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Biomedical subjects

S R Meshnick

Publications and source records attributed to S R Meshnick.

At least 37 records · Page 2Linked to original sources
PubMed1999

Malaria and pregnancy: placental cytokine expression and its relationship to intrauterine growth retardation.

Malaria infections during pregnancy can lead to the delivery of low-birth-weight infants. In this study, cytokine mRNA was measured in placentas from 23 malaria-infected and 21 uninfected primigravid women who had delivered in Mangochi, Malawi, a region with a high rate of transmission of falciparum malaria. Significantly increased expression of interleukin (IL)-1beta, IL-8, and tumor necrosis factor (TNF)-alpha and decreased expression of IL-6 and transforming growth factor-beta1 were found in malaria-infected compared with uninfected placentas. TNF-alpha and IL-8 were produced by maternally derived hemozoin-laden placental macrophages. Increased TNF-alpha expression was associated with increased placental hemozoin concentrations. Increased TNF-alpha or IL-8 expression in the placenta was associated with intrauterine growth retardation but not with preterm delivery. The results suggest that malaria infections induce a potentially harmful proinflammatory response in the placenta.

Cytokines↗
PubMed1999

Splenic and hepatic hemozoin in mice after malaria parasite clearance.

Hemozoin (malaria pigment) is found in many tissues during malaria infections. In mice that have self-cured from Plasmodium yoelii and Plasmodium chabaudi infections, liver hemozoin concentration and total content decreased for 6-9 mo after parasite clearance. However, both spleen hemozoin concentration and total hemozoin content increased dramatically during this time period. Thus, hemozoin or hemozoin-laden macrophages continue to accumulate in murine spleens for at least several months after malaria parasitemia becomes undetectable.

Animals↗
PubMed1998

The Plasmodium falciparum translationally controlled tumor protein homolog and its reaction with the antimalarial drug artemisinin.

Artemisinin and its derivatives are important new antimalarial drugs. When Plasmodium falciparum-infected erythrocytes are incubated with [10-3H]dihydroartemisinin, several malaria-specific proteins become labeled. One of these proteins is the P. falciparum translationally controlled tumor protein (TCTP) homolog. In vitro, dihydroartemisinin reacts covalently with recombinant TCTP in the presence of hemin. The association between drug and protein increases with increasing drug concentration, plateauing at approximately 1 drug/TCTP molecule. By Scatchard analysis, there appear to be 2 hemin binding sites on TCTP with dissociation constants of approximately 18 microM. When the single cysteine moiety is blocked by pretreatment with iodoacetamide, hemin binding is not affected, whereas drug binding is reduced by two-thirds. Thus, TCTP reacts with artemisinin in situ and in vitro in the presence of hemin and appears to bind to hemin. The function of the malarial TCTP and the role of this reaction in the mechanism of action of artemisinin await elucidation.

Amino Acid Sequence↗
PubMed1998

Pneumocystis carinii mutations associated with sulfa and sulfone prophylaxis failures in AIDS patients.

BACKGROUND: Failures of prophylaxis against Pneumocystis carinii pneumonia (PCP) in AIDS patients do occur, but no evidence for drug resistance has yet been presented. OBJECTIVE: To determine whether mutations in the sulfa and sulfone drug target are associated with failure of prophylaxis using a sulfa-containing agent. METHODS: Portions of the gene for P. carinii dihydropteroate synthase (DHPS), the sulfa and sulfone target, from 27 patients (20 of whom had AIDS) diagnosed with PCP between 1976 and 1997 were amplified using polymerase chain reaction and sequenced. Seven of the 27 patients (all of whom had AIDS) were receiving sulfa or sulfone drugs as prophylaxis for PCP. RESULTS: Mutations were found at only two amino-acid positions and were significantly more common in patients who received sulfa/sulfone prophylaxis. Mutations were observed in five (71%) out of seven isolates from AIDS patients receiving sulfa/sulfone as prophylaxis compared with only two (15%) out of 13 specimens from AIDS patients who did not (P = 0.022). No mutations were seen in isolates from seven non-HIV-infected patients, none of whom were on prophylaxis. Mutations were only observed in specimens obtained in 1995-1997. CONCLUSIONS: Mutations in two amino-acid positions were significantly more common in AIDS patients with PCP who failed sulfa/sulfone prophylaxis. These amino acids appeared to be directly involved in both substrate and sulfa binding, based on homology to the Escherichia coli DHPS crystal structure. Thus, the results were consistent with the possibility that mutations in the P. carinii DHPS are responsible for some of the failures of sulfa/sulfone prophylaxis in AIDS patients.

