Combination Treatment with Sclerostin and Dkk1 Antibodies Synergizes with Tibial Loading to Stimulate Bone Formation in Aged Mice

Aging is associated with decreased bone formation and bone mass and increased fracture risk. Wnt pathway activation by mechanical loading is a potent strategy to improve bone mass, however, load-induced bone formation is diminished with aging. Neutralizing antibody (Ab) therapies targeting Wnt pathway inhibitors Sclerostin (Scl) and Dickkopf-related protein 1 (Dkk1) have proven successful in preclinical and clinical osteoporotic conditions. We asked whether treatment combining Scl-Ab and Dkk1-Ab can increase load-induced bone formation in a preclinical model of osteoporosis. Aged (22-month) C57BL/6N female mice underwent combination Scl-Ab plus Dkk1-Ab therapy (15 mg/kg each; subcutaneously; 2x/wk; saline control) for 2 weeks, concomitant with a mechanical loading regimen previously shown to induce modest bone formation in tibias of aged mice (-2200 {varepsilon}, 1200 cycles/day, 5 day/wk). Changes in bone morphology and formation were assessed by longitudinal microCT and dynamic histomorphometry, respectively. Molecular indices of bone formation and Wnt pathway activation were assessed by qPCR of cortical bone. Treatment with Scl-Ab plus Dkk1-Ab induced significant improvements in cancellous (BV/TV +50%) and cortical morphology (Ct.Th +25%) in non-loaded limbs of antibody-treated mice vs. vehicle control mice. Importantly, periosteal bone formation rate was 10-fold higher in loaded limbs of antibody versus vehicle treated mice, indicating a synergistic effect. Gene expression analysis showed that antibody treatment and loading synergistically upregulated Wnt1 expression, which may have contributed to the observed synergistic effect on bone formation. These results confirm the potent anabolic effect of combination Scl plus Dkk1 antibody treatment. Moreover, they show that antibody treatment and skeletal loading are more effective at increasing periosteal bone formation in aged mice than either treatment alone. These findings support the concept that combinatorial therapy using dual Scl and Dkk1 antibodies plus weight-bearing exercise may be an effective treatment for age-related osteoporosis.