Synthesis and biological evaluation of a series of liver-selective phosphonic acid thyroid hormone receptor agonists and their prodrugs

New triazinephosphonate dopants for Nafion proton exchange membranes (PEM)

A new paradigm for energy is underway demanding decarbonized energy systems. Some of them rely on emerging electrochemical devices, crucial in hydrogen technologies, including fuel cells, CO2 and water electrolysers, whose applications and performances depend on key components such as their separators/ion-exchange membranes. The most studied and already commercialized Nafion membrane shows great chemical stability, but its water content limits its high proton conduction to a limited range of operating temperatures. Here, we report the synthesis of a new series of triazinephosphonate derivatives and their use as dopants in the preparation of new modified Nafion membranes. The triazinephosphonate derivatives were prepared by substitution of chlorine atoms in cyanuric chloride. Diverse conditions were used to obtain the trisubstituted (4-hydroxyphenyl)triazinephosphonate derivatives and the (4-aminophenyl)triazinephosphonate derivatives, but with these amino counterparts, only the disubstituted compounds were obtained. The new modified Nafion membranes were prepared by casting incorporation of the synthesized 1,3,5-triazinephosphonate (TPs) derivatives. The evaluation of the proton conduction properties of the new membranes and relative humidity (RH) conditions and at 60 °C, showed that they present higher proton conductivities than the prepared Nafion membrane and similar or better proton conductivities than commercial Nafion N115, in the same experimental conditions. The Nafion-doped membrane with compound TP2 with a 1.0 wt % loading showed the highest proton conductivity with 84 mS·cm-1.

Thyroid receptor β: A promising target for developing novel anti-androgenetic alopecia drugs

Androgenetic alopecia (AGA) significantly impacts the self-confidence and mental well-being of people. Recent research has revealed that thyroid receptor β (TRβ) agonists can activate hair follicles and effectively stimulate hair growth. This review aims to comprehensively elucidate the specific mechanism of action of TRβ in treating AGA from various perspectives, highlighting its potential as a drug target for combating AGA. Moreover, this review provides a thorough summary of the research advances in TRβ agonist candidates with anti-AGA efficacy and outlines the structure-activity relationships (SARs) of TRβ agonists. We hope that this review will provide practical information for the development of effective anti-alopecia drugs.

Molecular Dynamic Simulation To Reveal the Mechanism Underlying MGL-3196 Resistance to Thyroxine Receptor Beta

Thyroxine receptor beta (TRβ) is a ligand-dependent nuclear receptor that participates in regulating multiple biological processes, particularly playing an important role in lipid metabolism regulation. TRβ is currently a popular therapeutic target for nonalcoholic steatohepatitis (NASH), while no drugs have been approved to treat this disease. MGL-3196 (Resmetirom) is the first TRβ agonist that has succeeded in phase III clinical trials for the treatment of NASH; therefore, studying its molecular mechanism of action is of great significance. In this study, we employed molecular dynamic simulation to investigate the interaction mode between MGL-3196 and TRβ at the all-atom level. More importantly, by comparing the binding patterns of MGL-3196 in several prevalent TRβ mutants, it was identified that the mutations R243Q and H435R located, respectively, around and within the ligand-binding pocket of TRβ cause TRβ to be insensitive to MGL-3196. This indicates that patients with NASH carrying these two mutations may exhibit resistance to the medication of MGL-3196, thereby highlighting the potential impact of TRβ mutations on TRβ-targeted treatment of NASH and beyond.

Discovery of a potent and selective human AC2 inhibitor based on 7-deazapurine analogues of adefovir

Adefovir based acyclic nucleoside phosphonates were previously shown to modulate bacterial and, to a certain extent, human adenylate cyclases (mACs). In this work, a series of 24 novel 7-substituted 7-deazaadefovir analogues were synthesized in the form of prodrugs. Twelve analogues were single-digit micromolar inhibitors of Bordetella pertussis adenylate cyclase toxin with no cytotoxicity to J774A.1 macrophages. In HEK293 cell-based assays, compound 14 was identified as a potent (IC50 = 4.45 μM), non-toxic, and selective mAC2 inhibitor (vs. mAC1 and mAC5). Such a compound represents a valuable addition to a limited number of small-molecule probes to study the biological functions of individual endogenous mAC isoforms.

