Simple and selective high-performance liquid chromatographic method for estimating plasma quinidine levels

Area of Science:

• Analytical Chemistry
• Pharmacokinetics
• Clinical Chemistry

Background:

• Accurate quantification of quinidine and its metabolites is crucial for therapeutic drug monitoring and understanding patient response.
• Existing analytical methods may lack the speed, sensitivity, or precision required for routine clinical application.
• Development of a robust assay is needed to assess plasma levels of quinidine, dihydroquinidine, and 3-hydroxyquinidine.

Purpose of the Study:

• To develop and validate a rapid, precise, and accurate reversed-phase, high-performance liquid chromatographic (HPLC) method for quantifying quinidine, dihydroquinidine, and 3-hydroxyquinidine in human plasma.
• To establish a simplified method for preparing plasma standards using isolated 3-hydroxyquinidine.
• To evaluate the method's performance characteristics, including precision, accuracy, and detection limits.

Main Methods:

• Reversed-phase HPLC with fluorescence detection.
• Sample preparation involved protein precipitation with acetonitrile followed by direct injection.
• Mobile phase: 1.5 mM aqueous phosphoric acid and acetonitrile (90:10) at a flow rate of 2 ml/min.
• 3-hydroxyquinidine isolation from urine via thin-layer chromatography for standard preparation.

Main Results:

• The HPLC method achieved rapid quantification of quinidine, dihydroquinidine, and 3-hydroxyquinidine in plasma.
• Intra-assay coefficients of variation were <1% for quinidine and 3-hydroxyquinidine (2.5–20 µmol/l).
• Inter-assay coefficients of variation were 3.5% for quinidine (10 µmol/l) and 4.0% for 3-hydroxyquinidine (5 µmol/l).
• Detection limits were 50 µmol/l for quinidine and 25 µmol/l for 3-hydroxyquinidine.
• Excellent correlation (r² = 0.96) was observed with a gas-liquid chromatographic assay for quinidine.
• Poor correlation (r² = 0.78) was noted when compared to a published protein precipitation-fluorescence assay.

Conclusions:

• The developed HPLC method provides a rapid, precise, and accurate means for quantifying quinidine and its major metabolites in plasma.
• This assay is suitable for clinical applications, including therapeutic drug monitoring.
• The method's performance surpasses that of some existing assays, offering improved reliability.