Abstract
e19050 Background: Tafasitamab (tafa) is a CD19-targeting immunotherapy that induces enhanced antibody dependent cellular cytotoxicity and phagocytosis. Tafa received accelerated FDA approval in 2020 for treating relapsed or refractory DLBCL in combination with lenalidomide in patients ineligible for autologous stem cell transplant, based on results from L-MIND, a multicenter, open-label, single-arm, phase 2 trial. FirmMIND (NCT05429268) is an ongoing identical study, further evaluating efficacy and safety of tafa in this setting. Several other CD19 targeting agents are available; however, CD19 antigen loss following treatment with chimeric antigen receptor T-cell therapy targeting CD19 (CART19) has raised concerns over CD19-targeted therapy sequencing. Loss of CD19 has not been observed to date in tafa clinical trials or practice. Methods: To evaluate CD19 expression at screening and after disease progression in firmMIND, we developed an immunohistochemistry (IHC) assay using a mAb that binds to an intracellular epitope of CD19 clone LE-CD19. To validate that CD19 expression is detectable after tafa treatment, we treated cultured SU-DHL-4 or Jurkat cells with tafa at a saturating concentration for 24 hours. After washing out unbound tafa, cells were harvested at specific timepoints. Resulting cell line blocks were assessed for CD19 expression by IHC. We also monitored CD19 trafficking in response to fluorescently labeled tafa by confocal microscopy. Results: By IHC, CD19 expression was comparable between treatment conditions, suggesting the assay can effectively measure CD19 expression in lymphoma biopsy samples post-tafa. Confocal microscopy showed a fraction of tafa-bound CD19 internalized and colocalized with a CD19 intracellular pool labeled with anti-CD19 mAb clone EPR5906; this mAb also detected CD19 that remained localized on the cell surface. The data demonstrated that CD19 persists on tumor cells after tafa treatment at levels that could be bound by other CD19-targeting therapies. As of the January 25, 2024 data cutoff, we received 6 end-of-treatment (EOT) samples from patients in firmMIND. Biopsies were collected 14-26 days after the last tafa dose; 5 of 6 samples were biopsies from lymph nodes or liver and showed ≥80% of tumor cells staining CD19+ on membranes with 3+ intensity by IHC, except 1 that was collected only 6 days after last dose and showed 50% positivity. The sixth biopsy (extranodal subcutaneous lesion) showed no CD19 expression at baseline or EOT. Conclusions: Patient biopsy analysis and preclinical data suggest that following tafa treatment, CD19 antigen is preserved on the surface of tumor cells after short and transient internalization and could be targeted by other CD19 agents, such as CART therapy. This conclusion is supported by sequential treatment with tafa followed by CART in vivo (Sakemura, 2024). Analyses are ongoing to confirm these findings.
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