Goetz, Matthew P.

Abstract Background: The first in-human phase-I clinical trial of the tamoxifen metabolite, Z-endoxifen, demonstrated promising antitumor activity in both aromatase inhibitor (AI) and tamoxifen resistant ER+ breast cancer patients (Goetz M.P., et al; JCO, 2017). By utilizing a letrozole-sensitive MCF7 aromatase expressing (MCF7AC1) cell line, and its corresponding letrozole-resistant (MCF7LR) cell line, we have shown superior antitumor and antiestrogenic activity of Z-endoxifen over tamoxifen in the AI-sensitive and resistant settings in vivo. This study also revealed that Z-endoxifen may target protein kinases in addition to estrogen receptor alpha. Follow-up studies demonstrated that Z-endoxifen, but not tamoxifen, binds to protein kinase C beta 1 (PKCβ1), a member of the serine/threonine-specific protein kinase family that regulates signaling pathways involved in cell proliferation and tumorigenic transformation (Guo C., et al; SABCS, 2017). We have further shown that PKCβ1 silencing, similar to Z-endoxifen, potently inhibits ER+ cell proliferation and additionally causes ERα turnover. Here, we report a potential mechanism by which PKCβ1 may mediate its anti-proliferative effects in ER+ breast cancer cells. Methods: The effects of siRNA-mediated PKCβ1-silencing on gene transcription were evaluated by RNAseq in MCF7AC1 cells. RNAseq results were further validated by quantitative real-time polymerase chain reaction (qRT-PCR) in PKCβ1-silenced MCF7AC1 cells as well as in ER+ T47D cells. The effects of Z-endoxifen on the mRNA expression of genes altered by PKCβ1-silencing in MCF7AC1 cells were evaluated by qRT-PCR analysis. PKCβ1 silencing effects on protein expression in MCF7AC1 cells were evaluated by immunoblot analysis. Results: RNAseq analysis of PKCβ1-silenced MCF7AC1 cells compared to non-silenced cells revealed NF-κB signaling pathway as the top biological pathway significantly altered by PKCβ1 silencing (p=5.93e-08). Enhanced mRNA expression of multiple NF-κB downstream target genes including CXCL10, RELB, MMP1 and IL6 were observed following PKCβ1 silencing. qRT-PCR analyses validated increased mRNA expression of these genes in PKCβ1-silenced MCF7AC1 and T47D cells. Notably, Z-endoxifen treatment in MCF7AC1 cells mirrored PKCβ1 silencing effects, resulting in enhanced mRNA expression of the aforementioned genes. Evaluation of the protein expression of RelA and RelB, the canonical and noncanonical NF-κB transcription factors respectively, in PKCβ1-silenced MCF7AC1 cells revealed significant upregulation of RelA (p=0.0203) but not RelB (p=0.1360) expression. ERα silencing in MCF7AC1 cells did not affect RelA protein expression. Consistent with prior reports, treatment of MCF7 cells with TNFα inhibited cell proliferation. Finally, targeted siRNA-mediated silencing of RelA in PKCβ1-silenced MCF7AC1 cells significantly attenuated PKCβ1-mediated growth inhibition (p=0.0002). Conclusion: Our findings demonstrate that both Z-endoxifen treatment and PKCβ1-silencing commonly activates a NF-κB-mediated gene signature in ER+ breast cancer cells. Notably, PKCβ1-silencing induces the protein expression of the NF-κB transcription factor RelA in an ERα-independent manner, which appears to contribute to PKCβ1-mediated growth inhibition. Current studies are ongoing to evaluate the impact of RelA silencing on Z-endoxifen mediated growth inhibition. This information will foster our understanding of the estrogenic and non-estrogenic mechanisms by which Z-endoxifen mediates its antitumor activity in ER+ breast cancer. Citation Format: Swaathi Jayaraman, Mary J Kuffel, Krishna R Kalari, Kevin J Thompson, Xiaojia Tang, Elizabeth S Bruinsma, John R Hawse, Matthew P Goetz. Protein kinase C beta 1 (PKCβ1), a novel drug target of Z-endoxifen, inhibits growth of estrogen receptor positive (ER+) breast cancer via activation of the NF-kB transcription factor RelA [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 P6-04-06.

