Abstract
Several second- and third-line immunotherapeutic options for patients with relapsed or refractory diffuse large B-cell lymphoma ineligible for autologous stem cell transplant are directed against the B-cell antigen cluster of differentiation 19 (CD19). The anti-CD19 monoclonal antibody tafasitamab, paired with the immunomodulator lenalidomide, mediates antibody-dependent cellular toxicity and phagocytosis; the antibody-drug conjugate loncastuximab tesirine delivers the DNA cross-linking agent tesirine via CD19 binding and internalization; and CD19-directed chimeric antigen receptor T-cell therapy (CAR-T) products are engineered from autologous T cells. Although CD19 expression is assessed at diagnosis, clinically relevant thresholds of CD19 expression-which may not be detectable using current routine methodologies-have not been defined and may vary between CD19-directed treatment modalities. Determining optimal treatment sequencing strategies for CD19-directed therapy is hampered by the exclusion of patients who have received prior CD19-directed therapies from major clinical trials. Antigen escape, which is attributed to mechanisms including epitope loss and defective cell surface trafficking of CD19, is an important cause of CAR-T failure. Limited data suggest that CD19 expression may be maintained after non-CAR-T CD19-directed therapy, and retrospective analyses indicate that some patients with disease relapse after CAR-T may benefit from subsequent CD19-directed therapy. To date, clinical evidence on the effect of anti-CD19 therapy prior to CAR-T has been limited to small case series. Prospective studies and detailed analyses are needed to understand how pretreatment and posttreatment CD19 expression correlates with clinical responses to subsequent CD19-directed therapy to fully maximize treatment strategies.