Background: Tafasitamab is a humanized, Fc-modified anti-CD19 monoclonal antibody that enhances antibody-dependent cellular cytotoxicity and phagocytosis. It is FDA-approved with LEN for adult patients (pts) with relapsed/refractory (R/R) DLBCL ineligible for autologous stem cell transplantation. First-MIND (NCT04134936) is a Phase Ib, open-label, randomized study of tafa + R-CHOP or tafa + LEN + R-CHOP in newly diagnosed DLBCL. Methods: Eligible pts were ≥18 years, treatment-naïve, with histologically confirmed DLBCL not otherwise specified, international prognostic index (IPI) 2–5 and ECOG performance status (PS) 0–2. Pts with known double- or triple-hit and transformed lymphoma were excluded. Treatment (Tx) comprised six 21-day cycles of tafa (12 mg/kg IV, Day [D] 1, 8, 15) + R-CHOP (arm A) or tafa (12 mg/kg IV, D1, 8, 15) + LEN (25 mg orally, D1–10) + R-CHOP (arm B). G-CSF and VTE prophylaxis was mandatory. Primary objective is safety; secondary objectives are ORR, PET-CR rate at end of Tx, PFS, long-term safety, pharmacokinetics, immunogenicity. Results: From Dec 2019 to Aug 2020, 83 pts were screened in Europe and the US; 66 were randomized (33 per arm). Data cut-off for this analysis: 9 Dec 2020; study is ongoing. Median age was 64.5 years (range 20–86). Overall, 30% (20/66) of pts were ≥70 years and many had high-risk disease: IPI 2 29%, IPI 3 46%, IPI 4 26%. ECOG PS: 47% of pts were ECOG PS 0, 44% PS 1, 9% PS 2. Most pts had stage III/IV disease (92%); 46% had bulky disease. All pts experienced a treatment-emergent adverse event (TEAE). Grade ≥3 neutropenia and thrombocytopenia occurred in 54.5% and 12.1% (arm A) and 66.7% and 30.3% (arm B) of pts, respectively (Table). Serious TEAEs occurred in 42.4% (arm A) and 51.5% (arm B) of pts. There were three deaths, unrelated to tafa and/or LEN (sepsis, urosepsis, and COVID-19 pneumonia). R-CHOP dose intensity was maintained in both arms. Among 60 pts who completed tumor assessments after cycle 3, ORR was 89.7% (arm A) and 93.5% (arm B). Conclusions: These data suggest R-CHOP + tafa or tafa + LEN is tolerable in pts with Tx-naïve DLBCL and that R-CHOP dosing is not affected. Toxicities are similar to those expected with R-CHOP or R-CHOP + LEN. Updated safety and early efficacy data will be presented at the conference. Clinical trial information: NCT04134936

*Related to both rituximab and tafa; †Related to rituximab; ‡The majority of events were polyneuropathies related to vincristine.