Abstract 4030: In-vivo evaluation of the anti-metastatic efficacy of small molecule Cathepsin L inhibitor KGP94 in a prostate cancer model

It is estimated that ∼85% of patients with advanced prostate cancer develop skeletal metastases; an occurrence that results in a substantial reduction in the quality of life and a drastic worsening of the prognosis. The development of novel therapeutic strategies that impair the metastatic process and associated skeletal adversities is therefore critical to improving prostate cancer patient survival. Tumor cells rely on proteolytic enzymes for successful establishment of metastases. Cathepsin L (CTSL) is a lysosomal cysteine protease that is up regulated in a wide range of human cancers including prostate cancer. The elevated secretion of CTSL by aggressively metastasizing tumor cells plays a key role in metastatic spread of tumor cells through proteolytic degradation of extracellular matrix and basement membrane components. In addition, tumor secreted cytokines have been shown to increase osteoclastic secretion of CTSL to promote bone resorption. Intervention strategies targeting CTSL may therefore serve a dual purpose by inhibiting both metastatic dissemination as well as bone resorption. The goal of present studies was to evaluate the anti-metastatic and anti-bone resorptive efficacy of CTSL inhibition using the small molecule CTSL inhibitor 3-bromophenyl-3-hydroxyphenyl-ketone thiosemicarbazone (KGP94). KGP94 treatment significantly impaired the metastatic phenotype of highly metastatic prostate cancer PC-3ML cells. Treatment with 25µM KGP94 decreased the migratory and invasive capacities of PC-3ML cells by 78 and 53% respectively. To validate the anti-metastatic efficacy of KGP94 in-vivo, luciferase labelled PC-3ML cells were injected into the left ventricle of male nu/nu mice followed by daily treatment with 20mg/kg KGP94. Metastatic progression was monitored by weekly bioluminescence imaging using firefly luciferase reporter system. KGP94 treatment led to a significant reduction in metastatic tumor burden (65% reduction) and a reduction in metastatic incidence. Histological assessment of bones further confirmed the reduction in metastatic burden upon KGP94 treatment. CTSL inhibition also led to a significant increase in overall survival of tumor bearing mice (p=0.01). In vitro assessment of anti-resorptive function of KGP94 was performed using osteoclastogenesis and pit formation assays. KGP94 treatment of RAW264.7 osteoclast precursor cells led to a significant decrease (72%) in osteoclast formation as demonstrated by a decrease in the number of TRAP+ multinucleate cells. KGP94 also inhibited the bone resorptive activity of osteoclasts as indicated by a significant reduction in the number of resorptive pits formed by osteoclasts on bone slices. In conclusion, our findings suggest that CTSL inhibition using small molecule KGP94 can significantly impair metastatic dissemination of prostate cancer cells and alleviate bone resorption. Citation Format: Dhivya R. Sudhan, Dietmar W. Siemann. In-vivo evaluation of the anti-metastatic efficacy of small molecule Cathepsin L inhibitor KGP94 in a prostate cancer model. [abstract]. In: Proceedings of the 105th Annual Meeting of the American Association for Cancer Research; 2014 Apr 5-9; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2014;74(19 Suppl):Abstract nr 4030. doi:10.1158/1538-7445.AM2014-4030