Antiretroviral activity and pharmacokinetics in mice of oral bis(pivaloyloxymethyl)-9-(2-phosphonylmethoxyethyl)adenine, the bis(pivaloyloxymethyl) ester prodrug of 9-(2-phosphonylmethoxyethyl)adenine

Glycoconjugates of adefovir and tenofovir as asialoglycoprotein-mediated Anti-HBV prodrugs with enhanced liver targeting

Hepatitis B virus (HBV) infection remains a significant global health challenge, often leading to severe liver complications such as cirrhosis and cancer. Current treatments rely heavily on nucleos(t)ide analogues like adefovir and tenofovir due to their potent antiviral effects. However, their clinical utility is limited by insufficient liver targeting, leading to off-target side effects, particularly nephrotoxicity. To improve liver-specific drug delivery and reduce adverse effects, we designed novel liver-targeted prodrugs by conjugating adefovir and tenofovir with N-acetylgalactosamine (GalNAc) and tris-GalNAc ligands, which have high affinity for the asialoglycoprotein receptor (ASGPR) predominantly expressed in hepatocytes. Four prodrugs (A1, A2, T1, and T2) were synthesized and evaluated for cytotoxicity, maximum tolerated dose, anti-HBV activity, metabolic stability, pharmacokinetics, and liver-targeting properties. The prodrugs exhibited low cytotoxicity, robust anti-HBV activity, and enhanced selectivity compared to their parent drugs. Notably, the tris-GalNAc conjugates A2 and T2 demonstrated superior liver targeting, showing a threefold higher concentration in the liver compared to the kidneys, thus minimizing renal exposure. These findings suggest that GalNAc and tris-GalNAc conjugation is a promising strategy for enhancing the therapeutic efficacy and safety of adefovir and tenofovir, with potential for further optimization as liver-targeted anti-HBV prodrugs.

Phosphoryl Prodrugs: Characteristics to Improve Drug Development

Phosphoryl prodrugs are key compounds in drug development. Biologically active phosphoryl compounds often have negative charges on the phosphoryl group, and as a result, frequently have poor pharmacokinetic (PK) profiles. The use of lipophilic moieties bonded to the phosphorus (or attached oxygen atoms) masks the negative charge of the phosphoryl group, cleavage releasing the active molecule. The use of prodrugs to improve the PK of active parent molecules is an essential step in drug development. This review highlights promising trends in terminal elimination half-life, C_max, clearance, oral bioavailability, and cLogP in phosphoryl prodrugs. We focus on specific prodrug families: esters, amidates, and ProTides. We conclude that moderating lipophilicity is a key part of prodrug success. This type of evaluation is important for drug development, regardless of clinical application. It is our hope that this analysis, and future ones like it, will play a significant role in prodrug evolution.

An enolase inhibitor for the targeted treatment of ENO1-deleted cancers

Inhibiting glycolysis remains an aspirational approach for the treatment of cancer. We have previously identified a subset of cancers harbouring homozygous deletion of the glycolytic enzyme enolase (ENO1) that have exceptional sensitivity to inhibition of its redundant paralogue, ENO2, through a therapeutic strategy known as collateral lethality. Here, we show that a small-molecule enolase inhibitor, POMHEX, can selectively kill ENO1-deleted glioma cells at low-nanomolar concentrations and eradicate intracranial orthotopic ENO1-deleted tumours in mice at doses well-tolerated in non-human primates. Our data provide an in vivo proof of principle of the power of collateral lethality in precision oncology and demonstrate the utility of POMHEX for glycolysis inhibition with potential use across a range of therapeutic settings.

Investigation of the Structure and Dynamics of Antiviral Drug Adefovir Dipivoxil by Site-Specific Spin-Lattice Relaxation Time Measurements and Chemical Shift Anisotropy Tensor Measurements

Adefovir is regarded as a potential antiviral agent. However, it cannot be considered as a valuable drug candidate due to its high polarity that limits its permeability across the human intestinal mucosa. When the ribose phosphate group of adefovir is replaced by the isopolar phosphonomethyl ether functionality, it neutralizes the negative charge of the drug. This makes the drug lipid-soluble and potent to diffuse across the cell membrane. The prodrug adefovir dipivoxil is regarded as a potent antiviral drug against hepatitis B virus (HBV), human immunodeficiency virus (HIV), Rauscher murine leukemia virus (R-MuLV), murine cytomegalovirus (MCMV), herpes simplex virus (HSV), simian immunodeficiency virus (SIV), and feline immunodeficiency virus (FIV). The correlation between the structure and the dynamics of adefovir dipivoxil is determined by measuring the principal components of chemical shift anisotropy (CSA) tensor, site-specific spin-lattice relaxation time, and molecular correlation time at crystallographically different carbon nuclei sites. The CSA parameters, spin-lattice relaxation time, and molecular correlation time of phosphorous nucleus of the organophosphate group of adefovir dipivoxil molecule are also determined. The spin-lattice relaxation time of carbon nuclei varies from 1 to 107 s. The range of molecular correlation time also varies from 10-4 to 10-8 s. These remarkable diversities of motional dynamics of the molecules imply that there exist various motional degrees of freedom within this valuable drug and these motional degrees of freedom are independent of each other, which may be the reason for the biological activities exhibited by the drug. The correlation between structure and dynamics of such an important antiviral drug adefovir dipivoxil can be visualized by these types of extensive spectroscopic measurements, which will enlighten the path of inventing advanced medicine in the pharmaceutical industry, and it will also illuminate the understanding of the structure-activity relationships of antiviral drug.

Metabolic Efficacy of Phosphate Prodrugs and the Remdesivir Paradigm

Drugs that contain phosphates (and phosphonates or phosphinates) have intrinsic absorption issues and are therefore often delivered in prodrug forms to promote their uptake. Effective prodrug forms distribute their payload to the site of the intended target and release it efficiently with minimal byproduct toxicity. The ability to balance unwanted payload release during transit with desired release at the site of action is critical to prodrug efficacy. Despite decades of research on prodrug forms, choosing the ideal prodrug form remains a challenge which is often solved empirically. The recent emergency use authorization of the antiviral remdesivir for COVID-19 exemplifies a new approach for delivery of phosphate prodrugs by parenteral dosing, which minimizes payload release during transit and maximizes tissue payload distribution. This review focuses on the role of metabolic activation in efficacy during oral and parenteral dosing of phosphate, phosphonate, and phosphinate prodrugs. Through examining prior structure–activity studies on prodrug forms and the choices that led to development of remdesivir and other clinical drugs and drug candidates, a better understanding of their ability to distribute to the planned site of action, such as the liver, plasma, PBMCs, or peripheral tissues, can be gained. The structure–activity relationships described here will facilitate the rational design of future prodrugs.

