Cytotoxicity of the Recombinant Human Arginase (BCT-100) Against Acute Lymphoblastic Leukaemia (ALL)

Acute Lymphoblastic Leukaemia (ALL) is the most common leukaemia of childhood. Despite improvements in overall survival, significant numbers of children die from relapsed/ refractory disease or suffer the morbidities of multi-agent chemotherapy regimens. A major challenge is to find new drugs which are active against ALL blasts or which make blasts more sensitive to existing chemotherapy, without adding to toxicity. 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 and can be therapeutically targeted. Therefore we investigated if Acute Lymphoblastic Leukaemia is also sensitive to therapeutic arginine depletion.

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.

Arginine is recycled by cells from ornithine, through the enzymes OTC and ASS. These enzymes have been identified as biomarkers of sensitivity to arginine depletion in solid tumours. ASS and OTC expression pattern was characterised in leukaemic blasts from 37 ALL patients (19 adult, 18 paediatric), revealing the majority of ALL cases are deficient in one or more of these enzymes (Adults: ASS-: 84% OTC: 53%; Paediatric: ASS- 44% OTC 27%). Thus the majority of ALL blasts are auxotrophic for arginine.

We tested the cytotoxicity of BCT-100 against ALL. We demonstrated that ALL cell lines (n=4) are dependent on arginine for proliferation and survival. In vitro, BCT-100 leads to a complete depletion of extracellular arginine and induction of autophagy. BCT-100 was tested against primary blasts from ALL patients - IC₅₀S ranged from 17.5-2400ng/ml. Sensitivity to BCT-100 did not correlate with clinical or cytogenetic characteristics. The study is the first report on the arginine dependence of adult and paediatric ALL and provides the rationale for ongoing translation of BCT-100 for patients with relapsed ALL.