AIDS-Related Opportunistic Infections↗
PubMed1998

Drug resistance in Pneumocystis carinii: an emerging problem.

Pneumocystis carinii pneumonia (PCP) is a frequent opportunistic infection in AIDS patients. Large numbers of HIV-infected individuals take prophylactic medications to prevent this illness. The development of drug resistance, while expected, cannot be monitored by classical means, since the organism cannot be cultivated in vitro. Two drug target genes, dihydropteroate synthase (DHPS) and cytochrome b, have been cloned and sequenced from human-derived P. carinii. Mutations leading to amino acid substitutions in the active sites of both proteins have been detected in patients receiving prophylaxis with sulfonamides and sulfones (DHPS inhibitors) and with atovaquone (cytochrome b inhibitor), suggesting that drug resistance may indeed be developing.

Journal Article↗
PubMed1998

Sequence polymorphisms in the Pneumocystis carinii cytochrome b gene and their association with atovaquone prophylaxis failure.

Atovaquone (Mepron, 566c80) is an effective agent against Pneumocystis carinii, which probably acts by binding to cytochrome b and inhibiting electron transport. To assess the possibility that atovaquone resistance might be developing, the genes for the cytochrome b from P. carinii sp. f. carinii and P. carinii sp. f. hominis were partially sequenced. Eight of 10 patient isolates had cytochrome b genes with the same amino acid sequence. The P. carinii cytochrome b genes from 2 of 4 patients who had atovaquone prophylaxis failure contained mutations resulting in amino acid changes in one of the ubiquinone (coenzyme Q) binding sites (Qo). These mutations are homologous to mutations in other microorganisms that confer resistance to similar inhibitors. Variations in the sequence of the P. carinii cytochrome b gene suggest but do not prove the development of drug resistance.

Amino Acid Sequence↗
PubMed1998

Efficacy of sulfamethoxypyridazine in a murine model of Pneumocystis carinii pneumonia.

Sulfamethoxazole is the component of co-trimoxazole responsible for its efficacy against Pneumocystis carinii pneumonia, but this drug is associated with frequent adverse effects. Sulfamethoxypyridazine is significantly more effective than sulfamethoxazole against a murine model of P. carinii and might be a candidate for testing in infected patients.

Animals↗
PubMed1998

Quantitative structure-activity relationship studies of a series of sulfa drugs as inhibitors of Pneumocystis carinii dihydropteroate synthetase.

Sulfone and sulfanilamide sulfa drugs have been shown to inhibit dihydropteroate synthetase (DHPS) isolated from Pneumocystis carinii. In order to develop a pharmacophoric model for this inhibition, quantitative structure-activity relationships (QSAR) for sulfa drugs active against DHPS have been studied. Accurate 50% inhibitory concentrations were collected for 44 analogs, and other parameters, such as partition coefficients and molar refractivity, were calculated. Conventional multiple regression analysis of these data did not provide acceptable QSAR. However, three-dimensional QSAR provided by comparative molecular field analysis did give excellent results. Upon removal of poorly correlated analogs, a data set of 36 analogs, all having a common NHSO2 group, provided a cross-validated r2 value of 0.699 and conventional r2 value of 0.964. The resulting pharmacophore model should be useful for understanding and predicting the binding of DHPS by new sulfa drugs.

Dihydropteroate Synthase↗
PubMed1998

Effects of alpha-thalassemia on pharmacokinetics of the antimalarial agent artesunate.

