N2' -Branched Acyclic Nucleoside Phosphonates Containing 9-Deazahypoxanthine as Inhibitors of Plasmodium falciparum 6-Oxopurine Phosphoribosyltransferase

Twelve N2'-branched acyclic nucleoside phosphonates and bisphosphonates were synthesized as potential inhibitors of Plasmodium falciparum hypoxanthine-guanine-xanthine phosphoribosyltransferase (PfHGXPRT), the key enzyme in the purine salvage pathway for production of purine nucleotides. The chemical structures were designed with the aim to study selectivity of the inhibitors for PfHGXPRT over human HGPRT. The newly prepared compounds contain 9-deazahypoxanthine connected to a phosphonate group via a five-atom-linker bearing a nitrogen atom (N2') as a branching point. All compounds, with the additional phosphonate group(s) in the second aliphatic linker attached to N2' atom, were low micromolar inhibitors of PfHGXPRT with low to modest selectivity for the parasite enzyme over human HGPRT. The effect of the addition of different chemical groups/linkers to N2' atom on the inhibition constants and selectivity is discussed.

Synthesis and anti-trypanosomal evaluation of novel N-branched acyclic nucleoside phosphonates bearing 7-aryl-7-deazapurine nucleobase

A series of novel 7-aryl-7-deazaadenine-based N-branched acyclic nucleoside phosphonates (aza-ANPs) has been prepared using the optimized Suzuki cross-coupling reaction as the key synthetic step. The final free phosphonates 15a-h were inactive, due to their inefficient transport across cell membranes, but they inhibited Trypanosoma brucei adenine phosphoribosyltransferase (TbrAPRT1) with K_i values of 1.7-14.1 μM. The corresponding phosphonodiamidate prodrugs 14a-h exhibited anti-trypanosomal activity in the Trypanosoma brucei brucei cell-based assay with EC₅₀ values in the range of 0.58-6.8 μM. 7-(4-Methoxy)phenyl-7-deazapurine derivative 14h, containing two phosphonate moieties, was the most potent anti-trypanosomal agent from the series, with EC₅₀ = 0.58 μM and SI = 16. Finally, phosphonodiamidate prodrugs 14a-h exerted low micromolar cytotoxicity against leukemia and/or cancer cell lines tested.

Stereo-Defined Acyclic Nucleoside Phosphonates are Selective and Potent Inhibitors of Parasite 6-Oxopurine Phosphoribosyltransferases

Pathogens such as Plasmodium and Trypanosoma spp. are unable to synthesize purine nucleobases. They rely on the salvage of these purines and their nucleosides from the host cell to synthesize the purine nucleotides required for DNA/RNA production. The key enzymes in this pathway are purine phosphoribosyltransferases (PRTs). Here, we synthesized 16 novel acyclic nucleoside phosphonates, 12 with a chiral center at C-2', and eight bearing a second chiral center at C-6'. Of these, bisphosphonate (S,S)-48 is the most potent inhibitor of the Plasmodium falciparum and P. vivax 6-oxopurine PRTs and the most potent inhibitor of two Trypanosoma brucei (Tbr) 6-oxopurine PRTs yet discovered, with K_i values as low as 2 nM. Crystal structures of (S,S)-48 in complex with human and Tbr 6-oxopurine PRTs show that the inhibitor binds to the enzymes in different conformations, providing an explanation for its potency and selectivity (i.e., 35-fold in favor of the parasite enzymes).

C1'-Branched acyclic nucleoside phosphonates mimicking adenosine monophosphate: Potent inhibitors of Trypanosoma brucei adenine phosphoribosyltransferase

Some pathogens, including parasites of the genus Trypanosoma causing Human and Animal African Trypanosomiases, cannot synthesize purines de novo and they entirely rely on the purine salvage pathway (PSP) for their nucleotide generation. Thus, their PSP enzymes are considered as promising drug targets, sparsely explored so far. Recently, a significant role of acyclic nucleoside phosphonates (ANPs) as inhibitors of key enzymes of PSP, namely of 6-oxopurine phosphoribosyltransferases (PRTs), has been discovered. Herein, we designed and synthesized two series of new ANPs branched at the C1' position as mimics of adenosine monophosphate. The novel ANPs efficaciously inhibited Trypanosoma brucei adenine PRT (TbrAPRT1) activity in vitro and it was shown that the configuration on the C1' chiral centre strongly influenced their activity: the (R)-enantiomers proved to be more potent compared to the (S)-enantiomers. Two ANPs, with K_i values of 0.39 μM and 0.57 μM, represent the most potent TbrAPRT1 inhibitors reported to date and they are an important tool to further study purine metabolism in various parasites.

