The preparation of a strain of Trypanosoma rhodesiense resistant to stilbamidine and some observations on its nature

Functional expression of TcoAT1 reveals it to be a P1-type nucleoside transporter with no capacity for diminazene uptake

It has long been established that the Trypanosoma brucei TbAT1/P2 aminopurine transporter is involved in the uptake of diamidine and arsenical drugs including pentamidine, diminazene aceturate and melarsoprol. Accordingly, it was proposed that the closest Trypanosoma congolense paralogue, TcoAT1, might perform the same function in this parasite, and an apparent correlation between a Single Nucleotide Polymorphism (SNP) in that gene and diminazene tolerance was reported for the strains examined. Here, we report the functional cloning and expression of TcoAT1 and show that in fact it is the syntenic homologue of another T. brucei gene of the same Equilibrative Nucleoside Transporter (ENT) family: TbNT10. The T. congolense genome does not seem to contain a syntenic equivalent to TbAT1. Two TcoAT1 alleles, differentiated by three independent SNPs, were expressed in the T. brucei clone B48, a TbAT1-null strain that further lacks the High Affinity Pentamidine Transporter (HAPT1); TbAT1 was also expressed as a control. The TbAT1 and TcoAT1 transporters were functional and increased sensitivity to cytotoxic nucleoside analogues. However, only TbAT1 increased sensitivity to diamidines and to cymelarsan. Uptake of [³H]-diminazene was detectable only in the B48 cells expressing TbAT1 but not TcoAT1, whereas uptake of [³H]-inosine was increased by both TcoAT1 alleles but not by TbAT1. Uptake of [³H]-adenosine was increased by all three ENT genes. We conclude that TcoAT1 is a P1-type purine nucleoside transporter and the syntenic equivalent to the previously characterised TbNT10; it does not mediate diminazene uptake and is therefore unlikely to play a role in diminazene resistance in T. congolense.

Melarsoprol- and pentamidine-resistant Trypanosoma brucei rhodesiense populations and their cross-resistance

Resistance to melarsoprol and pentamidine was induced in bloodstream-form Trypanosoma brucei rhodesiense STIB 900 in vitro, and drug sensitivity was determined for melarsoprol, pentamidine and furamidine. The resistant populations were also inoculated into immunosuppressed mice to verify infectivity and to monitor whether rodent passage selects for clones with altered drug sensitivity. After proliferation in the mouse, trypanosomes were isolated and their IC(50) values to the three drugs were determined. To assess the stability of drug-induced resistance, drug pressure was ceased for 2 months and the drug sensitivity was determined again. Resistance was stable, with a few exceptions that are discussed. Drug IC(50)s indicated cross-resistance among all drugs, but to varying extents: resistance of the melarsoprol-selected and pentamidine-selected trypanosomes to pentamidine was the same, but the pentamidine-selected trypanosome population showed lower resistance to melarsoprol than the melarsoprol-selected trypanosomes. Interestingly, both resistant populations revealed the same intermediate cross-resistance to furamidine. Resistant trypanosome populations were characterised by molecular means, referring to the status of the TbAT1 gene. The melarsoprol-selected population apparently had lost TbAT1, whereas in the pentamidine-selected trypanosome population it was still present.

Loss of the high-affinity pentamidine transporter is responsible for high levels of cross-resistance between arsenical and diamidine drugs in African trypanosomes

Treatment of many infectious diseases is under threat from drug resistance. Understanding the mechanisms of resistance is as high a priority as the development of new drugs. We have investigated the basis for cross-resistance between the diamidine and melaminophenyl arsenical classes of drugs in African trypanosomes. We induced high levels of pentamidine resistance in a line without the tbat1 gene that encodes the P2 transporter previously implicated in drug uptake. We isolated independent clones that displayed very considerable cross-resistance with melarsen oxide but not phenylarsine oxide and reduced uptake of [(3)H]pentamidine. In particular, the high-affinity pentamidine transport (HAPT1) activity was absent in the pentamidine-adapted lines, whereas the low affinity pentamidine transport (LAPT1) activity was unchanged. The parental tbat1(-/-) line was sensitive to lysis by melarsen oxide, and this process was inhibited by low concentrations of pentamidine, indicating the involvement of HAPT1. This pentamidine-inhibitable lysis was absent in the adapted line KO-B48. Likewise, uptake of the fluorescent diamidine 4',6-diamidino-2-phenylindole dihydrochloride was much delayed in live KO-B48 cells and insensitive to competition with up to 10 muM pentamidine. No overexpression of the Trypanosoma brucei brucei ATP-binding cassette transporter TbMRPA could be detected in KO-B48. We also show that a laboratory line of Trypanosoma brucei gambiense, adapted to high levels of resistance for the melaminophenyl arsenical drug melarsamine hydrochloride (Cymelarsan), had similarly lost TbAT1 and HAPT1 activity while retaining LAPT1 activity. It seems therefore that selection for resistance to either pentamidine or arsenical drugs can result in a similar phenotype of reduced drug accumulation, explaining the occurrence of cross-resistance.

