New Antitumor Imidazole Derivative, 5-Carbamoyl-1H-imidazol-4-yl Piperonylate, as an Inhibitor of Purine Synthesis and Its Activation by Adenine Phosphoribosyltransferase

Abstract The mechanism of action of 5-carbamoyl-1 H -imidazol-4-yl piperonylate (SL-1250), which has a broad antitumor spectrum, was examined by in vitro cell culture and enzymatic studies. In the serum-containing culture medium, SL-1250 was rapidly deacylated to 4-carbamoylimidazolium 5-olate (SM-108). Thus, SL-1250 might be acting on the cells in the form of SM-108. The growth of L5178Y cells was completely inhibited by 10 -5 m SL-1250. It should be noted that this growth inhibition was significantly reversed in the presence of equimolar concentrations of guanine, guanosine, or guanosine 5′-monophosphate to those of SL-1250. However, hypoxanthine and xanthine were not effective. These effects of purine addition were observed to be quite similar in growth inhibition by SM-108. It was found that inosine 5′-monophosphate dehydrogenase (EC 1.2.1.14; IMP dehydrogenase), a key enzyme of de novo purine synthesis, from Ehrlich carcinoma cells was inhibited by SM-108 only when 5-phospho-α-d-ribose 1-diphosphate (PRPP) and MgCl 2 coexisted with SM-108. In contrast, a chemically synthetic ribonucleotide of SM-108 inhibited IMP dehydrogenase without PRPP and MgCl 2 , and the mode of inhibition was competitive with the K i value of 2 × 10 -8 m. On the other hand, the inhibition of either growth of L5178Y cells or IMP dehydrogenase in the presence of PRPP and MgCl 2 by these compounds was reversed by adenine. A nucleotide of SM-108 was chromatographically identified when [ 14 C]SM-108 was incubated in the enzyme solution with PRPP and MgCl 2 . This conversion by enzyme was also inhibited by adenine. Viewed together, these results strongly suggest that SL-1250 is, after being converted to SM-108, activated to its nucleotide form by adenine phosphoribosyltransferase (EC 2.4.2.7) and that this SM-108 nucleotide blocks de novo synthesis of guanosine 5′-monophosphate by inhibiting IMP dehydrogenase.