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TRILOSTANE UPDATE: FROM CONFUSION TO CLARITY
Rhett Nichols, DVM, ACVIM
INTRODUCTION
Since first approved for use in dogs by the FDA in 2009, trilostane (Vetoryl® - Dechra Pharmaceuticals)
has become a popular drug for the medical management of both pituitary-dependent
hyperadrenocorticism (PDH) and hyperadrenocorticism secondary to functional adrenal cortical tumors
(AT). Trilostane is a synthetic nonhormonal analog that acts to inhibit the adrenocortical enzyme 3-betahydroxysterid dehydrogenase, as well as 11-beta hydrolase. By blocking these enzymes, trilostane acts to
actively interfere with the adrenal’s metabolic pathways and decreases the synthesis of adrenal end
products, including both cortisol and aldosterone. The goal of this talk is to familiarize veterinarians with
dosing and monitoring strategies for trilostane in dogs with PDH, adrenocortical tumors, atypical
hyperadrenocorticism (AHAC), and diabetes mellitus with concurrent hyperadrenocorticism (HAC).
DIAGNOSING HYPERADRENOCORTICISM
There are three essential stages to establish the diagnosis of hyperadrenocorticism
1. A clinical index of suspicion
2. Supportive findings on standard laboratory testing (CBC, serum biochemical profile, UA)
3. A confirmatory test for HAC (e.g., low dose dexamethasone suppression test)
BEFORE STARTING TRILOSTANE
Identify and address concurrent disorders
Some dogs with HAC have concurrent problems, such as urinary tract infection (UTI), hypertension,
overt proteinuria or diabetes mellitus. It is important to identify these problems and begin appropriate
management of these disorders either before or at the same time that treatment with trilostane is started.
Differentiate between PDH and AT
Although trilostane (Vetoryl®) is licensed for dogs with both types of spontaneous HAC, it is important
to identify the small group of dogs with AT. Probably less than 15% of dogs with HAC have this form of
the disease, but they may require surgical intervention rather than long-term treatment with trilostane. The
easiest way to confirm an AT is with abdominal ultrasonography, although it can require some skill to be
a reliable diagnostic test. In classic cases, the affected adrenal gland has a large mass arising from one
pole and the contralateral gland is substantially atrophied. If an AT is found, it should be carefully
evaluated for evidence of invasion of the adjacent structures (kidneys and blood vessels) and the lymph
nodes and liver should be checked for signs of metastatic disease.
If ultrasonography is equivocal or unavailable, measurement of endogenous ACTH can be very helpful.
This hormone is labile, so careful handling is required. A high-dose dexamethasone suppression test
(HDDST) may also be performed; suppression of cortisol production rules out an AT. However, a
substantial proportion of dogs that fail to suppress on a HDDST have PDH, so this result may not be
conclusive.
If an adrenal tumor is identified, a careful assessment of operability is necessary. About half of these
tumors are malignant, and early metastasis may be evident. Removal of the gland is the treatment of
choice; successful surgery is curative with respect to the HAC and may be life-saving in dogs with
malignant lesions. However, if the AT is not operable, has already metastasized, or if the dog has other
serious concurrent problems, trilostane therapy should be started. This will not stop neoplastic growth,
vascular invasion or metastasis, but effectively controls the clinical signs of HAC and improves quality of
life for both pet and owner.
UNDERSTANDING TRILOSTANE
Trilostane has a very different mechanism of action than mitotane (Lysodren®), as it is a competitive
enzyme inhibitor which works within the adrenal cortex. Although the target enzymes are present in all
layers of the cortex, trilostane appears to be preferentially taken up by the zona fasciculata, so it primarily
impacts cortisol production. As trilostane is a competitive enzyme inhibitor, it does not directly damage
the adrenal cortex and its effect will diminish as the drug is metabolized.
The pharmacokinetics of trilostane is not fully understood, but we do know that intestinal absorption can
be variable, and appears to be enhanced by the presence of food. In most dogs, peak levels occur within a
few hours of oral administration and the drug is cleared by hepatic metabolism after about 18 hours or
sooner.
Trilostane therapy should not be administered to dogs with anemia or those intended for breeding. It
should be considered carefully in dogs with significant renal or hepatic disease. This does not include
dogs with the expected steroid hepatopathy (manifested by increased ALP activity and hepatomegaly). As
trilostane may inhibit maximal secretion of aldosterone, it should not be used concurrently with
spironolactone as this may lead to hyperkalemia. Electrolytes should be evaluated periodically in dogs
taking an angiotensin converting enzyme inhibitor (e.g., enalapril, benazepril). However, we have used
this combination many times with no clinical problems.
THE STARTING DOSE: CURRENT AND HISTORICAL PERSPECTIVES
The Vetoryl® package insert suggests an initial starting dose of 2.2-6.7 mg/kg. When this drug was first
introduced in Europe over a decade ago, the original starting dose was 4-10 mg/kg/day. However, as
experience with the drug grew, it became apparent that these doses were too high in many dogs and lower
doses were needed.
