Medical Treatment of Insulinomas

Dietary or medical management of insulinoma is recommended for animals that are showing signs of hypoglycemia and have previously undergone surgery and in those whose owners have declined surgery (1-6).

Specific chemotherapy can also be considered in animals in which all of the tumor cannot be resected and in those that have undergone previous surgery and again are showing signs of hypoglycemia. Just over half of dogs with insulinoma have metastases at the time of diagnosis (1-4), so it is reasonable to discuss the possibility of follow-up chemotherapy after surgery. Chemotherapy should be given only to patients with a confirmed histologic diagnosis of insulinoma.

Dietary management of hypoglycemia
Animals with insulinoma should be fed a diet that is high in protein, fat, and complex carbohydrates. Simple sugars, often contained in semimoist pet foods, should be avoided. Dogs should be fed small meals three to four times daily. Cats and ferrets may be fed free choice if they do not become obese. Exercise should be controlled and owners should attempt to limit excitement in these pets.

Glucocorticoids
Glucocorticoids are recommended when frequent feedings are no longer successful in controlling clinical signs of hypoglycemia. These drugs raise blood glucose by inhibiting glucose uptake in the peripheral tissues (creating insulin resistance) and stimulating hepatic glucose production (7-9).

Oral prednisone (or prednisolone) is started at the dosage of 0.25 mg/kg, twice daily (1-6). This dosage may be increased gradually as needed to control clinical signs or may be decreased if the disease is well controlled at the initial dosage. We should remember, however, that dosages of 1.1 mg/kg or higher given twice daily are considered immunosuppressive.

Diazoxide
Diazoxide (Proglycem, Teva Pharmaceuticals) is a nondiuretic benzothiadiazide that decreases insulin secretion, promotes gluconeogenesis and glycogenolysis, and inhibits the cellular uptake of glucose (10-12). Diazoxide can be difficult to obtain in the United States; however, reputable compounding pharmacies can often supply this drug.

The recommended starting dosage of diazoxide is 5 mg/kg, given orally twice daily (1-6,13). As with prednisone, the dosage may be increased as needed to control clinical signs. The maximal recommended dosage is 30 mg/kg twice daily.

The most common side effects of diazoxide are anorexia, vomiting, and diarrhea (1-6,13). These signs may be avoided or lessened by giving the medication with food. Ferrets find the diazoxide suspension distasteful, but because only small volumes are required, owners usually are able to administer it. Other potential side effects of diazoxide are hyperglycemia, bone marrow suppression, and sodium retention.

Octreotide acetate
Somatostatin is a polypeptide hormone that inhibits the secretion of insulin, glucagon, gastrin, secretin, and motilin. Octreotide acetate (Sandostatin, Novartis) is a long-acting somatostatin analogue that can be used in the management of patients with insulinoma (14).

Reports on the use of octreotide acetate in veterinary patients are limited and the response is mixed (4,12,15). About half of dogs with refractory hypoglycemia will show a response to octreotide acetate.  Ferrets refractory to other forms of treatment may show improvement in clinical signs in some, but certainly not all, cases.

The recommended dosage is 1 to 2 μg/kg given subcutaneously two to three times daily. This drug is relatively expensive, but may be practical for use in small dogs, cats, or ferrets due to their small size.

Currently, there is no way of predicting which patients will respond to octreotide acetate. Metastatic lesions may express fewer somatostatin receptors than the primary mass, so octreotide may be less effective in patients with advanced disease. This agent does appear to be safe and can be administered by owners at home. Thus, it should be considered for the treatment of animals with insulinoma that are refractory to or unable to tolerate traditional medical or surgical therapy (4,12).

Streptozotocin
Streptozotocin (Zanosar, Teva Pharmaceuticals) is a chemotherapeutic drug that selectively destroys pancreatic beta cells (16-19). When given alone, this drug may cause severe, acute renal failure in dogs. However, the drug can be administered safely if given with aggressive saline diuresis (17-19). Treatment is discontinued if there is clear tumor progression, resistant or recurrent hypoglycemia, or drug toxicity.

