Interaction of sulfonamide and sulfone compounds with Toxoplasma gondii dihydropteroate synthase

Combination of sulfonamides, silver antimicrobial agents and quorum sensing inhibitors as a preferred approach for improving antimicrobial efficacy against Bacillus subtilis

More and more antibacterial agents are used together to treat bacterial infections in diverse fields, but the overuse of antibacterial agents may cause the environmental pollution of antibiotic resistance genes (ARGs). In order to reduce the use of antimicrobial agents, the potential joint effects of quorum-sensing inhibitors (QSIs) and traditional antimicrobial agents have been proposed to be effective. In this study, the joint effects of traditional antimicrobial agents, represented by sulfonamides (SAs) and silver antibacterial agents (silver nitrate (AgNO₃) and nanosilver (AgNP, 5 nm)), and five potential QSIs, were investigated using B. subtilis. It was found that AgNP showed higher toxicity than AgNO₃, whereas the joint effects on B. subtilis showed no difference between AgNO₃ and AgNP when they combined with SAs or QSIs, respectively. In general, AgNO₃ and AgNP presented synergetic and additive effects with QSIs, but additive and antagonistic effects with SAs; SAs exhibited synergetic, additive and antagonistic effects with different QSIs whether in binary or ternary mixed system. Moreover, it was found that the use of antimicrobials was reduced and the synergistic combined toxicity of antimicrobial agents on B. subtilis was increased through the addition of the QSIs. This study can offer a valuable reference for the combined medication of the different antimicrobial agents, which will benefit the environmental and human health.

Perspective on current and emerging drugs in the treatment of acute and chronic toxoplasmosis

No new drugs for treatment of toxoplasmosis have been approved in over 60 years, despite the burden of toxoplasmosis on human society. The small selection of effective drugs is limited by important side effects, often limiting patient use. This perspective highlights promising late-stage drug candidates in the treatment of toxoplasmosis. Presently, drugs target the tachyzoite form of the parasite Toxoplasma gondii responsible for the acute infection but do not eradicate the tissue cyst form underlying chronic infection. Pyrimethamine - the first-line and only approved drug for treatment of toxoplasmosis in the United States - inhibits parasite DNA synthesis by inhibiting dihydrofolate reductase (DHFR). Two novel DHFR inhibitors with improved potency and selectivity for parasite DHFR over human DHFR are in clinical-stage development. One of the most advanced and promising therapeutic targets, demonstrating potential to treat both acute and chronic toxoplasmosis, is the calcium-dependent protein kinase 1 (CDPK1) which plays an essential role in the intracellular replicative cycle of the parasite, and has no direct mammalian homolog. Two CDPK1 inhibitor programs have identified potent and selective lead series, demonstrating acceptable systemic and CNS exposure, and in vivo efficacy in animal models of acute and chronic infection. Physicians need a better arsenal of parasiticidal drugs for the treatment of toxoplasmosis, particularly those active against tissue cysts.

Review of Experimental Compounds Demonstrating Anti-Toxoplasma Activity

Toxoplasma gondii is a ubiquitous apicomplexan parasite capable of infecting humans and other animals. Current treatment options for T. gondii infection are limited and most have drawbacks, including high toxicity and low tolerability. Additionally, no FDA-approved treatments are available for pregnant women, a high-risk population due to transplacental infection. Therefore, the development of novel treatment options is needed. To aid this effort, this review highlights experimental compounds that, at a minimum, demonstrate inhibition of in vitro growth of T. gondii When available, host cell toxicity and in vivo data are also discussed. The purpose of this review is to facilitate additional development of anti-Toxoplasma compounds and potentially to extend our knowledge of the parasite.

Clinically Available Medicines Demonstrating Anti-Toxoplasma Activity

Toxoplasma gondii is an apicomplexan parasite of humans and other mammals, including livestock and companion animals. While chemotherapeutic regimens, including pyrimethamine and sulfadiazine regimens, ameliorate acute or recrudescent disease such as toxoplasmic encephalitis or ocular toxoplasmosis, these drugs are often toxic to the host. Moreover, no approved options are available to treat infected women who are pregnant. Lastly, no drug regimen has shown the ability to eradicate the chronic stage of infection, which is characterized by chemoresistant intracellular cysts that persist for the life of the host. In an effort to promote additional chemotherapeutic options, we now evaluate clinically available drugs that have shown efficacy in disease models but which lack clinical case reports. Ideally, less-toxic treatments for the acute disease can be identified and developed, with an additional goal of cyst clearance from human and animal hosts.

Mycobacterium tuberculosis folate metabolism and the mechanistic basis for para-aminosalicylic acid susceptibility and resistance

para-Aminosalicylic acid (PAS) entered clinical use in 1946 as the second exclusive drug for the treatment of tuberculosis (TB). While PAS was initially a first-line TB drug, the introduction of more potent antitubercular agents relegated PAS to the second-line tier of agents used for the treatment of drug-resistant Mycobacterium tuberculosis infections. Despite the long history of PAS usage, an understanding of the molecular and biochemical mechanisms governing the susceptibility and resistance of M. tuberculosis to this drug has lagged behind that of most other TB drugs. Herein, we discuss previous studies that demonstrate PAS-mediated disruption of iron acquisition, as well as recent genetic, biochemical, and metabolomic studies that have revealed that PAS is a prodrug that ultimately corrupts one-carbon metabolism through inhibition of the formation of reduced folate species. We also discuss findings from laboratory and clinical isolates that link alterations in folate metabolism to PAS resistance. These advancements in our understanding of the basis of the susceptibility and resistance of M. tuberculosis to PAS will enable the development of novel strategies to revitalize this and other antimicrobial agents for use in the global effort to eradicate TB.