Stability and Efficiency of Mixed Aryl Phosphonate Prodrugs

A set of phosphonate prodrugs of a butyrophilin ligand was synthesized and evaluated for plasma stability and cellular activity. The mixed aryl acyloxy esters were prepared either via a standard sequence through the phosphonic acid chloride, or through the more recently reported, and more facile, triflate activation. In the best of cases, this class of prodrugs shows cellular potency similar to that of bis-acyloxyalkyl phosphonate prodrugs and plasma stability similar to that of aryl phosphonamidates. For example, {[((3E)-5-hydroxy-4-methylpent-3-en-1-yl) (naphthalen-2-yloxy)phosphoryl]oxy}methyl 2,2-dimethylpropanoate can activate BTN3A1 in K562 cells after just 15 minutes of exposure (at an EC50 value of 31 nm) and is only partially metabolized (60 % remaining) after 20 hours in human plasma. Other related novel analogues showed similar potency/stability profiles. Therefore, mixed aryl acyloxyalkyl phosphonate prodrugs are an exciting new strategy for the delivery of phosphonate-containing drugs.

A Clinical Drug-Drug Interaction Study Assessing a Novel Drug Transporter Phenotyping Cocktail With Adefovir, Sitagliptin, Metformin, Pitavastatin, and Digoxin

A new probe drug cocktail containing substrates of important drug transporters was tested for mutual interactions in a clinical trial. The cocktail consisted of (predominant transporter; primary phenotyping metric): 10 mg adefovir-dipivoxil (OAT1; renal clearance (CL_R )), 100 mg sitagliptin (OAT3; CL_R ), 500 mg metformin (several renal transporters; CL_R ), 2 mg pitavastatin (OATP1B1; clearance/F), and 0.5 mg digoxin (intestinal P-gp, renal P-gp, and OATP4C1; peak plasma concentration (C_max ) and CL_R ). Using a randomized six-period, open change-over design, single oral doses were administrated either concomitantly or separately to 24 healthy male and female volunteers. Phenotyping metrics were evaluated by noncompartmental analysis and compared between periods by the standard average bioequivalence approach (boundaries for ratios 0.80-1.25). Primary metrics supported the absence of relevant interactions, whereas secondary metrics suggested that mainly adefovir was a victim of minor drug-drug interactions (DDIs). All drugs were well tolerated. This cocktail may be another useful tool to assess transporter-based DDIs in vivo.

An improved synthesis of adefovir and related analogues

An improved synthesis of the antiviral drug adefovir is presented. Problems associated with current routes to adefovir include capricious yields and a reliance on problematic reagents and solvents, such as magnesium tert-butoxide and DMF, to achieve high conversions to the target. A systematic study within our laboratory led to the identification of an iodide reagent which affords higher yields than previous approaches and allows for reactions to be conducted up to 10 g in scale under milder conditions. The use of a novel tetrabutylammonium salt of adenine facilitates alkylations in solvents other than DMF. Additionally, we have investigated how regioselectivity is affected by the substitution pattern of the nucleobase. Finally, this chemistry was successfully applied to the synthesis of several new adefovir analogues, highlighting the versatility of our approach.

Highly convergent synthesis and antiviral activity of (E)-but-2-enyl nucleoside phosphonoamidates

Several hitherto unknown (E)-but-2-enyl nucleoside phosphonoamidate analogs (ANPs) were prepared directed with nitrogen reagents by cross-metathesis in water-under ultrasound irradiation. Two diastereoisomers were formally identified by X-ray diffraction. These compounds were evaluated against a large spectrum of DNA and RNA viruses. Among them, the phosphonoamidate thymine analogue 19 emerged as the best prodrug against varicella-zoster virus (VZV) with EC₅₀ values of 0.33 and 0.39 μM for wild-type and thymidine kinase deficient strains, respectively, and a selectivity index ≥200 μM. This breakthrough approach paves the way for new purine and pyrimidine (E)-but-2-enyl phosphonoamidate analogs.

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Phosphonoamidate prodrugs acyclic nucleosides were synthesized by convergent approach.

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Metathesis reaction in water was used between pyrimidic bases and a new phosphonoamidate synthons.

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EC₅₀ values of any molecules were in (sub)micromolar range against DNA viruses.

Current and evolving approaches for improving the oral permeability of BCS Class III or analogous molecules

The Biopharmaceutics Classification System (BCS) classifies pharmaceutical compounds based on their aqueous solubility and intestinal permeability. The BCS Class III compounds are hydrophilic molecules (high aqueous solubility) with low permeability across the biological membranes. While these compounds are pharmacologically effective, poor absorption due to low permeability becomes the rate-limiting step in achieving adequate bioavailability. Several approaches have been explored and utilized for improving the permeability profiles of these compounds. The approaches include traditional methods such as prodrugs, permeation enhancers, ion-pairing, etc., as well as relatively modern approaches such as nanoencapsulation and nanosizing. The most recent approaches include a combination/hybridization of one or more traditional approaches to improve drug permeability. While some of these approaches have been extremely successful, i.e. drug products utilizing the approach have progressed through the USFDA approval for marketing; others require further investigation to be applicable. This article discusses the commonly studied approaches for improving the permeability of BCS Class III compounds.