Abstract Background: Local antitumor immunity—as assessed by quantifying tumor-infiltrating immune cells—is increasingly recognized as a critical factor influencing prognosis and therapy response in TNBC. However, an understanding of systemic antitumor immune responses occurring in peripheral blood, and their influence on prognosis and chemotherapy response has not been rigorously studied. Methods: Cytometry by time-of-flight (CyTOFTM, Fluidigm) was used to examine viably cryopreserved peripheral blood mononuclear cell (PBMC) suspensions prospectively collected from patients with early-stage TNBC prior to initiation of standard neoadjuvant paclitaxel followed by doxorubicin and cyclophosphamide (NACT) as part of the BEAUTY study [1]. Samples were stained using a panel of metal-tagged antibodies, recognizing 30 surface proteins optimized for immune monitoring of human peripheral blood. Differential abundance analysis of immune cell subsets was carried out to evaluate differences between patients who achieved pCR versus those with residual disease after NACT, and between patients with known luminal androgen receptor (LAR) versus basal TNBC subtypes defined by bulk tumor RNA sequencing. Results: Viably cryopreserved PBMC samples from 40 treatment-naive TNBC patients were available for analysis. The median age was 52 years (range 32 - 73), with 6 (15%) patients having tumors classified as LAR TNBC, and the remaining 34 (85%) as basal TNBC. Overall, 21 (53%) patients achieved pCR after NACT. After acquisition on the mass cytometer, the median yield per sample was 626,815 single-cell events (range 42,786 - 1,035,575), with a median percent debris of 13.7% (range 14 - 58). Across the 40 PBMC samples, the total yield was 23,507,094 single-cell events. The median frequencies of major circulating immune cell subsets across the 40 TNBC patients were: T cells 53.9% (range 25.4 - 71.3), with 33.4% CD4+ T cells (range 11.4 - 46.7) and 10.3% CD8+ T cells (range 5.8 - 19.9); B cells 10.8% (3.3 - 32.6), NK cells 8.6% (1.7 - 17.0) and monocytes 10.6% (2.7 - 29.8). Examining pre-treatment blood samples, patients with residual disease after NACT exhibited a higher median frequency of baseline CD14+CD16- classical monocytes (7.5% vs. 4.1%, p=0.025) and a lower frequency of terminally-differentiated effector memory cytotoxic (CD8+) T cells (0.6% vs. 1.7%, p=0.038) compared to patients who achieved pCR. Patients with LAR TNBC also exhibited a higher frequency of CD14+CD16- classical monocytes (11.5% vs 4.3%, p=0.058), and in addition exhibited a lower frequency of central memory CD4+ T cells (10.4% vs 15.2%, p=0.048). No difference in CD8+ T cells was seen by LAR status. Additional associations of peripheral blood immune cell subsets and classic tumor pathological features will be presented at the meeting. Conclusion: To our knowledge, this is the first study focused on TNBC to demonstrate variation in peripheral blood immune cell populations by molecular TNBC subtype (LAR vs. basal), and by chemotherapy response. A higher frequency of circulating classical monocytes—which can infiltrate into tissues and give rise to macrophages—appears to be detrimental; whereas a higher frequency of circulating antigen-experienced memory CD8+ T cells seems to be protective, suggesting a putative role of this cell subset in TNBC anti-tumoral immunity. Reference: [1] Goetz MP et al. JNCI 2017, PMID:28376176 Citation Format: Roberto A Leon-Ferre, Kaitlyn McGrath, Jodi M Carter, Krishna R Kalari, Vera J Suman, Richard Weinshilboum, Liewei Wang, Keith L Knutson, Stephen M Ansell, Judy C Boughey, J C Villasboas, Matthew P Goetz. Deep phenotyping using CyTOF identifies peripheral blood immune signatures associated with clinical outcomes and molecular subtypes in patients with early-stage triple negative breast cancer (TNBC) [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 P5-04-09.

