An engineered cyst(e)inase enzyme is active against multiple tumor types without adverse side effects.
Major finding: An engineered cyst(e)inase enzyme is active against multiple tumor types without adverse side effects.
Mechanism: Cyst(e)inase systemically depletes both L-cystine and L-cysteine to reduce GSH and increase ROS.
Impact: Cyst(e)inase may enhance ROS to potentially treat patients with diverse tumor types.
Cancer cells are often dependent on the antioxidant glutathione (GSH) due to higher oxidative stress from reactive oxygen species (ROS) compared with normal cells. The amino acid L-cysteine (L-Cys) is required for the synthesis of GSH and its function in other antioxidant pathways. Oxidatively stressed cancer cells depend on the uptake of the disulfide form, L-cystine (CSSC), from the blood through the CSSC/glutamate transporter xCT(–). Thus, xCT(–) is a potential therapeutic target in cancer, and small-molecule inhibitors have demonstrated antitumor activity. However, these inhibitors do not eliminate free L-Cys, which can be imported by other transporters, suggesting that depleting both L-Cys and CSSC might be a superior approach. The cystathionine gamma-lyase enzyme can degrade both CSSC and L-Cys, but not quickly enough for clinical applications. Using a structure-guided mutagenesis approach, Cramer, Saha, Liu, and colleagues engineered a human enzyme variant, termed cyst(e)inase, that degrades both CSSC and L-Cys with high catalytic activity. It was conjugated to polyethylene glycol chains to prevent renal clearance and increase circulation persistence. No toxicities were observed when cyst(e)inase was administered to cynomolgus monkeys or mice. In prostate cancer cells, cyst(e)inase decreased GSH levels, increased ROS, and reduced cell survival via induction of autophagy and cell-cycle arrest. In vivo, cyst(e)inase depleted CSSC and L-Cys, blocked the growth of prostate cancer allografts and xenografts as well as breast cancer xenografts, and increased survival in mice with chronic lymphocytic leukemia, indicating widespread antitumor activity. Collectively, these results suggest that depletion of both CSSC and L-Cys results in broad activity against multiple tumor types, and support further investigation of cyst(e)inase, especially in combination with ROS-inducing drugs.