Chemical synthesis of the Plasmodium falciparum dihydrofolate reductase-thymidylate synthase gene

Trypanosoma brucei DHFR-TS Revisited: Characterisation of a Bifunctional and Highly Unstable Recombinant Dihydrofolate Reductase-Thymidylate Synthase

Bifunctional dihydrofolate reductase-thymidylate synthase (DHFR-TS) is a chemically and genetically validated target in African trypanosomes, causative agents of sleeping sickness in humans and nagana in cattle. Here we report the kinetic properties and sensitivity of recombinant enzyme to a range of lipophilic and classical antifolate drugs. The purified recombinant enzyme, expressed as a fusion protein with elongation factor Ts (Tsf) in ThyA- Escherichia coli, retains DHFR activity, but lacks any TS activity. TS activity was found to be extremely unstable (half-life of 28 s) following desalting of clarified bacterial lysates to remove small molecules. Stability could be improved 700-fold by inclusion of dUMP, but not by other pyrimidine or purine (deoxy)-nucleosides or nucleotides. Inclusion of dUMP during purification proved insufficient to prevent inactivation during the purification procedure. Methotrexate and trimetrexate were the most potent inhibitors of DHFR (Ki 0.1 and 0.6 nM, respectively) and FdUMP and nolatrexed of TS (Ki 14 and 39 nM, respectively). All inhibitors showed a marked drop-off in potency of 100- to 1,000-fold against trypanosomes grown in low folate medium lacking thymidine. The most potent inhibitors possessed a terminal glutamate moiety suggesting that transport or subsequent retention by polyglutamylation was important for biological activity. Supplementation of culture medium with folate markedly antagonised the potency of these folate-like inhibitors, as did thymidine in the case of the TS inhibitors raltitrexed and pemetrexed.

Expression of functional Plasmodium falciparum enzymes using a wheat germ cell-free system

One decade after the sequencing of the Plasmodium falciparum genome, 95% of malaria proteins in the genome cannot be expressed in traditional cell-based expression systems, and the targets of the best new leads for antimalarial drug discovery are either not known or not available in functional form. For a disease that kills up to 1 million people per year, routine expression of recombinant malaria proteins in functional form is needed both for the discovery of new therapeutics and for identification of targets of new drugs. We tested the general utility of cell-free systems for expressing malaria enzymes. Thirteen test enzyme sequences were reverse amplified from total RNA, cloned into a plant-like expression vector, and subjected to cell-free expression in a wheat germ system. Protein electrophoresis and autoradiography confirmed the synthesis of products of expected molecular masses. In rare problematic cases, truncated products were avoided by using synthetic genes carrying wheat codons. Scaled-up production generated 39 to 354 μg of soluble protein per 10 mg of translation lysate. Compared to rare proteins where cell-based systems do produce functional proteins, the cell-free yields are comparable or better. All 13 test products were enzymatically active, without failure. This general path to produce functional malaria proteins should now allow the community to access new tools, such as biologically active protein arrays, and lead to the discovery of new chemical functions, structures, and inhibitors of previously inaccessible malaria gene products.

Interactions between cycloguanil derivatives and wild type and resistance-associated mutant Plasmodium falciparum dihydrofolate reductases

Comparative molecular field analysis (CoMFA) and quantum chemical calculations were performed on cycloguanil (Cyc) derivatives of the wild type and the quadruple mutant (Asn51Ile, Cys59Arg, Ser108Asn, Ile164Leu) of Plasmodium falciparum dihydrofolate reductase (PfDHFR). The represented CoMFA models of wild type (r(2) = 0.727 and r(2) = 0.985) and mutant type (r(2) = 0.786 and r(2) = 0.979) can describe the differences of the Cyc structural requirements for the two types of PfDHFR enzymes and can be useful to guide the design of new inhibitors. Moreover, the obtained particular interaction energies between the Cyc and the surrounding residues in the binding pocket indicated that Asn108 of mutant enzyme was the cause of Cyc resistance by producing steric clash with p-Cl of Cyc. Consequently, comparing the energy contributions with the potent flexible WR99210 inhibitor, it was found that the key mutant residue, Asn108, demonstrates attractive interaction with this inhibitor and some residues, Leu46, Ile112, Pro113, Phe116, and Leu119, seem to perform as second binding site with WR99210. Therefore, quantum chemical calculations can be useful for investigating residue interactions to clarify the cause of drug resistance.

