Antiprotozoal activities of some constituents of Markhamia tomentosa (Bignoniaceae)

Phytochemical investigation of an ethyl-acetate extract of the stem bark of Markhamia tomentosa (Bignoniaceae), which had good antimalarial activity in vitro, resulted in the isolation of eight known compounds: 2-acetylnaphtho[2,3-b]furan-4,9-dione (1), 2-acetyl-6-methoxynaphtho[2,3-b]furan-4,9-dione (2), oleanolic acid (3), pomolic acid (4), 3-acetylpomolic acid (5), tormentic acid (6), beta-sitosterol (7) and beta-sitosterol-3-O-beta-D-glucopyranoside (8). The structures of these compounds were established by spectroscopic methods. Each of compounds 1, 2, 4 and 5 was evaluated in vitro for its antiprotozoal activities against the ring stages of two chloroquine-resistant strains of Plasmodium falciparum (K1 and W2), the amastigotes of Leishmania donovani, and the bloodstream trypomastigotes of Trypanosoma brucei rhodesiense (the species responsible for human malaria, visceral leishmaniasis and African trypanosomiasis, respectively). Although compounds 1 and 2 exhibited potent antiprotozoal activities, they also showed high toxicity against a mammalian (L-6) cell line.

Intermittent preventive therapy for malaria in pregnancy: is sulfadoxine-pyrimethamine the right drug?

Pregnant women are at particularly high risk for morbidity and mortality from malaria, and pregnancy can markedly affect drug pharmacokinetics, yet the pharmacokinetics of antimalarial drugs in pregnancy has been little studied. An important malaria-control measure in Africa is intermittent preventive therapy (IPT) with sulfadoxine-pyrimethamine (SP) during pregnancy. We discuss IPT with SP in light of several concerns and highlight recent findings from a pharmacokinetic study of SP in this population.

Amodiaquine metabolism is impaired by common polymorphisms in CYP2C8: implications for malaria treatment in Africa

Metabolism of the antimalarial drug amodiaquine (AQ) into its primary metabolite, N-desethylamodiaquine, is mediated by CYP2C8. We studied the frequency of CYP2C8 variants in 275 malaria-infected patients in Burkina Faso, the metabolism of AQ by CYP2C8 variants, and the impact of other drugs on AQ metabolism. The allele frequencies of CYP2C8*2 and CYP2C8*3 were 0.155 and 0.003, respectively. No evidence was seen for influence of CYP2C8 genotype on AQ efficacy or toxicity, but sample size limited these assessments. The variant most common in Africans, CYP2C8(*)2, showed defective metabolism of AQ (threefold higher K(m) and sixfold lower intrinsic clearance), and CYP2C8(*)3 had markedly decreased activity. Considering drugs likely to be coadministered with AQ, the antiretroviral drugs efavirenz, saquinavir, lopinavir, and tipranavir were potent CYP2C8 inhibitors at clinically relevant concentrations. Variable CYP2C8 activity owing to genetic variation and drug interactions may have important clinical implications for the efficacy and toxicity of AQ.

Expression and characterization of the Plasmodium falciparum haemoglobinase falcipain-3

In the malaria parasite Plasmodium falciparum, erythrocytic trophozoites hydrolyse haemoglobin to provide amino acids for parasite protein synthesis. Cysteine protease inhibitors block parasite haemoglobin hydrolysis and development, indicating that cysteine proteases are required for these processes. Three papain-family cysteine protease sequences have been identified in the P. falciparum genome, but the specific roles of their gene products and other plasmodial proteases in haemoglobin hydrolysis are uncertain. Falcipain-2 was recently identified as a principal trophozoite cysteine protease and potential drug target. The present study characterizes the related P. falciparum cysteine protease falcipain-3. As is the case with falcipain-2, falcipain-3 is expressed by trophozoites and appears to be located within the food vacuole, the site of haemoglobin hydrolysis. Both proteases require a reducing environment and acidic pH for optimal activity, and both prefer peptide substrates with leucine at the P(2) position. The proteases differ, however, in that falcipain-3 undergoes efficient processing to an active form only at acidic pH, is more active and stable at acidic pH, and has much lower specific activity against typical papain-family peptide substrates, but has greater activity against native haemoglobin. Thus falcipain-3 is a second P. falciparum haemoglobinase that is particularly suited for the hydrolysis of native haemoglobin in the acidic food vacuole. The redundancy of cysteine proteases may offer optimized hydrolysis of both native haemoglobin and globin peptides. Consideration of both proteases will be necessary to evaluate cysteine protease inhibitors as antimalarial drugs.

The impact of whole genome sequence data on drug discovery--a malaria case study

BACKGROUND

Identification and validation of a drug discovery target is a prominent step in drug development. In the post-genomic era it is possible to reevaluate the association of a gene with a specific biological function to see if a homologous gene can subsume this role. This concept has special relevance to drug discovery in human infectious diseases, like malaria. A trophozoite cysteine protease (falcipain-1) from the papain family, thought to be responsible for the degradation of erythrocyte hemoglobin, has been considered a promising target for drug discovery efforts owing to the antimalarial activity of peptide based covalent cysteine protease inhibitors. This led to the development of non-peptidic non-covalent inhibitors of falcipain-1 and their characterization as antimalarials. It is now clear from sequencing efforts that the malaria genome contains more than one cysteine protease and that falcipain-1 is not the most important contributor to hemoglobin degradation. Rather, falcipain-2 and falcipain-3 appear to account for the majority of cysteine hemoglobinase activity in the plasmodium trophozoite.

