Burosumab-twza or KRN23 is a monoclonal antibody approved by FDA on April 2018 to use as an injection for subcutaneous use in patients with X-linked hypophosphatemia.

Burosumab-twza or KRN23 is a monoclonal antibody prescribed to treat a rare hereditary disorder called X-linked hypophosphatemia (low levels of phosphate in the blood) in adult and pediatric patients who are one year and above. It has been successfully tested in children with X-linked hypophosphatemic rickets.

Use of burosumab-twza can be confirmed by the radiographic evidence of bone disease.

Burosumab-twza works by blocking the activity of FGF23 protein resulting in reabsorption of phosphate by the kidneys and thereby restoring the phosphate levels in the blood.  

Burosumab-twza should not be used in patients with

• Allergy to burosumab-twza
• Taking oral phosphate and vitamin D medications
• Normal or high phosphorus level in the blood
• Infants less than one year
• End-stage kidney disease

Caution is required while using burosumab-twza in pregnancy and breastfeeding mothers.

• The dose is recommended based on the patient’s phosphate level in blood and the individual body weight.
• The minimum dose is 10 mg, and the maximum dose is 90 mg

Patients aged from 1 year to 18 years:
The initial dose is 0.8 mg/kg of body weight given every two weeks

Patients aged above 18 years:
The recommended adult dose is 1 mg/kg of body weight given for every four weeks

• Fasting serum phosphorus level should be examined every month during first three months of burosumab-twza and treatment should be continued with the same dose if the phosphorus level is within the normal range.

• The dose can be adjusted based on the serum phosphorus level but never more frequently than every four weeks.

• If a dose is missed, take the missed dose as soon as it is remembered.

• Burosumab-twza is available in injection form to be given as subcutaneous or just under the skin which should be administered by a healthcare provider.

• Stop taking oral phosphate or vitamin D supplements at least 1 week before starting the treatment with burosumab-twza.

• Injection sites should be rotated after each injection. The most common injection sites are buttocks, upper thighs, any quadrant of the abdomen or upper arm.
• Never inject into tender, red, bruised or hard skin. Also, avoid injecting into moles or scars.
• Discontinue the treatment if severe injection site reactions such as pain, swelling, itching or irritation at the injection site occur.
• The injection should be inspected for the presence of any particulate matter, discoloration or cloudy appearance and should be discarded if such occur.
• Patients should be monitored for the presence of any allergic reactions and the treatment with burosumab-twza should be discontinued during such serious episodes.
• Phosphorus levels should be monitored regularly to avoid the risk of nephrocalcinosis
(a condition of calcium deposition in the kidneys).
• Consult the physician immediately if a new or worsening of the existing restless leg syndrome is reported.

• Headache
• Injection site reactions
• Vomiting
• Pain in the extremities
• Rashes
• Muscle pain
• Restless leg syndrome
• Fever
• Toothache or tooth abscess
• Low levels of vitamin D
• High phosphorus levels in the blood
• Dizziness

• Patients should be monitored for the level of parathyroid hormone regularly as there will be an increase in the hormone levels during treatment with burosumab-twza
• Dizziness may occur following burosumab-twza administration and activities involving driving or operating heavy machinery should be avoided as it may impair mental alertness.
• Injections should be allowed to come to room temperature before administering.

• Taking burosumab-twza with vitamin D or oral phosphate supplements leads to a risk of hypercalcemia or hyperphosphatemia and hence should be avoided.
• The physician should be informed if a patient is taking any prescription, non-prescription medicines or any herbal supplements.

• Store burosumab-twza injection in a refrigerator at 2°C to 8°C but do not freeze or shake the vial.
• Protect the injection from light by keeping it in its original package.