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Background: Abemaciclib plus nonsteroidal aromatase inhibitors (AI) demonstrated efficacy with a tolerable safety profile as initial treatment for postmenopausal women with HR+, HER2− advanced breast cancer in the MONARCH 3 trial. In the intent-to-treat (ITT) population, the updated progression-free survival (PFS) with 12 months of additional follow-up was 28.2 versus 14.8 months (HR [95% CI]: 0.525 [0.415, 0.665]; p Methods: In MONARCH 3, a randomized, double-blind, phase III trial, 493 postmenopausal women with HR+, HER2− advanced breast cancer and no prior systemic therapy in the advanced setting, received an AI (anastrozole or letrozole) plus abemaciclib or placebo. From the October 31, 2018 cutoff, secondary endpoints including objective response rate ([ORR], complete response [CR] + partial response [PR]), time to response (TTR), and duration of response ([DoR], time from a confirmed CR or PR until disease progression or death) were assessed in the ITT population and exploratory subgroups previously reported as significantly prognostic. Waterfall plots will be used to illustrate the magnitude of change in tumor size for each subgroup (to be presented). Results: In patients with measurable disease, the ORR was 62.5% (95% CI: 56.7, 68.2) in the abemaciclib plus AI arm and 44.7% (95% CI: 36.2, 53.2) in the placebo plus AI arm (p≤.001). Additional follow-up confirmed that all prognostic subgroups received benefit from the addition of abemaciclib to AI, consistent with the ITT population, as evidenced by change in ORR, with the largest effects observed in subgroups of patients with liver metastases, progesterone receptor-negative tumors, high grade tumors, or treatment-free interval of Conclusions: Extended follow-up in the MONARCH 3 trial further confirmed that the addition of abemaciclib to AI is associated with durable tumor responses, including those in patients with clinically poor prognostic characteristics. References: Di Leo A et al. NPJ Breast Cancer. 2018;4:41. Clinical trial information: NCT02246621 This study was funded by Eli Lilly and Company Table: Objective Response RateAbemaciclib + AIPlacebo + AIORR ChangeNn (%)Nn (%)%Measurable disease population269168 (62.5)13259 (44.7)17.8Liver metastasesYes4727 (57.45)306 (20.00)37.45No222141 (63.51)10253 (51.96)11.55Progesterone receptor statusNegative6137 (60.66)298 (27.59)33.07Positive206130 (63.11)10351 (49.51)13.60Treatment-free interval Citation Format: Joyce O’Shaughnessy, Stephen Johnston, Miguel Martin, Jens Huober, Masakazu Toi, Valerie AnneMarie Andre, Holly Rene Martin, Molly Catherine Hardebeck, Angelo Di Leo, Matthew Philip Goetz. Updated subgroup tumor response of abemaciclib plus aromatase inhibitor for hormone receptor positive (HR+), HER2 negative advanced breast cancer (MONARCH 3) [abstract]. In: Proceedings of the 2019 San Antonio Breast Cancer Symposium; 2019 Dec 10-14; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2020;80(4 Suppl):Abstract nr P1-19-09.

Background: Abemaciclib is an oral selective inhibitor of cyclin-dependent kinases 4 and 6 approved for hormone receptor (HR)+, human epidermal growth factor receptor 2 (HER2)- metastatic breast cancer. In the randomized, 3-arm, phase 2 study monarcHER (NCT02675231) for HR+, HER2+ advanced breast cancer (ABC), abemaciclib in combination with trastuzumab (T) and fulvestrant (F) significantly improved investigator-assessed progression-free survival (whereas abemaciclib + T did not) versus (vs) T + physician’s choice of chemotherapy and demonstrated a tolerable safety profile. Here, patient-reported HRQoL, functioning, and symptoms are reported. Methods: In monarcHER, 237 postmenopausal (surgical, natural, or chemical ovarian suppression) women with ABC and ≥2 prior HER2+ directed therapies in the advanced setting were randomized 1:1:1 to abemaciclib (150 mg PO Q12H every 21 days) + T (IV infusion on D1 every 21 days) with F (500 mg IM on Cycle 1 D1 and D15 and Cycle 2 D8, then Q4W; Arm A) or without F (Arm B) vs T + physician’s choice of chemotherapy (per label every 21 days; Arm C). Supportive measures to manage diarrhea were permitted. Patient-reported outcomes were measured at baseline and at each cycle using the modified Brief Pain Inventory-short form (mBPI-sf) and European Organization for Research and Treatment of Cancer Quality of Life Questionnaire-Core 30 (EORTC QLQ-C30). The EuroQol 5-Dimension 5 Level (EQ-5D 5L) questionnaire was also collected. Higher scores on EORTC QLQ-C30 functional and health status/QoL scales indicate improvement whereas higher scores on EORTC QLQ-C30 symptom scales and mBPI-sf indicate worsening of symptoms/pain. The EQ-5D 5L index score was calculated from a set of item weights to derive a score of 0-1, with 1 representing the best health status. Treatment arm comparisons of change from baseline (all post-baseline visits) were conducted using a mixed model repeated measure, with .05 considered statistically significant. Clinical meaningfulness was defined as a ≥10-point score change from baseline (on a 0-100 scale) for EORTC QLQ-C30 and a 2-point score change from baseline for mBPI-sf. Results: Patient-reported outcome compliance rates were ≥90% through Cycle 15; the range for median duration of each treatment component of each arm was 7.5-10.0 cycles. Overall, no statistically significant or clinically meaningful changes from baseline differences were observed between treatment arms for mBPI-sf pain scores or EORTC QLQ-C30 global health score, function scales, or for symptoms of fatigue, dyspnea, appetite loss, or financial difficulties. Least square (LS) mean change from baseline differences showed statistically significant improvements in Arm A vs C for EORTC QLQ-C30 symptoms of pain (-6.81; p=.026) and insomnia (-6.39; p=.041). Worsening for the symptom of nausea/vomiting was statistically significant but not clinically meaningful in Arm A vs C (4.08; p=.043). Diarrhea showed a statistically significant and clinically meaningful worsening in Arm A vs C (19.27; p Conclusions: Quality of life was maintained for patient-reported pain, global health, functioning, and most symptoms when abemaciclib was added to T + F compared with physician’s choice of chemotherapy in patients with HR+, HER2+ ABC. Gastrointestinal-related symptoms were transient and consistent with the manageable, reversible adverse event profile. Citation Format: Sara M Tolaney, Andrew M Wardley, Stefania Zambelli, John F. Hilton, Tiffany A Troso-Sandoval, Francesco Ricci, Seock-Ah Im, Sung-Bae Kim, Stephen RD Johnston, Arlene Chan, Shom Goel, Kristen Catron, Zhengyu Yang, M. Corona Gianford, Gregory L Price, Fabrice André. Health-related quality of life (HRQoL) in monarcHER: Abemaciclib plus trastuzumab with or without fulvestrant versus trastuzumab plus standard-of-care chemotherapy in HR+, HER2+ advanced breast cancer [abstract]. In: Proceedings of the 2019 San Antonio Breast Cancer Symposium; 2019 Dec 10-14; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2020;80(4 Suppl):Abstract nr P3-11-10.