Great efficacy of sulfachloropyrazine-sodium against acute murine toxoplasmosis

OBJECTIVE

To identify more effective and less toxic drugs to treat animal toxoplasmosis.

METHODS

Efficacy of seven kinds of sulfonamides against Toxoplasma gondii (T. gondii) in an acute murine model was evaluated. The mice used throughout the study were randomly assigned to many groups (10 mice each), which either remained uninfected or were infected intraperitoneally with tachyzoites of T. gondii (strains RH and CN). All groups were then treated with different sulfonamides and the optimal treatment protocol was determined candidates. Sulfadiazine-sodium (SD) was used for comparison.

RESULTS

The optimal therapy involved gavaging mice twice per day with 250 mg/kg bw of sulfachloropyrazine-sodium (SPZ) for five days. Using this protocol, the average survival time and the time-point of 50% fatalities were prolonged significantly compared with SD treatment. Treatment with SPZ protected 40% of mice from death, and the heart and kidney tissue of these animals was parasite-free, as determined by nested-PCR. SPZ showed excellent therapeutic effects in the treatment of T. gondii in an acute murine model and is therefore a promising drug candidate for the treatment and prevention of T. gondii in animals.

CONCLUSIONS

It can be concluded that the effective drug sulfachloropyrazine may be the new therapeutic options against animal toxoplasmosis.

Screening for small molecule inhibitors of Toxoplasma gondii

INTRODUCTION

Toxoplasma gondii, the agent that causes toxoplasmosis, is an opportunistic parasite that infects many mammalian species. It is an obligate intracellular parasite that causes severe congenital neurological and ocular disease mostly in immunocompromised humans. The current regimen of therapy includes only a few medications that often lead to hypersensitivity and toxicity. In addition, there are no vaccines available to prevent the transmission of this agent. Therefore, safer and more effective medicines to treat toxoplasmosis are urgently needed.

AREAS COVERED

The author presents in silico and in vitro strategies that are currently used to screen for novel targets and unique chemotypes against T. gondii. Furthermore, this review highlights the screening technologies and characterization of some novel targets and new chemical entities that could be developed into highly efficacious treatments for toxoplasmosis.

EXPERT OPINION

A number of diverse methods are being used to design inhibitors against T. gondii. These include ligand-based methods, in which drugs that have been shown to be efficacious against other Apicomplexa parasites can be repurposed to identify lead molecules against T. gondii. In addition, structure-based methods use currently available repertoire of structural information in various databases to rationally design small-molecule inhibitors of T. gondii. Whereas the screening methods have their advantages and limitations, a combination of methods is ideally suited to design small-molecule inhibitors of complex parasites such as T. gondii.

Mechanistic link between uptake of sulfonamides and bacteriostatic effect: model development and application to experimental data from two soil microorganisms

Sulfonamides (SA) are antibiotic compounds that are widely used as human and veterinary pharmaceuticals. They are not rapidly biodegradable and have been detected in various environmental compartments. Effects of sulfonamides on microbial endpoints in soil have been reported from laboratory incubation studies. Sulfonamides inhibit the growth of sensitive microorganisms by competitive binding to the dihydropteroate-synthase (DHPS) enzyme of folic acid production. A mathematical model was developed that relates the extracellular SA concentration to the inhibition of the relative bacterial growth rate. Two factors--the anionic accumulation factor (AAF) and the cellular affinity factor (CAF)--determine the effective concentration of an SA. The AAF describes the SA uptake into bacterial cells and varies with both the extra- and intracellular pH values and with the acidic pKa value of an SA. The CAF subsumes relevant cellular and enzyme properties, and is directly proportional to the DHPS affinity constant for an SA. Based on the model, a mechanistic dose-response relationship is developed and evaluated against previously published data, where differences in the responses of Pseudomonas aeruginosa and Panthoea agglomerans toward changing medium pH values were found, most likely as a result of their diverse pH regulation. The derived dose-response relationship explains the pH and pKa dependency of mean effective concentration values (EC50) of eight SA and two soil bacteria based on AAF and CAF values. The mathematical model can be used to extrapolate sulfonamide effects to other pH values and to calculate the CAF as a pH-independent measure for the SA effects on microbial growth.

Azithromycin reduces ocular infection during congenital transmission of toxoplasmosis in the Calomys callosus model

Toxoplasma gondii is a widely distributed obligatory intracellular parasite that causes severe disease to the fetus when transmitted during pregnancy. Drugs used to avoid congenital transmission have shown side effects, and their efficacy is controversial. The most widely used treatment for acute toxoplasmosis during pregnancy is pyrimethamine plus sulfadiazine, which has several side effects. In this work, we tested the efficacy of azithromycin in reducing congenital transmission of T. gondii in the large vesper mouse, Calomys callosus, a rodent. Females of C callosus were inoculated perorally with 20 cysts of ME49 strain of T. gondii on the day of fertilization, and fetuses were collected from the 15th to the 19th day of gestation. Azithromycin (300 mg/kg), in association with pyrimethamine (100 or 50 mg/kg) plus sulfadiazine (100 or 75 mg/kg) and folinic acid (15 mg/kg) (SPAf), or vehicle, were administered orally on different days after infection. Brain and ocular tissues were removed and processed for immunohistochemistry using a polyclonal antibody against T. gondii, or were processed for parasite DNA quantification. Toxoplasma gondii was detected in the brains of all females and in fetuses' eyes of C. callosus treated with SPAf. On the other hand, in females treated with azithromycin, there was a reduction of T. gondii in the brains of mothers, and no parasites were detected in eyes of fetuses, indicating that azithromycin may represent an alternative treatment for toxoplasmosis during pregnancy.

