Clinical pharmacodynamic effects of the growth hormone receptor antagonist pegvisomant: implications for cancer therapy

Growth hormone receptor agonists and antagonists: From protein expression and purification to long-acting formulations

Recombinant human growth hormone (rhGH) and GH receptor antagonists (GHAs) are used clinically to treat a range of disorders associated with GH deficiency or hypersecretion, respectively. However, these biotherapeutics can be difficult and expensive to manufacture with multiple challenges from recombinant protein generation through to the development of long-acting formulations required to improve the circulating half-life of the drug. In this review, we summarize methodologies and approaches used for making and purifying recombinant GH and GHA proteins, and strategies to improve pharmacokinetic and pharmacodynamic properties, including PEGylation and fusion proteins. Therapeutics that are in clinical use or are currently under development are also discussed.

Blockade of growth hormone receptor signaling by using pegvisomant: A functional therapeutic strategy in hepatocellular carcinoma

Hepatocellular carcinoma (HCC) is an aggressive neoplasm with poor clinical outcome because most patients present at an advanced stage, at which point curative surgical options, such as tumor excision or liver transplantation, are not feasible. Therefore, the majority of HCC patients require systemic therapy. Nonetheless, the currently approved systemic therapies have limited effects, particularly in patients with advanced and resistant disease. Hence, there is a critical need to identify new molecular targets and effective systemic therapies to improve HCC outcome. The liver is a major target of the growth hormone receptor (GHR) signaling, and accumulating evidence suggests that GHR signaling plays an important role in HCC pathogenesis. We tested the hypothesis that GHR could represent a potential therapeutic target in this aggressive neoplasm. We measured GH levels in 767 HCC patients and 200 healthy controls, and then carried out clinicopathological correlation analyses. Moreover, specific inhibition of GHR was performed in vitro using siRNA and pegvisomant (a small peptide that blocks GHR signaling and is currently approved by the FDA to treat acromegaly) and in vivo, also using pegvisomant. GH was significantly elevated in 49.5% of HCC patients, and these patients had a more aggressive disease and poorer clinical outcome (P<0.0001). Blockade of GHR signaling with siRNA or pegvisomant induced substantial inhibitory cellular effects in vitro. In addition, pegvisomant potentiated the effects of sorafenib (P<0.01) and overcame sorafenib resistance (P<0.0001) in vivo. Mechanistically, pegvisomant decreased the phosphorylation of GHR downstream survival proteins including JAK2, STAT3, STAT5, IRS-1, AKT, ERK, and IGF-IR. In two patients with advanced-stage HCC and high GH who developed sorafenib resistance, pegvisomant caused tumor stability. Our data show that GHR signaling represents a novel “druggable” target, and pegvisomant may function as an effective systemic therapy in HCC. Our findings could also lead to testing GHR inhibition in other aggressive cancers.

Genetic Code Expansion Enables Site-Specific PEGylation of a Human Growth Hormone Receptor Antagonist through Click Chemistry

Regulation of human growth hormone (GH) signaling has important applications in the remediation of several diseases including acromegaly and cancer. Growth hormone receptor (GHR) antagonists currently provide the most effective means for suppression of GH signaling. However, these small 22 kDa recombinantly engineered GH analogues exhibit short plasma circulation times. To improve clinical viability, between four and six molecules of 5 kDa poly(ethylene glycol) (PEG) are nonspecifically conjugated to the nine amines of the GHR antagonist designated as B2036 in the FDA-approved therapeutic pegvisomant. PEGylation increases the molecular weight of B2036 and considerably extends its circulation time, but also dramatically reduces its bioactivity, contributing to high dosing requirements and increased cost. As an alternative to nonspecific PEGylation, we report the use of genetic code expansion technology to site-specifically incorporate the unnatural amino acid propargyl tyrosine (pglY) into B2036 with the goal of producing site-specific protein-polymer conjugates. Substitution of tyrosine 35 with pglY yielded a B2036 variant containing an alkyne functional group without compromising bioactivity, as verified by a cellular assay. Subsequent conjugation of 5, 10, and 20 kDa azide-containing PEGs via the copper-catalyzed click reaction yielded high purity, site-specific conjugates with >89% conjugation efficiencies. Site-specific attachment of PEG to B2036 is associated with substantially improved in vitro bioactivity values compared to pegvisomant, with an inverse relationship between polymer size and activity observed. Notably, the B2036-20 kDa PEG conjugate has a molecular weight comparable to pegvisomant, while exhibiting a 12.5 fold improvement in half-maximal inhibitory concentration in GHR-expressing Ba/F3 cells (103.3 nM vs 1289 nM). We expect that this straightforward route to achieve site-specific GHR antagonists will be useful for GH signal regulation.

Endocytosis and Degradation of Pegvisomant and a Potential New Mechanism That Inhibits the Nuclear Translocation of GHR

CONTEXT

Pegvisomant, a growth hormone receptor (GHR) antagonist, is a well-known drug that was designed to treat acromegaly. However, recent studies have indicated that the GHR is a "moonlighting" protein that may exhibit dual functions based on its localization in the plasma membrane and nucleus. In light of this finding, we explored whether pegvisomant is a potential "moonlighting" GHR antagonist. In addition, the mechanisms of the endocytosis, postendocytic sorting, and degradation of pegvisomant are not fully understood.

OBJECTIVE

This study investigated whether pegvisomant is a "moonlighting" antagonist and explored the mechanisms of the endocytosis, postendocytic sorting, and degradation of pegvisomant.

METHODS

Indirect immunofluorescence and Western blot coupled with pharmacological inhibitors and gene silencing (small interfering RNA) were used to explore the mechanisms of the endocytosis, postendocytic sorting, and degradation of pegvisomant. Western blot, immunohistochemistry, and indirect immunofluorescence coupled with subcellular fractionation analysis were used to determine the effect of pegvisomant on GHR's nuclear localization in vitro and in vivo.

