ABSTRACT
In order to study structure-activity relationships among the derivatives and congeners of 5',9-anhydro-3-(beta-D-ribofuranosyl)xanthine for anti-hepatitis C virus activity, a series of 5',9-anhydro-purine-isonucleosides with a substituent (s) at 6- or/and 8-position of the purine moiety were synthesized, and their anti-hepatitis C virus activity and cytotoxicity were evaluated and discussed.
Subject(s)
Antiviral Agents/chemistry , Antiviral Agents/pharmacology , Hepacivirus/drug effects , Purine Nucleosides/chemistry , Purine Nucleosides/pharmacology , Antiviral Agents/chemical synthesis , Cells, Cultured , Humans , Purine Nucleosides/chemical synthesis , RNA, Viral/drug effects , Structure-Activity Relationship , Virus Replication/drug effectsABSTRACT
A series of purine nucleosides containing the 2'-deoxy-2'-fluoro-2'-C-methylribofuranosyl moiety were synthesized and evaluated as potential inhibitors of the hepatitis C virus in vitro. Of the nucleosides that were synthesized, only those possessing a 2-amino group on the purine base reduced the levels of HCV RNA in a subgenomic replicon assay.
Subject(s)
Antiviral Agents , Hepacivirus/drug effects , Purine Nucleosides , RNA, Viral , Virus Replication/drug effects , Antiviral Agents/chemical synthesis , Antiviral Agents/pharmacology , Cell Survival/drug effects , Cells, Cultured , Hepacivirus/genetics , Molecular Structure , Purine Nucleosides/chemical synthesis , Purine Nucleosides/pharmacology , RNA, Viral/drug effects , RNA, Viral/genetics , Replicon/drug effects , Structure-Activity RelationshipABSTRACT
Novel racemic, D- and L-beta-dioxolane N4-hydroxycytosine nucleosides have been synthesized and evaluated for their activity against hepatitis B virus. None of the synthesized nucleosides demonstrated selective anti-HBV activity.
Subject(s)
Antiviral Agents/pharmacology , Cytosine/pharmacology , Dioxolanes/pharmacology , Hepatitis B virus , Nucleosides/pharmacology , Virus Replication/drug effects , Antiviral Agents/chemical synthesis , Antiviral Agents/chemistry , Cell Line, Tumor , Cytosine/analogs & derivatives , Cytosine/chemical synthesis , Dioxolanes/chemical synthesis , Dioxolanes/chemistry , Humans , Nucleosides/chemical synthesis , Nucleosides/chemistryABSTRACT
A new approach to the synthesis of 2',3'-didehydro-2',3-dideoxynucleosides was described in excellent yield through unusual olefin formation by PhSe-F trans-elimination.
Subject(s)
Alkenes/chemistry , Dideoxynucleosides/chemistry , Biochemistry/methods , Dideoxynucleosides/chemical synthesisABSTRACT
Based on the discovery of beta-D-2'-deoxy-2'-fluorocytidine as a potent anti-hepatitis C virus (HCV) agent, a series of beta-D- and L-2'-deoxy-2'-fluoroibonucleosides with modifications at 5 and/or 4 positions were synthesized and evaluated for their in vitro activity against HCV and bovine viral diarrhea virus (BVDV). The introduction of the 2'-fluoro group was achieved by either fluorination of 2,2'-anhydronucleosides with hydrogen fluoride-pyridine or potassium fluoride, or a fluorination of arabinonucleosides with DAST. Among the 27 analogues synthesized, only the 5-fluoro compounds, namely beta-D-2'-deoxy-2',5-difluorocytidine (5), had anti-HCV activity in the subgenomic HCV replicon cell line, and inhibitory activity against ribosomal RNA. As beta-D-N4-hydroxycytidine (NHC) had previously shown potent anti-HCV activity, the two functionalities of the N4-hydroxyl and the 2'-fluoro were combined into one molecule, yielding beta-D-2'-deoxy-2'-fluoro-N4-hydroxycytidine (12). However, this nucleoside showed neither anti-HCV activity nor toxicity. All the L-forms of the analogues were devoid of anti-HCV activity. None of the compounds showed anti-BVDV activity, suggesting that the BVDV system cannot reliably predict anti-HCV activity in vitro.