Thyroid hormone action and liver disease, a complex interplay

Thyroid hormone action is involved in virtually all physiological processes. It is well known that the liver and thyroid are intimately linked, with thyroid hormone playing important roles in de novo lipogenesis, beta-oxidation (fatty acid oxidation), cholesterol metabolism, and carbohydrate metabolism. Clinical and mechanistic research studies have shown that thyroid hormone can be involved in chronic liver diseases, including alcohol-associated or NAFLD and HCC. Thyroid hormone action and synthetic thyroid hormone analogs can exert beneficial actions in terms of lowering lipids, preventing chronic liver disease and as liver anticancer agents. More recently, preclinical and clinical studies have indicated that some analogs of thyroid hormone could also play a role in the treatment of liver disease. These synthetic molecules, thyromimetics, can modulate lipid metabolism, particularly in NAFLD/NASH. In this review, we first summarize the thyroid hormone signaling axis in the context of liver biology, then we describe the changes in thyroid hormone signaling in liver disease and how liver diseases affect the thyroid hormone homeostasis, and finally we discuss the use of thyroid hormone-analog for the treatment of liver disease.

Thyroid Hormone Receptor-β Agonists in NAFLD Therapy: Possibilities and Challenges

Nonalcoholic fatty liver disease (NAFLD) is a progressive metabolic liver disease with an unknown pathogenesis and no FDA-approved drug treatment to date. Hypothyroidism has been identified as a risk factor for NAFLD as thyroxine is required for regulating metabolism in adults. Thyroxine has been shown to reduce fat in the livers of murine models with experimentally induced NAFLD. The use of synthetic thyroxine has been shown to increase lipid metabolism leading to weight loss; however, thyroxine has also been shown to cause many side effects, especially in the heart. Overcoming these cardiac side effects involves designing agonists specific to one of the 2 gene subtypes for the thyroid hormone (TH) receptor (TR), TRβ. While the other TH receptor subtype, TRα, is mainly expressed in the heart and is responsible for thyroxine's cardiac function, TRβ is mainly expressed in the liver and is involved in liver function. Using TRβ-specific agonists to treat NAFLD can prevent cardiac and other adverse side effects. Several TRβ-specific agonists have shown positive therapeutic effects in NAFLD animal models and have entered clinical trials. We seek to provide a comprehensive updated reference of TRβ-specific agonists in this review and explore the future therapeutic potential of TRβ-specific activation in the treatment of NAFLD.

Discovery of Highly Potent and Selective Thyroid Hormone Receptor β Agonists for the Treatment of Nonalcoholic Steatohepatitis

Nonalcoholic steatohepatitis (NASH) is a progressive stage of nonalcoholic fatty liver disease (NAFLD) and is characterized by steatosis, inflammation, hepatocyte ballooning, and fibrosis. While there are currently no approved therapies for NASH, the thyroid hormone receptor β (THR-β), primarily expressed in the liver, is emerging as an effective molecular target for the treatment of NASH. However, the adverse cardiac and bone effects mediated by thyroid hormone receptor α (THR-α) need to be minimized. Herein, we reported the discovery of a series of novel THR-β agonists featuring pyrrolo[3,2-b]pyridin-5-one skeletons based on structure-based drug design. Further optimization led to compound 15, which exhibited higher potency and selectivity for THR-β over THR-α compared to clinical drug MGL-3196. More significantly, an excellent liver-to-serum ratio of 93:1 was observed for compound 15. We believe that the high hepatic concentration of compound 15 may result in no cardiotoxicity.

Phosphonate Derivatives of Paracetamol and Valproic Acid

Paracetamol and valproic acid are standalone drugs with leading position in the world drug market. The phosphonate analogues of these drugs were synthesized and were tested in vivo. N-(4-hydroxyphenylcarbamoyl)phosphonic acid was four times more potent than paracetamol in preventing acetic acid-induced writhing. Phosphonate derivative of valproic acid, (2-propylpentanoyl)phosphonic acid, had similar in vivo activity to valproic acid in the pentylenetetrazole-induced kindling mouse model.

Discovery of a novel, liver-targeted thyroid hormone receptor-β agonist, CS271011, in the treatment of lipid metabolism disorders

INTRODUCTION

Thyroid hormone receptor β (THR-β) plays a critical role in metabolism regulation and has become an attractive target for treating lipid metabolism disorders in recent years. Thus, in this study, we discovered CS271011, a novel THR-β agonist, and assessed the safety and efficiency of CS271011 compared to MGL-3196 in vitro and in vivo.

