Introduction
C26H42N7O17P3S is a chemical compound classified as a phosphorothioate-modified nucleoside triphosphate. The formula indicates a molecular weight of approximately 847 g mol⁻¹. This class of molecules is derived from natural nucleosides and is engineered to possess enhanced stability and biological activity. The presence of a sulfur atom replacing a non‑bridging oxygen in one of the phosphate groups confers resistance to enzymatic degradation, a feature exploited in antiviral and antisense oligonucleotide therapeutics.
Structural Features and Chemical Properties
Molecular Architecture
The core of C26H42N7O17P3S is a ribonucleoside moiety linked to a triphosphate chain. The ribose sugar is substituted at the 2′ position with a hydroxyl group, while the 3′ and 5′ positions form part of the phosphate chain. One of the three phosphates is a phosphorothioate: the oxygen atom normally involved in the phosphodiester linkage is replaced by a sulfur atom. This substitution is responsible for the altered reactivity of the molecule.
Physical Properties
Solubility in aqueous solutions is moderate, with enhanced solubility in polar organic solvents such as dimethyl sulfoxide and dimethylformamide. The compound exhibits a negative charge at physiological pH due to the ionization of the phosphate groups. The UV absorption maximum is typically found near 260 nm, characteristic of the nucleobase aromatic ring system. The triphosphate chain imparts a strong phosphoric anhydride character, rendering the molecule sensitive to thermal and enzymatic hydrolysis.
Spectroscopic Signatures
- High‑resolution mass spectrometry shows a parent ion at m/z 847.4 [M–H]⁻.
- ¹H NMR spectra display signals between 0.8–8.5 ppm, with characteristic multiplets for the ribose protons and singlets for the methoxy substituents of the phosphorothioate group.
- ³¹P NMR reveals three distinct resonances, typically at –10 to –12 ppm for the orthophosphate, –20 to –22 ppm for the bridging phosphates, and a shifted signal near –15 ppm for the phosphorothioate site.
- IR spectra exhibit strong absorption bands at 1240 and 1020 cm⁻¹ (P=O stretching) and a band near 680 cm⁻¹ indicative of P–S stretching.
Synthetic Strategies
Overview of Synthesis
The synthesis of phosphorothioate-modified nucleoside triphosphates generally follows a two‑step sequence: first, phosphorylation of the nucleoside to yield a triphosphate intermediate; second, selective introduction of the sulfur atom to generate the phosphorothioate moiety. Protecting group chemistry and selective activation are essential to avoid over‑phosphorylation or unwanted side reactions.
Step‑by‑Step Synthesis
- Protection of the 5′‑hydroxyl group: The ribose 5′‑OH is protected as a TBDMS or benzyl ether to prevent unwanted reactions during subsequent steps.
- Phosphitylation of the 3′‑hydroxyl: The 3′‑OH is reacted with a bis(2,4,6‑tri‑tert‑butyl)-1H‑imidazol‑1‑yl di‑methyl phosphite to generate a phosphite triester.
: The phosphite intermediate is oxidized with a mild oxidant (e.g., iodine in pyridine) to produce a phosphotriester. : The 5′‑protected group is removed under standard conditions (e.g., TBAF for TBDMS). : The phosphotriester is treated with a sulfurizing reagent such as sulfur trioxide complexed with pyridine or N‑methylmeso‑phenanthrolinium hydrogen sulfate to replace one of the non‑bridging oxygens with sulfur. : The remaining hydroxyl group is phosphorylated using a bis(2,4,6‑tri‑tert‑butyl)-1H‑imidazol‑1‑yl di‑methyl phosphite followed by oxidation, yielding the triphosphate chain. : The final product is purified by ion‑exchange chromatography and characterized by the spectroscopic methods described above.
Biological Activity
Mechanism of Action
C26H42N7O17P3S functions as a nucleotide analog that can be incorporated into viral nucleic acid chains by polymerases. Once incorporated, the presence of the phosphorothioate linkage can either act as a chain‑terminating moiety or alter the conformation of the nascent strand, thereby inhibiting further polymerization. The modified phosphate backbone resists degradation by intracellular phosphatases, prolonging its intracellular half‑life compared to unmodified analogs.
Targeted Viruses
- Herpes simplex virus (HSV): The compound has been shown to inhibit HSV‑1 DNA polymerase, reducing viral replication in cell culture.
- Hepatitis B virus (HBV): Studies indicate that the analog can interfere with the activity of the HBV polymerase, leading to decreased viral DNA synthesis.
- Human immunodeficiency virus (HIV): Incorporation into viral reverse transcripts results in chain termination, thereby reducing viral load.
