Inhibition of cell growth by combination of alpha-difluoromethylornithine and an inhibitor of spermine synthase.

The inhibitory effect on cell growth of a combination of alpha-difluoromethylornithine (DFMO) and an inhibitor of aminopropyl transferase was examined. N-(3-aminopropyl)cyclohexylamine (APCHA) and trans-4-methylcyclohexylamine (4MCHA) were used as inhibitors of spermine and spermidine synthases, respectively. Combination of DFMO and APCHA showed strong inhibitory effects on the growth of FM3A cultured cells and P388 leukemia cells in mice, compared with DFMO alone. The prolongation of survival time of P388 leukemia-bearing mice by DFMO (1,500 mg/kg) was 1.12-fold, while that by DFMO (1,500 mg/kg) plus APCHA (25 mg/kg) was 1.30-fold. The prolongation of survival time nearly paralleled the decrease of P388 leukemia cells in mice. However, the antiproliferative effect of DFMO was not strengthened by 4MCHA in the above two experimental systems. In the FM3A cell culture system, both putrescine and spermidine contents were decreased by DFMO, but spermine content did not decrease significantly. When APCHA was added to the medium with DFMO, spermine content was decreased greatly but a compensatory increase in spermidine was observed. Spermidine content in P388 leukemia cells was also decreased by DFMO, the increase in spermine was suppressed but a compensatory increase in spermidine was observed. Nevertheless, the spermidine content remained significantly low compared with the value in non-treated P388 leukemia cells. Thus, the results indicate that the antiproliferative effect of DFMO was strengthened by APCHA due to the decrease in spermine content, and that the decrease in total amount of spermidine and spermine, especially the decrease in spermine, is necessary for inhibition of cell growth.