Determination of 1,25-dihydroxyvitamin D2 in rat serum using liquid chromatography with tandem mass spectrometry

Vitamin D therapy is widely used for the treatment of hyperparathyroidism associated with chronic renal failure in renal disease patients. The vitamin D prodrug, 1α-hydroxyvitamin D2 (1α(OH)D2), is used for the treatment of the end stage renal disease patients who as a result of impaired kidney function cannot convert the naturally occurring vitamin D to the active hormonal form namely 1,25-dihydroxyvitamin D2 (1,25(OH)2D2). The systemic circulating levels of this active form are in the pg/mL range and represent a significant bioanalytical challenge for therapeutic monitoring. Liquid chromatography with tandem mass spectrometry (LC-MS/MS) is considered the gold standard for the selective and sensitive determination of small molecule therapeutics in biological matrices. However, the reported LC-MS/MS bioanalytical assays for 1,25(OH)2D2 suffer from extensive sample preparation procedures or derivatization protocols to achieve the requisite sensitivity and selectivity. In this paper, we describe an assay that employs 96-well plate solid phase extraction sample preparation combined with highly sensitive LC-MS/MS instrumentation. The utility of ultra high pressure liquid chromatography to reduce the analytical run time was also demonstrated. Employing this assay a lower limit of quantitation of 25.0 pg/mL using 300 μL sample aliquot of rat serum was achieved with linearity obtained over the range of 25.0-1000 pg/mL. Both intra-day and inter-day coefficients of variation were <15% and accuracy across the assay range was within 100 ± 7.24%. The application of the assay was demonstrated for the analysis of 1,25(OH)2D2 rat serum samples to support pharmacokinetic studies conducted at doses down to sub-microgram per kilogram of 1α(OH)D2.