RESULTS

Here, we show that the endocytosis of pegvisomant is mainly mediated though the clathrin pathway. Further study of the postendocytic sorting of pegvisomant shows that pegvisomant enters into different types of endosomes under GHR mediation. In addition, GHR is slightly downregulated by pegvisomant; further study indicates that proteasomes and lysosomes may cooperate to regulate pegvisomant/GHR degradation. Most importantly, we show that pegvisomant inhibits the nuclear localization of GHR.

CONCLUSION

Our study showed that pegvisomant is a "moonlighting" antagonist. In addition, we revealed the mechanisms of the endocytosis, postendocytic sorting, and degradation of pegvisomant.

Anti-insulin-like growth factor therapy in breast cancer

Early preclinical and population data suggested a role for the type I insulin-like growth factor receptor (IGF1R) in the regulation of breast cancer growth and survival. To target this pathway, multiple monoclonal antibodies and tyrosine kinase inhibitors were developed and tested in clinical trials. While some of the early clinical trials suggested a benefit for these drugs, none of the attempts showed improved outcomes when compared to conventional therapy. This failure of the IGF1R inhibitors was pronounced in breast cancer; multiple trials testing IGF1R inhibition in estrogen receptor-positive breast cancer were conducted, none showed benefit. This review will evaluate the rationale for IGF1R inhibition, discuss results of the clinical trials and suggest a path forward.

MECHANISMS IN ENDOCRINOLOGY: Lessons from growth hormone receptor gene-disrupted mice: are there benefits of endocrine defects?

Growth hormone (GH) is produced primarily by anterior pituitary somatotroph cells. Numerous acute human (h) GH treatment and long-term follow-up studies and extensive use of animal models of GH action have shaped the body of GH research over the past 70 years. Work on the GH receptor (R)-knockout (GHRKO) mice and results of studies on GH-resistant Laron Syndrome (LS) patients have helped define many physiological actions of GH including those dealing with metabolism, obesity, cancer, diabetes, cognition and aging/longevity. In this review, we have discussed several issues dealing with these biological effects of GH and attempt to answer the question of whether decreased GH action may be beneficial.

Canine mammary tumors as a model for human disease

Animal models for examining human breast cancer (HBC) carcinogenesis have been extensively studied and proposed. With the recent advent of immunotherapy, significant attention has been focused on the dog as a model for human cancer. Dogs develop mammary tumors and other cancer types spontaneously with an intact immune system, which exhibit a number of clinical and molecular similarities to HBC. In addition to the spontaneous tumor presentation, the clinical similarities between human and canine mammary tumors (CMT) include the age at onset, hormonal etiology and course of the diseases. Furthermore, factors that affect the disease outcome, including tumor size, stage and lymph node invasion, are similar in HBC and CMT. Similarly, the molecular characteristics of steroid receptor, epidermal growth factor, proliferation marker, metalloproteinase and cyclooxygenase expression, and the mutation of the p53 tumor suppressor gene in CMT, mimic HBC. Furthermore, ductal carcinomas in situ in human and canine mammary glands are particularly similar in their pathological, molecular and visual characteristics. These CMT characteristics and their similarities to HBC indicate that the dog could be an excellent model for the study of human disease. These similarities are discussed in detail in the present review, and are compared with the in vitro and other in vivo animal models available.

Pegvisomant in acromegaly: an update

BACKGROUND

In 2007, we published an opinion document to review the role of pegvisomant (PEG) in the treatment of acromegaly. Since then, new evidence emerged on the biochemical and clinical effects of PEG and on its long-term efficacy and safety.

AIM

We here reviewed the emerging aspects of the use of PEG in clinical practice in the light of the most recent literature.

RESULTS

The clinical use of PEG is still suboptimal, considering that it remains the most powerful tool to control IGF-I in acromegaly allowing to obtain, with a pharmacological treatment, the most important clinical effects in terms of signs and symptoms, quality of life and comorbidities. The number of patients with acromegaly exposed to PEG worldwide has become quite elevated and the prolonged follow-up allows now to deal quite satisfactorily with many clinical issues including major safety issues, such as the concerns about possible tumour (re)growth under PEG. The positive or neutral impact of PEG on glucose metabolism has been highlighted, and the clinical experience, although limited, with sleep apnoea and pregnancy has been reviewed. Finally, the current concept of somatostatin receptor ligands (SRL) resistance has been addressed, in order to better define the acromegaly patients to whom the PEG option may be offered.

CONCLUSIONS

PEG increasingly appears to be an effective and safe medical option for many patients not controlled by SRL but its use still needs to be optimized.

Role of Growth Hormone in Breast Cancer

Breast cancer is one of the most common cancers diagnosed in women. Approximately two-thirds of all breast cancers diagnosed are classified as hormone dependent, which indicates that hormones are the key factors that drive the growth of these breast cancers. Ovarian and pituitary hormones play a major role in the growth and development of normal mammary glands and breast cancer. In particular, the effect of the ovarian hormone estrogen has received much attention in regard to breast cancer. Pituitary hormones prolactin and growth hormone have also been associated with breast cancer. Although the role of these pituitary hormones in breast cancers has been studied, it has not been investigated extensively. In this review, we attempt to compile basic information from most of the currently available literature to understand and demonstrate the significance of growth hormone in breast cancer. Based on the available literature, it is clear that growth hormone plays a significant role in the development, progression, and metastasis of breast cancer by influencing tumor angiogenesis, stemness, and chemoresistance.

Pegvisomant: a growth hormone receptor antagonist used in the treatment of acromegaly

PURPOSE

To review published data on pegvisomant and its therapeutic role in acromegaly.

METHODS

Electronic searches of the published literature were conducted using the keywords: acromegaly, growth hormone (GH) receptor (antagonist), pegvisomant, therapy. Relevant articles (n = 141) were retrieved and considered for inclusion in this manuscript.

