Purification, characterization, and drug susceptibility of tubulin from Leishmania

Isoflavonoids and other compounds from Psorothamnus arborescens with antiprotozoal activities

Bioactivity-guided fractionation of the root extract of Psorothamnus arborescens yielded the new isoflavone 5,7,3',4'-tetrahydroxy-2'-(3,3-dimethylallyl)isoflavone (1a) and the new 2-arylbenzofuran 2-(2'-hydroxy-4',5'-methylenedioxyphenyl)-6-methoxybenzofuran-3-carbaldehyde (2), together with seven known compounds, including three isoflavones, fremontin (3a), glycyrrhisoflavone (4a), and calycosin (5), two pterocarpans, maackiain (6) and 4-hydroxymaackiain (7), one triterpene, oleanolic acid (8), and one chalcone, isoliquiritigenin (9). In addition, the structure of the isoflavone fremontin was revised using spectroscopic and chemical methods and was assigned the new structure 3a. The isoflavone 1a and the chalcone 9 displayed leishmanicidal activity with IC50 values of 13.0 and 20.7 microM, respectively, against Leishmania donovani axenic amastigotes. Calycosin (5) exhibited selective toxicity against Trypanosoma brucei brucei (IC50 12.7 microM) compared to L. donovani amastigotes and Vero cells (IC50 100 and 159 microM, respectively). These results prompted us to test a small group of structurally related isoflavones for their antitrypanosomal activities. Genistein and 7,3',4'-trihydroxyisoflavone displayed promising activity (IC50 values 4.2 and 7.1 microM, respectively) and selectivity (IC50 versus Vero cells: 32.9 and 135 microM, respectively). These studies suggest that the isoflavone skeleton deserves further investigation as a template for novel antileishmanial and trypanocidal compounds.

Altered tubulin dynamics, localization and post-translational modifications in sodium arsenite resistant Leishmania donovani in response to paclitaxel, trifluralin and a combination of both and induction of apoptosis-like cell death

In this study the anti-leishmanial activity and anti-microtubule effects of paclitaxel, trifluralin and a combination of paclitaxel and trifluralin have been tested in a wild type and sodium arsenite-resistant strain of Leishmania donovani. Both paclitaxel and trifluralin have been shown to be effective in limiting parasite growth. Specific alterations in morphology, tubulin polymerization dynamics, post-translational modifications and cellular distribution of the tubulins have been confirmed to be a part of the intracellular anti-microtubule-events that occur in arsenite-resistant L. donovani in response to these agents, ultimately leading to death of the parasite. DNA analyses of the drug-treated wild type and arsenite-resistant strains revealed an apoptosis-like death in response to paclitaxel and the combination but not to trifluralin. Data provide valuable information for further development of chemotherapeutic strategies based on anti-microtubule agents against drug resistant Leishmania parasites.

Characterisation of benzimidazole binding with recombinant tubulin from Giardia duodenalis, Encephalitozoon intestinalis, and Cryptosporidium parvum

The binding kinetics of several benzimidazole compounds were determined with recombinant tubulin from benzimidazole-sensitive and -insensitive organisms. This study utilised the naturally occurring high efficacy of the benzimidazoles for the parasitic protozoa Giardia duodenalis and Encephalitozoon intestinalis, and low efficacy with Cryptosporidium parvum. Direct kinetic analysis of the benzimidazole-beta-tubulin interaction was performed using a fluorescence-based quenching method to determine the apparent association (k(on)) and dissociation (k(off)) rate constants from which the affinity constant (K(a)) was calculated. The binding kinetics were determined with recombinant alpha- and beta-tubulin from the parasitic protozoa with several benzimidazole R(2)-carbamate analogues. The affinity constant for the binding of several benzimidazoles with beta-tubulin from benzimidazole-sensitive protozoa was found to be significantly greater than binding to beta-tubulin from benzimidazole-insensitive protozoa. Additionally, the high affinity of several benzimidazole derivatives (albendazole, fenbendazole, mebendazole) for monomeric beta-tubulin and heterodimeric alphabeta-tubulin from benzimidazole-sensitive protozoa was also clearly demonstrated. The affinity constants determined with beta-tubulin from G. duodenalis and E. intestinalis also supported the observed in vitro efficacy of these compounds. The binding characteristics of the benzimidazoles with the highest in vitro efficacy (albendazole, fenbendazole, mebendazole) was reflected in their high association and slow dissociation rates with the beta-tubulin monomer or dimer from benzimidazole-sensitive protozoa compared with insensitive ones. Benzimidazole-bound alphabeta-tubulin heterodimers also had a significantly lower rate of microtubule assembly compared with benzimidazole-free alphabeta-heterodimers. The incorporation of benzimidazole-bound alphabeta-heterodimers into assembling microtubules was shown to arrest polymerisation in vitro although the addition of benzimidazole compounds to assembled microtubules did not result in depolymerisation. These findings indicate that a benzimidazole-beta-tubulin cap may be formed at the growing end of the microtubule and this cap prevents elongation of the microtubule.