Heterologous expression of plasmodial proteins for structural studies and functional annotation

Malaria remains the world's most devastating tropical infectious disease with as many as 40% of the world population living in risk areas. The widespread resistance of Plasmodium parasites to the cost-effective chloroquine and antifolates has forced the introduction of more costly drug combinations, such as Coartem^®. In the absence of a vaccine in the foreseeable future, one strategy to address the growing malaria problem is to identify and characterize new and durable antimalarial drug targets, the majority of which are parasite proteins. Biochemical and structure-activity analysis of these proteins is ultimately essential in the characterization of such targets but requires large amounts of functional protein. Even though heterologous protein production has now become a relatively routine endeavour for most proteins of diverse origins, the functional expression of soluble plasmodial proteins is highly problematic and slows the progress of antimalarial drug target discovery. Here the status quo of heterologous production of plasmodial proteins is presented, constraints are highlighted and alternative strategies and hosts for functional expression and annotation of plasmodial proteins are reviewed.

Codon optimization can improve expression of human genes in Escherichia coli: A multi-gene study

The efficiency of heterologous protein production in Escherichia coli (E. coli) can be diminished by biased codon usage. Approaches normally used to overcome this problem include targeted mutagenesis to remove rare codons or the addition of rare codon tRNAs in specific cell lines. Recently, improvements in technology have enabled cost-effective production of synthetic genes, making this a feasible alternative. To explore this option, the expression patterns in E. coli of 30 human short-chain dehydrogenase/reductase genes (SDRs) were analyzed in three independent experiments, comparing the native and synthetic (codon-optimized) versions of each gene. The constructs were prepared in a pET-derived vector that appends an N-terminal polyhistidine tag to the protein; expression was induced using IPTG and soluble proteins were isolated by Ni-NTA metal-affinity chromatography. Expression of the native and synthetic gene constructs was compared in two isogenic bacterial strains, one of which contained a plasmid (pRARE2) that carries seven tRNAs recognizing rare codons. Although we found some degree of variability between experiments, in normal E. coli synthetic genes could be expressed and purified more readily than the native version. In only one case was native gene expression better. Importantly, in most but not all cases, expression of the native genes in combination with rare codon tRNAs mimicked the behavior of the synthetic genes in the native strain. The trend is that heterologous expression of some proteins in bacteria can be improved by altering codon preference, but that this effect can be generally recapitulated by introducing rare codon tRNAs into the host cell.

Plasmodium vivax dihydrofolate reductase as a target of sulpha drugs

Sulpha drugs act as competitive inhibitors of p-amino benzoic acid, an intermediate in the de novo folate pathway. Dihydropteroate synthase condenses sulpha drugs into sulpha-dihydropteroate (sulpha-DHP), which competes with dihydrofolate, the dihydrofolate reductase (DHFR) substrate. This designates DHFR as a possible target of sulpha-DHP. We suggest here that Plasmodium vivax DHFR is indeed the in vivo target of sulpha drugs. The wild-type DHFR expressed in Saccharomyces cerevisiae leads to cell growth inhibition, while sensitivity to the drug is exacerbated in the mutants. Contrary to what is observed with sulphanilamide, methotrexate is less effective on P. vivax-DHFR mutants than on wild-type mutant.

Targeting purine and pyrimidine metabolism in human apicomplexan parasites

Synthesis de novo, acquisition by salvage and interconversion of purines and pyrimidines represent the fundamental requirements for their eventual assembly into nucleic acids as nucleotides and the deployment of their derivatives in other biochemical pathways. A small number of drugs targeted to nucleotide metabolism, by virtue of their effect on folate biosynthesis and recycling, have been successfully used against apicomplexan parasites such as Plasmodium and Toxoplasma for many years, although resistance is now a major problem in the prevention and treatment of malaria. Many targets not involving folate metabolism have also been explored at the experimental level. However, the unravelling of the genome sequences of these eukaryotic unicellular organisms, together with increasingly sophisticated molecular analyses, opens up possibilities of introducing new drugs that could interfere with these processes. This review examines the status of established drugs of this type and the potential for further exploiting the vulnerability of apicomplexan human pathogens to inhibition of this key area of metabolism.