Linking Pneumocystis jiroveci sulfamethoxazole resistance to the alleles of the DHPS gene using functional complementation in Saccharomyces cerevisiae

Curative and prophylactic therapy for Pneumocystis jiroveci pneumonia relies mainly on cotrimoxazole, an association of trimethoprim and sulfamethoxazole (SMX). SMX inhibits the folic acid pathway through competition with para-aminobenzoic acid (pABA), one of the two substrates of the dihydropteroate synthase (DHPS), a key enzyme in de novo folic acid synthesis. The most frequent non-synonymous single nucleotide polymorphisms (SNPs) in P. jiroveci DHPS are seen at positions 165 and 171, the combination leading to four possible different genetic alleles. A number of reports correlate prophylaxis failure and mutation in the P. jiroveci DHPS but, because of the impossibility of reliably cultivating P. jiroveci, the link between DHPS mutation(s) and SMX susceptibility is not definitively proven. To circumvent this limitation, the yeast Saccharomyces cerevisiae was used as a model. The introduction of the P. jiroveci DHPS gene, with or without point mutations, directly amplified from a clinical specimen and cloned in a centromeric plasmid into a DHPS-deleted yeast strain, allowed a fully effective complementation. However, in the presence of SMX at concentrations >250 mg/L, yeasts complemented with the double mutated allele showed a lower susceptibility compared with strains complemented with either a single mutated allele or wild-type alleles. These results confirm the need for prospective study of pneumocystosis, including systematic determination of the DHPS genotype, to clarify further the impact of mutations on clinical outcome. Additionally, the S. cerevisiae model proves to be useful for the study of still uninvestigated biological properties of P. jiroveci.

The continuing challenges of leprosy

Leprosy is best understood as two conjoined diseases. The first is a chronic mycobacterial infection that elicits an extraordinary range of cellular immune responses in humans. The second is a peripheral neuropathy that is initiated by the infection and the accompanying immunological events. The infection is curable but not preventable, and leprosy remains a major global health problem, especially in the developing world, publicity to the contrary notwithstanding. Mycobacterium leprae remains noncultivable, and for over a century leprosy has presented major challenges in the fields of microbiology, pathology, immunology, and genetics; it continues to do so today. This review focuses on recent advances in our understanding of M. leprae and the host response to it, especially concerning molecular identification of M. leprae, knowledge of its genome, transcriptome, and proteome, its mechanisms of microbial resistance, and recognition of strains by variable-number tandem repeat analysis. Advances in experimental models include studies in gene knockout mice and the development of molecular techniques to explore the armadillo model. In clinical studies, notable progress has been made concerning the immunology and immunopathology of leprosy, the genetics of human resistance, mechanisms of nerve injury, and chemotherapy. In nearly all of these areas, however, leprosy remains poorly understood compared to other major bacterial diseases.

Purification, properties, and crystallization of Saccharomyces cerevisiae dihydropterin pyrophosphokinase-dihydropteroate synthase

The tri-functional enzyme of Saccharomyces cerevisiae dihydroneopterin aldolase (DHNA)-dihydropterin pyrophosphokinase (PPPK)-dihydropteroate synthase (DHPS) catalyzes three sequential steps in folate biosynthesis. A cDNA encoding the PPPK and DHPS domains of the tri-functional enzyme has been cloned. This bi-functional enzyme was expressed as a His(6) fusion protein in Escherichia coli and the protein was purified to apparent homogeneity. The purified protein possesses both PPPK and DHPS activities as measured by the incorporation of [(3)H]p-ABA into the appropriate substrate. The pH optimum of the DHPS activity was determined to be 8.5. Gel filtration measurement indicates that the protein exists as a dimer in solution. A robotic screening method was used to identify crystallization conditions. Bi-pyramidal crystals of the enzyme formed with the protein in the presence of a pterin substrate analog in phosphate buffer (pH 6.3) and these diffracted to 2.3A. Structural information from these crystals could be used to design novel drugs to inhibit folate biosynthesis.

Molecular characterization of bifunctional hydroxymethyldihydropterin pyrophosphokinase-dihydropteroate synthase from Plasmodium falciparum

A 2118-base pair gene encoding the bifunctional hydroxymethyldihydropterin pyrophosphokinase-dihydropteroate syntheses of Plasmodium falciparum (pfPPPK-DHPS) was expressed under the control of the T5 promoter in a DHPS-deficient Escherichia coli strain. The enzyme was purified to near homogeneity using nickel affinity chromatography followed by gel filtration and migrates as an intense band on sodium dodecyl sulfate-polyacrylamide gel electrophoresis with apparent mass of approximately 83 kDa. Gel filtration suggested that the native pfPPPK-DHPS might exist as a tetramer of identical subunits. The enzyme was found to be Mg2+ - and ATP-dependent and had optimal temperature ranging from 37 to 45 degrees C with peak activity at pH 10. Sodium chloride and potassium chloride at 0.2 and 0.4 M, respectively, activated the activity of the enzyme but higher salt concentrations were inhibitory. Guanidine-HCl and urea inhibited the enzyme activity by 50% at 0.25 and 0.9 M, respectively. Kinetic properties of the recombinant pfPPPK-DHPS were investigated. Sulfathiazole and dapsone were potent inhibitors of pfPPPK-DHPS, whilst sulfadoxine, sulfanilamide, sulfacetamide and p-aminosalicylic acid were less inhibitory. Our construct provides an abundant source of recombinant pfPPPK-DHPS for crystallization and drug screening.

