What is the future for fluoride in the treatment of osteoporosis?

Treatment of osteoporosis: current status and recent advances

With the advent of new treatment choices, osteoporosis should no longer be considered an inevitable part of aging but rather a disease that can be treated. This article provides an overview of recent advances in treatment of osteoporosis.

Fluoride pharmacokinetics and changes in lumbar spine and hip bone mineral density

Debate about the use of fluoride for the treatment of vertebral osteoporosis has centered not only on whether fluoride treatment decreases vertebral fractures, but also the interindividual vertebral bone mineral density (BMD) response, the potential for nonvertebral fractures, as well as side effects and tolerability. These effects may be dose dependent and, in this study, we examine the pharmacokinetics of sodium monofluorophosphate (MFP) in osteoporotic patients and relate this to changes in BMD. Plasma fluoride absorption curves were measured from 0 to 6 h after ingestion of MFP at baseline and during long-term dosing in 21 patients with vertebral osteoporosis (T scores < or = 2). BMD was measured at baseline and at 12 months at the lumbar spine (LS), femoral neck (FN), trochanter, and Ward's triangle. We found that fluoride elimination was inversely related to creatinine clearance. LS BMD increased from a median of 0.77 g/cm2 (range 0.69 to 0.99) at baseline to 0.88 g/cm2 (0.75 to 1.13) (p < 0.001) after 12 months. This equates to a median increase of 12% (range -1.2 to 37). Median femoral neck BMD decreased from 0.75 g/cm2 (0.62 to 0.94) at baseline to 0.69 g/cm2 (0.62 to 0.92) (p = 0.13) after 12 months. This equates to a decrease of -2% (-19 to 10). BMD at the other hip sites also decreased slightly. Changes in LS and FN BMD were not significantly related (r = 0.28, p = 0.29). The various pharmacokinetic parameters measured were not related to changes in LS BMD; however, there was an inverse relationship between trough fluoride concentration during long-term dosing and change in FN BMD. Further studies are required to see if this relationship can be used to monitor osteoporotic patients treated with fluoride and prevent significant decreases in FN BMD and possibly fractures at this site.

Current and potential future drug treatments for osteoporosis

There has been a major interest in the drug treatment of osteoporosis and an increase in the number of drugs available in most countries. The ideal drug (one which increases or restores bone density and trabecular connectivity) is still not available. However, in patients with relatively preserved trabecular connectivity and moderately reduced bone density, several agents have shown substantial clinical benefit. Oestrogens are still the mainstay of drug treatment, but the risks of breast cancer versus the cardiovascular and skeletal benefits with long term use have to be assessed in the individual. Newer tissue specific oestrogens show some promise in this respect. The bisphosphonates and possibly fluoride are likely to be the major alternatives to oestrogens in the medium term. The newer bisphosphonates, alendronate and in the future risedronate, are likely to supersede etidronate. Calcitriol probably has a limited role, confined to those patients in whom HRT or bisphosphonates are not appropriate. Calcium supplementation, or an increase in dietary intake if deficient, irrespective of which agent is used, is also of benefit. In older patients there is considerable support for using a combination of calcium and vitamin D. Whether combination treatment, for example oestrogens, bisphosphonates, and calcium together, will result in greater efficacy remains to be conclusively shown, but may be an attractive option in younger patients with higher bone turnover. Apart from fluoride, bone formation stimulators are unlikely to have a major role until the next century, although it may be possible to use growth factors as part of an ADFR regimen (A = activate remodelling, D = depress resorption, F = free formation, and R = repeat). This is still an important theoretical approach and needs further work with newer agents to see if increased efficacy can be found. In addition sequential treatment may be necessary in view of the limited time periods over which particular agents, such as intermittent fluoride (four years), have been examined, and this will have to be individually tailored. Other approaches include trying to increase peak bone mineral density, although influencing the young to prevent a disease that may not manifest itself for half a century is daunting.

In corticosteroid-treated respiratory diseases, monofluorophosphate increases lumbar bone density: a double-masked randomized study

The efficacy of a monofluorophosphate-calcium combination (MFP-Ca) in increasing lumbar bone mineral density (BMD) was assessed in a prospective double-masked study. Patients (n = 35), who had been treated for 1 year or more with prednisone-equivalent doses > or = 7 mg/day for asthma or other respiratory diseases, were randomly assigned to receive twice a day, for 2 years, either one MFP-Ca tablet [100 mg sodium monofluorophosphate (13.2 mg F-) + 500.5 mg Ca2+] or one Ca tablet (500.5 mg Ca2+). BMD was measured from L2 to L4 using a dual photon absorptiometer. The eligible patients (7 premenopausal women, 21 men), who had no previous vertebral fractures and were aged 46.5 (21-65) years, had received 18 (7.5-60) mg prednisone-equivalent/day and had a mean lumbar BMD of 0.917 +/- 0.141 g/cm2 at baseline (MO); in these 28 patients, the mean increase in lumbar BMD at final assessment was significantly greater in the MFP-Ca group (p = 0.05; Mann-Whitney). There was also a significant difference after 2 years between the two groups (p = 0.05, ANOVA) in favour of MFP-Ca, with an increase in lumbar BMD of 11% (MFP-Ca) compared with 1% (Ca); thus, with MFP-Ca, lumbar BMD increased by an average of approximately 5.5%/year. There was no statistically significant difference between the two groups in doses of corticosteroids used during the 2 study years, rate of vertebral fractures, or frequency of side-effects (which were all minor). No bone fissure was observed. Thus, the daily dose of 200 mg monofluorophosphate (26.4 mg F-) combined with 1 g Ca2+ in patients with long-term corticosteroid-treated respiratory diseases appears to be a safe and efficient way of increasing lumbar BMD, suggesting that its use should be further studied in corticosteroid-induced osteoporosis.

Prevention and treatment of osteoporosis

Because the lifetime risk of fragility fracture for a 50-year-old Caucasian woman is about 40 per cent, a whole-life strategy of osteoporosis prevention is necessary. In childhood, primary prevention of osteoporosis is based on exercise and adequate dietary calcium. In women undergoing menopause, hormone replacement therapy administered for at least ten years remains the preventive treatment of choice, and is associated with a substantial reduction in vertebral and non-vertebral fractures. Intranasal salmon calcitonin and bisphosphonates are effective alternatives, but their effects on fracture rate and their long-term safety require further evaluation. Regarding the prevention of the late bone loss leading to senile osteoporosis, there is now evidence that the reduction of the secondary hyperparathyroidism induced by calcium and vitamin D insufficiencies through the administration of calcium and vitamin D supplements significantly decreases the hip fracture incidence. There is no general consensus about the efficacy of treatment for established osteoporosis with fractures. Fluoride salts have proven their direct stimulating effects on bone formation; dosage must be moderate, and the duration of treatment should be limited to 2-3 years in order not to impair the quality of the new bone. Cyclical therapy with etidronate induces beneficial effects on bone mass in the spine, but its effect on the vertebral fracture rate is not yet established. The new bisphosphonates seem to be promising for the management of osteoporosis. Several other agents such as growth factors, silicon derivatives and strontium salts are in various stages of testing. The new definition of osteoporosis proposed by a WHO study group, no longer based on the fracture but on a low bone mass, is of major interest, because it should make possible to have a more effective therapeutic approach, before the occurrence of an irreversible degree of bone loss.