VIP943 Is a Novel CD123 Antibody Drug Conjugate with in Vitro and In Vivo Efficacy in Acute Myeloid Leukemia (AML) Models

Abstract

Background: Current treatment options (cytarabine with anthracyclines) for patients with acute myeloid leukemia (AML) are often associated with severe and barely tolerable toxicities. In patients who tolerate induction therapy, the complete remission rate is 60%-80%; however, 50% of patients relapse due to the presence of leukemic stem cells (LSC). Antibody-drug conjugates (ADCs) are putatively designed to deliver potent drugs directly to cancer cells while sparing normal tissue; however, first-generation ADCs are associated with severe bone marrow and liver toxicity. VIP943 is a CD123-targeting ADC which uses a kinesin spindle protein inhibitor (KSPi) as a new payload class and a unique legumain linker designed to overcome liabilities of currently approved ADC therapies. The payload is delivered with a drug-antibody ratio of 6. It consists of the KSPi warhead which is modified with a CellTrapperTM moiety so it accumulates within the CD123-targeted tumor cell and cannot enter normal cells. Non-human primate toxicology studies demonstrated a highly favorable and differentiated safety profile for VIP943 (Kirchhoff et al, Cancers 2020). Here we characterize VIP943 in various patient-derived AML models. Methods: For co-localization experiments, MOLM-13 AML cell line was stained with labelled anti-CD123 antibody or isotype control and CytoPainter LysoGreen indicator reagent and analyzed by imaging flow cytometry. AML primary patient mononuclear cells from blood and bone marrow were treated with VIP943 for 24, 48, and 72 hours. Flow cytometry immunophenotyping for AML blasts and CD34+/CD38- LSCs was used to analyze these populations. An in vivo patient-derived AML PDX mouse model was treated with VIP943 (5 mg/kg IV every 7 days) or in combination with 5-azacytidine (2.5 mg/kg SC days 1-5 x 3) and venetoclax (50 mg/kg PO days 1-5 x 3). Cytokine release syndrome was analyzed comparing VIP943 to isotype control ADC, anti-CD123 antibody, and positive comparator antibodies by in vitro cytokine release assays (CRA) in two different assay formats using whole blood and isolated mononuclear cells from 10 healthy donors. Flow cytometric lymphocyte and leukemic immunophenotyping was performed from blood of non-human primates. A leukemia cell line panel drug screen with VIP943 was evaluated and baseline genomic features were correlated with response. Results: Co-localization studies confirm specific internalization and efficient intracellular trafficking to the lysosome of VIP943. In vitro treatment with VIP943 (66 nM, continuous exposure) delivers blast reductions of 37% after a 24h incubation with VIP943, 45% after 48h incubation and 83% after an 72h incubation. In the CD34+/CD38- cell population, a reduction of 71% was achieved. In a patient-derived AML model, VIP943 with 5-azacytidine and venetoclax led to complete tumor growth inhibition after 21 days showing superiority of this triplet therapy (complete regression: 8/9 mice) with high tolerability (maximum body weight loss -4.8 %). The immunotoxicological evaluation in both CRA formats and with the tested antibody concentrations (100µg, 10µg, 1µg) exerts no induction of cytokines by VIP943 compared to controls. Immunophenotyping demonstrates no impact of VIP943 treatment on lymphocytic subpopulations, whereas the CD123-positive basophils are reduced after 24h followed by an almost complete recovery. VIP943 IC50 ranges from 0.1nM-300nM in a leukemia cell line panel screen and features associated with sensitivity are investigated. Conclusions: VIP943 is a next-generation ADC with a differentiated safety profile from currently approved ADCs, including lack of in vitro cytokine release. In vitro and in vivo studies using patient-derived AML cells show VIP943 has favorable monotherapy and combination efficacy including targeting of leukemic stem cells, which drive relapse. These findings warrant evaluating VIP943 in clinical trials.