Abstract Background: BRCA1/2 carriers have higher scores in the 21-gene RS assay compared to non-carriers. Little is known about the 21-gene RS patterns in women harboring germline mutations in genes outside BRCA1/2. Methods: We utilized the Mayo Clinic Breast Cancer Study Registry to identify individuals with non-metastatic estrogen receptor positive breast cancer and clinical 21-gene RS results available. We cross-referenced this with an institutional registry of patients tested for hereditary cancer genes, either from the electronic health record or from a research study conducted using a custom QIAseq multiplex amplicon-based panel (Qiagen). The latter assay covered target regions of all coding regions for the following predisposition genes: ATM, BARD1, BLM, BRCA1, BRCA2, BRIP1, CDH1, CDKN2A, CHEK2, ERCC2, FANCC, FANCM, MLH1, MRE11A, MSH2, MSH6, MUTYH, NBN, NF1, PALB2, PMS2, PTEN, RAD50, RAD51C, RAD51D, RECQL, PRSS1, STK11, TP53, and XRCC2. Student’s t-test was used to compare 21-gene RS between carriers, non-carriers, and specific carrier genes (JMP v 14.0). Results: 282 patients were identified with early stage breast cancer and 21-gene RS available, including 85 known mutation carriers and 197 non-carriers (or unknown carrier status). Germline mutations were identified in CHEK2 (n=25), BRCA2 (n=18), ATM (n=10), BRCA1 (n=5), PALB2 (n=5), TP53 (n=3), FANCM (n=2), MUTYH (n=2), PMS2 (n=2), PPM1D (n=2), RECQL (n=2), BARD1 (n=1), BLM (n=1), CDH1 (n=1), ERCC2 (n=1), FANCC (n=1), MSH6 (n=1), NBN (n=1), PRSS1 (n=1), and PTEN (n=1). In the entire cohort, the mean 21-gene RS was 20.4 in carriers and 18.0 in non-carriers (p=0.11). We performed individual analyses on specific genes identified in ≥ 5 patients (table 1). Compared to the mean RS in non-carriers (18.0), higher mean RS were observed in patients harboring BRCA1 (38.8, p=0.002), PALB2 (26.6, p=0.0724), and BRCA2 (23.2, p=0.0453). The RS in CHEK2 and ATM carriers were comparable to those of non-carriers. Conclusions: We observed differences in RS score distribution based on specific mutated breast cancer predisposition genes. Further validation of the distribution and performance of the 21-gene RS in germline carriers is needed. 21-gene RS scorenMedian RSMean≤1516-25≥26n%n%n%CHEK2251616.61248728624BRCA2182123.2211.11161.1527.8ATM101716.9330550220BRCA153635.8120120360PALB253326.6120120360None1971518.09950.367343115.7 Citation Format: Karthik V Giridhar, Grace M Choong, Siddartha Yadav, Eric C Polley, Roberto A Leon-Ferre, Minetta C Liu, Matthew P Goetz, Fergus J Couch. Distribution of the 21-gene recurrence score (RS) assay (Oncotype DX) in carriers of germline mutations associated with breast cancer predisposition syndromes [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 P5-03-07.