Our recommended starting dose is either 2 mg/kg given once daily or 1 mg/kg given twice daily. We
would never start a dog on a dose at the higher end of the recommended dosage range (4-7 mg/kg),
although some dogs with HAC may require daily doses this high or even higher.
Whether to start with once-daily or twice-daily trilostane is controversial. While most dogs are controlled
clinically with once-daily dosing, trilostane may begin to lose its effectiveness 8-10 hours after
administration, so twice-daily trilostane may be necessary in this subset of dogs. In addition, it is very
possible that the efficacy of twice-daily trilostane might be more effective than once-daily dosing in
controlling complications associated with HAC (e.g., hypertension or proteinuria). However, the answer
to that question remains unclear, and additional studies are needed to resolve this issue.
Once-daily or twice-daily trilostane: and the winner is? Until it is proven whether SID or BID
treatment is better, we prefer to start with a twice-daily regimen, if feasible from a compliance and cost
perspective and the owner agrees. Why? Controlling cortisol concentrations as much as possible
throughout the day seems to make the most sense. For example, in diabetic dogs with concurrent HAC,
twice-daily administration of trilostane is essential in avoiding large fluctuations in serum cortisol
concentrations during the day. With once-daily trilostane administration, adequate diabetic control is
often difficult at best in dogs with concurrent HAC.
MONITORING AND GOALS OF THERAPY
The combination of the owner’s evaluation of the dog’s clinical response and the results of the ACTH
stimulation test are the keys to monitoring dogs on trilostane therapy. There are 3 published therapeutic
ranges in the veterinary literature. While the resting cortisol levels are similar, the post-ACTH cortisols
are 5.5, 7, and 9.1 mcg/dl. We recommend maintaining a post-ACTH cortisol between 2-7 mcg/dl when
tested 4-5 hours after the morning dose. If the dog looks great and the client does not report any clinical
signs, do not increase the dose even if the post ACTH cortisol is above this range. If the pre- and post ACTH cortisol concentrations are between 2 and 7 mg/dl and the dog is on once-daily trilostane but the
owner reports persistent polyuria and polydipsia or has other signs of moderate severity associated with
HAC, divide the present dose in half and administer it twice daily and recheck the ACTH response test in
two weeks.
If the post-ACTH cortisol is < 2 mcg/dl we recommend stopping the trilostane and repeating the ACTH
response test in 1- to 2 weeks in those dogs. When using trilostane, it has become increasingly clear that
low cortisol levels may indicate early or mild adrenal necrosis, Some of these dogs will require that the
drug be started at a lower dosage, others will maintain low to low normal cortisol concentrations for
prolonged periods of time, and some dogs will never need any further trilostane treatment to control the
signs of HAC.
EFFICACY WITH ADRENAL CORTICAL TUMORS
Trilostane appears to be an effective treatment for ADH (adrenal-dependent hyperadrenocorticism).
Evidence for efficacy is provided by one small series and a couple of case reports, a current series of 9
cases, and a recent review of 37 cases of ADH, seen at 4 UK centers over the last 12 years. This latter
study revealed the median survival time of 13 dogs treated with mitotane was 102 days (33-982 days)
whereas the median survival time for 22 dogs treated with trilostane was 353 days (range 4-1341 days); 2
dogs were treated with both drugs. There was no statistical difference in the survival time of the 13 dogs
treated with mitotane compared to the 22 dogs treated with trilostane. Metastatic disease was detected in
8 of 37 dogs. There was a significantly lower survival time in the dogs with metastatic disease compared
to dogs without metastatic disease. It is not known if the dose, frequency of dosing, or monitoring of
Vetoryl® treatment in ADH cases should be the same as in PDH cases or not. However, clinical success
can be achieved using doses as low as 1 mg/kg given twice daily in dogs with ADH.
EFFICACY WITH ATYPICAL HYPERADRENOCORTICISM
Atypical hyperadrenocorticism (AHAC) refers to a controversial, poorly understood syndrome where
dogs have clinical signs and laboratory findings consistent with HAC yet the recommended screening
tests (e.g., LDDS and ACTH response test) are normal. It is hypothesized that sex steroids are produced
in excess by the adrenal cortex and either cause or serve as markers for this disorder. Like traditional or
“typical” HAC, the underlying pathology is adrenal hyperplasia or AT. If AHAC is suspected, it is
suggested to perform an ACTH response test and submit serum to the University of Tennessee Veterinary
Medical Center Diagnostic Laboratory for an adrenal panel which includes cortisol, estradiol,
progesterone, 17-hydroxyprogesterone, androstenedione, and aldosterone. An abnormal profile (e.g.,
elevated sex steroids post-ACTH) is considered consistent with AHAC and a normal profile would tend
to rule out the disorder. The therapeutic use of melatonin and lignins, mitotane, and trilostane have all
been associated with clinical remission, However, it has been suggested that treatment with trilostane is
unlikely to cause clinical remission because some of the elevated sex steroids (e.g., 17hydroxyprogesterone) fail to decrease following therapy. Interestingly, in the first peer-reviewed article
that described this syndrome, all the dogs treated trilostane went into complete clinical remission despite
the fact that 17-hydroxyprogesterone levels remained elevated following therapy. Clearly, the etiology of
AHAC and why trilostane is an effective treatment for this disorder remains unclear.
