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.  

Top Clinical Endocrinology Research Abstracts, 2014 ACVIM Forum: Adrenal Part 1

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 whose day-job is senior member of the veterinarian consulting service for Antech Diagnostics, the world's largest laboratory dedicated to animal health.

In this post, we will review another of these "top 12" abstracts (starting with the adrenal gland abstracts). Next week, we will finish up the top clinical abstracts dealing with the adrenal gland, and then go on to disorders of the thyroid over the next 2 weeks.

Schrage A, Appleman E, Langston C. Iatrogenic Hypoadrenocorticism Following Trilostane Therapy for Pituitary-Dependent Hyperadrenocorticism in Dogs. J Vet Intern Med 2014;28:1035.

     This retrospective case series identified 13 dogs that developed iatrogenic hypoadrenocorticism (iHAC) following administration of trilostane for treatment of pituitary-dependent hyperadrenocorticism (PDH). Inclusion criteria required a previous diagnosis of PDH, monotherapy with trilostane (i.e., no other medications used for treatment of PDH), and a post- ACTH stimulated cortisol concentration of < 1 μg/dL while receiving trilostane. 
     Clinical signs of PDH resolved in 92% (12/13) of dogs prior to development of iHAC. At the time of diagnosis, 7/13 (53%) dogs had clinical signs consistent with iHAC. Lethargy and inappetence were the most common signs. Median age of dogs was 12 years with a median weight of 10 kilograms. No single breed was overrepresented. Dogs were treated with trilostane for a median of 8.5 months at a median dosage of 4.75 mg/kg/day prior to development of iHAC. Mineralocorticoid deficiency (hyperkalemia ± hyponatremia) was identified in 3/13 (23%) dogs. Trilostane was discontinued in all 7 dogs displaying clinical signs and later restarted at a lower dose in 2 dogs. Permanent hypoadrenocorticism developed in 4 dogs. No dog died or was euthanized as a result of iHAC. 
     This report illustrates that, while trilostane is an effective treatment for PDH, transient or permanent iatrogenic hypoadrenocorticism may occur. Development of mineralocorticoid deficiency is less common in comparison to glucocorticoid deficiency. These dogs were being closely evaluated and had received manufacturer-recommended doses of trilostane prior to development of iHAC. Close monitoring of dogs on trilostane therapy is warranted, with special emphasis on clinical signs, electrolyte levels, and cortisol concentrations.

Comments— This report emphasizes that trilostane (Vetoryl) is not a benign drug. Although safer to use than mitotane, trilostane can certainly result in hypoadrenocorticism (cortisol deficiency) and even complete hypoadrenocorticism (cortisol and mineralocorticoid deficiency; Addison's disease) (1-8). Therefore, it is imperative to use the lowest daily dose possible and to monitor the dog very closely while on treatment with this drug.

The median dose used in the dogs of this retrospective study (4.75 mg/kg/day) was indeed within the dosage recommended on the Vetoryl package insert (2.2-6.7 mg/kg/ day) (9). However, that dose is much higher than the starting dose we normally recommend (≈2 mg/kg/day) (10). The higher doses given to the dogs of this report was the likely reason for the very high rate of hypocortisolism (53% of dogs), as well as the high rate of concurrent mineralocorticoid deficiency seen in these dogs (23% of dogs). Over the years, we have learned that lower starting doses are generally much safer and result in fewer severe side effects (11-15), and we have not personally had a dog develop complete hypoadrenocorticism for the last decade. Such high rates of hypoadrenocorticism generally indicate drug overdosage and are not acceptable, at least in our opinion.

We do not know from this abstract what initial dose was given, when the dogs were rechecked, or exactly how the investigators decided that a dosage increase was indicated. We recommend that dogs on trilostane treatment should be evaluated at 14 days, 1 month, 3 months, and every 3 months thereafter (10). At each recheck, we collect a complete history, do a complete examination, and perform a serum biochemical panel with electrolytes. In addition, we do an ACTH stimulation test at each visit by collecting the basal cortisol sample and administering cosyntropin (Cortrosyn) ≈3-4 hours after the morning trilostane dose to evaluate the peak effect on lowering cortisol levels.

