What Is Somatrogon?

1. How does somatrogon work, and why only once a week?

Somatrogon is a fusion protein made in Chinese hamster ovary (CHO) cells. It contains the full amino acid sequence of human GH, with one copy of the C-terminal peptide (CTP) of hCG’s beta chain attached at the start of the chain and two copies in tandem at the end. The result weighs about 40 kDa, against 22 kDa for natural GH (FDA prescribing information).

The CTP tails nearly double the size of the molecule and slow its removal from the blood. The label states that the resulting effective half-life, covered in the next section, is what allows weekly dosing. Most daily somatropin, by contrast, is the plain 22 kDa hormone.

Once in circulation, somatrogon binds the GH receptor and activates the same STAT5b signaling pathway as natural GH, raising IGF-1 (insulin-like growth factor 1). GH and IGF-1 together drive bone growth in children and shift metabolism, increasing fat breakdown and reducing insulin sensitivity.

That makes somatrogon a replacement hormone, not a releaser. Peptides such as tesamorelin, sermorelin and CJC-1295 instead prompt the pituitary to release more of the body’s own GH, which requires a pituitary that can still respond.

2. How long does somatrogon last in the body?

The label’s pharmacokinetic data come from a population model built on 151 children aged 3 to 15.5 years with GH deficiency (Ngenla label):

Exposure rose in proportion to dose across 0.25, 0.48 and 0.66 mg/kg per week. It falls as body weight rises, but the label says the 0.66 mg/kg dose gave adequate exposure across the 10–54 kg weight range studied. Age, sex, race and ethnicity had no clinically meaningful effect. Somatrogon has not been studied in kidney or liver impairment.

The 2017 phase 2 dose-finding trial by Zelinska and colleagues estimated a half-life about five to 10 times longer than daily GH. In that study, IGF-1 SDS did not exceed +2 at any of the three doses. IGF-1 SDS is IGF-1 expressed as the number of standard deviations from the average for a child’s age and sex, so 0 is average and +2 is the upper edge of normal.

Because IGF-1 rises and falls across the week, a single result needs a timestamp. Nayak and colleagues used 16,213 dosing records from the phase 2 and 3 trials to build a correction table. A level taken 96 hours (day 4) after the injection best reflects the weekly average, and the table adjusts samples taken on other days to that reference.

A half-life chart built from the label’s figures shows how drug levels fall over the week. It cannot predict an individual’s IGF-1.

3. What did the pediatric trials find?

Phase 2: choosing the dose

Zelinska and colleagues randomized 53 prepubertal children with GH deficiency at 14 European centers to one of three weekly doses (0.25, 0.48 or 0.66 mg/kg per week) or daily GH at 0.24 mg/kg per week, for 12 months. All groups showed catch-up growth, and 0.66 mg/kg per week came closest to daily GH. No serious adverse events occurred on the study drug.

Phase 3: the pivotal trial

Deal and colleagues’ phase 3 trial (NCT02968004) randomized 228 treatment-naive prepubertal children, boys aged 3–11 and girls 3–10, to weekly somatrogon 0.66 mg/kg or daily somatropin 0.24 mg/kg per week for 12 months. The trial was open-label, meaning families knew which drug they were on, and ran at 84 sites in 24 countries (FDA Drug Trials Snapshot). In all, 224 children received at least one dose.

The lower end of the confidence interval, −0.24 cm/year, sat well above the prespecified noninferiority margin of −1.8 cm/year. In plain terms, weekly somatrogon was shown not to be meaningfully worse than daily GH. It was not shown to be better. Children started at an average height SDS of −2.9, far below average for their age (Ngenla label).

Long-term extension

Children who finished the main study could continue on somatrogon, and those on daily GH switched to it. The data now run to about five years:

Growth naturally slows after the catch-up phase, so falling velocity is expected. Mean height SDS rose every year and approached 0, the population average, by extension year 4. Mean IGF-1 SDS stayed positive and within the normal range. Up to one quarter of children had IGF-1 SDS above +2 on two consecutive checks in extension years 1 and 2, which met the protocol’s rule for a dose reduction. Children who switched from daily GH grew similarly to those who had been on somatrogon from the start.