Model Linking Plasma and Intracellular Tenofovir/Emtricitabine with Deoxynucleoside Triphosphates

The coformulation of the nucleos(t)ide analogs (NA) tenofovir (TFV) disoproxil fumarate (TDF) and emtricitabine (FTC) is approved for HIV-infection treatment and prevention. Plasma TFV and FTC undergo complicated hybrid processes to form, accumulate, and retain as their active intracellular anabolites: TFV-diphosphate (TFV-DP) and FTC-triphosphate (FTC-TP). Such complexities manifest in nonlinear intracellular pharmacokinetics (PK). In target cells, TFV-DP/FTC-TP compete with endogenous deoxynucleoside triphosphates (dNTP) at the active site of HIV reverse transcriptase, underscoring the importance of analog:dNTP ratios for antiviral efficacy. However, NA such as TFV and FTC have the potential to disturb the dNTP pool, which could augment or reduce their efficacies. We conducted a pharmacokinetics-pharmacodynamics (PKPD) study among forty subjects receiving daily TDF/FTC (300 mg/200 mg) from the first-dose to pharmacological intracellular steady-state (30 days). TFV/FTC in plasma, TFV-DP/FTC-TP and dNTPs in peripheral blood mononuclear cells (PBMC) were quantified using validated LC/MS/MS methodologies. Concentration-time data were analyzed using nonlinear mixed effects modeling (NONMEM). Formations and the accumulation of intracellular TFV-DP/FTC-TP was driven by plasma TFV/FTC, which was described by a hybrid of first-order formation and saturation. An indirect response link model described the interplay between TFV-DP/FTC-TP and the dNTP pool change. The EC₅₀ (interindividual variability, (%CV)) of TFV-DP and FTC-TP on the inhibition of deoxyadenosine triphosphate (dATP) and deoxycytidine triphosphate (dCTP) production were 1020 fmol/10⁶ cells (130%) and 44.4 pmol/10⁶ cells (82.5%), resulting in (90% prediction interval) 11% (0.45%, 53%) and 14% (2.6%, 35%) reductions. Model simulations of analog:dNTP molar ratios using IPERGAY dosing suggested that FTC significantly contributes to the protective effect of preexposure prophylaxis (PrEP). Simulation-based intracellular operational multiple dosing half-lives of TFV-DP and FTC-TP were 6.7 days and 33 hours. This model described the formation of intracellular TFV-DP/FTC-TP and the interaction with dNTPs, and can be used to simulate analog:dNTP time course for various dosing strategies.

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.

Remofovir Mesylate: A Prodrug of PMEA with Improved Liver-Targeting and Safety in Rats and Monkeys

Adefovir dipivoxil (Hepsera™), a first-line therapy for chronic hepatitis B, is an esterase-activated prodrug of PMEA. Dose-limiting nephrotoxicity necessitates suboptimal dosing at 10 mg/day. Remofovir mesylate (MB06866Q) (Hepavir B) is a CYP3A4-activated prodrug of PMEA based on the HepDirect™ technology that targets PMEA to the liver. In a whole body autoradiography study in rats after oral dosing (30 mg/kg) of [14C]adefovir dipivoxil or [14C]remofovir mesylate, remofovir yielded 15 times higher concentrations of radioactivity in the liver than adefovir dipivoxil, but only one-third of the concentrations in the kidney. After oral dosing (4 mg/kg) of the same radiolabelled agents in cynomolgus monkeys, remofovir mesylate yielded 60 times higher levels of total radioactivity in the liver, but only two-thirds of total radioactivity levels in the kidney. Thus, remofovir mesylate may provide better efficacy and reduced nephrotoxicity. In portal vein-cannulated rats (30 mg/kg) after a single oral dose of [14C]adefovir dipivoxil or [14C]remofovir mesylate, no PMEA was detectable in rat portal plasma early after dosing, indicating that intestinal CYP3A4 does not play a role in conversion of remofovir mesylate to PMEA. The portal/systemic extraction ratio was quite high in both models, suggesting good liver-targeting properties. Portal and systemic remofovir/PMEA ratio indicates that the liver is the site of conversion of remofovir to PMEA. 28-Day toxicity studies demonstrated renal toxicity in rats at doses of 100 mg/kg or higher with no safety concerns at 30 mg/kg and acceptable safety in monkeys at doses up to 60 mg/kg. Thus, in rats and non-human primates, remofovir mesylate has liver-targeting properties and is safer than adefovir dipivoxil.

Bisamidate Prodrugs of 2-Substituted 9-[2-(Phosphonomethoxy)ethyl]adenine (PMEA, adefovir) as Selective Inhibitors of Adenylate Cyclase Toxin from Bordetella pertussis

Novel small-molecule agents to treat Bordetella pertussis infections are highly desirable, as pertussis (whooping cough) remains a serious health threat worldwide. In this study, a series of 2-substituted derivatives of 9-[2-(phosphonomethoxy)ethyl]adenine (PMEA, adefovir), in their isopropyl ester bis(L-phenylalanine) prodrug form, were designed and synthesized as potent inhibitors of adenylate cyclase toxin (ACT) isolated from B. pertussis. The series consists of PMEA analogues bearing either a linear or branched aliphatic chain or a heteroatom at the C2 position of the purine moiety. Compounds with a small C2 substituent showed high potency against ACT without cytotoxic effects as well as good selectivity over human adenylate cyclase isoforms AC1, AC2, and AC5. The most potent ACT inhibitor was found to be the bisamidate prodrug of the 2-fluoro PMEA derivative (IC50 =0.145 μM). Although the bisamidate prodrugs reported herein exhibit overall lower activity than the bis(pivaloyloxymethyl) prodrug (adefovir dipivoxil), their toxicity and plasma stability profiles are superior. Furthermore, the bisamidate prodrug was shown to be more stable in plasma than in macrophage homogenate, indicating that the free phosphonate can be effectively distributed to target tissues, such as the lungs. Thus, ACT inhibitors based on acyclic nucleoside phosphonates may represent a new strategy to treat whooping cough.

Linking the population pharmacokinetics of tenofovir and its metabolites with its cellular uptake and metabolism

Empirical pharmacokinetic models are used to explain the pharmacokinetics of the antiviral drug tenofovir (TFV) and its metabolite TFV diphosphate (TFV-DP) in peripheral blood mononuclear cells. These empirical models lack the ability to explain differences between the disposition of TFV-DP in HIV-infected patients vs. healthy individuals. Such differences may lie in the mechanisms of TFV transport and phosphorylation. Therefore, we developed an exploratory model based on mechanistic mass transport principles and enzyme kinetics to examine the uptake and phosphorylation kinetics of TFV. TFV-DP median C_max from the model was 38.5 fmol/10⁶ cells, which is bracketed by two reported healthy volunteer studies (38 and 51 fmol/10⁶ cells). The model presented provides a foundation for exploration of TFV uptake and phosphorylation kinetics for various routes of TFV administration and can be updated as more is known on actual mechanisms of cellular transport of TFV.

