Toxicity evaluations of L-cysteine and Procysteine, a cysteine prodrug, given once intravenously to neonatal rats.

Decreased enzymatic production of cysteine in premature and newborn infants may limit the synthesis of glutathione. Unfortunately, cysteine supplementation is limited by associated toxicity and product instability. Procysteine (L-2-oxothiazolidine-4-carboxylate) is a prodrug of cysteine that is inert until metabolized to cysteine intracellularly, thus stimulating glutathione synthesis. The potential toxicities of cysteine and Procysteine were compared in two studies with neonatal rats (10 per group; 3 +/- 1 days of age) after a single intravenous administration. In one study, acute high dosage survivorship was compared for approximately equimolar cysteine dosages of L-cysteine and Procysteine. Mortality at 7 days after single intravenous dosages of L-cysteine at 1.52 or 1.14 g/kg or Procysteine at 1.80 or 1.35 g/kg was 80, 50, 10 and 0%, respectively. Clinical pathology parameters and body and organ weights were compared in a second study, following a moderate dosage of Procysteine or equimolar or lower dosages of L-cysteine. No differences were observed in clinical pathology parameters nor body or organ weights at 14 days following single intravenous dosages of L-cysteine at 369, 185 or 37 mg/kg or Procysteine at 450 mg/kg. Also, Procysteine solutions were considerably more stable than L-cysteine solutions (months vs. hours, respectively). These studies indicated that cysteine supplementation in infants may be enhanced by Procysteine administration.