METHODS

We conducted luciferase reporter gene assays to assess the activation of THR-β and α in vitro. C57BL/6J mice were fed a high-fat diet for 12 weeks, CS271011 was administered by gavage at the dose of 1 mg/kg and 3 mg/kg, and MGL-3196 was administered at the dose of 3 mg/kg for 10 weeks. Body weight, food intake, serum and hepatic parameters, histological analysis, pharmacokinetic studies, RNA sequencing of the liver and heart, and expression of hepatic lipid-metabolic genes were determined to evaluate the safety and efficiency of CS271011.

RESULTS

Compared with MGL-3196, CS271011 showed higher THR-β activation in vitro. In the diet-induced obesity mice model, CS271011 demonstrated favourable pharmacokinetic properties in mice and was enriched in the liver. Finally, CS271011 improved dyslipidaemia and reduced liver steatosis in the diet-induced obesity murine model. Mechanistically, CS271011 and MGL-3196 showed potent regulation of lipid metabolism-related genes.

CONCLUSIONS

CS271011 is a potent and liver-targeted THR-β agonist for treating lipid metabolism disorders.

Discovery of a Highly Selective and H435R-Sensitive Thyroid Hormone Receptor β Agonist

The design and development of agonists selectively targeting thyroid hormone receptor β (TRβ) and TRβ mutants remain challenging tasks. In this study, we first adopted the strategy of breaking the "His-Phe switch" to solve two problems, simultaneously. A structure-based design approach was successfully utilized to obtain compound 16g, which is a potent TRβ agonist (EC₅₀: 21.0 nM, 85.0% of the maximum efficacy of 1) with outstanding selectivity for TRβ over TRα and also effectively activates the TRβ^H435R mutant. Then, we developed a highly efficient synthetic method for 16g. Our serials of cocrystal structures revealed detailed structural mechanisms in overcoming subtype selectivity and rescuing the H435R mutation. 16g also showed excellent lipid metabolism, safety, metabolic stability, and pharmacokinetic properties. Collectively, 16g is a well-characterized selective and mutation-sensitive TRβ agonist for further investigating its function in treating dyslipidemia, nonalcoholic steatohepatitis (NASH), and resistance to thyroid hormone (RTH).

Regulation of gene transcription by thyroid hormone receptor β agonists in clinical development for the treatment of non-alcoholic steatohepatitis (NASH)

Thyroid hormones are important modulators of metabolic activity in mammals and alter cholesterol and fatty acid levels through activation of the nuclear thyroid hormone receptor (THR). Currently, there are several THRβ agonists in clinical trials for the treatment of non-alcoholic steatohepatitis (NASH) that have demonstrated the potential to reduce liver fat and restore liver function. In this study, we tested three THRβ-agonism-based NASH treatment candidates, GC-1 (sobetirome), MGL-3196 (resmetirom), and VK2809, and compared their selectivity for THRβ and their ability to modulate the expression of genes specific to cholesterol and fatty acid biosynthesis and metabolism in vitro using human hepatic cells and in vivo using a rat model. Treatment with GC-1 upregulated the transcription of CPT1A in the human hepatocyte-derived Huh-7 cell line with a dose-response comparable to that of the native THR ligand, triiodothyronine (T3). VK2809A (active parent of VK2809), MGL-3196, and VK2809 were approximately 30-fold, 1,000-fold, and 2,000-fold less potent than T3, respectively. Additionally, these relative potencies were confirmed by quantification of other direct gene targets of THR, namely, ANGPTL4 and DIO1. In primary human hepatocytes, potencies were conserved for every compound except for VK2809, which showed significantly increased potency that was comparable to that of its active counterpart, VK2809A. In high-fat diet fed rats, a single dose of T3 significantly reduced total cholesterol levels and concurrently increased liver Dio1 and Me1 RNA expression. MGL-3196 treatment resulted in concentration-dependent decreases in total and low-density lipoprotein cholesterol with corresponding increases in liver gene expression, but the compound was significantly less potent than T3. In conclusion, we have implemented a strategy to rank the efficacy of THRβ agonists by quantifying changes in the transcription of genes that lead to metabolic alterations, an effect that is directly downstream of THR binding and activation.

Selective thyroid hormone receptor β agonists with oxadiazolone acid isosteres

Use of the oxadiazolone acid isostere in triiodothyronine analogs yielded potent and selective agonists for the thyroid hormone receptor β. Selected examples showed good in-vivo efficacy in a rat hypercholesterolemic model. One compound was further profiled in a diet-induced mouse model of nonalcoholic steatohepatitis (NASH) and showed robust target engagement and significant histological improvements in both liver steatosis and fibrosis.

