Background: Ibrutinib inhibits CLL cell proliferation and results in prolonged remission, but MRD responses are rare. Obinutuzumab is a second generation anti-CD20 monoclonal antibody that is effective in CLL and can result in MRD responses. In the IcICLLe study (ISRCTN12695354), 40 participants with CLL requiring treatment (20 treatment-naïve, 20 with relapsed/refractory [R/R] disease) received ibrutinib until complete remission with <0.01% Minimal Residual Disease (MRD) in the bone marrow or disease progression. The IcICLLe Extension Study expanded IcICLLe to examine the efficacy and safety of the combination of obinutuzumab and ibrutinib in 40 patients with R/R CLL, of which 10/40 had received prior ibrutinib on the IcICLLe trial. Initial results after 1 month of combination treatment indicated that adding obinutuzumab to ibrutinib improved CLL depletion, and 18 month follow-up data is now available. Aim: to determine the MRD response rates for patients with R/R CLL treated with ibrutinib and obinutuzumab in ibrutinib-naïve trial participants compared to those treated with >1 year prior ibrutinib. Patients: The IcICLLe Extension Study recruited 40 participants with relapsed/refractory CLL requiring treatment. They received continuous ibrutinib (420mg OD) with 6 cycles of obinutuzumab given over 6 months (M). Ten participants had >1 year of prior ibrutinib monotherapy in IcICLLe and 30 were ibrutinib-naïve with obinutuzumab started 24 hours after first ibrutinib dose. Patient characteristics and Adverse Events (AEs, collected from registration until 30 days after treatment cessation and reported at 1, 3, and 6M, and 6-monthly thereafter using the Common Terminology Criteria for Adverse Events v4.0) are shown in Table 1. MRD assessment was performed according to ERIC guidelines with a maximum detection limit of 0.001%/10-5. Results: In the 20 R/R patients treated with ibrutinib monotherapy there were no IWCLL CR/CRi responses and no patients achieved <0.01% CLL in the PB or BM at the 6 month response assessment. PB MRD levels either remained stable or improved at subsequent timepoints, with 1/20 achieving <0.01% PB MRD at 18M. The addition of obinutuzumab did not have a discernible impact on safety but was associated with a higher response rates and greater depth of MRD depletion than observed in patients treated with ibrutinib monotherapy, particularly in patients who had received ibrutinib for >1 year prior to combination with obinutuzumab (see Table 1). Patients receiving obinutuzumab after >1 year prior ibrutinib monotherapy achieved a higher response rate compared to ibrutinib-naive patients (IWCLL CR/CRi 50% vs. 30%), with a higher proportion of patients achieving <0.01% BM MRD (50% vs. 6%) and a greater depth of disease depletion (3.1 vs. 1.5 log reduction). PB MRD levels continued to improve in ibrutinib-naïve patients after cessation of obinutuzumab with 30% (9/30 with 4/30 inevaluable) achieving PB MRD <0.01% rate at 12 months post-obinutuzumab, compared to 60% (6/10 with 2/10 inevaluable) of patients at the same timepoint (12 months post-obinutuzumab) who had received ibrutinib for >1 year prior to starting obinutuzumab. The difference in extent of disease depletion observed with obinutuzumab may be related to the pre-obinutuzumab disease bulk because the majority of patients (7/10) with >1 year prior ibrutinib treatment had already resolved any lymphadenopathy prior to receiving obinutuzumab. Conclusions: The results suggest that the addition of obinutuzumab to ibrutinib may result in a substantial improvement in the depletion of CLL cells from the PB and BM for ibrutinib-naïve patients. However, a greater impact in MRD response rate and depth of depletion was seen when obinutuzumab was introduced after >1 year of ibrutinib treatment and tumour bulk was low. For patients with persistent disease during/ following pathway inhibition treatments, the addition of anti-CD20 antibody therapy may be effective at improving MRD response rates. Disclosures Rawstron: Janssen: Membership on an entity's Board of Directors or advisory committees, Research Funding; Abbvie: Membership on an entity's Board of Directors or advisory committees, Research Funding; BD Bio-sciences: Research Funding; Beckman Coulter: Research Funding; Celgene: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding; Gilead: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding; Pharmacyclics: Consultancy, Research Funding; Roche: Membership on an entity's Board of Directors or advisory committees, Research Funding. Munir:MorphoSys: Membership on an entity's Board of Directors or advisory