Spectrophotometric detection of susceptibility to anti-malarial drugs

BACKGROUND

With malaria drug resistance increasing in prevalence and severity, new technologies are needed to aid and improve the accuracy and clinical relevance of laboratory or field testing for malaria drug resistance. This study presents a method based on simple and reagentless spectroscopic measurements coupled with comprehensive spectral interpretation analysis that provides valuable quantitative information on the morphological and compositional responses of Plasmodium falciparum and infected red blood cells (IRBCs) to anti-malarial treatment.

METHODS

The changes in the size, internal structure, nucleotide and haemozoin composition of the parasites as well as the morphology (size and shape) and haemoglobin composition of the IRBCs treated with dihydroartemisinin (DHA) and mefloquine (MFQ) were investigated using a spectral interpretation analysis.

RESULTS

DHA treatment reduced the sizes of the parasites and their structural organelles. The haemoglobin composition of the host IRBCs determined from spectroscopic analysis changed negligibly following DHA treatment. MFQ treated parasites grew to the same size as those from parallel non-treated cultures but lacked haemozoin. Lesser deformation of the cell shape and no haemoglobin depletion were detected for the IRBCs of MFQ treated cultures.

CONCLUSIONS

The spectroscopic analysis method proved to be sensitive for recognition of the effects of anti-malarial treatment on the structure and composition of the parasites and IRBCs. The method can have significant potential for research and clinical applications such as evaluating patient specimens for drug action, drug effects or for therapeutic monitoring.

The effects of α1-acid glycoprotein on the reversal of chloroquine resistance inPlasmodium falciparum

An in-vitro model based on the semi-automated microdilution technique has been developed for selecting compounds that might be used clinically for the reversal of chloroquine resistance. This was used initially to test the susceptibility of Plasmodium falciparum clone W2 to chloroquine (CQ). The model was then employed to investigate the effects of each of four resistance-reversing agents (verapamil, desipramine, chlorpheniramine and promethazine, at 1 microM) on this parasite's susceptibility to CQ, with and without alpha(1)-acid glycoprotein (AGP), at a patho-physiological concentration (1.25 g/litre), in the culture medium. In the absence of AGP, each of the resistance-reversing agents reduced the median inhibitory concentrations of CQ by 82%-97%, from a baseline value of about 94 ng/ml. In the presence of AGP, however, most of the resistance-reversing agents had much less effect. There appears to be competitive interaction between CQ, the resistance-reversing agents and AGP. The binding kinetics between CQ, resistance-reversing agents, AGP and other plasma proteins will clearly need to elucidated if clinically effective resistance-reversing agents are to be selected in vitro.

Antimalarial activity of 9a-N substituted 15-membered azalides with improved in vitro and in vivo activity over azithromycin

Novel classes of antimalarial drugs are needed due to emerging drug resistance. Azithromycin, the first macrolide investigated for malaria treatment and prophylaxis, failed as a single agent and thus novel analogues were envisaged as the next generation with improved activity. We synthesized 42 new 9a-N substituted 15-membered azalides with amide and amine functionalities via simple and inexpensive chemical procedures using easily available building blocks. These compounds exhibited marked advances over azithromycin in vitro in terms of potency against Plasmodium falciparum (over 100-fold) and high selectivity for the parasite and were characterized by moderate oral bioavailability in vivo. Two amines and one amide derivative showed improved in vivo potency in comparison to azithromycin when tested in a mouse efficacy model. Results obtained for compound 6u, including improved in vitro potency, good pharmacokinetic parameters, and in vivo efficacy higher than azithromycin and comparable to chloroquine, warrant its further development for malaria treatment and prophylaxis.

Synthesis, structure-activity relationship, and antimalarial activity of ureas and thioureas of 15-membered azalides

Azithromycin, a first member of the azalide family of macrolides, while having substantial antimalarial activity, failed as a single agent for malaria prophylaxis. In this paper we present the first analogue campaign to identify more potent compounds from this class. Ureas and thioureas of 15-membered azalides, N''-substituted 9a-(N'-carbamoyl-β-aminoethyl), 9a-(N'-thiocarbamoyl-β-aminoethyl), 9a-[N'-(β-cyanoethyl)-N'-(carbamoyl-β-aminoethyl)], 9a-[N'-(β-cyanoethyl)-N'-(thiocarbamoyl-β-aminoethyl)], 9a-{N'-[β-(ethoxycarbonyl)ethyl]-N'(carbamoyl-β-aminoethyl)}, and 9a-[N'-(β-amidoethyl)-N'-(carbamoyl-β-aminoethyl)] of 9-deoxo-9-dihydro-9a-aza-9a-homoerythromycin A, were synthesized and their biological properties evaluated. The results obtained indicate a substantial improvement of the in vitro activity against P. falciparum (up to 88 times over azithromycin), particularly for compounds containing both sugars on the macrocyclic ring and aromatic moiety on 9a-position. The improved in vitro activity was not confirmed in the mouse model, likely due to an increase in lipophilicity of these analogues leading to a higher volume of distribution. Overall, with increased in vitro activity, promising PK properties, and modest in vivo efficacy, this series of molecules represents a good starting platform for the design of novel antimalarial azalides.

Quantitative analysis of morphological alterations in Plasmodium falciparum infected red blood cells through theoretical interpretation of spectral measurements

Spectroscopic analysis can provide valuable insights into morphological and biochemical cellular transformations caused by diseases. However, traditional spectroscopic methods and the corresponding spectral interpretation approaches have been challenged by the complexities of the cell shape, orientation, and internal structure. Here we present an elegant spectral interpretation model that enables accurate quantitative analysis of the UV-visible spectra of red blood cells (RBCs) parasitized by the lethal human malaria parasite, Plasmodium falciparum. The model is based on the modified Mie theory (MMT) approach that incorporates the effects of the nonsphericity and orientation and multilayered cell structure to account for complex composition of the infected RBCs (IRBCs). We determine the structure and composition of the IRBCs and address unresolved matters over the alterations induced by the intraerythrocytic development of P. falciparum. The results indicate deformation and swelling of the IRBCs during the trophozoite stage of P. falciparum that is followed by substantial shrinkage during the schizont stages. We determine that up to 90% depletion of hemoglobin from the RBC cytosol does not lead to a net loss of iron from the infected cells. We quantitatively follow the morphological changes in the parasites during the intraerythrocytic development by applying the interpretation model to the UV-visible spectroscopic measurements of the IRBCs. We expect this method of quantitative spectroscopic characterization of the diseased cells to have practical clinical utility for rapid diagnosis, therapeutic monitoring, and drug susceptibility testing.

An outbreak of Plasmodium falciparum malaria in U.S. Marines deployed to Liberia

In 2003, 44 U.S. Marines were evacuated from Liberia with either confirmed or presumed Plasmodium falciparum malaria. An outbreak investigation showed that only 19 (45%) used insect repellent, 5 (12%) used permethrin-treated clothing, and none used bed netting. Adherence with weekly mefloquine (MQ) was reported by 23 (55%). However, only 4 (10%) had serum MQ levels high enough to correlate with protection (> 794 ng/mL), and 9 (22%) had evidence of steady-state kinetics (MQ carboxy metabolite/MQ > 3.79). Tablets collected from Marines met USP identity and dissolution specifications for MQ. Testing failed to identify P. falciparum isolates with MQ resistance. This outbreak resulted from under use of personal protective measures and inadequate adherence with chemophrophylaxis. It is essential that all international travelers make malaria prevention measures a priority, especially when embarking to regions of the world with high transmission intensity such as west Africa..

