Primary therapy for acromegaly with somatostatin analogs and a discussion of novel peptide analogs

Somatostatin analogues in acromegaly and gastroenteropancreatic neuroendocrine tumours: past, present and future

Acromegaly is a hormonal disorder that arises when the pituitary gland secretes excess growth hormone (GH), which in turn stimulates a concomitant increase in serum insulin-like growth factor 1 (IGF-1) levels. Gastroenteropancreatic neuroendocrine tumours (GEP-NET) constitute a heterogeneous group of tumours that can secrete serotonin and a variety of peptide hormones that may cause characteristic symptoms known as carcinoid syndrome or other symptoms and hormonal hypersecretion syndromes depending on the tumour's site of origin. Current medical therapy for the treatment of acromegaly and GEP-NET involves the administration of somatostatin analogues that effectively suppress excess hormone secretion. After its discovery in 1979, octreotide became the first synthetic biologically stable somatostatin analogue with a short-acting formulation of octreotide introduced into clinical practice in the late 1980s. Lanreotide, another somatostatin analogue, became available in the mid-1990s initially as a prolonged-release formulation administered every 10 or 14 days. Long-acting release formulations of both octreotide (Sandostatin LAR and Novartis) and lanreotide (Somatuline Autogel, Ipsen), based on microparticle and nanoparticle drug-delivery technologies, respectively, were later developed, which allowed for once-monthly administration and improved convenience. First-generation somatostatin analogues remain one of the cornerstones of medical therapy in the management of pituitary and GEP-NET hormone hypersecretion, with octreotide having the longest established efficacy and safety profile of the somatostatin analogue class. More recently, pasireotide (Signifor), a next-generation multireceptor-targeted somatostatin analogue, has emerged as an alternative therapeutic option for the treatment of acromegaly. This review summarizes the development and clinical success of somatostatin analogues.

Pasireotide: a novel treatment for patients with acromegaly

Morbidity and mortality rates in patients with active acromegaly are higher than the general population. Adequate biochemical control restores mortality to normal rates. Now, medical therapy has an increasingly important role in the treatment of patients with acromegaly. Somatostatin receptor ligands (SRLs) are considered the standard medical therapy, either after surgery or as a first-line therapy when surgery is deemed ineffective or is contraindicated. Overall, octreotide and lanreotide are first-generation SRLs and are effective in ~20%-70% of patients. Pegvisomant, a growth hormone receptor antagonist, controls insulin-like growth factor 1 in 65%-90% of cases. Consequently, a subset of patients (nonresponders) requires other treatment options. Drug combination therapy offers the potential for more efficacious disease control. However, the development of new medical therapies remains essential. Here, emphasis is placed on new medical therapies to control acromegaly. There is a focus on pasireotide long-acting release (LAR) (Signifor LAR®), which was approved in 2014 by the US Food and Drug Administration and the European Medicine Agency for the treatment of acromegaly. Pasireotide LAR is a long-acting somatostatin multireceptor ligand. In a Phase III clinical trial in patients with acromegaly (naïve to medical therapy or uncontrolled on a maximum dose of first-generation SRLs), 40 and 60 mg of intramuscular pasireotide LAR achieved better biochemical disease control than octreotide LAR, and tumor shrinkage was noted in both pasireotide groups. Pasireotide LAR tolerability was similar to other SRLs, except for a greater frequency and degree of hyperglycemia and diabetes mellitus. Baseline glucose may predict hyperglycemia occurrence after treatment, and careful monitoring of glycemic status and appropriate treatment is required. A precise definition of patients with acromegaly who will derive the greatest therapeutic benefit from pasireotide LAR remains to be established. Lastly, novel therapies and new potential delivery modalities (oral octreotide) are summarized.

A structural and functional acromegaly classification

CONTEXT

GH-secreting pituitary adenomas exhibit heterogeneous natural history ranging from small tumors to large aggressive adenomas.

OBJECTIVE

To rigorously classify an acromegaly patient cohort defined by clinical, radiological, histopathological, and outcome characteristics.

DESIGN

Cross-sectional study.

SETTING

Tertiary referral pituitary center.

PATIENTS

Subjects were selected from a pituitary tumor research registry that includes 1178 patients with pituitary disease. Cluster analysis was performed on 338 acromegaly patients.

INTERVENTIONS

None.

MAIN OUTCOME MEASURES

Biochemically active disease with elevated IGF-1 levels at follow-up.

RESULTS

Cluster analysis of all patients yielded 292 who were rigorously classified to three acromegaly types. Type 1 (50%) comprised older patients with the longest follow-up and most favorable outcomes, characterized by densely granulated, nonaggressive microadenomas and macroadenomas. Type 1 tumors extend to the sphenoid sinus more frequently than suprasellar extension (concave tumor image) and express abundant immunoreactive p21 and somatostatin receptor 2. Type 2 (19%) comprised noninvasive, densely or sparsely granulated macroadenomas, without significant extension (flat tumor image), with intermediate biochemical outcome. Type 3 (31%) was characterized by sparsely granulated aggressive macroadenomas and comprised patients with adverse therapeutic outcomes, despite receiving more treatments. These tumors extend to both the sphenoid sinus and suprasellar regions with commonly encountered optic chiasm compression ("peanut" magnetic resonance image), with low tumor p21 and somatostatin receptor 2 expression.