Eosin B as a novel antimalarial agent for drug-resistant Plasmodium falciparum

4',5'-Dibromo-2',7'-dinitrofluorescein, a red dye commonly referred to as eosin B, inhibits Toxoplasma gondii in both enzymatic and cell culture studies with a 50% inhibitory concentration (IC(50)) of 180 microM. As a non-active-site inhibitor of the bifunctional T. gondii dihydrofolate reductase-thymidylate synthase (DHFR-TS), eosin B offers a novel mechanism for inhibition of the parasitic folate biosynthesis pathway. In the present study, eosin B was further evaluated as a potential antiparasitic compound through in vitro and cell culture testing of its effects on Plasmodium falciparum. Our data revealed that eosin B is a highly selective, potent inhibitor of a variety of drug-resistant malarial strains, with an average IC(50) of 124 nM. Furthermore, there is no indication of cross-resistance with other clinically utilized compounds, suggesting that eosin B is acting via a novel mechanism. The antimalarial mode of action appears to be multifaceted and includes extensive damage to membranes, the alteration of intracellular organelles, and enzymatic inhibition not only of DHFR-TS but also of glutathione reductase and thioredoxin reductase. In addition, preliminary studies suggest that eosin B is also acting as a redox cycling compound. Overall, our data suggest that eosin B is an effective lead compound for the development of new, more effective antimalarial drugs.

Influence of glycosylphosphatidylinositol anchorage on the efficacy of DNA vaccines encoding Plasmodium yoelii merozoite surface protein 4/5

Immune responses induced to DNA vaccination vary considerably and depend on a variety of factors, including the physical form in which the antigen is expressed by target cells and presented to the immune system. Data on the effect of these factors will aid improved design of DNA vaccines and facilitate their further development. We examined the effect of different forms of surface anchoring on the immunogenicity of a DNA vaccine. A number of constructs were generated encoding Plasmodium yoelii merozoite surface protein 4/5 (PyMSP4/5) with or without its C-terminal glycosylphosphatidylinositol (GPI) attachment signal, replacing the endogenous GPI signal of PyMSP4/5 with that of mouse decay-accelerating factor (DAF), a well-established model for GPI-anchoring in mammalian cells, or the transmembrane anchor and cytoplasmic tail of mouse tissue factor (TF). All constructs were demonstrated to express the full-length PyMSP4/5 in transfected COS cells and induce PyMSP4/5-specific antibodies in mice. The GPI attachment signal of PyMSP4/5 was found to function poorly in mammalian cells and result in a much lower level of PyMSP4/5 expression in vitro than its mammalian counterpart. The DNA vaccine containing the mammalian GPI attachment signal induced the highest levels of antibodies and impacted Ig isotype distribution, consistent with the presence of a CD1-restricted pathway of Ig formation to GPI-anchored membrane proteins. Despite the induction of specific antibodies, none of these DNA vaccines induced sufficient levels of antibodies to protect mice against a lethal challenge with P. yoelii.

Enhanced expression of a recombinant malaria candidate vaccine in Escherichia coli by codon optimization

This study was conducted to compare the expression of three constructs of a multistage candidate vaccine (FALVAC-1) against Plasmodium falciparum in an Escherichia coli system: a synthetic gene with P. falciparum codons, a synthetic gene with optimized E. coli codons, and a synthetic gene with P. falciparum codons co-transformed with a RIG plasmid, which encodes three tRNAs (AG(A/G), ATA, GGA) that recognize rare E. coli codons. The expression of the protein increased at least threefold with codon optimization. The presence of the RIG plasmid in the co-transforming cells did not significantly increase the expression level of the gene with P. falciparum codons. The growth of cells transformed by the construct with P. falciparum codons was significantly slower than that of cells transformed by the construct with optimized E. coli codons after induction of protein expression with IPTG. The cells containing the non-codon optimized gene co-expressed with RIG plasmid had the slowest growth at all time points in culture. Thus, codon optimization significantly increases the yield of P. falciparum candidate vaccines in the E. coli expression system.