Characterization of the Saccharomyces cerevisiae Fol1 protein: starvation for C1 carrier induces pseudohyphal growth

Tetrahydrofolate (vitamin B9) and its folate derivatives are essential cofactors in one-carbon (C1) transfer reactions and absolutely required for the synthesis of a variety of different compounds including methionine and purines. Most plants, microbial eukaryotes, and prokaryotes synthesize folate de novo. We have characterized an important enzyme in this pathway, the Saccharomyces cerevisiae FOL1 gene. Expression of the budding yeast gene FOL1 in Escherichia coli identified the folate biosynthetic enzyme activities dihydroneopterin aldolase (DHNA), 7,8-dihydro-6-hydroxymethylpterin-pyrophosphokinase (HPPK), and dihydropteroate synthase (DHPS). All three enzyme activities were also detected in wild-type yeast strains, whereas fol1Delta deletion strains only showed background activities, thus demonstrating that Fol1p catalyzes three sequential steps of the tetrahydrofolate biosynthetic pathway and thus is the central enzyme of this pathway, which starting from GTP consists of seven enzymatic reactions in total. Fol1p is exclusively localized to mitochondria as shown by fluorescence microscopy and immune electronmicroscopy. FOL1 is an essential gene and the nongrowth phenotype of the fol1 deletion leads to a recessive auxotrophy for folinic acid (5'-formyltetrahydrofolate). Growth of the fol1Delta deletion strain on folinic acid-supplemented rich media induced a dimorphic switch with haploid invasive and filamentous pseudohyphal growth in the presence of glucose and ammonium, which are known suppressors of filamentous and invasive growth. The invasive growth phenotype induced by the depletion of C1 carrier is dependent on the transcription factor Ste12p and the flocullin/adhesin Flo11p, whereas the filamentation phenotype is independent of Ste12p, Tec1p, Phd1p, and Flo11p, suggesting other signaling pathways as well as other adhesion proteins.

Pneumocystisjiroveciidihydropteroate synthase genotypes in French patients with pneumocystosis: a 1998–2001 prospective study

Dihydropteroate synthase gene (DHPS) mutations at codons 55 and 57 have been associated with sulfa/sulfone resistance in Pneumocystis jirovecii strains from patients who previously received prophylaxis. To evaluate the prevalence of these mutations, a portion of P. jirovecii DHPS gene was analysed using PCR combined with restriction fragment length polymorphism (RFLP) analysis in 92 bronchoalveolar fluid samples collected between January 1998 and September 2001 from French patients with pulmonary pneumocystosis (PCP). Seventy-six samples contained the wild-type DHPS genotype (82.6%) and 16 contained a mutant genotype (17.4%). Twelve out of the 16 isolates with a mutant DHPS genotype corresponded to patients who had never received sulfa or sulfone prophylaxis, suggesting that DHPS mutants may be acquired de novo. There was no significant difference in favourable or adverse outcome in PCP caused by the wild or mutant DHPS genotypes (P = 0.34).

Folate synthesis in higher-plant mitochondria: coupling between the dihydropterin pyrophosphokinase and the dihydropteroate synthase activities

The plant enzyme 6-hydroxymethyl-7,8-dihydropterin pyrophosphokinase/7,8-dihydropteroate synthase (HPPK/DHPS) is a mitochondrial bifunctional protein involved in tetrahydrofolate synthesis. The first domain (HPPK) catalyses the pyrophosphorylation of 6-hydroxymethyl-7,8-dihydropterin (dihydropterin) by ATP, leading to 6-hydroxymethyl-7,8-dihydropterin pyrophosphate (dihydropterinPP(i)) and AMP. The second domain (DHPS) catalyses the next step, i.e. the condensation of p-aminobenzoic acid (p-ABA) with dihydropterinPP(i) to give 7,8-dihydropteroate (dihydropteroate) and PP(i). In the present article we studied the coupling between these two reactions. Kinetic data obtained for the HPPK domain are consistent with an ordered Bi Bi mechanism where ATP binds first and dihydropterinPP(i) is released last, as proposed previously for the monofunctional Escherichia coli enzyme. In the absence of p-ABA, AMP and dihydropterinPP(i) accumulate and negatively regulate the reaction. In the presence of p-ABA, the rates of AMP and dihydropteroate synthesis are similar, indicating a good coupling between the two reactions. DihydropterinPP(i), an intermediate of the two reactions, never accumulates in this situation. The high specific activity of DHPS relative to HPPK, rather than a preferential channelling of dihydropterinPP(i) between the two catalytic sites, could explain these kinetic data. The maximal velocity of the DHPS domain is limited by the availability of dihydropterinPP(i). It is strongly feedback-inhibited by dihydropteroate and also dihydrofolate and tetrahydrofolate monoglutamate, two intermediates synthesized downstream in the folate biosynthetic pathway. Thus the HPPK domain of this bifunctional protein is the limiting factor of the overall reaction, but the DHPS domain is a potential key regulatory point of the whole folate biosynthetic pathway.

Resistance as a tool in the study of old and new drug targets in Toxoplasma

Drug resistance generated in vitro in the protozoan parasite Toxoplasma gondii is described. We focus on drugs that are in use in patients, that show some promise for such use, or that represent lead compounds for further development. No instance has yet been reported where resistance to any of these drugs has arisen in a patient or in the field although different strains do show varying degrees of sensitivity. For many of these drugs, however, resistant lines have been generated in the laboratory and these have proven very useful for elucidating a given drug's target. These targets range from metabolic pathways in the cytosol to organellar functions encoded in the mitochondrion or plastid. Such information makes predictions about how fast resistance will arise in the field but more importantly, it helps identify targets that are crucial to the parasite and predicts which combinations of drugs should act synergistically.

