Systemic depletion of serum l-Cyst(e)ine with an engineered human enzyme induces production of reactive oxygen species and suppresses tumor growth in mice

Cancer cells experience higher oxidative stress from reactive oxygen species (ROS) than non-malignant cells due to genetic alterations and abnormal growth and as a result, maintenance of the anti-oxidant glutathione (GSH) is essential for their survival and proliferation(1–3). Under elevated ROS conditions endogenous l-Cysteine (l-Cys) production is insufficient for GSH synthesis, necessitating l-Cys uptake, predominantly in its disulfide form l-Cystine (CSSC) via the xCT(−) transporter. Here we show that administration of an engineered, pharmacologically optimized, human Cyst(e)inase enzyme mediates sustained depletion of the extracellular l-Cys and CSSC pool in mice and non-human primates, selectively causes cell cycle arrest and death (PI and Annexin-V staining) in cancer cells due to depletion of intracellular GSH and ensuing elevated ROS, yet results in no apparent toxicities in mice even after months of continuous treatment. Cyst(e)inase suppressed the growth of prostate carcinoma allografts, reduced tumor growth in prostate and breast cancer xenografts and doubled the median survival time of TCL1-Tg:p53(−/−) mice that develop disease resembling human chronic lymphocytic leukemia. The observation that enzyme-mediated depletion of the serum l-Cys and CSSC pool suppresses the growth of multiple tumors, yet is very well tolerated for prolonged periods suggests that Cyst(e)inase represents a safe and effective therapeutic modality for inactivating anti-oxidant cellular responses in a wide range of malignancies(4,5).