In vitro evaluation of 9-(2-phosphonylmethoxyethyl)adenine ester analogues, a series of anti-HBV structures with improved plasma stability and liver release

Chronic hepatitis B virus (HBV) infection may lead to liver cirrhosis and hepatocellular carcinoma, but few drugs are available for its treatment. Acyclic nucleoside phosphonates (ANPs) have remarkable antivirus activities but are not easily absorbed from the gastrointestinal tract and accumulate in the kidneys, resulting in nephrotoxicity. Therefore, there is a need to find effective liver site-specific prodrugs. The dipivaloyloxymethyl ester of 9-(2-phosphonylmethoxyethyl)adenine (PMEA)-adefovir dipivoxil (ADV)-is a first-line therapy drug for chronic hepatitis B with a low therapeutic index because of renal toxicity and low hepatic uptake. In this study, a series of PMEA derivatives were synthesized to enhance plasma stability and liver release. The metabolic stability of ADV (Chemical I) and its two analogues (Chemicals II and III) was evaluated in rat plasma and liver homogenate in vitro. An ion-pair reverse-phase HPLC-UV method and a hybrid ion trap and high-resolution time-of-flight mass spectrometry (LC-IT-TOF-MS) were used to evaluate the degradation rate of the analogues and to identify their intermediate metabolites, respectively. Chemicals I and II were hydrolyzed by cleavage of the C-O bond to give monoesters. Sufficient enzymatic activation in the liver homogenate through a relatively simple metabolic pathway, in addition to a favorable stability profile in rat plasma, made Chemical II an optimal candidate. Next, six analogues based on the structure of Chemical II were synthesized and evaluated in plasma and liver homogenate. Compared to Chemical II, these compounds generated less active PMEA levels in rat liver homogenate. Therefore, chemical modification of Chemical II may lead to new promising PMEA derivatives with enhanced plasma stability and liver activation.

Amidate prodrugs of 9-[2-(phosphonomethoxy)ethyl]adenine as inhibitors of adenylate cyclase toxin from Bordetella pertussis

Adenylate cyclase toxin (ACT) is the key virulence factor of Bordetella pertussis that facilitates its invasion into the mammalian body. 9-[2-(Phosphonomethoxy)ethyl]adenine diphosphate (PMEApp), the active metabolite of the antiviral drug bis(POM)PMEA (adefovir dipivoxil), has been shown to inhibit ACT. The objective of this study was to evaluate six novel amidate prodrugs of PMEA, both phenyloxy phosphonamidates and phosphonodiamidates, for their ability to inhibit ACT activity in the J774A.1 macrophage cell line. The two phenyloxy phosphonamidate prodrugs exhibited greater inhibitory activity (50% inhibitory concentration [IC50] = 22 and 46 nM) than the phosphonodiamidates (IC50 = 84 to 3,960 nM). The inhibitory activity of the prodrugs correlated with their lipophilicity and the degree of their hydrolysis into free PMEA in J774A.1 cells. Although the prodrugs did not inhibit ACT as effectively as bis(POM)PMEA (IC50 = 6 nM), they were significantly less cytotoxic. Moreover, they all reduced apoptotic effects of ACT and prevented an ACT-induced elevation of intracellular [Ca(2+)]i. The amidate prodrugs were less susceptible to degradation in Caco-2 cells compared to bis(POM)PMEA, while they exerted good transepithelial permeability in this assay. As a consequence, a large amount of intact amidate prodrug is expected to be available to target macrophages in vivo. This feature makes nontoxic amidate prodrugs attractive candidates for further investigation as novel antimicrobial agents.

Synthesis and anti-herpetic activity of phosphoramidate ProTides

Among the many prodrug approaches aimed at delivering nucleoside monophosphates into cells, the phosphoramidate ProTide approach is one that has shown success, which has made it possible for some of the phosphoramidates to enter into clinical trials. Herein, we report the synthesis and antiviral activity of a series of phosphoramidate ProTides designed to bypass the thymidine kinase (TK) dependence of the parent nucleoside analogues. Phosphoramidate derivatives of (E)-5-(2-bromovinyl)-2'-deoxyuridine (BVDU) that contain L-alanine or pivaloyloxymethyl iminodiacetate (IDA-POM) exhibit anti-HSV-1 and anti-VZV activity in cell cultures, but they largely lost antiviral potency against TK-deficient virus strains. Among deazapurine nucleosides and their phosphoramidate derivatives, the 7-deazaadenine containing nucleosides and their phosphoramidate triester derivatives showed weak antiviral activity against VZV. Apparently, intracellular nucleotide delivery with these phosphoramidates is partly successful. However, none of the compound prodrugs showed superior activity to their parent drugs.

Drug-Drug and Drug-Food Interactions Between Tenofovir Disoproxil Fumarate and Didanosine

The drug-drug and drug-food interactions between tenofovir DF and didanosine EC were evaluated in 2 pharmacokinetic studies in healthy adult subjects. When 400 mg was dosed with tenofovir DF, mean didanosine AUC was increased by 44% to 60% following fasted or fed administration. Staggered coadministration (2 hour, fasted) of a reduced didanosine dose of 250 mg resulted in equivalent didanosine exposure, while simultaneous administration with tenofovir DF in the fasted and fed state resulted in didanosine AUCs similar to that of the reference treatment of 400 mg alone in the fasted state. These data indicate that a dose reduction of didanosine is warranted when it is used with tenofovir DF. The drug-drug-food interaction of didanosine may offer more flexible dosing of didanosine EC when it is used with tenofovir DF. Patients receiving tenofovir DF and didanosine together should be carefully monitored for safety and efficacy.

Pharmacokinetics and pharmacodynamics of the reverse transcriptase inhibitor tenofovir and prophylactic efficacy against HIV-1 infection

