Immunization of mice with Plasmodium TCTP delays establishment of Plasmodium infection

Translationally controlled tumour protein (TCTP) may play an important role in the establishment or maintenance of parasitemia in a malarial infection. In this study, the potential of TCTP as a malaria vaccine was investigated in two trials. In the initial vaccine trial, Plasmodium falciparum TCTP (PfTCTP) was expressed in Saccharomyces cerevisiae and used to immunize BALB/c mice. Following challenge with Plasmodium yoelii YM, parasitemia was significantly reduced during the early stages of infection. In the second vaccine trial, the TCTP from P. yoelii and P. berghei was expressed in Escherichia coli and used in several mouse malaria models. A significant reduction in parasitemia in the early stages of infection was observed in BALB/c mice challenged with P. yoelii YM. A significantly reduced parasitemia at each day leading up to a delayed and reduced peak parasitemia was also observed in BALB/c mice challenged with the nonlethal Plasmodium chabaudi (P.c.) chabaudi AS. These results suggest that TCTP has an important role for parasite establishment and may be important for pathogenesis.

Ultrasound-derived fetal size nomogram for a sub-Saharan African population: a longitudinal study

OBJECTIVES

To create a fetal size nomogram for use in sub-Saharan Africa and compare the derived centiles with reference intervals from developed countries.

METHODS

Fetal biometric measurements were obtained at entry to antenatal care (11-22 weeks' gestation) and thereafter at 4-week intervals from pregnant women enrolled in a longitudinal ultrasound study in Kinshasa, Democratic Republic of Congo. The study population comprised 144 singleton gestations with ultrasound-derived gestational age within 14 days of the menstrual estimate. A total of 755 monthly ultrasound scans were included with a mean +/- SD of 5 +/- 1 (range, 2-8) scans per woman. Estimated fetal weight (EFW) was calculated at each ultrasound examination using the Hadlock algorithm. A general mixed-effects linear regression model that incorporated random effects for both the intercept and slope was fitted to log-transformed EFW to account for both mean growth and within-fetus variability in growth. Reference centiles (5(th), 10(th), 50(th), 90(th) and 95(th) centiles) were derived from this model.

RESULTS

Nomograms derived from developed populations consistently overestimated the 50(th) centile EFW value for Congolese fetuses by roughly 5-12%. Differences observed in the 10(th) and 90(th) centiles were inconsistent between nomograms, but generally followed a pattern of overestimation that decreased with advancing gestational age.

CONCLUSIONS

In low-resource settings, endemic malaria and maternal nutritional factors, including low prepregnancy weight and pregnancy weight gain, probably lead to lower fetal weight and utilization of nomograms derived from developed populations is not appropriate. This customized nomogram could provide more applicable reference intervals for diagnosis of intrauterine growth restriction in sub-Saharan African populations.

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.

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.

Prevalence of dihydropteroate synthase mutants in HIV-infected South African children with Pneumocystis jiroveci pneumonia

BACKGROUND

Pneumocystis jiroveci (formerly Pneumocystis carinii) pneumonia (PCP) is a major cause of mortality in human immunodeficiency virus (HIV)-infected infants in Africa, but the prevalence of mutations in the gene encoding dihydropteroate synthase (DHPS) in isolates from Africa has not been reported.

METHODS

This study investigated the prevalence of DHPS mutations in P. jiroveci isolates from South African HIV-infected children with PCP by amplifying DNA using 2 different polymerase chain reactions.

RESULTS

P. jiroveci DNA from 30 respiratory specimens was amplified; 26 specimens (86.7%) contained wild-type DHPS alleles. Of the 4 samples (13.3%) with DHPS mutations, 2 contained a homogenous population with single DHPS mutations, 1 contained a homogenous population with 2 DHPS mutations, and the fourth contained a heterogenous population of organisms with both wild-type and single-mutant DHPS genotypes. Only 1 child was receiving trimethoprim-sulphamethoxazole (TMP-SMZ) prophylaxis; this patient was infected with wild-type P. jiroveci. The mortality rate (overall, 20 [66.7%] of 30 children) was not significantly different between children infected with wild-type P. jiroveci (17 [65.4%] of 26) and those infected with mutant strains (3 [75%] of 4; P=.8).

