|Response from Gayle Nicholas Scott, PharmD, BCPS, ELS
Assistant Professor, Eastern Virginia Medical School, Norfolk, Virginia; Clinical Pharmacist, Chesapeake Regional Medical Center, Chesapeake, Virginia
Strontium is located on the periodic table in the alkali earth metal group, the same group that includes calcium. Strontium is used for a variety of indications ranging from an ingredient in toothpaste for sensitive teeth (Sensodyne®, GlaxoSmithKline, Research Triangle Park, North Carolina) to strontium-89 (Metastron™, GE Healthcare, Waukesha, Wisconsin) for metastatic bone pain. Strontium is widely promoted for treatment of osteoporosis. In Europe, strontium ranelate is available as a prescription drug. In the United States, strontium is a dietary supplement available as the carbonate, chloride, citrate, gluconate, and sulfate salts. The US Food and Drug Administration (FDA) granted strontium malonate investigational new drug (IND) status.
Most drugs for osteoporosis inhibit bone resorption. Available antiresorptive agents include bisphosphonates (alendronate, risedronate, ibandronate, and zoledronic acid), estrogen, selective estrogen modulators (raloxifene), and calcitonin. Parathyroid hormone (teriparatide) and fluoride stimulate bone formation. Strontium ranelate is unique in that it appears to act by both of these mechanisms.
Strontium ranelate is comprised of 2 atoms of strontium and the organic acid ranelic acid. Product information about strontium ranelate on the Website of the European Medicines Agency, the European Union equivalent of the FDA, explains the choice of the ranelate salt with “the organic part permit[s] the best compromise in terms of molecular weight, pharmacokinetics and acceptability of the medicinal product.” The bioavailability of strontium ranelate ranges from 19% to 27%; food and calcium products reduce its already low bioavailability by 60%-70%.
The safety and efficacy of strontium ranelate for osteoporosis have been investigated in 4 randomized, double-blind, placebo-controlled trials. Three trials evaluated strontium ranelate 0.5-2 g/day for osteoporosis treatment,[2-4] and 1 study evaluated 125 mg-1 g/day for osteoporosis prevention. For treatment, strontium ranelate 2 g/day for 3 years reduced vertebral fractures by 37% (relative risk [RR] 0.63; 95% confidence interval [CI] 0.56, 0.71) and nonvertebral fractures by 14% (RR 0.86; 95% CI 0.75, 0.98). Lower doses were superior to placebo, but reduction in vertebral fractures and increase in bone mineral density were greater with 2 g/day. A recent study of women with osteoporosis taking strontium ranelate 2 g daily for 8 years showed sustained increases in bone mineral density. In all studies, patients took supplemental calcium and vitamin D.
Diarrhea is the most common adverse effect associated with strontium ranelate, usually occurring with higher doses. Data suggest that patients receiving strontium 2 g/day for 3-4 years have a higher risk for vascular side effects (eg, blood clots) and nervous system side effects (eg, seizures).
All research published in peer-reviewed journals is on strontium ranelate. A phase 2 study on strontium malonate has been published as a meeting abstract. No credible research is available on other strontium salts, and the bioavailability and effectiveness of these products are unknown.
For patients concerned about osteoporosis prevention, emphasize the importance of weight-bearing exercises and adequate calcium and vitamin D intake. For patients with diagnosed osteoporosis, FDA-approved treatments are indicated. Tell patients that dietary supplements containing strontium are unproven and should be avoided.