Phthalimide analogs of CB 1954: synthesis and bioactivation

Four novel 4-substituted 5-nitrophthalimides (5-substituted-6-nitro-1,3-dih ydro-isoindol-1,3-diones), 6, 7, 10 and 11, and the known 5 are prepared as analogs of the dinitrobenzamide prodrug CB 1954, 1, and considered as pote ntial candidates for gene-directed enzyme prodrug therapy, All the phthalim ides are poor substrates for Escherichia coli nitroreductase compared to CB 1954, However, 6, 7, 10 and 11 are reduced by both the human and rat forms of DT-diaphorase; 10 is a particularly good substrate but 7 decomposes in phosphate buffer. A cell-line panel consisting of V79 cells that have been engineered to express various levels of either the human or rat forms of DT -diaphorase in an identical cellular background was used to evaluate these compounds as prodrugs activated by this enzyme. The cytotoxic effect of CB 1954 is proportional to the activity of either the rat or human enzyme but cells expressing the rat enzyme were much more sensitive (10 000-fold at hi gher levels of DT-diaphorase activity) than cells expressing comparable lev els of the human enzyme. These results demonstrate that the resistance of h uman tumors to CB 1954 can be accounted for solely by the kinetic propertie s of the enzyme for this prodrug, The nitrophthalimide analogs overcome thi s kinetic failing of CB 1954. However, these compounds are not activated to produce cytotoxicity in these DT-diaphorase-expressing cell lines. It is p ostulated their reduction products fail to undergo an acylation reaction in a manner analogous to CB 1954, Thus, reduction by DT diaphorase is not pre dictive of cytotoxicity in this class of prodrugs, [(C) 1999 Lippincott Wil liams & Wilkins].