Somapacitan

Summary

Somapacitan is a long‑acting growth‑hormone (GH) analogue approved for subcutaneous injection once a week. It is indicated for replacement therapy in adults and children with growth‑hormone deficiency (GHD), providing an alternative to the conventional daily GH injections.

Mechanism of Action

Somapacitan is a reversible albumin‑binding GH derivative that circulates bound to serum albumin, prolonging its half‑life. When released, it binds the growth‑hormone receptor (GHR) on target cells, activating the JAK2‑STAT5 pathway and stimulating hepatic production of insulin‑like growth factor‑1 (IGF‑I). The resulting rise in IGF‑I mediates the anabolic and growth‑promoting effects characteristic of GH therapy.

What the Research Shows

In the adult REAL 1 phase‑3 trial (n=301), once‑weekly somapacitan reduced truncal fat by 1.5% versus placebo and improved lean mass and IGF‑I SDS over 34 weeks; benefits were maintained to 86 weeks and safety matched daily GH. The pediatric REAL 4 trial (n=200) showed non‑inferior annualised height velocity (11.2 cm/yr) compared with daily GH over 52 weeks, with similar IGF‑I SDS and low injection‑site reactions. A 3‑year extension confirmed sustained growth (≈7.5 cm/yr) and tolerability. A 2024 network meta‑analysis placed somapacitan’s efficacy on par with daily GH and other long‑acting GHs, with comparable safety.

Reported Benefits

Weekly dosing simplifies the treatment regimen, potentially improving adherence for both adults and children. In adults, somapacitan improves body‑composition metrics such as reduced truncal fat and increased lean mass. In children, it supports normal height velocity and IGF‑I levels comparable to daily GH, while maintaining a low rate of injection‑site reactions. Overall safety appears similar to established daily GH therapy.

Limitations of the Evidence

Evidence of superiority over daily GH is lacking; trials demonstrate non‑inferiority but not greater efficacy. Long‑term data beyond three years are limited, especially regarding rare adverse events. The network meta‑analysis suggests somapacitan’s efficacy is comparable, not superior, to other long‑acting GH formulations. Most data derive from controlled trial populations, which may not reflect broader clinical practice.

Safety Considerations

Somapacitan was well tolerated in trials, with adverse‑event profiles mirroring daily GH. The most common events were mild injection‑site reactions (≈5% of patients). No new safety signals emerged over up to 86 weeks in adults or three years in children. Caution is advised in patients with known GH‑related adverse histories, such as intracranial hypertension or active malignancy, consistent with standard GH‑replacement precautions.

How It Is Administered

Somapacitan is administered subcutaneously once weekly. In clinical studies, pediatric dosing was 0.16 mg/kg per week, while adult dosing was titrated to achieve target IGF‑I standard‑deviation scores. The formulation includes a reversible albumin‑binding moiety that extends circulation time.

Routes of Administration

Goals & Uses

• Increase IGF-1 levelsBiomarkerHigh
• Improve body compositionMusculoskeletalHigh
• Reduce cardiovascular risk factorsCardiovascularModerate
• Improvement of quality of life in GHDPatient Reported OutcomesModerate
• Treatment of adult growth hormone deficiency (AGHD)Endocrinology / Hormone ReplacementHigh
• Treatment of pediatric growth hormone deficiencyEndocrinology / GrowthHigh
• Enhance quality of lifePatient ReportedModerate
• Improvement of body composition (lean mass / fat mass)Metabolic / Body CompositionHigh

Contraindications

• Active malignancyOncologyHighUse caution or avoid depending on agent and context
• Acute critical illnessCritical CareHigh
• Closed epiphyses with Prader-Willi syndrome and obesity/respiratory impairmentGenetic/RespiratoryHigh
• PregnancyPopulationModeratePotential fetal risk or insufficient safety data
• Diabetic retinopathyOphthalmologyModerate
• Hypersensitivity to somapacitan or excipientsAllergy / ImmunologyHigh

Adverse Effects

• EdemaFluid BalanceUncommonSwelling from fluid retention
• Peripheral edemaFluid BalanceCommonSwelling of the arms or legs
• HyperglycemiaMetabolicUncommonAbnormally high blood glucose
• Injection site reactionsLocalCommon
• HeadacheNeurologicCommonPain in the head or upper neck
• Intracranial hypertensionNeurologicalRare
• ArthralgiaMusculoskeletalUncommonJoint pain
• Hyperglycemia / insulin resistanceMetabolic / EndocrineUncommon
• Arthralgia / myalgiaMusculoskeletalCommon

Drug Interactions

• InsulinModerateMay increase risk of low blood sugar
• GlucocorticoidsLow
• Cytochrome P450-metabolized drugs (e.g., cyclosporine, sex steroids, anticonvulsants)Low
• Insulin / Oral antidiabeticsModerate
• Oral estrogenLow

Population Constraints

• PregnancyReproductive SafetyRelative
• History of malignancyOncologyRelative
• Pediatric patientsAgeRelative
• Diabetes mellitusMetabolicRelative
• Pediatric patients < 2 yearsAgeRelative
• Pregnant womenReproductiveRelative

Regulatory Status

• European UnionApprovedApproved: Adult growth hormone deficiencyApproved 2021.
• United StatesApprovedApproved: Adult growth hormone deficiencyApproved 2020; once‑weekly subcutaneous injection.
• United KingdomApprovedApproved: Adult growth hormone deficiencyApproved under MHRA.

Approved by FDA (2020) and EMA (2021) for adult GH deficiency; not approved for pediatric use.

Evidence & Sources

• Once-weekly Somapacitan is Effective and Well Tolerated in Adults with GH Deficiency: A Randomized Phase 3 Trial
  Journal ArticleModerateJohannsson G, et al.2020-01-01T00:00:00.000000Z
• Weekly Somapacitan is Effective and Well Tolerated in Children With GH Deficiency: The Randomized Phase 3 REAL4 Trial
  Journal ArticleModerateMiller BS, et al.2022-01-01T00:00:00.000000Z
• Efficacy, safety, and insulin-like growth factor I of weekly somapacitan in children with growth hormone deficiency: 3-year results from REAL4
  Journal ArticleModerateMiller BS, et al.2025-01-01T00:00:00.000000Z
• Long-acting growth hormone in the treatment of growth hormone deficiency in children: a systematic literature review and network meta-analysis
  Journal ArticleHighZhu J, et al.2024-01-01T00:00:00.000000Z

Frequently Asked Questions