Chemical synthesis of benzamide adenine dinucleotide: inhibition of inosine monophosphate dehydrogenase (types I and II)

General ADP-Ribosylation Mechanism Based on the Structure of ADP-Ribosyltransferase-Substrate Complexes

ADP-ribosylation is a ubiquitous modification of proteins and other targets, such as nucleic acids, that regulates various cellular functions in all kingdoms of life. Furthermore, these ADP-ribosyltransferases (ARTs) modify a variety of substrates and atoms. It has been almost 60 years since ADP-ribosylation was discovered. Various ART structures have been revealed with cofactors (NAD+ or NAD+ analog). However, we still do not know the molecular mechanisms of ART. It needs to be better understood how ART specifies the target amino acids or bases. For this purpose, more information is needed about the tripartite complex structures of ART, the cofactors, and the substrates. The tripartite complex is essential to understand the mechanism of ADP-ribosyltransferase. This review updates the general ADP-ribosylation mechanism based on ART tripartite complex structures.

Chemical Proteomics Approach for Profiling the NAD Interactome

Nicotinamide adenine dinucleotide (NAD⁺) is a multifunctional molecule. Beyond redox metabolism, NAD⁺ has an equally important function as a substrate for post-translational modification enzymes, the largest family being the poly-ADP-ribose polymerases (PARPs, 17 family members in humans). The recent surprising discoveries of noncanonical NAD (NAD⁺/NADH)-binding proteins suggests that the NAD interactome is likely larger than previously thought; yet, broadly useful chemical tools for profiling and discovering NAD-binding proteins do not exist. Here, we describe the design, synthesis, and validation of clickable, photoaffinity labeling (PAL) probes, 2- and 6-ad-BAD, for interrogating the NAD interactome. We found that 2-ad-BAD efficiently labels PARPs in a UV-dependent manner. Chemical proteomics experiments with 2- and 6-ad-BAD identified known and unknown NAD⁺/NADH-binding proteins. Together, our study shows the utility of 2- and 6-ad-BAD as clickable PAL NAD probes.

Chemoenzymatic Preparation of 4'-Thioribose NAD

This chemoenzymatic procedure describes a strategy for the preparation of 4'-thioribose nicotinamide adenine dinucleotide (S-NAD⁺ ), including chemical synthesis of nicotinamide 4'-riboside (S-NR), recombinant expression and purification of two NAD⁺ biosynthesis enzymes nicotinamide riboside kinase (NRK) and nicotinamide mononucleotide adenylyltransferase (NMNAT), and enzymatic synthesis of S-NAD⁺ . The first basic protocol describes the procedures for introduction of nicotinamide onto 4'-thioribose and subsequent deprotection to generate S-NR as the key intermediate for enzymatically synthesizing S-NAD⁺ . In the second basic protocol, experimental methods are detailed for the production of recombinant human NRK1 and NMNAT1 to catalyze conversion of S-NR to S-NAD⁺ . The third basic protocol presents the enzymatic approach for the generation of S-NAD⁺ from S-NR precursor. © 2019 by John Wiley & Sons, Inc.

Facile chemoenzymatic synthesis of a novel stable mimic of NAD. Chem Sci 2018;9:8337-8342. [PMID: 30568770 PMCID: PMC6256357 DOI: 10.1039/c8sc03899f]

Nicotinamide adenine dinucleotide (NAD+) is an essential cofactor participating in a variety of important enzyme-catalyzed physiological and pathophysiological processes. Analogues of NAD+ provide key and valuable agents for investigating NAD+-dependent enzymes. In this study, we report the preparation of a novel stable NAD+ mimic, 4'-thioribose NAD+ (S-NAD+), using a facile and efficient chemoenzymatic approach. Substrate activity assays indicated the resulting S-NAD+ is chemically inert to human CD38 and sirtuin 2 enzymes, but capable of participating in redox reactions in a manner similar to NAD+. X-ray crystallographic analysis revealed binding of S-NAD+ to the active site of human CD38 and critical residues involved in leaving group activation and catalysis. By more closely mimicking NAD+ in geometry and electrostatics, the generated S-NAD+ offers a unique and important tool that can be extended to study enzymes utilizing NAD+.