Background: In MONARCH 2 (M2), abemaciclib, an oral selective cyclin dependent kinase 4 & 6 inhibitor, demonstrated significant progression-free survival (PFS) improvement in hormone receptor positive (HR+), human epidermal growth factor receptor 2 negative (HER2-) advanced breast cancer when added to fulvestrant. Treatment with abemaciclib plus fulvestrant provided a statistically significant and clinically meaningful median overall survival (OS) benefit of 9.4 months (hazard ratio 0.757; 95% confidence interval, 0.606, 0.945; p=0.0137).1 High grade tumors, progesterone receptor (PgR) negativity, liver metastases, and absence of bone-only disease were identified as independent poor prognostic disease characteristics.2 In a retrospective analysis of M2, among subgroups analyzed, patients within those poor prognostic subgroups numerically received the largest improvement from the addition of abemaciclib to endocrine therapy (ET) with PFS hazard ratios in the range of 0.4 to 0.5.2 Methods: M2 (NCT02107703) was a global, randomized, double-blind Phase 3 trial of abemaciclib + fulvestrant (N=446) or placebo + fulvestrant (N=223) in women with ET resistant HR+, HER2- advanced breast cancer. Investigator-assessed PFS was the primary objective and OS was a key secondary endpoint. Here, we present exploratory OS results conducted within the previously described prognostic subgroups.2 Hazard ratios were estimated using Cox models with a test of interactions of subgroups with treatment performed. Results: As of the data cut-off (20 June 2019), OS prolongation was consistent across all subgroups; a numerically greater OS effect was observed in patients with previously identified negative prognostic factors including patients with high tumor grade, PgR negativity, baseline liver metastases, and absence of bone-only metastases, relative to their complementary subgroups (Table). However, no statistically significant interaction between these prognostic factors and treatment effect were observed. Similar to OS, a more pronounced effect for abemaciclib plus fulvestrant for patients with high tumor grade, PgR negativity, baseline liver metastases, and absence of bone-only metastases was also observed when examining the time to chemotherapy or death, whichever is earlier (chemotherapy free survival [CFS]). Conclusions: Consistent with the statistically significant and clinically meaningful benefit observed in the overall study population, abemaciclib plus fulvestrant improved OS across all exploratory subgroups, with the largest OS effects observed in patients with poor prognostic factors, consistent with previously reported PFS data. Refernces: 1. Sledge GW Jr, Toi M, Neven P, et al. MONARCH 2: Overall survival of abemaciclib plus fulvestrant in patients with HR+, HER2- advanced breast cancer. Abstract to be presented at: European Society for Medical Oncology 2019; Barcelona, Spain. 2. Di Leo A, O’Shaughnessy J, Sledge GW Jr, et al. Prognostic characteristics in hormone receptor-positive advanced breast cancer and characterization of abemaciclib efficacy. NPJ Breast Cancer. 2018;4:41. Abbreviations: CFS, chemotherapy free survival; HR, hazards ratio; N, number in overall study population; n, number in specified subgroup; OS, overall survival; PgR, progesterone receptor. n (%) N=669Overall Survival Hazard Ratio (95% CI)Chemotherapy Free Survival Hazard Ratio (95% CI)Tumor gradeHigh169 (25.26)0.66 (0.44, 1.01)0.52 (0.36, 0.75)Low/intermediate345 (51.57)0.79 (0.58, 1.08)0.78 (0.60, 1.02)Unknown155 (23.17)0.83 (0.52, 1.31)0.50 (0.34, 0.76)PgR statusNegative141 (21.08)0.66 (0.41, 1.05)0.59 (0.39, 0.89)Positive509 (76.08)0.79 (0.61, 1.01)0.68 (0.55, 0.85)Baseline liver metastasesYes174 (26.01)0.73 (0.49, 1.08)0.60 (0.42, 0.85)No495 (73.99)0.76 (0.58, 0.99)0.64 (0.51, 0.81)Bone-only metastasesYes180 (26.91)0.91 (0.56, 1.46)0.69 (0.46, 1.01)No486 (72.65)0.72 (0.57, 0.93)0.64 (0.52, 0.80) Citation Format: Patrick Neven, Stephen Johnston, Masakazu Toi, Joohyuk Sohn, Kenichi Inoue, Xavier Pivot, Olga Burdaeva, Meena Okera, Norikazu Masuda, Peter A Kaufman, Han Koh, Eva-Maria Grischke, PierFranco Conte, Yi Lu, Nadine Haddad, Karla Hurt, Antonio Llombart-Cussac, George W Sledge. MONARCH 2: Interim analysis of overall survival in patients with poor prognostic factors [abstract]. In: Proceedings of the 2019 San Antonio Breast Cancer Symposium; 2019 Dec 10-14; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2020;80(4 Suppl):Abstract nr P3-08-01.