Abstract Background: The combination of cyclin dependent 4/6 kinase inhibitors (CDK4/6i) with endocrine therapy (ET) has resulted in clinically significant improvements in progression-free survival (PFS) and overall survival (OS) in hormone-receptor (HR)-positive metastatic breast cancer (MBC). However, most patients’ disease ultimately progresses on CDK4/6i and ET. Therefore, further research is necessary to understand the mechanisms driving primary and secondary resistance. PROMISE is a multicenter prospective cohort study enrolling women with HR-positive MBC commencing treatment with palbociclib + letrozole (1st line) or palbociclib + fulvestrant (2nd Line). The study provides a comprehensive “omic” assessment of blood, tumor, urine and the fecal microbiome to identify molecular or cellular features associated with primary endocrine resistance (e.g. disease progression ≤ 12 months on treatment) and acquired resistance to CDK 4/6i. Additionally, patient derived xenografts and organoids are created to test new drug strategies designed to overcome resistance to CDK 4/6i and ET. Here, we present initial sequencing results from pretreatment biospecimens collected from PROMISE study participants. Methods: On-study tumor biopsies and blood samples were collected for DNA/RNA sequencing (TempusTM). The analyzed biospecimens were all obtained prior to initiation of palbociclib and ET. We correlated patient clinical characteristics (phenotypes) with molecular data and responses to protocol treatment. The data were analyzed using a series of cutting-edge bioinformatics pipelines for somatic and germline mutations in addition to copy number alterations (CNAs). The study database was locked for analysis on 06/20/2019. Results: We analyzed the somatic single nucleotide variants/INDELs (sSNV/INDEL) profiles across the tumor samples to determine the genes that were least likely to occur as a result of background mutation processes. Twenty-six patients had somatic copy number alterations (sCNA) and/or sSNV/INDEL in at least one of 18 genes with the most significant sSNV/INDEL profiles (p < 0.03) which included clinically and biologically relevant genes. The genes with the most statistically significant sSNV/INDEL mutation profiles were GATA3, PIK3CA, CDH1, and ESR1 (p < 0.0009). We observed a high percentage of tumors with somatic alterations in GATA3 (23% sSNV/INDEL, 15% sCNA), PIK3CA (38%, 12%), CDH1 (19%, 50%) and ESR1 (19%, 58%). ESR1 mutations were more frequent in patients receiving 2nd line treatment. Other frequently altered genes included TP53 (15%, 46%), MAP2K4 (8%, 50%), DNAAF1 (8%, 50%), and CDKN1B (8%, 35%). Further, ZNF317 and F3 were altered in 9 and 7 patients, respectively. Twenty-four samples had alterations in at least one of the CDK4/6 pathway genes (RB1, CCNE2, CCND1, CDK6, ESR1, CDKN2A, CCND3, CDK4, CDK2 and CCNE1). Four patients progressed on therapy; three of the four patients had mutations in PIK3CA, and one had a mutation in ESR1. Results of the RNA sequencing data (N=26) will be presented at the SABCS meeting. Conclusions: This is the first report of a prospective study designed to characterize the genomic landscape of ER+/HER2- MBC prior to palbociclib treatment. We observed high frequencies of known targetable alterations in PIK3CA and ESR1, including in patients that progress, which is consistent with previous reports. RNA sequencing data will be presented at the meeting. Citation Format: Ciara C O Sullivan, Krishna R Kalari, Vera J Suman, Peter T Vedell, Ann Moyer, Erin Carlson, Jason Sinnwell, Tejaswi Alaparthi, Xiaojia Tang, Kevin Thompson, Jaeyun Sung, Alvaro Moreno-Aspitia, Donald Northfelt, Minetta C Liu, Tufia C Haddad, Prema Peethambaram, Saranya Chumsri, Kathryn J Ruddy, Karthik V Giridhar, Roberto A Leon-Ferre, Paula Gill, Mohammad Ranginwala, Asad Javed, Sameer Batoo, Brendan P. McMenomy, Richard Weinshilboum, Liewei Wang, Matthew P Goetz. Comprehensive tumor sequencing to identify biomarkers of palbociclib response: First report of the PROMISE study [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 P2-11-07.

