Combinatorial Anti-Tumor Activity in Animal Models of a Novel CD123 x CD3 Bispecific Dart® Molecule (MGD024) with Cytarabine, Venetoclax or Azacitidine Supports Combination Therapy in Acute Myeloid Leukemia

Introduction: Notwithstanding recent progress, acute myeloid leukemia (AML) remains an incurable disease, particularly in patients (pts) with relapsing/refractory disorder or ineligible for intensive induction therapy (unfit pts). Redirected T-cell mediated killing via CD3-engaging bispecific molecules may offer an alternate therapeutic opportunity for aggressive disease or unfit pts. Flotetuzumab, a continuous infusion CD123 x CD3 DART molecule, has shown preliminary single-agent activity in pts refractory to induction therapy (Uy et al., Blood. 2021, 137:751). CD123, the IL-3 receptor alpha chain, is expressed by both leukemic blasts and leukemic stem cells and is a suitable therapeutic target in AML (Testa et al., Cancers (Basel). 2019, 11:1358). MGD024 is an Fc-bearing CD123 x CD3 DART molecule designed for prolonged circulating half-life and intermittent delivery. MGD024 was also designed with a CD3-binding arm with reduced affinity to diminish the propensity for cytokine release compared to flotetuzumab. The potentially improved tolerability and dosing convenience of MGD024 may provide a framework for introducing T-cell immunotherapy in early-stage AML or unfit pts. To explore whether MGD024 could complement AML standard of care (SOC), we investigated combination therapy in mouse models.

Materials and Methods: The DART molecules, flotetuzumab and MGD024, shared identical CD123 (humanized 7G3) and CD3 (humanized XR32) Fv arms, save for a mutation in the anti-CD3 arm of MGD024 that decreases its affinity for the CD3-epsilon chain. While flotetuzumab has no Fc domain, MGD024 includes an ala-ala-mutated human IgG1 Fc that extends its circulating half-life via the neonatal Fc receptor-mediated salvage pathway together with impairing binding to Fc-gamma receptors and complement. An IgG1-ala-ala Fc-bearing version of flotetuzumab (RES234M1.1) was also engineered to allow delivery at identical time intervals as MGD024 and avoid continuous infusion in experimental animals. MHC class I-null, NOD/SCID/IL2R-gamma-null mice were reconstituted with human PBMC (8x10 ⁶ cells/mouse, retro orbital). Two human AML cell lines expressing low or high levels of CD123 (KG1a < MOLM-13) were implanted SC at 2.5 x 10 ⁶ (KG1a) or 5 x 10 ⁶ (MOLM-13) cells/mouse. Treatments (IV, IP or PO by gavage, as indicated) were initiated when tumor volumes reached ~150 mm ³, with volumes recorded weekly or twice weekly thereafter.

Results and Conclusions: Consistent with its decreased affinity for CD3, MGD024 demonstrated reduced in vitro potency in killing CD123-positive target cells compared to flotetuzumab or RES234M1.1, but proportionally greater reduction in cytokine release. MGD024, however, achieved maximal cytolytic activity as flotetuzumab or RES234M1.1, albeit at increased concentrations. Similarly, MGD024 showed reduced potency in vivo against CD123-positive tumors compared to RES234M1.1; nevertheless, tumor growth reduction of the same magnitude as that observed with RES234M1.1 was attained at higher doses of MGD024 (0.5-1 mg/kg IV 2QW MGD024 vs. 0.05-0.1 mg/kg IV 2QW RES234M1.1, depending on the model). Reduced cytokine release was also observed with MGD024 compared to RES234M1.1 in vivo. To explore MGD024 suitability for combination therapy, sub-active doses of cytarabine (CYT, 10 mg/kg IV 2QW or 7.5-10 mg/kg IP QD), venetoclax (VEN, range 10-80 mg/kg PO QD), or azacitidine (AZA, 2 mg/kg PO QD) were co-administered with suboptimal regimens of MGD024 (range 0.005-0.1 mg/kg IV 2QW, depending on the model). Complete or near complete tumor elimination was observed with the combination of suboptimal MGD024 and CYT or VEN. In contrast, AZA, at the dose tested, did not contribute to the antitumor effect of MGD024. CYT, VEN or AZA did not inhibit a fully active dose of MGD024, confirming no detrimental impact of the SOC agents at the doses employed on the effector cell population engaged by the DART molecule. All treatments were well tolerated, as indicated by body weigh profiles across treatment groups. These data support clinical exploration of the combination of MGD024 with SOC in patients with AML. An investigational new drug (IND) application of MGD024 in pts with selected relapsed or refractory hematologic malignancies is planned.