RESULTS

Pegvisomant is a genetically engineered, recombinant growth hormone receptor antagonist, which is effective in normalizing serum insulin-like growth factor 1 (IGF-1) levels in the majority of patients with acromegaly and ameliorating symptoms and signs associated with GH excess. Pegvisomant does not have direct antiproliferative effects on the underlying somatotroph pituitary adenoma, which is the etiology of GH excess in the vast majority of patients with acromegaly. Therefore, patients receiving pegvisomant monotherapy require regular pituitary imaging in order to monitor for possible increase in tumor size. Adverse events in patients on pegvisomant therapy include skin rashes, lipohypertrophy at injection sites, and idiosyncratic liver toxicity (generally asymptomatic transaminitis that is reversible upon drug discontinuation), thus necessitating regular patient monitoring.

CONCLUSIONS

Pegvisomant is an effective therapeutic agent in patients with acromegaly who are not in remission after undergoing pituitary surgery. It mitigates excess GH action, as demonstrated by IGF-1 normalization, but has no direct effects on pituitary tumors causing acromegaly. Regular surveillance for possible tumor growth and adverse effects (hepatotoxicity, skin manifestations) is warranted.

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.

The insulin-like growth factor-I receptor (IGF-IR) in breast cancer: biology and treatment strategies

Breast cancer is the most common cancer and the second leading cause of cancer-related deaths among women worldwide. Although patients are often diagnosed in the early and curable stages, the treatment of metastatic breast cancer remains a major clinical challenge. The combination of chemotherapy with new targeting agents, such as bevacizumab, is helpful in improving patient survival; however, novel treatment strategies are required to improve clinical outcomes. The insulin-like growth factor-I receptor (IGF-IR) is a tyrosine kinase cell surface receptor which is involved in the regulation of cell growth and metabolism. Previous studies have shown that activation of the IGF-IR signaling pathway promotes proliferation, survival, and metastasis of breast cancer cells. Additionally, overexpression of IGF-IR is associated with breast cancer cell resistance to anticancer therapies. Recently, IGF-IR has been introduced as a marker of stemness in breast cancer cells and there is also accumulating evidence that IGF-IR contributes to the establishment and maintenance of breast cancer epithelial-mesenchymal transition (EMT). Therefore, pharmacological or molecular targeting of IGF-IR could be a promising strategy, in the treatment of patients with breast cancer, particularly in order to circumvent the therapeutic resistance and targeting breast cancer stem/progenitors. Currently, many strategies have been developed for targeting IGF-IR, some have entered clinical trials and some are in preclinical stages for breast cancer therapy. In this review, we will first discuss on the biology of IGF-IR in an attempt to find the role of this receptor in breast cancer and then discuss about therapeutic strategies to target this receptor.

PEGylation and its impact on the design of new protein-based medicines

PEGylation is the covalent conjugation of PEG to therapeutic molecules. Protein PEGylation is a clinically proven approach for extending the circulation half-life and reducing the immunogenicity of protein therapeutics. Most clinically used PEGylated proteins are heterogeneous mixtures of PEG positional isomers conjugated to different residues on the protein main chain. Current research is focused to reduce product heterogeneity and to preserve bioactivity. Recent advances and possible future directions in PEGylation are described in this review. So far protein PEGylation has yielded more than 10 marketed products and in view of the lack of equally successful alternatives to extend the circulation half-life of proteins, PEGylation will still play a major role in drug delivery for many years to come.

A male patient with acromegaly and breast cancer: treating acromegaly to control tumor progression

Background

Acromegaly is a rare disease associated with an increased risk of developing cancer.

Case presentation

We report the case of a 72-year-old man who was diagnosed with acromegaly (IGF-1 770 ng/ml) and breast cancer. Four years before he suffered from a colon-rectal cancer. Pituitary surgery and octreotide-LAR treatment failed to control acromegaly. Normalization of IGF-1 (97 ng/ml) was obtained with pegvisomant therapy. Four years after breast cancer surgery, 2 pulmonary metastases were detected at chest CT. The patient was started on anastrozole, but, contrary to medical advice, he stopped pegvisomant treatment (IGF-I 453 ng/ml). Four months later, chest CT revealed an increase in size of the metastatic lesion of the left lung. The patient was shifted from anastrozole to tamoxifen and was restarted on pegvisomant, with normalization of serum IGF-1 levels (90 ng/ml). Four months later, a reduction in size of the metastatic lesion of the left lung was detected by CT. Subsequent CT scans throughout a 24-month follow-up showed a further reduction in size and then a stabilization of the metastasis.

Conclusions

This is the first report of a male patient with acromegaly and breast cancer. The clinical course of breast cancer was closely related to the metabolic control of acromegaly. The rapid progression of metastatic lesion was temporally related to stopping pegvisomant treatment and paralleled a rise in serum IGF-1 levels. Normalization of IGF-1 after re-starting pegvisomant impressively reduced the progression of metastatic breast lesions. Control of acromegaly is mandatory in acromegalic patients with cancer.

Development of a growth-hormone-conjugated nanodiamond complex for cancer therapy

It is highly desirable to develop a therapeutic, observable nanoparticle complex for specific targeting in cancer therapy. Growth hormone (GH) and its antagonists have been explored as cancer cell-targeting molecules for both imaging and therapeutic applications. In this study, a low toxicity, biocompatible, therapeutic, and observable GH-nanoparticle complex for specifically targeting growth hormone receptor (GHR) in cancer cells was synthesized by conjugating GH with green fluorescence protein and carboxylated nanodiamond. Moreover, we have shown that this complex can be triggered by laser irradiation to create a "nanoblast" and induce cell death in the A549 non-small-cell lung cancer cell line via the apoptotic pathway. This laser-mediated, cancer-targeting platform can be widely used in cancer therapy.

Circulating levels of pegvisomant and endogenous growth hormone during prolonged pegvisomant therapy in patients with acromegaly

OBJECTIVE

To investigate whether pegvisomant treatment in acromegaly induces gradual elevations in endogenous serum growth hormone (GH) levels and whether serum pegvisomant levels predict the therapeutic outcome.

