Conventional and nonconventional uses of growth hormone

Efficacy and safety of a new ready-to-use recombinant human growth hormone solution

We report 24-month interim results of two multicenter phase III studies in previously untreated children with growth failure secondary to GH deficiency (GHD) that were paramount to the development of a new recombinant human GH (rh- GH, somatropin), approved as the first 'biosimilar' in Europe. Study 1 consisted of 3 parts performed in 89 children. The objective was to compare efficacy and safety of the lyophilized formulation of the new somatropin [Somatropin Powder (Sandoz)] with a licensed reference rhGH preparation and the liquid formulation of the new somatropin [Somatropin Solution (Sandoz)] and to assess long-term efficacy and safety of this ready-to-use Somatropin Solution. Study 2 was performed in 51 children and designed to demonstrate efficacy and safety of Somatropin Powder and to confirm its low immunogenic potential; rhGH was given sc at a daily dose of 0.03 mg/kg. Primary [body height, height SD score (HSDS), height velocity, and height velocity (HV) SD score (HVSDS)] and secondary [IGF-I and IGF binding protein 3 (IGFBP-3)] efficacy endpoints and safety parameters were assessed regularly. In study 1, all treatments showed comparable increases in growth. The baseline-adjusted difference between Somatropin Powder and the reference rhGH product in mean HV was -0.20 cm/yr (95% confidence interval (CI) [-1.34;0.94]) and in mean HVSDS was 0.76 (95% CI [-0.57;2.10]) after 9 months. These very small differences demonstrate comparable therapeutic efficacy between the two treatments. The results of study 2 were consistent with those seen in study 1. Equivalent therapeutic efficacy and clinical comparability in terms of safety and immunogenicity between Somatropin Powder and the reference rhGH product and between Somatropin Powder and Somatropin Solution was demonstrated. The safety and immunogenicity profiles were similar and as expected from experience with rhGH preparations.

Identification of residues outside the two binding sites that are critical for activation of the lactogenic activity of human growth hormone

Human growth hormone (hGH) binds lactogenic or somatotrophic receptors, creating active heterotrimeric complexes. Comparison of hGH structures, either free or bound to a single lactogenic or somatotrophic receptor, shows binding is associated with structural changes. Changes in hGH structure are unique when binding either lactogenic or somatotrophic receptors and they influence the spatial arrangement of residues constituting the second receptor-binding site. Using site-directed mutagenesis, we identified a contiguous set of largely hydrophobic residues that forms a motif communicating between the two receptor-binding sites of hGH. The residues are external to the receptor-binding epitopes and were identified when their mutation reduced site 2 function without changing site 1 function. The motif includes Phe44, Leu93, Tyr160, Leu163, and Tyr164, located in two hydrophobic clusters between the receptor-binding sites. Their mutation to Glu disrupts hydrophobic interactions and reduces lactogenic activity between 4.7- and 85-fold with little effect on somatotrophic activity or spectroscopic properties. These differential effects indicate that loss of lactogenic activity is not a result of global mis-folding. We propose the loss of lactogenic activity results from disruption of specific hydrophobic clusters that disables the site 1 binding-induced structuring of the second receptor-binding site.

Pharmacokinetic-pharmacodynamic modeling of ipamorelin, a growth hormone releasing peptide, in human volunteers

PURPOSE

To examine the pharmacokinetics (PK) and pharmacodynamics (PD) of ipamorelin, a growth hormone (GH) releasing peptide, in healthy volunteers.

METHODS

A trial was conducted with a dose escalation design comprising 5 different infusion rates (4.21, 14.02, 42.13, 84.27 and 140.45 nmol/kg over 15 minutes) with eight healthy male subjects at each dose level. Concentrations of ipamorelin and growth hormone were measured.

RESULTS

The PK parameters showed dose-proportionality, with a short terminal half-life of 2 hours, a clearance of 0.078 L/h/kg and a volume of distribution at steady-state of 0.22 L/kg. The time course of GH stimulation by ipamorelin showed a single episode of GH release with a peak at 0.67 hours and an exponential decline to negligible GH concentration at all doses. The ipamorelin-GH concentration relationship was characterized using an indirect response model and population fitting. The model employed a zero-order GH release rate over a finite duration of time to describe the episodic release of GH. Ipamorelin induces the release of GH at all dose levels with the concentration (SC50) required for half-maximal GH stimulation of 214 nmol/L and a maximal GH production rate of 694 mIU/L/h. The inter-individual variability of the PD parameters was larger than that of the PK parameters.

CONCLUSIONS

The proposed PK/PD model provides a useful characterization of ipamorelin disposition and GH responses across a range of doses.