Abstract Introduction: CDK 4/6 inhibitors (CDK4/6i) are FDA approved for treatment of hormone receptor (HR) positive, HER2 negative metastatic breast cancer (MBC). Recent work demonstrated that a rise in MCV was correlated with longer median progression free survival (Anampa et al, Haemoatologica 2018). We performed a single-institution retrospective analysis to evaluate whether palbociclib induced early changes in MCV serve as a pharmacodynamic biomarker to predict response to CDK4/6i. Methods: We identified a retrospective cohort of 81 patients with HR+, HER2- MBC treated with CDK4/6i as first line metastatic treatment at Mayo Clinic, Rochester. Hematologic indices were abstracted pretreatment and at the start of each cycle through the start of cycle 4 (C4). Wilcoxon signed test was used to examine the changes in MCV (ΔMCV) comparing pretreatment up to C4 and ΔMCV between cycles. Optimal cut points for ΔMCV were determined using Cutoff finder (Budczies et al., PLOS One 2012). Cox proportional hazard regression analysis was performed to evaluate associations of MCV and time to treatment failure (TTF), which was defined as the time from the start of the analyzed cycle to treatment discontinuation for any reason(in months). As eight comparisons were made, a Bonferroni correction established p-value <0.00625 as the adequate cutoff for statistical significance. Statistical analyses were performed using JMP (SAS Institute Inc, Cary, NC). Results: 60/81 patients had pretreatment and at least one subsequent lab data point that included MCV. The pretreatment mean hemoglobin (Hgb) was 12.8 [standard deviation (SD) 1.37 g/dL] and 6/60 (10.2%) patients with Hgb <11.0 g/dL. Mean pretreatment MCV was 89.0 femtoliters/cell [fl (SD 4.63)] and no macrocytosis (defined as MCV >100 fl) was observed. At C4, the mean Hgb was 12.1 g/dL (SD 1.11) with 4/49 (8.2%) with Hgb <11.0 g/dL. The C4 mean MCV was 96.6 fl (SD 5.23) and 8/29 (16.3%) developed macrocytosis. Wilcoxon signed analysis showed a significant increase in ΔMCV pretreatment to C2 [n=51, median 1.5 with interquartile range (IQR) -0.1-3.2, p<0.0001], C2 to C3 (n=44, median 2.9, IQR 1.4-5.2, p<0.0001), and C3 to C4 (n=41, median 3.4, IQR 1.9-4.9, p<0.0001). A rise in MCV≥8.15 from pretreatment to C4 was associated with a non-statistically significant improvement in TTF [n=49, hazard ratio (HR) 0.38, p = 0.06]. Between C2 and C3, a rise in MCV ≥2.9 was associated favorably with TTF (n=44, HR 0.36, p = 0.029). The estimated probability of remaining on treatment at 30 months was higher in those with C2-C3 ΔMCV ≥2.9 [44.1%; 95% Confidence Interval (CI) 17.3-68.2%] compared to those with a C2-C3 ΔMCV <2.9 (10.1%; 95%CI 1.5-28.7%). Discussion: We observed an increase in MCV in all patients receiving CDK4/6i by start of C4. The rise in MCV during CDK4/6i treatment may reflect drug induced cell-cycle arrest in hematopoietic cells. Evaluation in larger studies is needed to validate dynamic changes in MCV as a predictive biomarker of response to CDK4/6i therapy. Citation Format: Grace Mei Yee Choong, Roberto A Leon-Ferre, Ciara C O'Sullivan, Kathyrn J Ruddy, Tufia C Haddad, Timothy J Hobday, Prema P Peethambaram, Charles L Loprinizi, Minetta C Liu, Vera J Suman, Matthew P Goetz, Karthik V Giridhar. Evaluation of mean corpuscular volume (MCV) as a pharmacodynamic predictive biomarker in patients receiving CDK4/6 inhibitors for metastatic breast cancer (MBC) [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 P1-19-42.