NAD+ analog reveals PARP-1 substrate-blocking mechanism and allosteric communication from catalytic center to DNA-binding domains

PARP-1 cleaves NAD⁺ and transfers the resulting ADP-ribose moiety onto target proteins and onto subsequent polymers of ADP-ribose. An allosteric network connects PARP-1 multi-domain detection of DNA damage to catalytic domain structural changes that relieve catalytic autoinhibition; however, the mechanism of autoinhibition is undefined. Here, we show using the non-hydrolyzable NAD⁺ analog benzamide adenine dinucleotide (BAD) that PARP-1 autoinhibition results from a selective block on NAD⁺ binding. Following DNA damage detection, BAD binding to the catalytic domain leads to changes in PARP-1 dynamics at distant DNA-binding surfaces, resulting in increased affinity for DNA damage, and providing direct evidence of reverse allostery. Our findings reveal a two-step mechanism to activate and to then stabilize PARP-1 on a DNA break, indicate that PARP-1 allostery influences persistence on DNA damage, and have important implications for PARP inhibitors that engage the NAD⁺ binding site.

Poly(ADP-ribose) polymerases (PARPs) catalyse ADP-ribose posttranslational modifications using NAD⁺ as a substrate. Here, the authors present the crystal structure of PARP-1 bound to the non-hydrolyzable NAD⁺ analog BAD and provide insights into the mechanism of PARP-1 allosteric regulation.

Novel concept of enzyme selective nicotinamide adenine dinucleotide (NAD)-modified inhibitors based on enzyme taxonomy from the diphosphate conformation of NAD

The dihedral angle θ of the diphosphate part of NAD(P) were investigated to distinguish the differences in the binding-conformation of NAD(P) to enzymes and to create an enzyme taxonomy. Furthermore, new inhibitors with fixed dihedral angles showed that enzymes could recognize the differences in the dihedral angle θ. We suggest the taxonomy and the dihedral angle θ are important values for chemists to consider when designing inhibitors and drugs that target enzymes.

From ribavirin to NAD analogues and back to ribavirin in search for anticancer agents

Ribavirin, a broad-spectrum antiviral agent is used in the clinic alone or in combination with other antivirals and/or interferons. Numerous structural analogues of ribavirin have been developed, among them tiazofurin, which is inactive against viruses but is a potent anticancer drug. Tiazofurin was found to inhibit nicotinamide adenine dinucleotide (NAD)-dependent inosine monophosphate dehydrogenase (IMPDH) after metabolic conversion into tiazofurin adenine dinucleotide (TAD), which binds well but could not serve as IMPDH cofactor. TAD showed high selectivity against human IMPDH vs. other cellular dehydrogenases. Mycophenolic acid (MPA) was even more specific, binding at the cofactor-binding domain of IMPDH. Ribavirin adenine dinucleotide, however, did not show any significant inhibition at the enzymatic level. We synthesized numerous NAD analogues in which natural nicotinamide riboside was replaced by tiazofurin, MPA moiety, or benzamide riboside, and the adenosine moiety as well as the pyrophosphate linker were broadly modified. Some of these compounds were found to be low nanomolar inhibitors of the enzyme and sub-micromolar inhibitors of cancer cell line proliferation. The best were as potent as tyrosine kinase inhibitor gleevec heralded as a ‘magic bullet’ against chronic myelogenous leukemia. In recent years, ribavirin was rediscovered as a potential anticancer agent against number of tumors including leukemia. It was clearly established that its antitumor activity is related to the inhibition of an oncogene, the eukaryotic translation initiation factor (eIF4E).

