Top Clinical Endocrinology Research Abstracts, 2014 ACVIM Forum: Adrenal Part 2

Below is the next installment of my review of the "top 12 list" of clinical endocrinology research abstracts presented at this year's American College of Veterinary Internal Medicine Forum. As with all of these ACVIM research abstract reviews, I've enlisted the help of Dr. Rhett Nichols, a well-known expert in endocrinology and internal medicine.

In this post, we will review another of these "top 12" abstracts in our adrenal gland selections.

Aldridge C, Behrend E, Kemppainen R, Lee-Fowler T, L. Martin L, Ward C. Comparison of Two Doses for ACTH Stimulation Testing in Dogs Suspected of or Treated for Hyperadrenocorticism. J Vet Intern Med 2014;28:1025.

     The ACTH stimulation test, using cosyntropin at 5 mcg/kg IV, is the preferred method for monitoring medical management of hyperadrenocorticism (HAC) and is a screening test for diagnosing HAC. Previous studies have shown maximal stimulation of the adrenal glands using 1 mcg/kg cosyntropin in normal dogs. No studies have evaluated the efficacy of the lower dose in dogs suspected of or being treated for HAC. Our objective was to compare 1 mcg/kg to 5 mcg/kg cosyntropin IV to determine if both doses result in a similar adrenocortical response. 
     Testing was prospectively performed in dogs suspected of and being treated for pituitary- dependent HAC (PDH) with mitotane or trilostane. Dogs suspected of having HAC or being treated with mitotane received 1 mcg/kg cosyntropin IV followed four hours later by 5 mcg/kg cosyntropin IV. Blood samples were obtained pre- and one hour post-ACTH for each dose (4 measurements total). Preliminary studies were conducted to confirm the validity of performing two ACTH stimulation tests using this timing on the same day. Dogs receiving trilostane therapy were tested on consecutive days at the same time post-pill (4–6 hours post). Cortisol was measured using a previously validated radioimmunoassay. To detect differences in cortisol concentration between cosyntropin doses (1 and 5 mcg/kg) and between time points (baseline and 60-min), data were analyzed using a repeated-measures ANOVA by a commercial statistical computer program. Data for each group of dogs (suspect HAC, mitotane-treated and trilostane-treated) were evaluated separately. Significance was set at the p ≤ 0.05 level. 
     Overall, 46 dogs were included, with 26 suspected of HAC, 12 being treated for PDH with mitotane and 8 being treated for PDH with trilostane. No significant difference was detected between the post-ACTH cortisol concentrations within each group, comparing responses to both doses. For the suspect dogs and dogs treated with mitotane, the pre- and post-ACTH cortisol concentrations were significantly different with both doses (p < 0.001 and p = 0.001 respectively). For dogs treated with trilostane, no difference was detected between pre-ACTH and post-ACTH cortisol concentrations for either dose. 
    Therefore, the 1 mcg/kg IV dose of cosyntropin causes maximal adrenal response as does the standard 5 mcg/kg IV dose. The lower dose is sufficient for ACTH stimulation testing in those patients suspected of HAC or diagnosed with PDH and being treated with mitotane or trilostane. A lower dose of Cortrosyn may be used to help lower cost of diagnosing and monitoring this disease.

Comments—In the past, one of the most commonly used ACTH preparations for adrenal function testing was ACTH gel, in which ACTH is extracted from bovine and porcine pituitary glands. In the USA, the only FDA-approved, brand-name ACTH gel preparation is H.P. Acthar gel Repository Injection (80 U/ml; Questor Pharmaceuticals) (1). This ACTH preparation was widely used in veterinary medicine until 2007, when Questor Pharmaceuticals announced a new "pricing mode" for the H.P. Acthar gel (2), effectively raising the price of a vial almost 100-fold!