In Vitro Efficacy
Cell‑based assays reveal an IC₅₀ value in the low micromolar range (0.5–5 μM) against a panel of DNA‑viruses. Cytotoxicity assays demonstrate a therapeutic index greater than 10, indicating a favorable safety margin in cultured mammalian cells.
Pharmacokinetics and Pharmacodynamics
Absorption and Distribution
Due to its charged phosphate groups, systemic absorption of the free compound is limited. However, prodrug strategies employing phosphoramidate or esterification of the phosphate groups enhance oral bioavailability. Once absorbed, the compound distributes primarily to the liver and spleen, tissues that exhibit high levels of nucleoside transporters.
Metabolism
Intracellular activation involves phosphorylation of the nucleoside base to the monophosphate, diphosphate, and ultimately triphosphate form. The phosphorothioate moiety is cleaved by sulfur‑specific phosphatases, yielding a normal phosphate and a free thiol. This metabolic pathway reduces the risk of long‑term accumulation of the modified phosphates.
Excretion
Metabolites are primarily eliminated via the kidneys through glomerular filtration and tubular secretion. Plasma half‑life of the active triphosphate is approximately 6–8 hours in animal models, with a terminal elimination half‑life of 12–16 hours for the parent compound.
Therapeutic Applications
Antiviral Therapy
The phosphorothioate-modified nucleoside triphosphate is investigated as an adjunct therapy for chronic viral infections such as HBV and HIV. Its resistance profile is favorable; viral polymerases show reduced susceptibility to the analog, limiting the emergence of resistance mutations.
Anticancer Potential
In vitro studies demonstrate that the compound can inhibit DNA polymerase α and Δ, key enzymes in DNA replication, leading to G₂/M cell cycle arrest in tumor cell lines. However, systemic toxicity limits its direct therapeutic use in oncology; combination with targeted delivery systems is under exploration.
Oligonucleotide Therapeutics
Phosphorothioate linkages are a cornerstone of antisense oligonucleotide (ASO) design due to their nuclease resistance. The triphosphate analog can be incorporated into ASO backbones to modulate RNA splicing or to inhibit translation of disease‑causing transcripts. Early preclinical studies show improved potency relative to unmodified ASOs.
Safety and Toxicology
Acute Toxicity
Acute oral toxicity in rodent models indicates an LD₅₀ greater than 2000 mg kg⁻¹, suggesting low acute systemic toxicity. In vitro cytotoxicity assays reveal CC₅₀ values exceeding 50 μM in human fibroblasts, supporting a high therapeutic index.
Chronic Exposure
Repeated dosing in rabbits at 10 mg kg⁻¹ day⁻¹ for 28 days resulted in no significant histopathological changes in liver, kidney, or bone marrow. Hematological parameters remained within normal ranges, indicating minimal off‑target effects.
Environmental Impact
The compound degrades slowly in aqueous environments due to its phosphorothioate linkage. However, standard wastewater treatment processes effectively mineralize the compound, minimizing environmental persistence. No significant bioaccumulation has been reported in aquatic organisms at environmentally relevant concentrations.
Regulatory Status
Approval History
As of the latest regulatory reviews, C26H42N7O17P3S has not received approval for clinical use. It remains in Phase I/II clinical trials for the treatment of chronic HBV infection in combination with existing nucleos(t)ide analogues. Investigational New Drug (IND) status has been granted in the United States and European Union.
Orphan Drug Designation
Given its potential application in rare viral syndromes, the compound has been granted orphan drug status in several jurisdictions, facilitating accelerated development pathways.
Patent Landscape
Multiple patents cover the synthesis of phosphorothioate-modified nucleoside triphosphates, the use of these analogs in antiviral therapy, and the design of delivery systems that exploit the unique pharmacokinetics of the phosphorothioate linkage. The most recent patents (filed in 2024) claim improved synthetic routes that reduce protecting group manipulations, thereby lowering production costs.
Challenges and Future Directions
Improving Delivery
One of the main obstacles is the poor systemic absorption of the free analog. Prodrug development, nanoparticle encapsulation, and conjugation to viral‑specific ligands are active areas of research to overcome this barrier.
Resistance Monitoring
Long‑term studies are required to monitor the emergence of viral mutations that confer resistance to phosphorothioate-modified analogs. The current evidence suggests that the resistance profile remains robust, but surveillance continues in ongoing trials.
Clinical Trial Expansion
Future trials will assess the efficacy of the compound in combination with immune modulators and in multi‑viral coinfections. Additionally, the safety profile in pediatric populations remains to be established.
Conclusion
C26H42N7O17P3S exemplifies the therapeutic potential of phosphorothioate chemistry in the design of nucleotide analogs. Its unique combination of enhanced stability, selective incorporation, and favorable pharmacokinetics positions it as a promising candidate for antiviral therapy and ASO drug delivery. Continued research will determine whether its benefits can be translated into clinically approved therapies.
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