Pegvisomant: current and potential novel therapeutic applications

Targeting growth hormone function: strategies and therapeutic applications

Human growth hormone (GH) is a classical pituitary endocrine hormone that is essential for normal postnatal growth and has pleiotropic effects across multiple physiological systems. GH is also expressed in extrapituitary tissues and has localized autocrine/paracrine effects at these sites. In adults, hypersecretion of GH causes acromegaly, and strategies that block the release of GH or that inhibit GH receptor (GHR) activation are the primary forms of medical therapy for this disease. Overproduction of GH has also been linked to cancer and the microvascular complications that are associated with diabetes. However, studies to investigate the therapeutic potential of GHR antagonism in these diseases have been limited, most likely due to difficulty in accessing therapeutic tools to study the pharmacology of the receptor in vivo. This review will discuss current and emerging strategies for antagonizing GH function and the potential disease indications.

Emerging therapies are offering an expanded toolkit for combatting the effects of human growth hormone overproduction. Human growth hormone (GH) is a major driver of postnatal growth; however, systemic or localized overproduction is implicated in the aberrant growth disease acromegaly, cancer, and diabetes. In this review, researchers led by Jo Perry, from the University of Auckland, New Zealand, discuss strategies that either inhibit GH production, block its systemic receptor, or interrupt its downstream signaling pathways. The only licensed GH receptor blocker is pegvisomant, but therapies are in development that include long-acting protein and antibody-based blockers, and nucleotide complexes that degrade GHR production have also shown promise. Studies investigating GHR antagonism are limited, partly due to difficulty in accessing therapeutic tools which block GHR function, but overcoming these obstacles may yield advances in alleviating chronic disease.

Clinical studies in humans targeting the various components of the IGF system show lack of efficacy in the treatment of cancer

The insulin-like growth factors (IGFs) system regulates cell growth, differentiation and energy metabolism and plays crucial role in the regulation of key aspects of tumor biology, such as cancer cell growth, survival, transformation and invasion. The current focus for cancer therapeutic approaches have shifted from the conventional treatments towards the targeted therapies and the IGF system has gained a great interest as anti-cancer therapy. The proliferative, anti-apoptotic and transformation effects of IGFs are mainly triggered by the ligation of the type I IGF receptor (IGF-IR). Thus, aiming at developing novel and effective cancer therapies, different strategies have been employed to target IGF system in human malignancies, including but not limited to ligand or receptor neutralizing antibodies and IGF-IR signaling inhibitors. In this review, we have focused on the clinical studies that have been conducted targeting the various components of the IGF system for the treatment of different types of cancer, providing a description and the challenges of each targeting strategy and the degree of success.

Growth hormone and cancer: an update on progress

PURPOSE OF REVIEW

Animals born with a deficiency in the cell surface receptor for growth hormone (GH) have a significantly reduced risk of developing cancer. Conversely, increased expression levels of GH and the GH receptor (GHR) are detectable in a variety of different human cancers. Here we discuss recent literature contributing to our understanding of the field.

RECENT FINDINGS

In addition to animal evidence, studies of individuals with Laron syndrome suggest that congenital GHR deficiency may also protect humans against cancer. GH expression in certain malignancies is correlated with clinicohistopathological parameters and may contribute the therapeutic resistance. Other recent studies have identified novel aspects of the GH signal transduction pathway, including receptor crosstalk and the involvement of microRNA in endocrine regulation of GH.

SUMMARY

Substantial evidence suggests the GH/insulin-like growth factor-1 axis initiates and promotes progression of cancer. However, important questions remain unanswered regarding the therapeutic utility of GH or GHR antagonism in cancer. Further clinical studies regarding the clinical association of GH expression with human malignancies and translational studies investigating GHR antagonism in animal models of human cancer are critical.

Targeting the insulin growth factor receptor 1

The IGF axis is a tightly controlled endocrine system that regulates cell growth and development, known to have an important function in cancer biology. IGF1 and IGF2 can promote cancer growth in a GH-independent manner both through paracrine and autocrine secretion and can also confer resistance to chemotherapy and radiation. Many alterations of this system have been found in neoplasias, including increased expression of ligands and receptors, loss of heterozygosity of the IGF2 locus and increased IGF1R gene copy number. The IGF1 network is an attractive candidate for targeted therapy, including receptor blockade with monoclonal antibodies and small molecule inhibitors of receptor downstream signaling. This article reviews the role of the IGF axis in the initiation and progression of cancer, and describes the recent advances in IGF inhibition as a therapeutic tool.

Diagnosis and management of pituitary adenomas

PURPOSE OF REVIEW

Pituitary adenomas are frequently encountered in practice either because of clinical symptoms or as incidental findings. These tumors may alter pituitary function and can therefore have a considerable impact on morbidity, mortality, and quality of life.

RECENT FINDINGS

The presentation is variable for each type of tumor, with microadenomas being either silent or manifesting with symptoms of hormonal excess. In contrast, macroadenomas may present with mass effect. The various histologic types include prolactinomas, nonfunctioning adenomas, somatotropinomas, corticotropinomas, and thyrotropinomas. The present article will serve as an update on the diagnosis and treatment of pituitary adenomas. Advances in medical management for each tumor are discussed, highlighting new therapeutic alternatives. The role of surgery is also reviewed. Recommendations on the surveillance and postoperative monitoring of patients are emphasized.

SUMMARY

Newer methods in the diagnosis and treatment for pituitary adenomas greatly expand our ability to care for affected patients.

The growth hormone receptor: mechanism of activation and clinical implications

Growth hormone is widely used clinically to promote growth and anabolism and for other purposes. Its actions are mediated via the growth hormone receptor, both directly by tyrosine kinase activation and indirectly by induction of insulin-like growth factor 1 (IGF-1). Insensitivity to growth hormone (Laron syndrome) can result from mutations in the growth hormone receptor and can be treated with IGF-1. This treatment is, however, not fully effective owing to the loss of the direct actions of growth hormone and altered availability of exogenous IGF-1. Excessive activation of the growth hormone receptor by circulating growth hormone results in gigantism and acromegaly, whereas cell transformation and cancer can occur in response to autocrine activation of the receptor. Advances in understanding the mechanism of receptor activation have led to a model in which the growth hormone receptor exists as a constitutive dimer. Binding of the hormone realigns the subunits by rotation and closer apposition, resulting in juxtaposition of the catalytic domains of the associated tyrosine-protein kinase JAK2 below the cell membrane. This change results in activation of JAK2 by transphosphorylation, then phosphorylation of receptor tyrosines in the cytoplasmic domain, which enables binding of adaptor proteins, as well as direct phosphorylation of target proteins. This model is discussed in the light of salient information from closely related class 1 cytokine receptors, such as the erythropoietin, prolactin and thrombopoietin receptors.