Reactions of artemisinin and arteether with acid: implications for stability and mode of antimalarial action.

The currently accepted mechanism of trioxane antimalarial action involves generation of free radicals within or near susceptible sites probably arising from the production of distonic radical anions. An alternative mechanistic proposal involving the ionic scission of the peroxide group and consequent generation of a carbocation at C-4 has been suggested to account for antimalarial activity. We have investigated this latter mechanism using DFT (B3LYP/6-31+G* level) and established the preferred Lewis acid protonation sites (artemisinin O5a>>O4a approximately O3a>O2a>O1a; arteether O4a>or=O3a>O5b>>O2a>O1a; Figure 3) and the consequent decomposition pathways and hydrolysis sites. In neither molecule is protonation likely to occur on the peroxide bond O1-O2 and therefore lead to scission. Therefore, the alternative radical pathway remains the likeliest explanation for antimalarial action.

Endotoxin fever in granulocytopenic rats: evidence that brain cyclooxygenase-2 is more important than circulating prostaglandin E(2).

PGE(2) is a recognized mediator of many fevers, and cyclooxygenase (COX) is the major therapeutic target for antipyretic therapy. The source, as well as the site of action of PGE(2), as an endogenous pyrogen, is widely accepted as being central, but PGE(2) in the circulation, possibly from leukocytes, may also contribute to the development of fever. However, bacterial infections are important causes of high fever in patients receiving myelosuppressive chemotherapy, and such fevers persist despite the use of COX inhibitors. In the study reported here, the febrile response to bacterial LPS was measured in rats made leukopenic by cyclophosphamide. A striking increase in LPS fever occurred in these granulocytopenic rats when compared with febrile responses in normal animals. Unlike LPS fever in normal rats, fever in granulocytopenic rats was neither accompanied by an increase in blood PGE(2) nor inhibited by ibuprofen. Both leukopenic and normal rats showed LPS-induced COX-2-immunoreactivity in cells associated with brain blood vessels. Furthermore, LPS induced an increase of PGE(2) in cerebrospinal fluid. Induction of COX-2-expression and PGE(2) production was inhibited by ibuprofen in normal but not in leukopenic rats. Although the results presented are, in part, confirmatory, they add new information to this field and open a number of important questions as yet unresolved. Overall, the present results indicate that, in contrast to immunocompetent rats, leukocytes and/or other mechanisms other than PGE(2) are implicated in the mechanisms restricting and reducing the enhanced febrile response to endotoxin in immunosuppressed hosts.

Changes in mGlu5 receptor expression in the basal ganglia of reserpinised rats.

Dopamine depletion in Parkinson's disease results in a series of pathophysiological changes in the basal ganglia circuitry. Increased release of glutamate plays an important role in this motor disorder, therefore, agents interacting with glutamatergic transmission may have therapeutic potential. In this study we investigated changes in both mRNA expression and the number of binding sites of the mGlu5 receptor in a reserpinised rat model of Parkinson's disease. The in situ hybridisation demonstrated that acute reserpine treatment caused a significant decrease in the expression of mGlu5 receptor mRNA in the rostral and caudal parts of the rat striatum. At the same time, tritium-labelled 2-ethyl-6-(phenylethynyl)-pyridine ([(3)H]MPEP) ligand binding experiments detected a significant increase in the total number of mGlu5 receptors in the same region of the motor loop. These apparently contradictory data can be explained by mGlu5 receptor turnover being down-regulated in reserpinised rats, due possibly to an imbalance in the rates of synthesis/insertion and internalisation/degradation of the receptor. These findings suggest that changes such as these affecting mGlu5 receptors may be involved in the pathophysiological consequences of dopamine depletion in the brain.

Simultaneous increases in immune-competent cells and nitric oxide in the spleen during Plasmodium berghei infection in mice.

BACKGROUND AND PURPOSE: Nitric oxide and other reactive nitrogen intermediates (RNI) are thought to be important mediators of both immunological and pathological responses of the vertebrate host to malaria infection. The role of RNI has been studied most often by assay of stable RNI metabolites (nitrites, nitrates) in blood. This study evaluated the nature of the RNI response of mice to malaria by analyzing the subsets of immune-competent cells within the organ displaying increased RNI in vivo. METHODS: We measured RNI production indirectly, as stable metabolites of nitric oxide activity in tissue homogenates (brain, liver, spleen) from mice infected with Plasmodium berghei. Only spleen exhibited an RNI concentration response during rising parasitemia. Subsets of immune-competent cells (B cells, CD19+), macrophages/monocytes (MOMA2+) and T cells (CD4+, CD8+) in the spleen were assayed by fluorescence activated cell scan flow cytometry. RESULTS: The spleen was confirmed as a major source of RNI during mid-phase P. berghei infection. Significant increases in CD19+ and MOMA2+ spleen cells were evident during the mid-phase of P. berghei infection in MF1 mice when RNI are maximally elevated. CONCLUSIONS: The time courses of the cellular and RNI responses indicate that CD19+ and MOMA2+ cells may be responsible for the increase in RNI in the spleen. However, experiments in vitro are needed to make a definitive identification of the cell type(s) responsible for the increase in RNI in the mouse spleen during P. berghei infection.

Mapping antimalarial pharmacophores as a useful tool for the rapid discovery of drugs effective in vivo: design, construction, characterization, and pharmacology of metaquine.

