Growth hormone response induced by synthetic human growth hormone-releasing factor (1-44) in healthy dogs

Non-Clinical Pharmacology and Safety Evaluation of TH9507, a Human Growth Hormone-Releasing Factor Analogue

TH9507, an analogue of human growth hormone-releasing factor (hGRF1-44NH2) minimally modified by addition of a trans-3-hexenoyl moiety to Tyr1 of the amino acid sequence, was found to be resistant to dipeptidyl aminopeptidase-IV deactivation. Compared to natural hGRF1-44NH2, the modification slowed the in vitro degradation of the peptide in rat, dog and human plasma and prolonged the in vivo plasma elimination kinetics of immunoreactive TH9507. Plasma growth hormone and insulin-like growth factor-1 (IGF-1) markedly increased in pigs, rats and dogs after daily repeat intravenous or subcutaneous injections of TH9507 at doses up to 600 microg/kg. Subchronic toxicity studies in rats and dogs with TH9507 treatment for up to 4 months showed a significant, but not dose-related, increase in body weight gain associated with increased biomarker response. Although TH9507 was well tolerated by both rats and dogs, a more pronounced anabolic effect and more evident (reversible) adverse effects (liver and kidney findings, anaemia, clinical chemistry changes, organ weight effects) were observed in dogs after repeat daily subcutaneous injections, which were attributed to prolonged exposure to supraphysiological levels of growth hormone and/or IGF-1. In both rats and dogs, toxicokinetic evaluations indicated that exposure to immunoreactive TH9507 was dose related after both routes of administration. The apparent elimination t1/2 in dogs ranged from 21 to 45 min. In conclusion, TH9507 is a modified hGRF peptide having enhanced potency and duration of action. The adverse treatment-related effects in dogs appear to be associated with sustained exposure to supraphysiological levels of growth hormone and IGF-1 induced by prolonged TH9507 treatment.

Effects of plasmid-mediated growth hormone releasing hormone supplementation in young, healthy Beagle dogs

Our study focused on the evaluation of the pharmacological and toxicological effects of plasmid-mediated GHRH supplementation with electroporation in normal adult dogs over a 180-d period. Twenty-eight dogs (< 2 yr of age) were randomized to four groups. Three groups (four dogs/sex for each group) were treated with ascending doses of GHRH-expressing plasmid: 0.2, 0.6, and 1 mg. One group (two dogs of each sex) served as the control. Clinical observations and body weights were recorded. Hematological, serum biochemical, and urine analyses were performed. Serum IGF-I, ACTH, and insulin were determined. Necropsies were performed on d 93 and 180; organs were weighed and tissues were fixed and processed for light microscopy. Selected tissues were used to assess plasmid biodistribution on d 93. At all doses, plasmid GHRH caused increased weight gain (P < 0.001), without organomegaly. Serum glucose and insulin in fasted dogs remained within normal ranges at all time points. Adrenocorticotropic hormone was normal in all groups. Significant increases in number of red blood cells, hematocrit, and hemoglobin (P < 0.01) were observed. In conclusion, our study shows that plasmid-mediated GHRH supplementation is safe in electroporated doses up to 1.0 mg in young healthy dogs.

Effects of plasmid-mediated growth hormone-releasing hormone in severely debilitated dogs with cancer

Cachexia is a common manifestation of late stage malignancy and is characterized by anemia, anorexia, muscle wasting, loss of adipose tissue, and fatigue. Although cachexia is disabling and can diminish the life expectancy of cancer patients, there are still no effective therapies for this condition. We have examined the feasibility of using a myogenic plasmid to express growth hormone-releasing hormone (GHRH) in severely debilitated companion dogs with naturally occurring tumors. At a median of 16 days after intramuscular delivery of the plasmid, serum concentrations of insulin-like growth factor I (IGF-I), a measure of GHRH activity, were increased in 12 of 16 dogs (P < 0.01). These increases ranged from 21 to 120% (median, 49%) of the pretreatment values and were generally sustained or higher on the final evaluation. Anemia resolved posttreatment, as indicated by significant increases in mean red blood cell count, hematocrit, and hemoglobin concentrations, and there was also a significant rise in the percentage of circulating lymphocytes. Treated dogs maintained their weights over the 56-day study and did not show any adverse effects from the GHRH gene transfer. We conclude that intramuscular injection of a GHRH-expressing plasmid is both safe and capable of stimulating the release of growth hormone and IGF-I in large animals. The observed anabolic responses to a single dose of this therapy might be beneficial in patients with cancer-associated anemia and cachexia.

Growth hormone responses to growth hormone-releasing hormone and clonidine in dogs with Cushing's syndrome

The effects of clonidine, a growth hormone (GH) secretagogue, acting at the hypothalamic level and synthetic GH-releasing hormone (GHRH), a physiological stimulus of somatotrophs, on GH secretion were measured in 11 dogs with Cushing's syndrome. Eight healthy dogs served as controls. After the administration of GHRH the dogs with hyperadrenocorticism had a mean (SEM) GH peak of 11.2 (2.5) ng ml-1 which was significantly lower than the peak of 48.6 (13.4) ng ml-1 observed in the healthy dogs. Similarly, the GH response to clonidine was inhibited in the dogs with hyperadrenocorticism, the mean GH peak being 9.3 (3.3) ng ml-1 compared with 135.6 (43.3) ng ml-1 in the control dogs. No significant difference between the GH responses to GHRH and clonidine was observed in the dogs with Cushing's syndrome, the areas under the response curves being 567.9 (78.2) and 478.0 (102.6) ng.min ml-1, respectively. These results demonstrate that the function of somatotrophs is abnormal in dogs with Cushing's syndrome. There is evidence that the likely action of clonidine in dogs is to inhibit the release of somatostatin and the results therefore suggest that the effect of an excess of glucocorticoid in the dog is probably not mediated through an increase in somatostatin tone.

Effect of propranolol on growth hormone response to GH releasing hormone (GHRH 1-44) in the dog

The present study was undertaken to examine whether beta-adrenergic blockade with propranolol might influence and make less variable the growth hormone (GH) response to exogenous GH releasing hormone (GHRH) 1-44 in the dog. On four separate occasions eight healthy beagles, one to two years old, randomly received either propranolol (40 micrograms kg-1 intravenously) or an equivalent volume of saline, 30 minutes before either GHRH 1-44 (1 microgram kg-1 intravenously) or vehicle was injected. After propranolol alone, GH secretion did not differ from saline (area under the curve [AUC]: 649.5 +/- 128.3 v 633.2 +/- 87.7 ng min ml-1, respectively). GHRH alone elicited a significant increase in GH secretion (AUC: 1230.5 +/- 210.5 ng min ml-1) with a peak concentration of 16.7 +/- 4.8 ng ml-1. When GHRH was injected after propranolol the mean peak (59.1 +/- 14.7 ng ml-1) and secretory area (AUC: 2631.0 +/- 474.4 ng min ml-1) were greater than those observed after GHRH alone. However, from a clinical point of view propranolol pretreatment does not modify the great individual variability of the GH response to GHRH.