Replacing a single oxygen atom with sulfur in the backbone of a synthetic DNA or RNA strand dramatically changes how that strand behaves inside the body, and the phosphorothioate (PS) bond is the modification that makes this swap possible. In a natural nucleic acid, phosphorus sits at the center of each backbone linkage flanked by oxygen atoms. A PS bond substitutes one of those non-bridging oxygens with sulfur, and that small chemical change is the reason most approved antisense drugs survive long enough in the bloodstream to reach their targets.1PubMed Central. Sulfur modification in natural RNA and therapeutic oligonucleotides The modification touches nearly every aspect of how a therapeutic oligonucleotide performs, from stability and protein binding to immune activation and toxicity, and understanding those trade-offs is central to modern nucleic acid drug design.
Why Nucleases Normally Destroy Unmodified DNA
The body treats stray DNA and RNA as waste or as a pathogen signal. Enzymes called nucleases chew through the sugar-phosphate backbone in minutes. These enzymes typically rely on two metal ions, usually magnesium, positioned at precise distances from each other and from the phosphorus center. The metal ions help activate a water molecule that attacks the phosphorus and snaps the backbone apart. In an unmodified oligonucleotide injected into the bloodstream, this process is fast enough that most of the drug is degraded before it can reach target cells.
How the Sulfur Atom Disrupts Nuclease Activity
Sulfur is larger than oxygen and forms longer bonds with phosphorus. When it sits in the nuclease active site, it pushes things out of alignment. Structural studies of a bacterial DNA polymerase’s proofreading site show that the sulfur atom physically displaces the metal ions that the enzyme needs for catalysis. In one orientation of the PS bond, the zinc ions still bind but the substrate and the attacking water molecule are mispositioned; in the other orientation, the metal ions are displaced from the active site entirely.2PubMed. Structural principles for the inhibition of the 3′-5′ exonuclease activity of Escherichia coli DNA polymerase I by phosphorothioates
Molecular simulations of RNase H, the enzyme that antisense drugs deliberately recruit to destroy target RNA, tell a similar story from a different angle. When sulfur replaces one of the oxygens at the cleavage site, the catalytic water molecule shifts about half an angstrom away from where it needs to be, and the magnesium-to-magnesium distance stretches from its optimal range to around 4.8 angstroms. In the other stereochemical arrangement, the distortion is even worse: the metal ions can separate by more than 5 angstroms, a distance incompatible with catalysis.3Nucleic Acids Research. Controlled sulfur-based engineering confers mouldability to phosphorothioate antisense oligonucleotides The result is that the enzyme either works far more slowly or not at all when it encounters a PS linkage.
The Two Faces of the PS Bond
Swapping sulfur for oxygen creates a new center of asymmetry at the phosphorus atom. The sulfur can point in one of two directions, producing mirror-image forms called the Rp and Sp isomers. Standard chemical synthesis generates a random mixture of these two forms at every PS position, so a 20-unit oligonucleotide with PS bonds throughout has over a million possible stereochemical variants.4PubMed Central. Chirality matters: stereo-defined phosphorothioate linkages at the termini of small interfering RNAs improve pharmacology in vivo These two isomers behave differently in almost every way that matters for drug design.
Rp linkages tend to stabilize the duplex formed between an antisense strand and its complementary RNA, while Sp linkages are slightly destabilizing. When researchers systematically varied the ratio, melting temperatures shifted modestly, spanning a range of about negative three to positive three degrees Celsius across different stereochemical patterns, compared with roughly 64°C for the stereorandom version.5Nucleic Acids Research. Understanding the effect of controlling phosphorothioate chirality in the DNA gap on the potency and safety of gapmer antisense oligonucleotides Those few degrees may sound small, but the isomers also differ in nuclease sensitivity and in how they interact with the enzymes the drug needs to work. The Sp isomer is generally more resistant to nucleases, while the Rp isomer allows RNase H to function more easily. Getting the right mix at the right positions along the strand is one of the major optimization challenges in oligonucleotide drug development.
Advances in Stereocontrolled Synthesis
For decades, the random mixture of Rp and Sp isomers was accepted as a practical limitation. Separating desired stereoisomers after synthesis was possible for short sequences, and one approach used ion exchange chromatography to isolate single-stranded oligonucleotides containing the desired PS arrangements, which were then annealed into double-stranded drugs.6Nucleic Acids Research. Chirality matters: stereo-defined phosphorothioate linkages at the termini of small interfering RNAs improve pharmacology in vivo But purification-based methods do not scale well for longer sequences where the number of possible isomers is astronomical.