Background: Combination treatments of CDK4 & 6 inhibitors (CDK4 & 6i) with endocrine therapy (ET) have improved outcomes in patients with HR-positive (HR+), HER2-negative (HER2-) advanced breast cancer (ABC). Aside from estrogen receptor positivity (ER+), predictive biomarkers of clinical benefit from CDK4 & 6i in combination with ET remain elusive. We assessed clinical outcomes by genomic alterations detected in baseline circulating tumor DNA (ctDNA) from patients treated with abemaciclib plus a nonsteroidal aromatase inhibitor (NSAI) versus placebo plus NSAI in the Phase 3 study MONARCH 3 (NCT02246621). Methods: MONARCH 3 randomized 493 postmenopausal women with HR+, HER2- ABC who had no prior systemic therapy in the advanced setting to treatment with abemaciclib plus NSAI or placebo plus NSAI. Biomarker analysis of baseline ctDNA was an exploratory objective; plasma was analyzed by the Guardant360 next-generation sequencing (NGS)-based assay to identify potential tumor-related (i.e., somatic) genomic alterations including point mutations (SNV), indels, amplifications, and fusions in over 70 cancer-related genes. Genomic alterations detected in baseline ctDNA were associated with clinical outcomes including progression-free survival (PFS) and objective response rate (ORR). Results: Baseline ctDNA results were available for 295 patients (201 abemaciclib; 94 placebo) with 83% of patients harboring one or more detectable genomic alterations. Commonly altered genes of interest (% frequency) detected in baseline ctDNA included: PIK3CA (38%); TP53 (26%); EGFR (15%); FGFR1 (12%); NF1 (12%); MYC (9%); CCND1 (9%); ESR1 (5%).Overall, patients treated with placebo plus NSAI who harbored detectable ctDNA genomic alterations had shorter median PFS compared to patients without detectable genomic alterations, as shown in the Table (14.9 months [95% CI: 11.0-23.1] vs 19.2 months [95% CI 9.4-NR]). Moreover, genomic alterations in EGFR, FGFR1, MYC, and CCND1 were associated with median PFS Conclusions: In this exploratory subgroup analysis, the presence of detectable ctDNA genomic alterations was associated with shorter PFS with placebo plus NSAI. Consistently, abemaciclib added to NSAI improved outcomes for genomically identified subgroups. In contrast to prior studies, there was no subgroup (e.g., patients harboring FGFR1 alterations) that did not derive benefit from abemaciclib, supporting the efficacy of abemaciclib, including in difficult to treat tumors. These findings are hypothesis-generating and warrant further investigations in clinical studies of CDK4 & 6i in combination with ET. TableAbemaciclib + NSAIPlacebo + NSAIEvents, n/NMedian PFS (95% CI)Events, n/NMedian PFS (95% CI)Hazard Ratio (95% CI)OVERALLITT population170 / 32828.2 (23.7 - 33.9)123 / 16514.8 (11.2 - 19.2)0.52 (0.42 - 0.66)TR population93 / 20138.7 (31.1 - NR)71 / 9416.5 (11.7 - 23.1)0.45 (0.33 - 0.61)Any Gene AlterationDetected83 / 16634.1 (27.2 - 40.0)62 / 7914.9 (11.0 - 23.1)0.47 (0.34 - 0.66)Not detected10 / 35NR (36.4 - NR)9 / 1519.2 (9.40 - NR)0.31 (0.13 - 0.78)TP53Detected29 / 5327.0 (14.1 - NR)19 / 2315.4 (5.7 - 30.4)0.53 (0.30 - 0.95)Not detected64 / 14839.9 (34.1 - NR)52 / 7117.5 (11.7 - 24.2)0.42 (0.29 - 0.60)EGFRDetected10 / 26NR (32.4 - NR)16 / 1710.9 (2.1 - 24.7)0.20 (0.09 - 0.47)Not detected83 / 17536.4 (27.7 - NR)55 / 7717.6 (14.6 - 25.5)0.52 (0.37 - 0.73)FGFR1Detected15 / 2532.8 (10.1 - NR)10 / 117.6 (1.9 - 15.4)0.37 (0.16 - 0.85)Not detected78 / 17639.9 (31.1 - NR)61 / 8319.2 (14.6 - 26.5)0.45 (0.32 - 0.63)NF1Detected10 / 2435.9 (27 - NR)8 / 1014.6 (1.6 - 33.4)0.33 (0.13 - 0.84)Not detected83 / 17738.7 (30.8 - NR)63 / 8417.5 (11.7 - 24.2)0.47 (0.33 - 0.65)MYCDetected11 / 1720.1 (5.5 - NR)9 / 106.5 (1.2 - 15.7)0.33 (0.13 - 0.86)Not detected82 / 18438.9 (32.9 - NR)62 / 8419.2 (14.6 - 26.5)0.45 (0.32 - 0.63)CCND1Detected8 / 1632.8 (5.5 - NR)10 / 107.2 (3.6 - 9.0)0.28 (0.10 - 0.77)Not detected85 / 18538.7 (31.1 - NR)61 / 8417.6 (14.6 - 25.5)0.48 (0.34 - 0.67)n = number of patients with events; N = total number of patients in the corresponding population and treatment arm; NR = not reached. TR population consists of patients in the ITT population from whom a valid baseline ctDNA result has been obtained. Citation Format: Matthew P Goetz, J. Thaddeus Beck, Mario Campone, Sara Hurvitz, Seock-Ah Im, Stephen Johnston, Antonio Llombart-Cussac, Miguel Martin, Joohyuk Sohn, Masakazu Toi, Lacey M Litchfield, Hillary T Graham, Hong Wang, Sameera R Wijayawardana, Valerie M Jansen, Angelo Di Leo. Efficacy of abemaciclib based on genomic alterations detected in baseline circulating tumor DNA from the MONARCH 3 study of abemaciclib plus nonsteroidal aromatase inhibitor [abstract]. In: Proceedings of the 2019 San Antonio Breast Cancer Symposium; 2019 Dec 10-14; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2020;80(4 Suppl):Abstract nr PD2-06.

Introduction: Detection of circulating tumor DNA (ctDNA) presents a strategy to identify Molecular Residual Disease (MRD) in patients with breast cancer. Tools capable of detecting ctDNA at lower concentrations are needed to increase sensitivity and lengthen lead time between ctDNA detection and relapse. We present results from a highly sensitive personalized sequencing approach for ctDNA detection of MRD based on multiple patient specific mutations. Methods: 22 early breast cancer patients (12 hormone receptor positive HER2 negative (HR+HER2-), 7 HER2+ and 3 triple negative breast cancer (TNBC)) enrolled in the ChemoNEAR sample collection study were included. Tumor DNA from FFPE samples was Whole Exome Sequenced to identify patient specific mutations and design personalized Residual Disease and Recurrence (RaDaRTM) multiplex PCR assays. Cell free DNA was extracted from 147 plasma samples (median volume 4ml, range 0.5-5ml) and sequenced with RaDaR assays, with 10-61 variants (median 41) per panel, to 100,000x per locus. A matched single timepoint buffy coat was sequenced to identify confounding CHIP mutations. A proprietary algorithm was used to identify ctDNA. Tumor Sequencing of multiple biopsy timepoints was carried out for 14 patients (mean 2.8 samples per patient) and clonal populations estimated with Pyclone. For clusters of greater than 10 mutations, RaDaR panels were supplemented with additional variants for clonal tracking. Results: At a median follow-up of 24.6 months post-surgery, MRD was identified in 100% (17/17) of relapsed patients, and in none of the 54 time points in the 5 patients that did not relapse (p=0.0002, Log rank test). Detection of ctDNA levels ranged from 7.4 parts per million (ppm), equivalent to Allele Frequency (AF) of 0.0007%, to 13,195ppm (1.3%) (median 625ppm and 0.06% AF). Median lead-time from ctDNA detection to clinical relapse in patients with extracranial disease relapse was 12.89 months (range 3.72-26.04). In three patients with brain only relapse, ctDNA was detected prior to relapse in all patients (3/3, 100%) albeit with a reduced lead time over clinical relapse (3.85, 4.21 and 5.65 months), which was not previously achievable with single mutation dPCR MRD-detection assays. In 8/14 patients with multiple tumor samples sequenced, multiple clones (mean 3.4 clones/patient) were identified, with heterogenous polyclonal relapse in 4/8 patients, and a single clone detectable in 4/8 patients. Conclusions: In a retrospective, multi-center, proof-of-principle study of early stage breast cancer patients with personalized sequencing assays, ctDNA-detected MRD associates with relapse free survival and long lead time over clinical relapse. Sequencing based ctDNA testing can detect patients with brain-only relapses, with increased sensitivity over first generation dPCR-based ctDNA assays. Citation Format: Rosalind J. Cutts, Maria Coakley, Isaac Garcia-Murillas, Lara Ulrich, Karen Howarth, Warren Emmett, Malcolm Perry, Pete Ellis, Charlene Knape, Stephen R. Johnston, Alistair Ring, Simon Russell, Abigail Evans, Anthony Skene, Duncan Wheatley, Mitch Dowsett, Ian E. Smith, Nicholas C. Turner. Molecular residual disease detection in early stage breast cancer with a personalized sequencing approach [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 536.

