Mechanism of Action
Somatropin (as well as endogenous GH) binds to a dimeric GH receptor in the cell membrane of target cells resulting in intracellular signal transduction and a host of pharmacodynamic effects. Some of these pharmacodynamic effects are primarily mediated by IGF-I produced in the liver and also locally (e.g., skeletal growth, protein synthesis), while others are primarily a consequence of the direct effects of somatropin (e.g., lipolysis).
The primary and most intensively studied action of somatropin is the stimulation of linear growth. This effect is demonstrated in children with GHD.
The measurable increase in bone length after administration of somatropin results from its effect on the cartilaginous growth areas of long bones. Studies in vitro have shown that the incorporation of sulfate into proteoglycans is not due to a direct effect of somatropin, but rather is mediated by the somatomedins or insulin-like growth factors (IGFs). The somatomedins, among them IGF-I, are polypeptide hormones which are synthesized in the liver, kidney, and various other tissues. IGF-I levels are low in the serum of hypopituitary dwarfs and hypophysectomized humans or animals, and increase after treatment with somatropin.
It has been shown that the total number of skeletal muscle cells is markedly decreased in children with short stature lacking endogenous GH compared with normal children, and that treatment with somatropin results in an increase in both the number and size of muscle cells.
Somatropin influences the size of internal organs, and it also increases red cell mass.
Linear growth is facilitated in part by increased cellular protein synthesis. This synthesis and growth are reflected by nitrogen retention which can be quantitated by observing the decline in urinary nitrogen excretion and blood urea nitrogen following the initiation of somatropin therapy.
Hypopituitary children sometimes experience fasting hypoglycemia that may be improved by treatment with somatropin. In healthy subjects, large doses of somatropin may impair glucose tolerance. Although the precise mechanism of the diabetogenic effect of somatropin is not known, it is attributed to blocking the action of insulin rather than blocking insulin secretion. Insulin levels in serum actually increase as somatropin levels increase. Administration of human growth hormone to normal adults and patients with growth hormone deficiency results in increases in mean serum fasting and postprandial insulin levels, although mean values remain in the normal range. In addition, mean fasting and postprandial glucose and hemoglobin A1C levels remain in the normal range.
Somatropin stimulates intracellular lipolysis, and administration of somatropin leads to an increase in plasma free fatty acids and triglycerides. Untreated GHD is associated with increased body fat stores, including increased abdominal visceral and subcutaneous adipose tissue. Treatment of growth hormone deficient patients with somatropin results in a general reduction of fat stores, and decreased serum levels of low density lipoprotein (LDL) cholesterol.
Administration of somatropin results in an increase in total body potassium and phosphorus and to a lesser extent sodium. This retention is thought to be the result of cell growth. Serum levels of phosphate increase in children with GHD after somatropin therapy due to metabolic activity associated with bone growth. Serum calcium levels are not altered. Although calcium excretion in the urine is increased, there is a simultaneous increase in calcium absorption from the intestine. Negative calcium balance, however, may occasionally occur during somatropin treatment.
Connective Tissue Metabolism
A 180-min IV infusion of Norditropin (33 ng/kg/min) was administered to 9 GHD patients. A mean (±SD) hGH steady state serum level of approximately 23.1 (±15.0) ng/mL was reached at 150 min and a mean clearance rate of approximately 2.3 (±1.8) mL/min/kg or 139 (±105) mL/min for hGH was observed. Following infusion, serum hGH levels had a biexponential decay with a terminal elimination half-life (T1/2) of approximately 21.1 (±5.1) min.
In a study conducted in 18 GHD adult patients, where a SC dose of 0.024 mg/kg or 3 IU/m² was given in the thigh, mean (±SD) Cmax values of 13.8 (±5.8) and 17.1 (±10.0) ng/mL were observed for the 4 and 8 mg Norditropin vials, respectively, at approximately 4 to 5 hr. post dose. The mean apparent terminal T1/2 values were estimated to be approximately 7 to 10 hr. However, the absolute bioavailability for Norditropin after the SC route of administration is currently not known.
Short Stature in Children with Noonan Syndrome
A prospective, open label, randomized, parallel group trial with 21 children was conducted for 2 years to evaluate the efficacy and safety of Norditropin treatment for short stature in children with Noonan syndrome. An additional 6 children were not randomized, but did follow the protocol. After the initial two-year trial, children continued on Norditropin until final height. Retrospective final height and adverse event data were collected from 18 of the 21 subjects who were originally enrolled in the trial and the 6 who had followed the protocol without randomization. Historical reference materials of height velocity and adult height analyses of Noonan patients served as the controls.
