Phosphonomethoxy)ethyl]adenine Research Articles

Metal Ion‐Coordinating Properties in Aqueous Solutions of the Antivirally Active Nucleotide Analogue (S)‐9‐[3‐Hydroxy‐2‐(phosphonomethoxy)propyl]adenine (HPMPA) – Quantification of Complex Isomeric Equilibria

Acyclic nucleoside phosphonates are of medical relevance and deserve detailed chemical characterization. We focus here on (S)‐9‐[3‐hydroxy‐2‐(phosphonomethoxy)propyl]adenine (HPMPA) and include for comparison 9‐[2‐(phosphonomethoxy)ethyl]adenine (PMEA), as well as the nucleobase‐free (phosphonomethoxy)ethane (PME) and (R)‐hydroxy‐2‐(phosphonomethoxy)propane (HPMP). The acidity constants of H3(HPMPA)+ were determined and compared with those of the related phosph(on)ate derivatives; they are also needed to understand the properties of the metal ion complexes. Given that in vivo nucleotides and their analogues participate in reactions typically as divalent metal ion (M2+) complexes, the stability constants of the M(H;HPMPA)+ and M(HPMPA) species with M2+ = Mg2+, Ca2+, Sr2+, Ba2+, Mn2+, Co2+, Ni2+, Cu2+, Zn2+, and Cd2+ were measured. Comparisons between the results for HPMPA2– and the previous data for PMEA2–, HPMP2– and PME2– revealed that for most M(HPMPA) complexes the enhanced stability (the enhancement relative to the stability of a simple phosphonate‐M2+ coordination), can solely be explained by the formation of 5‐membered chelates involving the ether oxygen. These chelates occur in equilibrium with simple ′open′ phosphonate‐M2+ species, the phosphonate group being the primary binding site. The only exceptions are the M(HPMPA) complexes of Ni2+, Cu2+, and Zn2+, which show an additional stability enhancement; in these instances not only the indicated 5‐membered chelates are formed, but M2+ coordinates in addition to N3 of the adenine residue forming a 7‐membered chelate ring. This observation regarding N3 is important because it emphasizes the metal ion affinity of this site (which is often ignored). Note that in the DNA double helix N3 is exposed to the solvent in the minor groove. The stability data for the monoprotonated M(H;HPMPA)+ complexes suggest that these carry H+ at the phosphonate group whereas M2+ is partly at the nucleobase and partly also at the phosphonate group. The ratios of these isomers depend on the metal ion involved, e.g., for Cu(H;HPMPA) the ratio of the isomers is about 1:1.

Intramolecular π-stacks in mixed-ligand copper(II) complexes formed by heteroaromatic amines and antivirally active acyclic nucleotide analogs carrying a hydroxy-2-(phosphonomethoxy)propyl residue‡

Acyclic nucleoside phosphonates (ANPs) are of medical relevance and deserve detailed chemical characterization. We focus here on 1-[2-(phosphonomethoxy)ethyl]cytosine (PMEC), (S)-1-[3-hydroxy-2-(phosphonomethoxy)propyl]cytosine (HPMPC), 9-[2-(phosphonomethoxy)ethyl]adenine (PMEA), and (S)-9-[3-hydroxy-2-(phosphonomethoxy)propyl]adenine (HPMPA) and include for comparison the nucleobase-free (phosphonomethoxy)ethane (PME) and (R)-hydroxy-2-(phosphonomethoxy)propane (HPMP). The acidity constants of H3(ANP)+ and H2(NP) (NP2– = nucleoside phosph(on)ate derivative) are needed to understand the properties of the ternary neutral Cu(Arm)(ANP/NP) [Arm = 2,2′-bipyridine (Bpy) or 1,10-phenanthroline (Phen)] and the monoprotonated Cu(Arm)(H;ANP)+ complexes. The Cu(Arm)(ANP) species are considerably more stable than the corresponding Cu(Arm)(R-PO3), where R- represents a phosph(on)ate ligand with a non-coordinating group R. The observed stability enhancements are due to intramolecular stack formation (st) between the aromatic rings of Arm and the nucleobase residues and also to the formation of five-membered chelates involving the ether oxygen of the –CH(R)–O–CH2– residue (cl/O) (R = H or CH2–OH). In intramolecular equilibria, three structurally different Cu(Arm)(ANP) isomers occur; for example, of Cu(Phen)(HPMPA) about 5% exist as an open (op) Cu(Phen)(HPMPA)op isomer, 17% as Cu(Phen)(HPMPA)cl/O, and 78% as Cu(Phen)(HPMPA)st. In Cu(Arm)(ANP) the stacking tendency decreases in the order PMEA2– > HPMPA2– > PMEC2– > HPMPC2–. In monoprotonated Cu(Arm)(H;ANP)+ both H+ and Cu(Arm)2+ are at undergoing similar intramolecular equilibria as indicated above.