Novel 4-aminoquinolines active against chloroquine-resistant and sensitive P. falciparum strains that also inhibit botulinum serotype A

We report on the initial result of the coupling of 4-amino-7-chloroquinoline with steroidal and adamantane constituents to provide small molecules with excellent in vitro antimalarial activities (IC90 (W2) down to 6.74 nM). The same entities also inhibit the botulinum neurotoxin serotype A light chain metalloprotease at low micromolar levels (7-31 microM). Interestingly, structural features imparting increased antimalarial activity also provide increased metalloprotease inhibition, thus allowing for simultaneous compound optimizations against distinct targets.

Chemical stability of the peroxide bond enables diversified synthesis of potent tetraoxane antimalarials

Of 17 prepared 1,2,4,5-tetraoxacyclohexanes stable to reductive and acidic conditions, 3 of them were more active than artemisinin against CQ and MFQ resistant strain TM91C235 and all compounds were more active in vitro against W2 than against D6 strain. In vivo, amines 10 and 11a cured all mice at higher doses with MCD < or = 37.5 (mg/kg)/day. Triol 13 was exceptionally active against melanoma (LOX IMVI) and ovarian cancer (IGROV1), both with LC 50 = 60 nM.

Novel triazine JPC-2067-B inhibits Toxoplasma gondii in vitro and in vivo

BACKGROUND AND METHODOLOGY

Toxoplasma gondii causes substantial morbidity, mortality, and costs for healthcare in the developed and developing world. Current medicines are not well tolerated and cause hypersensitivity reactions. The dihydrotriazine JPC-2067-B (4, 6-diamino-1, 2-dihydro-2, 2-dimethyl-1-(3'(2-chloro-, 4-trifluoromethoxyphenoxy)propyloxy)-1, 3, 5-triazine), which inhibits dihydrofolate reductase (DHFR), is highly effective against Plasmodium falciparum, Plasmodium vivax, and apicomplexans related to T. gondii. JPC-2067-B is the primary metabolite of the orally active biguanide JPC-2056 1-(3'-(2-chloro-4-trifluoromethoxyphenyloxy)propyl oxy)- 5-isopropylbiguanide, which is being advanced to clinical trials for malaria. Efficacy of the prodrug JPC-2056 and the active metabolite JPC-2067-B against T. gondii and T. gondii DHFR as well as toxicity toward mammalian cells were tested.

PRINCIPAL FINDINGS AND CONCLUSIONS

Herein, we found that JPC-2067-B is highly effective against T. gondii. We demonstrate that JPC-2067-B inhibits T. gondii growth in culture (IC50 20 nM), inhibits the purified enzyme (IC50 6.5 nM), is more efficacious than pyrimethamine, and is cidal in vitro. JPC-2067-B administered parenterally and the orally administered pro-drug (JPC-2056) are also effective against T. gondii tachyzoites in vivo. A molecular model of T. gondii DHFR-TS complexed with JPC-2067-B was developed. We found that the three main parasite clonal types and isolates from South and Central America, the United States, Canada, China, and Sri Lanka have the same amino acid sequences preserving key binding sites for the triazine.

SIGNIFICANCE

JPC-2056/JPC-2067-B have potential to be more effective and possibly less toxic treatments for toxoplasmosis than currently available medicines.

Deoxycholic Acid-Derived Tetraoxane Antimalarials and Antiproliferatives

The synthesis of deoxycholic acid (DCA)- and cholic acid (CA)-derived mixed tetraoxanes revealed that N-(2-dimethylamino)ethyl derivatives are potent antimalarials in vitro and in vivo. The tetraoxanes presented in this paper are dual inhibitors: besides curing mice in vivo without observed toxic effects, they kill cancer cell lines at very low concentrations. For example, DCA and CA derivatives 16 and 25 cured 3/5 (160 mg/kg/day) and 2/5 (40 mg/kg/day, MTD >960 mg/kg), respectively, and they were extremely active against melanoma LOX IMVI cancer, LC50 = 22 nM and 69 nM, respectively.

Pharmacokinetics, safety, and hydrolysis of oral pyrroloquinazolinediamines administered in single and multiple doses in rats

Pyrroloquinazolinediamine (PQD) derivatives such as tetra-acetamide PQD (PQD-A4) and bis-ethylcarbamyl PQD (PQD-BE) were much safer (with therapeutic indices of 80 and 32, respectively) than their parent compound, PQD (therapeutic index, 10). Further evaluation of PQD-A4 and PQD-BE in single and multiple pharmacokinetic (PK) studies as well as corresponding toxicity studies was conducted with rats. PQD-A4 could be converted to two intermediate metabolites (monoacetamide PQD and bisacetamide PQD) first and then to the final metabolite, PQD, while PQD-BE was directly hydrolyzed to PQD without precursor and intermediate metabolites. Maximum tolerant doses showed that PQD-A4 and PQD-BE have only 1/12 and 1/6, respectively, of the toxicity of PQD after a single oral dose. Compared to the area under the concentration-time curve for PQD alone (2,965 ng.h/ml), values measured in animals treated with PQD-A4 and PQD-BE were one-third (1,047 ng.h/ml) and one-half (1,381 ng.h/ml) as high, respectively, after an equimolar dosage, suggesting that PQD was the only agent to induce the toxicity. Similar results were also shown in multiple treatments; PQD-A4 and PQD-BE generated two-fifths and three-fifths, respectively, of PQD concentrations, with 8.8-fold and 3.8-fold safety margins, respectively, over the parent drug. PK data indicated that the bioavailability of oral PQD-A4 was greatly limited at high dose levels, that PQD-A4 was slowly converted to PQD via a sequential three-step process of conversion, and that PQD-A4 was significantly less toxic than the one-step hydrolysis drug, PQD-BE. It was concluded that the slow and smaller release of PQD was the main reason for the reduction in toxicity and that the active intermediate metabolites can still maintain antimalarial potency. Therefore, the candidate with multiple-step hydrolysis of PQD could be developed as a safer potential agent for malaria treatment.

Confirmation of emergence of mutations associated with atovaquone-proguanil resistance in unexposed Plasmodium falciparum isolates from Africa

Background

In vitro and in vivo resistance of Plasmodium falciparum to atovaquone or atovaquone-proguanil hydrochloride combination has been associated to two point mutations in the parasite cytochrome b (cytb) gene (Tyr268Ser and Tyr268Asn). However, little is known about the prevalence of codon-268 mutations in natural populations of P. falciparum without previous exposure to the drug in Africa.

