Targeting of Radioactive Platinum-bisphosphonate Anticancer Drugs to Bone of High Metabolic Activity

Platinum-based chemotherapeutics exhibit excellent antitumor properties. However, these drugs cause severe side effects including toxicity, drug resistance, and lack of tumor selectivity. Tumor-targeted drug delivery has demonstrated great potential to overcome these drawbacks. Herein, we aimed to design radioactive bisphosphonate-functionalized platinum (Pt-BP) complexes to confirm preferential accumulation of these Pt-based drugs in metabolically active bone. In vitro NMR studies revealed that release of Pt from Pt BP complexes increased with decreasing pH. Upon systemic administration to mice, Pt-BP exhibited a 4.5-fold higher affinity to bone compared to platinum complexes lacking the bone-seeking bisphosphonate moiety. These Pt-BP complexes formed less Pt-DNA adducts compared to bisphosphonate-free platinum complexes, indicating that in vivo release of Pt from Pt-BP complexes proceeded relatively slow. Subsequently, radioactive Pt-BP complexes were synthesized using Pt(NO)(en) as precursor and injected intravenously into mice. Specific accumulation of Pt-BP was observed at skeletal sites with high metabolic activity using micro-SPECT/CT imaging. Furthermore, laser ablation-ICP-MS imaging of proximal tibia sections confirmed that Pt BP co-localized with calcium in the trabeculae of mice tibia.