Antiviral pre-exposure prophylaxis (PrEP) through daily drug administration can protect healthy individuals from HIV-1 infection. While PrEP was recently approved by the FDA, the potential long-term consequences of PrEP implementation remain entirely unclear. The aim of this study is to predict the efficacy of different prophylactic strategies with the pro-drug tenofovir-disoproxil-fumarate (TDF) and to assess the sensitivity towards timing- and mode of TDF administration (daily- vs. single dose), adherence and the number of transmitted viruses. We developed a pharmacokinetic model for TDF and its active anabolite tenofovir-diphosphate (TFV-DP) and validated it with data from 4 different trials, including 4 distinct dosing regimes. Pharmacokinetics were coupled to an HIV model and viral decay following TDF mono-therapy was predicted, consistent with available data. Subsequently, a stochastic approach was used to estimate the % infections prevented by (i) daily TDF-based PrEP, (ii) one week TDF started either shortly before, or -after viral exposure and (iii) a single dose oral TDF before viral challenge (sd-PrEP). Analytical solutions were derived to assess the relation between intracellular TFV-DP concentrations and prophylactic efficacy. The predicted efficacy of TDF was limited by a slow accumulation of active compound (TFV-DP) and variable TFV-DP half-life and decreased with increasing numbers of transmitted viruses. Once daily TDF-based PrEP yielded [Formula: see text]80% protection, if at least 40% of pills were taken. Sd-PrEP with 300 mg or 600 mg TDF could prevent [Formula: see text]50% infections, when given at least before virus exposure. The efficacy dropped to [Formula: see text]10%, when given 1 h before 24 h exposure. Efficacy could not be increased with increasing dosage or prolonged administration. Post-exposure prophylaxis poorly prevented infection. The use of drugs that accumulate more rapidly, or local application of tenofovir gel may overcome the need for drug administration long before virus exposure.

Antiviral Chemistry & Chemotherapy's Current Antiviral Agents FactFile (2nd Edition): Retroviruses and Hepadnaviruses

There are at present exactly 25 compounds that have been formally approved for the treatment of retrovirus (that is HIV) infections: seven nucleoside reverse transcriptase inhibitors (NRTIs), one nucleotide reverse transcriptase inhibitor (NtRTI), four non-nucleoside reverse transcriptase inhibitors (NNRTIs), 10 protease inhibitors (PIs), one core-ceptor inhibitor (CRI), one fusion inhibitor (FI) and one integrase inhibitor (INI). Other compounds expected to be approved for the treatment of HIV infections in the near future are the NNRTI rilpivirine, the CRI vicriviroc and the INI elvitegravir. To obtain synergistic activity, enable lower dosage levels, thus minimizing toxic side effects, and particularly to reduce the risk of drug resistance development, common wisdom dictates that the HIV inhibitors should be used in drug combination regimens. Although, given the number of compounds available, the drug combinations that could be concocted are uncountable, only one triple-drug combination has so far been formulated as single pill to be taken orally once daily, namely Atripla® containing the NtRTI tenofovir disoproxil fumarate, the NRTI emtricitabine and the NNRTI efavirenz. Here, we document these approved compounds along with other HIV-active compounds and, for the first time, compounds whose principal activity is against hepatitis B virus. The logic of this new division being the enzymatic similarity between the reverse transcriptase of HIV and hepatitis B virus; the strategies for the development of antiviral agents to combat them have much in common.

Gln151 of HIV-1 Reverse Transcriptase Acts as a Steric Gate Towards Clinically Relevant Acyclic Phosphonate Nucleotide Analogues

Background

In the treatment of HIV, the loose active site of the HIV-1 reverse transcriptase (RT) allows numerous nucleotide analogues to act as proviral DNA ‘chain-terminators’. Acyclic nucleotide phosphonate analogues (ANPs) represent a particular class of nucleotide analogue that does not possess a ribose moiety. The structural basis for their substrate efficiency regarding viral DNA polymerases is poorly understood.

Methods

Pre-steady-state kinetics on HIV-1 RT together with molecular modelling, were used to evaluate the relative characteristics of both the initial binding and incorporation into DNA of three different ANP diphosphates with progressively increasing steric demands on the acyclic linker: adefovir-diphosphate (DP), tenofovir-DP, and cidofovir-DP.

Results

The increase of steric demand in ANPs induced a proportional loss of the binding affinity to wild-type HIV-1 RT ( K d cidofovir-D P>> K d tenofovir-D P> K d adefovir-DP∼ K d dNTPs), consistent with the lack of HIV-1 inhibitory activity for cidofovir. We show that, starting from adefovir-DP, the steric constraints mainly map to Gln151, as its mutation to alanine provides cidofovir-DP sensitivity. Interactions between the Gln151 residue and the methyl group of tenofovir-DP further increase with the mutation Gln151Met, resulting in a specific discrimination and low-level resistance to tenofovir-DP. This alteration is the result of a dual decrease in the binding affinity ( K d) and the catalytic rate ( k pol) of incorporation of tenofovir-DP. By contrast, the tenofovir resistance mutation K65R induces a broad ‘ k pol-dependent’ nonspecific discrimination towards the three ANPs.

Conclusions

Overall, our results show that the efficiency of ANPs to compete against natural nucleotides as substrates for RT is determined by their close interaction with specific amino acids such as Gln151 within the RT active site. These results should help us to map and predict ANP sensitivity determinants in cellular and viral DNA polymerase active sites for which the understanding of different ANP sensitivity patterns are of medical importance.

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.

Antiviral prodrugs - the development of successful prodrug strategies for antiviral chemotherapy

Following the discovery of the first effective antiviral compound (idoxuridine) in 1959, nucleoside analogues, especially acyclovir (ACV) for the treatment of herpesvirus infections, have dominated antiviral therapy for several decades. However, ACV and similar acyclic nucleosides suffer from low aqueous solubility and low bioavailability following oral administration. Derivatives of acyclic nucleosides, typically esters, were developed to overcome this problem and valaciclovir, the valine ester of ACV, was among the first of a new series of compounds that were readily metabolized upon oral administration to produce the antiviral nucleoside in vivo, thus increasing the bioavailility by several fold. Concurrently, famciclovir was developed as an oral formulation of penciclovir. These antiviral 'prodrugs' thus established a principle that has led to many successful drugs including both nucleoside and nucleotide analogues for the control of several virus infections, notably those caused by herpes-, retro- and hepatitisviruses. This review will chart the origins and development of the most important of the antiviral prodrugs to date.

Simultaneous quantitation of the nucleotide analog adefovir, its phosphorylated anabolites and 2'-deoxyadenosine triphosphate by ion-pairing LC/MS/MS

The nucleotide analog adefovir is an important therapy for hepatitis B viral infection. The study of nucleoside/tide pharmacology has been hampered by difficulties encountered when trying to develop LC/MS/MS methods for these polar analytes. In an attempt to identify a more convenient, selective and sensitive alternative to the analysis of the metabolism of radiolabeled parent nucleotide traditionally used for in vitro cell culture studies, an LC/MS/MS method was developed for the quantitative detection of adefovir and its phosphorylated metabolites in cellular samples. Ion-pairing reversed phase LC using tetrabutylammonium (TBA) and ammonium phosphate had the best compromise between chromatographic separation and positive mode MS/MS detection. Using microbore reverse phase columns and a low flow acetonitrile gradient it was possible to quantitate adefovir, its metabolites and 2'-deoxyadenosine triphosphate. A cross-validation showed comparable levels of adefovir and its metabolites were determined using either LC/MS/MS or radioactivity detection. However, initial methods were conducted at high pH and utilized an acetonitrile step gradient causing unacceptable column life and unpredictable equilibration. Further method optimization lowered the concentration of TBA and phosphate, decreased pH and applied a linear gradient of acetonitrile. This work resulted in a method that was found to have sensitivity, accuracy and precision sufficient to be a useful tool in the study of the intracellular pharmacology of adefovir in vitro and may be more broadly applicable.