Methods

The prevalence of codon-268 mutations in the cytb gene of African P. falciparum isolates from Nigeria, Malawi and Senegal, where atovaquone-proguanil has not been introduced for treatment of malaria was assessed. Genotyping of the cytb gene in isolates of P. falciparum was performed by PCR-restriction fragment length polymorphism and confirmed by sequencing.

Results

295 samples from Nigeria (111), Malawi (91) and Senegal (93) were successfully analyzed for detection of either mutant Tyr268Ser or Tyr268Asn. No case of Ser268 or Asn268 was detected in cytb gene of parasites from Malawi or Senegal. However, Asn268 was detected in five out of 111 (4.5%) unexposed P. falciparum isolates from Nigeria. In addition, one out of these five mutant Asn268 isolates showed an additional cytb mutation leading to a Pro266Thr substitution inside the ubiquinone reduction site.

Conclusion

No Tyr268Ser mutation is found in cytb of P. falciparum isolates from Nigeria, Malawi or Senegal. This study reports for the first time cytb Tyr268Asn mutation in unexposed P. falciparum isolates from Nigeria. The emergence in Africa of P. falciparum isolates with cytb Tyr268Asn mutation is a matter of serious concern. Continuous monitoring of atovaquone-proguanil resistant P. falciparum in Africa is warranted for the rational use of this new antimalarial drug, especially in non-immune travelers.

Tetraoxane antimalarials and their reaction with Fe(II)

Mixed tetraoxanes 5a and 13 synthesized from cholic acid and 4-oxocyclohexanecarboxylic acid were as active as artemisinin against chloroquine-susceptible, chloroquine-resistant, and multidrug-resistant Plasmodium falciparum strains (IC50, IC90). Most active 13 is metabolically stable in in vitro metabolism studies. In vivo studies on tetraoxanes with a C(4' ') methyl group afforded compound 15, which cured 4/5 mice at 600 and 200 mg.kg-1.day-1, and 2/5 mice at 50 mg.kg-1.day-1, showing no toxic effects. Tetraoxane 19 was an extremely active antiproliferative with LC50 of 17 nM and maximum tolerated dose of 400 mg/kg. In Fe(II)-induced scission of tetraoxane antimalarials only RO* radicals were detected by EPR experiments. This finding and the indication of Fe(IV)=O species led us to propose that RO* radicals are probably capable of inducing the parasite's death. Our results suggest that C radicals are possibly not the only lethal species derived from peroxide prodrug antimalarials, as currently believed.

Triazine Inhibits Toxoplasma gondii tachyzoites in vitro and in vivo

The triazine WR99210 [4,6-diamino-1,2-dihydro-2,2-dimethyl-1-(2,4,5-trichlorophenoxypropyloxy)-1,3,5 triazine] inhibits Toxoplasma gondii in vitro at nanomolar levels (P < 0.05). The 50% inhibitory concentration (IC(50)) was approximately 50 nM. It is a potent inhibitor in vitro and is also effective in vivo. Administration of WR99210 parenterally (i.e., intraperitoneally) reduced the mean number of RH strain tachyzoites present in peritoneal fluid substantially 4 days after intraperitoneal infection of mice. There was a mean of approximately 35 million parasites in control mice as contrasted with approximately 2 million parasites in mice treated with 1.25 mg WR99210/kg of body weight in a representative experiment (P < 0.05). In addition the prodrug PS-15 N'-[3-(2,4, 5-trichlorophenoxy)propyloxy]-N9-(1-methylethyl) imidocarbonimidicdiamide is converted to 4,6-diamino-1,2-dihydro-2,2-dimethyl-1-(2,4,5-trichlorophenoxypropyloxy)-1,3,5 triazine in vivo when the prodrug is administered orally. PS-15 administered by gavage also reduced intraperitoneal RH strain T. gondii tachyzoite numbers. WR99210 has high efficacy and relatively low toxicity because of its substantial effect on T. gondii dihydrofolate reductase (DHFR) but not the mammalian host DHFR. Amino acid sequences of T. gondii, Plasmodium falciparum, and Homo sapiens DHFRs were compared. It is of interest that of the DHFR amino acids considered to be interacting with WR99210 in P. falciparum within interatomic distances within 3 to 5 A, four of eight were shared with T. gondii DHFR. H. sapiens also shared four amino acids thought to be interacting with WR99210. Efficacy of intraperitoneal administration of WR99210 and peroral administration of PS-15 demonstrate the potential usefulness of this class of compounds in treatment of toxoplasmosis administered either parenterally or perorally. The recent development program for this class of antimicrobials as antimalarials makes our proof of principle of improved efficacy of triazines (compared with the gold standard treatment, pyrimethamine) against T. gondii especially promising.

Structure identification and prophylactic antimalarial efficacy of 2-guanidinoimidazolidinedione derivatives

The reported synthetic procedure of WR182393, a 2-guanidinoimidazolidinedione derivative with high prophylactic antimalarial activity, was found to be a mixture of three closely related products. Poor solubility of WR182393 in both water and organic solvents and its impractical synthetic method have made the purification and structure identification of the reaction mixture a highly challenging task. The problems were circumvented by prodrug approach involving carbamate formation of the mixture, which enhances the solubility of the mixture in common organic solvents and facilitates the separation and structure determination of the two products. The structures of the two components were determined by X-ray crystallography and NMR of their corresponding carbamates 3a and 4a. Additional alkyl carbamates were prepared according to the same approach and two new carbamates 3b and 4d were found to possess higher intramuscular (im) efficacy than the parent compound WR182393 against Plasmodium cynomolgi in Rhesus monkey.

In vitro metabolism of phenoxypropoxybiguanide analogues in human liver microsomes to potent antimalarial dihydrotriazines

Phenoxypropoxybiguanides, such as 1 (PS-15), are prodrugs analogous to the relationship of proguanil and its active metabolite cycloguanil. Unlike cycloguanil, however, 1a (WR99210), the active metabolite of 1, has retained in vitro potency against newly emerging antifolate-resistant malaria parasites. Unfortunately, manufacturing processes and gastrointestinal intolerance have prevented the clinical development of 1. In vitro antimalarial activity and in vitro metabolism studies have been performed on newly synthesized phenoxypropoxybiguanide analogues. All of the active dihydrotriazine metabolites exhibited potent antimalarial activity with in vitro IC(50) values less than 0.04 ng/mL. In vitro metabolism studies in human liver microsomes identified the production of not only the active dihydrotriazine metabolite, but also a desalkylation on the carbonyl chain, and multiple hydroxylated metabolites. The V(max) for production of the active metabolites ranged from 10.8 to 27.7 pmol/min/mg protein with the K(m) ranging from 44.8 to 221 microM. The results of these studies will be used to guide the selection of a lead candidate.

Reduced resources applied to antimalarial drug development

This statement was made in 1984 (Ref. I): 'The Agency for International Development (AID) announced a major breakthrough in the development of a vaccine against the most deadly form of malaria in human beings. The vaccine should be ready for use around the world, especially in developing countries, within five years.' Since then, the spending on development of drugs against malaria has been on the decline. Brian Schuster and Wilbur Milhous wonder. did we declare victory too soon? Wouldn't the prudent approach have been to look at vaccines and drugs as complementary techniques rather than alternative approaches?

