CLINICAL PHARMACOLOGY
General
Linear Growth - Somatropin stimulates linear growth in pediatric
patients who lack adequate normal endogenous growth hormone. In vitro,
preclinical, and clinical testing have demonstrated that somatropin is therapeutically
equivalent to human growth hormone of pituitary origin and achieves equivalent
pharmacokinetic profiles in normal adults.
In addition, the following actions have been demonstrated for human growth hormone (somatropin and/or human growth hormone of pituitary origin).
Tissue Growth - 1. Skeletal Growth: Somatropin stimulates skeletal
growth in children with growth failure due to lack of adequate secretion of
endogenous GH (i.e. growth hormone deficiency), or in patients with Turner Syndrome.
The measurable increase in body length after administration of human growth
hormone results from an effect on the epiphysial plates of the long bones. Concentrations
of IGF-1, which may play a role in skeletal growth, are low in the serum of
growth hormone-deficient pediatric patients but increase during treatment with
somatropin. Elevations in mean serum alkaline phosphatase concentrations may
also be seen. 2. Cell Growth: It has been shown that there are fewer skeletal muscle cells in pediatric patients with short stature who lack endogenous growth
hormone as compared to the normal pediatric population. Treatment with human
growth hormone results in an increase in the size and number of skeletal muscle
cells.
Protein Metabolism- Linear growth is facilitated in part by
increased cellular protein synthesis. Nitrogen retention, as demonstrated by
decreased urinary nitrogen excretion and serum urea nitrogen, follows the initiation
of therapy with human growth hormone.
Carbohydrate Metabolism - Growth hormone is a modulator of carbohydrate
metabolism. Pediatric patients with hypopituitarism sometimes experience fasting
hypoglycemia that is improved by treatment with somatropin. Large doses of human
growth hormone may impair glucose tolerance (see PRECAUTIONS,
General).
Lipid Metabolism - In growth hormone-deficient patients, administration
of human growth hormone has resulted in lipid mobilization, reduction in body
fat stores, and increased plasma fatty acids.
Mineral Metabolism - Retention of sodium, potassium, and phosphorus
is induced by human growth hormone. Serum concentrations of inorganic phosphate
increased in patients with growth hormone deficiency after therapy with human
growth hormone. Serum calcium is not significantly altered in patients treated
with human growth hormone.
Pharmacokinetics
Absorption -Accretropin™ (somatropin injection) has been studied following subcutaneous
administration in adult volunteers. Bioavailability of Accretropin™ (somatropin injection) was
not determined. However, based on the bioavailability of other r-hGH products,
absolute bioavailability has been estimated at approximately 70% when administered
subcutaneously (Janssen et al., 1999; Zeisel et al., 1992).
Distribution - The volume of distribution of somatropin was not
determined for Accretropin™ (somatropin injection) .
Metabolism - Extensive metabolism studies have not been conducted.
Somatropin is metabolized in the liver and kidneys. In the kidneys, hGH is catabolized
to its constitutive amino acids, which are then returned to the systemic circulation.
Clearance was not determined for Accretropin™ (somatropin injection) . The mean half-life of subcutaneously
administered Accretropin™ (somatropin injection) is 3.63 hours (Table 1).
Excretion - Urinary excretion of intact somatropin has not been
measured.
Table 1: Summary of somatropin pharmacokinetic parameters
in the normal population following a 4 mg dose of Accretropin™ (somatropin injection) administered
subcutaneously*
|
AUC(0-t)
(ng•h/mL) |
AUC(0-inf)
(ng•h/mL) |
Cmax
(ng/mL) |
Tmax (h) |
t1/2 (h) |
mean ± SD |
238.09 ± 44.11 |
255.31 ± 43.03 |
29.49 ± 8.32 |
3.50 (2-6) |
3.63 ± 1.33 |
*Abbreviations: AUC0-t=area under the curve until
24 hours after administration; AUC0-inf=area under the curve
to infinity; Cmax=maximum concentration; t½=half-life; Tmax=time
to maximum concentration (given as the median value with range); SD=standard
deviation. |
Special Populations
Geriatric - The pharmacokinetics of Accretropin™ (somatropin injection) have not been
studied in patients greater than 65 years of age.
Pediatric - No formal pharmacokinetic studies of r-hGH in pediatric
patients have been conducted using Accretropin™ (somatropin injection) .
Gender - No studies have been performed to evaluate the effect
of gender on the pharmacokinetics of Accretropin™ (somatropin injection) .
Race - No data are available.
Renal, Hepatic insufficiency - No studies have been performed
with Accretropin™ (somatropin injection) in patients with renal or hepatic insufficiency.
Figure 1. Mean serum hGH levels over time following single
dose administration.
Figure 1 shows changes in mean hGH serum concentrations over time following
single dose administration of Accretropin™ (somatropin injection) (N= 20, data represent means
± Standard Error).
Clinical Trials
Pediatric Patients with GHD
The safety and efficacy of Accretropin™ (somatropin injection) in the treatment of pediatric patients with GHD was studied in a single-arm, open-label, multicenter trial conducted in 44 patients with GHD who were treated for up to 3 years with an Accretropin™ (somatropin injection) dose of 0.03 to 0.05 mg/kg/day (0.18 to 0.30 mg/kg/week) subcutaneously. The efficacy of Accretropin™ (somatropin injection) is displayed in Table 2.
Table 2: Height Velocity (cm/yr) and Height Velocity SDS
in patients with GHD*
|
Height Velocity (cm/yr)
N= number of patients
Mean (cm/yr) ± SD |
Height Velocity SDS
N= number of patients
Mean (SDS) ± SD |
Year 1 |
N=41
8.88 ± 2.29 |
N=41
3.60 ± 3.58 |
Year 2 |
N=34
7.64 ± 1.41 |
N=33
1.95 ± 2.32 |
Year 3 |
N=26
6.98 ± 1.62 |
N=26
1.76 ± 2.87 |
*Patients who entered puberty during the clinical trial were
discontinued as per protocol specifications. |
Height SD score calculated relative to population of normally growing children increased on Accretropin™ (somatropin injection) treatment from -3.04 at baseline to -2.46 at one year, -2.12 at two years, and -1.78 at three years.
Pediatric Patients with Turner Syndrome
The safety and efficacy of Accretropin™ (somatropin injection) in the treatment of children with short stature due to Turner Syndrome was evaluated in a single-arm, open-label, single-center trial conducted in 37 patients treated with an Accretropin™ (somatropin injection) dose of 0.06 mg/kg/day subcutaneously (0.36 mg/kg/week). The efficacy of Accretropin™ (somatropin injection) is shown in Table 3.
Table 3: Height Velocity (cm/yr) and Height Velocity SDS
in patients with Turner Syndrome
|
Height Velocity (cm/yr)
N= number of patients
Mean (cm/yr) ± SD |
Height Velocity SDS
N= number of patients
Mean (SDS) ± SD |
Year 1 |
N=37
8.56 ± 1.71 |
N=37
3.08 ± 2.56 |
Year 2 |
N=36
6.85 ± 1.21 |
N=36
1.50 ± 1.90 |
Year 3 |
N=35
5.84 ± 1.86 |
N=33
0.48 ± 3.28 |
Height SD score calculated relative to population of Turner Syndrome patients increased on Accretropin™ (somatropin injection) treatment from -3.17 at baseline to -2.67 at one year, -2.43 at two years, and -2.28 at three years.