Adefovir Dipivoxil

Twin hydrogen bonds with phosphine oxide: anticooperativity effects caused by competing proton donors

In this computational work we study complexes with two equivalent intermolecular hydrogen bonds formed between trimethyl phosphine oxide and two identical proton donors ("twin" hydrogen bonds) for a set of 70 proton donor molecules. The changes in the phosphorus chemical shift and stretching frequency of the PO group upon complexation correlate quite well with the total strength of two hydrogen bonds. A set of explicit numerical dependences is proposed for assessing interatomic distances and hydrogen bond strengths from spectral data. Comparison with the results obtained for analogous previously studied 1 : 1 complexes allowed us to analyze in detail anticooperativity effects on the geometry, energy and spectral parameters. Two hydrogen bonds compete for the PO acceptor group and their mutual weakening increases nonlinearly with the strengthening of the complex, reaching approximately 25% in energy (which corresponds to 0.1 Å lengthening for short strong H-bonds), which is clearly seen in NMR and IR spectra and correlates well with the changes in the spectral parameters.

Reaction Kinetics of the Benzylation of Adenine in DMSO: Regio-Selectivity Guided by Entropy

The factors governing the regio-selectivity of the alkylation of adenine have been of interest for many years due to the biological importance of adenine derivatives, however, no reaction kinetic studies have been conducted. Herein, we report the rate constants and activation parameters of the benzylation of adenine under basic conditions in DMSO in the absence and presence of 15-crown-5 ether using real-time 1H NMR spectroscopy. The reaction is second-order for the formation of the N9- and N3-benzyladenine products, with a regio-selectivity factor 2.3 in favour of the N9-adduct. The Gibbs free energy of activation amounts to 87±2 kJ mol-1 for both reactions. The formation of the N9-adduct is more activated by 7 kJ mol-1, but its effect is offset by a less negative activation entropy, demonstrating that the long-contested reason for the regioselectivity in the benzylation of adenine is dominated by compensation of entropy and enthalpy in the transition state. The kinetic parameters obtained in the presence of the 15-crown-5 ether indicate that the crown ether forms a complex with an adenine-sodium ion-pair, increasing the activation barrier. However, the Gibbs free energy in the absence and presence of the crown ether remains constant.

Synthetic Approaches, Properties, and Applications of Acylals in Preparative and Medicinal Chemistry

Diesters of geminal diols (R-CH(O-CO-R')2, RR'C(OCOR″)2, etc. with R = H, aryl or alkyl) are termed acylals according to IUPAC recommendations (Rule P-65.6.3.6 Acylals) if the acids involved are carboxylic acids. Similar condensation products can be obtained from various other acidic structures as well, but these related "non-classical acylals", as one might call them, differ in various aspects from classical acylals and will not be discussed in this article. Carboxylic acid diesters of geminal diols play a prominent role in organic chemistry, not only in their application as protective groups for aldehydes and ketones but also as precursors in the total synthesis of natural compounds and in a variety of organic reactions. What is more, acylals are useful as a key structural motif in clinically validated prodrug approaches. In this review, we summarise the syntheses and chemical properties of such classical acylals and show what potentially under-explored possibilities exist in the field of drug design, especially prodrugs, and classify this functional group in medicinal chemistry.

Recent Advances in the Application of P(III)-Nucleophiles to Create New P-C Bonds through Michaelis-Arbuzov-Type Rearrangement

Organophosphorus compounds have long been considered valuable in both organic synthesis and life science. P(III)-nucleophiles, such as phosphites, phosphonites, and diaryl/alkyl phosphines, are particularly noteworthy as phosphorylation reagents for their ability to form new P-C bonds, producing more stable, ecofriendly, and cost-effective organophosphorus compounds. These nucleophiles follow similar phosphorylation routes as in the functionalization of P-H bonds and P-OH bonds. Activation can occur through photocatalytic, electrocatalytic, or thermo-driven reactions, often in coordination with a Michaelis-Arbuzov-trpe rearrangement process, to produce the desired products. As such, this review offers a thorough overview of the phosphorylated transformation and potential mechanisms of P(III)-nucleophiles, specifically focusing on developments since 2010. Notably, this review may provide researchers with valuable insights into designing and synthesizing functionalized organophosphorus compounds from P(III)-nucleophiles, guiding future advancements in both research and practical applications.

MDSC-targeted liposomal all-trans retinoic acid suppresses mMdscs and improves immunotherapy in HBV infection

BACKGROUND

Myeloid-derived suppressor cells (MDSCs) are evolving as a prominent determinant in cancer occurrence and development and are functionally found to suppress T cells in cancer. Not much research is done regarding its involvement in viral infections. This research was designed to investigate the role of MDSCs in hepatitis B virus (HBV) infection and how targeting these cells with our novel all-trans retinoic acid encapsulated liposomal formulation could improve immunotherapy in C57BL/6 mice.

METHODS

Ten micrograms (10 μg) of plasmid adeno-associated virus (pAAV/HBV 1.2, genotype A) was injected hydrodynamically via the tail vein of C57BL/6 mice. An all-trans retinoic acid encapsulated liposomal formulation (L-ATRA) with sustained release properties was used in combination with tenofovir disoproxil fumarate (TDF), a nucleotide analog reverse transcriptase inhibitor (nRTI) to treat the HBV infection. The L-ATRA formulation was given at a dose of 5 mg/kg intravenously (IV) twice a week. The TDF was given orally at 30 mg/kg daily.

RESULTS

Our results revealed that L-ATRA suppresses MDSCs in HBV infected mice and enhanced T-cell proliferation in vitro. In vivo studies showed higher and improved immunotherapeutic effect in mice that received L-ATRA and TDF concurrently in comparison with the groups that received monotherapy. Lower HBV DNA copies, lower concentrations of HBsAg and HBeAg, lower levels of ALT and AST and less liver damage were seen in the mice that received the combination therapy of L-ATRA + TDF.

CONCLUSIONS

In effect, targeting MDSCs with the combination of L-ATRA and TDF effectively reduced mMDSC and improved immunotherapy in the HBV infected mice. Targeting MDSCs could provide a breakthrough in the fight against hepatitis B virus infection.