A general and efficient synthesis of pyridin-2-yl C-ribonucleosides bearing diverse alkyl, aryl, amino, and carbamoyl groups in position 6

An efficient and practical methodology of preparation of 6-substituted pyridin-2-yl C-ribonucleosides was developed. A one-pot two-step addition of 2-lithio-6-bromopyridine to TBS-protected ribonolactone followed by acetylation gave 1beta-(6-bromopyridin-2-yl)-1-O-acetyl-2,3,5-tri-O-(tert-butyldimethylsilyl)-D-ribofuranose in high yield. Its reduction with Et(3)SiH and BF(3) x Et(2)O afforded the desired TBS-protected 6-bromopyridine C-ribonucleoside as pure beta-anomer in good overall yield of 63%. This intermediate was then subjected to a series of palladium catalyzed cross-coupling reactions, aminations and aminocarbonylations to give a series of protected 1beta-(6-alkyl-, 6-aryl-, 6-amino-, and 6-carbamoylpyridin-2-yl)-C-ribonucleosides. Deprotection of silylated nucleosides by Et(3)N x 3HF gave a series of title free C-ribonucleosides (12 examples).

Selective inhibition of nicotinamide adenine dinucleotide kinases by dinucleoside disulfide mimics of nicotinamide adenine dinucleotide analogues

Diadenosine disulfide (5) was reported to inhibit NAD kinase from Listeria monocytogenes and the crystal structure of the enzyme-inhibitor complex has been solved. We have synthesized tiazofurin adenosine disulfide (4) and the disulfide 5, and found that these compounds were moderate inhibitors of human NAD kinase (IC(50)=110 microM and IC(50)=87 microM, respectively) and Mycobacterium tuberculosis NAD kinase (IC(50)=80 microM and IC(50)=45 microM, respectively). We also found that NAD mimics with a short disulfide (-S-S-) moiety were able to bind in the folded (compact) conformation but not in the common extended conformation, which requires the presence of a longer pyrophosphate (-O-P-O-P-O-) linkage. Since majority of NAD-dependent enzymes bind NAD in the extended conformation, selective inhibition of NAD kinases by disulfide analogues has been observed. Introduction of bromine at the C8 of the adenine ring restricted the adenosine moiety of diadenosine disulfides to the syn conformation making it even more compact. The 8-bromoadenosine adenosine disulfide (14) and its di(8-bromoadenosine) analogue (15) were found to be the most potent inhibitors of human (IC(50)=6 microM) and mycobacterium NAD kinase (IC(50)=14-19 microM reported so far. None of the disulfide analogues showed inhibition of lactate-, and inosine monophosphate-dehydrogenase (IMPDH), enzymes that bind NAD in the extended conformation.

Nucleoside-5'-phosphoimidazolides: reagents for facile synthesis of dinucleoside pyrophosphates

A facile method is presented for preparation of dinucleoside pyrophosphate derivatives based on reaction of a nucleoside 5'-monophosphate with carbonyldiimidazole followed by treatment of the resulting nucleoside 5'-phosphoimidazolide with a nucleoside 5'-phosphate. This method is suitable for preparation of pyrophosphates analogous to NAD, FAD, and related natural pyrophosphates. The resulting compounds are useful for mechanistic studies of enzymes that use natural pyrophosphates as co-factors or substrates, and in development of inhibitors that have potential applications as therapeutic agents.

Synthesis of pyrophosphates for in vitro selection of catalytic RNA molecules

Reported here are synthetic routes to pyrophosphates linking riboflavin with various nucleosides. The focus is on a flavin-uracil dinucleotide having a biotin tag on the uracil, a molecule that has potential value in the selection of RNA enzymes that catalyze the template-directed polymerization of RNA in the 3'-to-5' direction, which is the direction opposite that catalyzed by standard protein polymerases. Two detailed procedures are presented to prepare this new compound, as well as one procedure to prepare the new flavin-2,6-diaminopurine dinucleotide.

