Luis M. Ruiz-Pérez

Novel Azasterols as Potential Agents for Treatment of Leishmaniasis and Trypanosomiasis

ABSTRACT This paper describes the design and evaluation of novel azasterols as potential compounds for the treatment of leishmaniasis and other diseases caused by trypanosomatid parasites. Azasterols are a known class of (S)-adenosyl-l-methionine: Δ24-sterol methyltransferase(24-SMT) inhibitors in fungi, plants, and some parasitic protozoa. The compounds prepared showed activity at micromolar and nanomolar concentrations when tested against Leishmania spp. and Trypanosoma spp. The enzymatic and sterol composition studies indicated that the most active compounds acted by inhibiting 24-SMT. The role of the free hydroxyl group at position 3 of the sterol nucleus was also probed. When an acetate was attached to the 3β-OH, the compounds did not inhibit the enzyme but had an effect on parasite growth and the levels of sterols in the parasite, suggesting that the acetate group was removed in the organism. Thus, an acetate group on the 3β-OH may have application as a prodrug. However, there may be an additional mode(s) of action for these acetate derivatives. These compounds were shown to have ultrastructural effects on Leishmania amazonensis promastigote membranes, including the plasma membrane, the mitochondrial membrane, and the endoplasmic reticulum. The compounds were also found to be active against the bloodstream form (trypomastigotes) of Trypanosoma brucei rhodesiense, a causative agent of African trypanosomiasis.

Design, synthesis, and evaluation of inhibitors of trypanosomal and leishmanial dihydrofolate reductase

This paper concerns the design, synthesis, and evaluation of inhibitors of leishmanial and trypanosomal dihydrofolate reductase. Initially study was made of the structures of the leishmanial and human enzyme active sites to see if there were significant differences which could be exploited for selective drug design. Then a series of compounds were synthesized based on 5-benzyl-2, 4-diaminopyrimidines. These compounds were assayed against the protozoan and human enzymes and showed selectivity for the protozoan enzymes. The structural data was then used to rationalize the enzyme assay data. Compounds were also tested against the clinically relevant forms of the intact parasite. Activity was seen against the trypanosomes for a number of compounds. The compounds were in general less active against Leishmania. This latter result may be due to uptake problems. Two of the compounds also showed some in vivo activity in a model of African trypanosomiasis.

Kinetic Characterization of Squalene Synthase from Trypanosoma cruzi: Selective Inhibition by Quinuclidine Derivatives

ABSTRACT The biosynthesis of sterols is a major route for the development of antitrypanosomals. Squalene synthase (SQS) catalyzes the first step committed to the biosynthesis of sterols within the isoprenoid pathway, and several inhibitors of the enzyme have selective antitrypanosomal activity both in vivo and in vitro. The enzyme from Trypanosoma cruzi is a 404-amino-acid protein with a clearly identifiable membrane-spanning region. In an effort to generate soluble recombinant enzyme, we have expressed in Escherichia coli several truncated versions of T. cruzi SQS with a His tag attached to the amino terminus. Deletions of both the amino- and carboxyl-terminal regions generated active and soluble forms of the enzyme. The highest levels of soluble protein were achieved when 24 and 36 amino acids were eliminated from the amino and carboxyl regions, respectively, yielding a protein of 41.67 kDa. The Michaelis-Menten constants of the purified enzyme for farnesyl diphosphate and NAD (NADPH) were 5.25 and 23.34 μM, respectively, whereas the Vmax was 1,428.56 nmol min−1mg−1. Several quinuclidine derivatives with antiprotozoal activity in vitro were found to be selective inhibitors of recombinant T. cruzi SQS in comparative assays with the human enzyme, with 50% inhibitory concentration values in the nanomolar range. These data suggest that selective inhibition of T. cruzi SQS may be an efficient strategy for the development of new antitrypanosomal agents.

