Session VIII - Basic Science

Fri., 10/6/06 Basic Science, Paper #46, 4:30 pm

Molecular Mechanisms of Parathyroid Hormone-Mediated Enhancement of Fracture Repair

Sanjeev Kakar, MD (*); George L. Barnes, PhD (*); Maisa Al-Sebaei (*); Louis C. Gerstenfeld, PhD (*); Paul Tornetta III, MD (*); Thomas A. Einhorn, MD (*);
Boston University School of Medicine, Boston, Massachusetts, USA

Introduction/Hypothesis: Approximately 5% to 10% of fractures exhibit some degree of impaired healing. Currently no systemic treatment exists for enhancing bone repair. Recently we demonstrated that daily systemic parathyroid hormone (PTH) administration enhanced bone healing in a rat fracture model. The mechanisms by which this occurs, however, remains unknown. In this study we examine the early molecular responses of healing fractures to systemic PTH treatment. We hypothesized that enhanced PTH-mediated fracture repair is due to an increase in chondroprogenitor cell recruitment and maturation.

Methods: In order to define the stages of fracture repair enhanced by PTH treatment we analyzed the tissue, cellular, and molecular effects of PTH treatment (40 mg/Kg, PTH 1-34) during bone healing in a murine femoral fracture model. Calluses were harvested at sequential time points after fracture for radiographic, histologic, and molecular analyses.

Results: Radiographically, we observed that PTH treatment led to increased mineralization of the fracture callus by day 10 and an earlier bridging of the fracture site. Histologic analysis demonstrated that PTH increased the rate of cartilage recruitment, the total quantity of cartilage, and its rate of hypertrophic maturation. Molecular analysis confirmed these observations, showing that PTH-treated fractures expressed Col2A1 and Col10A1 on average 2 days earlier than in vehicle-treated animals. The earlier induction of cartilage hypertrophy was accompanied by an earlier induction of matrix metalloproteinases, specifically MMP9, associated with cartilage turnover and callus vascularization, critical steps in the endochondral repair process. Analysis of regulatory molecules associated with these early chondrogenic events demonstrated that PTH treatment led to enhanced and earlier expression of Wnts-4 and -5b, both shown in previous studies to be important regulators of chondroprogenitor proliferation and maturation.

Conclusion/Significance: Together these results support the conclusion that PTH treatment enhances fracture healing in part by inducing nascent mesenchymal cell recruitment into the chondrogenic lineage and by enhancing the subsequent maturation and expansion of these cells through a Wnt-mediated mechanism. Finally these results suggest that PTH may have efficacy as a systemic therapeutic agent to enhance fracture healing over a very short time course of treatment, thereby eliminating the need for local implantation.

If noted, the author indicates something of value received. The codes are identified as a-research or institutional support; b-miscellaneous funding; c-royalties; d-stock options; e-consultant or employee; n-no conflicts disclosed, and *disclosure not available at time of printing.
· The FDA has not cleared this drug and/or medical device for the use described in this presentation (i.e., the drug or medical device is being discussed for an "off label" use). · · FDA information not available at time of printing.