September 21, 2011 (San Diego, California) — Bisphosphonate therapy for osteoporosis is much more likely to be effective among patients whose blood serum levels of vitamin D are elevated, according to research presented here at the American Society for Bone and Mineral Research (ASBMR) 2011 Annual Meeting.
For most osteoporosis drug trials, participants are also placed on vitamin D supplementation, and some studies have suggested that the efficacy of bisphosphonates could depend on levels of circulating vitamin D.
To more closely examine the relationship in a real-world setting, researchers evaluated 210 postmenopausal women (mean age, 65 years) with low bone mineral density (BMD), at 2 ambulatory practices in New York City.
The women had been treated with bisphosphonates for approximately 5 years and were followed for drug and vitamin D adherence over at least 18 months, which was the time between their last 2 dual-energy x-ray absorptiometry (DEXA) scans.
About half of the patients were treated with alendronate, a quarter were treated with risedronate, and about 18% were treated with intravenous zoledronate. Vitamin D levels were measured as 25(OH)D serum levels.
Only 99 (47%) of the 210 patients had shown a favorable response to the prolonged treatment with bisphosphonates, and a comparison of the mean 25(OH)D levels between responders and nonresponders showed that patients with a mean 25(OH)D serum level of 33 ng/mL or higher had as much as a 4.5-fold greater odds of bisphosphonate response (estimated odds ratio, 4.5; P < .0001).
In addition, the results showed 25(OH)D level as a continuous variable to be significantly associated with response to bisphosphonates. One 1-ng/mL decrease in 25(OH)D, for instance, was associated with about a 5% decrease in odds of responding (odds ratio, 0.95; 95% confidence interval, 0.92 - 0.98; P = .0007).
"The current study is the first to identify a threshold level of 25(OH)D that defines improved outcome to bisphosphonate therapy such that patients with a mean 25(OH)D ≥ 33 ng/mL had a substantially greater likelihood of responding to bisphosphonates," the authors wrote.
"This threshold value of ≥ 33 ng/mL for 25(OH)D is higher than the level considered adequate by the Institute of Medicine report for the general population, arguing that higher levels may be required for specific therapeutic outcomes."
According to coauthor Richard S. Bockman, MD, PhD, the results may help explain discrepancies in responses to bisphosphonates seen in controlled studies compared with the real-world environment.
" 'Real world' patients have not been observed to respond to bisphosphonates at rates comparable to those seen in clinical trials," said Dr. Bockman, a professor of medicine at the Weill Medical College of Cornell University and head of endocrine at the Hospital for Special Surgery in New York, New York.
Yet, "there is a high prevalence of low 25(OH)D levels among 'real world' patients taking bisphosphonates."
Studies typically have other outcome measures, and may not look at bisphosphonates in terms of a favorable maintenance therapy, as the current study was designed to evaluate, he added.
"There are studies reporting no effect of vitamin D and studies showing a vitamin D benefit, but they are not specifically looking at maintenance of bisphosphonate effect, and none specifically correlate outcome based on direct measure of circulating 25(OH) vitamin D levels, which is the gold standard for assessing vitamin D status," Dr. Bockman noted.
Laura A.G. Armas, MD, an assistant professor at Creighton University's Osteoporosis Research Center in Omaha, Nebraska, agreed that the findings are not necessarily surprising considering that efficacy in drug trials is typically based on patients who take vitamin D supplements.
"All the drug trials for bone-saving medications use calcium and vitamin D supplements for all their subjects," explained Dr. Armas, who moderated the session. "We would expect the same level of supplementation would be needed in the 'real world' to have the same decrease in fracture rate."
She noted that previous studies have shown similar minimal thresholds for improvement. "We know there is an increase in calcium absorption with increased 25(OH)D levels. Heaney showed that increased 25(OH)D from 20 ng/mL to 32 ng/mL in postmenopausal women increased calcium absorption by 68%, so this clinical experience fits with our prior knowledge."
Dr. Bockman added that the findings from previous studies were the source for his research team's use of 33 ng/mL as a hypothetical cutoff, and the data validated that level.
"A 25(OH)D equal to or greater than 33 ng/mL was associated with about a 4.5- to 5.0-fold greater odds of maintaining a favorable response," he said.
Dr. Bockman and Dr. Armas have disclosed no relevant financial relationships.
American Society for Bone and Mineral Research (ASBMR) 2011 Annual Meeting; Abstract #1137. Presented September 18, 2011.
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