Pharmacokinetics of pradefovir and PMEA in healthy volunteers after oral dosing of pradefovir

The pharmacokinetics of pradefovir and adefovir, 9-(2-phosphonylmethoxyethyl) adenine (PMEA), was evaluated in healthy male volunteers after oral dosing of pradefovir (10, 30, or 60 mg). Pradefovir was absorbed rapidly. The maximum serum concentration, the area under the concentration-time curve between 0 and 96 hours after dosing (AUC(0-96)), and the area under the plasma concentration versus time curve from time 0 to infinity (AUC(0-infinity)) of pradefovir and PMEA increased with the dose of pradefovir. The ratio of PMEA to pradefovir for AUC(0-96) and AUC(0-infinity) ranged from 1.4 to 1.8. Renal clearance of pradefovir (18-31 L/h) increased with the dose of pradefovir and was greater than glomerular filtration. The fraction of total body clearance due to renal clearance was low (0.045 to 0.083), suggesting that metabolic clearance played a significant role in the clearance of pradefovir in man. In addition, an evaluation of the food effect was conducted at the 30-mg dose. The results indicate that food intake has no effect on the extent of exposure of pradefovir and PMEA but may decrease the rate of systemic availability of PMEA.

Cytotoxicity of pivoxil esters of antiviral acyclic nucleoside phosphonates: adefovir dipivoxil versus adefovir

Biological effectiveness of antiviral acyclic nucleoside phosphonate adefo vir, 9-[2-(phosphonomethoxy)ethy]ade nine (PMEA) and its more lipophilic (bis)pivaloyloxymethyl ester prodrug adefovir dipivoxil (bis-POM-PMEA) were compared under in vitro conditions in mammalian cell systems. Proliferation of murine splenocytes was inhibited in a concentration-dependent manner, the bis-POM-PMEA being more effective than PMEA. In contrast to PMEA, bis-POM-PMEA inhibited production of nitric oxide (NO) in macrophages activated with interferon-gamma (IFN-gamma) and lipopolysaccharide (LPS). Viability of both splenocytes and macrophages remained uninfluenced by PMEA, whereas pronounced cytocidal effects were exhibited by bis-POM-PMEA. The IC(50)s reached the values of 15 microM and 30 microM in cultures of macrophages and splenocytes, respectively (assayed at the interval of 24 hrs). The effects could partly be mimicked by formaldehyde, a decomposition product of the pivoxil moiety of bis-POM-PMEA. The other possible product, pivalic acid, was ineffective in this respect. The present data are consistent with the view that pivoxil prodrug of PMEA, bis-POM-PMEA possesses enhanced but also broader spectrum of biological effects than the parent compound.

Determination of adefovir in human plasma by liquid chromatography/tandem mass spectrometry: application to a pharmacokinetic study

A liquid chromatography/tandem mass spectrometry (LC/MS/MS) method was developed and validated to determine the concentrations of adefovir [9-(2-phosphonylmethoxyethyl)adenine, PMEA] in human plasma. After one-step protein precipitation of plasma samples by methanol, adefovir was analyzed by LC/MS/MS using positive electrospray ionization. Chromatography was performed on a C18 column. The extraction recoveries of adefovir were found to be 85.1-89.3%. Adefovir was stable under routine laboratory conditions. A minimal matrix effect resulting in a slight ionization enhancement of adefovir (<10.9%) was observed, which did not markedly affect the behavior of the calibrations curves and accuracy and precision data. The method had a chromatographic run time of 7.8 min and a linear calibration curve over the concentration range 1.5-90 ng/mL for adefovir. The lower limit of quantification of the method was 1.5 ng/mL. The intra- and inter-day precision was less than 8.4%. These results indicated that this LC/MS/MS method has high selectivity and efficiency, and acceptable accuracy, precision and sensitivity. The validated LC/MS/MS method has been successfully used in a pharmacokinetic study in healthy volunteers treated with oral adefovir dipivoxil at 10 and 20 mg.

Effective metabolism and long intracellular half life of the anti-hepatitis B agent adefovir in hepatic cells

Adefovir dipivoxil (ADV) is esterolytically cleaved to the 2'-deoxyadenosine monophosphate (dAMP) analog adefovir, subsequent phosphorylation leads to the formation of the anti-Hepatitis B virus (HBV) agent adefovir-DP. To better understand the mechanism of action of ADV, metabolism studies were done in Hep G2, Huh-7 and primary human hepatocytes. Separation of radiolabeled adefovir metabolites after incubation in Hep G2 cells suggested that adefovir in its mono- and di-phosphorylated forms are the only metabolites formed from adefovir. Incubation of 10 microM adefovir with hepatic cell lines and fresh monolayers of primary human hepatocytes from two donors and analysis of intracellular metabolites by liquid chromatography coupled to tandem mass spectrometry resulted in adefovir-DP levels of approximately 10 pmol/million cells. Adefovir was more efficiently phosphorylated in primary hepatocytes than cell lines with adefovir-DP accounting for 44% versus 26% of total intracellular adefovir after 24 h. Egress studies showed adefovir-DP to have a half-life of 33 +/- 3 h, 10 +/- 1 h, 48 +/- 3 h and 33 +/- 2 h in Hep G2, Huh-7, and primary hepatocytes from two separate donors, respectively. The markedly shorter half-life in Huh-7 cells was inferred to be transport dependent based on its sensitivity to the transport inhibitor MK-571. Effective phosphorylation coupled with a long intracellular half-life and small competing dATP pool sizes in primary hepatocytes forms the cellular metabolic basis for the efficacy of adefovir dipivoxil in the treatment of chronic hepatitis B.