Disposition of intravenous pyrimethamine in healthy volunteers

A proportion of patients with AIDS and toxoplasmic encephalitis (TE) sustain low plasma pyrimethamine concentrations during oral treatment, possibly because of incomplete and variable bioavailability. We wanted to develop a safe, practicable intravenous (i.v.) formulation of pyrimethamine and characterize its disposition in healthy volunteers. A neutral, aqueous, sterile solution of pyrimethamine was produced and presented in sealed glass ampoules. Pyrimethamine (1 mg/kg) was given to eight healthy male volunteers by i.v. infusion over 2 h, and blood was sampled over a 2 week period. Pyrimethamine levels in plasma were measured by high-performance liquid chromatography. The drug was well tolerated by all volunteers, and there were no changes in vital signs, electrocardiogram, hematology, or biochemical parameters. The maximum pyrimethamine concentration of 2,089+/-565 ng ml(-1) (mean +/- standard deviation) was achieved shortly after the end of the infusion; thereafter, concentrations declined in a log-linear manner, with a half-life of 140+/-31 h.

A controlled trial of dapsone versus pyrimethamine-sulfadoxine for primary prophylaxis of Pneumocystis carinii pneumonia and toxoplasmosis in patients with AIDS

Pneumocystis carinii pneumonia (PCP) is the most common opportunistic human immunodeficiency virus (HIV)-related infection, occurring in 85% of HIV infected patients without prophylaxis. Preventive treatment is required when CD4 cell count falls below 200 cells per cubic millimeter. Cotrimoxazole has been shown to be highly effective but alternative drug regimens are often necessary because of the frequent drug hypersensitivity exhibited by HIV infected patients. The aim of this prospective, open, randomized, one-site study, involving HIV-infected patients with a CD4 cell count below 200/mm3, or a percentage under 20%, randomly assigned to receive either dapsone 50 mg daily or Fansidar one tablet weekly, was to compare the efficacy and safety of these drugs in the primary prophylaxis of PCP. Both dapsone and Fansidar appear to be safe and effective alternative agents for the prevention of PCP. Their role in Toxoplasma gondii prophylaxis requires further evaluation.

Mutations in dihydropteroate synthase are responsible for sulfone and sulfonamide resistance in Plasmodium falciparum

Plasmodium falciparum causes the most severe form of malaria in humans. An important class of drugs in malaria treatment is the sulfone/sulfonamide group, of which sulfadoxine is the most commonly used. The target of sulfadoxine is the enzyme dihydropteroate synthase (DHPS), and sequencing of the DHPS gene has identified amino acid differences that may be involved in the mechanism of resistance to this drug. In this study we have sequenced the DHPS gene in 10 isolates from Thailand and identified a new allele of DHPS that has a previously unidentified amino acid difference. We have expressed eight alleles of P. falciparum PPPK-DHPS in Escherichia coli and purified the functional enzymes to homogeneity. Strikingly, the Ki for sulfadoxine varies by almost three orders of magnitude from 0.14 microM for the DHPS allele from sensitive isolates to 112 microM for an enzyme expressed in a highly resistant isolate. Comparison of the Ki of different sulfonamides and the sulfone dapsone has suggested that the amino acid differences in DHPS would confer cross-resistance to these compounds. These results show that the amino acid differences in the DHPS enzyme of sulfadoxine-resistant isolates of P. falciparum are central to the mechanism of resistance to sulfones and sulfonamides.

Identification of a class of sulfonamides highly active against dihydropteroate synthase form Toxoplasma gondii, Pneumocystis carinii, and Mycobacterium avium

Sulfanilanilides with 3',5'-halogen substitutions had Ki values 6- to 57-fold lower than the Ki of sulfamethoxazole when tested against dihydropteroate synthase from Toxoplasma gondii. The compounds acted as competitive inhibitors. These compounds were also active against dihydropteroate synthase from Pneumocystis carinii, Mycobacterium avium, and Escherichia coli but were not significantly more active than sulfamethoxazole. The compounds were significantly more active in culture than were standard agents. Against T. gondii in culture, 50% inhibitory concentrations were 7- to 30-fold lower than that of sulfadiazine; against P. carinii in culture, a concentration of 100 microM caused 33 to 95% inhibition of growth, compared with 9% inhibition with 100 microM sulfamethoxazole.

Pharmacokinetics and safety of weekly dapsone and dapsone plus pyrimethamine for prevention of pneumocystis pneumonia

The safety and pharmacokinetics of weekly dapsone and weekly dapsone plus pyrimethamine were examined in adult patients with human immunodeficiency virus infection who were at risk for pneumocystis pneumonia because of a prior episode or a CD4+ T-cell count less than 250 cells per mm3. Groups of patients received 100, 200, and 300 mg of dapsone as a single weekly dose. The maximum tolerated dose of weekly dapsone was established as 200 mg per week in patients receiving at least 500 mg of zidovudine concomitantly. This dose of dapsone was then found to be well tolerated when combined with pyrimethamine at 25 mg. Further patients were randomized to dapsone at 200 mg or dapsone at 200 mg plus pyrimethamine at 25 mg once weekly. Twenty-six patients each were followed for a median of 33 weeks on dapsone alone and 45 weeks on the combination. Seven patients in each group withdrew because of toxicity. Five patients receiving dapsone developed documented pneumocystis pneumonia, while four and two patients receiving dapsone plus pyrimethamine developed documented and presumptive pneumocystis pneumonia, respectively. To evaluate the tolerability of a higher dose of pyrimethamine, 11 patients had their regimen changed to dapsone at 200 mg plus pyrimethamine at 75 mg, which was well tolerated by 10 of the patients for a median period of 11 weeks. The pharmacokinetics of dapsone and pyrimethamine were examined by using a population pharmacokinetic model. Decreases in the apparent volume of the peripheral compartment were observed when multiple-dose regimens of dapsone were compared with single-dose dapsone and when multiple-dose regimens of dapsone with pyrimethamine were compared with multiple-dose dapsone alone. When administered weekly, dapsone at 200 mg and dapsone at 200 mg with pyrimethamine at 25 mg are both well-tolerated regimens. This preliminary study suggests that the efficacy of these regimens in preventing pneumocystis pneumonia, however, may be less than that of trimethoprim-sulfamethoxazole.