Antiviral activities of two nucleos(t)ide analogs against vaccinia, mpox, and cowpox viruses in primary human fibroblasts

Many poxviruses are significant human and animal pathogens, including viruses that cause smallpox and mpox (formerly monkeypox). Identifying novel and potent antiviral compounds is critical to successful drug development targeting poxviruses. Here we tested two compounds, nucleoside trifluridine, and nucleotide adefovir dipivoxil, for antiviral activities against vaccinia virus (VACV), mpox virus (MPXV), and cowpox virus (CPXV) in physiologically relevant primary human fibroblasts. Both compounds potently inhibited the replication of VACV, CPXV, and MPXV (MA001 2022 isolate) in plaque assays. In our recently developed assay based on a recombinant VACV expressing secreted Gaussia luciferase, they both exhibited high potency in inhibiting VACV replication with EC₅₀s in the low nanomolar range. In addition, both trifluridine and adefovir dipivoxil inhibited VACV DNA replication and downstream viral gene expression. Our results characterized trifluridine and adefovir dipivoxil as strong poxvirus antiviral compounds and further validate the VACV Gaussia luciferase assay as a highly efficient and reliable reporter tool for identifying poxvirus inhibitors. Given that both compounds are FDA-approved drugs, and trifluridine is already used to treat ocular vaccinia, further development of trifluridine and adefovir dipivoxil holds great promise in treating poxvirus infections, including mpox.

Adefovir Dipivoxil as a Therapeutic Candidate for Medullary Thyroid Carcinoma: Targeting RET and STAT3 Proto-Oncogenes

Aberrant gene expression is often linked to the progression of various cancers, making the targeting of oncogene transcriptional activation a potential strategy to control tumor growth and development. The RET proto-oncogene’s gain-of-function mutation is a major cause of medullary thyroid carcinoma (MTC), which is part of multiple endocrine neoplasia type 2 (MEN2) syndrome. In this study, we used a cell-based bioluminescence reporter system driven by the RET promoter to screen for small molecules that potentially suppress the RET gene transcription. We identified adefovir dipivoxil as a transcriptional inhibitor of the RET gene, which suppressed endogenous RET protein expression in MTC TT cells. Adefovir dipivoxil also interfered with STAT3 phosphorylation and showed high affinity to bind to STAT3. Additionally, it inhibited RET-dependent TT cell proliferation and increased apoptosis. These results demonstrate the potential of cell-based screening assays in identifying transcriptional inhibitors for other oncogenes.

Antiviral activities of two nucleos(t)ide analogs against vaccinia and mpox viruses in primary human fibroblasts

Many poxviruses are significant human and animal pathogens, including viruses that cause smallpox and mpox. Identification of inhibitors of poxvirus replication is critical for drug development to manage poxvirus threats. Here we tested two compounds, nucleoside trifluridine and nucleotide adefovir dipivoxil, for antiviral activities against vaccinia virus (VACV) and mpox virus (MPXV) in physiologically relevant primary human fibroblasts. Both trifluridine and adefovir dipivoxil potently inhibited replication of VACV and MPXV (MA001 2022 isolate) in a plaque assay. Upon further characterization, they both conferred high potency in inhibiting VACV replication with half maximal effective concentrations (EC 50 ) at low nanomolar levels in our recently developed assay based on a recombinant VACV secreted Gaussia luciferase. Our results further validated that the recombinant VACV with Gaussia luciferase secretion is a highly reliable, rapid, non-disruptive, and simple reporter tool for identification and chracterization of poxvirus inhibitors. Both compounds inhibited VACV DNA replication and downstream viral gene expression. Given that both compounds are FDA-approved drugs, and trifluridine is used to treat ocular vaccinia in medical practice due to its antiviral activity, our results suggest that it holds great promise to further test trifluridine and adefovir dipivoxil for countering poxvirus infection, including mpox.

Overview of the development of HBV small molecule inhibitors

Like tuberculosis and Acquired Immune Deficiency Syndrome (AIDS), hepatitis B is a globally recognized major public health threat. Although there are many small-molecule drugs for the treatment of hepatitis B, the approved drugs cannot eradicate the pathogenic culprit covalently closed circular DNA in patients, so the patients need long-term medication to control HBV amplification. Driven by a high unmet medical need, many pharmaceutical companies and research institutions have been engaged in the development of anti-HBV drugs to achieve a functional cure for chronic hepatitis B as soon as possible. This review summarizes the pathogenesis of hepatitis B virus and the research progress in the development of anti-HBV small molecule drugs, and introduces the cccDNA formation and transcription inhibitors and core inhibitors in detail, especially emphasizes the role of chinese herbal medicine in the treatment of chronic hepatitis B. Furthermore, this review proposes three potential strategies for cccDNA eradication in the future. We believe this review will provide meaningful guidance to achieve a functional cure for viral hepatitis B in the future.

Virtual Cocrystal Screening of Adefovir Dipivoxyl: Identification of New Solid Forms with Improved Dissolution and Permeation Profiles

The application of a computational screening methodology based on the calculation of intermolecular interaction energies has guided the discovery of new multicomponent solid forms of the oral antiviral Adefovir Dipivoxyl. Three new cocrystals with resorcinol, orcinol and hydroquinone have been synthesized and thoroughly characterized. They show improved dissolution profiles with respect to the single solid form, particularly the cocrystals of orcinol and resorcinol, which have 3.2- and 2-fold faster dissolution rates at stomach conditions (pH 1.5). Moreover, dynamic dissolution experiments that simultaneously mimic both the pH variation along the gastrointestinal tract and the partition into biological membranes show that, in addition to the faster initial dissolution, Adefovir Dipivoxyl also penetrates faster into the organic membranes in the form of resorcinol and orcinol cocrystals.

Heteroatom-Assisted Regio- and Stereoselective Palladium-Catalyzed Carboxylation of 9-Allyl Adenine

Strategy for the synthesis of acyclic nucleoside analogs of biological relevance via highly regio- and stereoselective C-H functionalization employing heteroatom-assisted palladium-catalyzed carboxylation of 9-allyl adenine is disclosed. Substrate scope with different carboxylic acids was performed giving decent to good yields of the desired products. The method also allowed for the synthesis of deuterated analogs.

Coordination Sites for Sodium and Potassium Ions in Nucleophilic Adeninate Contact ion-Pairs: A Molecular-Wide and Electron Density-Based (MOWED) Perspective

The adeninate anion (Ade-) is a useful nucleophile used in the synthesis of many prodrugs (including those for HIV AIDS treatment). It exists as a contact ion-pair (CIP) with Na+ and K+ (M+) but the site of coordination is not obvious from spectroscopic data. Herein, a molecular-wide and electron density-based (MOWED) computational approach implemented in the implicit solvation model showed a strong preference for bidentate ion coordination at the N3 and N9 atoms. The N3N9-CIP has (i) the strongest inter-ionic interaction, by -30 kcal mol-1, with a significant (10-15%) covalent contribution, (ii) the most stabilized bonding framework for Ade-, and (iii) displays the largest ion-induced polarization of Ade-, rendering the N3 and N9 the most negative and, hence, most nucleophilic atoms. Alkylation of the adeninate anion at these two positions can therefore be readily explained when the metal coordinated complex is considered as the nucleophile. The addition of explicit DMSO solvent molecules did not change the trend in most nucleophilic N-atoms of Ade- for the in-plane M-Ade complexes in M-Ade-(DMSO)4 molecular systems. MOWED-based studies of the strength and nature of interactions between DMSO solvent molecules and counter ions and Ade- revealed an interesting and unexpected chemistry of intermolecular chemical bonding.

