Inhibitory effect of 9-(2-phosphonylmethoxyethyl)adenine on visna virus infection in lambs: a model for in vivo testing of candidate anti-human immunodeficiency virus drugs

HPMPC (cidofovir), PMEA (adefovir) and Related Acyclic Nucleoside Phosphonate Analogues: A Review of their Pharmacology and Clinical Potential in the Treatment of Viral Infections

The acyclic nucleoside phosphonate (ANP) analogues are broad-spectrum antiviral agents, with potent and selective antiviral activity in vitro and in vivo. The prototype compounds are: ( S)-1-(3-hydroxy-2-phosphonylmethoxypropyl)cytosine (HPMPC, cidofovir), which is active against a wide variety of DNA viruses; 9-(2-phosphonylmethoxyethyl)adenine (PMEA, adefovir), which is active against retro-, herpes- and hepadnaviruses, and ( R)-9-(2-phosphonylmethoxypropyl) adenine (PMPA), which is active against retro- and hepadnaviruses. The antiviral action of the ANP analogues is based on a specific interaction of the active diphosphorylated metabolite with the viral DNA polymerase. The long intracellular half-life of the active metabolite accounts for the optimal efficacy in infrequent dosing schedules. The potential of HPMPC as a broad-spectrum anti-DNA virus agent, as originally observed in vitro and in vivo, has been confirmed in clinical trials. HPMPC has recently been commercially released in the USA for the treatment of cytomegalovirus retinitis in AIDS patients. In addition, topical systemic HPMPC is being (or will be) explored for use against other herpesviruses (i.e. herpes simplex virus, Epstein-Barr virus, or varicella-zoster virus), by adenoviruses, or by human papilloma- or polyomaviruses. Intravenous HPMPC is associated with dose-dependent nephrotoxicity, that should be counteracted by prehydration and concomitant administration of probenecid, and by the application of an infrequent dosing schedule. The oral prodrug of PMEA, bis(pivaloyloxymethyl)-PMEA, is currently being evaluated in patients infected with human immunodeficiency virus (HIV) or hepatitis B virus. Finally, preclinical data on the efficacy of PMPA in animal retrovirus models point to its potential usefulness against HIV infections, when given either prophylactically or therapeutically in the treatment of established HIV infections.

Virucidal effect of lipids on visna virus, a lentivirus related to HIV

Natural lipids and fatty alcohols show virucidal activities against enveloped viruses. A virucidal profile of these compounds against visna virus (VV), a lentivirus related to HIV, or against other viruses of the genus Lentivirus has not been established before and could help elucidate how lipids inactivate enveloped viruses and assist in the development of virucidal drugs. The activity profile for VV may not exactly reflect the profile for HIV or for the lentivirus subgroup in general, but the results for VV are in agreement with earlier studies, which have shown that lipids become generally more virucidal at low pH.

Immunobiological activity of N-[2-(phosphonomethoxy)alkyl] derivatives of N6-substituted adenines, and 2,6-diaminopurines