Synthesis of 4-phenoxybenzamide adenine dinucleotide as NAD analogue with inhibitory activity against enoyl-ACP reductase (InhA) of Mycobacterium tuberculosis

The chemical synthesis of 4-phenoxybenzamide adenine dinucleotide (3), a NAD analogue which mimics isoniazid-NAD adduct and inhibits Mycobacterium tuberculosis NAD-dependent enoyl-ACP reductase (InhA), is reported. The 4-phenoxy benzamide riboside (1) has been prepared as a key intermediate, converted into its 5'-mononucleotide (2), and coupled with AMP imidazolide to give the desired NAD analogue 3. It inhibits InhA with IC50 = 27 microM.

Probing binding requirements of NAD kinase with modified substrate (NAD) analogues

Synthesis of novel NAD(+) analogues that cannot be phosphorylated by NAD kinase is reported. In these analogues the C2' hydroxyl group of the adenosine moiety was replaced by fluorine in the ribo or arabino configuration (1 and 2, respectively) or was inverted into arabino configuration to give compound 3. Compounds 1 and 2 showed inhibition of human NAD kinase, whereas analogue 3 inhibited both the human and Mycobacterium tuberculosis NAD kinase. An uncharged benzamide adenine dinucleotide (BAD) was found to be the most potent competitive inhibitor (K(i)=90 microM) of the human enzyme reported so far.

Novel methylenephosphophosphonate analogues of mycophenolic adenine dinucleotide. Inhibition of inosine monophosphate dehydrogenase

Novel methylenephosphophosphonate analogues of mycophenolic adenine dinucleotide (MAD) have been prepared as potential inhibitors of IMP dehydrogenase. A coupling of the mycophenolic (hydroxymethyl)phosphonate 6 with the phosphitylated adenosine analogue 11 followed by oxidation and deprotection afforded the phosphophosphonate 8. A similar coupling between adenosine (hydroxymethyl)phosphonate 10 and phosphitylated mycophenolic alcohol 5 gave the corresponding phosphophosphonate 13. Both 8 and 13 (Ki = 20-87 nM) were found to be the most potent cofactor type inhibitors of IMP dehydrogenase.

Structure-activity relationships of dinucleotides: Potent and selective agonists of P2Y receptors

Dinucleoside polyphosphates act as agonists on purinergic P2Y receptors to mediate a variety of cellular processes. Symmetrical, naturally occurring purine dinucleotides are found in most living cells and their actions are generally known. Unsymmetrical purine dinucleotides and all pyrimidine containing dinucleotides, however, are not as common and therefore their actions are not well understood. To carry out a thorough examination of the activities and specificities of these dinucleotides, a robust method of synthesis was developed to allow manipulation of either nucleoside of the dinucleotide as well as the phosphate chain lengths. Adenosine containing dinucleotides exhibit some level of activity on P2Y₁ while uridine containing dinucleotides have some level of agonist response on P2Y₂ and P2Y₆. The length of the linking phosphate chain determines a different specificity; diphosphates are most accurately mimicked by dinucleoside triphosphates and triphosphates most resemble dinucleoside tetraphosphates. The pharmacological activities and relative metabolic stabilities of these dinucleotides are reported with their potential therapeutic applications being discussed.

Cytotoxicity and cellular differentiation activity of methylenebis(phosphonate) analogs of tiazofurin and mycophenolic acid adenine dinucleotide in human cancer cell lines

Mycophenolic acid (MPA) is a fungally-derived inhibitor of inosine 5'-monophosphate dehydrogenase (IMPDH). MPA binds IMPDH at the nicotinamide sub-site of the NAD cofactor binding domain leaving the adenosine sub-site empty. In order to improve the binding affinity we synthesized MPA analogs by linking adenosine 5'-methylenebis(phosphonate) with mycophenolic alcohols containing 2-, 4-, and 6-carbon atoms in their aliphatic side chain. Adenine dinucleotide analogs of tiazofurin, selenazofurin and benzamide riboside were synthesized as P1, P2-disubstituted pyrophosphates. Cytotoxicity of each analog was examined in human colon adenocarcinoma HT-29 and erythroleukemia K562 cells, and induction of differentiation in K562 cells by these agents was determined. Mycophenolic acid is currently used as an immunosuppressant but its anticancer action is limited by inactivation due to rapid glucuronidation. The new analogs show resistance to metabolism to inactive species and exhibit enhanced cytotoxicity in tumor cell lines, and therefore could be useful as anticancer agents.