Pharmacokinetic investigation on the therapeutic potential of artemotil (beta-arteether) in Thai patients with severe Plasmodium falciparum malaria

Pharmacokinetic data were obtained to evaluate the therapeutic potential of Artemotil (beta-arteether) in 56 Thai patients with severe Plasmodium falciparum malaria. Intramuscular administration was given at 1) a low dose of 3.2 mg/kg on day 0 and 1.6 mg/kg/day on days 1-4 and 2) a high dose of 4.8 mg/kg on day 0 at 0 hours, 1.6 mg/kg at 6 hours, and 1.6 mg/kg/day on days 1-4. Cmax values of 63.7 ng/mL at 6.1 hours and 140.8 ng/mL at 5.7 hours were reached in low-dose and high-dose patients, respectively. Drug concentrations decreased slowly with half-lives of 12.5-22.4 hours on day 0 and 31.6-40.7 hours on day 4 for both dosage regimens. Although the maintaining dosage on the last day was much lower than the loading dose on day 0, the area under the curve (AUC) and Cmax on day 4 were significantly increased (2.85-4.55 fold), suggesting drug accumulation in the blood. Dihydroartemisinin (DHA), an active metabolite of Artemotil, was detected in most patients. The mean ratios of DHA and Artemotil were 0.16-0.19 in both dosage regimens for the entire study period. Similar to previous reports, all patients showed a slow response to treatment with mean values of 77.2 hours for the fever clearance time (FCT) and 75.8 hours for the parasite clearance time (PCT) (low dose) and 70.1 hours for the FCT and 64.4 hours for the PCT (high dose). Interestingly, a very rapid response to the treatment was exhibited in patient 151, with an FCT of 4 hours and a PCT of 36 hours, with different pharmacokinetic data from others on day 0. The patient had a very high Cmax (2,407 ng/mL) and AUC (12,259 ng.hr/mL) values without an intramuscular absorption phase on the first day. These values were approximately 21.9 (Cmax) and 2.6 (AUC) times higher than in other patients; this patient may have been to be injected through a vessel at first dosing. In conclusion, the patients treated with the high dosage regimen had higher AUC values and higher antimalarial efficiency (cure rate = 48%) than the low-dose subjects (cure rate = 23%). Despite the high accumulation and longer exposure time (9-11 days) when compared with other artemisinin agents, due to the slow prolonged absorption of Artemotil from injection sites, the two dosage regimens did not show a better therapeutic effects than other artemisinin drugs, including alpha/beta-arteether dissolved in peanut oil used in Indian patients.

Structure-activity relationship study of antimalarial indolo [2,1-b]quinazoline-6,12-diones (tryptanthrins). Three dimensional pharmacophore modeling and identification of new antimalarial candidates

A widely applicable three-dimensional QSAR pharmacophore model for antimalarial activity was developed from a set of 17 substituted antimalarial indolo[2,1-b]quinazoline-6,12-diones (tryptanthrins) that exhibited remarkable in vitro activity (below 100 ng/mL) against sensitive and multidrug-resistant Plasmodium falciparum malaria. The pharmacophore, which contains two hydrogen bond acceptors (lipid) and two hydrophobic (aromatic) features, was found to map well onto many well-known antimalarial drug classes including quinolines, chalcones, rhodamine dyes, Pfmrk cyclin dependent kinase inhibitors, malarial FabH inhibitors, and plasmepsin inhibitors. The phamacophore allowed searches for new antimalarial candidates from multiconformer 3D databases and enabled custom designed synthesis of new potent analogues.

Antimalarial and antiproliferative evaluation of bis-steroidal tetraoxanes

Several cis and trans bis-steroidal 1,2,4,5-tetraoxanes possessing amide terminus were synthesised and evaluated as antimalarials and antiproliferatives. The compounds exhibited submicromolar antimalarial activity against Plasmodium falciparum D6 and W2 strains. The existence of HN-C(O) moiety was found necessary for pronounced antimalarial and antiproliferative activity. In antiproliferative screen, the trans tetraoxane 6 was found to exhibit a pronounced cytotoxicity on 14 cancer cell lines. In addition, tetraoxanes 11 and 12 exhibited significant cytotoxic activity too; microscopic examination of treated HeLa cells showed morphological appearance reminiscent for apoptosis (condensed and/or fragmented nuclei).

A 3D QSAR pharmacophore model and quantum chemical structure--activity analysis of chloroquine(CQ)-resistance reversal

Using CATALYST, a three-dimensional QSAR pharmacophore model for chloroquine(CQ)-resistance reversal was developed from a training set of 17 compounds. These included imipramine (1), desipramine (2), and 15 of their analogues (3-17), some of which fully reversed CQ-resistance, while others were without effect. The generated pharmacophore model indicates that two aromatic hydrophobic interaction sites on the tricyclic ring and a hydrogen bond acceptor (lipid) site at the side chain, preferably on a nitrogen atom, are necessary for potent activity. Stereoelectronic properties calculated by using AM1 semiempirical calculations were consistent with the model, particularly the electrostatic potential profiles characterized by a localized negative potential region by the side chain nitrogen atom and a large region covering the aromatic ring. The calculated data further revealed that aminoalkyl substitution at the N5-position of the heterocycle and a secondary or tertiary aliphatic aminoalkyl nitrogen atom with a two or three carbon bridge to the heteroaromatic nitrogen (N5) are required for potent "resistance reversal activity". Lowest energy conformers for 1-17 were determined and optimized to afford stereoelectronic properties such as molecular orbital energies, electrostatic potentials, atomic charges, proton affinities, octanol-water partition coefficients (log P), and structural parameters. For 1-17, fairly good correlation exists between resistance reversal activity and intrinsic basicity of the nitrogen atom at the tricyclic ring system, frontier orbital energies, and lipophilicity. Significantly, nine out of 11 of a group of structurally diverse CQ-resistance reversal agents mapped very well on the 3D QSAR pharmacophore model.

Mixed steroidal 1,2,4,5-tetraoxanes: antimalarial and antimycobacterial activity

Mixed 1,2,4,5-tetraoxanes possessing simple spirocycloalkane and spirocholic acid-derived substituents were prepared and shown to have significantly higher in vitro antimalarial activity than bis-substituted tetraoxanes. Out of 41 synthesized tetraoxanes, 12 were in vitro more potent against Plasmodium falciparum chloroquine-resistant W2 clone than artemisinin, and the most potent one was 2.4 times as active as arteether. In addition, 9 compounds exhibit higher activity than chloroquine against P. falciparum chloroquine-susceptible D6 clone. Cytotoxicity was assessed for most active compounds against the Vero cell line, showing a cytotoxicity/antimalarial potency ratio of 1/(1400-9500). For the first time, tetraoxanes were screened against Mycobacterium tuberculosis with MICs as low as 4.73 microM against H37Rv strain. Mixed tetraoxanes were synthesized in a simple procedure from cholic acid methyl esters by direct coupling of steroidal gem-dihydroperoxide to simple ketones and further transformed into corresponding acids and amides.

