Development of a novel poly bisphosphonate conjugate for treatment of skeletal metastasis and osteoporosis

Advanced stage prostate and breast cancer frequently metastasize to the skeleton (≈75%). An additional complication in these patients, that further affects the bones, is that their hormonal treatment, induces osteoporosis. Bisphosphonates (bpns) are standard drugs against osteoporosis and have been shown to have clinically significant anti-tumor effects. This study describes the development of a new polybisphosphonate conjugate (ODX) with enhanced dual efficacy i.e. with anti-bone resorption and anti-tumor properties. Zoledronic acid (Zometa®) was used as a positive control (at equimolar concentrations). Alendronic acid and aminoguanidine were conjugated to oxidized dextran with subsequent reductive amination (on average ≈8 alendronate and ≈50 guanidine moieties per conjugate). ODX was tested in a bone resorption assay for its capacity to inhibit bone resorbing osteoclasts (bone organ culture from neonatal mice, 45Ca labelled bone mineral). Tumor cell toxicity was studied on prostate (PC3) and breast cancer (MDA231, MDA453) cell cultures. Two methods were employed, a fluorescent cytotoxicity assay (FMCA) and an apoptosis assay (Annexin V assay). In the bone resorption assay, Zometa and ODX showed very similar potency with 50% osteoclast inhibition at ≈20 nM and 100% at 0.2 µM. In the FMCA, IC50 for ODX was at ≈2 µM and 25 µM for Zometa (PC3). In the apoptosis assay, ODX induced ≈85-97% apoptosis at 10 µM in both cell lines, while Zometa failed to induce any significant apoptosis in any of the cell lines at the tested concentration range (10 nM-10 µM). ODX appears to be a promising drug candidate with high dual efficacy for the treatment of bone metastasis and osteoporosis. It has both potent osteoclast inhibiting properties and enhanced anti-tumor efficacy.