Synthesis and antitumor activity of a heterodinucleotide of BVDU and gemcitabine.

A heterodinucleotide comprising BVDU and Gemcitabine bound together by a 5',5'-pyrophospate bridge (BVDUp(2)dFdC) has been synthesized and evaluated as antitumor agent against AH13 rat sarcoma cells. BVDUp(2)dFdC showed a cytotoxicity similar to that of Gemcitabine.

Inhibition of HIV-1 replication in macrophages by red blood cell-mediated delivery of a heterodinucleotide of lamivudine and tenofovir.

Homo- and heterodimers of nucleoside/nucleotide analogues as reverse transcriptase inhibitors are effective on HIV-1-infected human monocyte-derived macrophages (M/M) compared to the single drugs or their combination. Since the combined treatment of lamivudine (3TC) and tenofovir ((R)PMPA) has an antiretroviral efficacy and a synergic effect respect to separate drugs, the heterodinucleotide 3TCpPMPA was synthesized. A single administration of the dimer as free drug or 3TCpPMPA-loaded RBC selectively targeted to M/M was able to almost completely protect macrophages from "de novo" infection.

Adenosine deaminase from Saccharomyces cerevisiae: kinetics and interaction with transition and ground state inhibitors.

Several adenosine analogs, such as coformycin, 2'-deoxycoformycin and erythro-9-(3-nonyl-p-aminobenzyl)adenine (EHNA), which are strong inhibitors of mammalian adenosine deaminase, are much weaker inhibitors of the Saccharomyces cerevisiae enzyme. The specificity of the yeast enzyme is more restricted than that of mammalian adenosine deaminase, particularly towards the ribose moiety and around position 6 and 1 of the substrate. The sulphydryl group appears to be more masked in the yeast than in the mammalian enzyme. The kinetic effects of pH with adenosine substrate and with the inhibitor purine riboside are reported. The findings on specificity and pH kinetic effects can be interpreted in a model involving proton transfer from the -SH group of the enzyme to the N-1 atom of the substrate.

Nucleoside and non-nucleoside IMP dehydrogenase inhibitors as antitumor and antiviral agents.

IMP dehydrogenase (IMPDH) is an enzyme which catalyzes the NAD-dependent conversion of inosine 5 -monophosphate (IMP) to xanthosine 5 -monophosphate (XMP) at the metabolic branch point in the de novo purine nucleotide synthetic pathway. IMPDH was shown to be increased significantly in cancer cells and therefore considered to be a sensitive target for cancer chemotherapy. By blocking the conversion of IMP to XMP, IMPDH inhibitors lead to depletion of the guanylate (GMP, GDP, GTP and dGTP) pools. Two isoforms of human IMPDH, designed type I and type II, have been identified and sequenced. Type I is constitutively expressed and is the predominant isoform in normal cells, while type II is selectively up-regulated in neoplastic and replicating cells. Two types of IMPDH inhibitors, endowed with antineoplastic, antiviral and immunosoppressive activity, have been discovered so far: nucleoside inhibitors, such as ribavirin and tiazofurin, and non-nucleoside, such as mycophenolic acid. Ribavirin produces IMPDH inhibition via its anabolite 5 -monophosphate. Tiazofurin inhibits the enzyme after metabolic conversion into thiazole-4-carboxamide adenine dinucleotide (TAD), an analogue of the cofactor NAD. It was hypothesized that the inhibitory activity of tiazofurin is due to an attractive electrostatic interaction between the heterocyclic sulphur atom and the furanose oxygen 1 which constrain rotation about the C-glycosidic bond in tiazofurin and in its active anabolite TAD. To check this hypothesis, we studied several C-nucleosides related to tiazofurin and their NAD analogues. Non-nucleoside IMPDH inhibitors are also reviewed.

Structure-activity relationships in reactivators of organophosphorus-inhibited acetylcholinesterase. 9. N-Heterocyclic acraldoximes methiodides.

N-Heterocyclic acraldoximes methiodides, where the heterocyclic residues are 2-, 3-, and 4-pyridyl, 2-(1-methyl)imidazolyl or 4-pyrimidyl, were prepared and tested for their reactivating potency on acetylcholinesterase inhibited from diisopropylphosphorofluoridate (DFP). The in vitro testing revealed that the new compounds are good reactivators of the phosphorylated electric eel cholinesterase. The structure-activity relationships are briefly discussed.

Elucidation of the structure of the antineoplastic agents, 2-formylpyridine and 1-formylisoquinoline thiosemicarbazones.