PATIENTS AND METHODS

Seventeen patients (6 women), mean age 46·3 years (range: 23·2-76·2), were studied. For each patient, four hospital visits were identified including 'active disease' (no treatment) and last follow-up. At each visit, 12 blood samples were drawn during 3 h including an oral glucose tolerance test (OGTT). Eight patients received a somatostatin analogue in addition to pegvisomant on the last visit.

RESULTS

Median (range) pegvisomant doses (mg/day) were 10 (10-10), 15 (10-15) and 15 (10-15) at visits 2, 3 and 4, respectively, and the mean duration of pegvisomant treatment was 17·5 ± 3·2 (SEM) months. Serum IGF-I changed significantly during the treatment period with the highest level at baseline and lowest levels at visits 3 and 4. GH levels increased in a dose-dependent manner during pegvisomant treatment and decreased at visit 4. Changes in IGF-I levels correlated negatively with changes in serum pegvisomant levels between visits. Serum pegvisomant at each visit correlated with baseline growth hormone levels, whereas no associations between serum pegvisomant and either dose, gender, age or body weight were found.

CONCLUSIONS

(1) Serum GH levels increased initially, but remained stable during prolonged pegvisomant treatment in patients with acromegaly, (2) serum pegvisomant levels predicted the reduction in serum IGF-I during treatment and (3) the interindividual variation in serum pegvisomant levels seems not predicted by either age, gender or body composition.

siRNA-targeted inhibition of growth hormone receptor in human colon cancer SW480 cells

AIM: To determine the effects of RNAi-mediated inhibition of the growth hormone receptor (GHR) gene on tumors and colon cancer cells in vivo.

METHODS: Construction of a eukaryotic vector for human GHR expression, the pcDNA™6.2-GW/EmGFP-small interfering RNAs (siRNAs)-GHR plasmid, was used to inhibit GHR expression. Thirty-six BALB/c nude mice were randomly divided into groups and treated with normal saline (NS), recombinant plasmid (G₂), growth hormone (GH), 5-fluorouracil (FU), G₂+FU or G₂+FU+GH. Each nude mouse was subcutaneously inoculated with 1×10⁷ human colon cancer SW480 cells; the nude mice were weighed before inoculation and on the 2^nd, 5^th, 8^th, 11^th, 14^th and 17^th day after inoculation. All nude mice were sacrificed after 17 d. Each subcutaneous tumor was removed and studied. Tumor volume was measured on the 5^th, 8^th, 11^th, 14^th and 17^th day after inoculation. The expression of GHR protein in the tumor tissue was detected by Western blotting analysis, and the differences in GHR mRNA expression in the tumor tissue were detected by real-time quantitative reverse transcription-polymerase chain reaction.

RESULTS: Compared to the control group, the weights of the inoculated nude mice on the 17^th day after inoculation were: G₂: 21.60 ± 0.71 g, GH: 21.64 ± 0.45 g, FU: 18.94 ± 0.47 g, FU+G₂: 19.40 ± 0.60 g, G₂+FU+GH: 21.04 ± 0.78 g vs NS: 20.68 ± 0.66 g, P < 0.05; the tumor volumes after the subcutaneous inoculation were: G₂: 9.71 ± 3.82 mm³, FU: 11.54 ± 2.42 mm³, FU+G₂: 11.42 ± 1.11 mm³, G₂+FU+GH: 10.47 ± 1.02 mm³vs NS: 116.81 ± 10.61 mm³, P < 0.05. Compared to the GH group, the tumor volumes were significantly decreased in the experimental groups. The GHR protein expression (G₂: 0.39 ± 0.02, FU: 0.40 ± 0.02, FU+G₂: 0.38 ± 0.01, G₂+FU+GH: 0.39 ± 0.01 vs NS: 0.94 ± 0.02, P < 0.05) and the GHR mRNA expression (G₂: 14.12 ± 0.10, FU: 15.15 ± 0.44, FU+G₂: 16.46 ± 0.27, G₂+FU+GH: 15.37 ± 0.57 vs NS: 12.63 ± 0.14, P < 0.05) were significantly decreased and increased, respectively, in the experimental groups.

CONCLUSION: Inhibition of GHR in human colon cancer SW480 cells resulted in anti-tumor effects in nude mice.

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.

Pegvisomant bioavailability of single 30 mg/mL subcutaneous injection compared to two 15 mg/mL subcutaneous injections: a pharmacokinetic, safety and tolerability study

OBJECTIVE

The study was conducted to evaluate the pharmacokinetics (PK), relative bioavailability (relBA), safety and tolerability of two single-dose pegvisomant subcutaneous (SC) administrations: one injection of 30 mg/mL (1 × 30 mg/mL) versus two injections of two 15 mg/mL (2 × 15 mg/mL).

DESIGN

This was a 2-period, single-dose, crossover study in 14 healthy male and female subjects. All subjects received both administrations during the two treatment periods separated by a two-week washout. Serum samples were collected intensively up to 360 h post injection and were assayed by a validated enzyme linked immunosorbent assay (ELISA) for pegvisomant. PK parameters including AUC and Cmax were derived by noncompartmental analyses. Mixed effects model was used to obtain bioavailability estimates. Safety and tolerability were assessed by clinical monitoring, including adverse events, laboratory assessments and injection site reactions.

RESULTS

All subjects completed the study. The relBA of 1 × 30 mg/mL relative to 2 × 15 mg/mL was 123.89% with a 90% CI (112.91-135.93%). Adjusted for the difference in actual pegvisomant amounts in both formulations the dose-adjusted relBA reduced to 112.97% with a 90% CI (103.09-123.80%). Single injection with a higher drug concentration in injection solution might have a role in this 13% higher bioavailability for 1 × 30 mg/mL administration. Other PK parameters for the two administrations were comparable. No laboratory abnormalities, vital signs, ECG, or injection site reactions of clinical concern were observed in either treatment.

CONCLUSIONS

Comparable BA, safety and tolerability of the new 30 mg/mL strength to the currently marketed 15 mg/mL strength were established in this study.