Acyclic nucleoside phosphonates are novel class of virostatics effective against replication of both DNA-viruses and retroviruses. We found recently, that in addition to the antimetabolic mode of action, some acyclic nucleoside phosphonates such as 9-[2-(phosphonomethoxy)propyl]adenine [(R)-PMPA; tenofovir], which is used in treatment of human immunodeficiency virus (HIV) infection, possess immunostimulatory and immunomodulatory activities known to interfere with replication of viruses. The present experiments analyzed immunobiological effects of more than 70 novel derivatives of acyclic nucleoside phosphonates. They comprise substitutions at the N6-amino function of adenine (A) or 2,6-diaminopurine (DAP) by monoalkyl, dialkyl, cycloalkyl, alkenyl, alkynyl or substituted alkyl group, and at the N9-side chain represented by (R)- or (S)-enantiomeric 9-[2-(phosphonomethoxy)ethyl] (PME) and 9-[2-(phosphonomethoxy)propyl] (PMP) moieties. Their biological effects were investigated in vitro using mouse resident peritoneal macrophages. A number of the compounds under scrutiny, mainly the N6-cycloalkyl derivatives of 9-[2-(phosphonomethoxy)ethyl]2,6-diaminopurine (PMEDAP) and (R)-enantiomeric 9-[2-(phosphonomethoxy)propyl]adenine [(R)-PMPDAP] stimulate secretion of cytokines [tumor necrosis factor-alpha (TNF-alpha), interleukin-10 (IL-10)] and chemokines ["regulated-upon-activation, normal T expressed and secreted" (RANTES), macrophage inflammatory protein-1alpha (MIP-1alpha)]. Moreover, they substantially augment production of nitric oxide (NO) triggered by interferon-gamma. The effects are produced in a dose-dependent fashion. The most potent derivatives, i.e. N6-isobutyl-PMEDAP, N6-cyclopentyl-PMEDAP, N6-cyclooctyl-PMEDAP, N6-dimethylaminoethyl-(R)-PMPDAP, N6-cyclopropyl-(R)-PMPDAP, and N6-cyclopentyl-(R)-PMPDAP are more effective than (R)-PMPA (tenofovir) itself. They exhibit immunostimulatory effects at concentrations as low as 1 to 5 microM. It is suggested that these compounds might be prospective candidates for antiviral therapeutic exploitation.

Adenine and deazaadenine nucleoside and deoxynucleoside analogues: inhibition of viral replication of sheep MVV (in vitro model for HIV) and bovine BHV-1

A series of N(6)-cycloalkyl-2',3'-dideoxyadenosine derivatives has been prepared by coupling of 2,6-dichloropurine to protected 2,3-dideoxyribose, followed by reaction with appropriate cycloalkylamines. Synthesized compounds, along with other purine nucleoside analogues previously synthesized in our laboratory, have been tested for their antiviral activity against Bovine herpesvirus 1 (BHV-1) and sheep Maedi/Visna Virus (MVV), the latter being an in vitro and in vivo model of Human Immunodeficiency Virus (HIV). All compounds showed good antireplicative activity against MVV, with the N(6)-cycloheptyl-2',3'-dideoxyadenosine (5b) being the most active [effective concentration (EC(50)) causing 50% reduction of cytopatic effects (CPE)=27 nM]. All compounds showed also a from low to very low cell toxicity, resulting in a cytotoxic dose 50 (CD(50))/EC(50) ratio in some cases higher than 1000.

Antiretrovirus activity of a novel class of acyclic pyrimidine nucleoside phosphonates

A novel class of acyclic nucleoside phosphonates has been discovered in which the base consists of a pyrimidine preferably containing an amino group at C-2 and C-4 and a 2-(phosphonomethoxy)ethoxy (PMEO) or a 2-(phosphonomethoxy)propoxy (PMPO) group at C-6. The 6-PMEO 2,4-diaminopyrimidine (compound 1) and 6-PMPO 2,4-diaminopyrimidine (compound 11) derivatives showed potent activity against human immunodeficiency virus (HIV) in the laboratory (i.e., CEM and MT-4 cells) and in primary (i.e., peripheral blood lymphocyte and monocyte/macrophage) cell cultures and pronounced activity against Moloney murine sarcoma virus in newborn NMRI mice. Their in vitro and in vivo antiretroviral activity was comparable to that of reference compounds 9-[(2-phosphonomethoxy)ethyl]adenine (adefovir) and (R)-9-[(2-phosphonomethoxy)-propyl]adenine (tenofovir), and the enantiospecificity of (R)- and (S)-PMPO pyrimidine derivatives as regards their antiretroviral activity was identical to that of the classical (R)- and (S)-9-(2-phosphonomethoxy)propyl purine derivatives. The prototype PMEO and PMPO pyrimidine analogues were relatively nontoxic in cell culture and did not markedly interfere with host cell macromolecular (i.e., DNA, RNA, or protein) synthesis. Compounds 1 and 11 should be considered attractive novel pyrimidine nucleotide phosphonate analogues to be further pursued for their potential as antiretroviral agents in the clinical setting.

