Short report: floxacrine analog WR 243251 inhibits hematin polymerization

Floxacrine was a promising antimalarial compound that led to the identification of WR 243251. On the basis of their structures, we suspected that these compounds might be good inhibitors of hematin polymerization. Indeed, WR 243251 was as potent and floxacrine was only 2-fold less potent than chloroquine as inhibitors of this process. However, this hematin polymerization inhibition did not completely account for the increased antimalarial potency of WR 243251 versus chloroquine. The WR 243251 ketone hydrolysis product WR 243246 was without activity against hematin polymerization. These data also confirm that hematin polymerization inhibition can be quite sensitive to small changes in inhibitor structure.

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.

Molecular characterization of Plasmodium falciparum S-adenosylmethionine synthetase

S-Adenosylmethionine (AdoMet) synthetase (SAMS: EC 2.5.1.6) catalyses the formation of AdoMet from methionine and ATP. We have cloned a gene for Plasmodium falciparum AdoMet synthetase (PfSAMS) (GenBank accession no. AF097923), consisting of 1209 base pairs with no introns. The gene encodes a polypeptide (PfSAMS) of 402 amino acids with a molecular mass of 44844 Da, and has an overall base composition of 67% A+T. PfSAMS is probably a single copy gene, and was mapped to chromosome 9. The PfSAMS protein is highly homologous to all other SAMS, including a conserved motif for the phosphate-binding P-loop, HGGGAFSGKD, and the signature hexapeptide, GAGDQG. All the active-site amino acids for the binding of ADP, P(i) and metal ions are similarly preserved, matching entirely those of human hepatic SAMS and Escherichia coli SAMS. Molecular modelling of PfSAMS guided by the X-ray crystal structure of E. coli SAMS indicates that PfSAMS binds ATP/Mg(2+) in a manner similar to that seen in the E. coli SAMS structure. However, the PfSAMS model shows that it can not form tetramers as does E. coli SAMS, and is probably a dimer instead. There was a differential sensitivity towards the inhibition by cycloleucine between the expressed PfSAMS and the human hepatic SAMS with K(i) values of 17 and 10 mM, respectively. Based on phylogenetic analysis using protein parsimony and neighbour-joining algorithms, the malarial PfSAMS is closely related to SAMS of other protozoans and plants.

Synthesis and biological evaluation of 4-chloro-3, 5-dinitrobenzotrifluoride analogues as antileishmanial agents

Desnitro analogues of 4-chloro-3,5-dinitrobenzotrifluoride (chloralin) (2), an in vitro microtubule inhibitor of several Leishmania species, have been synthesized from 2-halo-5-(trifluoromethyl)benzenesulfonyl chlorides 4 and 5. The analogues exhibited moderate to excellent activity when tested against Leishmania donovani amastigotes in vitro. Two representative compounds, 7f and 8, were tested against the Khartoum strain of L. donovani in a hamster model using chloralin (2) and Glucantime (one of the current therapeutics of choice in the treatment of Leishmania) as standards, the results of which will be discussed herein.

Chagas' disease: a search for treatment and a question--should the disease be of military concern?

If military forces are required to operate in areas that are endemic for Chagas' disease, the occupation should be of critical concern. These areas, located in Central and South America, are many. The matter is of particular importance because no suitable drug exists to treat individuals who contract the disease. We examined 60 compounds of a chemical class, thiosemicarbazones, known to have some activity against the disease. The work was carried out using Trypanosoma cruzi-infected mice. Of the 60 potential drugs evaluated, 12 showed significant suppressive activity. One of these compounds was almost 50% greater than the reference drug used in the test system.

Simulation analysis of formycin 5'-monophosphate analog substrates in the ricin A-chain active site

Ricin is an RNA N-glycosidase that hydrolyzes a single adenine base from a conserved loop of 28S ribosomal RNA, thus inactivating protein synthesis. Molecular-dynamics simulation methods are used to analyze the structural interactions and thermodynamics that govern the binding of formycin 5'-monophosphate (FMP) and several of its analogs to the active site of ricin A-chain. Simulations are carried out initiated from the X-ray crystal structure of the ricin-FMP complex with the ligand modeled as a dianion, monoanion and zwitterion. Relative changes in binding free energies are estimated for FMP analogs constructed from amino substitutions at the 2- and 2'-positions, and from hydroxyl substitution at the 2'-position.