The twenty-four (24) (12 female, 12 male) children 3 – 14 years of age received either 0.033 mg/kg/day or 0.066 mg/kg/day of
Norditropin subcutaneously which, after the first 2 years, was adjusted based on growth response. In addition to a diagnosis of Noonan syndrome, key inclusion criteria included bone age determination showing no significant acceleration, prepubertal status, height SDS <-2, and HV SDS <1 during the 12 months pre-treatment. Exclusion criteria were previous or ongoing treatment with growth hormone, anabolic steroids or corticosteroids, congenital heart disease or other serious disease perceived to possibly have major impact on growth, FPG >6.7 mmol/L (>120 mg/dL), or growth hormone deficiency (peak GH levels <10 ng/mL).
Patients obtained a final height (FH) gain from baseline of 1.5 and 1.6 SDS estimated according to the national and the Noonan reference, respectively. A height gain of 1.5 SDS (national) corresponds to a mean height gain of 9.9 cm in boys and 9.1 cm in girls at 18 years of age, while a height gain of 1.6 SDS (Noonan) corresponds to a mean height gain of 11.5 cm in boys and 11.0 cm in girls at 18 years of age.
A comparison of HV between the two treatment groups during the first two years of treatment for the randomized subjects was 10.1 and 7.6 cm/year with 0.066 mg/kg/day versus 8.55 and 6.7 cm/year with 0.033 mg/kg/day, for Year 1 and Year 2, respectively. Age at start of treatment was a factor for change in height SDS (national reference). The younger the age at start of treatment, the larger the change in height SDS.
Examination of gender subgroups did not identify differences in response to Norditropin. Not all patients with Noonan syndrome have short stature; some will achieve a normal adult height without treatment. Therefore, prior to initiating Norditropin for a patient with Noonan syndrome, establish that the patient does have short stature.
Short Stature in Children with Turner Syndrome
Two randomized, parallel group, open label, multicenter studies were conducted in the Netherlands to evaluate the efficacy and safety of Norditropin for the treatment of children with short stature associated with Turner syndrome. Patients were treated to final height in both studies [height velocity (HV) < 2 cm/year]. Changes in height were expressed as standard deviation scores (SDS) utilizing reference data for untreated Turner syndrome patients as well as the national Dutch population.
In Study 1 (the primary study), 68 euthyroid Caucasian patients stratified based on age and baseline height SDS were randomized in a 1:1:1 ratio to three different Norditropin treatment regimens: 0.045 mg/kg/day (Dose A) for the entire study; 0.045 mg/kg/day for the first year and 0.067 mg/kg/day thereafter (Dose B); or 0.045 mg/kg/day for the first year, 0.067 for the second year, and 0.089 mg/kg/day thereafter (Dose C). Overall, at baseline, mean age was 6.5 years, mean height SDS (National standard) was -2.7, and mean HV during the previous year was 6.5 cm/year. Patients also received estrogen therapy after age 12 and following four years of Norditropin treatment if they did not have spontaneous puberty.
Patients were treated for a mean of 8.4 years. As seen in Table 3, overall mean final height was 161 cm in the 46 children who attained final height. Seventy percent of these children reached a final height within the normal range (height SDS > -2 using the National standard). A greater percentage of children in the two escalated dose groups reached normal final height. The mean changes from baseline to final height in height SDS after treatment with Dose B and Dose C were significantly greater than the mean changes observed after treatment with Dose A (utilizing both the National and Turner standards). The mean changes from baseline to final height in height SDS (Turner standard) in Table 3 correspond to mean height gains of 9.4, 14.1 and 14.4 cm after treatment with Doses A, B and C, respectively. The mean changes from baseline to final height in height SDS (National standard) in Table 3 correspond to mean height gains of 4.5, 9.1 and 9.4 cm after treatment with Doses A, B and C, respectively. In each treatment group, peak HV was observed during treatment Year 1, and then gradually decreased each year; during Year 4, HV was less than the pretreatment HV. However, between Year 2 and Year 6, a greater HV was observed in the two dose escalation groups compared to the 0.045 mg/kg/day group.