CONCLUSIONS

After validation, this classification may be useful to accurately identify acromegaly patients with distinctive patterns of disease aggressiveness and outcome, as well as to provide an accurate tool for selection criteria in clinical studies.

Medical therapy of acromegaly: efficacy and safety of somatostatin analogues

Acromegaly is a chronic disease with signs and symptoms due to growth hormone (GH) excess. The most frequent cause of acromegaly is a GH-producing pituitary adenoma. Chronic GH excess is accompanied by long-term complications of the locomotor (arthrosis) and cardiovascular (atherosclerosis, cardiomyopathy) systems and is, when untreated, associated with an increased mortality. The aim of treatment of acromegaly is to improve symptoms, to achieve local tumour mass control, and to decrease morbidity and mortality. Treatment options include surgery, medical therapy and radiotherapy. Transsphenoidal surgery is the first choice of treatment when a definitive cure can be achieved, particularly in the case of microadenomas and when decompression of surrounding structures (optic chiasm, ophthalmic motor nerves) is indicated. Primary medical therapy has been increasingly applied in recent years, especially when a priori chances of surgical cure are low (because of adenoma size and localization) and in patients with advanced age and/or serious co-morbidity. In addition, preoperative primary medical therapy may result in tumour shrinkage, facilitating tumour resection, and may reduce perioperative complications due to GH excess. Within the spectrum of medical therapy, long-acting somatostatin analogues (somatostatins) are considered as first-line treatment. Treatment with somatostatin analogues results in GH control in approximately 60% of patients. In addition, somatostatin analogues induce tumour shrinkage in 30-50% of patients, particularly when applied as primary therapy. Prolonged treatment with somatostatin analogues appears to be safe and is usually well tolerated. The currently available somatostatin analogues, octreotide and lanreotide, seem to be equally effective; however, this should still be evaluated in prospective, randomized trials evaluating efficacy with respect to GH control and tumour shrinkage. In patients with an insufficient clinical and biochemical response to somatostatin analogues, combination therapy with dopamine receptor agonists or the GH receptor antagonist pegvisomant usually leads to disease control. New developments in the medical therapy of acromegaly include the universal somatostatin receptor agonist pasireotide, which has a broader affinity for all somatostatin receptor (sst) subtypes compared with the currently available somatostatin analogues with preferential affinity for the sst2 receptor, and chimeric compounds that interact with both somatostatin and dopamine receptors with synergizing effects on GH secretion.

From somatostatin to octreotide LAR: evolution of a somatostatin analogue

BACKGROUND

Acromegaly is characterized by overproduction of growth hormone (GH) by the pituitary gland. GH stimulates the synthesis of insulin-like growth factor-I (IGF-I), and the somatic growth and metabolic dysfunction that characterize acromegaly are a consequence of elevated GH and IGF-I levels. Gastroenteropancreatic neuroendocrine tumors (GEP-NETs) are rare, slow-growing neoplasms that have usually metastasized by the time of diagnosis. The majority of GEP-NETs are carcinoid tumors whose syndrome is caused by the hypersecretion of biogenic amines, peptides and polypeptides responsible for the principal symptoms of diarrhea and flushing.

METHODS

The MEDLINE and EMBASE databases were searched for preclinical and clinical studies of octreotide (Sandostatin* ), a potent synthetic somatostatin analogue, in patients with acromegaly or GEP-NETs.

OBJECTIVE

This article reviews the 20 years of clinical experience with octreotide and the impact it has made in patients with acromegaly or GEP-NETs.

RESULTS

Octreotide has proven to be an essential component in the management strategy of acromegaly and GEP-NETs over the past 20 years. The multiple beneficial effects of octreotide throughout the body, combined with its established safety profile (the most common adverse effects are injection-site pain and gastrointestinal events), have made it an appealing option for clinicians. The advent of the long-acting release (LAR) formulation of octreotide provided additional benefits to patients through monthly administration, while maintaining the efficacy and tolerability profile of the daily subcutaneous formulation.

CONCLUSIONS

Octreotide is a potent synthetic somatostatin analogue that has become the mainstay of medical therapy for tumor control in neuroendocrine disorders such as acromegaly and GEP-NETs. The development of octreotide LAR offered a further advancement; less frequent dosing provided valuable benefits in quality of life to patients, with equivalent efficacy and tolerability. Moreover, recent results from the PROMID study have confirmed the antiproliferative effect of octreotide LAR in patients with well-differentiated metastatic GEP-NETs of the midgut. New therapeutic uses of octreotide are currently under investigation in a variety of clinical settings.