Helicobacter pylori Xanthine-Guanine-Hypoxanthine Phosphoribosyltransferase-A Putative Target for Drug Discovery against Gastrointestinal Tract Infections

Helicobacter pylori (Hp) is a human pathogen that lives in the gastric mucosa of approximately 50% of the world's population causing gastritis, peptic ulcers, and gastric cancer. An increase in resistance to current drugs has sparked the search for new Hp drug targets and therapeutics. One target is the disruption of nucleic acid production, which can be achieved by impeding the synthesis of 6-oxopurine nucleoside monophosphates, the precursors of DNA and RNA. These metabolites are synthesized by Hp xanthine-guanine-hypoxanthine phosphoribosyltransferase (XGHPRT). Here, nucleoside phosphonates have been evaluated, which inhibit the activity of this enzyme with K_i values as low as 200 nM. The prodrugs of these compounds arrest the growth of Hp at a concentration of 50 μM in cell-based assays. The kinetic properties of HpXGHPRT have been determined together with its X-ray crystal structure in the absence and presence of 9-[(N-3-phosphonopropyl)-aminomethyl-9-deazahypoxanthine, providing a basis for new antibiotic development.

Nucleotide analogues containing a pyrrolidine, piperidine or piperazine ring: Synthesis and evaluation of inhibition of plasmodial and human 6-oxopurine phosphoribosyltransferases and in vitro antimalarial activity

Parasites of the Plasmodium genus are unable to produce purine nucleotides de novo and depend completely on the salvage pathway. This fact makes plasmodial hypoxanthine-guanine-(xanthine) phosphoribosyltransferase [HG(X)PRT] a valuable target for development of antimalarial agents. A series of nucleotide analogues was designed, synthesized and evaluated as potential inhibitors of Plasmodium falciparum HGXPRT, P. vivax HGPRT and human HGPRT. These novel nucleoside phosphonates have a pyrrolidine, piperidine or piperazine ring incorporated into the linker connecting the purine base to a phosphonate group(s) and exhibited a broad range of K_i values between 0.15 and 72 μM. The corresponding phosphoramidate prodrugs, able to cross cell membranes, have been synthesized and evaluated in a P. falciparum infected human erythrocyte assay. Of the eight prodrugs evaluated seven exhibited in vitro antimalarial activity with IC₅₀ values within the range of 2.5-12.1 μM. The bis-phosphoramidate prodrug 13a with a mean (SD) IC₅₀ of 2.5 ± 0.7 μM against the chloroquine-resistant P. falciparum W2 strain exhibited low cytotoxicity in the human hepatocellular liver carcinoma (HepG2) and normal human dermal fibroblasts (NHDF) cell lines at a concentration of 100 μM suggesting good selectivity for further structure-activity relationship investigations.

Crystal structures of Trypanosoma brucei hypoxanthine - guanine - xanthine phosphoribosyltransferase in complex with IMP, GMP and XMP

The 6-oxopurine phosphoribosyltransferases (PRTs) are drug targets for the treatment of parasitic diseases. This is due to the fact that parasites are auxotrophic for the 6-oxopurine bases relying on salvage enzymes for the synthesis of their 6-oxopurine nucleoside monophosphates. In Trypanosoma brucei, the parasite that is the aetiological agent for sleeping sickness, there are three 6-oxopurine PRT isoforms. Two are specific for hypoxanthine and guanine, whilst the third, characterized here, uses all three naturally occurring bases with similar efficiency. Here, we have determined crystal structures for TbrHGXPRT in complex with GMP, XMP and IMP to investigate the structural basis for substrate specificity. The results show that Y201 and E208, not commonly observed within the purine binding pocket of 6-oxopurine PRTs, contribute to the versatility of this enzyme. The structures further show that a nearby water can act as an adaptor to facilitate the binding of XMP and GMP. When GMP binds, a water can accept a proton from the 2-amino group but when XMP binds, the equivalent water can donate its proton to the 2-oxo group. However, when IMP is bound, no water molecule is observed at that location. DATABASE: Coordinates and structure factors were submitted to the Protein Data Bank and have accession codes of 6MXB, 6MXC, 6MXD and 6MXG for the TbrHGXPRT.XMP complex, TbrHGXPRT.GMP complex, TbrHGXPRT.IMP complex, and TbrHGPRT.XMP complex, respectively.

