Drug-resistance in Plasmodium gallinaceum , and the persistence of paludrine-resistance after mosquito transmission

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.

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.

Resistance to diamino-diphenylsulphone in Plasmodium gallinaceum

A fourfold enhancement of resistance to diamino-diphenylsulphone has been produced over a period of 15 months in strain A of P. gallinaceum by subjecting the strain, maintained in a state of patent infection, to increasing doses of the drug.

The DDS-resistant strain was cross-resistant to the minimum effective dose of sulphadiazine and slightly resistant to twice this dose, but it showed only a slight enhancement of resistance to pyrimethamine and proguanil.

The antimalarial action of DDS was antagonized by p–A.B. in the ratio of 100 to 1. DDS, in the minimum effective dose, was antagonized completely by folic acid if given in equal doses, but not by smaller doses of the antagonist; the antagonism was not competitive.

The relationship of cross-resistance between p–A.B.-inhibited sulphonamides and proguanil and pyrimethamine is discussed.

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.

Problems concerned with gametogenesis in Haemosporidiidea, with particular reference to the genus Plasmodium

Although many interesting observations have been made upon gametogenesis in Haemosporidiidea many fundamental problems await solution. Our knowledge of the nuclear changes occurring during the development of the gametes and the fertilization of the female gamete is still incomplete. Little is known of the factors which control gametocyte production, though there is evidence that in some species, i.e. Leucocytozoon simondi and Haemoproteus columbae in the duck, it is influenced by the physiological stage of the host. The type of host and the method of transmission have also been shown to affect the numbers of gametocytes formed in some species. No correlation has been found in Plasmodium, the genus in which this subject has been studied extensively, between acquired immunity and gametocyte formation. Although a broad relationship has been demonstrated in Plasmodium between the density of gametocytes in the blood and the oocyst rate and density assessed in susceptible mosquitoes, hosts heavily infected with apparently ripe gametocytes may prove non-infective to susceptible mosquitoes. In birds heavily infected with P. gallinaceum, serum taken at the peak of parasitaemia has been shown to affect adversely the infectivity of gametocytes to mosquitoes. Pamaquin and primaquine, in small doses, inhibit the development of gametocytes completely, whereas proguanil may not prevent the formation of oocysts though it inhibits their subsequent development. High densities of gametocytes have been observed in patients treated with proguanil or sulphamethazine. An actual in crease in the number of gametocytes produced, as compared with the untreated control strain, was observed in strains of P. gallinaceum in process of developing resistance to sulphadiazine, proguanil or 2:4-diamino-6:7-diisopropylpteridine as a result of treatment with those drugs.

The action of 2:4-diamino-6:7-diisopropylpteridine upon Plasmodium gallinaceum and its relation to other compounds which are pteroylglutamic acid antagonists

1. Two strains ofPlasmodium gallinaceumwere made resistant to 2:4-diamino-6:7-diisopropylpteridine (0/129) by treatment with that drug.

2. The 0/129-resistant strains were resistant to proguanil, pyrimethamine, 2:4-diamino-6:7-diphenylpteridine (0/63) and 2:4-diamino-5-(p–chlorophenoxy)-6-methylpyrimidine (48–210), but not to sulphadiazine.

3. In one strain treated with 0/129, the development of resistance to that drug itself preceded resistance to proguanil, and resistance to proguanil preceded resis tance to pyrimethamine.

4. A strain ofP. gallinaceummade resistant to 0/63 was resistant to proguanil, pyrimethamine and 0/129, but not to sulphadiazine.

5. The action of 0/129 and proguanil uponP. gallinaceumwas not antagonized byp–A.B., though in the minimum effective dose their action was antagonized by relatively large doses of P.G.A.

6. Whereas the action of sulphadiazine uponP. gallinaceumwas antagonized competitively byp–A.B., it was antagonized by P.G.A. only when the sulphadiazine was given in small doses.

The effect of sulphadiazine, proguanil and 2:4-diamino-6:7-diisopro-Pylpteridine upon gametocyte production in Plasmodium gallinaceum (Brumpt, 1935)

In strains ofPlasmodium gallinaceumpassaged by blood-inoculation, through young birds treated with small, but effective doses of proguanil, sulphadiazine or 2:4-diamino-6:7-diisopropylpteridine, the numbers of gametocytes produced were much greater than in the parent strain passaged by the same method.

The effect of the drugs upon gametocyte production appears to be a long term one, since no change in gametocyte numbers was observed in birds treated with proguanil or sulphadiazine for only a few days.

The relationship of these drugs to nuclear division and the synthesis of nucleic acids is discussed in the light of the observed increase in gametocyte-production in drug-treated strains ofP. gallinaceum.

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.

The action of 2:4-diamino-6:7-diisopropylpteridine on normal, proguanil- and sulphadiazine-resistant strains of Plasmodium gallinaceum

1. 2:4-diamino-6:7-diisopropylpteridine has an antimalarial index of 1:40, when tested againstPlasmodium gallinaceum.

2. 2:4-diamino-1′-methylindolo-(2′:3′-6:7)-pteridine showed no activity againstP. gallinaceumin doses tolerated by the chick.

3. Proguanil- and sulphadiazine-proguanil-resistant strains ofP. gallinaceumwere cross-resistant to 2:4-diamino-6:7-diphenylpteridine.

4. Proguanil- and sulphadiazine-proguanil-resistant strains ofP. gallinaceumshowed no cross-resistance to 2:4-diamino-6:7-diisopropylpteridine, but were slightly hypersensitive to this compound.

I wish to thank Dr A. Bishop for her interest in this work, and for advice and criticism. The work has been financed by the Medical Research Council.