Update on the use of trilostane in dogs

Abstract

Many articles published in the past few years have contributed to a better understanding of the use of trilostane in dogs. Trilostane is a competitive inhibitor of 3β-hydroxysteroid dehydrogenase, the enzyme essential for synthesis of cortisol and all other steroids. Trilostane is reported to be safe and effective in the treatment of pituitary-dependent hyperadrenocorticism (HAC), adrenal-dependent HAC, and alopecia X. While trilostane controls most of the clinical signs associated with HAC, abnormalities such as hypertension, hypercoagulability, and proteinuria may persist despite therapy. Because the duration of cortisol suppression after a dose of trilostane is often less than 12 hours, many dogs with HAC could benefit from low dose trilostane treatment every 12 hours. Many controversies regarding trilostane still exist. This review provides a comprehensive commentary on trilostane’s indications, mode of action, dose, monitoring, efficacy, and adverse effects.

Introduction

Once a diagnosis of hyperadrenocorticism (HAC) has been made, treatment decisions are influenced by many factors including etiology of HAC, client financial constraints, the level of commitment involved in treatment, and a consideration of the risk/benefit ratio of the treatment compared with the consequences associated with the disease.

Three main categories of options are available for the treatment of pituitary-dependent and adrenal-dependent HAC (PDH and ADH) in dogs: medical, surgical, and radiation therapy. The optimal medical treatment should have minimal adverse effects while alleviating clinical signs associated with HAC, including polyuria-polydipsia (PUPD), polyphagia, haircoat and skin changes, truncal obesity, muscle weakness, and alopecia (1). Ideally, it should also improve deleterious physiological consequences of HAC including hypertension, thromboembolism, and proteinuria.

Trilostane is a synthetic steroid that selectively inhibits the enzyme 3β-hydroxysteroid dehydrogenase (3β-HSD) in the adrenal cortex. This inhibition blocks the conversion of pregnenolone to progesterone (2), thereby inhibiting the production of glucocorticoids and, to a lesser extent, mineralocorticoids and sex hormones.

Trilostane has been used in dogs with HAC for almost 20 y, and a veterinary approved product was first marketed in Canada in 2009. Control of clinical signs is gradual and variable among studies. Reports of good control ranged from < 50% to 100% of treated dogs after few weeks of treatment (3–13). However, after several months of treatment, partial to complete control of clinical signs occurred in > 75% of cases in published studies (3–13). Adverse effects are generally mild or moderate and are reported to occur in 0% to 40% of cases (3–13).

A recent article reported that 26 dogs with untreated PDH had shorter mean survival times (506 d) than 17 dogs that were treated with trilostane at 1 to 3 mg/kg body weight (BW) once (q24h) or twice a day (q12h), whose median survival time was not reached at the end of the study (14). This study suggests that withholding treatment for dogs with PDH could increase the risk of death. Therefore, it suggests a positive effect of trilostane in dogs with PDH. However, the cause of death in many of the untreated dogs was either unknown or did not seem to be related to their HAC. In addition, the retrospective nature of that study makes it difficult to accurately compare untreated dogs with dogs treated with trilostane.

Similar efficacy, similar or even longer survival times, and similar or lower rates of adverse effects have been reported with trilostane compared to mitotane in both PDH and ADH (13,15–18). Longer survival was found when trilostane was compared to mitotane in a non-selective adrenolytic protocol (18). However, the veterinary literature is lacking large prospective, randomized, controlled studies comparing trilostane and mitotane therapy to determine which is the superior treatment for dogs with HAC.

Many research articles detailing the use of trilostane in dogs with HAC have been published. This article will summarize the pharmacology of trilostane as well as the latest research on trilostane treatment in canine HAC.

Pharmacology

Trilostane is a competitive inhibitor of 3β-HSD. This blockade is reversible and appears to be dose-related (19). 3β-HSD converts pregnenolone to progesterone, and dehydroepian-drosterone (DHEA) to androstendione. The enzymatic action of 3β-HSD is thus essential for the synthesis of cortisol and all other steroids including mineralocorticoids, sex steroids, and other glucocorticoids. Inhibition of progesterone production reduces the synthesis of cortisol in the zona fasciculata of the adrenal glands. It also decreases the production of aldosterone in the zona glomerulosa and production of androstendione in the zona reticularis. In contradiction with a previous study (20), a later study did not find an effect of trilostane on DHEA concentration. This suggests that there might be more than one form of the enzyme 3β-HSD present in dogs, with a variable effect of trilostane on each isoenzyme (21). However, the first study was in vivo while the second study was in vitro (21). This could account for the different results between the 2 studies.

Trilostane may also inhibit the enzymes 11β-hydroxylase and 11β-hydroxysteroid dehydrogenase (11β-HSD) in dogs (20,22). 11β-HSD catalyzes the conversion of physiologically active cortisol to inactive cortisone.

Pharmacokinetic studies in healthy dogs showed that maximal plasma concentration of trilostane occurs 1.7 to 3.8 h following administration and plasma concentration returns to baseline by about 12 h [Vetoryl (trilostane) 2015 drug insert, Dechra Veterinary Products, Pointe-Claire, Quebec]. Studies in dogs with HAC have shown that trilostane’s activity can even be considerably shorter than 12 h in some dogs (23,24).

