Acquired Paludrine-Resistance inPlasmodium Gallinaceum

Malaria parasite evolution in a test tube

Experimental evolution studies reveal drug targets and resistance mechanisms

Cross-resistance between sulphanilamide and paludrine (proguanil) in a strain of Plasmodium gallinaceum resistant to sulphanilamide

1. Resistance to sulphanilamide, of a low order, has been developed in a strain of Plasmodium gallinaceum by the treatment of patent infections with the drug.

2. The sulphanilamide-resistant strain was resistant to paludrine over the whole range of doses tolerated by the host; it also proved to be resistant to sulphathiazole, sulphapyridine and sulpha-diazine, though the degree of resistance shown was much slighter than to paludrine.

3. Resistance to paludrine developed in the sulphanilamide-resistant strain before any resistance to sulphadiazine could be detected.

Failure to produce resistance to chloroquine in Plasmodium gallinaceum in chicks

A strain of Plasmodium gallinaceum has been treated for 16 months with chloroquine, under conditions which previously have produced strains resistant to proguanil, sulphonamides and pamaquin, but no change in reaction to the drug has occurred.

An analysis of the development of resistance to metachloridine in clones of Plasmodium gallinaceum

1. The development of resistance to metachloridine (3-metanilamido-5-chloropyrimidine) was studied in two clones of Plasmodium gallinaceum derived from single erythrocytic parasites and maintained by serial inoculation in young chicks. Resistance developed with equal facility and similarly in the two clones.

2. In thirteen strains of these clones an enhancement of resistance was obtained after three to five courses of treatment, each of seven doses of metachloridine over a period of 3½ days.

3. The effect of the size of the inoculum (2·5 × 107–109 parasites), and of the dose of drug upon the rate of development of resistance was studied. Although the number of courses of treatment required to produce an enhancement of resistance was not always related to the size of the inoculum, with the largest inoculum an enhancement of resistance was observed after the minimum number of courses of treatment (three), whereas with the smallest inoculum no enhancement of resistance was obtained. The rate of the development of resistance did not appear to be related to the size of the dose of drug.

4. In some strains the increase in resistance was sudden, whereas in others it was more gradual. Resistance was retained when parasites of a newly resistant strain were transmitted through Aëdes aegypti or maintained for 43 days in the absence of the drug.

5. A comparison of the development of resistance in populations of normal parasites, and of populations composed of mixtures of known numbers of resistant and normal parasites, indicated that the pattern of the development of resistance in normal populations could be explained by the selections of mutations of a frequency of less than 1 in 5 × 107, or probably less than 1 in 109 parasites.

An analysis of the development of resistance to proguanil and pyrimethamine in Plasmodium gallinaceum

1. The development of resistance to proguanil and pyrimethamine was studied in a clone of Plasmodium gallinaceum (strain A) derived from a single erythrocytic parasite and maintained by a standard method of serial inoculations in young chicks.

2. The rate of the development of resistance to 0·05 mg./20 g. doses of proguanil was compared in strains maintained by inocula of 5 × 107, 108 and 109 parasites. Some evidence was obtained that the rate of the development of resistance to the drug was related to the numbers of parasites in the inocula.

3. No correlation was observed between the rate of the development of resistance to pyrimethamine and the size of the population of parasites exposed to its action in strains treated with 0·001 g./20 g. doses of the drug and maintained by inocula of 5 × 107 parasites or 109 parasites; but a greater variability was observed in the rate in the strains maintained by the small inocula than in the strains maintained by the large inocula. Resistance appeared to develop sporadically.

4. Some evidence was obtained that strains treated with a high dose of pyrimethamine (0·05 mg./20 g.) might become more sensitive to the drug as a result of treatment. In three experiments the infections were eradicated by the second or third course of treatment with the drug. However, strains which withstood this dose, developed resistance as rapidly, or more rapidly than those treated with 0·001 mg./20 g. doses. There was therefore no evidence from these experiments with pyrimethamine that resistance to the drug could be produced more readily with small than with large doses of the drug.

5. Except for slight variations, resistance to proguanil developed concurrently with resistance to pyrimethamine in strains treated with that drug.

6. In many of the strains, whether treated with proguanil or with pyrimethamine, resistance appeared to develop quite suddenly, but in others it developed more gradually. The pattern of the development of resistance in strains treated with either of these drugs could be explained by the selection of spontaneously occurring mutants, though some of the data suggested that the mutants differed in degree of resistance to the drug.