Table 3 : Final
Height-Related Results After Treatment of Patients with Turner Syndrome with
Norditropin in a Randomized, Dose Escalating Study
|Dose A 0.045 mg/kg/day
(n = 19)
|Dose B up to 0.067 mg/kg/day
(n = 15)
|Dose C up to 0.089 mg/kg/day
(n = 12)
(n = 46)
|Baseline height (cm)1||105 (12)||108 (12.7)||107 (11.7)||106 (11.9)|
|Final height (cm)1||157 (6.7)||163 (6.0)||163 (4.9)||161 (6.5)|
|Number (%) of patients reaching normal height (height SDS >-2 using National standard)||10 (53%)||12 (80%)||10 (83%)||32 (70%)|
|Height SDS (Turner standard)2|
|Final [95% CI]||1.7 [1.4, 2.0]||2.5 [2.1, 2.8]3||2.5 [2.1, 2.9]4||NA|
|Change from baseline [95% CI]||1.5 [1.2, 1.8]||2.2 [1.9, 2.5]3||2.2 [1.9, 2.6]4||NA|
|Height SDS (National standard)2|
|Final [95% CI]||-1.9 [-2.2, -1.6]||-1.2 [-1.5, -0.9]4||-1.2 [-1.6, -0.8]5||NA|
|Change from baseline [95% CI]||0.7 [0.4, 1.0]||1.4 [1.1, 1.7]4||1.4 [1.1, 1.8]5||NA|
|Values are expressed as mean
(SD) unless otherwise indicated. SDS: Standard deviation score.
1Unadjusted (raw) means;
2Adjusted (least squares) means based on an ANCOVA model including terms for treatment, duration of treatment, age at baseline, bone age at baseline, height SDS at baseline, age at onset of puberty and mid-parental target height SDS;
3p=0.005 vs. Dose A;
4p=0.006 vs. Dose A;
5p=0.008 vs. Dose A
In Study 2 (a supportive study), 19 euthyroid Caucasian patients (with bone age ≤13.9 years) were randomized to treatment with 0.067 mg/kg/day of Norditropin as a single subcutaneous dose in the evening, or divided into two doses (1/3 morning and 2/3 evening). All subjects were treated with concomitant ethinyl estradiol. Overall, at baseline, mean age was 13.6 years, mean height SDS (National standard) was -3.5 and mean HV during the previous year was 4.3 cm/year. Patients were treated for a mean of 3.6 years. In that there were no significant differences between the two treatment groups for any linear growth variables, the data from all patients were pooled. Overall mean final height was 155 cm in the 17 children who attained final height. Height SDS changed significantly from -3.5 at baseline to -2.4 at final height (National standard), and from 0.7 to 1.3 at final height (Turner standard).
Short Stature in Children Born Small for Gestational Age (SGA) with No Catch-up Growth by Age 2-4 Years
A multi-center, randomized, double-blind, two-arm study to final height (Study 1) and a 2-year, multi-center, randomized, double-blind, parallel-group study (Study 2) were conducted to assess the efficacy and safety of Norditropin in children with short stature born SGA with no catch-up growth. Changes in height and height velocity were compared to a national reference population in both studies.
The pivotal study included 53 (38 male, 15 female) non-GHD, Dutch children 3-11 years of age with short stature born SGA with no catch-up growth. Catch-up growth was defined as obtaining a height of ≥ 3rd percentile within the first 2 years of life or at a later stage. These prepubertal children needed to meet the following additional inclusion criteria: birth length < 3rd percentile for gestational age, and height velocity (cm/year) for chronological age < 50th percentile. Exclusion criteria included chromosomal abnormalities, signs of a syndrome (except for Silver-Russell syndrome), serious/chronic co-morbid disease, malignancy, and previous rhGH therapy. Norditropin was administered subcutaneously daily at bedtime at a dose of approximately 0.033 (Dose A) or 0.067 mg/kg/day (Dose B) for the entire treatment period. Final height was defined as a height velocity below 2 cm/year. Treatment with Norditropin was continued to final height for up to 13 years. Mean duration of treatment was 9.5 years (boys) and 7.9 years (girls).
38 out of 53 children (72%) reached final height. Sixty-three percent (24 out of 38) of the children who reached final height were within the normal range of their healthy peers (Dutch national reference). For both doses combined, actual mean final height was 171 (SD 6.1) cm in boys and 159 (SD 4.3) cm in girls.