MATERIALS AND METHODS

We have modeled the falcipain-2 cysteine protease from one of the major human malaria species, Plasmodium falciparum and compared it to our original work on falcipain-1. As with falcipain-1, computa-tional screening of the falcipain-2 active site was conducted using DOCK. Using structural superpositions within the protease family and evolutionary analysis of substrate specificity sites, we focused on the commonalities and the protein specific features to direct our drug discovery effort.

RESULTS

Since 1993, the size of the Available Chemicals Directory had increased from 55313 to 195419 unique chemical structures. For falcipain-2, eight inhibitors were identified with IC50's against the enzyme between 1 and 7 microM. Application of three of these inhibitors to infected erythrocytes cured malaria in culture, but parasite death did not correlate with food vacuole abnormalities associated with the activity of mechanistic inhibitors of cysteine proteases like the epoxide E64.

CONCLUSIONS

Using plasmodial falcipain proteases, we show how a protein family perspective can influence target discovery and inhibitor design. We suspect that parallel drug discovery programs where a family of targets is considered, rather than serial programs built on a single therapeutic focus, will become the dominant industrial paradigm. Economies of scale in assay development and in compound synthesis are expected owing to the functional and structural features of individual family members. One of the remaining challenges in post-genomic drug discovery is that inhibitors of one target are likely to show some activity against other family members. This lack of specificity may lead to difficulties in functional assignments and target validation as well as a complex side effect profile.

Amodiaquine, sulfadoxine/pyrimethamine, and combination therapy for treatment of uncomplicated falciparum malaria in Kampala, Uganda: a randomised trial

BACKGROUND

Increasing Plasmodium falciparum resistance to chloroquine in sub-Saharan Africa necessitates use of alternative antimalarial agents. Affordable alternative treatments include sulfadoxine/pyrimethamine and amodiaquine. Combination of antimalarial agents can increase therapeutic efficacy and delay emergence of drug resistance. We compared the efficacy of sulfadoxine/pyrimethamine, amodiaquine, and an amodiaquine/sulfadoxine/pyrimethamine combination for treatment of uncomplicated malaria in a region of high chloroquine resistance.

METHODS

Patients with symptoms of uncomplicated falciparum malaria and confirmed disease in Kampala, Uganda, were randomly assigned to receive sulfadoxine/pyrimethamine (25 mg/kg sulfadoxine, and 1.25 mg/kg pyrimethamine) plus placebo; amodiaquine (25 mg/kg) plus placebo; or amodiaquine plus sulfadoxine/pyrimethamine. Patients were followed up for 14 days, and clinical and parasitological outcomes were assessed.

FINDINGS

90% (400/445) of patients enrolled in the study successfully completed 14 days of follow-up. Treatment failure based on clinical criteria occurred in 13 of 131 (10%) patients on sulfadoxine/ pyrimethamine, nine of 131 (7%) on amodiaquine, and four of 138 (3%) on amodiaquine/sulfadoxine/pyrimethamine. Based on parasitological criteria, treatment failed in 26%, 16%, and 10% of these patients, respectively. Amodiaquine/sulfadoxine/pyrimethamine was significantly more effective than sulfadoxine/pyrimethamine alone in children aged younger than 5 years (clinical failure in 3.5% vs 13.9%, respectively, risk difference 10.4% [95% CI, 1.6-19.3] p=0.021; parasitological failure in 12.8% vs 26.4%, risk difference 13.6% [1.2-26.0] p=0.041).

INTERPRETATION

Sulfadoxine/pyrimethamine, amodiaquine, and amodiaquine/sulfadoxine/pyrimethamine were all effective for treatment of uncomplicated falciparum malaria in Uganda. The amodiaquine/sulfadoxine/pyrimethamine combination was the most effective, and could be the optimum low-cost alternative to chloroquine in Africa.

Synthesis of quinolinyl chalcones and evaluation of their antimalarial activity

Quinolinyl chalcones were synthesized and evaluated for their inhibition of the Plasmodium falciparum cystein protease falcipain and their activity against cultured P. falciparum parasites. They were also tested for in vivo efficacy in a rodent P. berghei model. Their activity against falcipain and as antimalarials was moderate, but antimalarial activity was probably not due to the inhibition of falcipain and may follow a different mechanism. 1-(2,4-Dichlorophenyl)-3-[3-(2-chloro-6,7-dimethoxiquinolinyl)]-2-propen-1-one 3j was the most promising compound among those here reported (IC50 19.0 microM).