Codon optimization reveals critical factors for high level expression of two rare codon genes in Escherichia coli: RNA stability and secondary structure but not tRNA abundance

Expression patterns in Escherichia coli of two small archaeal proteins with a natural content of about 30% rare codons were analyzed. The proteins, a histone-like protein from Sulfolobus shibatae (Ssh10), and a glutaredoxin-like protein from Methanobacterium thermoautotrophicum (mtGrx), were produced with expression plasmids encoding wild-type genes, codon-optimized synthetic, and GST-fusion genes. These constructs were expressed in BL21 (DE3), its LysS derivative, and modified strains carrying copies for rare codon tRNAs or deletions in the RNAseE gene. Both Ssh10 and mtGrx expression levels were constitutively high in BL21(DE3) and its derivatives, with the exception of the LysS phenotype, which prevented high level expression of the Ssh10 wild-type gene. Surprisingly, a codon-optimized mtGrx gene construct displayed undetectable levels of protein production. The translational block observed with the synthetic mtGrx gene could be circumvented by using a synthetic mtGrx-glutathione S-transferase (GST) fusion construct or by in vitro translation. Taken together, the results underscore the importance of mRNA levels and RNA stability, but not necessarily tRNA abundance for efficient heterologous protein production in E. coli.

Analysis of the antimalarial drug resistance protein Pfcrt expressed in yeast

Mutations in the novel membrane protein Pfcrt were recently found to be essential for chloroquine resistance (CQR) in Plasmodium falciparum, the parasite responsible for most lethal human malaria (Fidock, D. A., Nomura, T., Talley, A. K., Cooper, R. A., Dzekunov, S. M., Ferdig, M. T., Ursos, L. M., Sidhu, A. B., Naude, B., Deitsch, K. W., Su, X. Z., Wootton, J. C., Roepe, P. D., and Wellems, T. E. (2000) Mol. Cell 6, 861-871). Pfcrt is localized to the digestive vacuolar membrane of the intraerythrocytic parasite and may function as a transporter. Study of this putative transport function would be greatly assisted by overexpression in yeast followed by characterization of membrane vesicles. Unfortunately, the very high AT content of malarial genes precludes efficient heterologous expression. Thus, we back-translated Pfcrt to design idealized genes with preferred yeast codons, no long poly(A) sequences, and minimal stem-loop structure. We synthesized a designed gene with a two-step PCR method, fused this to N- and C-terminal sequences to aid membrane insertion and purification, and now report efficient expression of wild type and mutant Pfcrt proteins in the plasma membrane of Saccharomyces cerevisiae and Pichia pastoris yeast. To our knowledge, this is the first successful expression of a full-length malarial parasite integral membrane protein in yeast. Purified membranes and inside-out plasma membrane vesicle preparations were used to analyze wild type versus CQR-conferring mutant Pfcrt function, which may include effects on H(+) transport (Dzekunov, S., Ursos, L. M. B., and Roepe, P. D. (2000) Mol. Biochem. Parasitol. 110, 107-124), and to perfect a rapid purification of biotinylated Pfcrt. These data expand on the role of Pfcrt in conferring CQR and define a productive route for analysis of important P. falciparum transport proteins and membrane associated vaccine candidates.

The circumsporozoite protein of Plasmodium falciparum is expressed and localized to the cell surface in the free-living ciliate Tetrahymena thermophila

Heterologous expression is an important tool for characterization of protein function, structural studies, and production of antigen. While many different host systems have been utilized for the expression of Plasmodium falciparum proteins, the extreme AT-richness of its genome represents an obstacle to efficient expression. In addition, primary sequence motifs such as glycosyl phosphatidyl-inositol (GPI) cleavage/attachment sites of P. falciparum are not recognized in currently used expression hosts. Recently, DNA-mediated transformation has been used for expression of heterologous genes in the ciliated protozoan Tetrahymena thermophila. We report the stable expression of full-length P. falciparum circumsporozoite (CS) protein in T. thermophila. The expressed gene utilized the native CS protein N-terminal secretory signal sequence and the C-terminal GPI anchoring signal. Immunofluorescence imaging demonstrated that the CS protein was localized to the cell surface of Tetrahymena. Metabolic labeling with tritiated myristate resulted in incorporation of label into the recombinant CS protein, indicating that the protein was bound to the cell surface via a GPI anchor. This is the first report of the recognition of targeting and GPI anchoring signals of the P. falciparum CS protein in a heterologous expression host.

