Immunomodulatory activity of 9-(2-phosphonylmethoxyethyl)adenine (PMEA), a potent anti-HIV nucleotide analogue, on in vivo murine models

Inhibitory Effects of Acyclic Nucleoside Phosphonate Analogues on Hepatitis B Virus DNA Synthesis in HB611 Cells

By using an assay system based on a human hepatoblastoma cell line (HB611) that continuously synthesizes hepatitis B virus (HBV) DNA, 56 acyclic nucleoside phosphonate analogues were examined for their inhibitory effects on HBV DNA synthesis. The following compounds were found to inhibit HBV DNA synthesis at concentrations that were significantly lower than their minimum cytotoxic concentrations; 9-(2-phosphonylmethoxyethyl)adenine (PMEA), 9-(2-phosphonylmethoxyethyl) guanine(PMEG), 9-(2-phosphonylmethoxyethyl) guanine ethyl ester (PMEGEE), 9 - (2 - phosphonylmethoxyethyl) - 1 - deazaadenine (PMEC1A), 9-(2-phosphonylmethoxyethyl)-2,6-diaminopurine (PMEDAP), ( S)-9-(3-hydroxy-2-phosphonylmethoxypropyl)adenine (HPMPA), 9-(3-isopropoxy-2-phosphonylmethoxypropyl)adenine (IPPMPA), 9-( RS)-(2-phosphonylmethoxypropyl)adenine (PMPA) and 9-(3-hydroxy-2-phosphonylmethoxypropyl)-2, 6-diaminopurine (HPMPDAP). The most selective compounds (with indexes greater than 100) were PMEDAP, PMEA, IPPMPA, and PMPA. Acyclic pyrimidine nucleoside phosphonate analogues did not prove markedly selective as anti-HBV agents. Diphosphoryl derivatives of some acyclic purine nucleoside phos-phonates (i.e. PMEA, PMEDAP, HPMPA) were prepared. They proved inhibitory to HBV DNA polymerase but not cellular DNA polymerase α.

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.

Acyclic Phosphonomethylether Nucleoside Inhibitors of Respiratory Viruses

A series of acyclic phosphonomethylether nucleosides were synthesized and then evaluated for inhibitory activity against respiratory viruses of clinical significance using CPE inhibition, neutral red uptake and virus yield reduction assays. Of the 20 compounds synthesized, none significantly inhibited influenza A or B viruses or respiratory syncytial virus strains A2, Long or 18537; the selective indices (SI) were less than 10. A new compound, GS-2128 (2R, 5R-9-[2,5-dihydro-5-(phosphonomethoxy)-2-furanyl]adenine; D4API), selectively inhibited adenovirus 5 (SI>10) as did GS-0577 (9-(3-hydroxy-2-phosphonylmethoxypropyl)-adenine; HPMPA) and GS-0504 [(S)-1-[3-hydroxy-2-(phosphonylmethoxypropyl)]-cytosine; HPMPC]. The 50% effective concentrations (EC50) ranged from 8–100 μg mL−1 and 50% cell inhibitory concentrations (CC50) from 40–1000 μg mL−1. All three compounds were also found to be active against laboratory strains and clinical isolates of adenovirus types 1, 2, 8 and 41 with EC50 values ranging from 0.2 to 10 μg mL−1. Two compounds, GS-438 (9-(2-phosphonylmethoxyethyl)guanine, PMEG) and GS-2542 (9-[3-phosphonomethoxy)methoxymethyl]guanine) inhibited parainfluenza virus 3 strain C243, with SI of 52 and >333, respectively. PMEG also inhibited measles virus strains CC, Halonen and Chicago with EC50 values ranging from 0.03–9 μg mL−1. These data suggest that these compounds should be considered for possible development as therapeutic agents for respiratory virus infections.

Dipyridamole Potentiates the Activity of Various Acyclic Nucleoside Phosphonates against Varicella-Zoster Virus, Herpes Simplex Virus and Human Cytomegalovirus

Dypiridamole (DPM) is widely used in the treatment of cardiovascular diseases as a coronary vasodilator and inhibitor of platelet aggregation. Phosphonylmethoxyethyl (PME) and 3-hydroxy-2-phosphonylmethoxypropyl (HPMP) derivatives of purines and pyrimidines are potent and selective inhibitors of varicella-zoster virus (VZV), herpes simplex virus (HSV) and human cytomegalovirus (HCMV). We have found that DPM markedly potentiates the antiviral effects of the PME derivatives of adenine (PMEA) and 2,6-diaminopurine (PMEDAP), and of the HPMP derivatives of adenine (HPMPA), 3-deazaadenine (HPMPc3A) and cyclic HPMPA (cHPMPA). This was reflected by a significant decrease in the 50% inhibitory concentration of the acyclic nucleoside phosphonates for VZV-, HSV- and HCMV-induced cytopathic effect or plaque formation. DPM did not enhance the activity of vidarabine, acyclovir or ganciclovir. These results were confirmed by virus yield assays (for HSV and HCMV) and flow cytometry (for VZV).

