Analogs of 2'-deoxyadenosine: facile enzymatic preparation and growth inhibitory effects on human cell lines

New trends in the biocatalytic production of nucleosidic active pharmaceutical ingredients using 2'-deoxyribosyltransferases

Nowadays, pharmaceutical industry demands competitive and eco-friendly processes for active pharmaceutical ingredients (APIs) manufacturing. In this context, enzyme and whole-cell mediated processes offer an efficient, sustainable and cost-effective alternative to the traditional multi-step and environmentally-harmful chemical processes. Particularly, 2'-deoxyribosyltransferases (NDTs) have emerged as a novel synthetic alternative, not only to chemical but also to other enzyme-mediated synthetic processes. This review describes recent findings in the development and scaling up of NDTs as industrial biocatalysts, including the most relevant and recent examples of single enzymatic steps, multienzyme cascades, chemo-enzymatic approaches, and engineered biocatalysts. Finally, to reflect the inventive and innovative steps of NDT-mediated bioprocesses, a detailed analysis of recently granted patents, with specific focus on industrial synthesis of nucleoside-based APIs, is hereunder presented.

Efficient Biocatalytic Synthesis of Dihalogenated Purine Nucleoside Analogues Applying Thermodynamic Calculations

The enzymatic synthesis of nucleoside analogues has been shown to be a sustainable and efficient alternative to chemical synthesis routes. In this study, dihalogenated nucleoside analogues were produced by thermostable nucleoside phosphorylases in transglycosylation reactions using uridine or thymidine as sugar donors. Prior to the enzymatic process, ideal maximum product yields were calculated after the determination of equilibrium constants through monitoring the equilibrium conversion in analytical-scale reactions. Equilibrium constants for dihalogenated nucleosides were comparable to known purine nucleosides, ranging between 0.071 and 0.081. To achieve 90% product yield in the enzymatic process, an approximately five-fold excess of sugar donor was needed. Nucleoside analogues were purified by semi-preparative HPLC, and yields of purified product were approximately 50% for all target compounds. To evaluate the impact of halogen atoms in positions 2 and 6 on the antiproliferative activity in leukemic cell lines, the cytotoxic potential of dihalogenated nucleoside analogues was studied in the leukemic cell line HL-60. Interestingly, the inhibition of HL-60 cells with dihalogenated nucleoside analogues was substantially lower than with monohalogenated cladribine, which is known to show high antiproliferative activity. Taken together, we demonstrate that thermodynamic calculations and small-scale experiments can be used to produce nucleoside analogues with high yields and purity on larger scales. The procedure can be used for the generation of new libraries of nucleoside analogues for screening experiments or to replace the chemical synthesis routes of marketed nucleoside drugs by enzymatic processes.

Synthesis of New Potential Lipophilic Co-Drugs of 2-Chloro-2'-deoxyadenosine (Cladribine, 2-CdA, Mavenclad®, Leustatin®) and 6-Azauridine (z6 U) with Valproic Acid

2-Chloro-2'-deoxyadenosine (cladribine, 1) was acylated with valproic acid (2) under various reaction conditions yielding 2-chloro-2'-deoxy-3',5'-O-divalproyladenosine (3) as well as the 3'-O- and 5'-O-monovalproylated derivatives, 2-chloro-2'-deoxy-3'-O-valproyladenosine (4) and 2-chloro-2'-deoxy-5'-O-valproyladenosine (5), as new co-drugs. In addition, 6-azauridine-2',3'-O-(ethyl levulinate) (8) was valproylated at the 5'-OH group (→9). All products were characterized by ¹ H- and ¹³ C-NMR spectroscopy and ESI mass spectrometry. The structure of the by-product 6 (N-cyclohexyl-N-(cyclohexylcarbamoyl)-2-propylpentanamide), formed upon valproylation of cladribine in the presence of N,N-dimethylaminopyridine and dicyclohexylcarbodiimide, was analyzed by X-ray crystallography. Cladribine as well as its valproylated co-drugs were tested upon their cancerostatic/cancerotoxic activity in human astrocytoma/oligodendroglioma GOS-3 cells, in rat malignant neuro ectodermal BT4Ca cells, as well as in phorbol-12-myristate 13-acetate (PMA)-differentiated human THP-1 macrophages. The most important result of these experiments is the finding that only the 3'-O-valproylated derivative 4 exhibits a significant antitumor activity while the 5'-O- as well as the 3',5'-O-divalproylated cladribine derivatives 3 and 5 proved to be inactive.