Effect of Coformer Selection on In Vitro and In Vivo Performance of Adefovir Dipivoxil Cocrystals

PURPOSE

This study aimed to improve the in vitro dissolution, permeability and oral bioavailability of adefovir dipivoxil (ADD) by cocrystal technology and clarify the important role of coformer selection on the cocrystal's properties.

METHODS

ADD was cocrystallized with three small molecules (i.e., paracetamol (PA), saccharin (SAC) and nicotinamide (NIC)), respectively. The obtained ADD-PA cocrystal was characterized by DSC, TGA, PXRD and FTIR. Comparative study on dissolution rates among the three ADD cocrystals were conducted in water and pH 6.8 phosphate buffer. Besides, effects of coformers on intestinal permeability of ADD were evaluated via in vitro Caco-2 cell model and in situ single-pass intestinal perfusion model in rats. Furthermore, in vivo pharmacokinetic study of ADD cocrystals was also compared.

RESULTS

Dissolution rates of ADD cocrystals were improved with the order of ADD-SAC cocrystal > ADD-PA cocrystal > ADD-NIC cocrystal. The permeability studies on Caco-2 cell model and single-pass intestinal perfusion model indicated that PA could enhance intestinal absorption of ADD by P-gp inhibition, while SAC and NIC did not. Further in vivo pharmacokinetic study showed that ADD-SAC cocrystal exhibited higher C_max (1.4-fold) and AUC_0-t (1.3-fold) of ADD than administration of ADD alone, and C_max and AUC_0-t of ADD-PA cocrystal were significantly enhanced by 2.1-fold and 2.2-fold, respectively, which was attributed to its higher dissolution and improved intestinal permeability.

CONCLUSION

Coformer selection had an important role on cocrystal's properties, and cocrystallization of ADD with a suitable coformer was an effective approach to enhance both dissolution and bioavailability of ADD.

An Overview of the Current Hepatitis B Treatment Strategies after Liver Transplantation

Currently, liver transplantation (LT) is considered as the only option for the treatment of patients with various causes of liver failure, including patients with chronic hepatitis B virus (HBV) infections. Overall, patients with HBV who undergo LT are at increased risk of hepatitis B infection recurrence. Although the current knowledge regarding the pathophysiology of this infection has been dramatically increased over the past few decades, it is still considered a complex disease process with varying degrees of clinical characteristics and changing patterns over time. There are various treatment strategies for preventing HBV recurrence in the LT setting. Generally, these regimens include oral nucleoside/ nucleotide analogues (NAs), hepatitis B immune globulin (HBIG), and vaccines or the combination of these drugs. The treatment strategy of choice should be based on cost-effectiveness, along with other patients underlying conditions. In this case, studies indicate that potent NAs are more cost-effective than HBIG in most case scenarios. In this article, we aimed to review the general medications used in the prophylaxis of the recurrence of HBV infection after LT.

SARS-CoV-2 RdRp Inhibitors Selected from a Cell-Based SARS-CoV-2 RdRp Activity Assay System

The COVID-19 pandemic, caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), urgently needs effective prophylactic and therapeutic drugs. RNA-dependent RNA polymerase (RdRp), essential for replicating and transcribing a viral RNA genome, is highly conserved in coronaviruses; thus, it is a potential target for inhibiting coronavirus infection. In this study, we generated the cell-based SARS-CoV-2 RdRp activity assay system by modifying a previously reported cell-based MERS-CoV RdRp activity assay system to screen for SARS-CoV-2 RdRp inhibitors. The assay system consisted of an expression plasmid encoding SARS-CoV-2 RdRp and an RdRp activity reporter plasmid. RdRp activity in the cells could be conveniently detected by luminescence after transfection. We confirmed that SARS-CoV-2 RdRp replicated double-stranded RNA using immunofluorescence staining and the inhibition of RdRp activity by remdesivir and lycorine using this system. Moreover, the Z-factor of this system was calculated to be 0.798, suggesting the reproducibility and reliability of the high-throughput screening system. Finally, we screened nucleoside and nucleotide analogs and identified adefovir dipivoxil, emtricitabine, telbivudine, entecavir hydrate, moroxydine and rifampin as novel SARS-CoV-2 RdRp inhibitors and therapeutic candidates for COVID-19 This system provides an effective high-throughput screening system platform for developing potential prophylactic and therapeutic drugs for COVID-19 and emerging coronavirus infections.

Synthesis and Characterization of a Series of Temozolomide Esters and Its Anti-glioma Study

Temozolomide is a first-line therapeutic drug for glioblastoma (GBM), and it has a low solubility, short biological half-life, and resistance to drug limits in clinical applications. Therefore, it is necessary to find more effective anti-tumor drugs to overcome drug resistance and enhance its anti-glioma activity. We therefore used n-butanol, n-hexanol, n-octanol, 1-dodecanol and 1-hexadecanol to synthesize a series of temozolomide ester compounds (TMZEs) and then investigated their physicochemical properties and anti-glioma efficacy. Our results showed that TMZEs had a higher lipophilicity compared to TMZ and could stably exist in plasma and brain homogenates. TMZEs had significantly increased cytotoxicity and cellular uptake in C6 glioma cells as chain lengths increased. Additionally, the IC₅₀ of TMZ-16E towards TMZ-resistant cells (T98G) was 85.9-fold lower than that of TMZ (p < 0.001), and Western blot results demonstrated that TMZ-16E could significantly reduce the expression of O⁶-methylguanine-DNA-methyltransferase (MGMT). The in vivo anti-glioma efficacy of TMZ-16E were then investigated in orthotopic and subcutaneous GBM models. TMZ-16E prolonged the survival time to 35 days in orthotopic glioma bearing rats, which was 1.94-fold longer than the survival time of rats treated with TMZ, and TMZ-16E increased tumor cell apoptosis based on TUNEL staining. Moreover, TMZ-16E (50 mg/kg) noticeably slowed the growth of T98G subcutaneous tumors by down-modulating MGMT expression in subcutaneous GBM-bearing mice, indicating that TMZ-16E could effectively reverse drug resistance. In conclusion, TMZEs improved the lipophilicity and stability of these drugs. Especially, TMZ-16E could reverse drug resistance and improve therapeutic effects of TMZ, which has clinical application potential for GBM treatment.

