Complete Responses in Relapsed/Refractory Acute Myeloid Leukemia (AML) Patients on a Weekly Dosing Schedule of XmAb14045, a CD123 x CD3 T Cell-Engaging Bispecific Antibody: Initial Results of a Phase 1 Study

Background: Blasts and leukemic stem cells of acute myeloid leukemia (AML) as well as several other hematologic malignancies express CD123, potentially providing a target for novel therapies. XmAb14045 (also known as SQZ622) is a potent bispecific antibody targeting both CD123 and CD3 that stimulates targeted T cell-mediated killing of CD123-expressing cells, regardless of T cell antigen specificity. It is a full-length immunoglobulin molecule designed to be dosed intermittently, in contrast to smaller constructs that are referred to as "DART" or "BiTE" bispecific antibodies that require a continuous infusion.

Methods: Patients with relapsed or refractory AML, B cell acute lymphoblastic leukemia (B-ALL), blast phase chronic myelogenous leukemia, or blastic plasmacytoid dendritic cell neoplasm were eligible to enroll on this first-in-human, multicenter, open-label phase 1 dose-escalation study (XmAb14045-01) with standard 3+3 design. The primary objective was to estimate the maximum tolerated dose (MTD) or recommended dose and schedule of XmAb14045. Secondary objectives included safety, preliminary antileukemic activity, and pharmacokinetics/pharmacodynamics of XmAb14045. Treatment was administered weekly in 28 day cycles, using a weight-based dose with a single dose level in Part A and, in Part B, an initial priming dose on Day 1 followed by an escalated dose on subsequent weeks. Premedication to prevent cytokine release syndrome (CRS) was instituted as needed and included a steroid, acetaminophen, and diphenhydramine. Patients were premedicated at all XmAb14045 doses ≥0.075 µg/kg. Therapy was continued for as long as tolerated and there was continuing evidence of therapeutic benefit in the opinion of the investigator. Treatment response was assessed by the 2017 European LeukemiaNet (ELN) criteria after Cycle 1 and after each odd-numbered cycle. CRS was graded using the CRS revised grading system (Lee et al., Blood, 2014).

Results: At data cut-off, 64 patients have been treated to date, 63 with relapsed/refractory AML and 1 with B-ALL. Patients had a median age of 61 years and were heavily pretreated (median of 3 prior therapies [range 1-8], including 19 [30%] who had undergone prior allogeneic stem cell transplantation). The recommended dose for Part A was 1.3 µg/kg after a single dose-limiting toxicity of Grade 4 CRS at 2.3 µg/kg; no MTD has been identified. CRS or its component symptoms was the most common treatment-emergent adverse event (TEAE). CRS episodes began within approximately 1-4 hours of the start of drug infusion and occurred in 49 of 64 patients (77%). Seven patients (11%) developed Grade ≥3 CRS, the majority of these on the first dose. There were no CRS-related deaths. Excluding CRS-related events, additional TEAEs occurring in >10% of patients included fatigue (31%), febrile neutropenia (30%), peripheral edema (30%), cough (23%), elevated hepatic transaminases (19%; all recovered without sequelae), pneumonia (17%), stomatitis (14%), hyperglycemia (13%), and sepsis (11%). No myelosuppression requiring dose modification or evidence of tumor lysis syndrome was seen. In Part A, single agent antileukemic activity was documented with a best response of CR (2) or CRi (1) in 3/13 AML patients (CR/CRi rate 23%) treated at the two highest dose levels studied to date (1.3 and 2.3 µg/kg weekly); no CR, CRi, or morphologic leukemia-free state (MLFS) responses were seen at lower doses. Antileukemic activity occurred quickly; all responders had achieved at least an MLFS response after 4 doses (1 cycle). Two responders were bridged to stem cell transplantation, and the third was ineligible for medical reasons but remains in remission at 14+ weeks after initiating therapy.

Conclusions: XmAb14045 demonstrated evidence of antileukemic activity in heavily pretreated patients with relapsed/refractory AML treated in Part A at the 1.3 and 2.3 µg/kg doses administered once weekly, with a 23% CR/CRi rate. CRS was the most common toxicity but was generally manageable with premedications. The study is ongoing with further optimization of dose, schedule, and premedication regimen for CRS anticipated during accrual to dose escalation cohorts in Part B.

ClinicalTrials.gov Identifier: NCT02730312