Subject(s)
Antiviral Agents/pharmacology , Deoxycytidine/analogs & derivatives , Fluorine/chemistry , Hepacivirus/metabolism , Ribonucleosides/chemistry , Animals , Cattle , Cell Line , Chemistry, Pharmaceutical/methods , Deoxycytidine/chemical synthesis , Deoxycytidine/pharmacology , Diarrhea Viruses, Bovine Viral/metabolism , Drug Design , Fluorides/pharmacology , Humans , Hydrofluoric Acid/chemistry , In Vitro Techniques , Liver/drug effects , Liver/virology , Models, Chemical , Molecular Biology/methods , Potassium Compounds/pharmacology , Pyrimidine Nucleosides/chemistry , RNA/chemistry , RNA, Ribosomal/chemistry , Ribonucleosides/pharmacology , StereoisomerismABSTRACT
We recently discovered a novel compound, identified as N3, 5-cyclo-4-(beta-D-ribofuranosyl)-vic-triazolo[4,5-b]pyridinin-5-one, with anti-hepatitis C virus (HCV) activity in vitro. The structure was confirmed by chemical synthesis from 2-hydroxy-5-nitropyridine. It showed anti-HCV activity with EC50= 19.7 microM in replicon cells. Its 3'-deoxy sugar analogue was also synthesized, but was inactive against HCV in vitro.
Subject(s)
Hepacivirus/metabolism , Hepatitis C/drug therapy , Nucleosides/chemical synthesis , Antiviral Agents/pharmacology , Carbohydrates/chemistry , DNA-Directed RNA Polymerases/chemistry , Deoxy Sugars/chemistry , Genome, Viral , Hepacivirus/genetics , Humans , Models, Chemical , Nucleosides/chemistry , Ribonucleosides/chemistry , Viral Nonstructural Proteins/chemistryABSTRACT
A series of N3, 5-Anhydro-4-(beta-D-ribofuranosyl)-8-azapurin-2-ones were prepared in multistep reactions from uridine as potential anti-hepatitis C virus (HCV) agents. The synthetic details as well as biological evaluations are discussed.
Subject(s)
Antiviral Agents/pharmacology , Chemistry, Pharmaceutical/methods , Hepacivirus/metabolism , Hepatitis C/drug therapy , Purine Nucleosides/chemistry , Cell Line , Cell-Free System , Drug Design , Humans , Models, Chemical , Purine Nucleosides/chemical synthesis , Purine Nucleosides/pharmacology , RNA/chemistry , RNA-Dependent RNA Polymerase/chemistry , Ribonucleosides , Uridine/chemistryABSTRACT
Several 6- and 7-monosubstituted N3,5'-cyclo-4-(beta-d-ribofuranosyl)-vic-triazolo[4,5-b]pyridin-5-one derivatives as well as the 5-thiono analogue were synthesized, providing structure-anti-hepatitis C virus (HCV) activity relationships for the series. Among the compounds synthesized, the 6-bromo, 7-methylamino, and 5-thiono analogues exhibited more potent anti-HCV activity in an HCV subgenomic replicon cell based assay (EC90 = 1.9, 7.4, and 10.0 microM, respectively) than the lead compound N3,5'-cyclo-4-(beta-D-ribofuranosyl)-vic-triazolo[4,5-b]pyridin-5-one (EC90 = 79.8 microM).
Subject(s)
Antiviral Agents/chemical synthesis , Bridged-Ring Compounds/chemical synthesis , Hepacivirus/drug effects , Nucleosides/chemical synthesis , Antiviral Agents/chemistry , Antiviral Agents/pharmacology , Bridged-Ring Compounds/chemistry , Bridged-Ring Compounds/pharmacology , Cell Line, Tumor , Hepacivirus/genetics , Humans , Nucleosides/chemistry , Nucleosides/pharmacology , RNA, Viral/antagonists & inhibitors , Structure-Activity RelationshipABSTRACT
The pyrimidine nucleoside beta-d-2'-deoxy-2'-fluoro-2'-C-methylcytidine (1) was designed as a hepatitis C virus RNA-dependent RNA polymerase (HCV RdRp) inhibitor. The title compound was obtained by a DAST fluorination of N(4)-benzoyl-1-(2-methyl-3,5-di-O-benzoyl-beta-d-arabinofuranosyl]cytosine to provide N(4)-benzoyl-1-[2-fluoro-2-methyl-3,5-di-O-benzoyl-beta-d-ribofuranosyl]cytosine. The protected 2'-C-methylcytidine was obtained as a byproduct from the DAST fluorination and allowed for the preparation of two biologically active compounds from a common precursor. Compound 1 and 2'-C-methylcytidine were assayed in a subgenomic HCV replicon assay system and found to be potent and selective inhibitors of HCV replication. Compound 1 shows increased inhibitory activity in the HCV replicon assay compared to 2'-C-methylcytidine and low cellular toxicity.
