Skeletal-Targeted, Osteolytic-Responsive Drug Delivery System Improves Anti-Tumor Efficacy in Multiple Myeloma.

Multiple myeloma (MM) cells often directly or indirectly induce the formation of osteoclasts, which enhance bone resorption by increasing secretion of a key protease (cathepsin K) to degrade bone matrix, leading to osteolytic lesions and serious skeletal complications. Hence, bone-targeted, osteolytic-responsive therapeutic modalities are greatly needed to improve clinical outcomes for MM.

A tri-block copolymer of peptide, poly(ethylene glycol) and poly(trimethylene carbonate) (Pep-b-PEG-b-PTMC) has been synthesized as a nanocarrier to improve treatment for MM. The functional peptide with the sequence of CKGHPGGPQAsp₈ was designed to possess a bone tropism octapeptide (Asp₈), a cathepsin K (CTSK)-cleavable substrate (HPGGPQ), multiple cationic residues and a terminal cysteine for site-specific conjugation. Maleimide-terminated diblock copolymer of PEG-b-PTMC was readily functionalized with the peptide to obtain Pep-b-PEG-b-PTMC that can spontaneously form micelles with the size of 75 nm in diameter. Sixty-six % of polymeric micelles were able to bind to hydroxyl apatite, showing high bone binding capability. The nanoparticles exhibited a negative-to-positive charge conversional profile upon exposure to cathepsin K, an overexpressed enzyme in osteolytic microenvironments. By using doxorubicin as a model drug, Pep-b-PEG-b-PTM showed 7.5 ± 0.5 % and 22.7 ± 1.5% for drug loading content and drug loading efficiency, respectively. More importantly, a unique characteristic of on-demand charge-conversional behaviour in a cathepsin K-rich condition led to enhanced cellular uptake of the nanotherapeutics, as demonstrated by confocal laser scanning microscopy. Enhanced tumor inhibition was observed in 5TGM1 MM cells when nanoparticles were pre-treated with 150 nM cathepsin K, demonstrating enzyme-triggered improved therapy. Efficacy of free DOX or DOX-loaded NPs in 5TGM1 mice bearing myeloma was further preliminarily tested. 5TGM1 mice bearing myeloma were established through injection of 5TGM1 cells (1 × 10⁶ cells in 100 μL PBS) via tail vein, and tumor was allowed to grow for a week before initiating treatment study. Mice (n=3) were injected twice weekly with different therapeutic formulations at equivalent DOX dose (0.75 mg/Kg) or PBS. Tumor burden in the mice was monitored by ELISA measurements of serum IgG2b. Drug-loaded nanoparticles from Pep-b-PEG-b-PTMC were more efficacious in terms of mice survival rate and tumor inhibition when compared to the groups with non-targeted nanoparticles from mPEG-PTMC, free DOX or PBS controls. This improved drug efficacy may be attributed to more selective delivery of DOX to bone metastatic tissues and/or responsiveness of the nanoparticles to cathepsin K, thus improving tumor uptake of DOX, enhancing therapeutic efficacy in terms of tumor reduction as well as MM mouse survival.

The promising results from this study may prompt the development of bone-targeted, enzyme-triggered drug delivery systems to improve their affinity to skeletal tissues, enhance selectivity for osteolytic regions and improve efficacy of anti-cancer agents, thus facilitating the development of effective nanotherapeutic modalities for multiple myeloma.

No relevant conflicts of interest to declare.