Background: CDK4/6 inhibitors are being used in combination with aromatase inhibitors as therapy for advanced ER+ breast cancer (BC) and are being explored for use in primary BC. Few mechanisms driving resistance to added CDK4/6 inhibitors have been defined. The PALLET phase II randomized neoadjuvant trial of letrozole (LET) ± palbociclib (PALBO) in postmenopausal ER+HER2- primary BC reported that clinical response rate over 14wks was not significantly increased by adding PALBO to LET but suppression of Ki67 was significantly increased (Johnston et al, JCO 2018, 37, 178): after 14wks complete cell cycle arrest (CCCA, Ki67 Methods: 307 patients were randomized to LET (n=103) or LET+PALBO (n=204) for 14 wks. The first 2wks of LET+PALBO patients were randomised to LET, PALBO, or LET+PALBO. Biopsies were taken at baseline, 2wks and 14wks. Biomarker data are presented here for baseline only, other than Ki67 at both baseline and 14wks. IHC analyses were conducted on FFPE biopsies for ER, PgR, RB1, cyclin-E1, and cyclin-D1 (also FISH). RNA-seq was performed on fresh frozen biopsies. Association of each biomarker with CCCA was determined by logistic regression. Differentially expressed genes (DEGs) were identified between patients sensitive (CCCA) (n=94) and resistant (non-CCCA) (n=10) to treatments with or without PALBO at 14wks by DESeq2. Fifty hallmark gene sets were tested for significant enrichment with DEGs and differential gene sets were identified by using Gene Set Enrichment Analysis (GSEA). Results: The association of IHC biomarkers with CCCA is shown in the table. Lower levels of ER, higher levels of cyclin-E1, and amplification of cyclin-D1 were each significantly associated with a greater chance of non-CCCA with LET+PALBO. High cyclin-E1 levels were also associated with reduced chance of CCCA with LET only. Patients with high baseline Ki67 also exhibited higher non-CCCA with LET+PALBO at 14wks (p=0.0002). In the RNAseq data we identified 1973 DEGs between the 14wk CCCA and non-CCCA patients for LET+PALBO. E2F and MYC targets, PI3K/AKT/MTOR signalling and interferon response gene sets were among the hallmark gene sets enriched for genes with higher expression in non-CCCA patients at 14wks for LET+PALBO (FDR Conclusion: Biomarkers associated with response/resistance to added PALBO were different from LET only. PALBO resistance was associated with higher baseline expression of cyclin-E1 (both IHC and RNA), CDK2, and genes related to E2F, MYC, interferon and MTOR signalling. These results suggest that multiple identifiable mechanisms of de novo resistance to PALBO are likely to exist in primary ER+ BC. On-going WES analyses will allow the significance of alterations at the DNA level to be presented. Table 1.Continuous measurement in a logistic regression for CCCA at 14 weeks; Oddsratio calculated separately for group A and groups B,C,D and were adjusted forregion (UK vs NA); * amplified vsnon-amplifiedContinuous measurement in a logistic regression for CCCA at 14 weeksBiomarkerLET LET+PALBOOdds ratio95% CIpOdds ratio95% CIpER1.120.36, 3.480.844.471.62, 12.380.004PgR4.381.03, 18.580.053.050.50, 18.560.23RB13.010.24, 38.560.400.420.05, 38.490.83Cyclin-E10.100.01, 0.840.030.020.00, 0.200.001Cyclin-D1 IHC3.090.51, 18.490.222.560.28, 23.330.40CyclinD1 FISH*1.470.43, 4.990.530.280.06, 0.860.03 Citation Format: Eugene F Schuster, Hui Xiao, Elena Lopez-Knowles, Lucy Kilburn, Mothaffar Rimawi, David A Wheeler, Katherine Pogue-Geile, Yuan Lui, Samuel A Jacobs, Chet Cornman, Shannon Puhalla, Maggie Cheang, Judith Bliss, Stephen Johnston, Mitch Dowsett, On behalf of the PALLET Trialists. Biomarkers of resistance to palbociclib in ER+ primary breast cancer in the PALLET trial [abstract]. In: Proceedings of the 2020 San Antonio Breast Cancer Virtual Symposium; 2020 Dec 8-11; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2021;81(4 Suppl):Abstract nr PS5-01.

Background: Abemaciclib is an oral selective inhibitor of CDK 4 & 6 administered on a continuous schedule and demonstrated clinically meaningful efficacy in patients with hormone-receptor-positive (HR+), human epidermal growth factor receptor 2-negative (HER2-) advanced breast cancer as monotherapy (MONARCH 1) and in combination with endocrine therapy (ET) in MONARCH 2 and MONARCH 3. Neoadjuvant abemaciclib + anastrozole (ANZ) (neoMONARCH) given for 2 weeks significantly reduced expression of Ki67 relative to ANZ alone. Endocrine monotherapy is the current endocrine standard of care (SOC) in the adjuvant setting, although risk of relapse remains a problem for patients with high-risk disease, identified by clinical and pathological characteristics of disease. Improving adjuvant therapy for patients with high-risk disease is an important need. Trial design: monarchE (NCT03155997) is a multicenter, randomized, open-label phase 3 trial evaluating adjuvant abemaciclib in patients with node-positive, high-risk, HR+, HER2- early advanced breast cancer. Patients are randomized 1:1 to abemaciclib 150 mg twice daily + SOC adjuvant ET vs SOC adjuvant ET alone and stratified by prior chemotherapy (neoadjuvant, adjuvant, or none), menopausal status (premenopausal or postmenopausal), and region (North America/Europe, Asia or Other). Patients in the investigation arm receive abemaciclib for up to 2 years or until discontinuation criteria are met. All patients receive ET for 5 years or as clinically indicated. Patients are followed for 10 years or until study completion, whichever comes first. The final evaluation for overall survival will mark study completion. Eligibility criteria: Patients must have early stage resected HR+, HER2- invasive advanced breast cancer with either ≥4 pathologically positive axillary lymph nodes (pALNs), or 1 to 3 pathological pALNs with at least one of the following high-risk factors: primary invasive tumor size ≥5 cm, histological grade 3 tumor, or central Ki67 index ≥20%. Patients must have completed definitive locoregional therapy (+/- [neo]adjuvant chemotherapy). Before randomization, patients may receive up to 12 weeks of ET after completing their last non-ET and must be randomized within 16 months of the definitive surgery. Specific aims: The primary objective is to evaluate invasive disease-free survival (IDFS) per the STEEP System.1 Secondary objectives include IDFS in patients with central Ki67 index ≥20%, distant relapse-free survival, overall survival, safety, pharmacokinetics, and health outcomes. Statistical methods: Assuming an IDFS hazard ratio of 0.73, the study is powered to ~85% to test the superiority of abemaciclib + standard ET at a 1-sided α=.025 using a stratified log-rank test. Target accrual: This study will be conducted in ~600 centers in 38 countries to enroll ~4580 patients. Accrual began in July 2017. Recruitment is complete. Contact information: 1-877-285-4559 Reference: 1. Hudis CA, Barlow WE, Costantino JP, et al. Proposal for standardized definitions for efficacy end points in adjuvant breast cancer trials: the STEEP system. J Clin Oncol. 2007;25(15):2127-32. Citation Format: Priya Rastogi, Masakazu Toi, Miguel Martin, Joyce O'Shaughnessy, Desiree Headley, Jennifer Wei, Joanne Cox, Nadia Harbeck, Stephen Johnston. MONARCH E: A phase 3 study of standard adjuvant endocrine therapy with or without abemaciclib in patients with high risk, node positive, hormone-receptor positive, human epidermal growth factor receptor 2-negative early-stage breast cancer [abstract]. In: Proceedings of the 2019 San Antonio Breast Cancer Symposium; 2019 Dec 10-14; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2020;80(4 Suppl):Abstract nr OT2-02-02.