Extent of intramolecular π stacks in aqueous solution in mixed-ligand copper(II) complexes formed by heteroaromatic amines and the anticancer and antivirally active 9-[2-(phosphonomethoxy)ethyl]guanine (PMEG). A comparison with related acyclic nucleotide analogues

The acyclic nucleoside phosphonate (ANP2−) 9-[2-(phosphonomethoxy)ethyl]guanine (PMEG) is anticancer and antivirally active. The acidity constants of the threefold protonated H3(PMEG)+ were determined by potentiometric pH titrations (aq. sol.; 25°C; I=0.1M, NaNO3). Under the same conditions and by the same method, the stability constants of the binary Cu(H;PMEG)+ and Cu(PMEG) complexes as well as those of the ternary ones containing a heteroaromatic N ligand (Arm), that is, of Cu(Arm)(H;PMEG)+ and Cu(Arm)(PMEG), where Arm=2,2′-bipyridine (Bpy) or 1,10-phenanthroline (Phen), were measured. The corresponding equilibrium constants, taken from our earlier work for the systems with 9-[2-(phosphonomethoxy)ethyl]adenine (PMEA) and 9-[2-(phosphonomethoxy)ethyl]-2,6-diaminopurine (PMEDAP) as well as those for Cu(PME) and Cu(Arm)(PME), where PME2−=(phosphonomethoxy)ethane=(ethoxymethyl)phosphonate, were used for comparisons. These reveal that in the monoprotonated ternary Cu(Arm)(H;PE)+ complexes, the proton and Cu(Arm)2+ are at the phosphonate group; the ether oxygen of the –CH2–O–CH2–P(O)2−(OH) residue also participates to some extent in Cu(Arm)2+ coordination. Furthermore, the coordinated Cu(Arm)2+ forms a bridge with the purine moiety undergoing π–π stacking which is more pronounced with H·PMEDAP− than with H·PMEA−. Most intense is π stack formation (st) with the guanine residue of H·PMEG−; here the bridged form Cu(Arm)(H·PMEG)st+ occurs next to an open (op), unbridged (binary) stack, formulated as Cu(Arm)2+/(H·PMEG)op−. The unprotonated and neutral ternary Cu(Arm)(PE) complexes are considerably more stable than the corresponding Cu(Arm)(R-PO3) species, where R-PO32− represents a phosph(on)ate ligand with a group R that is unable to participate in any intramolecular interaction. The observed stability enhancements are mainly due to intramolecular stack formation (st) between the aromatic rings of Arm and the purine residue in the Cu(Arm)(PE) complexes and also, to a smaller extent, to the formation of five-membered chelates involving the ether oxygen of the –CH2–O–CH2–PO32− residue (cl/O) of the PE2− species. The quantitative analysis of the intramolecular equilibria reveals three structurally different Cu(Arm)(PE) isomers; e.g., of Cu(Phen)(PMEG) ca. 1.1% exist as Cu(Phen)(PMEG)op, 3.5% as Cu(Phen)(PMEG)cl/O, and 95% as Cu(Phen)(PMEG)st. Comparison of the various formation degrees reveals that within a given Cu(Arm)(PE) series the stacking tendency decreases in the order PMEG2−⩾PMEDAP2−>PMEA2−. Furthermore, stacking is more pronounced in the acyclic Cu(Arm)(PE) complexes compared with that in the Cu(Arm)(NMP) species, where NMP2−=corresponding parent (2′-deoxy)nucleoside 5′-monophosphate. Here is possibly one of the reasons for the biological activity of the ANPs. One is tempted to speculate that the pronounced stacking tendency of PMEG2−, together with a different H-bonding pattern, leads to enhanced binding in the active site of nucleic acid polymerases, thus being responsible for the pronounced anticancer and antiviral activity of PMEG.

Extent of Intramolecular π‐Stacks in Aqueous Solution in Mixed‐Ligand Copper(II) Complexes Formed by Heteroaromatic Amines and Several 2‐Aminopurine Derivatives of the Antivirally Active Nucleotide Analog 9‐[2‐(Phosphonomethoxy)ethyl]adenine (PMEA)

The acidity constants of twofold protonated, antivirally active, acyclic nucleoside phosphonates (ANPs), H(2)(PE)(±), where PE(2-)=9-[2-(phosphonomethoxy)ethyl]adenine (PMEA(2-)), 2-amino-9-[2-(phosphonomethoxy)ethyl]purine (PME2AP(2-)), 2,6-diamino-9-[2-(phosphonomethoxy)ethyl]purine (PMEDAP(2-)), or 2-amino-6-(dimethylamino)-9-[2-(phosphonomethoxy)ethyl]purine (PME(2A6DMAP)(2-)), as well as the stability constants of the corresponding ternary Cu(Arm)(H;PE)(+) and Cu(Arm)(PE) complexes, where Arm=2,2'-bipyridine (bpy) or 1,10-phenanthroline (phen), are compared. The constants for the systems containing PE(2-)=PMEDAP(2-) and PME(2A6DMAP)(2-) have been determined now by potentiometric pH titrations in aqueous solution at I=0.1M (NaNO(3)) and 25°; the corresponding results for the other ANPs were taken from our earlier work. The basicity of the terminal phosphonate group is very similar for all the ANP(2-) species, whereas the addition of a second amino substituent at the pyrimidine ring of the purine moiety significantly increases the basicity of the N(1) site. Detailed stability-constant comparisons reveal that, in the monoprotonated ternary Cu(Arm)(H;PE)(+) complexes, the proton is at the phosphonate group, that the ether O-atom of the -CH(2)-O-CH(2)-P(O)(2)(-)(OH) residue participates, next to the P(O)(2)(-)(OH) group, to some extent in Cu(Arm)(2+) coordination, and that π-π stacking between the aromatic rings of Cu(Arm)(2+) and the purine moiety is rather important, especially for the H·PMEDAP(-) and H·PME(2A6DMAP)(-) ligands. There are indications that ternary Cu(Arm)(2+)-bridged stacks as well as unbridged (binary) stacks are formed. The ternary Cu(Arm)(PE) complexes are considerably more stable than the corresponding Cu(Arm)(R-PO(3)) species, where R-PO(3)(2-) represents a phosph(on)ate ligand with a group R that is unable to participate in any kind of intramolecular interaction within the complexes. The observed stability enhancements are mainly attributed to intramolecular-stack formation in the Cu(Arm)(PE) complexes and also, to a smaller extent, to the formation of five-membered chelates involving the ether O-atom present in the -CH(2)-O-CH(2)-PO(3)(2-) residue of the PE(2-) species. The quantitative analysis of the intramolecular equilibria involving three structurally different Cu(Arm)(PE) isomers shows that, e.g., ca. 1.5% of the Cu(phen)(PMEDAP) system exist with Cu(phen)(2+) solely coordinated to the phosphonate group, 4.5% as a five-membered chelate involving the ether O-atom of the -CH(2)-O-CH(2)-PO(3)(2-) residue, and 94% with an intramolecular π-π stack between the purine moiety of PMEDAP(2-) and the aromatic rings of phen. Comparison of the various formation degrees of the species formed reveals that, in the Cu(phen)(PE) complexes, intramolecular-stack formation is more pronounced than in the Cu(bpy)(PE) species. Within a given Cu(Arm)(2+) series the stacking intensity increases in the order PME2AP(2-) <PMEA(2-) <PMEDAP(2-) <PME(2A6DMAP)(2-). One could speculate that the reduced stacking intensity of PME2AP(2-), together with a different H-bonding pattern, could well lead to a different orientation of the 2-aminopurine moiety (compared to the adenine residue) in the active site of nucleic acid polymerases and thus be responsible for the reduced antiviral activity of PME2AP compared with that of PMEA and the other ANPs containing a 6-amino substituent.