Development of nanoparticle-delivery systems for antiviral agents: A review

The COVID-19 pandemic has resulted in unprecedented increases in sickness, death, economic disruption, and social disturbances globally. However, the virus (SARS-CoV-2) that caused this pandemic is only one of many viruses threatening public health. Consequently, it is important to have effective means of preventing viral transmission and reducing its devastating effects on human and animal health. Although many antivirals are already available, their efficacy is often limited because of factors such as poor solubility, low permeability, poor bioavailability, un-targeted release, adverse side effects, and antiviral resistance. Many of these problems can be overcome using advanced antiviral delivery systems constructed using nanotechnology principles. These delivery systems consist of antivirals loaded into nanoparticles, which may be fabricated from either synthetic or natural materials. Nevertheless, there is increasing emphasis on the development of antiviral delivery systems from natural substances, such as lipids, phospholipids, surfactants, proteins, and polysaccharides, due to health and environmental issues. The composition, morphology, dimensions, and interfacial characteristics of nanoparticles can be manipulated to improve the handling, stability, and potency of antivirals. This article outlines the major classes of antivirals, summarizes the challenges currently limiting their efficacy, and highlights how nanoparticles can be used to overcome these challenges. Recent studies on the application of antiviral nanoparticle-based delivery systems are reviewed and future directions are described.

Phosphine Oxides from a Medicinal Chemist's Perspective: Physicochemical and in Vitro Parameters Relevant for Drug Discovery

Phosphine oxides and related phosphorus-containing functional groups such as phosphonates and phosphinates are established structural motifs that are still underrepresented in today's drug discovery projects, and only few examples can be found among approved drugs. In this account, the physicochemical and in vitro properties of phosphine oxides and related phosphorus-containing functional groups are reported and compared to more commonly used structural motifs in drug discovery. Furthermore, the impact on the physicochemical properties of a real drug scaffold is exemplified by a series of phosphorus-containing analogs of imatinib. We demonstrate that phosphine oxides are highly polar functional groups leading to high solubility and metabolic stability but occasionally at the cost of reduced permeability. We conclude that phosphine oxides and related phosphorus-containing functional groups are valuable polar structural elements and that they deserve to be considered as a routine part of every medicinal chemist's toolbox.

Safety, efficacy, and pharmacokinetics of pradefovir for the treatment of chronic hepatitis B infection

BACKGROUND & AIMS

Pradefovir is a liver targeted novel prodrug of adefovir (PMEA) developed to provide higher antiviral activity with reduced systemic toxicities. This study evaluated the tolerability, pharmacokinetics, and antiviral activity of pradefovir in patients with chronic hepatitis B (CHB) virus infection.

METHODS

Non-cirrhotic, treatment-naïve subjects with CHB were divided into five groups (10 patients each) and randomized within each group in a ratio of 6:2:2 to receive an ascending dose of 30, 60, 75, 90, or 120 mg pradefovir, 10 mg adefovir dipivoxil (ADV), or 300 mg tenofovir disoproxil fumarate (TDF) once a day for 28 days.

RESULTS

A total of 51 subjects were randomized and 49 subjects completed the study. The groups were well matched and included 39 males, of whom 71% were hepatitis B e-antigen-negative with a mean hepatitis B virus (HBV) DNA level of 6.4-7.16 log10 IU/mL. No subject experienced a serious adverse event or nephrotoxicity. The most frequently reported adverse event was asymptomatic reduction in blood cholinesterase levels in the pradefovir group which recovered without any treatment about 13 ± 7 days after drug discontinuation. This adverse event was not observed in the ADV and TDF groups. The mean changes in serum HBV DNA were -2.78, -2.77, -3.08, -3.18, -3.44, -2.34, and -3.07 log10 IU/mL at 30, 60, 75, 90, and 120 mg pradefovir, 10 mg ADV and 300 mg TDF, respectively, with plateau levels reached with 60 mg pradefovir. Pradefovir and its metabolite PMEA showed linear pharmacokinetics proportional to the dose. The half-life of PMEA in the pradefovir group was 11.47-17.63 h.

CONCLUSIONS

Short-term use of pradefovir was well tolerated. A decline in HBV DNA levels was superior to TDF at higher doses of pradefovir. 30-60 mg pradefovir is recommended for CHB treatment.

CLINICAL TRIAL NUMBER

CTR20150224.

Enhanced Oral Bioavailability of 2-(Phosphonomethyl)-pentanedioic Acid (2-PMPA) from its (5-Methyl-2-oxo-1,3-dioxol-4-yl)methyl (ODOL)-Based Prodrugs

2-(Phosphonomethyl)-pentanedioic acid (2-PMPA) is a potent (IC₅₀ = 300 pM) and selective inhibitor of glutamate carboxypeptidase II (GCPII) with efficacy in multiple neurological and psychiatric disease preclinical models and more recently in models of inflammatory bowel disease (IBD) and cancer. 2-PMPA (1), however, has not been clinically developed due to its poor oral bioavailability (<1%) imparted by its four acidic functionalities (c Log P = -1.14). In an attempt to improve the oral bioavailability of 2-PMPA, we explored a prodrug approach using (5-methyl-2-oxo-1,3-dioxol-4-yl)methyl (ODOL), an FDA-approved promoiety, and systematically masked two (2), three (3), or all four (4) of its acidic groups. The prodrugs were evaluated for in vitro stability and in vivo pharmacokinetics in mice and dog. Prodrugs 2, 3, and 4 were found to be moderately stable at pH 7.4 in phosphate-buffered saline (57, 63, and 54% remaining at 1 h, respectively), but rapidly hydrolyzed in plasma and liver microsomes, across species. In vivo, in a single time-point screening study in mice, 10 mg/kg 2-PMPA equivalent doses of 2, 3, and 4 delivered significantly higher 2-PMPA plasma concentrations (3.65 ± 0.37, 3.56 ± 0.46, and 17.3 ± 5.03 nmol/mL, respectively) versus 2-PMPA (0.25 ± 0.02 nmol/mL). Given that prodrug 4 delivered the highest 2-PMPA levels, we next evaluated it in an extended time-course pharmacokinetic study in mice. 4 demonstrated an 80-fold enhancement in exposure versus oral 2-PMPA (AUC_0-t: 52.1 ± 5.9 versus 0.65 ± 0.13 h*nmol/mL) with a calculated absolute oral bioavailability of 50%. In mouse brain, 4 showed similar exposures to that achieved with the IV route (1.2 ± 0.2 versus 1.6 ± 0.2 h*nmol/g). Further, in dogs, relative to orally administered 2-PMPA, 4 delivered a 44-fold enhanced 2-PMPA plasma exposure (AUC_0-t for 4: 62.6 h*nmol/mL versus AUC_0-t for 2-PMPA: 1.44 h*nmol/mL). These results suggest that ODOL promoieties can serve as a promising strategy for enhancing the oral bioavailability of multiply charged compounds, such as 2-PMPA, and enable its clinical translation.

