DNA topoisomerases as targets for antiprotozoal therapy

Diseases caused by parasitic protozoa present a health problem of immense magnitude, and there is an urgent need for safe and effective new therapies. DNA topoisomerases are clinically relevant targets for anti-cancer and anti-bacterial agents. Inhibitor studies on parasite topoisomerases have revealed that these enzymes have great promise as molecular targets for anti-parasitics, and have helped to dissect the basic biology of DNA topoisomerases in these organisms. This review provides a brief introduction to DNA topoisomerases and anti-topoisomerase drugs, and an overview of studies on protozoal DNA topoisomerases and their inhibitors.

Antimalarial simplified 3-aryltrioxanes: synthesis and preclinical efficacy/toxicity testing in rodents

A streamlined five-step chemical synthesis of rationally designed, simplified 3-aryltrioxane 8a is described. A noteworthy feature of this synthetic scheme is use of air rather than expensive molecular oxygen as the source of the pharmacologically critical peroxide unit in trioxane 8a. This simplified acetal trioxane carboxylic acid 8a is thermally stable, and it is hydrolytically stable in water even at 40 degrees C and pH 7.4 for at least 7 days. Preclinical evaluation of this water-soluble synthetic trioxane 8a in rodents shows it to have at least as good a therapeutic index (efficacy/toxicity) as that of water-soluble semisynthetic trioxane artelinic acid (5).

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

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

Chemoprotective glucosinolates and isothiocyanates of broccoli sprouts: metabolism and excretion in humans

Broccoli sprouts are a rich source of glucosinolates and isothiocyanates that induce phase 2 detoxication enzymes, boost antioxidant status, and protect animals against chemically induced cancer. Glucosinolates are hydrolyzed by myrosinase (an enzyme found in plants and bowel microflora) to form isothiocyanates. In vivo, isothiocyanates are conjugated with glutathione and then sequentially metabolized to mercapturic acids. These metabolites are collectively designated dithiocarbamates. We studied the disposition of broccoli sprout glucosinolates and isothiocyanates in healthy volunteers. Broccoli sprouts were grown, processed, and analyzed for (a) inducer potency; (b) glucosinolate and isothiocyanate concentrations; (c) glucosinolate profiles; and (d) myrosinase activity. Dosing preparations included uncooked fresh sprouts (with active myrosinase) as well as homogenates of boiled sprouts that were devoid of myrosinase activity and contained either glucosinolates only or isothiocyanates only. In a crossover study, urinary dithiocarbamate excretion increased sharply after administration of broccoli sprout glucosinolates or isothiocyanates. Cumulative excretion of dithiocarbamates following 111-micromol doses of isothiocyanates was greater than that after glucosinolates (88.9 +/- 5.5 and 13.1 +/- 1.9 micromol, respectively; P < 0.0003). In subjects fed four repeated 50-micromol doses of isothiocyanates, the intra- and intersubject variation in dithiocarbamate excretion was very small (coefficient of variation, 9%), and after escalating doses, excretion was linear over a 25- to 200-micromol dose range. Dithiocarbamate excretion was higher when intact sprouts were chewed thoroughly rather than swallowed whole (42.4 +/- 7.5 and 28.8 +/- 2.6 micromol; P = 0.049). These studies indicate that isothiocyanates are about six times more bioavailable than glucosinolates, which must first be hydrolyzed. Thorough chewing of fresh sprouts exposes the glucosinolates to plant myrosinase and significantly increases dithiocarbamate excretion. These findings will assist in the design of dosing regimens for clinical studies of broccoli sprout efficacy.

Antimalarial sulfide, sulfone, and sulfonamide trioxanes

A series of trioxanes featuring sulfide, sulfone, and sulfonamide substituents in diverse positions has been prepared. Structure-activity relationship (SAR) generalizations highlight two major factors controlling the antimalarial potency of these new chemical entities: (1) the proximity of the sulfur-containing substituent to the crucial peroxide bond and (2) the oxidation state of the sulfur-containing substituent. Generally, sulfones are more antimalarially potent than the corresponding sulfides.

