Generation and profiling of fully human therapeutic antibodies against CD38 for the treatment of multiple myeloma

Abstract

6590 CD38 appears on lineage-committed progenitors of lymphoid, erythroid and myeloid cells, while the most primitive pluripotent stem cells of the hematopoietic system are CD38-negative. CD38-expression is also found on plasma cells and activated B- and T-cells. CD38-upregulation was detected on various cell-lines derived from B, T, and myeloid/monocytic tumors. Especially for the indication of multiple myeloma (MM), which remains an incurable malignancy with a median survival of 3–4 years, a strong expression has been reported in the majority of patient samples. Hence, over-expressed CD38 on malignant cells may provide an attractive therapeutic target for immunotherapy. CD38-specific human antibodies were selected from MorphoSys” proprietary HuCAL GOLD phage display library by cell panning strategies. Three antibodies recognizing different epitopes on CD38 were characterized in vitro in detail as follows: All affinities for CD38 were in the low nanomolar range with dissociation constants between 0.5 and 6.3 nM for bivalent IgG1 and between 2.4 and 56.0 nM for monovalent Fab fragments. IHC profiles with healthy and malignant tissue from MM-patients show that the antibodies were highly specific for human CD38 and correlated well with the tumor infiltration rates. Additionally, one candidate cross-reacted with non-human primate’s CD38. The human IgGs were able to kill efficiently CD38-expressing cell-lines and primary MM cells from patients by ADCC and CDC in a concentration dependent manner. EC50-values of the different antibodies in ADCC and CDC range from 40 pM to 280 pM and from 410 pM to 13.6 nM, respectively, demonstrating a similar or superior activity over the reference antibody OKT10, which is currently in phase I as immunotoxin-conjugate. Most importantly, early progenitor cells were not affected in ADCC as demonstrated by a clonogenic assay. Finally, one candidate was selected for in vivo efficacy profiling using the MM cell-line RPMI8226 in a SCID-mouse xenograft model. After the tumor became visible, the human IgG1 antibodies were given every other day over a period of 32 days at two different concentrations resulting in a significantly reduced tumor growth at 1 mg/kg and an even stronger effect at 5 mg/kg. In conclusion, CD38 appears to be a bona-fide target for immunotherapy of multiple myeloma with HuCAL anti-CD38 antibodies. No significant financial relationships to disclose.