We base dose adjustments on the dog's clinical response, routine blood tests, and cortisol testing. The ideal post-ACTH cortisol range that we recommend is 2.0-7.5 µg/dl (50-200 nmol/L). If`a dog continues to show clinical signs of hyperadrenocorticism and post-ACTH cortisol is above 7.5 µg/dl, we then increase the trilostane dose. If the signs of hyperadrenocorticism have resolved but the post-ACTH cortisol is above 7.5 µg/dl, we generally do not raise the daily dose but we would closely monitor for signs consistent with relapse.

If a Cushing's dog on trilostane is doing clinically well, but the serum cortisol values are low (post-ACTH cortisol less than 2 µg/dl [50 nmol/L]), we recommend that one stop the trilostane for 5-7 days and restart treatment at a 25-50% lower dose. Then, one should retest after 2 weeks of treatment on the new, lower dose. If the serum cortisol values remain subnormal on the reduced dosage, the trilostane should be discontinued indefinitely, with repeat ACTH stimulation testing scheduled for 1 month and every 3-6 months thereafter. The trilostane should only be restarted in these dogs if clinical signs of hyperadrenocorticism return and the post-ACTH cortisol concentrations once again become high. If Addison’s disease is confirmed (i.e., low cortisol concentrations with hyperkalemia, hyponatremia, or both), one should discontinue trilostane and treat the dog with glucocorticoids and mineralocorticoids, as needed.

Bottom line— The introduction of trilostane in many countries around the world has increased the options for the management of canine Cushing's disease. For most veterinarians, this drug has replaced the use of mitotane due to its greater safety. It is nearly as effective as mitotane and has a lower frequency of serious adverse reactions (15,16).

That all said, the drug can certainly lead to adverse side effects, including hypoadrenocorticism and adrenal necrosis (1-10). All of the side effects appear to be at least partially related to the dose given, so we recommend lower initial doses, close and frequent monitoring, and gradual increases in the daily dose as needed for control of clinical and biochemical signs of hyperadrenocorticism.

References:

