Lonapegsomatropin

A once-weekly prodrug of unmodified human growth hormone in which somatropin is transiently bound to a polyethylene glycol carrier, FDA approved as Skytrofa for pediatric growth hormone deficiency in 2021 and adult growth hormone deficiency in 2025.

Also referenced as: Skytrofa, TransCon hGH, ACP-001, Lonapegsomatropin-tcgd

Also appears in: Hormone

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What is Lonapegsomatropin?

Lonapegsomatropin is not a new hormone: the active molecule is the same 191-amino-acid, 22 kDa recombinant human growth hormone, somatropin, found in daily GH products. What is new is the packaging. In lonapegsomatropin, somatropin made in Escherichia coli is attached through a proprietary TransCon linker to an inert methoxypolyethylene glycol carrier (four 10 kDa mPEG arms), giving an inactive prodrug of about 63 kDa. After subcutaneous injection, the linker breaks on its own at body pH and temperature, releasing chemically unmodified somatropin over the course of a week. Developed by Ascendis Pharma under the codes ACP-001 and TransCon hGH, it is marketed as Skytrofa (US nonproprietary name lonapegsomatropin-tcgd) and was the first once-weekly growth hormone approved by the FDA for children.

It belongs to the clinic rather than the research-peptide market.

How it works

The pharmacology is that of somatropin; the engineering governs only how fast the hormone appears in circulation.

• Transient prodrug. The TransCon approach was designed to avoid the receptor-affinity loss and altered tissue distribution of permanently PEGylated or enlarged GH analogs. Unmodified GH is bound to an mPEG carrier through a linker that auto-hydrolyzes under physiologic conditions; while attached, the hormone is shielded from receptor binding and renal clearance, and once released it is indistinguishable from native somatropin (Sprogøe et al., Endocrine Connections, 2017, 6(8):R171–R181).
• Release kinetics. The approved label describes release of fully active somatropin by autocleavage of the linker. In children receiving 0.24 mg/kg/week, the prodrug reached peak concentration at a median of 25 hours with an observed half-life of about 30.7 hours, while released somatropin peaked at a median of 12 hours with an apparent half-life of about 25 hours; no significant accumulation of either species was seen on repeat dosing. The mPEG carrier is cleared by the kidneys, and somatropin itself is catabolized in liver and kidney as with any GH product.
• GH receptor action. Released somatropin binds the growth hormone receptor on target cells, with direct metabolic effects and indirect effects mediated by hepatic IGF-1, including chondrocyte proliferation at the growth plate, lipolysis and protein synthesis. In the first adult study, three weekly TransCon GH doses produced linear, dose-dependent GH exposure, with GH peak concentrations and IGF-1 exposure similar to the same weekly amount of daily somatropin (Omnitrope); no anti-GH antibodies, lipoatrophy or nodules were observed over four weeks (Höybye et al., Endocrine Connections, 2017, 6(3):129–138).
• Weekly IGF-1 cycle. In prepubertal children, weekly TransCon GH at 0.14, 0.21 or 0.30 mg/kg produced a dose response that brought IGF-1 standard deviation scores into the normal range at every dose, with GH exposure comparable to daily Genotropin (Chatelain et al., Journal of Clinical Endocrinology & Metabolism, 2017, 102(5):1673–1682). The label states that at steady state IGF-1 peaks about 2 days after a dose, with the weekly average occurring around day 4.5, and that levels remain in the normal range for GHD patients for most of the week.

Research status

Lonapegsomatropin has a completed registrational program in children and adults, multiple approvals, and an ongoing expansion into growth disorders without GH deficiency. All efficacy trials were active-comparator designs against daily somatropin; only the adult pivotal trial carried a placebo arm.

Phase 2. A four-week adult study (37 participants; NCT01247675) and a 26-week pediatric dose-finding study in 53 treatment-naive prepubertal children at 38 centers in 14 European countries and Egypt (NCT01947907) established dose-proportional exposure and IGF-1 responses comparable to daily GH. In the pediatric study, annualized height velocity ranged from 11.9 to 13.9 cm/year across the three weekly doses versus 11.6 cm/year on Genotropin, with one low-titer non-neutralizing antibody response (Chatelain et al., Journal of Clinical Endocrinology & Metabolism, 2017, 102(5):1673–1682). A dose of 0.24 mg/kg/week was carried into Phase 3.

heiGHt (NCT02781727). A randomized, open-label, 52-week Phase 3 trial in pediatric GHD at 73 sites in 15 countries dosed 161 treatment-naive prepubertal children 2:1 to weekly lonapegsomatropin (n = 105) or the same weekly amount of somatropin given daily (n = 56). Least-squares mean annualized height velocity at 52 weeks was 11.2 versus 10.3 cm/year (P = 0.009), meeting noninferiority and then superiority; height SDS rose by 1.10 versus 0.96 (P = 0.01), with similar bone-age advancement, adverse events and immunogenicity (Thornton et al., Journal of Clinical Endocrinology & Metabolism, 2021, 106(11):3184–3195). The approved label gives the treatment difference as 0.9 cm/year (95% CI 0.2 to 1.5). Letters to the same journal questioned how the modest difference and the near-identical adherence in both arms should be interpreted, and the authors responded.

fliGHt (NCT03305016). A 26-week open-label Phase 3 switch trial at 28 sites in the United States, Canada, Australia and New Zealand enrolled 146 children with GHD, 143 of whom switched from daily somatropin. The primary endpoint was safety; IGF-1 SDS rose by about 0.7 from the pre-switch baseline and stayed stable, and the adverse-event profile matched daily therapy (Maniatis et al., Hormone Research in Paediatrics, 2022, 95(3):233–243).

