Results from Phase 1/2 Trial of Tagraxofusp in Combination with Pomalidomide and Dexamethasone in Relapsed or Refractory Multiple Myeloma

Background: The bone marrow microenvironment of many multiple myeloma (MM) patients contains high levels of CD123-expressing plasmacytoid dendritic cells (pDCs). These pDCs have been shown to augment MM growth and contribute to drug resistance (Chauhan, et al., Cancer Cell, 2009). Tagraxofusp, a novel CD123 targeted therapy, has demonstrated high levels of anti-tumor activity in patients with blastic plasmacytoid dendritic cell neoplasm (BPDCN), an aggressive CD123+ malignancy of pDC origin. Tagraxofusp demonstrated potent in vitro and in vivo activity against MM cell lines and primary tumor samples via both a direct anti-MM effect and indirect pDC-targeting effect (Ray, et al., Leukemia, 2017), as well as demonstrating synergy in these systems when used in combination with traditional MM therapies including pomalidomide (POM). As such, targeting pDCs with tagraxofusp may offer a novel therapeutic approach in MM.

Methods: This multicenter, single arm Phase 1/2 trial enrolled patients with relapsed or refractory (r/r) MM and tested two different doses of tagraxofusp (7 or 9 mcg/kg). Patients received tagraxofusp as a daily IV infusion for days 1-5 of a 28-day cycle as a single agent for the initial run-in cycle (cycle 0) and in combination with standard doses/administration of POM and dexamethasone (DEX) in cycles 1 and beyond. Objectives included evaluation of safety and tolerability, identification of the maximum tolerated or tested dose, and efficacy.

Results: 9 patients with r/r MM received tagraxofusp (7 mcg/kg, n=7; 9 mcg/kg, n=2). 5 males, median age 65 years (range: 57-70), median 3 prior therapies (range 2-6). Median follow-up was 12 months (range: 7 - 19). The most common treatment-emergent AEs (TEAEs) were hypoalbuminemia 67% (6/9); chills, fatigue, insomnia, nausea and pyrexia each 56% (5/9); and dizziness, headache, hypophosphatemia, and thrombocytopenia each 44% (4/9). The most common grade 3 and 4 TEAEs were thrombocytopenia 44% (4/9) and neutropenia 33% (3/9). No grade 5 events reported. 5 patients treated with tagraxofusp and POM+DEX had a partial response (PR) after tumor evaluation. These patients demonstrated a rapid decrease in a set of myeloma-related laboratory values from pre-tagraxofusp treatment levels after the first combination cycle of tagraxofusp and POM+DEX. Additionally, these 5 patients demonstrated >50% decreases in peripheral blood pDC levels after both tagraxofusp monotherapy and combination therapy.

Conclusions: Tagraxofusp was well-tolerated, with a predictable and manageable safety profile, when dosed in combination with POM+DEX in patients with r/r MM. Evidence of pDC suppression in peripheral blood and BM was observed in this patient population. 5 patients that received tagraxofusp and POM+DEX combination had PRs and decreases in pDC levels while on treatment with tagraxofusp. Given CD123 expression on pDCs in the tumor microenvironment and the potential synergy of tagraxofusp with certain MM agents including POM, tagraxofusp may offer a novel mechanism of action in MM. NCT02661022.