In vitro, GHRP-6 stimulates GH secre- tion from pituitary cells by a mechanism not mediated by either growth hormone releasing hormone (GHRH) or opioid ...
22 Effects of Hexarelin on Growth Hormone Secretion in Short Normal Children, in Obese Children, and in Subjects with Growth Hormone Deficiency SANDRO LOCHE, PAOLA CAMBIASO, MARIA ROSARIA CASINI, BRUNO P. IMBIMBO, DANIELA CARTA, PATRIZIA BORRELLI, AND MARCO CAPPA
A series of small peptides [growth hormone releasing peptides (GHRPs)), analogues of enkephalin, have been recently synthesized that selectively stimulate growth hormone (GH) secretion (1). These peptides have potent GH releasing activity in all species tested so far, and are effective also after oral administration (2-4). One of these peptides, GHRP-6, has been extensively studied in vitro and in vivo. In vitro, GHRP-6 stimulates GH secretion from pituitary cells by a mechanism not mediated by either growth hormone releasing hormone (GHRH) or opioid receptors (5-9), and via signaling mechanisms distinct from those of GHRH (5, 10). In vivo, GHRP6 stimulates GH secretion in animals (11-13) and in humans (2, 3, 14-18). Interestingly, the GH releasing activity of GHRP-6 in vivo is more potent than that observed in in vitro experiments, indicating that the peptide may also have a hypothalamic site of action, a view supported by the observation that GHRP-6 activity is enhanced when the experiments are carried out on hypothalamic-pituitary incubates (12), and by the evidence of specific hypothalamic binding sites for the peptide (7, 8). Furthermore, GHRP-6 acts synergistically with GHRH to release GH both in vitro (5, 10, 12) and in vivo (3, 15). Hexarelin (Hex) is a new synthetic hexapeptide (His-D-2-Methyl-TrpAla-Trp-D-Phe-Lys-NH 2) analogue to GHRP-6, in which D-tryptophan has been replaced by its 2-methyl derivative (20). This new peptide is more stable to enzymatic, chemical, and oxidative degradation and is also more hydrophobic than the parent compound. These properties should theoretically increase the bioavailability of the peptide. Recently, Hex has been B. B. Bercu et al. (eds.), Growth Hormone Secretagogues © Springer-VerlaG New York, Inc. 1996
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shown to be slightly more effective than GHRP-6 in rats (21). Recent studies have also shown that Hex is a potent GH secretagogue in both adults (4) and children (22). This chapter presents our data on the GH releasing effect of Hex in short normal children, in subjects with organic hypopituitarism, in subjects with isolated GH deficiency (GHD), and in obese children.
Short Normal Children The study group comprised 45 children referred to our institutions for evaluation of short stature, and ultimately found to have familial short stature and/or constitutional delay of growth. Twenty-four children were prepubertal (11 boys and 13 girls, ages 5.9 to 13 years) and 21 were early pubertal (Tanner stages 2 to 3,13 boys and 8 girls, ages 10 to 14 years). All had normal insulin-like growth factor-I (IGF-I) levels and normal thyroid function tests, and none had taken long-term medications prior to the study. All children were tested on two occasions with GHRH 1-29 (Serono, Italy), 1 f,tg/kg iv, and with Hex (prepared and supplied by Europeptides, Argenteuil, France), 2 f,tg/kg iv. This dosage of Hex was chosen because it has been shown to be the calculated maximal dose in men (23). Blood samples were drawn from an indwelling catheter inserted in an antecubital vein 15 min before and immediately before the injection of GHRH or Hex and then every 15 min for 2 hours. All experiments started between 8 and 9 A.M. after the children fasted overnight. In five boys with constitutional growth delay, ages 12.0 to 13.7 years, the GH response to Hex was reevaluated one week after priming with testosterone (testosterone enanthate, 100mg intramuscularly). None of the subjects experienced adverse side effects after Hex administration. The GH response to GHRH and to Hex in the short normal children is shown in Figure 22.1. Mean peak GH and mean area under the curve (AVC) after GHRH were 26.6 ± 7.1 f,tg/L (AVe = 1,387 ± 317f,tg.minlL) and 23.2 ± 5.5f,tglL (AVe = 1,213 ± 217f,tg.min.L) in the prepubertal male and female subjects, respectively. In the pubertal subjects mean peak GH and mean Ave after GHRH were 19.0 ± 3.9f,tg/L (AVe = 975 ± 170f,tg.min.L) in boys and 21.1 ± 5.2f,tg/L (AVe = 1,164 ± 281 f,tg.min.L) in girls. In all children Hex caused a prompt a clear-cut increase of serum GH concentrations with peaks occurring between 15 and 30min from injection and ranging between 15 and 121 f,tglL. The GH response to Hex was significantly higher than that observed after GHRH in all groups of children evaluated both as maximum GH peak (prepubertal boys = 55.9 ± 7.2f,tg/L, p < .001 vs. GHRH; prepubertal girls = 40.9 ± 6.1 f,tg/L,p < .005 vs. GHRH; pubertal boys = 59.7 ± 6.9f,tg/L,p < .005 vs. GHRH; pubertal girls = 53.9 ± 11.5 f,tglL, P < .01 vs. GHRH) or as AVe (prepubertal boys = 2,934 ± 425 f,tg.min.L, p < .005 vs. GHRH; prepuber-
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22.2. Peak GH responses to iv bolus injection of hexarelin (2ltg/kg) in five males with constitutional growth delay before and after priming with testosterone (testosterone enanthate, 100mgim one week before the test).