As seen in Table 4, for boys and girls combined, both mean final height SDS (Dose A, -1.8 vs. Dose B, -1.3), and increase in height SDS from baseline to final height (Dose A, 1.4 vs. Dose B, 1.8), were significantly greater after treatment with Dose B (0.067 mg/kg/day). A similar dose response was observed for the increase in height SDS from baseline to Year 2 (Table 4).
Overall mean height velocity at baseline was 5.4 cm/y (SD 1.2; n=29). Height velocity was greatest during the first year of Norditropin treatment and was significantly greater after treatment with Dose B (mean 11.1 cm/y [SD 1.9; n=19]) compared with Dose A (mean 9.7 cm/y [SD 1.3; n=10]).
Table 4 : Study 1: Results for Final Height SDS and
Change from Baseline to Final Height in Height SDS Using National Standard
After Long-Term Treatment of SGA Children with Norditropin
|Raw Mean ± SD (N)|
|Dose A 0.033 mg/kg/day||Dose B 0.067 mg/kg/day||Total|
|Baseline Height SDS||-3.2 ± 0.7 (26)||-3.2 ± 0.7 (27)||-3.2 ± 0.7 (53)|
|Adjusted least-squares mean ± standard error (N) and [95% confidence intervals]|
|Height SDS: Change from Baseline at Year 22||1.4 ± 0.1 (26)
|1.8 ± 0.1 (26)
|Treatment Diff = 0.4
p-value = 0.002
|Height SDS: Change from Baseline at Final Height1||1.4 ± 0.2 (19)
|1.8 ± 0.2 (19)
|Treatment Diff = 0.5
p-value = 0.045
|Final Height SDS1||-1.8 ± 0.2 (19)
|-1.3 ± 0.2 (19)
|Final Height SDS > -2||13/19 (68%)||11/19 (58%)||24/38 (63%)|
|SDS: Standard deviation score.
1Adjusted (least-squares) means based on an ANCOVA model including terms for treatment, gender, age at baseline, bone age at baseline, height SDS at baseline, duration of treatment, peak GH after stimulation and baseline IGF-1.
2Adjusted (least-squares) means based on an ANCOVA model including terms for treatment, gender, age at baseline, height SDS at baseline, and pubertal status.
In this study, 84 randomized, prepubertal, non-GHD, Japanese children (age 3-8) with short stature born SGA with no catch-up growth were treated for 2 years with 0.033 or 0.067 mg/kg/day of Norditropin subcutaneously daily at bedtime or received no treatment for 1 year. Additional inclusion criteria included birth length or weight SDS ≤ -2 or < 10th percentile for gestational age, height SDS for chronological age ≤ -2, and height velocity SDS for chronological age < 0 within one year prior to Visit 1. Exclusion criteria included diabetes mellitus, history or presence of active malignancy, and serious co-morbid conditions.
As seen in Table 5, for boys and girls combined, there was a dose-dependent increase in height SDS at Year 1 and Year 2. The increase in height SDS from baseline to Year 2 (0.033 mg/kg/day, 0.8 vs. 0.067 mg/kg/day, 1.4) was significantly greater after treatment with 0.067 mg/kg/day. In addition, the increase in height SDS at Year 1 was significantly greater in both active treatment groups compared to the untreated control group.
Table 5 : Study 2: Results
for Change from Baseline in Height SDS At Year 1 and Year 2 Using National
Standard After Short-Term Treatment of SGA Children with Norditropin
|Raw Mean ± SD (N)|
|No Treatment||0.033 mg/kg/day||0.067 mg/kg/day||Total|
|Height SDS: Baseline||-2.9 ± 0.5 (15)||-3.0 ± 0.6 (35)||-2.9 ± 0.7 (34)||-2.9 ± 0.6 (84)|
|Height SDS: Year 1||-2.8 ± 0.5 (15)||-2.4 ± 0.6 (33)||-2.0 ± 0.8 (34)||-2.3 ± 0.7 (82)|
|Height SDS: Year 2||NA||-2.2 ± 0.7 (33)||-1.4 ± 0.7 (32)||-1.8 ± 0.8 (65)|
|Adjusted least-squares mean ± standard error (N) and [95% confidence intervals]|
|Height SDS: Change from Baseline at Year 11||0.1 ± 0.1 (15) [-0.1, 0.2]||0.6 ± 0.1 (33) [0.5, 0.7]||0.9 ± 0.1 (34) [0.8, 1.0]|
|0.033 vs. No Treatment: Treatment Diff = 0.5, [0.3, 0.7], p <0.0001
0.067 vs. No Treatment: Treatment Diff = 0.8, [0.6, 1.0], p < 0.0001
0.067 vs. 0.033: Treatment Diff = 0.3, [0.2, 0.5], p-value < 0.0001
|Height SDS: Change from Baseline at Year 21||NA||0.8 ± 0.1 (33) [0.7, 0.9]||1.4 ± 0.1 (32) [1.3, 1.6]|
|0.067 vs. 0.033: Treatment Diff = 0.6, [0.5, 0.8], p-value < 0.0001|
|SDS: Standard deviation score.