Maedi-visna virus, a model for in vitro testing of potential anti-HIV drugs

A series of beta-D- and beta-L-cytidine analogues were evaluated for their inhibitory effect on the replication of maedi-visna virus (MVV) strains KV1772 and MV1514 cultured on sheep choroid plexus cells and the sheep chondrocyte cell line G81092, respectively. Eleven cytidine analogues were selected for the anti-viral test. Five of them belong to the family of the 2',3'-dideoxycytidine analogues, well known for their activity against human immunodeficiency virus (HIV). The others, all newly synthesized, were potential anti-viral and/or anti-leukemic agents. None of the compounds under study had a toxic effect in both anti-viral assay systems up to a 300 microM concentration. Based on the cytopathic effects (CPE), the virus replication was completely inhibited by the five 2',3'-dideoxycytidine analogues at a concentration of 50 microM, whereas the others six newly synthesized compounds induced titre reductions of 4-5 log units. The effective concentration causing 50% reduction of CPE (EC50) was of 5 microM for the five 2',3'-dideooxycytidine analogues and for beta-L-XyloFc, whereas the value of 50 microM was found for the b-L-XyloC and the four 5-azacytidine compounds tested. All these data reveal a good correlation between inhibition of MVV replication by several nucleoside cytidine analogues and their reported anti-HIV activity.

Synthesis and Cytostatic Activity of N-[2-(Phosphonomethoxy)alkyl] Derivatives of N6-Substituted Adenines, 2,6-Diaminopurines and Related Compounds

N6-Substituted adenine and 2,6-diaminopurine derivatives of 9-[2-(phosphonomethoxy)- ethyl] (PME), 9-[(R)-2-(phosphonomethoxy)propyl] [(R)-PMP] and enantiomeric (S)-PMP series were synthesized by reactions of primary or secondary amines with 6-chloro-9-{[2-(diisopropoxyphosphoryl)methoxy]alkyl}purines (26-28) or 2-amino-6-chloro-9-{[2-(diisopropoxy- phosphoryl)methoxy]alkyl}purines (29-31) followed by treatment of the diester intermediates32with bromo(trimethyl)silane and hydrolysis. Diesters32were also obtained by reaction ofN6-substituted purines with synthons23-25bearing diisopropoxyphosphoryl group. Alkylation of 2-amino-6-chloropurine (9) with diethyl [2-(2-chloroethoxy)ethyl]phosphonate (148) gave the diester149which was analogously converted toN6-substituted 2,6-diamino- 9-[2-(2-phosphonoethoxy)ethyl]purines151-153. Alkylation ofN6-substituted 2,6-diaminopurines with (R)-[(trityloxy)methyl]oxirane (155) followed by reaction of thus-obtained intermediates156with dimethylformamide dimethylacetal and condensation with diisopropyl [(tosyloxy)methyl]phosphonate (158) followed by deprotection of the intermediates159gaveN6-substituted 2,6-diamino-9-[(S)-3-hydroxy-2-(phosphonomethoxy)propyl]purines160-163. The highest cytostatic activityin vitrowas exhibited by the followingN6-derivatives of 2,6-diamino-9-[2-(phosphonomethoxy)ethyl]purine (PMEDAP): 2,2,2-trifluoroethyl (53), allyl (54), [(2-dimethylamino)ethyl] (68), cyclopropyl (75) and dimethyl (91). In CCRF-CEM cells, the cyclopropyl derivative75is deaminated to the guanine derivative PMEG (3) which is then converted to its diphosphate.

Effect of PMPA and PMEA on the kinetics of viral load in simian immunodeficiency virus-infected macaques

In this study we compared the effect of postexposure treatment of the acyclic nucleoside analogs 9-(2-phosphonylmethoxyethyl)-adenine (PMEA) and 9-(2-phosphonylmethoxypropyl)-adenine (PMPA) on the kinetics of viral load in the blood and lymph nodes of rhesus macaques chronically infected with SIVmac251 for 18 weeks. Two of the four macaques treated with PMPA (20 mg/kg per day) for 28 consecutive days had demonstrable reductions in viral loads of 1.5 and 3 logs. Three of four macaques given the same dosing regimen of PMEA had viral load reductions ranging from 1.25 to 2.8 logs. Furthermore, treatment with either drug caused a reduction in virus burden in the lymph nodes by 2 weeks posttreatment. However, in both PMEA- and PMPA-treated animals, viral loads rebounded to day of treatment levels by 2 weeks after termination of treatment. The extent to which viral load was suppressed was similar for both drugs. In contrast, viral loads in three of four mock-treated animals remained persistently high throughout the study. This study has demonstrated that postexposure treatment with these acyclic nucleoside analogs could modulate the kinetics of viral load reduction in some animals.