Design and Synthesis of Novel 1′,3′-Dioxolane 5′-Deoxyphosphonic Acid Purine Analogues as Potent Antiviral Agents

Electronic parameters of 1′,3 ′-oxygen play significant roles in steering the conformation of nucleoside phosphonic acid analogues. To investigate the relationship of two oxygen atoms with antiviral enhancement, novel 1′,3 ′-dioxolane 5 ′-deoxyphosphonic acid purine analogues were synthesized via de novo acyclic stereoselective route from acrolein and glycolic acid. The synthesized nucleoside phosphonic acid analogues 14 and 19 were subjected to antiviral screening against several viruses, such as HIV-1, HSV-1, HSV-2, and HCMV. The guanine analogue 19 exhibits in vitro anti-HIV-1 activity similar to that of 9-[2-(phosphonomethoxy)ethyl]adenine (PMEA) in MT-4 cells.

Intramolecular π–π stacking interactions in aqueous solution in mixed-ligand copper(II) complexes formed by heteroaromatic amines and the nucleotide analogue 9-[2-(phosphonomethoxy)ethyl]-2-aminopurine (PME2AP), an isomer of the antivirally active 9-[2-(phosphonomethoxy)ethyl]adenine (PMEA)

The stability constants of the mixed-ligand complexes formed between Cu(Arm) 2+, where Arm = 2,2′-bipyridine (Bpy) or 1,10-phenanthroline (Phen), and the monoanion or the dianion of 9-[2-(phosphonomethoxy)ethyl]-2-aminopurine (PME2AP), a structural isomer of the antivirally active 9-[2-(phosphonomethoxy)ethyl]adenine (PMEA), were determined by potentiometric pH titrations in aqueous solution at 25 °C and I = 0.1 M (NaNO 3). Detailed stability constant comparisons reveal that in the monoprotonated ternary Cu(Arm)(H;PME2AP) + complexes the proton is at the phosphonate group and that stacking between Cu(Arm) 2+ and H(PME2AP) − plays a significant role. The ternary Cu(Arm)(PME2AP) complexes are considerably more stable than the corresponding Cu(Arm)(R–PO 3) species, where R – PO 3 2 - represents a phosph(on)ate ligand with a group R that is unable to participate in any kind of interaction within the complexes. The increased stability is attributed to intramolecular stack formation in the Cu(Arm)(PME2AP) complexes and also, to a smaller extent, to the formation of 5-membered chelates involving the ether–oxygen present in the – CH 2 – O – CH 2 – PO 3 2 - residue of PME2AP 2−. This latter interaction was previously quantified by studying ternary Cu(Arm)(PME) complexes (PME 2− = dianion of (phosphonomethoxy)ethane), which can form the 5-membered chelates but where no intramolecular ligand–ligand stacking is possible. Application of these results allows a quantitative analysis of the intramolecular equilibria involving three structurally different Cu(Arm)(PME2AP) species; e.g., about 5% of the Cu(Bpy)(PME2AP) system exist with the metal ion solely coordinated to the phosphonate group, 15% as a 5-membered chelate involving the ether–oxygen atom of the – CH 2 – O – CH 2 – PO 3 2 - residue, and 80% with an intramolecular π–π stack between the purine moiety of PME2AP 2− and the aromatic rings of Bpy. Finally, comparison of the stacking properties of PME2AP 2− and PMEA 2− in their ternary complexes reveals that stacking is somewhat more pronounced in the Cu(Arm)(PMEA) than in the Cu(Arm)(PME2AP) species. Speculatively, this reduced stacking intensity, together with a different hydrogen-bonding pattern, could well lead to a different positioning of the 2-aminopurine moiety (compared to the adenine residue) in the active site cavity of nucleic acid polymerases and thus be responsible for the reduced antiviral activity of PME2AP compared with that of PMEA.