The geometrical isomers of the antineoplastic agents 2-formylpyridine and 1-formylisoquinoline thiosemicarbazones were synthesized and their structure was studied by spectroscopic methods. It was found that the compounds previously described in the literature and tested for carcinostatic activity were isomers with E configuration which probably contained minor amounts of Z isomers.

3,7-Dideazapurine nucleosides. Synthesis and antitumor activity of 1-deazatubercidin and 2-chloro-2'-deoxy-3,7-dideazaadenosine.

1-Deazatubercidin (5) has been synthesized by glycosylation of the anion of 4,6-dichloro-1H-pyrrolo[3,2-c]pyridine (9) with 1-chloro-2,3-O-isopropylidene-5-O-(tert-butyldimethylsilyl)-alpha-D-r ibofuranos e (12). The reaction gave a mixture of blocked nucleosides with beta- and alpha-configuration (13a and 13b). Deprotection of 13a provided 4,6-dichloro-1-beta-D-ribofuranosylpyrrolo[3,2-c]pyridine (14), which on treatment with hydrazine, followed by reduction of the resulting 4-hydrazino compound with Raney nickel, gave 4-amino-6-chloro-1-beta-D-ribofuranosylpyrrolo[3,2-c]pyridine (15), 1-deazatubercidin, and a small quantity of 4,6-diamino-1-beta-D-ribofuranosylpyrrolo[3,2-c]pyridine (16). Dehalogenation of 15 provided another route to 5. 2-Chloro-2'-deoxy-3,7-dideazaadenosine (6) together with 2'-deoxy-3,7-dideazaadenosine (18) was obtained by hydrazinolysis of 4,6-dichloro-1-(2-deoxy-beta-D-erythro-pentofuranosyl)pyrrolo- [3,2-c]pyridine (17), followed by reduction of the resulting 4-hydrazino compound. Nucleosides 5, 6, 15, and 18 are devoid of any significant antitumor activity in vitro. Compound 16 showed significant activity against P388 leukemia in cell culture.

Structure-activity relationships in reactivators of organophosphorus-inhibited acetylcholinesterase. 10. Hydroxyiminomethylarylethenylpyridine methiodides.

The synthesis of styrylpyridine methiodides where a hydrogen of the pyridyl moiety was replaced by the hydroxyiminomethyl group produced highly effective inhibitors of acetylcholinesterase. As starting materials 4-methylpyridine-2-aldoxime and 2-methylpyridine-4-aldoxime methiodides were prepared which, together with 4-imidazolylethenyl-pyridine-2-aldoxime methiodide, were the only substances for which some activity as reactivators of phosphorylated electric eel cholinesterase in vitro could also be found.

Improved synthesis and antitumor activity of 1-deazaadenosine.

A more convenient synthetic route to 1-deazaadenosine (1) by reduction of the new nucleoside 7-nitro-3-beta-D-ribofuranosyl-3H-imidazo[4,5-b]pyridine (6) is reported. Compound 6 was obtained by reaction of 7-nitroimidazo-[4,5-b]pyridine with 1,2,3,5-tetra-O-acetyl-beta-D-ribofuranose in the presence of stannic chloride followed by treatment with methanolic ammonia. 1-Deazaadenosine (1) showed good activity in vitro as inhibitor of HeLa, KB, P388, and L1210 leukemia cell line growth, with ID50 values ranging from 0.34 microM (KB) to 1.8 microM (P388). The nitro derivative 6 demonstrated moderate activity against the same cell lines.

Heterocyclic quinones with potential antitumor activity. 2. Synthesis and antitumor activity of some benzimidazole-4,7-dione derivatives.

A series of benzimidazole-4,7-dione derivatives, bearing substituents at positions 1, 2, 5, and 6 of the benzimidazole ring, has been synthesized and tested for antitumor activity in vivo on P388 leukemia. Some of the synthesized compounds show significant antitumor activity, associated with high toxicity, however. Compounds 7, 18, and 27 show the highest antitumor activity in this series, whereas 17, 19, and 22 are scarcely active. Some hypothetical biological precursors of these quinones are devoid of antitumor activity. Some structure-activity relationships are discussed.

Adenosine receptor agonists: synthesis and biological evaluation of 1-deaza analogues of adenosine derivatives.