Dose

Trilostane is now supplied in 5-, 10-, 30-, 60-, and 120-mg capsules (Vetoryl). Currently, the manufacturer’s initial dose recommendation for trilostane is 2.2 to 6.6 mg/kg BW, PO, q24h based on body weight and capsule size. Previous label instructions for trilostane recommended initial dosing by body weight categories (< 5 kg, 30 mg; 5 to 20 kg; 60 mg; and > 20 kg; 120 mg; q24h). One study compared this dosing approach with dosing at 2 to 5 mg/kg BW, q24h. The study found that there was comparable clinical improvement and decrease in cortisol in both dosing groups, but a lower risk of side effects was associated with mg/kg BW dosing (25).

The use of compounded capsules, if necessary based on the patient size, should be considered carefully. If compounded doses are necessary, only the licensed trilostane product should be used, and compounding should be performed by trained pharmacists. A study evaluating trilostane products purchased from 8 compounding pharmacies found that actual concentrations in compounded capsules varied from 39% to 152.6% of the label claim. Additionally, dissolution of the compounded trilostane was lower in 20% of tested products (26). The potential variation in strength and dissolution of compounded trilostane could have negative impacts on management of patients with HAC. With the advent of licensed 5 mg and 10 mg capsules, the use of compounded capsules is rarely necessary.

Studies in dogs with HAC treated with trilostane reported a wide variation in the dose necessary to control the disease, from 0.42 to 50 mg/kg BW per day (3–13,19,23) (Tables 1 and 2). However, these studies had very different study designs. While most studies were prospective clinical trials, a few were retrospective studies (6,7,13) which carry more risk of selection bias, misclassification, and information bias than prospective studies. The number of dogs included varied from 9 to 78 with differences in dog breeds and body weight (3–13,19). The number of dogs can affect statistical results with higher risk of type 2 errors in smaller populations (3,8,9,11,13). Dogs > 25 to 30 kg, and possibly also dogs > 15 kg, have been found to require a lower dose/kg BW of trilostane to control clinical signs compared with dogs weighing ≤ 15 kg (5,10,27). Studies which only included dogs < 15 kg (3,9) or which segregated the smallest dogs (< 10 kg) to 1 group (7) had higher risk of bias. In addition, the diagnosis of HAC in dogs with supportive clinical and biochemical changes varied from a single confirmatory test (4–8,13) to at least 2 confirmatory tests (3,9–12). These tests consisted of 1 to 2 urine cortisol to creatinine ratio(s) (UCCR), an adrenocorticotropic hormone (ACTH) stimulation test, and/or a low dose dexamethasone suppression test. Studies which used fewer tests to confirm the diagnosis of HAC were more likely to include dogs that were falsely diagnosed with HAC.

Table 1.

Summary of clinical studies using once daily dosing of trilostane in dogs with hyperadrenocorticism.