EFFECT ON ADRENALS
In general, trilostane seems to be well tolerated by most dogs. If the numbers of dogs in the 7 published
clinical studies are combined, 44 out of the 291 dogs (15%) developed adverse signs. The most serious
side effect of trilostane is acute adrenal necrosis. This has been documented in two case reports, one fatal
and the other requiring permanent glucocorticoid therapy. Necrosis of the adrenal cortex cannot be
explained by competitive inhibition of steroidogenesis or be dismissed as an isolated idiosyncratic
reaction. Varying degrees of adrenal necrosis have been described in 5 of 7 non-randomly selected post
mortem examinations of dogs treated with trilostane. The severity of the lesions may have been related to
the dose of trilostane used and the duration of treatment.
Adrenal necrosis could be due to the hypersecretion of ACTH as it’s been established that trilostane
causes an increase in ACTH levels. This leads to the increased size of the adrenal glands that is observed
in many dogs that are treated with trilostane. Moreover, and paradoxically, short periods of ACTH
administration can also result in degeneration focal necrosis, and hemorrhage of human adrenal glands.
Adrenal necrosis does not explain most of the cases of adrenal insufficiency seen in trilostane treated
dogs, since most recover rapidly after cessation of the drug and continue to require the drug to control the
clinical signs. This suggests that these cases suffered from over-dosage rather than adrenal necrosis. Most
of the affected cases of adrenal necrosis have the typical electrolyte changes (hyponatremia,
hyperkalemia) consistent with hypoadrenocorticism. However, one case has been described which
developed isolated hypocortisolism.
Comment and clinical impact: Adrenal necrosis can be documented by performing an ACTH
stimulation test 48-72 hours after stopping the administration of trilostane; little or no cortisol
response to ACTH would be consistent with adrenal necrosis. Dogs with adrenal necrosis may not
need further treatment with trilostane.
SUMMARY
Trilostane has simplified the treatment of dogs with HAC. Presently Vetoryl®, which contains the active
ingredient trilostane, is the only licensed (FDA approved) treatment for both AT and PDH in dogs. The
safety and efficacy of this drug are well established, and there is abundant information about dosing and
monitoring strategies that allows the successful management and control of this potentially debilitating
disorder,
WHAT’S NEW AND AROUND THE CORNER?
A new trilostane monitoring strategy has been proposed. Below is the summary of an abstract presented at
the 2015 ACVIM Forum in Indianapolis, IN.
A Novel cortisol based method for monitoring trilostane therapy in dogs with HAC
Laura Cosgrove and Ian Ramsey, University of Glasgow, (Abstract summary) ACVIM 2015
The purpose of the study was to evaluate and compare a new cortisol based method to 1 hour postACTH cortisol measurements in the monitoring of dogs receiving trilostane
Why conduct such a study?
1. The cost of Synacthen (the cosyntropin product available in most European countries) has increased
10-fold and occasionally availability has been limited
2. Often the results of the post-ACTH cortisol measurements do not correlate with the clinical picture
Study design and selection of animals
Dogs with HAC being treated with trilostane were recruited from first opinion and referral practices.
-
Serum cortisol was measured at 3 time points: pre- trilostane (pre-pill), 3 hours post- trilostane
(post-pill), and 1 hour post-ACTH (post-ACTH)
Clinical and endocrine control was assessed by the following criteria:
o 1. Clinical control was assessed using responses of pet owners to questionnaires (9
questions related to clinical control such as panting, polyuria, polydipsia, polyphagia,
o
urinary accidents, etc). The questionnaires were scored and clinical control was divided
into 4 categories - poor, moderate, good, and over-controlled
2. Endocrine control was assessed by a newly devised novel algorithm that combined the
pre- and post-pill cortisol measurements (pre-post). Dogs were then placed into the same
four control categories as listed above based on the results of the post-ACTH or pre-post
method. Calculations were made to assess the level of agreement between the clinical
control which was based on the results of the owner questionnaires and the categorization
of endocrine control according to the post-ACTH or pre-post cortisol results
Results: A total of 116 tests (pre-post and post ACTH) were analyzed. Clinical control was correctly
categorized using the post- ACTH cortisol in 31% of the tests, compared to the pre-post method where
clinical control was correctly categorized in 50% of the tests.
Summary: The novel algorithm of pre-post cortisols better reflected clinical control of HAC (based on
the owner’s questionnaire scores) than the post-ACTH cortisol. The pre-post trilostane cortisol method
should be further investigated as a monitoring tool for dogs receiving trilostane.
What’s next?
Clinical researchers at the University of Utrecht and Zurich, after reviewing the results of the Glasgow
study, will start conducting their own studies to further investigate the novel cortisol based method for
monitoring trilostane therapy in dogs with hyperadrenocorticism.
References available on request