Aspartate Beta-Hydroxylase (ASPH) As a Novel Therapeutic Target in Acute Myeloid Leukemia

Introduction

Despite recent advances in treatment, acute myeloid leukemia (AML) remains difficult to cure with high rates of relapse. Relapsed/refractory AML patients who are elderly or unfit for cytotoxic chemotherapy and whose disease fails to respond to hypomethylating agents represent an unmet need, and new safe and effective treatment options are needed for this patient population. Aspartate β-hydroxylase (ASPH) is a transmembrane protein that hydroxylates aspartyl and asparaginyl residues of epidermal growth factor (EGF)-like protein domains, and promotes cellular motility, migration, and adhesion. ASPH is highly expressed during fetal development and in placental trophoblasts, but not in any other healthy adult human tissue. ASPH is uniquely upregulated in cancer cells and is reported to be overexpressed in over 20 different solid neoplasms, in which it propagates a malignant phenotype, and is associated with increased cell proliferation, invasiveness, and poor prognosis. ASPH is therapeutically targetable via a fully human monoclonal antibody against ASPH, SNS-622. Subsequently, SNS-622 antibody-drug conjugates (ADCs) and a vaccine have been developed and are under current preclinical and clinical testing, respectively. We have previously demonstrated ASPH overexpression on the MOLM-14 AML cell line and effective in vitro killing of these cells using SNS-622 ADCs. In an effort to further characterize ASPH as a therapeutic target in AML, we report here the first study of ASPH expression patterns in AML patient samples.

Methods

Bone marrow (BM) aspirate and peripheral blood (PB) samples were collected from AML patients during 2014-2018 on a University of Maryland Greenebaum Comprehensive Cancer Center (UMGCCC) tissue banking protocol. Mononuclear cells were isolated by density centrifugation and were viably cryopreserved at -80ᵒC. Samples were analyzed using 8-color multiparameter flow cytometry to assess cell surface expression of ASPH, using fluorescein isothiocyanate (FITC)-conjugated SNS-622. The remainder of the fluorophores were labeled with standard lineage-specific markers. Expression of ASPH was measured via FITC fluorescence for each cell population and analyzed by two blinded, independent reviewers using FlowLogic^TM software. Statistical analysis of ASPH expression was conducted using IBM^® SPSS^® Statistics for Windows, release 25.0.0 (IBM Corp., Armonk, N.Y., USA). Cohen's kappa (κ) was used to quantify the inter-observer agreement between two independent observers. Visual inspection of the expression data for the whole patient population showed a bimodal distribution, which was used to identify a robust cut-point separating high (i.e. positive) from low (i.e. negative) ASPH expression.

Results

Forty-two AML patient samples were evaluated (32 BM, 10 PB). Median patient age was 66 years, 48% (n=20) were female, 69% Caucasian and 21% African American. Disease status was 52% untreated de novo, 19% untreated secondary from antecedent MDS or MPN, and 29% relapsed/refractory. Samples were cytogenetically and molecularly diverse - with 50% exhibiting normal and 14% complex karyotype; 38% FLT3-ITD or FLT3-TKD, 29% NPM1, 19% IDH1/2, and 10% TP53 mutations. Full patient characteristics are shown in Table 1.

Myeloblast expression of ASPH was found in 38% of samples (n=16; 14 BM, 2 PB) with a mean fluorescent intensity (MFI) of 10 as a cutoff for ASPH surface expression positivity. ASPH expression was not seen on other non-neoplastic cells including CD34+ hematopoietic stem cells, B- or T-lymphocytes, and monocytes. A blinded, independent review of the data revealed a Cohen's kappa (κ) of 0.74 with standard error of the estimate of ± 0.11. Patients with AML with ASPH expression were clinically heterogeneous, with no correlation between ASPH expression and AML subtype, karyotype or mutation status (Table 1).

Conclusion

ASPH is overexpressed in approximately 40% of patients with AML and serves as a promising therapeutic target. An ASPH nanoparticle vaccine is currently under clinical investigation and has shown promising results in solid tumors. We plan to expand clinical testing of targeting ASPH to AML. The ASPH positivity cutoff established via this work will serve as the eligibility criterion for the planned phase Ib/IIa of anti-ASPH vaccination in AML.

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