QD + abemaciclib 150 mg BID) is proceeding. Recruitment is actively ongoing, enabling oral-oral combinations to determine a RP2D. Citation Format: Nuhad Ibrahim, Sung-Bae Kim, Nancy Lin, Ahmad Awada, Eva Ciruelos Gil, Giulia Tonini, Bartomeu Piza Vallespir, Kathy Puvana Theall, Erika Hamilton. ELECTRA: An open-label, multicenter, phase 1b/2 study of elacestrant in combination with abemaciclib in patients with brain metastasis (mets) from estrogen receptor-positive (ER+), HER2-negative (HER2-) breast cancer (BC) [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 PO2-05-05.

Abstract PO4-04-11: Exposure–response analyses of GO39932: a Phase Ia/b study of giredestrant in estrogen receptor-positive, HER2-negative, locally advanced or metastatic breast cancer

Abstract BACKGROUND Giredestrant (GIR) is a highly potent, nonsteroidal, oral, selective estrogen receptor antagonist and degrader (SERD). The Phase Ia/b GO39932 study (NCT03332797) investigated GIR ± palbociclib (palbo) and ± a luteinizing hormone-releasing hormone agonist for patients with ER-positive, HER2-negative, locally advanced or metastatic breast cancer who had disease progression on prior endocrine therapies. In the single-agent dose-escalation stage, patients received 10, 30, 90, or 250 mg of once-daily (QD) GIR. In the dose-expansion stage, patients received 30, 100, or 250 mg of QD GIR. Safety and tolerability of 100 mg GIR + 125 mg palbo was also explored in the dose-escalation and -expansion stages. Results showed that GIR was well tolerated and potentially clinically active as a single agent and in combination with palbo, including in patients with ESR1-mutated tumors. To inform the selection of the clinical dose for GIR late-stage development based on risk–benefit considerations, exposure–response (E–R) analyses for efficacy and safety endpoints were conducted using single-agent GIR data. METHODS Exposure metrics included maximum concentration at steady state and area under the concentration–time curve at steady state. The association between the exposure metrics and efficacy endpoints (objective response rate [ORR] and clinical benefit rate [CBR]) or safety endpoints (all adverse events [AEs], Grade ≥ 3 AEs, selected AEs of interest [sAEIs], including all-grade hepatotoxicity, and bradycardia) were evaluated. RESULTS Data cutoff was September 17, 2021. A total of 111 patients were enrolled in the single-agent GIR cohort. Of these 111 patients, 107 were pharmacokinetic-evaluable and included in the analysis. ORR in patients with measurable disease at baseline was 20% (16/81 patients) and CBR was 49% (54/111). No significant associations were observed between exposure and ORR or CBR, including in patients with ESR1-mutated tumors and those with no mutation detected (p > 0.05). There were 92/107 patients (86%) with AEs of any grade, overall. Seventy-two (67%) had sAEIs, 19 (18%) experienced hepatotoxicity (all events were reported as liver function test abnormalities), 11 (10%) had bradycardia, and 20 (19%) had Grade ≥ 3 AEs. Increasing exposure did not significantly increase the incidence of all AEs, Grade ≥ 3 AEs, sAEIs, or bradycardia at the dose range of 10 to 250 mg (p > 0.05). For hepatotoxicity, the analysis suggested that higher exposure may lead to a higher probability of events (p = 0.0065); however, at the clinically relevant exposure range for the 30 mg dose, the incidence of hepatotoxicity was low and the predicted exposures were at the shallower slope of the relationship. CONCLUSIONS Single-agent GIR did not exhibit a significant association between exposure and efficacy endpoints at a dose range of 10 to 250 mg, regardless of ESR1 mutation status, indicating that the efficacy of GIR might have reached a plateau at low dose levels. At the clinically relevant exposure range of the 30 mg dose, GIR exposure was not associated with an appreciable increase in the incidence of AEs. The Phase Ia/b study design and subsequent E–R results enabled dose selection to be based on risk–benefit considerations, rather than the traditional maximum tolerated dose paradigm, in line with the United States Food and Drug Administration’s oncology dose optimization recommendations. The E–R results also indicate that GIR may have a relevant wide therapeutic window. Overall, these data support the 30 mg clinical dose of GIR that was selected, with a favorable risk–benefit profile, for further clinical studies. Citation Format: Komal Jhaveri, Yi Ting (Kayla) Lien, Nicholas Turner, Elgene Lim, Maureen Cannon, Richard Anziano, Jurgen Langenhorst, Kenta Yoshida, Vikram Malhi, Mary Gates, Jennifer Eng-Wong, Chunze Li, Mona Shah, Pablo Perez-Moreno, Jiajie Yu, Aditya Bardia. Exposure–response analyses of GO39932: a Phase Ia/b study of giredestrant in estrogen receptor-positive, HER2-negative, locally advanced or metastatic breast cancer [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 PO4-04-11.