Efficacy of proton pump inhibitor drugs against Plasmodium falciparum in vitro and their probable pharmacophores

The substituted benzimidazoles omeprazole, lansoprazole, rabeprazole, and pantoprazole were found to have in vitro activity against three different isolates of Plasmodium falciparum: D6 (which is chloroquine and pyrimethamine sensitive), W2 (chloroquine and pyrimethamine resistant), and TM91C235 (multidrug resistant). Lansoprazole and rabeprazole were the most effective against all three isolates, with a 50% inhibitory concentration (IC(50)) range of 7 to 11 microM. Omeprazole showed intermediate activity against D6 and W2 isolates, with IC(50)s of 27 to 28 microM, but had poor activity against TM91C235, with an IC(50) of 76 microM. Pantoprazole was the least effective, with IC(50)s of 73 microM against D6, 53 microM against W2, and 39 microM against TM91C235. A pharmacophore model describing the important features responsible for potent activity of the drugs was developed using computational techniques of semiempirical quantum chemical methods and the three-dimensional QSAR procedure of the CATALYST software. The important features of the pharmacophore, according to the findings based on the CATALYST procedures, are the hydrogen bond acceptor and donor sites at the benzimidine nitrogen atoms and the two aromatic hydrophobic sites in the molecules. AM1 quantum chemical calculations identified the electrostatic potential surface surrounding the sulfoxide atom as crucial for potent activity.

Design, synthesis, and evaluation of new chemosensitizers in multi-drug-resistant Plasmodium falciparum

A series of new chemosensitizers (modulators) against chloroquine-resistant Plasmodium falciparum were designed and synthesized in an attempt to fabricate modulators with enhancing drug-resistant reversing efficacy and minimal side effects. Four aromatic amine ring systems-phenothiazine, iminodibenzyl, iminostilbene, and diphenylamine-were examined. Various tertiary amino groups including either noncyclic or cyclic aliphatic amines were introduced to explore the steric tolerance at the end of the side chain. The new compounds showed better drug-resistant reversing activity in chloroquine-resistant than in mefloquine-resistant cell lines and were generally more effective against chloroquine-resistant P. falciparum isolates from Southeast Asian (W2 and TM91C235) than those from South America (PC49 and RCS). Structure-activity relationship studies revealed that elongation of the alkyl side chain of the molecule retained the chemosensitizing activity, and analogues with four-carbon side chains showed superior activity. Furthermore, new modulators with phenothiazine ring exhibited the best chemosensitizing activity among the four different ring systems examined. Terminal amino function has limited steric tolerance as evidenced by the dramatic lose of the modulating activity, when the size of substituent at the amino group increases. The best new modulator synthesized in this study possesses all three optimized structural features, which consist of a phenothiazine ring and a pyrrolidinyl group joined by a four-carbon alkyl bridge. The fractional inhibitory concentration (FIC) index of the best compound is 0.21, which is superior to that of verapamil (0.51), one of the best-known multi-drug-resistant reversing agents. Some of the analogues displayed moderate intrinsic in vitro antimalarial activity against a W-2 clone of P. falciparum.

Neurotoxicity and efficacy of arteether related to its exposure times and exposure levels in rodents

The neurotoxicity of beta-arteether (AE) is related to drug accumulation in blood due to slow and prolonged absorption from the intramuscular injection sites. In this efficacy and toxicity study of AE, the traditional sesame oil vehicle was replaced with cremophore to decrease the accumulation and toxicity of AE. Dihydroartemisinin (DQHS), a more toxic and active metabolite of AE, was also analyzed. When administered at a daily dosage of 25 mg/kg for seven days, blood accumulation of AE with sesame oil (AESO) was used had a 7.5-fold higher area under the curve (AUC) (on last versus first day dosing), while AE with cremophore (AECM) had only a 1.8-fold higher AUC. Although the accumulation of AECM was greatly reduced, its total exposure level (46.29 microg x h/ml) was 2.7-fold higher than with AESO (16.92 microg x h/ml) due to a higher bioavailability of AECM (74.5%) compared with AESO (20.3%). Total exposure time (calculated at over the minimal detected neurotoxicity level of 41.32 ng/ml) of AECM was 103 hours during the whole treatment period (192 hours), which was more than one-third (37%) less than with AESO (162 hours). Similar pharmacokinetic results were also shown with the active metabolite, DQHS. Anorexia and gastrointestinal toxicity with AESO were significantly more severe than with AECM (P < 0.001). Histopathologic examination of the brain demonstrated neurotoxic changes; the AESO rat group was significantly more severe than the AECM rat group. The brain injury scores with AECM were mild to moderate (2.3-3.0), and with AESO they were moderate to severe (3.0-4.7) on day 7 and day 10, respectively. In addition, the results of a 50% cure dose (CD50) against Plasmodium berghei in mice were 34.1 mg/kg for AESO and 14.2 mg/kg for AECM, indicating a significant higher efficacy was found in the AECM animals. Toxicity and efficacy of DQHS were also dependent on its exposure time and level, which was the same as its parent drug (AE). In conclusion, following the seven-day treatment in rats, AE and DQHS exposure time and level varied based on the vehicle used. The extension of drug exposure time and the low peak level of AE and DQHS were more associated with severe neurotoxicity and lower antimalarial efficacy, whereas the high level and short exposure time of AE and DQHS resulted in higher efficacy and milder toxicity.

Efficacy of scopadulcic acid A against Plasmodium falciparum in vitro

Scoparia dulcis is a perennial herb widely distributed in many tropical countries. It is used as an herbal remedy for gastrointestinal and many other ailments, and in Nicaragua extracts are used to treat malaria. Phytochemical screening has shown that scopadulcic acid A (SDA), scopadulcic acid B (SDB), and semisynthetic analogues are pharmacologically active compounds from S. dulcis. SDB has antiviral activity against Herpes simplex virus type 1, antitumor activity in various human cell lines, and direct inhibitory activity against porcine gastric H(+), K(+)-ATPase. A methyl ester of scopadulcic acid B showed the most potent inhibitory activity against gastric proton pumps of 30 compounds tested in one study. Compounds with antiviral, antifungal, and antitumor activity often show activity against Plasmodium falciparum. In P. falciparum, the plasma membrane and food vacuole have H(+)-ATPases and the acidocalcisome has an H(+)-Ppase. These proton pumps are potential targets for antimalarial therapy and may have their function disrupted by compounds known to inhibit gastric proton pumps. We tested pure SDA and found in vitro activity against P. falciparum with an IC(50) of 27 and 19 microM against the D6 and W2 clones, respectively. The IC(50) against the multidrug-resistant isolate, TM91C235, was 23 microM.

The shikimate pathway and its branches in apicomplexan parasites

The shikimate pathway is essential for production of a plethora of aromatic compounds in plants, bacteria, and fungi. Seven enzymes of the shikimate pathway catalyze sequential conversion of erythrose 4-phosphate and phosphoenol pyruvate to chorismate. Chorismate is then used as a substrate for other pathways that culminate in production of folates, ubiquinone, napthoquinones, and the aromatic amino acids tryptophan, phenylalanine, and tyrosine. The shikimate pathway is absent from animals and present in the apicomplexan parasites Toxoplasma gondii, Plasmodium falciparum, and Cryptosporidium parvum. Inhibition of the pathway by glyphosate is effective in controlling growth of these parasites. These findings emphasize the potential benefits of developing additional effective inhibitors of the shikimate pathway. Such inhibitors may function as broad-spectrum antimicrobial agents that are effective against bacterial and fungal pathogens and apicomplexan parasites.