Baseline hepatitis B surface antigen (HBsAg) as predictor of sustained HBsAg loss in chronic hepatitis B patients treated with pegylated interferon-α2a and adefovir

BACKGROUND

In this study, we aimed to identify baseline predictors of response in chronic hepatitis B patients treated with a combination of pegylated interferon (PEG-IFN)-α2a and adefovir.

METHODS

We treated 92 chronic hepatitis B patients (44 hepatitis B e antigen [HBeAg]-positive and 48 HBeAg-negative) with HBV DNA > 100,000 copies/ml (> 17,182 IU/ml) with PEG-IFN and adefovir for 48 weeks and followed them up for 2 years. Baseline markers for HBeAg loss, combined response (HBeAg negativity, HBV DNA levels ≤ 2,000 IU/ml and alanine aminotransferase [ALT] normalization) and hepatitis B surface antigen (HBsAg) loss were evaluated.

RESULTS

Two years after the end of treatment, rates of HBeAg loss and HBsAg loss in HBeAg-positive patients were 18/44 (41%) and 5/44 (11%), respectively. In HBeAg-negative patients, rates of combined response and HBsAg loss were 12/48 (25%) and 8/48 (17%), respectively. HBeAg-negative patients with HBsAg loss had lower baseline HBsAg levels than those without HBsAg loss (mean HBsAg 2.35 versus 3.55 log10 IU/ml; P < 0.001). They also had lower HBV DNA levels and were more often (PEG-)IFN experienced. Baseline HBsAg was the only independent predictor of HBsAg loss (OR 0.02; P = 0.01).

CONCLUSIONS

With combination therapy of PEG-IFN and adefovir for 48 weeks, a high rate of HBsAg loss was observed in both HBeAg-positive (11%) and HBeAg-negative (17%) patients 2 years after treatment ended. In HBeAg-negative patients, a low baseline HBsAg level was a strong predictor for HBsAg loss.

Efficacy and adverse effects of the antiviral compound plerixafor in feline immunodeficiency virus-infected cats

BACKGROUND

Bicyclam derivatives inhibit feline immunodeficiency virus (FIV) replication through selective blockage of chemokine receptor CXCR4.

HYPOTHESIS/OBJECTIVES

CXCR4 antagonist plerixafor (AMD3100, 1,1'-bis-1,4,8,11-tetraazacyclotetradekan) alone or combination with adefovir (PMEA, 9-(2-phosphonylmethoxyethyl)adenine) safe and effective for treating FIV-infected cats.

ANIMALS

Forty naturally FIV-infected, privately owned cats.

MATERIALS AND METHODS

Prospective, placebo-controlled, double-blind clinical trial. Cats randomly classified into 4 treatment groups. Received AMD3100, PMEA, AMD3100 in combination with PMEA, or placebo for 6 weeks. Clinical and laboratory parameters, including CD4(+) and CD8(+) cell counts, FIV proviral and viral load measured by quantitative polymerase chain reaction (qPCR) evaluated. Additionally, FIV isolates from cats treated with AMD3100 tested for drug resistance.

RESULTS

FIV-infected cats treated with AMD3100 caused significant decrease in proviral load compared to placebo group (2.3 ± 3.8% to 1.9 ± 3.1%, of blood lymphocytes P < .05), but did not lead to improvement of clinical or immunological variables; it caused a decrease in serum magnesium concentration without clinical signs. No development of resistance of FIV isolates to AMD3100 found during treatment period. PMEA administration improved stomatitis (stomatitis score [degree 1 - 100] PMEA group: 23 ± 19 to 11 ± 10, P < .001; AMD3100 + PMEA group: 12 ± 17 to 3 ± 5, P < .05), but did not decrease proviral or viral load and caused anemia (RBC [× 10(6) /μL] PMEA group: 9.07 ± 1.60 to 6.22 ± 2.16, P < .05; AMD3100 ± PMEA group: 8.80 ± 1.23 to 5.84 ± 1.58, P < .001).