AIM: To evaluate the efficacy and functional consquences of combining AZD5363 with endocrine therapy in pre-clinical models of endocrine-sensitive and resistant ER+ BC. BACKGROUND: The PI3K/AKT/mTOR signalling pathway plays an important role in BC. Its close interaction with ER signalling becomes more complex and inter-dependent with acquisition of endocrine resistance. Targeting the mTOR pathway in combination with endocrine therapy has shown clinical utility. However, a negative feedback loop exists downstream of the PI3K/AKT/mTOR pathway with mTOR inhibition leading to increased activation of IGFR1-dependent AKT activity potentially negating long-term benefit. Direct blockade of AKT in combination with endocrine therapy, may provide a better rationale for treatment of endocrine-resistant BC, impacting on both cell survival/apoptosis and ER ligand-independent signaling. In this investigation, we assessed the efficacy of AZD5363, a pan-AKT inhibitor with endocrine therapies in pre-clinical models of endocrine sensitive and resistant ER+ BC and its impact on molecular and cellular response. METHODS: Inhibition of AKT using AZD5363 was examined in 5 ER+ BC lines before and after adaptation to long-term estrogen deprivation (LTED) or tamoxifen (TAMR). The effects of AZD5363 on cell proliferation were determined alone and in combination with endocrine treatment and feedback upregulation and activation of receptor tyrosine kinases (RTKs) was examined by western blotting. ER-transactivation was measured with an estrogen-response element (ERE)-linked luciferase reporter construct and confirmed using chromatin-immunoprecipitation. Global gene expression analysis was used to identify pathways associated with response. Xenografts were treated with AZD5363 ± fulvestrant to determine in vivo effects. RESULTS: AZD5363 caused a dose-dependent decrease in proliferation in all cell lines tested (GI50 CONCLUSION: These data suggest that AZD5363 plus fulvestrant may be effective in BC that is sensitive or resistant to E-deprivation or tamoxifen and that activated AKT is a determinant of response. These data strongly support the need for clinical evaluation. Citation Format: Lesley-Ann Martin, Stephanie K Guest, Sunil Pancholi, Ricardo Ribas, Qiong Gao, Nikiana Simigdala, Aradhana Rani, Barry Davies, Stephen Johnston, Mitch Dowsett. AKT antagonist AZD5363 targets estrogen receptor (ER) function in endocrine resistant breast cancer (BC) and synergises with fulvestrant in vivo [abstract]. In: Proceedings of the Thirty-Seventh Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2014 Dec 9-13; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2015;75(9 Suppl):Abstract nr P3-05-04.

Background: Despite treatment, many estrogen receptor positive (ER+) breast cancer (BC) patients, relapse with de novo or acquired endocrine resistant disease. Using a kinome siRNA library screen, we identified MPS1, which is required for recruitment of the spindle assembly checkpoint complex, as strongly associated with resistance to both endocrine therapy and CDK4/6 inhibitor palbociclib, a contemporary first-line combination for advanced ER+ BC. Until now, the target population for MPS1 inhibitors has been triple negative BC. Here we show for the first time, potential efficacy of MPS1 inhibitors in endocrine resistant BC models. Methods: A panel of ER+ BC cell lines adapted to estrogen independent growth (LTED) or after additional resistance to palbociclib (palboR), was subjected to a siRNA kinome screen targeting 709 genes. Kinases causative of resistance were identified using Z-scores. Cell viability upon MPS1 inhibition (MPS1i) with BOS172722 was assessed 2D and 3D, and the class effect confirmed with other compounds targeting MPS1. Impact of MPS1i on ER-mediated transcription was assessed using luciferase reporter assays. Effect of MPS1i on chromosomal alignment and time spent in mitosis was established by time lapse and confocal microscopy. Cell cycle and DNA content were analysed by FACS. Apoptosis was assessed using PARP cleavage. Efficacy of BOS172722 in a xenograft model of LTED was evaluated. Results: Kinome knockdown identified MPS1 as the top common hit in LTED and palboR cell lines. Increased MPS1 was evident in MCF7-LTED at the transcript and protein level. Notably, BOS172722 significantly reduced viability of the majority of LTED and palboR cell lines tested (IC50 between 25-100nM), an effect shown to be class specific using a second MPS1i. Upon MPS1i, cells demonstrated shorter time in mitosis, aberration of cell cycle, increased mitotic errors, culminating in apoptosis. Of note, as little as 48hr exposure to BOS172722 reduced colony formation over 3 weeks suggesting early accumulation of errors is sufficient to provide a long term anti-proliferative effect. To evaluate the clinical relevance of MPS1 in ER+ BC treated with endocrine therapy, we interrogated publicly available datasets from patients treated with neoajuvant AI therapy. In the anastrozole cohort, on-treatment MPS1 expression was significantly (p Conclusion: For the first time, we show that MPS1 inhibitors are capable of inducing mitotic aberrations and apoptosis in ER+ BC resistant to endocrine therapy and palbociclib providing a new therapeutic strategy. Citation Format: Joanna Nikitorowicz-Buniak, Sunil Pancholi, Nikiana Simigdala, Ricardo Ribas, Spiros Linardopoulos, Mitch Dowsett, Stephen Johnston, Lesley-Ann Martin. MPS1 as a novel target in endocrine- and palbociclib-resistant estrogen receptor positive breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 4950.