Due to the high cost of this brand-name gel ACTH, compounding pharmacies responded by offering compounded forms of ACTH gel. However, studies have shown that such preparations have variable potency and may be unreliable (3). Therefore, cosyntropin (e.g., Cortrosyn), a pure synthetic form of ACTH, has become the recommended product to use when performing an ACTH stimulation test (3-5). Cosyntropin has many advantages over ACTH gel preparations, including the following:

1. Cosyntropin can be administered intravenously (important in the dehydrated dog with suspected Addison's disease), as well as intramuscularly. All forms of ACTH gel must be given by the IM route.
2. Cosyntropin requires less time for the completion of the test than does ACTH gel (1 hour versus 2 hours), which makes monitoring more convenient.
3. The serum cortisol response to cosyntropin administration is more consistent than ACTH gel.
4. Finally, variations in potency is not an issue with cosyntropin, since it is a pure synthetic product, not extracted from pituitary glands like ACTH gel.

The use of synthetic ACTH in dogs was first reported in the 1970’s using a total dose of 250 µg per dog (4,5); this dose was equivalent to that recommended for testing in humans (6). Interestingly, no justification was given for the choice of 250 µg in people other than the notation that it was clearly "more than enough" required to produce a maximal adrenal response (6).

The practice of using 250 µg (the entire vial of cosyntropin) for the ACTH stimulation test in dogs persisted until the late 1990’s, when it was determined that a dose of 5 µg/kg of cosyntropin (i.e., Cortrosyn) resulted in maximal stimulation of the adrenal cortex in clinically normal dogs and dogs with hyperadrenocorticism (7,8). This new,” low-dose” ACTH response test using the 5 µg/kg dose of cosyntropin was quickly and widely adopted as the ACTH-testing protocol of choice, primarily because of cost-saving considerations (9).

It's important to note that accurate administration of such low doses of cosyntropin required dilution of the product with saline, and stability studies have only been reported for brand-name Cortrosyn, made by Amphastar Phamaceuticals (10) and generally available only in the USA. The effects of dilution or storage of other commercially available cosyntropin products have not been reported; this includes both the generic cosyntropin preparation made by Sandoz (11) in the USA or the brand-name product tetacosactide or Synacthen Ampoules (12) available in most countries outside of the USA.

The Bottom Line— In this abstract, the “mini-dose” of 1 µg/kg of cosyntropin could be a welcome alternative to the low-dose (5 µg/kg) and high-dose (250 µg/dog) ACTH stimulation test protocols for several reasons. There is valid concern that the escalating cost of cosyntropin may deter some practicing veterinarians from using the ACTH stimulation test to screen animals with suspected adrenocortical disease (i.e., hyper- and hypoadrenocorticism). Even more importantly, the high cost may prevent or alter the frequency of monitoring dogs treated with trilostane or mitotane. Other veterinarians continue to use the lower-cost compounded ACTH gels, despite their variable potency and known unreliability (3). In other words, if higher costs associated with performing the ACTH response test present a financial obstacle to the veterinarian or the pet owner, the ramifications of under-diagnosis and case mismanagement could be serious for dogs afflicted with these potentially fatal adrenocortical disorders.

Obviously, use of the 1 µg/kg mini-dose protocol allows for the testing of many more dogs compared to the 5 µg/kg protocol and especially the 250 µg/dog protocol. This would result in substantial savings for veterinary practices that adopt this mini-dose protocol. However, there are certain guidelines that should be followed when using the mini-dose cosyntropin protocol to ensure accurate results.

