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Purpose Radium-223 dichloride (radium-223, Xofigo®), a targeted alpha therapy, is currently used for the treatment of patients with castration-resistant prostate cancer (CRPC) with bone metastases. This study examines the mode-of-action and antitumor efficacy of radium-223 in two prostate cancer xenograft models. Experimental Design Mice bearing intratibial LNCaP or LuCaP 58 tumors were randomized to groups (n = 12–17) based on lesion grade and/or serum PSA level and administered with radium-223 (300 kBq/kg) or vehicle, twice at 4-week intervals. X-rays and serum samples were obtained biweekly. Soft tissue tumors were observed macroscopically at sacrifice. Tibiae were analyzed by gamma counter, micro-CT, autoradiography and histology. Results Radium-223 inhibited tumor-induced osteoblastic bone growth and protected normal bone architecture leading to reduced bone volume in LNCaP and abiraterone-resistant LuCaP 58 models. Furthermore, radium-223 resulted in lower PSA values and reduced total tissue and tumor areas, indicating that treatment constrains prostate cancer growth in bone. In addition, radium-223 suppressed abnormal bone metabolic activity as evidenced by decreased number of osteoblasts and osteoclasts and reduced level of the bone formation marker PINP. Mode-of-action studies revealed that radium-223 was deposited in the intratumoral bone matrix. DNA double-strand breaks were induced in cancer cells within 24 hours after radium-223 treatment and PSA levels were significantly lower 72 hours post treatment providing further evidence of the anti-tumor effects. Conclusion Taken together, radium-223 therapy exhibits a dual targeting mode-of-action that induces tumor cell death and suppresses tumor-induced pathological bone formation in tumor microenvironment in osseous CRPC growth in mice.

This study is based on a hypothesis that overexpression of an osteoclast enzyme, cathepsin K, causes an imbalance in bone remodeling toward bone loss. The hypothesis was tested in transgenic (TG) mice harboring additional copies of the murine cathepsin K gene (Ctsk) identifiable by a silent mutation engineered into the construct. For this study, three TG mouse lines harboring 3-25 copies of the transgene were selected. Tissue specificity of transgene expression was determined by Northern analysis, which revealed up to 6-fold increases in the levels of cathepsin K messenger RNA (

Objectives: To study the expression of cysteine proteinases, particularly cathepsin K, and their extracellular inhibitor cystatin C in articular cartilage of transgenic Del1 mice which harbour a short deletion mutation in a type II collagen transgene and are predisposed to early onset osteoarthritis. Methods: Northern analysis was used to measure mRNA levels of cathepsins B, H, K, L, and S, and cystatin C in total RNA extracted from knee joints of Del1 mice, using their non-transgenic litter mates as controls. Immunohistochemistry and morphometry was used to study the distribution of cathepsin K and cystatin C in the knee joints. Results: Up regulation of cathepsin K mRNA expression was seen in the knee joints of transgenic Del1 mice at the onset of cartilage degeneration. Cathepsin K was found near sites of matrix destruction in articular chondrocytes, particularly in clusters of proliferating cells, and in calcified cartilaginous matrix. In intact articular cartilage of control animals, cathepsin K was only seen in a small number of chondrocytes. Upon aging, control animals also developed osteoarthritis, which was accompanied by increased cathepsin K expression. Cystatin C was mostly localised in and around chondrocytes located in calcified cartilage, with no obvious association with the onset of cartilage degeneration. Conclusion: The temporospatial distribution of cathepsin K in osteoarthritic cartilage suggests a role for this enzyme in the pathogenesis of osteoarthritis. Because cathepsin K can digest cartilage matrix components it may contribute to the development of osteoarthritic lesions. These data may provide new clues for the development of treatments aimed at preventing cartilage degeneration.

Cysteine cathepsins are a large family of proteolytic enzymes active at acidic pH as found in lysosomes. Since its discovery in 1990's, cathepsin K has been shown to be a key enzyme in osteoclastic bone resorption through its activity in the resorption lacuna. Although characteristic to osteoclasts, the expression of cathepsin K has also been observed at other sites in skeleton. Several recent observations have demonstrated up-regulation of cathepsin K in osteoarthritic cartilage and inflamed synovial tissue. As cathepsin K is one of the few extracellular proteolytic enzymes capable of degrading native fibrillar collagen, it may play an important role in the progressive destruction of articular cartilage both in osteoarthritis and in inflammatory arthritides. Also transgenic mouse models have provided evidence supporting the important role of cathepsin K in both groups of arthritides. The aim of this chapter is to review the accumulating evidence for the role of cathepsin K in degradation of articular cartilage regardless of its pathogenic background, and to discuss the potential efficacy of cathepsin K inhibitors to slow down or prevent articular cartilage degradation.