Targeting insulin and insulin-like growth factor signaling in breast cancer

The insulin and insulin like growth factor (IGF) signaling systems are implicated in breast cancer biology. Thus, disrupting IGF/insulin signaling has been shown to have promise in a number of preclinical models. However, human clinical trials have been less promising. Despite evidence of some activity in early phase trials, randomized phase III studies have thus far been unable to show a benefit of blocking IGF signaling in combination with conventional strategies. In breast cancer, combination anti IGF/insulin signaling agents with hormone therapy has not yet proven to have benefit. This inability to translate the preclinical findings into useful clinical strategies calls attention to the need for a deeper understanding of this complex pathway. Development of predictive biomarkers and optimal inhibitory strategies of the IGF/insulin system should yield better clinical strategies. Furthermore, unraveling the interaction between the IGF/insulin pathway and other critical signaling pathways in breast cancer biology, namely estrogen receptor-α (ERα) and epidermal growth factor receptor (EGFR) pathways, provides additional new concepts in designing combination therapies. In this review, we will briefly summarize the current strategies targeting the IGF/insulin system, discuss the possible reasons of success or failure of the existing therapies, and provide potential future directions for research and clinical trials.

Autocrine human GH promotes radioresistance in mammary and endometrial carcinoma cells

Although recent advances in breast cancer treatment regimes have improved patient prognosis, resistance to breast cancer therapies, such as radiotherapy, is still a major clinical challenge. In the current study, we have investigated the role of autocrine human GH (hGH) in resistance to ionising radiation (IR)-based therapy. Cell viability and total cell number assays demonstrated that autocrine hGH promoted cell regrowth in the mammary carcinoma cell lines, MDA-MB-435S and T47D, and the endometrial carcinoma cell line, RL95-2, following treatment with IR. In addition, autocrine hGH enhanced MDA-MB-435S and T47D cell clonogenic survival following radiation exposure. The enhanced clonogenic survival afforded by autocrine hGH was mediated by JAK2 and Src kinases. Investigation into the DNA repair capacity demonstrated that autocrine hGH reduced IR-induced DNA damage in MDA-MB-435S and T47D cells. Functional antagonism of hGH increased RL95-2 sensitivity to IR in cell viability and total cell number assays, reduced clonogenic survival and enhanced the induction of DNA damage. Thus, autocrine hGH reduced sensitivity to treatment with IR in mammary and endometrial carcinoma cell lines in vitro, while functional antagonism of hGH sensitised endometrial carcinoma cells to IR. Functional antagonism of hGH, used in conjunction with radiotherapy, may therefore enhance treatment efficacy and improve the prognosis of patients with breast and endometrial cancer.

GH receptor blocker administration and muscle-tendon collagen synthesis in humans

CONTEXT

The growth hormone (GH)/insulin-like growth factor-I (IGF-I) axis stimulates collagen synthesis in tendon and skeletal muscle, but no studies have investigated the effect of reducing IGF-I on collagen synthesis in healthy humans.

OBJECTIVE

We hypothesised, that a GH blockade would decrease IGF-I and collagen synthesis in the connective tissue of skeletal muscle and tendon.

DESIGN

The study was randomised and double blinded.

PARTICIPANTS

20 healthy young males completed the study.

INTERVENTION

The participants were randomised to 2 weeks of GH receptor blocker supplementation (pegvisomant, 5 mg/day, n=9) or placebo (n=11).

MAIN OUTCOME MEASURES

Serum levels of GH, IGF-I and IGF-binding protein 3 (IGFBP-3) were measured before and after pegvisomant/placebo supplementation. Fractional synthesis rates (FSR) for collagen and myofibrillar protein were determined with stable isotopes in tendon and muscle, and mRNA for collagen (COL1A1 and COL3A1) as well as IGF-I isoforms (Ea and Ec) were measured in skeletal muscle.

RESULTS

Pegvisomant decreased serum IGF-I by 20% (p<0.01) and serum IGFBP-3 by 10% (p<0.05). Pegvisomant supplementation had no effect on collagen synthesis in tendon and skeletal muscle, nor was muscle myofibrillar protein synthesis affected. Similarly, pegvisomant supplementation had no effect on mRNA expression of IGF-I and collagen in skeletal muscle.

CONCLUSION

GH receptor blocker administration in healthy humans resulted in a moderate decrease in serum IGF-I. Collagen synthesis in tendon and skeletal muscle, as well as skeletal muscle IGF-I and collagen mRNA expression, was unaffected by GH receptor blocker supplementation.

[Growth hormone receptor antagonist in the treatment of acromegaly]

Exploration of construction, function and interaction of human growth hormone and growth hormone receptor in details resulted in the innovation of the new growth hormone receptor antagonist, pegvisomant. Pegvisomant with different mechanism of action extended the tools of medical management of acromegaly. Importance of the novel treatment modality is high. In one hand the necessity of the strict control of growth hormone/insulin-like growth factor-I axis has been proven regarding the mortality of the disease. On the other hand, despite the use of all current modes of treatment (surgery, radiotherapy, dopamine agonists, somatostatin analogs), a significant cohort of patients with acromegaly remains inadequately controlled. Pegvisomant has been registered in 2004. Since 2006, it has been used in Hungary for the treatment of acromegaly in patients who have had an inadequate response to surgery and/or radiation therapy and/or other medical therapies, or for whom these therapies are not appropriate. Clinical use of pegvisomant in the treatment of acromegaly is effective, well tolerated, and safe, based on international Acrostudy database. In order to improve the efficacy of therapy clinical trials started with pegvisomant and somatostatin analog combination treatment. Evidence of several further effects of the growth hormone/insulin-like growth factor-I axis suggests other potential uses of growth hormone receptor antagonists.

