Ultragenyx Pharmaceutical Inc. and Mereo BioPharma Group plc announced data from the dose-selection Phase 2 portion of the Phase 2/3 Orbit study showing that setrusumab rapidly induced bone production in OI-affected patients. Across all patients evaluated setrusumab demonstrated statistically significant increases in levels of serum P1NP, a sensitive marker of bone formation, and a substantial and significant improvement in bone mineral density (BMD) by 3 months. The rate of increasing bone mineralization we're observing on DXA scans is striking, unlike anything I have typically seen with bisphosphonate therapy.

This increase in bone mass underscores the potential to make denser and stronger bone, said Gary Gottesman, M.D., Professor of Pediatrics and Medicine, Washington University School of Medicine. As of the data cut off, serum P1NP levels through at least 1 month of treatment were available from all 24 patients enrolled in Orbit and demonstrated that treatment with setrusumab significantly increased serum P1NP in both dosing cohorts, peaking at one to two weeks and again, as expected, after the 2-month dosing timepoint. In the 20 mg/kg cohort, there was a mean serum P1NP increase of 57% from baseline over the first month.

Because of the higher baseline P1NP level in younger patients, this represents an approximate 8-fold greater increase in serum P1NP over 1 month in pediatric and adolescent patients when compared to adult OI patients. The absolute effect of setrusumab on increasing serum P1NP over the 1-month period with the 20 mg/kg dose, was approximately 80% of the effect achieved with the 40 mg/kg dose, demonstrating a dose response. Patients on placebo at the 1-month timepoint (n=4) showed no significant change in mean serum P1NP from baseline.

The large increase in BMD observed in the Orbit patient population over the first 3 months was consistent with the rapid increase in serum P1NP levels and was similar to results that took 1 year to achieve in the ASTEROID study in adult OI patients. Lumbar spine BMD data were available in 17 of 24 Orbit patients at the 3-month timepoint. Treatment with setrusumab for 3 months resulted in an increase in lumbar spine BMD from baseline of 9.4% at 20 mg/kg (n=10), which represents a substantial mean change in Z-score of +0.65.

Treatment with 40 mg/kg (n=7) resulted in a 9.8% BMD increase. Patients on placebo at the 3-month timepoint (n=2) showed no significant change in BMD or change in lumbar spine Z-score.