Levels of dapsone and pyrimethamine in serum during once-weekly dosing for prophylaxis of Pneumocystis carinii pneumonia and toxoplasmic encephalitis

Concentrations of dapsone, monoacetyldapsone, and pyrimethamine were determined in 36 serum samples from human immunodeficiency virus-infected patients on prophylaxis with once-weekly administration of dapsone-pyrimethamine (200 mg of dapsone-75 mg of pyrimethamine). During day 1 after ingestion, median levels of 1,038 ng of dapsone per ml and 356 ng of pyrimethamine per ml were found. During days 6 to 7, the dapsone level fell to < 20 ng/ml in five of nine serum samples, but the pyrimethamine level remained elevated (125 ng/ml). Concurrent, but separately ingested, didanosine administration did not seem to decrease the drug concentrations.

Randomized trial of dapsone and aerosolized pentamidine for the prophylaxis of Pneumocystis carinii pneumonia and toxoplasmic encephalitis

PURPOSE

Pneumocystis carinii pneumonia (PCP) and toxoplasmic encephalitis are the most frequent pulmonary and central nervous system opportunistic infections associated with human immunodeficiency virus (HIV) infection. We designed a prospective study to compare the effects of aerosolized pentamidine and dapsone in the prophylaxis of these infections in HIV-infected persons with CD4+ lymphocyte counts less than 250/mm3.

PATIENTS AND METHODS

Two hundred seventy-eight patients seropositive for HIV who had acquired immunodeficiency syndrome (AIDS) or advanced AIDS-related complex were randomly assigned to receive intermittent dapsone (100 mg twice weekly) or aerosolized pentamidine (100 mg every 2 weeks). The proportion of patients remaining free of PCP or toxoplasmosis was analyzed with the log-rank test as a function of time, as were the effects of zidovudine or prophylaxis on survival.

RESULTS

Dapsone and aerosolized pentamidine demonstrated similar efficacy in the primary and secondary prophylaxis of PCP, with 15 (18%) failures among patients receiving dapsone compared to 15 (14%) among those receiving aerosolized pentamidine (p = 0.4), after a mean length of follow-up of 42 and 44 weeks, respectively. Dapsone was more effective in the primary prophylaxis of toxoplasmic encephalitis, with six toxoplasmic encephalitis events occurring among those receiving aerosolized pentamidine, compared to none among those taking dapsone (p = 0.01). Primary prophylaxis for PCP was more effective than secondary prophylaxis with either therapy. Zidovudine therapy did not prevent PCP yet prolonged the PCP-free interval for those in whom either prophylactic therapy failed. Kaplan-Meier estimates did not show a difference in survival between the patients receiving either therapy, yet zidovudine use was associated with improved survival, independent of race and risk factor (Cox proportional hazards model, p = 0.001). The 1-month survival for patients developing PCP despite prophylaxis was better with those in whom dapsone failed than it was for those in whom aerosolized pentamidine failed (p = 0.08).

CONCLUSION

Dapsone is as effective as aerosolized pentamidine in preventing PCP and has the advantage of a lower cost, easier administration, and possibly an additional preventive effect against toxoplasmosis. Zidovudine prolongs the PCP-free interval for patients receiving prophylaxis, regardless of which prophylactic agent is used.

Identification of highly potent and selective inhibitors of Toxoplasma gondii dihydrofolate reductase

Toxoplasma gondii RH was obtained in high yield from culture in RPMI medium on a line of Chinese hamster ovary cells lacking dihydrofolate reductase activity (ATCC 3952 dhfr-; American Type Culture Collection). Dihydrofolate reductase preparations from harvested organisms had specific activities of 22.9 +/- 2.1 nmol/min/mg. The 50% inhibitory concentrations against reference compounds were 0.014 microM for methotrexate, 0.24 microM for pyrimethamine, 2.7 microM for trimethoprim, and 0.010 microM for trimetrexate. The Km value for NADPH was 11 microM and followed Michaelis-Menten kinetics; the Km for dihydrofolate was ca. 11 microM, but substrate inhibition appeared to occur at high substrate concentrations. Dihydrofolate reductase from T. gondii was used to screen 130 compounds from the National Cancer Institute repository. Thirteen compounds were > 100-fold more potent than pyrimethamine toward T. gondii dihydrofolate reductase; six compounds with various potencies were 8 to 46 times as selective as pyrimethamine for the protozoal form of the enzyme over the mammalian form. Four trimetrexate analogs were more potent than trimetrexate, and two were significantly more selective. Representative compounds were also tested in a culture model of T. gondii employing uracil incorporation as an index of growth. One pyrimethamine analog was as effective as pyrimethamine in inhibiting T. gondii in culture (50% inhibitory concentration, 0.45 microM). Three other compounds were also effective at micromolar concentrations.

