COMPARISON OF THE BIOLOGICAL EFFECTS OF 4 IRREVERSIBLE INHIBITORS OF ORNITHINE DECARBOXYLASE IN 2 MURINE LYMPHOCYTIC-LEUKEMIA CELL-LINES

Abstract

The effects of the enzyme-activated irreversible inhibitors of ornithine decarboxylase, .alpha.-difluoromethylornithine, .alpha.-(fluoromethyl)dehydroornithine, .alpha.-(fluoromethyl)dehydroornithine methyl ester, and (2R,5R)-6-heptyne-2,5-diamine (RR-MAP), on cell growth and parameters related to polyamine biosynthesis were compared in L5178Y and L1210 cells under identical culture conditions. The two lines are murine lymphocytic leukemia cells which differ in their ability to metabolize 5''-methylthioadenosine, the by-product of polyamine biosynthesis: L5178Y cells contain a specific 5''-methylthioadenosine phosphorylase; L1210 cells do not. In L1210 cells, the 50% inhibitory concentrations (IC50S) of the various analogues were 3.0 mM for .alpha.-difluoromethylornithine, 0.2 mM for .alpha.-(fluoromethyl)dehydroornithine, 0.1 mM for .alpha.-(fluoromethyl)dehydroornithine methyl ester, and 0.01 mM for RR-MAP. L5178Y cells were somewhat more sensitive to the inhibitors with IC50 values of 0.5 mM for .alpha.-difluoromethylornithine, 0.06 mM for .alpha.-(fluoromethyl)dehydroornithine, 0.03 mM for .alpha.(fluoromethyl)dehydroornithine methyl ester, and 0.002 mM for RR-MAP. In all cases, growth inhibition was fully prevented by exogenous putrescine. The effects of the inhibitors on parameters related to polyamine metabolism were compared at drug concentrations approximating the average of IC50 values for the two cell lines. Under these treatment conditions, polyamine pools were similarly affected by the various inhibitors. Typically, putrescine and spermidine were depleted, but effects on spermine pools differed according to the cell line, increasing slightly in L1210 cells and decreasing by about 50% in L5178Y cells. Spermine pools in L1210 cells could be reduced by RR-MAP at concentrations higher than the IC50 (i.e., 0.1 mM). Clonogenicity in soft agar was decreased about 50% by putrescine and spermidine depletion and was not further affected by spermine depletion. The inhibitors elevated S-adenosylmethinone decarboxylase activity in both cell lines with a 2-fold greater increase in L5178Y cells than in L1210 cells. Finally, the inhibitors decreased S-adenosylmethionine pools in L1210 cells by about 50% but had little effect on these pools in L5178Y cells with the exception of RR-MAP, which decreased S-adenosylmethionine pools by about 40%. Whether the different polyamine responses of the two cell lines are related to their ability to metabolize 5''-methylthioadenosine is uncertain. It is apparent, however, that the presence or absence of methylthioadenosine phosphorylase does not substantially modulate the antiproliferative activity of ornithine decarboxylase inhibitors. With respect to the new inhibitors of ornithine decarboxylase, the putrescine analogue, RR-MAP, was clearly more effective than the ornithine analogues in depleting intracellular polyamines and in inhibiting cell growth.