Development of new drugs for chemoprophylaxis of malaria

When U.S. troops first encountered drug resistant malaria during the Vietnam war, the United States Army responded by establishing a malaria drug research program. In 1988, the Walter Reed Army Institute of Research developed mefloquine (WR 149240) and halofantrine (WR 171669). Actually, in association with SmithKline Beecham, the WRAIR is developing tafenoquine (WR 238605), an analog of primaquine, which is expected to be effective in both preventing and treating malaria in deployed military personnel. Final phase III studies leading to U.S. Food and Drug Administration approval are planned for 2000. Applied research is also carried out with the association atovaquone-proguanil (Malarone) or with azithromycin, but also with primaquine, the associations paludrine-dapsone or lapudrine-dapsone, analogs of floxacrine (WR 243251), and a guanylhydrazone (WR 182393). The future scientific directions must focus on basic and applied research for a better understanding of the modes of action and mechanisms of resistance to standard and developmental drugs. Using new techniques, the design and synthesis of new drugs would hopefully result in the development of drugs that circumvent the malaria parasites elusive mechanisms of drug resistance.

Angiogenesis inhibitors specific for methionine aminopeptidase 2 as drugs for malaria and leishmaniasis

Methionine aminopeptidase 2 (MetAP2) is responsible for the hydrolysis of the initiator methionine molecule from the majority of newly synthesized proteins. We have cloned the MetAP2 gene from the malaria parasite Plasmodium falciparum (PfMetAP2; GenBank accession number AF348320). The cloned PfMetAP2 has no intron, consists of 1,544 bp and encodes a protein of 354 amino acids with a molecular mass of 40,537 D and an overall base composition of 72.54% A + T. PfMetAP2 has 40% sequence identity with human MetAP2 and 45% identity with yeast MetAP2, and is located in chromosome 14 of P. falciparum. The three-dimensional structure of Pf MetAP2 has been modeled based on the crystal structure of human MetAP2, and several amino acid side chains protruding into the binding pocket that differ between the plasmodial and human enzyme have been identified. The specific MetAP2 inhibitors, fumagillin and TNP-470, potently blocked in vitro growth of P. falciparum and Leishmania donavani, with IC(50) values similar to the prototype drugs. Furthermore, in the case of P. falciparum, the chloroquine-resistant strains are equally susceptible to these two compounds.

Plasmodium falciparum cross-resistance between trimethoprim and pyrimethamine

Trimethoprim-sulfamethoxazole has been recommended as part of the standard package of care for people with HIV and AIDS in Africa. A similar antifolate combination, sulfadoxine-pyrimethamine, is now the first-line antimalarial drug in several of the African countries with the highest rates of HIV infection. We present evidence of Plasmodium falciparum cross-resistance between trimethoprim and pyrimethamine at the molecular level. The impact of trimethoprim-sulfamethoxazole on the efficacy of sulfadoxine-pyrimethamine needs to be assessed urgently, and alternative antimalarial treatment should be considered for people on trimethoprim-sulfamethoxazole prophylaxis.

Triclosan inhibits the growth of Plasmodium falciparum and Toxoplasma gondii by inhibition of apicomplexan Fab I

Fab I, enoyl acyl carrier protein reductase (ENR), is an enzyme used in fatty acid synthesis. It is a single chain polypeptide in plants, bacteria, and mycobacteria, but is part of a complex polypeptide in animals and fungi. Certain other enzymes in fatty acid synthesis in apicomplexan parasites appear to have multiple forms, homologous to either a plastid, plant-like single chain enzyme or more like the animal complex polypeptide chain. We identified a plant-like Fab I in Plasmodium falciparum and modelled the structure on the Brassica napus and Escherichia coli structures, alone and complexed to triclosan (5-chloro-2-[2,4 dichlorophenoxy] phenol]), which confirmed all the requisite features of an ENR and its interactions with triclosan. Like the remarkable effect of triclosan on a wide variety of bacteria, this compound markedly inhibits growth and survival of the apicomplexan parasites P. falciparum and Toxoplasma gondii at low (i.e. IC50 congruent with150-2000 and 62 ng/ml, respectively) concentrations. Discovery and characterisation of an apicomplexan Fab I and discovery of triclosan as lead compound provide means to rationally design novel inhibitory compounds.

Malaria in Brazilian military personnel deployed to Angola

BACKGROUND

Malaria represents one of the most important infectious disease threats to deployed military forces; most personnel from developed countries are nonimmune personnel and are at high risk of infection and clinical malaria. This is especially true for forces deployed to highly-endemic areas in Africa and Southeast Asia where drug-resistant malaria is common.

METHODS

We conducted an outbreak investigation of malaria cases in Angola where a total of 439 nonimmune Brazilian troops were deployed for a 6-month period in 1995-1996. A post-travel medical evaluation was also performed on 338 (77%) of the 439 soldiers upon return to Brazil. Questionnaire, medical record, thick/thin smear, and serum anti-Plasmodium falciparum antibody titer (by IFA) data were obtained. Peak serum mefloquine (M) and methylmefloquine (MM) metabolite levels were measured in a subsample of 66 soldiers (42 cases, 24 nonmalaria controls) who were taking weekly mefloquine prophylaxis (250 mg).

RESULTS

Seventy-eight cases of malaria occurred among the 439 personnel initially interviewed in Angola (attack rate = 18%). Four soldiers were hospitalized, and 3 subsequently died of cerebral malaria. Upon return to Brazil, 63 (19%) of 338 soldiers evaluated were documented to have had clinical symptoms and a diagnosis of malaria while in Angola. In addition, 37 (11%) asymptomatically infected individuals were detected upon return (< 1% parasitemia). Elevated, post-travel anti-P. falciparum IFA titers (> or = 1:64) were seen in 101 (35%) of 292 soldiers tested, and was associated with a prior history of malaria in-country (OR = 3.67, 95% CI 1.98-6.82, p <.001). Noncompliance with weekly mefloquine prophylaxis (250 mg) was associated with a malaria diagnosis in Angola (OR = 3.75, 95% CI 0.97-17.41, p =.03) but not with recent P. falciparum infection (by IFA titer). Mean peak levels (and ratios) of serum M and MM were also found to be lower in those who gave a history of malaria while in Angola.

CONCLUSIONS

Malaria was a significant cause of morbidity among Brazilian Army military personnel deployed to Angola. Mefloquine prophylaxis appeared to protect soldiers from clinical, but not subclinical, P. falciparum infections. Mefloquine noncompliance and an erratic chemoprophylaxis prevention policy contributed to this large outbreak in nonimmune personnel. This report highlights the pressing need for development of newer, more efficacious and practical, prophylactic drug regimens that will reduce the malaria threat to military forces and travelers.

