What to Know About Strensiq (Asfotase Alfa)

An Enzyme Approved to Treat Severe Hypophosphatasia

Nurse prepares to give boy a shot

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Strensiq (asfotase alfa) is a treatment approved in 2015 for severe cases of hypophosphatasia (HPP), a rare, genetic metabolic disease. This disorder affects bone health and other bodily systems, and can lead to lifetime disability, deformity, and other health issues.

Indicated for those with perinatal, infantile, or juvenile HPP—cases arising in the womb, infancy, or during young childhood, respectively—this medication replaces the enzyme, alkaline phosphatase, which is critical for bone development.

Delivered via injection, Strensiq has been found to improve overall survival in HPP cases and helped spur growth and bone density. It’s also helped infants with the condition survive without the use of a ventilator.

In the face of this incurable condition, this type of enzyme replacement therapy (ERT) has become a standard approach. 


An inborn condition, HPP varies a great deal in severity, and only an estimated one in 100,000 people with the condition display the most severe perinatal and infantile forms. Those who show signs of the condition in the womb, within six months of birth, or in childhood tend to have more severe cases. Currently, Strensiq is only indicated for HPP in these populations.

At the core of this disorder is a mutation of the APLP gene, causing insufficient production of TNSALP. Essential for bone “mineralization”—the process by which bones incorporate minerals and become stronger—levels of this enzyme are replenished by Strensiq applications. In turn, this can slow or even stop the progression of bone malformation, ease symptoms, and assist with mobility.

Before Taking

Prior to Strensiq being indicated for perinatal, infantile, or juvenile HPP, careful assessment and evaluation is needed. Diagnosis may involve:

  • Clinical evaluation requires assessment of outward signs, such as bowed, short limbs, breathing difficulties, joint inflammation, and skeletal deformities. Many cases are first isolated by looking at family history of the condition.
  • Imaging such as X-ray is employed to assess the scope of damage and find signs of damage or malformation. Sonography, or ultrasound imaging, may also discover cases of perinatal HPP.
  • Bone densitometry employs multiple specialized X-rays to look inside bones and assess their density and strength. This test helps doctors assess the scope of damage or malformation.
  • ALP blood tests record alkalide phosphate levels in the blood. In many, but not all cases, low amounts are markers of HPP, so this test alone cannot conclusively diagnose the condition.
  • Pyridoxal phosphate (PLP) tests are blood tests looking at levels of the biologically active form of vitamin B6, or PLP. Elevated levels are a sign of insufficient TNSALP in the bloodstream.
  • Genetic testing can conclusively determine whether the ALPL gene is mutated. This testing is conducted using samples of blood.

Precautions and Indications

Another important aspect of prescribing Strensiq involves determining if it’s safe to take. Use of this drug is stopped or may be reconsidered for:

  • Those with allergy: In rare cases, use of Strensiq leads to allergic or “anaphylactic” shock, causing breathing problems, nausea, puffiness in the eyes, and dizziness.
  • Pregnant women: The safety of Strensiq use for this population has not been assessed. As such, careful consideration is taken in these cases.
  • Geriatric use: Since Strensiq is relatively novel and only indicated for cases of perinatal, infantile, or juvenile HPP, there is no data surrounding use of this therapy for those over 65.

Other Enzyme Replacement Therapies

Strensiq is the only therapy that boosts TNSALP levels specifically. However, several other ERTs are employed to take on a range of genetic conditions, including:

  • Fabry disease: This rare genetic disorder affects metabolism, leading to a wide range of effects on organs, such as the heart, lungs, and kidneys. This is treated with Fabrazyme (agalsidase alfa) or Replagal (agalsidase beta).
  • Pompe disease: Another rare condition, Pompe disease causes irregular accumulation of the protein glycogen, affecting cardiac activity, respiration, and skeletal muscles. The ERT alglucosidase, sold as Myozyme and Lumizyme, is indicated in these cases.
  • Gaucher disease: ERTs are used to fight another genetic disease, Gaucher disease, which also affects numerous organ systems. Ceredase (alglucerase), Cerezyme (imiglucerase), VPRIV (velaglucerase), and Elelyso (taliglucerase) may be indicated for this condition.


Strensiq comes in the form of pre-filled, single-use vials of solution in four different concentrations: 18 milligrams (mg)/0.45 milliliters (mL), 28 mg/0.7 mL, 40 mg/mL, and 80 mg/0.8 mL. The specific dosage is determined by the patient’s weight and can vary based on the individual case.

According to the Strensiq label, perinatal, infantile, and juvenile HPP treatment starts with doses of 6 mg per kilogram (mg/kg) of body weight a week, delivered either in three doses of 2 mg/kg each, or six of 1 mg/kg. This can be raised to 9 mg/kg a week—administered in three 3 mg/kg doses—if improvements aren’t seen.

Since HPP is a chronic condition, and since therapy is ongoing and individualized, adjustments may have to be made in the course of therapy. Make sure to stick closely to your doctor’s specific recommendations.


Oftentimes, especially in prolonged cases, doctors may have to fine-tune dosages based on weight status or other factors. It’s recommended that those under 40 kg (about 88 pounds) not be given 80 mg/0.8mL doses. Furthermore, those that experience redness, swelling, or other issues around injection sites should consider aiming for three instead of six shots a week.

Given the great variation in dosage, you’ll be told exactly what type of vials to use and how many injections are necessary. Some who require higher doses may need two concurrent injections of the medication.

