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

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.

Mechanisms of resistance of malaria parasites to antifolates

Antifolate antimalarial drugs interfere with folate metabolism, a pathway essential to malaria parasite survival. This class of drugs includes effective causal prophylactic and therapeutic agents, some of which act synergistically when used in combination. Unfortunately, the antifolates have proven susceptible to resistance in the malaria parasite. Resistance is caused by point mutations in dihydrofolate reductase and dihydropteroate synthase, the two key enzymes in the folate biosynthetic pathway that are targeted by the antifolates. Resistance to these drugs arises relatively rapidly in response to drug pressure and is now common worldwide. Nevertheless, antifolate drugs remain first-line agents in several sub-Saharan African countries where chloroquine resistance is widespread, at least partially because they remain the only affordable, effective alternative. New antifolate combinations that are more effective against resistant parasites are being developed and in one case, recently introduced into use. Combining these antifolates with drugs that act on different targets in the parasite should greatly enhance their effectiveness as well as deter the development of resistance. Molecular epidemiological techniques for monitoring parasite drug resistance may contribute to development of strategies for prolonging the useful therapeutic life of this important class of drugs.

Plasmodium falciparum: gene mutations and amplification of dihydrofolate reductase genes in parasites grown in vitro in presence of pyrimethamine

Samples of three pyrimethamine-sensitive clones of Plasmodium falciparum were grown for periods of 22-46 weeks in media containing stepwise increases in pyrimethamine concentrations and were seen to develop up to 1000-fold increases in resistance to the drug. With clone T9/94RC17, the dihydrofolate reductase (DHFR) gene was sequenced from 10 uncloned populations and 29 pure clones, all having increased resistance to pyrimethamine, and these sequences were compared with the sequence of the original pyrimethamine-sensitive clone. No changes in amino acid sequence were found to have occurred. Some resistant clones obtained by this method were then examined by pulsed-field gel electrophoresis, and the results indicated that there had been an increase in the size of chromosome 4. This was confirmed by hybridization of Southern blots with a chromosome 4-specific probe, the vacuolar ATPase subunit B gene, and a probe to DHFR. Dot-blotting with an oligonucleotide probe to DHFR confirmed that there had been increases up to 44-fold in copy number of the DHFR gene in the resistant strains. Resistant clones obtained by this procedure were then grown in medium lacking pyrimethamine for a period of nearly 2 years, and reversion nearly to the level of pyrimethamine sensitivity of the original clone T9/94RC17 was found to occur after about 16 months. Correspondingly, the chromosome 4 of the reverted population reverted to a size like that of the original sensitive clone T9/94RC17. The procedure of growing parasites in stepwise increases of pyrimethamine concentration was repeated with two other pyrimethamine-sensitive clones: TM4CB8-2.2.3 and G112CB1.1. (The DHFR gene of these clones encodes serine at position 108, in place of threonine as in clone T9/94RC17, and it was thought that this difference might conceivably affect the rate of mutation to asparagine at this position). Clones TM4CB8-2.2.3 and G112CB1.1 also responded by developing gradually increased resistance to pyrimethamine. However, in clone TM4CB8-2.2.3 a single mutation from Ile to Met at position 164 in the DHFR gene sequence was identified, and in clone G112CB1.1 there was a single mutation from Ala to Ser at position 16, but no mutations at position 108 were obtained in any of the clones studied here. In addition, chromosome 4 of clone TM4CB8-2.2.3 increased in size, presumably due to amplification of the DHFR gene. No increase in size was seen in clone G112CB1.1. We conclude that whereas some mutations producing changes in the amino acid sequence of the DHFR molecule may occur occasionally in clones or populations of P. falciparum grown in vitro in the presence of pyrimethamine, amplification of the DHFR gene following adaptation to growth in medium containing pyrimethamine occurs as a regular feature. The bearing of these findings on the development of pyrimethamine-resistant forms of malaria parasites in endemic areas is discussed.