Evaluation of the Trypanosoma brucei 6-oxopurine salvage pathway as a potential target for drug discovery

Due to toxicity and compliance issues and the emergence of resistance to current medications new drugs for the treatment of Human African Trypanosomiasis are needed. A potential approach to developing novel anti-trypanosomal drugs is by inhibition of the 6-oxopurine salvage pathways which synthesise the nucleoside monophosphates required for DNA/RNA production. This is in view of the fact that trypanosomes lack the machinery for de novo synthesis of the purine ring. To provide validation for this approach as a drug target, we have RNAi silenced the three 6-oxopurine phosphoribosyltransferase (PRTase) isoforms in the infectious stage of Trypanosoma brucei demonstrating that the combined activity of these enzymes is critical for the parasites’ viability. Furthermore, we have determined crystal structures of two of these isoforms in complex with several acyclic nucleoside phosphonates (ANPs), a class of compound previously shown to inhibit 6-oxopurine PRTases from several species including Plasmodium falciparum. The most potent of these compounds have K_i values as low as 60 nM, and IC₅₀ values in cell based assays as low as 4 μM. This data provides a solid platform for further investigations into the use of this pathway as a target for anti-trypanosomal drug discovery.

Human African Trypanosomiasis (HAT) is a life-threatening infectious disease caused by the protozoan parasite, Trypanosoma brucei. Current treatments suffer from low efficacy, toxicity issues and complex medication regimens. Moreover, an alarming number of these parasites are demonstrating resistance to current drugs. For these reasons, there is a renewed effort to develop new classes of modern therapeutics based upon the unique T. brucei cellular processes. One potential new drug target is 6-oxopurine phosphoribosyltransferase (PRTase), an enzyme central to the purine salvage pathway and whose activity is critical for the production of the nucleotides (GMP and IMP) required for DNA/RNA synthesis within this protozoan parasite. We demonstrated that T. brucei encodes two isoforms of hypoxanthine-guanine PRTases (HGPRT) and one hypoxanthine-guanine-xanthine PRTase (HGXPRT). The concurrent activity of these enzymes is required for the normal cell growth in vitro. Moreover, acyclic nucleoside phosphonates represent a promising class of potent and selective compounds as they inhibit the enzymes with K_i values in nanomolar range and exert cytotoxic effects on T. brucei cells grown in vitro with EC₅₀ values in the single digit micromolar range. Our results provide a new foundation for further investigations of these compounds in vivo and suggest that 6-oxopurine salvage pathway represents a possible target for future drug discovery efforts directed at eliminating HAT.

Design of Plasmodium vivax Hypoxanthine-Guanine Phosphoribosyltransferase Inhibitors as Potential Antimalarial Therapeutics

Plasmodium falciparum (Pf) and Plasmodium vivax (Pv) are the foremost causative agents of malaria. Due to the development of resistance to current antimalarial medications, new drugs for this parasitic disease need to be discovered. The activity of hypoxanthine-guanine-[xanthine]-phosphoribosyltransferase, HG[X]PRT, is reported to be essential for the growth of both of these parasites, making it an excellent target for antimalarial drug discovery. Here, we have used rational structure-based methods to design an inhibitor, [3R,4R]-4-guanin-9-yl-3-((S)-2-hydroxy-2-phosphonoethyl)oxy-1-N-(phosphonopropionyl)pyrrolidine, of PvHGPRT and PfHGXPRT that has K_i values of 8 and 7 nM, respectively, for these two enzymes. The crystal structure of PvHGPRT in complex with this compound has been determined to 2.85 Å resolution. The corresponding complex with human HGPRT was also obtained to allow a direct comparison of the binding modes of this compound with the two enzymes. The tetra-(ethyl l-phenylalanine) tetraamide prodrug of this compound was synthesized, and it has an IC₅₀ of 11.7 ± 3.2 μM against Pf lines grown in culture and a CC₅₀ in human A549 cell lines of 102 ± 11 μM, thus giving it a ∼10-fold selectivity index.

