The effect of treatment with the oral growth hormone (GH) secretagogue MK-677 on GH isoforms

Growth hormone isoforms

Human growth hormone (GH) is a heterogeneous protein hormone consisting of several isoforms. The sources of this heterogeneity reside at the level of the genome, mRNA splicing, post-translational modification and metabolism. The GH gene cluster on chromosome 17q contains 2 GH genes (GH1 or GH-N and GH2 or GH-V) in addition to 2(-3) genes encoding the related chorionic somatomammotropin. Alternative mRNA splicing of the GH1 transcript yields two products: 22K-GH (the principal pituitary GH form) and 20K-GH. Post-translationally modified GH forms include N(alpha)-acylated, deamidated and glycosylated monomeric GH forms, as well as both non-covalent and disulfide-linked oligomers up to at least pentameric GH. GH fragments generated in the course of peripheral metabolism may be measured in immunoassays for GH. The GH-N gene is expressed in the pituitary, the GH-V gene in the placenta. Secretion of pituitary GH forms is pulsatile under control from the hypothalamus, whereas secretion of placental GH-V is tonic and rises progressively in maternal blood during the 2nd and 3rd trimester. Pituitary GH forms are co-secreted during a secretory pulse; no isoform-specific stimuli have been identified. There are minor differences in somatogenic and metabolic bioactivity among the GH isoforms, depending on species and assay system used. Both 20K-GH and GH-V have poor lactogenic activity. Oligomeric GH forms have variably diminished bioactivity compared to monomeric forms. GH isoforms cross-react in most immunoassays, but assays specific for 22K-GH, 20K-GH and GH-V have been developed. The metabolic clearance of 20K-GH and GH oligomers is delayed compared to that of 22K-GH. The heterogeneous mixture of GH isoforms in blood is further complicated by the presence of two GH-binding proteins, which form complexes with GH; isoform proportions also vary depending on the lag time from a secretory pulse because of different half-lives. GH forms excreted in the urine reflect monomeric GH isoforms in blood, but constitute only a minute fraction of the GH production rate. The heterogeneity of GH is one important reason for the notorious disparity among assay results. It also presents an opportunity for distinguishing endogenous from exogenous GH.

Pharmacodynamic hormonal effects of anamorelin, a novel oral ghrelin mimetic and growth hormone secretagogue in healthy volunteers

OBJECTIVE

Activation of ghrelin receptors stimulates GH secretion and appetite, increasing lean body mass and body weight. However, clinical use of ghrelin is limited because it has a short half-life and must be administered parenterally. Anamorelin is a novel, orally active, non-peptidic ghrelin mimetic and growth hormone secretagogue. Our objective was to evaluate its hormonal effects in healthy subjects.

DESIGN

A double-blind, randomized, placebo-controlled study evaluated the short-term effects of anamorelin on GH, insulin-like growth factor-1 (IGF-1), insulin-like growth factor binding protein 3 (IGFBP-3), prolactin, ACTH, LH, FSH, TSH, cortisol, insulin and glucose. Normal healthy volunteers (n=32) recruited from the general population were administered escalating doses of anamorelin (25, 50, and 75 mg daily) vs. placebo.

RESULTS

Anamorelin significantly increased GH levels at all doses (p<or=0.01). Effects on the somatotropic axis were maintained, as evidenced by sustained increases in IGF-1 and IGFBP-3 compared to placebo following 5-6 days of treatment. Negligible effects on other anterior pituitary hormone profiles and on fasting glucose were noted and all mean hormone levels remained within normal range. Some degree of insulin resistance as assessed by HOMA-IR was evident after treatment with 75 mg dose but not with the 25 or the 50 mg doses. Significant dose-related increases in body weight were recorded. Changes in body weight directly correlated with changes in IGF-1 levels. Anamorelin was well tolerated.

CONCLUSIONS

Anamorelin increases GH, IGF-1, IGFBP-3 and body weight with good tolerability and selectivity, without affecting other anterior pituitary axes or fasting glucose levels.