Synthesis and in vitro antiviral activities of [(dihydrofuran-2-yl)oxy]methyl-phosphonate nucleosides with 2-substituted adenine as base

The synthesis of [(2',5'-dihydrofuran-2-yl)oxy]methyl-phosphonate nucleosides with a 2-substituted adenine base moiety starting from 2-deoxy-3,5-bis-O-(4-methylbenzoyl)-α-L-ribofuranosyl chloride and 2,6-dichloropurine is described. The key step is the regiospecific and stereoselective introduction of a phosphonate synthon at C(2) of the furan ring. None of the synthesized compounds showed significant in vitro activity against HIV, BVDV, and HBV.

Phosphodeoxyribosyltransferases, designed enzymes for deoxyribonucleotides synthesis

A large number of nucleoside analogues and 2'-deoxynucleoside triphosphates (dNTP) have been synthesized to interfere with DNA metabolism. However, in vivo the concentration and phosphorylation of these analogues are key limiting factors. In this context, we designed enzymes to switch nucleobases attached to a deoxyribose monophosphate. Active chimeras were made from two distantly related enzymes: a nucleoside deoxyribosyltransferase from lactobacilli and a 5'-monophosphate-2'-deoxyribonucleoside hydrolase from rat. Then their unprecedented activity was further extended to deoxyribose triphosphate, and in vitro biosyntheses could be successfully performed with several base analogues. These new enzymes provide new tools to synthesize dNTP analogues and to deliver them into cells.

Hoogsteen vs. Watson-Crick base pairing: incorporation of 2-substituted adenine- and 7-deazaadenine 2'-deoxy-beta-D-ribonucleosides into oligonucleotides

Various 2-substituted 2'-deoxyadenosines and 7-deazaadenosines have been synthesized. The phosphonate building block 9 of 2-chloro-7-deaza-2'-deoxyadenosine (7-deazacladribine; 2) was prepared by 4,4'-dimethoxytritylation of the parent nucleoside (-->7), followed by protection of the amino function with a formamidine residue (-->8). The latter was reacted with PCl3/N-methylmorpholine/1,2,4-triazole to give compound 9. Moreover, 2-methoxy-2'-deoxyadenosine (2'-deoxyspongosine; 1b) was converted into the fully protected derivative 12, which was then transformed into the 2-cyanoethyl phosphoramidite 14. Also the 2-(trifluoromethyl)-substituted 2'-deoxyadenosines 19-21 were prepared by glycosylation of the chromophore 16 with the halogenose 17, followed by one-pot deprotection and nucleophilic displacement of the 6-Cl substituent. The new DNA building blocks 9 and 14 were used--together with formerly prepared cladribine derivative 4--for solid-phase synthesis of a series of oligodeoxyribonucleotides. These were studied with respect to their thermal stability as well as of the base pairing mode (Watson-Crick vs. Hoogsteen) of modified bases.

Characterization of N-deoxyribosyltransferase from Lactococcus lactis subsp. lactis

A nucleoside N-deoxyribosyltransferase-homologous gene was detected by homological search in the genomic DNA of Lactococcus lactis subsp. lactis. The gene yejD is composed of 477 nucleotides encoding 159 amino acids with only 25% identity, which is low in comparison to the amino acid sequences of the N-deoxyribosyltransferases from other lactic acid bacteria, i.e. Lactobacillus leichmannii and Lactobacillus helveticus. The residues responsible for catalytic and substrate-binding sites in known enzymes are conserved at Gln49, Asp73, Asp93 (or Asp95), and Glu101, respectively. The recombinant YejD expressed in Escherichia coli shows a 2-deoxyribosyl transfer activity to and from both bases of purine and pyrimidine, showing that YejD should be categorized as a class II N-deoxyribosyltransferase. Interestingly, the base-exchange activity as well as the heat stability of YejD was enhanced by the presence of monovalent cations such as K(+), NH(4)(+), and Rb(+), indicating that the Lactococcus enzyme is a K(+)-activated Type II enzyme. However, divalent cations including Mg(2+) and Ca(2+) significantly inhibit the activity. Whether or not the yejD gene product actually participates in the nucleoside salvage pathway of Lc. lactis remains unclear, but the lactic acid bacterium possesses the gene coding for the nucleoside N-deoxyribosyltransferase activated by K(+) on its genome.

