Growth hormone (GH) receptor blockade with a PEG-modified GH (B2036-PEG) lowers serum insulin-like growth factor-I but does not acutely stimulate serum GH

B2036-PEG, a GH receptor (GH-R) antagonist, is an analog of GH that is PEG- modified to prolong its action. Nine mutations alter the binding properties of this molecule, preventing GH-R dimerization and GH action. A potential therapeutic role of B2036-PEG is to block GH action, e.g, in refractory acr omegaly. A phase I, placebo-controlled, single rising-dose study was perfor med in 36 normal young men (ages, 1837 yr; within 15% ideal body weight). F our groups received a single sc injection of either placebo (n = 3 in each group, total n = 12) or B2036-PEG (0.03, 0.1, 0.3, or 1.0 mg/kg; n = 6 each dose). B2036-PEG and GH concentrations were measured 0, 0.25, 0.5, 1, 3, 6 , 9, 12, 24, 36, 48, 72, 96, 120, and 144 h after dosing. Serum insulin-lik e growth factor-I was measured before and 1-7 days after dosing. All doses were well tolerated, with no serious or severe adverse reactions, B2036-PEG , at 1.0 mg/kg, reduced insulin-like growth factor-I by 49 +/- 6% on day 5 (P < 0.001 cs. placebo). GH was measured by two independent methods: 1) mod ified Nichols chemiluminescence assay (empirically corrected for B2036-PEG cross-reactivity); and 2) direct GH two-site immunoassay, using monoclonal antibodies that did not react with B2036-PEG. There was good agreement betw een the two methods. GH did not change substantially at any B2036-PEG dose, suggesting that B2036-PEG does not interact with hypothalamic GH-Rs to blo ck short-loop feedback. B2036-PEG may thus block peripheral GH action witho ut enhancing its secretion.