Strensiq (asfotase alfa)
An approved treatment for Hypophosphatasia.
The same compound appears under different names depending on the context. Here is how to identify Asfotase alfa wherever you encounter it, plus the key facts at a glance.
- Generic name
- Asfotase alfa
- Brand name
- Strensiq
- Development code
- ENB-0040
- Drug class
- Enzyme replacement therapy
- Manufacturer
- Alexion, AstraZeneca Rare Disease
- How it's taken
- Subcutaneous injection at home, 3 or 6 times a week, dosed by body weight
The only FDA-approved treatment for hypophosphatasia, an injection under the skin given 3 or 6 times a week that replaces the missing alkaline phosphatase enzyme. It is approved for people whose HPP began before birth, in infancy or in childhood, at any current age.
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Where Asfotase alfa fits
The first and only approved disease-modifying treatment for HPP since 2015. It is used for the severe perinatal and infantile forms, where it changed survival, and for childhood-onset disease in children and adults. Adult-onset HPP has no approved drug; care there is supportive, with NORD advising against bisphosphonates. Efzimfotase alfa, under FDA review with a decision expected in the first half of 2027, would be the first option for adult-onset patients and a 1-injection-every-2-weeks alternative for people already on Strensiq.
How Asfotase alfa works
People with HPP have too little tissue-nonspecific alkaline phosphatase (TNSALP) because of changes in the ALPL gene. Without enough of the enzyme, a substance called inorganic pyrophosphate builds up and blocks calcium and phosphate from forming the hard mineral in bone, so children develop rickets and adults develop soft bones (osteomalacia) and muscle weakness[1].
Asfotase alfa is a laboratory-made copy of the working part of TNSALP joined to 2 extra pieces: an antibody fragment that keeps it in the body longer and a short chain of 10 aspartate amino acids that sticks to bone mineral, so the enzyme goes where it is needed[1]. Once there it breaks down the excess pyrophosphate and bone can mineralize again. The label describes this as replacing the enzyme and reducing the substrate levels that drive the disease[1].
Mechanism: Bone-targeted recombinant tissue-nonspecific alkaline phosphatase (TNSALP) fused to an IgG1 Fc domain and a deca-aspartate bone-binding tail; replaces the missing enzyme and lowers inorganic pyrophosphate so bone can mineralize
Side effects and safety
- Hypersensitivity including anaphylaxis. Life-threatening reactions reported early and late in treatment; the drug is stopped and epinephrine given, and restarting is a case-by-case decision.
- Lipodystrophy at injection sites. Thickened or pitted fat under the skin after several months of injections; rotate sites and avoid red, swollen or inflamed areas.
- Ectopic calcification (eye and kidney). Calcium deposits on the conjunctiva or cornea and nephrocalcinosis or kidney stones were reported; HPP itself raises this risk, so eye exams and kidney ultrasounds are done at baseline and periodically.
- Possible immune-mediated loss of effect. Most patients develop anti-drug antibodies; some who first improved later worsened. Report new fractures, more difficulty walking or more breathing support. Antibody testing is arranged through Alexion Medical Information (1-888-765-4747).
- Laboratory interference. Serum ALP reads high on treatment and ALP-based lab assays can give wrong results; tell the lab the patient is on Strensiq.
- Observation under a healthcare provider when treatment starts
- Eye examinations at baseline and periodically
- Kidney ultrasounds at baseline and periodically
- Growth, X-rays, breathing status and PLP to judge response; worsening after a good response prompts antibody testing
- Injection sites for lipodystrophy
The label carries a boxed warning for severe allergic reactions including anaphylaxis, which have happened within minutes of an injection and also more than a year after starting, so treatment begins under medical supervision and families are taught the warning signs: trouble breathing, a choking feeling, nausea, swelling around the eyes and dizziness[1].
In the trials that supported approval, 99 patients aged 1 day to 58 years were treated, most for more than 2 years. Injection site reactions were by far the most common problem (63%), including redness, discoloration, pain, itching, swelling and hard lumps, and they reached 90% in the children with juvenile-onset HPP[1]. Lipodystrophy, a thickening or pitting of the fat under the skin at injection sites, affected 28% and usually appeared after several months, which is why rotating sites matters[1].
