The chemotherapy of trypanosome infections

Biochemical changes associated with alpha-difluoromethylornithine uptake and resistance in Trypanosoma brucei

Procyclic Trypanosoma brucei grown in semi-defined media are sensitive to alpha-difluoromethylornithine (DFMO) (EC50 100 microM), an inhibitor of ornithine decarboxylase (ODC), a key enzyme in polyamine biosynthesis. Organisms resistant to 5 mM DFMO (EC50 greater than 20 mM) were obtained by passage in incremental amounts of drug. Resistant and wild-type cells accumulated DFMO by passive diffusion with a consequent decrease in polyamine levels, indicating inhibition of ODC in both cell types. The resistant phenotype was stable in the absence of DFMO, in which state there was no increase in ODC abundance or activity. By kinetic analysis, the ODC of resistant cells appeared normal. In wild-type and resistant cells, [3H]DFMO equally and uniquely affinity-labelled a 50 kDa polypeptide corresponding to the ODC subunit. Levels of ODC and tubulin mRNAs were elevated 4-fold in resistant cells grown in the presence of DFMO, although there was no indication of gene amplification. The intracellular concentration of dihydrotrypanothione (N1,N8-bis(glutathionyl)-spermidine), a redox intermediate unique to kinetoplastids, was unchanged in resistant cells growing in DFMO but was halved in wild-type cells exposed to DFMO for 48 h. The exceptionally elevated levels of ornithine found in DFMO-treated resistant cells most likely play a crucial role in cell survival by maintaining intracellular concentrations of dihydrotrypanothione by competing with DFMO for ODC.

Impaired drug uptake in methotrexate resistant Crithidia fasciculata without changes in dihydrofolate reductase activity or gene amplification

Crithidia fasciculata cells grown in defined medium are sensitive to methotrexate (MTX), an inhibitor of dihydrofolate reductase (DHFR). When cells are challenged with 2-5 microM MTX, cell division ceases after 3-4 divisions and the cells become rounded and immotile for approximately 60 h, with a 40% decrease in cell viability occurring during this period. The cells then recover normal morphology and cell division resumes. Cells which undergo this treatment can be transferred directly into high levels of the drug (1-2 mM). The resistance phenotype is stable in the absence of the drug. Resistance correlates with impaired uptake of [3H]MTX, which in wild-type cells is taken up by a carrier-mediated process. There is no indication of gene amplification at the DNA level or at the level of DHFR activity, as occurs in the case of MTX-resistant Leishmania major. Several lines of MTX-resistant L. major which show gene amplification also exhibit impaired uptake of [3H]MTX.

Overproduction of a bifunctional thymidylate synthetase-dihydrofolate reductase and DNA amplification in methotrexate-resistant Leishmania tropica

Leishmania tropica promastigotes that are highly resistant to methotrexate, a dihydrofolate reductase inhibitor, have been developed. Organisms resistant to 1 mM methotrexate have a 40-fold increase in dihydrofolate reductase which is associated with thymidylate synthetase, and they contain amplified regions of DNA that may be directly visualized on stained gels of restriction digests. The amplified DNA in these organisms is about 56 kilobases in length, has a copy number about 80-fold higher than that of wild-type organisms, and constitutes about 10% of the nuclear DNA. When the methotrexate-resistant L. tropica are propagated in drug-free medium, the dihydrofolate reductase-thymidylate synthetase protein and the amplified DNA decrease in a parallel fashion until they are indistinguishable from the levels in wild-type organisms. However, when these apparent revertants are again challenged with 1 mM methotrexate, enzyme overproduction and DNA amplification occur rapidly. As in mammalian cells, it appears that drug resistance in parasitic protozoa may be mediated by gene amplification.