Determination of mesalazine, a low bioavailability olsalazine metabolite in human plasma by UHPLC–MS/MS: Application to a pharmacokinetic study

• Validated, sensitive and specific method for mesalazine by UHPLC–MS/MS.
• Labeled internal standard to track the analyte variability during analysis.
• Reproducible good recovery with negligible matrix effect.
• Resolved derivatized by-product peaks with gradient chromatography.
• Rapid analysis turnaround time for pharmacokinetic studies.

Olsalazine sodium, salicylate derivative (prodrug) is effectively bioconverted to mesalazine (5-aminosalicylic acid; 5-ASA), which has an anti-inflammatory activity in ulcerative colitis. In this article, a novel highly sensitive and selective method was developed and validated to determine mesalazine in human plasma using a derivatization technique to enhance the signal intensity by using ultra- high performance liquid chromatography coupled to tandem mass spectrometry (UHPLC–MS/MS) with an electrospray ionization interface. The sample preparation consisted of a derivatization with propionyl anhydride followed by liquid liquid extraction (LLE) to remove the interference and minimize the matrix effect of human plasma. The multiple reaction monitoring (MRM) mode of the negative ion was performed and the transitions of m/z 208.1 → 107.0 and m/z 211.1 → 110.1 were used to measure the derivative of mesalazine and mesalazine-d3. The chromatographic separation was achieved using kinetex XB-C18 (100 × 4.6 mm 2.6 μ) analytical column with 0.1% formic acid in water and acetonitrile as mobile phase with a gradient elution. Nominal retention times of mesalazine and IS were 3.08 and 3.07 min, respectively. Absolute recovery was found to be between 82–95% for analyte and about 78% for IS. The standard curves was linear (r2 > 0.995) in the concentration range 0.10 to 12.0 ng/mL with lower limit of quantification (LLOQ) in human plasma was 0.10 ng/mL. The average intra-day/inter-day precision values (%CV) were in the range from 0.6–2.9 % and 1.3–3.8 %, respectively, while the average accuracy value was 103.8–107.2%. This method has been successfully applied to the human pharmacokinetics of olsalazine sodium 250 mg capsules following single oral administration.