Calcium deposits in the eye or kidney (ectopic calcification) were reported in 14%, though HPP itself causes these and the label could not tell whether the drug added to the risk; eye exams and kidney ultrasounds are recommended before starting and periodically after[1]. Hypersensitivity reactions of any kind occurred in 12%[1].
Most patients develop antibodies to the drug, and after approval some patients who had responded well later worsened, with rising PLP and new bone changes, so the label asks families to report any return of HPP symptoms such as new fractures, more trouble walking or more breathing support[1]. One practical point: the drug is itself alkaline phosphatase, so blood tests that use ALP as a reagent can read wrong and a serum ALP result on treatment means nothing clinically[4].
This is not a complete list of side effects. Talk to your doctor or pharmacist about what to expect and when to seek medical attention.
Taking Asfotase alfa
Strensiq is injected under the skin (subcutaneously) at home after training, usually by a parent or the patient. The approved dose for both perinatal/infantile-onset and juvenile-onset HPP is 6 mg/kg a week, given either as 2 mg/kg 3 times a week or 1 mg/kg 6 times a week; the label notes that injection site reactions may make the 6-times-a-week schedule harder to tolerate[1].
For perinatal/infantile-onset HPP the dose can be raised to 9 mg/kg a week (3 mg/kg 3 times a week) if breathing, growth or X-rays are not improving[1]. It comes in single-use vials of 18 mg/0.45 mL, 28 mg/0.7 mL and 40 mg/mL, all at 40 mg per mL, plus an 80 mg/0.8 mL vial at 100 mg per mL that must not be used in children under 40 kg because it delivers less drug to the body[1].
Doses over 1 mL are split into 2 injections at 2 sites. Vials stay refrigerated at 2 to 8 C in the carton, are taken out 15 to 30 minutes before use, must be injected within 3 hours of leaving the refrigerator, and are never frozen or shaken[1]. Because dosing is by weight, the number of vials rises as a child grows.
Availability and cost
Only available as the brand-name product.
Strensiq is an orphan biologic dosed by body weight 3 or 6 times a week, so the amount of drug, and the cost, rises as a child grows and stays high for life. It is one of the more expensive rare disease medicines in the US; Alexion's copay program covers commercially insured patients, and NORD runs HPP-specific premium, copay and medical assistance funds for others.
Help paying for Strensiq
Pick your insurance to see which help fits. Drugmaker copay cards can't be used with Medicare, Medicaid or TRICARE; charity funds are the usual route there.
- Copay help
Alexion OneSource CoPay Program: eligible patients with commercial insurance can pay as little as $0 for Strensiq. You must be enrolled in OneSource, prescribed Strensiq for an FDA-approved indication and live in the US or its territories; annual maximums may apply and the full terms govern. Not for Medicare, Medicaid or other government plans.
For: private insurance · source - Insurance and case manager help
OneSource specialists help people start and stay on treatment, provide injection training and coordinate with PANTHERx Rare, the specialty pharmacy that fills Strensiq. Phone 1-888-765-4747 or OneSource@Alexion.com.
For: private insurance, Medicare, Medicaid, TRICARE, no insurance, underinsured · source - Other support
For people on Medicare, Medicaid or another government plan, or who do not qualify for the CoPay Program, OneSource connects patients to independent third-party foundations that may help with out-of-pocket costs. NORD's RareCare program lists HPP-specific Premium Copay Assistance, Medical Assistance and Emergency Relief funds (866-828-8902), shown as accepting applications on October 18, 2026.
For: Medicare, Medicaid, TRICARE, no insurance, underinsured · source
Good to know: Alexion's pages do not describe a free-drug program for Strensiq for uninsured patients; ask OneSource directly. The CoPay Program page (US/ALL/1077 V1 09/2024) says the program cannot be combined with other assistance and that moving from commercial to government insurance mid-year ends eligibility. NORD's HPP funds are not yet in assistance-funds.json; consider adding them there with the status text NORD shows.
Access and eligibility
The label covers HPP that began before birth, in infancy or in childhood (perinatal/infantile-onset and juvenile-onset), and NORD notes that in the US patients of any current age with pediatric-onset disease are eligible. Adults whose symptoms began only in adulthood are outside the approved indication, which is the group efzimfotase alfa's application is meant to add.