Background: Abemaciclib is an orally administered, selective CDK4 & 6 inhibitor dosed on a continuous schedule. In patients (pts) with hormone receptor-positive (HR+), human epidermal growth factor receptor 2-negative (HER2-) advanced breast cancer (ABC), abemaciclib demonstrated clinical activity as monotherapy in MONARCH 1 and efficacy with a generally tolerable safety profile in combination with fulvestrant in MONARCH 2. At the MONARCH 3 (NCT02246621) interim analysis, abemaciclib plus a nonsteroidal aromatase inhibitor (NSAI) significantly improved progression-free survival (PFS) (hazard ratio [HR], 0.543; 95% confidence interval [CI], 0.409, 0.723; p=.000021) and objective response rate (ORR) (measurable disease: abemaciclib arm, 59.2%; placebo arm, 43.8%; p=.004) with a generally tolerable safety profile as initial treatment for HR+, HER2- ABC. Here we present the MONARCH 3 results from the preplanned final PFS analysis. Methods: MONARCH 3 is a double-blind, Phase 3 study of abemaciclib/placebo (150 mg, twice daily continuous schedule) + NSAI (1 mg anastrozole or 2.5 mg letrozole, daily) in postmenopausal women with HR+, HER2- ABC who have had no prior systemic therapy for advanced disease. Endocrine therapy (ET) naïve pts or pts with disease relapse >12 months after (neo)adjuvant ET were randomized 2:1 and stratified by metastatic site (visceral, bone-only, or other) and prior ET (aromatase inhibitor, no ET, or other). The primary objective was investigator-assessed PFS. Additional objectives included ORR, clinical benefit rate (CBR), duration of response (DoR), overall survival (OS), and safety and tolerability. The study was powered to 80% at 1-sided α=.025 assuming a HR of 0.67 in favor of abemaciclib + NSAI, with the final analysis at 240 PFS events. Results: 493 pts were randomized to abemaciclib + NSAI (n=328) or placebo + NSAI (n=165). At the final PFS analysis, 246 PFS events had occurred. Abemaciclib + NSAI significantly extended PFS (HR, 0.540; 95% CI, 0.418, 0.698; p=.000002; median: abemaciclib arm, 28.18 vs placebo arm, 14.76 months). PFS was improved across all subgroups. For pts with measurable disease, the ORR was 61.0% in the abemaciclib arm and 45.5% in the placebo arm (p=.003), and the CBR was 79.0% in the abemaciclib arm and 69.7% in the placebo arm (p=.037). The median DoR was 27.39 months in the abemaciclib arm compared to 17.46 months in the placebo arm. OS was immature at the time of analysis. Adverse events were consistent with previous reports. Conclusions: Abemaciclib + NSAI demonstrated a generally tolerable safety profile and was an effective initial treatment for pts with HR+, HER2- ABC, significantly improving PFS and ORR. Citation Format: Matthew P. Goetz, Miguel Martin, Angelo Di Leo, Seock-Ah Im, Ahmad Awada, Tammy Forrester, Martin Frenzel, Joanne Cox, Susana Barriga, Masakazu Toi, Hiroji Iwata, Stephen Johnston. MONARCH 3: Abemaciclib as initial therapy for patients with HR+, HER2- advanced breast cancer - Results from the preplanned final PFS analysis [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr CT040.

Background: A recent exploratory subgroup analysis pooling data from over 1000 patients (pts) in MONARCH 2 (final analysis) and MONARCH 3 (interim analysis) demonstrated that pts with clinical characteristics that predict for worse outcome (e.g. liver metastases, progesterone receptor [PgR]-negative status, high-grade tumors) received the largest benefit from the addition of abemaciclib to endocrine therapy (ET). There is also interest in determining the extent to which abemaciclib improves progression-free survival (PFS) in pts with favorable prognostic factors (e.g. bone-only disease, long treatment-free interval [TFI]). Here we present an update of the subgroup analyses of the MONARCH 3 study using data from the preplanned final PFS analysis. Methods: Enrollment criteria and study designs were previously described (Sledge et al. J Clin Oncol. 2017; Goetz et al. J Clin Oncol. 2017). An exploratory pooled analysis was conducted using data from MONARCH 2 (NCT02107703) and the MONARCH 3 (NCT02246621) preplanned final PFS analysis to identify significant prognostic factors regardless of therapy received. Using the updated MONARCH 3 data, PFS and objective response rate (ORR) (in pts with measurable disease) were examined within the prognostic subgroups that had been previously identified as being statistically significant. Additionally, subpopulation treatment effect pattern plot (STEPP) methodology was used to evaluate the association between TFI following adjuvant ET and outcomes of NSAI alone or with abemaciclib. Results: Assessment of clinical factors from MONARCH 2 and MONARCH 3 (final PFS data) confirmed that the following factors remained significantly prognostic regardless of treatment received: bone-only disease, liver metastases, tumor grade, PgR status, and ECOG performance status. In MONARCH 3, prognosis was poor in pts with liver metastases, PgR-negative tumors, and high-grade tumors. While all subpopulations benefited from the addition of abemaciclib to ET regardless of prognosis, pts with these three poor prognostic factors received substantial benefit from abemaciclib, with PFS hazard ratios (HR) ranging from 0.4 to 0.5 and with differential improvement in ORR typically >30%. Furthermore, STEPP analysis of TFI (on the MONARCH 3 pts who had received adjuvant ET) demonstrated that pts who had a shorter TFI had a worse prognosis and received relatively greater benefit from abemaciclib plus NSAI than did pts with a longer TFI. Patients with bone-only disease or a long TFI also received benefit from abemaciclib, but to a relatively lesser extent (HR ranging from approximately 0.6 to 0.8). Conclusions: This exploratory analysis of the preplanned final PFS data from MONARCH 3 confirms previous findings that pts with poor prognostic factors may receive relatively greater benefit from the addition of abemaciclib to ET. Citation Format: Joyce O'Shaughnessy, Matthew P. Goetz, George W. Sledge, Miguel Martin, Yong Lin, Tammy Forrester, Ian C. Smith, Angelo Di Leo, Stephen Johnston. The benefit of abemaciclib in prognostic subgroups: An update to the pooled analysis of MONARCH 2 and 3 [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr CT099.

