The effect of hexarelin on growth hormone (GH) secretion in patients with GH deficiency

Impairment of GH responsiveness to GH-releasing hexapeptide (GHRP-6) in Prader-Willi syndrome

The aim of this study was to evaluate the GH-releasing activity of a synthetic hexapeptide, GHRP-6, in the Prader-Willi syndrome (PWS). Sixteen PWS patients (7 males and 9 females, aged 12.7-38.3 yr), 15 with essential obesity (OB) (7 males and 8 females, aged 12.9-42.9 yr), and 8 short normal children (SN; 3 males and 5 females, aged 10.2-14.3 yr) underwent 2 tests on separate occasions, being challenged with GHRP-6 (1 microg/kg, iv) or GHRH (1 microg/kg, iv)+PD (60 or 120 mg for children or adults, po). Moreover, in 11 patients with PWS and in the group of SN, the GH response to at least 2 stimulation tests had been previously determined. GH was analyzed either as mean peak values (GHp, mcg/l), or as the area under the curve (AUC, mcg/l/h) and the net incremental area under the curve (nAUC, mcg/l/h). In the group of PWS subjects, GH responses to both GHRP-6 (GHp: 11.4+/-2.0; AUC: 588+/-113; nAUC: 483+/-108) and GHRH+PD (GHp: 7.3+/-1.8; AUC: 486+/-122; nAUC: 371+/-250) were significantly lower than those observed either in OB (GHRP-6: GHp: 25.7+/-3.2, p<0.003; AUC: 1833+/-305, p<0.005; nAUC: 1640+/-263, p<0.0001. GHRH+PD: GHp: 15.1+/-2.4, p<0.009; AUC: 1249+/-248, p<0.003; nAUC: 918+/-230, p<0.006) or in SN patients (GHRP-6: GHp: 39.1+/-3.1, p<0.0001; AUC: 2792+/-158, p<0.0001; nAUC: 2705+/-165, p<0.00005. GHRH+PD: GHp: 27.5+/-3.7, p<0.0001; AUC: 1873+/-251, p<0.0001; nAUC: 1692+/-219, p<0.0005). Unlike control groups, in PWS patients GH levels after GHRP-6 did not differ from those obtained after GHRH+PD. Interestingly, low IGF-I values were present in all PWS subjects. Furthermore, no patient with PWS showed normal GH response to the previously performed GH stimulation tests. As already reported, GH release after GHRP-6 or GHRH+PD was significantly lower in OB than in SN subjects. In conclusion, our data indicate that: 1) GH response to GHRP-6 is clearly impaired in PWS; 2) the blunted GH responses to the provocative stimuli in PWS are not an artifact of obesity; 3) short stature in PWS is caused by a complex dysfunction of the hypothalamo-pituitary structures.

Hexarelin-induced growth hormone response in short stature. Comparison with growth hormone-releasing hormone plus pyridostigmine and arginine plus estrogen

Hexarelin (HEX) is a synthetic hexapeptide with strong GH-stimulating activity. We evaluated GH response (expressed as maximum value after stimulus [Cmax] and as area under the curve [AUC]) to HEX at the doses of 1 microg/kg i.v. (HEX 1) and 2 microg/kg i.v. (HEX 2), in comparison with the responses to GHRH (1 microg/kg i.v.) + pyridostigmine (PD, 60 mg po) and to arginine (ARG, 0.5 mg/kg i.v.) + ethinylestradiol (EE, 1 mg/day po for 3 days before the stimulation), in 5 subjects with familial short stature (FSS), 11 with constitutional growth delay (CGD), 6 with GH neurosecretory dysfunction (NSD), and 5 with isolated growth hormone deficiency (GHD). Cmax and AUC after HEX 1 were 26.8+/-10.5 ng/ml and 1448+/-514 ng/min x ml in FSS, 23.6+/-14.4 ng/ml and 1146+/-750 ng/min x ml in CGD, 36.9+/-21.5 ng/ml and 2048+/-1288 ng/min x ml in NSD, 9.4+/-5.8 ng/ml and 498+/-200 ng/min x ml in GHD (Cmax and AUC in FSS and CGD, p<0.05 vs GHD). Cmax and AUC after HEX 2 were 37.7+/-16 ng/ml and 1979+/-888 ng/min x ml in FSS, 32.5+/-16.2 ng/ml and 1613+/-237 ng/min x ml in CGD, 39.7+/-20.7 ng/ml and 2366+/-1569 ng/min xml in NSD, 13.4+/-4.2 ng/ml and 645+/-293 ng/min x ml in GHD (Cmax in FSS, CGD and NSD p<0.01 vs GHD; AUC in NSD, p<05 vs GHD). Cmax and AUC after GHRH+/-PD were 46.6+/-8.8 ng/ml and 3294+/-1031 ng/min x ml in FSS, 25.9+/-11.2 ng/ml and 1464+/-735 ng/min x ml in CGD, 38.8+/-21.7 ng/ml and 2428+/-1399 ng/min x ml in NSD, 8.4+/-6.2 ng/ml and 685+/-572 ng/min x ml in GHD (Cmax and AUC in FSS, p<0.001 vs CGD and GHD; Cmax in CGD and NSD, p<0.001 vs GHD). Cmax and AUC after ARG+EE were 21.3+/-4.2 ng/ml and 1432+/-514 ng/min x ml in FSS, 14.8+/-10 ng/ml and 805+/-489 ng/min x ml in CGD, 22.2+/-12.8 ng/ml and 1199+/-309 ng/min x ml in NSD, 4.6+/-2.5 ng/ml and 247+/-191 ng/min x ml in GHD (Cmax and AUC in FSS, CGD and NSD, p<0.01 vs GHD). Specificity was 62% for HEX 1 and 75% for HEX 2, GHRH+PD and ARG+EE. From a diagnostic point of view, HEX 1 + HEX 2 was the association with the largest percentage of false positives (20% in FSS, 27% in CGD and 33% in NSD), HEX 1 +GHRH+PD resulted in 9% in CGD, while the combined use of HEX 1 or HEX 2 with GHRH+PD or ARG+EE and of GHRH+PD with ARG+EE did not show false positive responses.

IN CONCLUSION

I) the most effective dose of HEX was 2 microg/kg i.v.; 2) HEX did not show more specificity than GHRH+PD and ARG+EE; 3) the association of GHRH+PD with ARG+EE could yield the best results at lower costs, confirming these tests as first-line tools in evaluating GH secretion.