New model for simulation of fracture repair in full-grown beagle dogs: Model characterization and results from a long-term study with ibandronate

Abstract

Introduction: Given that bisphosphonates reduce bone turnover, it is important to establish that their long-term administration does not impair bone quality. This paper describes a new model for simulation of fracture repair to evaluate several aspects of bone quality following long-term administration (34 or 36 weeks) of ibandronate in full-grown beagle dogs. Methods: The treatment schedule consisted of continuous daily subcutaneous administration of a pharmacologically active dose (1 μg/kg/day) and two cyclical intermittent regimens providing a similar total dose per animal at the end of the experiment. Seven or 8 weeks before study end, 10 holes were drilled in the left tibia and bone marrow ablation was performed in the ipsilateral femur. Serial measurements for blood biochemistry (osteocalcin and iso-alkaline phosphatase) and bone mineral density (BMD; whole body and L1–L7) by dual-energy X-ray absorptiometry (DEXA) were performed during the experiment. Bone quality was determined at the end of the experiment by assessing early and late stage defect healing and structural, cellular, and dynamic histomorphometry (femur, tibia, and lumbar vertebrae L3 and L4). Results: Healing of the drill hole defects, which simulate the first stage of fracture healing, was neither qualitatively nor quantitatively influenced by ibandronate. The same was true for the activation of cortical remodeling that occurs in the later stage of fracture healing, which started in Week 4 after surgery and declined after Week 8 in all groups. Additionally, no difference was found between the various regimens and the controls with respect to DEXA analyses, trabecular bone volume, cancellous bone tissue area, cancellous bone perimeter, osteoclast count, serum osteocalcin, or bone-specific alkaline phosphatase. Discussion: In conclusion, the presence of the first and second steps of fracture healing and the fact that the histological features closely resemble those of fracture repair validate the development and characterization of a new model for simulation of fracture repair. A long-term study with a therapeutically active dose of ibandronate shows that ibandronate does not impair BMD, bone structure, bone repair, coupling, and serum parameters for bone formation and turnover after long-term administration.

Introduction

Treatment of osteoporosis usually requires chronic therapy over several decades. Bisphosphonates have proven antifracture efficacy in osteoporosis and are currently the preferred treatment modality in this disease Bone et al., 1997, Harris et al., 1993, Mallette, 1994, Papapoulos, 2001, Storm et al., 1990, Thiébaud et al., 1994, Watts et al., 1990. Given that bisphosphonates specifically bind to and accumulate in the skeleton, where they prevent bone loss by suppressing bone remodeling, their long-term effects on bone quality are of clinical significance. In particular, it is important to ascertain that they do not have adverse effects on fracture healing. Another consideration is microdamage, which comprises microscopic cracks in cortical bone averaging 30–80 μm across caused by physiological repetitive loading during daily activity. Microdamage is repaired by remodeling (for a review, see Li, Mashiba, & Burr, 2001). Thus, it has been proposed that bisphosphonates may prevent the targeted repair of such microscopic cracks, thereby allowing their accumulation and resulting in increased bone fragility (Mashiba et al., 2000).

The bone resorptive potency of the bisphosphonates varies significantly, with the non-nitrogen-containing bisphosphonates, such as etidronate and clodronate, being least active. Thus, the etidronate dose required to inhibit bone resorption in the rat is high (>1 mg/kg/day) and is close to the dose that impairs normal mineralization. Studies in which high doses of the non-nitrogen-containing bisphosphonates, etidronate, and clodronate were administered to beagle dogs for 1–2 years demonstrated impaired bone mineralization and an increase in fractures Flora, 1982, Flora et al., 1981 or impaired fracture healing after 5 months of etidronate treatment (Lenehan, Balligand, Nunamaker, & Wood, 1985). These effects were, however, reversible following drug discontinuation. Similar results were found in growing rats Schenk et al., 1973, Schenk et al., 1982. Impaired mineralization has been described in long-term clinical trials of etidronate, although discontinuation of therapy or dose reduction resolved this effect (Fleisch, 1997). Unlike non-nitrogen-containing bisphosphonates, the nitrogen-containing bisphosphonates reduce bone turnover without impairment of mineralization (Papapoulos, 2001).

Some data are available on the influence of bisphosphonates on fracture repair in various animal models. A 22-week experiment conducted in a rat model using an artificial femoral fracture indicated that clodronate did not delay fracture healing (Hyvonen, Karhi, Kosma, Liimola-Luoma, & Hanhijarvi, 1994). Similarly, examination of the influence of pamidronate on fracture repair of the mid-diaphysis of the tibia in sheep revealed no adverse effects on the restoration of the mechanical integrity of a long bone after fracture (Goodship, Walker, McNally, Chambers, & Green, 1994). The effects of alendronate on fracture healing and remodeling in mature beagle dogs were assessed by inducing a transverse mid-diaphyseal fracture of the right radius (Peter et al., 1996). Alendronate 2 mg/kg/day before (9 weeks) or during fracture healing (16 weeks) or both (25 weeks) resulted in no adverse effects on the union, strength, or mineralization of bone.

