Enhanced Bioactivity of a Human GHR Antagonist Generated by Solid-Phase Site-Specific PEGylation

Growth hormone (GH) has been implicated in cancer progression andis a potential target for anticancer therapy. Currently, pegvisomant is the only GH receptor (GHR) antagonist approved for clinical use. Pegvisomant is a mutated GH molecule (B2036) which is PEGylated on amine groups to extend serum half-life. However, PEGylation significantly reduces the bioactivity of the antagonist in mice. To improve bioactivity, we generated a series of B2036 conjugates with the site-specific attachment of 20, 30, or 40 kDa methoxyPEG maleimide (mPEG maleimide) by introduction of a cysteine residue at amino acid 144 (S144C). Recombinant B2036-S144C was expressed in Escherichia coli, purified, and then PEGylated using cysteine-specific conjugation chemistry. To avoid issues with dimerization due to the introduced cysteine, B2036-S144C was PEGylated while immobilized on an Ni-nitrilotriacetic (Ni-NTA) acid column, which effectively reduced disulfide-mediated dimer formation and allowed efficient conjugation to mPEG maleimide. Following PEGylation, the IC₅₀ values for the 20, 30, and 40 kDa mPEG maleimide B2036-S144C conjugates were 66.2 ± 3.8, 106.1 ± 7.1, and 127.4 ± 3.6 nM, respectively. The circulating half-life of the 40 kDa mPEG conjugate was 58.3 h in mice. Subcutaneous administration of the 40 kDa mPEG conjugate (10 mg/kg/day) reduced serum insulin-like growth factor I (IGF-I) concentrations by 50.6%. This in vivo reduction in serum IGF-I was at a considerably lower dose compared to the higher doses required to observe comparable activity in studies with pegvisomant. In conclusion, we have generated a novel PEGylated GHR antagonist by the solid-phase site-specific attachment of mPEG maleimide at an introduced cysteine residue, which effectively reduces serum IGF-I in vivo.