We reported recently that administration of ([(Z)-4-amino-2-butenyl]methylamino)-5'-
deoxyadenosine (
MDL 73811), an
enzyme-activated irreversible inhibitor of
S-adenosyl-L-methionine decarboxylase (AdoMetDC; EC 4.1.1.50), a key
enzyme in the synthesis of
spermidine, cures African trypanosome
infections in mice. The precise mechanism of action of
MDL 73811 was not clear because a rapid disappearance of trypanosomes from the bloodstream of treated rats occurred before significant depletion of
spermidine. Administration of
MDL 73811 to Trypanosoma brucei brucei-infected rats resulted in a 70% decrease in parasitaemia within 1 h and a complete disappearance of parasites by 5 h. The reduction in parasitaemia was accompanied by complete inhibition of AdoMetDC activity by 10 min after injection of
MDL 73811; inhibition was sustained for at least 4 h.
Polyamine levels in trypanosomes were unaffected during the first 1 h in which the marked decrease in parasitaemia was observed, but parasite
AdoMet levels increased 20-fold within this time. In contrast, exposure of cultured mammalian cells to
MDL 73811 resulted in only a 1.5-2-fold increase in
AdoMet levels over a 6 h time course. Experiments with inhibitors of
ornithine decarboxylase (ODC) also suggested that the increased
AdoMet levels might be an important factor for antitrypanosomal efficacy. Trypanosomes taken from rats treated for 36 h with
eflornithine, an inhibitor of ODC, were depleted of
putrescine and had markedly decreased
spermidine levels. These organisms also had less than 10% of control AdoMetDC activity, and had elevated decarboxy
AdoMet (greater than 4000-fold) and
AdoMet (up to 50-fold) levels. The methyl
ester of alpha-monofluromethyl-3,4-dehydro-ornithine (delta-MFMO-CH3), which cures murine T. b. brucei
infections, and the ethyl
ester analogue of this compound (delta-MFMO-C2H5), which does not cure this
infection, become ODC inhibitors upon hydrolysis and thus were tested for their effects on trypanosomal
polyamines,
AdoMet and decarboxy
AdoMet levels. Although both
esters of delta-MFMO depleted trypanosomal
polyamines,
AdoMet and decarboxy
AdoMet levels were elevated in T. b. brucei from infected mice treated with delta-MFMO-CH3 but not in parasites from mice treated with the delta-MFMO-C2H5. These data suggest that inhibition of AdoMetDC, either directly with
MDL 73811 or indirectly with inhibitors of ODC, apparently leads to a trypanosome-specific elevation of
AdoMet. It is possible that major changes in
AdoMet, rather than changes in
polyamines, may be responsible for the antitrypanosomal effects of these drugs.