In a search for more selective A1 adenosine receptor agonists, N6-[(R)-(-)-1-methyl-2-phenethyl]-1-deazaadenosine (1-deaza-R-PIA, 3a), N6-cyclopentyl-1-deazaadenosine (1-deazaCPA, 3b), N6-cyclohexyl-1-deazaadenosine (1-deazaCHA, 3c), and the corresponding 2-chloro derivatives 2a-c were synthesized from 5,7-dichloro-3-beta-D-ribofuranosyl-3H-imidazo[4,5-b]pyridine. On the other hand, N-ethyl-1'-deoxy-1'-(1-deaza-6-amino-9H-purin-9-yl)-beta-D-ribofuranu ronamide (1-deazaNECA, 10) was prepared from 7-nitro-3-beta-D-ribofuranosyl-3H-imidazo[4,5-b]pyridine, in an attempt to find a more selective A2 agonist. The activity of all deaza analogues at adenosine receptors has been determined in adenylate cyclase and in radioligand binding studies. 1-DeazaNECA proved to be a nonselective agonist at both subtypes of the adenosine receptor. It is about 10-fold less active than NECA but clearly more active than the parent compound 1-deazaadenosine as an inhibitor of platelet aggregation and as a stimulator of cyclic AMP accumulation. The N6-substituted 1-deazaadenosines largely retain the A1 agonist activity of their parent compounds, but lose some of their A2 agonist activity. This results in A1-selective compounds, of which N6-cyclopentyl-2-chloro-1-deazaadenosine (1-deaza-2-Cl-CPA, 2b) was identified as the most selective agonist at A1 adenosine receptors so far known. The activity of all 1-deaza analogues confirms that the presence of the nitrogen atom at position 1 of the purine ring is not critical for A1 receptor mediated adenosine actions.

Adenosine deaminase inhibitors. Synthesis of deaza analogues of erythro-9-(2-hydroxy-3-nonyl)adenine.

Structural analogues of erythro-9-(2-hydroxy-3-nonyl)adenine (EHNA), in which the adenine moiety of the molecule was modified, were prepared in order to investigate the structural requirement of EHNA as an inhibitor of adenosine deaminase (ADA). Thus, 1- and 3-deaza-EHNA and their 6-deamino analogues were synthesized and evaluated as inhibitors of ADA from calf intestine. Inhibition studies indicated that isosteric substitution of pyrimidine nitrogens by carbons could be tolerated at the enzymatic binding site. In fact, 3-deaza-EHNA was found to have an inhibitory activity comparable to EHNA itself, and 1-deaza-EHNA, though less potent, is a good inhibitor. The 6-amino group gives an important contribution to the enzymatic binding if the N1 nitrogen is also present, conferring on the compound the characteristic of a semitight inhibitor.

N*-N*-S* tridentate ligand system as potential antitumor agents.

Compounds containing an N*-N*-S* tridentate ligand were synthesized and tested for antitumor activity against P-388 lymphocytic leukemia in mice. Of these, only 2,2'-bipyridyl-6-carbothioamide (1a) showed antitumor activity at relatively high dosage levels. Compound 1a was also evaluated against L-1210 and S180 cells in culture and found to have significant activity.

8-Azaxanthine derivatives as antagonists of adenosine receptors.

A series of 1,3-dimethyl- and 1,3-dipropyl-8-azaxanthines, substituted at the N8 or N7 position with substituents which usually increase the affinity of the xanthines for the adenosine receptors, was synthesized and studied in radioligand binding experiments. The substitution of CH with N at the 8-position of both theophylline and caffeine dramatically reduced the affinity, as demonstrated by the fact that 8-azatheophylline and 8-azacaffeine were inert. The introduction of a methyl group at 8-position of 8-azatheophylline restored the antagonistic activity at A2 receptors, while a 8-cycloalkyl substituent increased the affinity for both receptor subtypes. A more favorable effect on affinity was produced by the substitution of the 7-methyl group in 8-azacaffeine with cycloalkyl groups. 7-Cyclopentyl-1,3-dimethyl-8-azaxanthine was 3 times more potent than caffeine at A1 receptors and 6 times less active at A2 receptors. On the contrary, the 7-cyclohexyl-1,3-dimethyl-8-azaxanthine was more potent than caffeine at A2 receptors. The substitution of 1- and 3-methyl groups with propyl in both 7- and 8-substituted 8-azatheophylline increased remarkably the affinity for A1 receptors. The 7-cyclopentyl-1,3-dipropyl-8-azaxanthine appears to be one of the most potent and selective among 7-alkyl-substituted xanthines at A1 receptors so far known. Because the 8-aza analogues of 8-substituted 1,3-dialkylxanthine were in any case less active than the corresponding xanthine derivatives, it was confirmed that the hydrogen atom at the 7-position of xanthines plays an important role in the binding to adenosine receptors.