Targeting the insulin-like growth factor pathway in rhabdomyosarcomas: rationale and future perspectives

Rhabdomyosarcomas (RMS) are a heterogeneous group of tumors that share features of skeletal myogenesis and represent the most common pediatric soft tissue sarcoma. Even though significant advances have been achieved in RMS treatment, prognosis remains very poor for many patients. Several elements of the Insulin-like Growth Factor (IGF) pathway are involved in sarcomas, including RMS. The IGF2 ligand is highly expressed in most, if not all, RMS, and frequent overexpression of the receptor IGF1R is also found. This is confirmed here through mining expression profiling data of a large series of RMS samples. IGF signaling is implicated in the genesis, growth, proliferation, and metastasis of RMS. Blockade of this pathway is therefore a potential therapeutic strategy for the treatment of RMS. In this paper we examine the biological rationale for targeting the IGF pathway in RMS as well as the current associated preclinical and clinical experience.

Insulin-like growth factor receptor type I as a target for cancer therapy

In recent years, improvements in the understanding of oncogenesis has permitted the identification of new molecular targets for cancer therapy. Among all the different approaches, inhibition of tyrosine kinase receptor activity using small molecules or biomolecules for controlling cancer growth has been successful and has brought new therapeutic opportunities to the medical community. After more than 20 years of extensive work, insulin-like growth factor receptor I (IGF-IR) is becoming an attractive target for drug development. Owing to its close homology to insulin receptor, IGF-IR is of interest for antibody design while its specificity allows us to discriminate between the two receptors. Major efforts from a large number of pharmaceutical companies are invested in evaluating the efficacy of such molecules in humans. Discovery of biomarkers associated with efficacy and patient selection are the main challenges that we will have to deal with in order to target the appropriate patient population that will most benefit from anti-IGF-IR monoclonal antibodies and combined treatments. This review will provide an overview of the current knowledge on IGF-IR and ongoing clinical trials.

Immunogenicity, toxicology, pharmacokinetics and pharmacodynamics of growth hormone ligand-receptor fusions

A fundamental concern for all new biological therapeutics is the possibility of inducing an immune response. We have recently demonstrated that an LR-fusion (ligand-receptor fusion) of growth hormone generates a potent long-acting agonist; however, the immunogenicity and toxicity of these molecules have not been tested. To address these issues, we have designed molecules with low potential as immunogens and undertaken immunogenicity and toxicology studies in Macaca fascicularis and pharmacokinetic and pharmacodynamic studies in rats. Two variants of the LR-fusion, one with a flexible linker (GH-LRv2) and the other without (GH-LRv3), were tested. Comparison was made with native human GH (growth hormone). GH-LRv2 and GH-LRv3 demonstrated similar pharmacokinetics in rats, showing reduced clearance compared with native GH and potent agonist activity with respect to body weight gain in a hypophysectomized rat model. In M. fascicularis, a low level of antibodies to GH-LRv2 was found in one sample, but there was no other evidence of any immunogenic response to the other fusion protein. There were no toxic effects and specifically no changes in histology at injection sites after two repeated administrations. The pharmacokinetic profiles in monkeys confirmed long half-lives for both GH-LRv2 and GH-LRv3 representing exceptionally delayed clearance over rhGH (recombinant human GH). The results suggest that repeated administration of a GH LR-fusion is safe, non-toxic, and the pharmacokinetic profile suggests that two to three weekly administrations is a potential therapeutic regimen for humans.

A pegylated growth hormone receptor antagonist, pegvisomant, does not enter the brain in humans

BACKGROUND

GH receptors exist in the hippocampus, cerebral cortex, and hypothalamus, possibly influencing mood, cortical blood flow, and neuronal growth and mediating negative feedback.

RATIONALE

Pegvisomant is a recombinant mutated GH molecule with high affinity, but little or no activating capability, for the GH receptor. It is used clinically as a GH antagonist.

HYPOTHESIS

Systemic pegvisomant enters brain interstitial fluid via putative choroid-plexus GH receptors, thereby allowing for antagonism of central actions of GH. SUBJECTS AND LOCATION: Six adults requiring a cerebrospinal fluid (CSF) examination for nonneoplastic and noninflammatory syndromes participated at a tertiary medical center.

METHODS

Direct assays were conducted of serum and CSF pegvisomant concentrations 18-24 h after sc injection of pegvisomant (20 mg).

OUTCOMES

Median (range) concentrations of pegvisomant in serum were 215 (74-539) microg/liter and in CSF 0.035 (0.010-0.28) microg/liter (P=0.016). CSF drug levels were indistinguishable from assay threshold. Corresponding GH values were 0.29 (0.010-1.3) in serum and 0.075 microg/liter (0.01-0.13) in CSF. The geometric mean ratios of serum/CSF pegvisomant and GH concentrations were 5116:1 and 3.5:1, respectively, thus defining a more than 1400-fold difference between mutated and natural GH.

CONCLUSIONS

Based upon CSF measurements, a pegylated GH-receptor antagonist does not cross the human blood-brain barrier, thereby sparing inhibition of central nervous system GH actions. Thus, the capability of this antagonist to stimulate GH secretion predominantly reflects its actions outside the blood-brain barrier, such as via the median eminence and/or via suppression of IGF-I concentrations.

Synthesis of aryl-heteroaryl ureas (AHUs) based on 4-aminoquinoline and their evaluation against the insulin-like growth factor receptor (IGF-1R)

The insulin-like growth factor receptor (IGF-1R) is a receptor tyrosine kinase (RTK) involved in all stages of the development and propagation of breast and other cancers. The inhibition of IGF-1R by small molecules remains a promising strategy to treat cancer. Herein, we explore SAR around previously characterized lead compound (1), which is an aryl-heteroaryl urea (AHU) consisting of 4-aminoquinaldine and a substituted aromatic ring system. A library of novel AHU compounds was prepared based on derivatives of the 4-aminoquinoline heterocycle (including various 2-substituted derivatives, and naphthyridines). The compounds were screened for in vitro inhibitory activity against IGF-1R, and several compounds with improved activity (3-5 microM) were identified. Furthermore, a computational docking study was performed, which identifies a fairly consistent lowest energy mode of binding for the more-active set of inhibitors in this series, while the less-active inhibitors do not adopt a consistent mode of binding.