Resistant strains of Plasmodium falciparum and the unavailability of useful antimalarial vaccines reinforce the need to develop new efficacious antimalarials. This study details a pharmacophore model that has been used to identify a potent, soluble, orally bioavailable antimalarial bisquinoline, metaquine (N,N'-bis(7-chloroquinolin-4-yl)benzene-1,3-diamine) (dihydrochloride), which is active against Plasmodium berghei in vivo (oral ID(50) of 25 micromol/kg) and multidrug-resistant Plasmodium falciparum K1 in vitro (0.17 microM). Metaquine shows strong affinity for the putative antimalarial receptor, heme at pH 7.4 in aqueous DMSO. Both crystallographic analyses and quantum mechanical calculations (HF/6-31+G) reveal important regions of protonation and bonding thought to persist at parasitic vacuolar pH concordant with our receptor model. Formation of drug-heme adduct in solution was confirmed using high-resolution positive ion electrospray mass spectrometry. Metaquine showed strong binding with the receptor in a 1:1 ratio (log K = 5.7 +/- 0.1) that was predicted by molecular mechanics calculations. This study illustrates a rational multidisciplinary approach for the development of new 4-aminoquinoline antimalarials, with efficacy superior to chloroquine, based on the use of a pharmacophore model.

Pharmacological assessment of the role of nitric oxide in mice infected with lethal and nonlethal species of malaria.

This pharmacological investigation sought to determine whether nitric oxide (NO) had an antiparasitic effect and/or mediated pathology in mice infected with nonlethal P. chabaudi or lethal P. berghei. Nitric oxide synthase (NOS) inhibitors were evaluated for their ability to inhibit the rise in reactive nitrogen intermediates (RNI) induced by bacterial lipopolysaccharide (LPS) in mice. The more effective compound, aminoguanidine (AG) inhibited the rise in RNI induced by P. chabaudi and increased mortality, but had no effect on parasitaemia. Inducers and donors of NO were screened for their ability to increase RNI and the most effective agents evaluated for their ability to modify P. berghei infection. S-Nitrosoglutathione had little effect, but LPS decreased parasitaemia and mortality. An inconsistent relationship is evident between the abilities of these agents to modify NO activity and their effects on malaria in mice. Increased mortality in mice with P. chabaudi treated with AG indicates a reduction in resistance. The absence of an effect on parasitaemia by a NOS inhibitor or NO donor indicates either RNI have insignificant antimalarial action in vivo or the efficacy of the compounds is inadequade. Resistance to P. berghei in LPS-treated mice demonstrates an antiparasitic effect, but this may be attributable to factors other than NO.

The role of nitric oxide and its up/downstream molecules in malaria: cytotoxic or preventive?

The current study investigated the involvement of nitric oxide (NO) and related molecules in malaria target organs of outbred MF1 mice during lethal Plasmodium berghei and non-lethal P. c. chabaudi infections, in order to evaluate whether changes in NO production are beneficial or detrimental to the host. A number of methods have been applied to test this hypothesis, including Griess microassay, electrochemical assay, RT-PCR and Western blot. The results show that reactive nitrogen intermediate (RNI) accumulation, in vitro levels of endogenous NO production, inducible nitric oxide synthase (iNOS) mRNA induction and NOS protein expression altered during murine malaria. The changes depended upon the tissue, the day of infection, the degree of parasitemia, the strain of Plasmodia and the method of measuring NO biosynthesis. Differences in the pathology of two strains of Plasmodia appear to depend more on the strain of parasite rather than the strain of host. The involvement of NO and its up/downstream molecules in murine malaria are specified to host/parasite combinations and it is influenced by the method used to assess NO. The anti-parasitic function against Plasmodia did not relate only to NO in this study, but a complex process consisting of NO and other immune factors is required to resolve the parasite. Selective delivery of inhibitors and donors of NO synthesis in the tissues of the malarial host is indicated as a potential novel therapy to inhibit the parasite or prevent its pathological symptoms.

Expression of inducible nitric oxide synthase (iNOS) mRNA in target organs of lethal and non-lethal strains of murine malaria.

Nitric oxide (NO) is a putative mediator of the immunological and/or pathological responses to malaria, consequently it is a potential target for novel drug therapy. Numerous cell types increase expression of inducible nitric oxide synthase (iNOS) under inflammatory conditions, the most relevant stimuli being cytokines and endotoxins. In this study the expression of iNOS mRNA in several target organs (brain, liver, spleen) of malaria have been investigated in MF1 mice during lethal Plasmodium (P.) berghei and non-lethal P. c. chabaudi infection. In P. berghei malaria, iNOS mRNA decreased in liver and was unchanged in spleen during the period of rising parasitaemia, but increased in both organs late in the infection, when parasitaemia was high and death imminent. In mice infected with P. c. chabaudi, spleen iNOS mRNA increased progressively throughout the early, peak and recovery periods of parasitaemia, but decreased in liver. Brain iNOS mRNA decreased in samples collected throughout the time courses of both infections. Hence it is evident that changes in iNOS mRNA in murine malaria depend upon the tissue, day of infection, degree of parasitaemia and strain of Plasmodium. These data indicate induction of iNOS mRNA in the spleen has a role in combating these strains of Plasmodium in MF1 mice. Failure to clear lethal P. berghei parasitaemia was associated with increased iNOS mRNA expression in the liver, which may contribute to the pathology of this malaria.