Flexibility-Not just for yoga anymore!

Over the past few years, nucleosides have maintained a prominent role as one of the cornerstones of antiviral and anticancer therapeutics, and many approaches to nucleoside drug design have been pursued. One such approach involves flexibility in the sugar moiety of nucleosides, for example, in the highly successful anti-HIV and HBV drug tenofovir. In contrast, introduction of flexibility to the nucleobase scaffold has only more recently gained significance with the invention of our fleximers. The history, development, and some biological relevance for this innovative class of nucleosides are detailed herein.

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.

A novel multi-parametric high content screening assay in ciPTEC-OAT1 to predict drug-induced nephrotoxicity during drug discovery

Drug-induced nephrotoxicity is a major concern in the clinic and hampers the use of available treatments as well as the development of innovative medicines. It is typically discovered late during drug development, which reflects a lack of in vitro nephrotoxicity assays available that can be employed readily in early drug discovery, to identify and hence steer away from the risk. Here, we report the development of a high content screening assay in ciPTEC-OAT1, a proximal tubular cell line that expresses several relevant renal transporters, using five fluorescent dyes to quantify cell health parameters. We used a validation set of 62 drugs, tested across a relevant concentration range compared to their exposure in humans, to develop a model that integrates multi-parametric data and drug exposure information, which identified most proximal tubular toxic drugs tested (sensitivity 75%) without any false positives (specificity 100%). Due to the relatively high throughput (straight-forward assay protocol, 96-well format, cost-effective) the assay is compatible with the needs in the early drug discovery setting to enable identification, quantification and subsequent mitigation of the risk for nephrotoxicity.

The evolution of nucleoside analogue antivirals: A review for chemists and non-chemists. Part 1: Early structural modifications to the nucleoside scaffold

This is the first of two invited articles reviewing the development of nucleoside-analogue antiviral drugs, written for a target audience of virologists and other non-chemists, as well as chemists who may not be familiar with the field. Rather than providing a simple chronological account, we have examined and attempted to explain the thought processes, advances in synthetic chemistry and lessons learned from antiviral testing that led to a few molecules being moved forward to eventual approval for human therapies, while others were discarded. The present paper focuses on early, relatively simplistic changes made to the nucleoside scaffold, beginning with modifications of the nucleoside sugars of Ara-C and other arabinose-derived nucleoside analogues in the 1960's. A future paper will review more recent developments, focusing especially on more complex modifications, particularly those involving multiple changes to the nucleoside scaffold. We hope that these articles will help virologists and others outside the field of medicinal chemistry to understand why certain drugs were successfully developed, while the majority of candidate compounds encountered barriers due to low-yielding synthetic routes, toxicity or other problems that led to their abandonment.

N-Substituted Prodrugs of Mebendazole Provide Improved Aqueous Solubility and Oral Bioavailability in Mice and Dogs

Mebendazole (MBZ) was developed as a broad-spectrum anthelmintic but has recently shown efficacy as an anticancer agent. The use of MBZ for cancer, however, is challenging due to its poor solubility leading to poor bioavailability. Herein, we developed a prodrug approach with various N-linked promoieties including acyloxymethyl, aminoacyloxymethyl, and substituted phosphonooxymethyl in attempt to improve these characteristics. Compound 12, containing an (((((isopropoxycarbonyl)oxy)methoxy)phosphoryl)oxy)methyl promoiety, showed a >10 000-fold improvement in aqueous solubility. When evaluated in mice, 12 displayed a 2.2-fold higher plasma AUC_{0- t} and a 1.7-fold improvement in brain AUC_{0- t} with a calculated oral bioavailability of 52%, as compared to 24% for MBZ-polymorph C (MBZ-C), the most bioavailable polymorph. In dogs, 12 showed a 3.8-fold higher plasma AUC_{0- t} with oral bioavailability of 41% compared to 11% for MBZ-C. In summary, we have identified a prodrug of MBZ with better physicochemical properties and enhanced bioavailability in both mice and dog.

Synthesis and Physicochemical Evaluation of Entecavir-Fatty Acid Conjugates in Reducing Food Effect on Intestinal Absorption

The oral bioavailability of entecavir (EV), an anti-viral agent commonly prescribed to treat hepatitis B infections, is drastically reduced under a post-prandial state. This is primarily due to its low permeability in the gastrointestinal tract. To reduce the food effect on the intestinal absorption of the nucleotide analogue, four lipidic prodrugs were synthesized via the esterification of the primary alcohol of EV with fatty acids (hexanoic acid, octanoic acid, decanoic acid, and dodecanoic acid). EV-3-dodecanoate (or EV-C12) exhibited high solubility in a fed state simulated intestinal fluid (78.8 μg/mL), with the acceptable calculated logP value (3.62) and the lowest hydrolysis rate (22.5% for 12 h in simulated gastric fluid, pH 1.2). Therefore, it was chosen as a candidate to improve intestinal absorption of EV, especially under a fed state condition. Physical characterization using scanning electron microscopy, a differential scanning calorimeter, and X-ray powder diffraction revealed that EV-C12 had a rectangular-shaped crystalline form, with a melting point of about 170 °C. In a release test in biorelevant media, such as fasted and fed state-simulated intestinal and/or gastric fluid, more than 90% of the prodrug was released within 2 h in all media tested. These data suggest that this lipidic prodrug might have the potential to alleviate the negative food effect on the intestinal absorption of EV with increased therapeutic efficacy and patient compliance.