CONCLUSIONS AND CLINICAL IMPORTANCE

Administration of CXCR4 antagonists, as AMD3100, can induce reduction of proviral load and may represent viable treatment of FIV-infected cats. Combination treatment with PMEA not recommended.

The clinical benefits of tenofovir for simian immunodeficiency virus-infected macaques are larger than predicted by its effects on standard viral and immunologic parameters

Previous studies have demonstrated that tenofovir (9-[2-(phosphonomethoxy)propyl]adenine; PMPA) treatment is usually very effective in suppressing viremia in macaques infected with simian immunodeficiency virus (SIV). The present study focuses on a subset of infant macaques that were chronically infected with highly virulent SIVmac251, and for which prolonged tenofovir treatment failed to significantly suppress viral RNA levels in plasma despite the presence of tenofovirsusceptible virus at the onset of therapy. While untreated animals with similarly high viremia developed fatal immunodeficiency within 3-6 months, these tenofovir-treated animals had significantly improved survival (up to 3.5 years). This clinical benefit occurred even in animals for which tenofovir had little or no effect on CD4 and CD8 lymphocyte counts and antibody responses to SIV and test antigens. Thus, the clinical benefits of tenofovir were larger than predicted by plasma viral RNA levels and other routine laboratory parameters.

Viral kinetics in patients with lamivudine-resistant hepatitis B during adefovir-lamivudine combination therapy

BACKGROUND/AIMS

Mathematical analysis of viral kinetics during lamivudine-adefovir combination therapy has not yet been performed in patients with lamivudine-resistant hepatitis B.

METHODS

In 8 patients with lamivudine-resistant hepatitis B, adefovir dipivoxil (10 mg/day) was added to ongoing lamivudine. Viral decay during the first 8 weeks of combination therapy was described by a biphasic model to determine the efficacy: epsilon, of blocking viral production, the clearance: c, of free virus, and the loss of infected cells: delta.

RESULTS

Median epsilon was 98%, median c was 0.7/day, and median delta was 0.07/day. No significant association was found between viral kinetic and baseline parameters and virologic and biochemical treatment response. When compared with viral kinetic constants reported for higher dose adefovir dipivoxil monotherapy, epsilon was lower (P=0.026) and delta was higher (P=0.008) in this study whereas c did not differ between both studies.

CONCLUSIONS

Although a recent study did not show any differences in the reduction of HBV DNA comparing monotherapy with adefovir dipivoxil to adefovir-lamivudine combination therapy in patients with lamivudine-resistant chronic hepatitis B, mathematical analysis of early viral kinetics suggests an additional effect of lamivudine on the infected cell loss during adefovir-lamivudine combination therapy.

Tenofovir primes rhesus macaque cells in vitro for enhanced interleukin-12 secretion

We investigated if the antiviral drug tenofovir has immunomodulatory effects in macaques, similar to those described in murine models. While in vivo experiments were complicated by high individual and temporal variability of immune parameters, tenofovir primed macaque peripheral blood mononuclear cells in vitro for enhanced IL-12 secretion following exposure to bacterial antigens.

CD8+-cell-mediated suppression of virulent simian immunodeficiency virus during tenofovir treatment

The ability of tenofovir to suppress viremia in simian immunodeficiency virus (SIV)-infected macaques for years despite the presence of virulent viral mutants with reduced in vitro susceptibility is unprecedented in this animal model. In vivo cell depletion experiments demonstrate that tenofovir's ability to suppress viremia during acute and chronic infection is significantly dependent on the presence of CD8+ lymphocytes. Continuous tenofovir treatment was required to maintain low viremia. Although it is unclear whether this immune-mediated suppression of viremia is linked to tenofovir's direct antiviral efficacy or is due to independent immunomodulatory effects, these studies prove the concept that antiviral immune responses can play a crucial role in suppressing viremia during anti-human immunodeficiency virus drug therapy.

In vitro antihepadnaviral activities of combinations of penciclovir, lamivudine, and adefovir

