Optimal dosing of vobramitamab duocarmazine, a CD276 (B7-H3) antibody-drug conjugate, leads to durable eradication of Acute Myeloid Leukemia in preclinical xenograft models Free

CD276 (B7-H3) is an immune checkpoint inhibitor aberrantly expressed in a majority of solid tumors and hematologic malignancies, with limited data on the therapeutic potential of targeting CD276 in acute myeloid leukemia (AML). We previously characterized the prevalence of CD276 in adult and pediatric acute myeloid leukemia, notably identifying high CD276 expression across multiple high-risk subtypes of AML (Kirkey et al. ASH 2023). Given promising results of immunotherapeutic targeting of CD276 in solid tumors, we sought to evaluate the preclinical efficacy of targeting CD276 with vobramitamab duocarmazine (vobra duo), an investigational antibody-drug conjugate (ADC) with a humanized B7-H3 mAb conjugated via cleavable linker to the prodrug seco-DUocarmycin hydroxyBenzamide Azaindole. In vitro evaluation of vobra duo across multiple CD276+ AML cell lines demonstrated robust cytolytic activity with an IC50 less than 5nM (IC50 range: 0.41nM-4.012nM) supporting the potential efficacy of CD276 targeted therapies in AML.

To evaluate in vivo efficacy of vobra duo inmodels of high-risk AML, we engrafted OCI-AML2 cells, a high-risk KMT2A-rearranged (KMT2A-r) AML cell line, into NSG mice and confirmed engraftment via noninvasive bioluminescent imaging. Mice were then treated with a single dose of vobra duo (two dose levels: 5mg/kg vs. 10mg/kg) with anti-leukemic response evaluated compared to an untreated control cohort. Treatment with vobra duo rapidly eliminated leukemia without any residual leukemia detected within 1 week of treatment. Untreated control mice developed progressive leukemia requiring euthanasia within 3 weeks of leukemia engraftment. Treatment with vobra duo significantly improved survival (median survival 35 days vs. 14 days, p<0.0001), however treated mice developed extramedullary leukemia, predominantly in the liver without detectable leukemia in the bone marrow or peripheral blood. Despite the rapid and robust response of vobra duo against AML, we hypothesized a single dose was insufficient to fully eradicate leukemia either due to penetration into the liver or residual low level leukemia.

To optimize the dosing schema and evaluate efficacy in a more clinically-relavent model, we utilized an established high-risk KMT2A-MLLT4 AML patient-derived xenograft (PDX) model. We engrafted KMT2A-r PDX cells into NSG-SGM3 mice and confirmed engaftment via imaging. Mice were then treated with three weekly doses followed by three every other week dosing of vobra duo (10mg/kg). Control mice were either treated with a control ADC or untreated with progressive leukemia noted in all control mice. Treatment with 6 total doses of vobra duo eradicated AML PDX cells without evidence of leukemia in bone marrow aspirates at Day 140 and no evidence of extramedullary disease for the duration of the experiment. Treatment with this multi-dosing schema of vobra duo led to a durable response with all treated mice remaining alive and leukemia-free for >140 days compared to median survival 17 days for control ADC-treated mice and 107 days for untreated mice (p=0.0003).

In summary, CD276 (B7-H3) is a highly attractive immunotherapeutic target enriched in multiple high-risk subtypes of AML with a well-established safety profile from extensive immunotherapeutic evaluations in solid tumors. Therapeutic targeting of CD276 using an optimized dosing schema of the antibody-drug conjugate, vobra duo, resulted in remarkable anti-leukemia efficacy and durable responses in a clinically relavent high-risk patient derived xenograft model of AML. The established safety profile of vobra duo combined with the robust preclinical evaluations in AML reported here provides rationale for rapid translation of this targeted therapy for high-risk AML patients in dire need of novel targeted therapeutic approaches.