A sensitive bioanalytical assay for methylcobalamin, an endogenous and light-labile substance, in human plasma by liquid chromatography with tandem mass spectrometry and its application to a pharmacokinetic study

Methylcobalamin (mecobalamin) is a vitamin B12 coenzyme and is effective for the treatment of peripheral neuropathies. As mecobalamin is an endogenous substance in human plasma at low levels and is light-labile, a sensitive assay method has been developed by using liquid chromatography with tandem mass spectrometry with particular care taken for light exposure. Stability tests performed under light showed that mecobalamin was stable in plasma in amber tubes only when exposed to <10 lx light, and thus, all the assays used for the method validation and the in-study sample assays should be performed under red light with minimal light exposure. Mecobalamin and its stable isotope, which was used as an internal standard, were extracted from 0.1 mL plasma by protein precipitation with methanol and were quantifiable in a range from 0.05 to 20 ng/mL. The reproducibility assessment showed acceptable accuracy and precision (≤15 %). Mecobalamin was stable in plasma at room temperature for 21 h when exposed to <10 lx light in amber tubes and for 205 days when stored frozen. The validated method was successfully applied to an in-study sample assay by performing a clinical pharmacokinetic study of mecobalamin, in which a successful incurred sample reanalysis was performed.