Introduction The majority of breast cancers (BC) are estrogen (E) receptor positive (ER+). Endocrine therapies target E stimulation of tumour growth but resistance remains problematic, often a result of enhanced crosstalk between ER and growth factor pathways. Previously we reported the antiproliferative efficacy of combining everolimus (RAD001, mTORC1 inhibitor) with endocrine therapy in resistance models, but potential routes of escape from treatment via ERBB2/3 signalling were observed. We hypothesised that combined targeting of three cellular nodes (ER, ERBB and mTORC1) may target tumour rewiring and provide enhanced long-term clinical utility in endocrine resistant BCs. Methods Several ER+ BC lines adapted to long term E deprivation (LTED), modelling relapse to an aromatase inhibitor (AI), were treated in vitro with a combination of RAD001 and neratinib (pan ERBB inhibitor) in the presence or absence of estradiol (E2), tamoxifen or fulvestrant. Effects on proliferation, cell signalling, cell cycle and transcription were assessed. Additionally, an in vivo model of AI resistance was treated with monotherapies or combinations to evaluate efficacy in delaying tumour progression. Results All cell lines showed dose dependent decreases in proliferation in response to RAD001 (IC50 0.6-50nM without E2; 1-10nM with E2). Neratinib showed a wide range of IC50 values in the presence of E2 (300-1000nM). In the absence of E2, wild type (wt) cell lines showed IC50 values in excess of 1800nM with hormetic response curves, whilst in the LTED IC50 values ranged between 400-900nM. Combination of either agent with endocrine therapy caused a concentration dependent decrease in proliferation in both wt and LTED lines but the maximum effect was observed with a triple combination of RAD001, neratinib and endocrine therapy. Expression of pS6 was suppressed by RAD001 in all cells tested, whilst neratinib caused a cell specific reduction in expression of ERBB family proteins. Upregulation of pAKT was observed in all cell lines upon treatment with RAD001. Combination of RAD001 with neratinib suppressed the upregulation of pAKT and reduced cell cycle progression. In the absence of E2, RAD001 reduced ER mediated transcription and recruitment of ER and CREB binding protein to the TFF1 promoter, contrasting with neratinib, which caused a marked increase. In vivo study using LTED tumour xenografts showed the triple combination of RAD001, neratinib and fulvestrant was the most effective at reducing tumour volume (89%, p Conclusion The combination of RAD001, neratinib and endocrine agents may be effective in patients who have relapsed on endocrine therapy but retain a functional ER by combating ERBB pathway upregulation. Citation Format: Ricardo Ribas, Sunil Pancholi, Stephanie K. Guest, Aradhana Rani, Joanna Nikitorowicz-Buniak, Nikiana Simigdala, Allan Thornhill, Richard E. Cutler Jr, Alshad S. Lalani, Francesca Avogadri-Connors, Mitch Dowsett, Stephen R. Johnston, Lesley-Ann Martin. Co-blockade of mTORC1, ERBB and estrogen receptor signalling pathways in endocrine resistance breast cancer: Combating tumor plasticity [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 4157. doi:10.1158/1538-7445.AM2017-4157

Aromatase inhibitors (AI) are the mainstay for treatment of postmenopausal estrogen receptor positive (ER+) primary breast cancer (BC). However, many patients relapse whilst retaining a functional ER. The selective ER degrader (SERD) fulvestrant has been evaluated as a potential 2nd or 3rd line therapy for patients who relapse on AI treatment. Although exhibiting promising potential, its low bioavailability has limited its use to combination therapy in metastatic patients. Here, we assessed the efficacy of the orally available SERD, RAD1901, in a panel of BC cells with varying genetic backgrounds modelling patients both sensitive and resistant to AI therapy (LTED). Fulvestrant was used in parallel to allow relative responses to be compared. Cell proliferation assays in 2D and spheroids the presence of 0.01nM 17β-estradiol showed a concentration-dependent decrease in proliferation in response to RAD1901 and fulvestrant. GI50 values for RAD1901 in general were 10-fold higher than fulvestrant but equated to doses that are clinically achievable for RAD1901. Most importantly, RAD1901 effectively suppressed proliferation of two LTED models harbouring naturally occurring ESR1 mutations, MCF7 LTEDY537C (GI50 5nM) and SUM44-LTEDY537S (GI50 100nM). GI50 values of RAD1901 and fulvestrant showed similar reduction of ER, progesterone receptor (PGR) and cyclin D1 together with decreased phosphorylation of retinoblastoma (RB), concordant with cell cycle arrest. Furthermore, chromatin immunoprecipitation (ChIP) for ER in response to RAD1901 or fulvestrant showed a 70% reduction in recruitment of ER to TFF1, GREB1 and PGR promoters and concomitant reduction in mRNA expression of these genes. In MCF7-Arom cells, combination of letrozole with RAD1901 or fulvestrant showed enhanced antiproliferative effect compared to letrozole alone. The addition of RAD1901 to CDK4/6 inhibitor palbociclib or abemaciclib demonstrated additivity compared with monotherapy. In addition, RAD1901 effectively inhibited growth of palbociclib-resistant MCF7 LTED cells.These preclinical findings highlight the potential utility of RAD1901 as a potent drug in the treatment of ER+ BC. Combined with its bioavailability profile, RAD1901 warrants clinical testing versus fulvestrant after relapse on an AI, either alone or in combination with a CDK4/6 inhibitor. Citation Format: Sunil Pancholi, Joanna Nikitorowicz-Buniak, Ricardo Ribas, Nikiana Simigdala, Fiona Garner, Teeru Bihani, Stephen R. Johnston, Mitch Dowsett, Lesley-Ann Martin. Impact of oral selective estrogen receptor degrader RAD1901 in preclinical models of endocrine sensitive/resistant breast cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 4160. doi:10.1158/1538-7445.AM2017-4160

Background. ESR1 mutations are selected by prior aromatase inhibitor (AI) therapy for advanced breast cancer. We addressed the impact of ESR1 mutations on sensitivity to standard therapies in SoFEA and PALOMA3, two phase III randomised trials that represent the trends in the current therapy for estrogen receptor positive advanced breast cancer. Methods. We assessed plasma DNA ESR1 mutations in the SoFEA trial that compared exemestane with fulvestrant-containing regimens in patients with prior sensitivity to non-steroidal AI, and in the PALOMA3 trial that compared fulvestrant plus placebo with fulvestrant plus palbociclib in patients with progression on prior endocrine therapy. We developed and validated multiplex digital droplet PCR assays to detect ESR1 mutations in circulating free DNA. Plasma had been immediately processed in PALOMA3 but there was a delay of up to 9 days before processing of EDTA samples in the SoFEA trial. Results. In SoFEA we demonstrated that delayed processing of plasma samples had no effect on detection rate of ESR1 mutations (p = 0.71), which were found in 39.1%(63/161) patients. Patients with ESR1 mutations had improved progression free survival (PFS) on fulvestrant compared to exemestane (HR = 0.52, 95%CI 0.30 to 0.92, p = 0.02), while patients with wild type ESR1 had similar PFS on either treatment (HR = 1.07, 95%CI 0.68 to 1.67, p = 0.77). In PALOMA3, ESR1 mutations were found in the plasma of 25.3%(91/360) patients. ESR1 mutations were almost exclusively found in patients with prior AI exposure with or without tamoxifen, and were very rare with prior tamoxifen exposure only (28.9% (90/311) versus 2.0% (1/49) respectively, p endocrine therapy (sensitive to prior endocrine therapy, 29.8% (85/285) versus resistant 8.0% (6/75), p Conclusions. ESR1 mutation analysis in plasma after progression on prior AI therapy has clinical utility in directing choice of further endocrine therapy. Citation Format: Ben O’Leary, Charlotte Fribbens, Lucy Kilburn, Sarah Hrebein, Isaac Garcia-Murillas, Matthew Beaney, Massimo Cristofanilli, Fabrice Andre, Sherene Loi, Sibylle Loibl, John Jiang, Cynthia Huang Bartlett, Maria Koehler, Mitch Dowsett, Judith Bliss, Stephen Johnston, Nicholas Turner. ESR1 mutations in circulating tumour DNA predict outcome to endocrine treatment in patients with estrogen receptor positive advanced breast cancer; analysis of 521 patients in the SoFEA and PALOMA3 trials. [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 LB-069.