The multifunctional folic acid synthesis fas gene of Pneumocystis carinii encodes dihydroneopterin aldolase, hydroxymethyldihydropterin pyrophosphokinase and dihydropteroate synthase

The nucleotide sequence of a folic acid synthesis (fas) gene from Pneumocystis carinii contains an open reading frame (ORF) that predicts a protein of 740 amino acids with an M(r) of 83,979. A recombinant baculovirus was constructed which directed expression of the predicted Fas740 polypeptide in cultured Spodoptera frugiperda (SF9) insect cells. The overexpressed 'full-length' protein migrated anomalously in sodium dodecyl sulfate/polyacrylamide gels, with an apparent molecular mass of 71.5 kDa. An abundant 69-kDa species was also recognized by polyclonal sera specific for the Fas protein in immunoblotting analyses. Dihydroneopterin aldolase, dihydropterin pyrophosphokinase and dihydropteroate synthase activities were readily detected in SF9 extracts in which the 71.5/69-kDa immunoreactive species were overproduced, demonstrating that three enzyme functions involved in catalysing three sequential steps of the folate biosynthetic pathway are encoded by a single gene in P. carinii. Importantly, the polyclonal sera recognize a single 69-kDa species in P. carinii extracts suggesting that the three activities are indeed properties of a single polypeptide, although the nature of the suggested post-translational modification is unknown. Location of the individual enzyme domains with the Fas polypeptide based upon amino acid sequence similarity to their bacterial counterparts is discussed. Furthermore, expression of various truncated fas gene constructs demonstrates that the complete fas ORF, including the N-terminus of the predicted polypeptide (FasA domain) whose enzyme function is unknown, must be expressed for maximum dihydroneopterin aldolase (FasB domain) and dihydropteroate synthase (FasD domain) activities. This suggests interactions between the domains within the larger polypeptide to stabilize the functions of these two enzymes. The FasC domain, which contains 6-hydroxymethyl-7,8-dihydropterin pyrophosphokinase activity, is able to fold and function independently of the other domains. The requirement by mammalian cells for preformed folates, and the absence of dihydroneopterin aldolase, 6-hydroxymethyl-7,8-dihydropterin pyrophosphokinase and dihydropteroate synthase from these tissues opens up the possibility of designing highly selective drugs which inhibit these unique targets.

Inhibition of Pneumocystis carinii dihydropteroate synthetase by para-acetamidobenzoic acid: possible mechanism of action of isoprinosine in human immunodeficiency virus infection

Isoprinosine has been reported to decrease progression to AIDS, primarily by preventing Pneumocystis carinii pneumonia (PCP), in human immunodeficiency virus-infected patients, but the mechanism of action is unknown. para-Acetamidobenzoic acid (PAcBA), one component of isoprinosine, is structurally related to para-aminobenzoic acid (PABA), a precursor of de novo folate synthesis. This pathway is known to be important for P. carinii because sulfonamides, which are effective anti-P. carinii agents, inhibit incorporation of PABA into folate precursors by the enzyme dihydropteroate synthetase (DHPS). Inhibition of P. carinii DHPS by PAcBA was investigated by using two assays. In short-term cultures of P. carinii from rats, [3H]PABA incorporation into reduced folates was inhibited by both isoprinosine (mean +/- standard error 50% inhibitory concentration [IC50], 20 +/- 8.4 microM) and PAcBA free acid (IC50, 240 +/- 100 microM); a soluble PAcBA salt was more potent than PAcBA free acid alone (IC50, 29 +/- 48 microM). The activity of PAcBA free acid was confirmed in a cell-free DHPS inhibition assay (IC50, 120 +/- 120 microM). Inosine and dimethylaminopropanol, two other components of isoprinosine, were poor inhibitors of PABA incorporation (IC50, > 1,000 microM). PAcBA free acid also showed activity in inhibiting the DHPS of Toxoplasma gondii, but was a poor inhibitor of the DHPSs of Escherichia coli and Saccharomyces cerevisiae. In a rat model of PCP, the PAcBA salt administered intraperitoneally demonstrated no activity against established PCP either alone or when used in combination with trimethoprim; the lack of efficacy in this model may be due to the rapid metabolism of the drug. Prevention of PCP by PaCBA through inhibition of P. carinii DHPS may explain the activity of isoprinosine in decreasing the progression to AIDS in human immunodeficiency virus-infected patients.

Dapsone-pyrimethamine compared with aerosolized pentamidine as primary prophylaxis against Pneumocystis carinii pneumonia and toxoplasmosis in HIV infection. The PRIO Study Group

BACKGROUND

Pneumocystis carinii pneumonia and toxoplasmic encephalitis are frequent life-threatening opportunistic infections in patients with human immunodeficiency virus (HIV) infection. Primary prophylaxis against P. carinii pneumonia is now common, but there are few data on regimens for primary prophylaxis against toxoplasmosis.

METHODS

We conducted a randomized trial that compared two prophylactic regimens: dapsone (50 mg per day) plus pyrimethamine (50 mg per week) was compared with aerosolized pentamidine (300 mg per month). The patients had symptomatic HIV infection, no history of P. carinii pneumonia or symptomatic toxoplasmosis, and CD4+ counts below 200 per cubic millimeter (0.2 x 10(9) per liter).