Abstract Background: Breast cancer incidence and mortality rates are increased in patients’ older than 65 years. However, there are limited reports on age-specific efficacy, safety, and drug tolerability, particularly in elderly patients older than 75 years old. Abemaciclib demonstrated clinically meaningful progression-free survival (PFS) improvement in combination with endocrine therapy (ET) in HR+, HER2- advanced breast cancer patients in the Phase 3 studies, MONARCH 2 and 3. Here we report exploratory subgroup analyses of MONARCH 2 and 3 to provide age-specific outcomes. Methods: In MONARCH 2 (NCT02107703), patients progressing while on prior ET received abemaciclib (or placebo) plus fulvestrant; in MONARCH 3 (NCT02246621), postmenopausal women with no prior systemic therapy for recurrent or metastatic breast cancer received abemaciclib (or placebo) plus letrozole or anastrozole. Exploratory outcome analyses of MONARCH 2 and 3 studies were performed for three age groups (women aged <65, 65-74, and ≥75 years). Using pooled safety data, we performed age-specific subgroup analyses of the most common treatment-emergent adverse events (TEAEs) associated with either ET or ET plus abemaciclib. To assess the impact of age on efficacy, a subgroup analysis of PFS was performed using a Cox model for each trial independently. Results: Pooled safety data were available for 1152 patients treated in MONARCH 2 and 3 trials, including 59.7% pts <65 years, 28.7% pts between 65 and 74 years, and 11.5% pts ≥ 75 years. Of those, 768 women received abemaciclib plus ET and 384 women received placebo plus ET. The most frequent TEAE was diarrhea and the most common Grade ≥3 TEAE was neutropenia. While diarrhea (any grade) occurred with similar incidence in abemaciclib-treated patients across age groups, clinically relevant diarrhea (Grade 2/3) was higher in the elderly (<65, 39.5%; 65-74, 45.2%; ≥75, 55.4%); however, in the ≥75 group, diarrhea (Grade 2/3) incidence was also higher for placebo plus ET (<65, 6.8%; 65-74, 4.5%; ≥75, 16.0%). Similarly, nausea and decreased appetite were moderately higher (by 10-20%) in the 2 abemaciclib-treated elderly subgroups. Fatigue (any grade) was higher in the 2 abemaciclib-treated elderly subgroups (<65, 34.8%; 65-74, 48.4%; ≥75, 51.8%), while no differences were found across the 3 placebo subgroups. In contrast, the rates of Grade 3/4 neutropenia did not differ as a function of age in either the abemaciclib plus ET arm (<65, 25.8%; 65-74, 27.4%; ≥75, 18.1%) or placebo plus ET arm. For efficacy, a consistent PFS benefit was observed with abemaciclib plus ET versus placebo plus ET across all three age-specified subgroups in both studies (Table). Conclusion: Abemaciclib in combination with ET demonstrates a tolerable safety profile and consistent efficacy benefit across all age subgroups examined, supporting the use of this combination in elderly patient populations. Despite the limited number of patients in the ≥75 group with potentially confounding comorbidities, the safety data suggest that patient education and appropriate management of toxicities, including dose adjustments and use of supportive medication for gastrointestinal toxicities, could maximize the benefit of abemaciclib. Table: Age-specified PFS outcomes from MONARCH 2 and 3MONARCH 2MONARCH 3Age GroupAbemaciclib + ETPlacebo + ETHR (95% CI)P-value *Abemaciclib + ETPlacebo + ETHR (95% CI)P-value *Events/NEvents/NEvents/NEvents/N<65140/29192/1330.52 (0.40, 0.68)0.69581/18065/910.48 (0.35, 0.67)0.63465-7461/11442/600.63 (0.43, 0.94)40/10631/540.64 (0.40, 1.02)≥7521/4123/300.62 (0.34, 1.11)17/4212/200.54 (0.26, 1.13)Abbreviations: CI, confidence interval; ET, endocrine therapy; HR, hazard ratio; PFS, progression-free survival.*P-value corresponds to the interaction test between age and treatment. Citation Format: Matthew P. Goetz, Meena Okera, Hans Wildiers, Mario Campone, Eva-Maria Grischke, Luis Manso, Valerie Anne Marie Andre, Amy Lee Chong, Belen San Antonio, Masakazu Toi, George W Sledge Jr. Safety and efficacy of abemaciclib plus endocrine therapy (ET) in elderly patients with hormone receptor-positive/human epidermal growth factor receptor 2-negative (HR+, HER2-) advanced breast cancer: An age-specific subgroup analysis of MONARCH 2 and 3 trials [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 P1-19-10.