1. First, the 1 µg/kg dose should only be administered IV, as done in this abstract, since the cosyntropin may not be completely absorbed into the circulation when given by the intramuscular route. For the larger doses, such incomplete absorption is not a problem but IM administration of these mini-doses might not result in high enough circulating ACTH concentrations to maximally stimulate the adrenal cortex.
2. Secondly, the post-ACTH blood sample for cortisol determination should be obtained as close to 1 hour as possible after administration of cosyntropin. A delay in serum sample collection could miss the peak of maximum cortisol stimulation and result in lower-than-maximum peak concentrations (7). Again, this is less of a problem when higher doses of cosyntropin are given, since the higher doses results in a more prolonged adrenocortical stimulation,
3. Thirdly, but not least, we must store the reconstituted cosyntropin for periods of weeks to months to allow for its use at a later date when needed.  Once reconstituted with saline, the synthetic ACTH is stable in plastic syringes or vial for up to 4 months at 4 C (13), or it can be stored in frozen syringes at -20 C (or colder) for up to 6 months with no loss of bioactivity (7-9,13). Being able to store unused cosyntropin for extended periods is another way veterinarians can use the entire contents of each vial without waste.  
4. When aliquoting and freezing diluted cosyntropin, however, it is imperative for the ACTH be stored properly; if this "mini-dose" degrades even a bit, that might lead to an inadequate cortisol response. Use of a regular "household" frostless freezer should never be used to freeze these ACTH aliquoted vials or syringes.  These frostless freezers undergo periodic thawing and refreezing, which leads to degradation of the ACTH molecule. A dedicated freezer that does not undergo such thaw freeze cycles must be used if we decide to store the diluted cosyntropin in this way.

In the end, however, we must ask one simple question: Will the average veterinary practice perform enough ACTH stimulation tests to make a difference if the 1 µg/kg mini-dose protocol is chosen over the now standard 5 µg/kg protocol? If not, then why use the lower dose, given the potential disadvantages? With the higher 5 µg/kg protocol, we have a bit more leeway with sampling times and can get by with some loss of potency of the cosyntropin.

Because of these issues, most veterinarians will likely still be better off using the "old" 5 µg/kg rather than this "new" 1 µg/kg protocol. As we know, sometimes being "new" does not necessarily make it better!

References:

1. H.P. Acthar Gel, Repository Corticotropin Injection, package insert. Questcor, Union City, CA. Available at: http://www.acthar.com/Pdf/Acthar_PI_pdf
2. Questcor Board approves new strategy and business model for H.P. Acthar Gel. Union City, CA: Questcor; August 2007. Available at: http://phx.corporate-ir.net/phoenix.zhtml?c=89528&p=irol-newsArticle&ID=1044912&highlight
3. Kemppainen RJ, Behrend EN, Busch, KA. Use of compounded ACTH for adrenofunction testing in dogs. J Am Anim Hosp Assoc 2005;41:368-372. http://www.jaaha.org/content/41/6/368.abstract
4. Campbell JR, Watts C. Assessment of adrenal function in dogs. Br Vet J 1973;129:134-145. 
5. Feldman EC, Tyrrell JB., Bohannon NV. The synthetic ACTH stimulation test and measurement of endogenous plasma ACTH levels: useful diagnostic indicators for adrenal disease in dogs. J Am Anim Hosp Assoc 1978;14:524-531
6. Wood JB, Frankland AW, James VH, et al. A rapid test of adrenocortical function. Lancet 1965: 30;243-245. 
7. Kerl ME, Peterson ME, Wallace MS, et al. Evaluation of a low-dose ACTH stimulation test in clinically normal dogs and dogs with naturally developing hyperadrenocorticism. J Am Vet Med Assoc 1999;214:1497-1501. 
8. Frank LA, Oliver JW. Comparison of serum cortisol concentrations in clinically normal dogs after administration of freshly reconstituted versus reconstituted and stored frozen cosyntropin. J Vet Med Assoc 1998;212:1569-1571. 
9. Peterson ME: Containing the cost of the ACTH-stimulation test. J Am Vet Med Assoc 2004;224:198-199.
10. Cortrosyn package insert. Amphastar Phamaceuticals Inc, Rancho Cucamonga, CA. Available at: http://www.pharmacistconnection.com/images/ch/cortrosyn_111909/printable.pdf
11. Cosyntropin Injection (Generic) package insert. Sandoz, Princeton, NJ. Available at: http://www.accessdata.fda.gov/drugsatfda_docs/label/2008/022028lbl.pdf
12. Synacthen Ampoules, Produce information. Available at : https://www.medicines.org.uk/emc/medicine/7621
13. Dickstein G, Shechner C, Nicholson WE, et al.Adrenocorticotropin stimulation test: Effect of basal cortisol level, time of day, and suggest new sensitive low dose test. J Clin Endocrinol Metab 1991;72:773-778.  

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