Sequential therapy in the treatment of osteoporosis

Muscle-bone interactions: From experimental models to the clinic? A critical update

Bone is a biomechanical tissue shaped by forces from muscles and gravitation. Simultaneous bone and muscle decay and dysfunction (osteosarcopenia or sarco-osteoporosis) is seen in ageing, numerous clinical situations including after stroke or paralysis, in neuromuscular dystrophies, glucocorticoid excess, or in association with vitamin D, growth hormone/insulin like growth factor or sex steroid deficiency, as well as in spaceflight. Physical exercise may be beneficial in these situations, but further work is still needed to translate acceptable and effective biomechanical interventions like vibration therapy from animal models to humans. Novel antiresorptive and anabolic therapies are emerging for osteoporosis as well as drugs for sarcopenia, cancer cachexia or muscle wasting disorders, including antibodies against myostatin or activin receptor type IIA and IIB (e.g. bimagrumab). Ideally, increasing muscle mass would increase muscle strength and restore bone loss from disuse. However, the classical view that muscle is unidirectionally dominant over bone via mechanical loading is overly simplistic. Indeed, recent studies indicate a role for neuronal regulation of not only muscle but also bone metabolism, bone signaling pathways like receptor activator of nuclear factor kappa-B ligand (RANKL) implicated in muscle biology, myokines affecting bone and possible bone-to-muscle communication. Moreover, pharmacological strategies inducing isolated myocyte hypertrophy may not translate into increased muscle power because tendons, connective tissue, neurons and energy metabolism need to adapt as well. We aim here to critically review key musculoskeletal molecular pathways involved in mechanoregulation and their effect on the bone-muscle unit as a whole, as well as preclinical and emerging clinical evidence regarding the effects of sarcopenia therapies on osteoporosis and vice versa.

Sclerostin and Dickkopf-1 as therapeutic targets in bone diseases

The processes of bone growth, modeling, and remodeling determine the structure, mass, and biomechanical properties of the skeleton. Dysregulated bone resorption or bone formation may lead to metabolic bone diseases. The Wnt pathway plays an important role in bone formation and regeneration, and expression of two Wnt pathway inhibitors, sclerostin and Dickkopf-1 (DKK1), appears to be associated with changes in bone mass. Inactivation of sclerostin leads to substantially increased bone mass in humans and in genetically manipulated animals. Studies in various animal models of bone disease have shown that inhibition of sclerostin using a monoclonal antibody (Scl-Ab) increases bone formation, density, and strength. Additional studies show that Scl-Ab improves bone healing in models of bone repair. Inhibition of DKK1 by monoclonal antibody (DKK1-Ab) stimulates bone formation in younger animals and to a lesser extent in adult animals and enhances fracture healing. Thus, sclerostin and DKK1 are emerging as the leading new targets for anabolic therapies to treat bone diseases such as osteoporosis and for bone repair. Clinical trials are ongoing to evaluate the effects of Scl-Ab and DKK1-Ab in humans for the treatment of bone loss and for bone repair.

Combination anabolic and antiresorptive therapy for osteoporosis

Osteoanabolic agents directly stimulate bone formation to improve bone mass and skeletal microarchitecture. At present, parathyroid hormone (PTH), in the form of the full-length molecule (PTH(1-84)) and its fully active, but truncated amino-terminal fragment, PTH(1-34) (teriparatide), are the only medications that belong to the osteoanabolic class. It is appealing to consider simultaneous combination therapy with antiresorptive and osteoanabolic drugs as potentially more beneficial than monotherapy with either class, given that their mechanisms of action differ. This review focuses on the research that has been conducted on combination therapy with PTH and an antiresorptive drug.

New understanding and treatments for osteoporosis

To summarize promising areas of investigation in osteoporosis and to stimulate further research in this area, as discussed in a recent international conference. Over the recent years, there has been an improvement in the knowledge of molecular pathways involved in bone formation and resorption with the development of new drugs to treat osteoporosis. Intact parathyroid hormone, teriparatide, and anti-sclerostin monoclonal antibody are anabolic drugs, whereas denosumab and odanacatib are anti-resorptive drugs with more reversible effects as compared to bisphosphonates. Anabolic and anti-resorptive agents have different effects on bone, and research in this area includes the efficacy of combination and sequential therapies with them. New insights in the molecular pathways of bone remodeling have clarified the mechanisms responsible for skeletal fragility in several forms of secondary osteoporosis, such as that occurring in type 2 diabetes, following drug exposure and systemic inflammatory diseases. Future research is needed to address the efficacy of anti-osteoporotic drugs in these more recently recognized conditions of skeletal fragility. Osteoporosis continues to be an important field of biomedical research.

Combination antiresorptive and osteoanabolic therapy for osteoporosis: we are not there yet

Osteoanabolic therapy is theoretically and practically an appealing therapeutic option for men and postmenopausal women with osteoporosis because bone formation is directly stimulated, an action that is not shared by any antiresorptive agent. Parathyroid hormone (PTH), in the form of the full-length molecule (PTH[1-84]) and its fully active but truncated amino-terminal fragment teriparatide (PTH[1-34]), belong to this osteoanabolic class. Both formulations of PTH increase bone mineral density, increase biochemical markers of bone turnover, and reduce fracture incidence. They improve skeletal microstructure. While antiresorptive agents are considered by most to be first line for the treatment of osteoporosis, there are situations when anabolic therapy could be reasonably considered as first line. In most situations, however, treatment with PTH follows a course of antiresorptive therapy. Simultaneous combination therapy with PTH and an antiresorptive drug does not appear to provide any advantages over monotherapy. After the recommended 2-year period of PTH treatment, an antiresorptive should be used to maintain densitometric gains. The drugs are well tolerated. Early safety concerns about osteosarcoma in rats have not been borne out after almost 9 years experience with human subjects.