enliGHten (NCT03344458). The long-term open-label extension enrolled 298 children from heiGHt and fliGHt and ran from December 2017 to February 2023. At two years, height SDS in the heiGHt cohort had improved from −2.89 to −1.37 (original lonapegsomatropin arm) and from −3.0 to −1.52 (switched from daily GH), with a bone age/chronological age ratio of 0.8 and fewer local tolerability reactions after the move to the auto-injector (Maniatis et al., Journal of Clinical Endocrinology & Metabolism, 2022, 107(7):e2680–e2689). The final analysis reports mean height SDS of −0.39 at year 4 (n = 298) and treatment for up to six years; among 81 participants who completed treatment, 48 (59.3%) met or exceeded their average parental height SDS at the last visit, with adverse events consistent with daily somatropin and no evidence of accelerated skeletal maturation (Maniatis et al., Hormone Research in Paediatrics, 2026, 99(5):705–717). Near-adult height data for the whole cohort are therefore partial rather than complete.

foresiGHt (NCT04615273). A Phase 3 trial in adult GHD at 116 centers in North America, Europe and Asia-Pacific randomized 259 adults with GHD aged 23 to 81 (1:1:1) to double-blind weekly lonapegsomatropin (n = 89) or weekly placebo (n = 84), or open-label daily somatropin (n = 86), for 38 weeks. The primary endpoint, change in trunk percent fat by DXA, was −1.68% on lonapegsomatropin versus +0.37% on placebo (LS mean difference −2.04%; P < 0.001); total body lean mass rose by 1.60 kg versus −0.11 kg (difference 1.70 kg; P < 0.0001) and trunk fat mass fell by 0.48 kg versus a 0.22 kg gain (difference −0.70 kg; P = 0.0053). Safety and tolerability were comparable to daily somatropin (Biller et al., Journal of Clinical Endocrinology & Metabolism, 2026, 111(6):1640–1650; a correction notice to this article was issued on October 7, 2026). The label states that no formal statistical comparison between lonapegsomatropin and daily somatropin was made. Of the 248 who completed the trial, 220 entered a 52-week open-label extension (NCT05171855, completed December 23, 2024), reported as maintaining body-composition and IGF-1 effects without new safety signals (Biller et al., Endocrine Practice, 2026, online ahead of print).

Regulatory record. The FDA approved Skytrofa on August 25, 2021 for children aged 1 year and older weighing at least 11.5 kg with growth failure due to inadequate endogenous GH secretion, and extended the label to replacement of endogenous GH in adults with GHD on July 28, 2025; five lower-strength cartridges followed in October 2025. The European Commission granted marketing authorization on January 11, 2022 for children and adolescents aged 3 to 18 with GHD. In China, the National Medical Products Administration approved lonapegsomatropin for pediatric GHD on January 26, 2026 through Ascendis’s affiliate VISEN Pharmaceuticals. No complete response letter, withdrawal or discontinued indication is on record.

Ongoing work. An open-label Phase 3 basket trial (NCT07221851; 186 participants; started December 12, 2025) compares weekly lonapegsomatropin with daily somatropin in prepubertal children with Turner syndrome, SHOX deficiency, small-for-gestational-age short stature and idiopathic short stature, following the New InsiGHTS Phase 2 dose-ranging study in Turner syndrome (NCT05690386), whose 52-week results were published in October 2026 (Davis et al., Journal of Clinical Endocrinology & Metabolism, 2026, online ahead of print). A Phase 2 study combines lonapegsomatropin with the C-type natriuretic peptide prodrug navepegritide in children with achondroplasia (NCT06433557). A US non-interventional effectiveness and safety study (NCT05820672; 900 patients) and a post-authorization safety study (NCT05775523; 500 patients) run to 2033.

Common dosage forms

• Dual-chamber cartridges. Single-dose prefilled cartridges hold lyophilized lonapegsomatropin-tcgd in one chamber and Water for Injection in the other, supplied in cartons of four cartridges with six needles, in fourteen strengths from 0.7 to 13.3 mg somatropin-equivalent.
• Auto-injector. The cartridges are for use only with a reusable electromechanical auto-injector supplied separately by prescription.
• No vial or powder presentation. There is no multidose vial, pen or syringe presentation in any market.

Key considerations

• Regulatory status. The FDA Approved status reflects Skytrofa, approved August 25, 2021 for pediatric growth hormone deficiency and July 28, 2025 for adult growth hormone deficiency. The European Union (January 11, 2022) and China (January 26, 2026) have approved the pediatric indication. The label carries the somatropin-class contraindications and warnings, and nothing in the approval record extends to body-composition, anti-aging or performance use in people with intact GH secretion.
• Carrier, not hormone. Lonapegsomatropin delivers chemically unmodified somatropin, and the prodrug is inactive until the linker cleaves, which distinguishes it from somapacitan (hGH with a permanent albumin-binding side chain) and somatrogon (hGH fused to C-terminal peptide copies), which are themselves active, modified proteins. “PEG-hGH” and “pegylated growth hormone” are generic phrases that also describe permanently PEGylated analogs such as Jintrolong, a different molecule with a different clearance profile.
• Surrogate endpoints. The pediatric trials were open-label and powered on 52-week height velocity, and the one superiority finding is modest and debated; the adult trial was powered against placebo on a DXA measure with no formal test against daily somatropin. Near-adult height, fracture and cardiometabolic outcomes, safety beyond six years and the non-GHD pediatric indications remain under study.
• Safety and PEG exposure. Anti-drug antibodies were detected in 6.3% of children and 5.7% of adults in the registrational studies, none neutralizing, and peripheral edema was the most common adult adverse reaction (8% versus 1% on placebo). The mPEG carrier is renally cleared without observed accumulation over the trial durations, but lifelong weekly PEG dosing from early childhood has no long-term precedent and is being followed in the post-authorization safety studies.