More recently, chemistry has advanced to the point where each PS linkage can be installed with the desired handedness during synthesis rather than sorted out afterward. One platform uses phosphorus(V)-based reagents that allow programmable, stereocontrolled sulfur incorporation in a protocol described as efficient, inexpensive, and operationally simple.7PubMed Central. Unlocking P(V): Reagents for chiral phosphorothioate synthesis Another approach uses chiral building blocks derived from a natural compound, limonene, attached to a soluble support, allowing liquid-phase assembly of short stereopure PS oligonucleotides through a streamlined cycle of coupling and deprotection reactions.8Journal of Organic Chemistry. Stereo-Controlled Liquid Phase Synthesis of Phosphorothioate Oligonucleotides on a Soluble Support These methods are moving from academic demonstrations toward practical drug manufacturing, and several stereopure candidates are now in clinical trials.
Protein Binding and What It Means for Drug Distribution
PS oligonucleotides stick to proteins. More than 90% of a PS-backbone drug in the bloodstream is bound to plasma proteins like albumin through low-affinity, non-specific interactions.9Molecular Therapy. Phosphorothioate Bond: Nuclease Resistance and Drug Design This protein binding is a double-edged property. On one hand, it prevents the kidneys from filtering the drug out immediately, because protein-bound molecules are too large to pass through the kidney’s filtration barrier. On the other hand, the plasma half-life is still only about one to two hours, because the drug distributes quickly from blood into tissues, particularly the liver and kidneys. Once in tissue, PS oligonucleotides can persist for weeks, which is why many antisense drugs are dosed infrequently despite their short time in circulation.
Getting Inside Cells Without a Delivery Vehicle
One of the more surprising properties of PS oligonucleotides is their ability to enter cells without the help of transfection reagents or lipid nanoparticles. This process, sometimes called gymnosis, depends heavily on the PS backbone. When researchers compared oligonucleotides that were identical except for their backbone chemistry, those with normal phosphate linkages showed much lower uptake into muscle cells and essentially no activity against their nuclear RNA target. PS-backbone oligonucleotides accumulated in the cytoplasm and produced measurable gene-silencing activity, and that nuclear activity correlated directly with how much oligonucleotide the cells had taken up through this unassisted pathway.10PubMed Central. Intracellular Distribution and Nuclear Activity of Antisense Oligonucleotides After Unassisted Uptake in Myoblasts and Differentiated Myotubes In Vitro
The mechanism likely involves the PS backbone’s affinity for cell-surface proteins, which facilitates endocytosis. This same protein-stickiness that helps with tissue distribution also promotes cellular entry, making PS a kind of built-in delivery enhancer. The trade-off, as with many PS properties, is that the same stickiness can cause unwanted interactions with intracellular proteins, a topic that comes up repeatedly in toxicity discussions.
RNase H and the Gapmer Strategy
Most antisense drugs do not simply block their target RNA by sitting on it. Instead, they recruit RNase H1, a cellular enzyme that recognizes a DNA-RNA hybrid and cuts the RNA strand. The antisense oligonucleotide provides the DNA portion of the hybrid, guiding RNase H to destroy a specific messenger RNA. This is where PS chemistry creates a tension: the drug needs PS bonds for stability, but too many PS bonds at the cleavage site can impair RNase H activity, as described in the nuclease resistance section above.
The solution is the gapmer design, where a central stretch of DNA-like nucleotides (the “gap”) is flanked by wings of more heavily modified nucleotides. The gap is where RNase H does its cutting, while the wings provide extra nuclease resistance and binding affinity. PS bonds are typically used throughout the entire molecule but are most critical in the wings, where they protect against exonucleases that chew in from the ends.
Cellular RNase H1 concentration turns out to be the bottleneck for how fast these drugs work. When researchers reduced RNase H1 levels in cells, ASO-driven RNA degradation slowed to nearly the rate seen without any drug at all. Overexpressing the enzyme roughly doubled the degradation rate.11Nucleic Acids Research. The rates of the major steps in the molecular mechanism of RNase H1-dependent antisense oligonucleotide induced degradation of RNA This means that in tissues where RNase H1 is naturally abundant, antisense drugs tend to work better, and tissues with low levels of the enzyme may be harder to target with this approach.