Oligonucleotides containing pyrazolo[3,4-d]pyrimidines: 8-Aza-7-deazaadenines with bulky substituents in the 2- or 7-position

The synthesis of the 2'-deoxyadenosine analogues 1b, 2b, and 3c modified at the 7- and/or 2-position is described. The effect of 7-chloro and 2-methylthio groups on the duplex stability is evaluated. For that, the nucleosides 1b, 2b, and 3c were converted to the corresponding phosphoramidites 15, 19, and 22, which were employed in the solid-phase oligonucleotide synthesis. In oligonucleotide duplexes, compound 1b forms stable base pairs with dT, of which the separated 1b-dT base pairs contribute stronger than that of the consecutive base pairs. Compound 2b shows universal base pairing properties while its N8 isomer 3c forms duplexes with lower stability.

Functional cloning, heterologous expression, and purification of two different N-deoxyribosyltransferases from Lactobacillus helveticus

Lactobacillus helveticus contains two types of N-deoxyribosyltransferases: DRTase I catalyzes the transfer of 2'-deoxyribose between purine bases exclusively whereas DRTase II is able to transfer the 2'-deoxyribose between two pyrimidine or between pyrimidine and purine bases. An Escherichia coli strain, auxotrophic for guanine and unable to use deoxyguanosine as source of guanine, was constructed to clone the corresponding genes. By screening a genomic bank for the production of guanine, the L. helveticus ptd and ntd genes coding for DRTase I and II, respectively, were isolated. Although the two genes have no sequence similarity, the two deduced polypeptides display 25.6% identity, with most of the residues involved in substrate binding and the active site nucleophile Glu-98 being conserved. Overexpression and purification of the two proteins shows that DRTase I is specific for purines with a preference for deoxyinosine (dI) > deoxyadenosine > deoxyguanosine as donor substrates whereas DRTase II has a strong preference for pyrimidines as donor substrates and purines as base acceptors. Purine analogues were substrates as acceptor bases for both enzymes. Comparison of DRTase I and DRTase II activities with dI as donor or hypoxanthine as acceptor and colocalization of the ptd and add genes suggest a specific role for DRTase I in the metabolism of dI.

A convenient synthesis of 2'-deoxy-2-fluoroadenosine; a potential prodrug for suicide gene therapy

A convenient synthesis of 2'-deoxy-2-fluoro-adenosine (1) is described. Deaminative fluorination of 2-aminoadenosine (2) followed by silylation of the 3', 5'-hydroxyl groups gave the corresponding 2-fluoroadenosine derivative 4 in good yield. Thiocarbonylation of 4 to thiocarbonylimidazolyl derivative 5a followed by treatment with an excess of tris(trimethylsilyl)silane (TTMSS) and tert-butyl peroxide in toluene at 80 degrees C was found to affect an efficient deoxygenation to the corresponding 2'-deoxy derivative 6. Desilylation of 6 by Et4NF in CH3CN afforded 1 in high yield.

Rapid purification and characterization of two distinct N-deoxyribosyltransferases of Lactobacillus leichmannii

Two distinct N-deoxyribosyltransferases of Lactobacillus leichmannii, designated as DRTase I and DRTase II, were separated and purified almost to homogeneity by one-step affinity chromatography. DRTase I is distinguished by specifically catalyzing the direct transfer of 2-deoxyribosyl residues from purine deoxyribonucleosides to free purine bases, whereas DRTase II has a rather broad substrate specificity and is able to transfer the deoxyribosyl moiety between pyrimidines and between purines and pyrimidines. Furthermore, in addition to the different substrate spectrum, we clearly differentiated the two enzymes by comparing their varying temperature/activity and pH/activity profiles, their kinetic constants, their behaviour in Western blot analysis, and their N-terminal amino acid sequences. Denaturing and non-denaturing DISK-PAGE revealed strong evidence that both intact enzymes consist of hexamers with subunit molecular weights of approximately 20,000 for DRTase I and 18,000 for DRTase II.