committees; Alexion: Honoraria; Novartis: Honoraria; Gilead: Honoraria; Janssen: Honoraria; Abbvie: Honoraria. Brock:GlaxoSmithKline: Equity Ownership; AstraZeneca: Equity Ownership; Merck Sharp Dohme: Other: Reimbursement of conference fees; Roche: Other: Reimbursement of expenses; Lilly: Honoraria. Pettitt:AstraZeneca: Research Funding; Celgene: Research Funding; Chugai: Research Funding; Roche: Research Funding; GSK/Novartis: Research Funding; Gilead: Research Funding; Napp: Research Funding. Fox:Celgene: Consultancy, Other: Travel support, Speakers Bureau; Janssen: Consultancy, Other: Personal fees and non-financial support, Speakers Bureau; Gilead: Consultancy, Other: Travel support, Research Funding, Speakers Bureau; AbbVie: Consultancy, Other: Travel support, Research Funding, Speakers Bureau; Roche: Consultancy, Other: Travel support, Research Funding, Speakers Bureau; Sunesis: Consultancy. Devereux:Janssen: Other: Personal fees; Gilead: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Abbvie: Membership on an entity's Board of Directors or advisory committees; Novartis: Membership on an entity's Board of Directors or advisory committees. Fegan:Janssen: Honoraria; Gilead Sciences, Inc.: Honoraria; Abbvie: Honoraria; Roche: Honoraria; Napp: Honoraria. Bloor:Janssen: Research Funding; AbbVie: Research Funding. Hillmen:Alexion Pharmaceuticals, Inc: Consultancy, Honoraria; Acerta: Membership on an entity's Board of Directors or advisory committees; Celgene: Research Funding; Abbvie: Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; Pharmacyclics: Research Funding; Novartis: Research Funding; F. Hoffmann-La Roche Ltd: Research Funding; Gilead Sciences, Inc.: Honoraria, Research Funding; Janssen: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding.

TPS615 Background: Neoadjuvant chemotherapy (NACT) results in eradication of cancer in the axillary nodes in 40-70% of patients. This raises questions about the benefit of further axillary treatment in patients with no evidence of residual nodal disease (ypN0) post NACT. Methods: Design: ATNEC is a phase 3, randomized (1:1), multi-center UK trial, with embedded economic evaluation. Patients with proven axillary node metastases on needle biopsy receive NACT followed by sentinel node biopsy (SNB). If the sentinel nodes have converted to benign (ypN0), ATNEC randomly assigns patients to axillary treatment (nodal radiotherapy [ART] or axillary nodal clearance [ALND]) vs no further axillary treatment. Stratification: Institution, type of surgery (breast conserving surgery vs mastectomy), receptor status (triple negative vs HER2 positive vs ER positive and/or PR positive and HER2 negative).Inclusion criteria: Age ≥ 18; Male or female; T1-3N1M0 breast cancer at diagnosis (pre-NACT); FNA or core biopsy confirmed axillary nodal metastases at presentation; ER and HER2 status evaluated on primary tumor; Received standard NACT as per local guidelines; Imaging of the axilla to assess response to NACT;Dual tracer SNB post-NACT and at least 3 nodes removed (sentinel nodes and marked node): If a single tracer is used, the patient is eligible if the involved node is marked pre-NACT and at least 3 nodes removed (including the marked node), If axillary node sampling is performed following failed localization of sentinel nodes, patient is eligible if at least 3 nodes removed (including the marked node), If node is not marked, or marked node is not removed, patient is eligible if the histology report shows evidence of down-staging with complete pathological response in at least one node and at least 3 nodes removed; No evidence of nodal metastases post NACT (ypN0). Exclusion criteria: Bilateral invasive breast cancer; SNB prior to NACT; Previous ipsilateral axillary nodal surgery; Previous cancer within last 5 years or concomitant malignancy. Aims: To assess whether omitting further axillary treatment (ALND & ART) for patients with early-stage breast cancer and axillary nodal metastases on needle biopsy - who after NACT have no residual nodal disease on SNB (ypN0) - is non-inferior to axillary treatment in terms of disease-free survival, and reduces lymphoedema at 5 years. Statistical methods: All analyses will be carried out on an intention-to-treat basis to preserve randomization, avoid bias from exclusions and preserve statistical power. Radiotherapy Quality Assurance: Study has in-built radiotherapy QA program that will be coordinated by National Radiotherapy Trials QA (RTTQA) group. Target accrual: 1,900 Status: Recruiting. As of 11-Feb-2022, 39 sites open, 87 patients enrolled, 31 randomized. Clinical trial information: NCT04109079.

