Bupivacaine Metabolite Can Interfere with Norfentanyl Measurement by LC-MS/MS

Area of Science:

• Analytical Chemistry
• Clinical Chemistry
• Pharmacology

Background:

• Liquid chromatography coupled to tandem mass spectrometry (LC-MS/MS) is the preferred method for quantifying fentanyl and its metabolite, norfentanyl (NF), in urine due to high specificity.
• Norfentanyl (NF) is the primary urinary metabolite of fentanyl and is usually found at higher concentrations, making its accurate detection crucial for drug monitoring.
• The accuracy of LC-MS/MS assays for NF relies heavily on the precise identification and quantification of this metabolite.

Purpose of the Study:

• To investigate the interference of N-desbutyl bupivacaine (NDB), a metabolite of bupivacaine used in epidural analgesia, with the LC-MS/MS assay for urinary norfentanyl (NF).
• To identify analytical strategies to differentiate NDB from NF in urine specimens analyzed by LC-MS/MS.

Main Methods:

• Urine specimens from laboring women receiving epidural analgesia with bupivacaine and fentanyl were analyzed using LC-MS/MS.
• The interference of N-desbutyl bupivacaine (NDB) with the quantification of norfentanyl (NF) was assessed by examining shared precursor and product ions.
• A unique product ion specific to NF was identified to distinguish it from NDB.

Main Results:

• N-desbutyl bupivacaine (NDB) was found to interfere with the LC-MS/MS assay for norfentanyl (NF).
• NDB shares the same precursor ion (233 m/z) and key product ions (84 m/z, 150 m/z) as NF, hindering accurate quantification.
• A unique NF product ion (177 m/z) was identified, enabling differentiation between NDB and NF.

Conclusions:

• Laboratories must recognize the interference of N-desbutyl bupivacaine (NDB) in LC-MS/MS assays for norfentanyl (NF).
• Accurate identification of NF, facilitated by the 177 m/z product ion, is critical to prevent misidentification and ensure appropriate patient care.
• Failure to account for NDB interference can lead to significant misinterpretations of fentanyl metabolite levels.