Systematic optimization of expression and refolding of the Plasmodium falciparum cysteine protease falcipain-2

The Plasmodium falciparum cysteine protease falcipain-2 is a potential new target for antimalarial chemotherapy. In order to obtain large quantities of active falcipain-2 for biochemical and structural analysis, a systematic assessment of optimal parameters for the expression and refolding of the protease was carried out. High-yield expression was achieved using M15(pREP4) Escherichia coli transformed with the pQE-30 plasmid containing a truncated profalcipain-2 construct. Recombinant falcipain-2 was expressed as inclusion bodies, solubilized, and purified by nickel affinity chromatography. A systematic approach was then used to optimize refolding parameters. This approach utilized 100-fold dilutions of reduced and denatured falcipain-2 into 203 different buffers in a microtiter plate format. Refolding efficiency varied markedly. Optimal refolding was obtained in an alkaline buffer containing glycerol or sucrose and equal concentrations of reduced and oxidized glutathione. After optimization of the expression and refolding protocols and additional purification with anion-exchange chromatography, 12 mg of falcipain-2 was obtained from 5 liters of E. coli, and crystals of the protease were grown. The systematic approach described here allowed the rapid evaluation of a large number of expression and refolding conditions and provided milligram quantities of recombinant falcipain-2.

Polymorphisms in the Plasmodium falciparum pfcrt and pfmdr-1 genes and clinical response to chloroquine in Kampala, Uganda

The molecular mechanism of chloroquine resistance in Plasmodium falciparum remains uncertain. Polymorphisms in the pfcrt and pfmdr-1 genes have been associated with chloroquine resistance in vitro, although field studies are limited. In evaluations of known polymorphisms in parasites from patients with uncomplicated malaria in Kampala, Uganda, the presence of 8 pfcrt mutations and 2 pfmdr-1 mutations did not correlate with clinical response to therapy with chloroquine. Most notably, the pfcrt lysine-->threonine mutation at position 76, which recently correlated fully with chloroquine resistance in vitro, was present in 100% of 114 isolates, of which about half were from patients who recovered clinically after chloroquine therapy. These results suggest that, although key pfcrt polymorphisms may be necessary for the elaboration of resistance to chloroquine in areas with high levels of chloroquine resistance, other factors, such as host immunity, may contribute to clinical outcomes.

Comparison of efficacies of cysteine protease inhibitors against five strains of Plasmodium falciparum

Falcipain-2, a cysteine protease and essential hemoglobinase of Plasmodium falciparum, is a potential antimalarial drug target. We compared the falcipain-2 sequences and sensitivities to cysteine protease inhibitors of five parasite strains that differ markedly in sensitivity to established antimalarial drugs. The sequence of falcipain-2 was highly conserved, and the sensitivities of all of the strains to falcipain-2 inhibitors were very similar. Thus, cross-resistance between cysteine protease inhibitors and other antimalarial agents is not expected in parasites that are now circulating and falcipain-2 remains a promising chemotherapeutic target.

The comparative efficacy of chloroquine and sulfadoxine-pyrimethamine for the treatment of uncomplicated falciparum malaria in Kampala, Uganda

Chloroquine (CQ) remains the first-line treatment for uncomplicated malaria in much of Africa despite the growing problem of resistance to this drug. Sulfadoxine-pyrimethamine (SP) is often used after CQ treatment failure and has replaced CQ as the first-line treatment in parts of Africa. To compare the efficacy of these 2 regimens, we evaluated, in March-August 1999, clinical and parasitological responses over 28 days in 214 children and adults from Kampala, Uganda, with uncomplicated falciparum malaria. Compared to SP, significantly more patients treated with CQ developed early or late clinical failure (54% vs 11%, P < 0.001) and parasitological failure (72% vs 30%, P < 0.001) during 14 days of follow-up. The risk of treatment failure occurring after day 14 was similar between the 2 treatment groups. Among those treated with CQ, children aged < 5 years were at higher risk of clinical failure than older individuals (76% vs 28%, P < 0.001), an association not seen with SP (11% vs 10%, P = 0.91). Although early parasite clearance was significantly better in the SP group (P = 0.001), fever clearance at day 3 was the same (CQ 85%, SP 86%). These and other recent findings suggest that consideration be given to replacing CQ as the first-line therapy for uncomplicated malaria in Uganda, particularly in young children.

Characterization of native and recombinant falcipain-2, a principal trophozoite cysteine protease and essential hemoglobinase of Plasmodium falciparum

Trophozoites of the malaria parasite Plasmodium falciparum hydrolyze erythrocyte hemoglobin in an acidic food vacuole to provide amino acids for parasite protein synthesis. Cysteine protease inhibitors block hemoglobin degradation, indicating that a cysteine protease plays a key role in this process. A principal trophozoite cysteine protease was purified by affinity chromatography. Sequence analysis indicated that the protease is encoded by a previously unidentified gene, falcipain-2. Falcipain-2 was predominantly expressed in trophozoites, was concentrated in food vacuoles, and was responsible for at least 93% of trophozoite soluble cysteine protease activity. A construct encoding mature falcipain-2 and a small portion of the prodomain was expressed in Escherichia coli and refolded to active enzyme. Specificity for the hydrolysis of peptide substrates by native and recombinant falcipain-2 was very similar, and optimal at acid pH in a reducing environment. Under physiological conditions (pH 5.5, 1 mm glutathione), falcipain-2 hydrolyzed both native hemoglobin and denatured globin. Our results suggest that falcipain-2 can initiate cleavage of native hemoglobin in the P. falciparum food vacuole, that, following initial cleavages, the protease plays a key role in rapidly hydrolyzing globin fragments, and that a drug discovery effort targeted at this protease is appropriate.