Abstract 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<.0001) in the abemaciclib plus AI versus placebo plus AI arms, respectively. Here we report data from the additional 12 months of follow-up on tumor response for the ITT population and exploratory subgroups previously identified as significantly prognostic (Di Leo et al. 2018). 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 <36 months (Table). Median TTR was similar between the two treatment arms (abemaciclib plus AI: 3.62 months; placebo plus AI: 3.65 months), and this was generally consistent across subgroups. Responses were more durable in the abemaciclib plus AI arm (median DoR: 32.71 months; 95% CI: 25.74, -) versus the placebo plus AI arm (median DoR: 17.49 months; 95% CI: 11.61, 22.19), including in poor prognostic subgroups. 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<36 months3117 (54.84)225 (22.73)32.11≥36 months7442 (56.76)3216 (50.00)6.76Tumor gradeHigh5739 (68.42)2811 (39.29)29.13Intermediate/ Low13992 (66.19)7535 (46.67)19.52ECOG PS111275 (66.96)5022 (44.00)22.96015793 (59.24)8237 (45.12)14.12Bone-only diseaseNo256162 (63.28)12253 (43.44)19.84YesNA*NA*NA*NA*NA*N= total number of patients in each subgroup; n= number of responder patients; NA=not applicable*Response rate not reported for bone-only disease because majority of lesions were not measurable 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.

2596 Background: Docetaxel (D) is an anti-mitotic drug metabolized by the cytochrome P450 (CYP) 3A4 enzyme. While case reports have demonstrated a potential association of CYP3A inhibitor use with severe toxicity in D treated patients (pts), there are no data whether co-administration of CYP3A inhibitors with D increases the rate of hospitalization and/or death. Methods: A retrospective cohort of pts treated with D at the Mayo Clinic Rochester from 1/1/09-10/1/11 was identified. Information regarding D and concomitant chemotherapy (starting dose and dose reductions), type of CYP3A inhibitor, growth factor use, hospitalizations and deaths was abstracted. Time to event analyses (either hospitalization or death) were conducted using Kaplan-Meier estimates and proportional hazards modeling. Results: 384 pts (median age 63, range 20-93) received D as monotherapy (48%) or as multi-agent chemotherapy (52%) for a median of 4 cycles. 56 pts were co-administered CYP3A inhibitors (45 moderate, 11 strong). 90 pts were hospitalized during D chemotherapy and 15 died. The mean time to hospitalization was significantly shorter in pts treated with a mod/strong CYP3A inhibitor (186 days) vs not treated (408 days) (p=0.02) . While time to death was not significantly different based on inhibitor use (111 vs. 177 days, p=0.49), time to first event (hospitalization or death) varied significantly according to those on inhibitors (187 days) versus not (397 days) (HR: 1.78, p=0.02). Time dependent analyses indicated that use of multiage chemotherapy (combination vs D monotherapy; HR 1.96, p= 0.002) and chemotherapy dose reduction of either D (HR 0.54, p<0.01) or other chemotherapy (HR 0.61, p=0.02) were significantly associated with the risk of an event. In a multivariate analysis, moderate/strong CYP3A inhibitor use was associated with a higher risk of any event (HR 1.79, p<0.02), independent of the use or dose of either multi-agent chemotherapy or D dose. Conclusions: These are the first data to demonstrate that co-administration of CYP3A inhibitors with D increases the risk of hospitalization and/or death. Oncologists should reconcile medications prior to D administration and mod/strong CYP3A inhibitors should be avoided or used with caution.