Mechanisms of drug resistance in malaria

Plasmodium falciparum causes the most severe form of human malaria which directly results in over two million deaths per year. As there is not yet a useful vaccine against this disease the major form of treatment and control is the use of chemotherapeutic agents. Unfortunately the parasite has managed to devise mechanisms that allow it to evade the action of almost all the antimalarials in our arsenal. The antifolate drugs include the dihydrofolate inhibitors pyrimethamine and proguanil as well as the sulfones and sulfonamides. These antimalarials act on enzymes in the folate pathway. The mechanism of resistance to these compounds involve mutations in the target enzyme that decrease the affinity of binding of the drug. A second major group of antimalarials include the quinine-like compounds. Quinine was one of the first compounds used to treat malaria and the related drug chloroquine is the most important antimalarial. Mefloquine and halofantrine were developed in response to major problems with the spread of chloroquine resistance. Chloroquine resistance is due to the ability of the parasite to decrease the accumulation of the drug in the cell. The exact mechanism that allows this is still under investigation although at least one protein has been identified that affects the accumulation of this important antimalarial.

The mode of action and the mechanism of resistance to antimalarial drugs

The mechanism of action of the antifolate and quinoline antimalarials has been investigated over the last few decades, and recent advances should aid the development of new drugs to combat the increasingly refractile parasite. The molecular description of resistance to the antifolates has been well characterised and is due to structural changes in the target enzymes, but the factors involved in the parasite's ability to circumvent the action of the quinoline antimalarials have yet to be fully elucidated. This review discusses the mode of action of these drugs and the means used by the parasite to defeat our therapeutic ingenuity.

The resistance of falciparum malaria in Africa to 4-aminoquinolines and antifolates

Falciparum malaria cannot be eradicated from sub-Saharan Africa with present technology. The mainstay of malaria control in this situation is treatment of fever cases with chloroquine, aiming principally at reduction of mortality. The efficacy of this policy is now endangered because of the appearance and spread of chloroquine-resistance on the African continent. The present review examines laboratory and field research on the resistance of African P.falciparum to chloroquine, amodiaquine, pyrimethamine, proguanil, chlorproguanil and the combination sulfadoxine-pyrimethamine. Drug-resistance in malaria may be assessed with in vivo and in vitro technology. In vivo tests are simple, but the results are difficult to compare because of the influence of immunity. In vitro tests provide a more precise epidemiological tool, but their analysis should be undertaken with consideration of their technical limitations. For parasitological, immunological and epidemiological reasons, a one-to-one correlation between in vivo and in vivo grading of resistance is usually not found. Extended in vivo tests may be at least as sensitive as in vitro tests for detecting rare resistant parasites. On the other hand, the standardized grading of higher levels of in vivo resistance is arbitrary, and it is doubtful, whether such distinction has any clinical relevance. The 4-aminoquinolines (chloroquine and amodiaquine) presumably act by interfering with vital functions in the acid vesicles of parasites. Recent experiments indicate that resistance may be related to an increased rate of efflux of chloroquine from the parasite. It is caused by mutation, and at least three genetic levels of resistance have been identified. The blood stages of resistant plasmodia seem to have a biological advantage over sensitive ones, an observation that raises some hitherto unanswered questions. In the 1970s, a low degree of resistance to chloroquine was found in African P. falciparum in several localities. Resistance to the standard dose of chloroquine of 25 mg/kg was found in 1978 in tourists, who had sojourned in Kenya and Tanzania. Since then, chloroquine-resistance has spread centrifugally with increasing rapidity from an original focus in Northern Tanzania or Southern Kenya. The rate of increase in the proportion of resistant infections has generally been more rapid in the areas, where resistance has been introduced recently than in the original epifocus. The rate of increase is also generally more rapid in urban than in rural areas, an observation that can be ascribed to differences in drug pressure.(ABSTRACT TRUNCATED AT 400 WORDS)

Chemotherapy and drug resistance in malaria

Over recent years many antimalarial drugs have been rendered useless by the development of resistance by the malaria parasite. New antimalarials are rapidly suffering the same fate as the traditional therapies and yet a biological understanding of the mechanisms of resistance has, until recently, not been described. This review describes recent work which has identified the mechanism of resistance to the dihydrofolate reductase (DHFR) inhibitors as being due to point mutations within the DHFR gene that render the enzyme less susceptible to inhibition by the drugs. The relationship between chloroquine resistance and the recently described multidrug resistance gene is explored and the possibility that this is the main cause of chloroquine resistance by the parasite is discussed. Parasites have developed resistance against many of the quinine-like antimalarials over the past three decades and the possibility that this is linked to the appearance of chloroquine resistance must be considered.

