Effects of intermittent hPTH(1-34) alone and in combination with 1,25(OH)(2)D(3) or risedronate on endosteal bone remodeling in canine cancellous and cortical bone.

Therapies utilizing intermittent human parathyroid hormone(1-34) (hPTH[1-34]) in combination with other agents have recently been proposed as possible anabolic regimens for the treatment of osteoporosis. We conducted a 24 week study in aged beagle dogs to determine the effects of intermittent hPTH(1-34) administered alone or in combination with 1,25-dihydroxyvitamin D(3) (1,25(OH)(2)D(3)) on the endosteal remodeling in cancellous and cortical bone. Additionally, we tested the interaction between hPTH(1-34) and a new potent bisphosphonate, risedronate. The three treatment groups were compared with a vehicle control group. Kinetic reconstruction of the remodeling unit revealed substantial differences between the groups in resorption and formation at the basic multicellular unit level. Although the estimates of final erosion depth were unaffected by treatment, tunneling resorption was noted in six of the eight dogs administered hPTH(1-34) alone. These qualitative morphological changes in the resorption lacunae were attenuated or absent in dogs administered hPTH(1-34) in combination with either 1,25(OH)(2)D(3) or risedronate. Functional periods for resorption were significantly increased, and the resorption rates were significantly decreased in the hPTH(1-34) + risedronate group. Analyses of the formative site demonstrated that the wall thickness was significantly increased and the bone balance significantly more positive in all three hPTH(1-34) treatment groups. The most positive bone balance was achieved in the combined hPTH(1-34) + risedronate group (+ 15.6 + or - 14.2 mm, p <0.05). Increases in the mineral apposition rate in the early phases of the formative period suggest that an increase in osteoblastic activity (number or function) may contribute to the increase in wall thickness. Treatment with hPTH(1-34) alone or in combination with 1,25(OH)(2)D(3) caused an approximately 2-fold increase in the activation frequency in cancellous bone, which was essentially normalized to control values by the coadministration of risedronate. The impact of these changes on the cancellous bone microstructure was significant only in the combined hPTH(1-34) + risedronate group where normalized bone turnover in the face of a positive bone balance effected a significant increase in the trabecular thickness. Analyses of sequential fluorochrome labels, administered to reconstruct the temporal changes in intracortical activation, demonstrated the presence of an apparent cyclic pattern of activation in the cortex of placebo-treated dogs. Generally, activation was increased throughout the study in dogs administered hPTH(1-34) alone or in combination. However, in the hPTH(1-34) + risedronate group, activation was significantly blunted toward the end of the study, and the cyclic pattern of activation was modulated. These data suggest that intermittent hPTH(1-34) in combination with risedronate may be superior to hPTH(1-34) in combination with 1,25(OH)(2)D(3) as a therapeutic regimen for osteoporosis due to the protective effect of this bisphosphonate on the cortical and endocortical envelope.

Evaluation of the skeletal effects of combined mild dietary calcium restriction and ovariectomy in Sinclair S-1 minipigs: a pilot study.

A pilot study was conducted to investigate the combined effects of ovariectomy (OVX) with preceding and concomitant mild dietary calcium restriction on the minipig skeleton. Minipigs 4 months old were fed diets containing 0.9, 0.75, or 0.5% calcium (Ca). At 10 months, the 0.75 and 0.5% pigs were OVX and the 0.9% were either sham operated or OVX. All pigs were maintained on their respective diets for an additional 6 months. Excised lumbar vertebrae and long bones were evaluated by densitometry and histomorphometry, and vertebral cancellous bone samples were tested biomechanically. In pigs fed the 0.9% Ca diet, OVX alone effected decreases of 6% in vertebral bone mineral density (BMD), 15% in trabecular bone volume (BV/TV), and 13% in trabecular number (Tb.N), an increase of 15% in trabecular separation (Tb.Sp), and a nonsignificant increase (p < 0.056) in vertebral cancellous final erosion depth (F.E.De) compared with the 0.9% Ca sham-operated group. Decreasing dietary Ca to 0.5% in combination with OVX effected an 8% reduction in vertebral BMD that was not associated with any significant alterations in parameters of vertebral cancellous bone microstructure or remodeling compared with the 0.9% Ca sham-operated pigs. Increases in serum PTH noted in the 0.5% Ca OVX group were generally paralleled by increases in calcitriol. In OVX pigs fed a diet containing 0.75% Ca, a 10% reduction in vertebral BMD was observed. This was associated with significant increases in F.E.De and vertebral marrow star volume (Ma.St.V) compared with the 0.9% Ca sham-operated pigs and the other OVX groups. In addition, Tb.Sp was increased and Tb.N decreased compared with the 0.9% Ca sham-operated pigs. Increases in serum PTH in this group were not accompanied by increases in calcitriol. Midradial and midfemoral BMD values were reduced in the 0.75 and 0.5% Ca OVX groups compared with the 0.9% Ca sham-operated pigs. Histomorphometric analyses of cortical bone suggested the reduction in cortical bone mass in the 0.75% Ca OVX group may have been largely due to net loss on the endocortical surface versus possible failure to accrue bone in the 0.5% Ca OVX group. Ash density and biomechanical parameters for vertebral cancellous bone decreased progressively in the 0.9% sham-operated, 0.9% Ca OVX, and 0.75% Ca OVX groups and then increased in the 0.5% Ca OVX group. After normalization for bone mass (ash), mechanical changes were still apparent, particularly for the 0.75% Ca OVX group compared with other OVX groups, reflecting that structural changes had taken place in the trabecular network.(ABSTRACT TRUNCATED AT 400 WORDS)

Calcium-restricted ovariectomized Sinclair S-1 minipigs: an animal model of osteopenia and trabecular plate perforation.