Entecavir Combined With Adefovir Ameliorates Patients With Chronic Hepatitis B Who Fail to Respond to Nucleotide (Acid) Analog Monotherapy

The aim of this study was to evaluate the efficacy and safety of entecavir (ETV) combined treatment with adefovir (ADV) on chronic hepatitic B (CHB) patients who failed to respond to nucleotide (acid) analog (NA) treatment. On this basis, the possible factors in the combined treatment of these patients will be analyzed. The safety, biochemical index, and the possible factors that might affect the ETV and ADV combined treatment at different points in time were retrospectively analyzed. The biochemical index included the following: virological response, hepatitis B virus (HBV) DNA decline, primary nonresponse, biochemical response, and the hepatitis B virus E antigen/hepatitis B virus E antibody seroconversion rate. There were 94 CHB patients and compensated liver cirrhosis patients who received ETV plus ADV treatment for over 12 weeks after failure of treatment with NAs. The authors have also investigated 76 CHB patients (80.9%) and 18 hepatitis B cirrhosis patients (19.1%) in this study. The HBV DNA baseline was 4.4 ± 1.4 log10 IU/mL, and the positive rate of HBeAg before salvage treatment was 78.7% (74/94). The sample sizes were 94, 78, 42, 10, 6, and 1 for follow-up of 24, 48, 96, 144, 192, and 240 weeks, respectively. The virological responses (HBV DNA < 2 log10 IU/mL) and biochemical responses were 52.1%, 74.3%, and 90.4% and 63.1%, 61.6%, and 81.1%, respectively, at 24, 48, and 96 weeks, which showed significant differences (P < 0.001 and P < 0.005, respectively). The HBV DNA decline was presented as mean ± SEM, which were 1.53 ± 1.23, 1.75 ± 1.37, 2.07 ± 1.54, and 2.39 ± 1.77 log10 IU/mL at 12, 24, 48, and 96 weeks, respectively. They showed significant differences compared with the baseline (χ = 8.084, P < 0.05). The rate of primary nonresponse was 30.9% (29/94), and the primary treatment failure rates were 26.6% (25/94), 24.4% (19/78), and 4.8% (2/42) at 24, 48, and 96 weeks, respectively. They all have statistical difference (P = 0.011 < 0.05). There were 23 patients who experienced virological breakthrough after the HBV DNA levels were undetectable, whereas after follow-up for 12-24 weeks, the HBV DNA levels were back to undetectable again. ETV plus ADV treatment is an efficient and safe treatment for CHB and compensated liver cirrhosis patients who experienced NA treatment failure. The high quantity of baseline HBV DNA level is a risk factor for poor efficacy of salvage treatment.

Factors associated with the decrease in hepatitis B surface antigen titers following interferon therapy in patients with chronic hepatitis B: Is interferon and adefovir combination therapy effective?

The purpose of antiviral therapy in chronic hepatitis B (CHB) is generally to achieve a decrease and ultimately disappearance of HBs antigen (HBsAg). Interferon (IFN) therapy of CHB appears to be less effective in Asian countries than in European countries, and the advantage of IFN and nucleotide(s) analog (NA) combination therapy has yet to be fully investigated. The present study focused on the factors associated with a decrease in HBs antigen following IFN monotherapy or IFN + NA combination therapy. A total of 35 patients with CHB who received IFN-based therapy (mean ± standard deviation age 36.7±8.5 years; 27 males and 8 females) were enrolled in this study. Of the 35 patients, 21 patients received pegylated IFN monotherapy and 14 patients received IFN and adefovir (ADV) combination therapy. We examined the factors associated with reductions in the HBsAg titer of >1.0 log IU/ml from the initial HBsAg titer to the end of treatment and to 24 weeks after treatment. Although 13 patients (37%) had a reduction in HBsAg of >1.0 IU/ml at the end of treatment, it was only maintained to 24 weeks after treatment in 7 patients (20%). The HBV core-related antigen (HBcrAg) titer before treatment was significantly higher in patients with a decrease in HBsAg at the end of treatment than in patients without a decrease in HBsAg (6.56±0.78 vs. 5.30±1.66 log IU/ml, P<0.05). Moreover, an increase in alanine aminotransferase (ALT) of >2 times from baseline occurred significantly more frequently in patients with a decrease in HBsAg (62 vs. 14%, P<0.05). The proportion of patients with a decrease in HBsAg was significantly greater in patients who received IFN monotherapy than in patients who received IFN and ADV combination therapy (43 vs. 29%, P<0.05). The present results revealed that the HBcr antigen titer before therapy and an on-treatment elevation of ALT (indicative of host instruction flare) are important factors associated with a decrease in HBsAg titers after IFN-based therapy. The efficacy of IFN and ADV combination therapy was not apparent in terms of a reduction in the HBsAg titer.

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.

The Effect of Prophylactic Lamivudine plus Adefovir Therapy Compared with Lamivudine Alone in Preventing Hepatitis B Reactivation in Lymphoma Patients with High Baseline HBV DNA during Chemotherapy

Prophylactic antiviral therapy is essential for lymphoma patients with high baseline HBV DNA who undergo cytotoxic chemotherapy. However, there are limited data on the optimal options. The present study was designed to compare the efficacy of prophylactic lamivudine (LAM) with lamivudine plus adefovir dipivoxil (LAM+ADV) in preventing hepatitis B virus (HBV) reactivation in lymphoma with, pre-chemotherapy HBV DNA load ≥2000 IU/ml. We retrospectively analyzed the medical records of 86 lymphoma patients with baseline HBV DNA load ≥2000 IU/ml during chemotherapy and received LAM or LAM+ADV as prophylaxis between January 1, 2008 and November 30, 2014 at Sun Yat-sen University Cancer Center, China. Sixty-five patients received LAM and 21 received LAM+ADV. The rate was significantly lower in the LAM+ADV group compared with the LAM group for HBV reactivation (23.8% vs 55.4%; p = 0.012), while no difference was observed between the two groups in patients for HBV-related hepatitis (21.3% vs 33.3%; p   =  0.349), and chemotherapy disruption (10.9% vs 19.0%; p = 0.337). In a multivariate analysis of factors associated with HBV reactivation in these patients, LAM+ADV treatment and HBeAg negative were the independent protective factors. Therefore, LAM+ADV should be considered for antiviral prophylaxis in lymphoma patients with pre-chemotherapy HBV DNA load ≥2000 IU/ml. Further study is warranted to confirm these findings.

Selected organophosphorus compounds with biological activity. Applications in medicine

The purpose of this article is to provide an overview of the latest applications of organophosphorus compounds (OPs) that exhibit biological activity.

Phosphonylated Acyclic Guanosine Analogues with the 1,2,3-Triazole Linker

A novel series of {4-[(2-amino-6-chloro-9H-purin-9-yl)methyl]-1H-1,2,3-triazol-1-yl}alkylphosphonates and {4-[(2-amino-6-oxo-1,6-dihydro-9H-purin-9-yl)methyl]-1H-1,2,3-triazol-1-yl}alkylphosphonates as acyclic analogues of guanosine were synthesized and assessed for antiviral activity against a broad range of DNA and RNA viruses and for their cytostatic activity toward three cancerous cell lines (HeLa, L1210 and CEM). They were devoid of antiviral activity; however, several phosphonates were found slightly cytostatic against HeLa cells at an IC₅₀ in the 80–210 µM range. Compounds (1R,2S)-17k and (1S,2S)-17k showed the highest inhibitory effects (IC₅₀ = 15–30 µM) against the proliferation of murine leukemia (L1210) and human T-lymphocyte (CEM) cell lines.

