| Summary: | Accelerated osteoclastic bone resorption plays an important role in the pathogenesis of osteoporosis and other common bone diseases such as Paget's disease of bone, rheumatoid arthritis and cancer-associated bone disease. The prevention and treatment of these diseases is based on agents that inhibit osteoclast (OC) formation and resorption. Nitrated non-steroidal anti-flammatory drugs (NO-NSAID) are a recently developed group of compounds that contain a conventional NSAID linked to a nitric oxide (NO)-donor group. As prostaglandins and NO can both stimulate and inhibit OC formation and bone resorption, I investigated whether NO-NSAD may affect the bone loss observed in bone diseases. Both Flurbiprofen and its nitrated derivative NO-Flurbiprofen (HCT1026) inhibit IL-1-stimulated OC formation and resorption in mouse co-cultures. However, HCT1026 was much more potent than the parent compound Flurbiprofen. As resorption was completely blocked by HCT1026 even though there were still OC present, I studied the effect of this drug on actin ring formation as an indicator of OC activity HCT1026 significantly decreased the percentage of active OC within 4 hours and all actin rings had disappeared after 24 hours, whereas Flurbiprofen had no effects. HCT1027, a HCT1026 derivative that lacks the NO-donor properties, was a potent inhibitor of OC formation and resorption in both murine and human cultures. The fact that HCT1027 and HCT1026 were equipotent clearly demonstrates that the anti-resorptive effects of these compounds are not dependent on NO-donor properties. Both HCT1026 and HCT1027 were potent inducers of apoptosis in OC and J774 cells as evidenced by numbers of apoptotic nuclei and caspase-3 activation. Treatment of primary osteoblasts (OB) with HCT1026 for up to 72 hours failed to induce OB apoptosis, suggesting that this compound acts only on cells of the monocyte-macrophage lineage such as macrophages and OC. Intraperitoneal injection with ABD56 (10 mg/kg/day) completely prevented ovariectomy-induced bone loss in mice and maintained BMD at level comparable to those of non-ovariectomised Sham controls. Histomorphometrical analysis of the proximal tibial metaphysis indicated that ABD56 totally recovered ovariectomy-induced trabecular bone loss and significantly decreased OC number in both Sham- and ovariectomy-operated mice, whereas it had not significant effects on OB numbers. In conclusion, we successfully developed a novel class of anti-resorptive drugs using structure-based drug design. The non-nitrated biphenyl carboxylic acid derivatives ABD56 and ABD68 are presented here as anti-resorptive agents that inhibit osteoclast formation, survival and resorption by a mechanism that is independent of NO production, COX inhibition and formation of metabolites. ABD56 prevents ovariectomy-induced bone loss and induces apoptosis in mature OC in vitro by inhibiting NFkB and p42/44 activity. Although its exact mechanism of action is yet unclear, ABD56 is a promising therapeutic candidate for the treatment of bone diseases associated with accelerated bone loss due to osteoclast activation such as osteoporosis, RA and cancer associated bone disease.
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