CONCLUSIONS

DHPS mutations are uncommon in P. jiroveci isolates from South Africa. However, increasing use of TMP-SMZ prophylaxis may result in widespread development of mutations.

TNF-alpha promoter single nucleotide polymorphisms are markers of human ancestry

We present a map of single nucleotide polymorphisms (SNPs) in the human tumor necrosis factor (TNF)-alpha promoter based upon exploratory sequencing of 333 human TNF-alpha gene promoters from individuals of distinct ancestral backgrounds. We detect 10 TNF-alpha promoter SNPs that occur with distinct frequencies in populations of different ancestry. Consistent with these findings, we show that two TNF-alpha SNPs, the -243 SNP and the -856 SNP, are the first SNP markers of a sub-Saharan African-derived extended haplotype and an Amerindian HLA haplotype, respectively. Comparisons of TNF-alpha promoter SNP allele frequencies can thus help elucidate variation of HLA haplotypes and their distribution among existing ethnic groups and shed light into the history of human populations.

Immune mimicry in malaria: Plasmodium falciparum secretes a functional histamine-releasing factor homolog in vitro and in vivo

The Plasmodium falciparum translationally controlled tumor protein (TCTP) is a homolog of the mammalian histamine-releasing factor (HRF), which causes histamine release from human basophils and IL-8 secretion from eosinophils. Histamine, IL-8, and eosinophils have been reported to be elevated in patients with malaria. This study was undertaken to determine whether malarial TCTP is found in the plasma of malaria-infected patients and to determine whether it has HRF biologic activity. Malarial TCTP was found in lightly infected human volunteers and in heavily infected Malawian children, but not in uninfected patients. Recombinant malarial TCTP, like HRF, stimulated histamine release from basophils and IL-8 secretion from eosinophils in vitro. Whereas malarial TCTP was less active than HRF, the concentrations that were effective in vitro could be achievable in vivo. These data suggest that malarial TCTP, present in human plasma during a malarial illness, may affect host immune responses in vivo.

Immunoprecipitation of [(3)H]dihydroartemisinin translationally controlled tumor protein (TCTP) adducts from Plasmodium falciparum-infected erythrocytes by using anti-TCTP antibodies

Artemisinin and its derivatives are endoperoxide-containing antimalarial drugs that appear to form adducts in situ with the Plasmodium falciparum translationally controlled tumor protein (TCTP) homolog. Immunoprecipitation with antibody to recombinant TCTP suggests that adducts may form with both monomeric and dimeric TCTP.

Activities and conformational fitting of 1,4-naphthoquinone derivatives and other cyclic 1,4-diones tested in vitro against Pneumocystis carinii

Atovaquone is a chemotherapeutic agent used to treat pneumonia caused by Pneumocystis carinii in some immunocompromised patients. A set of cyclic 1,4-diones were tested in vitro for ability to inhibit growth of P. carinii, including 22 variously substituted 1,4-naphthoquinones, one bis-1,4-naphthoquinone, and three other quinones. For comparison, the antipneumocystic primaquine and its 5-hydroxy-6-desmethyl metabolite were also tested. At 1.0 microg/ml, seven compounds inhibited growth by at least 39%, with atovaquone at 92%; of these seven, five are 2-hydroxy-1,4-naphthoquinones, while one is a 2-chloro- and another is a 2-methyl-1,4-naphthoquinone. At 0.1 microg/ml, however, the most active compound tested was the primaquine metabolite, which inhibited growth by more than 42% at this concentration. To ascertain a structure-activity relationship, all 1,4-naphthoquinones were compared conformationally by means of computer-based molecular modeling (Spartan) incorporating the Sybyl force field. Without exception, for all 21 monomers tested, the substituent at position 3 of the 1,4-naphthoquinone favored activity most strongly when it simultaneously occupied (i) space centered at about 3 A from position 3, without projecting steric bulk from the area encompassed by atovaquone's cyclohexyl ring, and (ii) roughly planar space at about 7.3 A from position 3, without projecting steric bulk perpendicularly. This structure-activity relationship may prove useful in the rational design of better antipneumocystis agents.