N-Branched acyclic nucleoside phosphonates as monomers for the synthesis of modified oligonucleotides

Protected N-branched nucleoside phosphonates containing adenine and thymine bases were prepared as the monomers for the introduction of aza-acyclic nucleotide units into modified oligonucleotides. The phosphotriester and phosphoramidite methods were used for the incorporation of modified and natural units, respectively. The solid phase synthesis of a series of nonamers containing one central modified unit was successfully performed in both 3'→5' and 5'→3' directions. Hybridization properties of the prepared oligoribonucleotides and oligodeoxyribonucleotides were evaluated. The measurement of thermal characteristics of the complexes of modified nonamers with the complementary strand revealed a considerable destabilizing effect of the introduced units. We also examined the substrate/inhibitory properties of aza-acyclic nucleoside phosphono-diphosphate derivatives (analogues of nucleoside triphosphates) but neither inhibition of human and bacterial DNA polymerases nor polymerase-mediated incorporation of these triphosphate analogues into short DNA was observed.

Antimalarial activity of prodrugs of N-branched acyclic nucleoside phosphonate inhibitors of 6-oxopurine phosphoribosyltransferases

Acyclic nucleoside phosphonates (ANPs) that contain a 6-oxopurine base are good inhibitors of the human and Plasmodium falciparum 6-oxopurine phosphoribosyltransferases (PRTs), key enzymes of the purine salvage pathway. Chemical modifications, based on the crystal structures of several inhibitors in complex with the human PRTase, led to the design of a new class of inhibitors--the aza-ANPs. Because of the negative charges of the phosphonic acid moiety, their ability to cross cell membranes is, however, limited. Thus, phosphoramidate prodrugs of the aza-ANPs were prepared to improve permeability. These prodrugs arrest parasitemia with IC50 values in the micromolar range against Plasmodium falciparum-infected erythrocyte cultures (both chloroquine-sensitive and chloroquine-resistant Pf strains). The prodrugs exhibit low cytotoxicity in several human cell lines. Thus, they fulfill two essential criteria to qualify them as promising antimalarial drug leads.

First Crystal Structures of Mycobacterium tuberculosis 6-Oxopurine Phosphoribosyltransferase: Complexes with GMP and Pyrophosphate and with Acyclic Nucleoside Phosphonates Whose Prodrugs Have Antituberculosis Activity

Human tuberculosis is a chronic infectious disease affecting millions of lives. Because of emerging resistance to current medications, new therapeutic drugs are needed. One potential new target is hypoxanthine-guanine phosphoribosyltransferase (MtHGPRT), a key enzyme of the purine salvage pathway. Here, newly synthesized acyclic nucleoside phosphonates (ANPs) have been shown to be competitive inhibitors of MtHGPRT with Ki values as low as 0.69 μM. Prodrugs of these compounds arrest the growth of a virulent strain of M. tuberculosis with MIC50 values as low as 4.5 μM and possess low cytotoxicity in mammalian cells (CC50 values as high as >300 μM). In addition, the first crystal structures of MtHGPRT (2.03-2.76 Å resolution) have been determined, three of these in complex with novel ANPs and one with GMP and pyrophosphate. These data provide a solid foundation for the further development of ANPs as selective inhibitors of MtHGPRT and as antituberculosis agents.

Inhibition of the Escherichia coli 6-oxopurine phosphoribosyltransferases by nucleoside phosphonates: potential for new antibacterial agents

Escherichia coli (Ec) cells possess two purine salvage enzymes: xanthine-guanine phosphoribosyltransferase (XGPRT) and hypoxanthine phosphoribosyltransferase (HPRT). EcXGPRT shares a common structural feature with other members of this family, a flexible loop that closes over the active site during catalysis. The replacement of six of these amino acids by alanine has no effect on the Km for the two substrates. However, the Ki for the nucleoside monophosphate increases by 27-fold, and the kcat is reduced by ∼200-fold. Nucleoside phosphonates (NP) are good inhibitors of EcXGPRT and EcHPRT, with Ki values as low as 10 nM. In the absence of the flexible loop, these values increase by 5- to 30-fold, indicating the importance of the loop for high-affinity inhibition. Crystal structures of two NPs in complex with EcXGPRT explain the tight binding. Prodrugs of NPs with low Ki values for EcXGPRT or EcHPRT exhibit IC50 values between 5 and 23 μM against Mycobacterium tuberculosis in cell-based assays, suggesting that these compounds are therapeutic leads against pathogenic bacteria.

