In order to assess the tolerance of the target enzyme spermine synthase for alpha-substituents on the aminopropyl moiety of the substrate spermidine, 1-methylspermidine (MeSpd, 2) was synthesized. It was determined that MeSpd is a poor substrate for spermine synthase and is not a substrate for spermidine N1-acetyltransferase, suggesting that alpha-methylated polyamines might be metabolically stable and therefore useful tools for studying polyamine effects in intact cells. On the basis of initial cellular results with 2, 1-methylspermine (MeSpm, 3) and 1,12-dimethylspermine (Me2Spm, 4) were also synthesized. When added to cells (L1210, SV-3T3, or HT29) depleted of both putrescine and spermidine by prior treatment with alpha-(difluoromethyl)ornithine (DFMO), these alpha-methylated polyamines were able to restore cell growth to that observed in the absence of DFMO. In accord with the enzyme data noted above, metabolic studies indicated a slow conversion of 2 to 3, but no metabolism of 4 in these cells. It was concluded from these results that the alpha-methylated polyamines are able to substitute for the natural polyamines spermidine and spermine in critical biochemical processes which involve polyamines for continued cell growth. In accord with the hypothesis, preliminary data indicate that MeSpd and Me2Spm are as effective as spermidine and spermine, respectively, in promoting the conversion of B-DNA to Z-DNA.

The abilities of the natural polyamines, spermidine and spermine, and of the synthetic analogues, 1-methylspermidine and 1,12-dimethylspermine, to reverse the effects of the S-adenosyl-L-methionine decarboxylase inhibitor 5'-([(Z)-4-aminobut-2-enyl]methylamino)-5'-deoxyadenosine (AbeAdo) on L1210-cell growth were studied. L1210 cells were exposed to AbeAdo for 12 days to induce cytostasis and then exposed to spermidine, spermine, 1-methylspermidine or 1,12-dimethylspermine in the continued presence of AbeAdo. AbeAdo-induced cytostasis was overcome by the natural polyamines, spermidine and spermine. The cytostasis was also reversed by 1-methylspermidine. 1,12-Dimethylspermine had no effect on the AbeAdo-induced cytostasis of chronically treated cells, although it was active in permitting growth of cells treated with the ornithine decarboxylase inhibitor, alpha-difluoromethylornithine. The initial 12-day exposure to AbeAdo elevated intracellular putrescine levels, depleted intracellular spermidine and spermine, and resulted in the accumulation of unmodified eukaryotic translation initiation factor 5A (eIF-5A). Exposure of these cells to exogenous spermidine, which is the natural substrate for deoxyhypusine synthase, resulted in a decrease in the unmodified eIF-5A content. 1-Methylspermidine, which was found to be a substrate of deoxyhypusine synthase in vitro, also decreased the levels of unmodified eIF-5A in the AbeAdo-treated cells. Although spermine is not a substrate of deoxyhypusine synthase, spermine was converted into spermidine in the L1210 cells, and spermine addition to AbeAdo-treated cells resulted in the appearance of both intracellular spermine and spermidine and in the decrease in unmodified eIF-5A. Exogenous 1,12-dimethylspermine, which was not metabolized to spermine or to 1-methylspermidine and was not a substrate of deoxyhypusine synthase in vitro, did not decrease levels of unmodified eIF-5A. The finding that AbeAdo-induced cytostasis was only reversed by polyamines and polyamine analogues that result in the formation of hypusine or an analogue in eIF-5A is consistent with the hypothesis [Byers, Wiest, Wechter and Pegg (1993) Biochem. J. 290, 115-121] that AbeAdo-induced cytostasis is due to the depletion of the hypusine-containing form of eIF-5A, which is secondary to the depletion of spermidine by inhibition of S-adenosyl-L-methionine decarboxylase.

The synthesis of a new class of multisubstrate adduct inhibitors of polyamine biosynthesis has been investigated. The first target compound, designed to inhibit spermidine synthase, was obtained and proved to be a very potent inhibitor of that enzyme. Two synthetic routes to effect the coupling of the polyamine spermidine to the nucleoside adenosine were studied. The first route involved a proposed Wittig or Julia olefination reaction to form the critical 5'-6' carbon-carbon bond between the nucleoside and polyamine moieties. This route failed due to a facile beta-elimination of a portion of the side chain from a carbanion intermediate during either coupling reaction. A second route involved a reductive amination approach and proved to be successful. The new inhibitor, given the trivial name adenosylspermidine, is the most potent inhibitor of spermidine synthase prepared to date.

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