Malaria enhances expression of CC chemokine receptor 5 on placental macrophages

Malaria and human immunodeficiency virus (HIV) coinfections are common in pregnant women in sub-Saharan Africa. The current study shows that placentas of malaria-infected women contain 3 times as much CC chemokine receptor 5 (CCR5) RNA as placentas of women without malaria. By immunohistochemistry, CCR5(+) maternal macrophages were seen in placentas from malaria-infected women but not in placentas from malaria-uninfected women. In addition, CCR5 also was found on fetal Hofbauer cells in placentas from both groups. Thus, malaria infections increase the potential reservoir for HIV in the placenta by increasing the number of HIV target cells.

Pneumocystis carinii cytochrome b mutations are associated with atovaquone exposure in patients with AIDS

This retrospective cohort study was conducted to determine whether Pneumocystis carinii cytochrome b gene mutations in patients with AIDS and P. carinii pneumonia (PCP) are associated with atovaquone exposure. Portions of the P. carinii cytochrome b genes that were obtained from 60 patients with AIDS and PCP from 6 medical centers between 1995 and 1999 were amplified and sequenced by using polymerase chain reaction. Fifteen patients with previous atovaquone prophylaxis or treatment exposure were matched with 45 patients with no atovaquone exposure. Cytochrome b coenzyme Q binding site mutations were observed in 33% of isolates from patients exposed to atovaquone, compared with 6% from those who were not (P=.018). There was no difference in survival 1 month after treatment between patients with or without cytochrome b mutations (P=.14). Thus, cytochrome b mutations are significantly more common in patients with AIDS and PCP with atovaquone exposure, but the clinical significance of these mutations remains unknown.

Pneumocystis carinii mutations are associated with duration of sulfa or sulfone prophylaxis exposure in AIDS patients

This study was conducted to determine whether Pneumocystis carinii dyhydropteroate synthase (DHPS) gene mutations in AIDS patients with P. carinii pneumonia (PCP) are affected by duration of sulfa or sulfone prophylaxis and influence response to sulfa or sulfone therapy. The P. carinii DHPS genes from 97 AIDS patients with PCP between 1991 and 1999 from 4 medical centers were amplified, using polymerase chain reaction (PCR), and sequenced. Mutations were observed in 76% of isolates from patients exposed to sulfa or sulfone prophylaxis compared with 23% of isolates from patients not exposed (P=.001). Duration of prophylaxis increased the risk of mutations (relative risk [RR] for each exposure month, 1.06; P=.02). Twenty-eight percent of patients with mutations failed sulfa or sulfone treatment; mutations increased the risk of sulfa or sulfone treatment failure (RR, 2.1; P=0.01). Thus, an increased duration of sulfa or sulfone prophylaxis increases the chance of developing a P. carinii mutation. The majority of patients with mutations respond to sulfa or sulfone therapy.

Placental haemozoin and malaria in pregnancy

Malaria infections in pregnant women cause poor birth outcomes. Malaria pigment (haemozoin) accumulates in the placenta within macrophages and extracellularly, but its pathological significance is not understood. In order to study the potential role of haemozoin in malaria pathogenesis, we enrolled primigravid women at a Malawian government antenatal clinic and followed them through delivery. One hundred and thirteen women (71 per cent) out of 159 women followed through delivery were parasitaemic at least once. Mean placental haemozoin concentrations were significantly higher in women with delivery parasitaemias (223 ng/mg protein) than in women who never had a detectable parasitaemia (43 ng/mg protein; P<0.05), but were not significantly higher in women who were parasitaemic only during the antenatal period (67 ng/mg protein). Haemozoin was not associated with preterm delivery (PTD) or intrauterine growth retardation (IUGR) (P -values, 0.307-0.787). Thus, placental haemozoin is associated with malaria infection at the time of delivery and does not seem to be associated with poor birth outcome.

