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This study aimed to evaluate the pharmacokinetics (PK) of tranexamic acid (TXA) in horses and estimate its irrelevant plasma and urine concentrations using the pharmacokinetic/pharmacodynamic (PK/PD) approach by applying the Pierre‐Louis Toutain model. TXA was intravenously administered to eight thoroughbred mares, and plasma and urine TXA concentrations were quantified by liquid chromatography/tandem mass spectrometry. The quantified data were used to calculate the PK parameters of TXA in horses. The plasma elimination curves were best‐fitted to a three‐compartment model. Using the Toutain model approach, irrelevant plasma and urine TXA concentrations were estimated to be 0.0206 and 0.997 μg/mL, respectively. The typical values of clearance, steady‐state volume of distribution, and steady‐state urine‐to‐plasma ratio were 0.080 L/kg/h, 0.86 L/kg, and 49.0, respectively. The obtained irrelevant concentrations will be useful for establishing relevant regulatory screening limits for effective control of TXA use in horse racing and equestrian sports. [ABSTRACT FROM AUTHOR]

Vadadustat is a newly launched hypoxia-inducible factor stabilizer with anti-anemia and erythropoietic effects; however, its use in horses is expressly forbidden in both racing and equestrian competitions. Following our previous report on the pharmacokinetic study of vadadustat in horse plasma and urine, a long-term longitudinal analysis of vadadustat in horse hair after nasoesophageal administration (3 g/day for 3 days) to three thoroughbred mares is described in this study. Our main objective is to further extend the detection period of vadadustat for the purpose of doping control. Three bunches of mane hair from each horse were collected at 0 (pre), 1, 2, 3 and 6 month(s) post-administration. These hair samples were each cut into 2-cm segments and pulverized after decontamination of hair samples. The analyte in the powdered hair samples was extracted with liquid–liquid extraction followed by further purification by solid-phase extraction with strong anion exchange columns. The amount of vadadustat incorporated into the hair was quantified with a newly developed and validated method using liquid chromatography–high-resolution mass spectrometry. Our results show that vadadustat was confirmed in all post-administration hair samples, but its metabolites were not present. Thus, the detection window for vadadustat could be successfully extended up to 6 months post-administration. Interestingly, the 2-cm segmental analysis revealed that the tip of the drug band in the hair shifted along with the hair shafts in correspondence with the average hair growth rate (∼2.5 cm/month) but gradually diffused more widely from 2 cm at 1 month post-administration to up to 14 cm at 6 months post-administration. However, the loss in the total amount of vadadustat in hair over time was observed to most likely be due to the degradation of vadadustat. These findings will be useful for the control of abuse and/or misuse of vadadustat and the interpretation of positive doping cases. [ABSTRACT FROM AUTHOR]

IOX4 is a hypoxia‐inducible factor prolyl hydroxylase (HIF‐PHD) inhibitor, which was developed for the treatment of anemia by exerting hematopoietic effects. The administration of HIF‐PHD inhibitors such as IOX4 to horses is strictly prohibited by the International Federation of Horseracing Authorities and the Fédération Équestre Internationale. To the best of our knowledge, this is the first comprehensive metabolic study of IOX4 in horse plasma and urine after a nasoesophageal administration of IOX4 (500 mg/day, 3 days). A total of four metabolites (three mono‐hydroxylated IOX4 and one IOX4 glucuronide) were detected from the in vitro study using homogenized horse liver. As for the in vivo study, post‐administration plasma and urine samples were comprehensively analyzed with liquid chromatography/electrospray ionization high‐resolution mass spectrometry to identify potential metabolites and determine their corresponding detection times. A total of 10 metabolites (including IOX4 glucuronide, IOX4 glucoside, O‐desbutyl IOX4, O‐desbutyl IOX4 glucuronide, four mono‐hydroxylated IOX4, N‐oxidized IOX4, and N‐oxidized IOX4 glucoside) were found in urine and three metabolites (glucuronide, glucoside, and O‐desbutyl) in plasma. Thus, the respective quantification methods for the detection of free and conjugated IOX4 metabolites in urine and plasma with a biphase enzymatic hydrolysis were developed and applied to post‐administration samples for the establishment of elimination profiles of IOX4. The detection times of total IOX4 in urine and plasma could be successfully prolonged to at least 312 h. [ABSTRACT FROM AUTHOR]