Cholic acid derivatives as 1,2,4,5-tetraoxane carriers: structure and antimalarial and antiproliferative activity

Cholic acid-derived 1,2,4,5-tetraoxanes were synthesized in order to explore the influence of steroid carrier on its antimalarial and antiproliferative activity in vitro. Starting with chiral ketones, cis and trans series of diastereomeric tetraoxanes were obtained, and the cis series was found to be approximately 2 times as active as the trans against Plasmodium falciparum D6 and W2 clones. The same tendency was observed against human melanoma (Fem-X) and human cervix carcinoma (HeLa) cell lines. The amide C(24) termini, for the first time introduced into the carrier molecule of a tetraoxane pharmacophore, significantly enhanced both antimalarial and antiproliferative activity, as compared to the corresponding methyl esters, with cis-bis(N-propylamide) being most efficient against the chloroquine-susceptible D6 clone (IC(50) = 9.29 nM). cis- and trans-bis(N-propylamides) were also screened against PBMC, and PHA-stimulated PBMC, showing a cytotoxicity/antimalarial potency ratio of 1/10 000.

Synthesis and antimalarial activity of sixteen dispiro-1,2,4, 5-tetraoxanes: alkyl-substituted 7,8,15,16-tetraoxadispiro[5.2.5. 2]hexadecanes

Sixteen alkyl-substituted dispiro-1,2,4,5-tetraoxanes (7,8,15, 16-tetraoxadispiro[5.2.5.2]hexadecanes) were synthesized to explore dispiro-1,2,4,5-tetraoxane SAR and to identify tetraoxanes with better oral antimalarial activity than prototype tetraoxane 1 (WR 148999). The tetraoxanes were prepared either by peroxidation of the corresponding cyclohexanone derivatives in H(2)SO(4)/CH(3)CN or by ozonolysis of the corresponding cyclohexanone methyl oximes. Those tetraoxanes with alkyl substituents at the 1 and 10 positions were formed as single stereoisomers, whereas the five tetraoxanes formed without the stereochemical control provided by alkyl groups at the 1 and 10 positions were isolated as mixtures of diastereomers. Three of the sixteen tetraoxanes were inactive (IC(50)'s > 1000 nM), but five (2, 6, 10, 11, 12) had IC(50)'s between 10 and 30 nM against the chloroquine-sensitive D6 and chloroquine-resistant W2 clones of Plasmodium falciparum compared to corresponding IC(50)'s of 55 and 32 nM for 1 and 8.4 and 7.3 nM for artemisinin. We suggest that tetraoxanes 13, 16, and 17 were inactive and tetraoxanes 4 and 7 were weakly active due to steric effects preventing or hindering peroxide bond access to parasite heme. Tetraoxanes 1, 10, 11, and 14, along with artemisinin and arteether as controls, were administered po b.i.d. (128 mg/kg/day) to P. berghei-infected mice on days 3, 4, and 5 post-infection. At this dose, tetraoxanes 10, 11, and 14 cured between 40% and 60% of the infected animals. In comparison, artemisinin and tetraoxane 1 produced no cures, whereas arteether cured 100% of the infected animals. There was no apparent relationship between tetraoxane structure and in vitro neurotoxicity, nor was there any correlation between antimalarial activity and neurotoxicity for these seventeen tetraoxanes.

Methyl-substituted dispiro-1,2,4,5-tetraoxanes: correlations of structural studies with antimalarial activity

Two tetramethyl-substituted dispiro-1,2,4,5-tetraoxanes (7,8,15, 16-tetraoxadispiro[5.2.5.2]hexadecanes) 3 and 4 were designed as metabolically stable analogues of the dimethyl-substituted dispiro-1, 2,4,5-tetraoxane prototype WR 148999 (2). For a positive control we selected the sterically unhindered tetraoxane 5 (7,8,15, 16-tetraoxadispiro[5.2.5.2]hexadecane), devoid of any substituents. Tetraoxanes 3 and 4 were completely inactive in contrast to tetraoxanes 2 and 5. We hypothesize that the two inactive tetraoxanes possess sufficient steric hindrance about the tetraoxane ring due to the two additional axial methyl groups to prevent their activation to presumed parasiticidal carbon radicals by inhibiting electron transfer from heme or other iron(II) species. For each of the tetraoxanes 2-4, the tetraoxane and both spirocyclohexyl rings are in a chair conformation and the bond lengths and angles are all quite normal except for the C1-C2 bond which is slightly lengthened. Comparison of the modeled and X-ray structures for tetraoxanes 2-5 reveals that molecular mechanics (MMX and MM3) and 3-21G calculations each gave accurate structural parameters such as bond lengths, bond angles, and dihedral angles. In contrast, semiempirical methods such as AM1 gave poor results.

Randomized dose-ranging study of the safety and efficacy of WR 238605 (Tafenoquine) in the prevention of relapse of Plasmodium vivax malaria in Thailand

WR 238605 is an 8-aminoquinoline developed for the radical cure of Plasmodium vivax. Forty-four P. vivax-infected patients were randomly assigned to 1 of 4 treatment regimens: 3 groups received a blood schizonticidal dose of chloroquine followed by WR 238605: group A (n=15) received 300 mg daily for 7 days; group B (n=11), 500 mg daily for 3 days, repeated 1 week after the initial dose; group C (n=9), 1 dose of 500 mg. A fourth group (D; n=9) received chloroquine only. Among patients who completed 2-6 months of follow-up (n=23), there was 1 relapse in group B (day 120) and 1 in group C (day 112). Among patients treated with chloroquine only, there were 4 relapses (days 40, 43, 49, and 84). WR 238605 was safe, well tolerated, and effective in preventing P. vivax relapse.

8-Aminoquinolines active against blood stage Plasmodium falciparum in vitro inhibit hematin polymerization

From the Walter Reed Army Institute of Research (WRAIR) inventory, thirteen 8-aminoquinoline analogs of primaquine were selected for screening against a panel of seven Plasmodium falciparum clones and isolates. Six of the 13 8-aminoquinolines had average 50% inhibitory concentrations between 50 and 100 nM against these P. falciparum clones and were thus an order of magnitude more potent than primaquine. However, excluding chloroquine-resistant clones and isolates, these 8-aminoquinolines were all an order of magnitude less potent than chloroquine. None of the 8-aminoquinolines was cross resistant with either chloroquine or mefloquine. In contrast to the inactive primaquine prototype, 8 of the 13 8-aminoquinolines inhibited hematin polymerization more efficiently than did chloroquine. Although alkoxy or aryloxy substituents at position 5 uniquely endowed these 13 8-aminoquinolines with impressive schizontocidal activity, the structural specificity of inhibition of both parasite growth and hematin polymerization was low.