1. Neiger R, Ramsey I, O'Connor J, et al. Trilostane treatment of 78 dogs with pituitary-dependent hyperadrenocorticism. Vet Rec 2002;150:799-804. 
2. Braddock JA, Church DB, Robertson ID, et al. Trilostane treatment in dogs with pituitary-dependent hyperadrenocorticism. Aust Vet J 2003;81:600-607.
3. Wenger M, Sieber-Ruckstuhl NS, Muller C, et al. Effect of trilostane on serum concentrations of aldosterone, cortisol, and potassium in dogs with pituitary-dependent hyperadrenocorticism. Am J Vet Res 2004;65:1245-1250. h
4. Chapman PS, Kelly DF, Archer J, et al. Adrenal necrosis in a dog receiving trilostane for the treatment of hyperadrenocorticism. J Small Anim Pract 2004;45:307-310. 
5. Reusch CE, Sieber-Ruckstuhl N, Wenger M, et al. Histological evaluation of the adrenal glands of seven dogs with hyperadrenocorticism treated with trilostane. Vet Rec 2007;160:219-224.
6. Ramsey IK, Richardson J, Lenard Z, et al. Persistent isolated hypocortisolism following brief treatment with trilostane. Aust Vet J 2008;86:491-495. 
7. Richartz J, Neiger R. Hypoadrenocorticism without classic electrolyte abnormalities in seven dogs. Tierarztliche Praxis Ausgabe K, Kleintiere/Heimtiere 2011;39:163-169. 
8. Griebsch C, Lehnert C, Williams GJ, et al. Effect of trilostane on hormone and serum electrolyte concentrations in dogs with pituitary-dependent hyperadrenocorticism. J Vet Intern Med 2014;28:160-165. 
9. Dechra Animal Heath website. Veteryl Product Insert. 
10. Melián CM, Pérez-Alenza D, Peterson ME. Hyperadrenocorticism in dogs In: Ettinger SJ, Feldman EC, eds. Textbook of Veterinary Internal Medicine: Diseases of the Dog and Cat (Seventh Edition) Philadelphia, Saunders Elsevier, pp 1816-1840, 2010. Seventh ed. Philadelphia: Saunders Elsevier, 2010;1816-1840.
11. Vaughan MA, Feldman EC, Hoar BR, et al. Evaluation of twice-daily, low-dose trilostane treatment administered orally in dogs with naturally occurring hyperadrenocorticism. J Am Vet Med Assoc 2008;232:1321-1328. 
12. Arenas C, Melian C, Perez-Alenza MD. Evaluation of 2 trilostane protocols for the treatment of canine pituitary-dependent hyperadrenocorticism: twice daily versus once daily. J Vet Intern Med 2013;27:1478-1485. 
13. Braun C, Boretti FS, Reusch CE, et al. Comparison of two treatment regimens with trilostane in dogs with pituitary-dependent hyperadrenocorticism. Schweiz Arch Tierheilkd 2013;155:551-558. 
14. Feldman EC. Evaluation of twice-daily lower-dose trilostane treatment administered orally in dogs with naturally occurring hyperadrenocorticism. J Am Vet Med Assoc 2011;238:1441-1451. 
15. Clemente M1, De Andrés PJ, Arenas C, et al. Comparison of non-selective adrenocorticolysis with mitotane or trilostane for the treatment of dogs with pituitary-dependent hyperadrenocorticism. Vet Rec 2007;15;161:805-809.
16. Griffies JD. Old or new? A comparison of mitotane and trilostane for the management of hyperadrenocorticism. Compend Contin Educ Vet 2013;35:E3. 

Kool MMJ, Galac S, van der Helm N, Corradini S, Kooistram HS, Mol JA. Targeting Phosphatidylinositol-3-Kinase Signaling in Canine Cortisol-Secreting Adrenocortical Tumors - Novel Therapeutic Prospects? J Vet Intern Med 2014;28:1030.

     Hypercortisolism is one of the most common endocrinopathies in dogs, and is caused by cortisol- secreting adrenocortical adenomas or carcinomas in 15% of cases. The aim of this study was to investigate involvement of the insulin-like growth factor (IGF)-phosphatidylinositol-3-kinase (PI3K) signaling pathway in the pathogenesis of adrenocortical tumors (ATs), in order to identify components of this pathway that may hold promise as future therapeutic targets, prognostic and/or diagnostic markers.
     The tumor group consisted of histologically confirmed cortisol-secreting adenomas (n = 14) and carcinomas (n = 30). Whole tissue explants of normal adrenal glands (n = 10) were used as controls. Quantitative RT-PCR was used to assess the relative mRNA expression levels of IGF1 and 2, IGF- and EGF-receptors, IGF-binding proteins, PI3K inhibitor PTEN and downstream target genes of the PI3K signaling pathway. Localization of PTEN was immunohistochemically evaluated. Additionally, mutation analysis was performed on the full coding region of PTEN and the PI3K catalytic subunit, on mRNA level.
     When compared to normal adrenals, in carcinomas the differential expression of PI3K target genes indicated activation of the pathway. Also, carcinomas showed a decreased expression of PI3K inhibitor PTEN and an increased expression of the EGF receptor ErbB2. Gene expression levels in adenomas were mostly unchanged. Immunohistochemical staining of PTEN was predominantly negative in both ATs and normal adrenals. No missense mutations of PTEN and the PI3K catalytic subunit were detected.
     Based on gene function and reports in human ATs, the low expression of PTEN in carcinomas is of particular interest with regard to tumor pathogenesis. Target gene expression suggests PI3K activation in carcinomas, possibly due to decreased PTEN and/or increased ErbB2 expression. Based on these results, targeting of ErbB2, PI3K or its downstream effectors may have potential as a therapeutic option in canine cortisol-secreting adrenocortical carcinomas.