Ultra sensitive method for the determination of 9-(2-phosphonylmethoxyethyl)adenine in human serum by liquid chromatography–tandem mass spectrometry

An ultra sensitive method for the direct measurement of 9-(2-phosphonylmethoxyethyl)adenine (PMEA), an antiviral agent for hepatitis B, in human serum using high performance liquid chromatography/tandem mass spectrometry (LC-MS/MS) has been developed. This method involves the addition of [13C]PMEA (contains 5 13C) as internal standard, the purification and enrichment by a MCX solid phase extraction (SPE) cartridge, and quantitative analysis using LC-MS/MS. The MS/MS is selected to monitor the m/z 272 --> 134 and m/z 277 --> m/z 139 transitions for PMEA and [13C]PMEA, respectively, using negative electrospray ionization. The MS/MS response is linear over a concentration of 0.1-10 ng/ml with a lower limit of quantitation (LLOQ) of 0.1 ng/ml. The mean inter-assay accuracy (%Bias) for quality control (QC) at 0.1, 0.25, 1.0, and 10 ng/ml are 10, 1.6, -0.8, and 0.0%, respectively. The mean inter-assay precision (%CV) for the corresponding QCs is 3.9, 3.8, 5.3, and 3.4%, respectively. The method has been used to determine PMEA concentration in human serum following a single oral administration of a PMEA pro-drug at dose of 10 and 30 mg.

Tenofovir Disoproxil Fumarate

Tenofovir disoproxil fumarate (tenofovir DF) is an oral prodrug of tenofovir, a nucleotide (nucleoside monophosphate) analogue with activity against retroviruses, including HIV-1, HIV-2 and hepadnaviruses. Following absorption, tenofovir DF is rapidly converted to tenofovir, which is metabolised intracellularly to its active anabolite tenofovir diphosphate, which is a competitive inhibitor of HIV-1 reverse transcriptase and terminates the growing DNA chain. Tenofovir exerts antiviral effects in a variety of cell types, including resting cells. Tenofovir exhibits longer serum (17 hours) and intracellular (> or =60 hours) half-lives than those of nucleoside analogues, which supports a flexible once-daily administration schedule. The pharmacokinetics of tenofovir are dose-proportional and similar in healthy volunteers and HIV-infected individuals. The oral bioavailability of tenofovir is enhanced by administration with a high-fat meal, but is similar at steady state when administered with or without a typical meal. Tenofovir is not a substrate, inducer or inhibitor of human cytochrome P450 enzymes in vitro or in vivo. Tenofovir DF has been studied with 15 other antiretroviral and other concomitant medications frequently used in the HIV-1-infected population. With the exception of didanosine and atazanavir, which require dosage modifications, no clinically significant drug interactions have been observed with tenofovir DF. The recommended oral dosage of tenofovir DF in adults is 300 mg/day. Tenofovir is eliminated by renal elimination, including tubular secretion; dose-interval adjustments are necessary for tenofovir DF in patients with significant renal impairment. No dosage adjustment of tenofovir DF is necessary in patients with liver disease.

Pharmacokinetics and bioavailability of oral cephalosporins, KR-984055 and its prodrugs, KR-999001 and KR-999002, in the rat

KR-984055 is a new oral cephalosporin antibiotic with activity against both gram-positive and gram-negative bacteria. Lipophilic ester-type prodrugs of KR-984055, i.e., KR-999001 and KR-999002, have been synthesized in an attempt to increase the oral bioavailability of this broad-spectrum antibiotic agent. In this study we determined the oral bioavailability of KR-984055 and its prodrugs in the rat, and evaluated the pharmacokinetic model that best describes the plasma concentration behavior following single intravenous (i.v.) and oral single dose. In addition, concentrations in plasma as well as biliary and urinary recovery of KR-984055 were determined. Also, protein binding of KR-984055 in plasma was examined in vitro. The degree of protein binding of KR-984055 was in the range of 92.09-94.77%. KR-984055 exhibited poor oral bioavailability (7.02 +/- 1.58%). The observed oral bioavailabilities of KR-984055 from KR-999001 and KR-999002 were 38.77 +/- 2.81% and 39.81 +/- 5.25%, respectively. These data were calculated from the levels of free KR-984055 in plasma. Oral KR-999001 and KR-999002 were not recovered from plasma, suggesting that it was readily cleaved to free KR-984055. KR-999001 and KR-999002 appear to be an efficient oral prodrug of KR-984055 that deserved further clinical evaluation in human.

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.

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.

In vitro, ex vivo, and in situ intestinal absorption characteristics of the antiviral ester prodrug adefovir dipivoxil

Caco-2 monolayers (in vitro), rat intestinal sheets mounted in modified Ussing Chambers (ex vivo), and in situ intestinal perfusion of rat ileum were used as models to determine and compare the absorption characteristics of the antiviral agent 9-[2-(phosphonomethoxy)ethyl]adenine (PMEA, adefovir) and its bis(pivaloyloxymethyl)-ester prodrug [bis(POM)-PMEA, adefovir dipivoxil]. Although metabolism of adefovir dipivoxil was more pronounced in the ex vivo and in situ models than in the Caco-2 system, the transport of 'total adefovir' [= adefovir dipivoxil and its metabolites mono(POM)-PMEA and adefovir] was comparable in the three models. Compared with transport of the parent compound (adefovir), use of adefovir dipivoxil resulted in a significant increase in transport of total adefovir in the in vitro ( approximately 100-fold) and the in situ ( approximately 10-fold) models; in contrast, the ex vivo method failed to demonstrate a remarkable transport enhancement when using the ester prodrug. Similar to the results obtained in the Caco-2 model, the inclusion of the P-glycoprotein inhibitor verapamil resulted in transport enhancement during in situ perfusion of rat ileum with adefovir dipivoxil; however, no effect of verapamil could be observed in the ex vivo model. The results of this study confirm the utility of both the in vitro and in situ models to assess intestinal transport and metabolism of adefovir dipivoxil. The ex vivo model appeared to be less appropriate because of its inability to discriminate transport following administration of adefovir or adefovir dipivoxil and because of the absence of an effect of verapamil on transport when using adefovir dipivoxil.