Novel mycophenolic adenine bis(phosphonate) analogues as potential differentiation agents against human leukemia

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.

An examination of the role of asp-177 in the His-Asp catalytic dyad of Leuconostoc mesenteroides glucose 6-phosphate dehydrogenase: X-ray structure and pH dependence of kinetic parameters of the D177N mutant enzyme

The role of Asp-177 in the His-Asp catalytic dyad of glucose 6-phosphate dehydrogenase from Leuconostoc mesenteroides has been investigated by a structural and functional characterization of the D177N mutant enzyme. Its three-dimensional structure has been determined by X-ray cryocrystallography in the presence of NAD(+) and in the presence of glucose 6-phosphate plus NADPH. The structure of a glucose 6-phosphate complex of a mutant (Q365C) with normal enzyme activity has also been determined and substrate binding compared. To understand the effect of Asp-177 on the ionization properties of the catalytic base His-240, the pH dependence of kinetic parameters has been determined for the D177N mutant and compared to that of the wild-type enzyme. The structures give details of glucose 6-phosphate binding and show that replacement of the Asp-177 of the catalytic dyad with asparagine does not affect the overall structure of glucose 6-phosphate dehydrogenase. Additionally, the evidence suggests that the productive tautomer of His-240 in the D177N mutant enzyme is stabilized by a hydrogen bond with Asn-177; hence, the mutation does not affect tautomer stabilization. We conclude, therefore, that the absence of a negatively charged aspartate at 177 accounts for the decrease in catalytic activity at pH 7.8. Structural analysis suggests that the pH dependence of the kinetic parameters of D177N glucose 6-phosphate dehydrogenase results from an ionized water molecule replacing the missing negative charge of the mutated Asp-177 at high pH. Glucose 6-phosphate binding orders and orients His-178 in the D177N-glucose 6-phosphate-NADPH ternary complex and appears to be necessary to form this water-binding site.

The conformation of NADH bound to inosine 5'-monophosphate dehydrogenase determined by transferred nuclear Overhauser effect spectroscopy

Inosine 5'-monophosphate dehydrogenase (IMPDH) catalyzes the oxidation of inosine 5-monophosphate (IMP) to xanthosine 5'-monophosphate (XMP). The reaction proceeds with concomitant conversion of NAD+ to NADH and is the rate-limiting step in the de novo biosynthesis of guanosine nucleotides. IMPDH is a target for numerous chemotherapeutic agents. The conformations of enzyme-bound substrates, enzyme-bound products and enzyme-bound ligands in general, are of interest for the understanding of the catalytic mechanism of the enzyme and the design of new inhibitors. Although several of the chemotherapeutic inhibitors of IMPDH are NAD+ or NADH analogues, no structural data for IMPDH-bound NAD+ (or NADH) are available. In the present work, we have used transferred nuclear Overhauser effect spectroscopy (TRNOESY) to determine the conformation of NADH bound to the active site of human type II IMPDH (IMPDH-h2). The inter-proton distances determined from TRNOESY data indicate that NADH binds to the enzyme active site in an overall extended conformation. The adenosine moiety and the nicotinamide riboside moiety are both in the anti conformation about the glycosidic bond, and both ribose rings are in approximately C4'-exo conformations. The nicotinamide amide group was found to be in a cis conformation. The anti conformation of the nicotinamide riboside moiety is in accord with the preferred conformations of several potent and selective dinucleotide inhibitors and is consistent with that implied by the stereospecificity of hydride transfer in the enzymatic reaction. The implications of this conformation for the catalytic mechanism of IMPDH-h2 are discussed.

