Some of the drugs for osteoporosis have been covered elsewhere, but many of these drugs do not fit into other categories, so this chapter was created to pull this information together in one place.
Osteoporosis is the term used for a set of diseases characterized by the loss of bone mass. It is the most common of the metabolic bone diseases and is an important cause of morbidity in the elderly.
Pharmacologic therapy is targeted toward both prevention of bone loss and treatment of established osteoporosis (increasing bone mass and reducing fractures).
If you have not already done so, now is a good time to review the hormonal control of serum calcium and phosphorus concentrations. Which organs (kidney, parathyroid, intestine) produce which hormones that have what actions? Remember that bone is constantly forming and resorbing. The rates of the remodeling vary between people, between different bones, and at different ages. Diet and exercise play a major role in maintenance of bone mass. Intake of calcium and vitamin D are critical, and calcium supplements have been used for the treatment of osteoporosis. Calcium is not well absorbed from the gut and vitamin D improves absorption. Calcium supplementation has been shown to reduce bone loss in postmenopausal women, but not to increase bone density once lost. Calcium supplements are available in a variety of salts.
The bisphosphonates inhibit osteoclastic activity and decrease bone turnover and resorption. They have been shown to reduce the incidence of fractures.
These agents inhibit farnesyl pyrophosphate synthase, an enzyme that appears to be critical for osteoclast survival. They have been shown to improve bone mass in established osteoporosis. They are not well absorbed from the gastrointestinal (GI) tract, and absorption is decreased even more by the presence of food. They bind to bone, thus having a lasting effect from a single dose. This is the basis of the once weekly or once yearly dosing now available. The half-life of alendronate in bone has been measured to be at least 10 years.
Denosumab, a monoclonal antibody, is a RANK ligand inhibitor. RANK is “receptor activator of nuclear factor-κB.” Denosumab prevents the interaction of the ligand with the receptor on cells of osteoclastic lineage. Remember that osteoclasts break down bone and activation of the RANK receptor stimulates osteoclasts. Thus, blocking this receptor will inhibit bone resorption.
Teriparatide (1-34) and abaloparatide (full-length), recombinant parathyroid hormone, are effective in reducing the incidence of new fractures in patients with osteoporosis.
Parathyroid hormone was reviewed in Chapter 40. Unlike other treatments for osteoporosis, teriparatide stimulates the formation of new bone and increases bone mass. It must be given by injection.