Inhibition of adenylyl cyclase by acyclic nucleoside phosphonate antiviral agents

Acyclic derivatives of adenine, known as highly effective nucleotide analogs with broad spectrum antiviral activity, were evaluated for potential cross-reactivity with adenylyl cyclases, a family of membrane-bound enzymes that share putative topologies at their catalytic sites with oligonucleotide polymerases and reverse transcriptases. A series of derivatives of 9-(2-phosphonylmethoxyethyl)adenine (PMEA) inhibited a preparation of adenylyl cyclase derived from rat brain with IC(50) values that ranged from 66 microM (PMEA) to 175 nM for its diphosphate derivative (PMEApp) and mimics of it. PMEApp mimics included PMEAp(NH)p, PMEAp(CH(2))p, PMEAp(CX(2))p (X = fluorine, chlorine, or bromine), PMEAp(CHX)pp, and PMEAp(C(OH)CH(3)pp. The data suggest that inhibition of adenylyl cyclases may contribute to the therapeutic action of some of these or similar compounds or constitute part of their side effects in therapeutic settings.

Development and optimization of anti-HIV nucleoside analogs and prodrugs: A review of their cellular pharmacology, structure-activity relationships and pharmacokinetics

Significant improvements in antiviral therapy have been realized over the past 10 years. Numerous nucleoside analogs, as well as prodrugs of active compounds, have been synthesized and tested for anti-HIV activity. In addition to the five nucleoside analogs currently used clinically for the treatment of HIV infection, a broad spectrum of anti-HIV nucleoside analogs (including 2',3'-dideoxynucleoside analogs, oxathiolanyl 2',3'-dideoxynucleoside analogs, dioxolanyl 2',3'-dideoxynucleoside analogs, carbocyclic 2',3'-dideoxynucleoside analogs and acyclic nucleoside analogs) and their prodrugs (including ester prodrugs, phospholipid prodrugs, dihydropyridine prodrugs, pronucleotides and dinucleotide analogs), targeted at HIV reverse transcriptase, are reviewed with focus on structure-activity relationships, cellular pharmacology and pharmacokinetics. Several of these anti-viral agents show promise in the treatment of AIDS.

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.

Chemokines, nitric oxide and antiarthritic effects of 9-(2-phosphonomethoxyethyl)adenine (Adefovir)

Antiarthritic effects of two acyclic nucleoside phosphonates, 9-(2-phosphonomethoxyethyl)adenine (PMEA; Adefovir) and 9-(2-phosphonomethoxypropyl)adenine (PMPA), as well as their more bioavailable prodrugs, bis(pivaloyloxymethyl)ester of PMEA [bis(POM)-PMEA; Adefovir Dipivoxil] and bis(isopropyloxycarbonyloxymethyl)ester of PMPA [bis(POC)-PMPA], were investigated in a model of adjuvant-induced arthritis in Lewis rats. The drugs were injected subcutaneously at doses of 5-50 mg/kg. PMEA and its prodrug inhibited by > 80% arthritic paw swelling, splenomegaly and fibroadhesive perisplenitis. Both prophylactic and therapeutic dosing regimens were effective. Neither PMPA nor bis(POC)-PMPA suppressed development of arthritic lesions. Substantially reduced nitrite + nitrate levels were detected in serum and urine of PMEA-treated animals as compared to those of untreated diseased controls. Also, complete suppression of the disease-associated, greatly enhanced systemic levels of the chemokine, RANTES (regulated upon activation, normal T cell expressed and secreted), was observed in rats injected with PMEA. Additional in vitro studies showed that PMEA does not change, PMPA enhances, and both prodrugs inhibit the immune-activated NO production. Under the same conditions PMEA inhibits, while PMPA slightly stimulates, secretion of RANTES. Collectively, these data suggest that the in vivo-inhibited production of nitric oxide (NO) is a consequence rather than a mechanism of antiarthritic action of PMEA. Possible mechanisms for the anti-RANTES activity of PMEA remains to be firmly established.

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.

Carrier mechanisms involved in the transepithelial transport of bis(POM)-PMEA and its metabolites across Caco-2 monolayers

PURPOSE

To investigate the role of carrier mechanisms in: [1] the polarized transport of the bis(pivaloyloxymethyl)- [bis(POM)-] ester prodrug of the antiviral agent 9-(2-phosphonylmethoxyethyl)adenine [PMEA] and [2] the directional secretion of its metabolites.

METHODS

Caco-2 monolayers were used to study the modulation effect of carriers on the transport of bis(POM)-PMEA and the efflux of intracellularly formed metabolites mono(POM)-PMEA and PMEA from the cells. The interaction of bis(POM)-PMEA and its metabolites with the efflux mechanisms present in Caco-2 monolayers was investigated by testing the effect of various concentrations of verapamil (30, 100, 300, microns) or indomethacin (10-500 microns) on transport and efflux.

RESULTS

Polarity in transport of bis(POM)-PMEA (50 micron) across Caco-2 monolayers was noted: transport of total PMEA [=bis(POM)-PMEA, mono(POM)-PMEA and PMEA] was significantly higher in basolateral (BL) to apical (AP) direction (14.5 +/- 0.4%) than transport in the opposite (AP to BL) direction (1.7 +/- 0.2%). This difference was reduced in a concentration dependent way when verapamil (0-100 microns) was included in both AP and BL incubation media. After loading the cells with bis(POM)-PMEA (100 micron) for 1 hr, studies on efflux of PMEA and mono(POM)-PMEA from the Caco-2 monolayers over a 3 hr period, revealed that both metabolites were preferentially secreted towards the AP compartment. Efflux of PMEA toward AP and BL compartments amounted to 14.6 +/- 1.1% and 5.3 +/- 0.4, respectively, of the initial intracellular amount of total PMEA, while efflux of mono(POM)-PMEA towards AP and BL compartments was limited to 2.3 +/- 0.1% and 0.5 +/- 0.1%, respectively. When 10 micron indomethacin was included in the AP incubation medium, efflux of PMEA was decreased to 7.8 +/- 0.3% and 3.3 +/- 0.3% towards the AP and BL compartments, respectively. The decrease in efflux by indomethacin was concentration-dependent up to 100 micron. Transepithelial transport of total PMEA was also reduced in the presence of 30 micron indomethacin, as reflected in smaller concentrations of PMEA and mono(POM)-PMEA in the acceptor compartment, irrespective of the transport direction.

CONCLUSIONS

The data obtained in this study suggest that bis(POM)-PMEA is substrate for a P -glycoprotein-like carrier mechanism in Caco-2 monolayers, while its metabolites mono(POM)-PMEA and PMEA are transported by a non-P-glycoprotein efflux protein.