Reference numbers	3	4	5	6	7	8	9
Country	Switzerland	UK	Australia	Netherlands	UK	Spain	South Korea
Year of publication	2002	2002	2003	2008	2012	2013	2013
Number of dogs with PDH	11	78	30	63	47	16	7
Number of dogs with ADH	0	0	0	0	12	0	0
Starting dose, range in mg/kg BW, q24h	3.9 to 9.2	1.8 to 20.0	2.7 to 12.0	2.0 to 4.0	3.3 to 9.5	1.0 to 6.6	6 to 13.3
Starting dose, mean in mg/kg BW, q24h	NS	5.9	NS	NS	6.4	2.9	NS
Monitoring	1, 3 to 4, 6 to 7, 12 to 16, 24 to 28 wk	10 d, 1, 3, 6 mo, and then every 3 to 6 mo	10 d, 1, 3, then every 3 mo thereafter	3 wk then every 3 wk until stable, then every 3 mo	9 to 12 d, 1, 3, 6 mo	7d, 1, 3, 6 mo, 1 y	2 wk, 1, 2, 3, 4, 6 mo
Target cortisol in nmol/L	27 to 69	20 to 250	25–75 for tight control and 75–125 for acceptable control	30–190	40 to 120	55 to 248	55 to 150
Time of sampling	2 to 6 h	NS, most within a few hours	NS	2–4 h	2 to 6 h	8 to 12 h	NS
ACTH stimulation test protocol	0.25 mg of tetracosactide IM, post-ACTH cortisol 60 min later	NS	5 μg/kg of tetracosactrin IV, post-ACTH cortisol 60 min later	0.25 mg of tetracosactide IV, post-ACTH cortisol 90 min later	NS	0.25 mg of tetracosactide IM, post-ACTH cortisol 60 min later	0.25 mg of tetracosactide IV, post-ACTH cortisol 60 min later
Dose changes	Increased in 5/11 (45%), decreased in 3/11 (27%)	Increased in 24/78 (31%), decreased in 9/78 (11%)	NS	Increased in 22/63 and decreased in 4/63 at 3 wk	NS	In 12/16 dogs (75%). 9 increased, 2 decreased, 1 increased and decreased	Dose increased 12 times from 4th to 16th wk in total
Withdrawals	None	Discontinuation of trilostane in 2 dogs for prolonged hypoAC	None, but 2 dogs had trilostane discontinued for reasons unrelated to the medication	None	29/59 (49%). No withdrawals due to severe side effects of trilostane	2/16 (13%). Changed to q12h treatment	None
Final dose, range in mg/kg BW, q24h	4.1 to 15.6	NS	5.0 to 50.0	0.8–5.8	3.2 to 12	1.0–11.5	3.3 to 10.9
Final dose, mean in mg/kg BW, q24h	NS	7.3	19	2.8	7.6	4.6	7.1
Number of dogs with clinical improvement	9/11 (82%) at 6 mo	60/78 (77%) at 1 mo. Improvement alopecia in 24/39 dogs (62%) at 3 mo	Clinical improvement in all dogs at 3 mo	60/63 (95%)	Resolution of many clinical signs in all dogs at 3 mo. Improvement of the skin only at 6 mo	5/13 complete (38%) and 3/13 (23%) partial response at 1 mo, 5/12 (42%) complete or 5/12 (42%) partial response at 6 mo	2/7 (29%) at 1 mo, 5/7 (71%) 3 mo, 7/7 (100%) 4 and 6 mo
Number of dogs in target cortisol range	NS	59/73 (81%) at 1 mo	9/30 (30%) at 1 mo, 17/30 (57%) at 3 mo, 23/29 (79%) at 6 mo	60/63 (95%)	NS	5/13 (38%) at 1 mo, 9/12 (75%) at 6 mo	5/7 (71%) at 4 wk, 7/7 (100%) at 2 mo, 5/7 (71%) between 2 and 6 mo, 7/7 (100%) at 6 mo
Number of dogs with adverse effects	2/11 (18%), mild	15/78 (19.2%). 2 dogs with hypoAC (1 died). 13 dogs with minor adverse effects	No adverse effects for > 6 mo. 4/30 (13.3%) dogs had signs of hypoAC after > 12 mo of treatment	5/63 (7.9%). HypoAC including 2 dogs with permanent hypoAC	1/59 (1.7%), mild	6/16 (37.5%), mild	2/7 (29%), transient hypoAC
Survival	All alive at the end of the study	26 dogs dead at the end of the study, with 9 of unknown cause. 1 dog due to hypoAC. 2 deaths shortly after starting treatment	5 dogs dead or euthanized for causes unrelated to PDH	All alive at the end of the study	All alive at the end of the study	All alive at the end of the study	All alive at the end of the study
Other comments					Comparison q24h versus q12h treatment. Only dogs > 10 kg in q12h group. 8 centres. q24h and q12h groups studied over 2 separate time periods	Comparison q24h versus q12h treatment	Comparison q24h versus q12h treatment

BW — body weight; NS — not stated; PDH — pituitary-dependent hyperadrenocorticism; ADH — adrenal-dependent hyperadrenocorticism; HypoAC — hypoadrenocorticism; ACTH — adrenocorticotropic hormone.

Table 2.

Summary of clinical studies using multiple-daily dosing of trilostane in dogs with hyperadrenocorticism.

