General Information

Crysvita (burosumab-twza) is a fibroblast growth factor 23 (FGF23) blocking antibody.

Crysvita is specifically indicated for the treatment of X-linked hypophosphatemia (XLH) in adult and pediatric patients 1 year of age and older.

Crysvita is supplied as a subcutaneous injection. The recommended starting dose for pediatrics is 0.8 mg/kg of body weight, rounded to the nearest 10 mg, administered every two weeks. The minimum starting dose is 10 mg up to a maximum dose of 90 mg. After initiation of treatment with Crysvita, measure fasting serum phosphorus every 4 weeks for the first 3 months of treatment, and thereafter as appropriate. If serum phosphorus is above the lower limit of the reference range for age and below 5 mg/dL, continue treatment with the same dose. Follow dose adjustment schedule per the drug label. The recommended dose regimen in adults is 1 mg/kg body weight, rounded to the nearest 10 mg up to a maximum dose of 90 mg, administered every four weeks. After initiation of treatment with Crysvita, assess fasting serum phosphorus on a monthly basis, measured 2 weeks post-dose, for the first 3 months of treatment, and thereafter as appropriate. If serum phosphorus is within the normal range, continue with the same dose. See drug label for specific dose adjustments.

Clinical Results

FDA Approval

The FDA approval of Crysvita was based on the following trials:

Pediatrics with XLH: FDA approval is supported by 64-week data from Study CL201, a randomized, open-label study in 52 patients ages 5 to 12. Data showed that treatment with Crysvita improved rickets, increased serum phosphorus levels, decreased serum alkaline phosphatase activity, and increased growth. The indication is also supported by 40-week data from Study CL205, an open-label study in 13 patients ages 1 to 4. Crysvita improved rickets and lower-limb deformity, increased serum phosphorus levels and decreased serum alkaline phosphatase activity.

Adults with XLH: FDA approval is supported by 24-week data from Study CL303, a randomized, double-blind, placebo-controlled study in 134 adult XLH patients. Crysvita treatment resulted in a higher proportion of patients achieving serum phosphorus levels above the lower limit of normal, and a higher rate of complete healing of active fractures and pseudofractures, compared to placebo. The adult indication is also supported by data from the 48-week, open-label, single-arm bone biopsy study in 14 adult XLH patients, which showed healing of osteomalacia as demonstrated by decreases in osteoid volume/bone volume, osteoid thickness and mineralization lag time.

Side Effects

Adverse events associated with the use of Crysvita may include, but are not limited to, the following:

Pediatrics:

headache

injection site reaction

vomiting

pyrexia

pain in extremity

vitamin D decreased

Adults:

back pain

headache

tooth infection

restless leg syndrome

vitamin D decreased

dizziness

constipation

blood phosphorus increased

Mechanism of Action

Crysvita (burosumab-twza) is a fibroblast growth factor 23 (FGF23) blocking antibody. X-linked hypophosphatemia is caused by excess fibroblast growth factor 23 (FGF23) which suppresses renal tubular phosphate reabsorption and the renal production of 1,25 dihydroxy vitamin D. Burosumab-twza binds to and inhibits the biological activity of FGF23 restoring renal phosphate reabsorption and increasing the serum concentration of 1,25 dihydroxy vitamin D.