A 2 : 4-Diamino Pyrimidine in the Treatment of Proguanil-resistant Laboratory Malarial Strains

Absence of potentiation between quinine and pyrimethamine in infections of Plasmodium gallinaceum in chicks

The effect of combined doses of pyrimethamine and quinine has been studied in P. gallinaceum. No potentiation of the action of the drugs was observed.

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.

Of mice and malaria mutants: unravelling the genetics of drug resistance using rodent malaria models

It is well recognized that drug resistance is the most significant obstacle to gaining effective malaria control. Despite the enormous advances in the knowledge of the biochemistry and molecular biology of malaria parasites, only a few genes determining resistance to the commonly used drugs have been identified. The idea that rodent malaria parasites should be exploited more widely for such work, in view of the practical problems of studying this subject experimentally in human malaria, is presented.

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.

A study of resistance to cycloguanil in Plasmodium gallinaceum in chicks

A cycloguanil-resistant strain of Plasmodium gallinaceum was produced relatively rapidly by passage through chicks treated with low but effective doses of the drug, the dose being increased as resistance developed.

The strain was cross-resistant to proguanil but not to pyrimethamine or chloroquine.

A strain highly resistant to proguanil was resistant to cycloguanil but only slightly resistant to pyrimethamine.

A strain highly resistant to pyrimethamine was resistant to proguanil and cycloguanil.

Passage for 20 months through birds treated with doses of cycloguanil which suppressed infection for relatively long periods failed to change the sensitivity of the strain to this drug or to proguanil. Although the relatively large dose did not eradicate the infection in any of the birds, subinoculations demonstrated that parasites were absent from the blood for a period in some of the birds, though infections finally developed.

I am indebted to Parke Davis and Company for the supply of cycloguanil, to Imperial Chemical Industries Ltd. for the proguanil hydrochloride and chloroquine phosphate and to Burroughs Wellcome and Company for the pyrimethamine base.

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 chemotherapy of Plasmodium berghei. II. Antagonism of the action of drugs

The action of pyrimethamine, sulphadiazine, proguanil and its active metabolite CPT, and 2:4-diaminopteridines against infections ofPlasmodium bergheiin mice was antagonized byP–aminobenzoic acid and by pteroylglutamic acid. Antagonism was in some instances detected only whenP–aminobenzoic acid was given in solution in the drinking water as well as being injected subcutaneously. No antagonism was detected with a number of amino acids and nucleic acid derivatives.

As all of the above group of drugs can be antagonized byP–aminobenzoic acid and by pteroylglutamic acid, it would seem that they are alike in their mode of action. There must, however, be some differences between the mode of action or absorption of these drugs because species ofPlasmodiumthat are very sensitive to the action of one of these drugs are frequently not very sensitive to the action of others.

AsP. bergheiis dependent onp–aminobenzoic acid, it is suggested that it can utilize this compound in the synthesis of pteroylglutamic acid to a greater extent than canP. gallinaceum, and that it resemblesP. knowlesimore than other species ofPlasmodium.

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.