Antiprotozoal compounds from Psorothamnus polydenius

Bioactivity-guided fractionation of the methanolic extract of Psorothamnus polydenius yielded the new chalcone 2,2',4'-trihydroxy-6'-methoxy-3',5'-dimethylchalcone (2), together with six other known compounds, 2',4'-dihydroxy-6'-methoxy-3',5'-dimethylchalcone (1), dalrubone (3), demethoxymatteucinol (4), eriodictyol (5), and photodalrubone (6a and 6b). This is the first report of chalcones in P. polydenius. The extracts and isolated compounds were tested in vitro for their antiprotozoal activity against Leishmania donovani and Trypanosoma brucei. Chalcones 1 and 2 and dalrubone (3) exhibited leishmanicidal (IC(50) 5.0, 7.5, and 7.5 microg/mL, respectively) and trypanocidal (IC(50) 6.3, 6.8, and 21.6 microg/mL, respectively) properties. Dalrubone (3) displayed 6-fold selectivity for axenic L. donovani parasites over Vero cells. Furthermore, treatment of L. mexicana-preinfected macrophages with chalcones 1 and 2 and dalrubone (3) (12.5, 12.5, and 25 microg/mL, respectively) reduced the number of infected macrophages by at least 96% while posing no toxicity to the host cell.

Altered expression, polymerisation and cellular distribution of alpha-/beta-tubulins and apoptosis-like cell death in arsenite resistant Leishmania donovani promastigotes

Studies in mammalian systems have shown specific affinity of arsenite for tubulin proteins. The sodium m-arsenite (NaAsO2) resistant Leishmania donovani used in this study is resistant to 20 microM NaAsO2, which is a 13-fold increase in resistance compared to the wild type. Data presented in this study shows decreased expression of alpha- and beta-tubulin in wild type L. donovani promastigotes on exposure to NaAsO2 from 0.0016 to 5.0 microM (IC50 in the wild type strain) in a dose-dependent manner. alpha- and beta-tubulins in the resistant strain show decreased expression levels only at 65.0 microM NaAsO2 (IC50 in the resistant strain). Treatment with respective IC50 concentrations of NaAsO2 caused alterations in tubulin polymerisation dynamics and deregulated the cellular distribution of the microtubules in wild type and resistant strains. The NaAsO2-induced cell death exhibited characteristics of apoptosis-like DNA laddering and fragmentation in both the affected wild type and resistant cells. However, poly(ADP-ribose)polymerase cleavage was evident in the wild type strain but not in the resistant strain.

Antikinetoplastid activity of 3-aryl-5-thiocyanatomethyl-1,2,4-oxadiazoles

A series of 5-thiocyanatomethyl- and 5-alkyl-3-aryl-1,2,4-oxadiazoles were synthesized and evaluated for their activity against kinetoplastid parasites. Formation of the oxadiazole ring was accomplished through the reaction of benzamidoximes with acyl chlorides, while the thiocyanate group was inserted by reacting the appropriate 5-halomethyl oxadiazole with ammonium thiocyanate. The thiocyanate-containing compounds possessed low micromolar activity against Leishmania donovani and Trypanosoma brucei, while the 5-alkyl oxadiazoles were less active against these parasites. 3-(4-Chlorophenyl)-5-(thiocyanatomethyl)-1,2,4-oxadiazole (compound 4b) displayed modest selectivity for L. donovani axenic amastigote-like parasites over J774 macrophages, PC3 prostate cancer cells, and Vero cells (6.4-fold, 3.8-fold, and 9.1-fold, respectively), while 3-(3,4-dichlorophenyl)-5-(thiocyanatomethyl)-1,2,4-oxadiazole (compound 4 h) showed 30-fold selectivity against Vero cells but was not selective against PC3 cells. In a murine model of visceral leishmaniasis, compound 4b decreased liver parasitemia caused by L. donovani by 48% when given in five daily i.v. doses at 5mg/kg and by 61% when administered orally for 5 days at 50 mg/kg. These results indicate that aromatic thiocyanates hold promise for the treatment of leishmanial infections if the selectivity of these compounds can be improved.