Streptozotocin may induce diabetes in some dogs, but the chemotherapy drug may be given along with appropriate insulin therapy if gross disease is still present.  No reports have described the use of streptozotocin in cats or ferrets with insulinoma. Further study of this agent is needed in all species.

Prognosis

The short-term prognosis for dogs with insulinoma is good, although most will eventually die of this disease. While survival time depends on the stage of the disease and the success of surgery, it also depends on the owners’ willingness to treat aggressively and follow up with symptomatic therapy once signs of hypoglycemia return.

Approximately two-thirds to three-quarters of dogs survive 6 months or longer after surgery (often over a year) before intractable hypoglycemia recurs. Reported median survival time is much longer in dogs initially treated with surgery than in those treated with medical management alone (1-5,20,21). Individualizing therapy with the use of combinations of medical and surgical therapy based on the stage and extent of disease may improve prognosis and survival time in any given patient.  

References:

1. Nelson RW, Salisbury SK. Pancreatic beta cell neoplasia In: Birchard SJ, Sherding RJ, eds. Saunders’ Manual of Small Animal Practice. 2nd ed. Philadelphia: WB Saunders, 2000;288–294.
2. Feldman EC, Nelson RW. Beta-cell neoplasia: Insulinoma In: Feldman EC, Nelson RW, eds. Canine and Feline Endocrinology and Reproduction. Philadelphia: Saunders Elsevier, 2004;616-644.
3. Kintzer PP. Insulinoma and other gastrointestinal tract tumours In: Mooney CT, Peterson ME, eds. BSAVA Manual of Canine and Feline Endocrinology. Quedgeley, Gloucester: British Small Animal Veterinary Association, 2012;148-155.
4. Meleo KA, Peterson ME. Treatment of insulinoma in the dog, cat, and ferret In: Bonagura JD, Twedt DC, eds. Kirk's Current Veterinary Therapy, Volume XV. Philadelphia: Saunders Elsevier, 2013.
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13. Parker AJ, Musselman EM, O'Brien D. Diazoxide treatment of canine insulinoma. Vet Rec 1981;109:178-179. 
14. Maton PN. The use of the long-acting somatostatin analogue,octreotide acetate, in patients with islet cell tumors. Gastroenterol Clin North Am 1989;18:897-922. 
15. Simpson KW, Stepien RL, Elwood CM, et al. Evaluation of the long-acting somatostatin analogue octreotide in the management of insulinoma in three dogs. J Small Anim Pract 1995;36:161-165. 
16. Meyer DJ. Temporary remission of hypoglycemia in a dog with an insulinoma after treatment with streptozotocin. Am J Vet Res 1977;38:1201-1204. 
17. Moore AS, Nelson RW, Henry CJ, et al. Streptozocin for treatment of pancreatic islet cell tumors in dogs: 17 cases (1989-1999). J Am Vet Med Assoc 2002;221:811-818. 
18. Bell R, Mooney CT, Mansfield CS, et al. Treatment of insulinoma in a springer spaniel with streptozotocin. J Small Anim Pract 2005;46:247-250. 
19. Northrup NC, Rassnick KM, Gieger TL, et al. Prospective evaluation of biweekly streptozotocin in 19 dogs with insulinoma. J Vet Intern Med 2013;27:483-490. 
20. Tobin RL, Nelson RW, Lucroy MD, et al. Outcome of surgical versus medical treatment of dogs with beta cell neoplasia: 39 cases (1990-1997). J Am Vet Med Assoc 1999;215:226-230. 
21. Polton GA, White RN, Brearley MJ, et al. Improved survival in a retrospective cohort of 28 dogs with insulinoma. J Small Anim Pract 2007;48:151-156. 

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