7. The rate of development of resistance to pyrimethamine was studied in strains derived from primary inocula composed of known numbers of pyrimethamine resistant parasites and 5 × 107 parasites of the untreated strain A. The addition of at least 106 resistant parasites to the inoculum was required to produce an increase in parasitaemia during the subsequent course of treatment with the drug, but the addition of 102 resistant parasites or, in some experiments, fewer than 102 resistant parasites to the primary inoculum was sufficient to produce a heavy parasitaemia during the second serial course of treatment if the strain was maintained by the standard method (p. 496). When these results are compared with the pattern of the development of resistance to pyrimethamine in the strains treated with that drug, it can be concluded that only in one strain was there any evidence that resistant parasites were present in the primary inoculum when it consisted only of parasites of the untreated parent strain. The rate of development of resistance in many of the strains treated with either proguanil or pyrimethamine suggested a mutation rate of a low frequency.

Latent infections in avian malaria in relation to the production of drug-resistance

1. No resistance to paludrine or to sulphadiazine was obtained after treating latent infections ofPlasmodium gallinaceumin chickens with twice daily doses of 20 mg./20 g. of sulphadiazine over periods of 171, 178 and 190 days.

2. No resistance to paludrine was obtained after treating a latent infection ofP. relictumin a canary over a period of 1 year with doses of paludrine increasing from 0·05 mg./20 g. once daily to 1·0 mg./ 20 g. twice daily.

3. It is considered that if drug-resistance arises by mutation and selection, then resistance should arise more readily when a large number of rapidly multiplying parasites is treated with a drug than when the population treated is small, with a low reproduction rate, i.e. the failure to obtain resistant strains of malaria by prolonged treatment of latent infections with large amounts of drug, lends support to the theory of the origin of resistant strains of malaria by the selection of resistant mutants.

Pamaquin Resistance in a Strain of Plasmodium gallinaceum and its relationship to Other antimalarial Drugs

1. A four- to eight-fold increase in resistance to pamaquin has been developed in a strain of Plasmodium gallinaceum in chicks.

2. Pamaquin resistance conferred no resistance to proguanil, sulphadiazine, mepacrine, chloroquine or sontochin, but it conferred some resistance to pentaquine and to quinine.

3. An appreciable loss in resistance to pamaquin was observed in the pamaquin-resistant strain after it had been maintained in the absence of the drug, in a patent state of infection, for a period of 6 months.

4. No synergism was observed between pamaquin and quinine when these drugs were tested, in combined doses, upon active infections of P. gallinaceum.

The stability of paludrine-resistance inPlasmodium gallinaceumin the absence of the drug

Acquired resistance to paludrine inPlasmodium gallinaceumwas retained undiminished after passage at frequent intervals through untreated chicks for more than 1 year.

Sulphadiazine-resistance in Plasmodium gallinaceum and its relation to other antimalarial compounds

1. A thirty-two-fold increase in resistance to sulphadiazine has been induced in Plasmodium gallinaceum in chicks by treatment with that drug.

2. No loss in resistance to sulphadiazine occurred in the resistant strain during cyclical passage through Aëdes aegypti.

3. The sulphadiazine-resistant strain was resistant also to sulphathiazole, sulphanilamide and sulphapyridine, but not to mepacrine, quinine or pamaquin. An increase in sensitivity to pamaquin was observed.

4. The sulphadiazine-resistant strain was resistant to paludrine and its methyl homologue M 4430.

5. In strains maintained in a state of acute infection and treated with sulphadiazine, resistance to paludrine developed more rapidly than resistance to sulphadiazine, and in one strain a high degree of resistance to paludrine was obtained before any increase in resistance to sulphadiazine could be detected.

6. Resistance to paludrine as induced by sulphadiazine, develops rapidly and extends at once over the full range of doses which the chick will tolerate, whereas resistance to paludrine as induced by that drug itself, develops more slowly and by stages.

7. Whereas resistance to paludrine is induced readily by treatment with sulphadiazine, resistance to sulphadiazine is induced by paludrine only after treatment with high doses of the drug for a prolonged period.

8. In latent infections of Plasmodium gallinaceum resistance to paludrine or sulphadiazine was not induced by sulphadiazine during the period of the experiment (49–75 days), though the aggregate dosage of drug used was much greater than that with which resistance was induced in strains maintained in an acute state.

9. It is not considered probable that cross-resistance between sulphadiazine and paludrine is due to a similar mode of action of these drugs, as whereas sulphadiazine is antagonized by p-amino-benzoic acid paludrine is not.

