Canine Cushing's Syndrome: Prognostic Factors and New Treatment Options

Abstract

Hypercortisolism, often referred to as Cushing’s syndrome, is one of the most common endocrine disorders in dogs. It is caused by an ACTH-secreting pituitary adenoma in ~80-85% of cases, and by a cortisol-secreting adrenocortical tumor (adenoma or carcinoma) in ~15-20% of cases. To expand the currently available range of treatment options, new medical treatment options with high selectivity and tolerability are desired. To know which steroidogenic enzymes can be targeted for selective inhibition of cortisol production, first we had to know which steroidogenic enzymes are specific for cortisol production. The first part of this thesis therefore focuses on the zonal expression of steroidogenic enzymes in the adrenal cortex. We found that, contrary to previously assumed, the canine adrenal cortex expresses only one CYP11B enzyme, and that CYP17A1 is the only enzyme that is required for cortisol, but not for aldosterone production. CYP17A1 would therefore be an interesting target for specific inhibition of cortisol production, which we further explored in the second part of the thesis. The second part of this thesis focuses on new compounds as potential future treatment options. To test the potential of these compounds without using laboratory animals, we analyzed their efficacy in canine primary adrenocortical cell culture. We tested three groups of compounds: (1) abiraterone acetate, (2) melanocortin 2 receptor (MC2R) antagonists, and (3) Steroidogenic factor-1 (SF-1) inverse agonists. (1) Abiraterone acetate is an inhibitor of the aforementioned CYP17A1 enzyme, and therefore might selectively inhibit cortisol production. By measuring the concentration of multiple steroidogenic hormones, we determined that abiraterone acetate indeed inhibited cortisol but not aldosterone production. (2) MC2R antagonists are antagonists of the receptor for ACTH, which would therefore be an interesting treatment option for pituitary-dependent hypercortisolism. One compound, #299, was a potent inhibitor of ACTH-stimulated cortisol production in vitro. (3) SF-1 is an important regulator of both steroidogenesis and adrenal development, so inhibitors of SF-1 could potentially inhibit hormone production, but also tumor growth in case of an adrenocortical tumor. One compound, #31, significantly inhibited cortisol production and SF-1 target gene expression in vitro. These results were all very promising, and show much potential for these compound groups as future treatment options. Future research is required to determine their effects in vivo. The third part of this thesis focuses on prognostic factors of canine cortisol-secreting adrenocortical tumors. Prognostic factors could help to select dogs with high risk of recurrence that could benefit from adjuvant therapy after adrenalectomy. We therefore developed a histopathological scoring system: the Utrecht score, that was able to divide the dogs in groups with significantly different outcomes. Moreover, we identified three genes that were significantly associated with a poor prognosis, including SF-1. This emphasizes the importance of the SF-1 inverse agonists that we tested in the second part of the thesis, since this might improve the survival times of dogs after adrenalectomy. Due to the many similarities in Cushing’s syndrome between humans and dogs, the findings described in this thesis are not only useful for dogs, but possibly also for humans.