Synthesis and Antitubulin Activity of N- and N4-Substituted 3,5-Dinitro Sulfanilamides against African Trypanosomes and Leishmania

Thirty analogues of N(1)-phenyl-3,5-dinitro-N(4),N(4)-di-n-propylsulfanilamide (GB-II-5, compound 3), a new antikinetoplastid antimitotic agent, have been synthesized and evaluated. The addition of simple functional groups to the N1 aromatic ring generally decreases antiparasitic and antimitotic potency, but placement of a dibutyl substituent at the N4 nitrogen to give N(1)-phenyl-3,5-dinitro-N(4),N(4)-di-n-butylsulfanilamide (compound 35) augments antitrypanosomal and antileishmanial activity. Compound 35 possesses IC(50) values of 0.12 and 2.6 microM against cultured T. brucei and L. donovani amastigote-like forms, surpassing the activity of compound 3 against these parasites by 3.4- and 1.9-fold, respectively. Compound 35 inhibits the assembly of leishmanial tubulin with an IC(50) of 6.9 microM and displays antimitotic effects in cultured T. brucei as assessed by flow cytometry, but shows little effect on purified mammalian tubulin, and displays 100-fold selectivity for trypanosomes over two mammalian cell lines. Although 3 and 35 were not effective in initial in vivo antitrypanosomal assays, the in vitro potency and selectivity of these compounds make N(1)-aryl-3,5-dinitro-N(4),N(4)-dialkylsulfanilamides a promising new class of antikinetoplastid agents that act on parasite tubulin.

Selective antimicrotubule activity of N1-phenyl-3,5-dinitro-N4,N4-di-n-propylsulfanilamide (GB-II-5) against kinetoplastid parasites

Analogs of the antimitotic herbicide oryzalin (3,5-dinitro-N4,N4-di-n-propylsulfanilamide) were recently prepared that were more potent in vitro than the parent compound against the kinetoplastid parasite Leishmania donovani (Bioorg Med Chem Lett 12:2395-2398, 2002). In the present work, we show that the most active molecule in the group, N1-phenyl-3,5-dinitro-N4,N4-di-n-propylsulfanilamide (GB-II-5), is a potent, selective antimitotic agent against kinetoplastid parasites. GB-II-5 possesses IC50 values of 0.41 and 0.73 microM in vitro against two strains of the related parasite Trypanosoma brucei but is much less toxic to J774 murine macrophages and PC3 prostate cancer cells, exhibiting IC50 values of 29 and 35 microM against these lines, respectively. Selectivity is also observed for GB-II-5 with purified leishmanial and mammalian tubulin. The assembly of 15 microM leishmanial tubulin is completely inhibited by 10 microM GB-II-5, whereas 40 microM GB-II-5 inhibits the assembly of 15 microM porcine brain tubulin by only 17%. In cultured L. donovani and T. brucei, treatment with 5 and 0.5 microM GB-II-5, respectively, causes a striking increase in the fraction of G2M cells compared with control. Given the potency and selectivity of this agent against kinetoplastid tubulin, GB-II-5 emerges as an exciting new antitrypanosomal and antileishmanial lead compound.