Substrate Overlap between Mrp4 and Abcg2/Bcrp Affects Purine Analogue Drug Cytotoxicity and Tissue Distribution

The use of probe substrates and combinations of ATP-binding cassette (ABC) transporter knockout (KO) animals may facilitate the identification of common substrates between apparently unrelated ABC transporters. An unexpectedly low concentration of the purine nucleotide analogue, 9-(2-(phosphonomethoxy)ethyl)-adenine (PMEA), and up-regulation of Abcg2 in some tissues of the Mrp4 KO mouse prompted us to evaluate the possibility that Abcg2 might transport purine-derived drugs. Abcg2 transported and conferred resistance to PMEA. Moreover, a specific Abcg2 inhibitor, fumitremorgin C, both increased PMEA accumulation and reversed Abcg2-mediated PMEA resistance. We developed Mrp4 and Abcg2 double KO mice and used both single KOs of Abcg2 and Mrp4 mice to assess the role of these transporters in vivo. Abcg2 contributed to PMEA accumulation in a variety of tissues, but in some tissues, this contribution was only revealed by the concurrent absence of Mrp4. Abcg2 also transported and conferred resistance to additional purine analogues, such as the antineoplastic, 2-chloro-2'-deoxyadenosine (cladribine) and puromycin, a protein synthesis inhibitor that is often used as a dominant selectable marker. Purine analogues interact with ABCG2 by a site distinct from the prazosin binding site as shown by their inability to displace the substrate analogue and photoaffinity tag [(125)I]iodoarylazidoprazosin. These studies show that Abcg2, like Mrp4, transports and confers resistance to purine nucleoside analogues and suggest that these two transporters work in parallel to affect drug cytotoxicity and tissue distribution. This new knowledge will facilitate an understanding of how Abcg2 and Mrp4, separately and in combination, protect against purine analogue host toxicity as well as resistance to chemotherapy.

Design and synthesis of novel bis( l-amino acid) ester prodrugs of 9-[2-(phosphonomethoxy)ethyl]adenine (PMEA) with improved anti-HBV activity

A series of novel bis( l-amino acid) ester prodrugs of 9-[2-(phosphonomethoxy)ethyl] adenine (PMEA) was synthesized and their anti-HBV activity was evaluated in HepG 2 2.2.15 cells. Compounds 11, 12, 21, 22, 26, and 27 demonstrated more potent anti-HBV activity and higher selective index (SI) than adefovir dipivoxil, which was used as a positive control. Compound 11, which was found to be the most potent one, was five times more potent than adefovir dipivoxil with EC 50 value of 0.095 μM and CC 50 value of 6636 μM. The SI value (>69,000) of compound 11 was 60 times and 24 times higher than those of adefovir dipivoxil and lamivudine, respectively. In vitro stability studies showed that compound 11 was relatively more stable than adefovir dipivoxil with t 1/2 of 270 min. These findings suggested that compound 11 could be considered as a promising candidate for further in vivo studies.

In vivo effects of antiviral acyclic nucleoside phosphonate 9-[2-(phosphonomethoxy)ethyl]adenine (adefovir) on cytochrome P450 system of rat liver microsomes

Interference of antiviral agent adefovir, i.e. 9-[2-(phosphonomethoxy)ethyl]adenine (PMEA) with microsomal drug metabolizing system was investigated in rats. The content of total liver cytochrome P450 (CYP) was lowered while that of its denaturated form, P420, was elevated in animals intraperitoneally treated with PMEA (25 mg/kg). Similar effect was observed after treatment with a prodrug of adevofir, adefovir dipivoxil (bisPOM-PMEA). The CYP2E1-dependent formation of 4-nitrocatechol from p-nitrophenol was diminished, though the specific activity of p-nitrophenol hydroxylase remained unchanged. PMEA had no influence on expression of CYP2E1 protein and mRNA and mRNAs of other P450 isoenzymes (1A1, 1A2, 2C11, 3A1, 3A2, and 4A1). It may be concluded that repeated systemic administration of higher doses of PMEA results in a partial degradation of rat CYP protein to inactive P420.

Acid−Base and Metal-Ion-Binding Properties of 9-[2-(2-Phosphonoethoxy)ethyl]adenine (PEEA), a Relative of the Antiviral Nucleotide Analogue 9-[2-(Phosphonomethoxy)ethyl]adenine (PMEA). An Exercise on the Quantification of Isomeric Complex Equilibria in Solution

The acidity constants of 3-fold protonated 9-[2-(2-phosphonoethoxy)ethyl]adenine, H3(PEEA)+, and of 2-fold protonated (2-phosphonoethoxy)ethane, H2(PEE), and the stability constants of the M(H;PEEA)+, M(PEEA), and M(PEE) complexes with M2+ = Mg2+, Ca2+, Sr2+, Ba2+, Mn2+, Co2+, Ni2+, Cu2+, Zn2+, or Cd2+ have been determined (potentiometric pH titrations; aqueous solution; 25 degrees C; I = 0.1 M, NaNO3). It is concluded that in the M(H;PEEA)+ species, the proton is at the phosphonate group and the metal ion at the adenine residue. The application of previously determined straight-line plots of log K(M(R-PO3))M versus pK(H(R-PO3))H for simple phosph(on)ate ligands, R-PO3(2-), where R represents a residue that does not affect metal-ion binding, proves that the M(PEEA) complexes of Co2+, Ni2+, Cu2+, Zn2+, and Cd2+ as well as the M(PEE) complexes of Co2+, Cu2+, and Zn2+ have larger stabilities than is expected for a sole phosphonate coordination of M2+. For the M2+ complexes without an enhanced stability (e.g., Mg2+ or Mn2+), it is concluded that M2+ binds in a monodentate fashion to the phosphonate group of the two ligands. Combination of all of the results allows the following conclusions: (i) The increased stability of the Co(PEE), Cu(PEE), Zn(PEE), and Co(PEEA) complexes is due to the formation of six-membered chelates involving the ether-oxygen atom of the aliphatic residue (-CH2-O-CH2CH2-PO3(2-)) of the ligands with formation degrees of about 15-30%. (ii) Cd(PEEA) forms a macrochelate with N7 of the adenine residue (formation degree about 30%); Ni(PEEA) has similar properties. (iii) With Zn(PEEA), both mentioned types of chelates are observed, that is, Zn(PEEA)(cl/O) and Zn(PEEA)(cl/N7), with formation degrees of about 13 and 41%, respectively; the remaining 46% is due to the "open" isomer Zn(PEEA)(op) in which the metal ion binds only to the PO3(2-) group. (iv) Most remarkable is Cu(PEEA) because a fourth isomer, Cu(PEEA)(cl/O/N3), is formed that contains a six-membered ring involving the ether oxygen next to the phosphonate group and also a seven-membered ring involving N3 of the adenine residue with a very significant formation degree of about 50%. Hence, PEEA(2-) is a truly ambivalent ligand, its properties being strongly dependent on the kind of metal ion involved. Comparisons with M2+ complexes formed by the dianions of 9-[2-(phosphonomethoxy)ethyl]adenine (PMEA) and related ligands reveal that five-membered chelates involving an ether-oxygen atom are considerably more stable than the corresponding six-membered ones. This observation offers an explanation of why PMEA is a nucleotide analogue with excellent antiviral properties and PEEA is not.

