Development of a biomarker assay for 3‐indoxyl sulfate in mouse plasma and brain by liquid chromatography/tandem mass spectrometry

This paper describes the development and partial validation of a fast, sensitive and specific ultra‐performance liquid chromatography/tandem mass spectrometric method for the determination of 3‐indoxyl sulfate (3‐IS), an endogenous compound in mammals, in mouse plasma and brain samples. The analytical method involves direct dilution of samples with water and protein precipitation with acetonitrile containing an internal standard, followed by separation of 3‐IS on a MonoChrom C₁₈ column and detected by selected reaction monitoring (SRM) in negative ionization mode using turbo ion‐spray ionization. Due to high endogenous levels of 3‐IS in control mouse plasma and brain, blank guinea pig plasma and brain were used for the preparation of standard curves and quality controls (QCs). The compound of interest was well separated from interference peaks from the matrices with a total runtime of 2.7 min under a gradient condition. The method was partially validated. The linear concentration range was 0.1 to 100 µg/mL in mouse plasma and 10 to 10,000 ng/g in mouse brain. Inter‐assay mean bias and relative standard deviation (RSD) for plasma were in the range of ?4.8% to 3.1% and 2.5% to 3.2%, respectively. Intra‐assay mean bias and RSD for plasma were in the range of ?3.3% to 1.4% and 1.9% to 2.8%, respectively. Inter‐assay mean bias and RSD for brain were in the range of ?1.8% to 3.5% and 1.7% to 8.1%, respectively. Intra‐assay mean bias and RSD for brain were in the range of ?1.7% to 3.9% and 4.1% to 7.3%, respectively. The lower limit of quantitation (LLOQ) for this assay was 0.1 µg/mL for plasma and 10 ng/g for brain. The matrix effect was not observed in both guinea pig plasma and mouse plasma. Copyright © 2009 John Wiley & Sons, Ltd.