Difficulties in the prevention, diagnosis, and treatment of imported malaria

BACKGROUND

Imported malaria is quite common in the United States. Increasing antimalarial drug resistance and changes in travel patterns may have important implications for the prevention, clinical presentation, and management of this disease.

METHODS

Medical records were reviewed for 121 patients with microscopically confirmed malaria diagnosed at 2 university-affiliated hospitals in San Francisco, Calif, between 1988 and 1997.

RESULTS

Among 57 travelers from the United States, only 13 (23%) had been compliant with an appropriate chemoprophylactic regimen. No patients developed falciparum malaria after consistent chemoprophylactic therapy with mefloquine hydrochloride. However, 12 (19%) of US residents with imported malaria developed Plasmodium vivax or Plasmodium ovale infections despite an appropriate chemoprophylactic regimen, generally with a late onset suggestive of relapsing disease. Clinical presentations were similar between foreign residents and American travelers and between patients with falciparum and nonfalciparum infections; 98% of patients had a history of fever. Sixteen percent of patients had received previous evaluations during which the diagnosis of malaria was not considered. In 9% of patients, there were errors in treatment. Only 1 patient developed severe malaria.

CONCLUSIONS

Our results suggest that a standard chemoprophylactic regimen is highly effective in preventing falciparum malaria, but that many American travelers do not receive it. Also, relapsing P vivax or P ovale infection despite appropriate chemoprophylactic therapy was not uncommon among our cases. The presentation of imported malaria is nonspecific, highlighting the need to consider the diagnosis in any febrile patient who has been in a malaria-endemic area. Although errors in diagnosis and treatment were quite common in our study population, patient outcomes were good once the appropriate therapy was initiated.

Predictors of chloroquine treatment failure in children and adults with falciparum malaria in Kampala, Uganda

Chloroquine-resistant falciparum malaria is a serious problem in much of sub-Saharan Africa. However, it is desirable to continue to use chloroquine as first-line therapy for uncomplicated malaria where it remains clinically effective. To identify predictors of chloroquine treatment failure, a 14-day clinical study of chloroquine resistance in patients with uncomplicated falciparum malaria was performed in Kampala, Uganda. Among the 258 patients (88% follow-up), 47% were clinical failures (early or late treatment failure) and 70% had parasitological resistance (RI-RIII). Using multivariate analysis, an age less than five (odds ratio [OR] = 3.4, 95% CI = 1.8-6.3) and a presenting temperature over 38.0 degreesC (OR = 2.0, 95% CI = 1.1-3.7) were independent predictors of treatment failure. In addition, patients who last took chloroquine 3 to 14 days prior to study entry were significantly more likely to be treatment failures compared to patients with very recent (less than 3 days) or no recent chloroquine use. In areas with significant chloroquine resistance, easily identifiable predictors of chloroquine treatment failure might be used to stratify patients into those for whom chloroquine use is acceptable and those for whom alternative treatment should be used.

Proteases of protozoan parasites

Proteolytic enzymes seem to play important roles in the life cycles of all medically important protozoan parasites, including the organisms that cause malaria, trypanosomiasis, leishmaniasis, amebiasis, toxoplasmosis, giardiasis, cryptosporidiosis and trichomoniasis. Proteases from all four major proteolytic classes are utilized by protozoans for diverse functions, including the invasion of host cells and tissues, the degradation of mediators of the immune response and the hydrolysis of host proteins for nutritional purposes. The biochemical and molecular characterization of protozoan proteases is providing tools to improve our understanding of the functions of these enzymes. In addition, studies in multiple systems suggest that inhibitors of protozoan proteases have potent antiparasitic effects. This review will discuss recent advances in the identification and characterization of protozoan proteases, in the determination of the function of these enzymes, and in the evaluation of protease inhibitors as potential antiprotozoan drugs.

Antimalarial effects in mice of orally administered peptidyl cysteine protease inhibitors

The Plasmodium falciparum cysteine protease falcipain is required for the degradation of hemoglobin by erythrocytic malaria parasites. In prior studies, peptidyl inhibitors of falcipain blocked hemoglobin degradation and development by cultured parasites and one of these compounds, when administered parenterally, cured Plasmodium vinckei-infected mice. We now report an evaluation of orally administered peptidyl inhibitors of falcipain in a mouse malaria model. In studies with a fluoromethyl ketone, orally administered morpholine urea-phenylalanine-homophenylalanine-fluoromethyl ketone delayed the progression of murine malaria. In studies of a new series of vinyl sulfones, a set of related compounds demonstrated marked inhibition of falcipain and of parasite biological activities in vitro. One of these compounds, N-methyl piperazine urea-leucine-homophenylalanine-2-naphthalene vinyl sulfone, cured about 40% of mice when administered orally twice-a-day for four days. Our results suggest that peptidyl inhibitors of falcipain have promise as antimalarial chemotherapeutic agents.

