Analysis of the Trypanocidal Action of Trivalent Arsenicals and Acriflavine

The biochemical basis of arsenical-diamidine crossresistance in African trypanosomes

Resistance to currently used drugs is a serious problem in most fields of antimicrobial chemotherapy. Crossresistance between two of the major classes of drug used in the treatment of African trypanosomiasis, the melaminophenyl arsenicals and diamidines is easily selected in the laboratory. Here, Mike Barrett and Alan Fairlamb outline the mechanism underlying this crossresistance, which appears to arise as a result of alterations in an unusual adenosine transporter involved in the uptake of these drugs.

Drug resistance in trypanosomes: selective interference with trypanocidal action

Selective reversal of the trypanocidal action of carboxylated arsenicals by p-aminobenzoic acid and of melaminyl arsenicals and diamidines by melamine has been demonstrated in vivo and in vitro. The structural specificity of these reversal phenomena is high, and suggests preferential adsorption of the antagonist during a reversible primary drug fixation stage. Thiols antagonized neutral, carboxylated and melaminyl aromatic arsenicals equally, but not diamidines; p-aminobenzoic acid antagonism is specific for carboxylated arsenicals, and melamine antagonizes only the melaminyl arsenicals and the diamidines. These reversals reflect the pattern of crossresistance behaviour and suggest that cellular structures associated with a reversible stereospecific drug adsorption phase are modified during the development of resistance.

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.

Arsenical-resistant trypanosomes lack an unusual adenosine transporter

The melaminophenyl arsenical melarsoprol is still used to treat African sleeping sickness, a disease caused by parasitic protozoa of the Trypanosoma brucei subgroup. Based on the observation that melamine antagonizes the trypanocidal activity of this class of drugs, we investigated whether other physiological compounds could compete for the same receptor. Here we report that the in vitro trypanolytic effect of melarsen oxide can be specifically abrogated by adenine, adenosine and dipyridamole, all of which compete for uptake by an adenosine transporter. Melarsen-sensitive trypanosomes have two high-affinity adenosine transport systems: a P1 type, which also transports inosine; and a P2 type, which also transports adenine and the melaminophenyl arsenicals. Melarsen-resistant trypanosomes lack P2 adenosine transport, suggesting that resistance to these arsenicals is due to loss of uptake.

Characterisation of melarsen-resistant Trypanosoma brucei brucei with respect to cross-resistance to other drugs and trypanothione metabolism

An arsenical resistant cloned line of Trypanosoma brucei brucei was derived from a parent sensitive clone by repeated selection in vivo with the pentavalent melaminophenyl arsenical, sodium melarsen. The melarsen-resistant line was tested in vivo in mice against a range of trypanocidal compounds and found to be cross-resistant to the trivalent arsenicals, melarsen oxide, melarsoprol and trimelarsen (33, 67 and 122-fold, respectively). A similar pattern of cross-resistance was found in vitro using a spectrophotometric lysis assay (greater than 200-fold resistance to melarsen oxide and greater than 20-fold resistance to both trimelarsen and melarsoprol). Both lines were equally sensitive to lysis by the lipophilic analogue phenylarsine oxide in vitro, suggesting that the melamine moiety is involved in the resistance mechanism. Although trypanothione has been reported to be the primary target for trivalent arsenical drugs [1], levels of trypanothione and glutathione were not significantly different between the resistant and sensitive lines. Statistically significant differences were found in the levels of trypanothione reductase (50% lower in the resistant clone) and dihydrolipoamide dehydrogenase (38% higher in the resistant clone). However, the Km for trypanothione disulphide, the Ki for the competitive inhibitor Mel T (the melarsen oxide adduct with trypanothione) and the pseudo-first order inactivation rates with melarsen oxide were the same for trypanothione reductase purified from both clones. The melarsen-resistant line also showed varying degrees of cross-resistance to the diamidines: stilbamidine (38-fold), berenil (31.5-fold), propamidine (5.7-fold) and pentamidine (1.5-fold). Cross-resistance correlates with the maximum interatomic distance between the amidine groups of these drugs and suggests that the diamidines and melaminophenyl arsenicals are recognised by the same transport system.

The interactions between drugs and the parasite surface

The interrelationships between drugs and parasite surfaces are considered under the headings of (a) effects on membrane transport, (b) drug uptake mechanisms and (c) effects on surface morphology and function: praziquantel is discussed under a separate heading. The range of chemotherapeutic compounds that cause permeability changes and concomitant morphological disruption is discussed in terms of mode of drug action. Interpretation of the available data renders it difficult to identify the primary mode of action in the drugs considered. Drug uptake mechanisms are known for relatively few compounds; drug resistance as a function of drug acquisition is discussed. The role of the parasite surface as a specific drug target is argued.

Trypanosoma brucei: five commonly used trypanocides assayed in vitro with a mammalian feeder layer system for cultivation of bloodstream forms

The in vitro activity of five commonly used trypanocides on bloodstream forms of Trypanosoma brucei brucei TC 221 was examined in 24-well culture plates in the presence of bovine fibroblast feeder layer cells. The minimum effective concentrations determined were as follows: Berenil 1.0 microgram/ml; Samorin 10.0 micrograms/ml; Antrycide dimethylsulfate 0.1 microgram/ml; Arsobal 0.01 microgram/ml; Naganol 1.0 microgram/ml. Contrary to values obtained with other in vitro assays, minimum effective concentrations obtained here were within the range of drug levels reached in blood, plasma, or serum of humans and animals after treatment with curative doses. The trypanocidal activity of Naganol in this assay was of particular interest, since Naganol has been hitherto considered to be inactive in vitro.

Uptake of the trypanocidal drug suramin by bloodstream forms of Trypanosoma brucei and its effect on respiration and growth rate in vivo

After a single intravenous injection of suramin the rate of removal of the drug from the plasma into other tissue compartments of the rat is independent of initial concentration. The data can be fitted to the sum of two exponential functions, consistent with a two-compartment, open model system. Trypanosomes take up only small amounts of suramin in vivo and do not actively concentrate the drug within the cell. Uptake is apparently by a non-saturable process that decreases with time and is dependent on the amount of suramin already taken up. Once within the cell, suramin progressively inhibits respiration and glycolysis, such that, for a given exposure in vivo, inhibition of oxygen consumption is proportional to the total amount of suramin absorbed. It can be calculated that only a fraction (4--9%) of this total is required to inhibit respiration to the extent found in broken cell preparations. The combined inhibition of two key enzymes in glycolysis--the sn-glycerol-3-phosphate oxidase (EC unassigned) and the glycerol-3-phosphate dehydrogenase (NAD+) (sn-glycerol-3-phosphate: NAD+ 2-oxidoreductase, EC 1.1.1.8)--are sufficient to account for the differential inhibition of glucose and oxygen consumption and of pyruvate production, together with the small, but significant, production of glycerol. Even at the highest dose of suramin tolerated by the rat, trypanosomes continue to increase exponentially in the bloodstream for at least 6 h. The mean doubling time is increased from 4.6 h to a maximum of about 12.5 h in rats treated with doses of suramin in the range 25--150 mg/kg. In the light of these and other findings, it is concluded that part of the trypanocidal action of suramin results from the inhibition of ATP production by glycolysis.

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.