Reference numbers	10	11	12	7	8	9	13
Country	Spain	USA	USA	UK	Spain	South Korea	Spain
Year of publication	2006	2008	2011	2012	2013	2013	2014
Number of dogs with PDH	44	18	38	25	16	9	0
Number of dogs with ADH	0	4	9	5	0	0	12
Starting dose, range in mg/kg BW, q12h	1.2 to 7.5	0.5 to 2.5	0.21 to 1.1	1.5 to 2.9	1.25 to 2.75	0.5 to 1	NS
Starting dose, mean in mg/kg BW, q12h	3.1	1.4	0.86	2.2	1.8	0.78	3
Monitoring	7 d, 1, 3, 6 mo, then every 6 mo (up to 3.5 y)	1 to 2 wk, 1 to 2, 2 to 4 mo	1 to 2, 6 to 7, 15 to 17, 24 to 28 wk	9 to 12 d, 1, 3, 6 mo	7 d, 1, 3, 6 mo, 1 y	2 wk, 1, 2, 3, 4, 6 mo	7 d, 1, 3, 6 mo, then every 3 mo
Target cortisol in nmol/L	28 to 138 at 4 to 6 h and 27 to 248 at 8 to 12 h	< 150	40 to 150	40 to 120	55 to 138 at 4 to 6 h and 55 to 248 at 8 to 12 h	55 to 150	< 248
Time of sampling	4 to 6 h at 7 d, then 8 to 12 h	3 to 4 h	3 h	2 to 6 h	4 to 6 h at 7 d, then 8 to 12 h	NS	8 to 12 h
ACTH stimulation test protocol	5 μg/kg of tetracosactide IV, post-ACTH cortisol 60 min later	0.25 mg of cosyntropin IM, post-ACTH cortisol 60 min later	0.25 mg of cosyntropin IM, post-ACTH cortisol 60 min later	NS	0.25 mg of tetracosactide IM, post-ACTH cortisol 60 min later	0.25 mg of tetracosactide IV, post-ACTH cortisol 60 min later	0.25 mg of tetracosactide IM, post-ACTH time NS
Dose changes	Increased in 19/44 (43%), both increased and decreased in 10/44 (23%), and decreased in 5/44 (11%)	10 increased at 4 to 8 wk, 5 increased at 8 to 16 wk	18 increased at 1 to 2 wk, 17 increased at 6 to 7 wk, 1 increased + 2 decreased at 15 to 17 wk, 1 progressively increased over 4 mo	NS	In 11/16 dogs (69%). 5 increased, 6 increased and decreased	Dose increased twice and decreased 5 times in total	4 increased at 1 mo, 3 increased at 3, 6, and 12 mo
Withdrawals	5 for economic reasons. Discontinuation of trilostane in 5 dogs for prolonged hypoAC	4 dogs had surgery for ADH	2 no response, 2 dogs with signs of hypoAC, 9 dogs had surgery for ADH	4/30 (13%). No withdrawn due to severe side effects of trilostane	None	None	None
Final dose, range in mg/kg BW, q12h	1.3 to 9.05 at 6 mo	1.1 to 2.8	0.21 q12h to 10 q8h	1.4 to 4.4	1.4 to 4.45	0.9 to 1.9	NS
Final dose, mean in mg/kg BW, q12h	3.75 at 6 mo	1.7	NS	2.7	2.35	1.43	NS
Number of dogs receiving q8h treatment	0	3	7	0	0	0	0
Number of dogs with clinical efficacy	22/36 at 3 mo (61%), 20/30 at 6 mo (67%), 19/24 at 1 y (79%)	15/22 (68%) improved at 4 to 8 wk, 16/18 (89%) at 8 to 16 wk	9/9 ADH (100%) and 15/38 PDH (39%) good at 6 to 7 wk, 24/28 (86%) PDH good at 6 mo	Resolution of many clinical signs in all dogs at 9 to 12 d. Improvement skin only at 3 mo	8/11 complete (73%) and 2/11 (18%) partial response at 1 mo, 11/16 (69%) complete or 3/16 (19%) partial response at 6 mo	2/9 (22%) at 1 mo, 4/9 (44%) 3 mo, 8/9 (89%) 4 mo, 9/9 (100%) 6 mo	8/12 (67%) at 1 mo, then 9/12 (75%) at 3, 6, and 12 mo
Number of dogs in target cortisol range	25/36 (69%) at 3 mo	14/16 (88%) at 8 to 16 wk	7/9 (78%) and 15/38 (39%) at 6 to 7 wk	NS	7/11 (64%) at 1 mo, 9/16 (56%) at 6 mo	0/9 (0%) at 4 wk, 7/9 (78%) at 4 mo, 9/9 (100%) at 6 mo	8/12 (67%) at 1 mo, then 9/12 (75%) at 3, 6 and 12 mo
Number of dogs with adverse effects	11/44 (25%), mild in 3, mild to moderate in 4, severe in 4	2/22 (9%), severe	5/47 (10.6%), moderate in 4, severe in 1	5/30 (17%), mild to moderate	7/16 (44%), mild	None	4/12 (33%), mild in 3, severe in 1
Survival	930 d, 29 dogs still alive at the end of the study	All alive at the end of the study	1 dog euthanized for large pituitary mass	All alive at the end of the study	All alive at the end of the study	All alive at the end of the study	Mean survival of 17.7 +/− 4.2 mo (range: 3.3 to 55.0 mo; median, 14.0 mo)
Other comments				Comparison q24h versus q12h treatment. Only dogs > 10 kg in q12h group. 8 centers. q24h and q12h groups studied over 2 separate time periods	Comparison q24h versus q12h treatment	Comparison q24h versus q12h treatment	Comparison trilostane q12h versus mitotane

BW — body weight; NS — not stated; PDH — pituitary-dependent hyperadrenocorticism; ADH — adrenal-dependent hyperadrenocorticism; HypoAC — hypoadrenocorticism; ACTH — adrenocorticotropic hormone.

The most notable differences among studies were the time the ACTH stimulation test was performed after administration of trilostane and the definition of good disease control. Following administration of trilostane, the ACTH stimulation test was either performed variably within the day (4,5,9), 2 to 6 h later (3,7), 3 to 4 h later (11), 3 h later (12), 2 to 4 h later (6), 4 to 6 h later (8,10), or 8 to 12 h later (8,10,13). The upper limit of the acceptable post-ACTH cortisol concentration range varied from 69 nmol/L (3) to 250 nmol/L (4) a few hours after administration of trilostane. In some studies, the trilostane dose was mostly changed based on the results of the ACTH stimulation test (3,5,7). In other studies, the trilostane dose was left unchanged in dogs with high (and in some cases low) post-ACTH stimulation cortisol concentration as long as clinical signs were controlled (6,8,9,11,12).

Besides these differences in study design that could explain the wide differences in doses among the studies, some inter-individual variability could also be due to the low water solubility of trilostane, and thus variable absorption of the drug (15). In addition, it could be due to a variation in the 3β-HSD activity in the dogs’ adrenal glands, or variable conversion into active metabolites such as ketotrilostane (21). It is worth noting that to enhance absorption, trilostane should be given with food, including on the days recheck ACTH stimulation tests are being conducted.