Comments—To adequately understand the importance of this investigational study the molecular biology of PI3Ks, PTEN, IGF-1 and IGF-2, IGF- and EGF- receptors and IGF- binding proteins and their link to adrenocortical tumors is briefly reviewed.

Phosphoinositide 3-kinases (also called PI3Ks) are a family of enzymes involved in cellular functions such as cell growth, proliferation, differentiation, and survival (1). More specifically, PI3Ks phosphorylate cell membrane lipids to modulate the activity of intracellular protein effectors that regulate many aspects of cell function. For example, it is estimated that every cell has 50-100 “downstream” effectors of PI3Ks. In essence, the PI3K pathway is an intracellular signaling pathway important in apoptosis and hence cancer and longevity. In many cancers, this pathway is overactive, thus reducing apoptosis and allowing proliferation. Consequently, many experimental cancer drugs are designed to inhibit the signaling sequence at some point using PI3K inhibitors. Impact point: Expression of target genes suggests activation of the PI3K pathway in adrenocortical tumors.

Phosphatase and tensin homolog (PTEN) is a protein that, in humans, is encoded by the PTEN gene (2). PTEN acts as a tumor suppressor gene but is mutated in a large number of cancers with high frequency (3). When the PTEN protein is functioning properly, it acts as part of a chemical pathway that signals cells to stop dividing and can cause cells to undergo apoptosis when necessary. These functions prevent uncontrolled cell growth that can lead to the formation of tumors. Impact point: The low expression of PTEN and negative immunohistochemical staining with adrenocortical tumors suggests that lack of this PI3K inhibitor may play a role in tumor pathogenesis.

Epidermal growth factor or EGF is a growth factor that stimulates cell growth, proliferation, and differentiation by binding to its receptor EGFR (4). The epidermal growth factor receptor is a member of the ErbB family of receptors, a subfamily of four closely related receptor tyrosine kinases (ErbB1-4); receptor tyrosine kinases are the high-affinity cell surface receptors for many polypeptide growth factors, cytokines, and hormones (5). Increased activity of the receptor for EGF has been observed in certain types of cancer, often correlated with mutations in the receptor and abnormal function (8). Impact point: Adrenocortical tumors showed an increased expression of the ECG receptor ErbB-2.

Insulin-like growth factors (IGFs) are proteins with an amino acid sequence similar to insulin. IGFs are part of a complex system that cells use to communicate with their physiologic environment. This complex system (often referred to as the IGF "axis") consists of two cell-surface receptors (IGF1R and IGF2R), two ligands - insulin-like growth factor 1 (IGF-I) and insulin-like growth factor 2 (IGF-2), and a family of six high-affinity IGF-binding proteins which modulate IGF action in many ways (6). The IGF axis has been shown to play a key role in cancer cell proliferation, differentiation, and the inhibition of programmed cell death (apoptosis) using intracellular signaling through the PI3K pathway (see below). Impact point: Unlike in humans and for reasons that are unclear (see Bottom Line summary), the IGFs and IGF receptors apparently do not play a role in the pathogenesis of adrenocortical tumors in the dog.

The Bottom Line— To date, the most common treatment approach in dogs with an cortisol-secreting adrenal tumor is surgery and/or medical therapy with mitotane or trilostane (7-12). In human medicine, several novel approaches are under study for treatment of advanced adrenal carcinoma, many of which represent molecularly targeted therapies. For example, the finding that over 80% of adrenal tumors express the epidermal growth factor receptor (EGFR) (13, 14) provides a rationale for the study of agents that target the EGFR. In addition, approximately 80% of adrenocortical tumors also over express insulin-like growth factor type 2 (IGF-2), which is known to signal predominantly through the IGF-1 receptor (IGFR1).