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 (i.e., HIV) and hepadnaviruses (i.e., 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 (i.e., 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.

Quantification of isomeric equilibria formed by metal ion complexes of 8-[2-(phosphonomethoxy)ethyl]-8-azaadenine (8,8aPMEA) and 9-[2-(phosphonomethoxy)ethyl]-8-azaadenine (9,8aPMEA). Derivatives of the antiviral nucleotide analogue 9-[2-(phosphonomethoxy)ethyl]adenine (PMEA)

The acidity constants of the two-fold protonated acyclic 9-[2-(phosphonomethoxy)ethyl]-8-azaadenine, H2(9,8aPMEA)(+)(-), and its 8-isomer, 8-[2-(phosphonomethoxy)ethyl]-8-azaadenine, H2(8,8aPMEA)(+)(-), both abbreviated as H2(PA)(+)(-), as well as the stability constants of their M(H;PA)+ and M(PA) complexes with the metal ions M2+=Mg2+, Ca2+, Sr2+, Ba2+, Mn2+, Co2+, Ni2+, Cu2+, Zn2+ or Cd2+, have been determined by potentiometric pH titrations in aqueous solution at I=0.1 M (NaNO3) and 25 degrees C. Application of previously determined straight-line plots of log K(M)M(R-PO3) versus pK(H)H(R-PO3)for simple phosph(on)ate ligands, R-PO3(2-), where R represents a residue without an affinity for metal ions, proves that for all M(PA) complexes a larger stability is observed than is expected for a sole phosphonate coordination of the metal ion. This increased stability is attributed to the formation of five-membered chelates involving the ether oxygen present in the aliphatic residue (-CH2-O-CH2-PO3(2-)) of the ligands. The formation degrees of these chelates were calculated; they vary between about 13% for Ca(8,8aPMEA) and 71% for Cu(8,8aPMEA). The adenine residue has no influence on complex stability except in the Cu(9,8aPMEA) and Zn(9,8aPMEA) systems, where an additional stability increase attributable to the adenine residue is observed and equilibria between four different isomers exist. This means (1) an open isomer with a sole phosphonate coordination, M(PA)op, where PA(2-)=9,8aPMEA2-, (2) an isomer with a five-membered chelate involving the ether oxygen, M(PA)cl/O, (3) an isomer which contains five- and seven-membered chelates formed by coordination of the phosphonate group, the ether oxygen and the N3 site of the adenine residue, M(PA)cl/O/N3, and finally (4) a macrochelated isomer involving N7, M(PA)cl/N7. For Cu(9,8aPMEA) the formation degrees are 15, 30, 48 and 7% for Cu(PA)op, Cu(PA)cl/O, Cu(PA)cl/O/N3 and Cu(PA)cl/N7, respectively; this proves that the macrochelate involving N7 is a minority species. The situation for the Cu(PMEA) system, where PMEA2- represents the parent compound, i.e. the dianion of 9-[2-(phosphonomethoxy)ethyl]adenine, is quite similar. The relationship between the antiviral activity of acyclic nucleoside phosphonates and the structures of the various complexes is discussed and an explanation is offered why 9,8aPMEA is biologically active but 8,8aPMEA is not.

A novel class of phosphonate nucleosides. 9-[(1-phosphonomethoxycyclopropyl)methyl]guanine as a potent and selective anti-HBV agent.

9-[1-(Phosphonomethoxycyclopropyl)methyl]guanine (PMCG, 1), representative of a novel class of phosphonate nucleosides, blocks HBV replication with excellent potency (EC(50) = 0.5 microM) in a primary culture of HepG2 2.2.15 cells. It exhibits no significant cytotoxicity in several human cell lines up to 1.0 mM. It does not inhibit replication of human immunodeficiency virus (HIV-1) or herpes simplex virus (HSV-1) at 30 microM. Many purine base analogues of 1 also exhibit inhibitory activity against HBV, but at 30 microM, pyrimidine analogues do not. 1 is 4 times more potent than 9-[2-(phosphonomethoxy)ethyl]adenine (PMEA), which was used as a positive control (EC(50) = 2.0 microM). The characteristic cyclopropyl moiety at the 2'-position of 1 was prepared by titanium-mediated Kulinkovich cyclopropanation. 1 was modified to give the orally available drug candidate, PMCDG Dipivoxil (2). Compound 2 exhibited excellent efficacy when administered at 5 mg per kg per day in a study with woodchucks infected with woodchuck hepatitis B virus (WHBV). Drug candidate 2 has successfully completed phase I clinical trials and is currently undergoing phase II clinical studies for evaluation of efficacy.