In studies describing the use of trilostane q24h (Table 1) (3–9), the initial dose ranged from 1.0 to 20 mg/kg BW, q24h with a mean dose of 2.9 to 6.4 mg/kg BW, q24h. The final trilostane dose varied from 0.8 to 50 mg/kg BW, q24h with a mean dose of 2.8 to 19 mg/kg BW, q24h. However, as explained, the duration of cortisol suppression after a dose of trilostane is often less than 12 h. Therefore, several studies have examined the use and clinical efficacy of trilostane q12h for treatment of PDH and ADH (Table 2) (7–13).

In studies describing the use of trilostane q12h, the initial dose ranged from 0.5 to 7.5 mg/kg BW, q12h, with a mean dose of 0.78 to 3.1 mg/kg BW, q12h. The final trilostane dose varied from 0.21 to 9.05 mg/kg BW, q12h, with a mean dose of 1.43 to 3.75 mg/kg BW, q12h. Among the studies reported in Table 2, 10 out of 180 dogs (5.6%) required trilostane 3 times a day to successfully control their clinical signs (11,12).

In dogs treated with trilostane either q24h or q12h, frequent dose adjustments were necessary to control the disease (3–13). In these studies, clinical signs in dogs treated with trilostane q24h improved in > 75% of cases at 6 mo (3–9). However, the rates of improvement were variable and 1 study reported only 42% of the total cases (5/12 dogs) had complete resolution of clinical signs at 6 mo (8). Among studies reporting treatment with trilostane q12h, 67% to 100% of dogs were well-controlled at 6 mo (7–13). Between 2 to 4 mo, the cortisol was within the target range in 57% to 100% of the dogs treated with trilostane q24h and in 69% to 100% of the dogs treated with trilostane q12h.

Among the 11 studies listed in Tables 1 and 2, adverse effects were reported in 35/264 (13%) cases with q24h treatment and in 34/180 (19%) cases with q12h treatment. In both q12h and q24h treatment groups, most adverse effects were mild or moderate and included mild electrolyte abnormalities without clinical signs, transient decrease in appetite, vomiting, diarrhea, and lethargy.

To date, 3 studies have directly compared the efficacy of q24h versus q12h trilostane treatment in dogs with HAC. Augusto et al (7) concluded that dogs treated either q24h or q12h showed similar improvement in clinical scores. In this retrospective study, control of the clinical signs and cortisol concentrations was seen sooner in the q12h treatment group. The mean final dose was 7.6 mg/kg BW per day in the q24h group and 5.4 mg/kg BW per day in the q12h group (2.7 mg/kg BW, q12h). Reports of adverse effects, which were documented as mild to moderate in severity, were more common in the q12h versus the q24h treatment group. However, animals given trilostane q24h had a 15% increased chance of treatment discontinuation although no clear explanation to explain the difference was identified.

Cho et al (9) found fewer adverse effects with low dose (0.5 to 1 mg/kg BW) trilostane administered q12h compared with 30 mg/dog given q24h (6 to 12 mg/kg BW per day) in dogs < 5 kg BW. While 100% success in controlling clinical signs and cortisol concentrations was seen in both groups at 24 wk after initiation of trilostane treatment, improvement occurred more slowly in the q12h group. The mean final daily dose required for control of HAC was much lower in the q12h group compared with the q24h treatment group (2.9 mg/kg BW per day in the q12h group and 7.1 mg/kg BW per day in the q24h group). Additionally, results suggested fewer adverse effects in the q12h versus the q24h treatment group (9).

Arenas et al (8) compared dogs with PDH that were treated with trilostane either q12h or q24h. The mean final dose was 4.6 mg/kg BW per day in the q24h group and 4.7 mg/kg BW per day in the q12h group (2.4 mg/kg BW). This study found that more dogs in the q12h group had complete resolution of clinical signs than in the q24h group (69% had complete clinical recovery at 6 mo in the q12h group and 42% in the q24h group). While clinical response was more prevalent in the q12h treatment group, there was no significant difference in the mean post-ACTH cortisol concentration between groups. Both protocols were safe and generally well-tolerated.

Based on these studies, it appears that use of trilostane q12h, often given at total daily doses that are lower than the manufacturer’s recommendations, results in a safe and effective treatment for dogs with HAC. When using a twice-daily treatment protocol, it is recommended to start at doses of 0.5 to 1 mg/kg BW, q12h, with a maximum initial dose of 30 mg/dog for dogs > 30 kg. Indeed, complete control of clinical signs and cortisol concentrations can be achieved in some dogs using a very low dose of trilostane (12).

Monitoring

Choosing the best monitoring protocol in HAC dogs undergoing treatment can be challenging. The ACTH stimulation test is currently the gold standard test used to monitor the response to treatment of dogs with HAC. However, there are some limitations to this test which will be discussed later. It is recommended that the ACTH stimulation test be conducted at 10 to 14 d after initiating trilostane or changing doses, then at 1, 3, and subsequently every 3 to 6 mo after the trilostane dose becomes stable (15). It is generally accepted that clinical signs and cortisol concentrations continue to improve in some dogs in the first month after starting trilostane therapy (12). Therefore, the ACTH stimulation test performed after 10 to 14 d of onset of therapy should be used only to ensure that there is no excessive suppression of cortisol concentrations. Complete control of cortisol concentrations and clinical signs is not expected at 10 to 14 d after starting therapy, and the dose should not be increased at this time or hypoadrenocorticism could result (15).