Active site amino acids that participate in the catalytic mechanism of nucleoside 2'-deoxyribosyltransferase

The importance of eight nucleoside 2'-deoxyribosyltransferase residues for catalysis was investigated by site-directed mutagenesis. Each residue was selected because of its proximity to nucleophile Glu-98 or on its potential contribution to intrinsic protein fluorescence. Mutation of Asp-72, Asp-92, Tyr-7, Trp-12, and Met-125 resulted in over a 90% activity loss whereas mutation of Tyr-157, Trp-64, and Trp-127 produced less than a 80% activity loss. The magnitude of the perturbation on catalysis by mutation, however, was dependent on donor substrate. The kcat values for dIno hydrolysis by these mutants were greater than 25% of that for native enzyme. Although mutant and native enzymes bound substrate analogues with comparable affinities, Km values for dIno hydrolysis varied over a 1000-fold range. The pH dependence of Glu-98 esterification by dCyd suggested that amino acids with pK values of 4.2 and 7.5 were relevant for catalysis. The intrinsic protein fluorescence was attributed primarily to Trp-127 (approximately 80%). Pre-steady-state kinetic parameters for deoxyribosylation of mutant enzymes by dCyd, dThd, and dAdo were determined by monitoring changes in enzyme fluorescence. Collectively, results from mutagenesis suggest that, depending upon substrate, either Asp-92 or Asp-72 functions as the general acid catalyst, and that this enzyme undergoes a change in conformation upon Glu-98 deoxyribosylation.

Crystal structures of nucleoside 2-deoxyribosyltransferase in native and ligand-bound forms reveal architecture of the active site

BACKGROUND

Nucleoside 2-deoxyribosyltransferase plays an important role in the salvage pathway of nucleotide metabolism in certain organisms, catalyzing the cleavage of beta-2'-deoxyribonucleosides and the subsequent transfer of the deoxyribosyl moiety to an acceptor purine or pyrimidine base. The kinetics describe a ping-pong-bi-bi pathway involving the formation of a covalent enzyme-deoxyribose intermediate. The enzyme is produced by a limited number of microorganisms and its functions have been exploited in its use as a biocatalyst to synthesize nucleoside analogs of therapeutic interest.

RESULTS

We describe the crystal structure of the enzyme with and without bound ligand. The native structure was solved by the single isomorphous replacement with anomalous scattering method (SIRAS) and refined to 2.5 A resolution resulting in a crystallographic R factor of 16.6%. The enzyme comprises a single domain that belongs to the general class of doubly-wound alpha/beta proteins; it also exhibits a unique nucleoside-binding motif. X-ray analysis of enzyme-purine and enzyme-pyrimidine complexes presented here reveals that the active site lies in a cleft formed by the edge of the beta sheet and two alpha helices and contains side chains from two subunits.

CONCLUSIONS

These results indicate residues that may be important in substrate binding and catalysis and thus may serve as a framework for elucidating the mechanism of enzyme activity. In particular, the proposed nucleophile, Glu98, lies in the nucleoside-binding pocket at an appropriate position for nucleophilic attack. A comparison of the enzyme interactions with both a purine and pyrimidine ligand provides some insight into the structural basis for enzyme specificity.

Identification of the active site nucleophile in nucleoside 2-deoxyribosyltransferase as glutamic acid 98

2'-Fluoro-2'-deoxyarabinonucleosides are time-dependent inhibitors of nucleoside 2-deoxyribosyltransferase. 2,6-Diamino-9-(2'-deoxy-2'-fluoro-beta-D-arabinofuranosyl)-9H-purine (dFDAP) inhibited the enzyme by formation of a primary complex (Kd = 140 microM) that isomerized to a secondary complex with a first-order rate constant of 0.2 min-1. Inhibited enzyme contained stoichiometric amounts of covalently bound 2'-fluoro-2'-deoxyarabinosyl moiety, recovered less than 5% of its activity after storage for a week at 5 degrees C, but regained over 70% of the lost activity by treatment with 600 microM Ade. 6-Amino-9-(2'-deoxy-2'-fluoro-beta-D-arabinofuranosyl)-9H-purine (dFAdo) was a product of the reactivation reaction. Proteolysis of inhibited enzyme identified a modified fragment that spanned residues 82-107 which could not be sequenced past Gly-96. dFDAP-inhibited enzyme and enzyme reacted with normal substrates (i.e. dThd and dAdo) were hydrolyzed between Met-97 and Glu-98 by 0.1 M NaOH. These findings and model studies on the base lability of peptides containing glutamyl esters suggested that the gamma-carboxylate of Glu-98 was esterfied during catalysis. The role of Glu-98 was confirmed by changing this residue to alanine. The specific activity of wild-type enzyme was 3 orders of magnitude greater than that of the mutant enzyme. Collectively, chemical modification and mutagenesis studies have identified Glu-98 as the active site nucleophile of nucleoside 2-deoxyribosyltransferase.

