ESPE Abstracts

Background: Disproportionate short stature is seen in most individuals with X-linked hypophosphatemia (XLH). Vitamin D and phosphate supplementation can improve growth slightly. Burosumab showed minimal improvement of growth in older children. No growth data of XLH children that started burosumab at a very young age, i.e., between 1 and 4 years, are available.

Methods: We included 17 XLH children (11 boys) who started burosumab before the age of 4, treated for at least one year, and 31 treated with vitamin D analogues and phosphate supplements from diagnosis. Two children in the burosumab group additionally received treatment with recombinant human growth hormone, thus their growth was analyzed separately. Auxologic data such as height, weight, BMI and head circumference were analyzed every six months for the first two years of therapy within the two groups. We tested the statistical significance of the differences between timepoints by using a two-tailed t test (SPSS®, IMB version 29).

Results: In the burosumab- treated group, mean± SD for age at therapy baseline was 2.1± 0.7. They were treated conventionally before switching to burosumab for 1.7±0.8 years. Even though they were all eutrophic at birth, height SDS was already lower at burosumab start (mean± SD -0.3±0.7 and -1.4±0.8, respectively, P<0.001). In the burosumab-treated group, we did not see any statistical significant change in height SDS during the first two years of treatment: mean± SD 0.1±0.6 after one yer (P=0.16) and 0.0±0.7 SD after two years (P=0.54). Burosumab did not correct the height deficit during the observation period (still -1.5 SDS difference in height after two years of therapy when compared to birth length SDS, P=0.04). BMI SDS did not significantly change during the first two years of burosumab. In the conventionally treated group, even though therapy was started rather early (1.2±0.8 years old), height SDS decreased by 0.7±0.9 SDS during the first two years of therapy (P<0.001). BMI SDS improved by 0.5±0.9 SDS during the same period (P=0.006). In conclusion, we present data from the largest and youngest pediatric XLH cohort treated with burosumab over a period of two years. Our data suggest that burosumab does not correct the height deficit during the first years of life, but in contrast to conventional therapy, is not associated with further height loss. In addition, burosumab prevents the excessive weight gain, which is associated with the XLH disease development in children.