A Cathepsin K Inhibitor Reduces Breast Cancer–Induced Osteolysis and Skeletal Tumor Burden

Abstract

Osteoclasts mediate bone destruction in breast cancer skeletal metastases. Cathepsin K is a proteinase that is secreted by osteoclasts and degrades bone. Here, immunohistochemistry revealed that cathepsin K was expressed not only by osteoclasts but also by breast cancer cells that metastasize to bone. Following intratibial injection with cathepsin Kexpressing human BT474 breast cancer cells, tumor-bearing mice treated with a clinical dosing regimen of cathepsin K inhibitor (CKI; 50 mg/kg, twice daily) had osteolytic lesions that were 79% smaller than those of tumor-bearing mice treated with the vehicle. The effect of CKI was also studied in a mouse model in which the i.v. inoculation of human B02 breast cancer cells expressing cathepsin K leads to bone metastasis formation. Drug administration was started before (preventive protocol) or after (treatment protocol) the occurrence of osteolytic lesions. In treatment protocols, CKI (50 mg/kg, twice daily) or a single clinical dose of 100 Mg/kg zoledronic acid (osteoclast inhibitor) reduced the progression of osteolytic lesions by 59% to 66%. CKI therapy also reduced skeletal tumor burden by 62% compared with vehicle, whereas zoledronic acid did not decrease the tumor burden. The efficacy of CKI at inhibiting skeletal tumor burden was similar in the treatment and preventive protocols. By contrast, CKI did not block the growth of s.c. B02 tumor xenografts in animals. Thus, CKI may render the bone a less favorable microenvironment for tumor growth by inhibiting bone resorption. These findings raise the possibility that cathepsin K could be a therapeutic target for the treatment of bone metastases. [Cancer Res 2007;67(20):9894-902]

Efficacy of a Cathepsin K Inhibitor in a Preclinical Model for Prevention and Treatment of Breast Cancer Bone Metastasis

“…Because of species differences between the binding sites of human versus rodent CatK, there are usually significant reductions in potency against the rodent CatK with the inhibitors developed against the human enzyme. Even if the human osteoporosis dose of AFG-495 has not been determined, it was dosed via intraperitoneal injection twice daily to achieve sufficient high drug exposure in mice (15). Interestingly in that study, AFG-495 at 50 mg/kg, i.p., twice daily for 39 days reduced tumor burden by 62%, whereas a single high bolus subcutaneous injection of ZOL 100 mg/kg did not inhibit skeletal tumor burden (15).…”

“…Even if the human osteoporosis dose of AFG-495 has not been determined, it was dosed via intraperitoneal injection twice daily to achieve sufficient high drug exposure in mice (15). Interestingly in that study, AFG-495 at 50 mg/kg, i.p., twice daily for 39 days reduced tumor burden by 62%, whereas a single high bolus subcutaneous injection of ZOL 100 mg/kg did not inhibit skeletal tumor burden (15). Of note, Le Gall and colleagues also confirmed that AFG-495 did not inhibit subcutaneous growth of breast cancer B02 tumor xenografts in nude mice at a dosing regimen that was demonstrated to inhibit skeletal tumor burden, demonstrating that CatK inhibition does not affect breast cancer cell proliferation (15,24).…”

“…Human MDA-MB-231 breast carcinoma cells (ATCC) were maintained in RPMI-1640 medium containing 10% heat-inactivated FBS, 2 mmol/L glutamine, 100 U/mL penicillin-G sodium, 100 mg/mL streptomycin sulfate, 0.25 mg/mL amphotericin B, and 25 mg/mL gentamicin at 37 C and 5% CO 2 . The cell proliferation and invasion assays were done essentially as previously described (15).…”

“…Efficacy of L-235 in inhibiting the breast cancer MDA-MB-231 cell invasion in vitro was evaluated essentially as previously described (15). This CatKi potently inhibited the breast cancer Matrigel invasion with an IC 50 of 3.2 nmol/L (data not shown) as compared with its activity in blocking osteoclastic bone resorption (IC 50 ¼ 5 nmol/L; ref.…”

“…Expression of CatK mRNA or protein has been reported in breast and prostate cancers (13,14). Le Gall and colleagues previously reported the expression of CatK in breast tumors and its expression is further increased in breast cancer cells that metastasize to bone (15). In that study, a preclinical model of skeletal metastasis utilized the intratibially implanted CatK-expressing human BT474 breast cancer cells in nude mice, and intraperitoneal dosing of the CatK inhibitor (CatKi) AFG-495 was shown to reduce osteolytic lesions and skeletal tumor burden (15).…”