Divergent regulation of dihydrofolate reductase between malaria parasite and human host

For half a century, successful antifolate therapy against Plasmodium falciparum malaria has been attributed to host-parasite differences in drug binding to dihydrofolate reductase-thymidylate synthase (DHFR-TS). Selectivity may also arise through previously unappreciated differences in regulation of this drug target. The DHFR-TS of Plasmodium binds its cognate messenger RNA (mRNA) and inhibits its own translation. However, unlike translational regulation of DHFR or TS in humans, DHFR-TS mRNA binding is not coupled to enzyme active sites. Thus, antifolate treatment does not relieve translational inhibition and parasites cannot replenish dead enzyme.

Structure-based inhibitor screening: a family of sulfonated dye inhibitors for malaria parasite triosephosphate isomerase

The crystal structure of malaria triosephosphate isomerase (TIM) was screened against the National Cancer Institute database of three-dimensional molecular structures. Ten top-scoring commercially available compounds were analyzed for inhibition of recombinant TIM. Two anionic dyes showed inhibition of TIM at concentrations of <100 mM. Four related sulfonated dyes were identified from the literature, docked, and screened in vitro. All showed inhibition of malaria TIM. Models indicate that these compounds bind in two suggested conformations to the active site region of the TIM enzyme. These compounds may be used in rational modification procedures for the synthesis of lead anti-TIM drugs.

Overcoming codon bias: a method for high-level overexpression of Plasmodium and other AT-rich parasite genes in Escherichia coli

Parasite genes often use codons which are rarely used in the highly expressed genes of Escherichia coli, possibly resulting in translational stalling and lower yields of recombinant protein. We have constructed the "RIG" plasmid to overcome the potential codon-bias problem seen in Plasmodium genes. RIG contains the genes that encode three tRNAs (Arg, Ile, Gly), which recognise rare codons found in parasite genes. When co-transformed into E. coli along with expression plasmids containing parasite genes, RIG can greatly increase levels of overexpressed protein. Codon frequency analysis suggests that RIG may be applied to a variety of protozoan and helminth genes.

PCR-based gene synthesis as an efficient approach for expression of the A+T-rich malaria genome

The A+T-rich genome of the human malaria parasite Plasmodium falciparum encodes genes of biological importance that cannot be expressed efficiently in heterologous eukaryotic systems, owing to an extremely biased codon usage and the presence of numerous cryptic polyadenylation sites. In this work we have optimized an assembly polymerase chain reaction (PCR) method for the fast and extremely accurate synthesis of a 2.1 kb Plasmodium falciparum gene (pfsub-1) encoding a subtilisin-like protease. A total of 104 oligonucleotides, designed with the aid of dedicated computer software, were assembled in a single-step PCR. The assembly was then further amplified by PCR to produce a synthetic gene which has been cloned and successfully expressed in both Pichia pastoris and recombinant baculovirus-infected High Five(TM) cells. We believe this strategy to be of special interest as it is simple, accessible and has no limitation with respect to the size of the gene to be synthesized. Used as a systematic approach for the malarial genome or any other A + T-rich organism, the method allows the rapid synthesis of a nucleotide sequence optimized for expression in the system of choice and production of sufficiently large amounts of biological material for complete molecular and structural characterization.

An overview of chemotherapeutic targets for antimalarial drug discovery

The need for new antimalarials comes from the widespread resistance to those in current use. New antimalarial targets are required to allow the discovery of chemically diverse, effective drugs. The search for such new targets and new drug chemotypes will likely be helped by the advent of functional genomics and structure-based drug design. After validation of the putative targets as those capable of providing effective and safe drugs, targets can be used as the basis for screening compounds in order to identify new leads, which, in turn, will qualify for lead optimization work. The combined use of combinatorial chemistry--to generate large numbers of structurally diverse compounds--and of high throughput screening systems--to speed up the testing of compounds--hopefully will help to optimize the process. Potential chemotherapeutic targets in the malaria parasite can be broadly classified into three categories: those involved in processes occurring in the digestive vacuole, enzymes involved in macromolecular and metabolite synthesis, and those responsible for membrane processes and signalling. The processes occurring in the digestive vacuole include haemoglobin digestion, redox processes and free radical formation, and reactions accompanying haem release followed by its polymerization into haemozoin. Many enzymes in macromolecular and metabolite synthesis are promising potential targets, some of which have been established in other microorganisms, although not yet validated for Plasmodium, with very few exceptions (such as dihydrofolate reductase). Proteins responsible for membrane processes, including trafficking and drug transport and signalling, are potentially important also to identify compounds to be used in combination with antimalarial drugs to combat resistance.

