phosphorothioates

Phosphorothioates are chemical derivatives in which one of the non-bridging oxygen atoms in a phosphate diester backbone is replaced by sulfur, producing a phosphorothioate linkage. This modification is most commonly found in nucleic acids such as DNA and RNA and is a defining feature of phosphorothioate oligonucleotides used in research and therapeutic contexts.

The substitution of sulfur for oxygen changes several properties of the backbone. Phosphorothioate oligonucleotides are more resistant to nucleases, which slows degradation in biological environments. They also tend to bind more readily to plasma proteins, which can increase circulating half-life and affect distribution. The modification introduces a stereogenic center at phosphorus, yielding Rp and Sp diastereomers; most commercial preparations are mixtures, although stereodefined analogs can be synthesized for specific applications.

Synthesis of phosphorothioate backbones is typically achieved during solid-phase synthesis by sulfurizing phosphite intermediates to form the phosphorothioate linkage. This process generates a mixture of diastereomers at each modified linkage, and strategies to control stereochemistry or separate isomers add complexity and cost. The backbone can influence base-pairing properties and, in some contexts, the overall thermal stability of the nucleic acid.

Applications of phosphorothioates are most prominent in antisense oligonucleotides and related therapeutic modalities. The PS backbone enhances pharmacokinetics and cellular uptake, contributing to the efficacy of several approved drugs and clinical candidates. Phosphorothioates are also used in research probes and diagnostics, where increased stability is advantageous. Potential drawbacks include non-specific protein interactions and, at higher doses, immunostimulatory or off-target effects, requiring careful chemical design and dosing considerations.