Isosteric analogues of nicotinamide adenine dinucleotide derived from furanfurin, thiophenfurin, and selenophenfurin as mammalian inosine monophosphate dehydrogenase (type I and II) inhibitors

Dinucleotides TFAD (6), FFAD (7), and SFAD (8), isosteric NAD analogues derived, respectively, from C-nucleosides 5-beta-d-ribofuranosylthiophene-3-carboxamide (thiophenfurin, 1), 5-beta-d-ribofuranosylfuran-3-carboxamide (furanfurin, 2), and 5-beta-d-ribofuranosylselenophene-3-carboxamide (selenophenfurin, 5), were synthesized as human inosine monophosphate dehydrogenase (IMPDH) type I and II inhibitors. The synthesis was carried out by imidazole-catalyzed coupling of the 5'-monophosphate of 1, 2, and 5 with AMP. These dinucleotides, which are also analogues of thiazole-4-carboxamide adenine dinucleotide (TAD) and selenazole-4-carboxamide adenine dinucleotide (SAD), the active metabolites of the oncolytic C-nucleosides 2-beta-D-ribofuranosylthiazole-4-carboxamide (tiazofurin) and 2-beta-D-ribofuranosylselenazole-4-carboxamide (selenazofurin), were evaluated for their inhibitory potency against recombinant human IMPDH type I and II. The order of inhibitory potency found was SAD > SFAD = TFAD = TAD >> FFAD for both enzyme isoforms. No significant difference was found in inhibition of IMPDH type I and II.

Synthesis of a methylenebis(phosphonate) analogue of mycophenolic adenine dinucleotide: a glucuronidation-resistant MAD analogue of NAD

Mycophenolic alcohol (MPAlc), obtained by reduction of the carboxylic group of mycophenolic acid (MPA), was coupled with 2',3'-O-isopropylideneadenosine 5'-methylenebis(phosphonate) (4) in the presence of diisopropylcarbodiimide (DIC) to give P1-(2',3'-O-isopropylideneadenosin-5'-yl)-P2-(mycophenolic alcohol-6'-yl)methylenebis(phosphonate) (8) in 32% yield. Deisopropy-lidenation of 8 with CF3COOH/H2O afforded the methylenebis(phosphonate) analogue 3 of mycophenolic adenine dinucleotide (MAD). Compound 3, beta-methylene-MAD, was found to be a potent inhibitor of inosine monophosphate dehydrogenase (IMPDH) type II (Ki = 0.3 microM) as well as an inhibitor of growth of K562 cells (IC50 = 1.5 microM). In contrast to MPA and mycophenolic alcohol, beta-methylene-MAD was not converted into the glucuronide when incubated with uridine 5'-diphosphoglucuronyltransferase.

Novel nicotinamide adenine dinucleotide analogues as potential anticancer agents: quest for specific inhibition of inosine monophosphate dehydrogenase

Synthetic nicotinamide adenine dinucleotide (NAD) analogues containing 5-beta-D-ribofuranosylnicotinamide (C-NAD), 6-beta-D-ribofuranosylpicolinamide (C-PAD), 3-beta-D-ribofuranosylbenzamide (BAD), and 2-beta-D-ribofuranosylthiazole-4-carboxamide (TAD) in place of the nicotinamide riboside moiety are described and evaluated as potential inhibitors of inosine monophosphate dehydrogenase (IMPDH). TAD and BAD showed potent inhibitory activity against the enzyme in the form of pyrophosphates, as well as metabolically stable methylene- and difluoromethylenebis(phosphonate)s. Fluorination at the C2' (ribo and arabino configuration) and C3' (ribo) of the adenosine moiety of TAD afforded analogues highly potent against IMPDH, but weakly active against alcohol dehydrogenase. With the exception of the methylenebis(phosphonate) analogue of TAD compounds containing a methylene bridge were poor inhibitors of growth of K562 cells. On the other hand, NAD analogues containing difluoromethylene linkage were highly effective in inhibition of K562 cell growth, as well as potent inducers of K562 cell differentiation. Such compounds, therefore, may be of potential therapeutic interest.

