The antiresorptive mechanism of action of nitrogen-containing bisphosphonates (N-BPs) is multifactorial. It has become clear that the bone affinity which targets BPs to bone, and inhibition of farnesyl diphosphate synthase (FPPS) within osteoclasts and osteoclast precursor cells are the main determinants of their potency to inhibit resorption. Fluorescent-BPs provide information on localisation of BPs in cortical and cancellous bone, as well as penetration at bone surfaces and within osteocyte canaliculi. We further evaluated the relative mineral affinity of both parent BPs and fluorescent-BPs by measuring their binding on HAP columns and disks. Mineral affinity was determined relative to risedronate (RIS =1.0). Zoledronate (ZOL) had higher affinity than RIS, while minodronate (MIN) was similar. The addition of fluorescent groups to RIS can increase (rhodamine 2 (RhR) =1.45) or decrease (alexafluor 647 (AF647)=.75) mineral affinity relative to RIS in the order: ROX-RIS > ZOL > RhR-RIS > RIS = MIN > fluorescein (FAM)-RIS =800CW-ZOL > FAM-ZOL > AF647-RIS > deoxyRIS > RisPC. The impact of bone affinity in vivo on 24 hour urinary excretion after iv administration was also studied. Compounds were simultaneously injected in rats in this first head to head study of N-BPs. Approximately 90% of the amount excreted within 24 hours of both parent and fluorescent-BPs was excreted within 4 hours. The excretion within a 24 hour period relative to RIS, in order from high to low in vivo retention, was: alendronate > ZOL > RIS > etidronate > MIN > Ox-14 > neridronate > ibandronate = FAM-RIS > clodronate = AF647-RIS >> RisPC. This data indicates that binding of BPs to HAP strongly influences their excretion profiles, inversely reflecting skeletal retention. The ability to alter bone affinity among BPs with varied fluorescent tags is a useful tool for exploring the pharmacology of BPs.
Disclosure: Ebetino, Lundy: retiree P&G Pharmaceuticals, consultant BioVinc LLC; Russell, McKenna: former consultant to P&G Pharmaceuticals; Dobson, Jeans, Quijano: current employees of Procter & Gamble. Funding from Procter and Gamble Pharmaceuticals.