Nucleoside 2-deoxyribosyltransferase. Pre-steady-state kinetic analysis of native enzyme and mutant enzyme with an alanyl residue replacing Glu-98

Nucleoside 2-deoxyribosyltransferase catalyzes cleavage of a 2'-deoxyribosylnucleoside (A) to a nucleobase (P) with deoxyribosylation of the enzyme. Substrates quenched the intrinsic fluorescence of native enzyme (E) and a catalytically inactive mutant enzyme (E98A enzyme). The time courses of these reactions were analyzed in terms of the following scheme where EX is the 2-deoxyribosyl ester of Glu-98. [formula: see text] The initial complexes between E and dAdo, dGuo, dIno, and dCyd or those between EX and the corresponding nucleobases were formed in a rapid equilibrium step. Native enzyme and E98A enzyme bound 2'-deoxyribosylnucleosides with similar affinities (k-1/k1). From a comparison of the time-dependent fluorescence changes associated with the reaction of native enzyme or E98A enzyme with these substrate, the kinetic step for 2-deoxyribosylation of Glu-98 was identified (k2 and k-2). dThd and dUrd quenched the fluorescence of native enzyme in a biphasic process. The late phase of this reaction was associated with 2-deoxyribosylation of Glu-98. The pre-steady-state kinetic constants calculated from fluorescence quenching data for dAdo and Cyt were consistent with the experimental values for the steady-state kinetic coefficients and the equilibrium constant of the reaction.

2-Chloro-2'-deoxyadenosine monophosphate residues in DNA enhance susceptibility to 3'-->5' exonucleases

2-Chloro-2'-deoxyadenosine triphosphate, a purine nucleotide analogue and potent antileukaemic agent, was incorporated into double-stranded 36-mers in place of dATP to investigate the effects of 2-chloroadenine (ClAde) on DNA polymerase-associated 3'-->5' exonuclease activity. ClAde residues within one strand of duplex DNA did not inhibit exonuclease activity; on the contrary, ClAde-containing minus strands were digested to a greater extent than was control DNA in the absence of deoxyribonucleoside triphosphates by Escherichia coli Klenow fragment, yeast DNA polymerase II and T4 DNA polymerase. After a 30 min incubation with 5 units of Klenow fragment, approximately 65% of control DNA remained in DNA fragments of 26 bases or larger compared with only approximately 25% of ClAde-substituted substrates. Unsubstituted plus strands opposite a ClAde-containing strand were likewise digested more quickly by 3'-->5' exonuclease, but only in the vicinity of the ClAde sites. Approx. 63% of the plus strands from ClAde-containing oligomers were less than 24 bases in length after a 25 min digestion period with Klenow fragment compared with only approximately 32% of control DNA. Such results indicate that, unlike other base modifications such as pyrimidine dimers, methoxy psoralen adducts and certain nucleoside analogues, all of which inhibit or decrease the rate of strand degradation by 3'-->5' exonucleases, incorporated ClAde enhances strand degradation of duplex DNA.

Fludarabine phosphate. A new anticancer drug with significant activity in patients with chronic lymphocytic leukemia and in patients with lymphoma

Fludarabine phosphate is a purine antimetabolite approved for use in the management of patients with chronic lymphocytic leukemia. Fludarabine works primarily by inhibiting DNA synthesis. The compound also possesses lymphocytotoxic activity with preferential activity toward T-lymphocytes. Initial preclinical studies demonstrated antitumor activity with fludarabine against L1210 murine leukemia. In phase I studies, myelosuppression was identified as the dose-limiting toxicity in patients with solid tumors and fatal neurotoxicity as the dose-limiting toxicity in adult patients with acute hematologic malignancies. The recommended dose and schedule was determined to be 18-25 mg/m2/d for five days, repeated every 28 days. Unlike preclinical studies, phase II trials showed a lack of significant effect when fludarabine was given to patients with solid tumors. However, phase II investigations have confirmed the efficacy of fludarabine in lymphoid malignancies, including non-Hodgkin's lymphoma, mycosis fungoides, prolymphocytic leukemia, and chronic lymphocytic leukemia. The place of fludarabine in the management of leukemias in children is under investigation. Early results indicate an unusual degree of antitumor activity when the agent is used in combination chemotherapy for patients with refractory disease. Fludarabine is an effective antitumor agent in the management of lymphoid malignancies. Studies are ongoing to more completely define the role of fludarabine in these malignancies as well as in the pediatric leukemias. Additional studies evaluating the activity of fludarabine as an immunomodulator are warranted, due to the lymphocytotoxic properties associated with this agent.

