Complementary LC/MS and UV‑Vis Spectrophotometry for siRNA Quality Control and Thermal Stability Assessment

Importance of the Topic

This application note addresses analytical strategies for ensuring the structural integrity, mass confirmation, and thermal stability of therapeutic siRNA duplexes. Reliable characterization of siRNA is essential for development, formulation, comparability, and routine quality control in biopharmaceutical workflows because duplex integrity and thermal behavior strongly influence biological activity, delivery, and shelf life.

Objectives and Study Overview

The study demonstrates an integrated, orthogonal workflow combining ion-pair reversed-phase liquid chromatography with mass spectrometry (IP-RP LC/MS) and UV–Vis thermal melt (Tm) analysis to (1) separate and identify antisense (AS), sense (SS), and intact duplex species under non‑denaturing and denaturing chromatographic conditions, (2) confirm molecular masses of components by high-resolution MS, and (3) quantify duplex thermal stability in physiologically relevant and LC mobile‑phase buffers. The model analyte was inclisiran (commercial single strands and duplex).

Methodology and Experimental Summary

Sample preparation and concentrations:

• Inclisiran single strands and duplex resuspended in RNase‑free water (10 pmol/µL for LC/MS injections).
• UV‑Vis Tm samples prepared at ~20 µg/mL in either PBS (physiological buffer) or LC elution buffer (15 mM triethylamine (TEA), 100 mM hexafluoroisopropanol (HFIP), 25% methanol).

Chromatography and LC conditions (summary):

• IP‑RP column: Altura Oligo HPH‑C18, 2.1 × 50 mm, 2.7 µm.
• Mobile phases: Solvent A = 15 mM TEA + 100 mM HFIP in water; Solvent B = Solvent A : methanol 50:50.
• Flow rate 0.5 mL/min; gradient from 10% to 70% B over 10 min; injection volume 2 µL.
• Column temperature scanned at 30, 40, 50 and 60 °C to determine non‑denaturing conditions.

Mass spectrometry (summary):

• Instrument: Agilent InfinityLab Pro iQ Plus mass detector with JetStream ESI in negative polarity.
• Scan range m/z 700–3000 (profile); deconvolution used to recover neutral molecular weights in the ~7–18 kDa range.
• Data analysis performed with Agilent OpenLab CDS spectral deconvolution.

UV‑Vis thermal melt (Tm) measurement (summary):

• Instrument: Agilent Cary 3500 Multicell UV‑Vis; 260 nm monitoring, 10 mm pathlength quartz cuvettes, 800 µL volume.
• Temperature ramp: 25 → 90 °C at 5 °C/min; data interval 0.5 °C; experiments run in triplicate; mineral oil overlay used to limit evaporation.
• Tm derived from first derivative of absorbance vs. temperature.

Instrumentation Used

• Agilent 1290 Infinity II Bio LC System (pump, multisampler, multicolumn thermostat, variable wavelength detector).
• InfinityLab Pro iQ Plus mass detector with Agilent JetStream ESI source.
• Agilent Cary 3500 Multicell UV‑Vis Spectrophotometer with temperature probe and quartz semimicro cuvettes.
• Software: Agilent OpenLab CDS and Agilent Cary UV Workstation software for acquisition and analysis.

Key Results and Discussion

• Chromatographic behavior and duplex stability on column: At 30–40 °C the siRNA duplex remained intact under the tested IP‑RP conditions and AS, SS and duplex species were baseline resolved. Onset of on‑column melting occurred between 40 and 50 °C; at 60 °C the duplex was largely denatured to single strands. Increasing column temperature shortened retention times due to improved mass transfer and lower mobile‑phase viscosity.
• Strand-specific retention: The sense strand (SS) eluted later than the antisense (AS) because SS carried a tri‑antennary GalNAc conjugate that increased hydrophobicity and retention in reversed‑phase conditions.
• Mass confirmation: Deconvoluted MS results matched theoretical molecular weights for single strands with differences <2 Da (typical laboratory acceptance for 7–8 kDa oligos is ~1–5 Da depending on platform and algorithm). The intact duplex spectrum was congested because of overlapping charge states and adducts; advanced deconvolution algorithms resolved the components successfully.
• Thermal stability (UV‑Vis Tm): The measured Tm in PBS was 82.5 °C (triplicate identical), whereas Tm in the LC elution buffer (TEA/HFIP + 25% methanol) averaged 46.8 °C. The large Tm decrease shows that organic solvents and volatile ion‑pairing conditions used for LC substantially destabilize the duplex relative to physiological buffer.
• Cross‑validation: LC observations of duplex denaturation at elevated column temperatures agreed qualitatively with the UV‑Vis Tm results, supporting the orthogonality of the combined approach.

Benefits and Practical Applications

• The combined IP‑RP LC/MS plus UV‑Vis Tm workflow provides orthogonal confirmation of sequence identity (by mass), strand composition (by chromatographic separation), and functional stability (by Tm). This is valuable for method development, formulation screening, comparability studies, and routine QC of siRNA therapeutics.
• Determination of non‑denaturing chromatographic conditions (30–40 °C here) prevents on‑column duplex disruption and enables accurate purity assessment.
• UV‑Vis Tm offers a rapid, low‑resource way to compare stability across buffer and formulation conditions and to prioritize stabilizing modifications or excipients.

Future Trends and Applications

• Advances in native/soft‑ionization MS and improved deconvolution algorithms will increase confidence in intact duplex mass analysis and reduce spectral complexity from adducts and overlapping charge states.
• Automation and higher‑throughput UV‑Vis multicell platforms will accelerate formulation screens for stability and excipient selection.
• Standardized LC and MS acceptance criteria for oligonucleotide mass confirmation may emerge as regulatory frameworks converge, improving inter‑laboratory comparability.
• Integration of thermal profiling with biophysical readouts (e.g., calorimetry, fluorescence‑based melting) and computational prediction will enhance rational siRNA design and delivery optimization.

Conclusion

An orthogonal analytical strategy combining IP‑RP LC/MS and UV‑Vis thermal melt analysis yields a robust platform for siRNA characterization. Chromatography under controlled, non‑denaturing temperatures enables separation of AS, SS and duplex species for purity assessment; high‑resolution MS provides mass confirmation; and UV‑Vis melting quantifies thermal stability in relevant buffers. Together these methods support development, formulation, comparability and routine quality control of therapeutic siRNA products.

Reference

1. Gilar M., Redstone S., Gomes A. Impact of mobile and stationary phases on siRNA duplex stability in liquid chromatography. Journal of Chromatography A. 2024, 1733, 465285.
2. Huang T. Y., Liu J., Liang X. R., Hodges B. D. M., McLuckey S. A. Collision‑Induced Dissociation of Intact Duplex and Single‑Stranded siRNA Anions. Analytical Chemistry. 2008, 80(22), 8501–8508.
3. Berezhnoy A., Brenneman R., Bajgelman M., Seales D., Gilboa E. Thermal stability of siRNA modulates aptamer‑conjugated siRNA inhibition. Molecular Therapy — Nucleic Acids. 2012, 1, e51.
4. Agilent Technologies. Interview: Evaluating an Innovative Analytical ID Testing Strategy for Oligonucleotides. Agilent Technologies case study, publication number 5994‑5144EN, 2022.