Arzoxifene (ARZ) is a selective estrogen receptor (ER) modulator with greater bioavailability than raloxifene which is being developed as treatment for breast cancer. We have used an in vivo model of hormone-sensitive breast cancer to study the growth-inhibitory and pharmacodynamic effects of ARZ in comparison with the most widely used antiestrogen, tamoxifen (TAM). We compared the abilities of ARZ and TAM to antagonize the estrogen (E2)-dependent growth of MCF-7 human breast cancer xenografts in oophorectomized athymic mice. At four different time points over 28 days, we studied their time-related pharmacodynamic effects on biomarkers of tumor growth (cell proliferation/death measured by Ki-67 and apoptosis scores), cell cycle activity (cyclin D1 and p27(kip1)), and hormone-regulated gene expression (ERalpha, progesterone receptor, and pS2). Although maximal growth inhibition was seen after E2 withdrawal, ARZ and TAM induced significant and similar inhibition of E2-stimulated tumor growth. Inhibition of growth was reflected by changes in the tumor growth index (ratio posttreatment/pretreatment Ki-67/apoptosis scores). ARZ and TAM resulted in a significant (P0.001) increase in ER expression and reduction in progesterone receptor expression, whereas changes in cyclin D1 score were inversely related to p27(kip1) score. A significant but delayed biological effect was observed with a 10-fold lower dose of ARZ. These results show that ARZ is an effective antagonist of E2-stimulated breast cancer growth with similar growth-inhibitory and pharmacodynamic effects to TAM in this model.

Not all patients (pts) with HER2 positive (+) breast cancer (BC) benefit from treatment (tx) with EGFR/HER2 tyrosine kinase inhibitor lapatinib (L) & it is not known whether HER2 over-expression or amplification are obligate requirements for L response. Assessment of in vivo biomarkers of drug activity, e.g. change in Ki67, after short-term pre-surgical tx may elucidate the differential molecular profile of sensitive vs. resistant tumors. Aims: i) Determine whether L induces anti-proliferative effects in HER2+ & HER2 negative (-) BC & ii) identify biomarkers of sensitivity/resistance to anti-proliferative effects of L. Methods: Women with newly diagnosed BC were randomized (3:1) to receive 10-14 days of pre-operative L (1500mg/day) or placebo (P) in a multicentre phase II trial (ISRCTN68509377). Paired core biopsies before & after tx were analyzed for Ki67, TUNEL, HER2, EGFR, ER, PgR, pAkt, pErk, & stathmin (candidate marker of PI3K/Akt activation) by IHC +/− FISH. HER2, HER3 & ligands EREG, AREG & NRG1 mRNA was measured by RT-PCR. The primary endpoint was change in Ki67. HER2+ was defined as 3+ or 2+ by IHC and FISH+. Differences in geometric means were assessed by Mann-Whitney & correlations by Spearman rank. Results: 121 pts (L=94, P=27) were randomized between 12/2007-04/2011, 70% were ER+/PgR+, 13% ER+/PgR- & 17% ER-; 22% were HER2+ (including one 2+/FISH+) & 26% were EGFR+. L pts reported significantly more frequent G1-2 (64%) or G3 (6%) rash, & G1 diarrhea (56%); the only other G3 toxicity was infection in 1 pt. There were no delays in surgery. Paired samples containing tumor were obtained for 98% (118/121) of randomized pts. Ki67 fell significantly in the L group (relative reduction in post- vs. pre-tx samples = -31%, 95%CI: -44 to -16; p Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr LB-222. doi:1538-7445.AM2012-LB-222

CTRC-AACR San Antonio Breast Cancer Symposium: 2008 Abstracts Abstract #4114 Background: Evidence indicates a direct molecular link between HER2 amplification and up-regulation of VEGF in HER2+ breast cancer. Concurrent over-expression of HER2 and VEGF is associated with a poorer clinical outcome than over-expression of either alone. These data provide the rationale for simultaneous blockade of both pathways. Pazopanib (P) is an oral angiogenesis inhibitor targeting VEGFR, PDGFR, and c-kit. Lapatinib (Tykerb®) (L) is an oral tyrosine kinase inhibitor of EGFR (ErbB1) and HER2 (ErbB2). This study ([VEG20007][1]) evaluated the efficacy and safety of dual pathway inhibition in patients (pts) with HER2+ adv/met breast cancer. Methods: Eligible pts received no prior chemotherapy or HER2-directed therapy for met disease and had measurable disease per RECIST. Pts were enrolled in one of 2 cohorts. In cohort 1 (C1), pts were randomized to P 400 mg/d + L 1000 mg/d or L alone (1500 mg/d). Enrollment to cohort 2 (C2) began after completion of enrollment to C1. In C2 pts received the combination at doses of P 800 mg/d and L 1500 mg/d. Following disease assessment at week (wk) 12, pts with an objective response could continue on study treatment following re-consent; those with stable disease could continue only where trastuzumab was unavailable. The primary endpoint of C1 was progressive disease rate (PDR) at wk 12 in HER2 FISH+ pts. The primary endpoint of C2 is week 12 response rate in HER2 FISH+ pts. Results: 140 pts were included in the randomized cohort analysis which was previously reported [J Clin Onc 26:2008 (May 20 suppl; abstr 1016)]. Enrollment in C2 has been completed; 40 pts (36 confirmed FISH+) have been enrolled. A preliminary safety analysis has been performed. The most common adverse events of any grade were diarrhea, nausea, fatigue, hypertension, and rash. The most common laboratory abnormalities were ALT and AST increase. Efficacy data will be analyzed when the last pt has completed the week 12 assessment. Conclusion: This is the first Phase II trial to evaluate the combination of 2 oral targeted agents in first-line Her2+ adv/met breast cancer. The lower dose combination demonstrated improved response rate compared to L monotherapy. Previously unreported efficacy and safety data from the higher dose cohort will be presented. Citation Information: Cancer Res 2009;69(2 Suppl):Abstract nr 4114. [1]: /lookup/external-ref?link_type=GENPEPT&access_num=VEG20007&atom=%2Fcanres%2F69%2F2_Supplement%2F4114.atom