Thalassemia is common in Southeast Asia, where artemisinin derivatives are frequently used in the treatment of malaria. It has been previously reported that artemisinin derivatives can be concentrated by uninfected thalassemic erythrocytes in vitro but not by normal erythrocytes. As a follow-up to this report, we studied the antimalarial kinetics of intravascular artesunate (2.4 mg/kg of body weight) in 10 persons with normal hemoglobins and in 10 patients with thalassemia (2 with alpha-thalassemia type 1-hemoglobin Constant Spring and 8 with alpha-thalassemia type 1-alpha-thalassemia type 2). Concentrations of artesunate and its active metabolites in plasma were measured by bioassay and expressed relative to those of dihydroartemisinin, the major biologically active metabolite. Concentrations of intravascular artesunate in plasma peaked in both the normal individuals and the thalassemic individuals 15 min after injection (the first time point). Plasma drug concentrations at all time intervals, except that at 1 h, were significantly higher in thalassemic subjects than in normal subjects (P < 0.05). The area under the concentration-time curve was 9-fold higher (P < 0.001) and the volume of distribution at steady state was 15-fold lower (P < 0.001) in thalassemic than in normal subjects. In light of the potential neurotoxicity of artemisinin derivatives, these results suggest that thalassemic subjects may need a drug administration regimen different from that of normal patients.

Adolescent↗
PubMed1998

Interaction of rat Pneumocystis carinii and rat alveolar epithelial cells in vitro.

During Pneumocystis carinii pneumonia, P. carinii trophic forms adhere tightly to type I alveolar epithelial cells (AECs). However, the manner in which the interaction between P. carinii organisms and AECs results in clinical pneumonia has not been explored. To investigate this interaction in vitro, we established a culture system using rat P. carinii and primary cultures of rat AECs. We hypothesized that binding of P. carinii to AECs would alter the metabolic, structural, and barrier functions of confluent AECs. Using fluorescently labeled P. carinii, we demonstrated that P. carinii bound to AECs in a dose-dependent manner. During P. carinii-AEC interaction, both the AECs and the P. carinii organisms remained metabolically active. Immunofluorescent staining demonstrated that AEC expression of the junctional proteins E-cadherin and occludin and the structural protein cytokeratin 8 were unaffected by P. carinii binding. To evaluate the effect of P. carinii on AEC barrier function, transepithelial resistance across AEC monolayers was measured during interaction with organisms. Culture with P. carinii did not result in loss of AEC barrier function but in fact increased AEC transepithelial resistance in a dose- and time-dependent manner. We conclude that the direct interaction of P. carinii with AECs does not disrupt AEC metabolic, structural, or barrier function. Therefore, we speculate that additional inflammatory cells and/or their signals are required to induce the epithelial derangements characteristic of P. carinii pneumonia.

Animals↗
PubMed1998

Artemisinin antimalarials: mechanisms of action and resistance.

Artemisinin derivatives are an important new class of antimalarial agents. These compounds contain endoperoxide bridges which are essential for antimalarial activity. Artemisinin is believed to act via a two-step mechanism. Artemisinin is first activated by intraparasitic heme-iron which catalyzes the cleavage of this endoperoxide. A resulting free radical intermediate may then kill the parasite by alkylating and poisoning one or more essential malarial protein(s). No clinically relevant artemisinin-resistant human malaria has yet been reported. However, an artemisinin-resistant strain of murine malaria has been developed and may offer clues to the kinds of resistance that may someday develop in human malarias.

Animals↗
PubMed1997

Identification of hemoglobin degradation products in Plasmodium falciparum.

Malaria parasites break down human hemoglobin to its constituent amino acids by cysteine and aspartic proteinases. However, no one has previously been able to identify hemoglobin cleavage products in intact parasites. When isolated parasites were subjected to non-denaturing polyacrylamide gels electrophoresis, a unique protein band was found which contains heme and reacts with anti-human hemoglobin antibodies. This protein does not appear to represent oxidized or glycosylated hemoglobin, and is present in isolated parasites but not in the cytosol of infected or uninfected erythrocytes. When this band was eluted and subjected to SDS polyacrylamide gel electrophoresis, three bands were seen on Western blots. The proteins in these bands contain proteins with the N-terminal sequences of alpha- and beta-globin chains but molecular masses of only 13.2-13.4 kDa. These data suggest that hemoglobin alpha- and beta-chains are initially cleaved within the parasite phagolysosome to release peptides of 15-17 and 23-25 amino acids from the C-termini of alpha- and beta-globin chains, respectively. Production of the hemoglobin breakdown products was inhibited by E-64, a cysteine proteinase inhibitor, suggesting the involvement of a cysteine proteinase in an early step of hemoglobin degradation.

Amino Acid Sequence↗
PubMed1997

Dihydropteroate synthase polymorphisms in Pneumocystis carinii.