Prodrugs of reverse fosmidomycin analogues

Fosmidomycin inhibits IspC (Dxr, 1-deoxy-d-xylulose 5-phosphate reductoisomerase), a key enzyme in nonmevalonate isoprenoid biosynthesis that is essential in Plasmodium falciparum. The drug has been used successfully to treat malaria patients in clinical studies, thus validating IspC as an antimalarial target. However, improvement of the drug's pharmacodynamics and pharmacokinetics is desirable. Here, we show that the conversion of the phosphonate moiety into acyloxymethyl and alkoxycarbonyloxymethyl groups can increase the in vitro activity against asexual blood stages of P. falciparum by more than 1 order of magnitude. We also synthesized double prodrugs by additional esterification of the hydroxamate moiety. Prodrugs with modified hydroxamate moieties are subject to bioactivation in vitro. All prodrugs demonstrated improved antiplasmodial in vitro activity. Selected prodrugs and parent compounds were also tested for their cytotoxicity toward HeLa cells and in vivo in a Plasmodium berghei malaria model as well as in the SCID mouse P. falciparum model.

Association between adefovir dipivoxil treatment and the risk of renal insufficiency in patients with chronic hepatitis B: A meta-analysis

Adefovir dipivoxil (ADV) is an effective antiviral drug against hepatitis B virus. The renal tolerance of ADV at the currently approved dose of 10 mg daily for the treatment of chronic hepatitis B (CHB) remains controversial. The present meta-analysis was therefore performed to evaluate the renal safety of ADV treatment in patients with CHB. Two independent investigators searched MEDLINE, Embase and China National Knowledge Infrastructure databases for eligible studies published in English or Chinese until June 1, 2014. The Peto odds ratios (Peto ORs) or the rates of each study were analyzed. Seven randomized controlled trials (RCTs), four cohort studies and six single-arm studies were identified. ADV treatment was not associated with a higher incidence of nephrotoxicity in RCTs [Peto OR, 1.781; 95% confidence interval (CI), 0.637-4.979; P=0.271] but appeared to increase nephrotoxicity significantly in cohort studies (Peto OR, 2.682; 95% CI, 1.470-4.894; P=0.001); the significant increase was further observed in CHB patients receiving long-term ADV treatment in cohort studies (Peto OR, 2.275; 95% CI, 1.127-4.593; P=0.022). The analysis based on single-arm studies showed that the rate of renal dysfunction in the ADV-treated patients was 10.6% (95% CI, 0.059-0.185); the subgroup analysis with the standard of createnine levels showed a lower rate (6.9%, 95% CI, 0.013-0.298) than those in the overall studies. In conclusion, although current evidence indicated a positive link between treatment with ADV in CHB patients and an increased risk of renal dysfunction, optimally designed studies are required for definitive conclusions.

Comparison of adefovir dipivoxil and pegylated interferon alpha-2a treatment in chronic hepatitis B patients

OBJECTIVE:

In this study, we aimed to evaluate the efficacy of pegylated interferon alpha 2a and adefovir dipivoxil treatment in chronic hepatitis B patients.

METHODS:

This study was performed on patients treated for chronic hepatitis B in the Infectious Disease Clinic of Eskişehir Osmangazi University between 01.09.2005 and 31.03.2008. A total of 30 patients aged between 18 and 65 years constituted the study group. One of patient groups received (10 HBeAg negative, 4 HBeAg positive) PEG-IFN alpha 2a at a dose of 180 μg/once a week, whereas the other group (11 HBeAg negative, 5 HBeAg positive) received daily oral doses of 10 mg ADV. Treatment responses were evaluated at week 48.

RESULTS:

Reductions in serum HBV DNA levels at the end of 48 weeks were 4.8 log10 copy/ml and 4.2 log10 copy/ml in HBeAg negative patients who received ADV or PEG-IFN alpha 2a, respectively. Biochemical response rates were 60% and 91% in PEG-IFN alpha 2a and ADV groups, respectively. Among HBeAg positive patients, reductions in serum HBV DNA levels were 3. 2 log10 copy/ml and 4 log10 copy/ml in ADV and PEG-IFN alpha 2a groups, at week 48, respectively. Biochemical response rates were 50% and 40% in PEG-IFN alpha 2a and ADV groups, respectively. No significant difference was determined in biochemical and virological responses in HBeAg positive and negative patients between PEG-IFN alpha 2a and ADV groups, at week 48. When both treatment groups were evaluated for side effects, it was observed that side effects were significantly common in PEG-IFN alpha 2a group.

CONCLUSION:

When we compared PEG-IFN alpha 2a and ADV treatment in both HBeAg positive and negative patients, biochemical and virological response rates at 48 weeks were similar.

Adefovir dipivoxil in the treatment of chronic hepatitis B virus infection

Adefovir dipivoxil (Hepsera, Gilead Sciences) is a prodrug of adefovir, with potent antiviral activity against hepatitis B virus. Adefovir dipivoxil therapy, 10 mg daily for 48 weeks, is effective in hepatitis B e antigen-positive and -negative chronic hepatitis B. In hepatitis B e antigen-negative chronic hepatitis B, adefovir dipivoxil was recently found to maintain its efficacy even after 3 years of therapy. Adefovir dipivoxil is effective in patients with compensated or decompensated chronic viral B liver disease, and in pre- and post-transplant hepatitis B virus patients who develop resistance to lamivudine (Epivir, GlaxoSmithKline). It is well-tolerated and safe even after the third year of long-term therapy, and is associated with low rates of viral resistance. All these characteristics make adefovir dipivoxil an important drug for the treatment of hepatitis B virus infection and an excellent candidate for long-term maintenance therapy in chronic viral B liver disease.

Physicochemical property trends of marketed prodrugs

Many prodrug reviews describe specific examples of the successful application of prodrug technology to produce blockbuster drugs, such as simvastatin, omeprazole, acyclovir and enalapril. These reviews are helpful to understand the previous success stories and case histories of prodrug technology. The aim of the current review seeks to more clearly define quantitative trends in the changes in the physicochemical property parameters between the successful prodrug and the active parent molecule. This information can serve to guide medicinal chemists toward more successful pharmaceutical prodrugs in the future.

