Mechanism-based Design . of Inosine 5'-Monophosphate Dehydrogenase Inhibitors: Synthesis and Biological Activities of 5-Ethynyi-1- -D-ribofuranosylimidazole-4-carboxamide (EICAR) and its Derivatives

Inosine 5'-monophosphate dehydrogenase (IMPDH) catalyzes the nicotinamide adenine dinucleotide (NAD)-dependent oxidation of inosine 5'-monophosphate (IMP) to xanthosine 5'-monophosphate (XMP), and is one of the key rate-determining enzymes of de novo guanine nucleotide biosynthesis in mammalian systems. Based on the proposed catalytic mechanism of IMPDH, we designed and synthesized 5-ethynyi-1- ­ D-ribofuranosylimidazole-4-carboxamide (Sb, EICAR) from 5-amino-1- -D-ribofuranosylimida­ zole-4-carboxamide (AICAR) via palladium chemistry. EICAR is a potent cytostatic agent that inhibits various tumor cells in culture including human solid tumor cells in vitro and in vivo. EICAR also showed broad-spectrum antiviral activities, about 10- to 100-fold greater than those of ribavirin. An examination of the structure-activity relationships revealed that an alkynyl group, especially an ethynyl group at the 5-position, is important for its activity due to the inhibition of IMPDH. The mode of action of EICAR is also discussed.