Sex differences of burosumab in children with X-linked hypophosphataemic rickets

BACKGROUND

The severity of X-linked hypophosphataemic rickets (XLH) may be affected by genotype and sex. However, burosumab, a fully humanized monoclonal antibody against fibroblast growth factor 23, has the same pediatric dose recommendation for both sexes (0.8 mg/kg every 2 weeks).

PATIENTS AND METHODS

In a retrospective cohort study, we describe the burosumab response differences by sex in children with XLH.

RESULTS

We treated 10 children (5 females, mean age at initiation 4.2 ± 3.5 years) with XLH with burosumab. Initial mean serum phosphate was 0.69 ± 0.18 mmol/L in males and 0.86 ± 0.22 mmol/L in females (p = 0.108). The mean ratio of tubular maximum reabsorption rate of phosphate to glomerular filtration rate (TmP/GFR) was 0.55 ± 0.11 mmol/L in males and 0.76 ± 0.23 mmol/L in females (p = 0.06). The mean starting dose of burosumab was 0.83 ± 0.19 mg/kg subcutaneously every 14 days (males: 0.79 ± 0.19 mg/kg; females: 0.87 ± 0.21 mg/kg, n.s.). Two weeks after starting burosumab, serum phosphate differed significantly between males (0.90 ± 0.21 mmol/L) and females (1.27 ± 0.25 mmol/L) (p = 0.018). All males required a dose increase to try to normalize serum phosphate. On day 140 after starting, the average dose in males increased further to 1.24 ± 0.41 mg/kg to achieve a phosphate of 0.87 ± 0.11 mmol/L while females had a normal phosphate and alkaline phosphatase on the starting dose. After a mean of 458 ± 79 days, the mean burosumab dose/kg in males was 1.68 ± 0.61 mg/kg, mean serum phosphate was 1.08 ± 0.23 mmol/L, mean TmP/GFR was 1.01 ± 0.20, mean alkaline phosphatase had normalized to 303.6 ± 40.7U/L, and mean 1.25(OH)2 vitamin D level was 186.4 ± 16.6 nmol/L.

CONCLUSIONS

Our findings may suggest a sex difference in response to burosumab in XLH patients. Our data suggest that males may require higher doses.

Pulp chamber features, prevalence of abscesses, disease severity, and PHEX mutation in X-linked hypophosphatemic rickets

INTRODUCTION

Rickets, growth failure, and recurrent periapical abscesses with fistulae are main signs in patients with X-linked hypophosphatemic rickets (XLH). Prevalence of abscesses, pulp chamber features, biochemical findings, disease severity, and PHEX gene mutation were examined.

MATERIALS AND METHODS

Pulp chambers size, shape, and morphology were assessed by orthopantomography in XLH patients (n = 24, age 5.8 ± 1.6 years) and in sex and age-matched healthy controls (n = 23, age 6.2 ± 1.4 years). XLH patients received conventional treatment (3.5 ± 1.9 years). Pulp chamber features were assessed in teeth of primary dentition and in the permanent left mandibular first molar and compared with those of controls. Rickets severity score was assessed at wrist, knee, and ankle.

RESULTS

The mean pulp chamber area/tooth area ratio, mean pulp chamber height/pulp chamber width ratio, and prominence of pulp horns into the tooth crown in primary and secondary molars were significantly higher in patients than in controls and in patients suffered abscesses than in patients without abscesses. Sixteen patients (67%) had a history of abscesses; incisors were affected more than canines and molars. Severity of rickets and mean serum parathyroid hormone (PTH) levels were significantly higher, and mean serum 1,25-dihydroxyvitamin D [1,25(OH)₂D] levels significantly lower in patients suffered abscesses than in patients without abscesses. PHEX gene mutations were not correlated with dental phenotype and disease severity.

CONCLUSION

Enlarged pulp chambers with altered shape and morphology affected the majority of XLH patients predisposing to recurrent periapical abscesses with fistulae. Dental phenotype was associated with severity of rickets, high serum PTH, and low serum 1,25(OH)₂D levels.

X-linked hypophosphatemic rickets (PHEX mutation): A case report and literature review

Hypophosphatemic rickets is a rare form of rickets that affect children. The diagnosis requires high index of suspicion. We report a case of Hypophosphatemic rickets in 18-month-old Saudi boy presented with delayed walking and lower limb deformity. The diagnosis was confirmed by bone profile, radiological study and genetic testing, which reveled PHEX mutation. The patient was successfully treated by phosphate supplement.