Odanacatib, effects of 16-month treatment and discontinuation of therapy on bone mass, turnover and strength in the ovariectomized rabbit model of osteopenia

Odanacatib (ODN) a selective and reversible cathepsin K inhibitor, inhibits bone resorption, increases bone mass and reduces fracture risk in women with osteoporosis. A 16-month (~ 7-remodeling cycles) study was carried out in treatment mode to assess the effects of ODN versus ALN on bone mass, remodeling status and biomechanical properties of lumbar vertebrae (LV) and femur in ovariectomized (OVX) rabbits. This study also evaluated the impact of discontinuing ODN on these parameters. Rabbits at 7.5 months post-OVX were dosed for 16-months with ODN (7.5 μM·h0-24, in food) or ALN (0.2 mg/kg/wk, s.c.) and compared to vehicle-treated OVX- (OVX + Veh) or Sham-operated animals. After 8 months, treatment was discontinued in half of the ODN group. ODN treatment increased in vivo LV aBMD and trabecular (Tb) vBMD until reaching plateau at month 12 by 16% and 23% vs. baseline, respectively, comparable levels to that in Sham and significantly above OVX + Veh. LV BMD was also higher in ALN that plateaued around month 8 to levels below that in ODN or Sham. ODN treatment resulted in higher BMD, structure and improved biomechanical strength of LV and central femur (CF) to levels similar to Sham. ALN generally showed less robust efficacy compared to ODN. Neither ODN nor ALN influenced material properties at these bone sites following ODN or ALN treatment for 7 remodeling cycles in rabbits. ODN and ALN persistently reduced the bone resorption marker urinary helical peptide over study duration. While ALN reduced the bone formation marker BSAP, ODN treatment did not affect this marker. ODN also preserved histomorphometry-based bone formation indices in LV trabecular, CF endocortical and intracortical surfaces, at the levels of OVX + Veh. Discontinuation of ODN returned bone mass, structure and strength parameters to the comparable respective levels in OVX + Veh. Together, these data demonstrate efficacy and bone safety profile of ODN and suggests the potential long-term benefits of this agent over ALN with respect to accrued bone mass without long-term effects on bone formation.