Subject(s)
Antiviral Agents/chemical synthesis , Deoxycytidine/analogs & derivatives , Hepacivirus/drug effects , Antiviral Agents/chemistry , Antiviral Agents/pharmacology , Crystallography, X-Ray , Deoxycytidine/chemical synthesis , Deoxycytidine/chemistry , Deoxycytidine/pharmacology , Drug Design , Hepacivirus/physiology , Molecular Structure , Structure-Activity Relationship , Virus Replication/drug effectsABSTRACT
Based on the discovery of (2'R)-d-2'-deoxy-2'-fluorocytidine as a potent anti-hepatitis C virus (HCV) agent, a series of d- and l-2'-deoxy-2'-fluororibonucleosides with modifications at 5- and/or 4-positions were synthesized and evaluated for their in vitro activity against HCV and bovine viral diarrhea virus (BVDV). The key step in the synthesis, the introduction of 2'-fluoro group, was achieved by either fluorination of 2,2'-anhydronucleosides with hydrogen fluoride-pyridine or potassium fluoride, or a fluorination of arabinonucleosides with DAST. Among the 27 analogues synthesized, only the 5-fluoro compound, namely (2'R)-d-2'-deoxy-2',5-difluorocytidine (13), demonstrated potent anti-HCV activity and toxicity to ribosomal RNA. The replacement of the 4-amino group with a thiol group resulted in the loss of activity, while the 4-methylthio substituted analogue (25) exhibited inhibition of ribosomal RNA. As N(4)-hydroxycytidine (NHC) had previously shown potent anti-HCV activity, we combined the two functionalities of the N(4)-hydroxyl and the 2'-fluoro into one molecule, resulting (2'R)-d-2'-deoxy-2'-fluoro-N(4)-hydroxycytidine (23). However, this nucleoside showed neither anti-HCV activity nor toxicity. All the l-forms of the analogues were devoid of anti-HCV activity. None of the compounds showed anti-BVDV activity, suggesting that the BVDV system cannot always predict anti-HCV activity.
Subject(s)
Antiviral Agents/chemical synthesis , Antiviral Agents/pharmacology , Deoxyribonucleosides/chemical synthesis , Deoxyribonucleosides/pharmacology , Genome, Viral , Hepacivirus/drug effects , Replicon , Animals , Cattle , Hepacivirus/genetics , Hepacivirus/physiology , Magnetic Resonance Spectroscopy , Spectrometry, Mass, Fast Atom BombardmentABSTRACT
A number of 1- or 6-substituted N9,5'-cyclo-3-(beta-ribofuranosyl)-8-azapurin-2-one derivatives were synthesized in multi-step reactions. Their anti-hepatitis C virus activities were evaluated and some structure-activity relationship is discussed.
Subject(s)
Adenosine/chemical synthesis , Antiviral Agents/chemical synthesis , Hepacivirus/growth & development , Purines/chemical synthesis , Virus Replication/drug effects , Adenosine/analogs & derivatives , Adenosine/pharmacology , Antiviral Agents/pharmacology , Hepacivirus/drug effects , Humans , Purines/chemistry , Purines/pharmacology , Structure-Activity RelationshipABSTRACT
A novel anti-hepatitis C virus (HCV) agent, N(3),5'-cyclo-4-(beta-D-ribofuranosyl)-vic-triazolo[4,5-b]pyridinin-5-one, was identified, and the structure was confirmed by chemical synthesis from 2-hydroxy-5-nitropyridine.