The ovariectomized rat model has now been generally accepted as a useful model for screening different therapeutic agents, but there is a major requirement to identify reliable large animal models for osteoporosis research. In this study, the calcium restricted, ovariectomized minipig has been thoroughly investigated in order to define a large animal model with trabecular and cortical bone remodeling which would be reliable for further testing of agents that had shown promise of efficacy during the screening procedure. Twenty six female, 4-month old minipigs were randomized into four groups and fed either normal diet (0.90% calcium (Ca.)) or diet with restricted calcium content (0.75%, 0.50%). At the age of ten months, 3 groups were ovariectomized (OVX) while one group on normal diet was shamoperated. The groups were followed for six months after the operation. At death, bone mass was determined by densitometry and by ashing. Additionally, biomechanical competence was assessed in trabecular bone cores from the vertebral bodies. Finally, histomorphometry (static and dynamic parameters) and structural analyses (star volume) were performed on the vertebral bodies. The study revealed an OVX-related decline of 6% in vertebral bone mineral density (BMD) and a decline of 15% in trabecular bone volume (BV/TV). In contrast, a 15% increase in mean trabecular plate separation (Tb.Sp.) and a small increase in marrow space star volume (Ma. Star V.) were detected. The structural changes became more pronounced when OVX was combined with mild Ca. restriction (0.75% Ca.) with an increase in Ma. Star V. to 164%.(ABSTRACT TRUNCATED AT 250 WORDS)

Contrasting effects of parathyroid hormone and insulin-like growth factor I in an aged ovariectomized rat model of postmenopausal osteoporosis.

Agents that exert anabolic effects on bone have generally been tested in young or estrogen-replete animals. It is unclear whether these agents exert similar effects in older ovariectomized (Ovx) animals. In this single study we examined the effects of intermittent (daily) human PTH-(1-34) and continuous infusion of human recombinant IGF-I alone and in combination on bone resorption and formation over a 14 day period in an aged Ovx rat model of postmenopausal osteoporosis (2-year-old rats, Ovx at 1 year). Compared to Ovx controls, PTH treatment increased bone mineral content (BMC) and bone volume and stimulated bone formation but had no effect on bone resorption. In contrast, IGF-I treatment reduced BMC and stimulated resorptive activity as assessed by increases in marrow volume, cortical porosity, osteoclast-positive eroded surfaces, and urinary hydroxyproline excretion. IGF-I had no effect on bone formation, but when combined with PTH, IGF-I blunted the response to PTH on the periosteal and endocortical surfaces. In summary, PTH stimulated bone formation in a manner similar to that observed in younger animals and IGF-I stimulated bone resorption rather than formation and blunted the bone-forming response to PTH. The effects of IGF-I in older Ovx rats may differ from those observed in younger estrogen-replete animals.

Sequential histomorphometric changes in cancellous bone from ovariohysterectomized dogs.

To evaluate potential pharmacologic agents for the prevention or treatment of the bone loss associated with ovarian insufficiency, a predictable animal model is needed. To assess the potential utility of the ovariohysterectomized dog as a model of this condition, we characterized the sequential histomorphometric changes in canine cancellous bone in response to the loss of ovarian function. A group of 25 adult beagle dogs were ovariohysterectomized and terminated at 1, 3, 6, and 10 months following surgery. Iliac biopsies were performed following double-fluorochrome labeling at the time of surgery and at termination. Static and dynamic histomorphometry was performed on undecalcified sections. By 3 months postovariohysterectomy, there was activation of cancellous bone remodeling as indicated by significant increases in mineralizing surface and bone formation rate. Increases in osteoid surface, mineralizing surface, and bone formation rate were also apparent at 1 month postovariohysterectomy, and although not statistically significant, these trends suggest the skeletal response to acute loss of ovarian function was rapid. This increase in bone remodeling was transient. By 6 months, mineralizing surface and bone formation rate were depressed below presurgical levels. In addition to a reduction in bone formation, a reduction in osteoblast function characterized by reduced labeling of osteoid and a disproportionate increase in eroded surface also occurred. By 10 months postovariohysterectomy, cancellous bone remodeling was not significantly different from presurgical levels. At no time was a significant reduction in bone volume detected. These data suggest that the changes in cancellous bone remodeling in the ovariohysterectomized dog are a series of transient phenomena.(ABSTRACT TRUNCATED AT 250 WORDS)