Evaluation of Tagraxofusp (SL-401) Alone and in Combination with Ruxolitinib for the Treatment of Myeloproliferative Neoplasms

Background: The introduction of the JAK1/2 inhibitor Ruxolitinib has resulted in significant benefits for patients with Myelofibrosis (MF) and Polycythemia Vera, including reduction of splenomegaly and improvement in symptom burden. However, Ruxolitinib has limited ability to alter the natural history and biology of disease in myeloproliferative neoplasms (MPNs). More importantly, patients often lose response to ruxolitinib or suffer disease progression despite therapy with ruxolitinib. These observations have prompted efforts to introduce novel therapeutic approaches and devise novel combinatorial treatment strategies to improve the outcomes of patients with MPNs. CD123 (interleukin-3 receptor-a; IL-3R-a) has been identified as a therapeutic target in several myeloid malignancies. CD123 is expressed in a variety of myeloid malignancies, including AML, myelodysplastic syndrome (MDS) and CMML. Further, Tagraxofusp (ELZONRIS^®, SL-401), a targeted therapy directed to CD123 comprised of recombinant IL-3 fused to a truncated diphtheria toxin payload, was recently FDA approved for the treatment of blastic plasmacytoid dendritic cell neoplasm (BPDCN). In an ongoing Phase 1/2 trial, Tagraxofusp has demonstrated single agent clinical activity, with a predictable and manageable safety profile, in patients with relapsed/refractory MF. Thus, Tagraxofusp appears to have activity in MF. However, the utility of Tagraxofusp in more advanced forms of MPN (such as accelerated phase MPN), as well as the utility of combination Ruxolitinib and Tagraxofusp, have not been evaluated to date.

To address these questions, we first evaluated CD123 expression using flow cytometry analysis of peripheral blood samples from patients with MF with progression to accelerated-phase (>10%) or blast-phase (>20%) disease. CD123 expression was generally noted to be higher than that observed in normal control samples, by mean florescence intensity (Figure 1A). We next sought to determine the effect of treatment with Tagraxofusp alone and in combination with Ruxolitinib in leukemia cell lines and primary patient samples. We performed cell viability assays using the JAK2V617F mutant cell line UKE1. We first determined the IC50 of Tagraxofusp in UKE1 cells (range 2.95-3.57nM). Using this data, we then tested the impact of the addition of Tagraxofusp to Ruxolitinib on UKE1 cell viability using a fixed concentration of Tagraxofusp (5nM). The IC50 of range of single agent Ruxolitinib was 44.42-70.93nM. However, the addition of Tagraxofusp to Ruxolitinib resulted in a decrease of the IC50 range to 11.72-21.6nM, indicating an effect of combination therapy (Figure 1B). We then determined the impact of Tagraxofusp both alone and in combination with Ruxolitinib using primary patient MPN peripheral blood mononuclear cells in methylcellulose. Patient characteristics are described in Table 1. In most samples, CD123 expression was confirmed. In all samples studied, Tagraxofusp was able to significantly reduce colony formation when compared to vehicle, at a dose range of 2.5nM to 20nM. Notably, this includes two samples (151, 455) from patients with accelerated-phase disease. As well, Tagraxofusp demonstrated activity across genotypes, including in cases with TP53 and ASXL1 mutations. A reduction in colony formation in samples treated with combination Ruxolitinib and Tagraxofusp beyond that observed with either agent alone was observed in several cases (Figure 1C).

Conclusions: Current therapeutic options for patients with MF beyond Ruxolitinib are limited. This is particularly the case for patients with progression to accelerated and blast-phase MPN. Here, we demonstrate that CD123 expression is evident in many such cases. Further, therapeutic targeting of CD123 using Tagraxofusp either alone or in combination with Ruxolitinib has activity in primary patient samples, including those in accelerated-phase and with high molecular risk profiles. These data thus support further testing of Tagraxofusp in MPNs, and in advanced MPNs in particular.

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