Feline acromegaly: Treatment options
Medical options for treating acromegaly range from increasing a patient's insulin dosage to manage the diabetogenic effects of acromegaly to instituting treatment with a somatostatin analogue, dopamine agonist, or growth hormone receptor antagonist. Several of these treatments are common in human medicine but have not been studied widely in veterinary medicine.Somatostatin analogues
Somatostatin is a hypothalamic hormone that acts on the pituitary gland to inhibit growth hormone release. Somatostatin analogues are commonly administered in people with acromegaly and have efficacy rates of 50% to 60%. In addition to acting centrally by suppressing growth hormone release and peripherally by interfering with growth hormone receptor binding on hepatocytes, somatostatin analogues are also thought to result in tumor shrinkage of pituitary adenomas by promoting apoptosis.1
The somatostatin analogue octreotide has been evaluated in a few cats with acromegaly with limited success. In a study of four cats with acromegaly, no change in serum growth hormone concentration was noted after treatment with octreotide.2 Another study, which measured the short-term effects of octreotide in five cats with acromegaly, found a decrease in growth hormone concentrations for up to 90 minutes after octreotide administration.3 However, a recent study evaluating a long-acting somatostatin analogue (Sandostatin LAR Depot—Novartis) showed no benefit in cats treated for three to six months.4
The failure of these drugs to inhibit growth hormone release may be related to differences in somatostatin receptor subtypes found on pituitary adenomas. Future studies to identify the somatostatin receptor subtypes in feline growth hormone-secreting pituitary tumors are required to determine if these subtypes are similar to the ones found in people and if human somatostatin analogue therapy, at least in theory, may be beneficial in cats with acromegaly.
Dopamine agonists and growth hormone receptor antagonists
Dopamine agonists and, more recently, growth hormone receptor antagonists are also given to people to treat acromegaly.
Growth hormone receptor antagonist therapy has not been reported in cats, but in people, response rates have been reported to be as high as 90%.1 However, it has been noted that these medications have no effect on tumor size (do not result in tumor shrinkage) and, thus, would not benefit patients with neurologic signs.
A single case study on the treatment of feline acromegaly with a dopamine agonist (L-deprenyl) showed that the medication had no effect on reducing insulin requirements or clinical signs of disease.5 In people, dopamine agonists are typically only 10% to 20% effective but are often combined with other medications.1
Increasing the dosage of insulin to improve glycemic control and clinical signs of diabetes is the most conservative—and most common—method for managing insulin-resistant diabetic acromegalic cats. While helping to control the clinical signs of the diabetes, raising the insulin dose has no effect on growth hormone secretion, progression of the clinical signs of acromegaly, or continued growth of the pituitary tumor.
In addition, some patients treated with high doses of insulin unpredictably and inexplicably become sensitized to the effect of the insulin, resulting in hypoglycemic crises.6,7 The timing of the insulin sensitization and occurrence of hypoglycemic episodes was extremely variable. In one study, several acromegalic cats were euthanized after experiencing hypoglycemic comas.6