Source: Alexion OneSource
Access program details are provided for informational purposes and may vary based on insurance coverage, geographic location, and individual circumstances. Confirm current eligibility directly with the manufacturer or your specialty pharmacy.
How hypophosphatasia treatments compare
Hypophosphatasia has 1 FDA-approved medicine, Strensiq, and a successor enzyme from the same company, efzimfotase alfa, that the FDA is reviewing with a decision expected in the first half of 2027. Both replace the missing alkaline phosphatase enzyme; they differ in how often they are injected and in which patients they were studied. Your metabolic bone specialist will weigh when your HPP began, your age and your injection site history. Adults with adult-onset HPP have no approved drug today and are managed with supportive care.
Hypophosphatasia has 1 FDA-approved treatment: Strensiq (asfotase alfa, approved 2015). Also compared, not FDA-approved for hypophosphatasia: Efzimfotase alfa (no brand yet) (efzimfotase alfa (ALXN1850), Under FDA review (decision expected first half of 2027)). Strensiq is typically used as perinatal/infantile-onset and juvenile-onset hpp at any current age; not approved for adult-onset hpp.
| Drug | How it works | How it’s given | How often | Where you get it | Who it is for | FDA status |
|---|---|---|---|---|---|---|
Strensiq asfotase alfa You are here | Enzyme replacement: a bone-targeted copy of tissue-nonspecific alkaline phosphatase that clears the pyrophosphate blocking bone mineralization | Subcutaneous injection from a vial and syringe (self or caregiver injected after training) | 3 times a week (2 mg/kg) or 6 times a week (1 mg/kg); up to 3 times a week at 3 mg/kg in perinatal and infantile-onset HPP | Home, after supervised first doses | Perinatal/infantile-onset and juvenile-onset HPP at any current age; not approved for adult-onset HPP | 2015 |
Efzimfotase alfa (no brand yet) efzimfotase alfa (ALXN1850) | Enzyme replacement: a longer-acting second-generation alkaline phosphatase from the same company | Subcutaneous injection (studied as self-administered) | Once every 2 weeks at a fixed dose by weight band (20 mg, 35 mg or 50 mg in Phase 3) | Home (if approved as studied) | Under FDA review for all patients aged 2 and older, including adult-onset HPP; filed on the HICKORY, MULBERRY and CHESTNUT trials | Under FDA review (decision expected first half of 2027) |
This chart is for education only and is not medical advice. Efzimfotase alfa is investigational in the United States; its dosing, who it will be approved for and its side effects will be set by the FDA label if it is approved. Strensiq figures come from its prescribing information. Talk with a metabolic bone specialist about your own situation.
Clinical trial results
Approval rested on 4 open-label trials in 89 children plus extension studies, compared against untreated historical patients because a placebo arm was not considered ethical in a lethal infant disease[1].
In the pooled perinatal and infantile-onset studies (68 treated, 48 historical controls), 91% of treated patients were alive at last contact against 27% of controls, 97% versus 42% were alive at age 1, and 85% versus 25% were alive without invasive ventilation; 74% of treated infants met the X-ray definition of a rickets response, and in the long-term extension 69 of 78 (88%) had survived with follow-up of up to 7 years[1].
The first of these trials was published in the New England Journal of Medicine in 2012[3]. In juvenile-onset HPP, 8 children treated for at least 48 months gained height and weight against 32 untreated historical controls[1]. Alexion's follow-on enzyme, efzimfotase alfa, used Strensiq as the active comparator in the CHESTNUT switch trial, where children who switched had similar adverse event rates (90.5% versus 86.4%) and kept their bone gains[5].
Development history
Asfotase alfa was first studied under the code ENB-0040; the pivotal trials still carry Enobia-era study numbers in the FDA's letters, and Alexion brought it through approval. The FDA approved Strensiq on October 23, 2015 as the first treatment for perinatal, infantile and juvenile-onset HPP[2]. After approval, reports of patients losing their response prompted the FDA in June 2020 to require a long-term registry study of immune-mediated loss of effect and to add the possible immune-mediated clinical effects warning[2].