Antimalarial, antiproliferative, and antitumor activities of artemisinin-derived, chemically robust, trioxane dimers

Nine C-10 non-acetal derivatives of the natural trioxane artemisinin (1) were prepared as dimers using some novel chemistry. As designed, each dimer was stable chemically. C-10 Olefinic dimers 7 and C-10 saturated dimers 8-13 all showed good to excellent antimalarial and antiproliferative activities in vitro. Dimers 8, 10, and 12 were especially potent and selective at inhibiting growth of some human cancer cell lines in the NCI in vitro 60-cell line assay.

Antitrypanosomal activity of fluoroquinolones

Six fluoroquinolones presently in clinical use and four investigational tetracyclic fluoroquinolones were tested for in vitro activity against bloodstream-form Trypanosoma brucei brucei. All compounds had measurable activity, but the tetracyclic analogs were most potent, with 50% effective concentrations in the low micromolar range. In general, trypanosomes were more susceptible than L1210 leukemia cells. Consistent with the notion that they target type II topoisomerase in trypanosomes, the fluoroquinolones promote the formation of protein-DNA covalent complexes.

Prophylactic activity of atovaquone against Plasmodium falciparum in humans

The prophylactic antimalarial activity of atovaquone was determined in a randomized, double-blind, placebo-controlled study of healthy volunteers who were challenged by the bite of Plasmodium falciparum-infected Anopheles stephensi. Subjects were randomly assigned to one of three groups: six received seven daily doses of 750 mg of atovaquone, starting the day before challenge; six received a single dose of 250 mg of atovaquone the day before challenge; and four received placebo. Polymerase chain reaction- and culture-confirmed parasitemia developed in all four placebo recipients, but in none of the drug recipients, indicating that either of the atovaquone regimens provides effective prophylaxis (P = 0.005). However, in low-dose recipients, the drug levels by day 6.5 were profoundly subtherapeutic, indicating that parasites were eliminated prior to the establishment of erythrocytic infection. Atovaquone thus protects non-immune subjects against mosquito-transmitted falciparum malaria, and has causal prophylactic activity.

Atovaquone and proguanil hydrochloride for prophylaxis of malaria

BACKGROUND

The spread of drug-resistant malaria and appreciation of side effects associated with existing antimalarial drugs emphasize the need for new drugs to prevent malaria. The combination of atovaquone and proguanil hydrochloride was previously shown to be safe and highly effective for treatment of malaria, including multi-drug-resistant Plasmodium falciparum.

METHODS

We reviewed results of clinical trials that evaluated either a fixed-dose combination of atovaquone and proguanil hydrochloride for malaria prophylaxis or atovaquone alone for causal prophylactic activity against P. falciparum.

RESULTS

In three placebo-controlled trials, 331 subjects received 250 mg atovaquone and 100 mg proguanil hydrochloride (or an equivalent dose based on body weight in children) once daily for 10 to 12 weeks. The overall efficacy for preventing parasitemia was 98%. Among 175 nonimmune volunteers taking the same dose of atovaquone/proguanil once daily for 10 weeks while temporarily residing in a malaria-endemic area, malaria developed in one patient who was noncompliant with therapy. Results of volunteer challenge studies indicate that both atovaquone and proguanil have causal prophylactic activity directed against the liver stages of P. falciparum. Adverse events occurred with similar or lower frequencies in subjects treated with atovaquone/proguanil compared to placebo. Less than 1% of patients discontinued from these studies due to a treatment-related adverse event.

CONCLUSION

A fixed-dose combination of atovaquone and proguanil hydrocloride is a promising new alternative for malaria prophylaxis.

Orally active, hydrolytically stable, semisynthetic, antimalarial trioxanes in the artemisinin family

In only three chemical operations, natural trioxane lactone artemisinin (1) was converted into a series of C-10 carbon-substituted 10-deoxoartemisinin compounds 4-9. The three steps involved lactone reduction, replacement of the anomeric lactol OH by F using diethylaminosulfur trifluoride, and finally boron trifluoride-promoted substitution of F by aryl, heteroaryl, and acetylide nucleophiles. All of these C-10 nonacetal, chemically robust, enantiomerically pure compounds 4-9 have high antimalarial potencies in vitro against Plasmodium falciparum malaria parasites, and furans 5a and 5b and pyrrole 7a are antimalarially potent also in vivo even when administered to rodents orally.

