Risedronate is a commonly prescribed bisphosphonate for the treatment of bone disorders. Due to its high polarity and low oral bioavailability, low concentrations of risedronate are expected in human plasma and therefore a sensitive assay is required to serve in pharmacokinetic studies. Here, we describe the development and validation of an LC-MS/MS assay for the measurement of risedronate concentrations in human plasma. Risedronate and the internal standard, risedronate-d4, were derivatized on an anion exchange solid-phase extraction cartridge. Trimethylsilyl-diazomethane which is a thermally stable and relatively non-toxic derivatization agent was used to methylate the risedronate phosphonic acid groups and decrease analyte polarity. Following extraction, the analytes were separated on a Phenomenex Gemini C18 column (150 mm×2.0 mm, 5 μm), using a gradient of ammonium acetate 10 mM and acetonitrile with a flow rate of 300 μL/min. The assay calibration range was 0.2-25 ng/mL. The calibration curve of risedronate standards spiked in six individual plasma samples was linear (r²=0.9998). Accuracy (percent deviation from nominal) and precision (percent coefficient of variation) at concentrations 0.5, 5 and 20 ng/mL, and at the lower limit of quantification (LLOQ) of 0.2 ng/mL were excellent at <6%. Mean recovery was 54% for risedronate and 51% for the internal standard. Risedronate was stable in human plasma samples for at least 5 h at room temperature, 101 days frozen at -80°C, 72 h in an autosampler at 10°C, and for three freeze/thaw cycles. The validated assay method successfully quantified the concentrations of risedronate in plasma samples from informed consenting healthy volunteers administered a single 35 mg risedronate tablet.
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