A specific liquid chromatography-tandem mass spectrometry (LC-ESI-MS/MS) assay was developed and validated for simultaneous determination of two active metabolites of isosorbide dinitrate (ISDN), namely, isosorbide 2-mononitrate (IS 2-MN) and isosorbide 5-mononitrate (IS 5-MN). A simple protein precipitation extraction technique was employed using 13C6 isosorbide 5-mononitrate as the internal standard. The two isomers were separated on a chiral column and mass detection was carried out by electrospray ionization (ESI) in negative multiple reaction monitoring (MRM) mode (ESI -ve). As neutral organic nitrates do not ionize well in ESI ion source, adduct formation of IS 2-MN and IS 5-MN were evaluated. Acetate adduct ions of IS 2-MN and IS 5-MN were well ionized and fragmentable in the negative mode by liquid chromatography electrospray ionization/tandem mass spectrometry. These acetates adduct ions, of IS 2-MN and IS 5-MN were selected as parent mass for quantitation. The method was developed and validated in rat and human plasma with K2EDTA as an anticoagulant. This simultaneous quantitation method was shown to be linear over a working range of 25.0 ng/mL to 5050 ng/mL and 12.4 ng/mL to 2500 ng/mL for IS 2-MN (r2 > 0.99) and IS 5-MN (r2 > 0.99), respectively, in rat and human plasma. Sensitivity was determined as 25.0 ng/mL for IS 2-MN and 12.4 ng/mL for IS 5-MN in rat and human plasma. Inter- and intra-day accuracy and precision were within ±15% in both method validations. This validated method was subsequently applied to a pharmacokinetic (PK) study of ISDN in rat after oral administration.
Keywords: Human plasma; Isosorbide 2-mononitrite; Isosorbide 5-mononitrite; Isosorbide dinitrite; LC–MS/MS; Rat plasma.
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