Subject(s)
Antiviral Agents/chemical synthesis , Hepacivirus/drug effects , Nucleosides/chemical synthesis , Antiviral Agents/chemistry , Antiviral Agents/pharmacology , Cells, Cultured , Hepacivirus/genetics , Humans , Nucleosides/chemistry , Nucleosides/pharmacology , RNA, Viral/antagonists & inhibitors , RNA-Dependent RNA Polymerase/antagonists & inhibitors , Viral Nonstructural Proteins/antagonists & inhibitorsABSTRACT
We identified the previously unknown structures of ribosylated imidazoleacetic acids in rat, bovine, and human tissues to be imidazole-4-acetic acid-ribotide (IAA-RP) and its metabolite, imidazole-4-acetic acid-riboside. We also found that IAA-RP has physicochemical properties similar to those of an unidentified substance(s) extracted from mammalian tissues that interacts with imidazol(in)e receptors (I-Rs). ["Imidazoline," by consensus (International Union of Pharmacology), includes imidazole, imidazoline, and related compounds. We demonstrate that the imidazole IAA-RP acts at I-Rs, and because few (if any) imidazolines exist in vivo, we have adopted the term "imidazol(in)e-Rs."] The latter regulate multiple functions in the CNS and periphery. We now show that IAA-RP (i) is present in brain and tissue extracts that exhibit I-R activity; (ii) is present in neurons of brainstem areas, including the rostroventrolateral medulla, a region where drugs active at I-Rs are known to modulate blood pressure; (iii) is present within synaptosome-enriched fractions of brain where its release is Ca(2+)-dependent, consistent with transmitter function; (iv) produces I-R-linked effects in vitro (e.g., arachidonic acid and insulin release) that are blocked by relevant antagonists; and (v) produces hypertension when microinjected into the rostroventrolateral medulla. Our data also suggest that IAA-RP may interact with a novel imidazol(in)e-like receptor at this site. We propose that IAA-RP is a neuroregulator acting via I-Rs.
Subject(s)
Imidazoles/pharmacology , Receptors, Drug/agonists , Ribosemonophosphates/pharmacology , Adrenal Medulla/metabolism , Animals , Antibodies/immunology , Antibody Specificity , Arachidonic Acid/metabolism , Brain Stem/cytology , Calcium/metabolism , Enzyme-Linked Immunosorbent Assay , Humans , Hypertension/chemically induced , Imidazoles/chemistry , Imidazoles/immunology , Imidazoline Receptors , Insulin/metabolism , Insulin Secretion , Islets of Langerhans/metabolism , Isomerism , Ligands , Molecular Structure , Neurons/metabolism , PC12 Cells , Rats , Receptors, Drug/metabolism , Ribosemonophosphates/chemistry , Ribosemonophosphates/immunologyABSTRACT
Novel racemic 5'-C-methyl-1',3'-dioxolan-4'-yl nucleosides were synthesized from the key intermediate, 2-benzoyloxymethyl-4-oxo-5-C-methyl-1,3-dioxolane, which was prepared from racemic lactic acid.
Subject(s)
Dioxolanes/chemical synthesis , Nucleosides/chemical synthesisABSTRACT
2'-Deoxy-2'-fluorocytidine (FdC) is a potent inhibitor of the hepatitis C virus RNA replicon in culture, and FdC-5'-triphosphate is an effective inhibitor of the NS5B polymerase. Dynamic profiling of cell growth in an antiviral assay showed that FdC caused cytostasis due to an S-phase arrest. These observations demonstrate that FdC treatment is affecting both a viral target and a cellular target.
Subject(s)
Antiviral Agents/pharmacology , Deoxycytidine/analogs & derivatives , Deoxycytidine/pharmacology , Hepacivirus/drug effects , Hepacivirus/genetics , RNA, Viral/biosynthesis , RNA, Viral/genetics , Replicon/drug effects , Replicon/genetics , Animals , Cattle , Cell Division/drug effects , Cell Line , Diarrhea Viruses, Bovine Viral/genetics , Dose-Response Relationship, Drug , Enzyme Inhibitors/chemical synthesis , Enzyme Inhibitors/pharmacology , Humans , S Phase/drug effects , Viral Nonstructural Proteins/antagonists & inhibitorsABSTRACT
The synthesis and anti-HBV and anti-HIV activity of a number of 2',3'-dideoxy-2'-fluoro-3'-C-hydroxymethyl-beta-D-arabinofuranosyl pyrimidine nucleosides are reported.