A human monoclonal antibody against insulin-like growth factor-II blocks the growth of human hepatocellular carcinoma cell lines in vitro and in vivo

Elevated expression of insulin-like growth factor-II (IGF-II) is frequently observed in a variety of human malignancies, including breast, colon, and liver cancer. As IGF-II can deliver a mitogenic signal through both IGF-IR and an alternately spliced form of the insulin receptor (IR-A), neutralizing the biological activity of this growth factor directly is a potential alternative option to IGF-IR-directed agents. Using a Fab-displaying phage library and a biotinylated precursor form of IGF-II (1-104 amino acids) as a target, we isolated Fabs specific for the E-domain COOH-terminal extension form of IGF-II and for mature IGF-II. One of these Fabs that bound to both forms of IGF-II was reformatted into a full-length IgG, expressed, purified, and subjected to further analysis. This antibody (DX-2647) displayed a very high affinity for IGF-II/IGF-IIE (K(D) value of 49 and 10 pmol/L, respectively) compared with IGF-I (approximately 10 nmol/L) and blocked binding of IGF-II to IGF-IR, IR-A, a panel of insulin-like growth factor-binding proteins, and the mannose-6-phosphate receptor. A crystal complex of the parental Fab of DX-2647 bound to IGF-II was resolved to 2.2 A. DX-2647 inhibited IGF-II and, to a lesser extent, IGF-I-induced receptor tyrosine phosphorylation, cellular proliferation, and both anchorage-dependent and anchorage-independent colony formation in various cell lines. In addition, DX-2647 slowed tumor progression in the Hep3B xenograft model, causing decreased tumoral CD31 staining as well as reduced IGF-IIE and IGF-IR phosphorylation levels. Therefore, DX-2647 offers an alternative approach to targeting IGF-IR, blocking IGF-II signaling through both IGF-IR and IR-A.

Pegvisomant: current and potential novel therapeutic applications

BACKGROUND

Pegvisomant is a genetically engineered molecule, which exhibits specific growth hormone (GH) antagonism by directly interacting with the GH receptor. It is currently licensed for the treatment of acromegaly where surgery and medical therapy with somatostatin analogues have failed.

OBJECTIVE

To delineate the role of pegvisomant in the treatment of acromegaly and its novel application in other areas of clinical medicine where suppression of GH action may be of therapeutic benefit.

METHODS

A literature review from PubMed- and EMBASE-listed publications and the web-sites of licensing organisations for medicinal products.

CONCLUSION

Pegvisomant is currently used as a second line therapy in the management of acromegaly. It is highly effective in suppressing the metabolic effects of elevated GH levels when used alone or in combination with somatostatin analogues. However, its long term efficacy and safety for this indication has yet to be established. Preliminary data indicate that pegvisomant may have a role in management of type 1 diabetes with beneficial effects on insulin sensitivity and in preventing the progression of microvascular complications. Additional roles as an adjunct to cancer chemotherapy regimens and for the diagnosis of GH deficiency have been proposed, but have yet to be confirmed.

Targeting insulin-like growth factor type 1 receptor in cancer therapy

It is believed that the insulin-like growth factor receptor type 1 (IGF-1R) signaling pathway plays a pivotal role in cancer growth, progression, and resistance to anticancer therapies. Strategies are being developed to block IGF-1R as an anticancer treatment. We reviewed several potential strategies for disrupting the IGF axis. We also reviewed the effects of two drugs that target the IGF-1R: monoclonal antibodies and tyrosine kinase inhibitors. Preliminary results of studies involving these agents provided a foundation for ongoing clinical trials, whose results in the near future will help us understand how to incorporate anti IGF-1R strategies into the current anticancer armamentarium.

Growth hormone receptor modulators

Growth hormone (GH) regulates somatic growth, substrate metabolism and body composition. Its actions are elaborated through the GH receptor (GHR). GHR signalling involves the role of at least three major pathways, STATs, MAPK, and PI3-kinase/Akt. GH receptor function can be modulated by changes to the ligand, to the receptor or by factors regulating signal transduction. Insights on the physico-chemical basis of the binding of GH to its receptor and the stoichiometry required for activation of the GH receptor-dimer has led to the development of novel GH agonists and antagonists. Owing to the fact that GH has short half-life, several approaches have been taken to create long-acting GHR agonists. This includes the pegylation, sustained release formulations, and ligand-receptor fusion proteins. Pegylation of a GH analogue (pegvisomant) which binds but not activate signal transduction forms the basis of a new successful approach to the treatment of acromegaly. GH receptors can be regulated at a number of levels, by modifying receptor expression, surface availability and signalling. Insulin, thyroid hormones and sex hormones are among hormones that modulate GHR through some of these mechanisms. Estrogens inhibit GH signalling by stimulating the expression of SOCS proteins which are negative regulators of cytokine receptor signalling. This review of GHR modulators will cover the effects of ligand modification, and of factors regulating receptor expression and signalling.

Use of a GH receptor antagonist (GHRA) to explore the relationship between GH and IGF-I in adults with severe GH deficiency (GHD)

OBJECTIVE

At diagnosis, approximately 50% of adults with severe GH deficiency (GHD) have an IGF-I within the reference range. It is unclear whether in such patients serum IGF-I levels are regulated by factors other than GH.

DESIGN AND PATIENTS

We performed a double-blind, randomized, placebo-controlled, cross-over study to investigate the effect of the GH receptor antagonist - pegvisomant (20 mg daily for 14 days) on GH and IGF-I levels in three cohorts: patients with GHD and a normal IGF-I (NORMS); patients with GHD and a low IGF-I (LOWS) and healthy volunteers (CONS).