Penciclovir (9-[2-hydroxy-1-(hydroxymethyl)-ethoxymethyl]guanine [PCV]), lamivudine ([-]-beta-L-2',3'-dideoxy-3'-thiacytidine [3TC]), and adefovir (9-[2-phosphonylmethoxyethyl]-adenine [PMEA]) are potent inhibitors of hepatitis B virus (HBV) replication. Lamivudine has recently received approval for clinical use against chronic human HBV infection, and both PCV and PMEA have undergone clinical trials against HBV in their respective prodrug forms (famciclovir and adefovir dipivoxil [bis-(POM)-PMEA]). Since multidrug combinations are likely to be used to control HBV infection, investigation of potential interactions between PCV, 3TC, and PMEA is important. Primary duck hepatocyte cultures which were either acutely or congenitally infected with the duck hepatitis B virus (DHBV) were used to investigate in vitro interactions between PCV, 3TC, and PMEA. Here we show that the anti-DHBV effects of all the combinations containing PCV, 3TC, and PMEA are greater than that of each of the individual components and that their combined activities are approximately additive or synergistic. These results may underestimate the potential in vivo usefulness of PMEA-containing combinations, since there is evidence that PMEA has immunomodulatory activity and, at least in the duck model of chronic HBV infection, is capable of inhibiting DHBV replication in cells other than hepatocytes, the latter being unaffected by treatment with either PCV or 3TC. Further investigation of the antiviral activities of these drug combinations is therefore required, particularly since each of the component drugs is already in clinical use.

Adefovir dipivoxil

Adefovir dipivoxil is an ester prodrug of the nucleoside reverse transcriptase inhibitor adefovir (PMEA), the prototype compound of the acyclic nucleoside phosphonates. It has better oral bioavailability than the parent compound. Adefovir dipivoxil 120mg once daily significantly reduced viral load compared with placebo when added to standard antiretroviral therapy in a 6-month, double-blind study in patients with HIV infection. Viral suppression was maintained during an additional 6-month nonblind extension phase. The drug was most effective in patients with baseline isolates containing the M184V lamivudine resistance mutation according to data from a virological substudy of a large placebo-controlled trial. Adefovir dipivoxil 60mg was as effective as 120mg (both once daily) after 20 weeks' treatment in a randomised double-blind study in antiretroviral-experienced (protease inhibitor-naive) patients. Viral suppression was generally maintained in patients who developed new reverse transcriptase mutations during adefovir dipivoxil monotherapy or combination therapy for up to 12 months. No clear pattern of particular clinical resistance mutations has emerged. GI disturbances, hepatic effects and delayed renal abnormalities are the principal adverse events seen with adefovir dipivoxil. Reductions in serum free carnitine levels may occur and coadministration of L-carnitine is recommended.

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.

9-[2-(Phosphonomethoxy)propyl]adenine (PMPA) therapy prolongs survival of infant macaques inoculated with simian immunodeficiency virus with reduced susceptibility to PMPA

Simian immunodeficiency virus (SIV) infection of newborn rhesus macaques is a useful animal model of human immunodeficiency virus infection for the study of the emergence and clinical implications of drug-resistant viral mutants. We previously demonstrated that SIV-infected infant macaques receiving prolonged treatment with 9-[2-(phosphonomethoxy)propyl]adenine (PMPA) developed viral mutants with fivefold reduced susceptibility to PMPA in vitro and that the development of these mutants was associated with the development of a K65R mutation and additional compensatory mutations in reverse transcriptase (RT). To study directly the virulence and clinical implications of these SIV mutants, two uncloned SIVmac isolates with similar fivefold reduced in vitro susceptibilities to PMPA but distinct RT genotypes, SIVmac055 (K65R, N69T, R82K A158S,S211N) and SIVmac385 (K65R, N69S, I118V), were each inoculated intravenously into six newborn rhesus macaques; 3 weeks later, three animals of each group were started on PMPA treatment. All six untreated animals developed persistently high levels of viremia and fatal immunodeficiency within 4 months. In contrast, the six PMPA-treated animals, despite having persistently high virus levels, survived significantly longer: 5 to 9 months for the three SIVmac055-infected infants and > or = 21 months for the three SIVmac385-infected infants. Virus from only one untreated animal demonstrated reversion to wild-type susceptibility and loss of the K65R mutation. In several other animals, additional RT mutations, including K64R and Y115F, were detected, but the biological role of these mutations is unclear since they did not affect the in vitro susceptibility of the virus to PMPA. In conclusion, this study demonstrates that although SIVmac mutants with the PMPA-selected K65R mutation in RT were highly virulent, PMPA treatment still offered strong therapeutic benefits. These results suggest that the potential emergence of HIV mutants with reduced susceptibility to PMPA in patients during prolonged PMPA therapy may not eliminate its therapeutic benefits.

