Lonapegsomatropin (Skytrofa): The Once-Weekly Growth Hormone Prodrug Now Approved for Children and Adults (September 8, 2026)
Table of Contents
- A daily hormone, now given once a week
- What lonapegsomatropin is
- How the TransCon prodrug works
- The pediatric evidence: the heiGHt trial
- Durable growth over years: fliGHt and enliGHten
- The 2025 adult approval: the foresiGHt trial
- How it is given
- Safety and who should not take it
- How it fits with other growth hormone approaches
- Why lonapegsomatropin matters
A daily hormone, now given once a week
For decades, treating growth hormone deficiency (GHD) has meant a subcutaneous injection almost every day - six or seven times a week, often for years. Lonapegsomatropin, sold as Skytrofa and developed by Ascendis Pharma as TransCon hGH, changes the schedule without changing the medicine. It delivers the very same molecule as conventional growth hormone (unmodified somatropin) but releases it slowly from a single weekly injection. That is the core idea: keep the active hormone identical to what the body makes, and re-engineer only how and when it is released.
What lonapegsomatropin is
Lonapegsomatropin (generic name lonapegsomatropin-tcgd) is a prodrug of human growth hormone. It is built from three parts: unmodified somatropin, an inert methoxypolyethylene glycol (mPEG) carrier, and a proprietary TransCon (short for 'transient conjugation') linker that connects the two. While the hormone is attached to the carrier, it is shielded from binding its receptor and from being cleared by the kidneys, so it lingers in the body far longer than free growth hormone would. This is fundamentally different from an analog drug: nothing about the hormone's structure is permanently altered.
How the TransCon prodrug works
The clever part is the linker. At the body's normal pH and temperature, the TransCon linker slowly and predictably self-cleaves (autohydrolysis), releasing fully active, unmodified growth hormone at a controlled rate that follows first-order kinetics. In practice that means one weekly dose produces a steady release of native somatropin rather than the sharp peak-and-trough pattern of a daily injection. Once liberated, the hormone behaves exactly like endogenous growth hormone: it binds the growth hormone receptor and stimulates production of insulin-like growth factor 1 (IGF-1), the downstream signal that drives linear bone growth in children and anabolic and metabolic effects in adults.
The pediatric evidence: the heiGHt trial
Lonapegsomatropin's first approval was for children. In the pivotal Phase 3 heiGHt trial (NCT02781727), treatment-naive prepubertal children with GHD were randomized to once-weekly lonapegsomatropin or daily somatropin for 52 weeks. Weekly lonapegsomatropin met its non-inferiority goal and was numerically superior on annualized height velocity - roughly 11.2 cm/year versus about 10.3 cm/year for daily therapy - with a comparable safety profile. On that basis the FDA approved it in August 2021, making it the first once-weekly growth hormone cleared in the United States for children.
Durable growth over years: fliGHt and enliGHten
Two follow-on studies rounded out the pediatric picture. The fliGHt switch trial supported moving children already stabilized on daily growth hormone onto the weekly product, and the enliGHten long-term open-label extension (NCT03344458) followed children across 63 sites in 15 countries. enliGHten showed sustained catch-up growth, with mean height standard deviation score approaching the average-stature range (about -0.39 at year 4) and no new safety signals over multiple years of treatment. A separate Phase 3 trial, InsiGHTS, met its primary objective in girls with Turner syndrome, extending the evidence beyond classic GHD.
The 2025 adult approval: the foresiGHt trial
In July 2025 the FDA expanded lonapegsomatropin's label to adults with growth hormone deficiency. The decision rested on the Phase 3 foresiGHt trial, which randomized 259 adults with GHD (ages 23 to 80) in a 1:1:1 ratio to once-weekly lonapegsomatropin, once-weekly placebo, or daily somatropin. At week 38, weekly lonapegsomatropin was superior to placebo on the key body-composition endpoints - it significantly reduced trunk fat and increased total lean body mass - the metabolic effects that define successful growth hormone replacement in adults. An extension analysis reported continued benefit over longer follow-up.