Purine and 1-deazapurine ribonucleosides and deoxyribonucleosides: synthesis and biological activity.

A series of 6-(hydroxylamino)purine and -1-deazapurine nucleosides were synthesized and tested for their antitumor and adenosine deaminase inhibitory activity. All the examined molecules displayed an in vitro activity comparable to that of the reference compounds 6-(hydroxylamino)-9-beta-D-ribofuranosylpurine (HAPR) and ara-A, their ID50 ranging from 0.9 microM to approximately 100 microM. The 6-hydroxylamino derivatives of 1-deazapurine 9, 12, and 17 and also the blocked compound 13 are inhibitors of ADA whereas the purine derivatives 4 and 6 and the nitro compounds 11 and 16 are resistant to the enzyme. 7-(Hydroxylamino)-3-(2-deoxy-beta-D-erythro-pentofuranosyl)-3H-imi dazo[4,5- b]pyridine, the less cytotoxic but the most active ADA inhibitor in the series (Ki = 2.7 x 10(-7)), greatly potentiates the antitumor activity of ara-A in vitro.

Adenosine deaminase inhibitors: synthesis and structure-activity relationships of imidazole analogues of erythro-9-(2-hydroxy-3-nonyl)adenine.

A series of erythro-1-(2-hydroxy-3-nonyl)imidazole derivatives have been synthesized and evaluated for adenosine deaminase (ADA) inhibitory activity, in order to introduce simplifications in the ADA inhibitors erythro-9-(2-hydroxy-3-nonyl)adenine (EHNA, 1a) and 3-deaza-EHNA (1c). Opening the pyrimidine or pyridine ring of EHNA or 3-deaza-EHNA respectively led to compounds which are still ADA inhibitors. The most potent compound was erythro-1-(2-hydroxy-3-nonyl)imidazole-4-carboxamide (5, Ki = 3.53 x 10(-8) M), which provided potential donor and acceptor sites for hydrogen bonding. Lack of one of this sites could account for the order of potency of all compounds examined in this series. Opening the same ring in adenosine and in 3-deazaadenosine led to fully inactive compounds. These results support the hypothesis of the existence, at or near the enzyme active site, of a hydrophobic region able to bind the erythro-nonyl moiety.

Synthesis and antitumor activity of 2-beta-D-ribofuranosyloxazole-4-carboxamide (oxazofurin).

Condensation of 3,4,6-tri-O-benzoyl-2,5-anhydro-D-allonyl chloride (4) with ethyl 2-amino-2-cyanoacetate (5) provided 2-[(3',4',6'-tri-O-benzoyl-2',5'-anhydroallonyl)amino]-2-cyanoa cetate (6). Compound 6 was treated with hydrogen chloride gas to give ethyl 5-amino-2-(2',3',5'-tri-O-benzoyl-beta-D- ribofuranosyl)oxazole-4-carboxylate (8). Reductive dediazotization of blocked nucleoside 8 provided ethyl 2-(2',3',5'-tri-O- benzoyl-beta-D-ribofuranosyl)oxazole-4-carboxylate (10), which after deblocking with sodium methoxide and ammonolysis was converted to 2-beta-D-ribofuranosyl-oxazole-4-carboxamide (oxazofurin, 3), an analogue of the antitumor and antiviral C-nucleoside tiazofurin (1). Oxazofurin (3) was found to be cytotoxic toward B16 murine melanoma cells in culture but inactive against murine leukemia P388 and L1210.

Adenosine deaminase inhibitors. Synthesis and biological activity of deaza analogues of erythro-9-(2-hydroxy-3-nonyl)adenine.

Two new deaza analogues of erythro-9-(2-hydroxy-3-nonyl)adenine (EHNA, 1), 7-deaza-EHNA (6) and 1,3-dideaza-EHNA (11), were synthesized and evaluated for adenosine deaminase (ADA) inhibitory activity and compared with EHNA, 1-deaza-EHNA (2), and 3-deaza-EHNA (3). Substitution of a methine group for a nitrogen atom in the 7-position of the purine moiety of EHNA produces a dramatic drop in the inhibitory activity (Ki = 4 X 10(-4) M) whereas compounds 2 and 3 are still good inhibitors (Ki = 1.2 X 10(-7) M and 6.3 X 10(-9) M respectively). EHNA and its deaza analogues so far synthesized were also tested in vitro for their antiviral and antitumor activity in a range of cellular systems. EHNA and 1-deaza-EHNA are equiactive as inhibitors of human respiratory syncytial virus (HRSV) replication (MIC = 6.25 micrograms/mL) while the other compounds are inactive. On the other hand, all the examined compounds displayed an antitumor activity comparable to that of the reference compound 1-beta-D-arabinofuranosyladenine (ara-A), 7-deaza-EHNA being the most active of all. The results obtained showed that there is no correlation between adenosine deaminase inhibition and antiviral or antitumor activity in this series of compounds. 3-Deaza-EHNA, the most active inhibitor of ADA among the EHNA deaza analogues, greatly potentiates the antitumor activity of ara-A in vitro. In vivo activity was observed only when the two compounds were used in combination.