Background: For patients who are node positive to start at presentation and are found to have a complete nodal tumour response (ypN0) post neoadjuvant chemotherapy (NACT), we do not yet know whether local axillary therapy can be modified based on the response to NACT. ATNEC randomised trial specifically address axillary management following NACT in patients with proven axillary node metastases. Methods: Design: ATNEC is a phase III, randomised (1:1), multi-centre trial, with embedded economic evaluation. Patients with proven axillary node metastases on needle biopsy receive NACT followed by sentinel node biopsy (SNB). If the sentinel nodes have converted to benign (ypN0), ATNEC randomly assigns patients to axillary treatment (nodal radiotherapy or axillary nodal clearance) vs. no axillary treatment (without further surgery). Aims: To assess whether, omitting further axillary treatment (ALND and ART) for patients with early stage breast cancer and axillary nodal metastases on needle biopsy - who after NACT have no residual nodal disease on SNB - is non-inferior to axillary treatment in terms of disease free survival, and reduces the risk of lymphoedema at 5 years. Stratification: Institution, type of surgery (breast conserving surgery [BCS] vs mastectomy), receptor status (triple negative vs HER2 positive vs ER status positive and/or PR status positive and HER2 negative). Inclusion criteria:. • Age ≥ 18. • Male or female. • T1-3N1M0 breast cancer at diagnosis (pre-NACT). • FNA or core biopsy confirmed axillary nodal metastases at presentation. • ER and HER2 status evaluated on primary tumour. • Received standard NACT as per local guidelines. • Imaging assessment of the axilla at completion of NACT. • Dual tracer SNB after NACT and at least 3 nodes removed (sentinel nodes and marked node). o If a single tracer is used, the patient is eligible if the involved node is marked before NACT and at least 3 nodes removed (including the marked node). o If axillary node sampling is performed following failed localisation of sentinel nodes, patient is eligible if at least 3 nodes removed (including the marked node). o If node is not marked, or marked node is not removed, patient is eligible if the histology report shows evidence of down-staging with complete pathological response in at least one node and at least 3 nodes removed. • No evidence of nodal metastases post NACT (ypN0). Exclusion criteria:. • Bilateral invasive breast cancer. • SNB prior to NACT. • Previous ipsilateral axillary nodal surgery. • Previous cancer within last 5 years or concomitant malignancy. Radiotherapy Quality Assurance: ATNEC has in-built radiotherapy QA programme that is coordinated by National Radiotherapy Trials QA (RTTQA) group. Target accrual: 1900 Number of sites: 100 Trial Status: Recruiting. Trial is open to new sites. Disclaimer: This study is funded by the National Institute for Health Research (NIHR) Health Technology Assessment (HTA) programme (Reference - HTA NIHR128311). The views expressed are those of the author(s) and not necessarily those of the NIHR or the Department of Health and Social Care. Citation Format: Amit Goyal, Sophie Cramp, Andrea Marshall, Duncan Wheatley, Natalie Hammonds, Shama Puri, Tara Homer, Luke Vale, Roeum Butt, Romaana Mir, Janice Rose, Helen Teresa Edwards, Samreen Ahmed, Abeer Shaaban, Beatrix Elsberger, Julie Bruce, Sophie Gasson, Valerie Speirs, Jacqui Shaw, Helen Higgins, Janet Dunn. ATNEC: A multi-centre, randomised trial investigating whether axillary treatment can be avoided in T1-3N1M0 breast cancer patients with no residual cancer in the lymph glands after neoadjuvant chemotherapy (clinicaltrials.gov: nct04109079) [abstract]. In: Proceedings of the 2021 San Antonio Breast Cancer Symposium; 2021 Dec 7-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2022;82(4 Suppl):Abstract nr OT1-04-01.