Inhibitors of kynureninase

A kynureninase inhibitor selected from the group consisting of compounds having the formula:

• ##STR20## wherein X is S, PO.sub.2 H or PONH.sub.2 ;
• R.sub.A and R.sub.B, independently of one another are H, a halogen, CF.sub.3 or a small alkyl group having one to three carbon atoms;
• A is H or an acetyl group;
• R.sub.1 is H, NH.sub.2, NR.sub.6 R.sub.7, NO.sub.2, halogen, CF.sub.3 or a small alkyl group having from one to three carbon atoms, wherein:
• R.sub.6 and R.sub.7, independently of one another, are H, a formyl group or a small alkyl group having from one to three carbon atoms with the exception that only one of R.sub.6 or R.sub.7 can be a formyl group;
• R.sub.2 is OH, H, halogen, CF.sub.3 or a small alkyl group having from one to three carbon atoms;
• and
• R.sub.3, R.sub.4 and R.sub.5, independently of one another, are H, halogen, CF.sub.3, NO.sub.2, NH.sub.2 or small alkyl group having from one to three carbon atoms with the exception that the compound of the given formula is not S-(2-nitrophenyl)-L-cysteine, S-(4-nitrophenyl)-L-cysteine, S-(2,4-dinitrophenyl)-L-cysteine, S-(3,4-dinitrophenyl)-L-cysteine, S-(2,6-dinitrophenyl)-L-cysteine, S-(2-chloro-4-nitrophenyl)-L-cysteine, or an N-acetyl derivative thereof and when X=S, R.sub.1 -R.sub.5 cannot each be H or a halogen;
• when X=S, R.sub.1 cannot be CH.sub.3 or NH.sub.2 when R.sub.2 -R.sub.5 are each H;
• when X=S, R.sub.2 cannot be CH.sub.3 when R.sub.1 and R.sub.3 -R.sub.5 are each H;
• when X=S, R.sub.3 cannot be a halogen, CH.sub.3, NH.sub.2 or NO.sub.2 when X=S, R.sub.1 -R.sub.2 and R.sub.4 -R.sub.5 are each H.