RESULTS

In an intention-to-treat analysis, after a median follow-up of 539 days P. carinii pneumonia developed in 10 patients in each group, whereas toxoplasmosis developed in 32 of 176 patients in the pentamidine group and 19 of 173 patients in the dapsone-pyrimethamine group. Those assigned to pentamidine had a risk of P. carinii pneumonia that was similar to the risk in those assigned to dapsone-pyrimethamine (adjusted relative risk, 1.13; 95 percent confidence interval, 0.44 to 2.92; P = 0.79), but a higher risk of toxoplasmosis (adjusted relative risk, 1.81; 95 percent confidence interval, 1.12 to 2.94; P = 0.02). Among the 262 patients with serologic evidence of past exposure to Toxoplasma gondii, the relative risk of symptomatic toxoplasmosis was 2.37 times higher in those assigned to pentamidine (95 percent confidence interval, 1.3 to 4.4; P = 0.006). More patients discontinued dapsone-pyrimethamine than pentamidine because of toxicity (42 vs. 3; P < 0.001). Survival was similar in the two groups.

CONCLUSIONS

For primary prevention of P. carinii pneumonia, dapsone-pyrimethamine is as effective, though not as well tolerated, as aerosolized pentamidine. Dapsone-pyrimethamine also prevents first episodes of toxoplasmosis.

Cloning, sequence analysis, and overexpression of Escherichia coli folK, the gene coding for 7,8-dihydro-6-hydroxymethylpterin-pyrophosphokinase

The gene coding for the Escherichia coli enzyme 7,8-dihydro-6-hydroxymethylpterin-pyrophosphokinase has been cloned and sequenced. This gene, designated folK, codes for a protein of 159 amino acids, including an amino-terminal methionine. The protein was overexpressed in E. coli MC4100 by cloning the gene behind the lacUV5 promoter in a high-copy-number plasmid. The enzyme was purified to homogeneity. Amino-terminal analysis of the purified protein showed that the amino-terminal methionine had been removed. The compositional molecular mass (17,945 Da) was identical to the molecular mass determined by mass spectrometry. The enzyme was observed to have a large number of proline residues and migrated anomalously in sodium dodecyl sulfate-polyacrylamide gels, with an apparent molecular mass of 23,000 Da.

Toxoplasma gondii: characterization of a mutant resistant to sulfonamides

Sulfadiazine was a potent inhibitor of the in vitro growth of Toxoplasma gondii, although it had little effect during the first 24 hr of treatment. A mutant parasite (R-SulR-5) with a 300-fold increase in sulfadiazine resistance was selected by a combination of chemical mutagenesis and growth in gradually increased sulfadiazine concentrations. This mutant was completely cross-resistant to several other sulfonamides and to dapsone. The same concentration of p-aminobenzoic acid reversed the sulfadiazine inhibition of both mutant and wild-type parasites even though much higher concentrations of sulfadiazine were used to inhibit the mutant. Dihydropteroate synthase, a sulfonamide-sensitive enzyme in the pathway leading to dihydrofolic acid, had similar activities in wild-type and R-SulR-5 parasites. However, the mutant enzyme was 40-fold more resistant to sulfadiazine and had higher apparent Kms for both substrates, p-aminobenzoic acid and dihydropteridine pyrophosphate. The mutant was slightly less active than the wild type in the uptake of sulfadiazine.

Synergistic combination of azithromycin and sulfadiazine for treatment of toxoplasmosis in mice

Experiments were performed in vivo in a mouse model of acute toxoplasmosis to evaluate the effectiveness of the combination azithromycin/sulfadiazine. Azithromycin alone or sulfadiazine alone, at doses that did not provide any protection against death due to toxoplasmosis, were remarkably and significantly synergistic against murine toxoplasmosis when administered in combination.

Purification and partial characterization of 7,8-dihydro-6-hydroxymethylpterin-pyrophosphokinase and 7,8-dihydropteroate synthase from Escherichia coli MC4100

The enzymes 7,8-dihydroxymethylpterin-pyrophosphokinase (HPPK) and 7,8-dihydropteroate synthase (DHPS), which act sequentially in the folate pathway, were purified to homogeneity from crude extracts of Escherichia coli MC4100. The enzymes represent less than 0.01% of the total soluble protein. HPPK was purified greater than 10,000-fold; the native enzyme appears to be a monomer with a molecular mass of 25 kDa and a pI of 5.2. DHPS was purified greater than 7,000-fold; the native enzyme has an apparent molecular mass of 52 to 54 kDa and is composed of two identical 30-kDa subunits. The amino-terminal sequences for both enzymes have been determined.

Anti-Toxoplasma effects of dapsone alone and combined with pyrimethamine

The efficacy of dapsone alone or combined with pyrimethamine against Toxoplasma gondii was investigated experimentally. For in vitro studies, a sensitive immunoassay was used for assessment of Toxoplasma growth in tissue cultures; dapsone was found to have a significant inhibitory effect at a concentration of 0.5 micrograms/ml in the cultures, and the 50% inhibitory concentration was estimated to be 0.55 micrograms/ml. When pyrimethamine and dapsone were combined, an important synergistic effect which was associated with morphological alterations of the parasites was observed. In vivo studies were performed in a murine model of acute toxoplasmosis in which a tissue culture method was used to estimate the parasite burden in the blood, lungs, and brains of infected mice. Dapsone alone, which was administered at 100 mg/kg/day for 10 days from day 1 after infection, was unable to prevent parasite dissemination and only delayed the time to death of treated mice compared with the time of death of untreated controls. When dapsone and pyrimethamine (18.5 mg/kg/day) were administered in combination from day 4 after infection, parasites were cleared from blood and organs within 6 days, but relapses were observed 15 days after the cessation of therapy. When treatment was started at day 1 after infection, 100% of mice survived and relapses were not observed, suggesting a good efficacy of this combination for preventive therapy.