The cytotoxicity of N-Pyridinyl and N-quinolinyl substituted derivatives of phthalimide and succinimide

The N-pyridinyl and N-quinolinyl substituted derivatives of phthalimides and succinimides demonstrated cytotoxicity against the growth of a number of cultured cell lines. The substituted succinimides were more effective than the unsubstituted succinimide derivative in reducing cell growth. On the other hand, phthalimide demonstrated more potent cytotoxicity than its N-substituted derivatives. Three representative examples N-[2-pyridinyl-1-oxide) methyl] phthalimide 8, 1-[N-2-phthalimidoethyl]-3,4-dihydroiso-quinoline 12, and 1-[N-(2-(1,2,3,4-tetrahydro-2-quinolinyl)] ethylphthalimide 14 were shown to inhibit L1210 leukemia DNA synthesis whereas RNA synthesis was not inhibited at 25-100 uM. All three agents inhibited the activities of DNA polymerase alpha, PRPP-amido transferase, nucleoside kinases, and dihydrofolate reductase. The cellular pool levels of d[GTP], d[CTP], and d[TTP] were reduced after 60 minutes incubation at 100 uM. The DNA molecule itself was not a target of these agents.

Drugs that show protective effects from ricin toxicity in in vitro protein synthesis assays

We used an in-vitro, inhibition of protein synthesis assay (PSI) to test a wide variety of drugs for possible therapeutic use against ricin, a toxic glycoprotein that causes death in animals by inhibiting protein synthesis. Selection of test drugs was based on possible interference with ricin activity at different stages of the toxic process. Most of the drugs tested had no effect on ricin-induced PSI, were toxic when tested alone, or enhanced the toxicity of ricin. The only ones showing protection were galactose, lactose, and several derivatives of these sugars, Brefeldin A (BFA), 3'-azido-3'-deoxythymidine (AZT), and a purine derivative (BM33203). THe sugar derivatives provided 50% protection against a PSI ED99 of ricin (0.1 micrograms/ml). Concentrations of BFA greater than 0.5 micro M caused about 50% PSI by itself, but blocked any further inhibitory effects of ricin. AZT, at optimum concentrations, reached a maximum protection level of about 40% in the presence of an ED99 dose of ricin, while the nucleoside derivative, BM33203 and AZT appeared to have an additive effect, showing up to 80% protection from an ED99 dose of ricin. Drugs showing protection in the PSI cell assay showed no protection from ricin in a cell-free translation assay used to determine if they would block ricin at the protein synthesis site.

2-Acetylpyridine thiosemicarbazones. 12. Derivatives of 3-acetylisoquinoline as potential antimalarial agents

A series of 3-acetylisoquinoline thiosemicarbazones and their related thiosemicarbazides was prepared for evaluation as potential antimalarial agents. The former were synthesized by the reaction of 3-acetylisoquinoline with methyl hydrazinecarbodithioate to give methyl 3-[1-(3-isoquinolinyl)ethylidene]hydrazinecarbodithioate, IV. Displacement of the S-methyl group of this intermediate by the requisite amines gave 3-acetylisoquinoline thiosemicarbazones, V. The corresponding thiosemicarbazides, in which the azomethine bond was reduced, were prepared by the reduction of IV with sodium borohydride to give methyl 3-[1-(3-isoquinolinyl)ethyl]hydrazinecarbodithioate, VI. Reaction of this dithioester with amines gave 1-[1-(3-isoquinolinyl)ethyl-3-thiosemicarbazides, VII. The antimalarial properties of series V and VII were evaluated in mice infected with Plasmodium berghei. Significant curative activity could be observed at doses as low as 40 mg/kg for 3 of 10 compounds in series V and at 160 mg/kg for 3 of 11 compounds in series VII.

Antibacterial properties of 2-acetylpyridine-1-oxide thiosemicarbazones

We have investigated the in vitro antibacterial activity of 13 2-acetylpyridine-1-oxide thiosemicarbazones and 5 thiosemicarbazides against 80 clinically significant bacterial cultures, including 13 isolates with known antibiotic resistance. Of the thiosemicarbazones tested, 5 had minimal inhibitory concentrations (MICs) of 0.25 microgram/ml for Neisseria gonorrhoeae isolates; 1 of these had an MIC range of 0.25-0.5 microgram/ml for the Neisseria meningitidis cultures, and 2 had MICs of 2 and 2-4 micrograms/ml for Staphylococcus aureus and Streptococcus faecalis isolates, respectively. Two of the thiosemicarbazides had MICs of 0.25 microgram/ml for N. gonorrhoeae, whereas 2 others had MICs of 2-4 and 4-8 micrograms/ml for S. aureus and S. faecalis isolates, respectively. The test compounds were ineffective against the gram-negative enteric cultures and the Pseudomonas isolates.

