The chemotherapy of rodent malaria, X

Antimalarial activity of the myxobacterial macrolide chlorotonil a

Myxobacteria are Gram-negative soil-dwelling bacteria belonging to the phylum Proteobacteria. They are a rich source of promising compounds for clinical application, such as epothilones for cancer therapy and several new antibiotics. In the course of a bioactivity screening program of secondary metabolites produced by Sorangium cellulosum strains, the macrolide chlorotonil A was found to exhibit promising antimalarial activity. Subsequently, we evaluated chlorotonil A against Plasmodium falciparum laboratory strains and clinical isolates from Gabon. Chlorotonil A was highly active, with a 50% inhibitory concentration between 4 and 32 nM; additionally, no correlations between the activities of chlorotonil A and artesunate (rho, 0.208) or chloroquine (rho, -0.046) were observed. Per os treatment of Plasmodium berghei-infected mice with four doses of as little as 36 mg of chlorotonil A per kg of body weight led to the suppression of parasitemia with no obvious signs of toxicity. Chlorotonil A acts against all stages of intraerythrocytic parasite development, including ring-stage parasites and stage IV to V gametocytes, and it requires only a very short exposure to the parasite to exert its antimalarial action. Conclusively, chlorotonil A has an exceptional and unprecedented profile of action and represents an urgently required novel antimalarial chemical scaffold. Therefore, we propose it as a lead structure for further development as an antimalarial chemotherapeutic.

The chemotherapy of rodent malaria. LXII. Drug combinations to impede the selection of drug resistance, part 5: rates of development of resistance to some inhibitors of folate metabolism and to artesunate

In recent years infection with chloroquine-resistant Plasmodium falciparum has been combatted with two long-acting antimalarials, pyrimethamine and sulfadoxine, in the combination known as Fansidar that exerts a strong, synergistic action on the asexual stages of the parasite. This second-line regimen, however, is failing increasingly because of the selection of resistant clones in endemic areas, and effective, safe, alternative drugs or drug combinations that are also affordable are urgently needed. Antimalarial drugs with shorter half-lives than those of pyrimethamine or sulfadoxine are likely to be slower to select resistant parasites. In the experiments reported here, the baseline in-vivo responses of rodent malarial parasites to chlorproguanil and proguanil and their active metabolites, chlorcycloguanil and cycloguanil, as well as those to dapsone and artesunate, were explored. In general, the most sensitive parasite to all of these compounds was P. chabaudi. When the drugs were used, individually, to select resistance via the '2%-relapse technique', relatively stable resistance to each was obtained in P. chabaudi as well as in P. berghei and P. yoelii ssp. NS, the last of these being also highly resistant to chloroquine. Of most concern was the rapidity and high level of resistance developed by P. chabaudi to artesunate. The experiments also validated the use of chlorcycloguanil or cycloguanil as surrogates for chlorproguanil or proguanil. Further studies to investigate the possible value of administering chlorproguanil-dapsone, with or without artesunate, are under way and will be reported separately.

The chemotherapy of rodent malaria. LXIII. Drug combinations to impede the selection of drug resistance, part 6: the potential value of chlorproguanil and dapsone in combination, and with the addition of artesunate