Structure-based design, synthesis and evaluation of conformationally constrained cysteine protease inhibitors

The inhibition of cysteine proteases is being studied as a strategy to combat parasitic diseases such as Chagas' disease, leishmaniasis, and malaria. Cruzain is the major cysteine protease of Trypanosoma cruzi, the etiologic agent of Chagas' disease. A crystal structure of cruzain, covalently inactivated by fluoromethyl ketone inhibitor 1 (Cbz-Phe-Ala-FMK), was used as a template to design potential inhibitors. Conformationally constrained gamma-lactams containing electrophilic aldehyde (12, 17, 18, 25, 26, and 29) or vinyl sulfone (43, 44, and 46) units were synthesized. Constrained lactam 26 had IC50 values of ca. 20 nM against the Leishmania major protease and ca. 50 nM versus falcipain, an important cysteine protease isolated from Plasmodium falciparum. However, all of the conformationally constrained inhibitors were weak inhibitors of cruzain, compared to unconstrained peptide aldehyde (e.g. 5 ) and vinyl sulfone inhibitors (e.g. 48, which proved to be an excellent inhibitor of cruzain with an apparent second order inhibition rate constant (k(inact)/Ki) of 634,000s(-1)M(-1). A significant reduction in activity was also observed with acyclic inhibitors 30 and 51 containing alpha-methyl phenylalanine residues at the P2 position. These data indicate that the pyrrolidinone ring, especially the quarternary center at P2, interferes with the normal substrate binding mode with cruzain, but not with falcipain or the leishmania protease.

Antimalarial synergy of cysteine and aspartic protease inhibitors

It has been proposed that the Plasmodium falciparum cysteine protease falcipain and aspartic proteases plasmepsin I and plasmepsin II act cooperatively to hydrolyze hemoglobin as a source of amino acids for erythrocytic parasites. Inhibitors of each of these proteases have potent antimalarial effects. We have now evaluated the antimalarial effects of combinations of cysteine and aspartic protease inhibitors. When incubated with cultured P. falciparum parasites, cysteine and aspartic protease inhibitors exhibited synergistic effects in blocking parasite metabolism and development. The inhibitors also demonstrated apparent synergistic inhibition of plasmodial hemoglobin degradation both in culture and in a murine malaria model. When evaluated for the treatment of murine malaria, a combination of cysteine and aspartic protease inhibitors was much more effective than higher concentrations of either compound used alone. These results support a model whereby plasmodial cysteine and aspartic proteases participate in the degradation of hemoglobin, and they suggest that combination antimalarial therapy with inhibitors of the two classes of proteases is worthy of further study.

Humoral immune responses of Africans to cysteine protease-related antigens of Plasmodium falciparum

The Plasmodium falciparum serine repeat antigen (SERA) and serine repeat protein homologue (SERPH) contain highly conserved domains that appear to encode cysteine proteases or related proteins. Humoral immune responses against the protease domains of SERA and SERPH were evaluated. Malaria-immune Africans, but not nonimmune controls, demonstrated potent humoral responses against the protease domains. As the SERA and SERPH protease domains are likely accessible to circulating antibody, these results suggest that humoral responses to the domains may contribute to antimalarial immunity.

Protective immune responses against protease-like antigens of the murine malaria parasite Plasmodium vinckei

The Plasmodium falciparum proteins serine repeat antigen (SERA) and serine repeat protein homologue (SERPH) have similarity in sequence with cysteine proteases in a well-conserved protease domain. We identified three SERA homologues from the murine malaria parasite Plasmodium vinckei and evaluated immune responses to the protease domains of these proteins. Mice that developed protective immunity to P. vinckei after serial infection and cure demonstrated humoral and cell-mediated responses against the SERA homologue protease domains. Mice immunized with Salmonella typhimurium expressing the protease domain of one of these antigens demonstrated cellular responses against the antigen and increased survival against lethal challenge with P. vinckei. Our results suggest that the protease domains of SERA and SERPH are worthy of additional study as potential components of a malaria vaccine.

Proteases of malaria parasites: new targets for chemotherapy

The increasing resistance of malaria parasites to antimalarial drugs is a major contributor to the reemergence of the disease as a major public health problem and its spread in new locations and populations. Among potential targets for new modes of chemotherapy are malarial proteases, which appear to mediate processes within the erythrocytic malarial life cycle, including the rupture and invasion of infected erythrocytes and the degradation of hemoglobin by trophozoites. Cysteine and aspartic protease inhibitors are now under study as potential antimalarials. Lead compounds have blocked in vitro parasite development at nanomolar concentrations and cured malaria-infected mice. This review discusses available antimalarial agents and summarizes experimental results that support development of protease inhibitors as antimalarial drugs.

Synthesis and antimalarial effects of phenothiazine inhibitors of a Plasmodium falciparum cysteine protease

Acridinediones have previously been shown to have potent antimalarial activity. A series of sulfur isosteres of acridinediones have been synthesized and evaluated for their inhibition of the Plasmodium falciparum cysteine protease falcipain and for their antimalarial activity. A number of these phenothiazines inhibited falcipain and demonstrated activity against cultured P. falciparum parasites at low micromolar concentrations. We propose that the compounds exerted their antimalarial effects by two mechanisms, one of which involves the inhibition of falcipain and a consequent block in parasite degradation of hemoglobin. These compounds and related phenothiazines are worthy of further study as potential antimalarial agents.