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tal girls = 1,916 ± 326 f,tg.min.L, p < .02 vs. GHRH; pubertal boys = 2,877 ± 415f,tg.min.L,p < .01 vs. GHRH; pubertal girls = 2,757 ± 509f,tg.min.L, p < .005 vs. GHRH). The GH responses to GHRH and to Hex were similar between boys and girls, either prepubertal or pubertal, as well as between prepubertal and pubertal subjects (Fig. 22.1). After priming with testosterone the GH response to Hex was increased in all five subjects studied (Fig. TABLE 22.1. Mean baseline and peak cortisol and prolactin concentrations following intravenous administration of hexarelin (2 ltg/kg) in 45 short normal, in 10 obese, and in 6 GHD children.
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22. Effects of Hexarelin on Growth Hormone Secretion
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22.2). Mean GH peak and AUe responses to Hex before priming were 57.4 ± 8.7f.tg/L and 2,448 ± 379f.tg.min.L, respectively, and rose to 89.2.1 ± l1.3f.tg/L (p < .05) and to 4,143 ± 590f.tg.min.L (p < .05) one week after testosterone administration. In all children Hex administration caused a significant increase from baseline of both cortisol (from 439 ± 30 to 646 ± 27 nmollL, p < .001) and prolactin (PRL) (from 8.0 ± 0.9 to 16.9 ± 1.8f.tglL, p < .001) concentrations, which returned to the baseline values within 2 hours. As no differences were found between boys and girls as well as between prepubertal and pubertal subjects, results of cortisol and PRL measurements after Hex administration have been pooled together and are shown in Table 22.1.
Obese Children Ten obese children (seven boys and three girls, ages 7.5 to 12.0 years, excess body weight 47% to 86.2 % above their ideal body weight derived from Tanner standards, body mass index 23.0 to 30.5, all prepubertal and of normal stature) were studied according to the same protocol described above. In the obese children the GH response to GHRH and to Hex were significantly lower than in the prepubertal children (GHRH: peak = 5.8 ± 0.8f.tglL, p < .02, AUe = 402 ± 51 f.tg.min.L, p < .01; Hex: peak = 19.7 ± 4.4f.tg/L,p < .001, Aue = 1,043 ± 239f.tg.min.L,p < .001) (Fig. 22.3). The GH response to Hex was significantly higher than that observed after GHRH evaluated both as maximum GH peak (p < .01) and as AUe (p < .02), and was similar to the response observed in the short normal children after GHRH. Also in the obese children Hex administration caused a significant increase over baseline of both cortisol and PRL concentrations.
Growth Hormone Deficiency Five subjects (four males and one female, ages 8.4 to 21 years) had organic hypopituitarism as a result of surgical operation for craniopharyngioma. At the time of the study they were on replacement therapy with hydrocortisone, L-thyroxine, and deamino-8-D-arginine vasopressin (DDAVP). Six subjects (four boys and two girls, ages 6.0 to 15.8 years) had isolated GHD, which was idiopathic in three and associated with anatomical abnormalities in the others. In particular, one patient had an arachnoid cyst, one had empty sella syndrome, and one had pituitary stalk interruption syndrome. The latter was diagnosed on magnetic resonance imaging on the basis of lack of a visible pituitary stalk and ectopic posterior pituitary lobe. The diagnosis of GHD was made by the classic criteria, i.e., short stature with subnormal growth rates and delayed bone age, plasma GH levels
