Preclinical safety and efficacy of 24R,25-dihydroxyvitamin D(3) or lactosylceramide treatment to enhance fracture repair

BACKGROUND/OBJECTIVE: Cyp24a1-null mice deficient in 24,25(OH)(2)D(3) display impaired callus formation during the endochondral phase of bone fracture repair. The 24,25(OH)(2)D(3) metabolite acted by binding to the TLC domain containing 3B isoform 2 (TLCD3B2, previously named FAM57B2) effector protein, which then synthesizes lactosylceramide (LacCer). Treatment with 24,25(OH)(2)D(3) or LacCer restored callus size and mechanical properties in Cyp24a1-null mice. METHODS: To assess the safety of these molecules and test their efficacy for bone healing in wild-type, non-genetically modified mice, we treated 12-week-old, osteotomized C57BL/6 female mice with each compound for up to 21 days post-osteotomy. Control cohorts were injected with vehicle. RESULTS: Neither compound was found to exhibit any nephro- nor hepato-toxicity. Calcemia remained stable throughout the experiment and was unaffected by either treatment. Supplementation with 24,25(OH)(2)D(3) increased circulating levels of this metabolite about 8-fold, decreased 1,25(OH)(2)D(3) levels, and significantly increased circulating 1,24,25(OH)(3)D(3) levels, suggesting 1?-hydroxylation of 24,25(OH)(2)D(3). TLCD3B2 was found to be expressed in fracture callus at the surface of unmineralized or pre-mineralized cartilage on day 10 and day 12 post-osteotomy and to progressively recede to become undetectable by day 18. Treatment with 24,25(OH)(2)D(3) or LacCer reduced the number of TLCD3B2-positive cells. Both treatments also significantly increased stiffness and elastic modulus of the healing bone callus. CONCLUSION: Exogenous administration of 24,25(OH)(2)D(3) or LacCer improved the biomechanical properties of repaired bones in wild-type animals without affecting circulating calcium levels or other blood parameters, demonstrating preclinical safety and efficacy. TRANSLATIONAL POTENTIAL: Our data suggest the use of 24R,25-dihydroxyvitamin D(3) or lactosylceramide for ameliorating fracture healing in clinical practice.