Accumulation of zoledronic acid in rabbit intervertebral discs

Low back pain is a major chronic musculoskeletal disorder, caused mainly due the degeneration of the intervertebral discs (IVDs). Bisphosphonates (BPs), like zoledronic acid (ZOL), are used in osteoporosis management; however, their accumulation in the IVDs and their physiological role has not been addressed so far. To this end, an SPE-liquid chromatography-tandem mass spectrometry (SPE-LC-MS/MS) analytical method, using on-cartridge derivatization, has been developed and validated. The analytical method presented excellent linearity (R² > 0.992), high recoveries (67.8-82.6%), increased repeatability (0.5-9.9%) and low LOD values (21 ng g^-1 in the case of IVDs) in all matrices studied. The injection of ZOL in a rabbit animal model resulted in rapid accumulation in blood plasma and the skin, followed by quick clearance. On the other hand, no ZOL was detected in nucleus pulposus, the core of the IVD, while in the peripheral annulus fibrosus a lower and delayed accumulation, as well as dispersal was found. These variations are most probably due to the avascular nature of IVD, allowing only the diffusion of small molecules in and out of the tissue, and/or to the unique physicochemical environment of IVDs. Finally, ZOL, at the concentrations found, did not affect cells' viability or the increase of reactive oxygen species (ROS).