Structure-antiviral activity relationship in the series of pyrimidine and purine N-[2-(2-phosphonomethoxy)ethyl] nucleotide analogues. 1. Derivatives substituted at the carbon atoms of the base

A series of dialkyl esters of purine and pyrimidine N-[2-(phosphonomethoxy)ethyl] derivatives substituted at position 2, 6, or 8 of the purine base or position 2, 4, or 5 of the pyrimidine base were prepared by alkylation of the appropriate heterocyclic base with 2-chloroethoxymethylphosphonate diester in the presence of sodium hydride, cesium carbonate, or 1,8-diazabicyclo[5,4, 0]undec-7-ene (DBU) in dimethylformamide. Additional derivatives were obtained by the transformations of the bases in the suitably modified intermediates bearing reactive functions at the base moiety. The diesters were converted to the corresponding monoesters by sodium azide treatment, while the free acids were obtained from the diester by successive treatment with bromotrimethylsilane and hydrolysis. None of the PME derivatives in the pyrimidine series, their 6-aza or 3-deaza analogues, exhibited any activity against DNA viruses or retroviruses tested, except for the 5-bromocytosine derivative. Substitution of the adenine ring in PMEA at position 2 by Cl, F, or OH group decreased the activity against all DNA viruses tested. PMEDAP was highly active against HSV-1, HSV-2, and VZV in the concentration range (EC50) of 0.07-2 microg/mL. Also the 2-amino-6-chloropurine derivative was strongly active (EC50 = 0.1-0. 4 microg/mL) against herpes simplex viruses and (EC50 = 0.006-0.3 microg/mL) against CMV and VZV. PMEG was the most active compound of the whole series against DNA viruses (EC50 approximately 0.01-0.02 microg/mL), though it exhibited significant toxicity against the host cells. The base-modified compounds did not show any appreciable activity against DNA viruses except for 7-deazaPMEA (IC50 approximately 7.5 microg/mL) against HIV-1 and MSV. The neutral (diisopropyl, diisooctyl) diesters of PMEA were active against CMV and VZV, while the corresponding monoesters were inactive. The diisopropyl ester of the 2-chloroadenine analogue of PMEA showed substantially (10-100x) higher activity against CMV and VZV than the parent phosphonate. Also, the diisopropyl and diisooctyl ester of PMEDAP inhibited CMV and VZV, but esterification of the phosphonate residue did not improve the activity against either MSV or HIV.

Treatment of visna virus infection in lambs with the acyclic nucleoside phosphonate analogue 9-(2-phosphonylmethoxyethyl)adenine (PMEA)

Nucleoside and nucleotide analogues, which are inhibitors of human immunodeficiency virus reverse transcriptase, are highly active inhibitors of visna virus replication in cell cultures. One such analogue, the acyclic nucleoside phosphonate PMEA, has also been found to have a prophylactic effect on visna virus infection in lambs. In the present study, lambs were injected subcutaneously with 10 mg/kg PMEA three times a week starting 4 weeks after inoculation with visna virus, when brain infection had been established. After 3 weeks of treatment there was a reduction in the amount of virus isolated from blood cells of PMEA-treated lambs compared to controls and during the remaining 7 months of drug treatment there was significantly less virus isolated from the blood cells of treated lambs than of controls. Antibody response against visna virus was also slower in the treated than in the untreated control group. On the other hand, there was no difference in the amount of virus isolated from various organs of the two groups and the severity of CNS lesions in sheep treated with PMEA for 8 months was comparable to that found in untreated controls, even though PMEA reached concentrations in the cerebrospinal fluid which were well in excess of the EC50 value of the drug for visna virus.

