CD276 (B7-H3) Is an Immunotherapeutic Target in Acute Myeloid Leukemia with Preclinical Efficacy of Vobramitamab Duocarmazine, an Investigational CD276 Antibody-Drug Conjugate

CD276 (B7-H3) is an immune checkpoint inhibitor expressed in a majority of solid tumors and hematologic malignancies, however it has not been well-described in acute myeloid leukemia (AML). A variety of immunotherapies targeting CD276 including CAR T cells and antibody-drug conjugates (ADCs) are being explored in solid tumors, however their role in leukemia has not been well elucidated. We interrogated the transcriptome and associated clinical and laboratory data from a large cohort of pediatric and adult AML patients to define the prevalence, frequency of co-occurring genetic alterations and outcomes associated with CD276 expression. Lastly, we evaluated preclinical in vitro efficacy in AML of vobramitamab duocarmazine (vobra duo), an investigational ADC with a humanized B7-H3 mAb conjugated via a cleavable linker to the prodrug seco-DUocarmycin hydroxyBenzamide Azaindole.

Ribo-depleted RNA sequencing data from 2,241 patients with AML (Pediatric ages 0-29 years N=1,412, from recent Phase 3 COG clinical trials; Adult ages 18-88 years BEAT AML N=451, SWOG N=199, TCGA-LAML N=179) with available cytogenetic, molecular, and clinical data were utilized. Conventional karyotyping and mutational analysis including NGS were used to determine additional cytomolecular aberrations. Response was measured by 5-year overall survival (OS), event-free survival (EFS) and relapse risk (RR).

Of the 1,412 pediatric patients evaluated, CD276 mRNA expression was positive (defined as mRNA transcripts per million (TPM) >2) in 289 patients (20%). Higher prevalence (58%) was seen in adult AML patients (BEAT AML N=209/451, SWOG N=191/199, TCGA-LAML N=85/179), Fig 1A. Flow cytometry validated cell surface expression in pediatric AML, with 22% of patients expressing CD276. Cell surface expression was also observed in AML cell lines. CD276 mRNA expression is absent in normal hematopoietic cells (TPM <1) with absent cell surface expression in normal bone marrow as assessed by flow cytometry, allowing for targeted CD276 therapies to be utilized with no/minimal on-target off-tumor hematopoietic toxicity.

Evaluation of co-occurring cytomolecular aberrations demonstrated a significant enrichment of CD276+ pediatric patients with high-risk genetic alterations, most notably KMT2A fusions (N=150/343, 43.7%), KAT6A-CREBBP (N=11/12, 91.6%) and CBFA2T3-GLIS2 (N=22/39, 56.4%). We evaluated the significance of CD276 mRNA expression on clinical outcomes in childhood AML. 5 year event free survival (EFS) for patients with CD276 expression was 35% compared to 44% for patients without CD276 expression (p=0.0022). Overall survival (OS) was 52% for patients with CD276 expression compared to 61% for patients without CD276 expression (p=0.0015). Relapse was seen in 40.5% of patients with CD276 expression compared to 35.8% of patients without CD276 expression and relapse occurred on average earlier in CD276 positive patients (0.89 years vs. 1.06 years, p=0.02).

Given the poor clinical outcomes for patients with high CD276 expression, novel targeted therapies are needed for this high risk cohort. With multiple CD276 immunotherapies under clinical evaluation in solid tumors, we tested vobra duo in OCI-AML2, a CD276 positive KMT2A-rearranged AML cell line, which demonstrated robust cytolytic activity with an IC50 less than 1nM (IC50=0.9306 nM) supporting the potential efficacy of CD276 targeted therapies in AML (Fig 1B).

In a large cohort of AML patients, we identified CD276 to be overexpressed in 20% of pediatric AML patients and 58% of adult AML patients with a significant enrichment in high risk fusion subtypes including KMT2A-rearranged AML, KAT6A-CREBBP and CBFA2T3-GLIS2 fusions. Patients with CD276 expression have poor outcomes with significantly worse EFS and OS compared to patients who do not have CD276 expression. With multiple CD276 immunotherapies under clinical evaluation for solid tumors, we hypothesized that targeting CD276 in AML would result in specific cytolytic activity. Vobra duo showed robust in vitro cytolytic activity against CD276 positive AML cells highlighting the need for ongoing preclinical evaluations of CD276 targeted therapies in AML. Given the established safety profile for vobra duo this provides a clear path for rapid translation to clinical use for high risk AML patients.