How it is given
Lonapegsomatropin is injected subcutaneously once weekly using the SKYTROFA Auto-Injector, which uses a single-dose, dual-chamber prefilled cartridge - the freeze-dried hormone sits in one chamber and water for injection in the other, mixing at the time of use. Dosing is individualized: pediatric dosing is roughly 0.24 mg/kg once weekly, adjusted to response, while adult dosing is titrated to the patient (for example, a starting dose around 2.1 mg/week in adults not taking oral estrogen), with dose adjustments guided by clinical response, side effects, and IGF-1 levels.
Safety and who should not take it
Because it releases ordinary growth hormone, lonapegsomatropin carries the established class safety profile. Common effects include fluid retention and swelling, joint and muscle pain, headache, reduced insulin sensitivity, and unmasking of low thyroid or adrenal function; benign intracranial hypertension and slipped capital femoral epiphysis have been reported in children. It is contraindicated in acute critical illness (after open-heart or abdominal surgery, serious trauma, or acute respiratory failure), in active malignancy, in children with closed growth plates, and in certain other conditions. Growth hormone can promote tumor growth or recurrence, so patients with a cancer history need careful monitoring. In trials a minority of children developed non-neutralizing anti-drug antibodies with no apparent effect on efficacy or safety.
How it fits with other growth hormone approaches
Lonapegsomatropin is one of three long-acting growth hormones now competing with daily somatropin, alongside somapacitan and somatrogon; all aim to improve adherence, and their differences lie in the release chemistry and dosing. It is important to separate this approved, regulated hormone from the growth hormone secretagogue peptides often discussed in research circles - sermorelin, CJC-1295, tesamorelin, and ipamorelin - which coax the pituitary into releasing its own growth hormone rather than supplying the hormone directly. Lonapegsomatropin supplies native growth hormone itself, on a weekly schedule, under a prescription.
Why lonapegsomatropin matters
Beyond GHD, lonapegsomatropin is a proof of concept. Its TransCon platform - the same transient-conjugation chemistry - now underpins two other approved or advancing hormone therapies from Ascendis: palopegteriparatide (TransCon PTH) for chronic hypoparathyroidism and navepegritide (TransCon CNP) for achondroplasia. That makes lonapegsomatropin the flagship demonstration that a sustained-release prodrug can turn a burdensome daily hormone into a convenient weekly one while delivering the identical active molecule. For patients, the practical payoff is fewer injections and potentially steadier hormone exposure; for the field, it is a template for extending the dosing interval of established biologics. As always, this is background information, not medical advice - growth hormone therapy should be prescribed and monitored by a qualified clinician.
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Citations
- [1] FDA Approves SKYTROFA (Lonapegsomatropin-tcgd) for the Once-Weekly Treatment of Adults with Growth Hormone Deficiency - Ascendis Pharma (July 2025) Source
- [2] FDA Approves Skytrofa (lonapegsomatropin-tcgd) for the Once-Weekly Treatment of Adults with Growth Hormone Deficiency - Drugs.com (2025) Source
- [3] SKYTROFA (lonapegsomatropin-tcgd) for injection - Full Prescribing Information (DailyMed) Source
- [4] A Phase 3 Trial of TransCon hGH Weekly Versus Daily hGH in Children With Growth Hormone Deficiency (heiGHt; NCT02781727) - ClinicalTrials.gov Source
- [5] Children with Growth Hormone Deficiency Treated with Lonapegsomatropin Demonstrated Sustained Height Improvements for up to 6 Years: enliGHten Trial Final Results - Hormone Research in Paediatrics (Karger) Source
- [6] Lonapegsomatropin Proves Long-Term Benefit for Adult GHD in Phase 3 foresiGHt Extension Trial - HCPLive Source
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