Inhibition of duck hepatitis B virus replication by 9-(2-phosphonylmethoxyethyl)adenine, an acyclic phosphonate nucleoside analogue

The use of regimens that use nucleoside analogues for the treatment of chronic hepatitis B virus infection is often limited because of their high relapse rates. This is thought to be due to the persistence of virus in nonhepatocyte reservoirs and/or the viral covalently closed circular (CCC) DNA species in the nucleus of infected hepatocytes. We have evaluated the novel nucleoside analogue 9-(2-phosphonylmethoxyethyl)adenine (PMEA) in the duck model of hepatitis B. Eight Pekin-Aylesbury ducks congenitally infected with the duck hepatitis B virus (DHBV) were treated with PMEA at a dosage of 15 mg/kg of body weight/day via the intraperitoneal route for 4 weeks. At the end of the treatment period, four animals were killed and the remainder were monitored for a further 4-week drug-free period before analysis. The results were compared with those for eight age-matched, untreated controls. The levels of viremia, the total intrahepatic DHBV load, and CCC DNA, viral RNA, and protein levels were measured by Southern hybridization, Northern hybridization, and immunoblotting of the appropriate specimen, respectively. Viral proteins and DNA were also measured by immunohistochemistry (IHC) and in situ hybridization (ISH) of sections of liver and pancreatic tissue. PMEA treatment reduced the viremia to undetectable levels, while the total viral DNA load in the liver was reduced by 95% compared to the control level. Viral RNA and protein levels decreased by approximately 30%. ISH and IHC confirmed the PMEA-related intrahepatic changes and established that the amount of virus in bile duct epithelial cells (BDEC) was reduced by 70% during therapy. During the follow-up period all parameters of active virological replication returned to those for the age-matched controls. PMEA had no significant effect upon the number of virus-infected islet or acinar cells in the pancreas. PMEA at a dosage of 15 mg/kg/day has potent activity against DHBV found within hepatocytes and BDEC and inhibits DHBV replication in BDEC.

Administration of 9-[2-(phosphonomethoxy)propyl]adenine (PMPA) for prevention of perinatal simian immunodeficiency virus infection in rhesus macaques

Simian immunodeficiency virus (SIV) infection of newborn macaques is a useful animal model to explore novel strategies to reduce perinatal human immunodeficiency virus (HIV) infection. The availability of two easily distinguishable virus isolates, SIVmac251 and the simian/human immunodeficiency virus chimera SHIV-SF33, allows tracing the source of infection following inoculation with both viruses by different routes. In the present study, we evaluated the efficacy of pre- and postinoculation treatment regimens with 9-[2-(phosphonomethoxy)propyl]adenine (PMPA) to protect newborn macaques against simultaneous oral SIVmac251 and intravenous SHIV-SF33 inoculation. Untreated newborns became persistently infected following virus inoculation. When three pregnant macaques were given a single subcutaneous dose of PMPA 2 hr before cesarean section, their newborns became SIV-infected following SIV and SHIV inoculation shortly after birth. In contrast, when four newborn macaques were inoculated simultaneously with SIV and SHIV, and started immediately on PMPA treatment for 2 weeks, only one animal became persistently SIV-infected; the remaining three PMPA-treated newborns, however, had some evidence of an initial transient virus infection but were seronegative and healthy at 8 months of age. Our data demonstrate that PMPA treatment can reduce perinatal SIV infection and suggest that similar strategies may also be effective against HIV.

Potent antitumor activity of the acyclic nucleoside phosphonate 9-(2-phosphonylmethoxyethyl)adenine in choriocarcinoma-bearing rats

The acyclic nucleoside phosphonate 9-(2-phosphonylmethoxyethyl)adenine (PMEA) is a potent and selective antiretroviral agent which is currently evaluated in its oral prodrug form, bis(POM)PMEA (adefovir dipivoxil), in phase II and III clinical trials in human hepatitis B virus (HBV)- and human immunodeficiency virus (HIV)-infected individuals, respectively. We have now found that PMEA is also a potent inhibitor of growth of the highly aggressive choriocarcinoma tumor arising from rat choriocarcinoma RCHO cells grafted under the kidney capsule of syngeneic WKA/H rats. In untreated rats, massive invasive RCHO tumors, covering the whole surface of the kidney and resulting in a marked enlargement of the kidney, were observed at day 10 after tumor cell grafting. Daily treatment with PMEA at 25 mg/kg/day afforded a marked reduction in tumor size (i.e., smaller tumors and slight, if any, enlargement of the kidney). Increasing the PMEA dose to 50, 100 or 250 mg/kglday resulted in a gradual increase of the antitumor effect of the compound. At the highest dose tested, i.e., 250 mg/kg/day, PMEA completely suppressed tumor growth. The antitumor activity of PMEA persisted for at least 10 days after termination of drug treatment. In addition, delayed treatment with PMEA at a dose of 200 mg/kg/day, started at a time point where choriocarcinoma tumors had already developed, stopped further growth and even induced regression of the tumors. PMPA, a closely related structural analogue of PMEA, failed to inhibit choriocarcinoma tumor growth. This observation points to the specificity of PMEA as an antitumor agent. In view of our findings, the therapeutic potential of PMEA for the treatment of neoplastic diseases appears to merit further investigation.