The inhibition by p-aminobenzoic acid of the development of paludrine-resistance as produced by sulphonamides in plasmodium gallinaceum

A strain of Plasmodium gallinaceum has been treated for 10 weeks with sulphadiazine (5 mg./20 g.) in the presence of p-aminobenzoic acid in doses (0·5 mg./ 20 g.) inhibiting the sulphadiazine. No resistance to paludrine or to sulphadiazine has developed.

The genetic basis of diversity in malaria parasites

The principal findings of the P. falciparum surveys are given below. Considerable diversity of enzymes, antigens, drug sensitivity and other characters is seen among P. falciparum isolates. Cloning studies show that certain isolates contain mixtures of parasites which may be diverse in one or more of these characters. No obvious regional distribution is seen in the enzymic and antigenic characters examined, although differences in the frequencies of certain enzymes appear to exist. Variations in drug sensitivity are seen among parasites from different regions, the occurrence of resistant forms usually being correlated with the extent of use of the drug concerned.

Resistance to primaquine in Plasmodium gallinaceum, and the problem of resistance to quinoline compounds in malaria parasites

A tenfold enhancement of resistance to primaquine was obtained by maintaining a strain of Plasmodium gallinaceum in a state of acute infection by serial passage of infected blood through young chicks treated with gradually increasing doses of the drug.

No loss in resistance to primaquine was observed when the resistant strain was transmitted through mosquitoes, though there was some loss in resistance to the maximum tolerated dose, but not to lower doses, when the strain was maintained in a state of acute infection through untreated chicks for 41 weeks.

The primaquine-resistant strain was cross-resistant to lower effective doses of pamaquin, and slightly less sensitive to quinine than the parent strain but the loss in sensitivity to chloroquine was only marginal. Sensitivity to proguanil, dihydrotriazine and pyrimethamine was normal.

Attempts were made to produce a chloroquine-resistant strain of P. gallinaceum using different doses of the drug but no change in sensitivity was observed though the experiments were continued for more than a year. An attempt to produce a chloroquine-resistant strain by treatment of a proguanil-resistant strain with chloroquine also failed.

The problem of resistance to quinoline compounds in different species of Plasmodium is discussed.

The chemotherapy of Plasmodium berghei. I. Resistance to drugs

1. Strains of P. berghei resistant to sulphadiazine, pyrimethamine, and methylene blue were produced by treating acute infections with low doses of drug.

2. The strain resistant to methylene blue was sensitive to pamaquin, mepacrine, sulphadiazine, proguanil, pyrimethamine, and 2 : 4-diamino-6 : 7-camphano-pteridine.

3. The pyrimethamine-resistant strain was cross-resistant to proguanil and its active metabolite CPT, 2 : 4-diamino-6 : 7-camphanopteridine, 2 : 4-diamino-6 : 7-(l′-ethylindolo)-pteridine, and 2 : 4-diamino-5-p-chlorophenylpyrimidine.

4. The sulphadiazine-resistant strain was cross-resistant to pyrimethamine, sulphanilamide, proguanil and its active metabolite CPT, 2 : 4-diamino-6 : 7-dinhexylpteridine, and 2 : 4-diamino-6 : 7-diisopropylpteridine. It was as sensitive as the parent strain to quinine, mepacrine, chloroquin, pamaquin, methylene blue, and M 3349.

5. The action of sulphadiazine against the sulphadiazine-resistant strain was inhibited by the same doses of p-aminobenzoic acid and folic acid as were required with the parent strain, although the dose of sulphadiazine was increased 30-fold.

The development of resistance to metachloridine in Plasmodium gallinaceum in chicks

1. An increase in resistance to metachloridine of more than 100-fold was obtained within a few weeks in a strain ofPlasmodium gallinaceumtreated with gradually increasing doses of the drug and maintained in young chicks by blood-inoculation at intervals of 2–3 days.

2. There was no evidence that the rapid development of resistance arose by the selection of highly resistant individuals present in the normal population.

3. Two strains ofP. gallinaceumpassaged through chicks treated with 0·025 mg. doses of the drug gradually became resistant to greater concentrations than that to which they had been exposed, though their growth rate decreased when they were inoculated into birds receiving higher doses of the drug.

4. In both strains maintained in birds treated with 0·025 mg. doses of the drug, resistance reached a maximum beyond which it did not increase.

5. Cross-resistance tests failed to show any relationship in mode of action between meta-chloridine and pamaquin, mepacrine, quinine or chloroquine. A strain ofP. gallinaceum, highly resistant to metachloridine, showed slight resistance to sulphadiazine, sulphapyridine and sulphathiazole, but none to sulphanilamide or proguanil.

We are indebted to the Cyanamid Products Ltd., London, for the gift of the Folvite used in these experiments.