2-Acetylpyridine thiosemicarbazones XI: 2-(alpha-Hydroxyacetyl)pyridine thiosemicarbazones as antimalarial and antibacterial agents

A series of 2-(alpha-hydroxyacetyl)pyridine thiosemicarbazones was synthesized as potential antimalarial and antibacterial agents. Their synthesis was achieved by the condensation of N4-mono- or N4,N4-disubstituted thiosemicarbazides with 2-(alpha-hydroxyacetyl)pyridine. The latter was prepared by selective bromine oxidation of (2-pyridinyl)-1,2-ethanediol. The new compounds show potent inhibitory activity against penicillin-sensitive as well as penicillin-resistant Neisseria gonorrhoeae (MIC, 0.5-0.004 micrograms/mL), against Neisseria meningitidis (MIC, 0.5-0.032 micrograms/mL), and Staphylococcus aureus (MIC, 0.5-2 micrograms/mL). Good in vitro antimalarial effects against Plasmodium falciparum (Smith strain; ID50, 6.7-38 ng/mL) were observed in most of these new agents, but only 3 of 12 compounds exhibit moderate in vivo activity against Plasmodium berghei. These new agents appear to be less toxic to the host and more water soluble than the corresponding 2-acetylpyridine thiosemicarbazones.

2-Acetylpyridine thiosemicarbazones and Mycobacterium leprae

Four 2-acetylpyridine thiosemicarbazones were tested in mice against Mycobacterium leprae by the kinetic method and found to be nearly inactive in a dosage of 0.05% in the diet. At the same dosage, thiacetazone, as a positive control, exhibited its expected activity.

2-Acetylpyridine thiosemicarbazones. 9. Derivatives of 2-acetylpyridine 1-oxide as potential antimalarial agents

In view of the antimalarial activity in mice of 2-acetylpyridine thiosemicarbazones, a series of analogous 1-oxides was prepared for evaluation. Their synthesis was achieved by the reaction of 2-acetylpyridine 1-oxide with methyl hydrazinecarbodithioate to give methyl 3-[1-(2-pyridinyl 1-oxide)ethylidene]hydrazinecarbodithioate (II). Reaction of the latter intermediate with secondary amines afforded the desired 2-acetylpyridine 1-oxide thiosemicarbazones (III). Reduction of the azomethine linkage of II with NaBH4 gave methyl 3-[1-(2-pyridinyl 1-oxide)ethyl]-hydrazinecarbodithioate (IV) whose S-methyl group was then displaced by amines to give a 1-[1-(2-pyridinyl 1-oxide)ethyl]thiosemicarbazide, V. Antimalarial activity of III was evaluated against both Plasmodium berghei in the mouse and Plasmodium falciparum in an automated in vitro test system. In both cases, 2-acetylpyridine 1-oxide thiosemicarbazones were found to be less active than the corresponding de-1-oxide analogues. When compounds V were evaluated against Plasmodium berghei in the mouse, a diminution of activity was similarly seen in comparison to the analogues not bearing the 1-oxide moiety.

2-Acetylpyridine thiosemicarbazones. 8. Derivatives of 1-acetylisoquinoline as potential antimalarial agents

A series of 1-acetylisoquinoline thiosemicarbazones was prepared in order to evaluate their antimalarial properties. This was achieved by the reaction of 1-acetylisoquinoline with methyl hydrazinecarbodithioate to give methyl 3-[1-(1-isoquinolinyl)ethylidene]hydrazinecarbodithioate (II). Displacement of the S-methyl group from this intermediate by various primary and secondary amines afforded the desired 1-acetylisoquinoline thiosemicarbazones (III). Thiosemicarbazides in which the azomethine moiety of the latter was reduced could be prepared by the reaction of II with NaBH4 to give methyl 3-[1-(1-isoquinolinyl)ethyl]hydrazinecarbodithioate (VIII). Reaction of VII with the appropriate amine gave 1-[1-(1-isoquinolinyl)ethyl]thiosemicarbazides (IX). Evaluation of the antimalarial activity of series III and IX in mice infected with Plasmodium berghei indicated that cures were attainable at dose levels of 40-160 mg/kg.

2-Acetylpyridine thiosemicarbazones. 6.2-Acetylpyridine and 2-butyrylpyridine thiosemicarbazones as antileukemic agents

N4-Monosubstituted and N4,N4-disubstituted thiosemicarbazones derived from 2-acetylpyridine, 2-acetyl-6-methylpyridine and 2-butyrylpyridine, and N4,N4-disubstituted selenosemicarbazones derived from 2-acetylpyridine were evaluated against leukemia P388 in the mouse. Significant antitumor activity (T/C greater than 125%) was observed for members of each class. Enhancement of antitumor activity resulted from increasing the size of the N4-substituent of the thiosemicarbazone moiety. Selenosemicarbazones were less active than the corresponding thiosemicarbazones.

