If an analyte is amenable to analysis, it’s difficult to ignore the benefits that LC-MS/MS can provide. There are numerous challenges associated with using LC-MS/MS for the bioanalysis of oligonucleotides that have slowed its utilization for these assays. These include:

  • adsorption throughout the extraction & LC-MS process
  • relatively poor sensitivity compared to ligand binding assays
  • the need for ion pairing agents for chromatography
  • possibility of high backgrounds in the mass spectrometer
  • chromatographic irreproducibility
  • carryover
  • incompatibility with other assays
  • operation in negative ion mode on the MS
  • need for project-specific instrument passivation

However, the increased adoption of LC-MS/MS for oligonucleotides and similar analytes has been aided by improvements in the chromatographic and mass spectrometry technology as well as learning what works best in a high-throughput environment. 

Experience Matters in Oligonucleotide Method Development

KCAS Bio has worked extensively on assays for oligonucleotides, SiRNAs and ARCs (Antibody siRNA conjugates) over the past few years so the experience gained from those projects directly leads to a more focused and efficient method development strategy for new entities.

Robust bioanalytical LC-MS/MS assays usually depend on sample clean up to reduce impact of other matrix components on the analyte of interest. Protein precipitation or liquid-liquid extraction would usually be our first approach for a new assay. However, this is a non-starter for oligonucleotides because of the complex nature of these molecules. Solid-phase extraction using weak anion exchange or custom phases developed specifically for oligonucleotides offer a much greater chance of success. Plasticware is critical too to minimize loss of analyte due to adsorption, so lo-bind materials are used throughout.

Optimizing Tissue and Surrogate Matrices

For tissues and matrices that are expensive or rare, we have found that a surrogate matrix approach can help. The choice of surrogate can depend on the type of oligonucleotide or the matrix of interest. We typically start with BSA or Sig matrix as a starting point for ARCs. A homogenate mix of multiple tissues can work for each of the tissues of interest. CSF and spinal cord typically show non-specific binding issues so use of a carrier protein or matrix with proteins, such as plasma, can help with recovery. Tissue homogenization also needs optimization with focus on type of homogenization buffer and ratio of tissue to homogenization buffer to help recovery and reduce matrix effects.

Chromatography Challenges and Ion Pairing Strategies

A stumbling block for oligonucleotide work in a lab where multiple projects are being worked on has been the need for ion pairing additives to help retention of the oligonucleotides. This makes it difficult to transition smoothly from an oligonucleotide project to another more routine assay. HILIC has been seen as a potential solution to this but in common with other research groups we have found that sensitivity of the HILIC approach is not adequate for project needs.

Why Instrument Passivation Is Important

Even moving between oligonucleotide projects can be challenging where project specific passivation is required. This is done with repeated injection of a system suitability sample prepared from the drug and IS where observing the typical performance assures we are ready to go and helps define the duration of passivation needed. When working on oligonucleotide projects it makes most practical sense to dedicate instrumentation to that assay until work is completed.

Choosing the Right Ion Pairing Reagent

Though ion pairing is the LC separation mode of choice, there is still no one-size-fits-all approach. However, we have seen correlation between the chain length and the ion pairing reagent that works best in terms of peak shape & sensitivity.

Selecting the Best Mass Spectrometer for Oligonucleotide Bioanalysis

Triple Quadrupole Performance

Historically, use of negative ionization could be unpredictable moving between instruments where one could show good sensitivity while a second showed minimal signal. The improvement in mass spectrometer performance has taken away much of that concern for analysis of oligonucleotides. Higher end triple quadrupoles such as the SCIEX API-6500 and API-7500 are both good options for sensitive assays for oligonucleotides. Although the API-7500 is a more sensitive instrument in terms of analyte signal it is not uncommon to get comparable performance from the API-6500 due to reduced signal to noise. With any new assay, it is worth testing both platforms.

When High-Resolution Mass Spectrometry Provides an Advantage

Even though oligonucleotides are relatively high molecular weight analytes and the LC-MS/MS transitions should be specific, we can see elevated background interference using triple quadrupole mass spectrometers. The availability of high-resolution mass spectrometers such as the Thermo Orbitrap platforms provides a valuable alternative. The increased resolution of the detector can help eliminate interference. Using HRMS can greatly reduce method development times for oligonucleotide-based analytes. Operation in full scan mode also provides the potential for interrogation of data sets for metabolites. This does come with the disadvantage of massive file sizes, so data storage becomes an additional challenge to consider.

Key Takeaways: LC-MS/MS Bioanalysis of Oligonucleotides

Oligonucleotides, siRNAs, and ARCs remain challenging analytes for LC-MS/MS bioanalysis. However, improvements in extraction strategies, analytical technologies, and experience gained from supporting these complex molecules have made successful bioanalysis of non-clinical and clinical studies more attainable.

Key considerations for successful LC-MS/MS bioanalysis of oligonucleotides include:

  • Optimize sample preparation strategies. Solid-phase extraction approaches and low-binding materials can help minimize analyte loss and improve assay performance.
  • Select matrices carefully. Surrogate matrices and optimized tissue processing approaches can help address challenges associated with complex or limited biological samples.
  • Tailor chromatography approaches. Ion pairing remains an important strategy, but reagent selection may depend on oligonucleotide characteristics such as chain length.
  • Evaluate instrumentation based on assay needs. Triple quadrupole and high-resolution mass spectrometry platforms each provide advantages depending on sensitivity, selectivity, and interference requirements.
  • Leverage experience to improve method development. Knowledge gained from previous oligonucleotide, siRNA, and ARC projects can support more efficient and robust analytical strategies.

Frequently Asked Questions About LC-MS/MS Bioanalysis of Oligonucleotides

What makes LC-MS/MS bioanalysis of oligonucleotides challenging?

Oligonucleotides present several analytical challenges, including adsorption during sample preparation and analysis, lower sensitivity compared to some ligand binding assays, the need for ion pairing reagents, chromatographic variability, carryover, and project-specific instrument passivation. Advances in chromatography, mass spectrometry, and laboratory experience have made these assays more practical and reliable.

What sample preparation methods work best for oligonucleotide LC-MS/MS assays?

Unlike many small molecule assays, oligonucleotides are generally not well suited for protein precipitation or liquid-liquid extraction. Solid-phase extraction (SPE), particularly weak anion exchange or oligonucleotide-specific phases, is often the preferred approach. Using low-binding plasticware throughout sample preparation also helps minimize analyte loss due to adsorption.

Why are ion pairing reagents important for oligonucleotide analysis?

Ion pairing reagents improve the chromatographic retention and separation of oligonucleotides during LC-MS/MS analysis. While ion pairing remains the preferred separation approach, no single reagent is ideal for every analyte. The optimal ion pairing strategy often depends on factors such as oligonucleotide chain length and assay performance requirements.

When should high-resolution mass spectrometry (HRMS) be considered for oligonucleotide bioanalysis?

High-resolution mass spectrometry can help reduce background interference that may occur when using triple quadrupole instruments. HRMS also offers the advantage of full-scan data acquisition, allowing researchers to investigate metabolites without additional analyses. These capabilities can shorten method development timelines for certain oligonucleotide-based therapeutics, although they require greater data storage capacity.

How can experience improve oligonucleotide method development?

Experience with oligonucleotide, siRNA, and antibody-siRNA conjugate (ARC) assays helps laboratories develop more efficient and robust analytical methods. Knowledge gained from previous projects can guide decisions around sample preparation, matrix selection, chromatography, instrument passivation, and mass spectrometry platform selection, ultimately supporting more reliable bioanalysis for both nonclinical and clinical studies.