Novel Inhibitors of Leishmanial Dihydrofolate Reductase

The program DOCK3.5 was used to search the Cambridge Structural Database for novel inhibitors of Leishmanial dihydrofolate reductase. A number of compounds were obtained and screened against the enzyme and against the intact parasite Leishmania donovani and the related organisms Trypanosoma brucei and Trypanosoma cruzi. The compounds screened showed weak activity in both the enzyme assays and the in vitro assays.

Kinetic properties and inhibition of the dimeric dUTPase‐dUDPase from Leishmania major

Kinetic properties of the dimeric enzyme dUTPase from Leishmania major were studied using a continuous spectrophotometric method. dUTP was the natural substrate and dUMP and PPi the products of the hydrolysis. The trypanosomatid enzyme exhibited a low Km value for dUTP (2.11 μM), a kcat of 49 s−1, strict Michaelis–Menten kinetics and is a potent catalyst of dUDP hydrolysis, whereas in other dUTPases described, this compound acts as a competitive inhibitor. Discrimination is achieved for the base and sugar moiety showing specificity constants for different dNTPs similar to those of bacterial, viral, and human enzymes. In the alkaline range, the Km for dUTP increases with the dissociation of ionizable groups showing pKa values of 8.8, identified as the uracil moiety of dUTP and 10, whereas in the acidic range, Km is regulated by an enzyme residue exhibiting a pKa of 7.1. Activity is strongly inhibited by the nucleoside triphosphate analog α‐β‐imido‐dUTP, indicating that the enzyme can bind triphosphate analogs. The existence of specific inhibition and the apparent structural and kinetic differences (reflected in different binding strength of dNTPs) with other eukaryotic dUTPases suggest that the present enzyme might be exploited as a target for new drugs against leishmaniasis.

New Azasterols against Trypanosoma brucei: Role of 24-Sterol Methyltransferase in Inhibitor Action

ABSTRACT A series of azasterol derivatives, designed as potential inhibitors of the Δ24-sterol methyltransferase enzyme (24-SMT), were synthesized and evaluated for their activities against parasitic protozoa. Values in the nanomolar range were obtained for 50% effective dose against the Trypanosoma brucei subsp. rhodesiense bloodstream form cultured in vitro. In order to investigate the mode of action, Trypanosoma brucei subsp. brucei 24-SMT was cloned and overexpressed and compounds were assayed for inhibitory activity. None of the inhibitors tested appeared to be active against the enzyme. Sterol composition analysis showed that only cholestane type sterols are present in membranes of bloodstream forms while ergosterol is a major component of procyclic sterol extracts. Interestingly, Northern blot analysis showed the presence of 24-SMT mRNA in both the procyclic and the bloodstream forms of the parasite, although levels of mRNA were threefold lower in the latter. Likewise, Western blot analysis and activity determinations evidenced the existence of active enzyme in both forms of the parasite. We conclude that the designed compounds act at sites other than 24-SMT in Trypanosoma brucei.

Design, synthesis and evaluation of novel uracil acetamide derivatives as potential inhibitors of Plasmodium falciparum dUTP nucleotidohydrolase.

The ubiquitous enzyme dUTP nucleotidohydrolase (dUTPase) catalyses the hydrolysis of dUTP to dUMP and can be considered as the first line of defence against incorporation of uracil into DNA. Inhibition of this enzyme results in over-incorporation of uracil into DNA, leading to DNA fragmentation and cell death and is therefore lethal. By taking advantage of structural differences between the human and Plasmodium dUTPase, selective inhibitors of the enzyme can be designed and synthesised with the aim of being developed into novel anti-parasitic drugs. Analogue based design was used to target the Plasmodium falciparum dUTPase (PfdUTPase). The structures of previously discovered selective inhibitors of the PfdUTPase were modified by insertion of an amide bond. A series of tritylated uracil acetamide derivatives were synthesised and assessed for inhibition of the enzyme and parasite growth in vitro. These compounds were weak inhibitors of the PfdUTPase.