How to Take and Store

Since Strensiq is delivered via injection, particular care needs to be taken when using it. Alongside the pre-filled vial or vials needed, an administration kit should include:

  • One or two sterile, single-use syringes with a half-inch needle and a 25 to 29 gauge tip
  • Alcohol wipes
  • A cotton ball or piece of gauze
  • A clean tabletop or surface
  • A sharps container or one suitable for safe disposal of used syringes

The procedure of administration will be demonstrated by the doctor or medical staff. Correct use first involves properly filling the syringe. This entails several steps:

  • Ensure proper storage: Strensiq vials should be stored in the refrigerator at temperatures between 36 F and 42 F. Check the expiration date before using, and throw out vials that are expired. Make sure to use the medicine within one hour of taking it out of the refrigerator.
  • Check the product: Make sure the liquid in the vial is clear or slightly yellow, and discard if it looks otherwise. Look out for sediment and lumps in the solution.
  • Unpack and assemble the needle: After taking the plastic cap off of the vial, prepare the needle by removing it from its packaging and assembling it. While twisting, push the needle onto the rest of the syringe until it’s tight, while keeping its cap on.
  • Access the medicine: While holding the needle pointed up, set the plunger to the line of your prescribed dose. Pull the cap off the needle and, with the Strensiq vial flat on a stable surface, slowly push it into the stopper. Lift and slowly flip the syringe over so that the syringe is pointing up, then slowly push it the rest of the way in.
  • Fill the vial: Once you see the tip of the needle in the solution, push the plunger of the syringe all the way in. Then, pull it back to a point one demarcation past your dosage amount, followed by smaller push to move it back to the proper setting. Don’t remove the syringe yet.
  • Ready the injection: Once the right amount is collected, turn everything back over so that the vial is flat on the surface. Slowly pull out the syringe, and with the needle pointing up, tap its barrel a couple times to remove any bubbles.

From there, a couple more steps are taken to administer the Strensiq:

  • Choose an injection site: First and foremost, make sure there is no redness, swelling, and heat on the site. This drug can be injected under the skin of abdomen, upper arms, or thighs. It’s important to rotate the sites of injection.
  • Prepare the site: Use an alcohol swab to clean the area off, and wait until it dries. Pinch the skin around the injection site.
  • Inject the medicine: Use the needle to pierce the area, coming in vertically at a 90-degree angle. If there is fat in the area, a diagonal 45-degree angle may be used. Push the plunger all the way in.
  • Finish up: Pull the needle out once all of the medicine is delivered. If there’s blood, push on the area with an alcohol swab, gauze, or piece of cotton. It’s important to make sure not to recap the needle and to dispose of it immediately.

Safe Sharps Disposal

When using injected drugs, it’s essential to immediately and safely dispose of used syringes. Because of the risk of needle-stick injury, you can’t just throw these out in the household trash. It’s worth investing in FDA-cleared sharps disposal containers, which you can get from hospitals, pharmacies, or even online. Failing that, the FDA recommends you use a container that is:

  • Made of sturdy, heavy-duty plastic
  • Has a lid that is tight fitting and puncture resistant
  • Remains upright during use
  • Leak resistant
  • Properly labeled as containing hazardous materials

Side Effects


Generally speaking, Strensiq is associated with few side effects and is well tolerated. The most frequently seen of these are:

  • Injection site reactions, which cause pain, swelling, and redness at the site of needle administration. This is highly treatable and can be avoided by varying the site of injection and carefully following administration directions.
  • Ectopic calcification is the development of calcium growths in the eyes or kidneys. It’s currently unknown if this results from using Strensiq itself or if it’s associated with the HPP.
  • Lipodystrophy, a problem storing fat, can occur near the site of injection. This causes either an indentation in the skin or a thickening and hardening of existing tissues.
  • Hypersensitivity to the medication can cause anaphylactic or allergic shock. This medical emergency leads to the swift onset of breathing difficulties, hives, itchiness, nausea, puffiness around the eyes, and dizziness. Discontinue use and get immediate medical help if you see or experience any of these symptoms.


Occurring in less than 1% of observed cases are a set of rare and often more adverse reactions, including:

Warnings and Interactions

Largely, Strensiq isn’t known to interact poorly with other medications, and there are no outright contraindications for its use. However, if you miss a dose, talk to your doctor about how best to get back to your routine. If you believe too much has been ingested, get emergency medical help or contact the Poison Control help line at (800) 222-1222.

Since Strensiq is a relatively new drug, there is more to learn about its long-term effects, and it’s possible some vitamins, herbs, or medications may affect how it functions. Keep track of what you’re taking and how you’re feeling, and share that information with your doctor.

As research continues, and as more and more people are helped by this ERT, there’s no doubt we’ll get a better sense of its safety and efficacy profile.

6 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Food and Drug Administration. Drug trials snapshots: STRENSIQ.

  2. MedlinePlus. Hypophosphatasia.

  3. Institute for Quality and Efficiency in Health Care. Asfotase alfa (Strensiq) for the treatment of hypophosphatasia: overview. InformedHealth.org.

  4. Food and Drug Administration. Highlights of prescribing information: STRENSIQ (asfotase alpha).

  5. Martos-Moreno G, Calzada J, Couce M, Argente J. Hypophosphatasia: clinical manifestations, diagnostic recommendations and therapeutic options. Anales de Pediatría (English Edition). 2018;88(6):356.e1-356.e11. doi:10.1016/j.anpede.2017.06.006

  6. Safary A, Akbarzadeh Khiavi M, Mousavi R, Barar J, Rafi M. Enzyme replacement therapies: what is the best option? BioImpacts. 2018;8(3):153-157. doi:10.15171/bi.2018.17

By Mark Gurarie
Mark Gurarie is a freelance writer, editor, and adjunct lecturer of writing composition at George Washington University.