Enhanced anti-cancer efficacy of arginine deaminase expressed by tumor-seeking Salmonella Gallinarum

Amino acid deprivation, particularly of nonessential amino acids that can be synthesized by normal cells but not by cancer cells with specific defects in the biosynthesis pathway, has emerged as a potential strategy in cancer therapeutics. In normal cells, arginine is synthesized from citrulline in two steps via two enzymes: argininosuccinate synthetase (ASS1) and argininosuccinate lyase. Several cancer cells exhibit arginine auxotrophy due to the loss or down-regulation of ASS1. These cells undergo starvation-induced cell death in the presence of arginine-degrading enzymes such as arginine deaminase (ADI). Thus, ADI has emerged as a potential therapeutic in cancer therapy. However, the use of ADI has two major disadvantages: ADI of bacterial origin is strongly antigenic in mammals, and ADI has a short circulation half-life (∼5 hours). In this study, we engineered tumor-targeting Salmonella Gallinarum to express and secrete ADI and deployed this strain into mice implanted with ASS1-defective mouse colorectal cancer (CT26) through an intravenous route. A notable antitumor effect was observed, suggesting that the disadvantages were overcome as ADI was expressed constitutively by tumor-targeting bacteria. A combination with chloroquine, which inhibits the induction of autophagy, further enhanced the effect.