The dihydrofolate reductase-thymidylate synthetase gene in the drug resistance of malaria parasites

Resistance to antifolate drugs such as pyrimethamine is widespread among malaria parasites of the most pathogenic species Plasmodium falciparum. These drugs inhibit the dihydrofolate reductase activity of the dihydrofolate reductase-thymidylate synthetase (DHFR-TS) bifunctional enzyme. This review examines work done to characterize the enzyme, the cloning of plasmodial DHFR-TS genes, chromosomal mapping studies of these genes by pulsed-field gel electrophoresis, and the structural insights into the mechanism of drug resistance that have been gained by comparing genes from drug-sensitive parasites with those from drug-resistant strains that have arisen in the field or after experimental induction.

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.

Drug-resistance transfer among rodent plasmodia

A study on transfer of drug resistance between species of malaria parasites was carried out in mixed infections ofP. bergheiandP. vinckei. The biological characters which served as contrasting markers for separation of the two species were the inability of the ‘old’ blood-passaged strain ofP. vinckeito develop in suitable mosquito vectors and its failure to produce parasitaemia in the adult golden hamster. Facility of sporogonic development and cyclical transmission and a pronounced virulence for the adult hamster marked the N.K. 65 strain ofP. berghei.

Development of a maximal and stable resistance to 200 mg/kg pyrimethamine in theP. vinckeistrain was achieved by a gradual increase in drug level in successive blood transfers.

Mixed, simultaneous infections of pyrimethamine-resistantP. vinckeiand sensitiveP. bergheiwere experimentally induced in white mice. Subinoculations of the mixed infection at the height of parasitaemia was carried out in adult hamsters. Suppression of theP. vinckeiinfection in the hamster by innate resistance and the reappearance ofP. bergheiin the blood was noted. Tests with the maximal concentration of pyrimethamine (200 mg/kg) showed the acquisition of drug resistance in some of theP. bergheiinfected hamsters. Persistence of the acquired resistance was demonstrated following fifteen successive blood transfers in the absence of drug pressure and after five cyclical transmissions.

Parallel control experiments with pyrimethamine-sensitiveP. vinckeiandP. bergheiin mixed infections failed to reveal any enhancement in drug resistance. No spontaneous drug-resistant mutant was found in the sensitiveP. bergheiafter exposure to a single course of treatment with the drug, and persistence of parasites in some treated animals was considered as individual variations in host reactions to the drug. This resistance never persisted after blood transfer or cyclical transmission. No loss in drug resistance had been found following transfer of the resistant R strain from mouse to hamster. A marked difference in the course of the plasmodial infection and in the length of survival of mice inoculated with the drug-resistantP. bergheiwas noted following treatment with pyrimethamine in various concentrations. It is suggested that these differences represent a selective action of the drug on a heterogenicP. bergheipopulation, consisting of drug-resistant and drug-sensitive parasites in the blood of the host.

Injection of sonified material from pyrimethamine-resistantP. vinckeiandP. bergheiinto mice inoculated with a drug-sensitiveP. bergheifailed to induce enhancement of resistance to the drug.

The phenomenon of drug-resistance transfer observed under experimental conditions in rodent malaria is viewed in the light of similar phenomena in micro-organisms.

We gratefully acknowledge the assistance rendered by Mr Jerold Sklarsh.

This paper is Contribution no. N 408 from the Army Research Programme on Malaria. The work was carried out under the sponsorship of the Commission on Malaria, Armed Forces Epidemiological Board, and supported in part by the United States Office of the Surgeon General, Department of the Army.

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.

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.