Nickel(II), Copper(II) and Zinc(II) Complexes of 9-[2- (Phosphonomethoxy)ethyl]-8-azaadenine (9,8aPMEA), the 8-Aza Derivative of the Antiviral Nucleotide Analogue 9-[2-(Phosphonomethoxy)ethyl] adenine (PMEA). Quantification of Four Isomeric Species in Aqueous Solution

The acidity constants of the twofold protonated acyclic nucleotide analogue 9-[2-(phosphonomethoxy)- ethyl]-8-azaadenine, H2(9,8aPMEA)±, as well as the stability constants of the M(H;9,8aPMEA)+ and M(9,8aPMEA) complexes with the metal ions M2+ =Ni2+, Cu2+ or Zn2+, have been determined by potentiometric pH titrations in aqueous solution at I=0.1 M (NaNO3) and 25℃. The result for the release of the first proton from H2(9,8aPMEA)+ (pKa= 2.73), which originates from the (N1)H+ site, was confirmed by UV-spectrophotometric measurements. Application of previously determined straight-line plots of log KMM(R-PO3) versus PKH3(R-HPO3)' for simple phosph(on)ate ligands, R- PO-, where R represents a residue without an affinity for metal ions, proves that the primary binding site of 9,8aPMEA2- is the phosphonate group for all three metal ions studied. By stability constant comparisons with related ligands it is shown, in agreement with conclusions reached earlier for the Cu(PMEA) system [PMEA2-=dianion of 9-[2- (phosphonomethoxy)ethyl]adenine], that in total four different isomers are in equilibrium with each other, i.e. (i) an open isomer with a sole phosphonate coordination, M(PA)op, where PA2-=PMEA2-or 9,8aPMEA2-, (ii) an isomer with a 5-membered chelate involving the ether oxygen, M(PA)cl/o, (iii) an isomer which contains 5- and 7-membered chelates formed by coordination of the phosphonate group, the ether oxygen and the N3 site of the adenine residue, M(PA)cl/O/N3, and finally (iv) a macrochelated isomer involving N7, M(PA)cl/]N7. The Cu2+ systems of PMEA2- and 9,8aPMEA2- behave quite alike; the formation degrees for Cu(PA)op, CuM(PA)cl/O, Cu(PA)cl/O/N3 and Cu(PA)cl/N3 are approximately 16, 32, 45 and 7%, respectively, which shows that Cu(PA)cl/N7 is a minority species. In the Ni2+ and Zn2+ systems the open isomer is the dominating one followed by M(PA)cl/O, but there are indications that the other two isomers also occur to some extent.

Solution Structures of Binary and Ternary Metal Ion Complexes of 9‐(5‐Phosphonopentyl)adenine (3′‐deoxa‐PEEA). A Nucleotide Analogue Related to the Antivirally Active 9‐[2‐(Phosphonomethoxy)ethyl]adenine (PMEA)

AbstractThe acidity constants of the twofold protonated acyclic 9‐(5‐phosphonopentyl)adenine, H2(dPEEA)±, as well as the stability constants of the M(H;dPEEA)+ and M(dPEEA) complexes with the metal ions M2+ = Mg2+, Ca2+, Sr2+, Ba2+, Mn2+, Co2+, Ni2+, Cu2+, Zn2+ or Cd2+, have been determined by potentiometric pH titrations in aqueous solution at I = 0.1 M (NaNO3) and 25 °C. Application of previously determined straight‐line plots of log KM versus pKH for simple phosph(on)ate ligands, R‐PO32−, where R represents a residue which cannot participate in the coordination process, proves that the primary binding site of dPEEA2− is the phosphonate group with all the metal ions studied. However, for the M(dPEEA) systems with Co2+, Ni2+, Cu2+, Zn2+ and Cd2+ a (in part rather small) stability increase is observed which is due to macrochelate formation with the adenine residue, i.e. most likely with N7. The formation degrees of the macrochelates are 17 ± 15%, 28 ± 10%, 46 ± 12%, 42 ± 28%, and 42 ± 9% (3σ), respectively. This means that in this respect dPEEA2− resembles the parent nucleotide adenosine 5′‐monophosphate (AMP2−) more than its chain‐shortened analogue 9‐(4‐phosphonobutyl)adenine (dPMEA2−); indeed, in a first approximation macrochelate formation increases for a given metal ion within the series M(dPMEA) &lt; M(dPEEA) &lt; M(AMP). However, the coordinating properties of all three mentioned ligands differ significantly from those of the antivirally active 9‐[2‐(phosphonomethoxy)ethyl]adenine (PMEA2−) which, due to the presence of an ether oxygen in the aliphatic side chain, has a different coordination chemistry which involves five‐membered chelates with the ether oxygen atom. In addition, the stability constants of the mixed ligand complexes formed between Cu(Arm)2+, where Arm = 2,2′‐bipyridine (Bpy) or 1,10‐phenanthroline (Phen), and the H(dPEEA)− or dPEEA2− species were also measured. Detailed stability constant comparisons reveal that in the monoprotonated ternary Cu(Arm)(H;dPEEA)+ complexes the proton is at the phosphonate group and that stacking between Cu(Arm)2+ and H(dPEEA)− plays a significant role. For the Cu(Arm)(dPEEA) complexes a large increase in complex stability (compared to the stability expected on the basis of the basicity of the phosphonate group) is observed, which is due to intramolecular stack formation between the aromatic ring systems of Phen or Bpy and the purine moiety of dPEEA2−. The formation degree of the stacked isomers is in the order of 65 to 80%. Comparisons of the Cu(Arm)(PA) systems, where PA2− = dPEEA2−, dPMEA2− or AMP2−, reveal that here dPEEA2− resembles its parent AMP2− less closely than dPMEA2− does. The biological implications of these results, including antiviral activities, are shortly discussed. (© Wiley‐VCH Verlag GmbH &amp; Co. KGaA, 69451 Weinheim, Germany, 2003)

Intrinsic acid-base properties of purine derivatives in aqueous solution and comparison of the acidifying effects of platinum(II) coordinated to N1 or N7: acidifying effects are reciprocal and the proton "outruns" divalent metal ions.