Hemoglobin catabolism and iron utilization by malaria parasites

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

Antimalarial effects of vinyl sulfone cysteine proteinase inhibitors

We evaluated the antimalarial effects of vinyl sulfone cysteine proteinase inhibitors. A number of vinyl sulfones strongly inhibited falcipain, a Plasmodium falciparum cysteine proteinase that is a critical hemoglobinase. In studies of cultured parasites, nanomolar concentrations of three vinyl sulfones inhibited parasite hemoglobin degradation, metabolic activity, and development. The antimalarial effects correlated with the inhibition of falcipain. Our results suggest that vinyl sulfones or related cysteine proteinase inhibitors may have promise as antimalarial agents.

Cysteine proteinase inhibitors block early steps in hemoglobin degradation by cultured malaria parasites

Erythrocytic malaria parasites degrade hemoglobin as a source of amino acids for parasite protein synthesis. Cysteine proteinase inhibitors have been shown to block the hydrolysis of globin by cultured parasites, indicating that a malarial cysteine proteinase is required for this process. In the present study, we have evaluated the role of parasite proteinases in earlier steps of hemoglobin degradation, namely the disassociation of the hemoglobin tetramer and the separation of heme from globin. Hemoglobin did not spontaneously denature or release heme under the pH and reducing conditions of the malarial food vacuole, suggesting that parasite enzymatic activity is necessary for early steps in hemoglobin degradation. The incubation of cultured parasites with cysteine proteinase inhibitors inhibited the denaturation of hemoglobin and the release of heme from globin. These results suggest that, in addition to its role in globin hydrolysis, a malarial cysteine proteinase participates in the dissociation of the hemoglobin tetramer and the release of heme from globin. Thus, the malarial cysteine proteinase is a promising target for antimalarial chemotherapy.

Functional expression of falcipain, a Plasmodium falciparum cysteine proteinase, supports its role as a malarial hemoglobinase

Erythrocytic malaria parasites degrade hemoglobin as a principal source of amino acids for parasite protein synthesis. We have previously shown that a Plasmodium falciparum trophozoite cysteine proteinase, now termed falcipain, is required for hemoglobin degradation, and we have hypothesized that this proteinase is responsible for initial cleavages of hemoglobin. To further evaluate the biological role of falcipain, we expressed the enzyme in bacterial and viral expression systems. After expression in the baculovirus system, falcipain was enzymatically active and had biochemical properties very similar to those of the native proteinase. Recombinant falcipain rapidly hydrolyzed both denatured and native hemoglobin. Hemoglobin hydrolysis was blocked by cysteine proteinase inhibitors but not by inhibitors of other classes of proteinases. Our results support our hypothesis that falcipain is a critical malarial hemoglobinase that is responsible for both initial cleavages of hemoglobin and the subsequent hydrolysis of globin into small peptides.

Plasmodium falciparum: effects of proteinase inhibitors on globin hydrolysis by cultured malaria parasites

The effects of peptide proteinase inhibitors on globin hydrolysis by cultured malaria parasites were studied. All of the four cysteine proteinase inhibitors evaluated blocked globin hydrolysis, as documented by the development of a morphological abnormality in which parasite food vacuoles filled with undegraded globin and by SDS-PAGE showing that the cysteine proteinase inhibitor-treated parasites accumulated large quantities of globin. The aspartic proteinase inhibitor pepstatin did not block globin hydrolysis by cultured parasites. None of seven antimalarial drugs tested elicited the food vacuole abnormality caused by cysteine proteinase inhibitors, indicating that this morphological alteration was not simply a sign of nonspecific parasite toxicity. Our results indicate that a trophozoite cysteine proteinase is required for initial cleavages of globin by intact malaria parasites.

Anti-malarial drug development using models of enzyme structure

BACKGROUND

The trophozoite stage of the malaria parasite infects red blood cells. During this phase of their life-cycle, the parasites use hemoglobin as their principal source of amino acids, using a cysteine protease to degrade it. We have previously reported a three-dimensional model of this cysteine protease, based on the structures of homologous proteases, and the use of the program DOCK to identify a ligand for the malaria protease.

RESULTS

Here we describe the design of improved ligands starting from this lead. Ligand design was based on the predicted configuration of the lead compound docked to the model three-dimensional structure of the protease. The lead compound has an IC50 of 6 microM, and our design/synthesis strategy has resulted in increasingly potent derivatives that block the ability of the parasites to infect and/or mature in red blood cells. The two best derivatives to date have IC50(s) of 450 nM and 150 nM.

CONCLUSIONS

A new class of anti-malarial chemotherapeutics has resulted from a computational search that was based on a model of the target protease. Despite the lack of a detailed experimental structure of the target enzyme or the enzyme-inhibitor complex, we have been able to identify compounds with increased potency. These compounds approach the activity of chloroquine (IC50 = 20 nM), but have a distinct mechanism of action. This series of compounds could thus lead to new therapies for chloroquine-resistant malaria.

