A Next-Generation Fc-Bearing CD3-Engaging Bispecific DART^® Platform with Extended Pharmacokinetic and Expanded Pharmacologic Window: Characterization As CD123 x CD3 and CD19 x CD3 DART Molecules

Background: CD3-engaging DART molecules direct T cells to tumor-expressed antigens. Flotetuzumab, a CD123 x CD3 bispecific DART molecule that targets the differential expression of the IL-3 receptor alpha chain (CD123) on acute myeloid leukemia (AML) blasts and leukemic stem cells, is currently being investigated in a phase 1 study in relapsed/refractory AML and myelodysplastic syndrome (MDS), with initial evidence of clinical activity¹. A common adverse event in redirected T-cell killing interventions is cytokine release syndrome (CRS), a C_max-associated event that may limit their therapeutic window. Owing to its short circulating half-life, flotetuzumab is administered as continuous infusion, which affords prolonged exposure with limited C_max excursions. In contrast, long-acting Fc-bearing, CD3-engaging molecules would eliminate the need for continuous dosing, but would be especially impacted by CRS, given the high C_max levels required to maintain adequate trough concentrations over several days or weeks. We previously reported the development of a CD3-engaging DART molecule with reduced affinity for CD3 that maintained maximal target cell killing and T-cell proliferation, albeit at higher concentrations than its wild-type (WT) counterpart, but with reduced cytokine release². We report here further optimization and validation of the approach.

Methods: Anti-CD3 variants spanning a range of affinities were formatted as Fc-bearing DART molecules with CD123- or CD19-targeting arms and evaluated for cell binding as well as for antigen-dependent induction of T-cell proliferation, cytolytic activity and cytokine release. A therapeutic index (TI) was determined from the ratio of in vitro cell killing activity to that of target cell-induced cytokine release. In vivo anti-tumor activity was assessed in human immune cell-reconstituted mouse tumor models or in mice expressing the epitope recognized by the CD3 arm of the DART proteins (human CD3 k/i mice). Safety, pharmacokinetic (PK) and biological activity of selected molecules was evaluated in cynomolgus monkeys.

Results: Six Fc-bearing DART molecules (from a panel of 23 CD123 x CD3 DART variants) were selected for characterization. These molecules spanned a relatively small interval of low CD3 affinities, but varied greatly in activity, from inactive at all concentrations tested to being capable of lysing tumor cells, including primary leukemic and MDS blasts, to the same maximal level of killing observed with the parental WT DART molecule. Higher concentrations of the active variants were needed, owing to a deliberate design incorporating decreased CD3 binding. At levels capable of complete tumor cell lysis, active DART variants showed greatly reduced pro-inflammatory cytokine release (e.g., IL-2, IFN-γ and TNF-α) in vitro and in human PBMC-reconstituted mice compared to the WT version. Molecules were ranked based on their TI and a version with a highly favorable index was engineered as a B-cell targeting CD19 x CD3 DART molecule to conveniently ascertain its biological activity in cynomolgus monkeys. The CD19 x CD3 DART variant demonstrated good tolerability at single doses up to 30 mg/kg. B-cell depletion in the circulation and lymphoid tissues occurred at 1 mg/kg and achieved maximal depletion at ≥10 mg/kg, equivalent to the degree of reduction observed with 0.1 mg/kg of the WT molecule. However, only minimal circulating pro-inflammatory cytokines (including IL-6) were observed at all doses of the DART variant, while a substantial increase in IL-2, IL-6 and IFN-γ (among others) was observed with 0.1 mg/kg of the WT counterpart. The PK profile of the DART variant in cynomolgus monkeys appeared linear in the 1-30 mg/kg interval, with a half-life of 5 to 10 days, consistent with possible clinical dosing at weekly or longer intervals.

Conclusions: A next-generation CD3-engaging Fc-bearing DART platform was generated through CD3 arm affinity modulation. These DART variants feature extended PK and an expanded TI suitable for potential clinical development in hematological malignancies.

1. Uy et al, Blood 2017, vol. 130, Suppl 1 637

2. Huang et al, Keystone Symposia: Antibodies as Drugs, 2/25-3/1/2018, Whistler, BC, Canada