Thiosemicarbazones of 2-acetylpyridine, 2-acetylquinoline, 1- and 3-acetylisoquinoline and related compounds as inhibitors of clinically significant bacteria in vitro

Antibacterial activity of 64 thiosemicarbazones of 2-acetylpyridine, 2-acetylquinoline, 1- and 3-acetylisoquinoline and related compounds was determined by testing clinical isolates of ten bacterial genera, including some which were antibiotic resistant. Minimal inhibitory concentrations (MICs) of 0.016 to 0.125 microgram/ml were obtained with 26% of the compounds for Neisseria meningitidis and 0.002 to 0.25 microgram/ml with 19% of the compounds for N. gonorrhoeae. Staphylococcus aureus was inhibited in the MIC range of 0.062 to 0.5 microgram/ml by 34% of the thiosemicarbazones, whereas 26% inhibited Streptococcus faecalis with an MIC range of 0.25 to 1.0 microgram/ml. Less antibacterial activity was shown toward the Gram-negative bacilli, i.e., Pseudomonas, Klebsiella-Enterobacter, Shigella, Escherichia coli, Serratia marcescens and Proteus.

2-Acetylpyridine thiosemicarbazones. 5. 1-[1-(2-Pyridyl)ethyl]-3-thiosemicarbazides as potential antimalarial agents

Reduction of the azomethine bond of 2-acetylpyridine thio- and selenosemicarbazones with sodium borohydride readily afforded the corresponding thio- or selenosemicarbazides when they were N4,N4-disubstituted. This conversion failed, however, when the thio- or selenosemicarbazones were N4-substituted or unsubstituted. A more general route to the desired thio- or selenosemicarbazides consisted of reduction with sodium borohydride of methyl 3-[1-(2-pyridyl)ethylidene]hydrazinecarbodithioate to give the 2-pyridylethyl derivative. Displacement of methyl mercaptan from the thio ester moiety of the latter by amines produced 1-[1-(2-pyridyl)ethyl]-3-thiosemicarbazides. These compounds were somewhat more active as antimalarial agents in Plasmodium berghei infected mice than the corresponding thiosemicarbazones; however, the enhancement of activity was accompanied by an increase in toxicity. Compound 7, 3-azabicyclo[3.2.2]nonane-3-carbothioic acid 2-[1-(2-pyridyl)ethyl]hydrazide, is the most potent derivative of 2-acetylpyridine we have evaluated to date.

In vitro assessment of 2-acetylpyridine thiosemicarbazones against chloroquine-resistant Plasmodium falciparum

A series of 2-acylpyridine thiosemicarbazones was evaluated in vitro against a chloroquine-resistant Plasmodium falciparum strain. Antimalarial activity was assessed by the inhibition of uptake of [G-3H]hypoxanthine by the parasites. Among the mono- and disubstituted derivatives tested, 13 of 17 had 50% inhibitory doses of less than 10 ng/ml. Increasing the size of the ring at N4 from four to five, six, and seven members produced concomitant decreases in activity. Similarly, increasing the size of the aliphatic substituent on the azomethine carbon reduced activity. Selected compounds were also tested against a chloroquine-susceptible strain. The results suggested that the activities of these agents were not modified significantly by resistance to chloroquine. In general, in vitro activities correlate poorly with the in vivo activities in mice infected with Plasmodium berghei.

2-Acetylpyridine thiosemicarbazones. 4. Complexes with transition metals as antimalarial and antileukemic agents

Reaction of the 2-acetylpyridine thiosemicarbazones, 3-azabicyclo[3.2.2]nonane-3-thiocarboxylic acid 2-[1-(2-pyridyl)ethylidene]hydrazide (IIIa), its selenium analogue (IIIb), 1H-hexahydroazepine-1-thiocarboxylic acid 2-[1-(2-pyridyl)ethylidene]hydrazide (IV), and 1H-octahydroazocine-1-thiocarboxylic acid 2-[1-(2-pyridyl)ethylidene]hydrazide (V) with Cu(II), Ni(II), Fe(III), and Mn(II) salts gave crystalline complexes. Relative to the free ligands, these complexes show reduced antimalarial activity in mice infected with Plasmodium berghei; however, antileukemic properties are enhanced by coordination with the above-mentioned metals.

