Average Degree of Substitution of Betadex Sulfobutyl Ether Sodium Using the Agilent 7100 Capillary Electrophoresis System

Importance of the Topic

The degree of substitution of Betadex sulfobutyl ether sodium (SBE-b-CD Na) determines its inclusion complex formation, solubility and interaction with drug molecules. Accurate characterization of individual isomers enhances formulation stability, bioavailability and safety in pharmaceutical applications.

Objectives and Study Overview

This study introduces a capillary electrophoresis method on the Agilent 7100 CE system to separate and quantify positional and regional sulfobutyl ether substituents on β-cyclodextrin. The goal is to resolve ten discrete isomeric peaks and determine the average degree of substitution, in compliance with USP criteria (6.2–6.9).

Methodology and Instrumentation

• Instrumentation: Agilent 7100 Capillary Electrophoresis, bare fused silica capillary (50 µm id × 56 cm effective length).
• Running electrolyte: 30 mM benzoic acid in CE-grade water, adjusted to pH 8.0 with 100 mM Tris buffer.
• Detection: Indirect UV at 360 nm (BW 20 nm), reference 205 nm (BW 10 nm).
• Operating conditions: Linear voltage ramp to 30 kV over 10 min, then 30 kV for 20 min; capillary temperature 25 °C; injection by pressure (34 mbar, 10 s).
• Sample preparation: 10 mg/mL SBE-b-CD Na in CE water, filtered through 0.45 µm syringe filter.

Results and Discussion

The optimized method resolved ten distinct SBE-b-CD Na peaks with relative migration times ranging from 0.58 to 1.30, closely matching USP reference values. Normalized peak areas for each isomer fell within USP limits. The calculated resolution between peaks IX and X was 1.51, exceeding the minimum requirement of 0.9. The average degree of substitution was 6.44 for the reference standard and 6.28 for the sample, both within the USP range of 6.2–6.9.

Benefits and Practical Applications

This CE method offers rapid, high-resolution separation of SBE-b-CD Na isomers, enabling precise quantification of substitution patterns. It supports QA/QC workflows in drug development and manufacturing, providing an efficient alternative to NMR for routine analysis.

Future Trends and Opportunities

Advances may include coupling CE with mass spectrometry for structural confirmation, deployment of microfluidic CE platforms for higher throughput, and expanding the approach to other functionalized cyclodextrin derivatives. Integration with automated sample handling can further enhance laboratory productivity.

Conclusion

The Agilent 7100 CE method successfully resolves and quantifies individual sulfobutyl ether β-cyclodextrin isomers, delivering accurate average substitution values in line with USP standards. Its robustness and precision make it well suited for pharmaceutical quality control.

References

1. Betadex Sulfobutyl Ether Sodium USP NF 33 Official Monograph, Betadex 6546-6550.