Human metabolism and excretion of cancer chemoprotective glucosinolates and isothiocyanates of cruciferous vegetables

Isothiocyanates and their naturally occurring glucosinolate precursors are widely consumed as part of a diet rich in cruciferous vegetables. When plant cells are damaged, glucosinolates are released and converted to isothiocyanates by the enzyme myrosinase. Many isothiocyanates inhibit the neoplastic effects of various carcinogens at a number of organ sites. Consequently, these agents are attracting attention as potential chemoprotectors against cancer. As a prerequisite to understanding the mechanism of the protective effects of these compounds, which is thought to involve the modulation of carcinogen metabolism by the induction of phase 2 detoxication enzymes and the inhibition of phase 1 carcinogen-activating enzymes, we examined the fate of ingested isothiocyanates and glucosinolates in humans. Recently developed novel methods for quantifying isothiocyanates (and glucosinolates after their quantitative conversion to isothiocyanates by purified myrosinase) and their urinary metabolites (largely dithiocarbamates) have made possible a detailed examination of the fates of isothiocyanates and glucosinolates of dietary crucifers. In a series of studies in normal volunteers, we made these findings. First, in nonsmokers, urinary dithiocarbamates were detected only after the consumption of cruciferous vegetables and condiments rich in isothiocyanates and/or glucosinolates. In sharp contrast, the consumption of noncrucifers (corn, tomatoes, green beans, and carrots) did not lead to the excretion of dithiocarbamates. Moreover, the quantities of dithiocarbamates excreted were related to the glucosinolate/isothiocyanate profiles of the cruciferous vegetables administered (kale, broccoli, green cabbage, and turnip roots). Second, eating prepared horseradish containing graded doses of isothiocyanates (12.3-74 micromol; mostly allyl isothiocyanate) led to a rapid excretion of proportionate amounts (42-44%) of urinary dithiocarbamates with first-order kinetics. The ingestion of broccoli in which myrosinase had been heat-inactivated also led to proportionate but low (10-20%) recoveries of urinary dithiocarbamates. Broccoli samples subsequently treated with myrosinase to produce the cognate isothiocyanates were much more completely (47%) converted to dithiocarbamates. Finally, when bowel microflora were reduced by mechanical cleansing and antibiotics, the conversion of glucosinolates became negligible. These results establish that humans convert substantial amounts of isothiocyanates and glucosinolates to urinary dithiocarbamates that can be easily quantified, thus paving the way for meaningful studies of phase 2 enzyme induction in humans.

DNA topoisomerases: a new twist for antiparasitic chemotherapy?

The parasitic protozoa are notorious for their bizarre cellular structures and metabolic pathways, a characteristic also true for their nucleic acids. Despite these florid differences from mammalian cells, however, it has proven surprisingly difficult to devise novel chemotherapy against these pathogens. In recent years, the DNA topoisomerases from parasites have been the focus of considerable study, not only because they are intrinsically interesting, but also because they may provide a target for much-needed new antiparasitic chemotherapy.

Antimalarial cyclic peroxy ketals

Over 20 new, cyclic, peroxy ketals have been prepared via a two-step protocol starting with readily available aryl methyl ketones. Structure-activity correlations using in vitro antimalarial data as a guide for optimization of potency have led to the design and synthesis of seven new peroxides that have IC50 values of 31-85 nM (artemisinin IC50 = 8.4 nM). Some SAR generalizations are discussed.

Synthesis and in vitro antimalarial activity of sulfone endoperoxides

A series of 4,8-dimethyl-4-phenylsulfonylmethyl-2,3-dioxabicyclo[3.3.1]+ ++nonanes, carrying a variety of substituents at position-8 (4) were prepared by a short and efficient method from R-(+)-limonene. Key reactions include thiol oxygen cooxidation, and alkylation and acylation of a sterically hindered tertiary alcohol compatible with the endoperoxy functionality. Some of compounds 4, which are structurally related to yingzhaosu A (2), were found to exhibit in vitro antimalarial activity comparable to that of artemisinin (1) and superior to that of arteflene (3).