Resistance is readily produced in rodent malaria using the single-dose, '2%-relapse technique' (2%RT) against the individual compounds chlorproguanil (CPG), chlorcycloguanil (CCG), cycloguanil, dapsone (DDS) and artesunate (ASN). Using the '4-day test', a low level of synergism or a simple additional action between CPG and DDS was observed with multiple dosing of these two compounds in a combination. Resistance to a 1 : 3 combination of CPG-DDS was selected in each of three parasite lines: Plasmodium berghei NK65, P. yoelii ssp. NS and P. chabaudi AS. Of these lines, P. chabaudi AS was found to be the most sensitive to the 1 : 3 combination in the 2%RT (and was also previously found to be the most sensitive when the compounds were used individually). Plasmodium chabaudi AS was also the line found most sensitive to a 7 : 21 : 300 combination of CPG-DDS-ASN (CDA). In mice infected with P. chabaudi AS, compared with the use of the individual components, the CPG-DDS combination only a gave a modest level of protection (as indicated by the increase in the time required to select resistance in the 2%RT) but the triple CDA combination was totally effective over the duration of the experiment. New pharmacokinetic data to be reported elsewhere indicate, however, that the antimalarial action of CPG in mice is exerted by a mechanism that is not associated with the drug's conversion to the antifolate triazine, CCG. The question thus arises as to how, in the present model, the protective action of CDA was effected. The present results nevertheless reinforce the hypothesis that a CDA combination, appropriately proportioned for human use, should be of practical value, in protecting the individual components, when used for the treatment of multidrug-resistant P. falciparum, and possibly other Plasmodium species, in endemic areas. Clinical trials, both with a CPG-DDS combination (Lapdap) and CDA, are currently under way in tropical Africa. Further studies are now required to determine whether DDS, CPG or an as-yet unidentified metabolite of CPG interact with ASN, and whether a simple double combination of ASN with one or other of these would be as protective, against the selection of resistance, as CDA.

The antimalarial ferroquine: role of the metal and intramolecular hydrogen bond in activity and resistance

Inhibition of hemozoin biocrystallization is considered the main mechanism of action of 4-aminoquinoline antimalarials including chloroquine (CQ) but cannot fully explain the activity of ferroquine (FQ) which has been related to redox properties and intramolecular hydrogen bonding. Analogues of FQ, methylferroquine (Me-FQ), ruthenoquine (RQ), and methylruthenoquine (Me-RQ), were prepared. Combination of physicochemical and molecular modeling methods showed that FQ and RQ favor intramolecular hydrogen bonding between the 4-aminoquinoline NH group and the terminal amino group in the absence of water, suggesting that this structure may enhance its passage through the membrane. This was further supported by the use of Me-FQ and Me-RQ where the intramolecular hydrogen bond cannot be formed. Docking studies suggest that FQ can interact specifically with the {0,0,1} and {1,0,0} faces of hemozoin, blocking crystal growth. With respect to the structure-activity relationship, the antimalarial activity on 15 different P. falciparum strains showed that the activity of FQ and RQ were correlated with each other but not with CQ, confirming lack of cross resistance. Conversely, Me-FQ and Me-RQ showed significant cross-resistance with CQ. Mutations or copy number of pfcrt, pfmrp, pfmdr1, pfmdr2, or pfnhe-1 did not exhibit significant correlations with the IC(50) of FQ or RQ. We next showed that FQ and Me-FQ were able to generate hydroxyl radicals, whereas RQ and me-RQ did not. Ultrastructural studies revealed that FQ and Me-FQ but not RQ or Me-RQ break down the parasite digestive vacuole membrane, which could be related to the ability of the former to generate hydroxyl radicals.

Stage independent chloroquine resistance and chloroquine toxicity revealed via spinning disk confocal microscopy

We previously customized a Nipkow spinning disk confocal microscope (SDCM) to acquire 4D data for live, intraerythrocytic malarial parasites [Gligorijevic B, McAllister R, Urbach JS, Roepe, PD. Spinning disk confocal microscopy of live, intraerythrocytic malarial parasites. 1. Quantification of hemozoin development for drug sensitive versus resistant malaria. Biochemistry 2006;45:12400-10]. We reported that chloroquine (CQ) treatment did not appear to affect progress through the cell cycle, and suggested that toxicity may be manifested post-schizogony. We now use SDCM, synchronized cell culture and continuous vs. bolus drug dosing to investigate stage specific CQ effects in detail. We develop a novel, extremely rapid method for counting schizont nuclei in 3D. We then quantify schizont nuclei and hemozoin (Hz) production for live parasite cultures pulsed with CQ at different stages in the cell cycle and find that bolus treatment of rings affects the multiplicity of nuclear division. We quantify parasitemia and merozoite development in subsequent cycles following bolus CQ exposure and find that a portion of CQ toxicity is manifested post-schizogony as "delayed death". Using these methods and others we compare CQ sensitive (CQS) vs. resistant (CQR) strains as well as transfectants that are CQR via introduction of mutant PfCRT. Surprisingly, we find that PfCRT confers resistance to CQ administered at the very early ring stage of development, wherein a digestive vacuole is not yet formed, as well as at the schizont stage, wherein Hz production is thought to plateau. Taken together, these data force a rethinking of CQ pharmacology and the mechanism of CQR.

