LC-MS Analysis of Two mRNA Critical Quality Attributes: 5’-Capping Efficiency and Average Poly(A) Tail Length and Heterogeneity Using the Xevo™ MRT Mass Spectrometer

Significance of the Topic

Recent advances in mRNA therapeutics and vaccines have elevated the need for robust analytical methods to measure critical quality attributes (CQAs) that directly impact efficacy and safety. Two CQAs of high regulatory and process interest are 5-capping efficiency and the average length and heterogeneity of the 3 poly(A) tail. Accurate, high-throughput characterization of these attributes supports process development, batch release, and comparability studies in mRNA manufacturing.

Objectives and Study Overview

This application study describes an optimized single LC-MS assay and informatics workflow capable of simultaneously characterizing two mRNA CQAs: 5-capping efficiency and average poly(A) tail length/heterogeneity. The workflow was demonstrated on a firefly luciferase (Fluc) mRNA (~1,813 nt plus cap and poly(A) tail) and emphasizes reduced runtimes, improved mass accuracy/resolution, and streamlined data processing using dedicated software tools.

Methodology

Key aspects of the analytical approach:

• Sample: In vitro transcribed Fluc mRNA containing an intended Cap-1 structure and a polyadenylated tail; aliquots digested enzymatically prior to LC-MS.
• Enzymatic digestion: Two digestion strategies were used depending on the target CQA:
  • hRNase4 digestion (37C): produced 5-end oligonucleotides suitable for cap detection; digestion example: 90 min with hRNase4 for some samples.
  • RNase T1 digestion (37C, 15 min): generated longer poly(A)-containing fragments enabling poly(A) tail measurement; a denaturation step at 90C for 2 min preceded enzyme addition.
• Chromatography: Ion-pair reversed-phase UPLC using an ACQUITY Premier Oligonucleotide BEH C18 column (2.1 x 150 mm, 1.7 m) at 60C, 300 L/min. Mobile phases contained n-dipropylamine (DPA) and HFIP to enhance oligonucleotide retention and ESI response.
• Mass spectrometry: Negative-mode electrospray ionization with data-independent acquisition (MSE). High-resolution, high-accuracy analysis targeted m/z 50 4000 with acquisition at 2 Hz and optimized collision energy ramps for fragment information.
• Data processing: Automated workflows in the waters_connect platform using three apps: SYNTHETIC Library (to define expected digestion products and custom nucleotides), MAP Sequence (sequence mapping and cap identification), and INTACT Mass (deconvolution and mass-based identification of poly(A) variants).

Instrumentation Used

• Xevo MRT Multi-Reflecting Time-of-Flight (QTOF) Mass Spectrometer
• ACQUITY Premier UPLC System
• ACQUITY Premier Oligonucleotide BEH C18 Column
• waters_connect informatics platform with INTACT Mass App v1.9, SYNTHETIC Library App v2.0, MAP Sequence App v2.0

Main Results and Discussion

5-Capping efficiency (CQA1):

• hRNase4 digestion yielded two distinct 5-end Cap-1 containing 9-mer digestion products chromatographically separated and identified by MAP Sequence: m7GpppAm-GGG AAA U cP and m7GpppGm-GGG AAA U cP.
• The uncapped 7-mer product was not observed, consistent with enzymatic dephosphorylation of uncapped transcripts; overall capping efficiency was therefore measured as 100% for this material.
• Relative quantification based on extracted ion chromatogram peak areas showed a mixture of Cap-1 second-nucleotide variants: 62.7% m7GpppAm and 37.3% m7GpppGm. Detection of two distinct Cap-1 variants within a single preparation highlights the assays sensitivity to cap composition heterogeneity.

Poly(A) tail length and heterogeneity (CQA2):

• RNase T1 digestion produced a late-eluting, well-resolved poly(A) peak in IP-RP LC that coalesced multiple polyadenosine species.
• High charge-state ions (observed roughly from 24 to 36) were isotopically resolved using the Xevo MRT at ~100,000 resolution, enabling accurate mass determination (<10 ppm) even for high molecular weight oligonucleotides (~35 kDa).
• Deconvolution (MaxEnt1 algorithm via INTACT Mass) identified nine poly(A) species ranging from 98 to 106 nt. The most abundant component was a 102-mer (composition C(A)101) at 19.3% relative abundance.
• Weighted by relative abundance, the average poly(A) tail length was calculated as 102.3 nt with an average mass of 33,608.0 Da. The workflow demonstrated robust detection of poly(A) heterogeneity without prior enrichment steps.

Benefits and Practical Applications

The combined LC-MS/informatics workflow offers several practical advantages:

• Single assay capability: simultaneous measurement of capping efficiency and poly(A) tail characteristics reduces instrument time, sample consumption, and method complexity compared with two separate assays.
• High resolution and mass accuracy: isotopic resolution of highly charged oligonucleotides enables confident mass assignments and low-ppm mass errors for large oligos.
• Automated data processing: waters_connect apps reduce manual interpretation, accelerate result turnaround, and provide standardized outputs useful for quality control and regulatory documentation.
• Actionable metrics: direct measurement of cap composition heterogeneity and poly(A) length distribution supports process optimization, comparability assessments, and release criteria definition.

Future Trends and Opportunities

Anticipated developments and applications include:

• Integration into QC workflows: further validation and robustness testing could enable routine release testing for mRNA therapeutics as instrumentation and software mature.
• Extension to diverse constructs: adaptation for longer or chemically modified mRNAs, alternative cap chemistries, and different tail compositions.
• Higher throughput and automation: coupling sample preparation automation (e.g., bead-based enrichment when needed) with batch processing in informatics to scale for manufacturing labs.
• Complementary orthogonal assays: combining LC-MS data with sequencing-based poly(A) tail methods or enzymatic cap assays to provide comprehensive CQA profiles.

Conclusion

This application demonstrates a single, streamlined LC-MS method—enabled by a high-resolution Xevo MRT mass spectrometer and dedicated waters_connect informatics—that accurately measures two critical mRNA quality attributes: 5-capping efficiency (including cap composition heterogeneity) and average poly(A) tail length/heterogeneity. The approach provides high mass accuracy, resolves high-charge oligonucleotide isotopic envelopes, and uses automated software to yield reproducible, actionable CQA metrics suited for development and potential QC use.

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