Characterization of a Plasmodium vivax cysteine proteinase gene identifies uniquely conserved amino acids that may mediate the substrate specificity of malarial hemoglobinases

The gene encoding a cysteine proteinase of the human malaria parasite Plasmodium vivax has been identified and characterized. The sequence predicted by the proteinase gene shares several unique features with the sequences of two recently characterized cysteine proteinases of other malarial species. These features include the conservation of a number of amino acids that are predicted, based on a recently devised model for the related Plasmodium falciparum cystine proteinase, to be located near the enzyme's active site. We hypothesize that these residues have been conserved to maintain optimal proteolytic specificity in the hydrolysis of globin by malaria parasites.

The proteases and pathogenicity of parasitic protozoa

Protozoan parasites are among the most prevalent pathogens worldwide. Diseases like malaria, leishmaniasis, amebiasis, and trypanosomiasis affect hundreds of millions of people. Recent advances in our understanding of the biochemistry and molecular biology of these organisms has focused attention on specific parasite molecules that are key to the parasite life cycle or the pathogenesis of the diseases they produce. One group of enzymes that plays myriad roles in these processes are the parasite-derived proteases. Different types of proteases are frequently expressed at different stages of the parasite life cycle to support parasite replication and metamorphosis. Intracellular parasites such as those that produce malaria and Chagas' disease express high levels of protease activity to efficiently degrade host proteins like hemoglobin. In other instances, such as infection with Entamoeba histolytica, the causative agent of amebiasis, proteases released by the parasite can damage host cells and tissues, contributing to host tissue damage and parasite invasion. Detailed studies of these enzymes have led to model systems for the study of parasite gene regulation, parasite metabolism, and the host-parasite interplay. In some instances, proteases appear to be promising targets for the development of new antiparasitic chemotherapy.

A Plasmodium vinckei cysteine proteinase shares unique features with its Plasmodium falciparum analogue

The gene encoding a cysteine proteinase of the murine malaria parasite Plasmodium vinckei has been identified and characterized. The gene encodes a papain-family proteinase that shares unique features with a previously described P. falciparum cysteine proteinase. We hypothesize that both enzymes mediate the hydrolysis of hemoglobin, and perhaps other Plasmodium-specific functions.

Structure-based inhibitor design by using protein models for the development of antiparasitic agents

The lack of an experimentally determined structure of a target protein frequently limits the application of structure-based drug design methods. In an effort to overcome this limitation, we have investigated the use of computer model-built structures for the identification of previously unknown inhibitors of enzymes from two major protease families, serine and cysteine proteases. We have successfully used our model-built structures to identify computationally and to confirm experimentally the activity of nonpeptidic inhibitors directed against important enzymes in the schistosome [2-(4-methoxybenzoyl)-1-naphthoic acid, Ki = 3 microM] and malaria (oxalic bis[(2-hydroxy-1-naphthylmethylene)hydrazide], IC50 = 6 microM) parasite life cycles.

Inhibition of a Plasmodium vinckei cysteine proteinase cures murine malaria

Intraerythrocytic malaria parasites degrade hemoglobin as a principal source of amino acids for parasite protein synthesis. We have previously identified a Plasmodium falciparum trophozoite cysteine proteinase as a putative hemoglobinase and shown that specific inhibitors of this proteinase block the hydrolysis of globin and the development of cultured parasites. We now show that the murine malaria parasite Plasmodium vinckei has an analogous cysteine proteinase with similar biochemical properties to the P. falciparum proteinase, including an acid pH optimum, a preference for the peptide proteolytic substrate benzyloxycarbonyl (Z)-Phe-Arg-7-amino-4-methylcoumarin, and nonomolar inhibition by seven peptide fluoromethyl ketone proteinase inhibitors. Thus, P. vinckei offers a model system for the in vivo testing of the antimalarial properties of cysteine proteinase inhibitors. One of the proteinase inhibitors studied, morpholine urea (Mu)-Phe-Homophenylalanine (HPhe)-CH2F strongly inhibited the P. vinckei cysteine proteinase in vitro and rapidly blocked parasite cysteine proteinase activity in vivo. When administered four times a day for 4 d to P. vinckei-infected mice, Mu-Phe-HPhe-CH2F elicited long-term cures in 80% of the treated animals. These results show that peptide proteinase inhibitors can be effective antimalarial compounds in vivo and suggest that the P. falciparum cysteine proteinase is a promising target for chemotherapy.

Isolation and characterization of a cysteine proteinase gene of Plasmodium falciparum

We have previously identified a 28-kDa cysteine proteinase of Plasmodium falciparum trophozoites that appears to be an essential malarial hemoglobinase and a potential target for antimalarial chemotherapy. The trophozoite cysteine proteinase (TCP) shares a number of biochemical properties with the lysosomal cysteine proteinase cathepsin L. To isolate the gene encoding TCP, we synthesized degenerate oligonucleotides based on two amino acid sequences of cathepsin L that are well conserved among papain-family cysteine proteinases, and used the oligonucleotides to prime the polymerase chain reaction (PCR) with P. falciparum genomic DNA. A 549-bp DNA fragment was amplified by PCR. This fragment was used as a hybridization probe to screen a lambda gt11 library of P. falciparum genomic DNA and isolate a 1.8-kb genomic clone (C1.8) that encoded an intact malarial cysteine proteinase gene. The sequence of C1.8 predicted a 67-kDa protein containing a typical signal sequence, a large pro sequence, and a 26.8-kDa mature proteinase with 37% amino acid identity to cathepsin L. Antisera directed against a peptide encoded by C1.8 recognized a 28-kDa trophozoite protein on immunoblots. In a Northern analysis, C1.8 hybridized predominantly with RNA from rings, the life-cycle stage immediately preceding the trophozoite stage. Taken together, these results strongly suggest that the P. falciparum cysteine proteinase gene we have isolated and characterized encodes TCP.