Synthesis and evaluation of sulfonylurea derivatives as novel antimalarials

We have synthesized a series of sulfonylureas and have tested their antimalarial activities, including inhibition of in vitro development of a chloroquine-resistant strain of Plasmodium falciparum, in vitro hemoglobin hydrolysis, hemozoin formation, and development of Plasmodium berghei in murine malaria. The most active antimalarial compound was (E)-1-[4'-(3-(2,4-difluorophenyl)acryloyl)phenyl]-3-tosylurea (22) with an IC(50) of 1.2microM against cultured P. falciparum parasites. Biological results suggest a fairly potent antimalarial activity for this derivative, but also imply that its activity may arise from an unknown mechanism. Indeed, these compounds may act against malaria parasites through multiple mechanisms.

PLASMODIUM YOELII: IDENTIFICATION AND PARTIAL CHARACTERIZATION OF ANMDR1GENE IN AN ARTEMISININ-RESISTANT LINE

The molecular mechanisms by which the malarial parasite has managed to develop resistance to many antimalarial drugs remain to be completely elucidated. Mutations in the pfmdr1 gene of Plasmodium falciparum, as well as an increase in pfmdr1 copy number, have been associated with resistance to the quinoline-containing antimalarial drugs. We investigated the mechanisms of drug resistance in Plasmodium using a collection of P. yoelii lines with different drug resistance profiles. The mdr1 gene of P. yoelii (pymdr1) was identified and characterized. A 2- to 3-fold increase in the pymdr1 gene copy number was observed in the P. yoelii ART line (artemisinin resistant) when compared with the NS parental line. The pymdr1 gene was mapped to a chromosome of 2.1 Mb in all lines analyzed. Reverse transcriptase-polymerase chain reaction and Western blot experiments confirmed the expression of the gene at the RNA and protein levels.

Qinghaosu resistance in rodent malaria

Resistance to qinghaosu (artemisinin) developed rapidly in a chloroquine-resistant line of Plasmodium yoelii (NS) passaged in mice, but was not produced in chloroquine-sensitive P. berghei. Development of resistance took place in an apparently stepwise fashion. After removal of drug selection pressure some resistance was lost which was regained rapidly within three passages when drug pressure was reapplied. The resistant QS line was cross-resistant to two reduced derivatives of artemisinin but not to propoxycarbonyl dihydroartemisinin or artesunate. No significant resistance was shown against primaquine, pyrimethamine, cycloguanil or pyrimethamine-sulphadoxine, but resistance to chloroquine was enhanced and marked resistance to quinine, mefloquine and amodiaquine was noted. It is suggested that the unusual cross-resistance pattern of the strain relates to changes in membrane characteristics.

The chemotherapy of rodent malaria, XXVI. The potential value of WR 122,455 (a 9-phenanthrenemethanol) against drug-resistant malaria parasites

The phenanthrenemethanol compound WR 122,455 is an effective blood schizontocide against lines of Plasmodium berghei that are highly resistant to primaquine, sulphonamides, pyrimethamine and cycloguanil. It is also active against the NS line that is moderately resistant to chloroquine. WR 122,455 is inactive against the RC line which is highly resistant to chloroquine. Resistance to WR 122,455 is fairly readily developed by the drug-sensitive N strain of P. berghei, using a relapse technique. Resistance develops very readily to the NS line of P. berghei. Both resistant lines exhibit cross-resistance to quinine, but a roughly normal response to chloroquine, primaquine, sulphonamides, dapsone, pyrimethamine and cycloguanil. Resistance to WR 122,455 is stable through cyclical transmission and through cryopreservation, as well as in the absence of drug selection pressure. The resistant parasites have an essentially normal morphology and virulence. A warning is given against the widescale use of WR 122,455 or similar new drugs for human malaria other than in a suitable combination, in order to minimize the danger of the development of resistance to them.