Abstract A12: Combination of the anti-CD38 monoclonal antibody daratumumab and all-trans retinoic acid.

Abstract

Abstract Background: Daratumumab (DARA) is an anti-CD38 monoclonal antibody (mAb) with lytic activity on multiple myeloma (MM) cells, including ADCC (antibody-dependent cellular cytotoxicity) and CDC (complement-dependent cytotoxicity). In current clinical phase I/II trials, DARA induced anti-MM activity; however, the depth of the response varied between patients. Further improvement of DARA-treatment can be achieved by modulating the mechanisms hampering DARA responsiveness. Results: In a sub-analysis of 16 MM patients treated with DARA monotherapy (GEN501 trial), we observed that response to DARA was associated with baseline CD38 expression levels on the MM cells (R2= 0.40; P= 0.009). We also observed a significant decrease of CD38 expression on MM cells during DARA-treatment (median MFI decreased from 900 to 83). Consistent with this idea, in vitro experiments with isogenic MM cell lines expressing different levels of CD38 have revealed that the level of CD38 expression correlates with DARA-mediated ADCC and CDC. Similarly, in bone marrow samples from 125 and 56 MM patients, we observed a significant correlation between CD38 expression and DARA-mediated ADCC (Pearson R= 0.34; P= 0.<0001) as well as CDC (Pearson R= 0.35; P= 0.0038) respectively. Altogether, this suggested that upregulation of CD38 expression, before and during DARA-treatment, may improve the anti-MM activity of DARA. Indeed, all-trans retinoic acid (ATRA) induced a ~5-fold increase in CD38 expression both in 4 MM cell lines and primary MM cells from 5 DARA-naive patients, which resulted in synergistic improvement of DARA-mediated ADCC and CDC. The synergy between ATRA and DARA was observed not only in DARA-naive patients, but also in 2 DARA-treated patients, whose residual MM cells had lower CD38 expression following therapy. Conclusion: Our results provide evidence that CD38 expression levels may predict response to DARA. Furthermore, we show that ATRA increases CD38 expression on MM cells, resulting in enhanced DARA-mediated lysis of MM cells. Our results provide the preclinical rationale for further evaluation of DARA combined with ATRA in MM patients. Citation Format: Inger S. Nijhof, Henk M. Lokhorst, Berris van Kessel, Parul Doshi, Kate Sasser, Tuna Mutis, Niels WCJ van de Donk. Combination of the anti-CD38 monoclonal antibody daratumumab and all-trans retinoic acid. [abstract]. In: Proceedings of the AACR Special Conference on Hematologic Malignancies: Translating Discoveries to Novel Therapies; Sep 20-23, 2014; Philadelphia, PA. Philadelphia (PA): AACR; Clin Cancer Res 2015;21(17 Suppl):Abstract nr A12.