A boxed warning for anaphylaxis was added in July 2024[1]. Alexion became AstraZeneca's rare disease unit in 2021. In 2026 AstraZeneca reported Phase 3 results for a successor enzyme, efzimfotase alfa, dosed every 2 weeks, and the FDA accepted its application under priority review on September 18, 2026, with a decision expected in the first half of 2027[4].
Explore Hypophosphatasia trials
Other Hypophosphatasia treatments
Hypophosphatasia has 1 approved drug today, Strensiq, which replaces the missing enzyme. The next step is a longer-acting version of the same idea: efzimfotase alfa, injected every 2 weeks instead of 3 to 6 times a week and studied in adults with adult-onset disease as well as children. A different approach, BioMarin's oral ALE1, aims to lower pyrophosphate with a pill and is in an early safety study. Supportive care such as pain control, dental care and fracture management continues alongside any of these.
Common questions about Asfotase alfa
▸What is Asfotase alfa (Strensiq)?
The only FDA-approved treatment for hypophosphatasia, an injection under the skin given 3 or 6 times a week that replaces the missing alkaline phosphatase enzyme. It is approved for people whose HPP began before birth, in infancy or in childhood, at any current age.
▸How does Asfotase alfa work?
People with HPP have too little tissue-nonspecific alkaline phosphatase (TNSALP) because of changes in the ALPL gene. Without enough of the enzyme, a substance called inorganic pyrophosphate builds up and blocks calcium and phosphate from forming the hard mineral in bone, so children develop rickets and adults develop soft bones (osteomalacia) and muscle weakness[1]. Asfotase alfa is a laboratory-made copy of the working part of TNSALP joined to 2 extra pieces: an antibody fragment that keeps it in the body longer and a short chain of 10 aspartate amino acids that sticks to bone mineral, so the enzyme goes where it is needed[1]. Once there it breaks down the excess pyrophosphate and bone can mineralize again. The label describes this as replacing the enzyme and reducing the substrate levels that drive the disease[1].
▸What are the side effects of Asfotase alfa?
The label carries a boxed warning for severe allergic reactions including anaphylaxis, which have happened within minutes of an injection and also more than a year after starting, so treatment begins under medical supervision and families are taught the warning signs: trouble breathing, a choking feeling, nausea, swelling around the eyes and dizziness[1]. In the trials that supported approval, 99 patients aged 1 day to 58 years were treated, most for more than 2 years. Injection site reactions were by far the most common problem (63%), including redness, discoloration, pain, itching, swelling and hard lumps, and they reached 90% in the children with juvenile-onset HPP[1]. Lipodystrophy, a thickening or pitting of the fat under the skin at injection sites, affected 28% and usually appeared after several months, which is why rotating sites matters[1]. Calcium deposits in the eye or kidney (ectopic calcification) were reported in 14%, though HPP itself causes these and the label could not tell whether the drug added to the risk; eye exams and kidney ultrasounds are recommended before starting and periodically after[1]. Hypersensitivity reactions of any kind occurred in 12%[1]. Most patients develop antibodies to the drug, and after approval some patients who had responded well later worsened, with rising PLP and new bone changes, so the label asks families to report any return of HPP symptoms such as new fractures, more trouble walking or more breathing support[1]. One practical point: the drug is itself alkaline phosphatase, so blood tests that use ALP as a reagent can read wrong and a serum ALP result on treatment means nothing clinically[4].
▸How is Asfotase alfa taken?
Strensiq is injected under the skin (subcutaneously) at home after training, usually by a parent or the patient. The approved dose for both perinatal/infantile-onset and juvenile-onset HPP is 6 mg/kg a week, given either as 2 mg/kg 3 times a week or 1 mg/kg 6 times a week; the label notes that injection site reactions may make the 6-times-a-week schedule harder to tolerate[1]. For perinatal/infantile-onset HPP the dose can be raised to 9 mg/kg a week (3 mg/kg 3 times a week) if breathing, growth or X-rays are not improving[1]. It comes in single-use vials of 18 mg/0.45 mL, 28 mg/0.7 mL and 40 mg/mL, all at 40 mg per mL, plus an 80 mg/0.8 mL vial at 100 mg per mL that must not be used in children under 40 kg because it delivers less drug to the body[1]. Doses over 1 mL are split into 2 injections at 2 sites. Vials stay refrigerated at 2 to 8 C in the carton, are taken out 15 to 30 minutes before use, must be injected within 3 hours of leaving the refrigerator, and are never frozen or shaken[1]. Because dosing is by weight, the number of vials rises as a child grows.