Review: Present and future directions in the treatment of chronic hepatitis B infection

The last decade has witnessed substantial progress in the development of chemotherapeutic agents for chronic hepatitis B. However, the only currently licensed treatment in Australia, interferon-alpha, has low initial response rates and the adverse effects are often unacceptable. Of the newer agents in the class of nucleoside analogues, famciclovir and lamivudine are in phase III clinical trials with encouraging preliminary results, while other agents, such as bis-POM PMEA (Adefovir), are at phase I/II development. Future approaches to therapy will be governed by an understanding of the effects of nucleoside analogues on the natural history of the disease as well as on the hepatitis B virus hepatocyte interaction. Combination antiviral therapy should theoretically offer improved response rates, decrease the development of viral resistance, and provide the greatest reduction in viral load, but it has not yet been widely examined in the clinical setting. In this article, we review the currently available strategies, discuss potential problem areas, and speculate on promising approaches with combination chemotherapy and the features of agents soon to be trialed.

Immunomodulatory properties of antiviral acyclic nucleotide analogues: cytokine stimulatory and nitric oxide costimulatory effects

Acyclic nucleotide analogues exhibit strong activity against a broad range of viruses, including HIV-1 and -2. We have investigated their effects on in vitro secretion of cytokines and production of nitric oxide (NO) by murine peritoneal macrophages, factors known to play a role in virus replication. Included in the study were the most prominent compounds of the series: 9-(2-phosphonomethoxyethyl)adenine, 9-(2-phosphonomethoxyethyl)-2,6-diaminopurine, the (R)- and (S)-enantiomers of 9-(2-phosphonomethoxypropyl) adenine [(R)- or (S)-PMPA], (R)- and (S)-enantiomers of 9-(2-phosphonomethoxypropyl)-2,6-diaminopurine [(R)- or (S)-PMPDAP], 9-(2-phosphonomethoxyethyl)guanine (PMEG), and (S)-enantiomer of 1-(3-hydroxy-2-phosphonomethoxypropyl)cytosine [(S)-HPMPC]. PMEG, (R)-PMPA, and (S)-PMPA greatly enhanced the secretion of both tumour necrosis factor-alpha (TNF-alpha) and interleukin-10 (IL-10), (R)-PMPDAP stimulated only TNF-alpha, other test compounds were ineffective. None of them influenced the secretion of IL-2 or interferon-gamma (IFN-gamma). Both TNF-alpha and IL-10 have been found to be major factors determining enhancing effects of PMEG, (R)-PMPA, and (S)-PMPA on production of NO generated by exogenous IFN-gamma. The study points to a possible implication of immunomodulatory properties in the antiviral effects of some acyclic nucleotide analogues. In addition, our data support the view that endogenous IL-10 can stimulate certain macrophage functions.

Red blood cells mediated delivery of 9-(2-phosphonylmethoxyethyl)adenine to primary macrophages: efficiency metabolism and activity against human immunodeficiency virus or herpes simplex virus

Red blood cells (RBC) may act as selective carriers of drugs to macrophages, an important reservoir of viruses such as human immunodeficiency virus (HIV) and herpes simplex virus type 1 (HSV-1). We therefore assessed the incorporation of 9-(2-phosphonylmethoxyethyl)adenine (PMEA), a potent inhibitor of HIV and HSV-1) into RBC, its delivery to macrophages and its activity against HIV or HSV-1. Loading of PMEA in artificially aged opsonized RBC affords significant levels of intracellular PMEA. RBC metabolize PMEA to its active congener PMEA-diphosphate, although with low efficiency. Exposure of macrophages to RBC-encapsulated PMEA inhibits the replication of both HIV and HSV-1 (about 90% inhibition at the highest RBC:macrophages ratios) even if RBC were removed before virus challenge. By contrast, the antiviral activity of free PMEA removed before virus challenge was irrelevant at concentrations up to 150-fold higher than the 50% effective concentration (EC50). Finally, the antiviral effect of RBC-encapsulated PMEA correlates with PMEA levels in macrophages about 500-fold higher than those achieved by free PMEA (at concentrations 10-fold higher than the EC50). The efficacy of RBC-mediated delivery to macrophages of PMEA (and perhaps of compounds with shorter intracellular half-lives) warrants further studies in infectious diseases involving phagocytizing cells as main targets of the pathogen.

