Background: Patients (pts) with refractory or relapsed (R/R) diffuse large B-cell lymphoma (DLBCL) have poor outcomes with standard treatment. Loncastuximab tesirine (loncastuximab tesirine-lpyl; Lonca), an antibody-drug conjugate (ADC) comprising a humanized anti-CD19 monoclonal antibody conjugated to a pyrrolobenzodiazepine dimer toxin, is approved in R/R DLBCL based on data from the phase 2 LOTIS-2 trial (Caimi et al. Lancet Oncol 2021). Rituximab (R) is part of standard immunochemotherapy for DLBCL, both as frontline therapy and in subsequent treatments. Preclinical evidence suggests that the addition of rituximab to anti-CD19 ADC therapy may result in prolonged tumor control (Ryan et al. Blood 2017). LOTIS-5 aims to evaluate Lonca-R vs. standard immunochemotherapy of R + gemcitabine + oxaliplatin (R-GemOx) in pts with R/R DLBCL. Methods: This is a phase 3 randomized open-label, two-part, two-arm multicenter study of Lonca-R in pts with R/R DLBCL (NCT04384484). A review of safety data from the nonrandomized safety run-in (Part 1), comparing the safety of Lonca-R to previous Lonca safety data, was completed in January 2022. The trial is now continuing to the randomized phase (Part 2). In Part 2, approximately 330 pts will be randomized 1:1 to receive Lonca-R or R-GemOx. The primary objective of the study is to evaluate the efficacy of Lonca-R versus R-GemOx. The primary endpoint is progression-free survival by independent central review (ICR). Secondary endpoints include overall survival, overall response rate (by ICR using 2014 Lugano classification), complete response rate by ICR, duration of response by ICR, frequency and severity of adverse events, changes from baseline in safety laboratory and clinical variables, concentration and pharmacokinetic parameters of Lonca (conjugated and total antibody and unconjugated warhead), immunogenicity, and changes in patient-reported outcomes. The time to event endpoints will be analyzed based on the intent-to-treat population using a stratified log-rank test. The dosing regimen for Lonca-R: Lonca at 0.15 mg/kg + rituximab at 375 mg/m² every 3 weeks (Q3W) for 2 cycles and then Lonca at 0.075 mg/kg + rituximab at 375 mg/m² Q3W for up to 6 cycles. The dose regimen of R-GemOx: rituximab at 375 mg/m², gemcitabine at 1000 mg/m² Gem, and oxaliplatin at 100 mg/m² every 2 weeks for up to 8 cycles. Key inclusion criteria include age ≥18 years, pathologic diagnosis of DLBCL (including pts with DLBCL transformed from indolent lymphoma) or high-grade B-cell lymphoma with MYC and BCL2 and/or BCL6 rearrangements, ≥1 line of prior systemic therapy, not being a candidate for stem cell transplantation, and measurable disease per the 2014 Lugano classification. The randomized part of LOTIS-5 commenced in January 2022, and recruitment is ongoing. Funding: ADC Therapeutics SA; medical writing: CiTRUS Health Group. Clinical trial information: NCT04384484.