The Race to Bash NASH: Emerging Targets and Drug Development in a Complex Liver Disease

Nonalcoholic steatohepatitis (NASH) is a severe form of nonalcoholic fatty liver disease (NAFLD) characterized by liver steatosis, inflammation, and hepatocellular damage. NASH is a serious condition that can progress to cirrhosis, liver failure, and hepatocellular carcinoma. The association of NASH with obesity, type 2 diabetes mellitus, and dyslipidemia has led to an emerging picture of NASH as the liver manifestation of metabolic syndrome. Although diet and exercise can dramatically improve NASH outcomes, significant lifestyle changes can be challenging to sustain. Pharmaceutical therapies could be an important addition to care, but currently none are approved for NASH. Here, we review the most promising targets for NASH treatment, along with the most advanced therapeutics in development. These include targets involved in metabolism (e.g., sugar, lipid, and cholesterol metabolism), inflammation, and fibrosis. Ultimately, combination therapies addressing multiple aspects of NASH pathogenesis are expected to provide benefit for patients.

Phosphonate prodrugs: an overview and recent advances

Phosphonates, often used as isosteric replacements for phosphates, can provide important interactions with an enzyme. Due to their high charge at physiological pH, however, permeation into cells can be a challenge. Protecting phosphonates as prodrugs has shown promise in drug delivery. Thus, a variety of structures and cleavage/activation mechanisms exist, enabling release of the active compound. This review describes the structural diversity of these pro-moieties, relevant cleavage mechanisms and recent advances in the design of phosphonate prodrugs.

Quantification of Thyromimetic Sobetirome Concentration in Biological Tissue Samples

Thyroid hormone is a principal regulator of essential processes in vertebrate physiology and homeostasis. Synthetic derivatives of thyroid hormone, known as thyromimetics, display desirable therapeutic properties. Thoroughly understanding how thyromimetics distribute throughout the body is crucial for their development and this requires appropriate bioanalytical techniques to quantify drug levels in different tissues. Here, we describe a detailed protocol for the quantification of the thyromimetic sobetirome using liquid chromatography tandem-mass spectrometry (LC-MS/MS).

Thyroid hormone analogs for the treatment of dyslipidemia: past, present, and future

OBJECTIVE

Treatment of dyslipidemia is a major burden for public health. Thyroid hormone regulates lipid metabolism by binding the thyroid hormone receptor (TR), but the use of thyroid hormone to treat dyslipidemia is not indicated due to its deleterious effects on heart, bone, and muscle. Thyroid hormone analogs have been conceived to selectively activate TR in the liver, thus reducing potential side-effects.

METHODS

The authors searched the PubMed database to review TR and the action of thyromimetics in vitro and in animal models. Then, all double-blind, placebo controlled trials that analyzed the use of thyroid hormone analog for the treatment of dyslipidemia in humans were included. Finally, the ongoing research on the use of TR agonists was searched, searching the US National Institutes of Health Registry and the WHO International Clinical Trial Registry Platform (ICTRP).

RESULTS

Thyromimetics were tested in humans for the treatment of dyslipidemia, as a single therapeutic agent or as an add-on therapy to the traditional lipid-lowering drugs. In most trials, thyromimetics lowered total cholesterol, low-density lipoprotein cholesterol, and triglycerides, but their use has been associated with adverse side-effects, both in pre-clinical studies and in humans.

CONCLUSIONS

The use of thyromimetics for the treatment of dyslipidemia is not presently recommended. Future possible clinical applications might include their use to promote weight reduction. Thyromimetics might also represent an interesting alternative, both for the treatment of non-alcoholic steatohepatitis, and type 2 diabetes due to their positive effects on insulin sensitivity. Finally, additional experimental and clinical studies are needed for a better comprehension of the effect(s) of a long-term therapy.

Sustained zero-order delivery of GC-1 from a nanochannel membrane device alleviates metabolic syndrome

BACKGROUND/OBJECTIVES

Our objective was to assess the sustained, low-dose and constant administration of the thyroid receptor-β (TRβ)-selective agonist GC-1 (sobetirome) from a novel nanochannel membrane device (NMD) for drug delivery. As it known to speed up metabolism, accomplish weight loss, improve cholesterol levels and possess anti-diabetic effects, GC-1 was steadily administered by our NMD, consisting of an implantable nanochannel membrane, as an alternative to conventional daily administration, which is subject to compliance issues in clinical settings.