Human growth hormone site 2 lactogenic activity requires a distant tyrosine164

Comparison of crystallographic structures of human growth hormone, either bound to the prolactin receptor or free of receptors, reveals that human growth hormone binding to the prolactin receptor at site 1 is associated with a structural change in human growth hormone that influences the organization of residues which constitute site 2. We have identified Tyr164 as a residue that is critical for the propagation of this structural rearrangement. Tyr164 is a structural epitope for site 1 and is distal to site 2. Mutation of Tyr164 to glutamic acid (Y164E) does not affect the somatotrophic activity, absorption or fluorescence spectra or binding to the human prolactin receptor when compared to wild-type human growth hormone, indicating the subtle effects of the mutation. Lactogenic assays using extended concentrations of Y164E human growth hormone produce dose-response curves that are characterized by a right-shifted agonist phase and an unchanged antagonist phase when compared to wild-type human growth hormone. These results indicate that Tyr164 is required for the lactogenic activity of human growth hormone and that mutation to glutamic acid disrupts the lactogenic function of site 2.

Effects of troglitazone on hepatic and peripheral insulin resistance induced by growth hormone excess in rats

It is well known that short-term growth hormone (GH) administration in humans and animals induces insulin resistance and glucose intolerance. The purpose of the present study was to clarify whether troglitazone, a new insulin-sensitizing drug of the thiazolidinedione class, counteracts the insulin antagonistic effects of recombinant human (rh) GH on glucose metabolism in rats. Male Wistar rats weighing 184 to 226 g were treated either with rhGH (n = 8) or rhGH plus troglitazone (n = 8). rhGH (20 IU/kg body weight/d) was given by subcutaneous injection twice daily for 2 days. Troglitazone was given at 100 mg/kg/d orally for 5 days before and 2 days during rhGH. Saline was injected to the control rats (n = 7). Euglycemic clamp studies with an insulin infusion rate of 8 mU/kg/min were performed in these rats after an overnight fast. Hepatic glucose output (HGO), glucose infusion rate (GIR), and glucose disappearance rate (GDR) were measured. Fasting levels of plasma glucose (6.6 +/- 0.1, 6.1 +/- 0.3, 6.5 +/- 0.2 mmol/L), insulin (187.5 +/- 24.1, 206.4 +/- 24.1, 182.3 +/- 31.0 pmol/L), and serum free fatty acid (FFA) (1.58 +/- 0.18, 1.43 +/- 0.16, 1.61 +/- 0.25 mEq/L) were comparable among rats treated with rhGH, rhGH plus troglitazone, and controls, respectively. Basal HGO was also comparable among the three treatment groups. HGO was suppressed significantly during the hyperinsulinemic glucose clamp in control rats, but not in rhGH rats. When troglitazone was coadministered with rhGH, suppressibility of HGO during the glucose clamp was comparable to that of controls. GIR (13.5 +/- 4.5 v 24.1 +/- 4.1 mg/kg/min) and GDR (18.1 +/- 5.8 v 30.3 +/- 5.2 mg/kg/min) were decreased by rhGH treatment compared with control values. They returned to normal levels in rats treated with both rhGH and troglitazone (GIR, 22.4 +/- 5.9; GDR, 24.7 +/- 7.1). From these results, it is evident that rhGH treatment impaired insulin's ability to suppress HGO and stimulate peripheral glucose utilization. Troglitazone could block the insulin antagonistic effects of GH on hepatic glucose output and peripheral glucose utilization.

Efficacy and safety of one year of growth hormone therapy in steroid-dependent nephrotic syndrome

OBJECTIVES

To study the efficacy and safety of 1 year of growth hormone (GH) therapy in children with steroid-dependent nephrotic syndrome.

STUDY DESIGN

A prospective pilot, open study in which GH (mean dose 0.32 mg/kg per week) was administered for 1 year to 8 children with steroid-dependent nephrotic syndrome requiring prednisolone (mean dose 0.46 mg/kg per day) to maintain remission. Steroid dependence was defined as recurrence of proteinuria within 2 weeks of discontinuation of prednisolone, or when the dose was lowered below a critical level. At entry, all patients had been steroid dependent for at least 1 year. Anthropometric and bone mineral density measurements after treatment were compared with 1-year pretreatment data.

RESULTS

Pretreatment mean (+/-SD) chronologic age was 12.6 (+/-3.1) years, with a mean bone age of 9.1 (+/-2.0) years, with delayed puberty in five patients. The mean height velocity increased from 3.7 (+/-1.4) to 9.4 (+/-2.1) cm/yr after 1 year of treatment (p < 0.05). The mean height standard deviation score increased from -1.4 (+/-1.6) to -0.3 (+/-1.1), (p < 0.05). In the spine, the mean bone mineral density increased from 0.50 to 0.64 gm/cm2 (p < 0.05), and in the femoral neck, from 0.55 to 0.64 gm/cm2 (p < 0.05) after 1 year of treatment. Mean lean body mass increased from 58.1% to 62.6% (p < 0.01). There were no significant changes in creatinine clearance, fasting glucose, fasting insulin, or glycosylated hemoglobin levels. The mean bone age increased to 11.4 (+/-2.4) years, and pubertal stage advanced in 2 patients.

CONCLUSIONS

One year of GH therapy is effective in improving the height standard deviation score, height velocity, bone mineral density, and lean body mass of children with steroid-dependent nephrotic syndrome. There were no significant adverse effects. However, the bone age accelerated at a greater pace than the height age, and further studies are required to define the role of GH therapy in steroid-dependent nephrotic syndrome.

