Optimized clinical performance of growth hormone with an expanded genetic code

Abstract

The ribosomal incorporation of nonnative amino acids into polypeptides in living cells provides the opportunity to endow therapeutic proteins with unique pharmacological properties. We report here the first clinical study of a biosynthetic protein produced using an expanded genetic code. Incorporation of p-acetylphenylalanine (pAcF) at distinct locations in human growth hormone (hGH) allowed site-specific conjugation with polyethylene glycol (PEG) to produce homogeneous hGH variants. A mono-PEGylated mutant hGH modified at residue 35 demonstrated favorable pharmacodynamic properties in GH-deficient rats. Clinical studies in GH-deficient adults demonstrated efficacy and safety comparable to native human growth hormone therapy but with increased potency and reduced injection frequency. This example illustrates the utility of nonnative amino acids to optimize protein therapeutics in an analogous fashion to the use of medicinal chemistry to optimize conventional natural products, low molecular weight drugs, and peptides.

Fig. 1.

PEGylated His₆hGH analog-induced growth in hypophysectomized rats. Six site-selective PEGylated His₆hGH analogs were compared for their ability to induce weight gain in hypophysectomized rats (N = 5–11 per group). Error bars represent standard deviation. Unless otherwise indicated, hGH, His₆hGH, and the PEGylated His₆hGH analogs were dosed once on day 0 at 2.1 mg/kg. PEGylated Y35pAcF-His₆hGH produced the largest body weight increase but was not significantly better than daily hGH administration at 0.3 mg/kg when all mean pairs were compared using a Tukey–Kramer honestly significant difference test.

Fig. 2.

Dose-dependent increase in tibia length with PEGylated Y35pAcF-hGH. Rats were treated with placebo daily or on days 0 and 7, with 0.3 mg/kg genotropin, or with increasing doses of PEGylated Y35pAcF-hGH on days 0 and 7. Solid bars indicate the mean of measured tibia bone length. Error bars represent standard deviation. N = 10 rats per treatment group, one tibia measurement per rat.

Fig. 3.

Dose-dependent weight gain in rats treated with PEGylated Y35pAcF-hGH. Rats were treated with placebo daily or on days 0 and 7, with 0.3 mg/kg genotropin, or with increasing dose of PEGylated Y35pAcF-hGH on days 0 and 7. Solid lines indicate the mean percentage change in body weight. Error bars represent standard deviation. N = 10 rats per treatment group, one daily weight measurement per rat.

Fig. 4.

ARX201-induced increase in IGF-1 standard deviation scores (SDS) in GH-deficient human clinical trial subjects. ARX201 (starting dose 0.02 and 0.06 mg/kg) was administered weekly by subcutaneous injection for up to 26 weeks, and induced an IGF-1 SDS increase into the normal range within one day.