Antigenicity of recombinant proteins derived from rhoptry-associated protein 1 of Plasmodium falciparum

Rhoptry-associated protein 1 (RAP1) of Plasmodium falciparum is a potential component of a malaria vaccine. We have expressed in Escherichia coli eight recombinant RAP1 proteins representing almost the entire sequence of the mature protein and assessed the antigenicity of the proteins by immunization of mice. Antisera to six of the recombinant proteins reacted specifically with parasite-derived RAP1 (PfRAP1), as determined by indirect immunofluorescence and by immunoblotting. These proteins were then used in enzyme-linked immunosorbent assays to evaluate human antibody responses to RAP1 during naturally transmitted infections in The Gambia. Immunoglobulin G (IgG) antibodies specifically reactive with the recombinant RAP1 proteins are directed mostly towards fragments containing the N-terminal sequences of mature PfRAP1. The most N-terminal segment (residues 23 to 175) contains only minor epitopes, while major epitopes are outside this region. Antibodies from malaria patients do not compete for a linear epitope recognized by an inhibitory anti-RAP1 monoclonal antibody. Analysis of IgG subclass distribution shows that human anti-RAP1 antibodies are predominantly IgG1.

A biochemical and genetic model for parasite resistance to antifolates. Toxoplasma gondii provides insights into pyrimethamine and cycloguanil resistance in Plasmodium falciparum

We have exploited the experimental accessibility of the protozoan parasite Toxoplasma gondii and its similarity to Plasmodium falciparum to investigate the influence of specific dihydrofolate reductase polymorphisms known from field isolates of drug-resistant malaria. By engineering appropriate recombinant shuttle vectors, it is feasible to examine mutations by transient or stable transformation of T. gondii parasites, in bacterial and yeast complementation assays, and through biochemical analysis of purified enzyme. A series of mutant alleles that mirror P. falciparum variants reveals that the key mutation Asn-108 (Asn-83 in T. gondii) probably confers resistance to pyrimethamine by affecting critical interactions in the ternary complex. Mutations such as Arg-59 (T. gondii 36) have limited effect in isolation, but in combination with other mutations they enhance the competitive ability of folate by increasing the speed of product turnover. Val-16 (T. gondii 10) confers low level resistance to cycloguanil but hypersensitivity to pyrimethamine. This mutation precludes Asn-108, probably because compression of the folate binding pocket introduced by this combination is incompatible with enzyme function. These studies permit detailed biochemical, kinetic, and structural analysis of drug resistance mutations and reconstruction of the probable phylogeny of antifolate resistance in malaria.

Plasmodium falciparum: asparagine mutant at residue 108 of dihydrofolate reductase is an optimal antifolate-resistant single mutant

The codon for serine residue 108 of the Plasmodium falciparum dihydrofolate reductase gene was replaced with those for the other 19 amino acids. Except for the Lys108 mutant, which was not expressed, all other substitutions yielded DHFR mutants which were expressed in Escherichia coli as inactive inclusion bodies. Nine of the mutants--Asn108, Thr108, Gly108, Ala108, Gln108, Cys108, Val108, Leu108, and Met108--yielded active DHFR upon refolding of the protein from the inclusion bodies. The remaining mutants--IIe108, Arg108, Pro108, Asp108, His108, Tyr108, Phe108, Trp108, and Glu108--did not exhibit detectable DHFR activity on refolding. The Asn108 mutant had almost unperturbed kinetic parameters but conferred resistance to pyrimethamine and cycloguanil; other active mutants showed poorer DHFR activity. We purified and characterized four mutants which produced highest DHFR activity, i.e., the Gln108, Gly108, Cys108, and Ala108 mutants. These mutant enzymes had kcat/K(m) values ranging from 7 to 22% of the wild-type enzyme. While DHFRs from Gly108, Cys108, and Ala108 mutants were as susceptible to pyrimethamine and cycloguanil as the wild type, the Gln108 mutation conferred high resistance to both inhibitors. Our data suggest that residue 108 is important for antifolate binding, and that the Ser108 to Asn108 mutation was selected in nature because of (i) the need for only a single base change, (ii) its good activity, and (iii) its resistance to antifolates.