PS Bonds in siRNA and CRISPR Guide RNAs
The PS modification is not limited to single-stranded antisense drugs. In double-stranded siRNA therapeutics, which silence genes through a different cellular pathway, PS bonds are placed strategically at the ends of each strand. Modifying just the two terminal linkages on each strand is generally enough to protect against the exonucleases that would otherwise rapidly degrade the siRNA in vivo.6Nucleic Acids Research. Chirality matters: stereo-defined phosphorothioate linkages at the termini of small interfering RNAs improve pharmacology in vivo Researchers have found that the specific stereochemistry of these terminal PS bonds matters for the drug’s performance in animals, with certain Sp or Rp configurations at particular positions improving potency or duration of effect.
CRISPR gene-editing tools face a similar challenge. The guide RNAs that direct Cas9 to its target are vulnerable to cellular nucleases, and synthetic guide RNAs with PS modifications at their ends show improved stability and can modestly boost editing efficiency, in some cases by more than 1.5-fold compared with unmodified guides.12PLoS ONE. Minimal 2′-O-methyl phosphorothioate linkage modification pattern of synthetic guide RNAs for increased stability and efficient CRISPR-Cas9 gene editing avoiding cellular toxicity As with siRNA, the pattern matters: too many modifications in the wrong places can interfere with Cas9 binding or activity, while too few leave the guide RNA unprotected. Analytical methods for characterizing the stereochemistry of PS bonds within guide RNAs are an active area of development.13PubMed Central. Quantitative Analysis of Phosphorothioate Isomers in CRISPR sgRNA at Single-Residue Resolution Using Endonuclease Digestion Coupled with Liquid Chromatography Cyclic Ion Mobility Mass Spectrometry (LC/cIMS)
The Immune System Notices Sulfur
The PS backbone does not just evade nucleases; it actively engages the innate immune system. Toll-like receptor 9, a sensor that normally detects microbial DNA inside endosomes, responds to PS oligonucleotides. This happens even when the sequences lack unmethylated CpG motifs, which are the classical trigger for TLR9 activation. PS oligonucleotides without CpG motifs can still activate TLR9, and those containing CpG motifs produce a robust stimulatory response that overrides other inhibitory effects of the PS backbone.14Molecular Therapy Nucleic Acids. Cooperative Activation of Human Toll-like Receptor 9 by Phosphorothioate Oligonucleotides
The interplay between backbone chemistry and sequence is nuanced. The PS-modified 2′-deoxyribose backbone appears to be the dominant feature that determines whether an oligonucleotide engages TLR9 competitively, while CpG motifs within the sequence can flip the outcome from inhibition to strong activation.15Immunity. The DNA Sugar Backbone 2′ Deoxyribose Determines Toll-like Receptor 9 Activation Drug designers exploit this deliberately in some contexts, using CpG PS oligonucleotides as immune stimulants for vaccines or cancer immunotherapy. But for antisense or siRNA drugs where immune activation is unwanted, careful sequence design and sugar modifications in the wings help minimize TLR9 engagement.
Toxicity Trade-Offs
The protein-binding tendency that gives PS oligonucleotides their favorable tissue distribution also creates safety liabilities. At high intravenous doses in monkeys, PS oligonucleotides triggered complement activation, an arm of the immune system that can cause drops in blood pressure and changes in blood cell counts. The mechanism involves direct interaction between the oligonucleotide and Factor H, a protein that normally keeps complement in check. Displacing Factor H from its regulatory role allows the alternative complement pathway to fire inappropriately, and the effect correlates with how much oligonucleotide is in the plasma at any given moment.16PubMed. Activation of the alternative pathway of complement by a phosphorothioate oligonucleotide: potential mechanism of action This is one reason why many antisense drugs are given by subcutaneous injection rather than IV: slower absorption avoids the sharp plasma spikes that trigger complement.
Liver toxicity is another concern, particularly with certain sugar modifications combined with PS backbones. Gapmer oligonucleotides that use 2′-fluoro modifications in their wings bind tightly to a family of nuclear proteins called DBHS proteins, leading to their rapid degradation. When these proteins are depleted, cells become sick. In mice, 2′-fluoro PS gapmers of various sequences caused reductions in DBHS protein levels and liver damage.17Nucleic Acids Research. Acute hepatotoxicity of 2′ fluoro-modified 5–10–5 gapmer phosphorothioate oligonucleotides in mice correlates with intracellular protein binding and the loss of DBHS proteins This finding helped steer the field toward other wing chemistries that interact less aggressively with intracellular proteins.