Bisquinolines. 2. Antimalarial N,N-bis(7-chloroquinolin-4-yl)heteroalkanediamines

N,N-Bis(7-chloroquinolin-4-yl)heteroalkanediamines 1-11 were synthesized and screened against Plasmodium falciparum in vitro and Plasmodium berghei in vivo. These bisquinolines had IC50 values from 1 to 100 nM against P. falciparum in vitro. Six of the 11 bisquinolines were significantly more potent against the chloroquine-resistant W2 clone compared to the chloroquine-sensitive D6 clone. For bisquinolines 1-11 there was no relationship between the length of the bisquinoline heteroalkane bridge and antimalarial activity and no correlation between in vitro and in vivo antimalarial activities. Bisquinolines with alkyl ether and piperazine bridges were substantially more effective than bisquinolines with alkylamine bridges against P. berghei in vivo. Bisquinolines 1-10 were potent inhibitors of hematin polymerization with IC50 values falling in the narrow range of 5-20 microM, and there was a correlation between potency of inhibition of hematin polymerization and inhibition of parasite growth. Compared to alkane-bridged bisquinolines (Vennerstrom et al., 1992), none of these heteroalkane-bridged bisquinolines had sufficient antimalarial activity to warrant further investigation of the series.

Lactate Dehydrogenase and the Diagnosis of Malaria

Over the past five years, several methods have been developed that exploit the differences between Plasmodium lactate dehydrogenase (pLDH) and the human LDH isoforms for the purposes of measuring pLDH in blood and in in vitro cultures. These methods have been incorporated into an easy screening method for the identification and quantitation of parasite growth in in vitro cultures using a Malstattrade mark reagent. In addition, another quantitative microplate method, the immunocapture pLDH (IcpLDH) assay, has been developed that utilizes monoclonal antibodies (mAbs) to capture the pLDH and then to measure the captured enzyme by its ability to reduce 3 acetyl pyridine adenine dinucleotide (APAD). In addition, a rapid immunochromatographic method, the OptiMAL® assay, has been formatted to capture pLDH as an antigen, and then to signal the presence of this captured antigen (enzyme) with a colloid conjugated antibody. The microplate IcpLDH assay, and the dipstick OptiMAL® assays, are both being used for the diagnosis and monitoring of malaria infections, as described here by Michael Makler, Rob Piper and Wil Milhous.

Evidence for the shikimate pathway in apicomplexan parasites

Parasites of the phylum Apicomplexa cause substantial morbidity, mortality and economic losses, and new medicines to treat them are needed urgently. The shikimate pathway is an attractive target for herbicides and antimicrobial agents because it is essential in algae, higher plants, bacteria and fungi, but absent from mammals. Here we present biochemical, genetic and chemotherapeutic evidence for the presence of enzymes of the shikimate pathway in apicomplexan parasites. In vitro growth of Toxoplasma gondii, Plasmodium falciparum (malaria) and Cryptosporidium parvum was inhibited by the herbicide glyphosate, a well-characterized inhibitor of the shikimate pathway enzyme 5-enolpyruvyl shikimate 3-phosphate synthase. This effect on T. gondii and P. falciparum was reversed by treatment with p-aminobenzoate, which suggests that the shikimate pathway supplies folate precursors for their growth. Glyphosate in combination with pyrimethamine limited T. gondii infection in mice. Four shikimate pathway enzymes were detected in extracts of T. gondii and glyphosate inhibited 5-enolpyruvyl shikimate 3-phosphate synthase activity. Genes encoding chorismate synthase, the final shikimate pathway enzyme, were cloned from T. gondii and P. falciparum. This discovery of a functional shikimate pathway in apicomplexan parasites provides several targets for the development of new antiparasite agents.

In vitro and in vivo reversal of chloroquine resistance in Plasmodium falciparum with promethazine

The effect of combining promethazine with chloroquine was examined against Plasmodium falciparum in vitro in the Aotus-P. falciparum model and in bioassays from volunteers given promethazine. The combination of chloroquine plus promethazine (1 x 10(-6) M) reversed chloroquine resistance in standard P. falciparum clones and patient parasite isolates from Nigeria. The combination reduced the 50% inhibitory concentrations (IC50s) for chloroquine against resistant parasites by 32-92%. Coadministration of promethazine with chloroquine also demonstrated a dose-dependent effect in Aotus monkeys infected with chloroquine-resistant P. falciparum. Monkeys were given a chloroquine dose (20 mg/kg of body weight for seven days), which normally has no effect on parasitemia, plus 10, 20, 40, or 80 mg of promethazine/kg of body weight. In one monkey, parasitemia was suppressed at the lowest promethazine dose, but re-treatment with 20 mg/kg resulted in clearance of parasitemia. Initial treatment with chloroquine and 20 or 40 mg/kg of promethazine cleared parasitemia in some animals followed by recrudescence. Re-treatment at higher doses cured one monkey and resulted in initial clearance and delayed recrudescence 28 or 63 days after treatment in two monkeys. Recrudescent parasitemia in the two monkeys was low (10 parasites/microl of blood) and subsequently cleared without re-treatment. An in vitro bioassay model was developed to examine the effects of clinically achievable doses of promethazine on parasites susceptibilities in vitro. Plasma samples taken at hourly intervals from patients given a single oral dose of 25 mg of promethazine decreased the IC50 values for chloroquine by 20-58% with the most significant reductions occurring in plasma obtained from volunteers 3-4 hr after ingestion. Plasma obtained from two volunteers 6 hr after ingestion of the drug demonstrated no effect on chloroquine susceptibility, suggesting that study of the pharmacokinetic disposition and potential interaction is warranted to optimize the dose regimen in patients for antimalarial efficacy. Historic use of this drug combination for treatment or prevention of chloroquine-associated pruritus or as an antiemetic suggest that the combination is safe and effective when used at standard dosages. The results from this study demonstrate that promethazine is a potent modulator of chloroquine resistance. Clinical evaluation of therapeutic regimens is required to validate clinical efficacy of this promising combination for treatment of uncomplicated chloroquine-resistant malaria.

Characterization of Plasmodium falciparum isolated from the Amazon region of Brazil: evidence for quinine resistance

The prevalence and severity of drug-resistant malaria is emerging rapidly in the Amazon basin of Brazil. In support of clinical trials using the new antimalarial drug combination of atovaquone and proguanil, we performed in vitro drug sensitivities, molecular characterization of parasite populations using the circumsporozoite protein, merozoite surface antigen-1 (MSA-1), and MSA-2 markers, and an analysis of the Plasmodium falciparum multidrug resistance (pfmdr1) gene sequence and copy number in 26 isolates of P. falciparum obtained in a gold-mining endemic area in Peixoto de Azevedo, Mato Grosso State. All 26 isolates were found to be resistant to chloroquine (50% inhibitory concentration [IC50] = 100-620 nM) and sensitive to mefloquine (IC50 < 23 nM) and halofantrine (IC50 < 6 nM). The isolates also show reduced susceptibility to quinine (IC50 = 48-280 nM). Sequence analysis of the pfmdr1 gene revealed Asn, Phe, Cys, Asp, and Tyr in positions 86, 184, 1034, 1042, and 1246, respectively. These point mutations were similar to that previously described in other Brazilian isolates. Southern blot analysis revealed no amplification of the pfmdr1 gene. These results suggest that three different mechanisms for drug resistance exist for chloroquine, mefloquine, and quinine.