Antiprotozoal activities of symmetrical bishydroxamic acids

Symmetrical bishydroxamic acids along with their sodium salts containing an alkyl spacer between two aromatic rings were synthesized, and their antiparasitic activities were evaluated. Bishydroxamic acids were conveniently prepared from the alkylation of methyl 4-hydroxybenzoate with various dihalo-alkane, -alkene, and -ether followed by reaction with hydroxylamine. Surprisingly, the bishydroxamic acids and their sodium salts possess strong inhibitory activities against Plasmodium falciparum parasites with IC50 values in the range of 0.26-3.2 microM. Bishydroxamic acid 3 and its sodium salt 12 also inhibit the growth of Leishmania donovani, albeit at higher concentrations. The corresponding biscarboxylic acids and bismethyl esters are inactive. Presumably, the ability of bishydroxamic acids to complex with metallic iron in hemoglobin may be responsible for antimalarial activity of these compounds.

The activity of diguanidino and 'reversed' diamidino 2,5-diarylfurans versus Trypanosoma cruzi and Leishmania donovani

The in vitro activity of 20 dicationic molecules containing either diguanidino or reversed amidine cationic groups were evaluated versus Trypanosoma cruzi and Leishmania donovani. The most active compounds were in the reversed amidine series and six exhibited IC(50) values of less than 1 micro mol versus T. cruzi and five gave similar values versus L. donovani.

Boon and bane of metal ions in medicine

In biological systems metal ions promote responses that range from deficiency to toxicity. Some, such as iron and zinc, have a known optimal intake range for normal, healthy individuals. Metal ions contained within well-designed molecules already constitute a great boon for the medicinal pharmacopoeia. However, whether essential or not, the threshold for toxicity can be very low. One of the challenges of designing metal-based drugs is to balance the potential toxicity of an active formulation with the substantial positive impact of these increasingly common therapeutic and diagnostic aids.

Microtubules: dynamics, drug interaction and drug resistance in Leishmania

Microtubules are cytoskeletal polymers essential for the survival of all eukaryotes. These proteins are the proposed cellular targets of many anticancerous, antifungal and antihelminthic drugs. Sufficient differences exist between the microtubules of kinetoplastid parasites like Leishmania and humans to explore the selective targeting of these proteins for therapeutic purposes. This review describes the basic structure of microtubules and its dynamics in general, with specific insights into leishmanial microtubules, the salient features of microtubule-drug interactions including the specificity of certain drugs for parasitic microtubules. Chemotherapy against leishmanial parasites is failing because of the emergence of drug resistant strains. The possible mechanisms of resistance to antimicrotubule agents along with insights into the role of microtubules in mediating drug resistance in Leishmania are discussed.

Antileishmanial dinitroaniline sulfonamides with activity against parasite tubulin

Novel dinitroaniline sulfonamides based on the herbicide oryzalin 3 were synthesized and evaluated for activity against the parasitic protozoan Leishmania donovani and against leishmanial tubulin, the putative antiparasitic target of oryzalin. A subset of these compounds possess more activity against both Leishmania and the target protein in vitro. Compound 20 displays improved potency against leishmanial tubulin and is 13.4-fold more active against L. donovani axenic amastigotes than oryzalin.

Synthesis and antiparasitic activity of 1H-benzimidazole derivatives

Compounds 1-18 have been synthesized and tested in vitro against the protozoa Giardia lamblia, Entamoeba histolytica and the helminth Trichinella spiralis. Inhibition of rat brain tubulin polymerization was also measured and compared for each compound. Results indicate that most of the compounds tested were more active as antiprotozoal agents than Metronidazole and Albendazole. None of the compounds was as active as Albendazole against T. spiralis. Although only compounds 3, 9 and 15 (2-methoxycarbonylamino derivatives) inhibited tubulin polymerization, these were not the most potent antiparasitic compounds.

Promising therapeutic targets for antileishmanial drugs

Current treatments for the parasitic disease leishmaniasis are unsatisfactory due to their route of administration, toxicity and expense. Resistance is also developing to first-line antimonial drugs. Fortunately, a handful of antileishmanial agents, such as the orally available compound miltefosine, are currently in clinical trials. In addition, several promising drug targets and lead molecules are being studied with the goal of developing new antileishmanial agents. Drug candidates have been identified through the continued investigation of parasite sterol metabolism and parasite proteases. New antileishmanial molecules have also been discovered through the study of novel targets and pathways, such as the bisphosphonate inhibitors of isoprenoid biosynthesis. This review presents a synopsis of the drug targets and lead compounds that have been investigated over the last few years against leishmaniasis, gives a perspective on the chemotherapeutic potential of each and discusses some of the obstacles to antileishmanial drug development.

