Results from Ongoing Phase 1/2 Trial of SL-401 in Combination with Pomalidomide and Dexamethasone in Relapsed or Refractory Multiple Myeloma

Background: The bone marrow microenvironment of many multiple myeloma (MM) patients harbors high quantities of plasmacytoid dendritic cells (pDCs), which are specialized immune cells that express the interleukin-3 receptor (CD123). These pDCs have been shown to augment MM growth and contribute to drug resistance, suggesting that targeting pDCs may offer clinical benefit for MM patients. SL-401, a novel targeted therapy directed to CD123, has previously demonstrated potent preclinical in vitro and in vivo activity against MM cell lines and primary tumor samples via both a direct anti-MM effect and an indirect effect by targeting neighboring pDCs. SL-401 has also demonstrated synergy in these systems when used in combination with traditional MM therapies including pomalidomide (POM). Clinically, SL-401 has demonstrated high levels of anti-tumor activity in patients with an aggressive CD123⁺ malignancy of pDC origin, namely blastic plasmacytoid dendritic cell neoplasm (BPDCN). SL-401 is currently being evaluated in combination with POM and dexamethasone (DEX) in relapsed or refractory (r/r) MM patients. Preliminary results are reported here.

Methods and Results: This multicenter, single arm Phase 1/2 trial of patients with r/r MM includes a lead-in (stage 1) and expansion (stage 2). In stage 1, patients receive SL-401 as a daily IV infusion at 7, 9, or 12 ug/kg/day 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+DEX in cycles 1 and beyond, in a 3x3 design. In stage 2, patients receive SL-401 in combination with POM+DEX at the dose and regimen determined in stage 1. Objectives include characterization of the safety profile of SL-401 in combination with POM+DEX, including determination of the maximum tolerated or tested dose, and detection of efficacy signals including evaluation of tumor response based on International Myeloma Working Group criteria, duration of response, progression-free survival, and translational evaluation of changes in BM microenvironmental pDCs. As of 7-25-16, 2 patients with r/r MM received SL-401 at 7 ug/kg in combination with POM+DEX. The median age was 65 years (range: 63-67 years). The most common treatment-related AEs, all grades, were thrombocytopenia (2/2, both grade 1) and hypoalbuminemia (2/2, both grade 2); there has been no DLT. Rapid onset decrease in a set of myeloma-related laboratory values from pre-SL-401 treatment was observed in both patients after the first combination cycle of SL-401 and POM+DEX. In one patient, serum M-protein decreased from 2.34 to 1.19 g/dL (cycle 1), free light chain kappa decreased from 40.1 to 8.27 mg/dL (cycle 1), and free light chain kappa/lambda ratio decreased from 58.12 to 41.35 (cycle 1). In the other patient, serum M-protein decreased from 1.88 to 0.87 (cycle 1) and then was 0.96 (cycle 3) g/dL, free light chain kappa decreased from 134 to 49.4 (cycle 1) and then was 92.5 (cycle 3) mg/dL, and free light chain kappa/lambda ratio decreased from 638.1 to 76 (cycle 1) and then was 111.45 (cycle 3). Both patients remain on study receiving ongoing SL-401 at 2⁺ and 4⁺ months. Dose escalation to 9 ug/kg is planned if a third patient clears the 7 ug/kg cohort.

Conclusions:This is the first clinical study to evaluate SL-401 in combination with other agents. SL-401 thus far has been well-tolerated in combination with POM+DEX in r/r MM patients, with no unexpected AEs observed. After the first cycle of SL-401 and POM+DEX combination therapy, 2 of 2 patients experienced a rapid decrease in serum M-protein and remain on SL-401 therapy. Given CD123 expression on microenvironmental immune pDCs and the potential synergy of SL-401 with certain current MM agents including POM, SL-401 may offer a novel therapeutic approach in MM. This Phase 1/2 trial continues to enroll and updated data will be presented. Clinical trial information: NCT02661022.