Cathepsin K inhibitors prevent bone loss in estrogen-deficient rabbits

Abstract

Two cathepsin K inhibitors (CatKIs) were compared with alendronate (ALN) for their effects on bone resorption and formation in ovariectomized (OVX) rabbits. The OVX model was validated by demonstrating significant loss (9.8% to 12.8%) in lumbar vertebral bone mineral density (LV BMD) in rabbits at 13-weeks after surgery, which was prevented by estrogen or ALN. A potent CatKI, L-006235 (L-235), dosed at 10 mg/kg per day for 27 weeks, significantly decreased LV BMD loss ( p < .01) versus OVX-vehicle control. ALN reduced spine cancellous mineralizing surface by 70%, whereas L-235 had no effect. Similarly, endocortical bone-formation rate and the number of double-labeled Haversian canals in the femoral diaphysis were not affected by L-235. To confirm the sparing effects of CatKI on bone formation, odanacatib (ODN) was dosed in food to achieve steady-state exposures of 4 or 9 mM/day in OVX rabbits for 27 weeks. ODN at both doses prevented LV BMD loss ( p < .05 and p < .001, respectively) versus OVX-vehicle control to levels comparable with sham or ALN. ODN also dose-dependently increased BMD at the proximal femur, femoral neck, and trochanter. Similar to L-235, ODN did not reduce bone formation at any bone sites studied. The positive and highly correlative relationship of peak load to bone mineral content in the central femur and spine suggested that ODN treatment preserved normal biomechanical properties of relevant skeletal sites. Although CatKIs had similar efficacy to ALN in preventing bone loss in adult OVX rabbits, this novel class of antiresorptives differs from ALN by sparing bone formation, potentially via uncoupling bone formation from resorption. ß

Alteration of Type I collagen microstructure induced by estrogen depletion can be prevented with drug treatment

“…ALN treatment was previously observed to function as an antiresorptive leading to a BMD higher than that in the vehicle-treated group. 12 In this study, ALN treatment resulted in a frequency of collagen fibril observations intermediate between Sham and OVX values, although the differences in proportions of fibrils in Parallel and Oblique regions in these treatments were not significant. The bone resorption inhibitor CatKI also gave intermediate values for the frequency of collagen fibril observations and showed even less shift from the initial Sham values, with none of the comparisons versus other treatments being significant.…”

“…8,9 Discussion OVX is a common model used to study the effects of estrogen depletion. 8,9,[15][16][17] Pennypacker et al 12 applied the OVX model to study osteopenia in rabbits using the samples described herein and demonstrated osteoclastic bone resorption resulting from estrogen depletion. On the basis of the strong positive correlations between the experimental strength parameters and the tissue bone mineral content from animals from the same study, it was demonstrated that there were no obvious treatment-related changes in the material properties of vertebrae and femurs in the OVX rabbits treated with odanacatib or ALN for 27 weeks.…”

“…11 In order to explore the effect of estrogen depletion and compare efficacy of CatKI and alendronate (ALN) drug treatments, the rabbit OVX model was developed by Merck Inc., as human CatKI's exhibit similar potency for the rabbit enzyme and the adult rabbit skeleton undergoes substantial cortical Haversian remodeling (unlike the rodent). 12 Nine-month-old New Zealand white female rabbits (Data Set I) underwent the following treatments: sham operation þ vehicle (n ¼ 11), ovariectomy þ vehicle (OVX; n ¼ 12), OVX þ 200 mg kg À 1 ALN, 3 Â a week for 27 weeks (n ¼ 12) and OVX þ 10 mg kg À 1 CatKI L-235 daily for 27 weeks. Treatments were initiated 3 days after surgery.…”

“…11 Furthermore, biglycan and decorin knockout mice have abnormal collagen fibril morphologies. 12 We hypothesize that reduced proteoglycan levels perturb the interactions between adjacent fibrils, resulting in a change to the microstructure. Cathepsin-K, a collagenase secreted by osteoclasts, cleaves proteoglycans from the fibril exterior before degrading the primary fibril structure.…”

“…12 Sample preparation and the AFM imaging methods employed in this work have also been described in detail. 6 All imaging was carried out in air using a PicoPlus 5500 AFM (Agilent, Santa Clara, CA, USA) employing tapping mode with VistaProbes T300R probes (NanoScience, Phoenix, AZ, USA; nominal radius 10 nm, force constant 40 N m À 1 , resonance frequency 300 kHz).…”