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Because norcantharidin (NCTD) is unstable and subject to ring opening and hydrolysis, the diacid metabolite of norcantharidin (DM-NCTD) is the stable form of NCTD found in normal saline solution. Conversion of NCTD to DM-NCTD is almost 100 %, making it possible to determine and investigate the pharmacokinetics of DM-NCTD converted from NCTD. In this paper, a sensitive, simple and selective liquid chromatographic-tandem mass spectrometric method was developed and validated for determination of DM-NCTD in beagle plasma. DM-NCTD was detected in multiple-reaction monitoring (MRM) mode by using the dehydrated ion 169.3 as precursor ion and its product ion 123.1 as the detected ion. Ribavirin was used as internal standard and detected in MRM mode by use of precursor ions, resulting in a product ion transition of m/z 267.1 → 135.1. This method was successfully used for a pharmacokinetic study of DM-NCTD in beagles after intravenous administration of DM-NCTD in normal saline solution at doses of 0.39, 0.78, and 1.6 mg kg. DM-NCTD had dose-dependent kinetics across the dosage range investigated, with enhanced T and AUC and apparently decreasing V and CL with increasing dosage. After single-dose administration, T ranged from 0.20 to 0.55 h, AUC from 1.81 to 43.6 μg mL h, V from 228 to 55.9 mL kg, and CL from 220 to 36.5 mL kg h ( P < 0.01). The results indicated nonlinear pharmacokinetic behavior of DM-NCTD in beagles, suggesting that the risk of DM-NCTD in normal saline solution intoxication may be non-proportionally increased at higher doses.[Figure not available: see fulltext.] [ABSTRACT FROM AUTHOR]