Orally active antimalarial 3-substituted trioxanes: new synthetic methodology and biological evaluation

On the basis of a mechanistic understanding of the mode of action of artemisinin-like antimalarials, a series of structurally simple 3-aryl-1,2,4-trioxanes 5 was designed and was prepared in three to five operations from commercial reactants. The 3-aryl group was attached in each case as a nucleophile. In an electronically complementary fashion, 3-(fluoroalkyl)-trioxanes 6 were prepared via attachment of electrophilic fluoroalkyl esters. Both in vitro and in vivo antimalarial evaluations of these new trioxanes showed 12 beta-methoxy-3-aryltrioxanes 5g, 5j, 5k, and 51 to be highly potent, with crystalline fluorobenzyl ether trioxane 5k especially potent even when administered to rodents orally. As shown by rearrangement of hexamethyl Dewar benzene into hexamethylbenzene, iron-induced degradation of some of these 3-aryltrioxanes 5 involves generation of high-valent iron oxo species that might kill malaria parasites.

Design, synthesis, derivatization, and structure-activity relationships of simplified, tricyclic, 1,2,4-trioxane alcohol analogues of the antimalarial artemisinin

Novel C4-(hydroxyalkyl)trioxanes 5d and 5e were designed and synthesized based on an understanding of the molecular mechanism of action of similar 1,2,4-trioxanes structurally related to the antimalarial natural product artemisinin (1). In vitro efficacies of these two new pairs of C4-diastereomers against chloroquine-sensitive Plasmodium falciparum support conclusions about the importance to antimalarial activity of formation of a C4 radical by a 1,5-hydrogen atom abstraction. Derivatives 6, 7, and 21 of C4 beta-substituted trioxane alcohols 4a, 5d, and 5e were prepared, each in a single-step, high-yielding transformation. Four of these new analogues, 6a-c and 7, are potent in vitro antimalarials, having 140 to 50% of the efficacy of the natural trioxane artemisinin (1).

Effects of camptothecin, a topoisomerase I inhibitor, on Plasmodium falciparum

Currently, the treatment of falciparum malaria is seriously compromised by spreading drug resistance. We studied the effects of camptothecin, a potent and specific topoisomerase I inhibitor, on erythrocytic malaria parasites in vitro. In Plasmodium falciparum, camptothecin trapped protein-DNA complexes, inhibited nucleic acid biosynthesis, and was cytotoxic. These results provide proof for the concept that topoisomerase I is a vulnerable target for new antimalarial drug development.

Trioxane dimers have potent antimalarial, antiproliferative and antitumor activities in vitro

A series of tetracyclic and tricyclic trioxane dimers has been prepared with ether and ester tethers of varying length and flexibility. Several of these trioxane dimers have been found to have potent and potentially therapeutically valuable antimalarial, antiproliferative, and antitumor activities in vitro.

Diagnostic testing of cardiac function

The electrocardiogram's value in detecting atrial abnormalities, ventricular hypertrophy, and myocardial injury is described, with attention to ambulatory monitoring and exercise stress testing. Perfusion imaging, assessment of resting cardiac function with echocardiography, and other techniques also are addressed.

Biomechanical properties of human tibias in long-term spinal cord injury

Long-term spinal cord injury (SCI) profoundly alters skeletal structure and function. In this study, the biomechanical properties of tibias from persons with SCI and from individuals closely matched in age and size but without SCI were quantified at both the structural and material levels. Nondestructive torsion tests were performed to determine apparent shear moduli for the tibia. The cortical thicknesses and polar moment of inertia were determined numerically. Four-point bending tests were performed to determine flexural modulus of elasticity on cortical bone specimens of the tibia. The apparent shear moduli of the SCI tibias were found to be lower than the non-SCI tibias (p < 0.05). The cortical thicknesses of the SCI tibias were significantly thinner than the control tibias (p < 0.05), while the polar moment of inertia showed no significant differences between control and SCI tibial cross sections (p > 0.05). The flexural modulus of elasticity of the cortical bone specimens were lower in the SCI tibias than the controls (p < 0.05). These differences suggest that tibias may undergo micro-structural changes as well as structural adaptation following SCI, which alter their mechanical properties.

Topoisomerases in kinetoplastids

Topoisomerases are enzymes that mediate topological changes in DNA that are essential for nucleic acid biosynthesis and for cell survival. The kinetoplastid protozoa, which include pathogenic trypanosomes and Leishmania, have yielded an interesting variety of purified topoisomerase activities as well as several topoisomerase genes. In these parasites, topoisomerases are involved in the metabolism of both nuclear and mitochondrial (kinetoplast) DNA. In this review, Christian Burri, Armette Bodley and Theresa Shapiro summarize what is known about topoisomerases in kinetoplastids, and consider the intriguing possibility that these enzymes may act as valuable antiparasite drug targets.

