Alendronate-conjugated liposome for targeting of osteoblastic prostate cancer: evaluation of binding to bone matrix and <i>in vivo</i> targeting in a mouse model

Prostate cancer bone metastases are predominantly osteoblastic, placing them in a unique position for bone-targeted drug delivery. Bone-targeted nanoparticles may be able to accumulate in aberrant bone formation inside these osteoblastic tumours, but this has not been explored. Here, we developed an alendronate-conjugated liposome (Lip-Cy5.5-Aln) encapsulating the near-infra-red dye Cy5.5 as a model payload to evaluate targeting. Binding to bone matrix was assessed <i>in vitro</i> using hydroxyapatite, MC3T3-E1-derived osteoblasts and 2H11-derived osteoblast-like cells. <i>In vivo</i> targeting of bone and bone-forming tumours was assessed in male nude mice with or without subcutaneous C4-2B (non-osteoblastic) or C4-2B-BMP4 (osteoblastic) tumours using optical imaging and non-targeted liposomes as control. Lip-Cy5.5-Aln selectively bound to bone matrix <i>in vitro</i> as opposed to control (57.4% vs. 0.6%, <i>p</i> < 0.0001). <i>In vivo</i>, Lip-Cy5.5-Aln exhibited increased musculoskeletal accumulation (51.0% vs. 39.8%, <i>p</i> < 0.0001) and reduced kidney localisation (18.4% vs. 29.9%, <i>p</i> < 0.0001). However, there was no significant difference in Cy5.5 signal between Lip-Cy5.5-Aln and control in bone-forming C4-2B-BMP4 tumours, nor was localisation greater in osteoblastic versus non-osteoblastic tumours. Overall, the alendronate-conjugated liposome showed enhanced musculoskeletal accumulation, but it does not seem to effectively target pathologic intratumour bone formation in metastatic prostate cancer.