Acyclic nucleoside phosphonates with a branched 2-(2-phosphonoethoxy)ethyl chain: efficient synthesis and antiviral activity

Series of novel acyclic nucleoside phosphonates (ANPs) with various nucleobases and 2-(2-phosphonoethoxy)ethyl (PEE) chain bearing various substituents in β-position to the phosphonate moiety were prepared. The influence of structural alternations on antiviral activity was studied. Several derivatives exhibit antiviral activity against HIV and vaccinia virus (middle micromolar range), HSV-1 and HSV-2 (lower micromolar range) and VZV and CMV (nanomolar range), although the parent unbranched PEE-ANPs are inactive. Adenine as a nucleobase and the methyl group attached to the PEE chain proved to be a prerequisite to afford pronounced antiviral activity.

Plasmodium vivax hypoxanthine-guanine phosphoribosyltransferase: a target for anti-malarial chemotherapy

The malarial parasite, Plasmodium vivax (Pv), causes a serious infectious disease found primarily in Asia and the Americas. For protozoan parasites, 6-oxopurine phosphoribosyltransferases (PRTases) provide the only metabolic pathway to synthesize the purine nucleoside monophosphates essential for DNA/RNA production. We have purified the recombinant Pv 6-oxopurine (PRTase) and compared its properties with the human and Pf enzymes. The Pv enzyme uses hypoxanthine and guanine with similar catalytic efficiency to the Pf enzyme but xanthine is not a substrate, hence we identify this enzyme as PvHGPRT. Mass spectrometry suggests that PvHGPRT contains bound magnesium ions that are removed by EDTA resulting in loss of activity. However, the addition of Mg(2+) restores activity. Acyclic nucleoside phosphonates (ANPs) are good inhibitors of PvHGPRT having K(i) values as low as 3 microM. These compounds can form the basis for the design of new drugs aimed at combating malaria caused by Pv.

Inhibition of hypoxanthine-guanine phosphoribosyltransferase by acyclic nucleoside phosphonates: a new class of antimalarial therapeutics

The purine salvage enzyme hypoxanthine-guanine-xanthine phosphoribosyltransferase (HGXPRT) is essential for purine nucleotide and hence nucleic acid synthesis in the malaria parasite, Plasmodium falciparum. Acyclic nucleoside phosphonates (ANPs) are analogues of the nucleotide product of the reaction, comprising a purine base joined by a linker to a phosphonate moiety. K(i) values for 19 ANPs were determined for Pf HGXPRT and the corresponding human enzyme, HGPRT. Values for Pf HGXPRT were as low as 100 nM, with selectivity for the parasite enzyme of up to 58. Structures of human HGPRT in complex with three ANPs are reported. On binding, a large mobile loop in the free enzyme moves to partly cover the active site. For three ANPs, the IC(50) values for Pf grown in cell culture were 1, 14, and 46 microM, while the cytotoxic concentration for the first compound was 489 microM. These results provide a basis for the design of potent and selective ANP inhibitors of Pf HGXPRT as antimalarial drug leads.

Pronounced in vitro and in vivo antiretroviral activity of 5-substituted 2,4-diamino-6-[2-(phosphonomethoxy)ethoxy] pyrimidines

OBJECTIVES

To discover new potent and selective anti-human immunodeficiency virus (HIV) acyclic nucleoside phosphonate (ANP) drugs with in vivo antiretroviral activity.

METHODS

New acyclic pyrimidine nucleoside phosphonate derivatives that mimic the structure of the anti-HIV purine nucleoside phosphonates 9-(2-phosphonylmethoxyethyl)adenine (PMEA, adefovir) and (R)-9-(2-phosphonylmethoxypropyl)adenine (PMPA, tenofovir) were designed by linking the acyclic side chain of the ANPs through an ether bond to the C-6 position instead of the N-1 position of the pyrimidine ring. The compounds were evaluated against HIV and Moloney murine sarcoma virus (MSV) in cell culture, including a broad variety of HIV-1 clade clinical isolates and relevant mutant (drug-resistant) HIV-1 isolates. Their antiviral activities were correlated and investigated in an in vivo model consisting of MSV-infected newborn mice. MSV-induced tumour formation and associated death were recorded in drug-treated animals.