Japan and real-world data

A separate Japanese trial (NCT03874013) followed 42 children into a 3-year extension, for up to 4 years of treatment in total. Height SDS rose steadily in both original groups, and no child stopped or needed a dose cut because of an adverse event (Horikawa et al., 2026).

A 2026 matched comparison by Deal and colleagues paired 155 children from the phase 3 program with 155 similar children treated with daily somatropin in KIGS, a long-running Pfizer growth registry. Height velocity was similar in years 1 to 3 (10.03 vs 9.57, 7.70 vs 7.33 and 7.15 vs 6.56 cm/year). In a Turkish real-world cohort of 39 older children (mean age 12.9 years), annual height gain averaged 10.07 cm over about 11 months, and 5 children (12.8%) needed a dose reduction for IGF-1 SDS above +2. The study’s authors note IGF-1 may run higher in children going through puberty.

Treatment burden

The case for weekly GH is easier adherence. Maniatis and colleagues ran a randomized crossover study in 87 children already on daily somatropin: 12 weeks on each regimen, in random order. Life interference scores, where higher means more disruption, were 8.63 after somatrogon and 24.13 after daily injections, a difference of −15.49 (95% CI −19.71 to −11.27). Adverse events were reported by 54.0% on somatrogon and 44.2% on daily somatropin, with none severe or serious. A 2026 narrative review cautions that real-world adherence data for weekly GH are still very limited.

4. What happened with adults?

Adults with GH deficiency were part of the plan from the start, but the program has not produced an approval.

Fleseriu and colleagues’ adult phase 3 trial (NCT01909479) randomized 202 adults 2:1 to weekly somatrogon or placebo for 26 weeks, double-blind, followed by open-label somatrogon for everyone. Doses were set by sex, age and estrogen use. Of these, 198 were treated: 133 on somatrogon and 65 on placebo.

The primary endpoint was missed. Several secondary measures of lean mass and fat distribution favored somatrogon, and IGF-1 SDS rose from a baseline of about −2.6 into the normal range. Adverse events were similar between groups. The authors pointed to outliers, more women with adult-onset GH deficiency and estrogen use in the somatrogon arm, and possibly dosing as reasons. The trial was sponsored by OPKO Biologics.

The European Medicines Agency’s withdrawal notice says Pfizer applied to extend Ngenla to adults with GH deficiency and withdrew the application on December 20, 2024. At that point the agency’s provisional view was that it could not have been authorized for adults, because the study did not show a benefit on trunk fat mass. Pfizer said it withdrew to allow additional analyses in that population. The current US label, updated in 2026, remains pediatric only. As of November 2026, I found no public record of a new adult filing.

By contrast, Sogroya and Skytrofa both carry US approvals for adults with GH deficiency, as covered on the somapacitan and lonapegsomatropin pages.

5. Who is Ngenla approved for, and is it legal?

The FDA approved Ngenla on June 27, 2023, based on the single phase 3 trial above (FDA Drug Trials Snapshot). The label’s indication is children aged 3 years and older with growth failure due to inadequate secretion of endogenous GH. It specifically says Ngenla is not indicated for growth failure due to genetically confirmed Prader-Willi syndrome.

The EMA authorized it on February 14, 2022, for the long-term treatment of children and adolescents from 3 years of age with growth disturbance due to insufficient GH secretion. It is also approved in Japan, according to the Japanese extension study, and OPKO’s product page, checked in November 2026, says it is approved in 40 markets for pediatric patients.

In the US it is a prescription biologic and not a controlled substance. The WADA 2026 Prohibited List names somatrogon, with lonapegsomatropin and somapacitan, as examples of GH analogs in class S2.2.3, prohibited at all times as non-Specified Substances. Anti-doping scientists published a urine test for somatrogon in 2025.