Antileishmanial activities of several classes of aromatic dications

Aromatic dicationic molecules possess impressive activity against a broad spectrum of microbial pathogens, including Pneumocystis carinii, Cryptosporidium parvum, and Candida albicans. In this work, 58 aromatic cations were examined for inhibitory activity against axenic amastigote-like Leishmania donovani parasites. In general, the most potent of the compounds were substituted diphenyl furan and thiophene dications. 2,5-Bis-(4-amidinophenyl)thiophene was the most active compound. This agent displayed a 50% inhibitory concentration (IC50) of 0.42 +/- 0.08 microM against L. donovani and an in vitro antileishmanial potency 6.2-fold greater than that of the clinical antileishmanial dication pentamidine and was 155-fold more toxic to the parasites than to a mouse macrophage cell line. 2,4-Bis-(4-amidinopheny)furan was twice as active as pentamidine (IC50), 1.30 +/- 0.21 microM), while 2,5-bis-(4-amidinopheny)furan and pentamidine were essentially equipotent in our in vitro antileishmanial assay. Carbazoles, dibenzofurans, dibenzothiophenes, and benzimidazoles containing amidine or substituted amidine groups were generally less active than the diphenyl furans and thiophenes. In all cases, aromatic dications possessing strong antileishmanial activity were severalfold more toxic to the parasites than to a cultured mouse macrophage cell line. These structure-activity relationships demonstrate the potent antileishmanial activity of several aromatic dications and provide valuable information for the future design and synthesis of more potent antiparasitic agents.

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.

Cell cycle-dependent changes in localization of a 210-kDa microtubule-interacting protein in Leishmania

Using the monoclonal antibody MA-01, a new 210-kDa microtubule-interacting protein was identified in Leishmania promastigotes by immunoblotting and by immunoprecipitation. The protein was thermostable and was located on microtubules prepared by taxol-driven polymerization in vitro. On fixed cells the antibody gave specific staining of flagellum, flagellar pocket, and mitotic spindle. Subpellicular microtubules were basically not decorated but posterior poles of the cells were labeled in a cell-cycle-dependent manner. In anterior and posterior poles of cells the 210-kDa protein codistributed with the 57-kDa protein, immunodetected with anti-vimentin antibody, that was located only on cell poles. Immunolocalization of the 57-kDa protein was most prominent in dividing cells. The presented data suggest that the 210-kDa protein is a newly identified microtubule-interacting protein of Leishmania that could be involved in anchoring the microtubules in posterior poles of these cells. The striking codistribution of the microtubule-interacting protein and the 57-kDa protein in protozoa is described for the first time.

Dose-dependent differential effect of hemin on protein synthesis and cell proliferation in Leishmania donovani promastigotes cultured in vitro

Leishmania donovani requires an exogenous source of heme for growth and transformation. In in vitro culture of the free-living promastigotes, exogenously added hemin enhances cell proliferation. In this investigation, the question of the function of heme with particular reference to protein synthesis and cell proliferation has been addressed. The results of in vitro cell culture experiments demonstrated that hemin (10 microM) alone is suitable for supporting optimum level of protein synthesis, and thereby cell proliferation of promastigotes to an extent that it can replace fetal bovine serum. However, in situ labelling experiments along with Western blots revealed that high concentration of hemin (50 microM) reduced the level of protein synthesis in general and of beta-tubulin in particular with a concomitant induction of hsp90, and induced consequent morphological changes that are observed during in situ transformation of promastigotes in mammalian macrophages. These results therefore suggest that sudden exposure to high concentration of heme in mammalian macrophages may be one of the key factors that trigger promastigote to amastigote transformation in L. donovani. Furthermore, hemin with its dual characteristic could be used as a tool to understand molecular mechanism of cell proliferation and transformation in these parasites.

Dinitroaniline herbicides against protozoan parasites: the case of Trypanosoma cruzi

The drugs presently in use against Chagas disease are very toxic, inducing a great number of side effects. Alternative treatments are necessary, not only for Chagas disease but also for other diseases caused by protozoan parasites where current drugs pose toxicity problems. The plant microtubule inhibitor trifluralin has previously been tested with success against Leishmania, Trypanosoma brucei and several other protozoan parasites. Trypanosoma cruzi, the causative agent of Chagas disease, is also sensitive to the drug. This sensitivity has been correlated with the deduced amino acid sequences of alpha- and beta-tubulin of T. cruzi as compared with plant, mammal and other parasite sequences.