Mechanisms of arsenical and diamidine uptake and resistance in Trypanosoma brucei

Sleeping sickness, caused by Trypanosoma brucei spp., has become resurgent in sub-Saharan Africa. Moreover, there is an alarming increase in treatment failures with melarsoprol, the principal agent used against late-stage sleeping sickness. In T. brucei, the uptake of melarsoprol as well as diamidines is thought to be mediated by the P2 aminopurine transporter, and loss of P2 function has been implicated in resistance to these agents. The trypanosomal gene TbAT1 has been found to encode a P2-type transporter when expressed in yeast. Here we investigate the role of TbAT1 in drug uptake and drug resistance in T. brucei by genetic knockout of TbAT1. Tbat1-null trypanosomes were deficient in P2-type adenosine transport and lacked adenosine-sensitive transport of pentamidine and melaminophenyl arsenicals. However, the null mutants were only slightly resistant to melaminophenyl arsenicals and pentamidine, while resistance to other diamidines such as diminazene was more pronounced. Nevertheless, the reduction in drug sensitivity might be of clinical significance, since mice infected with tbat1-null trypanosomes could not be cured with 2 mg of melarsoprol/kg of body weight for four consecutive days, whereas mice infected with the parental line were all cured by using this protocol. Two additional pentamidine transporters, HAPT1 and LAPT1, were still present in the null mutant, and evidence is presented that HAPT1 may be responsible for the residual uptake of melaminophenyl arsenicals. High-level arsenical resistance therefore appears to involve the loss of more than one transporter.

Tracer studies of the distribution and trypanocidal action of stilbamidine in rats

[(14)C]Stilbamidine was used to study the distribution of the drug in the organs and tissues of rats following intravenous injection. The prophylactic action of stilbamidine was shown to depend upon the unchanged drug retained in tissues, especially the liver. Only the parent drug was extracted from trypanosomes when an infection was treated during the acute phase, as shown by the use of the fluorescent properties of stilbamidine in conjunction with scanning and chromatographic techniques. The action of stilbamidine on trypanosomes is therefore a direct one. A method for the synthesis of [(14)C]stilbamidine in much improved yield is also described.

Drug resistance in trapanososmes: cross-resitance analyses

Eight strains of Trypanosoma rhodesiense, made resistant respectively to atoxyl, butarsen, acriflavine, stilbamidine, Surfen C, suramin, and pontamine sky blue 5BX, have been examined for cross-resistance to representatives of nine structurally dissimilar groups of trypanocide. On the basis of their predominant ionic form at blood pH, these groups are considered in three main classes: (a) feebly ionized (neutral aromatic arsenicals), (b) ionized as cations (melaminyl arsenicals and antimonials, acridine derivatives, diguanidines and diamidines, 6-aminoquinoline and 6-aminocinnoline derivatives, phenanthridinium derivatives, triphenylmethane dyes), and (c) ionized as anions (carboxylated aromatic arsenicals and sulphonated naphthylamine derivatives). The results are discussed in relation to those of other workers and to possible modes of trypanocidal drug action. Cross-resistance behaviour is not wholly explicable on an ionic basis; the results suggest that stereospecific structural changes associated with initial drug uptake occur in resistant trypanosomes.

The trypanocidal action of homidium, quinapyramine and suramin

Homidium, quinapyramine, and suramin (Group II compounds) produce their trypanocidal effect in vivo only after a latent period of 24 hr. or more, during which time the trypanosomes may continue to multiply; this is in contrast to trivalent arsenical and diamidine compounds (Group I compounds), which begin to act immediately. Group II compounds also differ from Group I compounds in that (a) they have only a slight tendency to combine with trypanosomes, (b) they have little trypanocidal action in vitro, but (c) they make trypanosomes non-infective to fresh subinoculated mice. To explain these features it is postulated that homidium, quinapyramine, and suramin first combine in small amounts with some receptor on the trypanosome and then block some biochemical system which produces a hypothetical substance X which is needed for cell division of the trypanosome; the trypanosome is supposed to contain a preformed store of this substance X sufficient for several divisions to take place; and it is only when this store is exhausted that cell division is prevented and the trypanosome eventually dies.

The absorption of prothidium by Trypanosoma rhodesiense

When rats, heavily infected with Trypanosoma rhodesiense, were injected with Prothidium and killed 1 to 5 hours later, measurable amounts of the drug could be extracted from the parasites: a million trypanosomes have been shown to absorb 0.01 to 0.06 mug. of Prothidium in vivo. When viewed with the fluorescence microscope, treated trypanosomes appeared to concentrate Prothidium particularly in the blepharoplast and other cytoplasmic granules. Prothidium was absorbed by trypanosomes in vitro in less than 30 min. When equilibrium had been reached the concentration of the drug inside the trypanosome was approximately 400 times the concentration outside, in the range of concentrations studied.

