Short-term administration of intranasal or oral Hexarelin, a synthetic hexapeptide, does not desensitize the growth hormone responsiveness in human aging.

The function of the growth hormone-insulin-like growth factor I (GH-IGF-I) axis is reduced in aging, although the secretory capacity of somatotrope cells is preserved. Previous studies have suggested that continuous administration of GH-releasing peptides (GHRPs) results in homologous desensitization to the GH-releasing effect of the peptides. In the present study we have studied whether healthy elderly subjects would remain responsive to short-term, intermittent treatment with Hexarelin (HEX), a GHRP, and whether this treatment would result in an increase in serum IGF-I. In study I, the effect of an 8-day treatment with intranasal administration of 1.25 mg (about 18 micrograms/kg) t.i.d. HEX on the acute GH response to the hexapeptide and on serum IGF-I, IGF binding protein 3 (IGFBP-3), prolactin and cortisol levels was studied in seven elderly subjects (four males and three females, aged 67-80 years). In study II, the same parameters were studied before and after a 15-day treatment with oral administration of 20 mg (about 300 micrograms/kg) t.i.d. HEX in seven elderly women (aged 63-80 years). The GH response to the intranasal HEX administration was not significantly higher than that induced by 1 microgram/kg iv GHRH (229.4 +/- 35.9 vs 145.8 +/- 26.9 micrograms.l-1.h-1) and was maintained with a trend towards increase after an 8-day treatment with the peptide (342.5 +/- 199.3 micrograms.l-1.h-1). On the other hand, HEX treatment did not significantly modify IGF-I (138.7 +/- 11.1 vs 122.4 +/- 14.1 micrograms/l) but increased IGFBP-3 levels (2.4 +/- 0.2 vs 1.6 +/- 0.2 mg/l, p < 0.02). The GH response to the oral HEX administration was also not significantly higher than that to iv GHRH (257.6 +/- 72.0 vs 179.0 +/- 42.8 micrograms.l-1.h-1) and did not change after a 15-day treatment with the peptide (237.8 +/- 42.8 micrograms.l-1.h-1). Both IGF-I and IGFBP-3 levels were slightly but significantly increased by oral HEX treatment (156.0 +/- 10.7 vs 141.6 +/- 13.6 micrograms/l, p < 0.03, 3.4 +/- 0.2 vs 3.1 +/- 0.2 mg/l, p < 0.03, respectively). Neither intranasal nor oral HEX treatment modified PRL or cortisol levels and did not induce any side effect. In conclusion, these results indicate that chronic but intermittent treatment with HEX, administered either by intranasal or oral route, does not desensitize the GH response to the peptide. Moreover, after HEX treatment a trend towards increase was shown for IGF-I and IGFBP-3 levels. Thus, our findings strengthen the hypothesis that prolonged treatment with HEX may restore the reduced GH release in aging.