The effect of Pt(2+) coordination, in particular of (dien)Pt(2+) or cis-(NH(3))(2)Pt(2+), on the acid-base properties of the purine ligands 9-ethylguanine (9EtG), 9-methylhypoxanthine (9MeHx), inosine (Ino), 9-methyladenine (9MeA), and N6',N6',N9-trimethyladenine (TriMeA) is quantitatively evaluated. The corresponding acidity constants of the complexes are calculated by curve-fitting procedures using previously published (1)H NMR shift data which had been measured in aqueous solution (D(2)O) in dependence on pH (pD). Comparison of the pK(a) values of the ligands with those of the Pt(2+) complexes reveals the expected behavior for the (N7)-platinated complexes; i.e., the (N1)H(0/+) sites are acidified due to charge repulsion. However, Pt(2+) coordination at (N1)(-)(/0) sites leads to an (already previously observed) apparent increase in the basicity of the N7 sites for the guanine, hypoxanthine, and adenine residues; this is also the case if Pt(2+) is bound to N3. Coordination of Pt(2+) to both the (N1)(-) and N7 sites of 9EtG results apparently in an enhanced basicity of N3 if compared with the release of the proton from the (N3)H(+) site in H(2)(9EtG)(2+). For the former cases in aqueous solution (H(2)O) it is now proven for a comprehensive set of data (seven examples), by taking into account the intrinsic basicities of the various N7 sites via micro acidity constants, that the acidifications are reciprocal and identical. This means Pt(2+) coordinated to (N1)(-)(/0) sites in guanine, hypoxanthine, or adenine residues acidifies the (N7)H(+) unit to the same extent as (N7)-coordinated Pt(2+) acidifies the (N1)H(0/+) site. In other words, the apparently increased basicity of N7 upon Pt(2+) coordination at (N1)(-)(/0) sites disappears if the micro acidity constants of the appropriate isocharged tautomers of the ligand are properly taken into account. It is further proven, on the basis of the evaluations of the nucleotide analogue 9-[2-(phosphonomethoxy)ethyl]adenine (PMEA), that these given conclusions are also valid for nucleotides. In addition, it is shown that the mentioned apparent basicity increase, which results from the use of macro acidity constants, has its origin in the fact that the proton-metal ion (Pt(2+)) interaction (the extent of which depends on the kind of metal ion involved) is less pronounced than the proton-proton interaction. Finally, the proven reciprocal behavior will now allow one to determine micro acidity constants of ligands by studying complexes formed with kinetically inert metal ions. A further result of interest is the proof that the competition of Pt(2+) (or Pd(2+)) with the proton for the (N1)(-) and N7 binding sites of inosinate results in the isomer where the metal ion is at N7 with the proton relegated to (N1)(-); this isomer is favored by a factor of about 2000 compared with the one having the metal ion at (N1)(-) and the proton at N7.

2-Amino-6-arylthio-9-[2-(phosphonomethoxy)ethyl]purine bis(2,2,2-trifluoroethyl) esters as novel HBV-specific antiviral reagents.

Novel 2-amino-6-arylthio-9-[2-(phosphonomethoxy)ethyl]purine bis(2,2,2-trifluoroethyl) esters were synthesized and evaluated for antihepatitis B virus (HBV) activity in vitro using HB611, HuH-6 cell line, stably transfected with the HBV genome. Among the compounds synthesized, 2-amino-6-phenylthio-9-[2-(phosphonomethoxy)ethyl]purine bis(2,2,2-trifluoroethyl) ester (8), 2-amino-6-(4-methoxyphenylthio)-9-[2-(phosphonomethoxy)ethyl]purine bis(2,2,2-trifluoroethyl) ester (16), 2-amino-6-(3-methoxyphenylthio)-9-[2-(phosphonomethoxy)ethyl]purine bis(2,2,2-trifluoroethyl) ester (17), and 2-amino-6-(2-methoxyphenylthio)-9-[2-(phosphonomethoxy)ethyl]purine bis(2,2,2-trifluoroethyl) ester (18) showed considerably high anti-HBV activity, as represented by IC(50) values of 0.05, 0.03, 0.04, and 0.08 microM, respectively, and exhibited low cytotoxicity, as represented by CC(50) values of more than 1000 microM. It was suggested that these compounds did not have anti-HIV activity, and compound 8 showed only weak anti-HSV-1 activity. An antiviral agent, 9-[2-(phosphonomethoxy)ethyl]adenine (PMEA), which was used as a control in the present study, showed moderate anti-HBV activity, as represented by an IC(50) value of 0.2 microM. Furthermore, compound 16 was administered orally to mice at a dose of 100 mg/kg in order to examine its gastrointestinal absorbability. Consequently, the main active metabolite was observed in mouse plasma, with especially high concentrations in the liver.

