Strontium and drugs
The experiments of Ringer showed, more than one hundred years ago, that strontium could replace calcium for allow the beating of an isolated frog heart. The ion radius of strontium, Sr2+, is 1.12 Angström, i.e. a little larger than that of Ca2+ which is 0.99 Angström.
Strontium is absorbed by the digestive tract by the same mechanisms that calcium but the absorption of calcium is preferential.
The plasma concentration of strontium in normal subjects is approximately 20 micrograms per liter; it rises when its food intake increases. Thus, the plasma concentration of calcium is controlled, but that of strontium is not.
As calcium, strontium has a great affinity for bone in which it accumulates.
Strontium is eliminated by urinary route.
The many studies devoted to the effects of strontium on bone can be summarized as follows:
- taken alone by oral route, in replacement of calcium or in greater quantity than calcium, strontium causes bone disorders pointing out those of the rickets and hypocalcemia.
- taken at the same time as calcium, it induces histogenesis and could reduce the development of osteoporosis.
Strontium ranelate (Protelos*) is marketed for the treatment of postmenopausal osteoporosis where it reduced the risk of vertebral and hip fractures.
Since strontium has high affinity for bone, radioactive strontium, Metastron*, is used for the treatment of bone metastases. The radioactive isotope used is Sr 89 whose half-life is 50 days. It accumulates in bone metastases which are thus preferentially irradiated. Its principal therapeutic use is the palliative treatment of the pain due to bone metastases from prostate cancer.
To increase uptake and binding of radioactive strontium by bones, it is necessary to stop any calcium supplementation during at least two weeks before strontium administration.
The adverse effects of radioactive strontium can be a transitory exacerbation of the bone pain, thrombopenia and leukopenia.
Samarium lexidronam (Quadramet*) is a complex formed of radioactive samarium and a tetraphosphonate, having a great affinity for bone. It is used, like radioactive strontium, in the treatment of the bone pains of metastatic origin.