Endocrine Abstracts

Background: The UK acromegaly register reported that <60% of acromegalics on medical therapy had controlled disease (1). Pegvisomant, a growth hormone antagonist (GHA), controls disease in >95% cases, but is not cost-effective and requires high dose daily injections (2). We have developed a fusion technology for making a cost-effective long-acting GH molecule (3), and generated a GHA by linking mutated growth hormone to its binding protein (GHBP).

Design of GHA: A mutation (G120R) within site 2 of GH produces a receptor antagonist and mutations in site 1 enhance binding creating a super antagonist. Linking to GHBP delays clearance but site 1 in GH can bind to GHBP reducing activity. We hypothesised that the W104A mutation in GHBP would prevent intramolecular binding and generate a potent antagonist.

Methods: Four target molecules were gene synthesised to include either site 2 mutation (GHA1), site 1 & 2 mutations (GHA2), site 2+W104A mutations (GHA3) and site 1 & 2+W104A mutations (GHA4). In vitro bio-potency of CHO purified protein was measured using GH-specific bioactivity assays.

Pharmacokinetics: Rats were given 1 nMole/kg (i.v) of GHA1-4. GHA from serum was quantified using an in-house GH ELISA.

Pharmacodynamics: Rabbits were given 2 mg/kg (s.c). GHA from serum was quantified using an in-house GH ELISA. IGF-I levels were analysed using an automated assay.

Results: Median IC50s of 45 nM (GHA1); 133 nM (GHA2); 40 nM (GHA3) and 16 nM (GHA4) were obtained. GHA4 antagonist activity was comparable to Pegvisomant. GHA1, 2, & 4 had similar terminal half-life’s >20 h. Only GHA4 induced a reduction in IGF-I in rabbits of 14% with an associated reduction in weight gain.

Conclusions: GHA4 has the potential to be a long-acting potent GHA with no requirement for post-translational modification and is likely to be a cost-effective treatment for acromegaly.