Marked inhibitory activity of masked aryloxy aminoacyl phosphoramidate derivatives of dideoxynucleoside analogues against visna virus infection

Lipophilic masked aryloxyaminoacylphosphoramidate derivatives of 2',3'-dideoxynucleoside (ddN) analogues with potent anti-HIV activity (i.e., stavudine [d4T], azidothymidine [AZT], dideoxycytidine [ddC], 3'thio-2',3'-dideoxy cytidine [3TC], dideoxyadenosine [ddA], and 2',3'-didehydro-2',3'-dideoxyadenosine [d4A]) activity were evaluated for their activity against visna virus (VV) in sheep choroid plexus (SCP) cells. The activity of several prodrug derivatives against VV proved markedly superior to that of the corresponding free ddN analogues. In particular, the d4A and ddA prodrug derivatives were exquisitely inhibitory in this model system (50% effective concentration [EC50], < or = 0.003 microM), and their anti-VV potency exceeded by at least 200-fold the antiviral potency of the corresponding free nucleosides. Marked differences were noted in the anti-VV potencies of several of the test compounds depending on the nature of the amino acid linked to the 5'-phosphate moiety, the nature of the nucleoside, or both. In view of the stability of the prodrugs in lamb serum, the VV infection model in lambs may be considered highly useful for investigating the in vivo antiretroviral efficacy of these type of drugs, particularly the d4T, ddA, and d4A prodrug derivatives.

In search of a selective antiviral chemotherapy

This article describes several approaches to a selective therapy of virus infections: (E)-5-(2-bromovinyl)-2'-deoxyuridine (BVDU [brivudin]) for the therapy of herpes simplex virus type 1 and varicella-zoster virus infections: (S)-9-(3-hydroxy-2-phosphonylmethoxypropyl)cytosine (HPMPC [cidofovir]) for the therapy of various DNA virus (i.e., herpesvirus, adenovirus, papillomavirus, polyomavirus, and poxvirus) infections; 9-(2-phosphonylmethoxyethyl)adenine (PMEA [adefovir]) for the therapy of retrovirus, hepadnavirus, and herpesvirus infections; (R)-9-(2-phosphonylmethoxypropyl)adenine (PMPA) for the therapy and prophylaxis of retrovirus and hepadnavirus infections; and nonnucleoside reverse transcriptase inhibitors (NNRTIs), such as tetrahydroimidazo[4,5,1-jk][1,4]-benzodiazepin-2(IH)-one and -thione (TIBO), 1-[(2-hydroxyethoxy)methyl]-6-(phenylthio)thymine (HEPT), alpha-anilinophenylacetamide (alpha-APA), and 2',5'bis-O-(tert-butyldimethylsilyl)-3'-spiro-5"-(4"-amino-1",2"-oxat hiole- 2",2"-dioxide)pyrimidine (TSAO) derivatives, and thiocarboxanilides for the treatment of human immunodeficiency virus type 1 (HIV-1) infections. For the clinical use of NNRTIs, some guidelines have been elaborated, such as starting treatment with combinations of different compounds at sufficiently high concentrations to effect a pronounced and sustained suppression of the virus. Despite the diversity of the compounds described here and the different viruses at which they are targeted, they have a number of characteristics in common. As they interact with specific viral proteins, the compounds achieve a selective inhibition of the replication of the virus, which, in turn, should be able to develop resistance to the compounds. However, as has been established for the NNRTIs, the problem of viral resistance may be overcome if the compounds are used from the start at sufficiently high doses, which could be reduced if different compounds are combined. For HIV infections, drug treatment regimens should be aimed at reducing the viral load to such an extent that the risk for progression to AIDS will be minimized, if not avoided entirely. This may result in a real "cure" of the disease but not necessarily of the virus infection, and in this sense, HIV disease may be reduced to a dormant infection, reminiscent of the latent herpesvirus infections. Should virus replication resume after a certain time, the armamentarium of effective anti-HIV and anti-herpesvirus compounds now available, if applied at the appropriate dosage regimens, should make the virus return to its dormant state before it has any chance to damage the host. It is unlikely that this strategy would eradicate the virus and thus "cure" the viral infection, but it definitely qualifies as a cure of the disease.