▸Is Asfotase alfa FDA approved?
Yes, Asfotase alfa (Strensiq) is FDA approved (2015) for the treatment of Hypophosphatasia.
▸Who can get Strensiq?
The FDA label covers patients with perinatal/infantile-onset and juvenile-onset HPP, meaning the disease started before birth, in infancy or in childhood. Age today does not matter; an adult whose first symptoms were in childhood qualifies, according to NORD. Someone whose HPP first appeared in adulthood is outside the approved use.
▸How often is Strensiq injected?
Either 3 times a week at 2 mg/kg or 6 times a week at 1 mg/kg, both adding up to 6 mg/kg a week. Infants and young children with perinatal or infantile-onset disease can go up to 9 mg/kg a week if they are not improving. It is given under the skin at home after training.
▸What are the most common side effects of Strensiq?
Injection site reactions, seen in 63% of trial patients overall and 90% of those with juvenile-onset HPP, followed by lipodystrophy at injection sites (28%), calcium deposits in the eye or kidney (14%) and allergic-type reactions (12%). The label has a boxed warning for anaphylaxis.
▸Is there help paying for Strensiq?
Alexion OneSource runs a CoPay Program that can bring the cost to as little as $0 a month for eligible people with commercial insurance, and refers people on Medicare or Medicaid to independent foundations. NORD's RareCare program lists HPP-specific premium, copay, medical and emergency assistance funds (866-828-8902).
▸Will Strensiq be replaced by efzimfotase alfa?
Not yet decided. Efzimfotase alfa is Alexion's longer-acting enzyme, given every 2 weeks, and the FDA accepted its application under priority review on September 18, 2026 with a decision expected in the first half of 2027. In the CHESTNUT trial children who switched from Strensiq kept their bone gains with similar side effect rates. Until a decision, Strensiq remains the only approved option.
Sources and references
Every factual claim on this page is drawn from the public sources listed below. Click any reference to open the original document.
- Alexion Pharmaceuticals via openFDA · 2025-12-12. STRENSIQ (asfotase alfa) injection, prescribing information. https://api.fda.gov/drug/label.json?search=set_id:%223387574f-5eaa-4501-a71d-4cbfbd563031%22
- U.S. Food and Drug Administration · 2020-06-12. BLA 125513/S-018 supplement approval letter (approval date October 23, 2015; postmarketing requirements). https://www.accessdata.fda.gov/drugsatfda_docs/appletter/2020/125513Orig1s018ltr.pdf
- New England Journal of Medicine · 2012-03-08. Enzyme-replacement therapy in life-threatening hypophosphatasia. https://pubmed.ncbi.nlm.nih.gov/22397652/
- AstraZeneca · 2026-09-18. Efzimfotase alfa granted Priority Review in the US as treatment for patients with hypophosphatasia aged 2 years and older. https://www.astrazeneca-us.com/media/press-releases/2026/Efzimfotase-alfa-granted-Priority-Review-in-the-US-as-treatment-for-patients-with-hypophosphatasia-aged-2-years-and-older.html
- AstraZeneca · 2026-06-28. Efzimfotase alfa demonstrated improvements in bone health in treatment-naive paediatric patients with hypophosphatasia at week 25 in MULBERRY Phase III trial. https://www.astrazeneca.com/media-centre/press-releases/2026/efzimfotase-alfa-demonstrated-improvements-in-bone-health-in-treatment-naive-patients-with-hypophosphatasia-at-week-25-in-mulberry-phase-iii-trial.html
- Alexion Pharmaceuticals. Alexion OneSource, STRENSIQ financial assistance. https://alexiononesource.com/strensiq/financial-assistance
- National Organization for Rare Disorders · 2021-02-16. Hypophosphatasia, Rare Disease Report. https://rarediseases.org/rare-diseases/hypophosphatasia/