Activity of 2-acetylpyridine thiosemicarbazones against Trypanosoma rhodesiense in vitro

Twenty-seven 2-acetylpyridine thiosemicarbazones and analogs were tested for antitrypanosomal activity against Trypanosoma rhodesiense using a semiautomated in vitro assay system. Activity was determined by relative inhibition of uptake of two radiolabeled macromolecular precursors, [methyl-3H]thymidine and L-[U-14C]leucine, as compared to untreated controls. It was observed that the nitrogen atom of the pyridyl moiety of the 2-acetylpyridine thiosemicarbazones was essential for antitrypanosomal activity. The 2-acetylpyridine thiosemicarbazones generally inhibited the uptake of L-[U-14C]leucine to a greater extent than they inhibited [methyl-3H]thymidine uptake. Twenty-four of the 27 compounds tested exhibited activity comparable to that found for the antitrypanosomal agent ethidium bromide.

Inhibition of clinically significant bacterial organisms in vitro by 2-acetylpyridine thiosemicarbazones

Antibacterial activity of 65 2-acetylpyridine thiosemicarbazones and related compounds was determined by using clinical isolates of nine bacterial genera. Minimal inhibitory concentrations (MICs) of 0.002 to 0.062 micrograms/ml were obtained with 23% of the compounds for Neisseria gonorrhoeae and 0.016 to 0.062 micrograms/ml with 17% of the compounds for N. meningitidis. Staphylococcus aureus was inhibited in the MIC range of 0.125 to 0.5 micrograms/ml by 18% of the thiosemicarbazones, whereas 26% inhibited group D enterococcus with an MIC of 0.25 to 2.0 micrograms/ml. Poor antibacterial activity was shown toward the gram-negative bacilli, i.e., Pseudomonas, Klebsiella-Enterobacter, Shigella, Escherichia coli, and Proteus.

2-Acetylpyridine thiosemicarbazones. 2. N4,N4-Disubstituted derivatives as potential antimalarial agents

The most effective antimalarial agents among the N4-monosubstituted 2-acetylpyridine thiosemicarbazones recently described by us have a cyclohexyl or a phenyl substituent and produce cures in Plasmodium berghei infected mice at a dose of 160 and 320 mg/kg, respectively. We report here on a related series of N4,N4-disubstituted 2-acetylpyridine thiosemicarbazones. Several members of this group bearing alkyl or cycloalkyl substituents at N4 show activity superior to the most active monosubstituted 2-acetylpyridine thiosemicarbazones. However, the greatest improvement in potency was seen when the N4-nitrogen atom was incorporated into a six- or seven-membered ring, such as the piperidine, piperazine, or azabicyclo[3.2.2]nonane systems, to give compounds with curative properties at a dose level as low as 20 mg/kg.

Primaquine analogues: derivatives of 4-amino-2-methoxyacridine

Based on the antimalarial activity of primaquine (1a) and its 4-methyl analogue 1b, 4-aminoacridinyl analogues, 4-[(4-amino-1-methylbutyl)amino]-2-methoxyacridine (2a) and 4-[(4-amino-1-methylbutyl)amino]-2-methoxy-9-methylacridine (2b), were prepared and evaluated as potential tissue schizonticidal agents. These compounds were found to be substantially less active than primaquine against Plasmodium cynomolgi in the rhesus monkey. The antileishmanial activity in hamsters of 4-[[6-(diethylamino)hexyl]amino]-2-methoxy-9-methylacridine (2d) was found to be considerably less than that of 8-[[6-(diethylamino)hexyl]amino]-6-methoxy-4-methylquinoline (1c).

2-Acetylpyridine thiosemicarbazones. 1. A new class of potential antimalarial agents

Based on the antimalarial properties observed for 2-acetylpyridine 4-phenyl-3-thiosemicarbazone (1), an extensive series of related thiosemicarbazones was prepared and tested against Plasmodium berghei in mice. Screening results indicated that the presence of the 2-pyridylethylidene group was critical and that certain phenyl, benzyl, phenethyl, or cycloalkyl groups at N4 of the thiosemicarbazone moiety also contribute to antimalarial activity.

Synthesis of 5-substituted aminomethyluracils via the Mannich reaction

An extension of the Mannich reaction, in which aminomethylation of the five position of uracil, is reported. Thus, primary and secondary alkylamines and primary aromatic amines containing ring-activating groups led to the title compounds 3-10. Compound 11 in which the aromatic ring contains the ring-deactivating nitro group was synthesized in an alternative way. All compounds were characterized by their elemental and spectral properties.