Substrate specificity of the purine-2'-deoxyribonucleosidase of Crithidia luciliae

The purine-2'-deoxyribonucleosidase of Crithidia luciliae catalyses an efficient deoxyribosyl transfer between a variety of purine bases, benzimidazole and 5,6-dimethylbenzimidazole. Since the deoxyriboside of a deoxyribosyl acceptor is necessarily also a substrate, the trans-N-deoxyribosylase activity of the enzyme allows a study of its specificity to be extended to a large number of purines and purine analogues. Amongst 27 different deoxyribosyl acceptors, only hypoxanthine gave rise to isomeric products. The introduction of methyl groups at appropriate positions in either purine or benzimidazole lowered the Michaelis constant, KB, for deoxyribosyl acceptors: by about 10-fold for 6-methylpurine (KB 351 +/- 87 microM) compared with purine (KB 3.91 +/- 0.8 mM) and by about 10(3)-fold for 5,6-dimethylbenzimidazole (KB 7.0 +/- 0.79 microM) compared with benzimidazole (Km,app. 7.8 +/- 2.4 mM). The maximal rates of deoxyribosyl transfer to different acceptors, on the other hand, varied by only 4.5-fold, and can be ascribed to decreases in the rate of release of the newly formed purine deoxyriboside from the enzyme.

Polymerase chain reaction amplification of single-stranded DNA containing a base analog, 2-chloradenine

By utilization of polymerase chain reaction techniques, single-stranded DNA of defined length and sequence containing a purine analog, 2-chloroadenine, in place of adenine was synthesized. This was accomplished by a combination of standard polymerase chain amplification reactions with Thermus aquaticus DNA polymerase in the presence of four normal deoxynucleoside triphosphates, M13 duplex DNA as template, and two primers to generate double-stranded DNA 118 bases in length. An asymmetric polymerase chain reaction, which produced an excess of single-stranded 98-base DNA, was then conducted with 2-chloro-2'-deoxy-adenosine 5'-triphosphate in place of dATP and with only one primer that annealed internal to the original two primers. Standard polymerase chain reaction techniques alone conducted in the presence of the analog as the fourth nucleotide did not produce duplex DNA that was modified within both strands. This asymmetric technique allows the incorporation of an altered nucleotide at specific sites into large quantities of single-stranded DNA without using chemical phosphoramidite synthesis procedures and circumvents the apparent inability of DNA polymerase to synthesize fully substituted double-stranded DNA during standard amplification reactions. The described method will permit the study of the effects of modified bases in template DNA on a variety of protein-DNA interactions and enzymes.

The toxicity of adenosine analogues against Babesia bovis in vitro

The toxicities of 20 analogues of deoxyadenosine or adenosine were tested in vitro against the intraerythrocytic parasite Babesia bovis. IC37 values (the concentration of compound required to reduce cell survival to 37%) were determined for each compound. Tubercidin (7-deaza-adenosine), 2-bromo-adenosine, 8-bromo-3-ribosyl adenine and 6-phenylamino-deoxyadenosine were shown to be the most toxic towards B. bovis. Comparison of the toxicity results for these compounds in B. bovis with those in human melanoma cell lines indicated a differential toxicity, in that many of the compounds were toxic towards B. bovis but were relatively non-toxic towards human melanoma cell lines and vice versa. These results suggest that the mechanism of toxicity of the deoxyadenosine and adenosine analogues, whose normal metabolism involves transport, metabolism and incorporation into nucleic acids, may vary significantly between B. bovis and mammalian cells, allowing such drugs to be considered for parasite chemotherapy.

Metabolism and action of purine nucleoside analogs

Recent investigations have identified many new purine nucleoside analogs that act as antimetabolites. This article focuses on the metabolism and mechanisms of action of tiazofurin, 3-deazaguanosine, neplanocin A, arabinosyladenine in combination with inhibitors of adenosine deaminase, arabinosyl-2-fluoroadenine, and 2-chloro-2'-deoxyadenosine, drugs that are either currently being evaluated in clinical trials or are close to that stage. The diverse metabolic requirements for activation, unique mechanisms of action, and differential biological activities of these compounds are characterized and evaluated for prospective therapeutic application.

