Cytotoxicity of the Recombinant Human Arginase (BCT-100) Against Acute Myeloid Leukaemia (AML)

Acute Myeloid Leukaemia (AML) is one of the most common leukaemias in both adults and children however improvements in overall survival have plateaued resulting in significant numbers of patients dying of disease. A major challenge is to find new drugs which are active against AML blasts or which make blasts more sensitive to existing chemotherapy. Arginine, is a semi-essential amino acid, required for protein synthesis, and cell viability. Some cancers are dependent on arginine for proliferation and survival - ‘arginine auxotrophism’. We have recently identified that arginine metabolism plays a key role in AML pathogenesis (Mussai et al. Blood. 2013). Therefore therapeutic depletion of arginine could be an Achilles-heel in AML survival.

A clinical grade, FDA approved, pegylated-recombinant human arginase, PEG-rhArgI molecule BCT-100, has been developed by Bio-Cancer Treatment International (BCT, Hong Kong). A Phase I and II clinical trial has been successfully completed in adult hepatocellular carcinoma. The Phase I trial demonstrated that at 1600U/kg BCT-100 (Observed Biological Dose), plasma arginine falls below 8mM (Adequate Arginine Depletion - ADD) and is maintained for up to 166 hours, with no evidence for neutralising immunogenicity. Only grade 1 or 2 reversible transaminitis was seen and patients experienced significant improvements in overall survival.

In this study we tested the cytotoxicity of BCT-100 against AML. We demonstrated that AML cell lines (n=4) are dependent on arginine for proliferation and survival. In vitro, BCT-100 leads to a complete depletion of arginine within 8 hours at doses significantly below those achievable in humans. In AML cell lines, BCT-100 arginine depletion causes an inhibition of cell division and G0/1 phase cell cycle arrest, leading to necrotic cell death (IC₅₀ 50-225mU/ml), confirmed by flow cytometry staining and electron microscopy. NOD-SCID AML xenografts treated with BCT-100 (50mU/kg iv twice weekly for 4 weeks) had a significant reduction in AML bone marrow engraftment (p=0.05).

The in vitro activity of BCT-100 was tested against primary blasts from 17 AML patients (n=14 adults, n=3 paediatric). The IC₅₀ ranged from 85-800mU/ml in 10/15 patients. Necrotic cell death was confirmed by flow cytometry and electron microscopy. Cytotoxicity of BCT-100 is additive in combination with low dose cytarabine in vitro (p=0.002). In this small cohort, sensitivity to BCT-100 did not correlate with clinical or cytogenetic characteristics. Resistance to cytarabine does not correlate with resistance to BCT-100 (p=0.06).

The enzymes OTC and ASS have been identified as biomarkers of sensitivity to arginine depletion in solid tumours. ASS and OTC expression pattern was characterised in leukaemic blasts from 30 (20 adult, 10 paediatric) AML patients, revealing the majority of AML cases are deficient in one or more of these enzymes (Adults: ASS-: 55% OTC: 34%; Paediatric: ASS- 80% OTC 20%). Importantly we demonstrated that BCT-100 is active against AML blasts expressing both ASS and OTC. RNA-sequencing of BCT-100 resistant and sensitive blasts identified a number of putative, predictive pathways involved in resistance to arginine depletion. The study is the first report on the cytotoxicity of pegylated human recombinant arginase BCT-100 against adult and paediatric AML. The data provides the rationale for an upcoming Phase I/II trial of BCT-100 for relapsed/treatment resistant AML.