RESULTS

Several 5-substituted 6-[2-(phosphonomethoxy)ethoxy]-2,4-diaminopyrimidine (PMEO-DAPy) analogues were found to inhibit a broad variety of HIV-1 clinical isolates. They showed a more favourable cross-resistance profile to mutant virus isolates than adefovir and tenofovir. There was a close correlation between inhibition of MSV in C3H/3T3 cells and inhibition of HIV-1 in CEM cells. The PMEO-DAPy derivatives potently inhibited MSV-induced tumour cell formation in newborn mice. The 5-methyl analogue PMEO-5-Me-DAPy proved markedly more inhibitory to MSV-induced tumour cell formation and associated animal death than its unsubstituted parent PMEO-DAPy derivative. When compared with adefovir, PMEO-5-Me-DAPy was less toxic and more antivirally active in MSV-infected mice.

CONCLUSIONS

PMEO-5-Me-DAPy deserves further (pre)clinical investigations as a candidate anti-HIV drug.

Antiviral potential of a new generation of acyclic nucleoside phosphonates, the 6-[2-(phosphonomethoxy)alkoxy]-2,4-diaminopyrimidines

Three acyclic nucleoside phosphonates (ANPs) have been formally approved for clinical use in the treatment of 1) cytomegalovirus retinitis in AIDS patients (cidofovir, by the intravenous route), 2) chronic hepatitis B virus (HBV) infections (adefovir dipivoxil, by the oral route), and 3) human immunodeficiency virus (HIV) infections (tenofovir disoproxil fumarate, by the oral route). The activity spectrum of cidofovir {(S)- 1-[3-hydroxy-2-(phosphonomethoxy)propyl]cytosine [(S)-HPMPC)]}, like that of (S)-HPMPA [(S)-9-[3-hydroxy-2-(phosphonomethoxy)propyl]adenine) and (S)-HPMPDAP [(S)-9-[3-hydroxy-2-(phosphonomethoxy)propyl]-2, 6-diaminopurine), encompasses a broad spectrum of DNA viruses, including polyoma-, papilloma-, adeno-, herpes-, and poxviruses. Adefovir {9-[2-(phosphonomethoxy)ethyl]adenine (PMEA)} and tenofovir [(R)-9-[2-(phosphonomethoxy) propyl]adenine [(R)-PMPA)]} are particularly active against retroviruses (ie., HIV) and hepadnaviruses (ie., HBV); additionally, PMEA also shows activity against herpes- and poxviruses. We have recently identified a new class of ANPs, namely 6-[2-(phosphonomethoxy)alkoxy]-2,4-diaminopyrimidines, named, in analogy with their alkylpurine counterparts, HPMPO-DAPy, PMEO-DAPy, and (R)-PMPO-DAPy. These compounds exhibit an antiviral activity spectrum and potency that is similar to that of (S)-HPMPDAP, PMEA, and (R)-PMPA, respectively. Thus, PMEO-DAPy and (R)-PMPO-DAPy, akin to PMEA and (R)-PMPA, proved particularly active against HIV- 1, HIV-2, and the murine retrovirus Moloney sarcoma virus (MSV). PMEO-DAPy and (R)-PMPO-DAPy also showed potent activity against both wild-type and lamivudine-resistant strains of HBV. HPMPO-DAPy was found to inhibit different poxviruses (ie., vaccinia, cowpox, and orf) at a similar potency as cidofovir. HPMPO-DAPy also proved active against adenoviruses. In vivo, HPMPO-DAPy proved equipotent to cidofovir in suppressing vaccinia virus infection (tail lesion formation) in immunocompetent mice and promoting healing of disseminated vaccinia lesions in athymic-nude mice. The 6-[2-(phosphonomethoxy)alkoxy]-2,4-diaminopyrimidines offer substantial potential for the treatment of a broad range of retro-, hepadna-, herpes-, adeno-, and poxvirus infections.

Novel acyclic nucleoside phosphonate analogues with potent anti-hepatitis B virus activities

Novel acyclic nucleoside phosphonates with a pyrimidine base preferentially containing an amino group at C-2 and C-4 and a 2-(phosphonomethoxy)ethoxy or (R)-2-(phosphonomethoxy)propoxy group at C-6 selectively inhibit the replication of wild-type and lamivudine-resistant hepatitis B viruses. The activity of the most potent compounds was comparable to that of adefovir.

