PTH and interactions with bisphosphonates

We report that a therapeutic dose of the antiresorptive bisphosphonate alendronate administered to skeletally mature rats for the duration of 16 weeks significantly blunted the anabolic response to a high dose SDZ PTS 893 in the tibia and femur but not in lumbar vertebra. Effects were seen at the level of bone mass (DEXA, pQCT) as well as in biomechanical tests. In one arm of this study, rats were switched to vehicle injections after 8 weeks on alendronate for another 8 weeks before being challenged with the anabolic stimulus (washout). This recovery period was insufficient for full recovery and the response to SDZ PTS 893 was still greatly reduced after this procedure. Serial pQCT-measurements suggest that part of the interaction happened during the first two weeks of PTH treatment when bone-lining cells are activated by the anabolic drug. In addition bisphosphonate pretreated rats failed to catch up with the vehicle control at all time points suggesting a second level of drug interaction. The failure of the 'washout' period to restore the normal response to PTH is suggestive of a physico-chemical interaction on the level of the matrix embedded bisphosphonate with the overlaying bone lining cells, rather than of direct effects of the drug on osteoblasts or their precursor cells. Overall the data raises the possibility, that bisphosphonate treated patients respond to PTH and SDZ PTS 893 with a delay which could affect the shorter bone mass measurements carried out at 6 months to 1 year. Additionally, bisphosphonate pre-treated rats did not develop the full anabolic response over time. Clinical investigators studying anabolic drugs such as PTH should be aware of potential long-term interactions of bisphosphonates when assessing the outcome of their experiments. However, the beneficial effect of bisphosphonates like alendronate on PTH-induced bone remodeling, as well as its potent action in the protection of bone loss after cessation of anabolic therapy might outweigh the worries about a small delay in the bone response to parathyroid hormone.