Sulfa drugs are widely used in the treatment and prophylaxis of Pneumocystis carinii pneumonia. The nucleotide sequences of the sulfa target enzyme, dihydropteroate synthase (DHPS), differed substantially in human-, rat-, and mouse-derived P. carinii. Sequence variation also existed in the DHPSs from human-derived isolates. Six nucleotide changes were found in 6 human isolates; each was nonsynonymous and resulted in an amino acid change. Several of these changes were in highly conserved regions and are similar to those that cause sulfa resistance in other organisms. These data suggest that the human-derived P. carinii DHPS may be evolving under positive selective pressure from sulfa drugs.

Amino Acid Sequence↗
PubMed1997

Artemisinin neurotoxicity: neuropathology in rats and mechanistic studies in vitro.

Despite the wide use of artermisinin and its derivatives, concerns have been raised about their potential neurotoxicity. Accordingly, studies were undertaken on rats treated with high doses of arteether and on mouse neuroblastoma cells (Neu2a) treated with 3H-dihydroartemisinin. Rats uniformly developed neurologic symptoms following intramuscular administration of 50 mg/kg/day of arteether for 5-6 days. Acute neuronal necrosis associated with vacuolization and focal axonal swelling in the neuropil was observed in specific areas of the brain, especially the vestibular nuclei and red nuclei. Scattered swollen neurons were also evident in the cerebellar nuclei and the reticular formation. No neurologic symptoms, neuronal nuclei necrosis, nor gliosis was observed in rats administered 25 or 30 mg/kg/day for six or eight days. In vitro, Neu2a cells took up much less 3H-dihydroartemisinin than Plasmodium falciparum-infected red blood cells when incubated under identical conditions for 4 hr with 4.2 microM 3H-dihydroartemisinin. This selective uptake may explain why the artemisinin derivatives are selectively toxic to malaria parasites. Autoradiograms of sodium dodecyl sulfate-polyacrylamide gels run from 3H-dihydroartemisinin-treated cells showed that neuronal proteins with molecular weights of 27, 32, 40, and 81 kD were alkylated, although not nearly as strongly or rapidly as the P. falciparum proteins. The results indicate that while artemisinin derivatives have neurotoxic effects in rats and alkylate proteins in neuroblastoma cells, these effects only occur at high doses or after prolonged exposure.

Alkylation↗
PubMed1996

Hemoglobin catabolism and iron utilization by malaria parasites.

Erythrocytic malaria parasites transport large quantities of erythrocyte cytoplasm to an acidic food vacuole, where hemoglobin is degraded. Globin is hydrolysed to free amino acids, which are subsequently incorporated into parasite proteins. Potentially toxic heme moieties are polymerized to hemozoin and also probably provide necessary parasite iron. Our understanding of the precise mechanisms of hemoglobin processing is incomplete. However, it is clear that hemoglobin catabolism and related events in the malarial food vacuole are the likely targets of both important antimalarial drugs and of promising new compounds. Thus, a more precise characterization of the metabolism of hemoglobin and iron by malaria parasites should expedite the development of new modes of antimalarial chemotherapy.

Animals↗
PubMed1996

Haemozoin: identification and quantification.

Haemozoin (malaria pigment) is a haem polymer resulting from the breakdown of haemoglobin by Plasmodium spp. This refractory substance has been the focus of many studies and of much debate, mainly because of its role in the pharmacological activity of certain antimalarials. Haemozoin is also important because its presence in tissues serves as an indicator of malaria infections, and may itself be a mediator of malaria pathogenesis. In this article, Amy Sullivan and Steven Meshnick review the structure and synthesis of haemozoin, and then focus on methods of haemozoin identification in tissue. This latter aspect has implications for the study of haemozoin both as an indicator of malaria infection and as a possible mediator of malaria pathogenesis.

Journal Article↗
PubMed1996

Second-generation antimalarial endoperoxides.

Artemisinin, derived from a Chinese herbal remedy, is a potent peroxide-containing antimalarial. New types of peroxides, derived from this structure, as well as other naturally occurring antimalarial peroxides, have been synthesized and found to have potent antimalarial activities. Studies on the activities, modes of action, and toxicities of these compounds are discussed here by Steven Meshnick and colleagues.

Journal Article↗