The structure and replication of kinetoplast DNA

The mitochondrial DNA of trypanosomatid protozoa, termed kinetoplast DNA (kDNA), is unique in its structure, function, and mode of replication. kDNA is a massive network, composed of thousands of topologically interlocked DNA circles, which resembles the chain mail of medieval armor. Each cell contains one network condensed into a disk-shaped structure within the matrix of its single mitochondrion. The kDNA circles are of two types, maxicircles present in a few dozen copies and minicircles present in several thousand copies. The maxicircles, which encode ribosomal RNAs and a few mitochondrial proteins, are similar in structure and genetic function to the mitochondrial DNA of other eukaryotes. Many maxicircle transcripts undergo editing, a remarkable process involving the insertion or deletion of uridine residues at specific sites. The minicircles encode small guide RNAs that control the specificity of editing. During kDNA replication, covalently closed minicircles are released from the network by a topoisomerase II. The free minicircles replicate as theta-structures within one of two complexes of replication proteins that are positioned on opposite sides of the kinetoplast disk. The progeny minicircles, which contain nicks or gaps, are attached to the network periphery. Maxicircles also replicate as theta-structures, but they remain linked to the network. As replication proceeds, the number of minicircles and maxicircles increases. When the network has doubled in size, all of the minicircle nicks and gaps are repaired, and the network splits in two. The two progeny networks then segregate into the daughter cells.

Drug cytotoxicity assay for African trypanosomes and Leishmania species

The trypanosomes and Leishmania species are parasitic protozoa that afflict millions of people throughout the world. If not treated, African trypanosomiasis and visceral leishmaniasis are fatal. The available drugs are severely limited by toxicity, marginal efficacy, the requirement for parenteral administration, and spreading drug resistance. In this study, a spectrophotometric assay was developed and validated for measuring the cytotoxicity of test compounds against axenically cultured bloodstream-form Trypanosoma brucei (African trypanosomes) and promastigotes of Leishmania donovani. Enzymatic hydrolysis of p-nitrophenyl phosphate, monitored by a microtiter plate reader, is a reliable surrogate for parasite cell counts. The assay is simple, inexpensive, and highly reproducible. The coefficient of variation for EC50 values is < 10% for determinations obtained over several months. This method permits the rapid screening of candidates for much-needed new drugs against these parasites.

Antitrypanosomal activity of camptothecin analogs. Structure-activity correlations

African trypanosomes (Trypanosoma brucei species) are parasitic protozoa that cause lethal diseases in humans and cattle. Previous studies showed that camptothecin, a potent and specific inhibitor of DNA topoisomerase I, is cytotoxic to African trypanosomes and related pathogenic hemoflagellates (Bodley AL and Shapiro TA, Proc Natl Acad Sci USA 92: 3726-3730, 1995). In this study, a series of camptothecin analogs was tested against axenically cultured, bloodstream form, T. brucei. Modifications to the pentacyclic nucleus of camptothecin ablated antiparasitic activity. In contrast, activity could be increased by substituents added to the parent ring system (e.g. 10,11-methylenedioxy or ethylenedioxy groups; alkyl additions to carbon 7; or 9-amino or 9-chloro substituents). Cytotoxicity was correlated with the level of cleavable complexes in trypanosomes, implicating topoisomerase I as the intracellular target for these compounds. To obtain some indication of selective toxicity, ten compounds were also tested against L1210 mouse leukemia cells. The 9-substituted-10,11-methylenedioxy analogs caused a disproportionate increase in antiparasitic activity, compared with mammalian cell toxicity. These findings provide a basis for designing further structural modifications and for selecting camptothecin analogs to test in animal models of trypanosomiasis.