Use of synthetic ACTH [Cortrosyn (cosyntropin) and Synacthen (tetracosactrin or tetracosactide)] for ACTH stimulation tests has been reported for diagnosing and monitoring HAC in dogs. However, the potency of both drugs has not been compared. Samples for cortisol measurement are obtained at 0 and 60 min after IV or IM injection of standard non-absorbed products. Both IV and IM administration of cosyntropin produce similar ACTH-stimulated cortisol concentrations in healthy dogs and in dogs with HAC (1,28). Doses of 1 μg/kg BW and 5 μg/kg BW of cosyntropin were also recently found to produce similar results when reassessing dogs with HAC treated with mitotane or trilostane. Therefore, the lower dose appears to be a safe and more cost-effective way to use synthetic ACTH for monitoring HAC in dogs undergoing treatment. However, the same study found a significant difference between the 2 doses in dogs suspected of having HAC. Therefore a dose of 5 μg/kg BW is still recommended when being used to diagnose HAC (29).

Use of depot tetracosactide (250 μg total dose or 5 μg/kg BW, IM) was evaluated in healthy dogs and in dogs with HAC (30,31). However, since peak cortisol is only obtained 180 min after IM injection, it is recommended to collect blood samples at 0 and 180 min for cortisol measurement (30,31).

Only 1 small study evaluated the use of compounded ACTH in 5 healthy dogs and found no difference in serum cortisol concentrations at 60 min for 4 compounded products (32). While compounded ACTH preparations may be less expensive, they are not recommended because their potency and the time that maximal concentration of cortisol occurs may vary significantly among formulations, even potentially from bottle to bottle at a specific compounding pharmacy (33).

There is currently no standardization regarding the time when an ACTH stimulation test should be performed, nor the optimal cortisol concentration target range. The manufacturer recommends that the ACTH stimulation test be performed 4 to 6 h after trilostane is administered. Studies report variable timing for performance of ACTH stimulation for monitoring of trilostane treatment. Reported times include 2 to 6 h (3,7), 3 to 4 h (11), 3 h (12), 2 to 4 h (6), 4 to 6 h (8,10), or 8 to 12 h (8,10,13) after administration of trilostane. Post-ACTH cortisol concentrations vary with the time interval between dosing and testing (11). One study demonstrated that a difference of only 2 h between the time the ACTH stimulation test was started significantly changed the post-ACTH serum cortisol concentrations (34). An ACTH stimulation test should therefore always be started at or about the same time after trilostane administration in each individual patient. The lowest cortisol concentrations usually occur 2 to 4 h after trilostane administration (35), so if the goal of the ACTH stimulation is to determine the cortisol concentration when trilostane is maximally effective, an ACTH stimulation test should be performed during this time interval.

The literature reports various optimal target cortisol concentrations for dogs undergoing treatment of HAC. Neiger et al (4) found that good clinical control of PDH was usually associated with post-ACTH cortisol concentrations of 20 to 250 nmol/L, regardless of the time of the test. Others have suggested target post-ACTH cortisol concentrations of < 70 to 75 nmol/L (3,5), between 40 and 120 nmol/L (15), or < 150 nmol/L (9,11,12,36), for control of HAC. The authors of the present article recommend targeting a post-ACTH cortisol concentration < 150 nmol/L. However, if good clinical control is achieved when post-ACTH cortisol concentrations are < 250 nmol/L, the dose can remain unchanged. It is worth mentioning that cortisol concentration can be affected by the assay methodology and can vary among laboratories (37). Therefore, the cut-offs reported in the literature may be difficult to apply directly to data from another laboratory. Ideally, laboratory specific cut-offs should be used.

There is a lack of clear recommendations of how to adjust the trilostane dose when dogs have well-regulated HAC, but low cortisol concentrations before and after ACTH stimulation. One recent study examined a small group of dogs with well-regulated HAC and cortisol concentrations < 55 nmol/L before and after ACTH stimulation performed 3 to 6 h after trilostane was administered. A second ACTH stimulation test revealed that the dogs had significantly increased cortisol concentrations 9 to 12 h after the dose of trilostane was given (38). The authors suggested that a second ACTH stimulation test performed later in the day could support continued treatment with the same trilostane dose, although further study is needed. In animals with low pre- and post-ACTH cortisol concentrations (< 40 to 55 nmol/L), the authors of the present article recommend that the trilostane dose be decreased or discontinued (especially if cortisol concentration is < 30 nmol/L). If trilostane is discontinued, it should be restarted at a lower dose only after an ACTH stimulation test shows that hypocortisolemia has resolved.

Despite the study findings previously discussed, it remains unclear whether the ACTH stimulation test is the best method for monitoring treatment with trilostane (23). In some dogs, the results of the ACTH stimulation test do not parallel the control of clinical signs in dogs treated for HAC in both q12h and q24h treatments (8,12).