SUBJECTS/METHODS

Diet-induced obese C57BL/J6 male mice were fed a very high-fat diet (VHFD) and received NMD implants subcutaneously. Ten mice per group received capsules containing GC-1 or phosphate-buffered saline (control). Weight, lean and fat mass, as well as cholesterol, triglycerides, insulin and glucose, were monitored for 24 days. After treatment, plasma levels of thyroid-stimulating hormone (TSH) and thyroxine were compared. mRNA levels of a panel of thermogenic markers were examined using real-time PCR in white adipose tissue (WAT) and brown adipose tissue (BAT). Adipose tissue, liver and local inflammatory response to the implant were examined histologically. Pancreatic islet number and β-cell area were assessed.

RESULTS

GC-1 released from the NMD reversed VHFD-induced obesity and normalized serum cholesterol and glycemia. Significant reductions in body weight and fat mass were observed within 10 days, whereas reductions in serum cholesterol and glucose levels were seen within 7 days. The significant decrease in TSH was consistent with TRβ selectivity for GC-1. Levels of transcript for Ucp1 and thermogenic genes PGC1a, Cidea, Dio2 and Cox5a showed significant upregulation in WAT in NMD-GC-1-treated mice, but decreased in BAT. Although mice treated by NMD-GC-1 showed a similar number of pancreatic islets, they exhibited significant increase in β-cell area.

CONCLUSIONS

Our data demonstrate that the NMD implant achieves steady administration of GC-1, offering an effective and tightly controlled molecular delivery system for treatment of obesity and metabolic disease, thereby addressing compliance.

Prodrugs of phosphonates and phosphates: crossing the membrane barrier

A substantial portion of metabolism involves transformation of phosphate esters, including pathways leading to nucleotides and oligonucleotides, carbohydrates, isoprenoids and steroids, and phosphorylated proteins. Because the natural substrates bear one or more negative charges, drugs that target these enzymes generally must be charged as well, but small charged molecules can have difficulty traversing the cell membrane by means other than endocytosis. The resulting dichotomy has stimulated a great deal of effort to develop effective prodrugs, compounds that carry little or no charge to enable them to transit biological membranes, but able to release the parent drug once inside the target cell. This chapter presents recent studies on advances in prodrug forms, along with representative examples of their application to marketed and developmental drugs.

Discovery of 2-[3,5-dichloro-4-(5-isopropyl-6-oxo-1,6-dihydropyridazin-3-yloxy)phenyl]-3,5-dioxo-2,3,4,5-tetrahydro[1,2,4]triazine-6-carbonitrile (MGL-3196), a Highly Selective Thyroid Hormone Receptor β agonist in clinical trials for the treatment of dyslipidemia

The beneficial effects of thyroid hormone (TH) on lipid levels are primarily due to its action at the thyroid hormone receptor β (THR-β) in the liver, while adverse effects, including cardiac effects, are mediated by thyroid hormone receptor α (THR-α). A pyridazinone series has been identified that is significantly more THR-β selective than earlier analogues. Optimization of this series by the addition of a cyanoazauracil substituent improved both the potency and selectivity and led to MGL-3196 (53), which is 28-fold selective for THR-β over THR-α in a functional assay. Compound 53 showed outstanding safety in a rat heart model and was efficacious in a preclinical model at doses that showed no impact on the central thyroid axis. In reported studies in healthy volunteers, 53 exhibited an excellent safety profile and decreased LDL cholesterol (LDL-C) and triglycerides (TG) at once daily oral doses of 50 mg or higher given for 2 weeks.

Thyroid hormone analogues and derivatives: Actions in fatty liver

Fatty liver or nonalcoholic fatty liver disease (NAFLD), a problem of increasing clinical significance and prevalence worldwide, is associated with increased risk for the development of cirrhosis and hepatocellular carcinoma. Although several therapeutic approaches can be used in the context of NAFLD, dietary and physical activities are still the most frequently used strategies. Some pharmacological agents show promising results although no conclusions can be drawn from recent clinical trials. Thyroid hormones [THs; thyroxine (T4) and 3,3′,5-triiodo-L-thyronine (T3)] coordinate a diverse array of physiological events during development and lipid/energy homeostasis and have some potentially therapeutic actions which include inducing weight loss, and lowering plasma cholesterol levels and tissue adiposity. The thyroid hormones exert their physiological effects by binding to specific nuclear receptors [thyroid hormone receptors (TR)] of which the TRβ isoform is liver specific and has been considered a putative target for the treatment of dyslipidemia and fatty liver. In view of this, the aim of the review is (1) to provide an overview of the action of T3 on lipid metabolism with implications for liver steatosis and (2) to provide an update on the current knowledge concerning the administration of TRβ selective thyromimetics (GC-1 and MB07811), as well as of 3,5-diiodo-L-thyronine and its novel functional analogue TRC150094 in animal models of overweight and related disorders including primarily fatty liver.