6-[2-phosphonomethoxy)alkoxy]-2,4-diaminopyrimidines: a new class of acyclic pyrimidine nucleoside phosphonates with antiviral activity

Acyclic nucleoside phosphonate derivatives containing a pyrimidine base preferably bearing amino groups at C-2 and C-4 (DAPym), and linked at the C-6 position to (S)-[3-hydroxy-2-(phosphonomethoxy)propoxy] (HPMPO), 2-(phosphonomethoxy) ethoxy (PMEO) or (R)-[2-(phosphonomethoxy)propoxy] (PMPO), display an antiviral sensitivity spectrum that closely mimic that of the parental (S)-HPMP-, PME- and (R)-PMP-purine derivatives. Several PMEO-DAPym derivatives proved as potent as PMEA (adefovir) and (R)-PMPA (tenofovir) in inhibiting Moloney murine sarcoma virus (MSV)-induced tumor formation in newborn NMRI mice. The HPMPO-, PMEO- and PMPO-DAPym derivatives represent a novel well-defined subclass among the acyclic nucleoside phosphonates endowed with potent and selective antiviral activity.

Synthesis and antiviral activity of 2,4-diamino-5-cyano-6-[2-(phosphonomethoxy)ethoxy]pyrimidine and related compounds

Synthesis of 2,4-diamino-5-cyano-6-[[(diisopropoxyphosphoryl)methoxy]ethoxy]pyrimidine was based on the formation of the pyrimidine ring by cyclization followed by modification of the side chain by alkylation. The 5-cyano group was also transformed to a 5-formyl and 5-hydroxymethyl group by reduction. As a side product an unexpected dimer was formed. Resulting compounds were converted to the free phosphonic acids by treatment with bromotrimethylsilane followed by hydrolysis. The 5-cyano and 5-formyl derivatives showed pronounced antiretroviral activity, comparable to that of the reference drugs adefovir and tenofovir.

5-Substituted-2,4-diamino-6-[2-(phosphonomethoxy)ethoxy]pyrimidinesAcyclic Nucleoside Phosphonate Analogues with Antiviral Activity

2,4-Diamino-6-hydroxypyrimidines substituted in position 5 by an allyl, benzyl, cyanomethyl, ethoxycarbonylmethyl, phenyl, cyclopropyl, or methyl group were prepared either by C5-alkylation or by formation of the pyrimidine ring by cyclization. Their alkylation with 2-[(diisopropoxyphosphoryl)methoxy]ethyl tosylate afforded N1- and O6-regioisomers that were separated and converted to the free phosphonic acids by treatment with bromotrimethylsilane followed by hydrolysis. Reaction of 2,4-diamino-6-[[(diisopropoxyphosphoryl)methoxy]ethoxy]pyrimidine with elemental bromine, N-chloro-, or N-iodosuccinimide gave the corresponding phosphorus-protected 5-bromo-, 5-chloro-, and 5-iodo derivatives, respectively. Their deprotection afforded 2,4-diamino-5-bromo- and -5-chloro-6-[2-(phosphonomethoxy)ethoxy]pyrimidines. 2,4-Diamino-5-methyl-6-[2-(phosphonomethoxy)ethoxy]pyrimidine was synthesized also by cross-coupling of the 5-bromo compound with AlMe(3), followed by deprotection. The compounds showed poor, if any, inhibitory activity against DNA viruses such as herpes simplex virus type 1 and type 2, cytomegalovirus, varicella-zoster virus, and vaccinia virus. In contrast, several 5-substituted 2,4-diaminopyrimidine derivatives markedly inhibited retrovirus replication in cell culture. The 5-methyl derivative was exquisitely inhibitory to human immunodeficiency virus and Moloney murine sarcoma virus-induced cytopathicity in cell culture (EC(50) approximately 0.00018 mumol/mL) but also cytostatic to CEM cell cultures. In contrast, the 5-halogen-substituted derivatives showed pronounced antiretroviral activity (EC(50) = 0.0023-0.0110 mumol/mL), comparable to that of the reference drugs adefovir and tenofovir, but were devoid of measurable toxicity at 0.3 mumol/mL.

Cytostatic 6-Arylpurine Nucleosides V. Synthesis of 8-Substituted 6-Phenylpurine Ribonucleosides

Regioselective Suzuki-Miyaura reaction of 8-bromo-6-iodo-9-(2,3,5-tri-O-acetyl-β-D-ribofuranosyl)purine with phenylboronic acid gave 8-bromo-6-phenylpurine derivative that was used for cross-coupling reactions (with PhB(OH)2, Me3Al, Et3Al, BnZnCl) or nucleophilic substitutions (with NaOH, NaOMe, NH3, NHMe2 or thiourea). A series of 8-X-substituted 6-phenyl-9-(β-D-ribofuranosyl)purines (X = Ph, Me, Et, Bn, OH, OMe, NH2, NMe2, SH) was prepared in this way directly or after deprotection. None of the title nucleosides exhibited any considerable cytostatic activity.