RESULTS

Pegvisomant decreased IGF-I in CONS and NORMS [158.5 (101-206) vs. 103 (77-125) microg/l, P < 0.01; 124 (81-136) vs. 95 (51-113) microg/l, P < 0.01 respectively], but not in LOWS [31 (< 31-32) vs. 34.5 (< 31-38) microg/l], and this was associated with an increase in mean 24 h GH in CONS [0.49 (0.12-0.89) to 1.38 (0.22-2.45) microg/l (P = 0.03)] and in NORMS [69 (0-320)% from 0.1 (< 0.1-0.13) to 0.17 (0.11-0.42) microg/l (P = 0.03)], but not in the LOWS. The peak GH response to arginine was increased by pegvisomant in CONS and NORMS [6.1 (0.8-9) vs. 20.4 (13.1-28.8) microg/l, P = 0.03; 0.4 (0.1-0.5) vs. 0.5 (0.3-0.6) microg/l, respectively], but not in LOWS.

CONCLUSIONS

These data indicate that patients with severe GHD with a normal IGF-I are able to increase GH secretion in response to a pegvisomant-induced fall in IGF-I, whereas those with low IGF-I levels are unable to increase GH secretion. Therefore circulating IGF-I appears to be GH-independent in GHD patients with a low IGF-I, but remains partially GH-dependent in GHD patients with a normal IGF-I.

The role of IGF-1R in pediatric malignancies

The insulin-like growth factor (IGF) family consists of ligands (IGF-I, IGF-II, insulin), several receptors (including IGF-1R), and six binding proteins (IGFBP-1 through IGFBP-6). Members of this family regulate key cellular activities and they also play an important role in the development and progression of both adult and childhood cancers. Binding of a ligand to the receptor leads to its activation, followed by signal transduction along several pathways. In some childhood malignancies, IGF-1R can be activated by endocrine, autocrine, or paracrine mechanisms. Although mutations in IGF-1R have not been identified, this signaling pathway is upregulated in many childhood cancers. These findings have led to the development of a host of IGF-1R signaling modulators that are currently being tested in clinical trials. This review explores the role of IGF-1R in a range of childhood malignancies.

Early drug development of inhibitors of the insulin-like growth factor-I receptor pathway: lessons from the first clinical trials

The insulin-like growth factor-I receptor (IGF-IR) was first cloned in 1986. Since then, intense work has defined classic phosphorelays activated via the IGF-IR, which regulate cell proliferation, apoptosis, motility, and fate. The understanding of the roles of hormones in cancer and the growth hormone-IGF-IGF-binding protein axis specifically has yield to a second wave of development: the design of specific inhibitors that interrupt the signaling associated with this axis. The ability to manipulate these pathways holds not only significant therapeutic implications but also increase the chance of deeper insight about the role of the axis in carcinogenesis and metastasis. Nowadays, >25 molecules with the same goal are at different stages of development. Here, we review the clinical and preclinical experience with the two most-investigated strategies, tyrosine kinase inhibitors and monoclonal antibodies, and the advantages and disadvantages of each strategy, as well as other alternatives and possible drug combinations. We also review the biomarkers explored in the first clinical trials, the strategies that have been explored thus far, and the clinical trials that are going to explore their role in cancer treatment.

Autocrine human growth hormone stimulates oncogenicity of endometrial carcinoma cells

Recent published data have demonstrated elevated levels of human GH (hGH) in endometriosis and endometrial adenocarcinoma. Herein, we demonstrate that autocrine production of hGH can enhance the in vitro and in vivo oncogenic potential of endometrial carcinoma cells. Forced expression of hGH in endometrial carcinoma cell lines RL95-2 and AN3 resulted in an increased total cell number through enhanced cell cycle progression and decreased apoptotic cell death. In addition, autocrine hGH expression in endometrial carcinoma cells promoted anchorage-independent growth and increased cell migration/invasion in vitro. In a xenograft model of human endometrial carcinoma, autocrine hGH enhanced tumor size and progression. Changes in endometrial carcinoma cell gene expression stimulated by autocrine hGH was consistent with the altered in vitro and in vivo behavior. Functional antagonism of hGH in wild-type RL95-2 cells significantly reduced cell proliferation, cell survival, and anchorage-independent cell growth. These studies demonstrate a functional role for autocrine hGH in the development and progression of endometrial carcinoma and indicate potential therapeutic relevance of hGH antagonism in the treatment of endometrial carcinoma.

Growth hormone excess and the development of growth hormone receptor antagonists

In 1990, a single amino acid substitution in the growth hormone (GH) gene at position 119 was found to transform the consequent protein from an agonist to an antagonist at the growth hormone receptor (GHR). Further amino acid substitutions plus prolongation of the half-life of the protein by pegylation resulted in the first clinically effective GHR antagonist, pegvisomant. Following extensive clinical trials, this medication has emerged as the most efficacious therapy for treatment-resistant acromegaly. Subsequent advances in our understanding of GH-GHR interactions and downstream GH signalling pathways suggest that pegvisomant binds to preformed GHR dimers and prevents rotational changes within the receptor-GH complex necessary for intracellular signalling to occur. This article reviews the discovery of pegvisomant, from initial experimental data to successful licensing of the drug for treatment-resistant acromegaly, and discusses its other potential therapeutic uses in diseases with abnormalities in the GH-IGF-I axis.

Targeting the type 1 insulin-like growth factor receptor as a treatment for cancer

BACKGROUND

The type 1 insulin-like growth factor receptor (IGF1R) plays a critical role in transformation, invasion and apoptosis protection, and is an attractive cancer treatment target.

OBJECTIVE

To review IGF1R antibodies and kinase inhibitors that are in preclinical and clinical development, and to discuss questions that will influence the success of this approach in clinical practice.

METHODS

This review is drawn from published literature, meeting abstracts and online resources.

RESULTS/CONCLUSION

IGF1R blockade is generally well tolerated although it can induce hyperglycaemia. Single-agent activity has been documented in Ewing's sarcoma but not thus far in common solid tumours. Key issues include identification of factors that influence sensitivity to IGF1R blockade, and how most effectively to combine IGF1R inhibitors with other treatments.