In breast cancer (BC) the employment of sequencing technologies for metagenomic analyses has allowed not only the description of the overall metagenomic landscape but also the specific microbial changes and their functional implications. Most of the available data suggest that BC is related to bacterial dysbiosis in both the gut microenvironment and breast tissue. It is hypothesized that changes in the composition and functions of several breast and gut bacterial taxa may contribute to BC development and progression through several pathways. One of the most prominent roles of gut microbiota is the regulation of steroid-hormone metabolism, such as estrogens, a component playing an important role as risk factor in BC development, especially in postmenopausal women. On the other hand, breast and gut resident microbiota are the link in the reciprocal interactions between cancer cells and their local environment, since microbiota are capable of modulating mucosal and systemic immune responses. Several in vivo and in vitro studies show remarkable evidence that diet, probiotics and prebiotics could exert important anticarcinogenic effects in BC. Moreover, gut microbiota have an important role in the metabolism of chemotherapeutic drugs and in the activity of immunogenic chemotherapies since they are a potential dominant mediator in the response to cancer therapy. Then, the microbiome impact in BC is multi-factorial, and the gut and breast tissue bacteria population could be important in regulating the local immune system, in tumor formation and progression and in therapy response and/or resistance.

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201 Background: Dostarlimab is a programmed death receptor-1 (PD-1) blocking antibody that is approved in the US as a monotherapy in adult patients (pts) with mismatch repair deficient (dMMR) recurrent or advanced endometrial cancer that has progressed on or following prior treatment with a platinum-containing regimen or dMMR recurrent or advanced solid tumors that have progressed on or following prior treatment and who have no satisfactory alternative treatment options. Here, we report on the antitumor activity and safety of dostarlimab monotherapy in pts with dMMR colorectal cancer (CRC), a post-hoc subgroup analysis of cohort F of the GARNET trial. Methods: GARNET is a phase 1, multicenter, open-label, single-arm study of dostarlimab monotherapy in pts with advanced or recurrent solid tumors. Cohort F of the GARNET expansion cohorts enrolled pts with dMMR/MSI-H or POLε mutated non-endometrial solid tumors, including pts with CRC. Pts must have progressed per blinded independent central review (BICR) following prior systemic therapy for advanced disease. Pts with CRC were required to have progressive disease after or been intolerant to fluoropyrimidine, oxaliplatin, and irinotecan. Pts received 500 mg intravenous dostarlimab every 3 weeks for 4 cycles, followed by 1000 mg every 6 weeks until discontinuation. Primary endpoints were objective response rate (ORR) and duration of response by BICR per RECIST v1.1. Pts were included in the efficacy analysis if they received ≥1 dose of dostarlimab, had measurable disease at baseline, and had ≥24 weeks of follow-up. All pts who received ≥1 dose of dostarlimab were included in the safety analysis. Results: As of the March 01, 2020 interim analysis data cut, 141 pts with dMMR non-endometrial solid tumors were included in the safety analysis, with 106 in the efficacy analysis. Of the pts in the efficacy analysis, 69 (65.1%) had CRC. Confirmed ORR by BICR per RECIST v1.1 in pts with dMMR CRC was 36.2% (95% CI, 25.0%–48.7%). There were 3 complete responses and 22 partial responses. At the data cut, 23 pts (92%) were still on treatment. Median duration of response had not been reached for pts with CRC. In the overall dMMR non-endometrial solid tumor population, treatment-related adverse events (TRAEs) were reported in 68.1% of pts, with 8.5% of pts experiencing at least 1 grade ≥3 TRAE. The most common grade ≥3 TRAE was lipase increased in 2 (1.4%) of pts. Only 5 pts (3.5%) discontinued dostarlimab due to at TRAE. Treatment-related serious AEs were reported in 9 (6.4%) pts. Conclusions: Dostarlimab demonstrated durable clinically meaningful antitumor activity in pts with dMMR CRC, which was consistent with that seen in patients with dMMR non-CRC solid tumors. No new safety signals were detected in patients with dMMR non-endometrial solid tumors. Clinical trial information: NCT02715284.

burden, including a partial response in one patient who has been on treatment for w 1 year. Graphic illustrations of data, including 'waterfall' and 'swimmer' plots, will be presented.Conclusions: Samuraciclib has demonstrated an acceptable safety profile with evidence of anti-tumour activity in combination with fulvestrant for patients with advanced HR+BC who have progressed on their prior CDK4/6i.

