Prevention of naphthalene-induced cataract and hepatic glutathione loss by the L-cysteine prodrugs, MTCA and PTCA.

Rapid-onset cataracts were induced in SPF C57 bl/6 mice by intraperitoneal administration of naphthalene following cytochrome P-450 isozyme induction with phenobarbital. Several L-cysteine prodrugs with masked sulfhydryl groups in the form of thiazolidine-4-carboxylic acids, as well as N-acetyl-L-cysteine, N,S-bis-acetyl-L-cysteine and glutathione ethyl ester, were evaluated for their ability to maintain hepatic and lenticular glutathione at near-normal levels and to prevent naphthalene-induced cataract formation. Each prodrug was administered at three specified times to a cumulative total of 1.5 mole equivalents of the single dose of naphthalene. Three L-cysteine prodrugs delayed but did not prevent cataract formation in 40-60% of the mice over a 72-hr period, while eight of the 13 compounds produced cataract yields similar to the naphthalene control animals, i.e. 83% in 72 hr. However, two L-cysteine prodrugs, 2(R,S)-methylthiazolidine-4(R)-carboxylic acid (MTCA) and 2(R,S)-n-propylthiazolidine-4(R)-carboxylic acid (PTCA), prevented cataract formation in 20 of 21 and 12 of 12 mice, respectively, and maintained hepatic reduced glutathione levels at 82% and 51% of untreated controls. In contrast, glutathione was depressed to 3% of the normal value in those animals treated with naphthalene alone. Lenticular glutathione values were depressed, albeit minimally, in all naphthalene-treated mice regardless of administration of either MTCA or PTCA. The mice protected with either MTCA or PTCA showed no visible effects of naphthalene toxicity or lens opacities at any time. It can be concluded that these L-cysteine prodrugs were effective in preventing naphthalene-induced cataract and maintaining near-normal hepatic glutathione levels.