TPS7566 Background: Relapsed DLBCL in the rituximab era portends a poor prognosis with only about 25% of patients achieving long-term disease control following 2^ndline therapy and AutoHCT. Patients with the ABC subtype have an inferior prognosis at diagnosis than those with GC and are overrepresented at relapse. In order to improve outcomes in ABC-DLBCL, we designed a study targeting disease pathobiology at the time of AutoHCT. Ibr has a safety profile allowing combination with cytotoxic chemotherapy and has single-agent activity with a 37% response rate in patients with relapsed/refractory ABC-DLBCL. Methods: This is an intergroup, randomized, placebo-controlled phase III study combining Ibr or placebo with high-dose chemotherapy during conditioning with AutoHCT and for 12 months following AutoHCT. Pts with relapsed or refractory DLBCL have tissue submitted centrally for real-time review and subtype assignment by GEP (Frederick Laboratory). Key eligibility criteria include no more than 3 prior regimens, no active CNS involvement, no need for long-term anticoagulation, and no progression on prior Ibr. Pts with chemosensitive ABC-DLBCL are randomized to Ibr 560 mg or placebo with BEAM or CBV chemotherapy until day 0. After engraftment, pts receive Ibr 560 mg daily or placebo for 12 additional cycles. Pts with progressive disease on placebo can cross over to Ibr monotherapy. An initial safety cohort of 6 pts is being enrolled to evaluate the tolerability of Ibr with concurrent BEAM and CBV therapy. The primary endpoint is superior 2-year PFS (Ha/H0: 67% vs 50%). Secondary endpoints include time to count recovery, post-transplant response rates, OS, PFS, and incidence of 2^arymalignancies. In correlative studies, we will assess the prognostic and predictive role of pre-transplant FDG-PET in the setting of Ibr or placebo therapy, the role of emergent BCR pathway mutations, double hit genetics, and pharmacogenetic determinants on treatment outcome and toxicities. We expect to accrue 296 patients over 4 years. An Alliance/BMT-CTN study: NCT02443077 Clinical trial information: NCT02443077.