Neiger et al (4) proposed that the administration of exogenous ACTH during stimulation testing may override the reversible inhibition of cortisol synthesis by trilostane, particularly as the concentration of the drug decreases, such that the results of the ACTH stimulation test may be inaccurate. Another possible explanation would be that the action of trilostane is too short in these dogs to fully control their clinical signs. The frequency of administration may need to be increased in these dogs. Lastly, undiagnosed concurrent diseases could mimic some of the clinical signs associated with uncontrolled HAC. For instance, a persistence of PUPD in dogs with early chronic kidney disease may be falsely interpreted as clinical signs of uncontrolled HAC. Measuring multiple cortisol concentrations over time, while expensive and time consuming, may be more accurate than a stimulation test.

Investigators have assessed other possible options for monitoring treatment response in dogs with HAC. These have included the UCCR (5,11,12,39), the baseline cortisol (40,41) with or without combination with an endogenous ACTH and the cortisol/ACTH ratio (6,42), the pre-trilostane cortisol concentration alone or in combination with the 3 h post-trilostane cortisol concentration (43). In many of these studies, results of these ancillary tests have been evaluated to determine whether they are predictive of the ACTH stimulation test results, and therefore if they could be less expensive monitoring tools. The UCCR, baseline cortisol, endogenous ACTH, and cortisol/ACTH ratio have not been found to be very reliable for this purpose. However, as described, the ACTH stimulation test has its own limitations making it challenging to assess utility of these other tests for the monitoring of HAC.

Macfarlane et al (43) evaluated the utility of measuring a pre-trilostane cortisol concentration (considered peak cortisol concentration) and a 3-hour post-trilostane cortisol concentration (considered trough cortisol concentration). In this study, the pre-trilostane cortisol concentration as well as the 3-hour post-trilostane cortisol concentration better reflected the level of clinical control in dogs with uncontrolled HAC than did the post-ACTH cortisol concentration performed 3 h after trilostane administration. Clinical control was determined by detailed questionnaires filled by the owners. However, 94/110 tests (85%) were performed on dogs receiving q24h trilostane treatment; the results may be different with q12h trilostane treatment.

Efficacy of trilostane

Clinical signs of HAC, such as PUPD, polyphagia, or lethargy, improve gradually over the first months of treatment in most dogs treated with trilostane q12h or q24h (Tables 1 and 2). Resolution of dermatological abnormalities can take several more months after starting trilostane (7).

Trilostane is effective for both PDH and ADH. In ADH cases, it can be used as a short-term treatment for the control of cortisol concentrations in preparation for adrenalectomy, or as a long-term medical therapy (8,11,12). Similar survival times but fewer side effects were reported with trilostane compared to mitotane in the treatment of HAC due to ADH (8).

The typical laboratory abnormalities associated with HAC, such as increase in liver enzymes, hypercholesterolemia, low urine specific gravity, secondary hyperparathyroidism and increased phosphorus concentration have all improved with trilostane treatment (3,7,9,44–46).

Along with the typical clinical signs associated with HAC, this disorder can also create severe physiological abnormalities. Sequelae of HAC can include hypertension, proteinuria, hypercoagulability, immunosuppression, insulin resistance, and soft tissue calcification including calcinosis cutis, muscle wasting, and cranial cruciate rupture (1). Over half of HAC patients have hypertension at the time of diagnosis, and hypertension does not resolve in many dogs treated with trilostane, even after a year of treatment (13,45,46). Persisting hypertension could be explained by the fact that after trilostane treatment is initiated, plasma renin activity increases (6), which may lead to activation of the renin-angiotensin-aldosterone system and renal vasoconstriction. Proteinuria, as determined by the urinary protein-to-creatinine ratio, improves over time with trilostane treatment, but some dogs remain proteinuric after a year of treatment (45,46). However, hypercoagulability, as measured by thromboelastographic variables, increased platelet count and fibrinogen concentration, did not improve following trilostane treatment in dogs with PDH over 6 mo (46). The persistence of these physiologic abnormalities associated with HAC may be due to the serum cortisol concentration still exceeding physiological concentrations during a certain period of the day, even when trilostane is administered twice daily (45).

Hyperadrenocorticism is associated with calcium dysregulation that can lead to the development of calcium-oxalate uroliths, soft tissue mineralization, such as calcification of the skin (calcinosis cutis), or perihilar bronchial mineralization. Dogs with HAC have higher parathyroid hormone and phosphate concentrations than healthy dogs, and these abnormalities improve in some dogs with HAC following treatment with trilostane (44). Some patients had resolution of calcinosis cutis with trilostane treatment (4,47).

Little information is available on the use of trilostane in dogs with concurrent HAC and diabetes mellitus. Excessive cortisol concentration causes insulin resistance, making it more difficult to regulate diabetes mellitus in dogs with concurrent uncontrolled HAC (1). Insulin resistance is expected to at least partially improve with trilostane treatment, and therefore, careful monitoring of blood glucose concentration is needed in dogs treated for both disorders. In a small study of 8 dogs with diabetes mellitus and HAC, insulin requirements and fructosamine concentrations were not consistently reduced during trilostane treatment for HAC (48). However, the number of dogs included in the study was small and results may have been different if a larger population had been used. Interestingly, and likely only coincidentally, 10% of 103 dogs treated with trilostane developed diabetes mellitus after trilostane was initiated in 1 retrospective study (40% of them within the first 4 mo after starting trilostane) (49).

