Urinary cortisol metabolites are reduced in MDR1 mutant dogs in a pilot targeted GC‐MS urinary steroid hormone metabolome analysis

P‐glycoprotein (P‐gp) is the gene product of the multidrug resistance gene (MDR1, syn. ABCB1) that normally restricts the transfer of cortisol across the blood–brain barrier. In the absence of P‐gp, cortisol access to the hypothalamus is increased and, by feedback inhibition, this finally leads to lower endogenous plasma cortisol levels in dogs with homozygous nt230(del4) MDR1 mutation (MDR1−/− mutant dogs). While a previous study only focused on plasma cortisol levels, the present study used urinary steroid hormone metabolites to analyze cortisol metabolism in MDR1−/− mutant dogs. Morning void urine was collected from 23 MDR1−/− mutant and 16 MDR1+/+ normal dogs and was subjected to targeted GC‐MS steroid hormone metabolome analysis. Seven cortisol metabolites, cortisol itself, and 13 other steroid metabolites were detected. In general, all cortisol metabolites were lower in the urine of the MDR1−/− mutant dogs, with allo‐tetrahydro‐cortisol and β‐cortol reaching the level of significance. In addition, 11‐keto‐pregnanetriol levels were significantly lower in the urine of the MDR1−/− mutant dogs, indicating that also the 17alpha‐OH‐progesterone‐derived metabolism was altered. In conclusion, the present study provides the first steroid hormone metabolome analysis in the urine of MDR1−/− mutant dogs. Significant differences in the steroid metabolome of MDR1−/− mutant dogs point to a significant role of P‐gp for cortisol metabolism and excretion and so indirectly also for hypothalamic–pituitary–adrenal axis regulation in dogs.