No abstract

Eight prodrugs of L-cysteine (1a-h) were synthesized by the condensation of the sulfhydryl amino acid with naturally occurring aldose monosaccharides containing three, five, and six carbon atoms. The resulting 2-(polyhydroxyalkyl)thiazolidine-4(R)-carboxylic acids (TCAs) are capable of releasing L-cysteine and the sugars by nonenzymatic ring opening and hydrolysis. Thus, when added to rat hepatocyte preparations in vitro, these TCAs (1.0 mM) raised cellular glutathione (GSH) levels 1.2-2.1-fold relative to controls. On the basis of this finding, the cysteine prodrugs were tested as protective agents against acetaminophen-induced hepatotoxicity in a mouse model. The TCA derived from D-ribose and L-cysteine (RibCys, 1d) showed the greatest therapeutic promise of the series, with a 100% (12/12) survival profile compared to 17% without treatment. However, the degree of stimulation of GSH production in rat hepatocytes by these prodrugs did not correlate with the extent of protection afforded in mice, suggesting that pharmacokinetic parameters must supervene in vivo. To evaluate the effect of increased lipid solubility, we prepared prodrugs 2a-c by using peracetylated aldehydic sugars in the condensation reaction. These compounds, however, displayed acute toxicity to mice, possibly due to liberation of the acetylated sugars themselves. Nevertheless, the efficacy of the unacetylated TCAs, and RibCys (1d) in particular, suggests that the prodrug approach for the delivery of L-cysteine to the liver represents a viable means of augmenting existing detoxication mechanisms in protecting cells against xenobiotic substances that are bioactivated to toxic, reactive metabolites.

A series of prodrugs of 3-mercaptopyruvate (3-MP), the substrate for the enzyme 3-mercaptopyruvate/cyanide sulfurtransferase (3-MPST) that converts cyanide to the nontoxic thiocyanate, which are highly effective cyanide antidotes, have been developed. These prodrugs of 3-MP are unique in being not only orally bioavailable, but may be administered up to an hour prior to cyanide as a prophylactic agent and are both rapid-or slow-acting when given parenterally.

A number of 2-alkyl- and 2-aryl-substituted thiazolidine-4(R)-carboxylic acids were evaluated for their protective effect against hepatotoxic deaths produced in mice by LD90 doses of acetaminophen. 2(RS)-Methyl-, 2(RS)-n-propyl-, and 2(RS)-n- pentylthiazolidine -4(R)-carboxylic acids (compounds 1b,d,e, respectively) were nearly equipotent in their protective effect based on the number of surviving animals at 48 h as well as by histological criteria. 2(RS)-Ethyl-, 2(RS)-phenyl-, and 2(RS)-(4-pyridyl)thiazolidine-4(R)-carboxylic acids (compounds 1c,f,g) were less protective. The enantiomer of 1b, viz., 2(RS)- methylthiazolidine -4(S)-carboxylic acid (2b), was totally ineffective in this regard. Thiazolidine-4(R)-carboxylic acid (1a), but not its enantiomer, 2a, was a good substrate for a solubilized preparation of rat liver mitochondrial proline oxidase [Km = 1.1 x 10(-4) M; Vmax = 5.4 mumol min-1 (mg of protein)-1]. Compound 1b was not a substrate for proline oxidase but dissociated to L-cysteine in this system. At physiological pH and temperature, the hydrogens on the methyl group of 1b underwent deuterium exchange with solvent D2O (k1 = 2.5 X 10(-5) s), suggesting that opening of the thiazolidine ring must have taken place. Indeed, 1b labeled with 14C in the 2 and methyl positions was rapidly metabolized by the rat to produce 14CO2, 80% of the dose being excreted in this form in the expired air after 24 h. It is suggested that these 2-substituted thiazolidine-4(R)-carboxylic acids are prodrugs of L-cysteine that liberate this sulfhydryl amino acid in vivo by nonenzymatic ring opening, followed by solvolysis.