Effect of Plasmodium falciparum parasitemia density on hemoglobin concentrations among full-term, normal birth weight children in western Kenya, IV. The Asembo Bay Cohort Project

The relative importance of acute high-density versus persistent low-density Plasmodium falciparum parasitemia in contributing to the public health problem of malarial anemia remains unclear. The Asembo Bay Cohort Project in western Kenya collected monthly hemoglobin (Hb) and parasitologic measurements and biweekly assessments of antimalarial drug use among 942 singleton live births between 1992 and 1996. A mixed-model analysis appropriate for repeated measures data was used to study how time-varying parasitemia and antimalarial drug exposures influenced mean Hb profiles. Incidence of World Health Organization-defined severe malarial anemia was 28.1 per 1,000 person-years. Among children aged less than 24 months, concurrent parasitemia was significantly associated with lower mean Hb, especially when compared to children with no concurrent parasitemia. Increased densities of the 90-day history of parasitemia preceding Hb measurement was more strongly associated with mean Hb levels than concurrent parasitemia density. While the highest quartile of 90-day parasitemia history was associated with lowest mean Hb levels, children in the lowest 90-day exposure quartile still experienced significantly lower Hb levels when compared to children who remained parasitemia-free for the same 90-day period. The results highlight the importance of collecting and analyzing longitudinal Hb and parasitologic data when studying the natural history of malarial anemia.

Mechanisms of artemisinin resistance in the rodent malaria pathogen Plasmodium yoelii

Artemisinin and its derivatives are important new antimalarials which are now used widely in Southeast Asia. Clinically relevant artemisinin resistance has not yet been reported but is likely to occur. In order to understand how the malaria parasite might become resistant to this drug, we studied artemisinin resistance in the murine malaria parasite Plasmodium yoelii. The artemisinin-resistant strain (ART), which is approximately fourfold less sensitive to artemisinin than the sensitive NS strain, accumulated 43% less radiolabeled drug in vitro (P < 0.01). Within the parasite, the drug appeared to react with the same parasite proteins in both strains. The translationally controlled tumor protein, one of the artemisinin target proteins, did not differ between the strains. No DNA sequence difference was found, but the resistant strain was found to express 2.5-fold-more protein than the sensitive strain (P < 0.01). Thus, the phenotype of artemisinin resistance in P. yoelii appears to be multifactorial.

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.

The Plasmodium falciparum translationally controlled tumor protein: subcellular localization and calcium binding

Artemisinin derivatives are endoperoxide antimalarials widely used to treat falciparum malaria in areas where drug resistance is common. In Plasmodium falciparum-infected erythrocytes, radiolabeled artemisinin derivatives have been shown to react with malarial proteins, one of which is the Translationally Controlled Tumor Protein (TCTP). The P. falciparum TCTP was found by immunofluorescence to be located in both the cytoplasm and food vacuoles. Immunoelectron microscopy shows that it is present in the parasite cytoplasm as well as in its food vacuolar and limiting membranes. Like other TCTPs, the P. falciparum protein binds to calcium. Further studies on the physiological role of TCTP may aid in understanding the mechanism of action of endoperoxide antimalarials.

Malaria infection during pregnancy: intrauterine growth retardation and preterm delivery in Malawi

In sub-Saharan Africa, malaria infection in pregnancy contributes to low birth weight through intrauterine growth retardation (IUGR) and preterm delivery (PTD). It was hypothesized that malaria-associated PTD and IUGR have differing etiologies due to timing of infection. In a prospective cohort of primigravid women enrolled at the antenatal clinic of Mangochi District Hospital in Malawi, the associations were investigated between antenatal or delivery parasitemias and IUGR or PTD. Among 178 singleton deliveries, 35% of infants were preterm or had IUGR. Cord blood parasitemia (odds ratio [OR]=3.34; 95% confidence interval [CI], 1.3-8.8], placental parasitemia (OR=2.43; 95% CI, 1.2-5.1), and postdelivery maternal peripheral parasitemia (OR=2.78; 95% CI, 1.3-6.1) were associated with PTD. Parasitemia and/or clinically diagnosed malaria in the antenatal period was associated with IUGR (OR=5.13; 95% CI, 1.4-19.4). Delivery parasitemias had borderline associations with IUGR. The risk patterns observed suggest that the timing and severity of infection influences the occurrence of IUGR or PTD.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

Is haemozoin a target for antimalarial drugs?