The stereoselective enzymatic synthesis of 9-beta-D-2'-deoxyribofuranosyl 1-deazapurine

The transfer of 2-deoxyribose from thymidine to 1-deazapurine which is catalysed by N-deoxyribosyl transferases from Lactobacillus leichmanii occurs in high yield. This is a very stereoselective process and only one product, 9-beta-D-2'-deoxyribofuranosyl 1-deazapurine, is formed. Nmr spectroscopy, and in particular, nuclear Overhauser enhancement experiments, confirm that the 2-deoxyribose moiety is bound to N-9 rather than N-7 and that the glycosidic link has the beta-configuration.

Metabolic activation of 2,6-diaminopurine and 2,6-diaminopurine-2'-deoxyriboside to antitumor agents

2,6-Diaminopurine (DAP) and 2,6-diaminopurine 2'-deoxyriboside (DAPdR) are analogs of adenine and deoxyadenosine, respectively. It was the purpose of this study to compare these analogs under identical conditions in order to define their inhibitory properties and the underlying mechanism in L1210 mouse leukemia cells. In a 5-day cell growth experiment, DAP exerted a significantly stronger antiproliferative effect than DAPdR. Correspondingly, colony formation of L1210 cells in soft agarose was inhibited by DAP to a greater extent than by DAPdR. A differential distribution of L1210 cells in the cell cycle resulted from an exposure to DAP and DAPdR. While DAPdR arrested cells in the G1/G0 phase of the cell cycle, DAP appeared to lead to an accumulation of G2/M cells. The diaminopurines were combined with modulatory agents to test the antiproliferative action of the combinations. Deoxycytidine partially rescued the cells from the growth inhibitory action of DAPdR without affecting the growth of DAP-treated cells. When adenine was used, the antiproliferative effect of DAPdR was slightly enhanced while the effect of DAP was completely abolished. 8-Aminoguanosine, a specific inhibitor of purine nucleoside phosphorylase, synergistically potentiated the cytostatic effect of DAPdR. However, this inhibitor did not alter DAP effects. At the biochemical level, the target of DAPdR was ribonucleotide reductase which was in line with a drastic expansion of the dGTP pool in DAPdR-treated cells. In cells exposed to DAP, high levels of DAP riboside triphosphate were measured; concomitantly, the ATP level dropped markedly. Enzymological studies revealed that DAPdR is an excellent substrate of adenosine deaminase giving rise to the formation of deoxyguanosine. DAP was found to be activated in the purine nucleoside phosphorylase reaction and in a phosphoribosyl-pyrophosphate-dependent reaction. The data from this comparative study suggest that DAPdR and DAP possess different toxicity mechanisms. DAPdR and DAP possess different toxicity mechanisms. DAPdR acts as a precursor of deoxyguanosine, and DAP is metabolically activated to DAP-containing ribonucleotide analogs. These different metabolic routes seem to account for the different effects of DAP and DAPdR at the cellular level.

Lack of cross-resistance between cytosine arabinoside and a new halogenated nucleoside analogue, 2-bromo-2'-deoxyadenosine in human acute myeloid leukaemia cells

2-Bromo-2'-deoxyadenosine (BdA) is one of a group of recently synthesised halogenated deoxyadenosine analogues that are relatively resistant to inactivation by adenosine deaminase (ADA). Its activity has been studied in human acute myeloid leukemia (AML) in vitro. In these studies BdA behaved as a cycle-active, phase-active agent that blocked cells at the G1-S transition. It did not exhibit significant cross-resistance with cytosine arabinoside (Ara-C) in either clinical AML samples (from patients who exhibited Ara-C resistance in vivo) or in HL60 in which Ara-C resistance had been induced in vitro. Deoxycytidine kinase levels were not reduced in resistant lines. Erythro-9-(2-hydroxy-3-nonyl)adenine (EHNA), an adenosine deaminase (ADA) inhibitor, with BdA produced a simple additive response without the dramatic synergism reported when it is used with deoxyadenosine. This is consistent with the idea that BdA is a poor substrate for ADA. This group of compounds warrants further investigation to determine their suitability for clinical use, especially in situations where Ara-C resistance is likely to be a problem.

