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Lonapegsomatropin (Skytrofa)
Lonapegsomatropin isn't a new hormone — it's the same somatropin already used daily for decades, re-engineered as a once-weekly prodrug. Real Phase 3 trials in both children and adults back both of its approvals, and it now has six years of published follow-up data behind it.
On this page
In brief
Should you care? Relevant if you or your child are being treated for growth hormone deficiency and weighing once-weekly against daily injections — not a different drug, a different delivery schedule for the same hormone this site's own Human Growth Hormone page covers.The short version
- A prodrug, not a new molecule: releases ordinary, unmodified somatropin at a controlled rate from an inert carrier.
- Approved first for children (2021), then adults (2025) — each on its own placebo/active-controlled Phase 3 trial.
- The first FDA-approved once-weekly hGH product for children — somapacitan (Sogroya) reached adults a year earlier, in 2020.
A once-weekly version of an old hormone
Lonapegsomatropin (Skytrofa, Ascendis Pharma) is a long-acting prodrug that releases unmodified recombinant human growth hormone (somatropin) at a steady rate over about a week, using what the company calls its TransCon ("transient conjugation") technology — an inert polyethylene-glycol carrier chemically attached to somatropin that gradually, predictably breaks off in the body, freeing the exact same hormone molecule already used in daily hGH products. It is not a new or modified hormone; it's a new way of releasing an old one on a slower schedule.
The heiGHt trial
The FDA approved lonapegsomatropin on 25 August 2021 for pediatric patients one year and older, weighing at least 11.5 kg (25.4 lb), with growth failure due to inadequate endogenous growth hormone — the first FDA-approved once-weekly hGH product for children. Approval rested on the Phase 3 heiGHt trial (Thornton PS, et al., J Clin Endocrinol Metab 2021;106(11):3184-3195, PMID 34272849, DOI 10.1210/clinem/dgab529), which randomized 161 treatment-naive children with growth hormone deficiency to once-weekly lonapegsomatropin or daily somatropin for 52 weeks. The trial met its primary non-inferiority objective and further showed statistically superior annualized height velocity for the weekly product: 11.2 cm/year versus 10.3 cm/year for daily somatropin, with a similar safety profile between arms.
Six years of follow-up, and an adult approval
The enliGHten open-label extension study has continued following heiGHt trial participants on lonapegsomatropin, with published results reporting sustained height gains through 6 years of treatment — a genuinely long follow-up window for a pediatric growth therapy. Separately, the FDA approved lonapegsomatropin for adult growth hormone deficiency on 28 July 2025, letting adults initiating GH therapy — or switching from a daily product — use the same once-weekly option already available to children.
How the weekly release actually works
TransCon's carrier is designed to be biologically inert on its own: the PEG-hGH conjugate circulates without hormonal activity until a slow, predictable hydrolysis reaction releases free, unmodified somatropin, which then acts exactly like standard daily hGH at the growth hormone receptor. That's a different chemical strategy from Somapacitan (Sogroya), this site's other once-weekly hGH page, which instead attaches a small albumin-binding group that lets the intact somapacitan molecule itself bind and unbind from circulating albumin — two different engineering answers to the same problem of making an inherently short-acting hormone last a week.
Same deficiency-state evidence base as daily hGH
Everything on this site's Human Growth Hormone page about approved indications, the historical cadaver-derived-GH safety chapter, and the lack of trial evidence for off-label anti-aging/bodybuilding use applies to the underlying hormone here too — lonapegsomatropin changes the release schedule, not the indication or the evidence base for unapproved uses.
References
- Thornton PS, Maniatis AK, Aghajanova E, et al., "Weekly Lonapegsomatropin in Treatment-Naïve Children With Growth Hormone Deficiency: The Phase 3 heiGHt Trial," J Clin Endocrinol Metab 2021;106(11):3184-3195, PMID 34272849, DOI: 10.1210/clinem/dgab529.
- FDA approval of Skytrofa (lonapegsomatropin-tcgd) for pediatric growth hormone deficiency, 25 August 2021; FDA approval for adult growth hormone deficiency, 28 July 2025 (Ascendis Pharma press releases and FDA approval records).
- enliGHten open-label extension study of heiGHt trial participants — published 6-year sustained height-velocity follow-up data (Ascendis Pharma/investigator reporting).