Antimalarial effects of peptide inhibitors of a Plasmodium falciparum cysteine proteinase

We previously identified a Plasmodium falciparum trophozoite cysteine proteinase (TCP) and hypothesized that it is required for the degradation of host hemoglobin by intraerythrocytic malaria parasites. To test this hypothesis and to evaluate TCP as a chemotherapeutic target, we examined the antimalarial effects of a panel of peptide fluoromethyl ketone proteinase inhibitors. For each inhibitor, effectiveness at inhibiting the activity of TCP correlated with effectiveness at both blocking hemoglobin degradation and killing cultured parasites. Benzyloxycarbonyl (Z)-Phe-Arg-CH2F, the most potent inhibitor, inhibited TCP at picomolar concentrations and blocked hemoglobin degradation and killed parasites at nanomolar concentrations. Micromolar concentrations of the inhibitor were nontoxic to cultured mammalian cells. These results support the hypothesis that TCP is a necessary hemoglobinase and suggest that it is a promising chemotherapeutic target.

Plasmodium falciparum: inhibitors of lysosomal cysteine proteinases inhibit a trophozoite proteinase and block parasite development

Trophozoites of Plasmodium falciparum obtain free amino acids for protein synthesis by degrading host erythrocyte hemoglobin in an acidic food vacuole. We previously reported that leupeptin and L-trans-epoxysuccinyl-leucylamido(4-guanidino)butane (E-64), two inhibitors of the cysteine class of proteinases, blocked hemoglobin degradation in the trophozoite food vacuole, and we identified a 28-kDa trophozoite cysteine proteinase as a potential food vacuole hemoglobinase. We now report that the biochemical properties of the trophozoite cysteine proteinase closely resembled those of the lysosomal cysteine proteinases cathepsin B and cathepsin L. The trophozoite proteinase had a pH optimum of 5.5-6.0, near that of both lysosomal proteinases, and it was efficiently inhibited by highly specific diazomethylketone and fluoromethylketone inhibitors of cathepsin B and cathepsin L. The trophozoite proteinase preferred peptide substrates with arginine adjacent to hydrophobic amino acids, as does cathepsin L. Micromolar concentrations of the fluoromethylketone inhibitor Z-Phe-Ala-Ch2F blocked the degradation of hemoglobin in the trophozoite food vacuole and prevented parasite multiplication. In previous studies much higher concentrations of the inhibitor were not toxic for mice. Our results provide additional evidence that the 28-kDa trophozoite proteinase is a food vacuole hemoglobinase and suggest that specific inhibitors of the enzyme may have potential as antimalarial drugs.

A malarial cysteine proteinase is necessary for hemoglobin degradation by Plasmodium falciparum

To obtain free amino acids for protein synthesis, trophozoite stage malaria parasites feed on the cytoplasm of host erythrocytes and degrade hemoglobin within an acid food vacuole. The food vacuole appears to be analogous to the secondary lysosomes of mammalian cells. To determine the enzymatic mechanism of hemoglobin degradation, we incubated trophozoite-infected erythrocytes with peptide inhibitors of different classes of proteinases. Leupeptin and L-transepoxy-succinyl-leucyl-amido-(4-guanidino)-butane (E-64), two peptide inhibitors of cysteine proteinases, inhibited the proteolysis of globin and caused the accumulation of undegraded erythrocyte cytoplasm in parasite food vacuoles, suggesting that a food vacuole cysteine proteinase is necessary for hemoglobin degradation. Proteinase assays of trophozoites demonstrated cysteine proteinase activity with a pH optimum similar to that of the food vacuole and the substrate specificity of lysosomal cathepsin L. We also identified an Mr 28,000 proteinase that was trophozoite stage-specific and was inhibited by leupeptin and E-64. We conclude that the Mr 28,000 cysteine proteinase has a critical, perhaps rate-limiting, role in hemoglobin degradation within the food vacuole of Plasmodium falciparum. Specific inhibitors of this enzyme might provide new means of antimalarial chemotherapy.

Rectal leishmaniasis in a patient with acquired immunodeficiency syndrome

A severe rectal lesion due to Leishmania infection is described in an American-born homosexual man with the acquired immunodeficiency syndrome. The infection, which may have been venereally transmitted, responded to treatment with amphotericin B. There was no evidence of visceral leishmaniasis. The contribution of the patient's immunodeficiency to the development of the atypical cutaneous leishmanial lesion is unclear. The case may foretell increasing problems with protozoan infections in AIDS as the epidemic spreads to areas with endemic protozoan diseases.