Panobinostat Induces Upregulation of CD38 and Augments the Anti-Myeloma Efficacy of Daratumumab in Pre-Clinical Models

Background:

Immunotherapy with monoclonal antibodies (mAbs) has recently entered the clinical arena in multiple myeloma, including Daratumumab that targets CD38 on malignant plasma cells. The efficacy of mAbs depends on antigen density and expression of accessory ligands on target cells to initiate cell- and complement-dependent effector mechanisms. Here, we investigate the use of the histone deacetylase inhibitor (HDACi) Panobinostat to modulate target antigen expression and ligand profile on myeloma in favor of potent mAb-mediated recognition and destruction. We show that Panobinostat augments CD38 expression specifically on myeloma cells and demonstrate powerful synergy with anti-CD38 mAb Daratumumab in pre-clinical models.

Methods:

The myeloma cell line MM1.S and primary myeloma cells were treated with titrated doses of Panobinostat (0, 10, 25 nM) and expression of CD38 and a panel of additional target molecules including B-cell maturation antigen (BCMA) and SLAMF7, as well as accessory ligands analyzed by flow cytometry at 24, 48 and 72 hours. Antibody-dependent cellular cytotoxicity (ADCC) against Panobinostat treated and untreated myeloma cells was analyzed at 4 and 20 hours after addition of PBMC at an effector to target ratio of 25:1 in the presence of Daratumumab (1, 10, 50 ug/mL) or an isotype control antibody.

Results:

We first treated the myeloma cell line MM1.S with Panobinostat and analyzed its direct cytotoxic anti-myeloma effect. Consistent with previous work, the percentage of live MM1.S myeloma cells had decreased to 85% and 50% after 48 hours of exposure to 10 and 25 nM respectively. We analyzed expression of CD38 on residual live, i.e. 7-AAD negative MM1.S cells by flow cytometry and observed a 1.5 (10 nM) and 2-fold (25 nM) increase of CD38 expression by mean fluorescence intensity (MFI) compared to baseline levels and untreated control cells. The increase in CD38 expression was already detectable after 24 hours and plateaued between 48 and 72 hours. We confirmed our observation in primary myeloma cells from multiple donors (n=4) and detected an even stronger increase to 2 (10 nM) and 4-fold (25 nM) higher CD38 expression compared to untreated cells at 48 hours. Interestingly, expression of BCMA and SLAMF7 was not increased after Panobinostat treatment at all tested concentrations and time points in both MM1.S and primary myeloma. We confirmed that Panobinostat-induced upregulation of CD38 specifically occurred in myeloma, and neither observed this phenomenon in a panel of leukemia and lymphoma cell lines including Raji (Burkitt) and JeKo-1 (mantle cell), nor on resting/activated primary CD8+ and CD4+ T cells that we isolated from peripheral blood of several donors (n=3).

Next, we were interested in determining whether the increase in CD38 expression enabled superior anti-myeloma activity of the anti-CD38 mAb Daratumumab. Panobinostat pre-treatment was done for 48 hours at 10 nM as this is a clinically achievable serum level with currently approved regimens. Indeed, significantly higher ADCC was mediated by Daratumumab at all tested concentrations (1, 10 and 50 ug/mL) against MM1.S that we had exposed to Panobinostat. At 4 hours, ADCC was 45% and 25% in Panobinostat-treated and untreated MM1.S respectively, and at 20 hours, near-complete, >90% ADCC of Panobinostat-pre-treated MM1.S had occurred, whereas only 65% of MM1.S were eliminated by Daratumumab without Panobinostat pre-treatment. These data were confirmed in multiple experiments with MM1.S and PBMC from different donors, and with primary myeloma cells. Experiments to evaluate synergy of Panobinostat and Daratumumab therapy in a xenograft model (NSG/MM1.S) are ongoing.

Conclusions:

Our data demonstrate that the HDACi Panobinostat induces upregulation of CD38 on myeloma and a subsequent dramatic increase of Daratumumab-mediated ADCC in pre-clinical models. These data suggest that Panobinostat could be used synergistically with Daratumumab in a clinical setting to increase response rates and extend duration of responses to Daratumumab. Panobinostat has a known ability to modulate the transcriptional profile of myeloma cells and our data demonstrate for the first time that this ability can be utilized to augment the therapeutic index of antibody-based immunotherapy in multiple myeloma.