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Pamidronate functionalized nanoconjugates for targeted therapy of focal skeletal malignant osteolysis

Significance Malignant osteolysis associated with inoperable primary bone tumors and multifocal skeletal metastases remains a challenging clinical problem in cancer patients. The outgrowth of residual cancer cells within the bone microenvironment despite aggressive multimodality therapies necessitates the discovery and validation of novel strategies for treating osteotropic solid tumors. We report on pamidronate functionalized polylactide nanoparticles for the targeted treatment of focal malignant osteolysis by delivering doxorubicin specifically to the bone tumor microenvironment. Improved efficacy was demonstrated in a preclinical orthotopic mouse model of osteosarcoma. Most importantly, through the inclusion of dogs with naturally developing osteosarcoma, biocompatibility, biodistribution, and anticancer activities at clinically relevant dosages were demonstrated in a large mammalian modeling system. Malignant osteolysis associated with inoperable primary bone tumors and multifocal skeletal metastases remains a challenging clinical problem in cancer patients. Nanomedicine that is able to target and deliver therapeutic agents to diseased bone sites could potentially provide an effective treatment option for different types of skeletal cancers. Here, we report the development of polylactide nanoparticles (NPs) loaded with doxorubicin (Doxo) and coated with bone-seeking pamidronate (Pam) for the targeted treatment of malignant skeletal tumors. In vivo biodistribution of radiolabeled targeted Pam-NPs demonstrated enhanced bone tumor accumulation and prolonged retention compared with nontargeted NPs. In a murine model of focal malignant osteolysis, Pam-functionalized, Doxo-loaded NPs (Pam-Doxo-NPs) significantly attenuated localized osteosarcoma (OS) progression compared with nontargeted Doxo-NPs. Importantly, we report on the first evaluation to our knowlege of Pam-Doxo-NPs in dogs with OS, which possess tumors of anatomic size and physiology comparable to those in humans. The repeat dosing of Pam-Doxo-NPs in dogs with naturally occurring OS indicated the therapeutic was well tolerated without hematologic, nonhematologic, and cardiac toxicities. By nuclear scintigraphy, the biodistribution of Pam-Doxo-NPs demonstrated malignant bone-targeting capability and exerted measurable anticancer activities as confirmed with percent tumor necrosis histopathology assessment.