The effects of bisphosphonate-induced suppression of bone turnover on microdamage accumulation and mechanical properties of dog rib were assessed by Mashiba et al. (2000). Suppression of bone turnover by high doses (five times clinical doses) of alendronate and risedronate was associated with microdamage accumulation and a reduction in some mechanical properties of the bone after 1 year (Mashiba et al., 2000). The authors concluded that intracortical bone remodeling can be suppressed by alendronate and risedronate, without impacting on bone mineralization but is associated with accumulation of microdamage. Further investigation in the same animal model indicated that these bisphosphonates suppress remodeling targeted towards the repair of microdamage as well as nontargeted (or stochastic) remodeling when used at doses considerably higher those clinically recommended (Li et al., 2001).

Ibandronate (ibandronic acid monosodium salt, monohydrate, formerly BM 21.0955) is a highly potent, nitrogen-containing bisphosphonate that inhibits bone resorption at considerably lower doses than currently licensed bisphosphonates, without impairment of mineralization at doses at least several orders of magnitude above its therapeutically active dose (Mühlbauer et al., 1991). A number of preclinical studies support the positive effects of ibandronate on the biomechanical properties of bone. The optimal dose for preventing bone loss due to cessation of ovarian function is ≥1 μg/kg/day sc in dogs and rats Bauss et al., 2002, Monier-Faugere et al., 1993, Monier-Faugere et al., 1999. In aged ovariectomized rats, ibandronate prevented reduction in bone mass and deterioration in architecture and strength when considerable bone loss had already been demonstrated Bauss et al., 2002, Bauss et al., 2002. Interestingly, the effects of ibandronate on the biomechanical endpoints of long bones and vertebrae were equivalent, whether the same total dose was given as a daily or cyclical intermittent regimen Bauss et al., 2002, Bauss et al., 2002. Furthermore, the high level of correlation between bone mass and strength was consistent with that seen in intact rats after 2 years of daily oral ibandronate treatment at doses far in excess of any therapeutically intended dose (Lalla, Hothorn, Haag, Bader, & Bauss, 1998). Recent data from a study conducted in ovariectomized cynomolgus monkeys also support the efficacy of intermittent intravenous ibandronate in preventing bone loss and maintaining bone strength and quality (Smith, Recker, Hannan, Müller, & Bauss, 2003).

Clinically, ibandronate has proven efficacy in the management of hypercalcemia of malignancy Pecherstorfer et al., 1996, Ralston et al., 1997 and metastatic bone disease (Tripathy, Budde, & Bergstrom, 2002), and evidence is rapidly mounting to support its value in the treatment and prevention of osteoporosis Adami et al., 2002, Cooper et al., 2002, Delmas et al., 2002, Felsenberg et al., 2002, Ravn et al., 1996, Stakkestad, et al., 2003, Thiébaud et al., 1996. The potential of bisphosphonate dosing regimens with extended between-dose intervals is currently the subject of considerable interest among osteoporosis physicians. Significantly, ibandronate is the only bisphosphonate to deliver lasting efficacy from unique and simple oral and intravenous regimens featuring extended between-dose intervals Adami et al., 2002, Delmas et al., 2002, Schimmer & Bauss, 2003, Stakkestad, et al., 2003. Furthermore, recent data have shown ibandronate to be the first osteoporosis medication proven to offer lasting antifracture efficacy in regimens with a between-dose interval of >2 months (Delmas et al., 2002). Clinical trials are underway to assess simplified intermittent ibandronate regimens that aim to promote therapy adherence and optimize patient management. It is therefore important to understand the long-term suppressive effects of highly potent bisphosphonates, such as ibandronate, on bone quality.

In this paper, we describe the development and characterization of a new model in beagle dogs that combines defect healing, stimulation of bone turnover, mechanical properties, bone architecture, bone density, and blood chemistry. In this model, the influence of long-term ibandronate on bone quality was investigated using various administration schedules (continuous and intermittent cyclical) that provided a similar cumulative total dose per animal. Evidence suggests that the cumulative dose of bisphosphonates may be more important than the daily dose Bauss et al., 2002, Bauss et al., 2002, Fleisch, 1996, Monier-Faugere et al., 1999, O'Rourke et al., 1993 with respect to efficacy. Thus, in addition to the development and characterization of a new model for simulation of fracture repair, we were also able to examine the influence of treatment schedule on bone quality by this experimental approach.