A High Throughput Quantitative Seroproteomics Analysis of Multiple Myeloma Patients on Tagraxofusp Therapy Identifies Novel Cytokine-Assisted Mechanism of Action

Background We showed a tumor-promoting and immunosuppressive role of plasmacytoid dendritic cells (pDCs; CD123/IL-3Rα⁺) in multiple myeloma (MM) pathogenesis (Chauhan et al. Cancer Cell, 2009;Ray et al, Leukemia, 2018). Importantly, tagraxofusp, an FDA-approved (for patients with blastic plasmacytoid dendritic cell neoplasm) novel targeted therapy directed against CD123, triggers anti-MM activity by reducing the viability of MM-promoting pDCs. These observations led to a recently completed phase 1 clinical trial of tagraxofusp and pomalidomide/dexamethasone in relapsed and refractory MM patients (NCT02661022). The treatment regimen demonstrated preliminary safety and efficacy, with 5 of 9 heavily pretreated patients achieving durable partial response (PR) (ASH 2019). Here, we report the initial results of our correlative science studies using bone marrow (BM), peripheral blood (PB), and serum from the study cohort.

Materials and Methods Tagraxofusp is a bioengineered therapeutic protein developed by fusing human IL-3 to the catalytic translocation domain of truncated diphtheria toxin (DT) via a Met-His linker (Stemline Therapeutics, NY). pDCs and patient MM cells were purified from BM/PB samples after informed consent, and quantified using FACS, as described (Ray et al, Leukemia, 2018). A novel 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 screening 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, tagraxofusp treatment reduced pDC-related soluble proteins including IFN-α (fold change: 0.8, treated vs untreated; p < 0.05). Our earlier study showed that pDC-MM interactions triggered secretion of IL-3, which in turn promotes both pDC survival and MM cell growth. Importantly, tagraxofusp decreased serum IL-3 (fold change 0.75, treated vs untreated; p < 0.05), consistent with tagraxofusp decreasing survival of tumor-promoting pDCs.

Conclusions Our current correlative science studies validate target specificity of tagraxofusp against MM pDCs in relapsed and refractory MM patients enrolled in a phase 1 clinical trial and support further evaluation for this novel therapeutic to improve the clinical outcome of patients with MM. Further combination studies are planned.