9-[2-(Phosphonomethoxy)propyl]adenine therapy of established simian immunodeficiency virus infection in infant rhesus macaques

The long-term therapeutic and toxic effects of 9-[2-(phosphonomethoxy)propyl]adenine (PMPA) were evaluated in simian immunodeficiency virus (SIV)-infected newborn rhesus macaques. Four untreated SIV-infected newborn macaques developed persistently high levels of viremia, and three of the four animals had rapidly fatal disease within 3 months. In contrast, long-term PMPA treatment of four newborn macaques starting 3 weeks after virus inoculation resulted in a rapid, pronounced, and persistent reduction of viremia in three of the four animals. Emergence of virus with fivefold-decreased susceptibility to PMPA occurred in all four PMPA-treated animals and was associated with the development of a lysine-to-arginine substitution at amino acid 65 (K65R mutation) and additional mutations in the reverse transcriptase; however, the clinical implications of this low-level drug resistance are nuclear. No toxic side effects have been seen, and all PMPA-treated animals have remained disease-free for more than 13 months. Our data suggest that PMPA holds much promise for the treatment of human immunodeficiency virus-infected human infants and adults.

Safety of 9-(2-phosphonylmethoxyethyl)adenine (PMEA) in patients with human immunodeficiency virus infection: a pilot study

The compound 9-(2-phosphonylmethoxyethyl)adenine (PMEA) is a potent inhibitor of a number of viruses in vitro such as human immunodeficiency virus types 1 and 2, herpes simplex virus types 1 and 2, hepatitis B virus, cytomegalovirus, and Epstein-Barr virus. PMEA also proved to be effective in vivo against feline immunodeficiency virus in cats and simian immunodeficiency virus in rhesus monkeys. In an open, non-placebo-controlled trial, the safety of weekly doses of PMEA in 10 patients with acquired immunodeficiency syndrome (AIDS) or AIDS-related complex was studied for a period of 11 weeks. CD4+ T-cell counts at baseline were between 10 and 450/mm(3). The drug was administered intravenously at a dose of 1000 mg. No serious side-effects were seen. On one occasion one patient showed alanine aminotransferase and aspartate aminotransferase levels 5 times higher than the upper limit of normal and another patient showed on one occasion aspartate aminotransferase levels 5 times higher than the upper limit of normal. In another patient serum amalyse levels increased, on one occasion 1.5 times above the upper limit of normal. An improvement in general well-being was reported by all patients. For patients with a CD4+ T-cell count > 100/mm(3) at baseline, the CD4+ T-cell count increased from a mean of 283/mm(3) at baseline to a mean of 448/mm(3) at the end of the study. Repeat infusions of PMEA at a dose of 1000 mg were safe and well tolerated. Our results suggest that PMEA, administrated according to this treatment schedule, may be effective in treating patients with human immunodeficiency virus infection.

(R)-9-(2-phosphonylmethoxypropyl)-2,6-diaminopurine is a potent inhibitor of feline immunodeficiency virus infection

The antiviral efficacy of acyclic nucleoside phosphonates, including 9-(2-phosphonylmethoxyethyl)adenine (PMEA) and (R)-9-(2-phosphonylmethoxypropyl)-2,6-diaminopurine [(R)-PMPDAP] against feline immunodeficiency virus (FIV) infection was determined. (R)-PMPDAP showed the highest selectivity index (> 2,000) in vitro. Treatment of experimentally FIV-infected asymptomatic cats with PMEA or (R)-PMPDAP had no effect on the CD4+/CD8+ ratio. However, mean plasma viral RNA concentrations decreased significantly in the (R)-PMPDAP-treated cats. Our data show that, in comparison to PMEA, (R)-PMPDAP is a more potent and less toxic inhibitor of FIV replication both in vitro and in vivo.

9-(2-Phosphonylmethoxyethyl) adenine increases the survival of influenza virus-infected mice by an enhancement of the immune system

PMEA (9-(2-phosphonylmethoxyethyl)adenine) is a potent inhibitor of DNA viruses and retroviruses able to enhance natural immune functions such as natural killer cell activity and interferon production. The results reported in this paper show that the treatment with PMEA significatively decreased the mortality of mice challenged with influenza A/PR8 virus (an RNA virus, non sensitive to the antiviral effect of PMEA) compared to untreated, infected controls (median survival 8.64 days and 7.61 days, respectively), and reduced lung weight and consolidation (two surrogate markers of virus infection). Furthermore, virus titer obtained from lung homogenates was substantially decreased in PMEA-treated mice compared to controls. Finally, enhancement of natural killer cell activity was achieved in PMEA-treated A/PR8-infected mice compared to A/PR8-infected controls. Overall, results suggest that PMEA decreases the influenza virus-related mortality and morbidity through the enhancement of some immune functions, and that this effect might be additive or even synergystic with the direct inhibitory effect of DNA viruses or retroviruses induced by PMEA itself. This supports the importance of evaluating this drug in patients with diseases related to herpesviruses or to human immunodeficiency virus.