Subject(s)
Antiviral Agents/chemical synthesis , Pyrimidine Nucleosides/chemical synthesis , Pyrimidine Nucleosides/pharmacology , Antiviral Agents/chemistry , Antiviral Agents/pharmacology , HIV/drug effects , Hepatitis B virus/drug effects , Indicators and Reagents , Molecular Conformation , Molecular Structure , Pyrimidine Nucleosides/chemistryABSTRACT
A series of 2',3'-dideoxy (D2) and 2',3'-didehydro-2',3'-dideoxy (D4) 5-fluorocytosine nucleosides modified with substituted benzoyl, heteroaromatic carbonyl, cycloalkylcarbonyl and alkanoyl at the N4-position were synthesized and evaluated for anti-human immunodeficiency virus type 1 (HIV-1) and anti-hepatitis B virus (HBV) activity in vitro. For most D2-nucleosides, N4-substitutions improved the anti-HIV-1 activity markedly without increasing the cytotoxicity. In the D4-nucleosides series, some of the substituents at the N4-position enhanced the anti-HIV-1 activity with a modest increase in the cytotoxicity. The most potent and selective N4-modified nucleoside for the D2-series was N4-p-iodobenzoyl-D2FC, which had a 46-fold increase in anti-HIV-1 potency in MT-2 cells compared to the parent nucleoside D-D2FC. In the D4-series, N4-p-bromobenzoyl-D4FC was 12-fold more potent in MT-2 cells compared to the parent nucleoside D-D4FC. All eight N4-p-halobenzoyl-substituted D2- and D4-nucleosides evaluated against HBV in HepAD38 cells demonstrated equal or greater potency than the two parental compounds, D-D2FC and D-D4FC. The N4-modification especially in the D2-nucleoside series containing the N4-nicotinoyl, o-nitrobenzoyl and n-butyryl showed a significant reduction in mitochondrial toxicity relative to the parent nucleoside analogue. Although the 5'-triphosphate of the parent compound (D-D4FC-TP) was formed from the N4-acyl-D4FC analogues in different cells, the levels of the 5'-triphosphate nucleotide did not correlate with the cell-derived 90% effective antiviral concentrations (EC90), suggesting that a direct interaction of the triphosphates of these N4-acyl nucleosides was involved in the antiviral activity.
Subject(s)
Antiviral Agents/pharmacology , Zalcitabine/pharmacology , Animals , Anti-HIV Agents/chemical synthesis , Anti-HIV Agents/chemistry , Anti-HIV Agents/pharmacology , Antiviral Agents/chemical synthesis , Antiviral Agents/chemistry , Cell Division/drug effects , Cell Line , Cell Line, Tumor , Chlorocebus aethiops , Dose-Response Relationship, Drug , HIV-1/drug effects , Hepatitis B virus/drug effects , Humans , Inhibitory Concentration 50 , Models, Chemical , Vero Cells , Zalcitabine/analogs & derivatives , Zalcitabine/chemical synthesis , Zalcitabine/chemistryABSTRACT
A base-modified nucleoside analogue, beta-D-N(4)-hydroxycytidine (NHC), was found to have antipestivirus and antihepacivirus activities. This compound inhibited the production of cytopathic bovine viral diarrhea virus (BVDV) RNA in a dose-dependant manner with a 90% effective concentration (EC(90)) of 5.4 microM, an observation that was confirmed by virus yield assays (EC(90) = 2 microM). When tested for hepatitis C virus (HCV) replicon RNA reduction in Huh7 cells, NHC had an EC(90) of 5 microM on day 4. The HCV RNA reduction was incubation time and nucleoside concentration dependent. The in vitro antiviral effect of NHC was additive with recombinant alpha interferon-2a and could be prevented by the addition of exogenous cytidine and uridine but not of other natural ribo- or 2'-deoxynucleosides. When HCV RNA replicon cells were cultured in the presence of increasing concentrations of NHC (up to 40 micro M) for up to 45 cell passages, no resistant replicon was selected. Similarly, resistant BVDV could not be selected after 20 passages. NHC was phosphorylated to the triphosphate form in Huh7 cells, but in cell-free HCV NS5B assays, synthetic NHC-triphosphate (NHC-TP) did not inhibit the polymerization reaction. Instead, NHC-TP appeared to serve as a weak alternative substrate for the viral polymerase, thereby changing the mobility of the product in polyacrylamide electrophoresis gels. We speculate that incorporated nucleoside analogues with the capacity of changing the thermodynamics of regulatory secondary structures (with or without introducing mutations) may represent an important class of new antiviral agents for the treatment of RNA virus infections, especially HCV.