Drug resistance in trypanosomes: effects of metabolic inhibitiors, pH and oxidation-reduction potential on normal and resistant Trypanosoma rhodesiense

A wide variety of metabolic inhibitors tested in vitro for trypanocidal activity on normal and drug-resistant strains of Trypanosoma rhodesiense showed no relation between acquired drug resistance and changes in specific enzymatic function. Oxidation-reduction potential is an important factor in trypanocidal action but is not obviously related to the development of resistance. The dependence on pH of the trypanocidal action of ionizing drugs against both normal and resistant trypanosomes supports the postulate that the development of resistance involves physical changes in cell structures associated with the uptake of drug.

Trypanosoma congolense: manifestation of resistance to Berenil and Samorin in cloned trypanosomes isolated from Zambian cattle

Four Trypanosoma congolense clones derived from a Mumbwa field isolate proved to be resistant to Berenil with a minimum curative dose (MCD) value of 40 mg/kg and to Samorin with an MCD of 4 mg/kg for mice. Two other clones, one being resistant to Berenil with an MCD of 45 mg/kg but susceptible to 1 mg/kg Samorin, and the other being resistant to Samorin with MCD of 16 mg/kg but susceptible to 7 mg/kg Berenil, were experimentally rendered resistant to each of the respective drugs they were susceptible to by subcurative treatments in mice. The original trypanosome strains and their derivative clones were then screened for their sensitivity to Berenil or Samorin. Three clones derived from the Mumbwa isolate were resistant to Berenil, with MCD's of 14 to 28 mg/kg, and to Samorin, with MCD's of 4 mg/kg. A single Mumbwa derivative clone was relatively sensitive to both Berenil with an MCD of 7 mg/kg and to Samorin with an MCD of 2 mg/kg. The reciprocal drug induction results confirmed that although trypanosomes can acquire tolerance to both Berenil and Samorin, no cross-resistance between the two was evident.

Trypanosoma congolense: drug resistance during cyclical transmissions in tsetse flies and syringe passages in mice

A drug-resistant Trypanosoma congolense strain with predetermined curative doses (CD50 and CD90) of samorin at 13.9 +/- 1.02 and 20.3 +/- 1.13 mg/kg body weight, respectively, was cyclically transmitted through tsetse flies and by syringe passages in mice in the absence of drug pressure. The changing levels of drug sensitivity were determined after every 3rd cyclic and 5th syringe passage intervals. It was noted that when the strain was maintained in tsetse flies through 12 cyclical transmissions, the CD50 and CD90 dropped slightly from 13.9 to 11.9 +/- 1.06 and from 20.3 to 18.0 +/- 1.08 mg/kg body weight, respectively. This decrease in the level of resistance was not significant (P greater than 0.05). However, when the trypanosomes were maintained by syringe passages in mice, there was a significant reduction (P less than 0.05) in the degree of resistance (CD50 from 13.9 to 11.4 +/- 1.07 and CD90 from 20.3 to 16.7 +/- 1.16 mg/kg), by the 15th syringe passage.

Flow cytofluorimetric analysis of drug accumulation by multidrug-resistant Trypanosoma brucei brucei and T. b. rhodesiense

Dual laser flow cytofluorimetry has been used to compare accumulation of compounds representing three major classes of trypanocidal drugs by drug sensitive and drug resistant clones of Trypanosoma brucei brucei and T. b. rhodesiense. Clones selected for resistance to melarsoprol were shown to be cross-resistant in vivo to two diamidines, pentamidine and Berenil, but not to suramin. At 35 degrees C, bloodstream forms of these multidrug-resistant clones accumulated lower intracellular concentrations of the diamidines 4',6-diamidino-2-phenyl-indole (DAPI) and Hoechst 33342, the phenanthridine ethidium bromide, and the acridine acriflavine than drug sensitive parasites. Accumulation of all four drugs was saturable. Drug concentrations giving half-maximal rates of accumulation were increased in the resistant clones relative to the sensitive parent clones. The rate of DAPI accumulation by both resistant and sensitive parasites was strongly temperature dependent and increased sharply above 27 degrees C. Two distinct populations were resolved in mixtures of sensitive and resistant clones exposed to DAPI. Resistant and sensitive cells accumulated identical intracellular concentrations of DAPI following brief treatment with the detergent Triton X-100. The results suggest that alterations in the surface membrane of multidrug-resistant trypanosomes reduce accumulation of several drugs relative to drug sensitive parasites.

Pentamidine transport and sensitivity in brucei-group trypanosomes

Sensitivity to pentamidine of bloodstream forms and culture forms of Trypanosoma brucei brucei, strains of this subspecies, and strains of T. brucei rhodesiense characteristically differs in vitro. Analyses of transport parameters for pentamidine uptake in these organisms show differences that correspond with drug sensitivity. Long slender bloodstream forms of T. b. brucei have a high affinity for the drug and high rates of uptake at indicated by Km and Vmax values for [3H]pentamidine transport. Although pentamidine and stilbamidine resistance is associated with dyskinetoplasty, this condition does not itself confer resistance to pentamidine nor does it affect pentamidine transport. However, drug-resistant strains show lower rates for pentamidine transport as does T. b. rhodesiense, which is characteristically less sensitive to the drug. Of all the forms and strains studied, procyclic trypomastigotes were least sensitive to pentamidine and had a remarkable ability to exclude the drug.