Enhancement of natural killer activity and interferon induction by different acyclic nucleoside phosphonates

Acyclic nucleoside phosphonate (ANP) analogues are a class of compounds with potent activity against herpesviruses and/or retroviruses. Our preliminary experiments have shown that 9-(2-phosphonylmethoxyethyl)adenine (PMEA), a prototype of the ANP family, enhances some parameters of natural immunity. In this paper we have evaluated the effect of different schedules of administration of PMEA and other ANP analogues of clinical interest upon natural killer (NK) activity and interferon (IFN) production in a mouse model. The results show that PMEA significantly enhances NK activity and interferon production. Other ANP analogues tested in our system, i.e., 9-(2-phosphonylmethoxyethyl)-2,6-diaminopurine (PMEDAP), and 9-(3-fluoro-2-phosphonylmethoxypropyl)adenine (FPMPA), similarly induced enhancement of natural immunity. The immunomodulating effect of PMEA was even more pronounced with a single administration compared to repeated administrations of the drug. Dose-dependent enhancement of NK activity and IFN production could also be demonstrated during chronic administration of PMEA (more resembling to what will be the schedule of administration of this drug in patients). Overall, the data here presented suggest that the enhancement of some natural immune functions induced by ANP analogues may add to the direct antiviral activity of these drugs against retroviruses and herpesviruses, and thus may be able to increase the host resistance against viral infections.

Inhibitory effect of 9-(2-phosphonylmethoxyethyl)-adenine (PMEA) on human and duck hepatitis B virus infection

9-(2-Phosphonylmethoxyethyl)adenine (PMEA) was evaluated for its inhibitory effect on hepadnavirus replication in three different cell systems, i.e., human hepatoma cell lines HepG2 2.2.15 and HB611 (transfected with human hepatitis B virus (HBV)) and primary cultures of duck hepatocytes infected with duck hepatitis B virus (DHBV). PMEA inhibited HBV release from HepG2 2.2.15 cells and HB611 cells at a 50% inhibitory concentration (IC50) of 0.7 and 1.2 microM, respectively. Intracellular viral DNA synthesis was inhibited at concentrations equivalent to those required to inhibit virus release from the cells. DHBV secretion from duck hepatocytes was inhibited by PMEA at an IC50 of 0.2 microM. HBsAg secretion was inhibited by PMEA in a concentration-dependent manner in HB611 cells and DHBV-infected duck hepatocytes but not HepG2 2.2.15 cells. The 50% cytotoxic concentration, as measured by inhibition of [3H-methyl]deoxythymidine incorporation was 150 microM for the two human hepatoma cell lines and 40 microM for the duck hepatocyte cultures. In a pilot experiment PMEA was found to reduce the amounts of DHBV DNA in the serum of Pekin ducks.

Anti-AIDS drug development: challenges and strategies

A myriad of chemical derivatives has been shown to inhibit in vitro replication of the AIDS virus at concentrations that are nontoxic to the host cells. The majority of these agents acts by either (i) inhibiting enzymes such as reverse transcriptase (RT), protease, or glucosidase, (ii) arresting expression of genes or gene products, or (iii) inhibiting viral processes such as giant cell (syncytia) formation or viral binding to the target cell. The nucleoside RT inhibitors are the most widely studied agents at both the preclinical and the clinical levels. Their inability to cure AIDS has stimulated the discovery of several novel nonnucleoside RT inhibitors, possessing varied structures and demonstrating activity at nanomolar concentrations. These agents demonstrate a unique mode of binding to RT and show a high specificity for HIV-1. Protease inhibitors, soluble CD4 derivatives, oligonucleotides, and many anionic derivatives also demonstrate potent anti-HIV-1 activities. These derivatives possess mechanisms of action different to the nucleosides and exhibit selectivity as exemplified by their high in vitro therapeutic indices. This article discusses the structural parameters that govern activity in these agents, the pros and cons regarding the development of these compounds as putative anti-AIDS agents, and the future promise of searching for newer agents directed at novel targets to inhibit the AIDS virus.