Evidence for a wide occurrence of proton-translocating pyrophosphatase genes in parasitic and free-living protozoa

Proton-translocating inorganic pyrophosphatases (H(+)-PPase, EC 3.6.1.1) are integral membrane proteins that have been extensively studied in higher plants, the photosynthetic bacterium Rhodospirillum rubrum and, more recently, in some human pathogenic protozoa. By using a PCR-based approach, fragments of genes coding for H(+)-PPases in a number of protists, both free-living and parasites of animals and plants, that belong to diverse taxonomic groups (trypanosomatids, ciliates, apicomplexans, euglenoids, amoeboid mycetozoa, heterokonts) have been isolated. The experimental procedure involved the use of degenerate oligonucleotides designed from protein domains conserved in H(+)-PPases from plants and bacteria. The PCR-amplified DNA fragments exhibited the characteristic genomic structure and codon usage of the corresponding protozoan group. Paralogous genes were found in some species suggesting the occurrence of protein isoforms. These results indicate that H(+)-PPases are more widely distributed among protozoa than previously thought.

High-Throughput Screening Platform for Natural Product–Based Drug Discovery Against 3 Neglected Tropical Diseases Human African Trypanosomiasis, Leishmaniasis, and Chagas Disease

African trypanosomiasis, leishmaniasis, and Chagas disease are 3 neglected tropical diseases for which current therapeutic interventions are inadequate or toxic. There is an urgent need to find new lead compounds against these diseases. Most drug discovery strategies rely on high-throughput screening (HTS) of synthetic chemical libraries using phenotypic and target-based approaches. Combinatorial chemistry libraries contain hundreds of thousands of compounds; however, they lack the structural diversity required to find entirely novel chemotypes. Natural products, in contrast, are a highly underexplored pool of unique chemical diversity that can serve as excellent templates for the synthesis of novel, biologically active molecules. We report here a validated HTS platform for the screening of microbial extracts against the 3 diseases. We have used this platform in a pilot project to screen a subset (5976) of microbial extracts from the MEDINA Natural Products library. Tandem liquid chromatography–mass spectrometry showed that 48 extracts contain potentially new compounds that are currently undergoing de-replication for future isolation and characterization. Known active components included actinomycin D, bafilomycin B1, chromomycin A3, echinomycin, hygrolidin, and nonactins, among others. The report here is, to our knowledge, the first HTS of microbial natural product extracts against the above-mentioned kinetoplastid parasites.

Cloning and expression of the dihydrofolate reductase-thymidylate synthase gene from Trypanosoma cruzi

We have cloned, sequenced and expressed the Trypanosoma cruzi gene encoding the bifunctional protein dihydrofolate reductase-thymidylate synthase (DHFR-TS). The strategy followed for the isolation of positive clones from a genomic library was based on the construction of a probe by the amplification of highly conserved sequences of the TS domain by the polymerase chain reaction. Translation of the open reading frame of 1563 bp yields a polypeptide of 521 amino acids with a molecular mass of 58829 Da. For heterologous expression of T. cruzi DHFR-TS in Escherichia coli, the entire coding sequence was amplified by polymerase chain reaction and cloned into the plasmid vector pKK223.3. The presence of catalytically active DHFR-TS was demonstrated by complementation of the Thy- E. coli strain chi 2913 and the DHFR- Thy- E. coli strain PA414. The gene is expressed as an active protein which constitutes approximately 2% of the total cell soluble protein. Recombinant bifunctional enzyme and the DHFR domain have been purified by methotrexate-Sepharose chromatography to yield 1-2 mg of active DHFR-TS per litre of culture. Southern and electrophoretic analyses using the coding sequence as probe indicated that the T. cruzi enzyme is encoded by a single copy gene which maps to two bands of approximately 990 kb and 1047 kb. It appears that T. cruzi is diploid for the DHFR-TS gene which is located on two different-sized homologous chromosomes.