Diastereomer Characterization of PS-Modified Synthetic Oligonucleotides Using Cyclic IMS-MS.

Synthetic oligonucleotides have emerged as effective therapeutics that regulate gene expression to treat and prevent diseases. Oligonucleotide therapeutics are often modified with a substitution of a phosphorothioate (PS) linkage along the phosphodiester backbone to improve the drug performance and stability. The PS modification creates a mixture of diastereomer structures, increasing by a factor of 2n where n is the number of PS linkages. Despite recent draft guidances highlighting the importance of their characterization, analytical methods to measure the resulting diastereomers are currently lacking. Here, we present a method combining tandem mass spectrometry (MS) and tandem ion mobility spectrometry (IMS) using a cyclic IMS-MS instrument to study diastereomers in PS-modified oligonucleotides. This approach requires no enzymatic digestion as the intact oligonucleotides are directly injected into the MS instrument. Analogous to top-down proteomics, MS fragmentation of the intact oligonucleotide results in 3' and 5' fragment ends that have fewer diastereomers than their intact counterpart. Tandem IMS allows for mobility resolution of the diastereomers at the terminal ends. We tested four model oligonucleotides that differ in either the number of PS bonds or sequence to demonstrate the capability of this method to elucidate diastereomer structures on modified oligonucleotides.

Nuclease P1 Digestion for Bottom-Up RNA Sequencing of Modified siRNA Therapeutics.

siRNA therapeutics provide a selective and powerful approach to reduce the expression of disease-causing genes. For regulatory approval, these modalities require sequence confirmation which is typically achieved by intact tandem mass spectrometry sequencing. However, this process produces highly complex spectra which are difficult to interpret and typically results in less than full sequence coverage. We sought to develop a bottom-up siRNA sequencing platform to ease sequencing data analysis and provide full sequence coverage. Analogous to bottom-up proteomics, this process requires chemical or enzymatic digestion to reduce the oligonucleotide length down to analyzable lengths, but siRNAs commonly contain modifications that inhibit the degradation process. We tested six digestion schemes for their feasibility to digest the 2' modified siRNAs and identified that nuclease P1 provides an effective digestion workflow. Using a partial digestion, nuclease P1 provides high 5' and 3' end sequence coverage with multiple overlapping digestion products. Additionally, this enzyme provides high-quality and highly reproducible RNA sequencing no matter the RNA phosphorothioate content, 2'-fluorination status, sequence, or length. Overall, we developed a robust enzymatic digestion scheme for bottom-up siRNA sequencing using nuclease P1, which can be implemented into existing sequence confirmation workflows.