The Place of Pegvisomant in the Management of Acromegaly:

Aggressive pituitary adenomas--diagnosis and emerging treatments

The WHO categorizes pituitary tumours as typical adenomas, atypical adenomas and pituitary carcinomas, with typical adenomas constituting the major class. However, the WHO classification does not provide an accurate correlation between histopathological findings and clinical behaviour. Tumours lacking typical histological features are classified as atypical, but not all are clinically atypical or exhibit aggressive behaviour. Pituitary carcinomas, by definition, have craniospinal or systemic metastases, although not all display classical cytological features of malignancy. Aggressive pituitary adenomas, defined from a clinical perspective, have earlier and more frequent recurrences and can be resistant to conventional treatments. Specific biomarkers have not yet been identified that can distinguish between clinically aggressive and nonaggressive pituitary adenomas, although the antigen Ki-67 proliferation index might be of value. This Review highlights the need to develop new biomarkers to facilitate the early detection of clinically aggressive pituitary adenomas and discusses emerging markers that hold promise for their identification. Defining aggressiveness is of crucial importance for improving the management of patients by enhancing prognostic predictions and effectiveness of treatment. New drugs, such as temozolomide, have potential use in the management of these patients; anti-VEGF therapy, mTOR and tyrosine kinase inhibitors are also potentially useful in managing selected patients.

Pathology of acromegaly

This review summarizes current knowledge on pituitary changes in patients with acromegaly. The histologic, immunohistochemical and electron microscopic study provided conclusive evidence that a marked diversity exists between the tumors which secrete growth hormone (GH) in excess, such as densely and sparsely granulated GH cell adenoma, the mixed GH prolactin cell adenoma and the mammosomatotrope adenoma. The latter two tumors produce GH and prolactin simultaneously. Densely granulated GH cell tumors may produce thyrotropin and alpha subunit as well. Somatotrope carcinomas are extremely rare. GH cell hyperplasia can also be associated with acromegaly in patients with extrapituitary GH-releasing hormone secreting tumors. The medical therapy of acromegaly is reviewed briefly, including long-acting somatostatin analogs and pegvisomant, a GH receptor blocker.

Effects of medical therapy on pituitary tumors

Previously surgery and irradiation were the only available procedures to treat patients with pituitary tumors. During the last few decades, novel drugs such as dopamine agonists and long-acting somatostatin analogs were developed and, an alternative medical therapy emerged. This paper summarizes the effect of medical therapy on the morphologic features of pituitary tumors and illustrates the ultrastructural alterations on electron micrographs. Currently drugs can be used in the management of pituitary tumors secreting GH, PRL, and/or TSH in excess. No medical therapy is available so far for ACTH-, FSH-, LH-, or alpha-subunit-secreting tumors as well as non-hormone-secreting pituitary tumors. Dopamine agonists are effective in the management of PRL-secreting tumors; they cause marked reversible tumor shrinkage in the substantial majority of patients. Long-acting somatostatin analogs are useful in the management of GH- and TSH-secreting pituitary tumors; they lead to mild to moderate tumor shrinkage in approximately 50% of cases. In patients treated with these drugs reduction of elevated blood hormone levels and amelioration of clinical symptoms ensue. It should be emphasized that no permanent cure is obtained. Blood hormone levels increase and the clinical symptoms reappear after discontinuation of treatment. Recently GH receptor blockers (pegvisomant) were introduced in the treatment of GH-producing pituitary adenomas. To the authors' knowledge the effect of these drugs on the morphology of pituitary tumors has not been revealed so far.

Treatment of Acromegaly

Mortality is increased in individuals with acromegaly unless serum growth hormone (GH) levels are below 2 microg/l and serum insulin-like growth factor (IGF)-I levels are normal following treatment. These combined criteria have been used to define remission of the disorder in this review. Transsphenoidal surgery achieves remission targets in an average of 55% of patients. For those not in remission following surgery, options include repeat surgery or use of adjuvant therapy. Fractionated external beam pituitary radiotherapy achieves 10-year remission rates of 47% but leaves patients exposed to excess GH until remission occurs. Stereotactic radiotherapy and gamma knife radiosurgery achieve remission rates of 40% over 3 years, and dopamine agonists produce remission in about 20% of patients. Somatostatin analogues induce remission in 59% of patients within the first year of treatment. The GH receptor antagonist pegvisomant leads to remission in 90% of patients, using IGF-I levels for assessment. Optimal treatment for a patient with acromegaly thus depends on the likely efficacy of treatment, cost, surgical skill, severity of side effects, tolerability, control of tumour growth, and improvement in complications related to tumour mass. A primary surgical approach, followed by medical therapy for those not in remission, remains the preferred option in most centres.

Second-generation biopharmaceuticals

The majority of first generation biopharmaceuticals are unengineered murine monoclonal antibodies or simple replacement proteins displaying an identical amino acid sequence to a native human protein. While some such products continue to be approved, an increasing number of modern biopharmaceuticals are engineered, second-generation products. Engineering can entail alteration of amino acid sequence, alteration of the glycocomponent of a glycosylated protein, or the covalent attachment of chemical moieties such as polyethylene glycol. Engineering has been applied in order to alter a protein's immunological or pharmacokinetic profile, or in order to generate novel fusion products. Better understanding of the links between protein structure and function will underpin the development of an increasing number of engineered biopharmaceuticals in the future.