Selective toxicity of deoxyadenosine analogues in human melanoma cell lines

The in vitro toxicities of 19 analogues of deoxyadenosine were tested using a panel of human melanoma cell lines including two lines sensitive to deoxyadenosine and deoxyinosine. The 2-fluoro-, 2-chloro-, 2-bromo- and 2-amino-8-aza derivatives were the most toxic and showed selectivity against deoxyadenosine-sensitive cells. 2-Bromodeoxyadenosine (BrdAdo) and its 5'-phosphate were less potent than the chloro compound but showed the greatest selectivity. In further studies of BrdAdo a third sensitive melanoma line was identified of the eight tested. A treatment time of 24 hr or more was required to develop toxicity to BrAdo; this could be prevented by deoxycytidine or cytidine added to the medium but not by other nucleosides. Flow cytometry showed that BrdAdo blocked cells in the G1 and S phases of the cell cycle. DNA synthesis as judged by thymidine incorporation was rapidly inhibited by BrdAdo to an extent which reflected the sensitivity of the particular cell line; RNA synthesis was less affected. Exposure to BrdAdo for 48 hr induced breaks in the preformed DNA of sensitive but not resistant cells. The results suggest that the toxicity of BrdAdo is associated with prolonged inhibition of DNA synthesis and subsequent DNA fragmentation.

Cytotoxicity of 2-chlorodeoxyadenosine in a human tumor colony-forming assay

We have utilized a human tumor colony forming assay to test the antitumor activity of 2-chlorodeoxyadenosine and to compare its activity with that of 9-beta-D-arabinofuranosyl-2-fluoroadenine, a related analog now in phase I/II clinical trials. The overall in vitro response rate (defined as less than 50% survival of tumor colony forming units) for 2-chlorodeoxyadenosine was: 8% and 23% at 1.0 and 10.0 micrograms/ml as a 1 hour pulse exposure, respectively; 11% and 31% at 1.0 and 10.0 micrograms/ml, as a continuous exposure, respectively. 2-Chlorodeoxyadenosine and 9-beta-D-arabinofuranosyl-2-fluoroadenine did not have identical spectra of antitumor activities in vitro, suggesting that both may be worthy of further clinical trial.

Formation of 3-(2'-deoxyribofuranosyl) and 9-(2'-deoxyribofuranosyl) nucleosides of 8-substituted purines by nucleoside deoxyribosyltransferase

Earlier results suggested that although the N-deoxyribosyltransferase from lactobacilli is a convenient tool for the preparation of analogs of 2'-deoxyadenosine, 8-substituted purines do not act as substrates. However, eight of nine 8-substituted purines that were examined proved to be substrates for the transferase from Lactobacillus leichmannii, and deoxyribonucleosides of four of these bases have been prepared. The substituents at C-8 of the purine greatly affect the rate of deoxyribosyl transfer to the base, and in all cases the rate is slower than transfer to purines lacking an 8-substituent. The 8-substituent also affects the nature of the nucleoside formed. With the electron-donating methyl group at position 8 of adenine, the transferase forms the expected 8-methyl-9-(2'-deoxyribofuranosyl)adenine. However, when purines bearing an electron-withdrawing substituent at the 8-position are used as substrates, the deoxyribosyl moiety is preferentially transferred to N-3 of the base. In the case of 8-trifluoromethyladenine the 3-deoxyribonucleoside is the only product detectable. With 8-bromo or 8-chloroadenine as substrate the 3- and 9-deoxyribonucleosides can both be isolated from the enzymatic reaction mixture. Time course studies indicated that with thymidine and 8-bromoadenine as substrates the 3-deoxyribonucleoside is initially the major product, but that the 9-deoxyribonucleoside becomes the major product after long incubation periods. Negligible interconversion of these nucleosides occurs in the absence of transferase, but conversion in either direction occurs readily in the presence of the enzyme. Significant hydrolysis of pyrimidine and purine deoxyribonucleosides occurs in the presence of the transferase. This was more obvious during the course of reactions involving 8-substituted purines because the slowness of deoxyribosyl transfer required longer incubation periods and larger amounts of enzyme. The hydrolysis is proportional to enzyme concentration, little affected by the nature of the base and is attributed to hydrolysis of a deoxyribosyl derivative of the transferase which is an obligatory intermediate of deoxyribosyl transfer. 8-Trifluoromethyl-3-(2'-deoxyribofuranosyl)adenine, 8-methyl-9-(2'-deoxyribofuranosyl)adenine, and 8-bromo-9-(2'-deoxyribofuranosyl)adenine were tested for their ability to inhibit the growth of CCRF-CEM cells in culture. Unlike the potent 2-halogeno-2'-deoxyadenosine derivatives, these three nucleosides cause less than 50% inhibition at concentrations up to 100 microM.