Molecular and cytotoxic effects of camptothecin, a topoisomerase I inhibitor, on trypanosomes and Leishmania

Parasites pose a threat to the health and lives of many millions of human beings. Among the pathogenic protozoa, Trypanosoma brucei, Trypanosoma cruzi, and Leishmania donovani are hemoflagellates that cause particularly serious diseases (sleeping sickness, Chagas disease, and leishmaniasis, respectively). The drugs currently available to treat these infections are limited by marginal efficacy, severe toxicity, and spreading drug resistance. Camptothecin is an established antitumor drug and a well-characterized inhibitor of eukaryotic DNA topoisomerase I. When trypanosomes or leishmania are treated with camptothecin and then lysed with SDS, both nuclear and mitochondrial DNA are cleaved and covalently linked to protein. This is consistent with the existence of drug-sensitive topoisomerase I activity in both compartments. Camptothecin also inhibits the incorporation of [3H]thymidine in these parasites. These molecular effects are cytotoxic to cells in vitro, with EC50 values for T. brucei, T. cruzi, and L. donovani, of 1.5, 1.6, and 3.2 microM, respectively. For these parasites, camptothecin is an important lead for much-needed new chemotherapy, as well as a valuable tool for studying topoisomerase I activity.

In vivo inhibition of trypanosome mitochondrial topoisomerase II: effects on kinetoplast DNA maxicircles

Kinetoplast DNA, the mitochondrial DNA of trypanosomes, is a topologically complex structure composed of interlocked minicircles and maxicircles. We previously reported that etoposide, a potent inhibitor of topoisomerase II, promotes the cleavage of about 20% of network minicircle DNA (T. A. Shapiro, V. A. Klein, and P. T. Englund, J. Biol. Chem. 264:4173-4178, 1989). We now find that virtually all maxicircles are released from kinetoplast DNA networks after trypanosomes are treated with etoposide. As expected for a topoisomerase II cleavage product, the linearized maxicircles have protein bound to both 5' ends. After etoposide treatment, the residual minicircle catenanes have a sedimentation coefficient which is only 70% that of controls, and by electron microscopy the networks are less compact. Double-size networks, the characteristic dumbbell-shape forms that normally arise in the final stages of network replication, are replaced by aberrant unit-size forms.

Mitochondrial topoisomerase II activity is essential for kinetoplast DNA minicircle segregation

Etoposide, a nonintercalating antitumor drug, is a potent inhibitor of topoisomerase II activity. When Trypanosoma equiperdum is treated with etoposide, cleavable complexes are stabilized between topoisomerase II and kinetoplast DNA minicircles, a component of trypanosome mitochondrial DNA (T. A. Shapiro, V. A. Klein, and P. T. Englund, J. Biol. Chem. 264:4173-4178, 1989). Etoposide also promotes the time-dependent accumulation of small minicircle catenanes. These catenanes are radiolabeled in vivo with [3H]thymidine. Dimers are most abundant, but novel structures containing up to five noncovalently closed minicircles are detectable. Analysis by two-dimensional gel electrophoresis and electron microscopy indicates that dimers joined by up to six interlocks are late replication intermediates that accumulate when topoisomerase II activity is blocked. The requirement for topoisomerase II is particularly interesting because minicircles do not share the features postulated to make this enzyme essential in other systems: for minicircles, the replication fork is unidirectional, access to the DNA is not blocked by nucleosomes, and daughter circles are extensively nicked and (or) gapped.

Prolonged heparin therapy for occlusive intracoronary thrombus

The presence of intracoronary thrombus is associated with increased complications during coronary angioplasty. Such thrombus may also mimic the appearance of a critical stenosis. We report a case of nearly occlusive intracoronary thrombus which resolved after prolonged heparin therapy, revealing only a minimal underlying stenosis. The recognition and treatment of this entity is discussed.

Inhibition of topoisomerases in African trypanosomes

African trypanosomiasis continues to pose a challenge for the development of new chemotherapy. Type II topoisomerases, essential enzymes in nucleic acid metabolism, have proven highly suitable as targets for antibacterial and antitumor therapy. Well-characterized topoisomerase II inhibitors affect the cognate nuclear and mitochondrial enzymes in Trypanosoma equiperdum. Inhibition is accompanied by extensive fragmentation and structural alteration in nuclear and mitochondrial DNA. Some clinically important antitrypanosomal drugs bind to DNA (i.e., pentamidine, isometamidium, diminazene). These agents inhibit the mitochondrial, but not nuclear, topoisomerase II of trypanosomes. These studies suggest that type II topoisomerase inhibitors may prove to be effective and safe new antitrypanosomal drugs.