Somatostatin receptor subtypes mRNA in TSH-secreting pituitary adenomas: a case showing a dramatic reduction in tumor size during short octreotide treatment

TSH-secreting adenoma is a rare pituitary adenoma, and the expression levels of the specific subtypes of somatostatin receptors (sstr) mRNAs have remained obscure. To determine the quantitative expression of the sstr1-5 mRNAs in TSH-secreting adenomas that may be related to the efficacy of treatment with a somatostatin analogue, expression of the sstr1-5 mRNAs was examined and compared in TSH-secreting adenomas and other pituitary adenomas. The pituitary adenomas were obtained at transsphenoidal surgery from 4 cases of TSH-secreting adenoma, including 1 patient showing a significant shrinkage of the tumor size after only 10 days of octreotide treatment, 2 patients without tumor size reduction and 1 patient without treatment, and 5 GH-secreting adenomas, 6 prolactinomas, 5 nonfunctioning adenomas, 4 ACTH-secreting adenomas and normal pituitaries at autopsy from 4 normal subjects. In comparison to the normal pituitary, sstr2A>sstr1>sstr5>sstr3 mRNAs were expressed in the TSH-secreting adenomas examined. No expression of sstr2B or sstr4 mRNA was observed. The expression level of sstr2 mRNA was significantly higher than those in normal pituitary, prolactinomas, ACTH-secreting and nonfunctioning pituitary adenomas. The patient with marked shrinkage of the tumor showed the highest expression of both sstr2 and sstr5 mRNAs among all the cases of pituitary adenoma. A TSH-secreting tumor without shrinkage showed a similar expression level of sstr2 mRNA. These findings demonstrated that TSH-secreting adenomas express sstr1, 2A, 3 and 5 mRNAs, predominantly sstr2A, and in addition to the expression of sstr2 mRNA, the expression level of sstr5 mRNA may be a factor affecting the tumor shrinkage by somatostatin analogues against TSH-secreting adenomas.

Changing paradigms in the surgical and medical management of acromegaly

Acromegaly is a serious, but rare, form of pituitary tumor. This review will discuss recent progress in surgical and medical care, highlight data on choosing the order for primary and secondary therapy and preview emerging drugs. Several controversial areas will be discussed, including the role of preselecting subjects for drug responsiveness, whether or not a brief course of medical therapy before surgery improves outcomes, and trials of combination drug therapy. It is hoped that earlier diagnosis, referral of patients to dedicated pituitary surgeons or neuroendocrinology teams, development of more effective and affordable drugs, and further advances in surgery and radiotherapy will continue to improve the morbidity and mortality from acromegaly.

Treatment of pituitary tumors: surgery

Following a century of technical developments and refinements, a variety of standard operation techniques to date are available for the surgical treatment of pituitary tumors. The vast majority of the lesions can be dealt with satisfactorily utilizing transsphenoidal approaches. The goal of surgical treatment is rapid eradication of the tumor mass, decompression of visual pathways, and elimination of hormonal oversecretion while preserving the normal gland and avoiding potential surgical complications. The tumor's size, extension, and configuration and the magnitude of hormonal oversecretion, are the essential factors that decide whether all the goals can be reached. Another important factor is the individual skill and experience of the surgeon. Still, several lesions that are mainly developed outside of the sella require transcranial approaches, of which the pterional and subfrontal routes are the most widely used. With microsurgical techniques and standard approaches, mortality is far below 1% and morbidity is remarkably low. The most favorable surgical results are obtained with microadenomas, which in the MR image are depicted as distinct low intensity lesions. Only recently has the recovery of pituitary function following surgery been convincingly demonstrated. With the extended transsphenoidal approaches, lesions become accessible that previously have been considered contraindications for transsphenoidal surgery. The introduction of new technical gadgets such as neuronavigation, endoscopy, and intraoperative imaging open new avenues and, even more, widen the spectrum of accessible lesions. Indications for surgery, the preoperative workup, surgical techniques, results, limitations, and new technical developments are briefly reviewed in this article.

The role of somatostatin analogs in Cushing's disease

Somatostatin (SRIF) has been proposed to be of therapeutic interest in the medical treatment of Cushing's disease. While in vitro data demonstrate the presence of SRIF-receptor subtype (sst) expression in corticotroph adenomas, the current clinically available SRIF-analog Octreotide, predominantly targeting sst(2), is ineffective in lowering ACTH levels in Cushing's disease and only appears to inhibit the release of ACTH in Nelson's syndrome. In the present review, current knowledge on the physiological role of SRIF in the regulation of ACTH secretion by the anterior pituitary gland, as well as by corticotroph tumor cells is summarized. In addition, the role of glucocorticoids in regulating sst-mediated inhibition of ACTH release by corticotroph adenoma cells is discussed. Recently, it was reported that the novel multiligand SRIF-analog SOM230 might have the potential to be of therapeutic interest for Cushing's disease. On the basis of the potent suppressive effects on ACTH release by SRIF-analogs with high binding affinity to sst(5) and the observation that sst(5) expression and action is relatively resistant to glucocorticoid treatment, including the recent observation that sst(5) is the predominant sst expressed in human corticotroph adenomas, it is hypothesized that sst(5) may be a new therapeutic target for the control of ACTH and cortisol hypersecretion in patients with pituitary dependent Cushing's disease.