Subject(s)
Antiviral Agents/pharmacology , Cytidine/analogs & derivatives , Cytidine/pharmacology , Diarrhea Viruses, Bovine Viral/drug effects , Hepacivirus/drug effects , Virus Replication/drug effects , Animals , Antiviral Agents/chemical synthesis , Antiviral Agents/pharmacokinetics , Cattle , Cells, Cultured , Cytidine/chemical synthesis , Cytidine/pharmacokinetics , Diarrhea Viruses, Bovine Viral/genetics , Female , Mice , RNA, Viral/drug effectsABSTRACT
Oncolytic C-nucleosides, tiazofurin (2-beta-D-ribofuranosylthiazole-4-carboxamide) and benzamide riboside (3-beta-D-ribofuranosylbenzamide) are converted in cell into active metabolites thiazole-4-carboxamide- and benzamide adenine dinucleotide, TAD and BAD, respectively. TAD and BAD as NAD analogues were found to bind at the nicotinamide adenine dinucleotide (cofactor NAD) site of inosine monophosphate dehydrogenase (IMPDH), an important target in cancer treatment. The synthesis and evaluation of anticancer activity of a number of C-nucleosides related to tiazofurin and nicotinamide riboside then followed and are reviewed herein. Interestingly, pyridine C-nucleosides (such as C-nicotinamide riboside) are not metabolized into the corresponding NAD analogues in cell. Their conversion by chemical methods is described. As dinucleotides these compounds show inhibition of IMPDH in low micromolar level. Also, the synthesis of BAD in metabolically stable bis(phosphonate) form is discussed indicating the usefulness of such preformed inhibitors in drug development. Among tiazofurin analogues, Franchetti and Grifantini found, that the replacement of the sulfur by oxygen (as in oxazafurin) but not the removal of nitrogen (tiophenfurin) of the thiazole ring resulted in inactive compounds. The anti cancer activity of their synthetic dinucleotide analogues indicate that inactive compounds are not only poorly metabolized in cell but also are weak inhibitors of IMPDH as dinucleotides.
Subject(s)
Antineoplastic Agents/pharmacology , IMP Dehydrogenase/antagonists & inhibitors , NAD/analogs & derivatives , NAD/chemistry , Niacinamide/analogs & derivatives , Niacinamide/pharmacology , Cell Survival/drug effects , Humans , NAD/pharmacology , Organoselenium Compounds , Pyridinium Compounds , Ribavirin/analogs & derivatives , Ribavirin/pharmacology , Ribonucleosides , Ribonucleotides/analysis , Tumor Cells, Cultured/drug effectsABSTRACT
Novel mycophenolic adenine dinucleotide (MAD) analogues have been prepared as potential inhibitors of inosine monophosphate dehydrogenase (IMPDH). MAD analogues resemble nicotinamide adenine dinucleotide binding at the cofactor binding domain of IMPDH; however, they cannot participate in hydride transfer and therefore inhibit the enzyme. The methylenebis(phosphonate) analogues C2-MAD and C4-MAD were obtained by coupling 2',3'-O-isopropylideneadenosine 5'-methylenebis(phosphonate) (22) with mycophenolic alcohols 20 and 21 in the presence of diisopropylcarbodiimide followed by deprotection. C2-MAD was also prepared by coupling of mycophenolic methylenebis(phosphonate) derivative 30 with 2',3'-O-isopropylideneadenosine. Compound 30 was conveniently synthesized by the treatment of benzyl-protected mycophenolic alcohol 27 with a commercially available methylenebis(phosphonic dichloride) under Yoshikawa's reaction conditions. C2-MAD and C4-MAD were found to inhibit the growth of K562 cells (IC(50) = 0.7 microM and IC(50) = 0.1 microM, respectively) as potently as mycophenolic acid (IC(50) = 0.3 microM). In addition, C2-MAD and C4-MAD triggered vigorous differentiation of K562 cells an order of magnitude more potently than tiazofurin, and MAD analogues were resistant to glucuronidation in vitro. These results show that C2-MAD and C4-MAD may be of therapeutic interest in the treatment of human leukemias.