Kinetoplast DNA maxicircles: networks within networks

Kinetoplast DNA (kDNA), the mitochondrial DNA of trypanosomes, is an enormous network of interlocked minicircles and maxicircles. We selectively removed minicircles from Trypanosoma equiperdum kDNA networks by restriction enzyme cleavage. Maxicircles remained in aggregates that were resistant to protease or RNase and contained no residual minicircles, but were resolved into circular monomers by topoisomerase II. Maxicircles thus form independent catenanes within kDNA networks. Heterogeneity in the size, composition, and organization of maxicircle catenanes reflects changes that occur during kDNA replication. The rosette-like arrangement of maxicircle catenanes is distinctly different from that of minicircle catenanes. Trypanosome kDNA networks reveal unique topological complexity: they are composed of entirely dissimilar catenanes that are in turn extensively interlocked with one another.

Effects of probenecid on the pharmacokinetics of allopurinol riboside

Allopurinol riboside is an experimental agent for the treatment of leishmaniasis and American trypanosomiasis. Previous studies showed that after oral administration, unexpectedly low levels of allopurinol riboside in plasma are attributable to incomplete absorption and rapid renal clearance. In this randomized, crossover evaluation in healthy volunteers, probenecid reduces the renal clearance of allopurinol riboside, extends the half-life of allopurinol riboside in plasma, and triples the levels of allopurinol riboside in plasma.

Coronary angiography and interventional cardiology

Percutaneous transluminal coronary angioplasty has become the most commonly applied coronary revascularization procedure. Its growth is due to expanding indications and new patient populations including patients with multivessel coronary artery disease, acute myocardial infarction or cardiogenic shock, and elderly patients. Success rates and complications for percutaneous transluminal coronary angioplasty have remained the same or improved despite the expanding indications and the persistent problem of restenosis. Several recent studies have examined new biologic approaches to restenosis. However, the most exciting area of interventional cardiology continues to be the use and development of new mechanical devices including stents, lasers, and atherectomy for unfavorable anatomic lesions, acute occlusion after angioplasty, and to prevent restenosis. The gold standard in the assessment of coronary disease is still angiography, but newer imaging techniques including intravascular ultrasound and angioscopy have added to our understanding of angioplasty, unstable angina, and the use of new devices.

Pharmacokinetics and metabolism of allopurinol riboside

There are no safe and effective oral drugs to treat leishmaniasis and Chagas' disease. The safety, pharmacokinetics, and metabolism of single and multiple oral doses of allopurinol riboside, an investigational antiparasitic agent, were evaluated in a randomized, double-blinded, placebo-controlled study in 32 healthy male volunteers, at levels up to 25 mg/kg q.i.d. for 13 doses. No significant toxicity was detected. Allopurinol riboside peaks in plasma 1.6 hours after administration, has an elimination half-life of 3 hours, and steady-state concentrations in the therapeutic range. However, in contrast to preclinical studies in dogs (plasma levels proportional to oral doses up to 200 mg/kg), we found that plasma levels were unexpectedly low and did not rise with increasing dose. Furthermore, allopurinol and oxypurinol (unanticipated metabolites) were detected at levels proportional to the dose of allopurinol riboside. We present a model that includes incomplete absorption, metabolism of residual drug by enteric flora, and absorption of bacterial metabolites to explain these findings in humans.

Selective cleavage of kinetoplast DNA minicircles promoted by antitrypanosomal drugs

Pentamidine, diminazene aceturate (Berenil), isometamidium chloride (Samorin), and ethidium bromide, which are important antitrypanosomal drugs, promote linearization of Trypanosoma equiperdum minicircle DNA (the principal component of kinetoplast DNA, the mitochondrial DNA in these parasites). This effect occurs at therapeutically relevant concentrations. The linearized minicircles are protease sensitive and are not digested by lambda exonuclease (a 5' to 3' exonuclease), indicating that the break is double stranded and that protein is bound to both 5' ends of the molecule. The cleavage sites map to discrete positions in the minicircle sequence, and the cleavage pattern varies with different drugs. These findings are characteristic for type II topoisomerase inhibitors, and they mimic the effects of the antitumor drug etoposide (VP16-213, a semisynthetic podophyllotoxin analog) on T. equiperdum minicircles. However, the antitrypanosomal drugs differ dramatically from etoposide in that they do not promote detectable formation of nuclear DNA-protein complexes or of strand breaks in nuclear DNA. Selective inhibition of a mitochondrial type II topoisomerase may explain why these antitrypanosomal drugs preferentially disrupt mitochondrial DNA structure and generate dyskinetoplastic trypanosomes (which lack mitochondrial DNA).