Safety of trilostane

Trilostane has a relatively low incidence of adverse effects, especially when low doses are used. Adverse effects were reported in 35/264 (13%) dogs undergoing q24h treatment and in 34/180 (19%) dogs receiving q12h treatment in a total of 11 studies (3–13). Reported adverse effects include anorexia, lethargy, vomiting, diarrhea, and other events typical of a hypoadrenal state (hyperkalemia, hyponatremia, and hypovolemic shock) which may or may not be associated with adrenal necrosis and hemorrhage. Chronic trilostane use is also associated with the development of diffuse and/or nodular hyperplasia (50), enlargement (up to 60% of the original size) and change of echotexture of the adrenal glands on ultrasound (3). This may be secondary to the increased endogenous ACTH concentration that is noted with trilostane treatment (51).

Although more frequent with higher doses, adverse reactions to trilostane can occur with any dose and any frequency. These include adverse reactions in dogs treated with trilostane doses < 1 mg/kg BW, q12h (12), suggesting that factors other than dose or frequency of administration may contribute to the development of adverse effects. For instance, since trilostane is metabolized by the liver and excreted in bile and urine, its use is not recommended in dogs with liver or renal insufficiency as dose accumulation may occur. Other factors could include a particularly good absorption of the drug in some individuals, as well as individual or breed susceptibility to the effect of trilostane (21). Another possibility is that some dogs could have been wrongly diagnosed with HAC.

Although in theory the effects of trilostane as an enzyme inhibitor should be rapidly reversible, prolonged adrenocortical suppression can occur, and cases of permanent hypoadrenocorticism or adrenal necrosis following trilostane therapy have been described (5,6,10). In most of the described cases, prolonged iatrogenic hypoadrenocorticism occurred after several months of treatment (5,10). However, 2 dogs developed prolonged hypoadrenocorticism or isolated hypocortisolism following short courses (3 and 13 d) of trilostane given at 2 to 5 mg/kg BW, q24h (52,53). Based on a study performed in rats, adrenal degeneration in this species is likely caused by the effect of increased ACTH concentration on the adrenal glands rather than the direct effect of trilostane on the adrenal glands (54).

Periodic monitoring of electrolyte concentrations during trilostane treatment is also recommended since both hyperkalemia and hyponatremia are common (40). However, most dogs have mild electrolyte derangements and ACTH stimulation testing is typically not indicative of hypoadrenocorticism. The mechanism of mild hyperkalemia or other electrolyte changes in dogs treated with trilostane is unknown. Usually trilostane inhibits the production of glucocorticoids more than the production of mineralocorticoids. However, it is possible that in some dogs, trilostane may more effectively block the synthesis of mineralocorticoids than glucocorticoids (11,12,55). If electrolyte changes are observed with trilostane treatment, a careful client interview to help detect clinical signs of hypoadrenocorticism and an ACTH stimulation test are recommended. It is possible that a short episode of hypocortisolemia (although not enough to induce clinical signs) may not be detected by the ACTH stimulation test (35).

Use in alopecia X

Alopecia X is a form of canine adult-onset alopecia that mainly affects nordic breeds such as the Alaskan malamute, chow chow, keeshond, Pomeranian, Samoyed, and Siberian husky, and also other breeds including the miniature poodle. This form of alopecia resembles the alopecia seen with HAC but other clinical signs of HAC, such as PUPD and polyphagia, are absent and traditional tests to diagnose HAC (low dose dexamethasone suppression test and ACTH stimulation) are normal. Despite these findings, trilostane has been shown to be effective in the treatment of alopecia X. Currently, there are few data regarding the treatment of alopecia X. The doses used in 3 published studies were highly variable (56–58).

The first effective reported doses for treatment of alopecia X in Pomeranians and miniature poodles ranged from < 6 to > 23 mg/kg BW, q24h. Some dogs were treated with pulse therapy (2 to 3 treatments per wk) after initial stabilization (57). Three Alaskan malamutes were successfully treated in another study with doses of 3 to 3.6 mg/kg BW, q12h (58). The frequency of administration was reduced to twice weekly after initial stabilization. More recently, successful treatment has also been reported with a dose of 1 mg/kg BW, q12h in Pomeranians (56). Alopecia X has been postulated to be due to increased concentration of 17-hydroxyprogesterone (17-OHP). However, values of 17-OHP have been found to further increase during successful treatment with trilostane (57).

In conclusion, there have been many studies published in the past few years contributing to a better understanding of the use of trilostane in dogs. One of the most clinically relevant findings that has been recently reported is the use of significantly lower doses, often given twice daily, which have effectively controlled HAC in dogs with fewer side effects.

Further study of the use of trilostane in dogs with HAC is needed to answer some ongoing clinical questions. First, the results of an ACTH stimulation test, which is the current gold standard test for monitoring the effect of treatment with trilostane, do not always correlate with clinical signs, making it challenging to monitor dogs undergoing treatment. Second, there is no consensus on the optimal post-ACTH serum cortisol concentration, the ideal timing of sampling after trilostane administration, or the appropriate starting dose and frequency.

Overall, trilostane is an effective and safe method of treating PDH and ADH in many dogs, and is also reported to be effective in the treatment of alopecia X. However, it is important to note that trilostane might not correct physiological changes associated with HAC, including hypertension and proteinuria, and these conditions may require specific therapy. CVJ