Synthesis of nonhydrolyzable analogues of thiazole-4-carboxamide and benzamide adenine dinucleotide containing fluorine atom at the C2' of adenine nucleoside: induction of K562 differentiation and inosine monophosphate dehydrogenase inhibitory activity

Thiazole-4-carboxamide adenine dinucleotide (TAD) analogue 7 containing a fluorine atom at the C2' arabino configuration of the adenine nucleoside moiety was found to be a potent inducer of differentiation of K562 erythroid leukemia cells. This finding prompted us to synthesize its hydrolysis-resistant methylenebis(phosphonate) and difluoromethylenebis(phosphonate) analogues 8 and 9, respectively. Since both TAD and benzamide adenine dinucleotide (BAD) are potent inhibitors of inosine monophosphate dehydrogenase (IMPDH), the corresponding fluorine-substituted methylenebis(phosphonate) analogue 12 of BAD was also synthesized. Thus, 9-(2-deoxy-2-fluoro-beta-D-arabinofuranosyl)adenine (13) was converted in five steps into the corresponding methylenebis(phosphonate) analogue 18. Dehydration of 18 with DCC led to the formation of the bicyclic trisanhydride intermediate 19a, which upon reaction with 2',3'-O-isopropylidenetiazofurin (20) or -benzamide riboside (21) followed by hydrolysis and deprotection afforded the desired methylene-bridged dinucleotides 8 and 12, respectively. The similar displacement of the 5'-mesyl function of 2',3'-O-isopropylidene-5'-O-mesyltiazofurin (24) with the difluoromethylenebis(phosphonic acid) derivative gave the phosphonate 25 which was coupled with 13 to afford 26. The desired difluoromethylenebis(phosphonate) analogue 9 was obtained by deprotection with Dowex 50/H+. This compound as well as beta-CF2-TAD (4) showed improved differentiation-inducing activity over beta-CH2-TAD (3), whereas analogues containing the -CH2-linkage (8 and 12) were inactive.

The practical synthesis of a methylenebisphosphonate analogue of benzamide adenine dinucleotide: inhibition of human inosine monophosphate dehydrogenase (type I and II)

beta-Methylene-BAD (8), a nonhydrolyzable analogue of benzamide adenine dinucleotide (BAD), was synthesized as potential inhibitor of human inosine monophosphate dehydrogenase (IMPDH). Treatment of 2',3'-O-isopropylideneadenosine 5'-methylenebisphosphonate (15) with DCC afforded P1,P4-bis(2',3'-O-isopropylideneadenosine) 5'-P1,P2:P3,P4-dimethylenetetrakisphosphonate (17). This compound was further converted with DCC to an active intermediate 18 which upon reaction with 3-(2',3'-O-isopropylidene-beta-D-ribofuranosyl)benzamide (19) gave, after hydrolysis and deisopropylidenation, the desired beta-methylene-BAD (8) in 95% yield. In a similar manner, treatment of 18 with 2',3'-O-isopropylidenetiazofurin (21) followed by hydrolysis and deprotection afforded beta-methylene-TAD (5) in 91% yield. Compound 8 (IC50 = 0.665 microM) was found to be a 6-8 times less potent inhibitor of IMPDH than 5 (IC50 = 0.107 microM) and was almost equally potent against IMPDH type I and type II. Although TAD and beta-methylene-TAD were bound by LADH with the same affinity, the binding affinity of 8 toward LADH (Ki = 333 microM) was found to be 50-fold lower than that of the parent pyrophosphate 7 (Ki = 6.3 microM).