Drug-promoted cleavage of kinetoplast DNA minicircles. Evidence for type II topoisomerase activity in trypanosome mitochondria

Minicircle DNA, the major component of the mitochondrial DNA of trypanosomes (kinetoplast DNA), is linearized when living Trypanosoma equiperdum cells are treated with inhibitors of mammalian type II topoisomerases and then lysed with sodium dodecyl sulfate. A variety of intercalating and nonintercalating compounds (the epipodophyllotoxins, 4'-(9-acridinylamino)-methanesulfon-m-anisidine, 2-methyl-9-hydroxyellipticine, and acriflavine) are active, but novobiocin and specific gyrase inhibitors (the quinolones) are not. The linearized minicircles are in a DNA-protein complex, as their electrophoretic mobility is increased by Proteinase K treatment. They are digested by exonuclease III but not by lambda exonuclease, indicating that the protein must be linked to both 5' ends. Drug-induced cleavage sites vary with different compounds and are found throughout the minicircle sequence. These results indicate that trypanosome mitochondria contain a type II topoisomerase with some properties similar to those of type II topoisomerases in the nucleus of higher eukaryotes. A maximum of 12% of all minicircles is cleaved in the presence of VP16-213, indicating there are at least 600 molecules of mitochondrial type II topoisomerase/cell or about one enzyme/8 kilobases of minicircle DNA.

A knotted free minicircle in kinetoplast DNA

Kinetoplast DNA, the mitochondrial DNA of trypanosomes, is a network containing thousands of minicircles that are topologically interlocked. The minicircle replication intermediates are free molecules that have been released from the network. We report here that one form of free minicircles is a trefoil knot. Identification of this knotted structure is based on its electrophoretic and sedimentation properties, its response to treatments with restriction enzymes or topoisomerase II, and its appearance by electron microscopy. Except for its topology, the knotted minicircle closely resembles a previously described replication intermediate with a unique gap in the newly synthesized L strand.

Ribonucleotides associated with a gap in newly replicated kinetoplast DNA minicircles from Trypanosoma equiperdum

In Trypanosoma equiperdum, some newly replicated kinetoplast DNA minicircles contain a single gap at a unique location in their newly synthesized strand (Ntambi, J. M., and Englund, P. T. (1985) J. Biol. Chem. 260, 5574-5579). We now report that ribonucleotides are associated with this gap, with one or two covalently attached to the 5' terminus of the newly synthesized strand. There appear to be two possible RNA/DNA junctions at adjacent positions in the sequence. The ribonucleotides may be remnants of a replication primer, and their presence strongly implies that the gap is at the site of a replication origin.

Clinical evaluation of amoscanate in healthy male volunteers

Single oral doses of amoscanate (4-isothiocyanato-4'-nitrodiphenylamine), an experimental antiparasitic agent, are highly effective in animals against the four major species of schistosomes which infect humans. Two prospective, randomized, double blinded, placebo controlled Phase I studies were designed to evaluate the tolerance and safety of the 5% aqueous suspension of 2-mu particles of amoscanate administered to healthy male volunteers. In addition to routine safety monitoring, particular attention was directed toward detecting hepatic, neurological, cardiovascular or ocular toxicity. Three of four men who received 3.5 mg/kg of amoscanate developed mild, reversible hepatotoxicity, which could not be unequivocably attributed to the drug. In the second study, of 1 mg/kg amoscanate, there was no statistically significant evidence of hepatotoxicity, although 1 of 12 drug recipients developed transient liver chemistry changes. Despite intensive monitoring, there was no evidence in either study of significant symptomatic complaints, or of neurological, cardiovascular or ocular toxicity. No mutagenic activity attributable to amoscanate was detectable in the urine. These results suggest that this formulation of amoscanate, at 1 mg/kg, is sufficiently well tolerated and safe to justify evaluation for efficacy in patients with schistosomiasis.