6. How is Ngenla dosed?

The figures below are the label’s; choosing and adjusting the dose is the prescriber’s job (Ngenla label).

If one injection cannot deliver the full dose, the label says to split it across different sites. Pens are refrigerated before and between uses and discarded 28 days after first use.

Milligram doses are not interchangeable between weekly products. Somatrogon is a heavier molecule than somapacitan or the somatropin released by Skytrofa, and each label sets its own mg/kg dose from its own trials. The general idea of stepwise dose changes is covered in the dose titration guide.

7. What are the side effects and warnings?

Ngenla has no boxed warning. Its warnings mirror those of daily GH. In the 12-month pivotal trial, these adverse reactions occurred in at least 5% of children in either group (Ngenla label):

Within injection site reactions, pain was reported by 39% versus 25%, swelling or inflammation 10% versus 1%, and redness 8% versus none. Families recorded reactions in a diary after every injection. Blood eosinophils, a type of white cell, moved from normal to high in 29% of children on somatrogon versus 12% on daily GH.

Later studies confirm the pattern. A 2026 network meta-analysis of 18 randomized trials and 3,137 children found somatrogon carried a much higher risk of injection site redness (risk ratio 10.55) and pain than daily GH, while overall discontinuation rates did not differ. In a small Japanese study, 19 children who reported pain on somatrogon switched to somapacitan; their pain scores fell from 6.5 to 1.6 out of 10, and all preferred to stay on the new drug (Iwayama et al., 2026). That study was single-arm and unblinded.

Antibodies against the drug were common. In the main study, 77.1% of children tested positive for anti-drug antibodies, mostly directed at the GH part of the molecule, and 3.7% developed neutralizing antibodies, all transient. The label reports no clinically significant effect on safety or growth over up to 42 months.

Other labeled warnings and contraindications include:

• higher death rates when GH is given during acute critical illness after heart or abdominal surgery, major trauma or acute respiratory failure;
• possible tumor progression; active cancer is a contraindication, and childhood cancer survivors who had head radiation face a higher risk of second tumors;
• raised pressure inside the skull, with headache and vision changes, usually within the first 8 weeks;
• fluid retention and carpal tunnel symptoms;
• falling cortisol or thyroid function after GH is started;
• slipped hip growth plate, worsening scoliosis and pancreatitis;
• sudden death reported in children with Prader-Willi syndrome who are severely obese or have breathing problems;
• closed growth plates and active diabetic eye disease, both contraindications.

One warning is unique to this product. Because the molecule carries hCG fragments, the label says it may interfere with hCG-based blood and urine pregnancy tests, giving false positive or false negative results, and recommends methods that do not rely on hCG. No human pregnancy data exist.

8. How does somatrogon compare with other GH options?

The somapacitan and lonapegsomatropin figures come from their labels and trials as summarized on the somapacitan and lonapegsomatropin pages. The pivotal trials used different designs and comparator doses, so the cm/year figures cannot be ranked against each other directly. The network meta-analysis by Abadi and colleagues found no significant differences in height SDS across products, and its authors suggest choosing among them based on the individual patient, with the higher rate of local reactions on somatrogon in mind.

9. Blood sugar, interactions and GLP-1 drugs

Growth hormone works against insulin. The Ngenla label warns that it may reduce insulin sensitivity, particularly at higher doses, that new type 2 diabetes has been reported with GH, and that glucose should be checked periodically, especially with obesity or a family history of diabetes.

The label’s interaction list:

GLP-1 drugs are not named, and no trial has paired somatrogon with semaglutide or tirzepatide. For a young person with diabetes who is also on a GLP-1 drug and insulin, the label’s diabetes-drug warning is the relevant one. Starting both drugs at once also makes it hard to tell which caused a change in glucose, appetite or weight.