7-(1-Cyanobut-3-en-1-yl)-N6-[(N,N-dimethylamino)methylene]adenine as a Starting Compound for the Synthesis of α-Branched Acyclic Analogues of N7-Isomer of Adenosine

7-(1-Cyanobut-3-en-1-yl)-N6-[(N,N-dimethylamino)methylene]adenine (2) was used as a multipurpose starting material for the synthesis of a series of α-branched acyclic analogues of N7-isomer of adenosine by further reactions on allyl (hydroxylation) and/or cyano group (hydrolysis, methanolysis or tetrazole formation).

Synthesis of Some "Abbreviated" NAD+ Analogues

Four new "abbreviated" NAD+ analogues with hydroxymethyl or carboxyl function as a substituent of an aliphatic chain linking the adenine and nicotinamide moieties were prepared using the Zincke reaction as the key step. As intermediates several new acyclic nucleoside analogues containing hydroxy, carboxyl, azido and amino group were prepared.

"Abbreviated" NAD+ Analogues Containinga Phosphonate Function

"Abbreviated" NAD+ analogues with anionic phosphonate function as a part of the link between the adenine and nicotinamide moieties, 9-(2-phosphonomethoxyethyl)adenine 2-(3-carbamoylpyridinium)ethyl ester (1a), (R)- and (S)-9-(2-phosphonomethoxypropyl)adenine 2-(3-carbamoylpyridinium)ethyl ester (1b and 1c), (RS)-9-(3-hydroxy-2-phosphonomethoxypropyl)adenine 2-(3-carbamoylpyridinium)ethyl ester (1d), and (S)- and (R)-1-[3-(adenin-9-yl)-2-phosphonatomethoxypropyl]-3-carbamoylpyridinium (2a and 2b), were prepared by multistep syntheses using the Zincke reaction in the last step.

Acyclic Nucleoside and Nucleotide Analogs Derived from 2-Azaadenine

Alkylation of cesium salt of 2-azaadenine with appropriate synthons afforded the following acyclic analogs of nucleosides and nucleotides: (S)-9-(2,3-dihydroxypropyl)-2-azaadenine (VI), 3-(2-azaadenin-9-yl)-2-hydroxypropanoic acid (VIII), 9-(2-phosphonomethoxyethyl)-2-azaadenine (Xb), (R)-9-(2-phosphonomethoxypropyl)-2-azaadenine (XIb) and (S)-9-(3-hydroxy-2-phosphonomethoxypropyl)-2-azaadenine (XVIb). In some cases the N-2 isomers were also isolated.

Synthesis of 2-Deoxy-β-D-ribonucleosides and 2,3-Dideoxy-β-D-pentofuranosides on Immobilized Bacterial Cells

Alginate gel-entrapped cells of auxotrophic thymine-dependent strain of E. coli catalyze the transfer of 2-deoxy-D-ribofuranosyl moiety of 2'-deoxyuridine to purine and pyrimidine bases as well as their aza and deaza analogs. All experiments invariably gave β-anomers; in most cases, the reaction was regiospecific, affording N9-isomers in the purine and N1-isomers in the pyrimidine series. Also a 2,3-dideoxynucleoside can serve as donor of the glycosyl moiety. The acceptor activity of purine bases depends only little on substitution, the only condition being the presence of N7-nitrogen atom. On the other hand, in the pyrimidine series the activity is limited to only a narrow choice of mostly short 5-alkyl and 5-halogeno uracil derivatives. Heterocyclic bases containing amino groups are deaminated; this can be avoided by conversion of the base to the corresponding N-dimethylaminomethylene derivative which is then ammonolyzed. The method was verified by isolation of 9-(2-deoxy-β-D-ribofuranosyl) derivatives of adenine, guanine, 2-chloroadenine, 6-methylpurine, 8-azaadenine, 8-azaguanine, 1-deazaadenine, 3-deazaadenine, 1-(2-deoxy-β-D-ribofuranosyl) derivatives of 5-ethyluracil, 5-fluorouracil, and 9-(2,3-dideoxy-β-D-pentofuranosyl)hypoxanthine, 9-(2,3-dideoxy-β-D-pentofuranosyl)-6-methylpurine, and other nucleosides.