The regulation of growth hormone secretion

The regulation of GH secretion involves finely balanced systems with multiple components. As our knowledge of the physiology of GH regulation expands, so does our understanding of the bases for GH diseases. We now can identify several cellular loci that cause GH deficiency or GH excess. In addition, the recent increased understanding of GH physiology has resulted in an increase in potential therapies for growth disorders.

Partial growth hormone insensitivity in childhood

There appears to be a continuum of GH-responsiveness, ranging from complete GH-resistance in Laron syndrome to normal GH-sensitivity. Partial GHI is suggested by findings at both ends of this spectrum; some patients with Laron-type dwarfism are less severely affected by their GHI than others (Savage et al, 1993; for a review, see Savage et al, 1995) and some short, non-GH-deficient, children have reduced responsiveness to GH. Among children with ISS, we have identified a subgroup where defects at the level of the GH receptor lead to a partial-GHI syndrome (Carlsson et al, 1994; Attie et al, 1995; Goddard et al, 1995). Partial-GHI may explain the growth failure in some children who do not meet the criteria for GHD. So far, the clinical evaluation of short children has been focused on the exclusion or demonstration of GHD. The diagnosis of GHD at present requires that stimulated or spontaneous GH concentrations should fail to reach a certain, arbitrarily determined level. This assumes that GH-sensitivity is equal in all subjects, with the exception of rare cases with Laron syndrome. The diagnosis of GHD is in itself controversial, and it has been suggested that GH testing should be supplemented by other measures such as auxological evaluation and measurement of other components of the GH/IGF-I axis (Rosenfeld et al, 1995). However, the fact that some short children have partial-GHI suggests that both GH secretion and GH responsiveness should be taken into consideration when investigating the cause of short stature (Figure 5). Many short children do not have GHD per se, but may be short due to inadequate GH stimulation because of reduced GH-sensitivity.

Mutations of the growth hormone receptor in children with idiopathic short stature. The Growth Hormone Insensitivity Study Group

BACKGROUND

Short stature in children who are not deficient in growth hormone (GH) is probably caused by a variety of defects. Some children with idiopathic short stature have low serum concentrations of GH-binding protein, which is derived from the GH receptor. The possibility that low serum concentrations of GH-binding protein might indicate partial insensitivity to GH led us to investigate possible defects in the gene for the GH receptor in children with idiopathic short stature and low serum concentrations of GH-binding protein.

METHODS

We studied 14 children with idiopathic short stature who were selected on the basis of normal GH secretion and low serum concentrations of GH-binding protein. Analysis of single-strand conformation polymorphisms and DNA sequencing were both used to identify mutations in the GH-receptor gene.

RESULTS

Mutations in the region of the GH-receptor gene that codes for the extracellular domain of the receptor were found in 4 of the 14 children, but in none of 24 normal subjects. One of the four children with mutations was a compound heterozygote, with one mutation that reduced the affinity of the receptor for GH and a second mutation that may affect a function other than ligand binding. The remaining three children had single mutations in one allele of the gene. One mutation introduced a premature termination codon, and two caused substitutions of single amino acids in a structurally conserved domain of the receptor.

CONCLUSIONS

Some children with idiopathic short stature may have partial insensitivity to GH due to mutations in the GH-receptor gene.

Evidence for partial growth hormone insensitivity among patients with idiopathic short stature. The National Cooperative Growth Study

OBJECTIVE

To determine whether some patients with idiopathic short stature have partial resistance to growth hormone (GH). Patients with idiopathic short stature have decreased serum levels of the GH receptor-related GH-binding protein (GHBP), and low GHBP levels are associated with complete GH insensitivity (Laron) syndrome. We hypothesized that patients with idiopathic short stature and low GHBP levels may also have a degree of GH insensitivity.

DESIGN

Retrospective analysis of patients in a multicenter study.

SETTING

Ninety-six National Cooperative Growth Study centers in the United States and Canada.

SUBJECTS

Five hundred eleven patients with idiopathic short stature who were treated with GH. All patients had a baseline height standard deviation score of less than -2 and a maximum stimulated GH level greater than 10 micrograms/L. Of these, 101 (20%) had a baseline GHBP standard deviation score of -2 or less.

RESULTS

The patients with low GHBP levels, in comparison with those with normal GHBP levels, had a lower mean extracted standard deviation score for insulin-like growth factor I (-3.3 +/- 1.1 vs -2.5 +/- 1.4; p < 0.0001) but mean 12-hour GH values (2.8 +/- 1.1 vs 2.3 +/- 1.1 micrograms/L; p <0.0001). The differences between groups were statistically significant after control for age and weight-for-height standard deviation score. Among prepubertal patients, there was no significant difference between the low and normal GHBP groups in mean pretreatment or first-year growth rate (p = 0.74, 0.61 respectively) with comparable doses of GH.

CONCLUSIONS

Patients with idiopathic short stature and low GHBP levels, compared with those with normal GHBP levels, had significantly lower standardized levels of insulin-like growth factor I, and higher mean 12-hour GH levels, which suggest partial GH insensitivity. There was no significant correlation of GHBP levels with the growth response to exogenous GH.