Raman spectroscopy study of acid-base and structural properties of 9-[2-(phosphonomethoxy)ethyl]adenine in aqueous solutions.

The acid-base properties of the acyclic antiviral nucleotide analogue 9- [2-(phosphonomethoxy)ethyl] adenine (PMEA) in aqueous solutions are studied by means of Raman spectroscopy in a pH range of 1-11 and compared with the properties of its common adenosine monophosphate counterparts (5'-AMP, 3'-AMP, and 2'-AMP). Factor analysis is used to separate the spectra of pure ionic species (PMEA2-, HPMEA-, H2PMEA, H3PMEA+) in order to determine their abundance, sites of protonation, and corresponding spectroscopic pK(a) values. The characteristic Raman features of the neutral adenine moiety in PMEA2- and HPMEA- species resemble those of neutral adenine in the AMPs, whereas significant differences are observed between the Raman spectra of the N1-protonated adenine of the solute zwitterionic H2PMEA and its N1-protonated AMP counterparts. On the contrary, the spectrum of crystalline H2PMEA, adopting an "anti-like" conformation, is found to be similar to the N1-protonated AMPs in solution. To explain peculiar Raman features a "syn-like" conformation is suggested for N1-protonated PMEA species in aqueous solutions instead of an anti-like one adopted by H2PMEA in crystals or by common AMPs in aqueous solutions. A physical mechanism of the anti-like to syn-like conformational transition of the solute PMEA that is due to adenine protonation and the flexibility of the (phosphonomethoxy)ethyl group is proposed and discussed.

Intramolecular stacking interactions in ternary copper(II) complexes formed with 2,2′-bipyridine or 1,10-phenanthroline and 9-(4-phosphonobutyl)adenine (dPMEA), the carba relative of the antiviral nucleotide analogue 9-[2-(phosphonomethoxy)ethyl]adenine (PMEA)

The stability constants of the mixed ligand complexes formed between Cu(Arm) 2+, where Arm=2,2′-bipyridine (Bpy) or 1,10-phenanthroline (Phen), and the monoanion or the dianion of 9-(4-phosphonobutyl)adenine (dPMEA=3′-deoxa-PMEA), which is the carba analogue of the antivirally active 9-[2-(phosphonomethoxy)ethyl]adenine (PMEA), were determined by potentiometric pH titrations in aqueous solution at 25°C and I=0.1 M (NaNO 3). Detailed stability constant comparisons reveal that in the monoprotonated ternary Cu(Arm)(H;dPMEA) + complexes the proton is at the phosphonate group and that stacking between Cu(Arm) 2+ and H(dPMEA) − plays a significant role. For the Cu(Arm)(dPMEA) complexes a large increase in complex stability (compared to the stability expected on the basis of the basicity of the phosphonate group) is observed, which is due to intramolecular stack formation between the aromatic ring systems of Phen or Bpy and the purine moiety of dPMEA 2−. The formation degree of the stacked isomer in the Cu(Arm)(dPMEA) systems is on the order of 90%, though it is somewhat more pronounced with Phen than with Bpy. Comparisons of the Cu(Arm)(N) systems, where N=dPMEA 2− and PMEA 2− or adenosine 5′-monophosphate (AMP 2−), reveal that the stacking properties of dPMEA 2− and PMEA 2− resemble closely those of their parent nucleotide AMP 2−.

Metal ion-binding properties of 9-(4-phosphonobutyl)adenine (dPMEA), a sister compound of the antiviral nucleotide analogue 9-[2-(phosphonomethoxy)ethyl]adenine (PMEA), and quantification of the equilibria involving four Cu(PMEA) isomers

The acidity constants of the threefold protonated acyclic 9-(4-phosphonobutyl)adenine, H3(dPMEA)+, as well as the stability constants of the M(H;dPMEA)+ and M(dPMEA) complexes with the metal ions M2+ = Mg2+, Ca2+, Sr2+, Ba2+, Mn2+, Co2+, Ni2+, Cu2+, Zn2+ or Cd2+, have been determined by potentiometric pH titrations, in aqueous solution at I = 0.1 M (NaNO3) and 25 °C. Application of previously determined straight-line plots of log KMM(R-PO3)versus pKHH(R-PO3) for simple phosph(on)ate ligands, R-PO32−, where R represents a residue without an affinity for metal ions, proves that the primary binding site of dPMEA2− is the phosphonate group with all the metal ions studied; in fact, in most instances the stability is solely determined by the basicity of the phosphonate residue. Only for the Ni(dPMEA), Cu(dPMEA) and Cd(dPMEA) systems a stability increase due to macrochelate formation with the adenine residue occurs; the formation degrees are 21 ± 15%, 31 ± 14% and 29 ± 18%, respectively. In these three instances the additional interaction of the phosphonate-coordinated M2+ occurs most probably with N7; hence, dPMEA2− is more similar in its metal ion-binding properties to the parent nucleotide adenosine 5′-monophosphate (AMP2−) than to the antivirally active and structurally more related dianion of 9-[2-(phosphonomethoxy)ethyl]adenine (PMEA2−). This result agrees with the observation that replacement of the ether O atom in PMEA by a CH2 unit leads to a compound, i.e. dPMEA, devoid of any biological activity. In addition, use is made of the stability enhancement obtained for the Cu(dPMEA) system due to macrochelate formation to analyze the equilibria regarding the four isomeric complex species possibly formed in the Cu(PMEA) system. It is shown that a macrochelated isomer involving N7 of the adenine residue occurs with Cu(PMEA) only in trace amounts; the important isomers in this system involve the ether oxygen (formation degree ca. 34%) and also N3 of the adenine moiety (ca. 41%).