1Adjusted (least-squares) means based on an ANCOVA model including terms for treatment, gender, age at baseline, and height SDS at baseline. All children remained prepubertal during the study.
Adult Growth Hormone Deficiency (GHD)
A total of six randomized, double-blind, placebo-controlled studies were performed. Two representative studies, one in adult onset (AO) GHD patients and a second in childhood onset (CO) GHD patients, are described below.
A single center, randomized, double-blind, placebo-controlled, parallel-group, six month clinical trial was conducted in 31 adults with AO GHD comparing the effects of Norditropin [somatropin (rDNA origin) for injection] and placebo on body composition. Patients in the active treatment arm were treated with Norditropin 0.017 mg/kg/day (not to exceed 1.33 mg/day). The changes from baseline in lean body mass (LBM) and percent total body fat (TBF) were measured by total body potassium (TBP) after 6 months.
Treatment with Norditropin produced a significant (p=0.0028) increase from baseline in LBM compared to placebo (Table 6).
Table 6 : Lean Body Mass
(kg) by TBP
|Change from baseline at 6 months (mean)||1.12||-0.63|
|Treatment difference (mean) 95% confidence interval p-value||1.74 (0.65, 2.83) p=0.0028|
Analysis of the treatment difference on the change from baseline in percent TBF revealed a significant decrease (p=0.0004) in the Norditropin-treated group compared to the placebo group (Table 7).
Table 7 : Total Body Fat (%) by TBP
|Change from baseline at 6 months (mean)||-2.83||1.92|
|Treatment difference (mean) 95% confidence interval p-value||-4.74 (-7.18, -2.30) p=0.0004|
Fifteen (48.4%) of the 31 randomized patients were male. The adjusted mean treatment differences on the increase in LBM and decrease in percent TBF from baseline were larger in males compared to females.
Norditropin also significantly increased serum osteocalcin (a marker of osteoblastic activity).
A single center, randomized, double-blind, placebo-controlled, parallel-group, dose-finding, six month clinical trial was conducted in 49 men with CO GHD comparing the effects of Norditropin and placebo on body composition. Patients were randomized to placebo or one of three active treatment groups (0.008, 0.016, and 0.024 mg/kg/day). Thirty three percent of the total dose to which each patient was randomized was administered during weeks 1-4, 67% during weeks 5-8, and 100% for the remainder of the study. The changes from baseline in LBM and percent TBF were measured by TBP after 6 months.
Treatment with Norditropin produced a significant (p=0.0079) increase from baseline in LBM compared to placebo (pooled data) (Table 8).
Table 8 : Lean Body Mass
(kg) by TBP
|Change from baseline at 6 months (mean)||2.06||0.7|
|Treatment difference (mean) 95% confidence interval p-value||1.40 (0.39, 2.41) p=0.0079|
Analysis of the treatment difference on the change from baseline in percent TBF revealed a significant decrease (p=0.0048) in the Norditropin-treated groups (pooled data) compared to the placebo group (Table 9).
Table 9 : Total Body Fat (%)
|Change from baseline at 6 months (mean)||-6||-1.78|
|Treatment difference (mean) 95% confidence interval p-value||-4.24 (-7.11, -1.37) p=0.0048|
Norditropin also significantly reduced intraabdominal, extraperitoneal and total abdominal fat volume, waist/hip ratio and LDL cholesterol, and significantly increased serum osteocalcin.
Forty four men were enrolled in an open label follow up study and treated with Norditropin for as long as 30 additional months. During this period, the reduction in waist/hip ratio achieved during the initial six months of treatment was maintained.
Last reviewed on RxList: 10/29/2013
This monograph has been modified to include the generic and brand name in many instances.
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