Pituitary tumor enlargement in two patients with acromegaly during pegvisomant therapy.
BACKGROUND
Several classes of pharmacological agents are approved for the medical therapy of acromegaly, including dopamine agonists, somatostatin analogs and a growth hormone receptor antagonist. Pegvisomant, a growth hormone receptor antagonist, suppresses IGF-1 levels into the normal range in over ninety percent of patients. However, increased tumor volume was reported in patients receiving pegvisomant who had not received prior radiotherapy.


OBJECTIVE
To describe two patients with acromegaly who developed significant tumor enlargement on pegvisomant and discuss the potential mechanisms involved.


METHODS
Both patients received long-acting somatostatin analog (octreotide) therapy subsequent to incomplete transsphenoidal tumor resection. Octreotide did not normalize GH/IGF-1 levels in either patient but did control tumor mass size. Pegvisomant therapy was initiated after discontinuing octreotide.


RESULTS
Both patients exhibited suppression of IGF-1 into the normal range during pegvisomant therapy. However, significant tumor enlargement occurred in both. Potential mechanisms for tumor enlargement include the natural tendency of the tumor to grow with time, discontinuation of tumor suppressive effects of the somatostatin analog, or a direct effect of pegvisomant. The presumed mechanism of tumor enlargement is by loss of the inhibitory effect on tumor growth when IGF-1 levels are reduced. This could also explain the increase in circulating GH levels in patients with acromegaly receiving pegvisomant; patient 1 demonstrated an 18-fold increase in circulating GH levels while receiving the drug.


CONCLUSIONS
The mechanisms of tumor enlargement in patients with acromegaly on pegvisomant are likely multifactorial. Patients, especially those who have not received prior radiotherapy, should be closely monitored for tumor enlargement.
Frohman
Lawrence A
LA
Section of Endocrinology, Diabetes and Metabolism, University of Illinois at Chicago, Chicago, IL 60608, USA. frohman@uic.edu
Bonert
Vivien
V
D000172:Acromegaly / Q000188:drug therapy* / Q000473:pathology*; D000328:Adult; D018931:Antineoplastic Agents, Hormonal / Q000627:therapeutic use; D005260:Female; D019382:Human Growth Hormone / Q000009:adverse effects / Q000031:analogs & derivatives* / Q000097:blood / Q000627:therapeutic use; D006801:Humans; D007334:Insulin-Like Growth Factor I / Q000378:metabolism; D008279:Magnetic Resonance Imaging; D015282:Octreotide / Q000627:therapeutic use; D010911:Pituitary Neoplasms / Q000188:drug therapy* / Q000473:pathology* / Q000601:surgery
D018931:Antineoplastic Agents, Hormonal; D019382:Human Growth Hormone; D007334:Insulin-Like Growth Factor I; C406545:pegvisomant; D015282:Octreotide