Strontium ranelate for the treatment of osteoporosis is useful, but changes in bone mineral density need careful interpretation. Nowadays the main treatment options are anti-resorptive agents that prevent bone breakdown.
For patients with previous fractures, treatment with an anabolic agent that enhances bone formation is preferable, and this is now possible since the introduction of recombinant parathyroid hormone (PTH). However, the use of PTH is limited by its high cost and the need for subcutaneous self-injection.
Recently strontium ranelate has been licensed in the United Kingdom for the treatment of osteoporosis. The mechanism of action of strontium ranelate is not yet fully understood, compared with other treatments, but it seems to have a unique effect in that it inhibits bone resorption as well as stimulating bone formation. Recent studies have shown its efficacy at preventing both vertebral and non-vertebral fractures, including hip fractures in older women. Strontium ranelate was well tolerated in these trials and the most common side effects reported were nausea and diarrhea.
Strontium ranelate is composed of two atoms of stable strontium combined with organic ranelic acid. The ranelic acid is a carrier that makes the treatment palatable, and the strontium is the active component with regard to the skeleton. As an alkaline earth element, strontium is similar to calcium in its absorption in the gut, incorporation in bone, and elimination from the body through the kidneys.
Strontium is naturally present in trace amounts with around 100 µg in every gram of bone, so treatment with strontium ranelate is simply making more strontium available for incorporation into bone. In the short term the strontium atoms are adsorbed on to the surface of hydroxyapatite crystals, and in the longer term some strontium will exchange with calcium in the bone mineral and may remain bound in the skeleton for years.
Strontium not incorporated into bone is excreted through the kidneys and feces. After three years' treatment with strontium ranelate, bone tissue will contain around one strontium atom for every 100 calcium atoms. No human studies have yet reported how quickly bone strontium is washed out once treatment is stopped.
An impressive increase in bone mineral density occurred in the spine (14.4%) and hip (8.3%).7. However, some caution is necessary in interpreting these figures because much of this effect is due to the higher atomic number of strontium (Z = 38) compared with calcium (Z = 20).
Bone biopsy was performed in a subset of patients after the adjustment for bone strontium, the measured effect of treatment on bone mineral density in the spine of 14.4% was corrected to 8.1%.7.
The important issue for treatment for osteoporosis is prevention of fractures and not changes in bone mineral density. Caution is therefore needed in predicting efficacy against fractures simply on the basis of a change in bone mineral density.
An increase of 5% in spine bone mineral density is required to confirm a notable response to treatment. With strontium ranelate, large increases in bone mineral density are often seen, and if this is the case one can be confident that the patient is complying with treatment. The issue of patients' long-term use and compliance with treatment is important for all types of treatment for osteoporosis.