Thyromimetics: a journey from bench to bed-side

Selective thyromimetics are synthetic analogs of thyroid hormones with tissue-specific thyroid hormone actions. Tissue selectivity is partly mediated by selectivity for the thyroid hormone receptor-β isoform, but is also enhanced by tissue-selective uptake. Several preclinical animal models and recent human clinical trials have provided sound evidence that thyromimetics can serve as pharmacological tools to improve serum lipids without affecting heart rate. Thyromimetics consistently and efficiently lowered low-density lipoprotein cholesterol and lipoprotein (a) plasma levels without positive chronotropic effects. Most importantly, thyromimetics had a synergistic action when used in addition to 3-hydroxy-3-methylglutaryl CoA reductase inhibitors. Animal data have further suggested that thyromimetics might be useful in the treatment of obesity, hepatic steatosis and atherosclerosis. However, only long-term phase III clinical trials will tell if the observed lipid lowering effects of thyromimetics will improve cardiovascular outcome in humans, too. At the moment, the treatment of dyslipidemia seems to be the major indication for the therapeutic use of thyromimetics, which are now rapidly moving from bench to bed-side.

Lipid lowering with thyroid hormone and thyromimetics

PURPOSE OF REVIEW

To summarize how thyroid hormones exert their effects on lipid metabolism through specific interaction with their nuclear receptors, to review studies of the effects of new and selective thyromimetic drugs in animals and humans and to identify important questions for future research.

RECENT FINDINGS

Thyroid hormones exert their effects by stimulation of thyroid hormone receptors that have different tissue distribution and metabolic targets. TRβ is predominant in liver and mainly responsible for effects on cholesterol and lipoprotein metabolism, whereas TRα is most important in fat, muscle, and heart. Thyroid hormone analogs (thyromimetics, tiromes) have been developed that activate TRβ and are selectively taken up and/or activated by the liver. Such compounds stimulate hepatic LDL receptors, cholesterol elimination as bile acids and cholesterol, and presumably promote reverse cholesterol transport. In animals, they retard atherosclerosis progression. In humans, eprotirome exerts favorable lipid-modulating effects while lacking thyroid hormone-related side-effects and maintaining normal hypothalamic-pituitary-thyroid feedback. When added to statins, it reduces LDL and non-HDL cholesterol, apolipoprotein B, and triglycerides as well as lipoprotein (a).

SUMMARY

Liver-specific and β-selective thyroid hormone analogs activate a spectrum of favorable thyroid hormone actions that optimize lipid metabolism and promote cholesterol elimination. Further studies should establish long-term safety and potential clinical usefulness of thyromimetics.

Synthesis of 3'-amino-3'-deoxyguanosine and 3'-amino-3'-deoxyxyloguanosine monophosphate HepDirect prodrugs from guanosine

The synthesis of 3'-amino-3'-deoxyguanosine and 3'-amino-3'-deoxyxyloguanosine monophosphate HepDirect prodrugs from guanosine is reported. Initial incorporation of N,N-dibenzylformamidino protection of the C2-amino of guanosine masked the reactivity of that group and simplified purification of subsequent analogues. The first key intermediate, 9-(2,5-bis-O-tert-butyldimethylsilyl-beta-D-ribofuranosyl)-2-N-(N,N-dibenzylformamidino)guanine (3a), was prepared in 60% yield after recycling of the undesired 3',5'-bis-O-protected byproduct (4a) by simple equilibration in methanol to a mixture of the two bis-O-protected compounds. Thus, protected, the 3'-position was manipulated to form the 3'-deoxyribo- or 3'-deoxyxylo-3'-azido derivatives (9 or 16, respectively). Further selective manipulations provided the cis-5'-monophosphate (3-chlorophenyl)-1,3-propanyl diester prodrugs (HepDirect prodrugs), 15 and 21. These HepDirect prodrugs were demonstrated to activate to their respective NTPs in rat hepatocytes.