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TPS1101 Background: MEN1611 is a potent, selective class I inhibitor of PI3K, a key enzyme in the transduction of various extracellular growth factor signals essential for cell survival and apoptosis. The discovery in human cancers of frequent PIK3CA mutations, that have been linked to a worse outcome in advanced HER2-positive breast cancer, makes PI3K an attractive therapeutic target. Preclinical and clinical evidences support the development of MEN1611 in combination with other agents in the context of solid tumors. Methods: B-PRECISE-01 is an open-label, multicenter, phase Ib dose escalation study in patients with PIK3CA mutant tumors, HER2-positive advanced or metastatic breast cancer which has progressed after at least 2 lines of anti-HER2 based therapy. PIK3CA mutations are assessed centrally by real-time PCR assay in DNA derived from archived tumor samples. MEN1611 will be administered orally BID for continuous 28-day cycles until disease progression, in combination with weekly IV infusions of trastuzumab. In addition, HR-positive postmenopausal patients will also be treated with fulvestrant. After the completion of the dose escalation phase (Step 1), the study will continue in an expansion cohort (Step 2) testing the Recommended Phase 2 dose (RP2D) in a total of 15 patients in each of the treatment groups. The primary study objective is to assess combination safety and select RP2D. Secondary objectives include assessment of pharmacokinetics and pharmacodynamics, preliminary clinical activity of MEN1611 in combination with trastuzumab +/- fulvestrant, and correlation with PIK3CA mutations and other relevant cancer genes mutational status. Adverse events will be graded according to NCI CTCAE v4.03. Responses will be evaluated according to RECIST v1.1. Study variables will be presented by dose-cohort and overall using appropriate descriptive statistics. The enrollment began in July 2018 at European sites and US sites will shortly participate; up to date the first dose cohort level has been achieved. Clinical trial information: NCT03767335.

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Background: The role of the HER2-enriched (HER2E) subtype determined by the Prosigna Assay in the neoadjuvant setting has remained largely uncharacterized. In this study, we examine whether Prosigna can identify a subgroup of HER2+ patients for whom combination neoadjuvant therapy that includes trastuzumab (Herceptin) is associated with a greater likelihood of pathological complete response (pCR). Methods: In this single-arm retrospective analysis, 75 patients determined to be HER2+ by IHC were treated with a neoadjuvant regimen (NAC) consisting of 8-12 cycles of anthracyclines and taxanes as well as Herceptin. The Prosigna Assay was performed on the NanoString nCounter® Dx Analysis System at HU Virgende la Victoria de Málaga/CIMES-UMA. pCR was used as the endpoint for this study and was determined using the Miller & Payne scoring criteria. Results: Mean patient age for this study population was 49 (±11.1yr) and all patients were determined to be HER2+ by IHC. The overall pCR rate in this patient population was 46.2%. Of the 75 patient samples analyzed for this study, 59 (78.6%) were HER2E, 4 (5.3%) were Luminal A and 12 (16.1%) were Luminal B, as identified by the Prosigna Assay. Of the 16 tumors classified as Luminal (A or B) by Prosigna within this HER2+ population, only 2 (12.5%) responders were observed. Categorical analysis revealed that Prosigna subtype predicted response to a NAC regimen combined with Herceptin (Odds ratio [Her2E vs. non-Her2E]=6.4, p=0.023). Further analysis of the Her2E subtype revealed that tumors with profile expression that correlated well with the prototypical Her2E centroid were significantly more likely to respond to combination NAC and Herceptin (Odds ratio [Unit increase of 1 in Her2E correlation]=88.2, p=0.004). Conclusions: The results of this study indicate that HER2+ patients with greater correlations to the HER2E subtype have an increased likelihood of response to combination neoadjuvant regimens that included HER2-targeted therapy. Citation Format: Santonja Á, Ribelles N, Jiménez-Rodríguez B, Sánchez Rovira P, Álvarez M, Vicioso L, Isabel Fernandez A, de Luque V, Fernández de Sousa C, Villar E, Zarcos I, Ramírez C, González-Hermoso C, Jeiranian A, Dowidar N, Schaper C, Buckingham W, Ferree S, Jiménez A, Prat A, Alba E. Prosigna® intrinsic subtyping predicts response to neoadjuvant combination therapy in study that includes herceptin within HER2+ (IHC) patients. [abstract]. In: Proceedings of the Thirty-Eighth Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2015 Dec 8-12; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2016;76(4 Suppl):Abstract nr P3-07-14.