10. Can somatrogon offset muscle loss on GLP-1 drugs?

There is no evidence for this. Lean-mass loss during weight loss is a real concern, covered in the GLP-1 and muscle loss guide, but somatrogon is approved only for children, and no trial has enrolled people taking GLP-1 drugs or adults with normal GH levels.

The adult trial is the closest data point, and it enrolled adults with diagnosed GH deficiency whose IGF-1 started far below normal. Lean mass improved, but the trial did not measure strength or physical function, and it missed its fat-mass goal. In someone with normal GH, the label’s risks become risks of GH excess: the overdose section lists low and then high blood sugar, fluid retention and, long term, signs of gigantism. The HGH guide reviews what GH has and has not shown in healthy adults.

11. What is worth recording on weekly GH?

Weekly dosing makes timing matter. An IGF-1 result only makes sense alongside the number of days since the last injection, and the published correction method assumes that number is known (Nayak et al., 2025). What the label asks clinicians to monitor:

A useful record keeps the injection day, the dose, the pen used, the injection site and the date and time of each IGF-1 draw together. Injection site pain and redness belong in the same log, since they were the most common treatment-related adverse events in the long-term extension (final extension report, 2026). The peptide tracker vs spreadsheet guide covers ways to keep those records in one place.

12. Somatrogon FAQ

• Is somatrogon the same as HGH?

  It contains the full amino acid sequence of human growth hormone, but it is a larger fusion protein of about 40 kDa. One copy of a C-terminal peptide (CTP) from hCG is attached at one end and two copies at the other. It acts on the same GH receptor, but stays in circulation much longer, so it is given weekly.

• What is Ngenla approved for?

  In the US, Ngenla is approved only for children aged 3 years and older who have growth failure due to inadequate secretion of their own growth hormone. The EU indication is the long-term treatment of children and adolescents from 3 years with growth disturbance due to insufficient GH secretion. It is not approved for adults.

• Is somatrogon approved for adults with growth hormone deficiency?

  No. The adult phase 3 trial did not show a significant difference from placebo in trunk fat mass, its primary endpoint, although lean mass improved. Pfizer withdrew its EU application to add adults on December 20, 2024, after the EMA's provisional view was that it could not be authorized. The current US label, revised in 2026, still covers children only.

• How long does somatrogon stay in your system?

  The FDA label gives an effective half-life of about 37.7 hours in children with GH deficiency and says the drug is present in circulation for about 8 days after the last dose. IGF-1 peaks about 2 days after each injection, and the average weekly IGF-1 level occurs about 4 days after the dose.

• Does Ngenla hurt more than daily growth hormone?

  In the pivotal trial, injection site pain was reported by 39% of children on somatrogon versus 25% on daily somatropin, using a diary after every injection. A 2026 network meta-analysis also found more injection site redness and pain with somatrogon than with daily GH. Most reactions were mild to moderate.

• How does Ngenla compare with Sogroya and Skytrofa?

  All three are weekly GH products for children with GH deficiency, and each was noninferior to or better than daily somatropin for growth in its pivotal trial. They differ in design: somatrogon is a CTP fusion, somapacitan (Sogroya) binds albumin, and lonapegsomatropin (Skytrofa) releases unmodified somatropin from a PEG carrier. Sogroya and Skytrofa are also FDA-approved for adults; Ngenla is not.

• Can somatrogon be used with semaglutide or tirzepatide?

  No trial has studied that combination, and the Ngenla label does not mention GLP-1 drugs. It does warn that growth hormone can reduce insulin sensitivity and that doses of insulin and other glucose-lowering drugs may need adjustment, so any combination belongs under a prescriber's supervision.

• Is somatrogon banned in sport?

  Yes. The WADA 2026 Prohibited List names somatrogon as an example of a growth hormone analog in class S2.2.3, which is prohibited at all times, in and out of competition. A urine test that detects it was published in 2025.

13. Sources