Activity and tolerability of SL-401, a targeted therapy directed to the interleukin-3 receptor on cancer stem cells and tumor bulk, as a single agent in patients with advanced hematologic malignancies.

7029^ Background: SL-401 is a novel biologic targeted therapy directed to the interleukin-3 receptor (IL-3R). IL-3R is overexpressed on cancer stem cells (CSCs) and tumor bulk relative to normal hematopoietic cells in a wide range of hematologic malignancies including AML and blastic plasmacytoid dendritic cell neoplasm (BPDCN). Since SL-401 targets both leukemia blasts and CSCs, tumor regression and improvement in long-term outcome is expected. The clinical activity and side effect profile of SL-401 were evaluated in a multicenter Phase I/II trial of patients with advanced hematologic cancers. Methods: Eighty-one patients with advanced hematologic cancers, including relapsed or refractory AML (n = 59) and heavily pretreated BPDCN (n = 4), have been enrolled. Patients received a single cycle of SL-401 via 15-minute IV infusion to determine the maximum tolerated dose (MTD) and assess antitumor activity. Results: A single cycle of SL-401 demonstrated single agent activity in relapsed or refractory AML patients, including 2 durable CRs of 8 and 25+ months duration and multiple cases of blast reductions. SL-401, when delivered at therapeutically relevant doses, was associated with > 3-fold greater median overall survival (OS) in AML patients who received 2+ prior lines of treatment relative to historical results. In addition, 3 heavily pre-treated patients with BPDCN, an uncommon malignancy that expresses high levels of IL-3R and is ultrasensitive to SL-401 (IC50 values in the femtomolar [10-15 M] range), had CRs, with durations of 5, 3+ and 1+ months. The MTD was 16.6 µg/kg/day; the dose-limiting toxicities of hypoalbuminemia and edema, which are manifestations of capillary leak, occurred at 22.1 µg/kg/day. Other ≥ Grade 3 adverse events included transient transaminase elevations. There was no treatment-related myelosuppression. Conclusions: SL-401 was well tolerated and demonstrated single agent activity in patients with relapsed or refractory AML and BPDCN. Based on these findings, single agent SL-401 given in multiple cycles will be advanced into pivotal studies of AML (3rd-line) and BPDCN. Clinical trial information: NCT00397579.