2562 Background: Empiric chemotherapy (taxane/platinum) is standard for CUP. Because prognosis is poor, novel approaches are needed. The PI3K/mTOR pathway is frequently dysregulated in cancer. Everolimus (E), an mTOR inhibitor, is approved for multiple malignancies. We performed a phase II study of Pac + Carb + E as first-line therapy in metastatic CUP patients (pts). We additionally determined if a gene expression profiling (GEP) test that identifies tissue of origin (Pathwork Tissue of Origin) could identify responsive pts. (NCT00936702) Methods: Newly diagnosed, untreated CUP pts were eligible. Central pathology review confirmed CUP prior to registration; GEP was performed on formalin fixed tumor tissue. Pac (200 mg/m2), Carb (AUC=6) and E (30 mg once weekly) were delivered every 3 wks until progression or intolerable adverse events (AEs). The primary endpoint was confirmed response, with ≥11 of 50 responses (22%) needed for trial success. Secondary endpoints were OS, progression-free survival (PFS) and AEs. Results: 46 pts (median age 61) received a median 4 cycles (range: 1-33). 39 (85%), 21 (46%) and 1 (2%) experienced ≥1 grade (gr) 3+, 4+, or 5 (sepsis) AE, with gr 3+ hematologic AE most common (74%). Of 44 evaluable pts, 15 had a confirmed response (RR 34%, 95% CI: 21-50%), with a median PFS and OS of 4.1 and 10.1 mos, respectively. Adequate tissue for GEP was available in 36 pts and predicted 10 different sites of origin. In pts with a predicted tissue of origin in which taxane/platinum is standardly used, higher RR and significantly longer PFS and OS were observed compared with pts whose GEP identified a malignancy where taxane/platinum is not standard (Table). Conclusions: In pts with untreated CUP, Carb +Pac +E demonstrated promising antitumor activity. The GEP test identified patients clinically responsive to Carb/Pac/E therapy, and may be useful to select CUP pts for specific antitumor regimens. Clinical trial information: NCT00936702. [Table: see text]

1024 Background: The combination of platinum agents and PARP inhibitors may benefit patients (pts) with BRCA-associated metastatic breast cancer (MBC). We report on the response and clinical benefit rates when combining the PARP inhibitor veliparib (V) and carboplatin (carb) in a phase I trial. Methods: BRCA carriers with MBC were eligible. Carb starting at an AUC of 6 was given IV in 21-day cycles (C) and V was given orally twice daily (BID) at dose levels (L) L1 through L5. Results: Twenty-eight pts (26 eligible) carrying BRCA1 (12) or BRCA2 (15), or both (1) mutations were accrued between June 2010 and June 2012. The median age (32-66) was 45 years. The number (#) of prior chemotherapy regimens given for MBC was 1 (0-5); 70% of BCs were ER+, and 7% were HER2+. The schema, dose limiting toxicities (DLT) during C 1, median # of Cs on trial, and maximum tolerated dose (MTD) are shown. There were 3 (12%) complete and 9 (35%) partial responses (PR). Unconfirmed PR or stable MBC (median duration: 8 months [6-10+]) were seen in 7 pts (27%); the clinical benefit rate was74%. The median progression-free survival (PFS) is 7.8 months (95% CI 7.3-9.5). The pt with Fallopian tube cancer had a CR. DLTs with C 1 were seen in 2/6 evaluable pts at L1 (1 pt w/grade 3 hyponatremia and dehydration, and 1pt w/grade 4 thrombocytopenia [PLT]), leading to de-escalation of carb. At L2, 1 pt had grade 4 PLT. At L5, 1 pt had grade 4 PLT, and 2 pts grade 3 PLT (1 pt also experienced grade 4 granulocytopenia [ANC]), defining the MTD at carb AUC 5 and V 150 mg BID (L4). Dose delays and/or dose adjustments due to grade ≥ 2 toxicities for ANC or PLT were seen during the first 3 Cs at L1 (100%), L2 (50 %), L3 (67%), L4 (83 %), and at L5 (67%). Conclusions: The combination of carb and V is active, and is associated with substantial clinical benefit rate and managable hematologic toxicities in BRCA carriers with MBC. Further definition of the role of V is warranted. [Table: see text]