Maintenance of CD19 Expression after Tafasitamab Treatment in Patients with Relapsed/Refractory Diffuse Large B-Cell Lymphoma (R/R DLBCL) from Clinical Trial and Real-World Settings

Introduction: Tafasitamab, a CD19-directed monoclonal antibody (mAb), in combination with lenalidomide, is a chemotherapy-free treatment approved for adults with R/R DLBCL ineligible for autologous stem cell transplant. The availability of other CD19-targeting agents, such as CD19-directed chimeric antigen receptor T-cell (CAR-T) therapy, allows for sequential targeting of the CD19 antigen in R/R DLBCL. However, CD19-negative relapse has been reported in ~30% of patients following the CAR-T therapy, axicabtagene ciloleucel (Plaks V, et al. Blood. 2021). In contrast, several studies have reported that CD19 expression is retained after tafasitamab treatment (Boxhammer R, et al. Blood. ASH 2019; Duell J, et al. Leuk Lymphoma. 2022), although the assays used for detecting CD19 expression were not standardized and were limited by small sample sizes. Here, we used validated assays to evaluate the impact of tafasitamab on tumor CD19 expression and mutations in the CD19 gene, using biopsies collected from patients receiving tafasitamab for the treatment of R/R DLBCL enrolled in the L-MIND (NCT02399085) and firmMIND (NCT05429268) clinical trials, and from patients treated with tafasitamab in a real-world setting. Additionally, blood samples from patients in the topMIND (NCT04809467) basket study were included for sequencing.

Methods: Tumor and blood samples were collected before tafasitamab treatment (baseline) and at end of treatment (EOT) from anti-CD19–naive patients. Tumor CD19 expression was assessed using immunohistochemistry (IHC) assays with a validated mAb that binds to an intracellular epitope of CD19 clone LE-CD19 (Volgina A, et al. EHA 2024). In addition, next-generation sequencing (NGS) was performed with DNA extracted from patient samples.

Results: As of the June 30, 2024, data cutoff, EOT samples from a total of 29 patients with R/R DLBCL were analyzed by IHC following tafasitamab treatment (L-MIND, n=6; firmMIND, n=6; real world, n=17). All samples were CD19+ at EOT, except 1 that was also negative at baseline. The length of time between last tafasitamab dose and EOT biopsy collection ranged from 1 to 200 days. These samples showed ≥60% of tumor cells staining CD19 positive, with an intensity score of 1-3+ by IHC. NGS was performed on 6 EOT biopsies (L-MIND) and 8 peripheral blood samples from patients who were positive for minimal residual disease at EOT (topMIND). Sequencing revealed no likely somatic mutations in the CD19 gene.

Preclinical analysis in the SU-DHL4 lymphoma cell line suggests that CD19 is internalized upon tafasitamab binding and this complex is trafficked into the lysosomes. However, some CD19 is retained on the cell surface and CD19 returns to the baseline level within 24 hours after removing tafasitamab from culture.

Conclusions: The results of this study of EOT tumor and blood samples collected from patients treated with tafasitamab in clinical trials or in the real world demonstrate that the CD19 antigen is retained on tumor cells and no somatic mutations in the CD19 gene are observed following tafasitamab therapy. These results suggest the potential to treat patients who received tafasitamab with subsequent CD19-targeting agents, such as CAR-T therapy. In addition, these findings support the adoption of the LE-CD19 clone as the standard antibody for assessing CD19 expression in clinical practice. Analyses with larger sample sizes are ongoing to confirm these findings.