C-glycosyl bond conformation in oxazofurin: crystallographic and computational studies of the oxazole analogue of tiazofurin.

Oxazofurin is the inactive oxazole analogue of the C-glycosyl thiazole antitumor agent tiazofurin. Replacement of the thiazole sulfur in tiazofurin with the oxazole oxygen in oxazofurin produces conformational effects that are examined using crystallographic and computational methods. The crystal structure of oxazofurin contains six molecules in the asymmetric unit and has been refined to a standard R value of 6.8% for all data. The six oxazofurin conformers show an average C-glycosidic torsion angle of 70(9) degrees. This value is significantly higher than the average absolute C-glycosidic torsion angle of 24(10) degrees obtained from previous thiazole nucleoside structures. Previous studies suggest that, in tiazofurin, an electrostatic interaction between a positively charged thiazole sulfur and negatively charged furanose oxygen constrains the C-glycosidic torsion angle to a relatively small value. Ab initio molecular orbital studies presented here suggest that the higher C-glycosidic angles observed in the oxazofurin structures result from a repulsive interaction between negatively charged oxazole and furanose oxygens. Thus, it is likely that differences in activity between oxazo- and tiazofurin are either (1) due directly to differences in electronic properties between the thiazole and oxazole rings or (2) due to the variation in C-glycosidic bond conformation resulting from the alteration in the charge distribution of the heterocycle.

Synthesis, conformational analysis, and biological activity of C-thioribonucleosides related to tiazofurin.

The syntheses of furanthiofurin [5beta-D-(4'-thioribofuranosyl)furan-3-carboxamide, 1] and thiophenthiofurin [5beta-D-(4'-thioribofuranosyl)thiophene-3-carboxamide, 2], two C-thioribonucleoside analogues of tiazofurin, are described. Direct trifluoroacetic acid-catalyzed C-glycosylation of ethyl furan-3-carboxylate with 1-O-acetyl-2,3,5-tri-O-benzyl-4-thio-D-ribofuranose gave 2- and 5-glycosylated regioisomers, as a mixture of alpha and beta anomers. Ethyl 5-(2,3,5-tri-O-benzyl)-beta-D-(4'-thioribofuranosyl)furan-3-carboxylate (6beta) was debenzylated and then converted into the corresponding amide (furanthiofurin) by reaction with ammonium hydroxide. A similar C-glycosylation of ethyl thiophene-3-carboxylate with 1,2,3,5-tetra-O-acetyl-4-thio-D-ribofuranose catalyzed by stannic chloride afforded an anomeric mixture of 2- and 5-glycosylated regioisomers. Deacetylation of ethyl 5-(2,3,5-tri-O-acetyl)-beta-D-(4'-thioribofuranosyl)thiophene-3-carboxylate (13beta) with methanolic ammonia and treatment of the ethyl ester with ammonium hydroxide gave thiophenthiofurin. The glycosylation site and anomeric configuration were established by (1)H NMR spectroscopy. Thiophenthiofurin was found to be cytotoxic in vitro toward human myelogenous leukemia K562, albeit 39-fold less than thiophenfurin, while furanthiofurin proved to be inactive. K562 cells incubated with thiophenthiofurin resulted in inhibition of inosine 5'-monophosphate dehydrogenase (IMPDH) and an increase in IMP pools with a concurrent decrease in GTP levels. From computational studies it was deduced that, among the C-nucleoside analogues of tiazofurin, activity requires an electrophilic sulfur adjacent to the C-glycosidic bond and an energetically favorable conformer around chi = 0 degrees. Among these, the more constrained (least flexible) compounds (tiazofurin and thiophenfurin) are more active than the less constrained thiophenthiofurin. Those compounds which contain a nucleophilic oxygen in place of the thiazole or thiophene (oxazofurin, furanfurin, and furanthiofurin) show the least activity.