Blood-related side effects add another layer of complexity. PS oligonucleotides can bind to glycoprotein VI, a receptor on platelets, activating them and triggering the formation of platelet-leukocyte aggregates. In whole blood treated with certain PS oligonucleotides, the percentage of these aggregates rose from about 12% in untreated samples to over 30% and in some cases nearly 70%, depending on the sequence. A kinase inhibitor completely reversed the effect, pointing to a specific signaling pathway rather than a generic stickiness problem.18Haematologica. Sequence-specific 2′-O-methoxyethyl antisense oligonucleotides activate human platelets through glycoprotein VI, triggering formation of platelet-leukocyte aggregates Low platelet counts have been observed clinically with some antisense drugs, and this mechanism helps explain why.
How PS Bonds Affect Target Binding Strength
An antisense drug needs to bind tightly to its complementary RNA to be effective. PS bonds weaken this binding slightly. High-resolution melting studies across a range of sequences found that each PS linkage reduced duplex stability by an average of about 0.12 kcal/mol.19PLoS ONE. Developing predictive hybridization models for phosphorothioate oligonucleotides using high-resolution melting In practical terms, this translates to a drop in melting temperature of roughly half a degree per modification for typical sequences. The effect is also sequence-dependent: AT-rich regions show a larger depression in melting temperature than GC-rich regions.20Nucleic Acids Research. Physicochemical properties of phospborothioate oligodeoxynucleotides
This is one reason why PS bonds are almost always combined with sugar modifications like 2′-O-methoxyethyl or locked nucleic acids in the wings of gapmers. The sugar modifications boost RNA-binding affinity enough to more than compensate for the destabilization caused by the PS backbone. The net result is a molecule that binds its target more tightly than unmodified DNA while also resisting degradation.
Manufacturing the PS Backbone at Scale
Commercially, PS oligonucleotides are made on automated synthesizers using the phosphoramidite approach, a stepwise method where each nucleotide is added one at a time. The key extra step, compared with making normal DNA, is a sulfurization reaction that converts the freshly formed phosphite linkage into a phosphorothioate. Early sulfurizing agents were slow or required special handling. The development of reagents like benzyltriethylammonium tetrathiomolybdate provided a rapid, efficient, and reliable sulfur-transfer step compatible with automated instruments.21Tetrahedron Letters. Solid phase synthesis of phosphorothioate oligonucleotides using benzyltriethylammonium tetrathiomolybdate as a rapid sulfur transfer reagent Another family of reagents based on disulfide compounds can complete the sulfurization within one to two minutes and produce oligonucleotides with efficiency comparable to standard unmodified synthesis.22PubMed Central. New efficient sulfurizing reagents for the preparation of oligodeoxyribonucleotide phosphorothioate analogues The availability of fast, cheap sulfurizing chemistry has been essential to making PS oligonucleotides practical as drugs manufactured at kilogram scale.
Beyond PS: the Phosphorodithioate and Extended Backbones
If replacing one oxygen with sulfur helps, what about replacing both non-bridging oxygens? Phosphorodithioate linkages, where two sulfur atoms flank the phosphorus, provide even greater nuclease resistance. Oligomers with alternating phosphorodithioate and normal phosphate linkages were highly resistant to degradation in human serum, in cellular extracts, and against purified nucleases, while still forming stable duplexes and directing RNase H cleavage of target RNA.23PubMed Central. Biochemical and physicochemical properties of phosphorodithioate DNA The catch is that having more than one unmodified linkage between phosphorodithioates left the molecule vulnerable to certain endonucleases, so the spacing pattern has to be carefully designed.
Another emerging approach bypasses the PS bond entirely for terminal protection. Extended nucleic acid (exNA) backbones, which add an extra carbon to the sugar-phosphate framework, showed dramatic improvements in nuclease resistance in laboratory tests. An exNA-modified PS bond had a half-life against one exonuclease roughly 36 times longer than PS alone and about a thousand times longer than an unmodified linkage.24PubMed Central. Enhancing siRNA efficacy in vivo with extended nucleic acid backbones Combining exNA with PS could produce the most stable oligonucleotide ends reported so far, potentially reducing the number of PS bonds needed and thereby mitigating some of the toxicity associated with a fully PS backbone. These chemistries remain in early development, but they illustrate the direction the field is heading: using PS bonds where they are needed most and replacing them with cleaner alternatives where possible.