N-substituted 2-(2,6-dinitrophenylamino)propanamides: Novel prodrugs that release a primary amine via nitroreduction and intramolecular cyclization

A series of N-dinitrophenylamino acid amides [(4-CONHZ-2,6-diNO(2)Ph)N(R)C( X,Y)CONHPhOMe] were prepared as potential bioreductive prodrugs and reduced radiolytically to study their rates of subsequent intramolecular cyclizati on. Compounds bearing a free NH group (R = H) underwent rapid cyclization i n neutral aqueous buffers (t(1/2) < 1 min) following 4-electron reduction, with the generation of a N-hydroxydihydroquinoxalinone and concomitant rele ase of 4-methoxyaniline. Amine release from analogous N-methyl analogues (R = Me) was relatively slow. These results are consistent-with intramolecula r cyclization of a monohydroxylamine intermediate. The high rates of cycliz ation/extrusion by these very electron-deficient hydroxylamines suggest tha t the process is greatly accelerated by the presence of an H-bonding "confo rmational lock" between the anilino NH group and the adjacent o-nitro group (Kirk and Cohen, 1972). Changes in the phenylcarboxamide side chain or in C-methylation in the linking chain had little effect-on the rate of cycliza tion. The model compounds had 1-electron reduction potentials in the range appropriate for cellular reduction (-373 mV for a measured example) and app eared suitable for development as prodrugs that release amine-based effecte rs following enzymic or radiolytic reduction. Prodrug examples containing 4 -aminoaniline mustard and 5-amino-1-(chloromethyl)benz[e]indoline alkylatin g units were evaluated but were not activated efficiently by cellular nitro reductases. However, cell killing by the radiation-induced reduction of the latter prodrug was demonstrated.