Anti-HBV agents derived from botanical origin

There are 350,000 hepatitis B virus (HBV) carriers all over the world. Chronic HBV infection is at a high risk of developing liver cirrhosis and hepatocelluar carcinoma (HCC), and heavily threatened people's health. Two kinds of drugs approved by FDA for anti-HBV therapy are immunomodulators (interferon α, pegylated-interferon α) and nucleos(t)ide analogues (lamivudine, adefovir dipivoxil, entecavir, telbivudine, and tenofovir disoproxil fumarate). These drugs have been proved to be far from being satisfactory due to their low specificity, side effects, and high rate of drug resistance. There is an urgent need to discover and develop novel effective anti-HBV drugs. With vast resources, various structures, diverse biological activities and action mechanisms, as well as abundant clinical experiences, botanical agents become a promising source of finding new anti-HBV drugs. This review summarizes the recent research and development of anti-HBV agents derived from botanical origin on their sources and active components, inhibitory effects and possible toxicities, as well as action targets and mechanisms, and also addresses the advantages and the existing shortcomings in the development of botanical inhibitors. This information may not only broaden the knowledge of anti-HBV therapy, and offer possible alternative or substitutive drugs for CHB patients, but also provides considerable information for developing new safe and effective anti-HBV drugs.

Renal function during treatment with adefovir plus peginterferon alfa-2a vs either drug alone in hepatitis B/D co-infection

Long-term safety of treatment with hepatitis B virus (HBV) polymerase inhibitors is a concern. Adefovir dipivoxil (ADV) therapy has previously been associated with impairment of renal function. Limited data are available on the safety of combination therapy with nucleos(t)ide analogues and interferon alfa (IFNα). The aim of this analysis was to assess the renal function during combination therapy with peginterferon alfa-2a (PegIFNα-2a) plus ADV vs either drug alone in patients with hepatitis B/D co-infection. We performed a retrospective analysis of renal function data of patients treated in the Hep-Net/International Delta Hepatitis Intervention Trial 1(HIDIT-1-trial), a European multicenter study to investigate the efficacy of 48 weeks of therapy with PegIFNα-2a+ADV vs either drug alone in 90 patients with chronic hepatitis B/D co-infection. Glomerular filtration rates (GFR) were calculated by Cockcroft-Gault (CG), abbreviated Modification of Diet in Renal Disease (MDRD) study and Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) equation. After 48 weeks of therapy GFR values were significantly lower in patients receiving adefovir-containing treatment vs PegIFNα-2a alone [mean difference 16.1 mL/min (CG) and 10.2 mL/min (MDRD), respectively, P < 0.05] while no differences were observed between patients receiving adefovir alone vs combination treatment. Twenty-four weeks after treatment GFR values did not differ between treatment arms. A decrease in GFR ≥ 20% was observed more often in patients during adefovir-containing treatment vs PegIFNα-2a alone (P < 0.05) which was confirmed by Kaplan-Meier analysis. Adefovir-containing but not PegIFNα-2a treatment was associated with a decrease in GFR values in about one-fifth of patients. Combination treatment of PegIFNα-2a+ADV in chronic hepatitis B/D co-infection did not lead to any further impairment of kidney function.

Current Antiviral Therapy of Chronic Hepatitis B: Efficacy and Safety

The treatment of chronic hepatitis B is in constant evolution. Interferon, the first agent licensed for chronic hepatitis B treatment, has been superseded by the growing popularity of nucleoside/nucleotide analogues (NA). However, resistance to these agents is a major challenge. Newer NAs, such as entecavir and tenofovir dipivoxil fumarate, have very low resistance rates and favorable safety profiles. Long-term use of these agents can effectively suppress hepatitis B virus DNA, leading to decrease in incidence of hepatitic flares, as well as in the development of cirrhosis and hepatocellular carcinoma. The efficacy and safety of various antiviral agents is discussed in this review.

Antiviral drugs for viruses other than human immunodeficiency virus

Most viral diseases, with the exception of those caused by human immunodeficiency virus, are self-limited illnesses that do not require specific antiviral therapy. The currently available antiviral drugs target 3 main groups of viruses: herpes, hepatitis, and influenza viruses. With the exception of the antisense molecule fomivirsen, all antiherpes drugs inhibit viral replication by serving as competitive substrates for viral DNA polymerase. Drugs for the treatment of influenza inhibit the ion channel M(2) protein or the enzyme neuraminidase. Combination therapy with Interferon-α and ribavirin remains the backbone treatment for chronic hepatitis C; the addition of serine protease inhibitors improves the treatment outcome of patients infected with hepatitis C virus genotype 1. Chronic hepatitis B can be treated with interferon or a combination of nucleos(t)ide analogues. Notably, almost all the nucleos(t) ide analogues for the treatment of chronic hepatitis B possess anti-human immunodeficiency virus properties, and they inhibit replication of hepatitis B virus by serving as competitive substrates for its DNA polymerase. Some antiviral drugs possess multiple potential clinical applications, such as ribavirin for the treatment of chronic hepatitis C and respiratory syncytial virus and cidofovir for the treatment of cytomegalovirus and other DNA viruses. Drug resistance is an emerging threat to the clinical utility of antiviral drugs. The major mechanisms for drug resistance are mutations in the viral DNA polymerase gene or in genes that encode for the viral kinases required for the activation of certain drugs such as acyclovir and ganciclovir. Widespread antiviral resistance has limited the clinical utility of M(2) inhibitors for the prevention and treatment of influenza infections. This article provides an overview of clinically available antiviral drugs for the primary care physician, with a special focus on pharmacology, clinical uses, and adverse effects.

Prodrugs--from serendipity to rational design

The prodrug concept has been used to improve undesirable properties of drugs since the late 19th century, although it was only at the end of the 1950s that the actual term prodrug was introduced for the first time. Prodrugs are inactive, bioreversible derivatives of active drug molecules that must undergo an enzymatic and/or chemical transformation in vivo to release the active parent drug, which can then elicit its desired pharmacological effect in the body. In most cases, prodrugs are simple chemical derivatives that are only one or two chemical or enzymatic steps away from the active parent drug. However, some prodrugs lack an obvious carrier or promoiety but instead result from a molecular modification of the prodrug itself, which generates a new active compound. Numerous prodrugs designed to overcome formulation, delivery, and toxicity barriers to drug utilization have reached the market. In fact, approximately 20% of all small molecular drugs approved during the period 2000 to 2008 were prodrugs. Although the development of a prodrug can be very challenging, the prodrug approach represents a feasible way to improve the erratic properties of investigational drugs or drugs already on the market. This review introduces in depth the rationale behind the use of the prodrug approach from past to present, and also considers the possible problems that can arise from inadequate activation of prodrugs.