Cellular effects of leishmanial tubulin inhibitors on L. donovani

To aid our investigation of tubulin as an antileishmanial drug target, the effects of the mammalian antimicrotubule agents ansamitocin P3, taxol, and hemiasterlin on Leishmania donovani promastigotes were described. These drugs affected the assembly of purified leishmanial tubulin and inhibited the growth of L. donovani promastigotes at micromolar concentrations. When promastigotes were treated with these agents, mitotic partitioning of nuclear DNA and cytokinesis were usually inhibited. The spatial orientation of kinetoplasts was often disturbed, suggesting a role for microtubules in the segregation of these organelles during mitosis. Aberrant cell types produced in drug-treated cultures included parasites with one nucleus and two geometrically distinct kinetoplasts, parasites with multiple kinetoplasts, and cytoplasts containing a kinetoplast but no nucleus. A subset of unique cell types, parasites containing two nuclei, a spindle fiber, and two geometrically distinct kinetoplasts, were observed in hemiasterlin-treated cultures. Flow cytometric analysis of L. donovani promastigotes treated with these three drugs indicated a dramatic shift toward the G2 + M phase of the cell cycle, with some cells containing four times the amount of DNA present in G1. These results were used to evaluate the cellular effects of WR85915, an aromatic thiocyanate with in vitro antileishmanial and anti-tubulin activity, on L. donovani. Treatment of parasites with WR85915 did not produce the unusual cell types described above and did not cause the accumulation of parasites in G2 + M, suggesting that WR85915 acts on target(s) in Leishmania in addition to tubulin. These studies validate tubulin as a suitable antileishmanial drug target and provide criteria to assess the cellular mechanism of action of new candidate antileishmanial agents.

Analysis of stereoelectronic properties of camptothecin analogues in relation to biological activity

Camptothecin and four of its 10,11-methylenedioxy analogues were examined for their activity against the pathogenic protozoan Leishmania donovani in vitro. The methylenedioxy analogues were 36- to 180-fold more potent than the parent camptothecin, possessing IC50 values ranging from 160 to 32 nM against the parasite. Our finding that the methylenedioxy camptothecins possess greater activity than camptothecin, which is also the case for other cell types and for the generation of cleavable complex in the presence of DNA and purified mammalian topoisomerase I, prompted us to examine the molecular features of camptothecin and methylenedioxy camptothecin analogues. A delocalization of positive potential was observed in the methylenedioxy camptothecin analogues, which could increase the affinity of these molecules for DNA. In addition, geometrical and electronic differences between the E ring of camptothecin and its methylenedioxy analogues were noted. One or both of these factors may contribute to the superior biological activity of the methylenedioxy camptothecin analogues.

Evidence for novel cell cycle checkpoints in trypanosomes: kinetoplast segregation and cytokinesis in the absence of mitosis

Trypanosoma brucei has a single nucleus and a single kinetoplast (the mitochondrial genome). Each of these organelles has a distinct S phase, which is followed by a segregation period, prior to cell division. The segregation of the two genomes takes place in a specific temporal order by interaction with microtubule-based structures, the spindle for nuclear DNA and the flagellum basal bodies for the kinetoplast DNA. We used rhizoxin, the anti-microtubule agent and polymerisation inhibitor, or the nuclear DNA synthesis inhibitor aphidicolin, to interfere with cell cycle events in order to study how such events are co-ordinated. We show that T. brucei cytokinesis is not dependent upon either mitosis or nuclear DNA synthesis, suggesting that there are novel cell cycle checkpoints in this organism. Moreover, use of monoclonal antibodies to reveal cytoplasmic events such as basal body duplication shows that some aphidicolin treated cells appear to be in G(1) phase (1K1N) but have activated some cytoplasmic events characteristic of G(2) phase (basal body segregation). We discuss a possible dominant role in trypanosomes for kinetoplast/basal body segregation in control of later cell cycle events such as cytokinesis