e20537 Background: We have previously described the pivotal pathogenetic role of plasmacytoid dendritic cells (pDCs; CD123/IL-3Ra+) in multiple myeloma (MM) (Chauhan et al Cancer Cell, 2009,16:309). In preclinical MM models, we demonstrated that tagraxofusp, a novel targeted therapy directed to CD123, triggers anti-MM activity by reducing the viability of MM-promoting pDCs. These observations led to an ongoing phase 1/2 clinical trial of tagraxofusp in MM patients (NCT02661022). The treatment regimen demonstrated safety and efficacy, with 5 of 9 heavily pretreated patients achieving durable partial response (PR). Here, we report the initial results of our translational exploratory studies using bone marrow (BM), peripheral blood (PB), and serum from the study cohort. Methods: pDCs and MM tumor cells were purified from BM/PB samples and quantified using FACS, as described ( Ray et al Leukemia, 2018,32:843). A high-throughput seroproteomics platform SOMAscan was utilized to analyze 1,310 protein analytes in serum samples from MM patients (N = 9). SOMAscan data were subjected to meta-analysis to generate heatmaps, followed by hierarchical cluster analysis. SOMAscan results were validated with ELISA using supernatants from MM patient pDCs cultured with or without tagraxofusp. Results: Analysis of BM/PB samples from MM patients receiving tagraxofusp therapy showed a marked reduction in the frequency of viable pDCs [average 2% at screen vs 0.75% post-tagraxofusp; N = 6; p = 0.036]. pDCs isolated from tagraxofusp-treated patients showed decreased ability to trigger MM cell growth. Seroproteomics analysis of MM patient serum before and after tagraxofusp therapy showed alterations in the levels of 100 proteins [Median Fold Change in expression: 0.39 to 4.5; n = 6; 3 each; p < 0.05]. Importantly, we found that tagraxofusp treatment reduced pDC-related soluble proteins, in particular, IFN-a and IL-3Rα. Additionally, our earlier study showed that pDC-MM interactions triggered secretion of MM cell growth including IL-3, which serves a dual role of promoting pDC survival and MM cell growth. Importantly, tagraxofusp decreased serum IL-3, suggesting that tagraxofusp attenuates survival mechanisms for tumor-promoting pDCs. Conclusions: Our current correlative studies validate target specificity of tagraxofusp and support further evaluation for this novel therapeutic to improve the clinical outcome of patients with MM.
SAT0284 CD123+ PLASMACYTOID DENDRITIC CELLS (PDCS) FROM SYSTEMIC SCLEROSISPATIENTS ARE SUSCEPTIBLE TO THE CYTOTOXIC ACTIVITY OF TAGRAXOFUSP, A CD123-TARGETED THERAPY