Malarial parasites utilize a unique pathway to degrade haemoglobin. Most of the haem which is released during haemoglobin degradation is incorporated into haemozoin. The mechanism of haemozoin synthesis and the structure of haemozoin are controversial. Neither chloroquine nor artemisinin appear to affect haemozoin synthesis in vivo. Artemisinin is activated by intraparasitic haem and iron into a free radical which then alkylates specific malarial proteins.

Artemisinin and the antimalarial endoperoxides: from herbal remedy to targeted chemotherapy

Artemisinin and its derivatives are endoperoxide-containing compounds which represent a promising new class of antimalarial drugs. In the presence of intraparasitic iron, these drugs are converted into free radicals and other electrophilic intermediates which then alkylate specific malaria target proteins. Combinations of available derivatives and other antimalarial agents show promise both as first-line agents and in the treatment of severe disease.

The mode of action of the antimalarial artemisinin and its derivatives

1. Atremisinin (qinghaosu) is a sesquiterpene endoperoxide derived from a plant which was used in Chinese herbal medicine for thousands of years. 2. Artemisinin and its derivatives have potent antimalarial activity, and are now being used clinically in much of the world. 3. The artemisinin derivatives have an unusual mode of action involving the iron-catalyzed generation of a carbon-centered free radical followed by the alkylation of malaria-specific proteins.

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.

Patterns of haemozoin accumulation in tissue

A sensitive fluorometric method for assaying malarial pigment, haemozoin, has been developed and used to determine the haemozoin content of blood and tissue samples. Plasmodium falciparum rings and trophozoites were found to contain 23 and 339 ng haemozoin/10(6) parasitized red blood cells (PRBCs), respectively. Unsynchronized Plasmodium berghei NK65 or ANKA parasites from infected mice contained 27 and 61 ng haemozoin/10(6) PRBCs, respectively. An exponential accumulation of haemozoin within 18 days after infection was demonstrated in liver and spleen tissue, representing up to 0.2% of the tissue by wet weight by day 18. Histology indicated that the accumulation occurred predominantly in the tissue monocytes. In the brain, the levels of haemozoin after 8 days of infection were considerably lower than they were in the liver or spleen, and most of the pigment appeared to be that present inside parasitized red blood cells. CBA/Ca mice infected with P. berghei ANKA (a cerebral malaria model) had significantly higher amounts of haemozoin in the brain than did ICR mice infected with P. berghei NK65. Thus, haemozoin levels in tissue increase with the duration of infection, and its presence may be associated with cerebral pathology.

Syntheses and antimalarial activities of N-substituted 11-azaartemisinins

A two-step reaction sequence between artemisinin and methanolic ammonia followed by treatment with Amberlyst 15 yielded 11-azaartemisinin in 65% yield. Substituting a variety of primary alkyl- and heteroaromatic amines for ammonia in the reaction sequence yields N-substituted 11-azaartemisinins in similar or greater yield. When Amberlyst 15 is replaced by a mixture of sulfuric acid/silica gel, both 11-azaartemisinin and the expected metabolite, 10-azadesoxyartemisinin, are formed in 45% and 15% yields, respectively. In vitro and in vivo test data for a number of novel N-substituted 11-azaartemisinins, against drug-resistant strains of Plasmodium falciparum, show they possess antimalarial activities equal to or greater than that of artemisinin. The most active derivative, N-(2'-acetaldehydo)-11-azaartemisinin, 17, was 26 times more active in vitro and 4 times more active in vivo than artemisinin.

Pteroylpolyglutamate synthesis by lung- and culture-derived Pneumocystis carinii

Pneumocystis carinii synthesizes folates de novo from exogenous p-aminobenzoic acid (pABA). Lung-derived organisms take up [3H]pABA in vitro except in the presence of sulfamethoxazole. Supernatants from spinner-flask cultures take up [3H]pABA if they were inoculated with lungs from infected rats, but not if they were inoculated with lungs from uninfected rats. P. carinii folates consist primarily of pteroylpentaglutamates. Plasmodium falciparum, in contrast, contains primarily pteroyltetraglutamates. Culture-derived organisms synthesize folates at a four-fold higher specific activity than lung-derived organisms, possibly because they contain less contaminating lung debris. These data suggest that P. carinii remains metabolically active in culture for at least 4 days.