Not all anti-CD38 antibodies are created equal: The epitope of felzartamab is partially DTT-resistant.

OBJECTIVES: To evaluate anti-CD38 antibody candidates for their ability to bind to DTT-treated CD38 and red blood cells (RBCs). BACKGROUND: Dithiothreitol (DTT) treatment of RBCs is routinely used to denature CD38 and mitigate interference from anti-CD38 antibodies, such as daratumumab and isatuximab, in immunohematology testing. However, new anti-CD38 antibodies are in development, some of which may be directed against linear or otherwise DTT-resistant epitopes. METHODS/MATERIALS: Anti-CD38 antibody candidates were titrated in an enzyme-linked immunosorbent assay (ELISA) to assess their ability to bind to untreated and DTT-treated recombinant CD38. Antibody binding was subsequently confirmed by flow cytometry of RBCs. Daratumumab and felzartamab were then tested on a panel of native and DTT-treated RBCs in the indirect antiglobulin test (IAT). RESULTS: All antibodies except felzartamab failed to bind to DTT-denatured CD38 in ELISA at concentrations up to 1.25 μg/mL. Flow cytometry confirmed this binding pattern. In the IAT, some DTT-treated cells remained positive with felzartamab, despite being negative with daratumumab. CONCLUSION: Not all epitopes of anti-CD38 antibodies are fully DTT-sensitive. This has important implications for handling of anti-CD38 antibody induced interference in immunohematology, and raises the question of whether DTT is an adequate solution for interference mitigation.