Preclinical studies targeting the IGF-1 receptor (15) and two phase I trials have shown promising results (16, 17), and ongoing phase II and III trials are close to completion. The recent work by Kool et al in dogs, would suggest that targeting a specific EGFR (i.e., ErbB2) or PK3K or its downstream effectors may have potential as a therapeutic option in canine cortisol-secreting adrenal tumors. Clearly more investigational studies are needed to determine the efficacy, adverse effects, and cost of molecular targeting therapy before it becomes an accepted form of treatment for adrenocortical tumors in the dog.

References:

1. Vanhaesebroeck B, Stephens L, Hawkins P. PI3K signaling: the path to discovery and understanding. Nat Rev Mol Cell Biol 2012;13:195-203.
2. Steck PA, Pershouse MA, Jasser SA, et al. Identification of a candidate for a tumor suppressor gene that is mutated in multiple advanced cancers. Nat Genet 1997;15: 356–62.
3. Chu EC, Tarnawski AS. PTEN regulatory functions in tumor suppression and cell biology. Med Sci Monit 2004; 10:235–41.
4. Herbst RS. Review of epidermal growth factor receptor biology. Int J Radiat Oncol Biol Phys 2004;59 (2 Suppl): 21–26.
5. Zhang H, Berezov A, Wang Q, et al. ErbB receptors: from oncogenes to targeted cancer therapies. J Clin Invest 2007;117:2051–2058.
6. Le Roith D. Insulin-like growth factors. N Engl J Med 1997; 336; 633-640.
7. van Sluijs FJ1, Sjollema BE, Voorhout G, et al. Results of adrenalectomy in 36 dogs with hyperadrenocorticism caused by adrenocortical tumor. Vet Q 1995;17:113-116.
8. Anderson CR, Birchard SJ, Powers BE, et al. Surgical treatment of adrenocortical tumors: 21 cases (1990-1996). J Am Anim Hosp Assoc 2001;37:93-97.
9. Scavelli TD, Peterson ME, Matthiesen DT. Results of surgical treatment for hyperadrenocorticism caused by adrenocortical neoplasia in the dog: 25 cases (1980-1984). J Am Vet Med Assoc 1986;189:1360-1364.
10. Kintzer PP, Peterson ME. Mitotane treatment of 32 dogs with cortisol-secreting adrenocortical neoplasms. J Am Vet Med Assoc 1994;205;54-60.
11. Feldman EC, Nelson RW, Feldman MS, et al. Comparison of mitotane treatment for adrenal tumor versus pituitary-dependent hyperadrenocorticism in dogs. J Am Vet Med Assoc 1992;200:1642-1647
12. Helm JR, McLauchlan G, Boden LA, et al. Comparison of factors that influence survival in dogs treated with mitotane and trilostane with adrenal-dependent hyperadrenocorticism. J Vet Intern Med 2011;25:251-260.
13. Edgren M, Eriksson B, Wilander E, et al. Biological characteristics of adrenocortical carcinoma: a study of p53, IGF, EGF-r, Ki-67 and PCNA in 17 adrenocortical carcinomas. Anticancer Res 1997;17:1303-1309.
14. Kamio T, Shigematsu K, Sou H, et al. Immunohistochemical expression of epidermal growth factor receptors in human adrenocortical carcinoma. Hum Pathol 1990; 21:277-282.
15. Barlaskar FM, Spalding AC, Heaton JH, et al. Preclinical targeting of the type I insulin-like growth factor receptor in adrenocortical carcinoma. J Clin Endocrinol Metab 2009;94:204-212.
16. Haluska P, Worden F, Olmos D, et al. Safety, tolerability, and pharmacokinetics of the anti-IGF-1R monoclonal antibody figitumumab in patients with refractory adrenocortical carcinoma. Cancer Chemother Pharmacol 2010;65:765–773.
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