Neplanocin A

The mechanism of action of the adenosine analog, neplanocin A (NPC), was investigated in human colon carcinoma cell line HT-29. Cell viability was reduced to 38 and 17% of control by 24-h exposure to 10(-5) and 10(-4) M NPC, respectively. Cytocidal activity was not affected by inhibition of adenosine deaminase with 2'-deoxycoformycin. Concomitant with decreased cell viability was the reduced incorporation of [14C]dThd and [3H]Leu, and to a lesser extent [3H]Urd, into acid-precipitable material. Labeling of rRNA and tRNA during drug treatment for 24 h with [methyl-3H]Met and [14C]Urd revealed that NPC primarily inhibited RNA methylation, and to a lesser extent, RNA synthesis. RNase T2 digests of total RNA indicated that base and 2'-O-methylation were inhibited to approximately the same degree. Metabolites of NPC were measured by reverse-phase high-performance liquid chromatography and it was found that the major drug metabolite was the drug analog of S-adenosylmethionine with little formation of the respective, S-adenosylhomocysteine metabolite. NPC was utilized to a very small degree for RNA synthesis where only 2 and 30 pmol of NPC/A260 were incorporated into rRNA and tRNA after 24-h exposure to 10(-5) and 10(-4) M NPC, respectively. These results indicate that NPC is metabolized to a metabolite of S-adenosylmethionine which is a poor methyl donor for RNA methyltransferases, and that the accompanying decrease in RNA methylation and protein synthesis appears to be related to its cytocidal activity.

Stereoselective formal synthesis of (+/-)-neplanocin A from a cyclopentane derivative employing an elegant strategy involving reiterative usage of an already existing acetonide protecting group is reported. The acetonide protecting group that is carried forward intact right from the starting adduct to an advanced intermediate is shuffled around twice as in a "relay race" through the synthetic sequence, thus avoiding unnecessary employment of additional protecting groups.

Neplanocin A, a cyclopentenyl analog of adenosine, is a naturally occurring antibiotic possessing potent inhibitory activity toward the enzyme S-adenosylhomocysteine (AdoHcy) hydrolase. In the present study, we examined whether there was a correlation between the inhibition of prokaryotic AdoHcy hydrolase and the reported antibacterial activity of neplanocin A, e.g. Alcaligenes faecalis (Yaginuma et al., J. Antibiotics 34: 359 approximately 366, 1981). Of 16 bacterial species screened, only 2 organisms (both of which contained AdoHcy hydrolase) were sensitive to 10 nM neplanocin A when grown on agar plates. None of the 16 strains showed any growth sensitivity in broth culture to concentrations of the antibiotic as high as 4 mM. However, treatment of A. faecalis in broth culture with 14 microM neplanocin A resulted in complete inhibition of cellular AdoHcy hydrolase and subsequent elevation of intracellular AdoHcy. No alternative method for degrading or removing the excess AdoHcy from these cells was detected. Bacillus subtilis, which exhibited no AdoHcy hydrolase activity showed no alteration of AdoHcy metabolism when treated with the same concentration of the antibiotic. These results indicate that inhibition of AdoHcy hydrolase is not related to the antibacterial activity of neplanocin A and suggest that using this enzyme as a target for the design of antimicrobial agents is not likely to prove a productive approach.